Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 5M

Year: Revised 2012

• In wireless communication limited or finite numbers of channels are available for the communication thus multiple access techniques are used to allow many mobile users to share simultaneously a finite amount of radio spectrum.

• The sharing spectrum is required to achieve high capacity by simultaneously allocating the available bandwidth to multiple users. For high quality communication this must be done without severe degradation in the performance of the system.

• Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) are the three major access techniques used to share the available bandwidth in a wireless communication system.

• These techniques can be grouped as narrowband and wideband systems depending upon how the available bandwidth is allocated to the users.

A. Narrowband Systems:

• The term narrowband is used to relate the bandwidth of a single channel to the expected coherence bandwidth of the channel. In a narrowband multiple access system the available radio spectrum is divided into a large number of narrowband channels. The channels are usually operated using FDD.

• In Narrowband FDMA, a user is assigned a particular channel which is not shared by other users in the vicinity, and if FDD is used then the system is called FDMA/FDD.

• Narrowband TDMA, allows users to share the same radio channel but allocates a unique time slot to each user in a cyclical fashion on the channel, thus separating a small number of users in time on a single channel. For narrowband TDMA systems there are a large number of radio channels allocated using either FDD or TDD, and each channel is shared using TDMA. Such systems are called TDMA/FDD or TDMA/TDD .access systems.

B. Wideband systems:

• In wideband systems the transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel is much larger than the coherence bandwidth of the channel. Thus multipath fading does not greatly vary the received signal power within a wideband channel and frequency selective fades occur in only a small fraction of the signal bandwidth at any instance of time.

• In wideband multiple access system a large number of transmitters are allowed to transmit on the same channel. TDMA allocates time slots to the many transmitters on the same channel and allows only one transmitter to access the channel at any instance of time whereas CDMA allows all of the users to access the channel at the same time.

• In addition to FDMA, TDMA, CDMA two other multiple access techniques will soon be used for wireless communication. These are Packet Radio (PR) and Space Division Multiple Access (SDMA).