Analysis of Inter Carrier Interference Cancellation Technique and Channel Capacity of OFDM Communication System

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique, which is widely used in various communication systems and standards due to its high-data rate, high-spectral efficiency and robustness to the frequency selective channels. However, one of the major disadvantages of OFDM is its sensitivity against carrier frequency offset, which causes intercarrier interference (ICI) and degrades the performance of system. The carrier frequency offset is caused by the mismatch of frequencies between the oscillators of transmitter and receiver, or from the Doppler spread due to the relative movement between the transmitter and receiver as well as phase noise arises predominantly due to imperfections of the local oscillator in the transceiver. However, the ICI induced by phase noise and timing offset can be completely compensated or corrected. Since the Doppler spread or frequency shift is random, so we can only mitigate its impact in the system. A simple and most effective method, which is known as selfcancellation scheme, reduces the ICI at the cost of transmission rate with little additional computational complexity. The main idea used in this scheme is to modulate the input data symbol onto a group of subcarriers with predefined coefficients such that the generated ICI signals within that group cancel each other, hence named as self-cancellation. Due to this reason, the bandwidth efficiency becomes half, which is the major drawback of this technique. Frequency domain equalization for ICI cancellation, which is very simple but the short coming of this technique is that it can only reduce the ICI caused by multi-path fading distortion. Moreover, it is only suitable for flat-fading channels but in the mobile communication the channels are frequency selective fading in nature. Another effective technique is conjugate cancellation, in which two sequences are transmitted in each data symbol. First sequence is original received sequence and another sequence is conjugate of the original sequence.