c3.0 multiple access techniques

8/13/2019 C3.0 Multiple Access Techniques

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C3.0 Multiple Access Techniques

, Jeng-Han Tsai

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Ref: Behzad Razavi, RF Microelectronics, Prentice Hall, 1997.

Introduction

The modulation and detection techniques provide the basis for

communication between only one transmitter and one receiver. For a large

number of transceivers in a network, additional methods are required to

ensure proper communication among multiple users. Such methods are called

multiple access techniques.

The complexity of wireless systems mandates that they conform to a

standard. In addition to modulation and multiple access techniques, a

wireless standard includes such details as frequency bands, timing, and data

coding, while defining precise tests for the performance of transceiver.

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Mobile RF Communication

A mobile is one in which users can physically move while

communicating with one another. Ex: pager, cellular phones, and

cordless phones.

The transceiver carried by the user is called the mobile unit, the

terminal, or the hand-held unit.

The wireless infrastructure is called the base station.

Each mobile receiver and transmits information from and to the base

station via two RF channels called the forward channel ordownlink and the reverse channel or uplink, respectively.

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Cellular System

Each cell is configured as a hexagon and surrounded by six other cells.

If the center cell uses a frequency f1 for communication, the six neighboringcells cannot utilize this frequency but the cells beyond the immediateneighbors may.

More efficient frequency assignment leads to the 7-cell reuse pattern. Each

cell utilizes a group of frequencies.

The mobile units in each cell are served by a base station, and all of the basestation are controlled by a mobile telephone switching office (MTSO).

Simple cellular system 7-cell reuse pattern

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Co-Channel Interference

An important issue in a cellular system is how much two cells that use thesame frequency interfere with each other, which is called co-channelinterference (CCI).

The effect depends on the ratio of the distance between two co-channel cellsto the cell radius and is independent of the transmitted power.

Given by the frequency reuse plan, this ratio is approximately equal to 4.6 forthe 7-cell pattern.

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Hand-off

What happens when a mobile unit roams from cell A to cell B?

Since the power level received from the base station in cell A is insufficient tomaintain communication, the mobile must change its server to the basestation in cell B. Since adjacent cells do not use the same group of frequencies,the channel must also change, which is called hand-off.

Hand-off is performed by the MTSO. Once the level received by the basestation in cell A drops below a threshold, the MTSO hands off the mobile tothe base station in cell B.

To improve the hand-off process, second-generation cellular systems allowthe mobile unit to measure the received signal level from different basestation.

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Path Loss and Multipath Fading

Propagation of signals in a mobile communication

environment is quite complex.

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Multiple Access Techniques

Time-Division Duplexing (TDD)

Frequency-Division Duplexing (FDD)

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Frequency-Division Multiple Access (FDMA)

Time-Division Multiple Access (TDMA)

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Code-Division Multiple Access (CDMA)

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Direct-sequence spread spectrum (DSSS)

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TRANSMITTER BASEBAND SIGNAL BEFORE SPREADING TRANSMITTER BASEBAND SIGNAL AFTER SPREADING

RECEIVER BASEBAND SIGNAL BEFORE DESPREAD RECEIVER BASEBAND SIGNAL AFTER DESPREAD

c3.0 multiple access techniques

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