ed into 256 subcarriers (OFDMA256), SO that the subcarrier spacing depends on the channel width and varies from 6.8 to 78.1 kHz. In mobile applications, varying Doppler shift and multipath delays lead to a degradation of performance, in terms of SNR and BER, particularly for subcarrier spacings at the low end of this range. Conversely, channel capacity can be increased by using more subcarriers at higher channel widths. Scalable OFDMA addresses these issues by using a constant subcarrier spacing of 11.2 kHz and varying the number of subcarriers depending on the channel width, from 128 for a 1.25 MHz channel to 2048 for a 20 MHz channel. This maximises channel capacity for the wider channels and ensures that all channel widths are equally tolerant of the delay spread resulting from moving stations. Chapter Twelve Full channel of OFDM carriers Uplink subcarrier power if all tones are used Pilot Subcarrier power tones for adjacent subchannelisation Carrier power for distributed subchannelisation Figure 12-2: WiMax Subchannelization S-OFDMA and OFDMA256 will not be compatible, SO that WMAN equipment will have to be replaced in order to support mobile applications based on S-OFDMA. Subchannelization Subchannelization refers to the use of a subset of available OFDM subcarriers. By concentrating the available transmitter power into fewer subcarriers, each subcarrier can be transmitted with higher individual power. This additional link margin can be used either to extend the link range, to overcome penetration losses allowing mobile devices to be located indoors, or to reduce the power consumed by the transmitting device. These benefits come at a cost of reduced link capacity, since only a subset of subcarriers are used to carry data, allowing a trade-off between throughput and mobility. Various methods can be used for allocating subcarriers to subchannels - the main ones being the use of adjacent or distributed subcarriers (Figure 12-2). Distributed subcarrier allocation is used for mobile applications mainly because th