On the performance of joint rate/power control with adaptive modulation in wireless CDMA networks

Adaptive rate/power control schemes have great potential to increase the throughput of wireless CDMA networks. In this paper, we investigate the additional gains achieved through adaptation of the orthogonal modulation (OM) order. We show that adaptive orthogonal modulation (AOM) can significantly increase network throughput while simultaneously reducing the per-bit energy consumption (compared to variable-rate, variable-power, fixed-order OM schemes). We study the problem of joint rate/power control in AOM under two different objective functions: minimizing the maximum service time and maximizing the sum of users rates. For the first objective function, we show that the optimization problem can be formulated as a generalized geometric program (GGP), which can be transformed into a nonlinear convex problem and solved optimally and efficiently. In the case of the second objective function, we obtain a lower bound on the performance gain of AOM over fixed-order OM schemes. Unlike previous works on adaptive transmission, which have focused mainly on cellular networks, ours is applicable to both ad hoc and cellular networks. Numerical results indicate that relative to a variable-rate, variablepower fixed-order OM scheme, the proposed AOM scheme achieves significant throughput and energy gains.