Resource Optimization And Cooperation Strategies In Cooperative UWB Systems

In recent years, UWB (Ultra-wideband) radios became a fast developing technology in WPAN with uniquely attractive features as high-speed rate, huge space capacity, low cost, low power, short range and so on.Cooperative diversity has emerged as a new diversity technology which based on relay transmission and get diversity gain through the cooperation between the users. It can improve the system performance, extend the transmission coverage and reduce the transmit power.In this thesis, cooperative diversity is employed to MB-OFDM (Multiband Orthogonal Frequency Division Multiplexing) UWB systems. Based on the low power, multi-subbands and TFI (Time-Frequency-Interleaved) characteristics of MB-OFDM UWB, we research on some key issues as follows: how to select the transmit subband and how to reduce the transmit power while assuring an adequate system performance; how to realize rate control when power optimization is not efficient and what are the cooperation stratigies between the selfish UWB devices.Our research object is MB-OFDM UWB system with DF (Decode-and-Forward) cooperative scheme. According to the characteristic of TFI, UWB system can provide different transmit rates. So how to assign subbands and allocate power to the simultaneous devices while assuring their different transmit requirements must be considered. According to the cooperative characteristic, how to allocate the power between the source and relay under the overall power limitation must be considered too. First, the SNR (Signal-to-Noise Ratio) and SER (Symbol Error Rate) performance in DF cooperative MB-OFDM UWB system is analyzed, the closed-form formulation is given. A joint allocation of subband and power is discussed, and a computationally inexpensive suboptimal approach is proposed. The algorithm takes two steps, the power was optimized and subband was assigned for the network regarding each user pair as a user, then the power was optimized in each user pair. Simulation results show that the suboptimal approach can save up the transmit power greatly.When the resource allocation algorithm is not efficient, the rate control scheme can be employed. Rate control adapts the transmission rate via introducing more or less redundancy. An effective way of realizing rate control is to systematically puncture a low rate code, a so-called mother code, and get a set of codes called RCPC (Rate Compatible Punctured Code). The beauty of RCPC lies in the fact that only one encoder/decoder pair is needed for the entire rage of rates. Furthermore, only the redundant bits are needed to be transmitted in ARQ, which can increase the overall system throughput. Here an algorithm of rate control using RCP-LDPC code in DF MB-OFDM UWB systems is proposed, and a suitable RCP-LDPC code is constructed. Simulation results show that the proposed encoding scheme outperform the scheme in the UWB proposal.In the preceding research, cooperation is associated with a node's willingness to sacrifice resources for the benefit of other nodes in the networks. But in WAPN applications especially in power limited UWB networks, nodes are independent selfish individuals and they are not voluntary to help other nodes. Cooperation strategy based on the game theory method is studied here. An unsymmetrical system model is constructed, and a price game based on a new payoff function is proposed to describe the cooperation in this model. The proposed pricing game is shown to converge to Nash equilibrium by the iteration algorithm, which effectively increases the payoffs of the users and the system.