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Wireless& Mobile Networking:Multiple Division Techniques

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Multiple Division Techniques

To accommodate a number of users, many traffic

channels need to be made available.

In principle, there are three basic ways to have

many channels within an allocated bandwidth:

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)

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Concepts and Models of FDMA,

TDMA and CDMA

In one BS radio service range, may be many MSs arelocated.

MS must distinguish which signal meant for itself amongmany signals being transmitted by other users.

BS should be able to recognize the signal sent by a particularuser.

In cellular system, MS not only can distinguish a signalfrom a serving BS but also can discriminate the signal fromadjacent BS. – Multiple access techniques important inmobile cellular system!

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TDMA and CDMA (cont.)

A Radio Signal can be presented as a function of

frequency, time and code.

s (f,t,c) = s(f,t)c(t)

Where

s(f,t) – a function of frequency and time

c(t) – a function of code When c(t) = 1 then

s (f,t,c) = s(f,t)

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TDMA and CDMA (cont.)

System employs different carrier frequency – FDMAsystem.

System uses distinct time – TDMA system. System uses different code – CDMA system.

In wireless communications, it is necessary to utilize limitedfrequency bands at the same time, allowing multipleusers(MSs) to share radio channel simultaneously.

To provide simultaneous two-way communication (duplexcommunication) :

Frequency division duplexing (FDD)

Time Division Duplexing (TDD)

FDMA uses FDD, TDMA & CDMA uses TDD @FDD

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FDMA

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TDMA

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CDMA

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CDMA

CDMA is a system based on spread-spectrum

technology.

Spread-spectrum – transmission technique wherein

data occupy a larger bandwidth than necessary.

There are two basic types of implementation

methodologies: Direct Sequence (DS)

Frequency Hoping (FH)

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TD-CDMA

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SDMA(SPACE DIVISION MULTIPLE ACCESS)

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CDMA: Spread Spectrum

Spread spectrum – a transmission techniquewherein data occupy a larger bandwidth than

necessary Bandwidth spreading is a accomplished before

transmission using a code that is independent of thetransmitted data. The same code is used to

demodulated the data at receiving end. Originally designed for military used to avoid

jamming.

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Direct Sequence Spread Spectrum (DSSS)

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CDMA: Spread Spectrum (cont.)

Signal transmission consists of the following steps:

1. A pseudo-random code is generated, different for each channel and eachsuccessive connection.

2. The Information data modulates the pseudo-random code (the Informationdata is “spread”).

3. The resulting signal modulates a carrier.

4. The modulated carrier is amplified and broadcast.

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Signal reception consists of the following steps:

1. The carrier is received and amplified.

2. The received signal is mixed with a local carrier to recover the spread digitalsignal.

3. A pseudo-random code is generated, matching the anticipated signal.

4. The receiver acquires the received code and phase locks its own code to it.

5. The received signal is correlated with the generated code, extracting theInformation data.

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Direct Sequence Spread Spectrum (DSSS)

In this method, the radio signal is multiplied by a pseudorandom

sequence whose bandwidth is much greater than that signal itself – spreading its bandwidth.

Pseudorandom sequence directly phase modulates a carrier –

increase the bandwidth of transmission and lowering the

spectral power density.

The resulting RF signal has a noiselike spectrum. Noise to othersbut not to the intended receiver.

The received signal is despread by correlating it with the local

identical pseudorandom sequence to spread the carrier at the

receiver.

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Frequency Hopping Spread Spectrum (FHSS)

A pseudorandom sequence is used to change the radio signal

frequency across a broad frequency band in a random fashion. The modulation technique implies that the radio transmitter

frequency hops from channel to channel in a predetermined.

The RF signal is dehopped at the receiver end using a frequency

synthesizer control by a pseudorandom sequence generator.

A frequency hopper may be:

Fast hopped: multiple hops per data bit

Slow hopped: multiple bits per hop

Multiple simultaneous from several users is possible using FH as

long as each uses different FH sequences and not collide.

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CDMA: Near Far Problem

Stems from a wide range of signal levelsreceived in wireless mobile communication.

When two MSs are communicating with BSwith same transmission power and differentpath length – the received signal levels will

be different. When MSs used adjacent channel.. There

will be Inteference.

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Modulation Techniques

AM (Amplitude Modulation)

First method used to transfer voice information. The amplitude of a carrier signal with constant

frequency.

FM (Frequency Modulation)

Method that integrate information signal with an

alternating current(ac) wave.

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