2.2 frequency division multiple access

2.2 frequency division multiple access

1. FREQUENCY
DIVISION
MULTIPLE
ACCESS

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3. Multiplexing VsMultiple Access
Multiplexing(ormuxing)isawayofsendingmultiple
signalsorstreamsofinformationoveracommunications
linkatthesametimeintheformofasingle,complex
signal;
MultipleAccessisatechniqueusedtoallowmultiple
userstoaccessthechannelatthesametimewithoutany
interference.Inotherwords,
Multiple Access is a application of Multiplexing

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

5. FDMA
•FDMAAssignsindividualchannelstoindividualusers
•Eachuserisallocatedauniquefrequencybandorchannel
•Thechannelsareassignedondemandbytheusers
•Thesamefrequencyisnotallottedfor2differentusers.
•ThebandwidthofFDMAchannelisrelativelynarrow(30Khz)
•FDMAareusuallyimplementedinnarrowbandsystems
•TheamountofintersymbolinterferenceislowandthusNoeuqalizationisrequiredinFDMA
narrowbandsystems
•FDMAiscontinuoustransmissionsystemswhereasitrequiredlessOverheadcomparedtoTDMA
•FDMAusesDuplexerssinceboththetransmitterandreceiveroperatesatthesametime.This
resultsinincreaseinCOSTofFDMA
•FDMArequirestightRFfilteringtominimizeadjacentchannelinterference

6. Non Linear Effects in FDMA
•InFDMA,Manychannelssharethesame
antennaatthebasestation.
•InordertogettheMaximumPower
EfficiencythePowerAmplifiersand
PowerCombinersareoperatedat(or)
nearSaturationRegion
•At(or)nearSaturationRegionthePower
amplifiersandpowercombinersareNon
Linear.
•Thesenonlinearitiescausessignal
spreadinginthefrequencydomainand
generate Intermodulation(IM)
Frequencies.
•TheseSpreadingwillinterferewith
adjacentchannelscausingadjacent
channelinterference.

7. Capacity of FDMA
•Thenumberofchannelsthatcanbesimultaneouslysupportedin
FDMAsystemsisgivenby
N=(B
t–2 B
guard) / B
c
•Where
•B
tisthetotalspectrumallocation
•B
guardistheguardbandallocationattheedgeofallocatedspectrum
•B
cisthechannelbandwidth

8. Summary
•Multiplexing VsMultiple Access
•Multiple Access is an application of Multiplexing
•Type of Multiple access Techniques
•Frequency Division Multiple Access Technique
•Principle of Operation
•Pros and Cons
•Non Linear Effects of FDMA
•Channel Capacity of FDMA systems