CURRENT DEVELOPMENTS IN WiMAX TECHNOLOGY

WiMAX technology encompasses broadband wireless equipment which is designed in compliance with the IEEE 802.16 standard and certified by the WiMAX Forum. The IEEE 802.16e standard leverages several differences and enhancements over the 802.16-2004 standard to support mobile subscribers.

Scalable orthogonal frequency division multiple access (SOFDMA): Introduced in the 802.16e amendment over fixed WiMAX’s OFDM, SOFDMA supports scalable channel bandwidths from 1.25 to 20 MHz, using quadrature amplitude modulation (QAM; 16QAM or 64QAM) or quaternary phase shift keying (QPSK) modulation. SOFDMA enables additional resource allocation flexibility and adaptively optimized multiuser performance.

Advanced antenna technologies: MIMO PHY layer techniques have the potential to significantly increase bandwidth efficiency based on the premise that operation occurs in a rich scattering multipath environment. 802.16e defines optional support for such advanced antenna technologies. Major advantages of MIMO include diversity gains, multiplexing gains, interference suppression, and array gains. The inclusion of MIMO techniques alongside flexible subchannelization and adaptive modulation and coding (AMC) enables mobile WiMAX technology to improve system coverage and capacity.

In addition, 802.16e presents many advanced features for performance enhancements, such as handover support, quality-of-service (QoS) support, and energy savings mechanisms for handheld support, etc.

Multihop relay: Another milestone in the development of WiMAX was the introduction of multihop relay running as the 802.16j multihop relay project, which targets on OFDMA PHY layer and Medium Access Control (MAC) layer enhancements for licensed bands to enable the operation of RSs. The objectives of 802.16j are to enhance coverage, throughput, and system capacity by specifying 802.16 multihop relay capabilities and functionalities of interoperable RSs and BSs. Several technical topics were focused on, mainly including relay concepts, frame structure, network entry, bandwidth request, security, mobility management, routing, path management, interference control and radio resource management, etc.

The concept of multihop relaying is already well developed in the fixed telecommunications world. Microwave radio relays have been widely used to transmit digital and analog signals over long distances, with examples including telephony and broadcast television. With the evolution of mobile networks, wireless relays have been further developed for cellular transmission. Analog repeaters are sometimes used in cellular systems to extend coverage into regions that are uncovered by the standard network. Digital relaying for cellular applications was initially investigated to enhance coverage for delay-insensitive traffic. More recently, Streaming21 of the United States announced the availability of a 3G relay server in mid-2006, a carrier-grade mobile streaming solution that allows mobile operators and content providers to deliver multimedia contents to mobile phone subscribers over GPRS and 3G networks. Now, the relay concept is being further developed in 802.16j to supporting both digital repeater and decode-forward (DF) relaying with various techniques, such as cooperative relaying, intelligent radio resource management (RRM) for radio resource reuse, smart antenna on RSs for direction-controlled transmission, relay grouping, etc.

While the IEEE and the WiMAX Forum strive to address the technological challenges of high mobile, NLOS WiMAX services, large throughput and coverage, etc., commercial service providers face additional operational challenges including spectrum limitations, security vulnerabilities, and QoS implementations. Many researchers have been working toward developing mechanisms that provide highly efficient mobile WiMAX, which is the core topic in this chapter. Further development is expected in a newly approved project from the IEEE, namely 802.16m—Advanced Air Interface to meet IMT-advance requirements. It is targeting data rates of 100 Mbps for mobile applications and 1 Gbps for fixed applications, cellular, macro- and microcell coverage, with currently no restrictions on the RF bandwidth.

Relay-Assisted Mobile WiMAX

There are two major technological and social trends significantly changing people’s lives: wireless communications and the Internet. Leveraging these two trends, worldwide interoperability for microwave access (WiMAX) creates a new utility enabling the development of new services and new Internet business models. In particular, the full potential of WiMAX will be realized when it is used for innovative nomadic and mobile broadband applications. With the finalization of the IEEE 802.16e standard and upcoming test and certification of WiMAX products, mobile broadband services are becoming a reality. The 802.16e standard provides broadband wireless Internet Protocol (IP) access to support a variety of services (such as voice, data, and multimedia) on virtually any device. The operation of WiMAX is currently limited to a number of licensed frequency bands below 6 GHz for reliably supporting non-line-of-sight (NLoS) operations. The 802.16e standard has also become a part of the IMT-2000 family.

WiMAX is often quoted as combining long transmission ranges (e.g., in a macrocell) with high data capacities (multi megabit per second throughput to end users). Power and spectral efficiency is key to a successful WiMAX deployment. The mobile WiMAX physical (PHY) layer is based on scalable orthogonal frequency division multiple access (SOFDMA) technology, which enables flexible channelization. The new technologies employed by mobile WiMAX result in higher data transfer rates, simpler mobility management, and lower infrastructure costs compared to current 3G systems. The underlying scenario for mobile WiMAX is an outdoor environment with multiple users within a cell. Hence scheduling (allowing a fair and efficient distribution of resources) and interference (from intracell and intercell) become important issues.

Radio relaying can address many of the challenges faced in the deployment of mobile WiMAX and its potential benefits. The relay system targets one of the biggest challenges in next generation mobile wireless access (MWA), namely the provision of high data rate coverage in a cost-effective and ubiquitous manner. Within a multihop relay network, a multihop link can be formed between the base station (BS) and a distant mobile station (MS) using a number of intermediate relay stations (RS). To avoid interference between the relay links, the simplest approach is to assign unique radio resources to each link. Using this approach, the multihop users will rapidly drain the system of valuable radio resource. The wireless medium is a precious infrastructure commodity and the situation is especially acute at lower frequencies (where the radio signal propagation characteristics are more favorable) when a significant amount of radio spectrum is needed to provide ubiquitous wireless broadband connectivity. Spectrum predictions for future cellular networks indicate large shortfalls by 2010, if not before. Hence the above approach can only be used for a very small number of very high-value MSs, or in applications where spectral efficiency is not vital, such as military or disaster relief communication networks. Given its commercial applications, relaying in the context of WiMAX must conserve radio spectrum and emphasize the need for high spectral efficiency. Enhancing radio resource efficiency is a key challenge in a competitive business development.

We focuses on the efficiency of relay transmission. We present leading edge techniques, and merge both theoretical analysis and practical application for a mobile WiMAX system with multihop relay. Directional distributed relaying is then proposed to achieve high data throughput with reduced demands on radio resource.

QUANTITATIVE PERFORMANCE STUDY | Multimedia over Mobile WiMAX

The delay-distortion performance of the position-value based resource allocation is compared in this section with traditional layer-based resource allocation in mobile WiMAX. The parameters of the simulation study are listed as follows. Default time frame duration T = 0.004 second, default channel BER is 0.0001, MAC header is 6 bytes, and fragmentation subheader is 2 bytes. Frequency bandwidth is 20 MHz, and 64-QAM with 3/4 coding rate and 1/4 cyclic prefix are used.

Figures 1 and 2 indicate the average loss ratio and expected delay trade-off for delivering a typical SDU with 1400 bytes using different fragmentation thresholds and SR-ARQ retry limit strategies. From these figures it is clear to see, with larger fragmentation number (lower fragmentation threshold and shorter PDU length accordingly) and higher SR-ARQ retry limit, the SDU packet loss ratio is decreased considerably. However, the penalty of such packet loss ratio decreasing is the prolonged delay of successful SDU delivery, mainly because of the retransmission latency. This is because mobile WiMAX is basically TDMA based scheduling, retransmission has to be reissued in the next frame duration. Thus, quality and latency form the trade-off that can be fine-tuned in mobile WiMAX transmission strategies optimization.

 

Figure 1: SDU loss ratio for an application layer packet with 1400-byte length, at channel BER 1e-4.

Figure 2: SDU delay expectation for an application layer packet with 1400-byte length, at channel BER 1e-4.

Figure 3 depict the delay-distortion performance comparison of the position-value based approach and layer-based approaches. For both of these approaches, image qualities with loose delay constraints are better than those with strict delay constraints. This is because with loose delay constraints, more network resource especially the SR-ARQ retransmissions can be allocated to the code stream, which improves the packet delivery ratio and thus the picture quality considerably. The position-value based approach achieves better delay-distortion performance than layer-based approach with the same latency budget constraint. Layer-based UEP approaches allocate resource according to the importance of different layers in code stream, and important layers containing coarse image information are more effectively protected while unimportant layers containing imagefine details are less protected. The position oriented approach allocates resource more efficiently by considering not only different layers’ unequal importance, but also the unequal importance of position and value information in each layer. With the position-value based resource allocation, the p-segments especially those in the coarse image quality layers are more effectively protected to improve image quality; and the v-segments especially those in fine details enhancement layers are less protected to reduce delay penalty. From this figure we can see, with 1e-4 channel BER the position-value based approach shows quality improvement up to 7–8 dB in terms of PSNR over traditional layer based approach with the same delay constraint. In the worst cases, i.e., with ultra tight or ultra loose delay constraints, the position-based approach has similar performance as layer based approach. This is because it has either not enough network resource or over excessive network resource for allocation. In those situations, the performance is not confined by efficiency of resource allocation, but the amount of network resource itself.

Figure 3: Image quality and delay-bound for different resource allocation schemes at BER 1e-4.