Multi-user wireless communication systems with rate constraints

In this thesis, we will make a comprehensive study on multi-user wireless communication systems where every user in the system must transmit a certain amount of information within each frame. This hard-fairness scenario is applicable to delay-sensitive services such as voice and video. We will discuss both the theoretical and practical aspects of multi-user wireless systems under such hard rate constraints. The main contributions of this thesis are listed below. In the first contribution, the closed-form expressions are derived for the average unconstrained minimum transmitted sum power (MTSP) of multi-user singleinput single-output (SISO) systems over multiple access channels (MACs) and broadcast channels (BCs). It is shown that significant performance gain, which is referred to as multi-user gain (MUG) in this thesis, can be achieved by allowing multi-user concurrent transmission. The MUG in SISO systems mainly comes from the near-far diversity among users and a large portion of MUG can be achieved with a small number of simultaneous users. The practical implementation aspect of multi-user SISO systems is also considered. We adopt interleave-division multiple-access (IDMA) as a platform and propose several power allocation methods to enhance the system performance. Both evolution and simulation results show that considerable MUG that is predicted in theory can indeed be achieved in practical environments. In the second contribution, we extend the results for the SISO scenario to the multiple-input multiple-output (MIMO) scenario. We avoid the complicated computation in finding the exact unconstrained MTSP of multi-user MIMO systems for each channel realization by adopting bounding techniques and derive the closed-form expression for the corresponding asymptotical average unconstrained MTSPs. We point out that, besides the near-far diversity, MUG in MIMO systems also comes from direction diversity that is provided by multiple antennas at the base station (BS), and the number of antennas at the BS has a more significant effect on the system capacity than that at the user side. In the meanwhile, we propose a low-cost but asymptotically optimal technique, i.e., the maximum eigenmode beamforming (MEB) technique, to realize the aforementioned MUG in multi-user MIMO systems with practical coding. In the third contribution, we consider the capacity analysis of cellular systems with various BS cooperation strategies. Based on the results in the first two contributions, some lower and upper bounds are derived for cellular systems with