Cooperative Space-Time Coded OFDM with Timing Errors and Carrier Frequency Offsets

Abstract—The use of distributed space-time codes in cooperative communications promises to increase the rate and reliability of data transmission. These gains were mostly demonstrated for ideal scenarios, where all the nodes are perfectly synchronized. Considering a cooperative uplink scenario with asynchronous nodes, the system suffers from two effects: timing errors and individual carrier frequency offsets. In effect, timing errors can completely cancel the advantages introduced by space-time codes, while individual carrier frequency offsets provide a great challenge to receivers. Indeed, in cooperative communications, frequency offsets are perceived as a time-variant channel, even if the individual links are static. We show that using OFDM, space-time codes (STCs) become robust to timing errors. Channel estimation and tracking takes care of frequency offsets. Our simulations demonstrate that the bit error rate (BER) performance improves by an order of magnitude, when using a cooperative system design, which takes these two effects into account. Index Terms—OFDM, Block codes, distributed space-time code, virtual MIMO, diversity methods, cooperative systems