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.

NETWORK RESOURCE ALLOCATION

Network resources can be dynamically adjusted and adapted in MAC or PHY layers for mobile WiMAX, for example, physical layer FEC strategy, modulation scheme, transmission power control, link layer scheduling strategy, fragmentation threshold, and ARQ retry limit. Here, we focus our discussion on the resource management strategies that can be easily applied to mobile WiMAX networks. In WiMAX networks, each higher layer SDU usually consists of multiple link layer Protocol Data Units (PDUs). Each SDU of a traffic flow, for example, a JPEG2000 coded image stream, is dispatched to a specific 802.16e connection by SDU classifier in convergence sublayer. The connection is associated with a set of QoS requirement parameters, and the delay budget T_max for transmitting the whole JPEG2000 image stream. We specifically consider the transmission strategy optimization within an IEEE 802.16e connection, where the multiple connection management overhead is effectively obviated. We specifically consider the MAC layer delay performance of different fragmentation and retransmission strategies, which can be seamlessly applied to mobile WiMAX without violating what has already been defined in the IEEE 802.16e standard. In mobile WiMAX and the IEEE 802.16e standard, Selective Repeat based Automatic Repeat reQuest (SR-ARQ) is defined as the default ARQ strategy for optional performance enhancement, where the characteristics of SR-ARQ for mobile WiMAX are summarized as follows:

1. The SR-ARQ in mobile WiMAX is enabled per connection basis.
2. A WiMAX connection must have SR-ARQ enabled or not, but it cannot have a mixture mode of both SR-ARQ and non-SR-ARQ.
3. During connection establishment process, SR-ARQ is negotiated using dynamic service addition (DSA) and dynamic service change (DSC) messages. The fragmentation threshold ARQ_BLOCK_SIZE is negotiated and the smaller one provided by BS and SS is chosen for the SR-ARQ enabled connection between BS and SS.
4. The SR-ARQ feedback bitmap is sent in the MAC management message via basic management connection between BS and SS, or in the piggyback message via the reverse link of data connection.
5. SR-ARQ feedback bitmap cannot be re-fragmented.

The SR-ARQ operation in mobile WiMAX is described in Figure 1. Without losing generality, we use downlink transmission in TDD mode as the example to describe the SR-ARQ process. The bandwidth resource is divided into fix-sized time frames with duration T, and the frame duration T is further divided into downlink and uplink subframes with an adaptive boundary separated by a transmit/receive transition gap (TTG). The time frames are separated by a receive/transmit transition gap (RTG). The downlink subframe is composed of preambles, DL_MAP, UL_MAP, DCD, UCD control messages as well as burst transmission opportunities allocated for each SS. The Uplink subframe is further composed of ranging and bandwidth request slots, as well as transmission opportunity grants for each SS. In each upper layer SDU transmission, the SDU is fragmented into fix-sized SR-ARQ blocks and these blocks are dispatched into a specific connection queue. During the downlink transmission opportunity to the destination SS, these SR-ARQ blocks are transmitted in the time-varying and error-prone wireless channel and some of them may be lost due to bit errors. It is worth noting that the chance of collision is minimal in the time slot scheduling based WiMAX networks, and the major packet loss is due to physical layer bit or symbol errors. The receiver SS responds with an SR-ARQ ACK bitmap to provide the receiving status of the SDU during the uplink transmission opportunity in the same frame duration, and those erroneous or lost blocks are negatively acknowledged. In the next frame duration T, the BS retransmits only negatively acknowledged blocks as well as the new data blocks, until the SDU is successfully delivered to the SS.

 

Figure 1: SR-ARQ operations for mobile WiMAX. The detailed concept is explained in the 802.16(e) standards.