Throughput Maximization for User Cooperative Wireless Systems with Adaptive Modulation

Adaptive modulation has been widely adopted in broadband wireless communication systems to improve spectrum efficiency. On the other hand, user cooperative diversity has been investigated to improve system coverage and efficiency. How to take the advantage of adaptive modulation for user cooperative transmissions to maximize network throughput under the constraint of the bit error rate (BER) requirement is an open issue. In this paper, a simple user-cooperation strategy with adaptive M-ary Quadrature Amplitude Modulation (M-QAM) is proposed to fill the gap. We use an approximate BER expression of M-QAM modulation to formulate an easy-to-solve optimization problem, so the modulation types for the source node and the relay node can be optimized in real time to maximize the throughput under the BER constraint. To maximize the throughput for the whole network, we further use a worst-link-first (WLF) matching algorithm for selecting appropriate cooperators. Numerical results show that the proposed adaptive cooperative protocol can effectively improve system spectral efficiency.