Saturday, May 7, 2011


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 Tmax 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 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.
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