In this paper, we proposed methods to compensate the effects of multiple carrier frequency offsets(CFO) for orthogonal frequency division multiplexing(OFDM) based systems. We consider two types of multiple CFOs. One is multiple CFOs in OFDMA(orthogonal frequency division multiple access) uplink system and another is in cooperative communication system.
The OFDMA is a multiuser version of the OFDM modulation scheme and has attracted much attention because of the inheriting advantages of OFDM. In OFDMA, all the orthogonal subcarriers are divided into a number of mutually exclusive subsets and these subsets are assigned to different users. OFDMA is weak to the CFO as OFDM. Especially, in uplink OFDMA systems where several users access to a base station simultaneously, multiple CFOs may appear in the received signals of all users. These multiple CFO bring multiuser interference(MUI) which is the incoming interference from other users’ signals.
In the proposed method for OFDMA system, the MUI cancellation scheme adopting the parallel interference cancellation(PIC) on a few subcarriers located at both edges are used to reduce computational complexity.
The cooperative communication systems reduces a radio shadow area by installing some relays in the middle of cell. In these schemes, a subscriber receives the same signal from several transmitters simultaneously, and multiple CFOs of different meaning to OFDMA may appear at the receiver. The these multiple CFOs are caused by several spatially separated frequency-unsynchronized transmitters.
In proposed scheme for cooperative communication, we compensate the multiple CFOs of received signal by the predesigned value, which results in cancellation of the opposite interchannel interference(ICI) by each other. this is based on the property that the spectrum of ICIs shown in a subcarrier has similar magnitudes and opposite signs from the fact that spectrum shapes of ICI are of sinc function.
Simulation results are shown to demonstrate the effectiveness of the proposed methods for compensating the multiple CFOs on both OFDMA and cooperative communications systems.