Cognitive Radio Communication and Networking: Principles and Practice

Chapter 9

Agile Transmission Techniques (II): Orthogonal Frequency Division Multiplexing

OFDM is a technique of digital data transmission based on multicarrier modulation [68, 1097–1099]. The history of OFDM can be traced back to the middle 1960s [1097, 1100–1103]. OFDM is the extension of the frequency division multiplexing scheme. In frequency domain, though the signals of subchannels or subcarriers are overlapped, they are orthogonal after demodulation. Hence, OFDM improves efficiency of spectrum utilization compared to the frequency division scheme which assigns nonoverlapping frequency bands to different signals [68].

OFDM is an effective tool to handle ISI without using equalization at the receiver. The high-data-rate data stream is divided into many low-data-rate substreams and these substreams are sent over many different subchannels [68]. The bandwidth of each subchannel is much smaller than the total system bandwidth [68]. Meanwhile, the bandwidth of each subchannel is less than the coherent bandwidth of the radio channel. Hence, the effect of ISI on each sub-channel is small and flat fading can be assumed for each subchannel [68].

OFDM is the core technology in the current wireless and wired communications. OFDM is widely used by 3GPP LTE, WLAN, WiMAX, digital TV [1104], power line communication, ADSL, VDSL, and HDSL [1105].

9.1 OFDM Implementation

OFDM can use DFT or FFT for efficient implementation. If N subcarriers are used in OFDM, the continuous-time baseband OFDM ...