OVSF Code Assignment Schemes at the Forward Link of wcdm a Systems

Abstract In this paper we discuss code assignment schemes in wideband code division multiple access (WCDMA) systems based on orthogonal variable spreading factor (OVSF) codes. The OVSF codes are the limited resources in the forward link and their usage is critical for higher efficiency. The paper describes single code and multi code schemes for different requirements. In single code assignment category, adaptive code assignment (ADA) uses arrival distribution to reduce the number of code searches prior to finding a vacant candidate code. The time multiplexing in single code design makes code blocking zero with additional time slot management requirement. It provides additional benefit of handling non-quantized data rates. The next code precedence high (NCPH) scheme uses precedence numbers to make code assignment compact. The multi code scheme uses multiple codes to handle each call. Two multi code schemes are described, one aims to reduce complexity and the other aims to minimize code scattering and code blocking. The simulation results are demonstrated to show the number of code searches and call blocking variation for various code assignment schemes.