| Mobile communication systems need to meet users'demands for various wireless services including voice, video and data. Therefore it is necessary to devise a method of increasing bandwidth efficiency in order to provide high-speed multi-media communication services with CDMA systems under limited radio bandwidth resources. Combining CDMA technology with MC modulation, high-rate data transmissions are easily implemented without increasing complexities in a transmitter and a receiver. MC-CDMA (Multi-carrier-CDMA) systems have been chosen for RTT (Radio Transmission technology) of IMT-2000/UMTS by the proposal"CDMA 2000". The multi-carrier approach is motivated by a spectrum overlay with an existing IS-95 carrier, and is a strong candidate for RTT due to low complexities in a transmitter and a receiver. This paper highlights the limitations of ECPC (Each Carrier Power Control) concept - originally proposed for OFDM and DS-CDMA - when extended to MC-CDMA (Multi-Carrier Code Division Multiple Access) systems: first, its impractical signaling overhead of 80%, secondly, its inability to be used as an uplink power control mechanism. Then we propose BBPC (Band Based Power Control) as a practical alternative to ECPC for MC-CDMA systems. Unlike, ECPC that controls power on each carrier basis, BBPC assigns same power level to a band of carriers (lying within coherence bandwidth of channel). It has been shown that with a nominal performance loss, BBPC reduces the signaling overheads to 2.5% and by employing the control index estimator after de-spreading; it can be used as an uplink power control mechanism for MC-CDMA. We have used SIR (Signal to Interference Ratio) as a power control index, BER and standard deviation of power control error as performance metric. And, more we propose a new idea to improve the performance of BBPC . |
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