Simulation of Third Generation CDMA Systems - Virginia Tech

of the power amplifier. Figures 2.7(a) and 2.7(b) show the probability density function of thetransitions in the signal constellation of the scrambled signal. The transition of π indicates adiagonal transition in the signal constellation. As we can observe, proper selection of thespreading code can reduce the diagonal transitions in the signal constellation by a significantamount.2.5 Uplink Scrambling CodesUplink Scrambling codes help maintain separation among different mobile stations.Either short or long scrambling codes can be used in the uplink. Short scrambling codes arerecommended for base stations equipped with advanced receivers employing multiuserdetection or interference cancellation. Since we employed a simple rake receiver, we usedlong scrambling codes in the simulator.Scrambling codes (both short and long) can be defined with the help of the followingequationCsc = C w + jw C ‡1( 1 2 2) (2.4)Here,C 1 is a real chip rate code;C 2 ’ is a decimated version of a real chip rate code C 2 .The usual decimation factor is 2 so that,C ‡( 2k) = C ‡( 2k + 1) = C ( 2k) (2.5)2 2 2w 1 is a repetition of {1 1} at the chip ratew 2 is a repetition of {1 -1} at the chip rateSo we can writeC C jw CCsc= + ‡1 2 1 2(2.6)18