Tradeoffs between Spreading Factor, Symbol Constellation Size and Rake Fingers in UMTS

As an example of a systematic procedure for evaluating UMTS downlink DSP functions for adaptivity we study bit-error-rate performance tradeoffs between the spreading factor, the symbol constellation size and the number of used Rake fingers in UMTS despreading and demodulation. After an overview of the DSP functions that are used in UMTS downlink transmission and reception we present a usage scenario for studying the described tradeoffs. In the usage scenario three UMTS physical channel configurations, with different spreading factors and constellation sizes, are used to transport packet-data with a rate of 960 kbps over the ITU Pedestrian B multipath channel. The bit-error-rate performance of the different physical channel configurations is evaluated for a range of signal-to-noise ratios by means of simulations. The simulation results indicate that the physical channel configuration that is used, and thus the spreading factor and constellation size, should be chosen based on the required bit-error-rate and the requirements of the other data streams in the cell. The simulation results also show that the number of used Rake fingers can be chosen adaptively to optimize a tradeoff between bit-error-rate and required processing power in the low signal-to-noise ratio region. Keywords— variable spreading factor spreading, higherorder modulation, Rake receiver, adaptivity, UMTS