02 - Physical Layer Concepts

Physical Layer ConceptsPhysical Layer Concepts

Physical Layer Concepts 2

TopicsTopics1. Multiple Access Approaches2. Code Correlation3. Spread Spectrum Multiple Access (SSMA) Using PN Codes4. Orthogonal Data Channelisation


Multiple Access ApproachesMultiple Access Approaches


From 2G to 3GFrom 2G to 3G

Circuit-Switched Voice

Circuit-Switched Data

Short Message Service (SMS)

2G 9.6, 14.4 kbps

Circuit-Switched Voice

Low-Speed Circuit-Switched Data

Packet Data

High-Speed Circuit-Switched Data

3G up to 2 Mbps ?


Multiple Access ApproachesMultiple Access ApproachesFrequency Division Multiple Access

Each User has a unique frequency

(1 voice channel per user)

All users transmit at the same time

AMPS, NMT, TACS

Each Transmitter has a unique spreading code

Each Data Channel has a uniqueorthogonal code

Many users share the same frequency and time

IS-95, cdma2000, WCDMA

CodeDivision Multiple Access

SpreadSpectrumMultipleAccess

Multiple Transmittersand Data Channels

Each User has a unique time slot

Each Data Channel has a uniqueposition within the time slot

Several users share the same frequency

IS-136, GSM, PDC

Time Division Multiple Access

TIME

FREQUENCY

POWER

TIME

FREQUENCY

POWER

TIME

FREQUENCY

POWER


Channel CapacityChannel Capacity• In 1948, Claude Shannon of Bell Laboratories

proved the following remarkable formula:

• Capacity measured in bits/second− one bit = yes/no, or on/off, or 0/1

• Bandwidth is in hertz• Signal power and noise power measured

in same units, e.g. watts• Shannon’s result is the best that can ever be achieved—it’s the job

of engineers to design systems which approach this capacity

⎟⎟⎠

⎞⎜⎜⎝

⎛+×=

power noisepower signal1logbandwidthcapacity 2

Claude Shannon (1916—2001)


CCode Correlationode Correlation


OrthogonalityOrthogonality and Correlation of Signalsand Correlation of Signals• In communication receivers,

need to quantify the degree to which discrete waveforms or digital symbols 'differ' from one another.

• This can be accomplished by correlation operation

• Two signals s1(t) and s2(t) are said to be orthogonal or in the interval t1 to t2 if:

( ) ( ) 0 2

1

21 =⋅∫ dttstst

t

s (t)1

t0

1 2

1

Signallinginterval, T

t0

1 2

s (t)2

1

-1

-1

( ) ( )dttstsSt

txy 2

1

21∫ ⋅=


Optimum ReceiversOptimum Receivers• Optimum receivers minimize the probability of making a decision

error by averaging out the noise contribution in the channel− need to make a couple of assumptions, most importantly, channel

noise is additive white Gaussian noise (AWGN)• Optimum receiver must compare received signal with the two known

symbols s0(t) and s1(t)• Optimum detection is therefore a two-stage process:

− a correlator produces a number which indicates how close the received signal r(t) is to s0(t) and s1(t)

− a detector decides which symbol is more likely

×

)(tsn

( )∫T

dt0( )∫

T

dt0

)()( tntr + nr

Sampleat t = T

Correlator hTThreshold

ka


Correlation ValuesCorrelation Values

( ) ( ) 1 1

0 21 +=⋅∫ dttsts

s (t)1

t0

+1

SignallingInterval, T

t0

s (t)2

+1

-1

-1

0.5 1

0.5 1

( ) ( ) 1 1

0 21 −=⋅∫ dttsts ( ) ( ) 0 1

0 21 =⋅∫ dttsts

s (t)1

t0

+1


t0

0.5 1

s (t)2

+1

-1

-1

0.5 1

s (t)1

t0

+1


t0

s (t)2

+1

-1

-10.5 1

0.5 1

Similar Signals Dissimilar Signals Orthogonal Signals


DS Spread Spectrum SystemDS Spread Spectrum System

t

t

t

t

t

m (t)

m (t)p (t)

p (t)

p (t)

x (t)

T c

Tb

u (t)

LPF

Coherentdetection

2cosω t c

Correlator

p(t)

Recovereddata

signalDatasignal m(t)

Rc = 1Tc

Rb = 1Tb

p(t)s(t)

ModulatorMultiplierChannel

r(t) u(t) x(t)dt

TbT

b

)(10∫ v(t)

Codegenerator

Carriercosω t c

Carrierrecovery

Codegenerator

Codesync


Code CorrelationCode CorrelationCase I: Autocorrelation using a PN Code

Receiver and Transmitter use identical code at same time offset

Input Data +1 -1 +1

+1 -1 +1Divide byCode Length

+1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1PN code used

in Transmitter

x x x

+8 -8 +8

IntegrateResult

Integrate Integrate Integrate

+1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1TransmittedSequence

= = =

+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 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1PN Code usedin Receiver

x x x

Transmitter

Receiver

(a) (b) (c)


Code CorrelationCode CorrelationCase II: Cross-Correlation using PN CodesReceiver and Transmitter use different codes

Input Data +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 +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 -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

PN code usedin Transmitter

TransmittedSequence

PN Code usedin Receiver

-4 0 2

IntegrateResult

-0.5 0 0.25Divide by

Code Length

x x x


= = =

x x x

= = =

Transmitter

Receiver

(a) (b) (c)


Code CorrelationCode CorrelationCase III: Correlation using Orthogonal Codes

(a) Same Orthogonal code; (b) Different Orthogonal codes; (c) Same code with non-zero time offset

Input Data +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 +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 +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

Orthogonal codein Transmitter

TransmittedSequence

Orthogonal Codeused in Receiver

8 0 -4

IntegrateResult

+1 0 -0.5Divide byCode Length

x x x


= = =

x x x

= = =

Transmitter

Receiver

(a) (b) (c)


Code Correlation: Key PointsCode Correlation: Key Points• TX, RX use same codes, at the same time offset

− PN Codes: 100% correlation− Orthogonal Codes: 100% correlation

• TX, RX use different codes− PN Codes: “Low” (noise-like) correlation at any time

offset Average correlation level proportional to 1/(code length)

− Orthogonal Codes: 0% Correlation

• TX, RX use same codes, but at different time offsets− PN Codes: “Low” (noise-like) correlation for any

offset > +1 chip− Orthogonal Codes: Unpredictable results


Cellular CDMACellular CDMA

“Cellular CDMA” = SSMA + Orthogonal Coding(IS-95, cdma2000, WCDMA)

Spread Spectrum Multiple Access (SSMA):Allows multiple CDMA transmitters to share the same Radio Frequency

Orthogonal Coding:Allows multiple data channels to co-exist within each CDMA transmission


Spread Spectrum Multiple AccessSpread Spectrum Multiple Access(SSMA) Using PN Codes(SSMA) Using PN Codes


Spread Spectrum Multiple AccessSpread Spectrum Multiple Access

RFDemod

PN 3Receiver

In this example, the receiver correlates the composite received signal using PN code 3.

The result is the recovered transmission from Transmitter #3, plus some spread spectrum interference from transmitters #1, #2, and #4

PN 1

RFModulation

Transmitter 1

PN 3

RFModulation

Transmitter 3

PN 4

RFModulation

Transmitter 4

PN 2

RFModulation

Transmitter 2


Why is it called Why is it called ““Spread SpectrumSpread Spectrum””??

• Direct-Sequence Spread Spectrum Transmitter

PN Code Generator

Chip ClockFc >> Fd

RF Modulator

cos(ωrf*t)

Nulls @ N*Rc Frf

Filter

PN Code Mask

“Bits”

“Chips”

0 0.1 0.2 0.3 0.4 0.5 0.6-50

-40

-30

-20

-10

0

10

Frequency

Pow

er S

pect

rum

Mag

nitu

de (d

B)

0 0.1 0.2 0.3 0.4 0.5 0.6-60

-50

-40

-30

-20

-10

0

10

Frequency

Pow

er S

pect

rum

Mag

nitu

de (d

B)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

x 107

-40

-20

0

20

40

60

80

Frequency

Pow

er S

pect

rum

Mag

nitu

de (d

B)


Spread Spectrum Multiple AccessSpread Spectrum Multiple Access

⎟⎠⎞

⎜⎝⎛=

Rate DataRate Code PN

Both signals combinedin the air interface

PN Code 1Frequency

Am

plit

ude

Signal 1

PN Code 2Frequency

Am

plit

ude

Signal 2

Spread SpectrumSpreading Factor

PN Code 1 Signal 1 is reconstructedSignal 2 looks like noise

Both signals arereceived together

AT THE RECEIVER...

Two Transmitters at the same frequency


SSMA PN Code PlanningSSMA PN Code Planning

Uplink: PN Code used to distinguish each Mobile StationDownlink: PN Code used to distinguish each Base Station

PN3 PN4

PN1 PN1

Cell Site “1” transmits using PN code 1

PN5 PN6

PN2 PN2

Cell Site “2” transmits using PN code 2


SSMA PN Code PlanningSSMA PN Code Planning

Spread Spectrum Code Planning ExampleN

S

W

PN1

PN2

PN3PN7

PN6 PN4

PN5

PN7

PN6 PN4

PN5

PN1

PN2

PN3

PN1

PN2

PN3PN7

PN6 PN4

PN5

PN1

PN2

PN3PN7

PN6 PN4

PN5

PN1

PN2

PN3PN7

PN6 PN4

PN5 PN1

PN2

PN3PN7

PN6 PN4

PN5

E


Tx 1

Tx 2

Tx 3

...

SSMA: SummarySSMA: Summary

Each Transmitterhas a unique

PN spreading code

Several Transmitters share the same frequency

and time

Frequency


• SSMA Utilization− Used to distinguish transmission source

(Base Station or Mobile Station) in cellular CDMA systems

Provides good (but not 100%) separation between multiple transmissions in same geographic area, on same frequency

− Works regardless of time-of-arrival delays− Code Planning instead of Frequency

Planning− Frequency Reuse = 1

• SSMA Limitations− Imperfect signal separation− Number of simultaneous transmitters in one

area is limited by the Spreading Factor• Not good for transmitting multiple data streams

from one transmitter


Orthogonal Data Orthogonal Data ChannelisationChannelisation


Orthogonal Data ChannelizationOrthogonal Data Channelization

OC 4

OC 3

OC 2

OC 1

RFModulation

RFDemod

OC 3

Data Channel 1

Data Channel 2

Data Channel 3

Data Channel 4

Receiver

In this example, receiver correlates composite received signal using Orthogonal Code 3.

Result is a perfect reconstruction of Data Channel #3, with no interference from other data channels.

To realize this perfect cross-correlation property, it is essential that orthogonal codes be received in perfect timing relation to each other.

Linear Addition

Transmitter

PN 1 PN 1


Orthogonal CodesOrthogonal Codes

Uplink: Orthogonal Codes used to distinguish data channelscoming from each Mobile Station

Downlink: Orthogonal Codes used to distinguish data channelsComing from each Base Station

OC1, OC2OC3, OC4

OC5, OC6, OC7

OC1 , OC2, OC3OC1, OC2

OC1, OC2, OC3, OC4


Orthogonal CDMA: SummaryOrthogonal CDMA: Summary

• CDMA allows multiple data streams to be sent on the same RF carrier

Perfect isolation between data streamsTiming between data streams must be exactMaximum number of data channels = orthogonal code length

The longer the code, the slower the data rate

• Code space can be rapidly re-allocated to match user data rate requirements

• CDMA advantages are limited in practiceMultipath, small timing errors, and motion-related effects diminish the usable code space

Data 1

Data 2

Data 3

...

Each Data Stream has a unique

Orthogonal spreading code

Many users share the same frequency and time

IS-95, cdma2000, WCDMA

Frequency



Cellular CDMA (SSMA + OC)Cellular CDMA (SSMA + OC)

• Cellular CDMA (IS-95, cdma2000, WCDMA)

PN Codes are used:To distinguish between Mobile StationsTo distinguish between Base Stations

Orthogonal Codes are used:To distinguish between data channels coming from each MSTo distinguish between data channels from each BS

User 1

User 2

User 3

...

PN Spreading Codes

and

Orthogonal Codes

are simultaneously utilized

Frequency




Cellular CDMA (SSMA + OC)Cellular CDMA (SSMA + OC)

2 data channels(voice, control)

PN3 + OC1 + OC2

2 data channels(14 kbps data, control)

PN4 + OC1 + OC2

2 data channels(voice, control)

PN1 + OC1 + OC2

1 data channels(control)

PN1 + OC3Voice

Conversation Uplink Packet Data

Pilot, BroadcastPN1 + OCP + OCB

3 data channels(voice, video, control)

PN2 + OC1 + OC2 + OC3

3 data channels(voice, video, control)

PN5 + OC1 + OC2 + OC34 data channels

(384 kbps data, voice, video, control)PN6 + OC1 + OC2 + OC3 + OC4

4 data channels(384 kbps data, voice, video, control)

PN2 + OC4 + OC5 + OC6 + OC7Video

conference

Videoconference with Data

Pilot, BroadcastPN2 + OCP + OCB


Cellular CDMA Code LayeringCellular CDMA Code Layering

Function IS-95A/B cdma2000(1x, RC 3~9)

WCDMA

DataChannelization

Orthogonal64 chip

OVSF4 ~ 256 chip

OVSF4 ~ 512 Chip

BTS Separation “Short PN Code”215 chip

Same as IS-95

38,400 chipsof 2 18 Gold codeDownlink

Data Encryption

“Long PN Code”(242 –1) chip

Same as IS-95

None*

DataChannelization

None(only one data channel at a time)

OVSF4 ~ 256 chip

OVSF4 ~ 256 Chip

MSSeparation

“Long PN Code”(242 –1) chip

Same as IS-95

38,400 chipsof 225 Gold codeUplink

DataScrambling

Due to MS Separation Code

Same as IS-95

None*

*WCDMA implements encryption at higher layers


OC-N

OC-2

OC-1

RFModulation

RFDemod

OC-2

Data Channel 1

Data Channel 2

Data Channel 3

Receiver1

Σ

Transmitter 1

Codes in WCDMA

SC-2

RFModulation

Transmitter 2

SC-M

RFModulation

Transmitter M

SC-1..

..

SC-1

Scrambling Code (SC)Used to distinguish transmission source (Base Station or Mobile Station)

Orthogonal Code (OC)allows multiple data streams to be sent on the same RF carrier


Practical Issues with Codes• UEs near to cell edge experiences higher interference due to

− DL channels from neighbour cells (iother)− Degraded orthorgonality of own cell’s channels (iown)

• When there is little multipath with a cell, there is little mutual interference between DL channels

• Antenna downtilt is used to manage interference from other cells

UE3

PN 1

PN 2

OC 1OC 2

UE2

UE1

CPICH+CCH


SummarySummary1. Multiple Access Approaches

− FDMA, TDMA, CDMA (SSMA)− Channel Capacity

2. Code Correlation− Correlation and Orthogonality− DS Spread Spectrum System− Code Correlation using PN Codes and Orthogonal Codes

3. Spread Spectrum Multiple Access (SSMA) Using PN Codes4. Orthogonal Data Channelisation

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02 - Physical Layer Concepts