FDMA

Frequency Division Multiple Access

M.S.P.V.L Polytechnic College, Pavoorchatram

Introduction

• In FDMA the total bandwidth is divided among

multiple simultaneous users (as shown in figure)

such that each user is allocated a channel with a

bandwidth of WFDMA = W/M Hz. The capacity of

each channel is

• CFDMA = (W/M) log2 (1+S/N) = C/MM.S.P.V.L Polytechnic College,

Pavoorchatram

Cont..,

• Where,

C is the channel capacity in bits per second

W is the bandwidth in Hz

S is the mean signal power

N is the noise varianceM.S.P.V.L Polytechnic College,

Pavoorchatram

Cont.., • For a constant S/N, the capacity of the total bandwidth is divided

among the users.

• In practice, each user occupies a bandwidth slightly narrower than

W/M so that interference between channels will be acceptable in a

system using realizable filters, which have a frequency response that

does not go to zero abruptly outside the channel bandwidth.

• This approach is applicable to both digital and analog modulation

formats.

M.S.P.V.L Polytechnic College, Pavoorchatram

FDMA(Frequency Division Multiple Access)

User 1 User 2 User 3

Time

power

30kHzFrequency

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M.S.P.V.L Polytechnic College, Pavoorchatram

• Assign different frequency bands to individual

users or circuits

– Frequency band(“channel”) assigned on demand to

users who request service.

– No sharing of the frequency bands: idle if not used.

– Usually available spectrum divided into number of

“narrowband” channels symbol time >> average

delay spread, little or no equalization required.M.S.P.V.L Polytechnic College,

Pavoorchatram

Cont..,

• Continuous transmission implies no framing or

synchronization bits needed.

• Tight RF filtering to minimize adjacent band interference.

• Costly bandpass filters at base station to eliminate spurious

radiation.

• Usually combined with FDD for duplexing.

M.S.P.V.L Polytechnic College, Pavoorchatram

FDMA assigning different frequency bands to individual users

Time

Freq

uenc

y

F2’

F2

F1

F1

’

F2’

F1’

F2

F1

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M.S.P.V.L Polytechnic College, Pavoorchatram

Uses FDMA/FDD

• A channel is a pair of frequency duplexed simplex channels.

• Each simplex channel is 30 KHz.

• Simplex channels are separated by 45 MHz (allows cheap

RF duplexers).

• Forward link 869-894 MHz, reverse link 824-849 MHz.

• Two carriers per market share the channels.

M.S.P.V.L Polytechnic College, Pavoorchatram

Advantages

• Very simple to design.

• Narrowband (no ISI).

• Synchronization is easy.

• No interference among users in a cell.

M.S.P.V.L Polytechnic College, Pavoorchatram

Disadvantages

• Narrow band interference

• Static spectrum allocation.

• Frequency reuse is a problem.

• High Q analog filters or large guard band

required.M.S.P.V.L Polytechnic College,

Pavoorchatram

The End

.…. Thank You…..

M.S.P.V.L Polytechnic College, Pavoorchatram