Multiple Division Techniques - National Tsing Hua...

transcript

Chapter 7

Multiple Division Techniques

Outline

Introduction

Concepts and Models for Multiple Divisions Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

Orthogonal Frequency Division Multiplexing (OFDM)

Space Division Multiple Access (SDMA)

Comparison of FDMA, TDMA, and CDMA

Modulation Techniques Amplitude Modulation (AM)

Frequency Modulation (FM)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

Quadrature Phase Shift Keying (QPSK)

p/4QPSK

Quadrature Amplitude Modulation (QAM)

Multiple access techniques are based on orthogonalization of signals

A radio signal is a function of frequency, time and code as;

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

where s(f,t) is the function of frequency and time and c(t) is the function of code

Use of different frequencies to transmit a signal: FDMA

Distinct time slot: TDMA

Different codes CDMA

Multiple simultaneous channels: OFDM

Specially separable sectors: SDMA

Concepts and Models for Multiple Divisions

Frequency Division Multiple Access (FDMA)

User 1

User 2

User n …

Time • Single channel per carrier

• All first generation systems use FDMA

jidftfstfs j

i ,...,2 ,1,,0

1),(),(

Orthogonality conditions of two signals in FDMA:

Reverse channels

(Uplink)

Forward channels

(Downlink)

Basic Structure of FDMA

Forward and Reverse channels in FDMA and Guard Band

Band Wg

1 2 3 …

Frequency Total Bandwidth W =

Sub Band

f1’ f2’ fn’

f1 f2 fn

Reverse channels Forward channels

Protecting bandwidth

Time Division Multiple Access (TDMA)

• Multiple channels per carrier

• Most of second generation systems use TDMA

Orthogonality conditions of two signals in TDMA:

jidttfstfs j

i ,...,2,1,,0

1),(),(

Reverse channels

(Uplink)

Frequency f ’ #1

… #n

The Concept of TDMA

Forward channels

(Downlink)

Frequency f

TDMA: Channel Structure

(a). Forward channel

… t #1

… … #1

Frame Frame Frame

(b). Reverse channel

… #1

Frame Frame Frame

Channels in TDMA/FDD

Forward and Reverse Channels in TDMA

Frequency f = f ’

Frame Frame

Forward

channel

Reverse

channel

Forward

channel

Reverse

channel

Channels in TDMA/TDD

Frame Structure of TDMA

… … #1

Frame Frame Frame

Head Data Guard

Code Division Multiple Access (CDMA)

Frequency

• Users share bandwidth by using code sequences that are orthogonal to each other

• Some second generation systems use CDMA

• Most of third generation systems use CDMA U

Orthogonality conditions of two signals in CDMA:

jidttsts j

i ,...,2,1,,0

CDMA Encode/Decode

Slot 1 Slot 0

d1 = -1

1 1 1 1

1 - 1 - 1 - 1 -

Zi,m= di.cm d0 = 1

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

slot 0

Channel

output

slot 1

Channel

output

Channel output Zi,m

Render

Slot 1 Slot 0

d1 = -1

d0 = 1

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

1 1 1 1

1 - 1 - 1 - 1 -

Slot 0

channel

output

Slot 1

channel

output Receiver

Received

Di = S Zi,m.cm m=1

CDMA: two-sender interference

Structure of a CDMA System

… …

Reverse channels (Uplink)

Forward channels (Downlink)

Frequency f ’

Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes,

Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes

Frequency f

Digital

signal

Code c(t)

Spreading

signal m(t) Spreading

Frequency

Spread Spectrum

)()()( tctstm

Spreading of data signal s(t) by the code signal c(t)

to result in message signal m(t) as:

Spreading

signal m(t)

Spreading

Transmitter

Digital

signal s(t)

Despread

Receiver

Direct Sequence Spread Spectrum (DSSS)

Digital

signal s(t)

Frequency Frequency

Frequency

Orthogonal Codes

Orthogonal codes

All pairwise cross correlations are zero

Fixed- and variable-length codes used in CDMA systems

For CDMA application, each mobile user uses one sequence in the set as a spreading code

Provides zero cross correlation among all users

Walsh codes

Variable-length Orthogonal codes

Walsh Codes

Set of Walsh codes of length n consists of

the n rows of an n x n Walsh matrix:

W1 = (0)

where n = dimension of the matrix

Every row is orthogonal to every other row

Requires tight synchronization

Cross correlation between different shifts of Walsh

sequences is not zero

Example:

Frequency

An Example of Frequency Hopping Pattern

Hopping

pattern

Spreading

signal

Spreading

Transmitter

Digital

signal

Frequency Frequency

Frequency Hopping Spread Spectrum (FHSS)

Digital

signal s(t)

Despread

Receiver

Hopping

pattern

Frequency

MS1 MS2 BS

Distance Distance 0

Received signal strength

MS1 MS2 BS

Near-far Problem

Adjacent Channel Interference

Frequency f1 f2

Interference in Spread Spectrum

Frequency

Baseband

signal

Interference signals

Frequency

Despread signal

Frequency

Interference

baseband signals

Spectrum spreading signal

Power Control in CDMA

Controlling transmitted power affects the CIR

Pr = Received power in free space

Pt = Transmitted power

d = Distance between receiver and transmitter

f = Frequency of transmission

c = Speed of light

= Attenuation constant (2 to 4)

jidttfstfs ji

,....,2,1,,,0

,1),(),( *

Orthogonal Frequency Division Multiplexing (OFDM)

Spectrunm of an

OFDM signal with

multiple subchannels

Orthogonality of two signals in OFDM can be given by

a complex congugate relation indicated by *:

Spectrum of a single

OFDM subchannel

Divide a channels into multiple sub-channels and do parallel transmission

Modulation / Demodulation steps in OFDM

Modulation operation at the OFDM transmitter

High speed

Serial to parallel

conversion

IDFT Guard interval

insertion

data stream

Low speed bit stream

N2 ….

Transmission of

OFDM signal

Demodulation steps at the OFDM receiver

Guard interval

removal DFT

Parallel to serial

conversion

High speed

data stream

Received

OFDM signal

Space Division Multiple Access (SDMA)

The concept

of SDMA

Beam n Beam 1

Beam 2

Beam 3

Beam i

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

s(f,t,c)

Space divided into spatially separate sectors

Omni-directional

transmission

Transmission in SDMA

The basic structure of a SDMA system.

MS2 MS3 BS

Beam 1 Beam 2 Beam 3

Comparison of Various Multiple Division Techniques

Technique FDMA TDMA CDMA SDMA

Concept

Divide the frequency

band into disjoint

subbands

Divide the time into

non-overlapping

time slots

Spread the signal

with orthogonal

Divide the space in to

sectors

Active terminals

All terminals active

on their specified

frequencies

Terminals are active

in their specified

slot on same

frequency

All terminals active

on same frequency

Number of terminals

per beam depends on

TDMA/CDMA

Signal separation

Filtering in

frequency

Synchronization in

Code separation Spatial separation

using smart antennas

Handoff Hard handoff Hard handoff Soft handoff Hard and soft

handoffs

Advantages Simple and robust Flexible Flexible Very simple, increases

system capacity

Disadvantages

Inflexible, available

frequencies are fixed,

requires guard

Requires guard

space,

synchronization

problem

Complex receivers,

requires power

control to avoid

near-far problem

Inflexible, requires

network monitoring to

avoid intracell

handoffs

Current

applications

Radio, TV and

analog cellular

GSM and PDC 2.5G and 3G Satellite systems,

other being explored

Modulation Techniques

Why need modulation?

Small antenna size

Antenna size is inversely proportional to frequency

(wavelength)

e.g., 3 kHz 50 km antenna

3 GHz 5 cm antenna

Limits noise and interference,

e.g., FM (Frequency Modulation)

Multiplexing techniques,

e.g., FDM, TDM, CDMA

Analog and Digital Signals

Analog Signal (Continuous signal)

Amplitude

Digital Signal (Discrete signal)

Amplitude

1 1 1 1 0 0 +

Amplitude Modulation (AM)

Message signal

Carrier signal

AM signal

The modulated carrier signal s(t) is:

)2(cos)]([)( tftxAts cp Where fc is the carrier frequency and A its amplitude

Frequency Modulation (FM)

Carrier signal

Message signal

FM signal

The modulated carrier signal s(t) is:

c dxftfAts

0)(22(cos)( pp Where f is the peak frequency deviation from the original

frequency and f << fc

BW=2(b1)fm with b= f/fm; fm is the

maximum modulating frequency used

Frequency Shift Keying (FSK)

1/0 represented by two different frequencies

Carrier signal ‘1’

for message signal ‘1’

1 0 1 1 0 1

Message signal x(t)

FSK signal s(t)

Carrier signal 2

for message signal ‘0’

Phase Shift Keying (PSK)

• Use alternative sine wave phases to encode bits

Carrier signal

)2sin( pp tfc

Message signal x(t)

)2sin( tfcp

1 0 1 1 0 1

PSK signal s(t)

Quadrature Phase Shift Keying (QPSK)

Four different phase shifts used are:

I (in-phase) and Q (quadrature) modulation used

QPSK Signal Constellation

I 0,0 1,1

(a) BPSK

(Binary Phase Shift Keying)

(b) QPSK

(Quadrature Phase Shift Keying)

p/4 QPSK

All possible states in p/4 QPSK

The phase of the carrier is: Where k is carrier phase shift corresponding to input bit pairs. If 0=0, input bit stream is [1011], then:

Quadrature Amplitude Modulation

(QAM) Combination of AM and PSK: modulate signals using two

measures of amplitude and four possible phase shifts

Bit sequence represented

Amplitude Phase shift

000 1 0

001 2 0

010 1 p/2

011 2 p/2

100 1 p

101 2 p

110 1 3p/2

111 2 3p/2

A representative QAM Table

Quadrature Amplitude Modulation

Two carriers out of phase by 90 deg are amplitude modulated

Rectangular constellation of 16QAM

0000 0100 1100 1000

0001 0101 1101 1001

0011 0111 1111 1011

0010 0110 1110 1010

Homework

Exercises: 7.2, 7.17

Practice at home: 7.3, 7.6, 7.8

Quiz 2

What is the difference between collision detection and collision avoidance?

What are the differences between adjacent channel interference and co-channel interference?

Multiple Division Techniques - National Tsing Hua...

Documents