Research On Assigning Algorithms Of OVSF Code In DS-CDMA Systems

Direct sequence code division multiple access (DS-CDMA) is a basic technology of air interface in the third Gerneration (3G) mobile communication systems, and the key for implementation of multimedia communication. It employs orthogonal variable spreading factor (OVSF) codes as channelization code. Code blocking is caused by the orthogonality constraint and the limited number of OVSF codes, which leads to waste of code resource and unfairness for high-rate calls. Therefore, OVSF codes assigning algorithms should make full use of code resource by alleviating even eliminating code blocking and help system being fair to each kind of calls.We classified tens of existing assigning algorithms into two kinds: (1) aiming to alleviate code blocking for system, including single-code assigning algorithm (SAA) and multi-code assigning algorithm (MAA); (2) aiming to make system fair to each kind of calls, including reserved assigning algorithm (RAA). Thruoughput, blocking probability (BP), code blocking probability (CBP) and fairness index were used to evaluate all three kinds of algorithms quantitatively. We studied SAA, MAA and RAA with traffic model and OPNET simulation model designed according to the features of OVSF code and assignnment process.Four existing SAA algorithms were presented at first. They are random, leftmost, nonarrangeable compact assignment (NCA) and weight algorithms. After analyzing their performances by comparing the simulation results of throughput, BP and CBP, we can conclude that the weight algorithm is the best to alleviate code blocking. Then a new SAA named leftmost-fragment assignment (LFA) was proposed after investigating the weakness of existing algorithms. Comparing with the weight scheme, LFA has much lower computational complexity with nearly CBP and fairness. By employing LFA the system can make assignment faster to help users wait less, ulilize OVSF code resource thoroughly, and be fair to any types of calls.When studying MAA, we divided the whole assignment into two phases: (1) getting assigning codeword (GAC); and (2) getting codes' location (GCL). Existing schemes in GAC and GCL were all introduced. It was found that MMA is better than SAA in alleviating or eliminating code blocking, at the cost of hardware complexity. Moreover, we evaluated all MAA algorithms in different phases or steps by simulation, and noticed that code blocking appeared when the user equipment can support only two codes as specified in 3GPP2. As a result of that, the optimum algorithm for the whole MAA can be obtained by combining every best algorithm in all phases and steps.When it comes to RAA, firstly we explained existing approaches including solo partition (SP), hybrid partition 1(HP1) and hybrid partition 2(HP2), and investigated their performances in fairness by simulation. A new RAA named partition&borrow (P&B) was proposed, which is fairer than old schemes. Besides preserving code resource, it tries to assign a code for the call with hightest BP borrowed from another partition with lower BP, and make a fairer assignment and higher throughput.