enic 2002 -ofdm tutorial - luc deneire 1 wireless local networks are emerging wireless lan...
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ENIC 2002 -OFDM tutorial - Luc Deneire 1
Wireless Local Networks are Emerging
Wireless LAN
Hiperlan-2, IEEE802.11a, MMAC
ENIC 2002-OFDM tutorial - Luc Deneire 2
Wireless OFDM Transceivers
Luc Deneire
deneire@i3s.unice.fr
Laboratoire I3S
http://www.i3s.unice.fr/
ENIC 2002 -OFDM tutorial - Luc Deneire 3
“Future” Broadband Wireless Networks will be OFDM based
100M
10M
1M
100k
10k
802.11b
802.11aHiperlan-II
MMAC
BluetoothHomeRF
time
Spread spectrum
OFDMlink adaptation6 to 54 Mbit/s
for multimediacommunication
1999 2000 2001 2002
ENIC 2002 -OFDM tutorial - Luc Deneire 4
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 5
Indoor Propagation Model
ENIC 2002 -OFDM tutorial - Luc Deneire 6
Office tables Metal cupboards
Multipath
RX
TX
Direct path
Multipath channel in an office room.
ENIC 2002 -OFDM tutorial - Luc Deneire 7
The channel : a collection of delayed, attenuated and dephased diracs
Channel
Power delay profile
∑−
=
−=1
0
)()(N
kk
jk teth k τδβ θ
∑−
=
−==1
0
2* )()()()(N
kkk tththtp τδβ
P
t
ENIC 2002 -OFDM tutorial - Luc Deneire 8
Channels differ in time and frequency behaviorCoherence 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 9
Delay spread
ENIC 2002 -OFDM tutorial - Luc Deneire 10
∑
∑−
=
−
=
−= 1
0
2
1
0
2)(
N
kk
N
kkk
RMS
β
βτττ
∑
∑−
=
−
== 1
0
2
1
0
2
N
kk
N
kkk
β
βττ
Delay spread measures the “length” of the channel
RMS delay spread is measure of the amount of dispersion
10 to 100ns correspond to paths 3 to 30m
ENIC 2002 -OFDM tutorial - Luc Deneire 11
Coherence bandwidth is where the channel is “similar”(correlated)
Autocorrelation of channel response
Bcoh is defined as f for which
{ });();(5.0);( * tththtc Δ+Ε=Δ τττϕ
2
1
)0(
)(=
ΦΦ
c
c f
{ } (.))();();(5.0),( *cc FouriertffHtfHf ϕττ =+Δ+Ε=ΔΦ
ENIC 2002 -OFDM tutorial - Luc Deneire 12
Coherence bandwith is inversely proportional to delay spread
rmscohB
τ1
≈ rmsτ
)(τϕ c)( fc Φ
ENIC 2002 -OFDM tutorial - Luc Deneire 13
Compared to the coherence bandwith •W Bcoh frequency selective channel
•W Bcoh frequency nonselective channel
Frequency selective fading…. where bandwith is large ….
Bcoh
W
Bcoh
W
ENIC 2002 -OFDM tutorial - Luc Deneire 14
Frequency selective channelsintroduce Inter Symbol Interference
Incoming signal
Outcoming signal
Channel impulseresponse
ENIC 2002 -OFDM tutorial - Luc Deneire 15
Coherence Time (10-50 ms indoor):
time in which a channel is “stable”
dc B
t1
)( ≈dB
S(f)
Signals sent at these instantssee uncorrelated channels
)( tc ΦFourier
ENIC 2002 -OFDM tutorial - Luc Deneire 16
Subsequent symbols see different channels in fast fading Tb (t)c fast-fading channel
Tb (t)c slow-fading channel
ENIC 2002 -OFDM tutorial - Luc Deneire 17
Propagation overviewSummary of channel properties
Fre
qu
ency
dis
per
sio
n
cohS
dTX
tT
BB
>>
<<or
cohS
dTX
tT
BB
<<
>>or
RMSS
cohTX
T
BB
τ>>
<<or
RMSS
cohTX
T
BB
τ<<
>>or
Slo
w fa
ding
Low
Dop
pler
Fas
t fad
ing
Hig
h D
oppl
er
Time dispersion
Frequency flatShort channel
Frequency selectiveLong channel
ISI-free and
flat-fading
channel
ISI and
flat-fading
channel
ISI-free and
fast-fading
channel
ISI and
fast-fading
channel
Using the previousmeasures oncharacteristics wecan place radiochannels in fourgroups.
NOTE that theclassificationis in relation tothe transmissionbandwidth/symbol-time.
Using the previousmeasures oncharacteristics wecan place radiochannels in fourgroups.
NOTE that theclassificationis in relation tothe transmissionbandwidth/symbol-time.
This is wherewe will try tofit the sub-carriers inOFDM.
ENIC 2002 -OFDM tutorial - Luc Deneire 18
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 19
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 20
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
ENIC 2002 -OFDM tutorial - Luc Deneire 21
An Equalizer is a costly filter
f
f
f
Signal (channel) spectrum
Equalizer
Equalized signal
t
t
t
ENIC 2002 -OFDM tutorial - Luc Deneire 22
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 23
OFDMbasic principles
ENIC 2002 -OFDM tutorial - Luc Deneire 24
OFDM is Multi-Carrierand lowers the symbol rate : less ISI
f1
f2
fnf
...T / sec
T/n / sec
ENIC 2002 -OFDM tutorial - Luc Deneire 25
OFDM : Overlapping spectra to save bandwith
(b)
(a)
ENIC 2002 -OFDM tutorial - Luc Deneire 26
Overlapping spectra are orthogonalto enable proper reception of individual carriers
(a) (b)
ijj
T
i dttftf δ=∫ )(sinc)(sinc0
Orthogonality to avoidinter carrier interference:
signal design + frequencies
ENIC 2002 -OFDM tutorial - Luc Deneire 27
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 28
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 29
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 30
Parameters for designing an OFDM Systemnumber 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 31
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 32
SerialTo
Parallel
QAMData
exp(-jπNs( -t ts)/T)
(expjπ(Ns-2)( -tts)/T)
+ OFDMSignal
OFDM modulator block diagram
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).
ENIC 2002 -OFDM tutorial - Luc Deneire 33
Example of 4 subcarriers within one OFDM symbol.
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.
ENIC 2002 -OFDM tutorial - Luc Deneire 34
The OFDM spectrum fulfills Nyquist’s criterium for an inter-symbol interference free pulse shape
ENIC 2002 -OFDM tutorial - Luc Deneire 35
Impact of channel on OFDM ReceptionMultipath 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 36
Principle of guard interval
ENIC 2002 -OFDM tutorial - Luc Deneire 37
CP CP
Transmitters and receivers... through the channel ...
( )thch( )ts
( )tn
( ) ( ) ( ) ( )tnthtstr ch += *
t
CP CP
( )ts ( )tr
t
}
chT
}
chTt}
sampLT
As long as the CP is longer than the delay spread of thechannel, the CP will absorb the ISI.
Channel Noise
ENIC 2002 -OFDM tutorial - Luc Deneire 38
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 39
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 40
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.
FFT Integration Time = 1/Carrier SpacingGuard Time
OFDM Symbol Time
Delayed subcarrier #2
Subcarrier #1
Part of subcarrier #2 causingICI on subcarrier #1
ENIC 2002 -OFDM tutorial - Luc Deneire 41
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
FFT Integration Time = 1/Carrier SpacingGuard Time / Cyclic Prefix
OFDM Symbol Time
Guard time with cyclic extension
ENIC 2002 -OFDM tutorial - Luc Deneire 42
Example of an OFDM signal with 3 subcarriers in a 2-ray multipath channel. The dashed line represents a delayed multipath component.
FFT Integration Time Guard Time
OFDM Symbol Time First arriving path
Reflection
Reflection delay Phase Transitions
No crosstalk (ICI) between carriers, but distortion per carrier.Freq domain equalization needed.
ENIC 2002 -OFDM tutorial - Luc Deneire 43
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.
ENIC 2002 -OFDM tutorial - Luc Deneire 44
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.
ENIC 2002-OFDM tutorial - Luc Deneire 45
OFDM Performance
ENIC 2002 -OFDM tutorial - Luc Deneire 46
OFDM Performance: AssumptionsThe 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 47
OFDM Performance: Transmitter
IDFTIDFTPtoS
PtoS
AddCyclicPrefix
AddCyclicPrefix
s(t)x0,k
x1,k
xN-1,k
s0,k
s1,k
sN-1,k
HtrHtr
xk
ENIC 2002 -OFDM tutorial - Luc Deneire 48
OFDM Performance: Channel
HchHch +
n(t)
s(t)r(t)
( ) ( ) ( ) ( )( ) chch
ch
Tttth
tntsthtr
><=
+⊗=
and 0for 0with
Cyclicprefix IFFT
ChannelInput
ChannelOutput
Tch
ENIC 2002 -OFDM tutorial - Luc Deneire 49
OFDM Performance: Receiver
StoP
StoP
DFT
DFT
RemoveCyclicPrefix
RemoveCyclicPrefix
r(t)y0,q
y1,q
yN-1,q
HreHre
q(N+v)T+pTr0,q
r1,q
rN-1,q
yq
( ) ( ) ( )( )( )
index symbol : index,carrier : index, sample :
2exp
)1
0,,
,
qnp
N
npjry
pTTvNqrr
trthtr
N
pqpqn
qp
re
∑−
=
⎟⎠
⎞⎜⎝
⎛−=
++′=
⊗=′
π
r’(t)
ENIC 2002 -OFDM tutorial - Luc Deneire 50
OFDM Performance: Combined ModelCombine transmit, channel and
receive filters
Received Signal:
( ) ( )fNfHfN
fHfHfHfH
re
rechtr
.)(
)().().()(
=′
=
( ) ( )( ) ( )
( )( )
( )pTTvNqn
mTTvNkpTTvNqhsr
tnmTTvNkthstr
k
N
vmkmqp
k
N
vmkm
++′+
−+−++=
′+−+−=′
∑ ∑
∑ ∑∞
−∞=
−
−=
∞
−∞=
−
−=
)(
)(.
.
1
,,
1
,
ENIC 2002 -OFDM tutorial - Luc Deneire 51
OFDM Performance: Combined Model Impulse response of h < v
Substitute in received signal( ) qp
p
vpmqmqp nmTpThsr ,,, . ′+−= ∑
−=
( )( )
pvpmkq
Np
vmTTvmNkpTTvNq
)...( and
1...0for
)(0
−==⇒
−=
≤−+−++≤
ENIC 2002 -OFDM tutorial - Luc Deneire 52
OFDM Performance: Combined ModelExpand sm,q
Vector Notation
( )
( )
2exp1
.2exp2exp1
.2exp1
,
1
0,
,
1
0 0,
,
1
0,,
qp
N
nnqn
qp
N
n
v
zqn
qp
p
vpm
N
nqnqp
nhN
npjx
N
nzThN
nzj
N
npjx
N
nmTpThN
nmjx
Nr
′+⎟⎠
⎞⎜⎝
⎛=
′+⎟⎠
⎞⎜⎝
⎛−⎟
⎠
⎞⎜⎝
⎛=
′+−⎟⎠
⎞⎜⎝
⎛=
∑
∑ ∑
∑ ∑
−
=
−
= =
−=
−
=
π
ππ
π
( ) qqq nxhr ′+= .IDFT
ENIC 2002 -OFDM tutorial - Luc Deneire 53
( )( )( )( )
nxh
nxh
nxh
ry
+=
′+=
′+=
=
.
DFT.
.IDFTDFT
DFT
OFDM performance: Combined ModelCalculate yq:
ENIC 2002 -OFDM tutorial - Luc Deneire 54
2 4 6 8 10 12 1410
-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
Eb/No
Pe
idealN=256,v=16
OFDM Performance: AWGN
⎟⎠
⎞⎜⎝
⎛ +=
N
vNloss 10log
ENIC 2002 -OFDM tutorial - Luc Deneire 55
OFDM Performance: Rayleigh ChannelRayleigh received SNR per bit PDF:
Calculate BER() with standard formula.
Calculate E{BER}:
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−=
exp
MP
{ }( ) ( )( )∫
∫=
dP
dPBERBERE
ENIC 2002 -OFDM tutorial - Luc Deneire 56
OFDM performance: Rayleigh Channel
5 10 15 20 25 3010
-4
10-3
10-2
10-1
100
Eb/No
Pe
ENIC 2002 -OFDM tutorial - Luc Deneire 57
Coded OFDM
Block codes
Convolutional codes
Concatenated codes
Trellis coded modulation of the carriers
transmittransmitStoP
StoP
errorcoding
errorcoding channelchannel receivereceive
PtoS
PtoS
errordecoding
errordecoding
ENIC 2002 -OFDM tutorial - Luc Deneire 58
Coded OFDM: interleaving
Time interleaver: block interleaver
Frequency interleaver:permutation over carriers
transmittransmitStoP
StoP
errorcoding
errorcoding
frequencyinter-leaver
frequencyinter-leaver
timeinterleaver
timeinterleaver
ENIC 2002 -OFDM tutorial - Luc Deneire 59
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 60
Summary 2
OFDM alone has bad performance on fading channel.
Additional technique needed to exploit diversity:•error coding,
•adaptive loading.
5 10 15 20 25 3010-4
10-3
10-2
10-1
100
Eb/No
Pe
ENIC 2002-OFDM tutorial - Luc Deneire 61
Adaptive Loading
ENIC 2002 -OFDM tutorial - Luc Deneire 62
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).
ENIC 2002 -OFDM tutorial - Luc Deneire 63
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:
Take the FFT of each received FFT-symbol separately, for the jth symbol resulting in:
)()()()( inihisir +⊗=
Adaptive Loading: Channel Estimation
)()().()( kNkxkHky jj +=
ENIC 2002 -OFDM tutorial - Luc Deneire 64
Multiply yj(k) with the reference symbol:
Average over the n FFT-symbols:
Apply low-pass filter over carriers to obtain Hf(k).
∑ ∑= =
+==n
j
n
j
jj kNkxn
kHkzn
kH1 1
)().(.1
)()(.1
)(~
Adaptive Loading: Channel Estimation
)().()()( kxkNkHkz jj +=
ENIC 2002 -OFDM tutorial - Luc Deneire 65
Adaptive Loading: Noise EstimationSame reference sequence.Uses filtered channel estimation to
estimate noise signal:
Average over all carriers (assuming white Gaussian noise):
∑=
=N
ia iNN
1
)(~~
∑=
−=n
jf
j kxkHkyn
kN1
2|)().(~
)(|.1
)(~
ENIC 2002 -OFDM tutorial - Luc Deneire 66
Adaptive Loading: Loading AlgorithmsHughes-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 67
Adaptive Loading: Loading AlgorithmsChow et al.:
•Assign the bits according to:
•Algorithmic Complexity: O(Niter x Nc + 2Nc)
∑ =
Γ
⎟⎟⎠
⎞⎜⎜⎝
⎛
+Γ+=
iTimargin
i
i
inm
ii
RR
iSNR
iR
SNRR
guarantee y toiterativel determined
usage actual theandcapacity channel ebetween th difference the
carrier for ratio noise tosignal the
carrier for rate data the
with1logarg
2
γ
γ
ENIC 2002 -OFDM tutorial - Luc Deneire 68
Adaptive Loading: Loading AlgorithmsFischer et al.:
•Minimizes BER for a constant data rate RT
(application) and a constant total transmit power ST
•Assign the bits according to:
iR
iN
D
N
N
DD
RR
i
i
Di
D
ll
Ti
carrier for rate data the
carrier for power noise equivalent the
iteration specificin carriers ofnumber
with
)(log1 1
2
∏=⋅+=
ENIC 2002 -OFDM tutorial - Luc Deneire 69
Adaptive Loading: Loading Algorithms
•Iterate until all Ri 0 (exclude negative rate carriers)
•Quantize Ri and assure
•Adapt power per carrier to compensate for quantization
T
D
ii RR =∑
=1
ENIC 2002 -OFDM tutorial - Luc Deneire 70
Adaptive loading: Operation
ENIC 2002 -OFDM tutorial - Luc Deneire 71
Adaptive Loading: Performance
ENIC 2002 -OFDM tutorial - Luc Deneire 72
Adaptive Loading: OFDMA PrincipleEstimate 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 73
Adaptive Loading: OFDMA Operation
ENIC 2002-OFDM tutorial - Luc Deneire 74
Hiperlan/2 case study
ENIC 2002 -OFDM tutorial - Luc Deneire 75
Hiperlan/2 Positioning - Mobility vs. Bitrate
Mbps1 10 1000,1
Ou
tdo
or
Stationary
Walk
Vehicle
Ind
oo
r
Stationary/Desktop
WalkMo
bili
ty
HIPERLAN/2
User Bitrates
LAN
W-CDMA/ EDGE
Bluetooth
ENIC 2002 -OFDM tutorial - Luc Deneire 76
W-CDMA/EDGE
Hiperlan/2 Positioning - Cost vs. Bitrate
Mbps1 10 1000,1
Use
r C
ost /
bit
HIPERLAN/2
User Bitrates
LANBluetooth
Low
Medium
High
Very Low
$
ENIC 2002 -OFDM tutorial - Luc Deneire 77
Hiperlan/2 protocol architecture
Physical Layer
Convergence Layer
Control Plane User Plane
DLC Control SAP
DLCConnection
Control
AssociationControl
RadioResource
Control
RLC
DLC User SAP
Error Control
Radio Link Controlsublayer
Medium Access Control
Data Link Control -Basic Data Transport Function
One instance per MAC ID
One instance per AP
One instance per DLCUser Connection,identified by DUC ID(MAC ID + DLCC ID)
Higher Layers
Scope ofHIPERLAN/2standards
CL SAPs
ENIC 2002 -OFDM tutorial - Luc Deneire 78
Basic MAC frame structure
BCH FCH DL phase UL phase RCHs
MAC-Frame MAC-Frame MAC-Frame MAC-Frame
ACH
SCH
DiL phase
SCH LCH LCH SCH LCH... ... ...
2ms
DL to one MT
One DLC connection
One PDU train mapped one PHY burst
ENIC 2002 -OFDM tutorial - Luc Deneire 79
Hiperlan-2 Transmitter PHY Model
scramblerscrambler FECcoder
FECcoder interleaverinterleaver mappermapper OFDM
mod
OFDMmod
burstformatter
burstformatter
radiotransmitter
radiotransmitter
ENIC 2002 -OFDM tutorial - Luc Deneire 80
Data scramblerS(x) = X7 + X4 + 1
n4n3n2n1: frame counter, first 4 bits of broadcast channel (BCH)
Initialization sequence
n4 n3 n2 n11 1 1
Scrambled PDU train
out
PDU train in
X7 X6 X5 X4 X3 X2 X1
ENIC 2002 -OFDM tutorial - Luc Deneire 81
FEC coder
Y
XChannel coded
PDU trainScrambled PDU train
Append six tail bits
Convolutional encoder
Puncturing P1 with serial
output
Puncturing P2
Output data X
Output data Y
Input data Tb Tb Tb Tb Tb Tb
ENIC 2002 -OFDM tutorial - Luc Deneire 82
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 83
Mapper
Gray coded constellation mapping
10 0011 0001 0000 00
10 01
10 11
11 0101 0100 01
10 1011 10
00 11
+301 1000 10
11 11
-3
-3
+3
01 11
-1
-1 +1
+1
16QAMb1b2b3b4
Normalization to achieve same average power for all constellations•BPSK: 1
•QPSK: 1/sqrt(2)
•16QAM: 1/ sqrt(10)
•64QAM: 1/ sqrt(42)
ENIC 2002 -OFDM tutorial - Luc Deneire 84
Modulation Coding rateR Nominal bit rate[Mbit/s]
Coded bits persub-carrier
NBPSC
Coded bits perOFDM symbol
NCBPS
Data bits perOFDM symbol
NDBPS
BPSK 1/2 6 1 48 24BPSK 3/4 9 1 48 36QPSK 1/2 12 2 96 48QPSK 3/4 18 2 96 72
16QAM 9/16 27 4 192 10816QAM 3/4 36 4 192 14464QAM 3/4 54 6 288 216
OFDM Modulation parameters
Different modulation schemes allow variable bitrate and QoS
ENIC 2002 -OFDM tutorial - Luc Deneire 85
OFDM modulation parameters
Parameter ValueSampling rate fs=1/T 20 MHzUseful symbol part duration TU 64*T
3.2 μsCyclic pr efix dura tion TCP 16*T
0.8 μs (mandatory)8*T0.4 μs (optional)
Symbol inte rva l TS 80*T4.0 μs (TU+TCP)
72*T3.6 μs (TU+TCP)
Nu mbe r of data sub -carrie rs NSD 48Nu mbe r of pilot sub -carrie rs NSP 4To tal numbe r of sub -carrie rs NST 52 (NSD+ NSP )
Sub -carrie r s pac ing Δf 0.3125 MHz (1/TU)
Spa cing be tween the t wo outmos t sub -carrie rs 16 .25 MHz (NST*Δf)
Copy
TCP TU
Data nCP
Allows delay spread of 250ns
Compromise betweencoherency time and bandwidth
ENIC 2002 -OFDM tutorial - Luc Deneire 86
OFDM sub-carrier frequency allocation and guard interval
D0,n D4,n D5,n D17,n D18,n D23,n D24,n D29,n D30,n D42,nD43,n D47,n
P0,n P1,n P2,n P3,n
DC
-26 -21 -7 0 7 21 26
Copy
TCP TU
Data nCP
IFFT
ENIC 2002 -OFDM tutorial - Luc Deneire 87
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 88
Broadcast burst preamble
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
Copy
tPREAMBLE=16.0μs
Section 1 Section 2 Section 3 5*0.8μs=4.0μs 5*0.8μs=4.0μs 2*0.8μs+2*3.2μs=8.0μs
A IA A IA IA B B B B IB CP C C
ENIC 2002 -OFDM tutorial - Luc Deneire 89
Time synchronization based on auto correlation of A and B-fields
A IA A IA IA B B B B IB CP C C
A IA A IA IA B B B B IB CP C C
Correlation window
conjMoving average
A IA A IA IA B B B B IB CP C C
A IA A IA IA B B B B IB CP C C
Correlation window
conjMoving average
ENIC 2002 -OFDM tutorial - Luc Deneire 90
Frame time synchronization
Robustnessagainstmultipath,CFO
Accuracy Implementaitoncost
Autocorrelation High low low
Crosscorrelation low high high
ENIC 2002 -OFDM tutorial - Luc Deneire 91
Automatic gain control
Based on non-coherent energy measurement at baseband• (input . input*)
Gain is kept constant during burst because signal is non constant envelope
ENIC 2002 -OFDM tutorial - Luc Deneire 92
Carrier Frequency SynchronisationEstimate CFO (f) with C-field
Apply correction to the input datastream
Cyclicprefix C C
conj.
angle
Tf ..2 = πφ
ENIC 2002 -OFDM tutorial - Luc Deneire 93
Channel estimation and equalization based on C-fieldcompensate with a rotator per
carrier, i.e. a frequency domain equalizer.
a
Known transmitted
received
Cyclicprefix C
X
Cyclicprefix data...
αje−
training symbol data symbols
ENIC 2002 -OFDM tutorial - Luc Deneire 94
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 95
Spectrum Allocation at 5 GHz
5.200 5.300 5.400 5.500 5.600 5.700 5.800 5.9005.100
Europe
USA
Japan
Freq./GHz
5.350
5.150
5.150 5.250
5.350 5.725 5.825
5.150 5.470 5.725
Outdoor 1W EIRP Indoor 200 mW EIRP
Indoor 200 mW / Outdoor 1 W EIRP
Max mean Tx power
Outdoor 4 W EIRP
Max peak Tx power
DFS & PC DFS & PC
DFS: Dynamic Frequency Selection
PC: Power Control
ENIC 2002 -OFDM tutorial - Luc Deneire 96
Hiperlan-2 receiver block diagram
descrambler
descrambler
FECdecoder
FECdecoder
deinterleaver
deinterleaver
demapper
demapper
radioreceiver
radioreceiver
timedomain sync &AGC
timedomain sync &AGC
FFTFFTfreq
domain EQU
freqdomain
EQU
ENIC 2002 -OFDM tutorial - Luc Deneire 97
Main differences between H2 and 802.11aProtocol
•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 98
Implementation of Hiperlan2
transceivers
ENIC 2002 -OFDM tutorial - Luc Deneire 99
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 100
In half-duplex operation the DFT/IDFT can be shared
(D)PSK or QAM Mo-
dulation
(D)PSK or QAM Mo-
dulationIDFTIDFT
CyclicExten-
sion
CyclicExten-
sion
(D)PSK or QAM Demo-
dulation
(D)PSK or QAM Demo-
dulationDFTDFT
RemoveExten-
sion
RemoveExten-
sion
Equa-lizer
Equa-lizer
Time &Carrier
Synchro
Time &Carrier
Synchro
shared
ENIC 2002 -OFDM tutorial - Luc Deneire 102
DFT/IDFT implementation based on hierarchical decompositionDFT/IDFT definition:
hierarchical decomposition:•decimation in frequency (DIF)
•decimation in time (DIT)
1,...,1,0 and
110,2
expwith
1 and
1
0
1
0
−=
−=⎟⎠
⎞⎜⎝
⎛−=
== ∑∑−
=
−−
=
Nk
,...,N,mNj
W
WXN
xWxX
N
N
k
mkNkm
mkN
N
mmk
π
ENIC 2002 -OFDM tutorial - Luc Deneire 103
DIF of a DFT results in two smaller DFTs
( )
( ) ( )
( )
( )m
NmNmmmr
N
N
mmr
mNmmmr
N
N
mmr
N
mmN
km
mkN
mkN
N
mmN
kN
Nmk
N
N
mmk
WxxxWxX
xxxWxX
xxW
WxWWxX
. with
with
1.
22
12
012
22
12
02
12
0 2
12
0 2
2.
12
0
⎟⎠⎞⎜
⎝⎛ −=′′′′=
+=′′=
⎟⎠⎞⎜
⎝⎛ −+=
+=
+
−
=+
+
−
=
−
=+
−
=+
−
=
∑
∑
∑
∑∑
ENIC 2002 -OFDM tutorial - Luc Deneire 104
We get a recursive DFT structure based on butterfly operations
N/2-taps FFT
x0
x1
xN/2
xN/2+1
xN-1
...
...
...
...N/2-taps
FFT...
...
W0
-
+
+
+
butterfly operation
105ENIC 2002 -OFDM tutorial - Luc Deneire 105
Radix-4 decomposition:
Remap indexes according to radix-2:
Apply these steps recursively
( )
( )( )
( ) ( )∑ ∑
∑
= =++
+
−
=++
=
=
1
0
1
0242
224
121121211
14/
0412112121142
2 1
111
221
21211212211
2
22
21211
...
BFwith
.,,,,BF
u u
kunuNuN
nkkN
ukuk
N
n
nkNkkk
WxWWW
,k,k,n,nn
WkknnnX
Recursive Radix Algorithm is based on radix-4 decomposition
∑ ∑−
= =++ =
14/
0
3
0444
2 1
11
214
2221
21..
N
n n
nknn
nkN
nkNkk WxWWX N
ENIC 2002 -OFDM tutorial - Luc Deneire 106
The basic structure is a radix-4 butterfly
x r
x N/4+r
x N/2+r
x 3N/4+r
WN1
WN3
WN2
0NW
-j
-
ENIC 2002 -OFDM tutorial - Luc Deneire 108
Optimizing wordlengths results in a considerable gain
ENIC 2002 -OFDM tutorial - Luc Deneire 109
OFDM Modem: 256 point (I)FFT 0.5μ 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 110
Implementation: (I)FFT
Organisation #points clock Power Tech Area #transistorsCNET 8192 20 MHz 600 mW 0.5 μm 100 _mm 1.500.000CNET 2048 20 MHz 300 mW 0.5 μm 100 _mm 1.500.000CNET 1024 20 Mhz - 0.5 μm 40 mm_ 21 Kcells
Stanford Uni .v 1024 16 MHz 9.5 mw 0.7 μm 50 mm_ 460.000Stanford Uni .v 1024 173 MHz 845 mW 0.7 μm 50 mm_ 460.000Macquari e Uni .v 16 50 MHz 80 mW 0.6 μm 6 _mm 70.000
IMEC 256 50 MHz - 0.35 μm 20 mm_ 200.000
ENIC 2002 -OFDM tutorial - Luc Deneire 111
Fast acquisition is crucial in a burst mode system
Cyclicprefix
OFDMdata
sequence
symboltiming
sequence
carrieroffset
sequence
OFDMdata
sequence...
OFDMdata
sequence
Time-domainacquisitionsequence
OFDMreferencesymbol
OFDMdata
symbol...
OFDMdata
symbol
ENIC 2002 -OFDM tutorial - Luc Deneire 112
Time synchronisation is based on repetition of known sequence
COS-1 COS-2
Frame start
TS1 TS1 ….TS1TS2 TS2 TS1 TS1
RTS ATS
…. ….
ENIC 2002 -OFDM tutorial - Luc Deneire 113
Initial carrier frequency com-pensation is in time-domain
ENIC 2002 -OFDM tutorial - Luc Deneire 114
Centralized data re-ordering minimizes memories
IFFTFFT
equalizer demapper r=2 (7kb)
refsym
mapperr=2
(4kb)
SYNCr=1
(8kb)
r=2 (4kb)RAM 1 RAM 2
SSR
r=2 (8kb)
: Tx datapath : Rx datapath : reference signal
ENIC 2002 -OFDM tutorial - Luc Deneire 115
Adaptive equalizer has low implementation cost
Gain control
yi
Refsym RAM
COEF RAMS
conjugate
Decide & rotate
Integrate & dump
conjugate
y’i
mode (feedback, reference)
mode (single, average)
ENIC 2002 -OFDM tutorial - Luc Deneire 116
Programmability is a must for OFDM modems
Parameter Options
Number of carriers 64, 128, 256
Guard interval 0:4:28Modulation QPSK (BPSK)
Equalizer modes REF-FF, SC-FB, AC-FBreference sequence
Spectral mask Complex, per carrier
FFT clipping 5-8b, MSB or LSB aligned
Spreading 1, 2, 4, 8; code sequence
Acquisition sequence, length,confidence factors
Number of zerocarriers
Low: 0:1:3, left and rightHigh: 0:2:30, left and right
ENIC 2002 -OFDM tutorial - Luc Deneire 117
Token flow control: integration = communication
Token flow produced: Soft initial token arrival time specification: Tmin < T < Tmax
=> boundary check only=> supports IP block strategy
‘smart’receiver
‘smart’senderTrigger at ‘1’
1
0
1
0
Repeatedtoken
Singletoken
ENIC 2002 -OFDM tutorial - Luc Deneire 118
Clock gating is essential for low-power operation
sleep RXSYNC
RXdemod
AREQ ACQ
EOB
2%6%
19%73%
6%
TRX
CFO
‘IEEE’ mode @ 50 MHz Power GOPS #acc/s Gbit/s
Tx mode 670 mW 3.8 1.32 G 21.3Rx mode 570 mW 6.8 1.28 G 20.3Sleep 150 mW n/a n/a n/a
ENIC 2002 -OFDM tutorial - Luc Deneire 119
Putting it all together
0.35μ CMOS, 3.3V, 5LM
50MHz
144pins
210 kgates
10 RAMs
20mm2
670 mW
ENIC 2002 -OFDM tutorial - Luc Deneire 120
Improved equalizer for better performance with QAM
QAM64 BER performance
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
12 13 14 15 16 17 18
SNR per bit (dB)
bit
err
or
rate
Improved equalizer
Standard equalizer
ENIC 2002 -OFDM tutorial - Luc Deneire 121
Second generation OFDM Modem supports QAM-64
0.18μ CMOS 5LM
20MHz
160pins
500 kgates
19 RAMs
25mm2
ENIC 2002-OFDM tutorial - Luc Deneire 122
Crest Factor Reduction
ENIC 2002 -OFDM tutorial - Luc Deneire 123
Crest Factor: Definitions
Peak-to-Average Power Ratio:
Crest factor
Example
( ){ }
N
sE
sPAPR
km
km
=
= 2,
max2
,
PAPRCF =
dBCFdBPAPRN 12,23256 ==⇒=
ENIC 2002 -OFDM tutorial - Luc Deneire 124
We get a peak amplitude if all tones add in phase
0 50 100 150 200 250 3000
10
20
30
40
50
60
70
ENIC 2002 -OFDM tutorial - Luc Deneire 125
Large peaks do not occur very often (Gaussian distribution)
4 5 6 7 8 9 10 11 12 13 140
2000
4000
6000
8000
10000
12000
PAPR(dB)
ENIC 2002 -OFDM tutorial - Luc Deneire 126
The Crest Factor has major impact on implementationLarge 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 127
PAPR is bottleneck for low cost, low power front-end design
PAPR<19Large back-off (4)low efficiency (2%)30W DC power for 600mW RF Power
Pin
Pout
IFFT
.
.
.
.
.
.
channel ...
FFT
.
.
.
PA
ENIC 2002 -OFDM tutorial - Luc Deneire 128
IFFT
.
.
.
H(n) ...
.
.
.FFT
.
.
.FFT
.
.
.IFFT
.
.
.
H(n) ...
.
.
.FFT IFFT
.
.
.
.
.
.Equ
Single carrier Tx with freq domain processing
ENIC 2002 -OFDM tutorial - Luc Deneire 129
Several techniques try to reduce the Crest FactorClip 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 130
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 131
Crest Factor Reduction: Selected Mapping
Options for Ai:
• random rotation vectors
•M-sequences
log2(D-1) bits of side information are needed
IDFTIDFT
Selectionof best
yk
Selectionof best
yk...
yk,1
xk
IDFTIDFT
IDFTIDFT
x
x
x
A1
A2
AD
yk,2
yk,D
yk
ENIC 2002 -OFDM tutorial - Luc Deneire 132
Crest Factor Reduction: Partial Transmit Sequence
Select Ai from set of size W
(D-1).log2(W) bits of side information are needed.
IDFTIDFT
++
...
xk
IDFTIDFT
IDFTIDFT
x
x
A1
AD-1
ykPartition
intoSub-
blocks
PartitionintoSub-
blocks
Optimize ykOptimize yk
ENIC 2002 -OFDM tutorial - Luc Deneire 133
Crest Factor Reduction: Performance
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