Reducing nternal and external fragmentations of OVSF codes in WCDMA systems with multiple codes

Chih Min Chao*, Yu-Chee Tseng, Li-Chun Wang

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

46 Scopus citations


In the third-generation partnership project technical specification, orthogonal variable spreading factor (OVSF) codes are used as channelization codes. The use of OVSF codes can provide variable data rates to flexibly support applications with different bandwidth requirements. Most works in the literature assume that only one single OVSF code is used to support one connection. This may sometimes waste the scarce wireless bandwidth since the allocated bandwidth will increase exponentially as the spreading factor decreases, i.e., a user may be "overserved." In this paper, we consider the possibility of using multiple OVSF codes to support a connection. We show how using multiple codes can reduce the internal and external fragmentations of an OVSF code tree. The tradeoff between bandwidth utilization and hardware complexity of a multicode system is analyzed. The result shows that using two or three codes will be quite cost effective. Several multicode assignment and reassignment strategies, namely, random, leftmost, crowded-first-space, and crowded-first-code, are proposed based on such an environment. Our simulation results show significant increase in code tree utilization and significant reduction in code blocking probability by using the crowded-firstspace and crowded-first-code schemes.

Original languageEnglish
Article number1512112
Pages (from-to)1516-1526
Number of pages11
JournalIEEE Transactions on Wireless Communications
Issue number4
StatePublished - 1 Jul 2005


  • Mobile communication
  • OVSF
  • Third generation (3G)
  • Wireless communication
  • Personal communication services

Fingerprint Dive into the research topics of 'Reducing nternal and external fragmentations of OVSF codes in WCDMA systems with multiple codes'. Together they form a unique fingerprint.

Cite this