*Phase Noise in OFDM DOI: http://dx.doi.org/10.5772/intechopen.105551*

#### **Figure 7.**

*16-QAM OFDM BER performance in AWGN channel with phase noise.*


#### **Table 2.**

*OFDM modeling parameters.*

subcarrier, is the finest slot, which or group of which is allocated to one of the multiple users. **Figure 8** shows the time frequency and power grid of OFDMA [24, 25].

Practically, in frequency domain, the allocation is not done at the level of subcarriers but on the group of subcarriers. This subcarrier's allocation is known as subchannelization. To explain the basic principle of OFDMA transceiver, we are considering here that one user is using one subcarrier in the given time slot, i.e., number of users (*U*)= *N*. With this the simplest OFDMA uplink scheme is illustrated in **Figure 9**. At the transmitter side (mobile terminal), each user is having individual transmitters. At the receiver side (base station), the received signal is the sum of U users'signal, which acts as an OFDM signal. Because of this in OFDMA receiver, a single MC demodulator (OFDM demodulator) is required than U demodulators as in case of conventional frequency division multiple access (FDMA) system. At the transmitter side, a single transmitter consists of symbol generator and OFDMA modulator. The symbol is generated with applicable channel coding and mapping. These symbols are then OFDMA modulated with subchannelization and SC modulator (in case of *U* ¼ *N*) or OFDM modulator in case a single user is using group of the subcarriers.

An exact clock and carrier synchronization is must for an OFDMA system to ensure orthogonality between the umodulated signals from different mobile terminals. This is achieved by transmitting synchronization signals from the receiver to all mobile terminals instantly. Each terminal OFDM modulator drives the carrier frequency and clock signal from these downlink signals. In case of coherent detection,

**Figure 8.** *OFDMA time-frequency power grids [24].*

**Figure 9.** *OFDMA uplink.*

simple carrier and clock recovery circuits are sufficient in the demodulator to extract this information from the received signal as the clock and carrier frequencies are available at the base station. This factor simplifies the OFDMA demodulator.
