Cooperative OFDMA in the Presence of Frequency Offsets

Besides frequency diversity, spatial diversity gain can be realized by employing a space-time code and/or multiple co-located transmit antennas [11, 12]. However, in cooperative diversity, a distributed virtual multiple antenna system is created to extract diversity gain [13-20]. A relay is used to improve the transmission quality between node S (the source) and node D (the destination), and the relay can operate in either the amplify-and-forward (AF) or the decode-and-forward (DF) mode. Joint optimization in the relay-precoders and decoders in a cooperative network is discussed in Ref. [21], with either full channel state information (CSI) or partial CSI being available at the destination and the relays. A hybrid forwarding scheme for cooperative relaying in orthogonal frequency-division multiplex (OFDM)-based networks is proposed in Ref. [22], and based on the current signal-tonoise ratio (SNR) of each subcarrier between the source, the relay and the destination, the relay can adaptively determine which relaying mode to use. Outage capacity of the slow-fading relay channel that focuses on the low SNR and low outage probability regime is discussed in Ref. [23]. It is shown that the Bursty Amplify-Forward protocol proposed in Ref. [23] achieves the optimal outage capacity per unit energy in the low SNR regime. An efficient algorithm for performing joint optimization in relay selection and resource allocation in a cellular system employing OFDMA is proposed in Ref. [24], where the optimization is performed not only by considering the power and bandwidth allocation, but also by selecting the best relay as well as the best relay strategy. The performance degradation of a cooperative OFDMA uplink due to the frequency offset is analyzed in Refs. [25, 26].