riations. Channel estimation is required for high-order QAM modulations. As a multi-carrier transmission technique, OFDM is less sensitive to shift in sampling time in comparison to serial transmission techniques. The bandwidth efficiency of an OFDM system approaches the Nyquist rate as the FFT size increases (this, however, increases system complexity and is subject to an upper-limit due to channel variations and carrier frequency offset). For fixed bandwidth efficiency, the complexity (multiplications per symbol) of an FFT-based OFDM system grows logarithmically with the increase in the channel multipath spread, in comparison with a linear increase of complexity for equalizers for a single carrier system. A properly coded and interleaved OFDM system can exceed the performance of other practical systems. Higher transmitter output back-off is required in comparison to a single carrier system, because of the high peak-to-average power ratio of an OFDM system. As a parallel transmission technique, OFDM is more sensitive to carrier frequency offset and tone interference than a single carrier system. Orthogonal Frequency Division Multiple Access (OFDMA) is the multi-user variant of the OFDM scheme where multiple-access is achieved by assigning subsets of sub-carriers to different users, allowing simultaneous data transmission from several users. In OFDMA, the radio resources are two- dimensional regions over time (an integer number of OFDM symbols) and frequency (a number of contiguous or non-contiguous sub-carriers). The difference between OFDM and OFDMA is illustrated in Figure 9-9. Similarly to OFDM, OFDMA employs multiple closely spaced sub-carriers, but the sub- carriers are divided into groups of sub-carriers where each group is called a resource block. The grouping of sub-carriers into groups of resource blocks is referred to as subchannelization. The sub- carriers that form a resource block do not need to be physically adjacent. In the downlink, a resource block may be allocated to different users. In the upl