ADAPTIVE ANTENNA AND BEAMFORMING SYSTEMS IN WIMAX

1 Beamforming

Beamforming takes advantage of interference to change the directionality of an antenna array system. A beamformer controls the amplitude and phase of the signal at each transmitting antenna element, to create a pattern of constructive interference (beamspots) and destructive interference (null) in the wavefront. To create a beamspot, the beamformer uses an array of closely spaced antennas, often enclosed in a single enclosure as illustrated in Figure 1. λ/2 antenna spacing between the antenna elements is commonly used (where λ is the wavelength of the transmitted signals, given by λ = c/f; f is the frequency of the transmitted signals, c is the speed of light). By varying the amplitude and phase of each antenna element, the beamformer is able to focus electromagnetic energy (beam) in the desired directions. The beams are directed to intended users, while nulls are focused on other unintended users reducing interference to the unintended users while increasing received SNR for the intended user. This provides a stronger link to the intended user and improves reach and capacity.

 
Figure 1: Beamforming technique.

2 Adaptive Antenna System

AAS is one of the advanced antenna technologies specified in the WiMAX standard to improve performance and coverage. In the AAS system, the transmitter (base station, BS) adaptively tracks a mobile receiver as it moves around the coverage area of the transmitter (BS), and steers the focus of the beam (beam spot) on the receiver unit as it moves. The beam steering method can either be mechanical or electronic. Thus AAS creates narrow beams to communicate with desired user device, which helps to reduce interferences to unintended user devices and improves carrier-to-interference (C/I) and frequency reuse, giving rise to high spectral efficiency. Thus, through the use of adaptive processing (beam steering), AAS improves performance and coverage of the system significantly. In WiMAX networks, AAS will find wide applications both in the point-to-multipoint (PMP) as well as mesh network deployments. In the PMP mode, AAS operates in similar way as in the current 3G cellular system, and is used for enhancing coverage and performance. In the mesh mode, AAS is used to form physical or directed mesh links. Physical or directed mesh is a form of mesh where substantially directional antennas are used to create physical links between neighboring devices. Mesh nodes adaptively steer antennas towards other nodes in their neighborhood and direct the focus of the electromagnetic radiations accordingly, to create the physical link with the intended neighboring device. One of the main drawback of AAS however is the high complexity involved in designing antenna systems capable of adaptively switching (steering) antenna directionality toward users who may be highly mobile. The use of AAS technology in mobile network deployment is therefore very challenging from a complexity perspective. Another drawback of AAS technology is that in an urban environment, with rich scatterer, the beams get blurred at the receiver and are not focused as expected, due to the reflections of waves as it propagates from the transmitter to the receiver. This effect is known as angle spread and it impacts significantly the performance of AAS in urban areas with cluttered structures. The gains achieved using AAS in such places thus reduce considerably from the theoretical expectations. For example, an AAS system using an eight-column array would have an ideal gain of 6.9 dB but angle spread would reduce this to only 3.2 dB in an urban environment and 4.7 dB in a suburban environment. There are some techniques however to mitigate the effects of angle spread. Active research works are ongoing in this area

ANTENNA TECHNOLOGIES IN WIMAX

Advanced antenna technologies specified in the WiMAX system to mitigate the non-LOS propagation problems and ensure high quality signal receptions include Diversity and multiple-input multiple-output (MIMO) systems, adaptive antenna systems (AAS), as well as beamforming systems.

1 Diversity Systems

Diversity technique provides the receiver with multiple copies of the transmitted signal, each of them received over independently fading wireless channel. The notion of diversity relies on the fact that with M independently fading replicas of the transmitted signals available at the receiver, the probability of an error detection is improved to p^M, where p is the probability that each signal will fade below a usable level. The link error probability is therefore improved without increasing the transmitted power. Recently, the use of diversity technique at the transmitter side also gained wide attentions, and has resulted in the consideration of the more general case of multiple transmit–multiple receiving antennas or MIMO systems.

2 MIMO Systems

The two options for MIMO transmissions in the WiMAX standard are space-time codes and multiplexing. For space-time codes, both space-time trellis codes and Alamouti space-time block codes are specified. However, it is the Alamouti space-time block codes that has yet been implemented by vendors due to its reduced complexity (eventhough space-time trellis code has better link performance improvements). In the Alamouti scheme designed for two transmitting antennas, a pair of symbol is transmitted at a time instant, and a transformed version of the symbols are transmitted in the next time instant. At the receiver, the decoder detects the four symbols transmitted over two time slots and processes them to obtain 2-branch diversity gain. Thus the Alamouti scheme achieves full diversity, with a rate-1 code. For the multiplexing option, the multiple antennas are used for capacity increase. In this option, original high-rate stream is partitioned into N low-rate substreams and each substream is transmitted in parallel over the same channel, using different antennas. If there are enough scatterers between the transmitter and the receiver, adequate MIMO detection algorithms like zero-forcing, minimum mean-square error (MMSE), or vertical Bell labs Layered Architecture for space-time codes (V-BLAST), etc., can be designed to separate the substreams. Thus the link capacity (theoretic upper-bound on the throughput) is increased linearly with min(N,M), where N is the number of transmit and M is the number of receiving antennas.

3 MIMO Systems with Antenna Selection

For MIMO systems to be deployed on mobile WiMAX devices, the concept of antenna selection is very essential. Because RF chain dominates the link budget in wireless systems, mobile devices are unable to implement large numbers of RF chains to incorporate high order MIMO systems. For such systems therefore, a reduced numbers of RF chains are implemented and antennas with the best received energies are adaptively selected and switched on to the implemented RF chains for MIMO signal processings, as illustrated in Figure 1. The performance of such system has been studied quite elaborately in the literature, in comparison to the full complexity system that utilizes all available antennas. It was shown that the diversity gain performance is maintained in the reduced-complexity system despite the use of antenna selection, while the coding gain deteriorates proportional to the ratio of the selected antennas to the total available antennas.

 
Figure 1: MIMO subset antenna selection.

4 MIMO Technologies in IEEE 802.16m Standard

The IEEE 802.16 standards committee has recently initiated the process of extending the existing IEEE 802.16e standard (mobile WiMAX) for high capacity, high-QoS mobile application. The new standard was dubbed IEEE 802.16m at the IEEE January session in London, 2008. The working group tasked with the responsibility of producing the working documents for the new standard was named task group m (TGm). The group hopes to complete the specification for the new standard by the end of 2009. When completed, the standard will be backward compatible with IEEE 802.16e, and interoperable with 4G cellular standards supporting the IMT-advanced technologies. Although the details of the IEEE 802.16m standard is not available at the moment, the most important features being touted for the standard include

• Target downstream speed of 100 Mbps in highly mobile mode, and upto 1 Gbps in normadic mode (upstream rate are not yet known, but would be at least at par with 802.16e).
• Channel sizes upto 40 MHz (802.16e currently supports upto 20 MHz channel size).
• Use of TDD and FDD.
• Backward-compatibility with 802.16e.
• OFDMA radio (same as in 802.16e).
• Mandatory MIMO antenna technology of size 4 × 4 (four transmitting, and four receiving antennas).

In contrast to the IEEE 802.16e, which supports mandatory MIMO antenna technology of size 2 × 2 (two transmitting, and two receiving antennas), the use of mandatory higher-capacity MIMO technology in 802.16m will provide extra capacity to support the targeted high-speed in the downstream. Since downstream has been the bottleneck in wireless services, this improvement will provide significant boost in system capacity, to enable the system support wide range of multimedia services expected in 4-G compatible technologies.

WiMAX PHYSICAL AND MAC LAYERS

WiMAX PHY is responsible for the transmission of data over the air interface (physical medium). The PHY receives MAC layer data packets through its interface with the lowest MAC sublayer, and transmits them according to the MAC layer QoS scheduling. WiMAX MAC layer comprises of three sublayers, which interact through service access points (SAP) to provide the MAC layer services, as shown in Figure 1. The convergence sublayer (CS) interfaces the WiMAX network with other networks by mapping external network data (from ATM, Ethernet, IP, etc.) to the WiMAX system. MAC common part sublayer (MAC CPS) provides majority of the MAC layer services. The MAC CPS receives data from the CS as MAC service data unit (MAC SDU) and efficiently packs them on to the payload of the MAC packet data unit (MAC PDU) through the process of fragmentation and aggregations. Fragmented parts of MAC SDU are used to fill (aggregate) remnant portions of MAC PDU payloads that cannot accommodate full MAC SDU during package. As WiMAX provides connection-oriented service, MAC CPS is also responsible for bandwidth request/reservation for a requested connection, connection establishment, and maintenance. In the WiMAX standard, bandwidth request/reservation is an adaptive process that takes place on a frame-by-frame basis. This allows more efficient resource utilization and optimized performance. Thus the MAC CPS is required to provide up-to-date data on bandwidth request/reservation for each connection, on a frame-by-frame basis. The MAC CPS also provides connection ID for each established connection and marks all MAC PDUs traversing the MAC interface to the PHY with the respective connection ID. This sublayer also performs QoS scheduling by deciding the orders of packet transmissions on the PHY, based on the service flow decided during connection establishments. Privacy sublayer provides authentication to prevent theft of services, and encryption to provide security of services.

 
Figure 1: WiMAX Protocol stack.

The ensemble of the activities of the three sublayers of the WiMAX MAC layer constitutes the MAC layer services. MAC layer services can broadly be categorized into two: periodic and aperiodic activities. Periodic activities are fast- or delay-sensitive types of activities and are carried out to support ongoing communications, thus they must be completed in one frame duration. Examples include QoS scheduling, packing, and fragmentation. Aperiodic activities are slow- or delay-insensitive types of activities. They are executed when and as required by the system, and are not bounded by frame durations. Examples include ranging and authentications for network entry.

WIMAX NETWORK ARCHITECTURE

In Figure 1, we present the overall network architecture of a WiMAX network. The network can be logically partitioned into three components, user terminals, ASN, and CSN. User terminals capture the data origination points, could be using the fixed, mobile, or portable WiMAX technology. All the three variations can be supported using a common air interface. ASN spans the BS and the ASN-GW. BS receives the transmitted signal, processes it, and converts into an IP packet and sends to the GW on the outgoing IP transport link. GW receives and upon processing determines the destination on the network side and sends the packet. BS and GW are connected to each other using an IP transport. Typical implementations would have BS located in the field/coverage area and the GW will be centrally located in the switch centers. Therefore, the IP link between BS and GW forms the transport backhaul network. CSN contains many different commercial off-the shelf (COTS) components, which provide connectivity services to the WiMAX subscribers. Addressing, authentication, and availability (AAA) servers, mobile IP home agent (MIP HA), IP multimedia services (IMS), content services, etc. provide support for seamless services to subscribers. AAA servers ensure that a user is uniquely identified and authenticated as legitimate customer. MIP HA ensures that roaming across IP networks is handled and accurate routing of data packets is ensured. Call processing related services are provided by IMS entity. Billing and operational support systems help in managing the overall network.

 
Figure 1: Logical network architecture of a WiMAX network.

In Figure 2, we present typical implementation of a WiMAX network in a market. For example, say a carrier plans to lay down WiMAX network in Washington D.C. market. Typically, we would have more than 100 BSs connecting to a GW location, based on the anticipated traffic, each GW location might require a cluster of servers providing the functions of the GW. Each IP transport link would be leased from the local carrier and provisioned. Based upon the cost points and required capacity, the carrier can choose to directly lease a TDM segment, Ethernet link, fiber connectivity, etc. Components of the CSN located at each switch center might also be implemented using clusters and would have enough capacity to support the entire market. Switch centers could be connected to each other using a high speed IP network running on an OC-192 (or higher) SONET ring leased from local exchange carrier. Actual network would also include connectivity to the other markets, trunking with public switched telephone network (PSTN) via the end office (EO), tandem connections with other wireless carriers, etc.

 
Figure 2: Physical network architecture of a WiMAX network.

For most WiMAX networks, it is unlikely that the carriers would provision the IP transport based on the capacity of the WiMAX air interface. According to WiMAX forum, air interface built on 10 MHz channel with 2 × 2 MIMO can support peak downlink rate of 63 Mbps and peak uplink rate of 28 Mbps per sector. Assuming three sectors per BS, this would translate into close to 200 Mbps of backhaul transport for each BS. When we share the symbols 3:1 between DL and UL, it could provide data rates of 46 Mbps DL and 8 Mbps UL per sector. Even then it would require about 150 Mbps of capacity between BS and GW. Such a requirement would lead to an unmanageable backhaul cost, which might become a road block in the large-scale adoption of the WiMAX technology.

Our contention is that the service providers will only provision based on the anticipated demand. For example, they might provision just enough capacity for N voice calls, Mvideo calls, and few more Mbps for best effort. This would ensure that the initial cost of building the network is manageable, and as the users grow, more backhaul can be added to ensure acceptable QoS for the subscribers.

BENEFITS OF WiMAX

The WiMAX solution reflects the general trend in the communications industry toward unified packet-based voice and data networks. Fundamental benefits of this transition are reduced operation cost, improved network optimization, and better management of changes. The followings are some of the benefits of WiMAX.

Wireless. By using a WiMAX system, companies/residents no longer have to rip up buildings or streets or lay down expensive cables.

High bandwidth. WiMAX can provide shared data rates of up to 70 Mbps. This is enough bandwidth to support more than 60 businesses at once with T1-type connectivity. It can also support over a thousand homes at 1-Mbps DSL-level connectivity. Also, there will be a reduction in latency for all WiMAX communications.

Long range. The most significant benefit of WiMAX compared to existing wireless technologies is the range. WiMAX has a communication range of up to 40 km.

Multi-application. WiMAX uses the IP and is therefore capable of efficiently supporting all multimedia services from VoIP to high speed Internet and video transmission. It also supports a differentiated QoS enabling it to offer dynamic bandwidth allocation for different service types. WiMAX has the capacity to deliver services from households to small and medium enterprises, small office home office (SOHO), cybercafés, multimedia Tele-centers, schools and hospitals.

Flexible architecture. WiMAX supports several systems architectures, including point-to-point, point-to-multipoint, and ubiquitous coverage.

High security. The security of WiMAX is state of the art. WiMAX supports advanced encryption standard triple data encryption standard. WiMAX also has built-in VLAN support, which provides protection for data that is being transmitted by different users on the same BS. Both variants use privacy key management (PKM) for authentication between BS and SS station. WiMAX offers strong security measures to thwart a wide variety of security threats.

QoS. WiMAX can be dynamically optimized for a mix of traffic that is being carried.

Multilevel service. QoS is delivered generally based on the service-level agreement between the end user and the service provider.

Interoperability. WiMAX is based on international, vendor-neutral standard. This protects the early investment of an operator because it can select the equipments from different vendors.

Low cost and quick deployment. WiMAX requires little or no external plant construction compared with the deployment of wired solutions. BSs will cost under $20,000 but will still provide customers with T1-class connections.

Worldwide standardization. WiMAX is developed and supported by the WiMAX forum (more than 470 members). The WiMAX forum collaborates with different international standards organizations that are developing broadband wireless standards with the intent to provide interoperability among the standards. Some of the other broadband wireless standards include HiperMAN/HiperLAN (Europe) and WiBRO (South Korea). These standards are compatible with WiMAX at the physical layer. WiMAX will become a truly global technology-based standard for broadband and will guaranty interoperability, reliability, and evolving technology and will ensure equipment with very low cost.

VOIP AND IP | WIMAX APPLICATIONS

The WiMAX standard has been developed to address a wide range of applications. Based on its technical attributes and service classes, WiMAX is suited to supporting a large number of usage scenarios. Table 1 address a wide range of applications.

Table 1: Summary of WiMAX Applications 

Class Description	Real Time	Application Type	Bandwidth
Interactive gaming	Yes	Interactive gaming	50–85 Kbps
VoIP, video conferencing	Yes	VoIP	4–64 Kbps
		Videophone	32–384 Kbps
Streaming media	Yes	Music/speech	5–128 Kbps
		Video clips	20–384 Kbps
		Movies streaming	>2 Mbps
Information technology	No	Instant messaging	<250 byte messages
		Web browsing	>500 Kbps
		Email (with attachments)	>500 Kbps
Media content download (store and forward)	No	Bulk data, Movie download	>1 Mbps
		Peer to peer	>500 Kbps

Mobile WiMAX is an all-IP network. The use of OFDMA on the physical layer makes it capable of supporting IP applications. It is a wireless solution that not only offers competitive Internet access, but it can do the same for telephone service.

VoIP offers a wider range of voice services at reduced cost to subscribers and service providers alike. VoIP is expected to be one of the most popular WiMAX applications. Its value proposition is immediate to most users. Although WiMAX is not designed for switched cellular voice traffic as cellular technologies as are CDMA and WCDMA, it will provide full support for VoIP traffic because of QoS functionality and low latency. IPTV enables a WiMAX service provider to offer the same programming as cable or satellite TV service providers. IPTV, depending on compression algorithms, requires at least 1 Mbps of bandwidth between the WiMAX BS and the subscriber. In addition to IPTV programming, the service provider can also offer a variety of video on demand (VoD) services. IPTV over WiMAX also enables the service provider to offer local programming as well as revenue generating local advertising.

WIFI Comparison With Wimax

WiMAX is different from WiFi in many respects. The WiFi MAC layer uses contention access. This causes users to compete for data throughput to the access point. WiFi also has problems with distance, interference, and throughput and that is why triple play (voice, data, video) technologies cannot be hosted on traditional WiFi. In contrast, 802.16 uses a scheduling algorithm. This algorithm allows the user to only compete once for the access point. This gives WiMAX inherent advantages in throughput, latency, spectral efficiency, and advanced antenna support.

Companies developing radical innovations may adopt different stances not only based on the strategic interests of the company but also by taking into other considerations such as the market and its needs and requirements, as well as other products it may carry.

When comparing WiFi and WiMAX, one is comparing their substitutability and complementary to existing technologies and how different companies have and will view them. WiMAX and WiFi can offer some potentially significant cost savings for mobile network operators by providing an alternate means to backhaul BS traffic from cell site to the BS controllers. Mobile network operators typically utilize some type of wired infrastructure that they must buy from an incumbent operator. A WiFi or WiMAX mesh can offer a much more cost-effective backhaul capability for BSs in metropolitan environments.

Using WiFi and WiMAX open broadband wireless standards and implementing mobile computing, governments and partners can quickly and cost-effectively deploy broadband to areas not currently served, with little or no disruption to existing infrastructures. Standards-compliant WLANs and proprietary WiFi mesh infrastructures are being installed rapidly and widely throughout the world. Standards-compliant WiMAX products can provide NLOS backhaul solutions for these local networks and WiMAX subscriber stations can currently provide Internet access to customers such as schools and other educational institutions and campuses.

The results of the comparison show that mobile WiMAX has better performance in all the areas listed above (where it shares performance enhancing features with EVDO and HSDPA/HSPA). Furthermore, the technologies on which mobile WiMAX is based result in lower equipment complexity and simpler mobility management due to the all-IP core network. They also provide mobile WiMAX systems with many other advantages over CDMA-based systems such as

• Tolerance to multipath and self-interference
• Scalable channel bandwidth
• Orthogonal UL multiple access
• Support for spectrally-efficient TDD
• Frequency-selective scheduling
• Fractional frequency reuse
• Improved variable QoS
• Advanced antenna technology

1XEVDO Comparison With Wimax

This standard is developed by the third generation partnership project 2 (3GPP2), the body responsible for CDMA and EVDO. 1xEVDO is an enhanced version of CDMA2000-1x. There are four versions that have been released, namely, Rev. 0, Rev. A, Rev. B, and Rev. C.

1xEVDO is a high-speed data only specification for 1.25 MHz frequency division duplex (FDD) channels with a peak downlink (DL) data rate of 2.4 Mbps.

Improvements to CDMA2000-1x in the 1xEVDO Rev. 0 specification include:

• DL channel is changed from code division multiplexing (CDM) to time division multiplexing (TDM) to allow full transmission power to a single user.
• DL power control is replaced by closed-loop DL rate adaptation.
• Adaptive modulation and coding (AMC).
• HARQ.
• Fast DL scheduling.
• Soft handoff is replaced by a more bandwidth efficient “virtual” soft handoff.

1xEVDO Rev. 0, however, was designed to support only packet data services and not conversational services. In 1xEVDO Rev. A and EVDO Rev. C (also dubbed DORC), additional enhancements were added to the 1xEVDO specification. They include the following:

• DL: Smaller packet sizes, higher DL peak data rate (up to 3.1 Mbps), and multiplexing packets from multiple users in the MAC layer.
• Uplink (UL): Support of HARQ, AMC, higher peak rates of 1.8 Mbps, and smaller frame size

These enhancements in both the UL and DL of 1xEVDO Rev. A allow it to support conversational services.

WIMAX SERVICE PROVIDERS

As IP networks become faster (higher bandwidth) and more responsive (lower delay), the set of services implemented on IP-based networks has grown. This growth generates more revenue opportunities for service providers, and thus next-generation networks are all migrating toward IP technology. From an operator standpoint, services can be broken into four billable classes: 

(1) basic Internet services which are typically billed at a flat rate, 

(2) premium Internet services which are important not only to improve ARPU, but to add new services, 

(3) VPN services which can be billed by QoS level, and 

(4) operator premium services which are applications provided on the operator’s network.

The service providers are expected to gain profits through the sale of the different services and applications that WiMAX is capable of carrying. The different services that can be offered on WiMAX networks include best effort VoIP, carrier class IP telephony through the IP multimedia core, music, video conferencing, streaming video, interactive gaming, mobile instant messaging (IM), IP television (IPTV), basic broadband wireless Internet, and other application-based services to corporate customers. The concept of unbundling the network reduces the barriers of entry into the mobile telecommunications industry because a provider does not need to own the whole network.

The business aspect of the service providers can also be looked at from two perspectives. The first one is where the service provider owns the whole system including the core network and the access network. The second option is the unbundled option where the access network and core networks exist as independent business entities.

In emerging markets such as Africa, and South Asia where telecom investment is still nascent and 3G yet to be launched, WiMAX makes complete business sense even at equal cost, better speeds, better spectrum utilization, and the promise of broadband to a much sparsely spread population. For developed economies, the United States for instance, the 2.3 spectrum band is believed to be more capex efficient and hence better than 3G and high-speed uplink packet access (HSUPA). More importantly, the phase in the capex cycle of a telecom operator will determine each operator strategy—whether to embrace WiMAX or stick to its existing technology. The WiMAX industry entered the year 2007 as a year for ecosystem buildup in the preparation for regional and nationwide deployments of WiMAX services. It appears that 2008 will be a make-or-break period for WiMAX. Figure 1 shows a global WiMAX deployment by region.

 

Figure 1: Global WiMAX networks. APAC = Asia Pacific, CALA = Caribbean and Latin America.

WIMAX EQUIPMENT VENDORS

WiMAX, as with many new technologies, is based on an open standard. Although standards increasingly play a crucial role in driving adoption, they are not sufficient to guarantee success. A standard-based technology will success only if a solid ecosystem of operators, vendors, and solution and content providers emerge to support it, as is in the case of WiMAX. WiMAX enables intervendor interoperability which brings lower costs, greater flexibility and freedom, and faster innovation to operators.

Within the WiMAX industry there is a strong commitment to ensuring full interoperability through certification and ad-hoc testing between vendors. It is important for network operators to realize how interoperability is established and what it covers so that they understand how different products, solutions, and applications from different vendors can coexist in the same WiMAX network. The advantages that interoperability brings are multiple. Some of these advantages are the ability to choose among vendors, flexibility when choosing the appropriate network elements and components, success to the latest cutting-edge technology, and an open architecture which makes it easier for operators to roll out new revenue-generation services and applications as they can rely on wider pool of suppliers.

The two categories of equipment vendors include the network equipment vendors and the terminal equipment vendors. Network equipment includes ASN and CSN equipment, and vendors include companies such as Motorola and ZTE of China. They will gain their profits through the sale of the equipment and through installation of the equipment. They may further have after sales agreements with the customers who are the service providers. Terminal equipment includes mobile phones, CPE, modems, laptops, smart phones, and PDAs and they are manufactured by companies like Nokia, Blackberry, Motorola, and Intel. They will gain their profits through the sale of the terminal equipment. Nokia, the world’s top handset maker, expects to start selling cell phones using the WiMAX technology in 2008.

THE WiMAX BUSINESS MODEL

The biggest challenges to deploying WiMAX-based services are business related. Carriers need financial capability to implement infrastructure. Each operator has to carefully identify its own requirements, dictated by the type of services offered, the market segments targeted, the spectrum available, and the topography of the coverage area. There is no single solution that works for all, and operators need to make key choices about the management and core networks as they plan for their WiMAX networks.

An accurate business case analysis must take into account a wide variety of variables such as demographics, services, frequency band alternatives, capital expense items, operating expense items, and CPE equipment. The WiMAX business model can be looked from several perspectives. These include the equipment vendors, service providers and application providers, and customers. WiMAX will have a larger impact long term than we have seen from cellular phones in the past two decades. Initial rollouts of WiMAX will begin mostly by competitive local phone service carriers and rural Internet service providers. Larger carriers will utilize fixed WiMAX to deliver services to residential customers many of whom are in underserved markets. WiMAX adoption in these underserved markets will be high due to lack of availability of high-speed data access. These deployments will generate capital to be reinvested for future deployments. Larger customer base will begin driving both the cost of carrier and customer equipment down. As the economy of scale makes deployment less expensive, mobile platforms will begin to appear. This development will be spread between high population centers and the rural markets that already have fixed platforms deployed. Fixed platform will act as a springboard for mobile deployment. Then interconnections will begin to form between rural markets and metropolitan markets as carriers from cooperative agreements to share network resources. The economy of scale will increase exponentially at this point and we will notice a negative impact on traditional cellular, Internet, and voice services. Once the implementation of initial hot underserved rural markets and high-density metro areas is completed, springboard deployments will quickly take WiMAX coverage to the level of coverage offered by traditional wireless today. This process will move much faster than the deployment of cellular networks and devices for the following key reasons:

• Manufacturing process for WiMAX devices will be quite similar to that of wireless devices and mostly the changes will be in components and software.
• Readiness of the current wireless fixed and mobile market and waiting on new technology.
• As carriers built out wireless networks, most of the questions in this field have been answered and can now be applied to the development of a mirror network that provides WiMAX access.

TECHNOLOGIES EMPLOYED BY WiMAX

Mobile WiMAX operates in licensed frequency bands in the range of 2 to 6 MHz. The technologies employed by mobile WiMAX include the following:

• Scalable orthogonal frequency division multiple access (SOFDMA) on the physical layer
• MIMO
• IP
• Adaptive antenna systems (AAS)
• Adaptive modulation schemes (AMS)
• Advanced encryption standard (AES) encryption
  
  

PHYSICAL LAYER

Mobile WiMAX will initially operate in the 2.3, 2.5, 3.3, and 3.4–3.8 GHz spectrum bands using SOFDMA. OFDMA is perhaps the most important technology associated with WiMAX. SOFDMA is based on OFDMA which in turn is based on OFDM. OFDM is a form of frequency division multiplexing, but it has higher spectral efficiency and resistance to multipath fading and path loss compared to other multiplexing methods. It divides the allocated frequency spectrum into subcarriers which are at right angles to each other. This reduces the possibility of cross-channel interference thereby allowing the subcarriers to overlap. This reduces the amount of frequency spectrum required, hence the high spectral efficiency. The reduced data rate of each stream reduces the possibility of intersymbol interference because there is more time between the arrival of symbols from different paths. This feature of OFDM makes it resistant to multipath fading and ideal for nonline of sight (NLOS) applications. In OFDMA each frequency subcarrier is divided into subchannels which can be accessed by multiple users hence increasing the capacity of OFDM.

Scalable OFDMA is a form of OFDMA which allows variable channel bandwidth allocation from 1.25 to 20 MHz. SOFDMA has capabilities which make it ideal for the implementation of IP and hybrid automatic repeat request (HARQ). WiMAX also uses other features to enhance the performance of OFDMA. They include dynamic frequency shifting, MIMO, AAS, and software-defined radios. Dynamic frequency shifting monitors the signal and changes frequencies to avoid interference. Software-defined radios are controlled by changing software settings and this gives the equipment more flexibility when switching frequencies.

MIMO is a technology that has already found use in WiFi (IEEE 802.11n). MIMO multiplies the point-to-point spectral efficiency by using multiple antennas and RF chains at both the BS and the MS. MIMO achieves a multiplicative increase in throughput compared to single-input, single-output (SISO) architecture by carefully coding the transmitted signal across antennas, OFDM symbols, and frequency tones. These gains are achieved at no cost in bandwidth or transmit power.

AAS are spatial processing systems which combine antenna arrays with sophisticated signal processing. They reduce the effects of interference from multiple signal paths thereby also contributing to high capacity of the system and the use of mobile WiMAX in NLOS environments.

MAC SUBLAYER

The 802.16 MAC sublayer uses a scheduling algorithm for which the subscriber station only needs to compete for initial entry into the network. The scheduling algorithm also allows the BS to control QoS parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

WiMAX supports QoS differentiation for different types of applications. The 802.16 standard defines the following types of services:

• Unsolicited grant services (UGS): UGS is designed to support constant bit rate (CBR) services, such as T1/E1 emulation, and Voice-over-IP (VoIP) without silence suppression.
• Real-time polling services (rtPS): rtPS is designed to support real-time services that generate variable size data packets on a periodic basis, such as MPEG video or VoIP with silence suppression.
• Nonreal-time polling services (nrtPS): nrtPS is designed to support nonreal-time services that require variable size data grant burst types on a regular basis.
• Best effort (BE) services: BE services are typically provided by the Internet today for Web surfing.

WIMAX NETWORK ARCHITECTURE

The mobile WiMAX end-to-end network architecture is based on an All-Internet Protocol (IP) platform, all packet technology, and no circuit switch telephony. The end-to-end architecture makes the greatest possible use of IETF and IEEE standards and protocols along with the adoption of commonly available standard equipment.

The open IP architecture gives network operators great flexibility when selecting solutions that work with legacy networks or that use the most advanced technologies, and in determining what functionality they want their network to support. They can choose from a vertically integrated vendor that provides a turnkey solution or they can pick and choose from a dense ecosystem of best-of-breed players with a more narrow focus. The architecture allows modularity and flexibility to accommodate a broad range of deployment options such as small scale to large scale, urban, suburban, and rural coverage, mesh topologies, flat, hierarchical and their variant, and finally, coexistance of fixed, nomadic portable and mobile usage models.

Mobile WiMAX adds both the mobility and multiple-input multiple-output (MIMO) functionalities to the IEEE 802.16-2005 standard. It is one of two standards adopted by the WiMAX forum with the other one being the IEEE 802.16-2004. Mobile WiMAX network architecture mainly has three components. These include the access services network (ASN), the core services network (CSN), and the application services (AS) network. Figure 12.1 illustrates the interconnection of these networks. The WiMAX network supports the following key functions:

• All-IP access and core service networks
• Support for fixed, nomadic, and mobile access
• Interoperability with existing networks via internetworking functions
• Open interfaces between ASNs and between the ASN and the CSN
• Support for differential quality of service (QoS) depending on the application
• Unbundling of the access, core, and application service networks

 

Figure 1: WiMAX network architecture.

WiMAX—Architecture, Planning, and Business Model

The fixed/portable broadband wireless access is becoming a necessity for many residential and business subscribers worldwide. The demand is exploding as the pricing of broadband services is rapidly decreasing. The worldwide interoperability for microwave access (WiMAX) technology is an integral part of the portfolio by complementing 2G/3G mobile access, Digital Subscriber Line (DSL) broadband fixed access, and Wireless Fidelity (WiFi) hotspot access. An extended overview of WiMAX and its applications in higher generation wireless networks as a cost-effective solution to answering the challenges posed by the digital divide is presented. Technology behind WiMAX and its network architecture, design, and deployment are examined in addition to factors that impact WiMAX planning and performance. A WiMAX radio coverage simulation and analysis at different frequency bands for different demographic is presented. Furthermore, the WiMAX business models and a comparison with two enhanced third-generation (3G) technologies that are potential competitors to WiMAX are explored. 

Telecommunications has grown at a tremendous rate in the last ten to twenty years. Improved semiconductor and electronics manufacturing technology, and the growth of the Internet and mobile telecommunications have been some of the factors which have fueled this growth in telecommunications. The deployment of state-of-the-art telecommunications infrastructure and services has, however, been restricted to the developed world. The least developed countries have been left in the technological dark ages with few or none of the next-generation networks installed. Developed countries now boast high-speed connections with a large percentage of homes having access to the Internet and broadband services at an affordable fee. The underdeveloped countries are yet to enjoy such facilities. This is referred to as the digital divide. During the first World Summit on the Information Society (WSIS) held in Geneva in December 2003, the Digital Divide was defined as the unequal access to information and communication technologies (ICTs), where the least developed countries are separated from the developed countries because of a lack of technology particularly ICT.

The digital divide has persisted due to the relatively high cost of putting up modern telecommunications infrastructure. This is compounded by the fact that there are a number of different services available and each service requires its own technology and network. Therefore, existing technologies such as Wireless Fidelity (WiFi), Digital Subscriber Line (DSL), Global System for Mobile communications (GSM), Integrated Services Digital Network (ISDN), and the relatively new 3G technologies have not been able to provide a total solution to closing the digital divide. Figure 1 illustrates the main network types and the prevalent technologies associated with each, mapped against usage models and access modes.

 

Figure 1: Wireless network type and range. MAN—metropolitan area network (citywide, rural area), LAN—local area network (office, home, campus), and WAN—wide area network (countrywide, international).

WiMAX will boost today’s fragmented broadband wireless access market and mobile WiMAX promises to offer a solution to closing the existing digital divide. WiMAX can address the fixed wireless access and portable Internet market, complementing other broadband wireless technologies. Government initiatives to reduce the digital divide are making gains for broadband wireless countries such as Australia, South Korea, Taiwan, and the United States have programs in place today, and there has been a push by the European Commission for more flexible spectrum policies.

WiMAX access can be easily integrated within both fixed and mobile architectures, enabling operators to integrate it within a single converged core network, thereby providing new capabilities for a user-centric broadband world.

WiMAX addresses the following needs which may answer the question of closing the digital divide:

• Cost effective
• Offers high data rates
• Supports fixed, nomadic, and mobile applications thereby converging the fixed and mobile networks
• Easy to deploy and has flexible network architectures
• Supports interoperability with other networks
• Aimed at being the first truly a global wireless broadband network

WiMAX is a standard that is championed by the WiMAX forum which was formed in June 2001 to promote conformance to IEEE 802.16 standard. The WiMAX forum currently has more than 470 members comprising the majority of operators, component, and equipment companies in the communications ecosystem. The WiMAX forum promotes interoperability by working closely with IEEE and other standards groups such as the European Telecommunications Standards Institute (ETSI) which have their own versions of broadband wireless. Along these lines, the WiMAX forum works closely with service providers and regulators to ensure that WiMAX forum certified systems meet customer and government requirements.

The original WiMAX standard only catered for fixed and nomadic services. It was reviewed to address full mobility applications; hence, the mobile WiMAX standard, defined under the IEEE 802.16e specification was created. Mobile WiMAX supports full mobility, nomadic, and fixed systems to compete against DSL to cover isolated areas such as rural hot spots, private campus networks, and remote neighborhoods. Mobile WiMAX is more promising to be deployed as a cellular network that offers ubiquitous broadband services to mobile users to over large geographical areas. It can be deployed as a central office bypass to avoid using existing wired infrastructure for competitive local exchange carriers and wireless Internet service provider. Figure 2 shows the standard history for 802.16.

 

Figure 2: 802.16 standard evolution.