IS -95 is the underlying standard of CDMA systems. It is worth noting that CDMA is primarily designed and promoted by Qualcomm Inc., which holds key intellectual property rights related to CDMA technology. IS-95 is also commonly referred to as cdmaOne.

The forward link refers to the link from a base station to a mobile station, whereas the reverse link is the link from a mobile station to a base station. For both types of links, voice is encoded at a rate of 9600 bps after some error-correction code is added. In a forward link, both data and voice are encoded by a forward error-correction (FEC) scheme, resulting in a doubled bit rate of 19.2 Kbps. In a reverse link, because a different FEC scheme is used, the resulted data rate is 28.8 Kbps. For each forward link, 64 logical channels, each corresponding to a mobile station, are scrambled to prevent repetitive patterns. A reverse link comprises up to 32 logical access channels for paging and 62 logical traffi c channels. For both types of links, the DSSS function spreads data of the logical channels over the available frequency range, resulting in an overall 1.228-Mbps data rate. Specifi cally, a 42-bit-long mask code is used on a reverse link to identify logical traffi c channels that are dedicated to connecting mobile stations to a base station. The same mask code is also used to produce a bitstream that will be modulated onto the carrier using orthogonal QPSK or offset QPSK (OQPSK). OQPSK differs from QPSK in that in the implementation of OQPSK one of the two half-rate bitstreams of the original input signal is delayed for one-bit period to reduce phase shift at a time. Because of duplex communication, the total number of reverse-link logical channels for traffic must be the same as the total number of forward-link channels.

Source of Information :  Elsevier Wireless Networking Complete 2010


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