Multi-user sum-rate capacity for ultra-wideband radio

This paper studies the fundamental transmission limits for Ultra Wideband (UWB) downlink broadcast channels. The significant contribution of this article is demonstrating the effectiveness of adaptive frequency allocation in UWB systems. We derive an expression for the ergodic sum-rate capacity of an UWB channel with perfect channel knowledge, and show the advantage of an adaptive system that uses channel state information (CSI), over a conventional, non-adaptive system. A capacity achieving system is also proposed. In this system, the transmitter assigns each frequency band to the user that has the best channel gain, and adaptively changes the bandwidth allocation according to the known channel state. The sum-rate capacity can be achieved in a fast fading channel. For a slow fading channel the system must apply fair scheduling to ensure service to all users, and the optimization must be performed for each channel realization. In this case, we also take advantage of the wide bandwidth of the UWB system and show the advantage of an frequency adaptive systems over non frequency adaptive systems.