Multiple Access Techniques for Wireless Communications ppsx

Multiple Access Techniques for Wireless
Communications
2005/07/06
Weng Chien-Erh

Wireless Access
Tech. Lab.
CCU
Wireless Access Tech. Lab.
Table of Contents(1)

Introduction

FDMA

TDMA

CDMA

SS (Spread Spectrum)

FHSS

DSSS

Hybrid

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Wireless Access Tech. Lab.

Packet Radio

Pure ALOHA

Slotted ALOHA

CSMA

Reservation Protocol

Reservation-ALOHA

PRMA

NC-PRMA
Table of Contents(2)

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Introduction (1)

Multiple Access:

Enable many mobile users to share simultaneously radio spectrum.

Provide for the sharing of channel capacity between a number of

transmitters at different locations.

Aim to share a channel between two or more signals in such way that each
signal can be received without interference from another.

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Introduction (2)

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Introduction (3)

In conventional telephone systems, it is possible to talk and listen simultaneously,
called duplexing.

Duplexing

Allow the possibility of talking and listening simultaneously.

Frequency Division Duplex (FDD)

Provides two distinct bands of frequencies for every user

Time Division Duplex (TDD)


Multiple users share a signal channel by taking turns in time
domain

Each duplexing channel has both a forward time slot and a
reverse time slot to facilitate bidirectional communication.

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Introduction (4)

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
Each transmitter is allocated a channel with a particular bandwidth.

All transmitters are able to transmit simultaneously.
Frequency Division Multiple Access
(FDMA) (1)

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Frequency Division Multiple Access
(FDMA) (2)


Allocation of separate channels to FDMA signals

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
Time-frequency characteristic of FDMA
Frequency Division Multiple Access
(FDMA) (3)

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
Features of FDMA

If an FDMA channel is not in sue, then it sits idle and can’t be used by
other users.

Transmit simultaneously and continuously.

FDMA is usually implemented in narrowband systems.

Its symbol time is large as compared to the average delay
spread.
Frequency Division Multiple Access
(FDMA) (4)

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
Features of FDMA (Cont.)

For continuous transmission, fewer bits are needed for overhead
purposes (such as synchronization and framing bits) as compared to
TDMA.

FDMA uses duplexers since both TX and RX operate at the same time.
Frequency Division Multiple Access
(FDMA) (5)

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
Transmitter share a common channel.

Only one transmitter is allowed to transmit at a time.

Synchronous TDMA: access to the channel is restricted to regular.

Asynchronous TDMA: a station may transmit at any time that the channel
is free.
Time Division Multiple Access (TDMA) (1)

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
Allocation of time slot in TDMA
Time Division Multiple Access (TDMA) (2)

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
Time-frequency characteristic of synchronous TDMA
Time Division Multiple Access (TDMA) (3)

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Time Division Multiple Access (TDMA) (4)

Features of TDMA (Cont.)

TDMA systems divide the radio spectrum into time slots.

Each user occupies a cyclically repeating time slot.

Transmit data in a buffer-and-burst method, thus the transmission for
any user is not continuous.


TDMA has TDD and FDD modes.

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
TDMA Frame Structure
Time Division Multiple Access (TDMA) (5)

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
TDMA Frame Structure (Cont.)

In TDMA, the preamble contains the address and synchronization
information that both the base station and the mobiles use to identify
each other.

Different TDMA standards have different TDMA frame structures.
Time Division Multiple Access (TDMA) (6)

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

Features of TDMA (Cont.)

Share a single carrier frequency with several users.

Data transmission is not continuous, but occurs in bursts.

No duplexers is required since users employ different time slots for
transmission and reception.

TDMA can allocate different numbers of time slots per frame to
different users, allowing bandwidth be supplied on demand to different
users.
Time Division Multiple Access (TDMA) (7)

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
Combined used of synchronous TDMA and FDMA
Time Division Multiple Access (TDMA) (8)

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
Asynchronous TDMA: Carrier-Sense Multiple Access (CSMA)

Allows a transmitter to access the channel at any time that is not being

used by another transmitter.
Time Division Multiple Access (TDMA) (8)

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
Transmitter may transmit at the same time, in the same channel.

Each signal is modified by spreading it over a large bandwidth.

This spreading occurs by combining the transmitter signal with a
spreading sequence.
Code Division Multiple Access (CDMA) (1)

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Code Division Multiple Access (CDMA) (2)

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
example
Code Division Multiple Access (CDMA) (3)

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
Features of CDMA

Many users of a CDMA system share the same frequency.

The symbol (chip) duration is very short and usually much less than the
channel delay spread.

The near-far problem occurs at a CDMA RX if an undesired user has a
high detected power as compared to the desired user.
Code Division Multiple Access (CDMA) (4)