It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

In order to estimate the signal parameters accurately for mobile systems, it is necessary to estimate a system's propagation characteristics through a medium. Propagation analysis provides a good initial estimate of the signal characteristics. The ability to accurately predict radio-propagation behavior for wireless personal communication systems, such as cellular mobile radio, is becoming crucial to system design. Since site measurements are costly, propagation models have been developed as a suitable, low-cost, and convenient alternative. Channel modeling is required to predict path, loss and to characterize the impulse response of the propagating channel. The path loss is associated with the design of base stations, as this tells us how much a transmitter needs to radiate to service a given region. Channel characterization, on the other hand, deals with the fidelity of the received signals, and has to do with the nature of the waveform received at a receiver. The objective here is to design a suitable receiver that will receive the transmitted signal, distorted due to the multipath and dispersion effects of the channel, and that will decode the transmitted signal. An understanding of the various propagation models can actually address both problems. This paper begins with a review of the information available on the various propagation models for both indoor and outdoor environments. The existing models can be classified into two major classes: statistical models and site-specific models. The main characteristics of the radio channel - such as path loss, fading, and time-delay spread - are discussed. Currently, a third alternative, which includes many new numerical methods, is being introduced to propagation prediction. The advantages and disadvantages of some of these methods are summarized. In addition, an impulse-response characterization for the propagation path is also presented, including models for small-scale fading, Finally, it is shown that when two-way communication ports can be defined for a mobile system, it is possible to use reciprocity to focus the energy along the direction of an intended user without any explicit knowledge of the electromagnetic environment in which the system is operating, or knowledge of the spatial locations of the transmitter and the receiver.

A survey of various propagation models for mobile communication

For pt.I see ibid., vol 41, p.1626-34 (1993). Power control is essential in the use of direct-sequence code division multiple-access (CDMA) techniques. Early system-level performance analyses of a CDMA approach to wireless mobile and personal communications have assumed the ability of power control to equalize the absolute signal powers of CDMA users received at each base station. The present paper studies a more practical, although analytically more complicated, uplink power control technique that uses measurements of the received signal-to-interference ratio (SIR) instead. A combination of discrete-event link simulation and analysis of the obtained SIR statistics is used to explore the previously little-known behavior of a CDMA system using SIR-based power control and to obtain performance estimates for such a system under various operating assumptions. The overall results indicate that power control based on SIR has the potential for somewhat higher system performance than power control based on absolute signal strength assumed in the early analyses. >

Signal and interference statistics of a CDMA system with feedback power control. II

The authors present an overview of the characteristics of code division multiple access (CDMA) as it is currently being envisioned for use in wireless communications. There are many considerations in the design of such systems, and there are multiple designs being discussed. CDMA has been proposed for both terrestrial links and satellite links. However, there are key differences in the characteristics of the two types of links relative to the way they affect a CDMA system. The authors show how the ideas presented affect both terrestrial and satellite-based systems. >

Spread spectrum access methods for wireless communications

Signal-to-interference ratio (SIR)-based call admission control (CAC) algorithms are proposed and studied in a DS-CDMA cellular system. Residual capacity is introduced as the additional number of initial calls a base station can accept such that system-wide outage probability will be guaranteed to remain below a certain level. The residual capacity at each cell is updated dynamically according to the reverse-link SIR measurements at the base station. A 2/sup k/ factorial experimental design and analysis via computer simulations is used to study the impact of the parameters used in the algorithms. The influence of these parameters on system performance, namely blocking probability and outage probability, is then examined via simulation. The performance of the algorithms is compared together with that of a fixed call admission control scheme (fixed CAC) under both homogeneous and hot spot traffic loading. The results show that SIR-based CAC always outperforms fixed CAC even under overload situations, which is not the case in FDMA/TDMA cellular systems. The primary benefit of SIR-based CAC in DS-CDMA cellular systems, however, lies in improving the system performance under hot spot traffic. >

SIR-based call admission control for DS-CDMA cellular systems

Analysis techniques for quantitatively describing the impact of propagation path loss and user distribution on wireless direct-sequence code-division multiple-access (DS-CDMA) spread spectrum systems are presented. Conventional terrestrial propagation models which assume a d/sup 4/ path loss law are shown to describe modern cellular and personal communication system channels, poorly. A two-ray propagation model and path loss model derived from field measurements are used to analyze the impact of path loss on the frequency reuse efficiency of DS-CDMA cellular radio systems. The analysis shows that the frequency reuse efficiency (F) of the reverse channel with a single ring of adjacent cells can vary from a maximum of 71% in d/sup 4/ channels with a favorable distribution of users, to a minimum of 33% in d/sup 2/ channels with a worst-case user distribution. For three rings of adjacent users, F drops to 58% and 16%, respectively. Using the two-ray model, it is shown that F can vary over a wide range of values due to the fine structure of propagation path loss. >

http://www.faculty.poly.edu/~tsr/wp-content/uploads/CV/MTJ/1991-08-Effects%20of%20radio%20propagation%20path%20loss%20on%20DS-CDMA%20cellular%20frequency%20reuse%20efficiency%20for%20the%20reverse%20channel.pdf

Effects of radio propagation path loss on DS-CDMA cellular frequency reuse efficiency for the reverse channel

Expressions that quantitatively describe the impact of path loss and user distribution on CDMA (code division multiple access) cellular radio system performance are determined. Path loss in typical microcellular and cellular channels is shown to increase exponentially with distance, to between the second and third power, as opposed to the commonly quoted fourth power law. Two performance measures, frequency reuse efficiency and bit error rate probability, are evaluated for CDMA cellular systems. Values for frequency reuse efficiency are derived for the uplink using different propagation rules and spatial distributions of subscribers in adjacent cells. Bit error rate probabilities are found as a function of the number of users for a two-cell downlink model that assumes the subscriber is straddling the cell boundaries. >

L.B. Milstein

Papers

Effects of radio propagation path loss on DS-CDMA cellular frequency reuse efficiency for the reverse channel

Effects of path loss and fringe user distribution on CDMA cellular frequency reuse efficiency