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Overview of Digital Mobile Overview of Digital Mobile CommunicationsCommunications

Dong In Kim(dikim@ece.skku.ac.kr)

Wireless Communications Lab

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OutlineOutline

Digital CommunicationsDigital Communications

Multiple Access TechniquesMultiple Access Techniques

Power Control for CDMAPower Control for CDMA

IMTIMT--2000 System2000 System

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Why Digital? (1/3)Why Digital? (1/3)AdvantagesEasy to regenerate original function

Distance 1Distance 1OriginalOriginalpulse signalpulse signal

Distance 5Distance 5Amplification Amplification to regenerate pulseto regenerate pulse

Distance 2Distance 2Some signal Some signal distortiondistortion

Distance 3Distance 3DegradedDegradedsignalsignal

Distance 4Distance 4Signal is badlySignal is badlydegradeddegraded

1 2 3 4 5Propagation distance

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Why Digital? (2/3)Why Digital? (2/3)

Less subject to distortion & other interference

High signal fidelityHigh signal fidelity can be obtained

Digital circuits are more reliablereliable, lower costcost,

and more flexibleflexible to implementation in H/W

Protection against interference & jamming

Easy storageEasy storage & processingprocessing of data

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Why Digital? (3/3)Why Digital? (3/3)

Disadvantages

More technical complexitycomplexity

Require additional stepsadditional steps

Sampling

AD conversion

Require aa greater bandwidthgreater bandwidth

Require complicatedcomplicated synchronizationsynchronization issues

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Block Diagram of a Typical Digital Block Diagram of a Typical Digital Communication SystemCommunication System

Channel

InformationSource

InformationSink

ChannelBits

Synchron-ization

Digital Waveform

BitStream

FormatSourceencode Encrypt.

Channelencode

Multiplex ModulateMultipleAccess

FrequencySpread

XMT

FormatSourcedecode Decrypt. Channel

dncodeDemulti-

plexDemod-

lateMultipleAccess

FrequencyDepread

RCV

Digitalinput

Digitaloutput

SourceBits

ChannelBits

ChannelBits

SourceBits

ChannelBit

im∧

im

)t(s i

∧

)t(s i

EssentialOptional

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System ComponentsSystem ComponentsSystem components

Codec (Coder/Decoder)Modem (Modulator/Demodulator)RF (Transmitter/Receiver)

Data statusInformation source: digital, analog, textual informationBit stream

Digital waveform block diagramReciprocity between blocks in upper and lower branchesUpper block: transmitter (frequency up conversion stage, high power amp., antenna)Lower block: receiver (antenna, LNA, down-converter stage)

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Data StatusData StatusBit stream

Bandpass digital waveform

time

T T T

0 0 0 1 0 0 1 1 1 1 1 0 0 1 1 1 1 0 1 0 1

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Signal Flow in Digital CommunicationsSignal Flow in Digital Communications

FormattingFormatting: transforms the source information into digital symbolsSource codingSource coding: removes redundant or unneeded informationEncryptionEncryption: prevents unauthorized users fromunderstanding messages and from injecting false messages into the systemChannel codingChannel coding: reducing the probability of error, or SNR requirement, at the expense of bandwidth or decoder complexity

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CDMA Communication FlowCDMA Communication FlowConverting analog voice to digital

CODEC VOCODER 10011

Pulse CodedModulation

(PCM)

Vocodedvoice

Pulse Code Modulation (PCM) is a technique to convert analog voice to a digital representation of the voiceconvert analog voice to a digital representation of the voice. The Vocoder (Voice Coder) is used to compress the digital representations that came from the Codec (Code/Decoder). In other words, in compression, redundant digits are eliminated.Voice quality is not adversely affected.

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Example of FormattingExample of FormattingQuantization Level

CodeNumber X(t)(V)

Formatting Analog Information(Natural sample, quantized sample, pulse code modulation)

X(t)

t

001011100110111101134675

-2.5-0.50.52.53.51.5-2.4-0.80.523.31.3Natural sample value

PCM number

Code number

Quantized sample value-3.4-3.5

0

000

-2.5

3.5

0.5

-1.5

2.51.5

-0.5

-3.51

7

4

2

65

3

0-2

4

1

-1

32

0

-3-4

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Source coding, Encryption, Channel codingSource coding, Encryption, Channel coding

Three levels of codingThree levels of coding

TrashCan

Source Coding Source Coding (Data Compression)(Data Compression)

Encryption Encryption (Cryptography(Cryptography))

Channel Coding Channel Coding (Error Correction)(Error Correction)

ReducesReducesparcelparcelcostcost

PreventsPreventsthievesthieves

ProtectsProtectsagainst roughagainst roughhandinghanding

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Channel Encoding Example (1/2)Channel Encoding Example (1/2)

Actualmessage

Encodedtransmitted

message

Receivedmessage

Decodedmessage

D,D,DC,C,CB,B,BA,A,A

D,D,D?,?,CB,?,?A,A,A

DCBA

D??A

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Channel Encoding Example (2/2)Channel Encoding Example (2/2)A simple coding scheme is shown in this example. The actual

digit message consists of four letters (A,B,C,D). Each of the bits

are then encoded into 3 symbols to represent one bit.

The encoded transmitted message would be as follows:

AA,, AA, , AA, , BB,, BB, , BB, , CC, , CC, , CC, , DD,, DD, , DD..

Note that errors as “?” in this example. The received message is:

AA, , AA, , AA, , BB, ?, ?, ?, ?,, ?, ?, ?, ?, CC,, DD,, DD, , DD..

The decoder uses majority logic rule majority logic rule (two out of three wins).

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Interleaving Example (1/2)Interleaving Example (1/2)

A B C D Bit pattern

A A A B B B C C C D D D Encoded symbols

A B C D A B C D A B C D Interleaved symbolsInterleaved symbols

A B C ? ? ? ? D A B C D Burst errors

A ? A B ? B C ? C D ? D DeinterleavedDeinterleaved symbolssymbols

A B C D Decoded bits

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Interleaving Example (2/2)Interleaving Example (2/2)Interleaving is a simple, but powerful, method of reducing reducing errorserrors and recovering bitsrecovering bits when errors occur. In this example, the first 3 symbols for the 1st code (1,1,1) in the actual message are transmitted at locations 1, 5, and 9. The 3 symbols for the 3rd code (1,1,1) are transmitted at locations 3, 7, and 11.

Using the same error pattern as used in the previous example the received message is: 1?1,0?0,1?1,0?0.

What is the decoded message? Note that with the same error pattern as before, the original message is completely recovered using majority logic decodingmajority logic decoding.

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Modulation (1/3)Modulation (1/3)ModulationModulation: process by which the symbols areconverted to waveforms that are compatible with the transmission channel

PSKPSK (Phase Shift Keying): the carrier phasephase varies

with corresponding digital symbol.

FSKFSK (Frequency Shift Keying): the carrier frequencyfrequency

varies with corresponding digital symbol.

ASKASK (Amplitude Shift Keying): the carrier amplitudeamplitude

varies with corresponding digital symbol.

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Modulation (2/3)Modulation (2/3)

Note

ModulationModulation: translate a baseband message signal

to a bandpass signal at high frequencies

BasebandBaseband signalsignal: information-bearing signal

CarrierCarrier: high frequency signal

Modulated waveModulated wave: bandpass signal

Cellular systemCellular system: 800MHz, 1.7 ~1.8GHz (PCS)

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Modulation (3/3)Modulation (3/3)PSKPSK

FSKFSK

ASKASK

T TT

time

T TT

time

T TT

time

1

Methods to Improve Methods to Improve Frequency Efficiency (1/2)Frequency Efficiency (1/2)

Frequency reuse

Multiple access method

Multiple access technique permits the communication resources of the channel to be shared by a largeshared by a largenumber of usersnumber of users seeking to communicate with each other.

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Methods to Improve Methods to Improve Frequency Efficiency (2/2)Frequency Efficiency (2/2)

FDMAFDMA: disjoint subbands of frequencies are allocated to the different users on a continuous-time basis.

TDMATDMA: each user is allocated the full spectral occupancy of the channel, but only for a short duration of time called a time slot.

CDMACDMA: an application of spread-spectrum techniques (SS). using orthogonal codes permits multiple users to access the same frequency band at the same time.

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FDMAFDMAFFrequency DDivision MMultiple AAccess

ch2 ☆☆☆☆☆☆☆☆ ☆☆☆☆☆☆☆

Multiple Access

B

C ch3

ch1 ☆☆☆☆A

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FDMA (cont.)FDMA (cont.)

GOGO

HIHI

II HH

Frequency channel 1

Frequency channel 2

OO GG

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TDMATDMA

TTime DDivision MMultiple AAccess

Multiple Access

Time slot Time Gate

☆☆

B

C

☆☆A

☆

☆

☆

☆

ch3 ch2 ch1

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TDMA (cont.)TDMA (cont.)

Time

slot4

Time

slot3

Time

slot2

Time

slot1

GOGO

HIHI

II OO HH GG

HIHI

GOGO

Common Frequency ChannelCommon Frequency Channel

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CDMACDMACCode DDivision MMultiple AAccess

A ☆☆☆ 1010 1010 ☆☆☆ AB 1100 1100 B

X 1001 1001 X

11001100

11001100

11001100

10011001

10011001

10101010☆

10101010☆

Multiple Access / Spread Spectrum

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CDMA (cont.)CDMA (cont.)

Common ChannelCommon Channel

Red codeRed code

Green codeGreen code

HIHI

GOGO

HIHI

GOGO

OOGG

HHII

Green codeGreen code

Red codeRed code

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Codes for CDMACodes for CDMAOrthogonal Code (Walsh Code)Orthogonal Code (Walsh Code)

Generated by Hadamard MatrixHadamard MatrixIts important property is “orthogonal” among codes

PN CodePN CodeGenerated by hardware such as shift registershift registerIt is not perfectly orthogonalPseudo random noise codeShort code: length Long code: length

768,32215 ==

Billion44002 42 ==

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Orthogonal Code (Walsh Code)Orthogonal Code (Walsh Code)

Orthogonal functions have zero correlation !Orthogonal functions have zero correlation !

Two binary sequences are orthogonal if the process

of “XORingXORing” them results in an equal number of

1’s and 0’s.

Example:0 0 0 00 1 0 10 1 0 1

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Generation of Orthogonal CodeGeneration of Orthogonal CodeRepeat

Right ComponentBelow Component

InvertDiagonal Component

Procedure:0 0 0 00 1 0 10 0 1 10 1 1 0

0 00 1

0Seed

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Multiple access using SS technique (1/3)Multiple access using SS technique (1/3)( )tm1 ( )tc11 -1 1

bT b2T b3T

( ) ( )tctm 11

( )tm2

bT b2T b3T

-1

c2T c4T c12Tc8T( )tc21 1

( ) ( )tctm 22

c2T c4T c8T c12T

mi(t): Baseband messages

cj(t): Orthogonal codes

mi(t)cj(t): Spread messages

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( ) ( ) ( ) ( )tctmtctmA 2211 +=

( ) ( )tcAtB 11 ×=

( ) ( )tcAtB 22 ×=

bT b2T b3T

bT b2T b3T

bT b2T b3T

The waveforms for the signals at different points of the receiveThe waveforms for the signals at different points of the receiverr

--2 0 2 0 0 2 0 2 0 2 0 0 2 0 ––2 0 2 0 ––2 0 22 0 2

2 0 2 0 0 2 0 2 0 0 --2 0 2 0 ––2 0 2 0 22 0 2 0 2

2 0 2 0 0 2 0 2 0 2 0 2 0 0 2 0 2 0 --2 0 2 0 --22

Multiple access using SS technique (2/3)Multiple access using SS technique (2/3)

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( )tm 1~

2c

( )tm 2~

cT bT b2T b3T

cT bT b2T b3T

cT bT b2T b3T

cT bT b2T b3T

1cMultiple access using SS technique (3/3)Multiple access using SS technique (3/3)

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Basic Principle of CDMABasic Principle of CDMA0110100110010110011001101001100101100110

Base Mobile

User input User output

10011 10011

1001100110011001100110011001100110011001 1001100110011001100110011001100110011001Code Code

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CDMA techniqueCDMA technique(Direct(Direct--Sequence Spreading)Sequence Spreading)

1 0 0 1 1User Input

Spreading Sequence

1001 1001 10011001 1001 1001 10011001 10011001

0110 1001 10010110 1001 1001 01100110 01100110Tx Data

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CDMA techniqueCDMA technique(Decoding using Correct User Code)(Decoding using Correct User Code)

0110 1001 1001 01100110 1001 1001 0110 01100110Rx Data

Correct Code

1001 1001 10011001 1001 1001 10011001 10011001

1111 0000 0000 1111 1111

1 0 0 1 1User Output

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CDMA techniqueCDMA technique(Decoding with Incorrect User Code)(Decoding with Incorrect User Code)

0110 1001 1001 01100110 1001 1001 0110 01100110Rx Data

Incorrect Code

01010101 0101 0101 0101 0101 0101 0101 0101 0101

0011 1100 1100 0011 0011

? ? ? ? ?User Output

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CDMA Channel GenerationCDMA Channel Generation

Background

Noise

External

Interference

Other Cell

Interference (Ioc)

Other User

Noise(isc)

10KHz1.25MHz

f

1.25MHz

0

10KHz

f f

= -1.69dB/Hz

1.25MHz

Wideband

Spectrum

Data

DataData

(9.6Kbps)

Encoding &

Interleaving

PN

Source

1.25MHz 1.25MHz

BPF

1.25MHz

Digital

Filter

PN

Source

Deinterleave

& Decode

1.2288

McpsCarrier

Carrier

1.2288

Mcps

Correlator

f0

f f

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Receive Sensitivity in Free SpaceReceive Sensitivity in Free Space

Base station Mobile station

R (distance)

Receive sensitivity is in inverse proportion to Receive sensitivity is in inverse proportion to the square of distance in free space.the square of distance in free space.

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Receive Sensitivity in Cellular RadioReceive Sensitivity in Cellular Radio

Base station Mobile station

Multipath

Reflector

R (distance)

Receive sensitivity is in inverse proportion to Receive sensitivity is in inverse proportion to the third or fifth power of distance.the third or fifth power of distance.

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Receive SensitivityReceive SensitivityAverage path loss as a function of distanceAverage path loss as a function of distance

: reference distance

Free space20dB/decade pass loss

Cellular radio30~50dB/decade pass loss

( )n

ddd Pathloss ⎟⎟⎠

⎞⎜⎜⎝

⎛∝

0

0d

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NearNear––Far ProblemFar Problem

A B Signal B after despreading

Impossible for the receiver to detect signal B

A

B

A

~~ ~~

~~~~

BNearNear--far problemfar problem

Received power level at base

When the difference in distance from base station to mobile A and B is ten times, their received powers differ by the third or fifth power of 10 (30-50dB). Then, it is hard to detect the signal from mobile B.

100M 1000M0

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Need for Power ControlNeed for Power ControlTo avoid “nearnear--farfar” problemObservationsObservations:

Need to control the transmit power level to meetthe minimum required SIRminimum required SIR.Save the powerSave the power at the mobile station.

If Tx/Rx power level is too low, it will increase BER.If the power level is too high, undesired interferencewill degrade the BER of other mobiles.

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Forward Power ControlForward Power Control

OK!!OK!!

BaseStation

What? I can’t hear you

What? I can’t hear you

with forward with forward power controlpower control

Base station transmission power

AB C

DC

B

B

DA

C

D

A

without forward without forward power controlpower controlhigh BER in Chigh BER in C

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Reverse Power ControlReverse Power Control

Please raise your powerPlease raise your power

without reverse without reverse power controlpower control

with reverse with reverse power controlpower control

Base station received power

A B C

D AA

D

D

BC

B

C

BaseStation