Channel

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Wireless Local Networks are
Emerging
Wireless LAN
Hiperlan-2, IEEE802.11a, MMAC
ENIC 2002 -OFDM tutorial - Luc Deneire
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Wireless OFDM
Transceivers
Luc Deneire
[email protected]
Laboratoire I3S
http://www.i3s.unice.fr/
ENIC 2002-OFDM tutorial - Luc Deneire
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“Future” Broadband Wireless
Networks will be OFDM based
802.11a
Hiperlan-II
MMAC
100M
10M
1M
802.11b
Bluetooth
HomeRF
100k
Spread spectrum
OFDM
link adaptation
6 to 54 Mbit/s
for multimedia
communication
10k
time
1999
ENIC 2002 -OFDM tutorial - Luc Deneire
2000
2001
2002
3
What you will learn ...
 Indoor Propagation
 Basic OFDM concepts
 OFDM performance
 Adaptive loading
 The Hiperlan-2 OFDM system
 Implementation of Hiperlan-2 Transceivers
 Crest factor reduction
ENIC 2002 -OFDM tutorial - Luc Deneire
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Indoor Propagation
Model
ENIC 2002-OFDM tutorial - Luc Deneire
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Multipath channel in an office room.
Multipath
TX
RX
Office tables
ENIC 2002 -OFDM tutorial - Luc Deneire
Direct path
Metal
cupboards
6
The channel : a collection of delayed,
attenuated and dephased diracs
Channel
N 1
h (t ) 

ke
j k
 (t   k )
k 0
Power
delay
profile
N 1
p (t )  h (t )h (t ) 
*

 k  (t   k )
2
k 0
P
t
ENIC 2002 -OFDM tutorial - Luc Deneire
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Channels differ in
time and frequency behavior
 Coherence bandwith - delay spread
• spreading in time
• widening of impulse response due to multipath
 Coherence time - doppler spread
• spreading in frequency
• doppler effect of moving transmitter and/or
receiver
ENIC 2002 -OFDM tutorial - Luc Deneire
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Delay spread
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Delay spread measures the
“length” of the channel
 RMS delay spread is measure of the
amount of dispersion
N 1
N 1
 RMS 
 (
k
  )
k 0
N 1

k 0
2
k
2
k
kk
2
 
k 0
N 1

k
2
k 0
 10 to 100ns correspond to paths
3 to 30m
ENIC 2002 -OFDM tutorial - Luc Deneire
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Coherence bandwidth is where the
channel is “similar”(correlated)
 Autocorrelation of channel response
c
( ; D t )  0 . 5  h ( ; t ) h
*
( ; t  D t ) 
 c ( D f ,  )  0 . 5  H ( f ; t ) H ( f  D f ; t   )   Fourier ( c (.))
*
 Bcoh is defined as Df for which
 c (Df )
 c (0)
ENIC 2002 -OFDM tutorial - Luc Deneire

1
2
11
Coherence bandwith is inversely
proportional to delay spread
 c (Df )
B coh 
1
 c ( )
 rms
 rms
ENIC 2002 -OFDM tutorial - Luc Deneire
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Frequency selective fading
…. where bandwith is large ….
Compared to the coherence bandwith
• W  Bcoh  frequency selective channel
Bcoh
W
• W  Bcoh  frequency nonselective channel
Bcoh
ENIC 2002 -OFDM tutorial - Luc Deneire
W
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Frequency selective channels
introduce Inter Symbol Interference
Incoming signal
Channel impulse
response
Outcoming signal
ENIC 2002 -OFDM tutorial - Luc Deneire
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Coherence Time (10-50 ms indoor):
time in which a channel is “stable”
S(f)
Bd
 c (Dt)
Fourier
(Dt)c 
1
Bd
Signals sent at these instants
see uncorrelated channels
ENIC 2002 -OFDM tutorial - Luc Deneire
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Subsequent symbols see
different channels in fast fading
 Tb  (Dt)c 
fast-fading channel
 Tb  (Dt)c 
slow-fading channel
ENIC 2002 -OFDM tutorial - Luc Deneire
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Propagation overview
Summary of channel properties
Slow fading
Low Doppler
Fast fading
High Doppler
Frequency dispersion
Using the previous
measures on
characteristics we
can place radio
channels in four
groups.
NOTE that the
classification
is in relation to
the transmission
bandwidth/symboltime.
Time dispersion
This is where
we will try to
fit the subcarriers in
OFDM.
Frequency flat
Short channel
Frequency selective
Long channel
B TX  B d
B TX  B coh
or
or
T S  t coh
T S   RMS
B TX  B d
ISI-free and
ISI and
or
flat-fading
flat-fading
T S  t coh
channel
channel
B TX  B coh
ISI-free and
ISI and
or
fast-fading
fast-fading
T S   RMS
channel
channel
ENIC 2002 -OFDM tutorial - Luc Deneire
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Question
 Assume a wireless system making use of
BPSK modulation at 10Mbps.
 The system is used indoor. There are two
signal paths between Tx and Rx with a
relative distance of 10m.
 How many symbols are affected by the
channel?
 What happens if the relative distance
becomes 100m? What if the datarate
becomes 100Mbps?
ENIC 2002 -OFDM tutorial - Luc Deneire
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Answer
 Datarate 10Mbps
• Tsymbol=100ns
 Distance 10m
• delay = distance / c = 10 / 3.108 s = 30ns
• delay / Tsymbol = 0.3
 For 100m
• delay / Tsymbol = 3
 For 100m, 100Mbps
• delay / Tsymbol = 30
ENIC 2002 -OFDM tutorial - Luc Deneire
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What to do against ISI?
 Wideband signals:
• channel delay = many symbol periods
• heavy distortion of the received signal.
 Several techniques can be applied to
reduce or get rid of ISI in wideband signal
transmission
• equalization,
• spread-signal modulation,
• OFDM
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An Equalizer is a costly filter
Signal (channel) spectrum
t
Equalizer
t
Equalized signal
ENIC 2002 -OFDM tutorial - Luc Deneire
t
f
f
f
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OFDM avoids ISI by slowing pace
needs linear amp + sync
 Symbols of high bit rate signal are distributed
over a large number of subcarriers.
• Low symbol rate per carrier.
• Individual carrier signals see flat fading (no ISI).
 Promising technique for future high bit-rate
applications.
 However, it suffers from a number of problems:
• a very linear amplifier in the transmitter is required to prevent
signal distortion,
• accurate synchronization in the receiver is needed,
• in the transmitter and receiver real-time discrete Fourier
transform (DFT) operations have to be computed.
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM
basic principles
ENIC 2002-OFDM tutorial - Luc Deneire
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OFDM is Multi-Carrier
and lowers the symbol rate :
less ISI
f1
T / sec
f2
fn
...
f
T/n / sec
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM : Overlapping spectra to save bandwith
(a )
(b )
ENIC 2002 -OFDM tutorial - Luc Deneire
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Overlapping spectra are orthogonal
to enable proper reception
of individual carriers
(a)
Orthogonality to avoid
inter carrier interference:
signal design + frequencies
ENIC 2002 -OFDM tutorial - Luc Deneire
(b)
T
 sinc(
f i t ) sinc( f j t ) dt   ij
0
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Recent applications of OFDM
 high-bit-rate digital subscriber lines
(HDSL; 1.6 Mbps),
 asymmetric digital subscriber lines
(ADSL; up to 6 Mbps),
 very-high-speed digital subscriber lines
(VDSL; 100 Mbps),
 digital audio broadcasting (DAB),
 high definition television (HDTV)
terrestrial broadcasting,
 WLAN (6-54Mbps) indoor communication
(IEEE802.11a/g, ETSI Hiperlan/2)
ENIC 2002 -OFDM tutorial - Luc Deneire
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Advantages of OFDM
 OFDM deals with multipath
At low COST (implementation)
 OFDM enables adaptive loading :
Bit rate RISES with SNR
ON EACH carrier
 OFDM is robust against narrowband interference,
Inteference affects only part of the
carriers.
ENIC 2002 -OFDM tutorial - Luc Deneire
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Disadvantages of OFDM
 sensitive to frequency offset and
phase noise.
 large peak-to-average power ratio,
==> low power efficiency of the RF
amplifier.
ENIC 2002 -OFDM tutorial - Luc Deneire
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Parameters for designing an
OFDM System
 number of subcarriers,
 guard time,
 symbol duration,
 subcarrier spacing,
 modulation type per subcarrier,
 the type of forward error correction
coding
ENIC 2002 -OFDM tutorial - Luc Deneire
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Choice of parameters is
influenced by system
requirements
 available bandwidth,
 required bit rate,
 tolerable delay spread and
 Doppler values
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM modulation can be realized
with IFFT
 An OFDM signal consists of a sum of subcarriers
which are modulated by using Phase Shift
Keying (PSK) or Quadrature Amplitude
Modulated (QAM).
exp(- j  N s(t-t s)/T )
QAM
Data
Serial
To
Parallel
+
OFDM
Signal
exp( j  (N s-2)(t- t s)/T )
OFDM modulator block diagram
ENIC 2002 -OFDM tutorial - Luc Deneire
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Time domain view of OFDM
 All subcarriers have the same phase and
amplitude, but in practice the amplitudes and
phases may be modulated differently for each
subcarrier.
Example of 4 subcarriers within one OFDM symbol.
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The OFDM spectrum fulfills
Nyquist’s criterium for an intersymbol interference free pulse
shape
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Impact of channel on OFDM
Reception
 Multipath channel spreads energy of
one symbol into adjacent symbol.
Results in ISI between symbols
 Solutions
• make symbols longer by using more carriers,
ISI neglegible. But, negative impact due to
coherence time, FFT size and latency
• use guard interval between symbols
ENIC 2002 -OFDM tutorial - Luc Deneire
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Principle of guard interval
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Transmitters and receivers
... through the channel ...
Channel
Noise
n t 
hch t 
s t 
r t   s t  * hch t   n t 
}
t
Tch
s t 
CP
r t 
CP
CP
LT samp
}
}
t
CP
t
Tch
As long as the CP is longer than the delay spread of the
channel, the CP will absorb the ISI.
ENIC 2002 -OFDM tutorial - Luc Deneire
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Guard time reduces ISI
 The most important reasons to do OFDM
is the efficient way it deals with multipath
delay spread. By dividing the input data
stream in Ns subcarriers, the symbol
duration is made Ns times larger, which
also reduces the relative multipath delay
spread - relative to the symbol time - by
the same factor.
 To eliminate intersymbol interference
almost completely, a guard time is
introduced for each OFDM symbol.
ENIC 2002 -OFDM tutorial - Luc Deneire
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What to transmit during guard
interval?
 guard time > delay spread
• multipath components from one symbol
cannot interfere with the next symbol.
 The guard time could consist of no
signal at all. However, in that case
the problem of inter carrier
interference (ICI) would arise. ICI is
cross-talk between different
subcarriers, which means they are
no longer orthogonal.
ENIC 2002 -OFDM tutorial - Luc Deneire
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Effect of multipath with zero signal in
the guard time; the delayed subcarrier
#2 causes inter carrier interference
(ICI) on subcarrier #1 and vice-versa.
P ar t of sub carr ier # 2 ca usin g
IC I o n sub carr ier # 1
Su bcarr ie r #1
D elay ed sub carr ier # 2
G uar d Ti m e
FF T In te gra tio n Ti me = 1/Ca rr ier S pa cing
O FD M Sy mb ol Ti m e
ENIC 2002 -OFDM tutorial - Luc Deneire
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Guard time with cyclic extension
 Cyclic extension in
guard
 Delayed replicas of
OFDM
symbols
have
integer
number of cycles in
FFT interval
 No ICI if guard is
longer than signal
delay
G uard T im e / C yclic P refix
ENIC 2002 -OFDM tutorial - Luc Deneire
F F T Integration T im e = 1/C arrier S pacing
O F D M S ym bol T im e
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Example of an OFDM signal with 3 subcarriers
in a 2-ray multipath channel. The dashed line
represents a delayed multipath component.
First arriving path
R eflection
R eflection delay
O FD M Sym bol T im e
G uard T im e
FFT Integration T im e
Phase T ransitions
No crosstalk (ICI) between carriers, but distortion per carrier.
Freq domain equalization needed.
ENIC 2002 -OFDM tutorial - Luc Deneire
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Implementation complexity of
OFDM vs single carrier
modulation
 OFDM has the ability to deal with large
delay spreads with a reasonable
implementation complexity. Frequency
domain equalizer needed.
 In a single carrier system, the
implementation complexity is dominated
by equalization, which is necessary when
the delay spread is larger than about 10%
of the symbol duration.
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Implementation complexity of
OFDM vs single carrier
modulation (cont.)
 For Single carrier systems with equalizers, the
performance degrades abruptly if the delay spread exceeds
the value for which the equalizer is designed and because
of error propagation, the raw bit error probability increases
so quickly that introducing lower rate coding or a lower
constellation size does not significantly improve the delay
spread robustness.
 For OFDM, there are no such nonlinear effects as error
propagation, and coding and lower constellation sizes can
be employed to provide fallback rates that are significantly
more robust against delay spread. This enhances the
coverage area and avoids the situation that users in bad
spots cannot get any connection at all.
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OFDM Performance
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OFDM Performance:
Assumptions
 The impulse response of the channel
is shorter than the cyclic prefix
 Transmitter and receiver are
perfectly synchronised
 Channel noise is additive, white and
Gaussian
 The fading is slow enough to
consider the channel constant
during one OFDM symbol
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM Performance: Transmitter
x0,k
x1,k
IDFT
xN-1,k
s0,k
s1,k
sN-1,k
P
to
S
Add
Cyclic
Prefix
Htr
s(t)
xk
s m ,k 
1
N
N 1
x
n0
n ,k
nm 

exp  j 2

N 

 
s ( t )  h tr t    
 k  
N 1

 s m , k  t  k  N  v T  mT  
mv

m : sample index, n : carrier index, k : symbol index
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM Performance: Channel
n(t)
s(t)
Hch
+
r(t)
r t   h ch t   s t   n t 
with h ch t   0 for t  0 and t  T ch
Channel
Input
Cyclic
prefix
IFFT
Channel
Output
ENIC 2002 -OFDM tutorial - Luc Deneire
Tch
48
OFDM Performance: Receiver
q(N+v)T+pT
r(t)
Hre
r’(t)
r0,q
Remove
Cyclic
Prefix
S
D
to r1,q F
P
T
rN-1,q
y0,q
y1,q
yN-1,q
yq
r t   h re t   r t 
r p , q  r  q  N  v )T  pT
N 1
y n ,q 
r
p0
p ,q

np 

exp   j 2

N 

p : sample index, n : carrier index, q : symbol index
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM Performance: Combined
Model
 Combine transmit, channel and
receive filters
H ( f )  H tr ( f ). H ch ( f ). H re ( f )
N ( f )  H re  f . N  f

 Received Signal:
r t     s . h t  k  N

N 1
m ,k
 v T  mT   n t 
k   m   v

rp , q 
N 1
 s
m ,k
. h  q ( N  v ) T  pT  k  N  v T  mT
k   m   v
 n  q ( N  v ) T  pT
ENIC 2002 -OFDM tutorial - Luc Deneire

50

OFDM Performance: Combined
Model
 Impulse response of h < v
0   q ( N  v ) T  pT  k  N  vm T  mT

v
for p  0 ... N  1
 q  k and m  ( p  v )... p
 Substitute in received signal
p
rp , q 
s
m ,q
. h  pT  mT   n p , q
m  pv
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM Performance: Combined
Model
 Expand sm,q
p
rp ,q 

m  pv

1
N

1
N
1
N
N 1
x
n0
N 1
x
n ,q
n0
N 1
x
n0
n ,q
n ,q
nm 

exp  j 2
.h  pT  mT   n p , q
N 

np  v

exp  j 2
  exp
N  z0

nz 

  j 2
.h  zT   n p , q
N 

np 

exp  j 2
 h n  n p , q
N 

 Vector Notation
ENIC 2002 -OFDM tutorial - Luc Deneire
rq  IDFT h . x q   n q
52
OFDM performance: Combined
Model
 Calculate yq:
y q  DFT rq 
 DFT IDFT h . x q   n q 
 h . x q  DFT n q 
 h .x q  n q
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM Performance: AWGN
10
10
10
Pe
10
10
10
10
10
10
0
N v
loss  log 10 

 N 
-1
-2
-3
-4
-5
-6
-7
ideal
N=256,v=16
-8
2
4
ENIC 2002 -OFDM tutorial - Luc Deneire
6
8
Eb/No
10
12
14
54
OFDM Performance: Rayleigh
Channel
 Rayleigh received SNR per bit PDF:
 g 
Pg g  
exp   
g
 g 
M
 Calculate BER(g) with standard
formula.
 Calculate E{BER}:
BER g  Pg g d g

E BER  
 Pg g d g
ENIC 2002 -OFDM tutorial - Luc Deneire
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OFDM performance: Rayleigh
Channel
0
10
-1
Pe
10
-2
10
-3
10
-4
10
5
ENIC 2002 -OFDM tutorial - Luc Deneire
10
15
Eb/No
20
25
30
56
Coded OFDM
error
coding
S
to
P
transmit
channel
receive
P
to
S
error
decoding
 Block codes
 Convolutional codes
 Concatenated codes
 Trellis coded modulation of the
carriers
ENIC 2002 -OFDM tutorial - Luc Deneire
57
Coded OFDM: interleaving
time
interleaver
error
coding
S
to
P
frequency
interleaver
transmit
 Time interleaver: block interleaver
 Frequency interleaver:permutation
over carriers
ENIC 2002 -OFDM tutorial - Luc Deneire
58
Summary
 Channel charactersitics
• delay spread (50ns) - coherence bandwidth
- frequency (non) selective fading
• coherence time (50ms) - doppler spread
- fast fading Vs slow fading
 OFDM: orthogonal frequency division
multiplexing
• split high speed serial data in Nc lower rate parallel
streams
• less impact from ISI because symbols are longer
• guard interval with cyclic prefix is used to overcome
almost all ISI
ENIC 2002 -OFDM tutorial - Luc Deneire
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Summary 2
 OFDM alone has bad performance on
fading channel.
100
Pe
10-1
10-2
10-3
10-4
5
10
15
Eb/No
20
 Additional technique needed to
exploit diversity:
25
30
• error coding,
• adaptive loading.
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60
Adaptive Loading
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Adaptive Loading: Principle
 Estimate the attenuation per carrier
and the noise power.
 Adapt power and bit distribution per
carrier to the measured frequency
response.
 Achieves capacity (if waterfilling
distribution of power and infinite
number of carriers).
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Adaptive Loading: Channel
Estimation
 Reference symbol x(k) is an arbitrary BPSK
sequence of length N.
 The reference sequence s(i) is a n-fold repetition
of the IFFT of the reference symbol.
 The reference sequence is distorted in the
channel:
r (i )  s (i )  h (i )  n (i )
 Take the FFT of each received FFT-symbol
separately, for the jth symbol resulting in:
y ( k )  H ( k ). x ( k )  N ( k )
j
ENIC 2002 -OFDM tutorial - Luc Deneire
j
63
Adaptive Loading: Channel
Estimation
 Multiply yj(k) with the reference
symbol:
z ( k )  H ( k )  N ( k ). x ( k )
j
j
 Average over the n FFT-symbols:
n
1 n j
1
~
j
H ( k )  .  z ( k )  H ( k )  . x ( k ).  N ( k )
n j 1
n
j 1
 Apply low-pass filter over carriers to
obtain Hf(k).
ENIC 2002 -OFDM tutorial - Luc Deneire
64
Adaptive Loading: Noise
Estimation
 Same reference sequence.
 Uses filtered channel estimation to
estimate noise signal:
1 n
~
~
j
2
N ( k )  .  | y ( k )  H f ( k ). x ( k ) |
n j 1
 Average over all carriers (assuming
white Gaussian noise):
N
~
~
N a   N (i )
i 1
ENIC 2002 -OFDM tutorial - Luc Deneire
65
Adaptive Loading: Loading
Algorithms
 Hughes-Hartogs:
• Maximises the datarate for a given BER.
• Allocate bit by bit, each time selecting the
carrier with the smallest additional transmit
power for a requested BER.
• Algorithmic complexity: O(RT x Nc) with RT the
number of assigned bits.
ENIC 2002 -OFDM tutorial - Luc Deneire
66
Adaptive Loading: Loading
Algorithms
 Chow et al.:
• Assign the bits according to:

SNR i
R i  log 2  1 

  g m arg in


 with


R i the data rate for carrier i
SNR i the signal to noise ratio for carrier i
 the difference
between th e channel capacity and the actual usage
g margin determined
iterativel y to guarantee
R
i
 RT
i
• Algorithmic Complexity: O(Niter x Nc + 2Nc)
ENIC 2002 -OFDM tutorial - Luc Deneire
67
Adaptive Loading: Loading
Algorithms
 Fischer et al.:
• Minimizes BER for a constant data rate RT
(application) and a constant total transmit
power ST
• Assign the bits according to:
D
Ri 
RT
D

1
D
N
 log 2 (
l
l 1
D
Ni
)
with
D number of carriers in specific iteration
N i the equivalent noise power for carrier i
R i the data rate for carrier i
ENIC 2002 -OFDM tutorial - Luc Deneire
68
Adaptive Loading: Loading
Algorithms
• Iterate until all Ri  0 (exclude negative rate
carriers)
• Quantize Ri and assure
D

R i  RT
i 1
• Adapt power per carrier to compensate for
quantization
ENIC 2002 -OFDM tutorial - Luc Deneire
69
Adaptive loading: Operation
ENIC 2002 -OFDM tutorial - Luc Deneire
70
Adaptive Loading: Performance
ENIC 2002 -OFDM tutorial - Luc Deneire
71
Adaptive Loading: OFDMA
Principle
 Estimate for each user the
attenuation per carrier and the noise
power.
 Assign carriers to users based on
these estimates.
 Optionally, adapt the power and bit
distribution per carrier to the
measured frequency response.
ENIC 2002 -OFDM tutorial - Luc Deneire
72
Adaptive Loading: OFDMA
Operation
ENIC 2002 -OFDM tutorial - Luc Deneire
73
Hiperlan/2 case study
ENIC 2002-OFDM tutorial - Luc Deneire
74
Hiperlan/2 Positioning - Mobility
vs. Bitrate
Outdoor
Walk
Stationary
Indoor
Mobility
Vehicle
W-CDMA/
EDGE
HIPERLAN/2
Walk
Stationary/
Desktop
Bluetooth
0,1
LAN
1
10
100
Mbps
User Bitrates
ENIC 2002 -OFDM tutorial - Luc Deneire
75
Hiperlan/2 Positioning - Cost vs.
Bitrate
$
User Cost / bit
High
Medium
W-CDMA/
EDGE
HIPERLAN/2
Low
Very Low
Bluetooth
0,1
LAN
1
10
100
Mbps
User Bitrates
ENIC 2002 -OFDM tutorial - Luc Deneire
76
Hiperlan/2 protocol architecture
U se r Plan e
C on tr ol P lan e
H ig h e r La ye rs
C L S A Ps
C o nv er ge n c e L a ye r
D L C C on tr ol S A P
D L C U se r SAP
R ad io L in k Co n tro l s u b la ye r
O ne ins ta n ce pe r M A C ID
R ad io
R e s our c e
C ont rol
A s so c ia tion
C ont rol
D LC
C onne c tio n
C ont rol
O ne ins ta n ce pe r DLC
U ser C on n ec tio n ,
id en tifi e d b y DUC ID
(MA C I D + D LCC ID)
D ata L ink C o n tro l B as ic D ata T ran sp o rt Fu n c tion
E rro r C o ntro l
S co p e o f
H IP E RL A N /2
s ta nd ard s
R LC
O ne ins ta n ce pe r A P
M ed ium A cc es s Co ntro l
P h y sica l La ye r
ENIC 2002 -OFDM tutorial - Luc Deneire
77
Basic MAC frame structure
2ms
MAC-Frame
BCH
FCH
MAC-Frame
ACH
MAC-Frame
MAC-Frame
DL phase
UL phase
RCHs
DiL phase
SCH
...
SCH
DL to one MT
... LCH
LCH
SCH
...
LCH
One DLC connection
One PDU train mapped one PHY burst
ENIC 2002 -OFDM tutorial - Luc Deneire
78
Hiperlan-2 Transmitter PHY Model
scrambler
FEC
coder
interleaver
burst
formatter
ENIC 2002 -OFDM tutorial - Luc Deneire
mapper
OFDM
mod
radio
transmitter
79
Data scrambler
 S(x) = X7 + X4 + 1
 n4n3n2n1: frame counter, first 4 bits of
broadcast channel (BCH)
PDU train
in
Initialization
sequence
1 1 1
n4 n3 n2 n1
X7 X6 X5
X4 X3 X2 X1
Scrambled
PDU train
out
ENIC 2002 -OFDM tutorial - Luc Deneire
80
FEC coder
Scrambled
PDU train
Append six
tail bits
Convolutional
encoder
X
Y
Puncturing P1
with serial
output
Puncturing
P2
Channel coded
PDU train
Output data X
Input data
Tb
Tb
Tb
Tb
Tb
Tb
Output data Y
ENIC 2002 -OFDM tutorial - Luc Deneire
81
Data interleaver
 Block interleaver with size equal to
number of bits in OFDM symbol
 Two step permutation
1. adjacent coded bits (k) mapped onto non
adjacent subcarriers (i)
i = (NCBPS / 16) (k mod 16) + floor(k/16)
2. adjacent coded bits (i) mapped alternately
onto LSB and MSB of constellations (j)
j = s*floor(i/s) + (i+ NCBPS - floor(16*i/ NCBPS)) mod s
s = max(NBPSC / 2, 1)
ENIC 2002 -OFDM tutorial - Luc Deneire
82
Mapper
 Gray coded
constellation
mapping
16QAM
b1b2b3b4
00 10
01 10
11 10
 Normalization to
achieve same
average power for
all constellations
10 10
+3
00 11
01 11
11 11
10 11
• BPSK: 1
• QPSK: 1/sqrt(2)
• 16QAM: 1/ sqrt(10)
+1
-3
-1
+1
+3
00 01
01 01
11 01
10 01
11 00
10 00
• 64QAM: 1/ sqrt(42)
-1
00 00
01 00
-3
ENIC 2002 -OFDM tutorial - Luc Deneire
83
OFDM Modulation parameters
 Different modulation schemes allow
variable bitrate and QoS
Mo du la ti on
Co di ng r a teR
BPSK
BPSK
QPSK
QPSK
16QA M
16QA M
64QA M
1/2
3/4
1/2
3/4
9 / 16
3/4
3/4
No m i na l b it r a te
[Mb it/ s ]
6
9
12
18
27
36
54
ENIC 2002 -OFDM tutorial - Luc Deneire
Co de d b its pe r
s u b-ca r ri er
N BPS C
1
1
2
2
4
4
6
Co de d b its pe r
OF D Msym b ol
N CB PS
48
48
96
96
192
192
288
Dat a b its pe r
OF D Msym b ol
N DB PS
24
36
48
72
108
144
216
84
OFDM modulation parameters
Pa r ame ter
Sa m p ling ra t e f s =1 / T
Us e f u l s ym bo l pa rt du ra ti on T U
Va l ue
20 MH z
64 * T
3 . 2 s
Cy clic pr ef ix du ra ti on T CP
S y mbo l i nte rv a l T S
Nu mbe r of data sub -c a rri e rs N SD
Nu mbe r of p ilot sub -c a rri e rs N SP
To ta l numbe r of sub -c a rri e rs N S T
Sub -c a rri e r s pac ing D f
Spa ci ng be t ween the t
w o ou t mo s tsub -c a rri e rs
Compromise between
coherency time and bandwidth
16 * T
8*T
0 . 8 s (m anda tor y)
0 . 4 s (op ti ona l)
80 * T
72 * T
4 . 0 s (T U + T CP )
3 . 6 s (T U + T CP )
48
4
52 ( N SD + N SP )
0 . 3125 M Hz (1 / T U )
Allows delay spread of 250ns
16 . 25 M Hz (N S T * D f )
TCP
TU
CP
Data n
Copy
ENIC 2002 -OFDM tutorial - Luc Deneire
85
OFDM sub-carrier frequency
allocation and guard interval
D0,n
D4,n D5,n
P0,n
D17,n D18,n
D23,n D24,n
D29,n D30,n
P1,n
P2,n
D42,nD43,n
D47,n
P3,n
DC
-26
-21
-7
0
7
21
26
IFFT
TCP
TU
CP
Data n
Copy
ENIC 2002 -OFDM tutorial - Luc Deneire
86
Burst formatter
 Five different PHY bursts
• broadcast burst
• downlink burst
• uplink burst with short preamble
• uplink burst with long preamble
• direct link burst
 Each burst consists of preamble
followed by payload data
ENIC 2002 -OFDM tutorial - Luc Deneire
87
Broadcast burst preamble
tPREAMBLE=16.0s
Section 1
5*0.8s=4.0s
A
IA
A
IA
Section 2
5*0.8s=4.0s
IA
B
B
B
B
Section 3
2*0.8s+2*3.2s=8.0s
IB
CP
C
C
Copy
 Enables
• frame time synchronization
• automatic gain control
• carrier frequency synchronization
• channel estimation
 Preamble has low PAPR (3dB) so non
linearities of PA do not affect AGC
ENIC 2002 -OFDM tutorial - Luc Deneire
88
Time synchronization based on
auto correlation of A and B-fields
Correlation window
A
IA
A
IA
IA
B
B
B
B
IB
CP
C
C
conj
A
IA
A
IA
IA
B
B
B
B
IB
Moving average
CP
C
C
Correlation window
A
IA
A
IA
IA
B
B
B
B
IB
CP
C
C
conj
A
IA
A
IA
IA
B
B
B
ENIC 2002 -OFDM tutorial - Luc Deneire
B
IB
CP
Moving average
C
C
89
Frame time synchronization
Ro b u s tn ess
a g ai n s t
mu lti p a th ,
CF O
A cc u ra c y
Imp le m en tai to n
cost
A u to
c or re latio n
H igh
lo w
lo w
Cr os s
c or re latio n
lo w
h ig h
h ig h
ENIC 2002 -OFDM tutorial - Luc Deneire
90
Automatic gain control
 Based on non-coherent energy
measurement at baseband
• S (input . input*)
 Gain is kept constant during burst
because signal is non constant
envelope
ENIC 2002 -OFDM tutorial - Luc Deneire
91
Carrier Frequency
Synchronisation
 Estimate CFO (Df) with C-field
 Apply correction to the input
datastream
Cyclic
prefix
C
C
conj.
angle
ENIC 2002 -OFDM tutorial - Luc Deneire
  2  . D f .T
92
Channel estimation and
equalization based on C-field
 compensate with a rotator per
carrier, i.e. a frequency domain
equalizer.
training symbol
Cyclic
prefix
data symbols
C
...
Cyclic
prefix
data
Known
transmitted
a
e
 j
X
received
ENIC 2002 -OFDM tutorial - Luc Deneire
93
Radio transmission
 Frequencies
• 8 bands between 5.15 and 5.35GHz with
23dBm EIRP
• 11 bands between 5.47 and 5.725 with 30dBm
EIRP
• 20MHz carrier spacing
ENIC 2002 -OFDM tutorial - Luc Deneire
94
Spectrum Allocation at 5 GHz
5.150
5.250
Japan
5.150
5.350
5.725
5.825
USA
Indoor 200 mW / Outdoor 1 W EIRP
5.150 DFS & PC
5.350
Outdoor 4 W EIRP
DFS & PC
5.470
5.725
Max mean
Tx power
Europe
Indoor 200 mW EIRP
5.100
5.200
5.300
Max peak
Tx power
Outdoor 1W EIRP
5.400
5.500
5.600
5.700
5.800
5.900
Freq./GHz
DFS:
Dynamic Frequency Selection
PC:
Power Control
ENIC 2002 -OFDM tutorial - Luc Deneire
95
Hiperlan-2 receiver block diagram
de
scrambler
FEC
decoder
freq
domain
EQU
de
interleaver
FFT
ENIC 2002 -OFDM tutorial - Luc Deneire
time
domain
sync &
AGC
de
mapper
radio
receiver
96
Main differences between H2 and
802.11a
 Protocol
• 802.11a: MAC with CSMA/CA
• H2: centralized resource allocation
 Preamble
• 802.11a: only 1 preamble similar to long uplink
preamble of H2, because only a single type of
packets exists
 Modulation modes
• 802.11a does not foresee 27Mbps, instead it
has modes of 24Mbps, 48Mbps
ENIC 2002 -OFDM tutorial - Luc Deneire
97
Implementation of
Hiperlan2
transceivers
ENIC 2002-OFDM tutorial - Luc Deneire
98
The implementation of OFDM
modems is a challenge
 Is the implementation of the
IDFT/DFT a showstopper?
 What do we need extra in an OFDM
modem?
 What performance can be expected?
 How difficult is the radio front-end?
ENIC 2002 -OFDM tutorial - Luc Deneire
99
In half-duplex operation the
DFT/IDFT can be shared
(D)PSK or
QAM Modulation
Time &
Carrier
Synchro
Remove
Extension
IDFT
DFT
Cyclic
Extension
Equalizer
(D)PSK or
QAM Demodulation
shared
ENIC 2002 -OFDM tutorial - Luc Deneire
100
DFT/IDFT implementation based
on hierarchical decomposition
 DFT/IDFT definition:
N 1
Xk 
x
m
WN
mk
and x m 
m 0
with W N
  j 2
 exp 
 N
1
N
N 1

X kW N
 mk
k 0

 , m  0 ,1 ,...,N  1

and k  0 ,1,..., N  1
 hierarchical decomposition:
• decimation in frequency (DIF)
• decimation in time (DIT)
ENIC 2002 -OFDM tutorial - Luc Deneire
102
DIF of a DFT results in two
smaller DFTs
N
Xk 
2
1
N
x
m
WN
mk
WN
m 0
N

2

mk
m 0
N
X 2r 
2
X 2 r 1 
N
2
m
W N
k
. x m    1  x  N  m  
2


x m W N
mr
2
m 0
2
 x
mk
1

N
2
1
m 0
1
WN
2
N .k
1

x m W N
m 0
ENIC 2002 -OFDM tutorial - Luc Deneire
mr
2
with x m  x m  x  N  m 
2
m




with x m   x m  x  N  m  .W N
2


103
We get a recursive DFT structure
based on butterfly operations
butterfly operation
x0
x1
+
S
+
...
xN/2
xN/2+1
...
xN-1
...
N/2-taps
FFT
...
...
N/2-taps
FFT
...
+
-
S
ENIC 2002 -OFDM tutorial - Luc Deneire
W0
104
Recursive Radix Algorithm is
based on radix-4 decomposition
 Radix-4 decomposition:
N / 4 1
X k1  4 k 2 

3
W
k1 n 2
N
.W
k 2 n2
N 4
n2  0
x
n1  0
N
4
n1  n 2
.W
k 1 n1
4
 Remap indexes according to radix-2:
N / 4 1
X k11  2 k12  4 k 2 

BF  n11 , n12 , n 2 , k 11 , k 12 .W N 24 2
k n
n2  0
with BF  n11 ,n 12 ,n 2 ,k 11 ,k 12 
1

W
k 11u 2
4
.W
k 12 u 2
2
 k 11  2 k 12  n 2
.W N
u2 0
1

u1  0
x N
2 u1   N
u k
1 11
.
W
4 u 2  n 2
2
 Apply these steps recursively
ENIC 2002 -OFDM tutorial - Luc Deneire
105 105
The basic structure is a radix-4
butterfly
xr
x N/4+r
x N/2+r
x 3N/4+r
S
S
S
S
S
S
2
WN
1
WN
S
S
-j
ENIC 2002 -OFDM tutorial - Luc Deneire
0
WN
3
W N-
106
Optimizing wordlengths results in
a considerable gain
ENIC 2002 -OFDM tutorial - Luc Deneire
108
OFDM Modem: 256 point (I)FFT
 0.5 CMOS, TLM,
3V
 50 MHz
 Throughput 195
kFFT/s
 2.5 * 2.5 mm2
 31000 gates
 384 bytes RAM
ENIC 2002 -OFDM tutorial - Luc Deneire
109
Implementation: (I)FFT
O rga nisa tio n
C NE T
C NE T
# poi nts
8 192
2 048
clock
2 0 M Hz
2 0 M Hz
P o w er
6 00 m W
3 00 m W
T e ch
0 .5  m
0 .5  m
C NE T
St an for d U ni v.
1 024
1 024
2 0 M hz
1 6 M Hz
9 .5 mw
0 .5  m
0 .7  m
4 0 mm _
5 0 mm _
2 1 Kc e lls
4 60 .000
St an for d U ni v.
Ma cq uar ie Uni v.
1 024
16
1 73 MH z
5 0 M Hz
8 45 m W
80 m W
0 .7  m
0 .6  m
5 0 mm _
6 mm_
4 60 .000
7 0.000
IM E C
2 56
5 0 M Hz
-
0 .3 5  m
2 0 mm _
2 00 .000
ENIC 2002 -OFDM tutorial - Luc Deneire
A re a
1 00 mm_
1 00 mm_
# tra ns ist or s
1 .5 00 .000
1 .5 00 .000
110
Fast acquisition is crucial in a
burst mode system
carrier
symbol
OFDM OFDM
offset
timing
data
data
sequence sequence sequence sequence
...
OFDM
data
sequence
...
OFDM
data
symbol
Time-domain
acquisition
sequence
OFDM
reference
symbol
OFDM
data
symbol
Cyclic
prefix
ENIC 2002 -OFDM tutorial - Luc Deneire
111
Time synchronisation is based on
repetition of known sequence
TS1 TS2 TS1 TS2 TS1 …. TS1 TS1 ….
RTS
ENIC 2002 -OFDM tutorial - Luc Deneire
ATS
COS-1 COS-2 ….
Frame start
112
Initial carrier frequency compensation is in time-domain
ENIC 2002 -OFDM tutorial - Luc Deneire
113
Centralized data re-ordering
minimizes memories
mapper
r=2
(4kb)
IFFT
FFT
SSR
RAM 1 RAM 2
r=2 (4kb)
r=2 (8kb)
refsym
SYNC
r=1
(8kb)
equalizer
demapper
r=2 (7kb)
: Tx datapath
: Rx datapath
ENIC 2002 -OFDM tutorial - Luc Deneire
: reference signal
114
Adaptive equalizer has low
implementation cost
Refsym
RAM
mode (feedback, reference)
conjugate
Gain
control
yi
COEF
RAMS
conjugate
Decide
& rotate
y’i
ENIC 2002 -OFDM tutorial - Luc Deneire
Integrate
& dump
mode (single, average)
115
Programmability is a must for
OFDM modems
P a ra m ete r
O ption s
N u m ber of carrie rs
6 4, 12 8, 2 56
G uar d in terv al
0: 4:28
M odu lat ion
QPSK ( BPSK )
E q ual izer m o des
S p ectr al m as k
RE F-FF , SC -F B , AC -F B
ref eren ce seq uen ce
C o m pl ex, per carri er
FF T clipp ing
5- 8b, MSB or LSB al ign ed
S pr eadi ng
1, 2, 4, 8; co de s equ enc e
A cqui sit ion
sequ enc e, l engt h,
conf iden ce fac tors
N u m ber of zero
carrier s
L ow : 0:1 :3, left and r ight
H ig h: 0 :2:3 0, l eft a nd ri ght
ENIC 2002 -OFDM tutorial - Luc Deneire
116
Token flow control: integration =
communication
Repeated
token
Single
token
Token flow produced:
1
0
1
0
‘smart’
receiver
‘smart’
sender
Trigger
at
‘1’
ENIC 2002 -OFDM tutorial - Luc Deneire
Soft initial token arrival time
specification: Tmin < T < Tmax
=> boundary check only
=> supports IP block strategy
117
Clock gating is essential for lowpower operation
REQ ACQ
A
EOB
73%
19%
2%
6%
6%
sleep RX RX
RX
SYNC CFO demod
‘IEEE’ mode @ 50 MHz
Power
GOPS
Tx mode
670 mW
3.8
Rx mode
570 mW
6.8
Sleep
150 mW
n/a
ENIC 2002 -OFDM tutorial - Luc Deneire
T
#acc/s Gbit/s
1.32 G 21.3
1.28 G 20.3
n/a
n/a
118
Putting it all together
 0.35 CMOS,
3.3V, 5LM
 50MHz
 144pins
 210 kgates
 10 RAMs
 20mm2
 670 mW
ENIC 2002 -OFDM tutorial - Luc Deneire
119
Improved equalizer for better
performance with QAM
QAM64 BER performance
1.E+00
Improved equalizer
Standard equalizer
bit error rate
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
12 13 14 15 16 17 18
SNR per bit (dB)
ENIC 2002 -OFDM tutorial - Luc Deneire
120
Second generation OFDM Modem
supports QAM-64
 0.18 CMOS 5LM
 20MHz
 160pins
 500 kgates
 19 RAMs
 25mm2
ENIC 2002 -OFDM tutorial - Luc Deneire
121
Crest Factor
Reduction
ENIC 2002-OFDM tutorial - Luc Deneire
122
Crest Factor: Definitions
 Peak-to-Average Power Ratio:
PAPR

s

2
m ,k

max

E s m ,k
2

 N
 Crest factor
CF 
PAPR
 Example
N  256  PAPR  23 dB , CF  12 dB
ENIC 2002 -OFDM tutorial - Luc Deneire
123
We get a peak amplitude if all
tones add in phase
70
60
50
40
30
20
10
0 0
50
100
ENIC 2002 -OFDM tutorial - Luc Deneire
150
200
250
300
124
Large peaks do not occur very
often (Gaussian distribution)
12000
10000
8000
6000
4000
2000
0
4
5
6
7
8
9
10
11
12
13
14
PAPR(dB)
ENIC 2002 -OFDM tutorial - Luc Deneire
125
The Crest Factor has major
impact on implementation
 Large CF with non-linear power
amplifier
• in-band distortion
• spectral spreading
 Lineair power amplifier or operated
with large back-off:
• expensive
• power inefficient
ENIC 2002 -OFDM tutorial - Luc Deneire
126
PAPR is bottleneck for low cost,
low power front-end design
.
.
.
IFFT
.
.
.
PA
channel
.
.
.
FFT
.
.
.
Pout
PAPR<19
ENIC 2002 -OFDM tutorial - Luc Deneire
Large back-off (4s)
low efficiency (2%)
30W DC power for
600mW RF Power
Pin
127
.
.
.
FFT
.
.
.
IFFT
H(n)
.
.
.
.
.
.
FFT
.
.
.
.
.
.
IFFT
Single carrier Tx with freq domain processing
.
.
.
H(n)
ENIC 2002 -OFDM tutorial - Luc Deneire
.
.
.
FFT
.
.
Equ
.
IFFT
.
.
.
128
Several techniques try to reduce
the Crest Factor
 Clip the signal
• clipping noise = in-band distortion
• spectral spreading  filtering required  peak
regrowth
 Reduce the probability of clipping
by:
• coding of input data
• selected mapping
• partial transmit sequences
ENIC 2002 -OFDM tutorial - Luc Deneire
129
Crest Factor Reduction: Coding
 Map the transmitted sequence into a
larger sequence with limited PAPR.
 Good performance with little
overhead
 Approaches:
• Look-up tables  Only applicable for small
number of carriers and constellation sizes.
• Pseudo-noise codes
 Current work: systematic codes that
also offer error correction
ENIC 2002 -OFDM tutorial - Luc Deneire
130
Crest Factor Reduction: Selected
Mapping
A1
x
xk
A2
x
AD
x
IDFT
yk,1
Selection
IDFT yk,2
of best
yk
...
IDFT
yk
yk,D
 Options for Ai:
• random rotation vectors
• M-sequences
 log2(D-1) bits of side information are
needed
ENIC 2002 -OFDM tutorial - Luc Deneire
131
Crest Factor Reduction: Partial
Transmit Sequence
IDFT
xk
Partition
into
Subblocks
IDFT
...
yk
x
+
A1
x
IDFT
AD-1
Optimize yk
 Select Ai from set of size W
 (D-1).log2(W) bits of side information are
ENIC 2002 -OFDM tutorial - Luc Deneire
132
needed.
Crest Factor Reduction:
Performance
ENIC 2002 -OFDM tutorial - Luc Deneire
133
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