Issues in Wireless Physical Layer

Issues in Wireless Physical Layer

A. ChockalingamAssistant Professor

Indian Institute of Science, Bangalore-12

[email protected]://ece.iisc.ernet.in/~achockal

Dr. A. Chockalingam Dept of ECE, IISc, Bangalore 2

Outline

RF Spectrum IssuesRF Spectrum Issues Wireless Channel CharacteristicsWireless Channel Characteristics Combating Fading Combating Fading

– Diversity TechniquesDiversity Techniques– Transmit DiversityTransmit Diversity

Multiple AccessMultiple Access Power ControlPower Control Co-channel InterferenceCo-channel Interference Ultra Wideband TechniquesUltra Wideband Techniques


Radio Frequency Spectrum Radio Frequency Spectrum Communication through electromagnetic

wave propagation Frequency Spectrum

– Certain ranges of frequency Only certain frequency spectra are usable

– Limitations of atmospheric propagation effectsLimitations of atmospheric propagation effects– Technology/Device limitationsTechnology/Device limitations– Regulatory issuesRegulatory issues– Safety hazardsSafety hazards

Demand for spectrum far exceeds supply– Efficient use of RF spectrum is important


RF Spectrum - RF Spectrum - Some Current systemsSome Current systems 900 MHz Cellular Band900 MHz Cellular Band

– GSM: 890 - 915 MHz Uplink; 935 - 960 MHz Downlink– IS-54: 824 - 849 MHz Uplink; 869 - 894 MHz Downlink– PDC: 810 - 820 MHz and 1429 - 1453 MHz Uplink 940 - 960 MHz and 1477 - 1501 MHz Uplink– IS-95: 824 - 844 MHz Uplink; 869 - 889 MHz Downlink

1800 MHz PCS Band1800 MHz PCS Band– 1850 - 1910 MHz Uplink; 1930 - 1960 MHz Downlink– DECT: 1880 - 1900 MHz

C, Ku, L and S-Bands for SATCOMC, Ku, L and S-Bands for SATCOM– C-band: 5.9 - 6.2 GHz Uplink; 3.7- 4.2 GHz: Downlink– Ku-band: 14 GHz Uplink; 12 GHz Downlink – L-band: 1.61 - 1.6265 GHz; S-band: 2.4835 - 2.5 GHz


Unlicensed Radio Spectrum

26 MHz 26 MHz 83.5 MHz 83.5 MHz 200 MHz 200 MHz

902 902 MHz MHz

928 928 MHz MHz

2.4 2.4 GHz GHz

2.4835 2.4835 GHz GHz

5.15 5.15 GHz GHz

5.35 5.35 MHz MHz

• Wireless LANsWireless LANs• Cordless phonesCordless phones

• 802.11b 802.11b • BluetoothBluetooth• Microwave OvenMicrowave Oven

• 802.11a802.11a

CarrierCarrierwavelength:wavelength: 33 cm 33 cm 12 cm 12 cm 5 cm 5 cm


RF Spectrum RF Spectrum

Some forward looking developments

– 300 MHz BW in the 5 GHz band made available to stimulate Wireless LAN technologies and use

– Ultra wideband (UBW) technology

– 60 GHz band for high-speed, short-range communications


Physical Layer TasksPhysical Layer Tasks

Wireless systems need to overcome one or more of the following distortions: – AWGN (receiver thermal noise)– Receiver carrier frequency and phase offset– Receiver timing offset– Delay spread– Fading (without or with LOS component) – Co-channel and adjacent interference (CCI, ACI)– Nonlinear distortion, intermodulation, impulse

noise


Motivation for PHY Layer AdvancesMotivation for PHY Layer Advances

Increase channel capacity (spectral efficiency) - higher average bit rate

Increase Erlang Capacity - more users per square area

Increase reliability Reduce Tx power Increase range Increase coverage


Transmit Diversity

SectorisationSectorisation

Interference Interference SuppressionSuppression

Frequency Frequency HoppingHopping

Multi-user Detection

Power ControlPower Control

Voice Activity Voice Activity DetectionDetection

Receive DiversityReceive Diversity

Fixed BeamformingFixed Beamforming

LinkLinkAdaptationAdaptation

Range Range (Power Efficiency)(Power Efficiency)

Spectral Spectral EfficiencyEfficiency

Dynamic ChannelDynamic ChannelSelectionSelection

Space-Time CodingSpace-Time Coding

Turbo CodingTurbo Coding

Transmit DiversityTransmit Diversity

Spatial MultiplexingSpatial Multiplexing

Smart Beam-forming

Variable Bit-RateVariable Bit-Rate

DS-CDMA

OFDMOFDM

Erlang Erlang CapacityCapacity

PHY Layer Advances PHY Layer Advances


Wireless Channel CharacteristicsWireless Channel Characteristics

Free-space Transmission

( )( ) TxTx RxRx

dRGTG

TP RP

24

d

GGPP RTTR


Mobile Radio ChannelMobile Radio Channel

Characterized by

– Free space (distance) loss

– Long-term fading (shadowing)

– Short-term fading (multipath fading)


Mobile Radio ChannelMobile Radio Channel

Distance, d Distance, d

ReceivedReceivedPower Power

Distance Loss Distance Loss Long TermLong TermFading Fading

10 - 100 m (1 - 10 secs)

Short TermShort TermFading Fading

0.1 - 1 m (10 - 100 msecs)


Distance LossDistance Loss In line-of-sight AWGN channels In line-of-sight AWGN channels (AWGN: Additive White (AWGN: Additive White

Gaussian Noise)Gaussian Noise) – distance loss ,distance loss , : distance between Tx and Rx: distance between Tx and Rx

– loss exponent is 2loss exponent is 2 (i.e., 20 dB/decade loss)(i.e., 20 dB/decade loss) In urban mobile radio channelsIn urban mobile radio channels

– loss exponent varies between 2.5 to 5.5loss exponent varies between 2.5 to 5.5– 40 dB/decade loss (typ)40 dB/decade loss (typ) Rx Signal powerRx Signal power

(Based on field measurements)(Based on field measurements) Slowly varying compared Slowly varying compared to carrier wavelengthto carrier wavelength Fwd & Rev links impactedFwd & Rev links impacted in the same wayin the same way

2d d

d10 m10 m

40 dB/decade40 dB/decade

1 km1 km

40 dB40 dB

40 dB40 dB

100 m100 m


Shadowing

Signals are blocked by obstacles (e.g., bridges buildings, trees, etc)

Shadow loss variation - typ log-normally distributed (Std Dev of distribution: 4 to 12 dB)

Slowly varying compared to carrier wavelength Fwd & Rev links impacted in the same waybri


Multipath PropagationMultipath Propagation

Path nPath n

Base Base StationStation

MobileMobile

Path 1Path 1

Path 2Path 2

n

iii tstr

1

)()(

Channel

t

)(th

1 23

n

12

3

n

Tx. signal Rx. signal

ImpulseResponse

f

FrequencyResponse

)( fH


Multipath (Short term) FadingMultipath (Short term) Fading Time-varying impulse response

Fluctuations in received signal amplitude (typically Rayleigh distributed)

Time spread Doppler Spread Fade variations are fast

Rev link fading independent of Fwd link fading

Fwd link fade

Rev link fade

SignalStrength

time

1

)(2 ))(()();(i

itfj

i ttetth ic


Key Multipath Parameters Key Multipath Parameters

Delay / Frequency CharacterizationDelay / Frequency Characterization– Delay spread, Delay spread, – Coherence BW, Coherence BW,

Time variationsTime variations– Coherence time, Coherence time, – Doppler BW, Doppler BW,

mT

cB

cT

dB


Delay Spread / Coherence BWDelay Spread / Coherence BW Autocorrelation function ofAutocorrelation function of

If we let , If we let , gives the averagegives the average power output of the channel as a function of power output of the channel as a function of

tththEtc ;();();,( 2121 );( th

0 t )0;(c

)(ts

t t

)(tr

)(c

Autocorrelation

mT:mT Max. Delay Spread

f

)( fc

mc T

B 1

FT

:CB Coherence Bandwidth )(c)( fc

FT Pair


Delay / Frequency Characterization Delay / Frequency Characterization Delay Spread Delay Spread

– range of differential delay between different pathsrange of differential delay between different paths– jitter in Rx time of the signal, long echoesjitter in Rx time of the signal, long echoes– results in results in Inter-Symbol Interference (ISI). Inter-Symbol Interference (ISI). – Need equalization to combat ISINeed equalization to combat ISI (in unspread systems)(in unspread systems)– Provides “time Diversity” in spread systems (RAKE Provides “time Diversity” in spread systems (RAKE

Combining in CDMA)Combining in CDMA) Coherence BWCoherence BW

– BWBW over which fade remains constant or have over which fade remains constant or have strong amplitude correlationstrong amplitude correlation

)( mT

mc T

B 1

)( cB


Delay / Frequency Characterization Delay / Frequency Characterization

– Frequency non-selective fadingFrequency non-selective fading» Coherence BW > Signal BWCoherence BW > Signal BW

– Frequency selective fadingFrequency selective fading » Coherence BW < Signal BW:Coherence BW < Signal BW:

cB

W

cB

W

WBc

WBc

f

f


Time Variations Time Variations Coherence Time Coherence Time

– TimeTime over which fade remains constant or have over which fade remains constant or have strong amplitude correlation strong amplitude correlation – Coherence time > symbol time : Coherence time > symbol time : Slow fadingSlow fading– Coherence time < symbol time : Coherence time < symbol time : Fast fadingFast fading

Doppler BWDoppler BW– frequency shift on the carrier frequency due to frequency shift on the carrier frequency due to

relative motion between Tx and Rxrelative motion between Tx and Rx– depends on user velocity and carrier wavelengthdepends on user velocity and carrier wavelength

Note:Note:

)( cT

)( dB

cd T

B 1


Doppler BandwidthDoppler Bandwidth

vBd

v : : mobile velocitymobile velocity

fc

: : carrier wavelengthcarrier wavelength

f : : carrier frequencycarrier frequency ForFor MHz,MHz,900f

60v Km/hKm/h,,

HzHz50dB33.0 mm

• Larger Doppler Bandwidth necessitatesLarger Doppler Bandwidth necessitates• Larger power control control update Larger power control control update

rates in CDMArates in CDMA• Faster converging algorithms when Faster converging algorithms when adaptive receivers are employedadaptive receivers are employed


Effect of FadingEffect of Fading

0NEb

ep

1

1.0

01.0

001.0

0001.0

Non-fading AWGN Channel: falls Non-fading AWGN Channel: falls exponentiallyexponentially with increasing SNR with increasing SNR ep

Fading Channel: falls Fading Channel: falls linearlylinearly with increasing SNR with increasing SNR ep

AWGN

Fading


Combating Fading EffectsCombating Fading Effects

– Diversity techniques» Provide the receiver with multiple fade replicas of the

same information bearing signal» Assume independent diversity branches

» If denote the probability that the instantaneous SNR is below a given threshold on a particular diversity branch

» Then, the probability that the the instantaneous SNR is below the same threshold on diversity branches is

Lp

LLp


SISO to MIMOSISO to MIMO

– Single Input Single Output (SISO)» LOS point-to-point links

– Single Input Multiple Output (SIMO)» Receiver diversity

– Multiple Input Single Output (MISO)» Transmit diversity» Space time transmission

– Multiple Input Multiple Output (MIMO)» Multiple transmitting and multiple receiving

antennas


Receive Diversity TechniquesReceive Diversity Techniques

– Several methods by which receive diversity can be achieved include

» Space diversitySpace diversity» Time diversity (coding/interleaving can be viewed Time diversity (coding/interleaving can be viewed

as a efficient way of time diversity) as a efficient way of time diversity) » Frequency diversityFrequency diversity (multiple channels separated

by more than the coherence BW) » Multipath diversityMultipath diversity (obtained by resolving

multipath components at different delays)» Angle/Direction diversityAngle/Direction diversity (directional antennas)» Macro diversityMacro diversity


Receive Diversity CombiningReceive Diversity Combining

– Method by which signals from different diversity branches are combined

» Predetection CombiningPredetection Combining» Postdetection combiningPostdetection combining» With ideal coherent detection there is no difference

between pre- and postdetection combining» With differentially coherent detection, there is a

slight difference in performance


Receive Diversity CombiningReceive Diversity Combining– Maximal Ratio Combining (MRC)

For BPSK:

– Equal Gain Combining (EGC)

– Selection Combining (SC) where

– Generalized Selection Combining (GSC)– Switch and Stay Combining (SSC)

L

l

lk

lk

L

l

lkk

L

l

lk

lkk nxrr

1

)()(

1

2)(

1

)(

L

l

lk

lk

L

l

lkk

L

l

lkk nxrr

1

)()(

1

)(

1

)(

kkkk naxr )(2)1( ,...,,max Lkkkka


Diversity PerformanceDiversity Performance

ep

1

1.0

01.0

001.0

0001.0

• Diversity gain is maximum when the diversity branches are Diversity gain is maximum when the diversity branches are uncorrelated. uncorrelated. • Correlation between diversity branches reduces diversity gainCorrelation between diversity branches reduces diversity gain• Diversity gain is greater for Raleigh fading than for RiceanDiversity gain is greater for Raleigh fading than for Ricean

AWGN

Fading (L=1)

Average SNR

L=2 L=3

L=4



– Issue: Receive diversity at the mobile is difficult because of space limitations

– Using multiple transmit antennas at the base station with a single receive at the mobile can give same diversity benefits

– Tx. Diversity schemes» with feedback from the mobile» without feedback from the mobile



TxTx RxRx

)(),1( ksks

)1(),( ksks

)(),1( krkr 21h

11h


Spatial MultiplexingSpatial Multiplexing

TxTxRxRx

)(1 ks

)(2 ks

)(1 kr

23H

)(2 kr

)(3 krChannel Matrix

• Use N Tx antennas and M Rx antennas (N < M) by sending N symbols at a time


Co-channel InterferenceCo-channel Interference

Frequencies reused in different cells to Frequencies reused in different cells to increase capacity increase capacity

Reuse Distance: Reuse Distance: – Minimum distance between cells using Minimum distance between cells using same frequenciessame frequencies

Cell Radius:Cell Radius: Reuse Ratio: Reuse Ratio:

D

R

RD DR R


Co-channel InterferenceCo-channel Interference S/IS/I : Signal-to-Interference Ratio : Signal-to-Interference Ratio For same size cells, co-channel interference (CCI) For same size cells, co-channel interference (CCI) becomes a function of andbecomes a function of and Increasing reduces CCIIncreasing reduces CCI

: path loss exponent (=4 typ) : No. of co-channel cells: path loss exponent (=4 typ) : No. of co-channel cells S/IS/I required = 18 dB (typ) => cluster size required = 18 dB (typ) => cluster size NN > 6.49 > 6.49 For 7-cell reuse (For 7-cell reuse (NN = 7), = 7), S/IS/I = 18.7 dB = 18.7 dB

R D

RD

LN

LRD

D

R

I

SIS

L

ii

L

ii

3)/(

)(11

L


Co-Channel InterferenceCo-Channel Interference

– In FDMA/TDMA CCI determines the reuse In FDMA/TDMA CCI determines the reuse distance distance

– In CDMA, CCI affects the number of usersIn CDMA, CCI affects the number of users supported by a BSsupported by a BS– CCI can be reduced byCCI can be reduced by

» SectorizationSectorization» Power ControlPower Control» Discontinuous TransmissionDiscontinuous Transmission» Frequency HoppingFrequency Hopping» Multiuser detectionMultiuser detection


Multiple AccessMultiple Access

– FDMA FDMA » AMPSAMPS

– TDMATDMA» GSM, EDGE, DECT, PHSGSM, EDGE, DECT, PHS

– CDMACDMA» IS-95, WCDMA, cdma2000IS-95, WCDMA, cdma2000

– OFDM OFDM (can be viewed as a spectrally efficient FDMA)(can be viewed as a spectrally efficient FDMA)» 802.11a, 802.11g, HiperLAN, 802.16 802.11a, 802.11g, HiperLAN, 802.16


OFDMOFDM

Power

Time

Frequency

Time-slots

Carriers

Tones


DS-CDMA vs OFDMDS-CDMA vs OFDM

Channel

t

)(th

1 23

n

12

3

n

Tx. signal Rx. signal

ImpulseResponse

f

FrequencyResponse

)( fH

CDMA attempts to exploit CDMA attempts to exploit ““time-diversity” through time-diversity” through RAKE receiverRAKE receiver

OFDM attempts to exploit OFDM attempts to exploit ““frequency-diversity” by frequency-diversity” by frequency slicingfrequency slicing


RAKE ReceiverRAKE Receiver

90

Carrier

H*(f)

H*(f)

L-ParallelL-ParallelDemodulatorsDemodulators

1y

2y

Ly

Y


RAKE Finger

90

Carrier

H*(f)

H*(f)

nTc

nTc

Pilot SeqTracking Loop

(Early-Late Gate)

Initial timingfrom searcher Pilot

SequenceDespreader

)( Ipc

pN

1

pN

1 llc ˆsinˆ

ll ˆcosˆ

)(Qpc

)( Iuc

)( Iuc

N

1

l 0


Power Control

To combat the effect of fading, shadowing and distance losses

Transmit only the minimum required power to achieve a target link performance (e..g, FER)– Minimizes interference– Increases battery life

FL Power Control– To send enough power to reach users at cell edge

RL Power Control– To overcome “near-far” problem in DS-CDMA


Power Control Types of Power Control

– Open Loop Power Control– Closed loop Power Control

Open Loop Power Control (on FL)– Channel state on the FL is estimated by mobile– RL Transmit power made proportional to FL channel Loss– Works well if FL and RL are highly correlated

» which is generally true for slowly varying distance and shadow losses

» but not true with fast multipath Rayleigh fading

– So open loop power control can effectively compensate for

distance and shadow losses, and not for multipath fading


Power Control

Closed Loop Power Control (on RL)– Base station measures the received power– Compares it with the desired received power (target

Eb/No) – Sends up or down command to mobile asking it to

increase or decrease the transmit power– Must be performed fast enough a rate (approx. 10

times the max. Doppler BW) to track multipath fading

– Propagation and processing delays are critical to loop performance


Ultra wideband (UBW) Techniques Impulse Radio Tx (Marconi’s century old radio tx)

has now emerged under the banner `ultrawideband Reason:

– mature digital techniques– practicality low power impulse radio communications

UWB– Tx and Rx of ultra-short (sub-nanosecs)(sub-nanosecs) electromagnetic

energy impulses (or monocyclesmonocycles with few zero crossings) FCC’s definition of UWB: FCC’s definition of UWB:

– BW’s greater than 1.5 GHz orBW’s greater than 1.5 GHz or– or BW’s greater than 25% of the center frequency or BW’s greater than 25% of the center frequency

measured at 10 dB down pointsmeasured at 10 dB down points


UWB

Modern UWB radio is characterized by

– very low effective radiated power (sub-mW range)

– extremely low power spectral densities and wide bandwidths (> 1GHz)

– EIRP < -41.25 dBm/MHz, with restrictions in bands below 960 MHz, between 1.99 and 10.6 GHz


UWB Ways of generating signals having UWB

characteristics– TM-UWB

» Time modulated impulse stream

– DS-UWB» continuous streams of PN-coded impulses (resemble

CDMA signaling)» employ a chip rate commensurate with the emission

center frequency

– TRD-UWB» employs impulse pairs that are differentially polarity

encoded by the data


UWB Capabilities

High spatial capacity High channel capacity and scalability Robust multipath performance Very low transmit power Location awareness and tracking

Issues in Wireless Physical Layer

Documents

importantdept of ece

impulse noise dept of

bangalorerf spectrum

multipath fadingdept

wireless physical layer

certain frequency spectra

los component cochannel

wireless lan technologies