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

262 ApplicationsTable 5-26(OFDM)FEC coding and modulation parameters for uplink and downlinkPHY mode Modulation Inner Outer coding Overall Efficiencycoding coding rate (bit/s/Hz)0 BPSK CC 1/2 No 1/2 0.51 QPSK CC 2/3 RS(32,24,4) 1/2 1.02 QPSK CC 5/6 RS(40,36,2) 3/4 1.53 16-QAM CC 2/3 RS(60,48,8) 1/2 2.04 16-QAM CC 5/6 RS(80,72,4) 3/4 3.05 (optional) 64-QAM CC 3/4 RS(108,96,6) 2/3 4.06 (optional) 64-QAM CC 5/6 RS(120,108,6) 3/4 4.5Forward Error Correcting (FEC) Coding and ModulationThe FEC employed consists of the concatenation of a Reed Solomon (RS) outer code anda punctured convolutional inner code. Block and convolutional Turbo codes can also beused. The low density parity check code (LDPC) is supported optionally in IEEE 802.16e.Different modulation schemes with Gray mapping (QPSK, 16-QAM, and 64-QAM) areemployed.The outer RS code can be shortened and punctured to enable variable block sizes andvariable error-correction capability. The RS mother code is an RS (N,k,t) code, whereN is the code length, k represents the number of information bytes, and t is the number ofcorrectable error bytes. The inner convolutional code can be punctured as well to provideseveral inner code rates. The mother convolutional code is based on memory 6, rate 1/2,with zero tail biting. Eight tail bits are introduced at the end of each allocation. In the RSencoder, the redundant bits are sent before the input bits, keeping the tail bits at the endof the allocation.Table 5-26 and Table 5-27 show the detailed FEC parameters for the OFDM andOFDMA modes of IEEE 802.16d. The resulting number of bytes per FEC block matchesan integer number of OFDM symbols. As 64-QAM is optional, the codes for this modulationwill only be implemented if the modulation is implemented. As these tables show,different coding and modulation schemes are supported. The lowest concatenated codingscheme with code rate 1/2 will be used for control information.For S-OFDMA, support for QPSK, 16-QAM, and 64-QAM are mandatory in the downlink.In the uplink, 64-QAM is optional. Both the convolutional code (CC) with RS andthe convolutional Turbo code (CTC) with a variable code rate and repetition coding aresupported. The block Turbo code and low density parity check code (LDPC) are supportedas optional features. Table 5-28 summarizes the coding and modulation schemessupported in the mobile WiMAX profile.Hybrid-ARQ is supported by IEEE 802.16e. HARQ is enabled using the N channel‘stop and wait’ protocol, which provides a fast response to packet errors and improves celledgecoverage. Both combining and incremental redundancy are supported to improve the