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

EE5401 Cellular Mobile CommunicationsEE5401 Cellular Mobile CommunicationsDS spreading- Each code channel in a traffic channel and each accesschannel are identified by a different phase of a pseduorandomM-sequence with a length of 2 42 -1.Block interleaving is performed. Code symbols arewritten into the matrix by columns and read out by rows.- In theory, every chip-offset could be used as a differentuser code. This is the result of autocorrelationproperties of PN sequences. For a practical separationof 64 chips between consecutive offsets, there are 2 36possible offset. However, these shifted versions of thelong PN code are not perfectly orthogonal multipathand soft-handover further reduce the number of usersthat can suitably share a single physical channel in thereverse link. Usually it is limited to 16-24 users.Orthogonal modulationVariable data rate transmission- A 64-ary orthogonal modulation is used for the reverseCDMA channel. One of the 64 possible Walshfunctions is transmitted for each group of six coded bits.Example : if the interleaved data sequence is 000010, thisrepresents decimal number 2. As a result, Walsh code #2(64 Walsh chips) will be transmitted.- Note that Walsh functions are used for differentpurposes on the forward and reverse channels. On theforward channel, Walsh functions are used forspreading to denote a particular user channel, while onthe reverse channel, Walsh functions are used for datamodulation.- Repeated bits are gated-off and are not transmitted.The data burst randomizer generates a maskingpattern that masks out the redundant data producedby the repetition device.- The masking pattern is generated on a frame-by-framebasis and depends on the data rate of the frame beingtransmitted.- This is to reduce the average amount of reverse linkinterference and thereby increase user capacity.- Method to mask out the signal will not be mentioned.Institute for Infocomm Research 231National University of SingaporeInstitute for Infocomm Research 232National University of Singapore