Page 63 - Mobile Computing

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FDMA: In frequency-division multiple access (FDMA), the available bandwidth
is divided into frequency bands. Each station is allocated a band to send its data.

In other words, each band is reserved for a specific station, and it belongs to the
station all the time. Each station also uses a bandpass filter to confine the
transmitter frequencies. To prevent station interferences, the allocated bands are
separated from one another by small guard bands.

TDMA: In time-division multiple access (TDMA), the stations share the
bandwidth of the channel in time. Each station is allocated a time slot during
which it can send data. Each station transmits its data in is assigned time slot.

CDMA:

 In CDMA each user is given a unique code sequence or signature sequence.
This sequence allows the user to spread information signal across the
assigned frequency band.
 At the receiver the signal is recovered by using the same code sequence.

At the receiver, the signals received from various users are separated by
checking the cross-correlation of the received signal with each possible
user signature sequence.
 In CDMA the users access the channel in a random manner. Hence the
signals transmitted by multiple users will completely overlap both in time

and in frequency.
 The CDMA signals are spread in frequency. Therefore, the demodulation
and separation of these signals at the receiver can be achieved by using the
pseudorandom code sequence. CDMA is sometimes also called as spread
spectrum multiple access (SSMA).

 In CDMA as the bandwidth as well as time of the channel is being shared
by the users, it is necessary to introduce the guard times and guard bands.
 CDMA does not any synchronization, but the code sequences or signature
waveforms are required.