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Lessen 1Basic Communication Theory (基本通訊原理)

TransmitterInformation

Receiver

Information

Antenna

Antenna

TransmissionLine

TransmissionLine

Voltage

TimeFrequency

User 1 User 2 User 3 User 4 User 5

Time

User 1

Code 1XOR

XORCode 1

User 1

Composite

FDMA

TDMA

CDMA

1 2 3 4 5 6

Mobile Radio Communication Seminar

Lesson 1 - Basic Communication Theory 1 Dr. Sheng-Chou Lin

Lesson 1 Outline

Basic Radio Link structure and characteristicsAnalog Modulation (類比調變): AM, FM, PMDigital Modulation (數位調變): ASK, FSK, PSKAdvanced Modulation Concepts

•Occupied Bandwidth (頻寬)

Performance of Modulation Types used in WirelessTelephony•Analog FM, DQPSK, FSK, GMSK

Overview of Multiple Access Methods (多重存取) used inWireless Telephony•FDMA, TDMA, CDMA

Survey of Technologies used for Wireless Telephony



Conversion from Speech to RF

Speech coding

ConvolutionalEncoding

Interleaving

RF Transmission

De-Interleaving

ConvolutionalDecoding

SpeechCodec

SpeechCodec

ConvolutionalEncoding

ConvolutionalDecoding

Time

Interleaving Among Frames

De-Interleaving



Elements of a Radio (無線電) Link

Transmitter (發射機)•Generates RF energy on a

desired frequency•Modulates the RF energy to

convey information Antennas (天線)

•Convert RF energy intoelectromagnetic fields, vice versa

•Focus the energy into desireddirections (“Gain”)

Receiver (接收機)•filters out and ignores signals on

undesired frequencies•Amplifies the tiny signal

sufficiently to allow processing•De-modulates the signal to

recover the information


Receiver

Information

Antenna

Antenna

TransmissionLine

TransmissionLine



Some Basic Parameters ofRadio Link Elements

Transmitter•Power Output (ex: 10W=40dBm)

Antenna System•Gain (增益)•Beamwidth (波束寬)•Loss of transmission lines

Receiver•Sensitivity (靈敏度)

–required minimum signal powerlevel for acceptable performance

•Selectivity (選擇度)–degree to which strong signals on

nearby frequencies are ignored•Dynamic Range (動態範圍)

–range of signal strengths betweenunrecognizably low and thresholdof distortion


Receiver

Information

Antenna

Antenna

TransmissionLine

TransmissionLine



Radio Link Modes

Simplex•Uses only one channel, one-way-at-a-time•Only one talker speaks at a time; can’t interrupt•Example: CB Radio

Half Duplex•Uses two channels, but one at a time•Only one talker speaks at a time; can’t interrupt

•Example: Taxi Dispatcher

Duplex•Two channels are used•Both talkers can speak anytime & interrupt•Requires two totally independent channels•Examples: Telephone, Cellular, PCS



Analog Modulation (類比調變)

For example, let’s use this analogwaveform to modulate a signal.

The basic, unchanging, steady radiosignal without modulation is called a“carrier”Characteristics of thecarrier which we could modulate:

Amplitude (i.e., strength) (振幅)example: AM radio broadcasting

Frequency (頻率)FM broadcasting,Voice transmissionin AMPS cellular

Phase (相位)

Modulation is the process of varying some characteristicof a radio signal in order to convey information

Voltage

Time

Notice thatfrequency and

phasemodulation lookvery similar with

this kind ofinput.



Digital Modulation (數位調變)

For example, let this digital waveformmodulate a signal. No morecontinuous analog variations, nowwe’re “shifting”between discretelevels. We call this “shift keying”

The steady radio signal withoutmodulation is called a “carrier”

Amplitude Shift Keying (振幅移鍵)ASK example: digital microwave

Frequency Shift Keying (頻率移鍵)FSK example: control messages in

AMPS cellular; TDMA cellularPhase Shift Keying (相位移鍵)

PSK examples: TDMA cellular,GSM & PCS-1900

The previous example showed modulation by an analogwaveform. What happens if we use a digital input?Voltage

Time1 0 1 0

1 0 1 0

1 0 1 0

1 0 1 0



Modulation and Occupied Bandwidth (頻寬)The bandwidth occupied by a signal

depends on:•input information bandwidth•modulation method

Information to be transmitted, called“input”or “baseband”•bandwidth usually is small, much

lower than frequency of carrier Unmodulated carrier

•the carrier itself has Zero bandwidth!! AM-modulated carrier (調幅)

•Notice the upper & lower sidebands•total bandwidth = 2 x baseband

FM-modulated carrier (調頻)•Many sidebands! bandwidth is a

complex mathematical function PM-modulated carrier (調相)

•Many sidebands! bandwidth is acomplex mathematical function

Voltage

Time

Time-Domain(as viewed on an

Oscilloscope)

Frequency-Domain(as viewed on a

Spectrum Analyzer)

Voltage

Frequency0

fc

fc

UpperSideband

LowerSideband

fc

fc



Advanced Modulation ConceptsPerformance of different Modulation Types

Each type of modulation has advantages and drawbacks:Necessary bandwidth (頻寬)

•How wide is the signal? How much spectrum is needed?•How big a “guard band”is needed between channels?

Relative vulnerability to interference (對抗干擾)•What C/I ratio is required for good system performance?

Relative difficulty of implementation (實現困難度)•Is complex equipment required?•Is it costly to implement?•Is it hard to maintain and adjust?

Let’s explore the different modulation methods used inmodern mobile telephony.



Analog FM is used onAMPS Cellular Voice Channels (類比蜂巢式)

Two signals simultaneously modulatethe AMPS cellular voice channel:

User’s voice waveform•complex, many frequencies approx.

300Hz to 3500 Hz.•peak deviation limited to +/- 12 KHz

Supervisory Audio Tone (“SAT”)•tone frequency 5970, 6000, or 6030 Hz.•peak deviation set as +/- 2.0 KHz.

The resulting composite FM signal fitswithin the assigned 30 KHz.-widechannel

Signalling Tone at 10 KHz with +/- 8 kHz.deviation is also transmitted inoccasional bursts for call control

VoltageTime

Time-Domain(as viewed on an

Oscilloscope)

Frequency-Domain(as viewed on a

Spectrum Analyzer)

VoltageFrequency

0

fc

VoltageFrequency

0

Voltage

Time

Voltage

30 KHz. ChannelVoltage

6KHz

Voice

SAT



FSK (Frequency Shift Keying) forAMPS Cellular Control Messaging

Input signal is Manchester-encoded data(no DC component)•10 KB rate

Output Signal is FSK-modulated•+/- 8 KHz deviation•Binary 0 = fc - 8 KHz•Binary 1 = fc + 8 KHz.

On voice channels, when systemmessages must be sent, the FM voiceand SAT modulation is briefly muted andreplaced by FSK (this is called ”blankand burst”mode)

On control channels, FSK data istransmitted exclusively (no voice)

Time

Voltage

Voltage

Time

fc

Voltage 30 KHz. Channel

Frequency

Input Signal

Output Signal



Modified forms of FSK:MSK and GMSK

MSK and GMSK are forms of FSK• input signal is pre-filtered to eliminate abrupt

shifts• this reduces the spectrum occupied by the

output signal MSK

•The frequency shift never produces a phasediscontinuity; this reduces spectrum required

• the output spectrum still contains sidelobes GMSK: Used in GSM, DCS1800, PCS1900

•Side lobes in output spectrum are preventedby the Gaussian pre-filtering

•Generates narrow power spectrum•Spectrally efficient modulation technique•BER is slightly worse than MSK. This is a

worthwhile tradeoff since error control codingis available

MSK Minimum Shift Keying

GMSKGaussian Minimum Shift Keying

GaussianFilter

MSKModulator

GMSKOutput

Input:BinaryData

XFSK Modulator

Carrier

FSK ModulatedOutput

FILTER

1 00 T

0 T/2 T

1 0

1 0 1

NRZData



Power Spectral Density QPSK main lobe narrower than FSK

• QPSK better for high capacity applications such as voicechannels of TDMA cellular

• QPSK requires linear amplifier (greater cost, complexity;lower efficiency)

FSK side lobes roll of faster than QPSK• FSK better for control channel use• FSK constant amplitude allows use of class-C amplifier

(simple, high efficiency)

Other Observations:• FSK modulation simpler to implement.• With filtering, both methods meet EIA standard attenuation

>26 db on adj. ch.

BER Performance QPSK approx. 3dB better than FSK

Eb/No (dB) = (S/N)(BNR)Eb = Energy per bit No = Noise per bitN = Total noise power S = Signal powerBN = Noise bandwidth R = Bit Rate

Comparison of QPSK and FSK

0.010.0001

1E-61E-8

1E-101E-121E-141E-161E-181E-20

0

2 4 6 8 10 12 14 16 180 20

BER Performance ComparisonQPSK vs FSK

Eb/No (db)

BER

0

-10

dB

-50

-40

-30

-20

fc+1fbit

Power Spectral DensityQPSK vs. FSK

fc fc+2fb fc+3fb fc+4fbFrequency



What is Multiple Access (多重接取)

Since the beginning of telephony and radio,system operators have tried to squeeze themaximum amount of traffic over each circuit

Types of Media Examples:• Twisted pair - copper• Coaxial cable

• Fiber optic cable• Air interface (radio signals)

Advantages of Multiple Access• Increased capacity: serve more users• Reduced capital requirements since

fewer media can carry the traffic• Decreased per-user expense• Easier to manage and administer

Transmission

Medium

Each pair of usersenjoys a dedicated,

private circuit throughthe transmission

medium, unaware thatthe other users exist.

Multiple Access:by multiple, independent users .



What is a Channel (通道)

The physical transmission medium is a resource that can be subdividedinto individual channels according to different criteria depending on thetechnology used

Here’s how the three most popular technologies establishchannels:

Channel: An individually-assigned, dedicatedpathway through a transmissionmedium for one user information

Power

FrequencyTime

Power

FDMA

TDMA

CDMA

FrequencyTime

Power

FrequencyTime

FDMA (Frequency Division Multiple Access)• each user on a difficult frequency• a channel is a frequency

TDMA (Time Division Multiple Access)• each user on a different window period in time (time slot)• a channel is a special time slot on a special frequency

CDMA (Code division Multiple Access)• each user uses the same frequency all the time,but mixed with

different distinguishing code patterns• a channel is a unique code pattern



Access Methods

FDMA Frequency Division Multiple Access•each user has a separate radio frequency•radio receiver recovers user, ignores others

TDMA Time Division Multiple Access•each user occupies a time slot in a sequence which

repeats continuously•recover only user bits, ignore others

CDMA Code Division Multiple Access•input from user is coded into a composite bit

stream which occupies a large spectrum•composite bit stream received at other end•user’s code applied to composite bit stream•User’s input is recovered•Users’codes are orthogonal (non-interfering)

Frequency

User 1 User 2 User 3 User 4 User 5

Time

User 1

Code 1XOR

XORCode 1

User 1

Composite

FDMA

TDMA

CDMA

1 2 3 4 5 6



Modulation & Access SchemesUsed in Wireless Telephony

IS-54 TDMANADC

Cellular(voice channels)

AMPs AnalogCellular

VoiceChannels

Control &Messaging

IS-95CDMA

GSM-900DCS-1800PCS-1900GSM-Based

RF ChannelWidth

ModulationType

AccessStrategy

C/I Req

FDMA

FM

30 kHz

Users perRF channel

17 db

1

FDMA FDMA +TDMA

TDMA +CDMA

FDMA+ TDMA

30 kHz 30 kHz

17 db 17 db

1* 3(future 6?)

8(future 16?)

<11820 practical

200 kHz 1.23 MHz

PSK:/4 DQPSK

FSK: GMSKFSK OQPSK (bs)QPSK (ms)

7-9 db 6 db

System

SpeechCodec

n/a(analog) n/a VSELP

8kbRELP-LTP

13kbVar. Rate8kb, 13kb

ESMRMIRS

FDMA +TDMA

M16QAM

25 kHz

18 db

6

8kb



Circuit Switching (交換機)• Space-division switching• Digital switch: bit stream, CCS

– Analog link: Phone-Switch– Digital link: Switch-Switch (improve quality using digital repeater)

• Trunks: lines connecting switches– DS-1,..., DS-4, SONET

• Data Comm. over telephone Network -> Modem



Switching Systems Circuit Switching: Telephone Network

–A path is established for a conversation Package Switch: Message transmission

–Message is fragmented into smaller chunks (package)–Package is routed along different paths, reassembled at the receiving end

–Rules for network operations -> Protocols



Basic Definitions and CommonTerminology

Wireless versus Mobile•Both terms tend to be used interchangeably,but are not the same•LMDS and microwave are wireless, but not mobile (Fixed wireless)•Cellular/PCS phones and satellites are wireless and mobile

Degrees of Mobility•Cordless phones have low mobility, Cellular have higher mobility

Cellular•The term “cellular”describes a wireless infrastructure,geographical|y

deployed as a collection of “cells", in a honeycomb structure•“Cellular”often implies the 850-900MHz•personal mobile service, as oppose to higher band services such as PCS.

More dearly referred to as “Cellular Band',



FDD and TDD

Duplexing: Subscriber is allowed to send simultaneousinformation to BS while receiving information from BS•Frequency division duplexing (FDD:頻率多工): two distinct bands of

frequencies are provided•Time division duplexing (TDD:時間多工):time instead of frequency is used

tp provided both a forward and reverse link eliminate the need forsearate forward and reverse frequency band–TDD is limited to cordless phone due to rigid timing for time slotting or

short portable access and is effective for fixed wireless access as allusers are stationary

ReverseChannel

ForwardChannel

Freq. Separation

FDD

Freq.

ReverseChannel

ForwardChannel

Time Separation

TDD

Time



PCS Network

BSC

BSC

MSC

IWF

Internet/PSPDN

PSTN

Host Server

PC

Facsimile

Host Server

Circuit Data

Packet Data

PAD

Host Server

Modem

Low Speed Data Service Network

PCS Network Evolution



Circuit Data/G3-Fax Service Network Protocols

MSPC

BSC MSC IWF

RLP Protocol

AI, TCP/IP, PPP Protocol

User Remote Modem Control Modem Com.

HDLC FR

DATA Call Signaling DATA Call Signaling DATA Call Signaling

RS-232

Applications

Mobile to Land, Land to Mobile Communications

Data Network Interworking



Packet data transmission

BSMCC-SIM

ADP

Packet Data Transmission

64kbps ～384kbps,( ～2Mbps)

ADP

ATM

PC

ATM-LAN Internet

E-mail

Internet phoneInternet conference

WWW



Lesson 1 Complete

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