Praise for Fundamentals of WiMAX

8.6 Subchannel and Subcarrier Permutations 287After renumbering, the clusters are divided into six groups, with the first one-sixth of theclusters belonging to group 0, and so on. A subchannel is created using two clusters from thesame group, as shown in Figure 8.10.In PUSC, it is possible to allocate all or only a subset of the six groups to a given transmitter.By allocating disjoint subsets of the six available groups to neighboring transmitters, it is possibleto separate their signals in the subcarrier space, thus enabling a tighter frequency reuse at the costof data rate. Such a usage of subcarriers is referred to as segmentation. For example, in a BS withthree sectors using segmentation, it is possible to allocate two distinct groups to each sector, thusreusing the same RF frequency in all of them. By default, group 0 is always allocated to sector 1,group 2 is always allocated to sector 2, and group 4 is always allocated to sector 3. The distributionof the remaining groups can be done based on demand and can be implementation specific.By using such a segmentation scheme, all the sectors in a BS can use the same RF channel,while maintaining their orthogonality among subcarriers. This feature of WiMAX systems forOFDMA mode is very useful when the available spectrum is not large enough to permit anythingmore than a (1,1) frequency reuse. It should be noted that although segmentation can beused with PUSC, PUSC by itself does not demand segmentation.8.6.3 Uplink Partial Usage of SubcarriersIn UL PUSC, the subcarriers are first divided into various tiles, as shown in Figure 8.11. Eachtile consists of four subcarriers over three OFDM symbols. The subcarriers within a tile aredivided into eight data subcarriers and four pilot subcarriers. An optional PUSC mode is alsoallowed in the uplink, whereby each tile consists of three subcarriers over three OFDM symbolsas shown in Figure 8.12. In this case, the data subcarriers of a tile are divided into eight data subcarriersand one pilot subcarrier. The optional UL PUSC mode has a lower ratio of pilot subcarriersto data subcarriers, thus providing a higher effective data rate but poorer channel-trackingcapability. The two UL PUSC modes allow the system designer a trade-off between higher datarate and more accurate channel tracking depending on the Doppler spread and coherence bandwidthof the channel. The tiles are then renumbered, using a pseudorandom numberingsequence, and divided into six groups. Each subchannel is created using six tiles from a singlegroup. UL PUSC can be used with segmentation in order to allow the system to operate undertighter frequency reuse patterns.8.6.4 Tile Usage of SubcarriersThe TUSC (tile usage of subcarriers) is a downlink subcarrier permutation mode that is identicalto the uplink PUSC. As illustrated in the previous section, the creation of subchannels from theavailable subcarriers is done differently in the UL PUSC and DL PUSC modes. If closed loopadvanced antenna systems (AAS) are to be used with the PUSC mode, explicit feedback of thechannel state information (CSI) from the MS to the BS would be required even in the case ofTDD, since the UL and DL allocations are not symmetric, and channel reciprocity cannot beused. TUSC allows for a DL allocation that is symmetric to the UL PUSC, thus taking advantage