Wireless & Mobile Networking:

Multiple Division Techniques

Azizol Bin Abdullahazizol@fsktm.upm.edu.my (A2.04)

Rujukan: Text Book Chapter 7

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)

Concepts and Models of FDMA, TDMA and CDMA

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

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

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

In cellular system, MS not only can distinguish a signal from a serving BS but also can discriminate the signal from adjacent BS. – Multiple access techniques important in mobile cellular system!

Concepts and Models of FDMA, 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 thens (f,t,c) = s(f,t)

Concepts and Models of FDMA, TDMA and CDMA (cont.)

System employs different carrier frequency – FDMA system. System uses distinct time – TDMA system. System uses different code – CDMA system. In wireless communications, it is necessary to utilize limited

frequency bands at the same time, allowing multiple users(MSs) to share radio channel simultaneously.

To provide simultaneous two-way communication (duplex communication) : Frequency division duplexing (FDD) Time Division Duplexing (TDD)

FDMA uses FDD, TDMA & CDMA uses TDD @FDD

FDMA

FDMA

TDMA

TDMA

TDMA

CDMA

CDMA

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)

TD-CDMA

SDMA(SPACE DIVISION MULTIPLE ACCESS)

CDMA: Spread Spectrum

Spread spectrum – a transmission technique wherein data occupy a larger bandwidth than necessary

Bandwidth spreading is a accomplished before transmission using a code that is independent of the transmitted data. The same code is used to demodulated the data at receiving end.

Originally designed for military used to avoid jamming.

Direct Sequence Spread Spectrum (DSSS)

CDMA: Spread Spectrum (cont.)

Signal transmission consists of the following steps:

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

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

3. The resulting signal modulates a carrier.

4. The modulated carrier is amplified and broadcast.

CDMA: Spread Spectrum (cont.)

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 digital signal.

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 the Information data.

CDMA: Spread Spectrum (cont.)

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 others but 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.

CDMA: Spread Spectrum (cont.)

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.

CDMA: Near Far Problem

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

When two MSs are communicating with BS with same transmission power and different path length – the received signal levels will be different.

When MSs used adjacent channel.. There will be Inteference.

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.