Pilot-based individual forward and backward channel estimation in Amplify-and-Forward OFDM relay networks

In this paper, we propose an Amplify-and-Forward (AF) Orthogonal Frequency Division Multiplexing (OFDM) relay scheme that each relay superimposes its own pilot symbol (PS) such that, individual relay channels can be estimated at the destination. We derive optimal PS designs including both Modification Diagonal Matrix (MDM) and superimposed PS at each relay. These designs are based on channel Mean Square-Error (MSE) minimization and Inter-Relay Interference (IRI) elimination, equivalently. It is shown that the number of relays should satisfy the condition, M <; (N/2(2L-1)), where N and L are the OFDM size and channel order, respectively, and (a) is the maximum integer part of a. Both Least-Square (LS) and linear minimum MSE channel estimators are derived. Furthermore, the estimator according to uncertain data and approximate computations is derived from an efficient convex optimization problem.