In cellular wireless communication systems, transmitted power is regulated to provide each user an acceptable connection by limiting the interference caused by other users. Several models have been considered including: (1) fixed base station assignment where the assignment of users to base stations is fixed, (2) minimum power assignment where a user is iteratively assigned to the base station at which its signal to interference ratio is highest, and (3) diversity reception where a user's signal is combined from several or perhaps all base stations. For the above models, the uplink power control problem can be reduced to finding a vector p of users' transmitter powers satisfying p/spl ges/I(p) where the jth constraint p/sub j//spl ges/I/sub j/(p) describes the interference that user j must overcome to achieve an acceptable connection. This work unifies results found for these systems by identifying common properties of the interference constraints. It is also shown that systems in which transmitter powers are subject to maximum power limitations share these common properties. These properties permit a general proof of the synchronous and totally asynchronous convergence of the iteration p(t+1)=I(p(t)) to a unique fixed point at which total transmitted power is minimized. >

A framework for uplink power control in cellular radio systems

Most cellular radio systems provide for the use of transmitter power control to reduce cochannel interference for a given channel allocation. Efficient interference management aims at achieving acceptable carrier-to-interference ratios in all active communication links in the system. Such schemes for the control of cochannel interference are investigated. The effect of adjacent channel interference is neglected. As a performance measure, the interference (outage) probability is used, i.e., the probability that a randomly chosen link is subject to excessive interference. In order to derive upper performance bounds for transmitter power control schemes, algorithms that are optimum in the sense that the interference probability is minimized are suggested. Numerical results indicate that these upper bounds exceed the performance of conventional systems by an order of magnitude regarding interference suppression and by a factor of 3 to 4 regarding the system capacity. The structure of the optimum algorithm shows that efficient power control and dynamic channel assignment algorithms are closely related. >

Performance of optimum transmitter power control in cellular radio systems

Distributed power control algorithms that use only the carrier-to-interference ratios (C/I ratios) in those links actually in use are investigated. An algorithm that successfully approximates the behavior of the best known algorithms is proposed. The algorithm involves a novel distributed C/I-balancing scheme. Numerical results show that capacity gains on the order of 3-4 times can be reached also with these distributed schemes. Further, the effects of imperfect C/I estimates due to noise vehicle mobility, and fast multipath fading are considered. Results show that the balancing procedure is very robust to measurement noise, in particular if C/I requirements are low or moderate. However, for required high C/I levels or for a rapidly changing path loss matrix, convergence may be too slow to achieve substantial capacity improvements. >

Distributed cochannel interference control in cellular radio systems

We propose a new method of power control for interference-limited wireless networks with Rayleigh fading of both the desired and interference signals Our method explicitly takes into account the statistical variation of both the received signal and interference power and optimally allocates power subject to constraints on the probability of fading induced outage for each transmitter/receiver pair We establish several results for this type of problem We establish tight bounds that relate the outage probability caused by channel fading to the signal-to-interference margin calculated when the statistical variation of the signal and interference powers is ignored This allows us to show that well-known methods for allocating power, based on Perron-Frobenius eigenvalue theory, can be used to determine power allocations that are provably close to achieving optimal (ie, minimal) outage probability We show that the problems of minimizing the transmitter power subject to constraints on outage probability and minimizing outage probability subject to power constraints can be posed as a geometric program (GP) A GP is a special type of optimization problem that can be transformed to a nonlinear convex optimization problem by a change of variables and therefore solved globally and efficiently by interior-point methods We also give a fast iterative method for finding the optimal power allocation to minimize the outage probability

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Optimal power control in interference-limited fading wireless channels with outage-probability specifications

The interference reduction capability of antenna arrays and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. The minimum variance distortionless response beamformer maximizes the signal-to-interference-and-noise ratio (SINR) when it is employed in the receiver of a wireless link. In a system with omnidirectional antennas, power control algorithms are used to maximize the SINR as well. We consider a system with beamforming capabilities in the receiver, and power control. An iterative algorithm is proposed to jointly update the transmission powers and the beamformer weights so that it converges to the jointly optimal beamforming and transmission power vector. The algorithm is distributed and uses only local interference measurements. In an uplink transmission scenario, it is shown how base assignment can be incorporated in addition to beamforming and power control, such that a globally optimum solution is obtained. The network capacity and the saving in mobile power are evaluated through numerical study.

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Joint optimal power control and beamforming in wireless networks using antenna arrays

In two earlier papers (ICC'80 [3] and GLOBECOM'82 [6]) two power control schemes for spread spectrum cellular land mobile radio have been described, one for the upstream (mobile to base) link and one for the downstream link Here we compare the power control problem for the upstream and the downstream links and show that using simple algebra we can equalize the signal-to-interference ratio for each mobile in a given cell in both links and thereby increase the capacity by 30 to 100% compared with a system with no power control We also show that the SIR values can be equalized system-wide for both link directions by means of a nontrivial eigenvalue problem, which results in a further capacity improvement of 10 to 15% Denial statistics are also presented

Power control for a spread spectrum cellular mobile radio system

The problem of spectrally efficient communication in a cellular mobile radio system is discussed. Information-theoretical bounds are derived for the spectral efficiency of cellular systems using the channel-reuse method, where each user has a distinct channel that is also reused elsewhere in the system; and for the code-division multiple-access system, where all users use the same spectrum simultaneously and signature sequences are used to distinguish them. It is shown that the CDMA (spread spectrum) bounds are uniformly higher than the channel reuse bounds. Results for practical signalling techniques are compared with the bounds, showing that a real potential for improvement exists using either scheme, but that the potential improvement using spread spectrum is much greater.

R.W. Nettleton

Papers

Power control for a spread spectrum cellular mobile radio system

Spectrum efficient cellular mobile communications