h group is called a resource block. The grouping of subcarriers into groups of resource blocks is referred to as sub- channelization. The subcarriers 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 uplink, a user may be assigned to one or more resource blocks. Sub-channelization defines subchannels that can be allocated to mobile stations depending on their channel conditions and service requirements. Using sub-channelization, within the same time slot (i.e., an integer number of OFDM symbols) an OFDMA system can allocate more transmit power to user devices with lower SNR and less power to user devices with higher SNR. Sub- channelization also enables the base station to allocate higher power to sub-channels assigned to indoor mobile terminals resulting in better indoor coverage. In OFDMA, an OFDM symbol is constructed of subcarriers, the number of which is determined by the FFT size. There are several subcarrier types: (1) data subcarriers are used for data trans- mission, (2) pilot or reference-signal subcarriers are utilized for channel estimation and coherent detection, and (3) null subcarriers that are not used for pilot/data transmission. The null subcarriers including the DC subcarrier (if it exists) are used for guard bands. The number of used (or occupied) subcarriers is always less than the FFT size. The guard bands are used to allow spectrum sharing and to reduce the adjacent channel interference and OOB emissions. The sampling frequency is selected to be greater than or equal the channel bandwidth. The number of time samples in a radio frame is always an integer and to further simplify the design of analog transmit filter, the sampling frequency is scaled by a factor greater than one (e.g., in LTE, the sampling frequency for 20 MHz bandwidth is 30.72 MHz). In order to explain the signal processing concepts involved in an OFDMA transmission sys- tem, we use the generic transmitter model that is illustrated i