IEEE 802.20:
ent than any other current IEEE 802 standard in
that no other project supports full vehicular
Characteristic                                                     Target value

 Mobility of vehicular mobility classes  ...
that would seek to illegitimately discover the
MBWA data values are based on the design
                    1                                                            ...
hand, exhibits the inverse behavior. While it
does not perform well with regard to in-cell
eight mobile stations communicated via eight
 Parameter                                                                   ...
Network              802.16e                                            802.20                                            ...
IEEE 802.20 is                                    WHY MODIFICATIONS TO                                                    ...
architecture must be one of the following archi-
tectures: point-to-multipoint topology, mesh net-
work topology, and hybr...
From a technological
perspective, there
are several hurdles
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  1. 1. ACCEPTED FROM OPEN CALL IEEE 802.20: MOBILE BROADBAND WIRELESS ACCESS WALKER BOLTON, FEDEX SERVICES YANG XIAO, THE UNIVERSITY OF ALABAMA MOHSEN GUIZANI, WESTERN MICHIGAN UNIVERSITY ABSTRACT range of their coverage areas from a few hun- dred feet to 30 miles. The IEEE 802.20 MBWA, This article provides a survey of the emerging so-called MobileFi, may change the direction of IEEE 802.20 standard, also known as mobile wireless networking, and sits on existing cellular IEEE 802.20 tower broadband wireless access. It provides an intro- towers, promising the same coverage area as a duction to the activities with regard to this stan- IEEE 802.20 tower mobile phone system with the speed of a Wi-Fi Router Router dard, including purpose and scope that the connection. MobileFi is for truly mobile, high- IEEE 802.20 tower specification defines. The relationship with other speed data, and truly mobile rates of 20 Mb/s Router similar standards such as IEEE 802.16e and 3G are possible. Both IEEE 802.16e and IEEE are discussed as well. Various technical details of 802.20 aim to combine the benefits of mobility, Router IEEE 802.20 tower the standard are presented, including quality of standardization, and multivendor support. Router Router service parameters, data rates available to end MobileFi will increase the coverage or mobility users, application support, and security. Charac- compared with WLAN and WiMax. IEEE teristics that the air interface should provide, 802.11, IEEE 802.16, and IEEE 802.20 all define specifically in regard to the physical and medium Medium Access Control (MAC) protocols with access control layers, are detailed as well. several different physical (PHY) layer specifica- tions dependent on the spectrum of use and the INTRODUCTION associated regulations. The authors survey The IEEE 802.20 MBWA is an efficient pack- In March 2002, the Mobile Broadband Wireless et-based air interface that is optimized for the the emerging IEEE Access (MBWA) Study Group was formed with- transport of IP-based services to enable world- in the IEEE 802.16 committee to determine the wide deployment of affordable, ubiquitous, 802.20 standard, interest and feasibility of a new broadband wire- always-on and interoperable multivendor less standard. The study concluded that 802.16 MBWA networks. This will be an incredible also known as and MBWA addressed the interests of two differ- boost for users of laptops and PDAs as well as Mobile Broadband ent markets, and thus IEEE 802.20 as an official working group was born. On December 12, 2002, other wireless network devices. It also opens the field for integrating applications, such as target- Wireless Access the IEEE Standards Board approved the estab- ed advertising and video on demand, throughout lishment of IEEE 802.20 Working Group [1]. MBWA networks. (MBWA). They Three main wireless data technologies exist, The IEEE 802.20 standard draft began to be namely, the IEEE 802.11, IEEE 802.16, and developed after an evaluation by the IEEE work- provide an IEEE 802.20 technologies. The IEEE 802.11 ing group concluded that five necessary criteria wireless local area network (WLAN) technology, would be able to be met by the IEEE 802.20 introduction to the so-called Wi-Fi, has been widely deployed in a standard. The five criteria are broad market activities with regard range within 100 m. The IEEE 802.16 wireless metropolitan area network (WMAN) technolo- potential, compatibility, distinct identity, techni- cal feasibility, and economic feasibility [2]. to this standard, gy, so-called WiMax, only supports fixed broad- The working group believed that there was band wireless access system in which subscriber such a broad market potential due to the ubiqui- including purpose stations are in fixed locations. WiMax provides ty of narrowband wireless networks such as cel- data rates of about 10 Mb/s over longish dis- lular networks and the desire in the marketplace and scope that the tances, competing with wired DSL broadband for a technology that would enable mobile users services. The emerging IEEE 802.16e standard to have full access to all Internet-based services specification defines. enhances the original IEEE 802.16 standard with and information. In order to achieve the goal of mobility so that mobile subscriber stations can compatibility, the standard must conform to move during services. WiMax for longer-range IEEE 802.1D (MAC Bridges) and 802.1F (Vir- broadband services has been deployed recently. tual LAN Bridges). Distinct identity is achieved However, Wi-Fi and WiMAX are limited by the due to the fact that IEEE 802.20 is much differ- 84 1536-1284/07/$20.00 © 2007 IEEE IEEE Wireless Communications • February 2007 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  2. 2. ent than any other current IEEE 802 standard in that no other project supports full vehicular handoff, fast resource allocation for both uplink and downlink, quality of service (QoS) that is The purpose and mobility at high sustained data rates. The techni- policy-based and supports both IPv4 and IPv6, as scope of the IEEE cal feasibility has been proven in small-scale pro- well as the ability to be co-deployed with existing prietary systems that use widely available cellular systems [1]. 802.20 standard are components such as modems, radios, and anten- nas. Finally, economic feasibility was proven to TECHNICAL SPECIFICATION OVERVIEW very ambitious. The the working group since cost factors for mobile services and components are well documented. In this section we provide a technical specifica- IEEE 802.20 will fill Furthermore, investors are willing to invest large sums of money in such projects since they believe tion overview including overview, application support, user expectations and QoS, as well as the gap between that the return on investment will be good. Since security. cellular networks IEEE 802.20 is a packet-based access network, its designers believe that it is optimal for data OVERVIEW (low bandwidth and services, which are characterized by high peak There are many technical aspects related to an demand but bursty requirements overall [2]. IEEE 802.20 system, including peak user data high mobility) and This article provides a timely survey on IEEE rate, peak aggregate data rate, spectral efficien- 802.20, including its mission, technical specifica- cy, bandwidth maximum operating frequency, other IEEE 802 tion, system architecture, and MAC/PHY air interface characteristics. Furthermore, some support for security, and so forth. In Table 1 [6], which lists the most important aspects, DL, UL, wireless networks world testing experiments are described, and the FDD, TDD, AES, and RTT stand for downlink, (high bandwidth and relationship with other similar standards such as uplink, Frequency Division Duplexing, Time IEEE 802.16e and 3G are discussed as well. This Division Duplexing, Advanced Encryption Stan- low mobility) article provides a survey on wireless mesh net- dard, and round-trip time, respectively. works, particularly for IEEE 802.20 Mesh net- Although the peak data rate in Table 1 is currently in use, such works. Finally, we briefly introduce the PHY and based on 1.25 MHz channel bandwidth, Table 2 MAC layers in the initial draft of IEEE 802.20. shows that the data rates to the end user will as IEEE 802.11 scale accordingly for available bandwidth of an WLANs and IEEE MISSION MBWA base station [1]. Both 1.25 and 5 MHz channel bandwidths will be supported. These are 802.16 WMANs. The mission of the IEEE 802.20 working group peak data rates and the average data rates deliv- is to develop a specification for an efficient ered to the user will more than likely be substan- packet-based air interface optimized for IP- tially less, depending on various network factors. based services, with the goal of enabling world- Simulations show that the average user data wide deployment of affordable, ubiquitous, rates in a loaded system shall be > 512 kb/s always-on, and interoperable multivendor mobile downlink and 128 kb/s uplink. broadband wireless access networks. APPLICATION SUPPORT SCOPE The IEEE 802.20-based air interface shall sup- The scope of IEEE 802.20 is to define the PHY port applications such as video, web browsing and MAC layers for interoperable mobile broad- with full graphical capability, e-mail, file upload- band wireless access systems, operating in ing and downloading without size limitations licensed bands below 3.5 GHz, optimized for IP- (e.g., FTP), streaming video and streaming data transport, with peak data rates larger than 1 audio, Virtual Private Network (VPN) connec- Mb/s per user [5]. IEEE 802.20 supports various tions, Voice-Over-Internet Protocol (VoIP), vehicular mobility classes up to 250 km per hour instant messaging, and online multiplayer gam- in a MAN environment and targets spectral effi- ing, and all while maintaining an always-on con- ciencies, larger sustained user data rates, and nection similar to Cable modem or xDSL. With larger numbers of active users [5]. regard to voice services, the MBWA will support VoIP services by leveraging QoS features that PURPOSE deal with latency, jitter, and packet loss [6]. The purpose and scope of the IEEE 802.20 stan- dard are very ambitious. The IEEE 802.20 will USER EXPECTATIONS AND QOS fill the gap between cellular networks (low band- The MAC layer should be able to handle more width and high mobility) and other IEEE 802 than 100 simultaneous active sessions per sector. wireless networks (high bandwidth and low An active session is defined as a time duration mobility) currently in use, such as IEEE 802.11 during which a user can receive and/or transmit WLANs and IEEE 802.16 WMANs. It will pro- data with a short delay, less than 25 ms with vide seamless integration between the three probability of at least 0.9. Certain applications domains of work, home, and mobile [6], allowing such as VoIP, however, will have to be given a user to have a single connection that provides higher priority with respect to delay in order to for their networking needs wherever they go, by satisfy QoS requirements. laying the groundwork for a worldwide network Velocities, both on foot (or even stationary), that is cost effective, spectrum efficient, always as well as vehicular speeds, will undoubtedly play on, and interoperable. More specifically, it will a large role in the throughput that the user can offer transparent support of real-time and non- expect to see. As users’ speed increases, average real time applications, always-on connectivity, throughput available to the user degrades. Table universal frequency reuse, support of intertech- 3 illustrates the amount of degradation that the nology roaming and handoff (e.g., from MBWA IEEE 802.20 working group expects to see for a to WLANs), seamless intercell and intersector typical MBWA implementation. IEEE Wireless Communications • February 2007 85 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  3. 3. Characteristic Target value Mobility of vehicular mobility classes Up to 250 km/h Sustained spectral efficiency > 1 b/s/Hz/cell DL peak user data rate > 1 Mb/s * UL peak user data rate > 300 kb/s * DL peak aggregate data rate per cell > 4 Mb/s * UL peak aggregate data rate per cell > 800 kb/s * Airlink MAC frame RTT < 10 ms Bandwidth Examples: 1.25 MHz, 5 MHz Cell sizes Appropriate for ubiquitous MANs and capable of reusing existing infrastructure Spectrum (maximum operating frequency) < 3.5 Ghz Spectrum (frequency arrangements) Supports FDD and TDD frequency arrangements Spectrum allocations Licensed spectrum allocated to the mobile service Security support AES * Targets for 1.25 MHz channel bandwidth. This represents 2 × 1.25 MHz (paired) channels for FDD and a 2.5 MHz (unpaired) channel for TDD. For other bandwidths, the data rates may change. I Table 1. Specification overview. With regards to QoS, the IEEE 802.20 MAC the medium. Because wireless systems pass and PHY layers are the primary components information through the air, they are especially responsible for providing efficient QoS to users. vulnerable to the threat of interception, in which The system should be intelligent enough to rec- a malicious third party is able to gain access to ognize that a user may be using several different information that is being passed over the net- applications with differing QoS requirements at work. The IEEE 802.11-based systems in partic- the same time. For example, the user may be ular have been criticized by security experts due browsing the web and participating in a video to their lackluster security enforcement. Know- conference that has separate audio and video ing this, and knowing that a successful commer- streams associated with it. Clearly, the two ser- cial deployment of MBWA systems will depend vices differ enough as to need separate QoS on no small part to the level of trust people have negotiations. The system should be able to rec- in the security of the system, mechanisms that ognize and categorize various kinds of IP traffic provide for an optimal level of security are being based on specific packet flows associated with thoroughly studied and built into the IEEE each, such as delay, bit rate, error rate, and jit- 802.20 specification. ter. The bit rate, or data rate, should scale from Security in a MBWA system is centered the lowest allowable data rate to the maximum around three major factors: protecting against rate supported by the MAC/PHY. Delivery theft of service on behalf of the service provider, delay, also known as latency, should be in a protecting the privacy of the user, and deterring range from 10 ms to 10 sec. It should be noted, denial-of-service attacks. A mechanism for however, that 10 ms is the targeted objective for authenticating the mobile station as well as the round-trip delay time and that even 50 ms is base station that is mutually agreeable will be considered by the IEEE 802.20 working group to implemented as well. A method that protects the be way too high. The error rate, after corrections users’ privacy and data, possibly with the aid of have been made in the MAC and PHY, should encryption, shall be provided. be in the range from 10E-8 to 10E-1. Delay vari- AES is the encryption algorithm that will be ation, also known as jitter, should fall in the used due to the fact that it has been extensively range from 0 to 10 s [6]. evaluated and scrutinized by cryptographic experts worldwide and found to be a sound basis SECURITY for secure encryption. The encryption should be Wireless networking is inherently less secure done using either a stream cipher (i.e., AES in than wired networking. This is a proven fact that stream cipher mode) or a block cipher. The has been validated many times over by security encryption method should be implemented in a experts and is simply a result of the nature of way as to provide user anonymity from those 86 IEEE Wireless Communications • February 2007 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  4. 4. that would seek to illegitimately discover the 1.25 MHz 5 MHz identity of a particular user. Furthermore, the system should provide for the following four combinations of privacy and integrity: encryption Peak user data rate (downlink) > 1 Mb/s > 4 Mb/s and message integrity, encryption and no mes- sage integrity, message integrity and no encryp- Peak user data rate (uplink) > 300 kb/s > 1.2 Mb/s tion, and no message integrity and no encryption [6]. Peak aggregate data rate per cell (downlink) > 4 Mb/s > 16 Mb/s However, encrypting data is only one piece of the larger puzzle of securely transmitting data in Peak aggregate data rate per cell (uplink) > 800 kb/s > 3.2 Mb/s a network environment. While encryption is an important mechanism for transmitting data I Table 2. Available bandwidth at an MBWA base station. securely, equally important is for both parties to be confident that they are actually transmitting data with whom they think they are. In the par- ticular case of an MBWA system, the base sta- 3 km/h 120 km/h tion needs to be protected against unauthorized access by a rogue mobile station, and the mobile 1.25 MHz station needs to be sure they are talking to an actual base station and not a third party. This Downlink 2.5 1.25 process is known as authentication and is the basis upon which trust is formed in a networked Uplink 1.25 0.94 environment. Authentication for both the base station and 5 MHz mobile stations will be based on digital signa- tures that use the RSA algorithm as the signa- Downlink 10.0 5.0 ture primitive. The digital certificate contains information about the owner of the certificate Uplink 5.0 3.75 and its public key. RSA modulus ranges from a minimum of 1024 bits to a maximum of 2048 bits All values are for the average aggregate data [7]. The mechanism by which the parties will throughput (Mb/s/sector). exchange keys is public and will use a protocol known as elliptic curve cryptography [7]. The I Table 3. Channel bandwidth — FDD system. public keys are certified using the RSA-based digital certificates of the corresponding party. Elliptic curve cryptography was first proposed ing cells’ signals for handoff support, mainte- in the 1980s by Victor Miller and Neal Koblitz nance, and QoS monitoring. Some of these mea- [8]. It has been heavily scrutinized for close to 25 surements should be reported to the opposite years and no flaws have been found by crypto- side of the air link on a periodic basis, and/or graphic experts. Although the protocol has not upon request. gained widespread fame in the past, it is being considered more and more by designers of LAYERED ARCHITECTURE AND MAC STATES secure, modern systems. It has been standard- The air interface should support a layered archi- ized by the National Institute of Standards and tecture that will provide for a distinct separation Technology (NIST), the American National of functionality between the user, data, and con- Standards Institute (ANSI), as well as the IEEE trol planes. The air interface should support [8]. Elliptic curve cryptography is especially multiple MAC protocol states with fast and appropriate for a standard such as IEEE 802.20 dynamic transitions among them. The purpose because it requires less storage, less power, less of the aforementioned dynamic switching capa- memory, and less bandwidth than other systems bility is twofold: to improve system capacity and [8]. Furthermore, elliptic curve cryptography provide for a seamless networking experience on gives the implementer more for less in terms of the part of the user, that is, good end-to-end key sizes. While the current key-size recommen- TCP/IP performance. System capacity is dation for public schemes is 2048 bits, a vastly increased due to the fact that the dynamic smaller 224-bit elliptic curve cryptography key switching capability is much like to an operating offers the same level of security [8]. A smaller system that efficiently manages process manage- key size means less computational power is ment: when users are not actively sending or expended during the encryption and decryption receiving data, they are placed in a dormant process, thus saving both time and power con- state that requires fewer system resources to sumption. maintain [9]. RESOURCE ALLOCATION OVERVIEW OF AIR INTERFACE With regard to resource allocation, the air inter- CHARACTERISTICS face should support fast resource assignment and release procedures on both the uplink and The air interface shall support measurements in downlink channels. This is especially well suited the physical layer of both the base station and to the bursty requirements of IP applications the mobile station, including signal strength, sig- and, when combined with the dynamic transi- nal quality, error rates, access delays, session tioning between MAC protocol states, should interruption, effective throughput, and neighbor- allow for maximum system utilization [9]. IEEE Wireless Communications • February 2007 87 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  5. 5. MBWA data values are based on the design 1 objectives, not empirical testing data. The importance of designing a system with 0.9 high-spectral-efficiency characteristics cannot be 0.8 underestimated. Spectral efficiency measures the ability of a wireless system to deliver information 0.7 within a given amount of radio spectrum related b/s/Hz/Cell 0.6 to system capacity [10]. In many systems, espe- cially those that are highly populated, it is likely 0.5 to become one of the primary determinants of 0.4 system economics. 0.3 ArrayComm, a company that is actively work- ing on advanced antenna technologies to 0.2 improve spectral efficiency rates in future sys- 0.1 tems, notes that there are numerous benefits of high spectral efficiency, including high aggregate 0 capacity, higher per-user quality and service lev- EDGE 1X RTT els, higher subscriber density per base station, UMTS small spectrum requirements, as well as lower EV-DO MBWA capital and operational costs in deployment [10]. I Figure 1. Sustained spectral efficiency comparisons. USER DATA RATE MANAGEMENT The air interface should automatically select the optimized data rate that the user can achieve HANDOFF based on various network loading factors and in a manner that is consistent with and aware of Both intersector and intercell handoff proce- RF environment constraints. User data rates dures at stationary all the way up to full vehicu- should degrade gracefully so as to maintain an lar speeds should be supported. The concepts of appropriate frame error rate performance [9]. cell and sector are similar to those in cellular network. The handoff procedures should be OFDM designed and implemented in a way that mini- An IEEE 802.20 system should be designed such mizes both packet loss and latency and provides that it uses orthogonal frequency-division multi- for an uninterrupted IP packet transmission [9]. plexing (OFDM) to multiplex data signals Furthermore, the handover should be imple- together [11]. When using OFDM, different mented in a soft handoff way: the mobile station mobile stations within the same cell will be should have the ability to monitor the signal assigned different sinusoidal waveforms. These strength of surrounding base stations and, when waveforms are known within an OFDM-based the need for handoff occurs, register with the system as tones and are used due to the fact that new base station before breaking off from the they are the only functions that preserve the current base station [7]. orthogonality of the signals over multipath wire- less channels [11]. One of the major advantages LATENCY with using an OFDM approach is that there is As mentioned above, an MBWA system should no intracell interference. Also, OFDM allows be able to distinguish different types of packets much higher system capacities than those that and categorize them into certain classes, each of can be achieved using CDMA. The approach which has distinct QoS requirements in order to also allows for rapid transmission of short mes- function properly. Given these requirements, the sages, such as those that are used for control. air interface should minimize RTT and RTT This is crucial in an MBWA system, in which Acknowledgments for a given QoS class. The various user states at the mobile station need to RTT over the air link for a MAC data frame be employed and communication with the base should be no larger than 10 ms [9]. station needs to occur as rapidly as possible. This leads to efficient sharing of channel SPECTRUM resources among mobiles and a system that uses The air interface should be designed to operate fewer resources overall. For TDMA/FDMA, in the portion of the licensed spectrum below 3.5 users are orthogonal for in-cell interference, but GHz. The MBWA system frequency plan should must design for worst-case number of frequency provide for both paired and unpaired channel reuse for out-of-cell interference. For CDMA, plans with multiple bandwidths (e.g., 1.25 MHz users nonorthogonal for in-cell interference, and or 5 MHz), so as to allow co-deployment with users see average interference for out-of-cell existing cellular systems. interference. For OFDM, users are orthogonal for in-cell interference, and users see average SPECTRAL EFFICIENCY interference for out-of-cell interference. This is For a loaded network, the spectral efficiency why the IEEE 802.20 system designers decided should be larger than 1 b/s/Hz/cell. The air inter- to use an OFDM approach to multiplex data. face should also support universal frequency While traditional TDMA and FDMA systems reuse [9]. Figure 1 shows the sustained spectral achieve the desired characteristic of users being efficiency that the MBWA designers hope to orthogonal to one another for in-cell interfer- achieve relative to other competing technologies ence, their performance suffers with regard to [1]. It should be kept in mind, however, that the out-of-cell interference. CDMA, on the other 88 IEEE Wireless Communications • February 2007 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  6. 6. hand, exhibits the inverse behavior. While it does not perform well with regard to in-cell (for FDD) should be of the paired spectrum variety, consisting of two 1.25 MHz channels The IEEE 802.20 interference, it does provide for average-case that use FDD. The system should have the abili- standard is being out-of-cell interference characteristics. Thus, ty to support six or more sectors per cell, yet although OFDM is more complex and has more should be able to scale downward in order to designed with a overhead associated with it, it does provide for accommodate a more typical load of three sec- good in-cell and out-of-cell interference behav- tors per cell. A Doppler tolerance of > 400 Hz modular approach ioral patterns that are deemed important in a is needed in order to satisfy the requirement for working MBWA system. full vehicular mobility. At a carrier frequency of in mind. This 2 GHz, the system will be able to support a modularity means SYSTEM ARCHITECTURE Doppler tolerance of 400 Hz, thus enabling sup- port of vehicular speeds at 228 km/h [13]. A fre- that there should be An IEEE 802.20 MBWA system should operate quency reuse factor of one or less is needed so in a traditional cellular environment, meaning that the same frequencies can be reused in all a clear separation of that cell sizes of macro, micro, and pico should cells and sectors. Through the employment of be supported. The system should provide for directed adaptive antennas, it may be possible to functionality in the non-line-of-sight outdoor to indoor usage as well use the same frequency band more than once in as outdoor coverage [6]. Service attributes that a the same cell or sector [13]. system between the system should provide for, but are not limited to, low end-to-end latency, a high-frequency-reuse user, data, and network, high capacity per sector/per carrier, MAC-RELATED AIR INTERFACE control. and a fully mobile broadband user experience. CHARACTERISTICS The system designers hope to achieve a design that looks and functions at a high user level like Table 4 gives a summation of the characteristics that of a more traditional wired IP network, that the MAC layer is responsible for in the air while at the same time maintaining the obvious interface [13]. The air interface should be given advantage of full mobility. the capability to support multiple MAC protocol In order to increase the available coverage states with fast transitions among them. Three area, increase throughput available to the users, states should be supported: On, Hold, and Sleep. and enable a higher overall spectral efficiency, In the On state, the user is actively using system advanced antenna technologies such as multi- resources to transmit and receive data. In order antenna at the base station should be employed, to make better use of the system resources and although implementation at the mobile station provide for higher system efficiency, the Hold should be optional and determined by the mar- state should be initiated when the user is tem- ket, based upon economic factors [6]. The base porarily not using the system. When a mobile station should also support antenna diversity, but user is completely inactive, the Sleep state should just as in the case of multi-antenna technology, be initiated. implementation at the mobile station should not User states are extremely beneficial, since a be mandatory. significant amount of air-link resources related The IEEE 802.20 standard is being designed to power control, timing control, and traffic with a modular approach in mind. This modular- requests, are required to enable users (i.e., users ity means that there should be a clear separation that are in the On mode) to actively send and of functionality in the system between the user, receive traffic [14]. The fewer the air-link data, and control [6]. The PHY and MAC layers resources that are consumed by individual users, should each have a set of clearly defined respon- the more users can be added to a particular cell, sibilities that are encapsulated within the respec- ultimately decreasing total system costs for both tive layers. However, this does not mean that the the user and the operator, as well as increasing two layers should not be able to effectively com- total system efficiency. municate and pass information among them The air interface should support more than through a well-defined service interface [12]. 100 active users per sector or per cell, where The use of a layered architecture is consistent active is defined as a user being in either the On with the design methodologies employed in the or Hold state. Testing has shown that 100 con- current IEEE 802-based systems. Together, the current active users are required to fully utilize PHY and MAC layers, also known as layers 1 the 1 to 2 Mb/s bandwidth available to a given and 2, will make up the services that specify sector or cell [14]. This testing is based on the what should be delivered to an IP-based layer 3 observation that most users will be spending the or a switching layer [12]. The MAC layer should majority of their time using medium-bandwidth be optimized to support a specific PHY imple- http services such as web browsing. In the partic- mentation. If more than one PHY implementa- ular testing that the researchers did at Dart- tion is to be used, the MAC layer should be mouth College, they found as much as 53 percent designed so that it has a PHY-specific layer as of all web traffic to be http traffic [14]. well as a more general part [6]. The number of users that are inactive and in the sleep state can be almost unlimited [13]. In order to be utilized effectively and to minimize PHY RELATED AIR INTERFACE noticeable effects on behalf of the user, transi- CHARACTERISTICS tions between the states should be both fast and dynamic. Next, we provide an overview of the various A mechanism for waking users up from the characteristics that the PHY portion of the air Sleep state and bringing them into an active On interface should achieve. The channel bandwidth state must be implemented. The mechanism that IEEE Wireless Communications • February 2007 89 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  7. 7. eight mobile stations communicated via eight Parameter Proposed value carriers with a base station located approximate- ly 1 km away that was configured with an eight- Number of active users per sector/cell > 100 element adaptive antenna array [7]. Line of sight was present between the base station and the Transition from active on to active hold state < 100 ms mobile stations, and it seemed as if this would provide for an optimal test setup. However, the Transition time from active hold state to active on state < 50 ms engineers that were responsible for performing the test stated that line of sight existed between Transition time from active hold state to inactive sleep state < 100 ms the mobile stations and the base station, creating the most challenging special processing scenario Access time from inactive sleep state to active on state < 200 ms due to the scarcity of path diversity and the closeness of the mobile stations [7]. Although Paging signal periodicity < 100 ms specific measurements were not present con- cerning the distance between each of the mobile Paging signal duration < 1 ms stations, they are estimated to be approximately less than 1 to 2 km apart. Minimum scheduling interval < 2 ms In the experiment, data streams were gener- ated from each of the mobile stations to the UL request time < 10 ms base station, denoting an uplink data stream, as well as from the base station to each of the Intersector/cell handoff time < 200 ms mobile stations, denoting a downstream data channel. Average user data rates for the down- I Table 4. MAC related air interface characteristics. stream channel were 1 Mb/s, while average user data rates on the upstream channel were 330 kb/s [7]. These rates are in line with what the the designers of the MBWA standard are choos- MBWA system designers predicted and hoped ing to use is known as paging. The primary to be able to deliver upon commercial deploy- advantage that paging has to offer lies in its abil- ment. ity to allow a mobile station to conserve energy The only problem with the test data gener- by way of the Sleep state and still allows for the ated is that although it does use worst-case sce- mobile station to receive incoming packets as narios with regards to the line of sight and needed. This is especially important for real- positioning of the mobile stations, it is not rep- time applications such as voice and instant mes- resentative of a commercially deployed system saging in which the station needs to be due to the fact that it is highly unlikely there responsive at all times to incoming packets [13]. will be a designated carrier for each mobile In order to reduce the delay associated with station. Because of this, the team of testers waking a user up, the MBWA air interface decided to perform a second experiment in should support the ability to send paging signals which all the testing variables were kept the as often as once every 100 ms. The paging sig- same as the first except four, instead of eight, nals themselves should be very brief, however, carriers were employed. However, each carrier they should last no longer than 1 ms, so that a supported two spatial channels yielding a total mobile system in Sleep state can listen for a of eight virtual carriers. By only using four car- page very briefly and then be able to go back to riers, each of the carriers is reused twice within sleep again. This is especially important for a the same sector [7]. battery-powered mobile device that seeks to The second set of testing data shows that in a retain as much energy as possible, as it allows much more stringent testing environment, the the device to expend the least amount of energy system still performed admirably [7]. Average possible during the Sleep state [13]. user data rates on the downlink were on par The air interface should support both inter- with that seen in the first test, while average sector and intercell handoff in a time that is data rates on the uplink actually increased. Fur- comparable to the state transition time: 200 ms thermore, the system is being designed with [13]. This requirement is crucial for allowing much more than just user data rates in mind, minimal packet loss and latency, two properties and spectral efficiency plays a key role as well. that allow for seamless IP packet transmission. As noted above, the MBWA system designers are targeting a sustained spectral efficiency in a REAL WORLD TESTING loaded system of at least 1 b/s/Hz/cell. By com- bining the uplink and downlink data rates in the One of the promising things concerning the second set of testing data and accounting for the future potential of MBWA systems is that not fact that four carriers occupied 2.5 MHz, a spec- only is the technology behind it proven itself to tral efficiency of 4.3 b/s/Hz/sector is achieved [7]. be both sound and economical, as discussed A spectral efficiency of 4.3 b/s/Hz/sector is well above, but there have been real-world results in excess of the required 1 b/s/Hz/sector. Even that have validated the predictions of theoretical though the entire system consisted only of eight models. The proposed air interface has been mobile stations, the fact that the results achieved implemented and tested and performed accord- were consistent with expectations in regard to ing to expectations. Testing was done on Novem- both user data rate and spectral efficiency is ber 3, 2003 [7]. extremely promising. The testing was performed at a frequency of Commercial rollout of the technology has 2.3 GHz in a dense urban environment in which already begun to be rolled out in limited test- 90 IEEE Wireless Communications • February 2007 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  8. 8. Network 802.16e 802.20 3G High-data-rate fixed wireless user Fully mobile, high-throughput data Mobility Voice user requiring data services with adjunct mobility service user Data pattern Symmetric data services Symmetric data services Highly asymmetricdata services Support of low-latency data and Services Support of low-latency data services Lack of support for low-latency services real-time voice services Local/regional mobility and Global mobility and roaming sup- Roaming Global mobility and roaming support roaming support port Extensions to 802.16a MAC and New PHY and MAC optimized for MAC/PHY W-CDMA, cdma2000 PHY packet data and adaptive antennas Technology is optimized for and Technology is optimized for full Technology is an evolution of GSM or Technology backward compatible with fixed mobility IS-41 stations Bands Licensed bands, 2–6 GHz Licensed bands below 3.5 GHz Licensed bands below 2.7 GHz Bandwidth Typical Channel BW > 5 MHz Typical channel BW < 5 MHz Typical vhannel BW < 5 MHz I Table 5. Relationship to other standard activities. ing areas, although the services are based on RELATIONSHIP TO IEEE 802.16E AND 3G an unratified version of the IEEE 802.20 stan- Table 5 outlines many key differences among dard. Nextel is currently servicing a 1300- IEEE 802.20, IEEE 802.16e, and 3G cellular square-mile region of the Research Triangle networks [1]. The differences between IEEE Park (RTP) area of North Carolina with their 802.20 and IEEE 802.16e are less pronounced, version of the service, using what they call and therefore need to be investigated with FLASH-ODFM, which stands for Fast Low- greater scrutiny. In many ways, the IEEE 802.20 Latency Access with Seamless Handoff, specification is an attempt to bridge many of the Orthogonal Frequency Division Multiplexing technical differences between IEEE 802.16e and [15]. The service gives users a PC card for use 3G networks and combine them in a manner with their notebooks while providing a type of that is both powerful and accessible for the end modem for use in stationary PCs. The testing user. The standard is an attempt to combine the was performed between RTP and Syracuse, global mobility and roaming support of 3G with NY. A Chariot server located in Syracuse was the high-bandwidth and low-latency characteris- placed on a public IP address and executed tics of IEEE 802.16e. IEEE 802.16e handles low throughout tests directly between itself and mobility such as walking with PDA or laptop in IBM ThinkPad T41 with 512 MB of RAM and the 2 to 6 GHz licensed bands, whereas IEEE Windows XP Pro SP2, connected wirelessly in 802.20 handles high-speed mobility up 250 km RTP. TCP and UDP performance metrics were per hour in bands below 3.5 GHz. collected for both downstream and upstream flows [15]. The test results are clearly positive, IEEE 802.20 ADVANTAGES showing the wireless service to provide good One of the key advantages of the IEEE 802.20 upstream and downstream user rates while at standard is that it has been designed from the the same time delivering latency measures that ground up to be a fully mobile system without are almost on par with a broadband wired sys- the constraints of maintaining backwards com- tem. While the results are positive, they do not patibility, such as in the case of IEEE 802.16e. indicate how the system performs at vehicular This is a crucial characteristic of the IEEE 802.20 speeds, which is one of the primary advantages standard that cannot be emphasized enough. The that wireless broadband offers over standard IEEE 802.20 Executive Committee Study Group wired services. It is expected that the perfor- found that not only would it be extremely diffi- mance has clearly dropped. Furthermore, the cult to develop a fully mobile system while main- latency stayed relatively low, a factor that is taining backwards compatibility with existing immensely important if the service is to be suc- infrastructure, it would be impossible with today’s cessful. Just as the IEEE 802.20 system design- technology [3]. Enhancing existing fixed wireless ers envisioned, it looks as though the standard systems to support full vehicular mobility may is on target to provide ubiquitous web, e-mail, not be possible, since mobility support is a non- and instant messaging applications as well as trivial functionality of the system that goes unprecedented benefits to mobile-centric beyond just handoffs and fast multipath fading. industries such as police, fire, and emergency Therefore, IEEE 802.20, a new standard, should services. Nextel service plans will start at $34.99 be developed and optimized for data mobility a month [15]. and broadband wireless networks. IEEE Wireless Communications • February 2007 91 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  9. 9. IEEE 802.20 is WHY MODIFICATIONS TO WIRELESS MESH NETWORKS fundamentally data- 3G WILL NOT BE SUFFICIENT A wireless mesh network, as shown in Fig. 2, A great deal of work has been done and contin- includes wireless routers and mobile wireless sta- centric and addresses ues to be done in regard to bringing data ser- tions. Wireless routers communicate with each vices to users via 3G-based systems. Some people other, form a backbone of the wireless network, these issues by have believed that it would be possible to fulfill connect wired networks, and conduct multihop the markets’ need for mobile broadband access communications to forward mobile wireless sta- targeting significantly by making changes to the existing 3G infra- tions’ traffic to/from wired networks. Mobile higher spectral structure [4]. However, 3G is fundamentally a voice-based technology and is not optimized for users’ traffic travels over wireless routers and reaches wired networks such as the Internet. efficiency, lower packet-based delivery services. Many technical Each mobile station also acts as a router, for- deficiencies for carrying data exist, including, but warding packets for other mobile stations. Wire- latencies, and not limited to, spectral efficiency, latency, and less routers may have multiple different wireless the single-threaded call model structure of the interfaces to connect heterogeneous wireless improved user air interface [4]. Other issues include: networks. • Circuit-switched uplink: This is detrimental for Wireless mesh networks combine wired net- experience. This uplink data rates, capacity, and latency. works and wireless networks with wireless technical • Voice-centric channel parameters: Prevents effi- cient scheduling and power control, resulting routers as backbones and mobile stations as users. differentiation in higher latencies and poor performance for People love the convenience of not being bursty packet data. constrained by a wired line. Therefore, we have provides for wide • Contention-based access: For users with bursty remotes for TV, cordless phones, cellular data sessions to regain access to air-link phones, and so on. In a wireless mesh network, a market potential resources, the use of contention introduces mobile user can reach Internet via only a few latency and delay variations. hops of connections, showing much more reli- not adequately • Intracell interference: In 3G air interfaces, the able and practical than ad hoc networks. For addressed by negative impact of intracell interference on spectral efficiency, capacity and performance example, a wireless mesh network can be built on roofs of buildings to provide inexpensive 3G systems. makes it difficult to support high aggregate broadband Internet access, while outdoor cellu- data rates. lar Wi-Fi cells form a wireless mesh network. In • Insufficient support for QoS: QoS support for a wireless mesh network, users can connect to data delivery over the air is currently “best the Internet as long as they are in range of effort” in 3G. another device which connects Internet some- IEEE 802.20 is fundamentally data-centric how. A wireless mesh network for vehicles and addresses these issues by targeting signifi- enables people to access traffic information and cantly higher spectral efficiency, lower latencies, location-based services. and improved user experience. This technical differentiation provides for wide market poten- IEEE 802.20 MESH NETWORKS tial not adequately addressed by 3G systems [4]. The IEEE 802 working groups are working sev- eral wireless mesh standards such as IEEE 802.11s, IEEE 802.15.5, IEEE 802.16a, and WIRELESS MESH NETWORKS AND IEEE 802.20. The IEEE 802.11s extends the IEEE 802.20 MESH NETWORKS IEEE 802.11 standards to enable access points to form backbones of wireless mesh networks. Mobile ad hoc networks, although very popular The IEEE P802.15.5 Mesh Network Task Group in recent years, have failed to find many real was formed in November 2003 to define tech- applications, with the exception of military appli- nologies to support mesh networks of IEEE cations. Most research in wireless ad hoc net- 802.15 Wireless PANs. The IEEE 802.15.5 mesh work researches is far removed from those for networks increase WPANs coverage with short- realistic general-purpose applications. Although er-distance radio transmissions and higher IEEE 802.11 networks have been widely throughput, especially for UWB systems. The deployed, they are seldom used in ad hoc net- IEEE 802.16a supports optional mesh networks works, whereas, wireless mesh networks have a with TDMA-based MAC layer so that all the SSs great potential for general purpose commercial can have direct links with each other. Either the applications such as hotspots, Internet access, BSs can control resource allocations of SSs for public safety, transportation services, and so these direct links, or multiple BSs can coordi- forth. For example, MeshNetworks has imple- nate SSs’ transmissions within one or two hops. mented a system for transportation services, The IEEE 802.20 adopts a cellular architecture enabling users to obtain their own locations and with macrocells, microcells, and picocells. IEEE transportation information in real time at any 802.20 addresses high-speed mobility up to 250 place, any time [16]. A mesh network can be km per hour [18]. used to connect Wi-Fi hotspots in a scalable and Mobile stations in IEEE 802.20 can form a cost-effective manner. wireless mesh network. Furthermore, an IEEE In this section we provide a survey on wire- 802.20 mesh network can connect IEEE 802.16a, less mesh networks and IEEE 802.20 mesh net- IEEE 802.11s, IEEE 802.15.5, 3GPP, and 3GPP2 works. Finally, we present a summary on the to form heterogeneous wireless mesh networks. challenges for wireless mesh networks later. A The IEEE 802.20 systems must be designed to more comprehensive survey can be found in provide ubiquitous mobile broadband wireless [17]. access in a cellular architecture. The system 92 IEEE Wireless Communications • February 2007 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  10. 10. architecture must be one of the following archi- tectures: point-to-multipoint topology, mesh net- work topology, and hybrid of both mesh and point-to-multipoint. The requirements for a Internet Internet mesh network are completely different from those for point-to-multipoint networks. Figure 3 shows a heterogeneous wireless mesh network using both IEEE 802.20 and IEEE Wireless router 802.11, implementing both point-to-multipoint gateway communications and mesh communications, and Wireless router wireless routers are in IEEE 802.20 cover areas, gateway forming wireless backbones. Wireless router Wireless router CHALLENGES FOR WIRELESS MESH NETWORKS There are many challenges for building a large- Wireless router scale and high-performance multihop wireless mesh network, such as compatibility, coexis- tence, scalability, security, QoS, and so forth. The first and most important challenge is Wireless router compatibility. Building total new technologies to Wireless router replace old technologies may be not likely to success in reality based in previous history of networking fields. So far, many IEEE 802.11 products/networks have been deployed, and many more are waiting to be shipped to users. It I Figure 2. Wireless mesh network architecture. is unlikely that people will throw away what they have spent money on for something totally new. Therefore, designing wireless mesh networks compatibility with different architectures and should always take into consideration compati- technologies under different wireless networks. bility with old/current technologies in terms of Furthermore, there are many other issues, radio transmissions, medium access control such as high-speed radio interfaces, system (MAC), security, routing, and so on. resource management, range extension, new The second challenge is the coexistence issue. MAC and routing protocols (constrained by The IEEE 802.20 technologies should fit well compatibility), cross-layer design and optimiza- with AMPS, TDMA, GSM, IS-95, CDMA-2000, tion, mobility, power management, energy effi- WCDMA, 1xEV-DV, 1xEV-DO, HSDPA, ciency protocol, topology management, and EDGE, GPS, IEEE 802.16, IEEE 802.16e, IEEE applications. 802.16a, IEEE 802.15s, IEEE 802.11, IEEE 802.11s, and so on. Such coexistence should con- PHY AND MAC IN THE INITIAL DRAFT sider all aspects of technologies, which pose a great challenge, including radio technologies, During our final revision of this article, the ini- MAC layer, routing, security, QoS, smooth hand- tial draft of IEEE 802.20 was available for us to off, and so forth. This task also paces a road to reference [19]; hence, we summarize the PHY 4G cellular networks. and MAC features as follows. The third challenge is scalability. The perfor- mance of a wireless mesh network should not be PHY degraded significantly when the number of The physical (PHY) layer specification consists mobile stations and the network size increase of two different duplexing modes: Time Division significantly. The current MAC protocols are not Duplexing (TDD) and Frequency Division scalable, as evidenced by the fact that through- Duplexing (FDD), two different forward link put is significantly decreased as the number of hopping modes: SymbolRateHopping and Block- mobile stations increases. The current routing Hopping), two different synchronization modes: protocols are not scalable, as evidenced by the SemiSynchronous and Asynchronous, and two fact that performance is significantly degraded as different multicarrier modes: MultiCarrierOn the number of hops increases. However, design- and MultiCarrierOff. ing total new MAC and routing protocols should Modulation uses OFDM with QPSK, 8PSK, be also limited by the compatibility issue. Con- 16QAM, and 64QAM modulation formats sidering both scalability and compatibility togeth- er is a tough task to pursue. MAC The fourth challenge is security. Although The MAC layer includes session, convergence, security mechanisms and services are designed security, and lower MAC functions. The lower for different wireless networks, there is a great MAC sublayer controls operations of data chan- challenge when these networks coexist together. nels: Forward Traffic Channel and Reverse Traf- Security is especially difficult when compatibility, fic Channel [19]. It includes control channel efficiency, and QoS issues are also considered at MAC protocol, access channel MAC protocol, the same time. shared signaling MAC protocol, forward traffic The fifth challenge is QoS. QoS becomes a channel MAC protocol, reverse control channel very tough task in a heterogonous wireless mesh MAC protocol, and reverse traffic channel MAC network, and it involves many layers, such as protocol. Forward- and reverse-link transmis- MAC, routing, and application layers, as well as sions are divided into units of superframes, IEEE Wireless Communications • February 2007 93 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.
  11. 11. From a technological perspective, there are several hurdles Internet to such an implementation that will be difficult to IEEE 802.20 tower overcome. However, IEEE 802.20 tower the standard is Router Router IEEE 802.20 tower based in large part Router Router on proven, existing Router Router IEEE 802.20 tower techniques and Router Router products and it has fared well in preliminary testing. Fixed links 802.20 cover area Wi-Fi cell I Figure 3. IEEE 802.20 mesh network architecture. which are further divided into units of PHY the standard is based in large part on proven, frames. FDD and TDD superframe timing are existing techniques and products and it has fared used. well in preliminary testing. An FDD forward-link superframe consists of From a business perspective, it will be inter- a superframe preamble followed by several (e.g., esting to see how the service pans out. Is there 24) forward frames, and an FDD reverse-link really room for coexistence with a very similar superframe consists of several (e.g., 24) reversed IEEE 802.16e standard, or will one or the frames. other survive and prove to be the dominant A TDD forward-link superframe consists of a technology? Also, even though the IEEE superframe preamble and several forward 802.20 working group is claiming otherwise, it frames, and a TDD reverse-link superframe con- is our opinion that IEEE 802.20 and 802.16e sists of several reversed frames. will both have to compete with homes that use The default access channel MAC protocol traditional cable and DSL broadband tech- provides an access terminal to transmit by initial nologies and have IEEE 802.11-based local access or handoff via sending an access probe. area wireless networks. The IEEE 802.11 tech- After receiving the access probe, the network nology has improved significantly in recent responds with an Access Grant. years in terms of effective range, throughput, and security. CONCLUSION In conclusion, only the future will tell whether MBWA will prove to be feasible from technolog- The IEEE 802.20 MBWA standard is very ambi- ical and engineering standpoints, as well as from tious and wide-ranging in scope. It is trying to be the standpoint of being a viable business model. the best of all worlds — providing users with a Although many talented people from companies high bandwidth, low latency, always-on Internet all over the world are working hard to make the service at home that also has the capability to standard a success, there is little doubt that move with them whether they are on the road or many obstacles will have to be overcome. at work. Two of the important things the stan- dard has in its favor are the facts that it is being REFERENCES built from the ground up to serve a specific pur- [1] M. Klerer, “Introduction to IEEE 802.20: Technical and pose and it is unencumbered with the need to Procedural Orientation,” IEEE 802.20 PD-04, Mar. 10, maintain backwards compatibility with other 2003. ments.htm standards. From a technological perspective, [2] IEEE 802.20 WG, “Mobile Broadband Wireless Access there are several hurdles to such an implementa- Systems ‘Five Criteria’: Vehicular Mobility,” IEEE 802.20 tion that will be difficult to overcome. However, PD-03, Nov. 13, 2002. 94 IEEE Wireless Communications • February 2007 Authorized licensed use limited to: Biblioteka Glowna i OINT. Downloaded on March 13, 2009 at 05:59 from IEEE Xplore. Restrictions apply.