basic principles of wireless networks (ii)

Chalmers University of Technology

Basic Principles of Wireless Networks (II)

Nima Seifi

Division of Communication Systems,

Signals and Systems

Division of Communication Systems, Information Theory, and Antenna

SSY145, March 29, 2011

Based upon slides of P. Viswanath/Tse, A. Goldsmith, Shiv Kalyanaraman, Tae Hyun Kim, David Gesbert

& textbooks by Tse/Viswanath, A. Goldsmith, J. Andrews et al


How Do We Overcome TheLimitations Imposed By The

Wireless Channel?

• Flat Fading Countermeasures • Delay Spread Countermeasures

− Diversity

− Coding and Interleaving

− Adaptive Techniques

− Equalization

− Multicarrier Modulation

− Spread Spectrum

− Antenna Solution


• Independent signal paths have a low probability of experiencing

deep fades simultaneously.

0

-20

-40

-60

Received Signal Power

Diversity

• The basic concept is to send the same information over

independently fading radio

• Independent fading paths can be achieved by separating the

signal in time, frequency, space, polarization, etc.

The chance that two deep fades occur simultaneously is rare.

0

-80

-1004 8 12 16 dR

eceived Signal Power

(dBm)


Diversity Combining Techniques

αααα1 αααα2 αααα3 ααααM

• • •

• Selection Combining:

− picks the branch with the highest

SNR

• Equal-Gain Combining:

− all branches are coherently

Combiner

Output


combined with equal weights

• Maximal-Ratio Combining:


combined with weights which

depend on the branch SNR.


Diversity Performance

10-32

10-2

5

2

10-1

5

Pb

M = 1

Maximal

Ratio

Combining

• There is dramatic improvement even with two-branch selection combining.

5

2

10-6

10-5

5

2

10-4

5

2

5

10 15 20 25 30 35 40

γγγγb, SNR/bit, dB

M = 4

M = 2

M = 1

• The output SNR with Maximal-Ratio Combining improves linearly with the number of diversity branches, M ⇒ the complexity becomes prohibitive.


Channel Coding

For Pb = 10-6

Uncoded 10.5 dB

Hamming 10.0 dB

BCH 6.5 dB

Conv. 5.0 dB

10-2

10-4

10-3

5

2

5

2

BPSK

Bit error probability–AWGN channel

Pb

010-7

10-4

10-5

10-6

2 4 6 8 10 12 14

5

2

5

2

5

2

Uncoded

Hamming (7,4,1)

BCH (127,64,10)

Conv. 1/2 rate (k=7)

γγγγb, SNR/bit, dB

• Fading causes burst errors. If the fading is slow enough relative to the symbol rate, coding will not be effective.

• Channel coding reduces Pb by introducing redundancy in the transmitted bit stream.

• This improvement comes at the expense of increased signal bandwidth or a lower data rate


Coding Performance over Fading

1

10 --1

10 --2

--3

Pb

Uncoded 50 km/hr Coded 1 km/hr Coded 8 km/hr Coded 50 km/hr Coded 100 km/hr

• Pb performance for the IS-136 rate-1/2 convolutional code on a simulated mobile radio channel (hard-

decision decoding).

10 --3

10--48 10 12 14 16 18 20

100 km/hr

γγγγb, SNR/bit,

dB • Negligible coding gain if fading is slow compared

to bit rate ⇒ interleaving

[V. Iyengar and J. Michaelides, "Performance Evaluations of RLPs (Radio Link Protocols)

for TDMA Data Services," ITIA Contribution TR45.3.2.5/93.03.30.10, Chicago, March 30, 1993]


Coding and Interleaving• The basic principle is to spread the burst errors

over many code words.

1 2 3

9 10 11

5 6 7

read into

interleaver by rows

read out by

columns

1,5,9,2,6,10,3,7,

11,4,8,12

4

12

8

1 codeword

1 2 3

9 10 11

5 6 7reads out by rows

reads in by columns

1,5,9, 2 , 6 ,10 ,3,7,11,4,8,12

1, 2 ,3,4,5, 6 ,7,8,

9, 10 ,11,12

channel

4

12

8


Adaptive Techniques

• Adaptive Modulation

• Automatic Repeat Request


Adaptive Modulation

Adaptive Modulation and Coding

Power Control

Demodulation and Decoding

Channel Estimate +

TRANSMITTER RECEIVER

noise

Channel

Delay

FEEDBACK CHANNEL

• Power and/or data rate adapted

at transmitter to channel conditions

• Potential for large increase in

spectral efficiency

• Can be combined with adaptive

compression

– requires reliable feedback channel and accurate

channel estimation

– increases transmitter and receiver complexity


Automatic Repeat Request

• Method of "self-adapting" the data rate to

the channel conditions

• Used in combination with error-detecting code

• Types: Stop-and-Wait, Go-Back-N, Selective-• Types: Stop-and-Wait, Go-Back-N, Selective-

Repeat

– power and spectrally inefficient

– impacts higher layer protocols

– necessary for meeting stringent Pb

requirements or data


Delay Spread Countermeasures

• Signal Processing

– at the receiver, to alleviate the problems

caused by delay spread (equalization)

– at the transmitter, to make the signal less

sensitive to delay spread (multicarrier,

spread spectrum) spread spectrum)

• Antenna Solutions

– change the environment to reduce, or

eliminate, the delay spread (distributed

antenna system, small cells, directive

antennas)


EqualizationEqualization


Recap: Inter-symbol Interference (ISI) due to Multi-path Fading

Transmitted signal:

Received Signals:

Line-of-sight:

Reflected:

The symbols add up on the channel

� Distortion!

Delays


Equalization: Channel is a LTI Filter

• ISI due to filtering effect of the communications channel (e.g.

wireless channels)

• Channels behave like band-limited filters

)()()(

fj cefHfHθ= )(

)()(fj

cccefHfH

θ=

Non-linear phase

Phase distortion

Non-constant amplitude

Amplitude distortion


Equalization Principle

• Equalizer: enhance weak freq., dampen strong freq. to flatten

the spectrum

• Since the channel Hc(f) changes with time, we need adaptive

equalization, i.e. re-estimate channel & equalize

Tx filter Channel )(tr Rx. filter kz { }ka1a

)(th )(th)(th∑ −k

k kTta )(δ Equalizer)(th

)(tzTx filter Channel

)(tn

)(tr Rx. filter

DetectorkTt =

{ }ka

2a 3aT )(

)(

fH

th

t

t

)(

)(

fH

th

r

r

)(

)(

fH

th

c

c

∑k

Equalizer

)(

)(

fH

th

e

e

)(

1)(

fHfH

c

e = Taking care of ISI

caused by channel


Multicarrier Modulation and OFDMand OFDM


Multicarrier Modulation

• Key Idea: Since we avoid ISI if Ts > Tm, just send a large

number of narrowband carriers

• M subcarriers each with rate R/M, also have Ts’ = Ts*M. Total

data rate is unchanged.

subchannel

frequency

ma

gn

itu

de

carrier

channel


Multicarrier Modulation Cont’d

RF

Transmitter

R/N b/s

D(t)

f0

f1

fN-1

dN-1(t)

QAM filter

R/N b/sQAM filter

R/N b/sQAM filter

(t)d0

(t)d1

Bandlimitedsignals

f0 f1 f2

f0

fN-1

f1

N-1

Receiver

RF

filter

QAM

QAM

QAM

filterf1

f0

filterfN-1


Issues w/ Multicarrier Modulation

• 1. Large bandwidth penalty since the subcarriers can’t have

perfectly rectangular pulse shapes and still be time-limited.

• 2. Very high quality (expensive) low pass filters will be required to

maintain the orthogonality of the subcarriers at the receiver. maintain the orthogonality of the subcarriers at the receiver.

• 3. This scheme requires L independent RF units and demodulation

paths.

• OFDM overcomes these shortcomings!


Orthogonal Frequency Division Multiplexing (OFDM)

• OFDM uses a computational technique known as the Discrete Fourier Transform (DFT)

– … which lends itself to a highly efficient implementation commonly known as the Fast Fourier Transform (FFT).

– The FFT (and its inverse, the IFFT) are able to create a multitude of orthogonal subcarriers using just a single radio.orthogonal subcarriers using just a single radio.

Ch.1

Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10

Saving of bandwidthCh.3 Ch.5 Ch.7 Ch.9

Ch.2 Ch.4 Ch.6 Ch.8 Ch.10

Ch.1

Conventional multicarrier techniques

Orthogonal multicarrier techniques

50% bandwidth savingfrequency

frequency


Spectrum of the Modulated Data Symbols

• Rectangular Window of

duration T0

• Has a sinc-spectrum

with zeros at 1/ T0

Magnitude

with zeros at 1/ T0

• Other carriers are put in

these zeros

• � sub-carriers are

orthogonal Frequency


OFDM Symbols• Group L data symbols into a block known as an OFDM symbol.

– An OFDM symbol lasts for a duration of T seconds, where T = LTs.

– Guard period > delay spread

– OFDM transmissions allow ISI within an OFDM symbol, but by including a sufficiently large guard band, it is possible to guarantee that there is no interference between subsequent OFDM symbols.guarantee that there is no interference between subsequent OFDM symbols.

• The next task is to attempt to remove the ISI within each OFDM symbol


Cyclic Prefix: Eliminate intra-symbol interference!

• In order for the IFFT/FFT to create an ISI-free channel, the channel must appear to provide a circular convolution

• If a cyclic prefix is added to the transmitted signal, then this creates a signal that appears to be x[n]L, and so y[n] = x[n] * h[n].


Cyclic Prefix Cont’d

• The first v samples of ycp interference from preceding OFDM symbol

=> discarded. => discarded.

• The last v samples disperse into the subsequent OFDM symbol =>

discarded.

• This leaves exactly L samples for the desired output y, which is

precisely what is required to recover the L data symbols embedded in

x.


Spread SpectrumSpread Spectrum


Spread Spectrum

• Spread spectrum increases the transmit signal bandwidth to reduce

the effects of flat fading, ISI and interference.

• There are two SS methods: direct sequence and frequency hopping

− Direct sequence multiplies the data sequence by a faster chip

sequence

− Frequency hopping varies the carrier frequency by the same

chip sequence


Spread Spectrum

• Spread spectrum increases the transmit signal bandwidth to reduce

the effects of flat fading, ISI and interference.

• There are two SS methods: direct sequence and frequency hopping

− Direct sequence multiplies the data sequence by a faster chip

sequencesequence

− Frequency hopping varies the carrier frequency by the same

chip sequence


Direct Sequence Spread Spectrum

Modulator

Transmitter

Channel

Spreading (PN) Code Tc << T

Carrier Recovery

Demod

Receiver

Spreading (PN) Code

Data (T)

Data (T)

Synch

Interference

Transmitter Receiver

Original Data Signal

Narrowband Filter

Other SS Users

Demodulator Filtering

ISI

Modulated Data

Data Signal with Spreading

Narrowband Interference

Other SS Users

Receiver Input

ISI


Rake Receiver

Coherent Combiner

Demodulator

Data Output

Received

Signal

sc(t)

sc(t-Tc)

sc(t-2Tc)•sc(t-2Tc)

sc(t-TM)

•••

• When the chip time is much less than the rms delay spread, each branch has independent fading � equivalent to

diversity combining.

• When the chip time is greater than the rms delay spread, the paths cannot be resolved ⇒ no diversity gain.


Performance of Rake ReceiverFading Channel 0.5

10 -1

10 -2Rayleigh

DPSK

10 -3

10-4

10-5

Pb

0 5 10 15 γγγγb, SNR/bit, dB

AWGN

RAKE


Antenna SolutionsGoal: Reduce (or eliminate) delay spread

• Distributed Antenna System

• Very Small Cells ⇒ antenna in every room

• Sectorization

• Directive Antennas/Beam Steering

Omnidirectional Sectorized Directive

90

270

180

150

120

30

300240

210 330

60

0

90

270

180

150

120

30

300240

210 330

60

0

90

270

180

150

120

30

300240

210 330

60

0


Summary of Countermeasures

• Diversity

• Coding and Interleaving

• Adaptive Techniques

• Equalization

• Multicarrier

• Spread Spectrum

• Antenna Solutions

These techniques can be combined.


Multi-Antenna TechniquesMulti-Antenna Techniques


Multiple-Input Multiple-Output (MIMO)

• Expanding resources in space dimension

− key idea: different propagation path for each signal from different

transmit antennas

TX

RX

y1=h11x1+h12x2+h13x3

y2=h21x1+h22x2+h23x3

y3=h31x1+h32x2+h33x3

x1

x2

x3

y1

y2

y3

scatteringmatrix form: y=Hx


MIMO Leverages

• Array gain

– by combining multiple signals

• Diversity

– Transmitting same information redundantly

– Usually in a form of space-time (block) coding

• Spatial multiplexing• Spatial multiplexing

– Transmitting different info, creating multiple spatial streams

• Interference cancellation

– Two methods

• Decoding and discarding signals not destined to oneself

• Using spatial streams which are orthogonal to each other

– Can be seen as one method of spatial multiplexing


What Can We Do with MIMO?


Review Questions• Interleaving is one way to create diversity in fading

channels. Explain briefly how interleaving helps to create

diversity.

• There are several sources of interference in wireless

networks: intersymbol interference (AWGN channels networks: intersymbol interference (AWGN channels

due to poor pulse-shaping; multi-path frequency-selective

channels), inter-cell interference, intra-cell interference

(CDMA), and inter-carrier interference (in OFDM).

Briefly explain why each of these interference phenomena

arise? How do techniques like pulse-shaping, spread

spectrum/Rake, equalizers and OFDM deal with the ISI

problem?


Review Questions

• The cyclic prefix in multicarrier modulation serves as a

time gap (a guard interval) between consecutive data

blocks. Why is cyclic prefix used instead of a simpler

guard interval?

• Name two channel-related irreducible probability of error

phenomena, and briefly describe the nature of each one.

What is their relationship with the coherence bandwidth

and Doppler spread of the channel?

basic principles of wireless networks (ii)

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