What is CDMA

CDMA stands for “Code Division Multiple Access”. It is a multiple access technique that is used for sharing of a single frequency channel between multiple nodes in a network to facilitate their duplex communication with a single central node. This technique is used in CDMA mobile systems and also in Wireless LAN. IN CDMA mobile systems, the Base Station acts as the central node of the network while the mobile units in a cell are the multiple nodes sharing a single frequency channel between them for their communication with the Base Station. Normally Frequency Division Duplexing is used in these systems i.e. two channels are used, one for downlink from Base Station to the various mobile stations and second for uplink from the mobile stations to the Base Station. In case of a wireless LAN, CDMA is used to set up a similar communication scheme between the access point of the network and the various Wireless Clients.

Now a question arises on how does this technology work to allow for such a communication scheme? It allows for it via encoding the signals from different nodes using different 64 bit pseudorandom sequences. The signal from a source is first multiplied by a pseudorandom sequence which is unique for every device in the network which causes it to spread in the frequency spectrum and this signal is then transmitted in the allocated frequency band. At the receiver side, this signal is decoded using the same pseudorandom sequence. If the pseudorandom sequences don’t match, then the signal will just appear as useless noise.

For example: – Let three sources A, B, C are transmitting their signals to a central node D in a network using CDMA technique. Now if D wants to recognize the signal from A, then it will decode the whole received signal using the pseudorandom sequence unique to A. After decoding, only the original signal of A will be present while the signal from B and C will just appear as noise to the system. This noise puts an upper limit on the capacity of a CDMA system. In the reverse mode, If D wants to transmit a signal to A; it will encode the signal with the pseudorandom sequence unique to A. All the three nodes A, B, C will receive the signal as they are sharing the channel. But the signal on decoding will only appear useful to A while it will appear as noise to B and C.

Power Control is another important factor in CDMA. This is because a device transmitting at more power can over shadow a device transmitting at a lower power level in a CDMA network. Thus the power scheme in a CDMA network should be such that the power levels received at the central station from the various nodes in the network should be equal. In USA, CDMA is used in the IS-95 cellular network.

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