Aspects on Interference and Diversity in Wireless Networks

Abstract: This thesis deals with two topics in wireless communications: interference between frequency-hopping (FH) networks, and multiuser diversity. The work on FH treats a system where FH is used to provide simultaneous access to the channel for multiple uncoordinated wireless networks. This is achieved by performing frequency hops on a packet-by-packet basis, and letting the sequence of hopping frequencies be pseudo random. Furthermore, the networks in the considered system are allowed to use several packet types, which can be of different lengths. A throughput analysis of this system is performed, and this analysis is based mainly on two assumptions. Firstly, collisions, i.e., overlaps in time and frequency between packets, are assumed to always result in a total loss of the data in the colliding packets. Secondly, it is assumed that networks are asynchronous. Both exact and approximate expressions for the throughput are derived, and the dependence of the throughput on the number of networks and the available packet types is investigated for two existing wireless systems, namely Bluetooth and IEEE 802.11 FHSS. Furthermore, the accuracy of the collision assumption as a function of the distance between networks is determined for the Bluetooth system. The second topic of this thesis, multiuser diversity, is a scheme where a base station collects channel state information from a number of users and then transmits to the user with the instantaneously best channel. Although this scheme promises large gains, there are a number of practical aspects that reduce this gain. In this thesis two of these aspects are considered. Firstly, the impact of quantizing the feedback of the users' signal-to-noise ratio measurements is investigated for both variable- and constant-rate transmission. It is shown that even for a coarse quantization, the multiuser diversity gain is significant and good performance is achieved. Secondly, the impact of feedback delay is analyzed while also taking into account the additional diversity, referred to as link diversity, that Rake reception or transmit diversity (using space--time block codes) can provide. Expressions for the error rates and spectral efficiencies for uncoded constant- and variable-rate transmission are derived, and the dependence on the number of users, the feedback delay, and the link diversity order is investigated. These expressions also apply to Rake reception combined with transmit or receive antenna selection.