Praise for Fundamentals of WiMAX

312 Chapter 9 • MAC Layer of WiMAXIn order for PHS to work, the PHS rules at the transmitter and the receiver need to be synchronized.When it initiates or changes the PHS rule, the BS sends a dynamic service allocation(DSA) or a dynamic service change (DSC) message, respectively, with the PSHF, PHSI, orPHSM. When it initiates or changes PHS rule, the MS sends a DSA or DSC message, respectively,with all the elements except the PHSI. The BS, as a response to the DSA or DSC message,sends a DSC response with the PHSI to be used with this PHS rule. Like CID, PHSI isalways allocated by the BS. In order to delete a PHS rule, the BS or the MS sends a dynamic servicedelete (DSD) message.9.2 MAC PDU Construction and TransmissionAs the name suggests, the MAC common-part sublayer is independent of the higher-layer protocoland performs such operations as scheduling, ARQ, bandwidth allocations, modulation, andcode rate selection. The SDUs arriving at the MAC common-part sublayer from the higher layerare assembled to create the MAC PDU, the basic payload unit handled by the MAC and PHYlayers. Based on the size of the payload, multiple SDUs can be carried on a single MAC PDU, ora single SDU can be fragmented to be carried over multiple MAC PDUs. When an SDU is fragmented,the position of each fragment within the SDU is tagged by a sequence number. Thesequence number enables the MAC layer at the receiver to assemble the SDU from its fragmentsin the correct order.In order to efficiently use the PHY resources, multiple MAC PDUs destined to the samereceiver can be concatenated and carried over a single transmission opportunity or data region,as shown in Figure 9.4. In the UL and DL data regions of an MS is a contiguous set of slots 2reserved for its transmission opportunities. For non-ARQ-enabled connections, each fragmentof the SDU is transmitted in sequence. For ARQ-enabled connections, the SDU is first partitionedinto fixed-length ARQ blocks, and a block sequence number (BSN) is assigned to eachARQ block. The length of ARQ blocks is specified by the BS for each CID, using the ARQ-BLOCK-SIZE parameter. If the length of the SDU is not an integral multiple of the ARQ-BLOCK-SIZE, the last ARQ block is padded. Once the SDU is partitioned into ARQ blocks,the partitioning remains in effect until all the ARQ blocks have been received and acknowledgedby the receiver. After the ARQ block partitioning, the SDU is assembled into MACPDUs in a normal fashion, as shown in Figure 9.4. For ARQ-enabled connections, the fragmentationand packing subheader contains the BSN of the first ARQ block following the subheader.The ARQ feedback from the receiver comes in the form of ACK (acknowledgment),indicating proper reception of the ARQ blocks. This feedback is sent either as a stand-aloneMAC PDU or piggybacked on the payload of a regular MAC PDU. In WiMAX, the ARQ feedbackcan be in the form of selective ACK or cumulative ACK. A selective ACK for a given BSN2. A slot, the basic unit of PHY-layer resources, can be used for allocation and consists of one subchannelover one, two, or three OFDM symbols, depending on the subcarrier permutation. This isdiscussed more detail in Chapter 8.