Outdoor MIMO Wireless Networks

Outdoor MIMO Wireless Networks
Version 1.1
Chuck Lukaszewski
Jerrod Howard
Eric Johnson
Marcus Wehmeyer

Outdoor MIMO Wireless Networks Validated Reference Design
Aruba Networks, Inc. 2
Copyright
© Copyright 2015 Hewlett Packard Enterprise Development LP.
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Aruba Networks, Inc. Table of Contents | 3
Table of Contents
Chapter 1: Introduction 9
About the Outdoor MIMO Wireless Networks VRD 9
Outdoor Deployment Types 9
Campus Extension 10
Outdoor Mesh with AirMesh 11
Aruba Reference Architectures 12
Outdoor Wireless Integrators 13
Assumptions 13
Reference Documents 14
Icons Used in this Guide 15
Chapter 2: Outdoor Networking Deployment Methodology 17
Network Discovery 17
Preliminary (High-Level) System Design 18
Site Acquisition 18
Final (Low Level) System Design 19
Configuration and Installation 19
Coverage and Throughput Verification 20
Final Network Acceptance 20
Chapter 3: Outdoor Access Points and Multichannel Backhaul 21
Choosing the Deployment Type 21
Understanding Single-Channel and Multi-channel Backhaul 21
The Evolution of Mesh Technology 22
Comparing End-to-End Performance 24
ArubaOS or Instant AP for Campus Extension 26
AP-270 Family (Campus Extension) AP 26
AirMesh APs for Outdoor Mesh Networks 27
MSR4000 Quad-Radio Mesh Router 27
MSR2000 Dual-Radio Wireless Mesh Router 28
MST200 Single-Radio Wireless Mesh Router 29
AP Model Summary 30
Chapter 4: Outdoor Antennas and RF Coverage Strategies 31
Antenna Beamwidth, Pattern, and Gain 31
Omnidirectional Antenna Types 33
Directional Antenna Types 34

Effect of Mechanical Downtilt on Directional Antenna Coverage 35
Directional Antenna Conclusions 38
RF Coverage Strategies for Outdoor WLANs 39
Understanding Side and Overhead Coverage 39
Sparse Side Coverage 42
Dense Side Coverage 43
Dense Overhead Coverage 44
Selecting an Aruba Outdoor Antenna 45
Understanding Aruba MIMO Antenna Part Numbers 45
Access Layer Antennas 46
Backhaul Layer Antennas 49
Chapter 5: 802.11n and 802.11ac Multiple-In and Multiple-Out 52
Ratification and Compatibility 52
Understanding MIMO 52
802.11n and 802.11ac Spatial Streams 53
Other 802.11n and 802.11ac Technologies to Increase Throughput 53
40 MHz and 80 MHz Channels 53
Improved OFDM Subcarriers 55
Space Time Block Coding and Maximal Ratio Combining 56
Short Guard Interval 56
Understanding MAC Layer Improvements 56
A-MSDU 57
A-MPDU 57
Block Acknowledgement 58
802.11ac Transmit Beamforming (TxBF) 58
802.11 Terminology 59
Transmit, Receive, and Spatial Stream Designation 59
Modulation and Coding Scheme Index 60
2.4 and 5 GHz Support 61
Backward Compatibility 62
Maximizing Rate vs. Range with MIMO Outdoors 62
Direct vs. Indirect Multipath 63
Correlation and Decorrelation 64
Polarization 64
Leveraging Polarization Diversity to Improve Decorrelation 65
Chapter 6: AP Selection for Common Outdoor Topologies 66
Single-Radio Point-to-Point Bridge: MST200 66
Single-Radio Leaf Node: MST200 66

Dual-Radio Client Access: AP-270 Series and MSR2000 67
Single Hop Point-to-Point: AP-270 Series or MSR2000 67
Multi-hop Linear Mesh: MSR2000 68
Parallel Point-to-Multipoint: MSR2000 or MSR4000 High 68
Capacity Mesh Core: MSR4000 69
Remote Thin AP Endpoints Overlaid on AirMesh 70
Chapter 7: Aruba Software Technologies 71
Choosing an Outdoor Operating System for Campus Extension 71
AirMesh for Outdoor Mesh Networking 72
Radio Frequency Management 74
Adaptive Wireless Routing 76
Path Distance Factor 84
Active Video Transport
Virtual Private LAN over Mesh 91
MobileMatrix and Seamless Session Persistent Roaming 94
Chapter 8: Planning the Access Layer 99
Discovery 99
Define the Coverage Footprint 100
Identify Siting Constraints 101
Identify Quality-of-Service or Special Service Level Agreement Zones 101
Specify Key Network Design Parameters 101
Capacity Planning 102
Offered Loads of Typical Network Services 102
Bandwidth vs. Throughput 102
Client Throughput Requirements 103
Oversubscription Ratio 104
Strategic Throughput Reservation 104
Determining Cell Size 105
Matching Client and AP Power 105
Free-Space RF Propagation 106
Effect of Path Loss on Data Rate and Throughput 107
Estimate Path Losses 108
Link Budget Calculation and Link Balance 109
Path Loss Due to Cumulative RF Absorption 110
Path Loss Modeling for Indoor Coverage by Outdoor APs 112

Summary 113
Using the Aruba 3D Outdoor RF Planner 113
Finished RF Plan Examples 114
Chapter 9: Planning the Mesh Backhaul Layer 117
Identify Portal Candidates 117
Choose RF Backhaul Topology 118
Serial Point-to-Point Connections 118
Parallel Point-to-Multipoint Connections 118
Full Mesh in a Multi-Gateway Design 119
Choose Capacity Injection Topology 120
End-Fed Injection Topologies 120
Center-Fed Injection Topologies 122
Hybrid Topologies 123
Maximum Hop Count 124
Maximum Number of Children 124
Ratio of Mesh Portals to Mesh Points 124
Capacity Planning 125
Determine Number of Usable Backhaul Channels 125
Compute Ingress Load 127
Compute Egress Load 127
Estimate Bandwidth of Individual Mesh Links 128
Mesh Capacity Math for Single Channel Backhaul Systems 129
Model End-to-End Traffic Flows 131
RF Design 132
Planning Mesh Layers with the Aruba 3D Outdoor RF Planner 133
Chapter 10: Site Surveys for Large Outdoor Networks 135
Create a “Soft” RF Plan 135
General Considerations for Choosing Mounting Assets 137
Identifying RF Absorbers, Reflectors, and Interferers 138
Selecting Mounting Locations for Mesh Points 138
Performing the Survey 139
Choosing a Pole 140
Evaluating Pole Power From the Ground 141
Reading Pole Tags 142
Measuring Pole Dimensions 142
Radio LOS Path Planning 144
Antenna Height 144

Surveys for Mesh Portal Mounting Locations 146
Wired Backhaul Assessment 146
Antenna Position and Orientation 146
Radio Interference 147
Weather Conditions 147
Ethernet Cabling 147
Grounding 148
Civils Approvals 148
Final Network Design 149
Best Practices for Conducting Outdoor Surveys 149
Personal Safety & Security 149
Building a Complete Outdoor Survey Kit 150
Chapter 11: IP Planning for Aruba AirMesh 155
Configure a Router ID 155
Mesh Backhaul Links 155
Access Links and Client Devices 155
Wired Network Ethernet Link Parameters 155
IP Addressing and Networking 155
Chapter 12: Installation, Validation, and Optimization 157
MeshConfig 157
Staffing Expectations 159
Aruba Outdoor AP Antenna Weatherproofing 160
Installing Antennas 160
WeatherproofingConnections 160
RF Coverage Validation 169
Reconciling Drive Test Data with Predictive Models 170
Mesh Network Optimization 170
Appendix A: Allowed Wi-Fi Channels 171
2.4 GHz Band 171
4.9 GHz Band 171
5 GHz Band 172
Appendix B: DFS Operation 174
Behavior of 5 GHz DFS Radios in the Presence of Radar 174
Appendix C: Campus Extension Example 176

Appendix D: Intermodal Transportation Example 179
Application Types 179
Dense Overhead Coverage Strategy 180
Sparse Side Coverage Strategy 181
Appendix E: Terminal Doppler Weather Radars 182
Appendix F: Aruba Contact Information 186
Contacting Aruba Networks 186

Aruba Networks, Inc. Introduction | 9
Chapter 1: Introduction
This Solution Guide is designed to help customers understand the Aruba system architecture and the
individual components needed to deliver reliable, high-capacity outdoor networks using 802.11n and
802.11ac with multiple-in and multiple-out (MIMO) radios.
About the Outdoor MIMO Wireless Networks VRD
Aruba has extensive experience designing complex outdoor WLAN solutions for applications like
stadiums, outdoor transportation terminals, oil and gas facilities, municipalities, and large campus
environments. Aruba outdoor solutions meet the needs of emerging applications by increasing the
speed of each connection. This increase in speed is achieved using MIMO-based radio techniques
and mesh routing for very large outdoor areas.
This guide describes these main points:
 The lifecycle of an outdoor wireless network deployment
 Typical basic processes and tools that are used in outdoor wireless networking
 Products and technologies that meet the needs of a wireless network operator
 MIMO antenna selection and placement for maximum capacity
 Design recommendations for common deployment scenarios
 Regulatory rules that must be incorporated into all outdoor RF designs
Outdoor Deployment Types
This guide addresses two distinct types of deployments, each of which has its own technical
requirements, coverage strategies, and implementation methodologies:
 CampusextensionwithAP-270Series APs:Customersthathavestandardized ona
controller- basedthinAParchitecturefor indoor coverage often wanttoextendtherole-based
access control(RBAC)securitymodeltotheoutdoorenvironmentsontheirproperties.
 Outdoor wireless mesh with AirMesh: Some customers operate a wireless network that is
almostexclusivelyoutdoors.Indoorconnectionscanbeprovidedfromtheoutdoornetwork,
usuallyviaremotebridgelinksorspecial-purposeindoorrepeaters.
Aruba offers a choice of two different mesh-capable operating systems. The best choice typically
depends on which deployment type best fits the intended outdoor wireless network.
Both types of deployment use:
 Mesh portals: Connected to the high-speed wired network (also known as wired gateways).
 Mesh points: Unwired radios that connect to mesh portals using an RF backhaul link. Mesh
points are fully capable of multihopping over long distances.

Campus Extension
ArubaOS or Instant outdoor solutions extend secure indoor enterprise coverage to outdoor areas.
Some common examples of these applications include:
 Campus coverage for universities, hospitals, and large enterprises
 Manufacturing plants
 Industrial yards
 Ports, rail yards, and airports
 Stadiums, arenas, and other large public venues for Internet access or 3G offload
In these environments, controller-based or instant wireless LANs (WLANs) are generally running
indoors using a wired backbone to connect thin APs to an Aruba controller or the Instant Virtual
Controller (VC). For example, in the case of an intermodal transportation facility or manufacturing
plant, he business offices are often either using or migrating to a controller-based architecture. For
this reason, IT departments want to have the same security model for outdoor facilities. Also,
consistent equipment and configuration procedures can reduce IT operating costs.
From a hardware perspective, a campus extension network generally requires a rugged version of the
dual-radio access point (AP) that is used indoors. A campus extension network is illustrated in Figure
1. In this case, we assume an existing indoor ArubaOS or Instant WLAN, which is extended out via
mesh to cover the outdoor portions of the facility.
Figure 1 Campus extension network (Container Port)
For campus environments, both radios are often used to provide client access, with occasional short
mesh hops to connect remote buildings or provide spot coverage from utility poles nearby. Mounting
assets tend to be buildings; consequently, AP power is primarily power-over-Ethernet (PoE). PoE
leverages the existing indoor infrastructure and makes sense given the limited number of AC- or DC-
powered nodes.

Generally, campus extension networks should use ArubaOS or Instant, with outdoor APs managed
by the same controller(s) or Instant VC that supports the indoor network. ArubaOS and Instant is an
“overlay” network, which assumes that a reliable wired LAN or WAN interconnects the APs with their
controller or Instant VC.
Outdoor Mesh with AirMesh
When you consider a green-field outdoor wireless network, as shown in Figure 2, the driving
application may or may not include some indoor coverage. But these large area networks use mesh
routing technology instead of extending an indoor controller-based architecture.
Figure 2 Green-field outdoor wireless network topology (City Grid)
In the long-term, multiple applications and new users must be supported on these outdoor networks.
During the planning stage, consider how network capacity can be increased in the future. Examples of
common green-field wireless networks include:
 Municipal Wi-Fi® for video surveillance and public/private network access
 Mines, oil fields, and other large, outdoor, industrial facilities
 Emerging smart-grid applications
In these green-field wireless networks, the outdoor mesh network provides the backbone for delivering
all applications and services. These networks can cover extremely large areas, measured in square
kilometers (km2) or square miles (mi2). Any viable mounting asset in the vicinity of a desired mesh
node location must be supported. Therefore, a wide variety of single-, dual-, and quad-band radio
options are necessary to provide the wireless architect with maximum flexibility. AC- and DC-power
dominates outdoor mesh networks, with some PoE at mesh portals. The 4.9 GHz licensed band can
be used in countries that allow it.
Outdoor mesh networks should generally use Aruba AirMesh™ on standalone Multi-Service Router
(MSR) routers. MSR routers provide LAN-like layer 3 (L3) and layer 2 (L2) traffic forwarding across
Ethernet
DSL

Applications
Base Designs
Foundation
links of varying quality and availability. These routers also provide a range of other features to
maximize the performance of IP network services over a large area.
Aruba Reference Architectures
The Aruba Reference Design series is a collection of technology deployment guides that include
descriptions of Aruba technology, recommendations for product selections, network design decisions,
configuration procedures, and best practices for deployment. Together these guides comprise a
reference model for understanding Aruba technology and designs for common customer deployment
scenarios. Each Aruba VRD network design has been constructed in a lab environment and
thoroughly tested by Aruba engineers. Our customers use these proven designs to rapidly deploy
Aruba solutions in production with the assurance that they will perform and scale as expected.
The VRD series focuses on particular aspects of Aruba technologies and deployment models.
Together the guides provide a structured framework to understand and deploy Aruba wireless LANs
(WLANs). The VRD series has four types of guides:
 Foundation: These guides explain the core technologies of an Aruba WLAN. The guides also
describe different aspects of planning, operation, and troubleshooting deployments. This
Outdoor MIMO Wireless Networks VRD falls into the foundation category.
 Base Design: These guides describe the most common deployment models, recommendations,
and configurations.
 Applications: These guides are built on the base designs. These guides deliver specific
information that is relevant to deploying particular applications such as voice, video, or outdoor
campus extension.
 Specialty Deployments: These guides involve deployments in conditions that differ significantly
from the common base design deployment models, such as high-density WLAN deployments.
Specialty
Deployments
Figure 3 VRD core technologies
arun_0334

Outdoor Wireless Integrators
Outdoor wireless networks are the most labor-intensive and challenging type of WLAN to design and
deploy. Many different disciplines and trades must come together for a successful outdoor network,
including:
 Project management
 RF engineering
 LAN and IP network engineering
 Construction and fabrication
 Tower erection, climbing, and rigging
 Grounding and electrical safety
 AC, DC, battery-assist, and solar power systems
 Municipal attachment rights agreements and city council testimony
Few IT departments have access to experts in all of these areas. Therefore, Aruba strongly
recommends that every customer that intends to deploy an outdoor system of any size engage an
experienced outdoor wireless network integrator. These companies can provide any type of resource
required for a successful project, and can help navigate the many issues that inevitably come up
during an outdoor project.
Your local Aruba account manager can help direct you to a qualified outdoor integrator. You can also
explore the Aruba ServiceEdge™ provider network, which includes many integrators who specialize in
outdoor work: http://www.arubanetworks.com/support-services/professional-services/
Assumptions
In this guide we make several assumptions about the level of experience of a network planner. We
provide references to some basic material, but we make the following assumptions:
 Reader is familiar with unlicensed band plans.
 Reader understands RF link budget planning in outdoor environments.
 Reader understands MIMO fundamentals.
 Reader is experienced with physical installation of outdoor radio equipment.

Reference Documents
The following documents are recommended for further reading on 802.11n, MIMO, and outdoor
wireless networking technologies.
 ArubaNetwork's802.11acWhitePaper
 Aruba Networks' 802.11ac Migration Guide
 Designed for Speed: Network Infrastructure for an 802.11n World, Peter Thornycroft, Aruba,
2008
 Next Generation Wireless LANs: Throughput, Robustness, and Reliability in 802.11n, Eldad
PerahiaandRobertStacey,CambridgeUniversityPress,2008
 Hardware Installation Guides - Aruba AP-270 Series and MSR Outdoor APs
 Certified Wireless Network Administrator (CWNA) Study Guide, David A. Westcott & David
D.Coleman, John Wiley & Sons, 2006
 ArubaNetworks3DOutdoorRFPlanner
 ArubaAntennaMatrix
The following reference materials and discussion groups are recommended for learning about Aruba
products and solutions:
 For information on Aruba Mobility Controllers and deployment models, see the Aruba Mobility
ControllersandDeploymentModelsValidatedReferenceDesign,availableontheAruba
website at http://community.arubanetworks.com/t5/Validated-Reference-Design/tkb-p/Aruba-
VRDs.
 The complete suite of Aruba technical documentation is available for download from theAruba
support site. These documents present complete, detailed feature and functionality explanations
beyond the scope of the VRD series. The Aruba support site is located at
http://support.arubanetworks.com.
 For more training on Aruba products or to learn about Aruba certifications, visit the Aruba
trainingand certificationpageonour website. Thispage containslinksto class descriptions,
calendars, and test descriptions: http://www.arubanetworks.com/support-services/training-
services/
 Aruba hosts a user forum site and user meetings called Airheads. The forum contains
discussionsofdeployments,products,andtroubleshootingtips.AirheadsOnlineisan
invaluableresourcethatallowsnetworkadministratorstointeractwitheachother andAruba
experts.Announcements forAirheadsinpersonmeetingsarealsoavailable onthesite:
https://community.arubanetworks.com/
 The VRDseriesassumes a workingknowledge of Wi-Fi®,and more specifically dependent
AP, orcontrollerbased,architectures.Formoreinformationaboutwirelesstechnology
fundamentals,visittheCertifiedWirelessNetworkProfessional(CWNP)siteat
http://www.cwnp.com/
 For 802.11ac information, read Aruba Network's 802.11ac In-Depth white paper
(http://www.arubanetworks.com/pdf/technology/whitepapers/WP_80211acInDepth.pdf) and the
802.11ac Wave 1 Migration Guide
(http://www.arubanetworks.com/pdf/technology/MG_80211ac.pdf )

Icons Used in this Guide
Figure 4 shows the icons that are used in this guide to represent various components of the system.
MST200
(logical)
MSR 2K
or AP-270
series
MSR4000
(logical) AP with
camera & light
RAP5 Wired AP
MUX
MST200
(physical)
MSR2000
(physical)
Switch S3500
wired AP
Aruba
controller AirWave
server
Directional
antenna
Attenuator
Tunnels Mobile phone Smart phone Video camera
Server
Residence
Building
Surveillance
center
Laptop
Network cloud Router
Figure 4 VRD icon set
arun_0445

Aruba Networks, Inc. Outdoor Networking Deployment Methodology | 17
Chapter 2: Outdoor Networking Deployment Methodology
For many existing Aruba customers, an outdoor network is an extension of their indoor network that
delivers coverage across a large enterprise or hospital campus. After these customers select their
mounting locations, installation is like adding coverage indoors; select the right APs and antennas,
and make sure the controller supports the required licenses. For other customers who want to build
larger outdoor Wi-Fi networks, mesh radios are used and the selection of mounting locations
becomes more complex. This chapter describes a general methodology that is common to campus
extension and outdoor-mesh networks.
Whether you are extending an indoor network or building a large outdoor mesh network, the planning
process generally includes the steps displayed in Figure 5 to create a scalable, manageable network
with the required coverage and capacity:
Figure 5 Outdoor network deployment process
These steps can be completed quickly when an Aruba network is extended because customers are
familiar with existing locations for outdoor antennas and radios. However, large outdoor networks
often require very detailed plans and may require civil approvals and permits for mounting locations
that are not owned by the network operator.
Network Discovery
Like all IT projects, an outdoor wireless network begins with a discovery process. An outdoor discovery
includes these components:
 Map of the expected coverage area
 Statement of desired operating capacity
 List of potential mounting assets under the control of the network operator
 Primary network users, in order of priority
 Primary applications, in order of priority
 Desired project schedule, broken into relevant phases
 Available budget for initial construction and ongoing operation
Existing Aruba customers who plan campus extensions often can provide accurate mounting location
and terrain information that can be used during the outdoor planning process. These outdoor networks
may cover limited areas or be simple point-to-point solutions to bridge multiple buildings or locations
Step 1
Network
discover
y
Step 2
Preliminary
system
design
Step 3 Step 4
Site Final
Acquisition system
design
Step 5
Installation
and
configuration
Step 6
Coverage and
throughput
verification
Step 7
Final
network
acceptance
arun_0423

together. For these customers, the locations of radios identified in the preliminary system design and
the final system design can be very close.
For large outdoor mesh networks, the objective of the discovery step is to deliver a realistic overview of
the whole network, by outlining wired and wireless resources, which provides the foundation for more
meaningful planning during later steps.
Preliminary (High-Level) System Design
The preliminary system design establishes clear coverage and capacity expectations for each outdoor
area. After the high-level coverage area is identified, the area should be broken into smaller logical
sections of about 1-2 km2 or mi2 for further detailed planning. A preliminary design always includes
the initial site survey and an RF spectrum analysis. Depending on the size of the area to be covered,
these two tasks require the largest labor component of the preliminary design.
Large outdoor mesh networks rely on cells of coverage that communicate using layer 3 mesh routing.
First identify the number of active users that can be expected in each area and the peak bandwidth the
network is expected to deliver. Then use the following key metrics for further planning:
 Number of cells per kilometer or square mile
 The ratio of mesh points (unwired radios) to mesh portals (wired radios)
 For each area, identify mounting assets with access to usable power
The preliminary system design generally includes these components:
 Site survey and spectrum analysis report
 First draft of the RF design model for the network, possibly including IP design
 Preliminary bill of materials
 Proposed mounting locations and wired network access locations
 Radio propagation models and antenna selections for each mounting location
 Testing tools needed to verify coverage and capacity
 Preliminary budget estimate for integration and construction services
Site Acquisition
Site acquisition often involves two types of radio mounting assets:
 Assets that are owned or under the control of the network operator, like buildings
 Assets that may require permits and payment to a third party, like street lights
For example, a university that wants to expand the network to cover outdoor common areas can
generally assume that they can mount radios on the buildings and streetlamps within the campus. On
the other hand, if they prefer to mount radios on third-party building rooftops or city-owned lights, then
negotiations and timing can take longer. Site surveys that include these types of locations, should
identify alternate mounting locations in case the preferred sites are unavailable (which can be quite
common).
Each mounting site must support the weight of radios and any wind load, and have access to
continuous, unswitched electrical power. Each radio location must also have a suitable grounding

path. The antennas and mounting methods for each site are selected to provide the desired client
coverage and to complete a reliable RF path to other mesh points along the path to the mesh portal
and the wired network.
Final (Low Level) System Design
The final design should provide a detailed RF design and include detailed mounting location
information, such as GPS coordinates step-by-step cable pathway instructions to help with radio
installation planning. The final design must also include detailed IP addressing information and other
back-end system interfaces that may be required, such as captive portals for public networks. For
Aruba customers, outdoor networks are often simple extensions of the current role-based access
controls. However, new multiuse outdoor networks may require implementation of new authentication
models that should be carefully considered when planning the network.
The final design typically includes this information:
 Radio specifications for each validated mounting location
 User device characterization for network planning
 Clear coverage and capacity expectations by area
 Mesh portal radio locations and wired network connections
 Mesh point mounting locations and electrical powering plan
 RF frequency plan if required
 IP network design for the mesh network, wired network and back-office equipment
 An agreed-upon method of testing and validating coverage and capacity
 Deployment-related services and other resources
Configuration and Installation
To configure and install each radio, follow the steps in the hardware installation guides, as identified
in the final system design. It's a good best practice to configure all equipment on the bench or in the
lab, and test for general operations, before taking out for physical installation, as bench and lab time
are significantly less costly than truck and installation time.
As equipment is installed, carefully record the GPS coordinates of each radio and document these
for later use. Take pictures of each installation from multiple angles because each location may not
be visited for long periods of time. Aruba recommends labelling each cable and the port to which it is
attached. Sometimes it is necessary to affix customer-specific labels that identify the network owner
or operator or other asset tracking information. This information is invaluable for troubleshooting
elevated radios.
To simplify installation in the field, always preconfigure each remote radio. Be sure to follow the IP
network design to include the mesh radios and back-office equipment.
Aruba strongly recommends that only experienced outdoor wireless integrators install outdoor radio
equipment. A licensed electrician must complete all radio grounding, and must install low-voltage or
high-voltage power systems required by the network.

Coverage and Throughput Verification
While the network is being installed, it is common to measure coverage periodically using GPS-
enabled tools such as Air Magnet Survey Professional or Ekahau SiteSurvey Professional. When an
entire area or subarea is completely installed, drive tests are performed. Drive test results show “heat
maps” of the signal strength, which document the level of coverage. However, common best practice
is to measure the Receive Signal Strength (RSSI) using independent third-party tools. Doing so
ensures coverage in the required bands:
 2.4GHz802.11b/g/n/ac
 5GHz802.11a/n/ac
 Municipaluseofthe4.9GHzbands(optional)
Compare these results with the original system design to identify coverage gaps or holes. Address
these gaps by identifying additional mounting locations and adding equipment and installation
resources from a pool that is reserved for this purpose.
RF signal strength heatmaps only tell part of the coverage story, namely the AP-to-client radio
propagation. Properly done with the AP power matching the expected client power, it can also indicate
the likely return path. However, it does not necessarily tell you anything about actual two-way data
throughput. This is especially true because the capacity of the network may increase based on MIMO
spatial streams in each location. As you will learn in Chapter 5, the ability of radios to decorrelate
individual spatial streams does not necessarily depend on SNR.
To test two-way throughput, one must take performance measurements from sample points around
the area using active testing tools such as iperf or Ixia IxChariot. Aruba recommends a uniform test
suite at each test point:
 TCP upstream, downstream and bidirectional
 UDP upstream, downstream and bidirectional
 Repeat each of the above on each major type of client device to be used
In general it is important to use multiple streams (2-4 each way) whether using iperf or IxChariot to
generate sufficient load through the IP layer of the network driver stack. Once the throughput results
are obtained, additional optimization of the network may be advisable. It is also possible to test
different pathways across the network by using multiple traffic “sink” locations at various points in the
mesh.
During this phase, it is common to install monitoring systems and begin to measure the network
reliability. Additionally, the network operator is trained on how to use the monitoring systems.
Final Network Acceptance
During the final acceptance step, a coverage heat-map and throughput testing results from a drive test
are usually summarized and a final report is prepared with the assistance of the customer. The
network documentation should include the street address and GPS coordinates of every installed
radio, pictures of the majority of installations, and detailed IP network diagrams.

Aruba Networks, Inc. Outdoor Access Points and Multichannel Backhaul | 21
Chapter 3: Outdoor Access Points and Multichannel Backhaul
Aruba offers a wide range of APs, antennas, and related accessories to enable campus extension and
outdoor mesh wireless networks. The choice of which hardware and operating system to use for a
given network is driven by the deployment type and often by the need for single-channel or multi-
channel backhaul.
Choosing the Deployment Type
Aruba has two families of outdoor APs: the 11ac AP-270 series and the 802.11n MSR series. The
AP-270 series is further divided into two different operating systems. APs running ArubaOS (AP-270
series) use controllers to terminate and control the access points. APs running InstantOS (IAP-270
series) use a smaller Virtual Controller running on the AP itself, to run a “Virtual Cluster” of APs.
Generally, Instant APs (IAPs) are more restricted in terms of flexibility and capability in regards to
outdoor deployments. To read more, visit http://www.arubanetworks.com/products/networking/aruba-
instant/.
An outdoor area can be covered by extending an existing Aruba indoor network through the use of
AP-270 series outdoor APs. These APs run ArubaOS managed by a controller, or run the Instant
OS as part of the virtual controller cluster. The AP-270 series can interoperate with Aruba indoor
APs, can be used as mesh portals, and can be used with other ruggedized AP-270 series APs that
are operating as unwired mesh points. Role-based user access policies are preserved across the
combined indoor and outdoor network.
In large outdoor networks, the AirMesh MSR series of wireless mesh routers are mounted on rooftops,
radio towers, street lights, and even traffic lights to extend coverage across large areas. When
considering outdoor Wi-Fi networks, good coverage is generally equated to the availability of suitable
mounting assets in combination with Aruba hardware and antenna flexibility. The MSR series runs the
Aruba AirMesh operating system.
To provide scalable coverage over large outdoor areas, wireless networks use combinations of mesh
portals. Mesh portals are connected to the wired network and wireless mesh points. For each radio, its
role, frequency band, and channel are defined in the software configuration. Mesh links connect mesh
points to other mesh points and to mesh portals, which then connect to a high-speed wired network.
Table 1 lists the AP models that should be used for each deployment type.
Table 1 AP model based on deployment type
Deployment Type
Dual-Radio
Rugged
Quad-Radio
Rugged
Single-Radio
Rugged
Campus Extension
(ArubaOS/InstantOS)
AP-274, AP-275
AP-277
- -
Outdoor Mesh (AirMesh) MSR2000 MSR4000 MST200
Understanding Single-Channel and Multi-channel Backhaul
A key factor in choosing an AP family for your outdoor network is the number of radio channels that
will be used for backhaul. In general, campus extension networks with the AP-270 series tend to have
very few hops and utilize a single-channel for intramesh backhaul, where outdoor mesh networks

built with the AirMesh family typically have many hops and use multiple channel backhaul links to
extend capacity.
The Evolution of Mesh Technology
Mesh networking technology has been enabling production networks for many years. In that time, it
has gone through several generations, culminating in the fourth generation AirMesh solution from
Aruba. Figure 6 illustrates the progression of technology enhancements:
4th Generation
• Multi-radio 802.11n/ac
• Directional antennas
• Layer 3 routing
3rd Generation
• Multi-radio
• Directional antennas
Municipal coverage
HD-quality video
Voice, and mobility
Hot zones
Performance
&
scalability
• Layer 2 bridging
2nd Generation
• Dual radio
• Omni-directional antennas
Low-res video
Indoor & outdoor
Hot spots
1st Generation
• Single radio
• Omni-directional antenna
Indoor access
Technology evolution
Figure 6 Summary of wireless mesh technology evolution
To help put the value of the AirMesh solution into perspective, it is useful to consider how mesh
technology has evolved over the years:
 First generation - Single radio L2 mesh. The earliest mesh implementations used single radio
APs in the 2.4 GHz band for both client and backhaul service. Since there is only one radio, all
mesh nodes are on the same channel. This means that when one radio is transmitting, whether
a client or another mesh node, no other radio can transmit. This approach suffered from two
major performance limitations. First, client transmissions had to be received by the AP, and then
retransmitted on to the upstream mesh node(s). This meant that the offered load at the access
layer could not exceed 50% of the uplink bandwidth to avoid saturation. Second, if there was
more than one mesh hop, the same effect was experienced on the backhaul. This further
reduced the allowable offered load at the access layer. First generation meshes operated at
layer 2.
 Second generation - Dual radio L2 mesh. An obvious solution to the client performance
limitation was to use separate radios for client and backhaul service. Second generation mesh
APs typically used 2.4 GHz for client access and the 5 GHz band for backhaul. In this design,
all mesh radios share the same channel, though client radios can use typical 1, 6, 11
channelization. The AP could serve clients simultaneously with backhaul traffic. However,
when relaying frames between mesh nodes, the 50% throughput drop per hop is experienced
arun_0437

because each mesh node has to receive a transmission before repeating it upstream. Second
generation meshes also operate at layer 2.
 Third generation - Multichannel Layer 2 backhaul. Some vendors eliminated the first and
second generation intermittent send-receive-send cycle by using two radios for the backhaul.
These radios generally operate on separate non-interfering channels, and simultaneous send
and receive is possible. This dramatically improves latency over multiple mesh hops. However,
due to the layer 2 topology, the mesh has a fixed tree structure such that all traffic flowing
through the mesh must pass through the “root” node. For some traffic flows this is no problem.
However, for peer-to-peer applications such as connecting a mobile police car to a remote
video camera, the root node bottleneck imposes significant performance degradation. Also,
intra-mesh roaming of mobile vehicles was typically not possible due to IP address changes by
the client.
 Fourth generation - Multichannel Layer 3 backhaul. Aruba has delivered the industry's first
fourth generation mesh solution using AirMesh, combining the power of multiple backhaul
radios with an RF-aware layer 3 routing protocol inside the mesh. This allows the construction
of high-speed mesh “cores” which feed distribution and access tiers. Traffic flows directly where
it is needed inside the mesh, without imposing arbitrary paths or bottlenecks inside root nodes
that are not the least cost path. Further, AirMesh provides for seamless high speed roaming via
a MobileIP-like implementation. In addition, AirMesh includes the unique Virtual Private LAN
over Mesh (VPLM). VPLM presents a L2 appearance at the mesh ingress/egress points, while
allowing the mesh to operate internally in layer 3 mode. This combines the simplicity and
compatibility of L2 with the performance and efficiency of L3.
Realizing the potential of a fourth generation mesh is the subject of most of this Design Guide. In
Chapter 9: Planning the Mesh Backhaul Layer on page 119, you will learn how to create an RF design
for a multichannel backhaul. In Chapter 11: IP Planning for Aruba AirMesh on page 157, you will learn
about the IP planning for the L3 features of AirMesh.

Comparing End-to-End Performance
Single-channel backhaul was the dominant network design for most first and second generation
outdoor mesh networks. They remain an appropriate solution for campus extension use cases with low
hop counts, but their capacity limitations make them a poor choice for today’s mesh networks that
need to deliver high capacity for multiple HD video streams across multiple hops. Traditional single-
radio/single-channel multihop links experience a throughput decrease of 50% or greater for each
network hop. Throughput is decreased because a single channel radio must share the air and repeat
transmissions from upstream to downstream nodes and vice versa. Single channel outdoor networks
generally use omnidirectional antennas, as shown in Figure 7. Using this strategy, nodes are placed
much closer together than the required Wi-Fi coverage dictates due to the lower combined gain of the
omni antennas.
Internet
Radio 1 Radio 1 Radio 1 Radio 1
Ch. 149 Ch. 149 Ch. 149 Ch. 149
Throughput
100 Mb/s
50 Mb/s
25 Mb/s
12 Mb/s
Figure 7 50% per-hop throughput loss on single-channel mesh networks
By contrast, it is possible to maintain high end-to-end throughput with low latencies by employing
multiple channels in the backhaul network, as shown in Figure 8. This architecture is mandatory as
more mesh client devices use 802.11n and as fixed high-bandwidth sources such as video cameras or
vehicle-mounted digital video recorders become commonplace. Multichannel mesh networks generally
employ directional antennas between individual mesh nodes, creating a mesh from a large number of
discrete point-to-point or point-to-multipoint links.
Throughput
100 Mb/s 100 Mb/s 100 Mb/s 100 Mb/s
Figure 8 Throughput is maintained when using multiple backhaul channels
Internet
Ch. 149 Ch. 157 Ch. 153 Ch. 161
arun_0353arun_0354

Aruba has developed specific antennas, deployment practices, and software calibration controls that
work with the mesh routing algorithms to deliver reliable high-capacity RF coverage across very large
areas using a multi-channel backhaul.
The performance difference between single-channel and multichannel backhaul architectures can be
easily demonstrated with any IP load generation tool, such as iPerf or Ixia IxChariot. To illustrate the
point, Aruba measured end-to-end throughput across 4 hops using a single-channel and multi-channel
configuration. The single-channel testbed used 4 mesh nodes, each with a single backhaul radio using
omnidirectional antennas. Figure 9 illustrates the multichannel mesh testbed on which the data in
Figure 10 was obtained. Both tests were conducted inside a Faraday cage to eliminate outside
interference.
Attenuator Attenuator Attenuator Attenuator
R0 R0 R1 R0 R1 R0 R1 R0
MSR2k MSR2k MSR2k MSR2k
IXIA
MSR2k
Figure 9 Multichannel mesh testbed
Figure 10 clearly shows the early mesh generations have a performance limitation of 50% per hop,
and ability of AirMesh to maintain nearly constant end-to-end throughput and latency over large
distances.
Figure 10 Multi-channel vs. single-channel backhaul performance: four hops
arun_0430

Multichannel backhaul generally requires that directional antennas be used between radio pairs within
the mesh. This topology blends the best of outdoor mesh and point-to-point architectures into a single
platform. This is desirable for maintaining end-to-end throughput as shown these figures and also to
increase the allowable distance between mesh nodes. For the same range, a radio pair that uses
directional antennas can achieve a higher signal-to-noise (SNR) ratio in line-of-sight (LOS) and non-
line-of-sight (NLOS) conditions. Higher SNRs translate directly into higher physical-layer data rates
and more overall network capacity.
To keep the management overhead low, AirMesh allows automatic software configuration of each
radio using a feature called Radio Frequency Management (RFM). RFM ensures the flexibility to
deploy each system using the frequencies, channels, and maximum power that are allowed within
each country. AirMesh is a layer 3 system, and RFM is capable of automatically provisioning IP
addresses on all multichannel radio pairs. For more information on RFM, see Chapter 7: Aruba
Software Technologies on page 73.
ArubaOS and Instant AP for Campus Extension
This section presents the Aruba AP-270 series campus extension access points.
AP-270 series Campus Extension APs
The multifunction AP-270 series, shown in Figure 11 is an affordable, fully hardened, outdoor
802.11ac AP that provides maximum outdoor deployment flexibility. A high-performance AP-270
series AP delivers wire-like performance at data rates up to 1.3 Gb/s at 5 GHz. The AP-270 series is
the outdoor radio of choice for Aruba customers with installed ArubaOS controllers or Instant
deployments that are expanding coverage to adjacent outdoor areas.
Figure 11 AP-270 Series
The AP-270 series features two 3x3:3 MIMO radios, with one radio dedicated to 2.4 GHz and the
other dedicated to 5 GHz. The AP-274 has 6 connectors for external antennas, 3 for each band. The
AP-275 includes integrated dual-band omni-directional antennas. The AP-277 has integrated dual-
band directional antennas. The AP-270 series can be mounted on the wall or on a mast in any
outdoor area.
AP-274
AP-275
AP-277

The AP-270 series carries an IP66 and IP67 rating and has been engineered to operate in harsh
outdoor environments. The AP-270 series can withstand exposure to high and low temperatures,
persistent moisture and precipitation, and is fully sealed for protection from airborne contaminants.
As an 802.11ac AP, the AP-270 series work with centralized Aruba Mobility Controllers to enable
the use of existing role-based authentication systems. AP-270 series APs also support Instant OS.
The multifunction AP-270 series can be configured through to provide WLAN access with part-time
or dedicated air monitoring for wireless intrusion prevention systems.
The 802.11ac 3x3:3 AP-270 series comes in three different versions, and all support 802.3at PoE
and AC power (Aruba Networks sells DC-to-PoE to support DC-powered deployments):
 The AP-274 - Supports external antenna via 6 N-female connectors (3 per band)
 The AP-275 - Integrated dual-band omni-directional antennas
 The AP-277 - Integrated dual-band directional antennas
AirMesh APs for Outdoor Mesh Networks
MSR4000 Quad-Radio Mesh Router
The Aruba MSR4000 wireless mesh router, shown in Figure 12 delivers high-performance wireless
back haul and Wi-Fi access to outdoor environments where wired connectivity is impractical or
unavailable.
1 Radio 0 (Antenna 2) 6 Radio 1 (Antenna 2)
3 Ethernet Interface 8 Radio 3 (Antenna 1)
5 Radio 1 (Antenna 1) 10 Console Interface
Figure 12 MSR4000 quad-radio mesh router
The MSR4000 is ruggedized and hardened to withstand extreme environmental conditions, and it is
ideal for deployment in metro areas, oil and gas plants, retail centers, business parks, and
transportation hubs.

A multiradio, multifrequency architecture combined with adaptive layer 3 technology makes the
MSR4000 unique. Together, these features provide unparalleled speed and reliability, low latency, and
seamless handoffs for voice, video, and other real-time applications across long-distance, outdoor
wireless mesh networks.
The MSR4000 consists of four independent 802.11a/b/g/n radios to create flexible outdoor wireless
mesh topologies that can use the 2.4 GHz and 5 GHz bands as well as the 4.9 GHz public safety band.
Each radio is capable of providing a maximum throughput of 300 Mb/s.
Each individual radio can be configured to operate as a client access AP or as a point-to-point or point-
to-multipoint node to deliver full-mesh backhaul. This four-radio architecture separates client access
and mesh backhaul and optimizes radio resources for both types of traffic to ensure high throughput
and low latency. The MSR4000 fully participates in the Aruba Adaptive Wireless Routing™ (AWR)
algorithms, which automatically optimize traffic flow between multiple wireless mesh routers for
maximum user capacity.
MSR2000 Dual-Radio Wireless Mesh Router
The Aruba MSR2000 dual-radio mesh router, shown in Figure 13 provides unparalleled speed and
reliability at the edge of large-scale mesh networks. The two radios deliver low latency and seamless
handoffs for voice, video, and other real-time applications across long-distance, outdoor wireless mesh
networks.
Figure 13 MSR2000 dual-radio mesh router
The MSR2000 consists of two independent 802.11a/b/g/n radios to create flexible outdoor wireless
mesh deployments that use the 2.4 GHz and 5 GHz bands or the 4.9 GHz public safety band. Each
radio provides a maximum throughput of 300 Mb/s.

Each individual radio can be configured to operate as a client-access AP or as a point-to-point or point-
to-multipoint node to deliver full mesh backhaul. If necessary, both radios in the MSR2000 can be
configured for backhaul on different channels. This configuration allows the MSR2000 to serve as an
unwired relay in a multichannel architecture and maintain high end-to-end throughput and low latency.
The MSR2000 fully participates in the Aruba AWR algorithms, which automatically optimize traffic flow
between multiple wireless mesh routers for maximum user capacity.
MST200 Single-Radio Wireless Mesh Router
The Aruba MST200 wireless mesh access router is considered a true edge router and connects
devices such as video surveillance cameras and IP phones to high-performance Aruba outdoor
wireless mesh networks. The MST200 uses the AWR protocol to determine the best path for each
device to send data to the wired network.
1 Ethernet Interface (PoE In)
2 USB Console Interface
3 Status LEDs
4 Integrated Antenna (14dBi 60°x14°)
Figure 14 MST200 single-radio wireless mesh router
The MST200 is also an ideal solution for delivering wired network connectivity to buildings inside a
mesh or at the end of a mesh. MST200 routers can also be used in pairs to construct low-cost, high-
throughput point-to-point bridge links between two buildings when a full mesh is not required. The
integrated 14dBi dual-polarization 5 GHz MIMO antenna greatly simplifies the installation process while
providing a clean, attractive look.
The MST200 is ruggedized and hardened to withstand extreme environmental conditions. The
MST200 is ideal for deployments in outdoor environments to support applications like video and
perimeter surveillance, metro area networks, electronic billboards, and mass transit networks. The

MST200 is also ideal for public-safety monitoring along transportation corridors and for rapid
deployments at large-scale public events or during emergency response.
The MST200 provides a maximum throughput of 300 Mb/s and delivers unprecedented stability and
reliability. The MST200 and all MSR routers also employs Active Video Transport™ (AVT) traffic
shaping and load balancing algorithms for use across RF links. These algorithms enable the MST200
to deliver HD-quality video from fixed surveillance cameras to headquarters locations.
AP Model Summary
Table 2 presents a quick-reference summary of the entire family of Aruba Outdoor APs presented in
this chapter.
Table 2 Outdoor features on each Outdoor AP
Function / Model AP-270
Series
MSR4000 MSR2000 MST200
ArubaOS Controller-Managed or Instant 
Autonomous APs with AirMesh   
Number of Radios 2 4 2 1
4.9 GHz Public Safety Band  
Outdoor Rating IP66, IP67 IP66 IP66 IP66

Aruba Networks, Inc. Outdoor Antennas and RF Coverage Strategies | 31
Chapter 4: Outdoor Antennas and RF Coverage Strategies
The information in this section helps you understand antenna basics and Aruba best practices for
covering common outdoor environments. For those new to RF engineering, Aruba highly
recommends the vendor-neutral Certified Wireless Network Professional training classes and
certifications which provide in-depth education on RF fundamentals. For more information, visit
www.cwnp.com.
Antenna Beamwidth, Pattern, and Gain
Antenna gain is a relative measure of how the antenna compares to an ideal isotropic radiator. The
gain of an antenna is specified in dBi, which is the directional gain of the antenna compared to an
isotropic antenna. An isotropic antenna is an ideal (theoretical) antenna that spreads energy in all
directions (in a sphere) with equal power. You may think of the sun as a good analog for an isotropic
antenna.
Equal signal strength
radiated over a sphere
Figure 17 Isotropic antenna
Antenna gain is often confused with power because the gain of an antenna can increase the
transmitted or received signal levels. However, it is important to note that gain is usually only stated as
a maximum value and this value will increase signal levels only in a particular direction. This is
because antenna gain is achieved only by compressing the radiated power into a tighter region in 3D
space, and antennas (by themselves) do not create increased power. Antenna gain is more correctly
described as a focusing of radiated power rather than an amplification of it. This means that any
antenna with gain > 1 dBi will provide higher signal levels than the isotropic radiator in some directions,
but will actually reduce signal levels in other directions. With increasing maximum gain, the area in 3D
space with reduced signal level grows inversely with increasing gain. This means that higher gain
antennas focus the power into a tighter and tighter region of space, which can actually result in much
worse coverage for clients that are not in the region of higher gain.

To help visualize the idea of focusing energy in some directions at the expense of others, imagine that
the sphere in Figure 17 is a rubber ball. How could a ball with the same surface area be stretched
farther out? One way is to press down on the top of the ball and squash it down vertically. The same
basic shape is kept in the horizontal plane (round), but the ball is forced to stretch, which creates a
pancake shape in the vertical direction. Figure 18 represents the concept of the omnidirectional
antenna, which achieves a greater coverage distance in the horizontal direction at the expense of
coverage in the vertical areas of the radiating sphere.
Figure 18 Omni-directionalantenna
What would happen if the ball were pinched on one end instead of being squeezed? This concept is
illustrated in Figure 19. The ball is forced into a conical shape whose length depends on how much the
body of the cone is compressed. This represents the concept of a high gain directional antenna.
Figure 19 High gain directional antenna
It is not necessary for the cone to face sideways, parallel to the ground. It is also possible to pinch the
top of the ball and cause the cone to stretch down towards the ground. This is known as a “squint” or
“downtilt” pattern, and will be discussed extensively in the balance of this solution guide as it is Aruba’s
preferred antenna type for large outdoor yard and plant environments.

Omnidirectional Antenna Types
Each omnidirectional antenna (also known as an omni) falls into two categories. The classic omni -
known as the “stick” omni due to its appearance - is a tall, thin radome whose length varies with the
intended frequency band. Both vertically polarized and horizontally polarized stick omni antennas
are available, including 2X2 and 3X3 MIMO kits that include one of each from Aruba for use in
outdoor networks.
The other type of omni is known as the “squint” or “downtilt” omni. The squint is technically a
directional antenna because it faces down. However, the antenna is designed to provide standard
vertical polarization. It also operates as a full 360-degree omnidirectional antenna in the horizontal
plane. The antenna has a very low gain (3-5 dBi) depending on frequency, creating a tight, well formed
“cell” with the bulk of the signal focused down toward clients. See Figure 20 for an illustration of these
antenna patterns.
While squint antennas are common indoors, Aruba developed and brought to the market one of the
first outdoor models. This antenna is the result of our experience of providing coverage to intermodal
facilities that cover large areas and that require coverage behind and inside container stacks and
mobile equipment. However, this antenna is used in an increasing number of high-capacity outdoor
networks. This antenna is intended to be mounted high up—such as on a high mast, light pole, or tall
communications tower—where it has good LOS behind most obstructions. This antenna enables
wireless designers to use a “dense outdoor deployment” strategy in a manner similar to providing
consistent coverage indoors.
Pair of 8-dBi high-gain omnis
Figure 20 H-plane comparison of “stick” omni and down-tilt omni antenna patterns
The horizontal range of the squint antenna is much less than the high-gain antenna due to the lower
overall gain as well as the shape and directivity of the pattern.

However, the power of the squint antenna becomes obvious when we consider the E-plane pattern.
Figure 21 shows the vertical coverage of the same two antennas, which are mounted at a height of 12
meters (40 ft). One can immediately see that the -67dBm cell edge in the vertical plane does not even
reach the ground, whereas the squint omni not only reaches all the way but also has a clear LOS
behind any obstructions.
Pair of 8-dBi high-gain Omnis
Figure 21 E-Plane comparison of stick omni and squint omni antenna patterns
Directional Antenna Types
Though it is true that higher-gain antennas increase the range in the direction of the antenna gain, it is
not true that the signal strength is the same everywhere in that direction. High gain directional
antennas - also known as narrow vertical beamwidth antennas - achieve the range by “stretching” the
pattern. However, this stretch of the pattern also causes the area of reduced coverage that exists
between every antenna and the beginning of its main lobe to stretch out as well, as shown in Figure
22.
Figure 22 Null zone of a narrow vertical beamwidth antenna
This diagram is typical of a 12-14 dBi antenna with an 8-degree vertical beamwidth (hence the term
“narrow vertical beamwidth”). It is assumed to be mounted at 30 meters with no downtilt. In the area
before the main lobe hits the ground, there will be some illumination by side lobes of the antenna
pattern. While there may be some signal, it will be anywhere from 20dB to 40dB lower than inside the
main lobe.
50% radiated towards ground
50% radiated towards sky
447 m
Ground level
1,500 m Retail_139

Contrast the size of this area with that of a low-gain directional antenna - also known as a wide vertical
beamwidth antenna - as shown in Figure 23. In this case, a 5-dBi, 60-degree sector has a reduced
coverage zone of just 50 meters or so from the same mounting height.
Figure 23 Null zone of a wide vertical beamwidth antenna
Effect of Mechanical Downtilt on Directional Antenna Coverage
Mechanical downtilt is used on a directional antenna that is mounted high up to “aim” it toward its
intended coverage zone. Our experience is that professional wireless designers are often casual about
the actual angle of the mechanical downtilt. Generally they are content to estimate downtilt based on
a quick ground-based visual inspection of a site without fully considering the 3D implications on the
shape of the delivered coverage at ground level. However, the actual results of a high mounting height
and modest downtilt can often surprise even experienced wireless engineers. The following examples
show how important it is to use mechanical downtilt correctly, and where it is not suitable.
50% radiated towards ground
50% radiated towards sky
53 m Ground level
600 m
Retail_140

We begin by showing (in Figure 24) the relative horizontal and vertical beamwidths of two commonly
used directional antenna types. On the left is a 12 dBi antenna (Aruba ANT-82) and on the right is a 7
dBi antenna (Aruba ANT-83). Both offer 90 degrees of horizontal beamwidth. This makes it easy to
see how the increased gain of the higher-gain antenna comes at the expense of vertical beamwidth
(60 degrees on the 7 dBi antenna versus only 10 degrees for the 12 dBi antenna). In this example, the
antennas were modeled at a height of 30 meters.
The lighter area in the diagram in the upper right (and in the diagrams that follow in this section) shows
the main lobe of the antenna in contact with the ground.
Plan View
12 dBi gain
90 degrees horizontal beamwidth
10 degrees vertical beamwidth
7 dBi gain
90 degrees horizontal beamwidth
60 degrees vertical beamwidth
Elevation View
Figure 24 Effect of Higher gain on vertical beamwidth
Note the narrow the vertical beamwidth of the high-gain antenna, and how the main lobe does not
touch the ground. And while the wider vertical beamwidth of the lower-gain antenna does touch the
ground, only the bottom portion of the main lobe reaches the ground, meaning that most of the signal
is wasted overhead. Both antennas could benefit from mechanical downtilt.

In Figure 25, 10 degrees of mechanical downtilt is added to a narrow vertical beamwidth antenna on
the left (10 degrees) and an antenna with a wider vertical beamwidth antenna (60 degrees) on the
right.
12-dBi gain: 90 degree 7-dBi gain: 90 degree
Figure 25 Azimuth view with 10 degrees of mechanical downtilt
In Figure 26, the narrow vertical beamwidth antenna on the left sacrifices close-in coverage to achieve
greater range. Mechanical downtilt cannot fully compensate for this null area underneath the antenna
before the pattern hits the ground.
12-dBi gain: 90 degree 7-dBi gain: 90 degree
Figure 26 Elevation view with 10 degrees of mechanical downtilt
On the right, no null area exists, because more of the main lobe of the wide vertical beamwidth
antenna now hits the ground.

Figure 27 shows the results when the downtilt is further increased to 30 degrees for the narrow
vertical beamwidth antenna (the antenna on the left). This is done in an attempt to obtain better
coverage close to the AP. The result is a distorted and narrow coverage pattern with even less
coverage that actually reaches the ground.
12-dBi gain: 90 degree
horizontal view
12-dBi gain: 90 degree
vertical view
Figure 27 Narrow vertical beamwidth with 30 degrees mechanical downtilt
A common mounting height for outdoor networks is 12-15m (40–50 ft). Even at this relatively modest
mounting height, a small mechanical downtilt (10–30 degrees) creates a narrow vertical beamwidth
antenna that creates only a small “stripe” of coverage on the ground. This limited coverage is the
opposite of what the wireless designer intended, which was to provide uniform coverage throughout
the coverage area.
Directional Antenna Conclusions
This section describes why high-gain antennas are primarily intended for long-distance, point-to-point
connections, not close-in client coverage. We have further established that:
 Vertical beamwidth is more important than horizontal beamwidth in determining the experience
of clients.
 Mechanical downtilt is not a good solution to compensate for narrow vertical beamwidth. It
reduces the size of the main antenna lobe that reaches the ground.
 High mounting heights are not compatible with narrow vertical beamwidth antennas due to the
size of the null zone between antenna and the 3 dB point.
 Low mounting heights are easily obstructed by ground level equipment or buildings.

Assuming that the wireless designer is determined to use a narrow vertical beamwidth antenna for
client coverage, two methods are available to reduce the size of the null area:
 Use mechanical downtilt. However, as we have seen, a relatively small amount of downtilt (just
15 degrees) produces the “striping” affect and reduces the overall coverage area.
 Reduce the mounting height. The best way to maximize the coverage area of a narrow-vertical
beamwidth antenna and minimize the null is to reduce the mounting height. For this reason,
Aruba recommends that high-gain directional antennas that are used for client coverage (as
opposed to point-to-point links) should never be mounted higher than about 30 feet with a
maximum of about 5 degrees of mechanical downtilt.
It may seem that if you reduce the mounting height of a narrow vertical beamwidth directional antenna,
the coverage issues described here would be solved. Unfortunately, doing so renders the main lobe of
the signal more vulnerable to LOS obstructions that occur more often at lower mounting heights. The
network planner must constantly balance these trade-offs.
RF Coverage Strategies for Outdoor WLANs
A coverage strategy is a specific method or approach for locating APs inside a wireless service area.
Generally, any given coverage strategy will also call for a specific antenna pattern providing required
directionality (even if it is just using integrated antennas in an AP). Three basic coverage strategies are
generally used to provide 2.4 GHz and 5 GHz high capacity Wi-Fi coverage in outdoor environments:
 Sparse side coverage
 Dense side coverage
 Dense overhead coverage
Coverage is sparse when a relatively small number of irregularly-spaced locations cover a large
space, often using high-gain, narrow-vertical beamwidth directional antennas. Coverage is dense
when many APs are relatively evenly spaced to cover a large area from many locations and use
lower-gain, wide-vertical beamwidth antennas.
Understanding Side and Overhead Coverage
From a horizontal perspective, sparse and dense coverage are very easy to understand and to
visualize. Side and overhead coverage are more complex and will be considered in depth in this
section.
Side Coverage
Coverage is considered to be from the side when the main lobe of the antenna is approximately the
same elevation as the clients being served. If mechanical downtilt is in use, the elevation difference
may be greater, but it is still considered side coverage.

Viewed from the side, the main lobe of the antenna pattern spreads out to a precisely engineered limit
all around the AP. A common misconception is that each pole-mounted AP serves the area directly
below. However, a client standing immediately underneath such an AP using a stick omni will not
benefit from the antenna pattern because the main beam is passing overhead. Instead, the client may
well be associated to the next AP over. Also, the 50% of the signal that is directed upwards from a
typical stick omni antenna is immediately wasted, as illustrated in Figure 28.
Side Coverage
Figure 28 Side coverage
Overhead Coverage
Overhead coverage refers to the use of “squint” or “downtilt” omnidirectional antennas that face
downwards but are electrically designed to provide a full 360 degrees of coverage with standard
vertical polarization, as shown in Figure 29. All of the antenna gain is focused in the direction of the
clients underneath.
Figure 29 Overhead coverage
Viewed from the azimuth, or overhead, both antennas provide full 360 degree coverage in a circular
shape. However, the downtilt omni will have a smaller, tighter pattern, whereas the side coverage AP
will spread its signal further out.
Overhead
coverage
20 m
120°
120°
Wasted
signal
60°
3dB
beamwidth
10 m
Reduced coverage area outside
main antenna lobe
arun_0434arun_0433

Choosing Between Side and Overhead Coverage
Side coverage from low-gain directionals or omnis is recommended as the best and lowest-cost
solution for campus extension coverage at up to 9 meters (30 feet) of building height. In a standard
campus deployment, multiple APs on adjacent buildings work together to provide complete,
overlapping coverage of the target area.
For mounting positions higher than 12 m (40 ft), Aruba strongly recommends the use of squint omni
antennas. The reason for this is illustrated in the following diagram. For a standard 60 degree
directional antenna such as the ANT-3X3-D608 or ANT-3X3-D100, the -3 dB point where the main
lobe intersects the ground moves out 5.2 m (17 ft) from the AP for every additional 1 m (3.2 ft) of
mounting height. We have already shown that mechanical downtilt is limited in its ability to
compensate for increasing height.
40 m
25 m =
40 m
sin(30°)
= 80 m
MSR4K/2K
with
ANT-2x2-D607
10 m
60°
=
10 m
sin(30°)
=
25 m
= 50 m
sin(30°)
= 20 m
90°
30°
17 m
43 m
69 m
Figure 30 Effect of increasing AP height on main lobe reaching ground level
In summary, the low-gain squint omnidirectional antenna is idea for steep down angles and mounting
heights over 12 m (40 ft) in outdoor areas.
 It limits range to a predictable area around each AP and reduces AP-to-AP interference
 It reduces client density per AP by employing more, smaller cells
 Its antenna pattern provides users at ground level with a higher signal than APs see to each
other
 Adaptive radio management functionality is improved for auto-calibration of the RF network and
automation of ongoing operations.
arun_0435

Sparse Side Coverage
The sparse side coverage strategy is used when outdoor areas have very limited vertical mounting
assets and usable electrical service. We start by using these few existing buildings, towers, and
structures that have power and data services. These are also typical locations for other transmitters
such as two-way radios and even cellular telephone base stations, so we often co-locate AP-270s or
AirMesh routers in the same positions. Figure 30 is a real customer example of a 5 km2 (2 mi2) seaport
showing the handful of locations with wired backhaul. Note the uneven distribution of locations
throughout the yard, making it impossible to achieve uniform signal levels.
Figure 31 Sparse side coverage example
This deployment scenario uses very high-gain (≥ 13 dBi), 60-degree sector, moderate elevation (50
degree) antennas to cover as much range as possible from each radio position.
This strategy alone is unable to deliver reliable outdoor coverage for clients. Frequent LOS
obstructions cause signal drop-outs and a poor user experience. The exception to this observation is
that side coverage remains a good alternative for covering fixed wireless cameras, which are often at
similar elevations. This coverage strategy also does not comply with vendor RF design best practices
from Cisco®, SpectraLink®, or Vocera® when planning wireless Voice over IP (VoIP) networks
because it is not capable of delivering a consistent -67dBm signal level or predictable roaming
transitions throughout a coverage area.

Dense Side Coverage
Dense side coverage networks are most often seen in a campus environment where common areas
are surrounded by buildings that are accessible to the network operator. In a yard environment such as
pictured in Figure 32 below, dense side coverage can be achieved using existing light poles to mount
mesh radios at regular intervals. In these networks, AP-270 series APs or AirMesh routers are
deployed densely using omnidirectional or sectored side coverage from buildings or utility poles. In
dense side coverage networks the radio density is high and provides good RF reliability because
there is always another radio working nearby.
Figure 32 Dense side coverage example
Aruba typically recommends mid-gain (5 - 7 dBi) antennas rather than high-gain antennas in this
scenario to minimize close-in nulls. The mid-gain antennas deliver consistent client coverage
throughout as a result of delivering homogenous signal levels across large areas. These antennas
can also deliver good roaming performance. When AP-270s are deployed, ArubaOS or Instant can
utilize Adaptive Radio Management (ARM). Consistent AP spacing and the homogenous antennas
on the building walls enables the system to respond dynamically to ambient RF changes and is good
for delivering VoIP coverage.
Dense side coverage radio deployments can be consistent with voice handset vendor best practices
documented by Cisco, SpectraLink, and Vocera.

Dense Overhead Coverage
The dense overhead coverage strategy is often seen in transportation, manufacturing and industrial
deployments where antennas can be mounted overhead. But it can be equally well applied to
metropolitan networks, and offers some advantages in terms of decreasing the channel reuse
distance. In this strategy, AP-270 series APs or AirMesh routers are deployed densely and antennas
are mounted higher up, between 15-35m (50–120 ft) above ground level. Existing light poles, high
masts, and communication towers are used to mount AP-270 series APs or AirMesh routers every
200-300m (650-950ft), resulting in a high number of alternate paths and a very reliable system.
Figure 33 Dense overhead coverage example
Aruba sells a specialized low-gain (typically 3-5 dBi), squint, omnidirectional antenna that faces down
to create very uniform cells. These antennas work reliably and deliver consistent performance in
cluttered outdoor environments like container ports and rail yards because they usually have clear
LOS behind ground obstructions that would block side coverage solutions.
The dense overhead coverage strategy results in excellent voice support and a dense number of
radios with LOS to many APs. This strategy is consistent with voice handset vendor best practices.

Selecting an Aruba Outdoor Antenna
In outdoor networks, antenna types are always used for specific purposes. For example, directional
antennas are used for each backhaul link and omnidirectional antennas are used for access radios.
Aruba has invested heavily in research for MIMO antennas that deliver the highest possible
performance even in multipath-poor outdoor environments. The line of Aruba MIMO antenna products
represents the state of the art in rate-versus-range performance for outdoor extension and outdoor
mesh applications.
Aruba MIMO antennas contain special multiple-polarization arrays that have been
designed to maximize decorrelation of MIMO spatial streams, and minimize intra-
array coupling between antenna elements. Aruba does not warranty the
performance of outdoor networks using non-Aruba antennas. The use of third-
party antennas is at the customer’s own risk.
Understanding Aruba MIMO Antenna Part Numbers
Aruba has introduced a proprietary line of MIMO antennas for use with the AP-270 and MSR series
APs and mesh routers. To minimize cost and maximize performance, these antennas include multiple
elements with polarization diversity.
Be sure to check whether the models you choose require a separate low-loss RF
cable to connect to the AP. Some Aruba antennas include pigtail connectors and
may not need RF cables for attaching to the AP. Your Aruba representative can help
you determine what parts are necessary.
These antennas also use a part number system that makes it easy to select the right part and
understand existing networks with these antennas installed. The system is described in Figure 34:
Figure 34 Guide to Aruba outdoor antenna part numbers
NxM=
“2x2” for 2x2 MIMO
antennas, or "3x3"
for 3x3 MIMO
antennas
A =
 D for dual-band
 2 for 2.4 GHz single-band
 5 for 5 GHz single-band
B =
Single digit representing H-
plane
 0 = omnidirectional
 1 = 10 degrees or less
 2 = 20 degrees
 3 = 30 degrees
 4 = down-tilt
omnidirectional
 5 = {reserved}
 6 = 60 degrees
 7 = 70 degrees
 8 = 120 degrees
 9 = 90 degrees
CC =
Two digits that represent
gain in dBi
ANT - NxM - ABCC

The Aruba line of dual-band 3x3 and 2x2 MIMO antennas at the time of writing is as follows:
 ANT-3x3-2005: 2.4 GHz, Omnidirectional, 5 dBi, H/V Polarization (2x2 variant available)
 ANT-3x3-5005: 5 GHz, Omnidirectional, 5 dBi, H/V Polarization (2x2 variant available)
 ANT-3x3-5010: 5 GHz, Omnidirectional, 10 dBi, H/V Polarization (2x2 variant available)
 ANT-3x3-D100: 90 Degree Sector, 5 dBi, ±45/V Polarization (can be downward facing)
 ANT-3x3-D608: 60 Degree Sector, 7 dBi, ±45/V Polarization
 ANT-3x3-5712: 5 GHz, 70 Degree Sector, 12 dBi, ±45/V Polarization
 ANT-2x2-5314: 5 GHz, 30 Degree Sector, 14 dBi, H/V Polarization
 ANT-2x2-2314: 2.4 GHz, 30 Degree Sector, 14 dBi, H/V Polarization
 ANT-2x2-2714: 2.4 GHz, 70 Degree Sector, 14 dBi, H/V Polarization
For the latest listing of Aruba’s line of antenna products, visit our web site on
http://www.arubanetworks.com/products/networking/antennas/. From this page, you may also wish to
download the Aruba Antenna Matrix, which is a handy quick reference guide to the entire Aruba
antenna line in table format.
Access Layer Antennas
For access layer radios, omnidirectional antennas can provide good all-around coverage for client
devices. These antennas can be applied in outdoor extension or outdoor mesh networks when
mounting locations like street lights have a clear view in all directions. The Aruba ANT-2x2-2005 and
ANT-3x3-2005 is good for this purpose. They are kits of either two or three 5dBi 2.4 GHz antennas,
one horizontally polarized and one or two vertically polarized.
Aruba's squint antenna for outdoors is the ANT-3x3-D100. This is a 5dBi dual-band antenna with ±45
and vertical polarization.
For sectored coverage, Aruba offers a range of horizontal beamwidths such as the 5dBi 90 degree
ANT-3x3-D100 and the 8dBi 60 degree ANT-3x3-D608. A 14dBi 30 and 70 degree options are also
available. All Aruba directional antennas feature multiple polarizations.

Table 3 shows the Aruba MIMO antenna family typically used for client connectivity.
Table 3 Omnidirectional antenna typically for access connections
Vertical Horizontal
ANT-2x2-2005
ANT-3x3-2005
5 dBi
Vert. Beamwidth: 30°
2.4 GHz
ANT-2x2-5005
ANT-3x3-5005
5 dBi
5 GHz
ANT-3x3-D100
5 dBi
Horiz. Beamwidth: 360°
Dual-band
(downtilt orientation)

Table 3 Directional antenna typically for access connections (Continued)
Vertical Horizontal
ANT-3x3-D100
5 dBi
Dual-Band
ANT-3x3-D608
7.5 dBi
Dual-Band

Backhaul Layer Antennas
For backhaul radio links, narrow beamwidth MIMO antennas in 5 GHz are popular because more
channels are available and the 5 Ghz channels are generally much cleaner than 2.4 GHz. In addition,
narrow-beamwidth MIMO antennas have improved interference rejection and can achieve higher
SNRs based on good LOS. Omnidirectional antennas generally are not used for backhaul links
because they are exposed to interference from a full 360-degree radius. For client connections, the
Aruba MIMO-based omnidirectional antennas work particularly well because a “pair” includes one
vertical and one horizontally polarized antenna. These antennas should be mounted above and
below each other to maximize decorrelation of multiple spatial streams. Table 4 shows a typical
directional or sectored antenna, typically used for backhaul or point-to-point links.
Table 4 Directional antennas typically used for backhaul or mesh links
Vertical Horizontal
High Gain
Directional
ANT-3x3-5712
12 dBi
Vert. Beamwidth 25°
Horiz. Beamwidth 70°
5 GHz



 

Table 4 Directional antennas typically used for backhaul or mesh links (cont)
Vertical Horizontal
ANT-2x2-5314
14dBi
5 GHz
ANT-2x2-2314
14 dBi
2.4 GHz

When you select the specific antennas to be used for each site, consider both the horizontal and
vertical beamwidth for each frequency. Previous sections described the result of poor planning or poor
installations using even small amounts of mechanical downtilt. Also, remember that some Aruba Wi-Fi
antennas are dual-band and may combine horizontal and vertically polarized antenna elements for
improved performance and ease of installation.
Figure 35 Azimuth follows the visible beam of antenna gain
During planning, the antenna azimuth or direction, as shown in Figure 35 should be specified for each
location, including combinations of built-in antenna downtilt plus any mechanical downtilt to be added
by the installer using physical adjustments on the mounting brackets. In many cases, it may be
necessary to remotely locate the antennas from the AP or AirMesh router. For these sites, identify the
type and length of RF cable with the proper connectors and also adjust the RF link budget to account
for the added signal loss from this cable.
When selecting the antenna for each mounting location, refer to the Aruba Antenna Matrix for detailed
understanding of the antenna patterns and gain.

Aruba Networks, Inc. 802.11n and 802.11ac Multiple-In and Multiple-Out | 52
Chapter 5: 802.11n and 802.11ac Multiple-In and Multiple-Out
The promise of 802.11n and 802.11ac networks is their ability to provide “wire like” speeds to the end
user, eventually as much as 1300 Mb/s per radio. This speed is achievable by using multiple
technologies, including the use of multiple-input and multiple-output (MIMO) technology. MIMO
technology combines multiple send and receive antennas, and multiple streams of data sent at the
same time. In addition, the 802.11n specification adds new encoding algorithms and wider channels.
This all work together to significantly increase the data transfer rate.
Ratification and Compatibility
The IEEE ratified the 802.11n amendment in September of 2009, but by that time 802.11n APs and
clients based on an early draft of the 802.11n standard were already actively deployed. Many
organizations began to deploy 802.11n once the Wi-Fi Alliance® used an early draft of the amendment
and certified “draft-n” products as interoperable. Interoperability certification gave customers the
confidence to deploy the products, and also gave the vendors the ability to start actively producing and
deploying 802.11n capable devices.
802.11ac ratification came towards the end of 2013 and brought further enhancements, including
wider channel widths, PHY layer improvements, higher QAM rates, standards-based beamforming,
and other enhancements that fall outside the scope of this document.
Backward compatibility between 802.11n and 802.11ac APs and legacy clients is a key part of the
amendment. Backward compatibility means that stations that previously connected to 802.11a, 802.11b,
or 802.11g APs are still capable of connecting to 802.11n and 802.11ac APs.
Understanding MIMO
Unlike traditional 802.11a/b/g radios, which use single-input and single-output (SISO), 802.11n and
802.11ac radios use MIMO technology to increase throughput by increasing the number of radio
transmit and receive chains. An AP or client may have up to four transmit and four receive chains,
and it is possible to have a different number of transmit vs. receive chains. Figure 36 shows the
difference between a SISO and MIMO transmission.
Single in, single out Multiple in, multiple out
Transmitter Receiver
Transmitter Receiver
Figure 36 SISO vs. MIMO
SISO
AP
SISO
Client
Wireless
Channel
MIMO
AP
MIMO
Client
Wireless
Channel
arun_0312

Though many 802.11a/b/g APs have two antennas, they are not capable of using
both antennas at the same time. Instead, the two antennas provide diversity. Each
antenna receives a different receive signal strength and the AP selects the strongest
one to use for each reception. To send a signal, typically the AP uses the antenna
that was last used to receive a signal.
802.11n and 802.11ac Spatial Streams
The concept of spatial streams of data is related to the ability to transmit and receive on multiple
radios. More transmitters and receivers allow the AP to send independent streams of data. Much like
adding additional lanes to a road, multiple spatial streams allow the wireless AP to transmit more data
simultaneously. Spatial streams split data into multiple parts and forward them over different radios,
and the data takes different paths through the air. Figure 37 demonstrates the concept of multiple
spatial streams of data.
Stream 1
Stream 2
Client
Figure 37 A MIMO transmission with two spatial streams of data
Part of the advantage of MIMO and spatial streams is that APs can use multipath transmissions to
their advantage. SISO systems see performance degradation due to multipath transmissions because
the multipath may add to signal degradation. However, 11n and 11ac APs use multipath transmission
to reach their full speeds. The delay in the propagation of paths at different rates allows MIMO and
spatial streams to be received correctly at the other end of the transmission link. In a SISO system,
that delay can cause interference.
Multiple antennas are needed to transmit and receive multiple spatial streams. Depending on
hardware, an AP or client can transmit or receive spatial streams equal to the number of antennas it
has. However, the AP may have more antennas than spatial streams.
Other 802.11n and 802.11ac Technologies to Increase Throughput
Two spatial streams allow us to double the transmission rate. But this alone is not adequate to get us
from 54Mbps in 802.11a/g to 300Mbps with 802.11n or 1300Mbps with 802.11ac. The 802.11n
standard includes four other physical-layer technologies that work together to deliver 300Mbps. They
are 40 MHz channels, improved OFDM subcarriers, short guard interval, and space-time block
coding. 802.11ac goes further with 80 MHz channels and even more improved OFDM subcarriers,
along with Transmit Beamforming.
40 MHz and 80 MHz Channels
Previously, 802.11 transmissions were transmitted using 20 MHz data channels. Anyone who has
deployed an 802.11a/b/g AP has worked with 20 MHz channels, with each AP set to a single, non-
overlapping channel. With 802.11n and 802.11ac, two channels can be bonded, which actually more
arun_0313

than doubles the bandwidth because the guard channels in between also are used. Figure 38 shows
the difference is width for a 40 MHz spectral mask as opposed to the 20 MHz mask originally
specified for 802.11 transmissions.
-19 MHz
0 dBr
+19 MHz
0 dBr
-9 MHz
0 dBr
+9 MHz
0 dBr
-30 MHz
-28 dBr
-40 MHz
-40 dBr
-21 MHz
-20 dBr
+21 MHz
-20 dBr
+30 MHz
-28 dBr
+30 MHz
-40 dBr
-20 MHz
-28 dBr
-30 MHz
-40 dBr
-11 MHz
-20 dBr
+11 MHz
-20 dBr
+20 MHz
-28 dBr
+30 MHz
-40 dBr
-30MHz -20MHz fc +20 MHz +30 MHz -30MHz -10 MHz fc +10 MHz +30 MHz
-30MHz -10MHz +10 MHz +30 MHz -20MHz +20 MHz
Spectral mask for 40 MHz channel Spectral mask for 20 MHz channel
Figure 38 Spectral mask, 40 MHz vs. 20 MHz channels
In the 5 GHz band, multiple 40 MHz and 80 MHz channels are available, and depending on the
regulatory domain, additional channels are available with dynamic frequency selection (DFS)
enabled. Figure 39 outlines the available 40 MHz channels in the 5 GHz band. At the time of this
writing (January 2011), some channels have recently become unavailable for new AP models.
Figure 39 FCC and ETSI channels in the 5 GHz band
The limited number of channels in the 2.4 GHz band makes 40 MHz channels unsuitable for use. The
2.4 GHz band has only three 20 MHz non-overlapping channels available in most regulatory domains.
If a single 40 MHz channel is deployed in the 2.4 GHz band, the channel covers two of the three
usable channels. Aruba recommends that 40 MHz channels only be deployed in the 5 GHz band
where more non-overlapping channels are available for use. As you can see in Figure 40 a 40 MHz
ETSI
FCC
arun_0319

channel overlaps two of the three available channels in the 2.4 GHz frequency band.
3 4 5 6 7 8 9
1 6 11
Channel 1 2 3 4 5 6 7 8 9 10 11 12 13
Center frequency 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472
Figure 40 The 2.4 GHz band is not suitable for 40 MHz channels
Aruba recommends that customers do not use 40 MHz channels in the 2.4 GHz
band due to the lack of available bandwidth and high chance of interference with
legacy 802.11b/g networks. While it is possible to enable these channels, the
end result is fewer overall channels and a decrease in throughput.
Improved OFDM Subcarriers
Orthogonal frequency-division multiplexing (OFDM) is the encoding scheme used in Wi-Fi
transmissions. OFDM splits a single channel into very small subcarriers that can transport
independent pieces of data as symbols. Each symbol represents some amount of data, which
depends on the encoding scheme. The data subcarrier count has increased from the original 48 to 52
subcarriers in 20 MHz channels, 108 subcarriers in 40 MHz channels, and 234 subcarriers in 80 MHz
channels. This increase means that more data channels are available to carry traffic. Each additional
subcarrier can carry data over the channel, which increases throughput. In Figure 41 you can see the
difference in sub-carriers that 802.11n brings to 20 MHz channels, as well as the number of carriers
available with 40 and 80 MHz channels.
Figure 41 Increase in subcarriers increases throughput
arun_0345

To see how this directly affects data rates, Table 5 shows the difference between speeds in legacy
rates and high throughput (HT) rates. Wi-Fi engineers can use this information to compare rates
used under 802.11a/g to the new HT rates used in 802.11n. For more information about this
comparison, see Modulation and Coding Scheme Index.Modulation and Coding Scheme Index on
page 61.
Table 5 802.11a/g vs. 802.11n (one spatial stream)
HT rates with 800 ns guard interval
802.11a/g 802.11n (1 SS)
6  6.5
12  13.0
18  19.5
24  26.0
36  39.0
48  52.0
54  58.5
N/A  65.0
To read more about 802.11ac coding schemes and MCS Indexes, read the Aruba
Networks '802.11ac In-Depth' paper at:
http://www.arubanetworks.com/pdf/technology/whitepapers/WP_80211acInDepth.pdf
Space Time Block Coding and Maximal Ratio Combining
MIMO also uses diversity techniques to improve the performance. Between two communicating
stations, one station can have more antennas than the other. If there are more transmit antennas than
receive antennas, Space Time Block Coding (STBC) can be used to increase the signal-to-noise ratio
(SNR) and the range for a given data rate. For STBC, the number of transmit antennas must be
greater than the number of spatial streams.
The operation of Maximal Ratio Combining (MRC) is dependent on the number of available receive
radio chains. When there is more than one receive chain, the MRC technique combines the signals
received on multiple antennas. The signals can come from one or more transmit antennas. When the
signals are combined, the SNR is improved and the range for a given data rate is increased.
Short Guard Interval
The guard interval is the spacing between OFDM transmissions from a client. This interval prevents
frames that are taking a longer path from colliding with subsequent transmissions that are taking a
shorter path. A shorter OFDM guard interval between frames, from 800 ns to 400 ns, means that
transmissions can begin sooner in environments where the delay between frames is low.
Understanding MAC Layer Improvements
Moving up the OSI reference model, the 802.11n and 802.11ac standards also includes several
MAC-layer technologies to greatly improve the efficiency and throughput of wireless transmissions.
These are A-MSDU, A-MPDU and block acknowledgements.

A-MSDU
Aggregate MAC Service Data Unit (A-MSDU) allows stations that have multiple packets to send to a
single destination address and application to combine those frames into a single MAC frame. When
these frames are combined, less overhead is created and less airtime is spent on transmissions and
acknowledgements. A-MSDU has a maximum packet size of 7935 bytes. Figure 42 shows how A-
MSDU aggregation occurs.
Applications
MSDU (MAC Service Data Unit)
MAC processing
MPDU (MAC Protocol Data Unit)
Aggregated MSDU format (A-MSDU) PHY layer
Figure 42 A-MSDU aggregation
A-MPDU
Aggregate MAC Protocol Data Unit (A-MPDU) combines multiple packets that are destined for the
same address but different applications into a single wireless transmission. A-MPDU is not as efficient
as A-MSDU, but the airtime and overhead is reduced. The maximum packet size is 65535 bytes.
Figure 43 shows the operation of A-MPDU operation.
Applications
MSDU (MAC Service Data Unit)
MAC processing
MPDU (MAC Protocol Data Unit)
Aggregated MPDU format (A-MPDU) PHY layer
Figure 43 A-MPDU aggregation
MAC processing
MAC processing
P1 P2 P3
P1 P2 P3
arun_0315arun_0316
P1 P2 P3
MAC
header
P1 P2 P3
MAC
header
P1 MAC
header
P2 MAC
header
P3

Block Acknowledgement
Block acknowledgements confirm that a set of transmissions has been received, such as from an A-
MPDU. Only the single acknowledgement must be transmitted to the sender. Block
acknowledgements also can be used to acknowledge a number of frames from the same client that
are not aggregated. One acknowledgement for a set of frames consumes less airtime. The window
size for the block acknowledgement is negotiated between AP and client. Figure 44 shows the two
cases of block acknowledgement in action.
Aggregate MPDU is a special case requiring block acknowledgement
Figure 44 Block acknowledgement of multiple frames
Transmit Beamforming (802.11ac)
The 802.11ac standard introduces a standards-based Transmit Beamforming (TxBF) as part of the
11ac standard. Beamforming improves range and performance by using sounding frames between
the client and AP, by varying phase and amplitude across multiple streams to direct the RF energy
towards the client. Figure 45 shows a graphical example of how TxBF works.
Figure 45 Transmit Beamforming (TxBF)
header Ack P1, P2, ... P4
P2 headerP3 headerP4 header
Block acknowledgement covers many frames
in one acknowledgement
header Ack P1, P2, ... P3
P1 header
arun_0318
P3 P2 P1 header

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