Multiple Access Techniques-Wireless Networks-Lecture Slides, Slides of Wireless Networking

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2

Outlines

Review of previous lecture

FDMA

TDMA

CDMA

Random Access ►

ALOHA

Slotted ALOHA

Reservation-based ALOHA

Summary of today’s lecture

3

Last Lecture Review

Block Codes ►

Hamming

BCH

Reed Solmon

ARQ ►

Sliding window

Go-back-N

5

FDMA

f

FDMA was the initial multiple-accesstechnique for cellular systems 

Separates large band into smallerchannels. 

Each channel has the ability to supportuser. 

Guard bands are used to separatechannel preventing co-channelinterference 

Narrow bandwidth (30 khz).

User 1

User 2

User 3

User 4

Time

Code

6

Advantages ►

Simple to implement in terms of hardware.

Fairly efficient with a small base population and withconstant traffic.

Disadvantages ►

Network and spectrum planning are intensive andtime consuming.

Channels are dedicated for a single user, idlechannels add spectrum inefficiency.

8

How it works?

User presses Push-to-Talk (PTT) button

A control channel registers the radio to the closest basestation.

The BS assigns an available pair of channels.

Unlike FDMA, TDMA system also assigns an availabletime slot within the channel.

Data transmission is not continuous rather sent andreceived in bursts.

The bursts are reassembled and appear likecontinuous transmission.

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Advantages ►^

Extended battery life and talk time ►

More efficient use of spectrum, compared to FDMA ►

Will accommodate more users in the same spectrum space than anFDMA system

Disadvantages ►^

Network and spectrum planning are intensive ►

Multipath interference affects call quality ►

Dropped calls are possible when users switch in and out of differentcells. ►

Too few users result in idle channels (rural versus urban environment) ►

Higher costs due to greater equipment sophistication

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Code-Division Multiple Access (CDMA)

Basic Principles of CDMA ►

D

= rate of data signal

Break each bit into

k chips

•^

Chips are a user-specific fixed pattern

Chip data rate of new channel =

kD

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CDMA Example

If

k

=6 and code is a sequence of 1s and -1s

For a ‘1’ bit, A sends code as chip pattern•^

<c1, c2, c3, c4, c5, c6>

For a ‘0’ bit, A sends complement of code•^

<-c1, -c2, -c3, -c4, -c5, -c6>

Receiver knows sender’s code and performs electronicdecode function

-^

< d1, d2, d3, d4, d5, d6> = received chip pattern

-^

< c1, c2, c3, c4, c5, c6> = sender’s code

^

^

c d c d c d c d c d c d d

Su

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Advantages ►^

Greatest spectrum efficiency: ►

CDMA improves call quality by filtering out background noise, cross-talk, and interference ►

Simplified frequency planning - all users on a CDMA system use thesame radio frequency spectrum. ►

Random Walsh codes enhance user privacy; a spread-spectrumadvantage ►

Precise power control increases talk time and battery size for mobilephones

Disadvantages ►^

Backwards compatibility techniques are costly ►

Currently, base station equipment is expensive ►

Low traffic areas lead to inefficient use of spectrum and equipmentresources

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Random Access

Random Access Methods ►

more efficient way of managing medium accessfor communicating short bursty messages^ •

in contrast to fixed-access schemes, each user gainsaccess to medium only when needed -has some data tosend

-^

drawback: users must compete to access the medium(‘random access’)

-^

collision of contending transmissions

Random Access Methods in WirelessNetworks ►

can be divided into two groups:^ •

ALOHA based-no coordination between users

-^

carrier-sense based-indirect coordination -users senseavailability of medium before transmitting

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ALOHA-based Random Access

user accesses medium as soon as it has a packetready to transmit ►^

after transmission, user waits a length of time > round-tripdelay in the network, for an ACK from the receiver ►

if no ACK arrives, user waits a random interval of time (to avoidrepeated collision) and retransmits

advantages: ►^

simple, no synchronization among users required

disadvantages: ►^

low throughput under heavy load conditions ►

probability of collision increases as number of users increases

max throughput = 18% of channel capacity

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Pure-ALOHA

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Slotted ALOHA

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Example

slotted ALOHA in GSM ►

Two types of channels in GSM:^ •

Traffic channels (TCH):

used for transmission of user data

–based on FDMA/TDMA

-^

Signalling channels, used for control and management of acellular network

-^

Random Access Channel (RACH):

signalling channel for

establishing access to the network (i.e. BS)

-^

employs Slotted ALOHA

-^

only channel in GSM where contention can occur