Scope: DEFINE THE SERVICE Introduce Mobile Broadband – High Data Speeds Same end-user experience as encountered in the fixed networks High speed and low delay (up to 14 Mbit/s in WCDMA Evolved) Flexible bandwidth
PRACTICAL HANDOFF CONSIDERATION
Practical handoffconsiderations AJAL.A.J Assistant Professor –Dept of ECE, (FISAT) TM MAIL: firstname.lastname@example.org
A Taxonomy By Forms Of Energy Taxonomy is the practice and science of classification. . 2
Why Mobile Communication ? People now a days are demanding instant Communication, at the same time they do not want to stick to their Desk or Office. What they want is,Communicationat any place & at any time,even while they are on move.
Trunking Trunking: the channel is allocated on demand and recycle after usage Tradeoff between the number of channels and blocking probability Grade of service – Likelihood of a call is blocked or the delay greater than a threshold during the busiest time. Trunking theory – Erlang, a Danish Mathematician studied how a large population could be accommodated by a limited number of servers. – Erlang capacity: the percentage of line/channel occupied over time
Practical handoff considerations Practical handoffconsiderations / Problem in µcell system ►Umbrella cell ►Cell dragging
architecture to achieve a Single Frequency Network outdoor & indoorUmbrella cell per spot beam Terrestrial repeaters: gap filler coverage same carrier, same codes Co ve ra g e ho le Co ve ra g e ho le Terminal’s rake receiver combines the Satellite & terrestrial repeaters signals as echos of the same signal
Practical handoffs consideration Several problems arise to design a system for wide range of mobile velocities ► High speed vehicles pass through a cell in a matter of seconds • With micro cells addition, the MSC can quickly become burdened ► Pedestrian users may never need a handoff during a call ► Issues • Schemes to handle high speed and low speed users simultaneously • Ability to obtain new cell sites 13
Additional capacity is provided through addition of new cell sites, Difficult to obtain new cell sites Install additional channels and BS at same location of an existing cell By using different antenna heights and power levels, possible to provide large and small cells, which are co-located at single location called umbrella cell ► Provide large coverage area to high speed users minimizing number of handoffs ► Small coverage to slow speed users ► Speed can be estimated by BS or MSC by RSSI 14
CUST ME MER ORATITHI DEVO BHAVA @ holistic approach
WiMAX – Anytime, AnywhereEnterpri SOHO Mobile Resident se ial Confidential Information Locati Intern Securi on Vid Voic et Gamin Based Acce
Handoff (1/2)Handoff : • Cellular system tracks mobile stations in order to maintain their communication links. • When mobile station goes to neighbor cell, communication link switches from current cell to the neighbor cell.Hard Handoff : • In FDMA or TDMA cellular system, new communication establishes after breaking current communication at the moment doing handoff. Communication between MS and BS breaks at the moment switching frequency or time slot. switching Cell B Cell A Hard handoff : connect (new cell B) after break (old cell A) 21
Soft Handoff (2/2)Soft Handoff : • In CDMA cellular system, communication does not break even at the moment doing handoff, because switching frequency or time slot is not required. transmitting same signal from both BS A and BS B simultaneously to the MS Σ Cell B Cell A Soft handoff : break (old cell A) after connect (new cell B) 22
Mobility/Handoff in Umbrella CellsAvoids multiple handoffs. 23
Types of cellsDepending on density of population, various types of cells are used.MacrocellsMicrocellsSelective cellsUmbrella cells
Selective cells It is not always useful to define a cell with a full coverage of 360 degrees. In some cases, cells with a particular shape and coverage are needed. These cells are called selective cells. A typical example of selective cells are the cells that may be located at the entrances of tunnels where a coverage of 360 degrees is not needed. In this case, a selective cell with a coverage of 120 degrees is used.
Umbrella cells An umbrella cell covers several microcells. The power level inside an umbrella cell is increased comparing to the power levels used in the microcells that form the umbrella cell. A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the umbrella cell. This will reduce the number of handovers and the work of the network.
Umbrella cell - Defn By using1.Different Antenna Heights (often on same building / tower) &2. Different power levels It is possible to provide large and small cells which are co-located at a single location. This technique is called umbrella cell approach
Umbrella cells - advantages used to provide :1. large area coverage to high speed users2.small area coverage to low speed users
Smaller Cells: Macro-cell vs Micro-cell vs Pico-cell vs Femto-cell/Wifi offload 2-3 km 500m BS BS Micro-cell Macro-cell 200m 50m BS BS Femto-cell: home/SOHO operated: indoorPico-cell: enterprise-operated: indoor 30
What does this approach ensures ! Umbrella cell approach ensures that1. The No: of handoffs is minimized for High speed users &2.Provides additional microcell channels for pedestrian users 31
Practical handoff considerations•Using different antenna heights and differentpower levels it is possible to provide large andsmall cells which are co-located at a singlelocation. This technique is called umbrella cellapproach and is used to provide large areacoverage to high speed users while providingsmall area coverage to users traveling at lowspeeds.• The umbrella cell approach ensures that thenumber of handoffs in minimized for high speedusers and provides additional microcell channelsfor pedestrian users. 37
•Another practical handoff problemin microcell systems is known as cell dragging. Celldragging results from pedestrianusers that provide a very strong signal to thebase station. The signal strengthdoes not decay rapidly as the usertravels away from the base stationat a very low speed.•The IS-95 code division multipleaccess (CMDA) spread spectrumCellular system provides a uniquehandoff capability that cannotbe provided with other wirelesssystems.unlike channelized wireless systems that assign different radiochannels during a handoff, 39spread spectrum mobiles share thesame channel in every cell.
Leveraging the Ubiquity ofWireless Cell dragging
Cell dragging Cell dragging ► Problem in micro-cell due to high signal strength of pedestrian users. ► Occurs in urban areas when there is a LOS path ► Average signal strength does not decay rapidly even if a user travels well beyond the designed range of cell ► The RSSI may be above the handoff threshold and thus handoff is not made ► This creates potential interference since a user has traveled deep within a neighboring cell and creates Traffic management problem 42
How to solve Cell Dragging Problem ? ► Handoff parameters, threshold must be adjusted carefully ie :1. Handoff Thresholds2. Radio Coverage ParametersMust be adjusted carefully
Cell dragging If there is line of sight path between MS and BS, even when the user has travelled well beyond the designed range of the cell, the received signal at the base station may be above the handoff threshold, thus a handoff may not be made. This creates a potential interference and traffic management problem, since the user has meanwhile travelled deep within a neighboring cell.
@ IS-95 (CDMA) system In IS-95 (CDMA) system ► Provides unique handoff capability that can not be provided in with other wireless systems ► Unlike channelized (hard handoff), SS mobiles share the same channel in every cell. ► Thus handoff does not assign channel but a different BS handles a communication task ► By simultaneously evaluating RSSI from single user, MSC decides which version of the signal is best ► This ability selects between instantaneous received signals from a variety of BS is called soft handoff 45
CONCLUSION Practical handoff considerations There are several problems due to a wide range of mobile velocities. The umbrella cell approach: ► Different antenna heights and different power levels. ► “large” and “small” cells ► Used to provide large are coverage to high speed users while providing small are coverage to users traveling at low speed. Cell dragging problem in microcell systems. ► Results from pedestrians that provide a very strong signal in uplink (e.g., in urban environment when there is a LOS between the subscriber and the BS). ► Necessary handoff may not be made. ► For good solution, handoff thresholds must be adjusted carefully.
Prioritizing handoff – next session Dropping a call is more annoying than line busy Guard channel concept ► Reserve some channels for handoffs ► Waste of bandwidth ► But can be dynamically predicted Queuing of handoff requests ► There is a gap between time for handoff and time to drop. ► Better tradeoff between dropping call probability and network traffic. Reduce the burden for handoff ► Cell dragging ► Umbrella cell