- a basic introduction or tutorial about the basics of IS-95 A and B ( IS 95 ) marketed under the brand name cdmaOne
The reason that the uplink and downlink transmissions for IS-95 are generated in a different way results from the fact that it is difficult to synchronise the mobile handsets. Each one is a different distance away from the base station and the time delays will be different. As a result synchronisation is not possible. For the Walsh codes to maintain their orthogonality and to operate correctly they must be properly synchronised. PN codes do not require synchronisation and can be used more successfully under these circumstances.
One of the advantages of CDMA is the fact that handover can be made easier and more reliable. Normally when handing over from one from a base station in one cell to the base station in the next, it is necessary for the system to arrange for a new channel to be used. The mobile then changes channel and hopes to be able to receive the signal on the new one satisfactorily. Obviously there is a degree of risk, and occasionally a hand over does not proceed smoothly. With CDMA it is possible to use what is termed a soft hand over. As transmissions from the base stations in adjacent cells may be made on the same frequency, it is possible for a mobile to receive signals from two base stations at once. Normally the mobile would reject the signal from the second base station, but it is possible to arrange for it to receive signals from the two stations and this proves to be very useful during handover. During the period of the handover the two base stations transmit the same signal enabling the mobile to receive the signal via two routes at the same time. This means that during this handover phase the mobile should not loose the signal. Then as the mobile moves further into the second cell and the signal is firm, it can rely on one station only and the handover is complete.
This approach considerably reduces the risk of loosing the connection during handover, and it also minimises the risk of a short break in the speech during this period. However it is not free and there is an associated cost. The mobile needs two decoders to monitor and decode the two signals and this increases the complexity of the mobile. On the network side it means that two channels are used instead of one and this reduces the overall capacity. Some estimate this could be as high as 40%. This is dependent upon the speed of handover and the degree of overlap in the cells. The figure given is obviously a worst case scenario, but despite this the advantages are deemed to outweigh the reduction in capacity and increased mobile complexity.
IS-95 has been successfully installed in many areas of the world, chiefly in North America. IS 95 also has the advantage that it has an evolutionary migration path to 3G with CDMA2000 to give the higher data rates that are needed for video streaming and data transfer whilst retaining compatibility with the existing networks.
By Ian Poole
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