Advanced Topics for Wireless Communications

7/31/2019 Advanced Topics for Wireless Communications

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1. Spread Spectrum & CDMA


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n a app ca on: m ary

Benefitsn - amm ng

Robust to multipath fading

Multi le user access CDMA

High-resolution ranging (DS)

3

DATAd(t) r(t) r'(t) d'(t) Ts

Source

0

)( fSd

1/Ts

c c

d(t)T

s

)( fSc

)( fSr

1/Tcc(t)

)(' fSd

=r'(t)

d'(t)

4


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DS/SS makes noise-like waveforms

Maximal-length shift register makes the binary sequence thathave noise like properties

PN sequence

PN sequence is mapped to a chip spreading sequence of1s

The spreading comprises of chips of duration Tc


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Receiver: when the receiver knows the correct spreading sequence

Po

wer

De

nsity

received signalTIME

spreading sequence(spreading code)

10110100

0100101110110100 10110100

RadioFrequency

you can find thespreading timingwhich ives themaximum detectedpower, and

Accumulate for

gathering energy !

1011010010110100 10110100

one ura on

Demodulated data

1111111100000000 00000000

Base-bandFrequency

0 01

7

Receiver: when the receiver does not know the correct s readin se uence

received signalPower

Density

TIME

spreading sequence(spreading code) 01010101 01010101 01010101

0100101110110100 10110100

you cannot findthe spreadingtimin

a oFrequency

10101010 10101010 10101010

without correctspreading code,and

1011010010110100 10110100

Accumulate for

one bit duration

No data can be detected

- --

Base-bandFrequency

Demodulated data

8


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9

-

form of frequency diversity)

- If the maximal delay spread (due to multi-path) is Tm and if the

chip rate 1/Tc = W>> 1/Tm, then individual multi-path signal

components can be isolated

Amplitudes and phases of the multi-path components are

found by correlating the received waveform with delayed

versions of the signal Multi- ath with dela s less than 1/T cannot be resolved

10


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The carrier frequency is pseudo randomly hopped over

Slow hopping: Tsymbol < Thopping

Fast ho in : T > T

11

d t

DATA Mod Filter BPF BPF

Data

demod

Freq. Synthesizer Freq. Synthesizer

....Code Generator

....Code Generator

* Noncoherent detection is common

12


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Allows multi le users to share same bandwidth at the same

time

process

Matched filter pulls out desired users waveform, while

DS-SS is one popular way to make the noise-like waveforms

13

Freq.Freq.Freq.Freq.

BPFDespreader

Code A

Data A

Code A

BPF

MS-A

Data A

Freq.Freq.Freq.Freq.

BPFDespreader

Code B

Data B BPF

MS-B

Data B

BS

14


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Advanta es

Universal frequency reuse

Soft capacity

Soft handoff

Robust to multipath fading

o us o amm ng

Disadvantages

ear- ar pro em

Difficult synchronization

15

Objectives of a Wireless communication system

Deliver desired signal to a designated receiver

Minimize the interference that it receives

One way is to use disjoint slots in frequency or time in the same

cell as well as adjacent cells limited frequency reuse

In CDMA, universal frequency reuse (frequency reuse factor = 1)

app es no on y o users n e same ce u a so n a o er ce s

No frequency plan revision as more cells are added

esource a ocat on o eac user s c anne s energy nstea o

time and frequency)

16


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Handoffs between cells are supported while the mobile is in

traffic or idle MS continuously keeps searching for new cells as it moves across

the network

Target cell search

MS maintains active set, neighbor set, and remaining set as well

as candidate set

Two types of handoffs Hard handoff

Soft/softer handoff

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margin exceeds

Base A

T_ADDBase B

T_DROP

B_Active

candidate list

starts Drop timerresets

18


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o an o

Mobile commences communication with a new BS without interrupting

communication with old BS (make-before-break)

Same frequency assignment between old and new BSs

Provides different site selection diversity, called macro diversity

Neither the mobile nor the base station is required to change frequency

Handoff between sectors in a cell

19

-

P

CDMADATA A

Lp-a

CDMAReceiverCODE A

Demodulated DATA

P Lp-b

CDMADATA B

CODE A

Desired signal power = P/Lp-a

Transmitter

When user B is close to the receiver and user A is

Interference power = P/Lp-b/PG

far from the receiver, Lp-a could be much bigger

than Lp-b. In this case, desired signal power is

smaller than the interfered ower.

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Overcomes near-far problem

CDMA would not work without it

Copes with path loss and fading

tedPower

from A

from B

Time

Dete

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Power control is capable of compensating different path loss and

fading fluctuation

ac c anges transm t power ynam ca y so t at t e rece ved power at the BS from all MSs is controlled to be equal

dPower

from MS Bfrom MS A

Time

Detecte

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pen-Loop Power ontro ose -Loop Power ontro

transmitmeasuringreceived power decidetransmission

power

power controlcommand

est mat ng patloss about 1000 times

per second

transmission

power

transm t

received power

transmit receive

23

2. OFDM


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Multicarrier modulation/multi lexin techni ue

Available bandwidth is divided into several subcarriers

The subcarriers are overla in but ortho onal

Parallel data transmission

A high rate stream is partitioned into several low rate streams

25

Each subcarrier has exactly an integer number of cycles

Adjacent subcarriers have exactly one cycle difference

Transmit signal1

2( ) exp , 0

sN

k

ks t d j t t T

= 0

where : number of subcarriers

: transmit symbol for the th subcarrier

k

s

k

N

d k

=

Note:

: sym o urat on

0

2 2exp exp ( )

T kj t j t dt k

T T

=

ll

Subcarriers in time

( )01 2

exp

T

s t j t dt d T T

= l

l

1

Subcarriers in frequency

u carr er spac ng:T

26


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ransm tter an rece ver arc tecture

27

gna transm ss on an recept on

28


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2 2

2

1

( ) , 0

Ns

j it

TNs

Nsi

S t d e t T

++=

2

21

0

[ ] ( )s j in

N

N

i

i

S n S nT N d e

=

+

=

= = T

0

s

2 2 2

2 2 2 2

0 1 2

0 1 2

2

[0]

[1]j j

N N

j j

N N

S d d d

S d d e d e

= + +

= + + + L

L0 1 2

0

d

( )0S

1sND

IDFT P/S

( 1sS N

1d (1S

29

30


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Guard time

The guard time is chosen larger than the expected delay spread, so that

the delayed symbol cannot interfere with the next symbol (RemoveISI

Zero insertion or cyclic prefix

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Preserve orthogonality between subcarriers, once the largest

multipath delay is less than the duration of the cyclic prefix

No nter-carr er nter erence (I I) Even with long enough cyclic prefix, ICI can occur due to

0

2 2 ( ) (1 exp( 2 ))exp exp

T k k m T jj t j t dt

+ + = where denotes the normalized frequency offset

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FCC manages spectrum

Specifies power spectral density mask

Adjacent channel interference

Roll-off requirements

Implications to OFDM

Zero tones on edge of the band

Time domain windowing smoothes adjacent symbols

33

Shar si nal transitions at the OFDM s mbol boundariescan cause significant out-of-band emission

Windowing smoothens the transitions, making the power

spectrum decay faster Raised cosine window

0.5 0.5cos( ( ) ), 0 1

[ ] 1.0, (1 ) 1

n N N n N

w n N n N

+ +

= + . . ,

SYM sampleN T T

TT T

(1 )SYM SYM T T = +

time

34

SYM SYM


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Data (48) Pilots (4) Nulls (12)

Subcarrier Index (-32 ~ 31)

-25 -20 -15 -10 -5 -1-30 0 5 10 15 20 25 261-32 317 21-7-21-26 30

0 1 2 3 61 62 63NU

25 26 27 28NU

NU

36 37 38 39NU

NU

IFFT Block

LL

61 62 630 1 2 3

LL

LL

LL

LL

25 26 27 28 36 37 38 39

r0

r1

r2

r3

r4

r5

r62

r63

r0

r1

r56

r62

r63

Prefix Postfix

r2

r3

r54

r55

OFDM Symbol

Windowing Function

21 7 7 211, 1, 1, 1p p p p = = = = Pilot Symbols:

(the same as 802.11a)

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36


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Advanta es

Spectrally efficient

Conveniently implemented using IFFT and FFT

Easy to handle frequency-selective fading channel (wideband

transmission)

sa van ages

More complex than single carrier transmission

Long symbol duration vulnerable to frequency offset and fast fading

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Complexity

Single-carrier systems need equalizer when delay spread over

FFT does not need full multiplication but rather phase rotation

The complexity in OFDM grows slightly faster than linear

Robustness

Single-carrier system performance degrades abruptly when

delay spread exceeds the value for which the equalizer is

designed

OFDM s stems are robust a ainst dela s read

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Number of subcarriers

Guard time > delay spread

39

Carrier frequency: 900 MHz

Signal bandwidth 5 MHz

Channel characterization Frequency selectivity

RMS delay spread < 100 msec

90% coherence bandwidth (CB) = 1/(50m) = 209.2 kHz Time selectivity

Assumption for user mobility: 0-3 km/hr (maximum Doppler frequency

(fm) = 2.5 Hz at the carrier frequency of 900 MHz)

50% coherence time C = / 16 = 71.62 msec m

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su carr er spac ng s ou mee e o ow ng

relationship:1/CT= 13.9 Hz < f< CB = 209.2 kHz,

which is required for an OFDM system not to be vulnerable to both

frequency-selective and time-selective fading

We select a subcarrier spacing of 72.27 kHz

Set the number of subcarriers to 64

Total bandwidth = 72.27 kHz 64 = 4.625 MHz

41

Parameter Value

Total bandwidth (W) 4.625 MHz

Total number of subcarriers (NT) 64

Number data subcarriers (ND

) 48

Number pilot subcarriers (NP) 4

Number of guard or null subcarriers (NG) 12

Subcarrier frequency spacing (F) 72.27 kHz

IFFT/FFT period (TFFT

) 13.838 sec (64 samples)

uar n erva ura onGI

. sec samp es

OFDM symbol duration (TSYM

) 16.0 sec (TFFT+ TGI)

42


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Given sk, k pilot tone indices, solve for hl Find

2

0

kL jN

k k ky h e s n

=

= +l

l

l

2 kLj N

kH h e

= l

l

Interpolation for all subcarriers

More pilot tones give

0=l

Better noise resilience

Lower throughput

43

-

Frequency domain equalizer (FDE)

-

k k k k

Noise enhancement factor

Use MMSE to reduce noise enhancement

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Different channel fadin for different subcarriers

Bad subcarriers will cause many errors (channel-selective

errors)

Two approaches

Error correction coding across subcarriers

Adaptive modulation & coding and/or unequal power allocation

Coding across subcarriers realizes frequency diversity gain as

well as the coding gain

45

Two types of errors

Random errors: primarily caused by noise

Channel-selective errors: caused by magnitude distortion in channel

frequency response

Error correcting codes are effective for random errors

Interleaving is often used to scramble data bits so that standard

error correcting codes can be applied Interleaving and coding provide frequency diversity as well as the

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Transmitter Receiver

47

Multiple transmit antennas are separated geographically

Enables same radio/TV channel frequency throughout a country

Creates artificially large delay spread No problem in OFDM

48


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-

Effective to handle large bandwidth for high data rate

Takes advantage of multipaths through simple equalization

Resource allocation (AMC, power, etc)

Easy combination with MIMO

49

-

50


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. ransm vers y

MIMO

Fading

Fluctuation of received SNR

Diversity Reduce the fluctuation of received SNR

, , ,

Antenna diversity

, ,

Transmit diversity

52


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Space-Time Transmit Diversity (STTD)

Ortho onal Transmit Diversit (OTD

Time-Switched Transmit Diversity (TSTD)

Transmit Antenna Array (TxAA)

Classification

Open-loop

STTD, OTD, TSTD

Closed-loop Need feedback information

TxAA

53

-

P

2 h Alamouti STC

STTDEncoder

STTDDecoderh

1x 2x

0 T 2T

1x*

2x

0 T 2T

2x*

1x

0 T 2T

P

2

*

1 2*

x x Ant. 1Time 1 Time 2

( )1 1 1 2 2 12* *

Pr h x h x n= + +

2 1x xAnt. 2

STTD Decoder:2 2* *

2 1 2 2 1 22r x x n= + +

( )

1 1 1 2 2 1 2 1 12

2 2* *

2 1 2 2 1 1 2 2 22 P

x r r x

x r h r h h h x

= + = + +

= = + +

Diversity without BW

expansion!

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.

Ph1

2 h1

Source ReceiverEncoder Decoderh

2

2

2 P Tx. Power = P Tx. Power = P

1 : +h h2

1 2 SNR:

P 02

Avg. SNR: P 02

Avg. SNR:

z Average SNR gain = 0

z Diversity order = 2

z Average SNR gain = 0

z Diversit order = 1

55

P

2 h1

OTD

EncoderOTD

1x 2x

1x

0 T 2T

1x

20 T 2T

2x0 T 2T

P

2x

2

( )1 1 1 2 2 12Pr h x h x n= + +

Received Signal: OTD Encoder:

1 1x x Ant. 1Time 1 Time 2

( )2 1 1 2 2 22Pr h x h x n= +

OTD Decoder:

2 2x x Ant. 2

Different s mbol different fadin

( )( )

2*

1 1 1 2 1 1 1

2*

2 2 1 2 2 2 2

2

2

x h r r P h x

x h r r P h x

= + = += = +

Interleaving effects

Interleaving depth (diversity order) = 2

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-

P h1TSTD

TSTD1x 2x

1x

0 T 2T

0

20 T 2T

2x

0 T 2T

0

( )1 1 1 1r P h x n= + Received Signal: TSTD Encoder:

1 0x Ant. 1Time 1 Time 2

( )2 2 2 2r P h x n= +

TSTD Decoder: Different s mbol different fadin

20 x Ant. 2

2*

1 1 1 1 1 1

2*

2 2 2 2 2 2

x h r P h x

x h r P h x

= = +

= = +

Interleaving effects

Interleaving depth (diversity order) = 2

57

h1

Coded

w1 hm

Weight

Channelization &Scrambling Code

wm

Mi

wM Feedback

wh

mm=*

r w h x nm m

M

=F I

+

= hmm

M2

1

hii=

2

1

x h n

m

m

M

= +

=

1

2z Diversity order Mz Avera e SNR ain M

m=1

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10-1

No. of Tx antennas=1

No. of Tx antennas=2

No. of Tx antennas=4No. of Tx antennas=8

-3

10-2

edBER

10-4

Unco

-6 -4 -2 0 2 4 6 8 10 12 14 16 1810

-5

Eb/No (dB)

z 4 tx antennas: 5.2 dB gain at BER 10-2 compared with 2 antennas

z 8 tx antennas: 9.2dB gain

59

multiple receive antennas

Single I nput Single Output

Multiple I nput Single Output

Multiple I nput Multiple Output

s gna process ng n e space oma n can r ng

enormous capacity enhancement without bandwidth expansion,

60


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Mantennas Nantennas

RxTx

1x

1y

11 12 1

21 22 2

M

M

H H H

H H H

=H

L

L

Mx Ny

Received signal for flat fading channels

1 2N N NMH H H

M M O M

L

Total transmit powerE[xHx] = PT Noise covariance matrix at the receiver EnnH = 2I

= +y Hx n

Assumptions Receiver perfectly knows the channel H

-

61

General ca acit formula

2 2log det HT

PC

M

= +

I HH

SISO:M=N= 12

2 112log 1 T

SISO

PC H

= +

SIMO:M= 1,N 2

2NP

MISO:M 2,N= 1

2 121

ogSIMO nn =

= +

Logarithmical increase withM

SNRMISO = SNRSIMO/M

2

2 121

log 1

MT

MISO m

m

P

C HM =

= +

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MIMO:M 2,N 2

2 2

rank( )

log det HTMIMO

K

PC

M

P P

= +

H

I HH

where K= min MN and

2 22 21 1

og ogk k

k kM M = == + = +

ks are eigenvalues ofHHH that is ordered such that1 2 rank(H) rank(H)+1 = = K= 0

Interpretation:

12

rank(H)

Tx Rx

SISO channels

Capacity increases linearly with rank(H) at high SNR63

-

,

transformed into parallel SISO channels2 H

( : eigenvalues of )H=H UDV D HH

Maximize the capacity( )H H= + = +y U HVx n Y Dx U n

1s

HTn Rn

1

~S

min( , )

2

1min( , )

log (1 ),T R

T R

n n

k k

kn n

C p =

= +

VDecouplingTransform

kp

ksUH

DecouplingTransform k

S~1

kk

p P=

1 1

Tnp

Tns

TnS~,

0 elsewhere

k o

o kkp =

64


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i.i.d. Rayleigh channel: full rank

65

Linear detectors

Zero-Forcing (ZF)

Minimum Mean Square Error (MMSE)

Nonlinear detectors

ZF-OSIC, MMSE-OSIC (V-BLAST)

ML

Reduced-complexity ML (sphere decoding, QRM-MLD)

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ece ve s gna mo e

1 1 1N N M M N = +y H x n ZF

1 1 ( ) ( )H H H+ = = = +x H y H H H y x HH H n

MMSE12

arg min ( )H HMMSE M

E M = = +W x Wy H H I H

ML

( )1

( )H HMMSE M

M

= = +W

x W y H H I H y

2 arg min

c

c=

x

x y Hx

67

- -

:

1

Initialization

i

1

12

1

arg mink

+

+

==

= < >

y y

G H

H

:

i ik i k

Recursion

=< >=

w G

w

1

( )

[ ]

i i

i i

i i

k k

i i k k

x D z

x++

== y y H

1 2

1

2

1{ , , , }

arg min

i

i

i

i k

i jkj k k k

k

+

+

=

= < >H

+

68


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V-BLAST Exam le (3x2 s stem)

1 1

12 2

1 0.5

: 0.2 0.3

y n

xy n

= + = + y Hx n

First layer

2

3 30.2 0.7y n

1

. . .

0.2976 0.4762 1.0119

+ = = G H

2

= = =1 . , .jj j

[ ]1 1

0.8433 0.3175 0.4663k k=< > = w G

1 1 1 1 1

1

1 1

2

k k k k k

xz x

x= = + = +

w y w H w n w n

= =

69

1 11 1 1k k

Second layer

0.5

1 1 1 1

2 2. ,

0.7

0.5

k k k k

+

[ ]1

2[ ] 0.3 0.6024 0.3614 0.8434

0.7

k

+ = = =

G H

[ ]2

2

2

2

0.6024 0.3614 0.8434k

k =

= = w G

2 2 2 2

2 2 2 2

2

( ) ( )

k k k k

k k k k x D z D x

= =

= = + w n


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71

wireless systems

channel linear increase in capacity in contrast to

ogar m ca ncrease or an

Greatest capacity improvements are obtained under rich

scattering channels with full rank

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Advanced Topics for Wireless Communications