This paper investigates the resource allocation problem in the downlink of OFDMA cooperative relaying networks with one source (base station) and multiple relay and destination nodes. Assuming that the base station knows all the instantaneous channel gains of all links, we propose a dynamic joint subchannel and power allocation (SPA) scheme whose objective is to maximize the worst user's data rate with the constraint of total transmission power. Since the optimal solution to this combinatorial problem is extremely computationally complex, we propose a low-complexity suboptimal algorithm that allocates subchannel and power separately. In the proposed algorithm, subchannels are firstly paired by relay nodes according to their equivalent channel gains and then assigned to a specific user. Subsequently, an optimal power distribution algorithm is designed for the proposed subchannel allocation (SA) scheme. The simulation results show that the performance of the proposed algorithm approaches asymptotically to that of the optimal one, and maximal fairness is guaranteed for all users.

This paper proposes a novel channel estimation method to further improve the performance of the optimal pilot sequences in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. We first assume that the virtual pilot tones superimposed at the data locations over the specific sub-carriers are transmitted from all transmit antennas. We then obtain the virtual received pilot signals at the corresponding locations at the receive antennas by interpolation in the time domain. Finally, the channel parameters are obtained from the combination of the virtual and real received pilot signals over one OFDM symbol based on the least squares (LS) channel estimation. Simulation results show that the proposed channel estimation method provides much better performance than the previous method for the optimal pilot sequences over multiple OFDM symbols, especially in fast time- varying channels.