2. LET’S KNOW ABOUT MULTIPLE ACCESS……..
• Multiple access schemes are used to allow many mobile users to share
simultaneously a finite amount of radio spectrum.
• In wireless communication systems, it is often desirable to allow the subscriber
to send information simultaneously from the mobile station to the base station
while receiving information from the base station to the mobile station.
• A cellular system divides any given area into cells where a mobile unit in each
cell communicates with a base station. The main aim in the cellular system
design is to be able to increase the capacity of the channel, i.e., to handle as
many calls as possible in a given bandwidth with a sufficient level of quality of
service.
Three schemes have been devised for efficient sharing of a single channel under
these conditions; they are called-
3. TIME DIVISION MULTIPLE ACCESS (TDMA)
Time division multiple access (TDMA) is a channel access method (CAM) used to
facilitate channel sharing without interference.
TDMA allows multiple stations to share and use the same transmission channel by
dividing signals into different time slots. Users transmit in rapid succession, and
each one uses its own time slot. Thus, multiple stations (like mobiles) may share the
same frequency channel but only use part of its capacity.
TDMA allows a mobile station's radio component to listen and broadcast only in its
assigned time slot. During the remaining time period, the mobile station may apply
network measurements by detecting surrounding transmitters in different
frequencies.
TDMA is used in most 2G cellular systems, while 3G systems are based on CDMA.
However, TDMA remains relevant to modern systems. For example, combined
TDMA, CDMA and time division duplex (TDD) are universal terrestrial radio access
(UTRA) systems that allow multiple users to share one time slot.
5. FEATURES OF TDMA
Shares single carrier frequency with multiple users.
Non-continuous transmission makes handoff simpler.
Slots can be assigned on demand in dynamic TDMA.
Less stringent power control than CDMA due to reduced intra cell
interference.
Higher synchronization overhead than CDMA.
Advanced equalization may be necessary for high data rates if the
channel is "frequency selective" and creates Inter symbol
interference.
Frequency/slot allocation complexity.
6. Drawbacks or disadvantages of TDMA
Disadvantage using TDMA technology is that the users has a
predefined time slot. When moving from one cell site to other,
if all the time slots in this cell are full the user might be
disconnected.
Another problem in TDMA is that it is subjected to multipath
distortion. To overcome this distortion, a time limit can be used
on the system. Once the time limit is expired the signal is
ignored.
7. APPLICATIONS
There are some applications of time division multiplexing which are given
below,
It used in ISDN (Integrated Services Digital Network) telephone lines.
It is used in PSTN (public switched telephone network).
It is used for some telephone system.
It is used in wire line telephone lines.
