Information-theoretic considerations for symmetric, cellular, multiple-access fading channels - Part I

A simple idealized linear (and planar) uplink cellular multiple-access communication model, where only adjacent cell interference is present and all signals may experience fading, is considered. Shannon theoretic arguments are invoked to gain insight into the implications on performance of the main system parameters and multiple-access techniques. In Part I, we specialize to the linear model and address the case of practical importance where the cell-site receiver processes only the signals received at this cell site, and where the actively interfering users assigned to other cells (but not those within the cell) are interpreted as Gaussian noise (worst case assumption). We assume that the cell-site receiver is aware of the instantaneous signal-to-noise ratios for all users assigned to that cell while the users’ transmitters do not have access to this side information. We first investigate several different scenarios and focus on the effect of fading and intercell interference and then provide a general formulation for an achievable rate region (inner-bound) of which all the different scenarios are special cases. The features of TDMA and wideband (WB) intracell multiple-access techniques are examined as well as the role of (optimized) fractional intercell time-sharing (ICTS) protocol. With no fading, orthogonality within the cell is optimal (not unique, however) with or without intercell interference. When fading is present and intercell interference is not dominant, WB intracell access is found advantageous due to an inherent fading averaging effect. This conclusion may reverse when intercell interference takes place and exceeds a given threshold; and then intracell orthogonal signaling (TDMA) is preferable. With intercell interference present, it is demonstrated that fractional ICTS may prove beneficial as compared to full exploitation of time (no sharing) at each cell, where the latter is known to be optimal under no fading conditions and an ultimate receiver which optimally processes all the information received at all cell sites. In Part II, the model is extended to account for cell-site receivers that may process also the received signal at an adjacent cell site, compromising thus between the advantage of incorporating additional information from other cell sites on one hand and the associated excess processing complexity on the other. Various settings which include fading, TDMA, WB, and (optimized) fractional ICTS protocols are investigated and compared. In this case and for the WB approach and large number of users per cell it is found, surprisingly, that fading may enhance performance in terms of Shannon theoretic achievable rates. The linear model is extended to account for general linear Manuscript received May 10, 1996; revised May 15, 1997. The major part of this work was done while S. Shamai was on sabbatical with AT&T (now Lucent Technologies) Bell Laboratories, Murray Hill, NJ. The material in this paper was presented in part at ISITA ’94, Sydney, Australia, November 1994, and at the Information Theory Workshop, Information Theory, Multiple Access and Queueing, St. Louis, MO, April 1995. S. Shamai (Shitz) is with the Department of Electrical Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel. A. D. Wyner is with Lucent-Technologies, Bell Laboratories, Murray Hill, NJ 07974 USA. Publisher Item Identifier S 0018-9448(97)06720-5. and planar configurations. The effect of random number of users per cell is investigated and it is demonstrated that randomization is beneficial. Certain aspects of diversity as well as some features of TDMA and orthogonal code-division multiple-access (CDMA) techniques in the presence of fading are studied in an isolated cell scenario.