A sub-optimal practical computationally efficient transmit power allocation algorithm is proposed under the constraints of data rate and max bit error rate (BER) in OFDM systems. By exploiting the relation between the water-filling level and the data rate, the proposed algorithm derives an efficient adaptive coarse searching method for water-filling level without need of preset step and other initial parameters, then determines the required water-filling level by the dichotomy method. When the channel state changes, the proposed algorithm works out a small domain of required water-filling level based on the latest allocation and then tracks the channel quickly. Simulation results show that the proposed algorithm converges quickly and is suitable for time-varying channels.

This letter investigates a practical space time block coded (STBC) cooperative system to determine under what conditions a cooperative system outperforms a direct one, and then proposes two criteria for selecting proper relay based on outage probability and capacity, respectively. Performance analysis and numerical results indicate that the STBC cooperative system is superior in practical scenarios.