Tuesday, August 17, 2010

thesis on Adaptive Power and Rate Control in Third GenerationWideband CDMA Mobile Systems

This thesis proposes novel adaptive power control and rate change schemesand investigates the performance of a Wideband Code Division Multiple Access (W-CDMA)system in conjunction with these adaptive techniques. In these schemes, the transmit power and rate are adapted to the variations of the fading channel using adaptivethresholds based on the probability distribution function (pdf) of the predicted mobilechannel power values.

We define a policy similar to the traditional power control technique withthresholds except that the thresholds are set based on several regions of operation in ourAdaptive Transmitter Power Control (TPC) and Adaptive Seamless Rate Change (SRC)schemes. These regions are defined by means of the probability distribution function(pdf) of the total average channel power. The pdf is initially constructed based on thehistory of the predicted channel power values derived from the long-range predictionalgorithm. These regions can be defined such that the system operates at a constant ratioof energy per bit over noise power.In a 1-user model with one channel path, the pdf of the channel power would bean exponential or chi-square function with 2 degrees of freedom.

However, in a W-CDMAsystem, normally the rake receiver has several fingers. That is, at the receiver,the system either estimates or predicts the channel coefficients at each rake finger andperforms maximal ratio combining by multiplying each finger with its conjugate or chooses the ones with the highest energy and performs maximal ratio combining on theselected fingers.In a two-user system where the multi-access interference is modeled as the Standard Gaussian Approximation (SGA), the system performance and error probabilityof our W-CDMA system becomes similar to the one for our one-user system.Consequently, in a single user detector system, when all users adopt a similar policy fortheir adaptive power and rate control, the average total Multi-Access Interference (MAI)will be reduced. The resulting channel capacity of the system in this case will beincreased and the system may operate in a lower transmit power level.We evaluate the performance of these schemes using a detailed block diagramsimulation of a W-CDMA system.

We model and simulate all major components of thesystem including an accurate model for realistic mobile channels. We present simulationresults to verify that the proposed novel schemes are superior to the traditionalapproaches for transmitter power control and rate change. Furthermore, our simulationresults show that our proposed techniques reduce the effect of Multi Access Interferencein a multi-user system.

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