Jens Zander

Performance of optimum transmitter 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. >

Distributed cochannel interference 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. >

Centralized power control in cellular radio systems

This paper describes a centralized power control scheme for cellular mobile radio systems. The power for the mobiles in the proposed scheme is computed based on signal strength measurements. All the mobiles using the same channel in this scheme will attain a common carrier-to-interference ratio. The proposed scheme is analyzed and shown to have an optimal solution. >

Constrained power control

High system capacities can be achieved by controlling the transmitter power in multiuser radio systems. Power control with no constraint on the maximum power level has been studied extensively in earlier work [1–18]. Transmitter power is at a premium in radio systems such as cellular systems and PCS. There is a limit on the maximum transmitter power especially at the terminals (e.g. mobile units and handsets) since the power comes from a battery. In this paper we study power control that maximizes the minimum carrier to interference ratio (CIR), with a constraint on the maximum power. The optimal power vector solution lies on the boundary of the constrained power vector set and achieves a balance in the CIRs. Results indicate that the constraints do not induce any stability problems. A distributed scheme with favourable convergence properties and close to optimum performance is presented. Simulation results show that the algorithm tries to maximize the number of terminals served with CIR greater than or equal to the target CIR, while conserving power.

Gradual removals in cellular PCS with constrained power control and noise

In this paper we study themobile removal problem in a cellular PCS network where transmitter powers are constrained and controlled by a Distributed Constrained Power Control (DCPC) algorithm. Receivers are subject to non-negligible noise, and the DCPC attempts to bring each receivers CIR above a given target. To evaluate feasibility and computational complexity, we assume a paradigm where radio bandwidth is scarce and inter-base station connection is fast. We show that finding the optimal removal set is an NP-Complete problem, giving rise for heuristic algorithms. We study and compare among three classes of transmitter removal algorithms. Two classes consist of algorithms which are invoked only when reaching a stable power vector under DCPC. The third class consist of algorithms which combine transmitter removals with power control. These areOne-by-one Removals, Multiple Removals, andPower Control with Removals Combined. In the class of power control with removals combined, we also consider a distributed algorithm which uses the same local information as DCPC does. All removal algorithms are compared with respect to their outage probabilities and their time to converge to a stable state. Comparisons are made in a hexagonal macro-cellular system, and in two metropolitan micro-cellular systems. ThePower Control with Removals Combined algorithm emerges as practically the best approach with respect to both criteria.

Constrained power control in cellular radio systems

High system capacities can be achieved by controlling the transmitter power in multiuser radio systems. Power control with no constraint on the maximum power level has been studied extensively in earlier work. Transmitter power is at a premium in radio systems such as cellular systems and PCS. There is a limit on the maximum transmitter power especially at the terminals (e.g. mobile units and handsets) since the power comes from a battery. In this paper we study power control that maximizes the minimum carrier to interference ratio (CIR), with a constraint on the maximum power. The optimal power vector solution lies on the boundary of the constrained power vector set and achieves a balance in the CIRs. Results indicate that the constraints do not induce any stability problems. A distributed scheme with favourable convergence properties and close to optimum performance is presented. Simulation results show that the algorithm tries to maximize the number of terminals served with CIR greater than or equal to the target CIR, while conserving power.<<ETX>>

Combined power control and transmission rate selection in cellular networks

Emerging multimedia services in cellular radio systems introduce a variable transmission rate, which raises the problem of controlling transmission rates in a spectrally efficient way. We formulate the problem into a combined power and rate control, for which we suggest two different algorithms. In the first one, we derive an algorithm applying the Lagrangian relaxation technique. In the other method, called selective power control we extend a fixed rate power control algorithm to solve the problem. Computational experiments carried out on a CDMA system indicate that the proposed algorithms give satisfying performance in terms of system throughput, outage probability and transmission power consumption.

Relation between base station characteristics and cost structure in cellular systems

A simple method for estimating the costs of building and operating a cellular mobile network is proposed. Using the empirical data from a third generation mobile system (WCDMA), it is shown that the cost is driven by different factors depending on the characteristics of the base stations deployed. When the site density increased, the operational and transmission costs tend to dominate rather than the radio equipment and site costs. The results also show how, for different capacity requirements, the costs can be minimized by a proper selection of macro, micro and pico base stations. In many scenarios, the macro base stations yield the lowest cost, indicating that the coverage (cell range) is an important parameter when designing wireless systems.

Soft and safe admission control in cellular networks

We study the mobile admission control problem in a cellular PCS network where transmitter powers are constrained and controlled by a distributed constrained power control (DCPC) algorithm. Receivers are subject to nonnegligible noise, and the DCPC attempts to bring each receivers CIR (carrier-to-interference ratio) above a given quality target. Two classes of distributed admission control are considered. One is a noninteractive admission control (N-IAC), where an admission decision is instantaneously made based on the system state. The other is an interactive admission control (IAC), under which the new mobile is permitted to interact with one or more potential channels before a decision is made. The algorithms are evaluated with respect to their execution time and their decision errors. Two types of errors are examined: type I error, where a new mobile is erroneously accepted and results in outage; and type II error, where a new mobile is erroneously rejected and results in blocking. The algorithms in the N-IAC class accept a new mobile if and only if the uplink and the downlink interferences are below certain corresponding thresholds. These algorithms are subject to errors of type I and type II. In the IAC class, we derive a soft and safe (SAS) admission algorithm, which is type I and type II error free, and protects the CIRs of all active links at any moment of time. A fast-SAS version, which is only type I error-free, is proposed for practical implementation, and is evaluated in several case studies.

Joint power control and intracell scheduling of DS-CDMA nonreal time data

The performance of DS-CDMA systems depends on the success in managing interference arising from both intercell and intracell transmissions. Interference management in terms of power control for real time data services like voice has been widely studied and shown to be a crucial component for the functionality of such systems. In this work we consider the problem of supporting downlink nonreal time data services, where in addition to power control, there is also the possibility of controlling the interference by means of transmission scheduling. One such decentralized schedule is to use time division so that users transmit in a one-by-one fashion within each cell. We show that this has merits in terms of saving energy and increasing system capacity. We combine this form of intracell scheduling with a suggested distributed power control algorithm for the intercell interference management. We address its rate of convergence and show that the algorithm converges to a power allocation that supports the nonreal time data users, using the minimum required power while meeting requirements on average data rate. Numerical results indicate a big potential of increased capacity and that a significant amount of energy can be saved with the proposed transmission scheme.