Wireless Physical Layer Design

1
Wireless Physical Layer Design

• Y. Richard Yang
• 09/13/2006

2
Outline

• Recap
• Physical layer design

3
Recap Large-scale Fading

• Large-scale fading---signal strength is reduced

4
Recap Small-Scale Fading

• Channel characteristics change over location,
  time, and frequency

example
d2
d1
5
Recap Small-Scale Fading
time
Received
Signal
Power
(dB)
path loss
location
log (distance)
frequency
6
Recap Flat Fading Channel

Assume h is Gaussian random
BPSK
Conditional on h,
Averaged over h,
at high SNR.
7
Recap Comparison
small-scale fading
no small-scale fading
8
Recap Delay Spread
signal at sender
LOS pulse
multipath pulses
signal at receiver
LOS Line Of Sight
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Summary Challenges of Wireless Channels
received signal strength
use fade marginincrease power or reduce distance
bit/packet error rate at deep fade
diversity
ISI and irreducible error rate
equalization, OFDM
Remember there is also interference from other
sources
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Recap Main Story

• Communication over a wireless channel has poor
  performance due to significant probability that
  channel is in a deep fade
• increasing power is less effective
• Reliability is increased by using diversity
  more resolvable signal paths that fade
  independently
• Discussion how to get diversity?

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Diversity

• When one position (with d1 and d2) is in deep
  fade, another position (with d1 and d2) may not
• When one frequency is in deep fade (or has large
  interference), another frequency may be in good
  shape

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Recap Time Diversity

• Time diversity can be obtained by interleaving
  and coding over symbols across different coherent
  time periods

coherence time
interleave
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Simplest Code Repetition
After interleaving over L coherence time periods,

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Performance

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Beyond Repetition Coding

• Repetition coding gets full diversity, but sends
  only one symbol every L symbol times
• We can use other codes, e.g. Reed-Solomon code

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Example GSM

• Amount of time diversity limited by delay
  constraint and how fast channel varies
• In GSM, delay constraint is 40 ms (voice), slot
  size is 5 ms so interleave to 8 slots

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Space Diversity Antenna
Transmit
Both
Receive
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User Diversity Cooperative Diversity

• Different users can form a distributed antenna
  array to help each other in increasing diversity
• Interesting characteristics
• users have to exchange information and this
  consumes bandwidth
• broadcast nature of the wireless medium can be
  exploited

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Frequency Diversity FHSS (Frequency Hopping
Spread Spectrum)

• Discrete changes of carrier frequency
• sequence of frequency changes determined via
  pseudo random number sequence
• used in 802.11, GSM, etc
• Co-inventor Hedy Lamarr
• patent 2,292,387 issued on August 11, 1942
• intended to make radio-guided torpedoes harder
  for enemies to detect or jam
• used a piano roll to change between 88
  frequencies

http//en.wikipedia.org/wiki/Hedy_Lamarr
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Frequency Diversity FHSS (Frequency Hopping
Spread Spectrum)

• Two versions
• slow hopping several user bits per frequency
• fast hopping several frequencies per user bit

tb
user data
0
1
0
1
1
t
td
slow hopping (3 bits/hop)
t
td
fast hopping (3 hops/bit)
t
tb bit period td dwell time
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FHSS Advantages

• Frequency selective fading and interference
  limited to short period
• Simple implementation
• Uses only small portion of spectrum at any time
• explores frequency sequentially
• Considered a type of spread spectrum system

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Direct Sequence Spread Spectrum (DSSS)

• In DSSS
• one symbol is spread to multiple chips
• the increased rate provides frequency diversity
• the number of chips is called expansion factor
• examples
• IS-95 CDMA 1.25 Mcps 4,800 Sps
• 802.11 11 Mcps 1 Mbps

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Direct Sequence Spread Spectrum (DSSS)
tb
user data d(t)
1
-1
X
tc
chipping sequence c(t)
-1
1
1
-1
1
-1
1
-1
1
-1
-1
1
1
1

resulting signal
-1
1
1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
tb bit period tc chip period
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DSSS System Blocks
spread spectrum signal
transmit signal
user data
X
modulator
chipping sequence
radio carrier
transmitter
correlator
sampled sums
products
received signal
data
demodulator
X
low pass
decision
radio carrier
chipping sequence
receiver
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Example DSSS Using BPSK

• Assume BPSK modulation using carrier frequency f
  
• y(t) A x(t)c(t) cos(2? ft)
• A amplitude of signal
• f carrier frequency
• x(t) data 1, -1
• c(t) chipping 1, -1
• At receiver, incoming signal multiplied by c(t)
• since, c(t) c(t) 1, y(t)c(t) A x(t)
  cos(2? fct)

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DSSS

• Wider spectrum to reduce frequency selective
  fading and interference
• Provides frequency diversity

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Effects of Spreading on Interference

• Assume jamming at carrier frequency f
• Then received signal y(t) j(t) w(t)
• Spreads strength of jamming signal by 1/expansion
  factor

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Effects of Spreading and Interference
dP/df
dP/df
sender
i)
ii)
f
f
Intuition (high-level idea only) - multiply
data x(t) by chipping sequence c(t) spreads the
spectrum // this is i) to ii) - received
signal x(t) c(t) w(t), where w(t) is noise //
this is ii) to iii) - (x(t) c(t) w(t)) c(t)
x(t) w(t) c(t) // this is step (iv) - low pass
filtering // this is iv) to v)
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Multipath Diversity Rake Receiver

• Instead of considering delay spread as an issue,
  use multipath signals to recover the original
  signal
• Used in IS-95 CDMA, 3G CDMA, and 802.11
• Invented by Price and Green in 1958
• R. Price and P. E. Green, "A communication
  technique for multipath channels," Proc. of the
  IRE, pp. 555--570, 1958

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Multipath Diversity Rake Receiver
LOS pulse
multipath pulses

• Use several "sub-receivers" each delayed slightly
  to tune in to the individual multipath components
• Each component is decoded independently, but at a
  later stage combined
• this could very well result in higher SNR in a
  multipath environment than in a "clean"
  environment

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Rake Receiver Blocks
Correlator
Combiner
Finger 1
Finger 2
Finger 3
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Rake Receiver Matched Filter

• Impulse response measurement
• Tracks and monitors peaks with a measurement rate
  depending on speeds of mobile station and on
  propagation environment
• Allocate fingers largest peaks to RAKE fingers

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Rake Receiver Combiner

• The weighting coefficients are based on the
  power or the SNR from each correlator output
• If the power or SNR is small out of a particular
  finger, it will be assigned a smaller weight

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Handling Delay Spread
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Reducing to Transmit Diversity

• Delay spread is really a type of transmit
  diversity

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ISI Equalization

• The problem given received ym, m 1, , L
  y1 x1 h0 w1 y2 x2h0 x1 h1
  w2 y3 x3h0 x2h1 x3 h2
  w3 y4 x4h0 x3h1 x2 h2 w4
  y5 x5h0 x4h1 x3 h2 w5
• determine x1, x2, xL
• Solution using the Viterbi algorithm

http//en.wikipedia.org/wiki/Andrew_Viterbi
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Backup Slides
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Orthogonal Frequency Division Multiplexing
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Orthogonal Frequency Division Multiplexing

• Used in current Frequency diversity
• Reduce symbol rate

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Autocorrelation of Chipping Sequence

• Choose chipping sequence with good
  autocorrelation
• E.g., Barker code () used in 802.11