ls as well as different time intervals as shown in Fig. 7-3. Subchannels are combined from sub- carriers chosen either pseudo-randomly or sequentially. Each subscriber is treated separately according to location, distance from the base station, interference, and power requirements. Different modulations can be used for each of the carriers in the system to improve coverage and throughput. 240 Open Radio Access Network (O-RAN) Systems Architecture and Design Classic OFDM OFDMA FIGURE 7-3 OFDM versus OFDMA. OFDM, orthogonal frequency division multiplexing; OFDMA, orthogonal frequency division multiple access. The figure shows allocations to different users with different shades of gray for each user. Subchannels are represented as rows, OFDM symbols are represented as columns. OFDMA allows the transfer of data by variably allocating both subchannels and symbols, whereas OFDM is classically limited to only variably allocating symbols. The ability to variably allo- cate data on both the frequency and time domains is what allows OFDMA to be more flexi- ble and efficient. Orthogonal frequency division multiple access subcarrier allocation As previously stated, an OFDMA symbol is made up of multiple subcarriers. The size of the FFT used to convert frequency-domain to time-domain signals determines how many are present. Typically, FFT sizes of 512, 1024, 2048, and 4096 points are used, yielding corresponding numbers of subcarriers. However, not all subcarriers are used for data trans- mission. A subcarrier can be assigned one of three functions: transmission of data (data sub- carriers), channel estimation (pilot subcarriers), or guard band (null subcarriers). Data and pilot subcarriers are organized into subchannels (4G: resource block; 5G: resource element), where one subchannel may be assigned dozens of subcarriers. The subcarriers forming one subchannel may or may not be adjacent, depending on the air interface standard. 4G and 5G implement adjacent subcarrier allocation. The Medium Access Control (MAC) scheduler mus