Cooperative OFDM Channel Estimation with Frequency Offsets

This paper discusses channel estimation in a cooperative orthogonal frequency-division multiplexing (OFDM) network in the presence of frequency offsets. Both the amplify- and-forward (AF) and decode-and-forward (DF) relaying modes are analyzed. In order to eliminate the multiple access interference (MAI), the maximum number of active AF and DF relays are [N/2L-1] and [N/L], respectively, where N is the total number of subcarriers, L is the channel order and [a] is the maximum integer part of a. The pairwise error probability (PEP) of orthogonal space-time coding in cooperative OFDM due to both the frequency offset and channel estimation errors is also evaluated. The optimal power allocation ratio between the source and the set of the relays to minimize the PEP is derived for both the relay modes. When L < 16, the DF mode always outperforms the AF mode in terms of PEP. Given 16 active relays and with L = 4, the proposed DF mode outperforms the AF mode by about 9 dB for a frequency offset error variance of 10^-3, and this gap increases to about 11.3 dB when the variance increases to 10^-2.