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

WiMAX 257Total bandwidth (between 1.5 and 28 MHz)Sub-channel 1 Sub-channel 2 . . . Sub-channel KGuard bandDC Sub-carrierGuard bandFigure 5-34Example of OFDMA frequency allocation for K usershave good correlation properties, which eases the synchronization tasks. The power ofthe uplink and the downlink preambles is boosted by 3 dB compared to the data part.OFDMA ModeOFDMA in IEEE 802.16d was originally specified as an optional mode and later becamemandatory for mobile application in IEEE 802.16e. In the following the basic parametersas described in IEEE 802.16d are presented.As described in Chapter 3, in OFDMA only a part of all sub-carriers may be used fordata transmission. A set of sub-carriers, called a sub-channel, will be assigned to eachuser (see Figure 5-34). For both uplink and downlink the used sub-carriers are allocatedto pilot and data sub-carriers. However, there is small difference between the uplink andthe downlink sub-carrier allocation. In the downlink, there is one set of common pilotcarriers spread over all the bandwidth, whereas in the uplink each sub-channel contains itsown pilot sub-carriers. This is because the downlink is broadcasted to all terminal stationswhile in the uplink each sub-channel is transmitted from a different terminal station. Thegoal of these pilot sub-carriers is to estimate the channel characteristics.For OFDMA with FDD, the frame duration is an integer number of three OFDM symbols,where the actual frame duration is nearest to the nominal frame duration. In additionto the sub-channel dimension (set of sub-carriers), OFDMA uses the time dimension fordata transmission. An uplink or downlink burst in OFDMA has a two- dimensional allocation.A transmit burst is mapped on to a group of contiguous sub-channels and contiguousOFDM symbols. Each data packet is first segmented into blocks sized to fit into one FECblock. Then, each FEC block spans one OFDMA sub-channel in the sub-channel axis andthree OFDM symbols in the time axis. The FEC blocks are mapped such that the lowestnumbered FEC block occupies the lowest numbered sub-channel in the lowest numberedOFDM symbol. The mapping is continued such that the OFDMA sub-channel index isincreased for each FEC block mapped. When the edge of the data region is reached, themapping will be continued again from the lowest numbered OFDMA sub-channel in thenext OFDM symbol (see Figure 5-35).For the uplink transmission, a number of sub-channels over a number of OFDM symbolsis assigned per terminal station. The number of OFDM symbols is equal to 1 + 3N, where