Reducing Wastage Capacity in OVSF Based CDMA Networks using Dynamic Rake Combiners

Abstract

Orthogonal variable spreading factor (OVSF) codes in CDMA networks are designed to handle quantized data rates. Handling non-quantized data rates in such networks leads to code capacity wastage if traditional single code assignment is used. Although, the use of multiple codes reduce this wastage capacity but the use of large number of fixed rake combiners per call increase cost and complexity of the system. I propose zero wastage designs in which the rake combiner’s usage is made dynamic and the amount of rake combiners used depend upon the rate type, with more combiners given to the rate which deviates significantly from the quantized one. The average number of rakes per call is arbitrarily assumed, and if the rakes used for a particular call are less than the average (which happens for quantized or near quantized calls), the unused rakes can be used by future calls. The performance is significantly improved compared to the fixed rate systems. The amount of codes used on average is less than the codes required for existing multi code designs. In the reduced wastage capacity design, which is a special case of zero wastage designs, some wastage can be tolerated for simplicity and less equipment cost.