Search

Nummer

Nummer

Nummer

Nummer

Nummer

A method and system for quality-based transmission power control in a cellular communications system (100) are disclosed, whereby a network operator can specify the transmission quality requirements using a measurement that better reflects the actual quality perceived by the users. All transmitter power levels in the network can be controlled by identical power regulators (200), each of which can adapt to individual traffic situations in order to achieve the specified quality. For example, in a GSM frequency-hopping network, the FER together with the parameters estimated from the current traffic situation (54), are mapped onto a target C/I (56), which in turn, the power control algorithm strives (16) to achieve (58, 60). Consequently, the power regulators can adapt to the traffic situation experienced by each receiver.

NR 20140805

When the systems evolved from analog to digital, the performance was improved by the use of power control on the one hand and different modulations and coding schemes on the other. Condensing the available information we are able to propose a new concept of power control. The concept is applicable to real systems, since it uses the available measurements for estimating parameters necessary for the power control. It also supports the use of an adequate quality measure together with a quality specification supplied by the operator. We will use frequency hopping GSM as an example and the resulting control algorithm is ready for implementation in the software in the base stations where the output powers are computed. No modifications are needed in the GSM standard, the mobile terminals, the radio interfaces or in the base station transmitters. Finally we provide simulation results confirming the benefits of using the new concept for power control.

Stability is a fundamental property desirable for any controlled system. We briefly review the root locus and describing function techniques, which are tools for stability analysis, and show how they can be applied to power control algorithms in cellular radio networks. The root locus method is used to find stability limits on controller parameters, and describing functions for predicting the presence of oscillations in the system. Thus these methods can be used to support the design phase, when deciding upon the appropriate controller parameters. These tools are demonstrated for various control algorithms and when different smoothing filters are applied. The analysis reveals that the distributed power control (DPC) algorithm, which works fine under ideal circumstances, yields an unstable system when subject to a small time delay. Furthermore, it is concluded that the performance with respect to stability is better when the measurements are averaged by an exponential forgetting filter than by the moving average filter.

Time delays reduces the performance of any controlled system. If neglected in the design phase, the system may even become unstable when using the designed controller. Several power control strategies have been proposed in order to improve the capacity of cellular radio systems, but time delays are usually neglected. Here, it is shown that the problems can be handled by considering the time delays in the design phase in order to choose the appropriate parameter values. Most popular algorithms can be seen as special cases of an integrating controller. This structure is extended first to a proportional integrating (PI)-controller and then further on to a general linear controller of higher orders. Corresponding design procedures are outlined based on techniques, such as pole placement, from the field of automatic control. The PI-controller is a very appealing choice of structure, with better performance compared to an I-controller and less complex than a higher order controller. The benefits are further illuminated by network simulations.

The problem to track time-varying parameters in cellular radio systems is studied, and the focus is on estimation based only on the signals that are readily available. Previous work have demonstrated very good performance, but were relying on analog measurement that are not available. Most of the information is lost due to quantization and sampling at a rate that might be as low as 2 Hz (GSM case). For that matter a maximum likelihood estimator have been designed and exemplified in the case of GSM. Simulations indicate good performance both when most parameters are varying slowly, and when subject to fast variations as in realistic cases. Since most computations take place in the base stations, the estimator is ready for implementation in a second generation wireless system. No update of the software in the mobile stations is needed.

The problem to track time-varying parameters in cellular radio systems is studied. The focus is on estimation based only on the signals that are readily available. Previous work has demonstrated very good performance relying on analog measurement. In a real system most of the information is lost due to quantization and sampling at a rate that might be as low as 2 Hz (GSM case). Therefore a different approach is required and for that matter a maximum likelihood estimator has been designed and exemplified in the case of GSM. The needed probability functions of the measurements cannot be described analytically. Instead point-mass approximations can be obtained from Monte-Carlo simulations for each point in a grid covering the interesting parameter space. The proposed algorithm can be tuned to track both slowly and fast varying parameters individually. Since most computations take place in the base stations, the estimator is ready for implementation in a second generation wireless system. No update of the software in the mobile stations is needed.

When employing power control algorithms in real systems there are anumber of problems that have to be considered, of which some are addressed in this work. It is probably impossible to solve the problems using a linear controller, and as an extension, various nonlinear components and their applicability are discussed. Problems due to hardware and external constraints have been identified in previous work, and here anti-reset windup techniques are reviewed and employed. Previous proposals can be seen as special cases. Sometimes it is desirable to employ several algorithms in parallel, and then use a device to state priorities, i.e., to switch between the algorithms. The applicability of switches and selectors are described and exemplified as proposed solutions to the low interference problem, where a C/I balancing algorithm causes a user to use a too low a power. The benefits of using the proposed nonlinear components are further illustrated by network simulations.

The problem to track time-varying parameters in cellular radio systems is studied, and the focus is on estimation based only on the signals that are readily available. Previous work have demonstrated very good performance, but were relying on analog measurement that are not available. Most of the information is lost due to quantization and sampling at a rate that might be as low as 2 Hz (GSM case). For that matter a maximum likelihood estimator have been designed and exemplified in the case of GSM. Simulations indicate good performance both when most parameters are varying slowly, and when subject to fast variations as in realistic cases. Since most computations take place in the base stations, the estimator is ready for implementation in a second generation wireless system. No update of the software in the mobile stations is needed.

Time delays reduces the performance of any controlled system. If neglected in the design phase, the system may even become unstable when using the designed controller. Several power control strategies have been proposed in order to improve the capacity of cellular radio systems, but time delays are usually neglected. Here, it is shown that the problems can be handled by considering the time delays in the design phase in order to choose the appropriate parameter values. Most popular algorithms can be seen as special cases of an integrating controller. This structure is extended first to a proportional integrating (PI)-controller and then further on to a general linear controller of higher orders. Corresponding design procedures are outlined based on techniques, such as pole placement, from the field of automatic control. The PI-controller is a very appealing choice of structure, with better performance compared to an I-controller and less complex than a higher order controller. The benefits are further illuminated by network simulations.

The problem to track time-varying parameters in cellular radio systems is studied. The focus is on estimation based only on the signals that are readily available. Previous work have demonstrated very good performance relying on analog measurement. In a real system most of the information is lost due to quantization and sampling at a rate that might be as low as 2 Hz (GSM case). Therefore a different approach is required and for that matter a Maximum Likelihood Estimator has been designed and exemplified in the case of GSM. The needed probability functions of the measurements cannot be described analytically. Instead point-mass approximations can be obtained from Monte-Carlo simulations for each point in a grid covering the interesting parameter space. The proposed algorithm can be tuned to track both slowly and fastly varying parameters individually. Since most computations take place in the base stations, the estimator is ready for implementation in a second generation wireless system. No update of the software in the mobile stations is needed.

Many algorithms in communications systems can be considered as control loops, where the computed quantities depend on feedback information. A common scheme is to use increase/decrease signaling for bandwidth efficiency. Furthermore, the feedback information is delayed. The time delay itself, and even more pronounced in combination with nonlinearities, such as the increase/decrease mechanism, may cause oscillations and instabilities in the system. In this work, analysis methods based on root locus and describing functions are discussed. Design and tuning of algorithms are employed using pole placement techniques. Particular examples include clock synchronization in ADSL modems, control of available bit rate in data networks and, as studied in this project, distributed implementation of power control in cellular radio systems. The relevance of the methods is further illuminated by simulations.

The primary goal of cellular radio systems, is to provide communication services to a large number of mobile users. Due to the rapid expansion of the market in this area, the available resources have to be used efficiently. The main issue in this thesis is methods to assign appropriate transmission powers, given coarsely quantized measurements, in order to meet the quality requirements from the users despite various disturbances. We propose a concept of a power regulator comprising the steps of estimating relevant quantities, handling quality specications, and controlling the powers. With this setting, the power controlling component relates directly to the mainstream of the algorithms proposed to date. For practical reasons, it is necessary to control the powers in a distributed fashion, and these distributed algorithms can be seen as local control loops. The effects of time delays and power output constraints in these loops are analyzed with respect to stability, using root locus techniques and describing functions. We emphasize the importance of identifying these time delays and constraints in order to choose the appropriate controller parameters for stable operation. The relevance of the local stability results on the overall system level is discussed, and further analyzed in a simulation environment, which has been developed. The literature is surveyed, and the contributions are classified with respect to a common framework in order to stress their similarities and differences. We show that an integrating controller forms the basis for the most popular algorithms. Methods for convergence analysis are investigated and related to the theory of linear systems. These methods are applied when proving global convergence of the integrating controller. The power control strategies are evaluated under more realistic circumstances in an environment simulating the operation of a GSM system. Comparing the results when using different power control algorithms we note that t

Stability is a fundamental property desirable for any controlled system. We briefly review the root locus and describing function techniques, which are tools for stability analysis, and show how they can be applied to power control algorithms in cellular radio networks. The root locus method is used to find stability limits on controller parameters, and describing functions for predicting the presence of oscillations in the system. Thus these methods can be used to support the design phase, when deciding upon the appropriate controller parameters. These tools are demonstrated for various control algorithms and when different smoothing filters are applied. The analysis reveals that the distributed power control (DPC) algorithm, which works fine under ideal circumstances, yields an unstable system when subject to a small time delay. Furthermore, it is concluded that the performance with respect to stability is better when the measurements are averaged by an exponential forgetting filter than by the moving average filter.

When the systems evolved from analog to digital, the performance was improved by the use of power control on the one hand and different modulations and coding schemes on the other. Condensing the available information we are able to propose a new concept of power control. The concept is applicable to real systems, since it uses the available measurements for estimating parameters necessary for the power control. It also supports the use of an adequate quality measure together with a quality specification supplied by the operator. We will use frequency hopping GSM as an example and the resulting control algorithm is ready for implementation in the software in the base stations where the output powers are computed. No modifications are needed in the GSM standard, the mobile terminals, the radio interfaces or in the base station transmitters. Finally we provide simulation results confirming the benefits of using the new concept for power control.

Power control algorithms in cellular radio networks are analyzed. This is basically a MIMO problem where the controlled input is the powers used by the transmitters, and the measurements consist of signal to interference ratios for each receiver. These algorithms can, for each single connection and neglecting the cross couplings, be interpreted as a feedback system. The main contribution here is to analyze a generic control algorithm, which covers some important suggested classical control theory methods such as the root locus and describing function. The results include the important conclusion that unavoidable time delays and nonlinearities (saturation and quantization) in the feedback loop can cause a considerable loss in the stability margin. The results are supported by simulations.