In wireless communication, limited or finite numbers of radio channels are available and to share these channels simultaneously to many mobile users, Multiple Access Techniques are used. There are three major access techniques.
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Frequency Division Multiple Access (FDMA)
In FDMA technique individual or unique channels are assigning to those individual users, who request for service. On demand two distant bands of frequencies are allocated for every user. One is for forward channel and other is for reverse channel. Forward channel is used to transfer information from Base Station to Mobile and Reverse channel transfer information from Mobile to Base Station. FDMA includes the following features:
=>In FDMA, one phone circuit is carried by one FDMA channel at a time.
=>In FDMA, if an FDMA channel is not in use, it cannot be used by other users to increase the channel capacity. It remains idle.
=>FDMA is implemented in Narrowband systems, because, in FDMA, each channel supports only one circuit per carrier so the bandwidth of FDMA channels are relatively narrow (30 KHz).
=>Inter symbol Interference is low in Narrowband systems because Narrowband signal has large symbol time as compare to delay spread.
=>FDMA mobile systems are less complex as compare to TDMA mobile systems.
=>FDMA is a continuous transmission scheme as compare to TDMA because fewer bits are needed for synchronization and framing.
=>In FDMA, as unique channels are assigned to each user, so FDMA systems have higher cell site system cost as compared to TDMA system.
=>In FDMA, both the transmitter and receiver operates at the same time so FDMA
=>Mobile units require duplexers. These also increase the cost of FDMA subscriber units and base station.
Time Division Multiple Access (TDMA)
In TDMA, a radio spectrum is divided into time slots. These time slots are allocated for each user to transmit and receive information. The number of time slots is called a frame. Information is transferred and received in form of frame. A frame is consists a preamble, an information message and trial bits.
Preamble contains the address and synchronization information of both subscriber and Base Station to identify each other. Trial bits contain framing information.
TDMA includes the following features:
=>In TDMA, by using no overlapping time slots several users shares a single carrier frequency. The number of time slots per frame depends upon several factors, such as modulation techniques and bandwidth.
=>In TDMA, data transmission is not continuous and subscriber transmitter can be turned off which result in low battery consumption.
=>In TDMA, handoff process is much simpler for a subscriber because of discontinuous transmission.
=>In TDMA, duplexers are not required because different timeslots are used for transmission and reception.
=>In TDMA, the rate of transmission is very high as compare to FDMA.
=>In TDMA, synchronization is required and the time for synchronization or guard time should be minimum.
=>TDMA systems require large overheads (framing bits) as compare to FDMA because of discontinuous or burst transmission.
=>In TDMA, bandwidth can be utilized on demand of different users as different numbers of time slots are allocated to different users.
Code Division Multiple Access (CDMA)
In CDMA, all users transmit information simultaneously by using the same carrier frequency. Each user has its own codeword, which is orthogonal to other users. To detect the message, the receiver should know the codeword used by the transmitter. A time correlation operation is performed at receiver to detect the desired codeword and all other codeword appears as noise due to de-correlation and to minimize this noise power at receiver station power should be high. CDMA includes the following features:
=>In CDMA system many users share the same frequency.
=>In CDMA unlike FDMA and TDMA the number of users is not limited. It has a soft capacity. But due to large number of users its performance degrades.
=>In CDMA, the spread spectrum or bandwidth is large so multipart fading is reduced.
=>In CDMA, the channels data rates are very high.
=>CDMA systems provides soft handoff and this handoff is performed by the MSC.
=>In CDMA systems, if spreading sequences of different users are not exactly orthogonal, the Self-jamming problem is arises.
=>In CDMA systems, if a desired user has less power as compare to undesired users, the near-far problem occurs.
In CDMA, each user operates independently with no knowledge of the other users.
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