Carrier Frequency Offset Mitigation in Asynchronous Cooperative OFDM Transmissions

Carrier frequency offset (CFO) mitigation is critical for orthogonal frequency-division multiplexing (OFDM)-based cooperative transmissions because even small CFO per transmitter may lead to severe performance loss, especially when the number of cooperative transmitters is large. In this paper, we show that cyclic prefix (CP) can be exploited to mitigate or even remove completely the CFO. The mitigation performance increases along with the CP length. In particular, long CP with length proportional to NI, where N is the fast Fourier transform (FFT) block length and I is the number of cooperative transmitters, can guarantee a complete CFO removal. While this comes with a reduction in bandwidth efficiency, the long CP in the proposed scheme is exploited to enhance transmission power efficiency in a way similar to spread-spectrum systems, and thus is different from conventional CP that degrades both bandwidth and power efficiency. An efficient CFO-mitigation algorithm is developed that has complexity at most O(NI ²), or even linear in N approximately in some cases. Implemented as a preprocessing procedure independently from cooperative encoding/decoding details, this algorithm makes the CFO problem effectively transparent to and thus has general applications in OFDM-based transmissions.