Simulation of Third Generation CDMA Systems - Virginia Tech

2.4 Uplink Spreading CodesThe spreading code, as the name suggests, spreads the data to the chip rate of 3.84mega chips per second (Mcps). The most important purpose of the spreading codes is to helppreserve orthogonality among different physical channels of the uplink user. As mentioned insection 2.1.1, OVSF codes are employed as uplink spreading codes. OVSF codes can beexplained using the code tree shown in the next figure. The subscript here gives the spreadingfactor and the argument within the parenthesis provides the code number for that particularspreading factor.c 4 (1)= (1,1,1,1)c 2 (1)=(1,1)c 4 (2)= (1,1,-1,-1)c 1 (1)= (1)c 2 (2)= (1,-1)c 4 (3)= (1,-1,1,-1)c 4 (4)= (1,-1,-1,1)SF = 1 SF = 2SF = 4Figure 2.5: Code-tree for Generation of OVSF CodesEach level in the code tree defines spreading codes of length SF, corresponding to aparticular spreading factor of SF. The number of codes for a particular spreading factor isequal to the spreading factor itself. All the codes of the same level constitute a set and theyare orthogonal to each other. Any two codes of different levels are orthogonal to each otheras long as one of them is not the mother of the other code [8]. For example the codesc 16 (2),c 8 (1) and c 4 (1) are all mother codes of c 32 (3) and hence are not orthogonal to c 32 (32).Thus all the codes within the code tree can not be used simultaneously by a mobile station. A15