Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Sub-channel numbering (frequency)258 Applications0123:k-1 k k+1 k+2nn+1n+2n+3.n+11n+12n+13n+14n+15.n+23n+24n+25n+26n+27.n+35OFDM symbol numbering (time)313 OFDM symbolsNote: The numbers n, n+1, ... in the boxes indicate indices in the FEC block.Figure 5-35Mapping of FEC blocks to OFDMA sub-channels and symbolsN is a positive integer. The smallest number of allocated sub-channels per terminal stationis one sub-channel for a duration of four OFDM symbols, where the first OFDM symbolis a preamble.OFDMA Downlink Sub-Carrier AllocationAs shown in Figure 5-36, for the downlink the pilots will have both fixed and variablepositions [23]. The variable pilot location structure is repeated every four symbols. Theallocated data sub-carriers are partitioned into groups of contiguous sub-carriers. Thenumber of groups is therefore equal to the number of sub-carriers per sub-channel. InTable 5-21, the basic OFDMA downlink parameters are given.OFDMA Uplink Sub-Carrier AllocationThe total number of used sub-carriers is first partitioned into sub-channels (see Figure 5-37[23]). Within each sub-channel, there are 48 data sub-carriers, 1 fixed located pilot subcarrier,and 4 variable located pilot sub-carriers. The fixed located pilot is always atsub-carrier 26 within each sub-channel. The variable located pilot sub-carriers are repeatedevery 13 symbols. The fixed and the variable positioned pilots will never coincide.In Table 5-22 and 5-23 the OFDMA uplink parameters and guard times respectivelyare given. Note that for OFDMA with 2048 sub-carriers the symbol duration and guardtimes will be four times longer than in the case of the OFDM mode with 256 sub-carriers.Scalable OFDMAThe IEEE 802.16e OFDMA mode for mobile applications is based on the concept ofscalable OFDMA. This means that it supports a wide range of bandwidths in order tocope with different channel allocations. The scalability is supported by adjusting the FFTsize while keeping the sub-carrier frequency spacing constant at 10.94 kHz. Therefore,the so-called ‘resource unit’ (sub-carrier bandwidth and symbol duration) is constant.Hence, the impact of a higher protocol layer is minimal when scaling the bandwidth. The