802.11 Wireless Networks (PHY)

Kate Ching-Ju Lin (林靖茹) Academia Sinica

2016.03.18 CSIE, NTU

Reference 1.  OFDM Tutorial

online: http://home.iitj.ac.in/~ramana/ofdm-tutorial.pdf

2.  OFDM Wireless LWNs: A Theoretical and Practical Guide By John Terry, Juha Heiskala

3.  Next Generation Wireless LANs: 802.11n and 802.11ac By Eldad Perahia

We will cover … §  Medium Access Control

–  Infrastructure mode vs. Ad Hoc mode –  DCF vs. PCF –  CSMA/CA with exponential backoff –  Hidden terminal

§  Physical Layer Basics –  Packet Detection –  OFDM –  Synchronization

Packet Detection

§  Double sliding window packet detection §  Optimal threshold depends on the receiving power

Packet

An Bn

threshold Power ratio Mn=An/Bn

Packet Packet

Packet Detection

§  Use cross-correlation to detect the preamble

An Bn

preamble preamble

threshold

We will cover … §  Medium Access Control

–  Infrastructure mode vs. Ad Hoc mode –  DCF vs. PCF –  CSMA/CA with exponential backoff –  Hidden terminal

§  Physical Layer Basics –  Packet Detection –  OFDM (orthogonal frequency-division multiplexing) –  Synchronization

Why OFDM? §  Signal over wireless channels

–  y[n] = Hx[n]

§  Work only for narrow-band channels, but not for wide-band channels –  e.g., 20 MHz for 802.11

frequency 2.45GHz (Central frequency)

20MHz Capacity = BW * log(1+SNR)

Basic Concept of OFDM

Send a sample using the entire band

Send samples concurrently using multiple orthogonal sub-channels

Wide-band channel Multiple narrow-band channels

Why OFDM is Better?

§  Multiple sub-channels (sub-carriers) carry samples sent at a lower rate –  Almost same bandwidth with wide-band channel

§  Only some of the sub-channels are affected by interferers or multi-path effect

f f

t t

Wide-band Narrow-band0 1

1 00 0

1

0 1 1 0 0 0 1…........

Importance of Orthogonality

§  Why not just use FDM (frequency division multiplexing) –  Not orthogonal

§  Need guard bands between adjacent frequency bands à extra overhead and lower throughput

f

Individualsub-channel

Leakageinterferencefromadjacentsub-channels

f

guardbandGuardbandsprotectleakageinterference

Difference between FDM and OFDM

f

guardband

Frequency division multiplexing

Orthogonal sub-carriers in OFDM Don’t need guard bands

f

Orthogonal Frequency Division Modulation

Datacodedinfrequencydomain

fIFFT

*x[1]

*x[2]

*x[3]

… t

TransformaMontoMmedomain:eachfrequencyisasinewaveInMme,alladdedup

receive

Timedomainsignal Frequencydomainsignal

FFT

Decodeeachsubcarrierseparately

transmit

f

f

t

OFDM Transmitter and Receiver

Orthogonality of Sub-carriers

x(t) = X[k]e j2πkt Nk=−N 2

N 2−1

∑

Encode: frequency-domain samples à time-domain sample IFFT

Decode: time-domain samples à frequency-domain sample FFT

X[k]= 1N

x(t)e− j2πkt Nt=−N 2

N 2−1

∑

Orthogonality of any two bins : e j2πkt Ne− j2π pt Nt=−N 2

N 2−1

∑ = 0,∀p ≠ k

Time-domainsignals:x(t)Frequency-domainsignals:X[k]

Serial to Parallel Conversion §  Say we use BPSK and 4 sub-carriers to transmit a

stream of samples

§  Serial to parallel conversion of samples

§  Parallel to serial conversion, and transmit time-domain samples

c1c2c3c4symbol111-1-1symbol2111-1symbol31-1-1-1symbol4-11-1-1symbol5-111-1symbol6-1-111

Frequency-domainsignal Time-domainsignal

02-2i02+2i20-2i20+2i-2222-20-2i-20+2i0-2-2i0-2+2i0-2+2i0-2-2i

IFFT

0,2-2i,0,2+2i,2,0-2i,2,0+2i,-2,2,2,2,-2,0-2i,-2,0+2i,0,-2-2i,0,-2+2i,0,-2+2i,0,-2-2i,…

1,1,-1,-1,1,1,1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,1,1,-1,-1,-1,1,1

t2

symbol111-1-1symbol2111-1symbol31-1-1-1symbol4-11-1-1symbol5-111-1symbol6-1-111

subcarrier1

subcarrier2

subcarrier3

subcarrier4

t1 t3 t4 t5 t6

t1-4 t5-8 t9-12 t13-16t17-20 t21-24

symbol111-1-1symbol2111-1symbol31-1-1-1symbol4-11-1-1symbol5-111-1symbol6-1-111

bin1

bin2

bin3

bin4

symbol111-1-1symbol2111-1symbol31-1-1-1symbol4-11-1-1symbol5-111-1symbol6-1-111

bin1

bin2

bin3

bin4

SendthecombinedsignalastheMme-domainsignal

t1-4 t5-8 t9-12 t13-16t17-20 t21-24

Multi-Path Effect

y(t) = h(0)x(t)+ h(1)x(t −1)+ h(2)x(t − 2)+

= h(Δ

∑ Δ)x(t −Δ) = h(t)⊗ x(t)

time-domain

⟺ Y ( f ) = H ( f )X( f )

frequency-domain

Current symbol + delayed-version symbol à Signals are deconstructive in only certain frequencies

Frequency Selective Fading

Frequency selective fading: Only some sub-carriers get affected

Can be recovered by proper coding!

§  The delayed version of a symbol overlaps with the adjacent symbol

§  One simple solution to avoid this is to introduce a guard-band

Inter Symbol Interference (ISI)

Guardband

§  However, we don’t know the delay spread exactly –  The hardware doesn’t allow blank space because it needs

to send out signals continuously

§  Solution: Cyclic Prefix –  Make the symbol period longer by copying the tail and

glue it in the front

Cyclic Prefix (CP)

In802.11,CP:data=1:4

§  Because of the usage of FFT, the signal is periodic

§  Delay in the time domain corresponds to phase shift in in the frequency domain –  Can still obtain the correct signal in the frequency

domain by compensating this rotation

Cyclic Prefix (CP)

FFT()=exp(-2jπΔf)*FFT()delayedversion originalsignal

Cyclic Prefix (CP)

original signal

y(t)àFFT()àY[k]=H[k]X[k]w/omulMpath

wmulMpath

original signal + delayed-version signal

y(t)àFFT()àY[k] =α(1+exp(-2jπΔk))*X[k] =H’[k]X[k]

Lump the phase shift in H

Side Benefit of CP §  Allow the signal to be decoded even if the

packet is detected after some delay

decodable undecodable

OFDM Diagram

Mod

ulat

ion

S/P IFFT P/S Insert

CP D/A

channel

noise +

A/D

De-

mod

P/S FFT S/P remove

CP

Transmitter

Receiver

Unoccupied Subcarriers

•  Edge sub-carriers are more vulnerable �  Frequency might be shifted due to noise or multi-path

•  Leave them unused �  In 802.11, only 48 of 64 bins are occupied bins

•  Is it really worth to use OFDM when it costs so many overheads (CP, unoccupied bins)?

Mod

ulat

ion

S/P IFFT P/S Insert

CP D/A

channel

noise +

A/D

De-

mod

P/S FFT S/P remove

CP

Transmitter

Receiver

Osc

illat

or

Osc

illat

or

20MHz

20MHz

baseband

baseband

passband

passband

2.4GHz

2.4GHz

Carrier Frequency Offset (CFO)

§  The oscillators of Tx and Rx are not typically tuned to identical frequencies –  Up-convert baseband signal sn to passband signal

yn=sn*ej2πftxnTs

–  Down-convert passband signal yn back to rn=sn*ej2πftxnTs*e-j2πfrxnTs=sn*ej2πfΔnTs

Oscillator(Tx)

Oscillator(Rx)

ftx

frx Erroraccumulate!

Samplig Frequency Offset (SFO)

§  DAC (at Tx) and ADC (at Rx) never have exactly the same sampling period –  A slow shift of the symbol timing point, which

rotates subcarriers –  Intercarrier interference (ICI), which causes loss of

the orthogonality of the subcarriers

DAC(Tx)

ADC(Rx)

� =Trx

� Ttx

Ttx

Ttx

=1

ftx

Trx

=1

frx

Correct CFO in Time Domain

Symbol1 Symbol2

sn Sn+N

rn=sn*ej2πfΔnTs rn+N=sn+N*ej2πfΔ(n+N)Ts

rnrn+N* = sne

j2π fΔnTs sn+N* e− j2π fΔ (n+N )Ts

= e− j2π fΔNTs snsn+N*

= e− j2π fΔNTs sn2

z = rnrn+N*

n=1

L

∑

= e− j2π fΔNTs snsn+N*

n=1

L

∑

= e− j2π fΔNTs sn2

n=1

L

∑

fΔ =1

2πNTs∠z

Sampling Frequency Offset (SFO)

§  The transmitter and receiver may sample the signal at slightly different timing offset

§  All subcarriers experience the same sampling delay, but have different frequencies –  Each subcarrier is rotated by a constant phase shift

DAC(Tx)

ADC(Rx)

Yi=HiXi*ej2πδiNs/Nm

Sample Rotation due to SFO

I

Q

xxxx

xxx

xx xx xxxx

x

IdealBPSKsignals(NorotaMon)

θ

x

subcarrier1

subcarrier2

subcarrier3

SignalskeeprotaMng

θθ

Correct SFO in Frequency Domain

§  SFO: slope; residual CFO: intersection of y-axis

2πfΔTs(ResidualCFO)

12πδNs/Nm(SFO)

ChangeinphasebetweenTxandRxarerCFOcorrecMon

Subcarrierindex

phase

Data-aided Phase Tracking

§  Using pilot bits (known samples) to compute Hi*ej2πδiNs/Nfft=Yi/Xi

§  Find the phase change experienced by the pilot bits using regression

§  Update Hi = Hi*ej2πδiNs/Nfft for every symbol

2πδfTs(ResidualCFO)

12πtΔNs/Nm(SFO)

ChangeinphasebetweenTxandRxarerCFOcorrecMon

xx

x

x

regression

After Phase Tracking

I

Q

xxxx

xxx

xx xx xxxx

x

ArercorrecMon

OFDM Diagram

Mod

ulat

ion

S/P IFFT P/S Insert

CP D/A

channel

noise +

A/D

De-

mod

P/S FFT S/P remove

CP

Transmitter

Receiver Co

rrec

t CF

O

Phas

e tr

ack

Mme-domainfrequency-domain

Quiz §  Please explain what is the “multipath effect” §  Please explain what is ”frequency selective

fading” and what is its root cause