Fundamental Limits in Wireless Wideband Networking

Abstract: The rapid growth of the wireless communication industry recently does not only bring opportunities but also challenges on developing radio technologies and solutions that can support high data rate as well as reliable and efficient communications. Two fundamental factors that limit the transmission rate are the available transmit energy and the available bandwidth. In this thesis, we investigate fundamental limits on energy and bandwidth efficiencies in wireless wideband networking. The framework and results can be used for performance assessment, design, and development of practical cellular networks.First, we study the energy efficiency of a bidirectional broadcast channel in the wideband regime, i.e., where the bandwidth tends to infinity and the spectral efficiency is disregarded. In particular, we consider a transmit strategy for a Gaussian MIMO bidirectional broadcast channel that maximizes the energy efficiency, i.e., minimizes the energy required to reliably transmit one information bit. A closed-form solution of the optimal transmit covariance matrix is derived, which shows that using a single beam transmit strategy is optimal. Additionally, an extension to a multi-pair Gaussian MIMO bidirectional broadcast channel is studied, in which we propose a simple transmit strategy motivated from the optimal transmit strategy for the single user-pair setup. We show that serving a selected user-pair with full power is optimal in the sense of maximizing the achievable energy efficiency. Discussions on the optimality of the proposed transmit scheme for the multi-pair setup are also provided.Next, we study the bandwidth efficiency of another wireless wideband network, in which the available bandwidth is large but still finite. Accordingly, we consider the bandwidth efficiency limit of an uplink wideband CDMA channel. Various realistic assumptions such as asynchronous transmission, inter-symbol interference, continuous-time waveform transmitted signal, etc. are incorporated into the problem formulation. In order to tackle the problems that arise with those assumptions, we derive an equivalent discrete-time channel model based on sufficient statistics for optimal decoding of the transmitted messages with perfect channel knowledge. The capacity regions are then characterized using the equivalent channel model. In addition, an extension to a system with imperfect channel state information and mismatched filtering at the receiver is considered. Achievable rate regions are characterized considering two different assumptions on decoding strategy, i.e., the optimal decoding based on the actual statistics of channel estimation errors and the sub-optimal approach treating the estimation errors as additive worst-case noise. Moreover, we also present a low-complexity receiver for the uplink wideband CDMA channel, which is based on a decision feedback equalizer structure.