IS-95 CDMA CDMA offers some advantages over TDMA and FDMA ...

EE5401 Cellular Mobile CommunicationsEE5401 Cellular Mobile Communications- Convolutional encoder and repetition circuit : a halfrateconvolution encoder with constraint length 9 isused. The encoding process is described by generatorvectors 753 (octal) and 561 (octal) respectively. Inorder to keep a constant baseband symbol rate of19.2kbps, whenever the user rate is less than 9600bps,each symbol from the convolution encoder is repeated(till result in 19.2kbps) before block interleaving.- Block interleaver : After convolution coding andrepetition, symbols are sent to a 20ms blockinterleaver which is a 24 x 16 array.- Long PN sequence : DS is used for data scrambling.This is to encrypt the signal on the paging and trafficchannels. The long PN sequence is uniquely assignedto each user. Each periodic long code with period 2 42 -1.The 1.2288 MHz PN sequence is applied to a decimator,which keeps only the first chip out of every sixty-fourconsecutive PN chips, so that the output symbol ratefrom the decimator is 19.2 kbps. (scrambling has to bedone at the modulation symbol rate to preserve Walshsequence orthogonality among users)- Orthogonal channelization : Each traffic channel (MS)transmitted on the forward CDMA channel is spreadwith a Walsh funcion at a fixed chip rate of 1.2288Mcps. The Walsh codes are completely orthogonal toeach other and provide orthogonal channelization forall users on the forward link.- Quadrature modulation : Symbols are spread inquadrature. A short binary spreading sequence with aperiod of 2 15 -1 chips, is used for easy acquisition andsynchronization at each MS and is used for modulation.The short spreading sequence is called the pilot PNsequence. In-phase and quadrature PN codes are usedrespectively. QPSK modulation is used.- A pilot channel only consists of the quadrature PNspreading code.Institute for Infocomm Research 223National University of SingaporeInstitute for Infocomm Research 224National University of Singapore