t be aware of the proportion of data subcarriers to null and pilot subcarriers as this will affect how much data can be mapped onto the OFDMA frame. Adjacent subcarrier allocation results in different subchannel responses to different users, due to frequency-selective fading as shown in Fig. 7-4 the figure. As frequency-selective fad- ing rarely affects all users identically, adjacent subcarrier allocation allows users to be allo- cated the subchannels which give them best channel quality. This topic has been extensively discussed in literature, and performance gains of up to 200%-300% have been achieved Chapter 7 Wireless scheduling and Quality of Service optimization techniques 241 User channel response Resulting Subchannel Allocation User B Signal strength User A frequency signal strength frequency FIGURE 7-4 Simplified illustration of frequency-selective fading with adjacent subcarrier permutation. using this type of subcarrier allocation. 3,4 The downside of the adjacent subcarrier allocation is a higher level of complexity: In order to allocate better subchannels to a user, an up-to- date knowledge of the exact subchannel response is necessary. Note that Carrier Aggregation (CA), where a user is communicating to the base station over multiple allocated frequency channels concurrently, can be seen as a variety of OFDMA where each frequency channel can have a different channel characteristic/response and thus can benefit from scheduling/allocation schemes as shown above. Modulation The OFDMA air interface allows different modulation schemes to be applied to each subcar- rier (Fig. 7-5). Modulation schemes include quadrature phase shift keying (QPSK), 16 quad- rature amplitude modulation (QAM), and 64-QAM. The phase of a dot is indicated by the angle a line from it to the origin makes with the positive x-axis. The amplitude is the length of this line. Thus, for QPSK, four different phase-amplitude combinations are possible (2-bit). While 16 are possible for 16-QAM (4-bit), 64 are possible for 64-QAM (6-bit). T