2.3 time division multiple access

2.3 time division multiple access

1. TIME
DIVISION
MULTIPLE
ACCESS

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3. Frequency Division Multiple Access -FDMA
USER 1
USER 3
USER 2
GUARD BANDS
To avoid interference

4. Time Division Multiple Access -TDMA
USER 1
USER 3
USER 2

5. Time Division Multiple Access -TDMA
TDMA divides the entire radio spectrum in to time slots, and in each slot only one user
is allowed to either transmit or receive.
Each user occupies a cyclically repeating time slot
TDMA System transmits data in Buffer and Burst method, This makes the
transmission a discontinuous one.
Hence this makes the TDMA Systems more suitable for Digital Systems.
Systems use TDMA/TDD or TDMA/FDD methods for Duplexing and Multiple
Accessing.
In TDMA/TDD–The Single Frame will be used as forward link for half of the
duration and as reverse link on another half period.
In TDMA/FDD–2 Different Frames will be used for Forward and Reverse Link
but the carrier frequency will be different.

6. Understanding TDMA Frame Structure

7. Time Division Multiple Access -TDMA
Preamble:
Consists of Address and
Synchronisation bits which is
used by base stations
subscribers to identify each
others.
Information Message:
Contains multiple slots of user
information
Each slot will have
Trail Bits
Sync Bits
Information
Guard Bits
Trail Bits

8. Features of TDMA
1.IncaseofTDMAasinglecarrierfrequencyissharedamongseveralusers.Thetime
slotsusedbytheusersdependsonseveralfactorssuchasmodulationtechniquesand
theavailablebandwidth.
2.TheHandoffprocessismuchsimplerinthecaseofTDMAbecauseofitsdiscontinuity
anditisabletolistentotheotherbasestationsduringitsidleslottime.
3.TheTransmissionratesareveryhighinthecaseofTDMAascomparedtotheFDMA
becauseoftheAdaptiveEqualizationwhichisnecessaryinthecaseofTDMA.
4.TheguardtimepresentinthecaseofTDMAshouldbeminimum.
5.ThemainadvantageofTDMAisthatitisverycost-effective.
6.TDMAtechnologyisusedtoseparateusersintimeandensurethattherewillnotbe
anyinterferencewhichispresentedfromanyothersimultaneoustransmissions.

9. Efficiency in TDMA
•Calculation of Efficiency becomes trivial as TDMA uses high Overhead than
compared to FDMA.
•The efficiency of TDMA System is a measure of the percentage of transmitted
data that contains information as opposed to providing overhead for the scheme.
Frame Efficiency (η
??????
)is calculated by
η
??????=1−
??????
????????????
??????
??????
X100%
??????
????????????= N
rB
r+ N
tB
p+ N
tB
g+N
rB
g
N
r–Number of reference burst per frame
B
r–Number of overhead bits per reference burst
N
t–Number of Traffic Burst per Frame
B
p–Number of Overhead bits per Preamble in each slot
B
g–Number of Equalentbits in each guard time interval
??????
??????= T
fR
T
f–Frame Duration
R –Channel Bit Rate
??????
????????????-Number of overhead bits per frame
????????????–Total Number of Bits per Frame

10. Number of Channels in TDMA Systems
•ThenumberofTDMAChannelsslotsthatcanbeprovidedinaTDMAsystemis
foundbymultiplyingthenumberofTDMAslotsperchannelbythenumberof
channelsavailable.
??????=
??????(??????
�??????�−????????????
??????�????????????�)
??????
�
m–maximumnumberofTDMAuserssupportedinchannel
Itisalsoimportanttonotethatthe2GuardBands,oneatthelowerendandother
atthehigherendoftheallottedfrequencyarerequiredtoensuretheusersatthe
edgedonot“bleedover”intoanadjacentradiochannels

11. Summary
•Recap of FDMA
•Concepts and Working of TDMA
•Frame Structure of TDMA
•Features of TDMA
•Efficiency of TDMA Systems
•Capacity of TDMA Systems
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