Ph.D Thesis

Ph.D StudentZaidel Benjamin
SubjectInformation Theoretic Aspects of CDMA Systems
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Shlomo Shamai )Shitz(


In this thesis we investigate several information theoretic aspects of multicell communications. We start by focusing on the uplink and analyze system performance, under various assumptions, when optimally coded randomly spread direct sequence code-division multiple access (DS-CDMA) is employed. The setting adheres to

Wyner's (1994) infinite linear cell-array model, where only adjacent-cell interference is present, and characterized by a single parameter 0 £ a £ 1. The focus is on asymptotic analysis where both the number of users per cell and the processing gain go to infinity, while their ratio (the “cell load”) goes to some finite constant.

First, single cell-site processing is considered, and four multiuser detection strategies, differing in the information on multiuser interference utilized by the receiver, are analyzed and compared. The spectral efficiency, specifying the ultimate performance, is evaluated for non-fading channels, as well as for  slow flat-fading, while employing symbol-level interleaving. For the latter setting, a suboptimum practically oriented transmission and decoding strategy is also considered, assuming that all users employ equal rates and transmit powers, while considering a strongest-users-only decoding scheme. The total capacities under an outage constraint, derived as functions of the fraction of users that cannot be decoded, are analyzed and compared to the corresponding spectral efficiency results. We then proceed with the analysis the optimum and linear MMSE joint multicell receivers, while assuming flat-fading and chip-level interleaving. Dramatic performance enhancement as compared to single-cell-site processing is demonstrated. The impact of chip-interleaving vs. symbol-interleaving is also investigated. The final part of the thesis investigates the average per-cell sum-rate capacity with joint multicell processing, and no random spreading. The uplink, downlink, and both non-fading and flat-fading channels are addressed. The focus is on a simple Wyner-like model, where the system cells are arranged on a circle, assuming the cell-sites are located at the boundaries of the cells. Assuming individual per-cell power constraints, the sum-rate capacity of the downlink is shown to coincide with the corresponding explicitly obtained uplink result for non-fading channels. Introducing flat-fading, lower and upper bounds on the sum-rate capacity are derived, exhibiting an O(loge K) multi-user diversity factor for a number of users per-cell K >> 1, in addition to the array diversity gain. Joint multi-cell processing is shown to eliminate out-of-cell interference, which is traditionally considered to be a limiting factor in high-rate reliable communications.