Multiple Access Techniques for Wireless Communications
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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