UT EE 382C - Adaptive Power Control in Cellular Radio System

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EE 382CEmbedded Software SystemsAdaptive Power Control in Cellular Radio SystemFinal ReportJianhua GanMay 12, 19992Adaptive Power Control in Cellular Radio SystemJianhua GanAbstract Several power control methods in cellular radio systems are reviewed. Anadaptive power control scheme based on the adaptive optimization of transmitter powerand receiver filter coefficients is analyzed. We implement the power control algorithm,adaptive RAKE receiver and analog RF power amplifier in HP Advanced Design System.We perform the co-simulation in HP Advanced Design System to prove the effectivenessof the adaptive RAKE receiver and the adaptive power control algorithm.I. IntroductionAccurate power control can reduce the interference in both Global System Mobile(GSM) and Code Division Multiple Access (CDMA) systems. When the mobile is closeto the base station, it can use lower power since the signal loss is smaller. For GSM,interference to other cells using the same frequency is reduced when the power control isaccurate. For CDMA, accurate power control is vital because all CDMA signals interferewith each other. Lack of accurate power control reduces the capacity for CDMA systems.Moreover, power control can reduce battery drain. The major objective of power controlis to alleviate the co-channel and cross-channel interference [1]. Co-channel interferenceis generated during the resource sharing process. Cross-channel interference results fromimperfect technology, Doppler shift and multi-path propagation. The capacitymaximization and fair allocation of resources among different users largely depend on theeffectiveness of the power control scheme. Due to the effects of fast fading, shadowing3and distance loss, an adaptive power control (APC) scheme is needed [2]. Adaptivepower control algorithms are used to maintain a constant average performance among theusers, minimize the required transmitter power at each mobile, and reduce the multiple-access interference effect. Adaptive power control is a difficult problem to solve. Whilesimple power control algorithms are deployed in cellular radio systems and many quasi-analytical simulation results have been published [2-7], we review some of the importantresults and propose the implementation, co-simulation in HP Advanced DesignSystem(ADS) [8]. Using the highly integrated and flexible environment in ADS, we willevaluate the system performance using adaptive power control algorithms. We alsodiscuss the models of computation used in our design.II. Adaptive power controlThere are two types of adaptive power control: open-loop [2] and closed-loop [3].In closed-loop implementations, the reverse link (mobile-to-base station link) channelstate is estimated by the base station, and then the base station issues power controlcommand on the forward link (base station-to-mobile link) based on the estimation. Inopen-loop implementations, the channel state on the forward link is estimated by themobile itself, and this information is used by mobile as a measure of the channel state onthe reverse link. The mobile adjusts its transmitter power accordingly. This scheme workswell if the forward and reverse links are perfectly correlated. Closed-loop APCoutperforms open-loop APC in a terrestrial environment in which the round-trip delay issmall. The closed-loop APC is sensitive to round-trip delay and therefore it is noteffective in a land mobile satellite system. Closed-loop APC is more complex in4implementation and needs extra bandwidth for power control command. Open-loop APCcan compensate for large-scale variations and provides a fast, inexpensive solution. If the transmitter powers and receiver filters of the users are jointly optimized,the performance of a CDMA system can be greatly improved. We can use an iterativeand distributed algorithm [5]. At each iteration, the receiver parameters of the mobiles areupdated to suppress the interference optimally and then the transmitter powers of themobiles are determined so that each mobile creates the minimum possible interference toother mobiles while satisfying the quality of service requirements. It is shown that theresulting power control algorithm converges to a global fixed-point power vector whereall mobiles meet their SINR (signal to interference and noise ratio) based quality ofservice requirements and that the linear receiver converges to the MMSE multi-userdetector. The block diagram of the adaptive power control scheme is shown in Figure 1.The SINR information is obtained from the adaptive RAKE receiver. Based on thisinformation, each mobile adapts its receiver filter coefficients to suppress interferenceand performs iterative transmitter power level adjustment for fixed filter coefficients.Attenuation estimationusing RAKE receiver(SINR information)Receiver filterTransmitter amplifierIterative, distributed controlalgorithmFigure 1. Block diagram of the adaptive power control scheme5III. Models of computationIn order to evaluate the adaptive power control algorithm, we need model theentire communication signal path. The cellular mobile communication path consists ofsynchronous DSP (digital signal processing) components and analog RF (radiofrequency) circuits. Timed synchronous dataflow (TSDF) model in HP ADS is wellsuited for the modeling of mixed analog RF and digital systems [9]. HP Ptolemy SDF(synchronous dataflow) is extended to TSDF by adding the notion of time. Circuitenvelope and high frequency SPICE are used to model analog RF circuitry [8]. Withthese models in ADS, we can describe functional models of analog RF components andperform the co-simulation.IV. Implementation of adaptive power control in ADSWe implement the design hierarchy in ADS as shown in Figure 2. The cellularradio system is composed of mobiles, base stations and propagation channels. EachCellular Radio SystemMobilesPropagationchannelBasestationsAdaptiveRAKEReceiverTransmitterAntennaAttenuationestimatorRF poweramplifierFigure 2. The design hierarchy implemented in ADS6mobile or base station consists of transmitter, receiver and antenna. In the CDMA system,there are digital modulator (spreading), FIR filter, RF modulator (not shown) and poweramplifier in the transmitter. There are RF demodulator (not shown), RAKE receiver (de-spreading) and attenuation estimator in the receiver.In order to get a better estimation of the attenuation in the communication channelat the receiver, we use adaptive RAKE receiver [2] as shown in Figure 3. After


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