Multiple Rake Combiners and Performance Improvement in 3G and Beyond WCDMA Systems

We propose an efficient code-assignment scheme utilizing multiple rake combiners to avoid the code-blocking problem in third-generation (3G) and beyond wideband code-division multiple-access (WCDMA) systems based upon orthogonal variable spreading factor (OVSF) channelization codes. The multiple rake combiners are required to be equipped with each base station (BS) and user equipment (UE). The major benefit of the proposed assignment scheme is in handling nonquantized data rates, making internal code fragmentation approximately zero. The task of code assignment to handle quantized data rates is divided into three steps. In the first step, the number of terms satisfying the capacity equation is found. The code combinations for all the terms in step 1 are found in step 2, which can be further subdivided into terms producing unique combinations and multiple combinations of terms. The procedure for finding the optimal code combination according to the number of rake combiners available at the UE and BS is given in step 3. Simulation results show the performance improvement in terms of reduction in blocking probability compared with existing single-code assignment schemes.

I. Introduction

Third-Generation (3G) and beyond universal mobile telecommunications system terrestrial radio access–frequency-division duplexing [1] systems use wideband code-division multiple-access (WCDMA) [2] technology incorporating direct-sequence spreading [3] as a multiple-access method. The data and control channels in the uplink/downlink uses two sets of codes known as scrambling codes and channelization codes. The scrambling code is the identity of each base station (BS) and user equipment (UE) and is unique for each BS and UE. The channelization codes in WCDMA are orthogonal variable spreading factor (OVSF) codes. The OVSF codes are used for data spreading and rate matching in the WCDMA system. The use of OVSF codes leads to a constant channel chip rate for all the users, irrespective of the call bit rate.