10

IEEE802.16 and
WiMax
According to the applications, we define three “Area Networks”:
•
Personal Area Network (PAN),
for communications within a few meters. This is the
typical Bluetooth or
Zigbee
application between
between
personal devices such as
your cell phone, desktop, earpiece and so on;
•
Local Area Network (LAN),
for communications up 300 meters. Access points at
the airport, coffee shops, wireless networking at home. Typical standard is
IEEE802.11 (
WiFi
) or
HyperLan
in Europe. It is implemented by access points, but it
does not support mobility;
•
Wide Area Network (WAN),
for cellular communications, implemented by towers.
Mobility is fully supported, so you can move from one cell to the next without
interruption. Currently it is implemented by Spread Spectrum Technology via CDMA,
CDMA

2000, TD

SCDMA, EDGE and so on. The current technology, 3G, supports
voice and data on separate networks. For
current developments
, 4G technology will
be supporting both data and voice on the same network and the standard IEEE802.16
(
WiMax
)
and Long Term Evolution (LTE) are the candidates
Applications: various Area Networks
More Applications
1. WLAN (Wireless Local Area Network) standards and WiFi. In particular:
•
IEEE 802.11a in Europe and North America
•
HiperLAN /2 (High Performance LAN type 2) in Europe and North America
•
MMAC (Mobile Multimedia Access Communication) in Japan
2. WMAN (Wireless Metropolitan Network) and WiMax
•
IEEE 802.16
3. Digital Broadcasting
•
Digital Audio and Video Broadcasting (DAB, DVB) in Europe
4. Ultra Wide Band (UWB) Modulation
•
a very large bandwidth for a very short time.
5. Proposed for IEEE 802.20 (to come) for high mobility communications
(cars, trains …)
IEEE 802.16 Standard
IEEE 802.16
2004
(
http://www.ieee802.org/16/
):
Part 16: Air Interface for Fixed Broadband Wireless Access
Systems
From the Abstract:
•
It specifies air interface for
fixed
Broadband Wireless Access (BWA) systems
supporting multimedia services;
•
MAC supports point to multipoint with optional mesh topology;
•
multiple physical layer (PHY) each suited to a particular operational environment:
IEEE 802.16

2004 Standard
•
WirelessMAN

SC, Single Carrier (SC), Line of Sight (LOS), 10

66GHz, TDD/FDD
•
WirelessMAN

SCa, SC, 2

11GHz licensed bands,TDD/FDD
•
WirelessMAN OFDM, 2

11GHZ licensed bands,TDD/FDD
•
WirelessMAN

OFDMA, 2

11GHz licensed bands,TDD/FDD
•
WirelessHUMAN 2

11GHz, unlicensed,TDD
MAN: Metropolitan Area Network
HUMAN: High Speed Unlicensed MAN
Table 1
(Section 1.3.4)
Air Interface Nomenclature
:
IEEE 802.16e 2005:
Part 16: Air Interface for Fixed and Mobile Broadband
Wireless Access Systems
Amendment 2: Physical and Medium Access Control Layers
for Combined Fixed and Mobile Operation in Licensed
Bands
and
Corrigendum 1
Scope (
Section 1.1):
•
it enhances IEEE 802.16

2004 to support mobility at vehicular speed, for combined
fixed and mobile Broadband Wireless Access;
•
higher level handover between base stations;
•
licensed bands below 6GHz.
IEEE 802.16

2004: Reference Model (Section 1.4), Figure 1
By Layers:
Service Specific Convergence
Sublayer (CS)
CS

SAP
SAP=Service Access Point
MAC Common Part Convergence
Sublayer (CS)
MAC

SAP
Security Sublayer
Physical Layer
PHY

SAP
MAC
PHY
Section 5
Section 6
Section 7
Section 8
External Data
Parameters for IEEE 802.16 (OFDM only)
802.16

2004
802.16e

2005
Frequency Band
2GHz

11GHz
2GHz

11GHz fixed
2GHz

6GHz mobile
OFDM carriers
OFDM: 256
OFDMA: 2048
OFDM: 256
OFDMA: 128, 256, 512,1024,
2048
Modulation
QPSK, 16QAM, 64QAM
QPSK, 16QAM, 64QAM
Transmission Rate
1Mbps

75Mbps
1Mbps

75Mbps
Duplexing
TDD or FDD
TDD or FDD
Channel Bandwidth
(1,2,4,8)x1.75MHz
(1,4,8,12)x1.25MHz
8.75MHz
(1,2,4,8)x1.75MHz
(1,4,8,12)x1.25MHz
8.75MHz
randomization
data
Error
Correction
Coding
TX
IEEE802.16 Structure
M

QAM
mod
OFDM
mod
De

rand.
data
Error
Correction
Decoding
RX
M

QAM
dem
OFDM
dem
Coding rates
1/2
2/3
3/4
5/6
M

QAM
2
4
16
64
OFDM
carriers
256
512
1024
2048
Choices:
Channel
B/width
1.25 MHz
5 MHz
10 MHz
…
OFDM and OFDMA (Orthogonal Frequency Division Multiple Access)
•
Mobile WiMax is based on OFDMA;
•
OFDMA allows for subchannellization of data in both uplink and downlink;
•
Subchannels are just subsets of the OFDM carriers: they can use contiguous or
randomly allocated frequencies;
•
FUSC: Full Use of Subcarriers. Each subchannel has up to 48 subcarriers evenly
distributed through the entire band;
•
PUSC: Partial Use of Subcarriers. Each subchannel has subcarriers randomly
allocated within clusters (14 subcarriers per cluster) .
Section 8.3.2: OFDM Symbol Parameters and Transmitted Signal
OFDM Symbol
g
T
b
T
s
T
data
guard
(CP)
1 1 1 1
,,,
4 8 16 32
g
b
T
T
An OFDM Symbol is made of
•
Data Carriers: data
•
Pilot Carriers: synchronization and estimation
•
Null Carriers: guard frequency bands and DC (at the modulating carrier)
channel
frequency
pilots
data
Guard
band
Guard
band
to provide frequency guards between cha
nnels
1 (DC subcarrier is always zero)
pilots for channel tracking and synchr
onization
data subcarriers
guards
nulls guards
pilots
data
used pilots dat
N
N N
N
N
N N N
a
FFT size
256
128
512
1024
2048
N_used
200
108
426
850
1702
N_nulls
56
20
86
174
346
N_pilots
8
12
42
82
166
N_data
192
96
384
768
1536
OFDM Subcarrier Parameters:
Fixed
WiMax
Fixed and
Mobile
WiMax
IEEE 802.16, with
N=
256
0
100
155
255
13
38
88
63
168
218
193
243
101
]
[
L
n
x
]
[
k
X
0
255
IFFT
Data (192)
Pilots (8)
Nulls (56)
12
24
24
24
12
12
12
24
24
24
k
n
156
IEEE802.16 Implementation
In addition to OFDM Modulator/Demodulator and Coding we need
•
Time Synchronization: to detect when the packet begins
•
Channel Estimation: needed in OFDM demodulator
•
Channel Tracking: to track the time varying channel (for mobile only)
In addition we need
•
Frequency Offset Estimation: to compensate for phase errors and noise in the
oscillators
•
Offset tracking: to track synchronization errors
Basic Structure of the Receiver
WiMax Demodulator
Demodulated
Data
Received
Signal
Time Synchronization
:
detect the beginning of
the packet and OFDM
symbol
Channel Estimation
:
estimate the frequency
response of the channel
In IEEE802.16 (256 carriers, 64 CP) Time and Frequency Synchronization are
performed by the Preamble.
Long Preamble
: composed of 2 OFDM Symbols
Short Preamble:
only the Second OFDM Symbol
First OFDM Symbol
Second OFDM Symbol
320 samples
320 samples
4 repetitions of a short
pulse+CP
64
2 repetitions of a long
pulse + CP
64
64
64
64
128
128
d
T
g
T
d
T
g
T
64
Time Synchronization
The standard specifies the Down Link preamble as QPSK for subcarriers between

100
and +100:
otherwise
,
0
100
,...,
1
,
1
,...,
100
,
1
]
[
k
j
k
P
ALL
Using the periodicity of the FFT:
100
,...,
1
],
[
k
k
P
ALL
1
100
156
255
1
,...,
100
],
256
[
]
[
k
k
P
k
P
ALL
ALL
64
64
64
64
]
[
4
k
P
]
[
4
n
p
0
255
0
4
8
252
255
FFT
•
Short Preamble
, to obtain the 4 repetitions, choose only subcarriers multiple of 4:
otherwise
,
0
0
4
mod
if
],
[
2
]
[
*
4
k
k
P
k
P
ALL
Add Cyclic Prefix:
64
64
64
64
0
319
64
255
]
[
4
n
p
•
Long Preamble
: to obtain the 2 repetitions, choose only subcarriers multiple of 2 :
otherwise
,
0
0
2
mod
if
],
[
2
]
[
2
k
k
P
k
P
ALL
128
]
[
2
k
P
]
[
2
n
p
0
255
0
4
8
252
255
FFT
2
254
6
128
Add Cyclic Prefix:
64
0
319
]
[
2
n
p
128
128
CP
Several combinations for Up Link, Down Link and
Multiple Antennas
.
We can generate a number of preambles as follows:
otherwise
,
0
0
2
mod
if
],
[
2
]
[
0
2
k
k
P
k
P
ALL
otherwise
,
0
1
2
mod
if
],
[
2
]
[
1
2
k
k
P
k
P
ALL
otherwise
,
0
4
mod
if
],
2
[
]
[
*
4
m
k
m
k
P
k
P
ALL
m
otherwise
,
0
0
4
mod
if
],
[
2
]
[
*
0
4
k
k
P
k
P
ALL
3
,
2
,
1
m
0
m
With 2 Transmitting Antennas:
With 4 Transmitting Antennas:
Time Synchronization from Long Preamble
preamble
OFDM Symbols
64
128
128
Received signal:
128
z
xcorr
]
[
n
y
127
0
2
127
0
2
2
127
0
*
2
]
128
[
]
[
]
128
[
]
[
]
[
n
y
n
y
n
y
n
y
n
r
y
0
n
Compute Crosscorrelation Coefficient:
1. Coarse Time Synchronization using Signal Autocorrelation
1
]
[
2
n
r
y
0
n
n
MAX when
]
128
[
]
[
n
y
n
y
]
[
n
y
64
128
128
0
n
]
128
[
n
y
64
128
128
n
Effect of Periodicity on Autocorrelation (
no Multi Path
). Let
L
=64.
64
0
n
Max starts
at
….
64
0
n
n
Same signal
n
data
data
1
]
[
2
n
r
y
0
n
n
MAX when
]
128
[
]
[
n
y
n
y
]
[
n
y
64
128
128
0
n
]
128
[
n
y
64
128
128
n
Effect of Periodicity on Autocorrelation (
no Multi Path
):
64
0
n
… and ends at
0
n n
Same signal
n
data
data
1
]
[
2
n
r
y
0
n
n
MAX when
]
128
[
]
[
n
y
n
y
]
[
n
y
64
128
128
0
n
]
128
[
n
y
64
128
128
n
Effect of Periodicity on Autocorrelation (
with Multi Path of max length
):
0
64
C
n L
Max starts
at
….
0
64
C
n n L
Same signal
n
data
data
64
C
L L
L
L
C
1
]
[
2
n
r
y
0
n
n
MAX when
]
128
[
]
[
n
y
n
y
]
[
n
y
64
128
128
0
n
]
128
[
n
y
64
128
128
n
Effect of Periodicity on Autocorrelation (
with Multi Path of max length
):
0
64
C
n L
and ends
at
0
n n
Same signal
n
data
data
L
L
C
L
L
C
2
127
*
0 0
0
2
0
127 127
2 2
0 0
0 0
2
127
2
0
0
2
127
2 2
0 0
0
2
[ ] [ 128 ]
[ ]
[ ] [ 128 ]
[ ]
[ ] [ ]
1
y
R
R
y n y n
r n
y n y n
y n
y n w n
SNR
SNR
With Noise:
]
[
]
[
]
[
n
w
n
y
n
y
R
Then, at the maximum:
Information from Crosscorrelation coefficient:
]
[
n
r
y
Estimate of Beginning
of Data
Estimate of Channel Length
Estimate of SNR
0
n
L
L
C
1
MAX
MAX
r
SNR
r
2. Fine Time Synchronization using Cross Correlation with Preamble
xcorr
]
[
n
y
]
[
n
p
127
*
0
[ ] [ ] [ ]
yp
l
r n y n p
Since the preamble is random (almost like white noise), it has a short autocorrelation:
]
[
n
y
64
128
128
0
n
n
n
128
0
127
]
[
n
p
0
128
n
0
256
n
[ ]
yp
r n
… with dispersive channel
xcorr
]
[
n
y
]
[
n
p
Since the preamble is random, almost white, recall that the crosscorrelation yields the
impulse response of the channel
]
[
n
y
64
128
128
0
n
n
n
128
0
127
]
[
n
p
0
128
n
0
256
n
 [ ]
h n
127
*
0
[ ] [ ] [ ]
yp
l
r n y n p
[ ]
yp
r n
127
*
0
127
*
0
[ ] [ ] [ ]
[ 127 ] [127 ]
[ 127]
yp
l
l
yp
r n y n p
y n p
r n
However this expression is non causal.
It can be written as (change index ):
127
]
[
~
*
]
[
]
[
*
n
p
n
y
n
r
yp
*
[ ]
p n
]
[
n
y
Which van be computed as the output of an FIR Filter with impulse response:
127
,...,
0
],
127
[
]
[
~
*
*
n
n
p
n
p
Taking the time delay into account we obtain:
Since the preamble is random, almost white, recall that the crosscorrelation yields the
impulse response of the channel
]
[
n
y
64
128
128
0
n
n
n
128
0
127
]
[
n
p
0
1
n
0
129
n
 [ ]
h n
[ ]
yp
r n
[ ]
yp
r n
*
[ ]
p n
]
[
n
y
Compare the two (non dispersive channel):
y
r
yp
r
Autocorrelation of
received data
Crosscorrelation with
preamble
0
n
0
64
n
0
128
n
Synchronization with Dispersive Channel
Channel impulse
response
y
r
yp
r
Autocorrelation of
received data
Crosscorrelation with
preamble
0
n
Start of Data
Synchronization with Dispersive Channel
Let
be the length of the channel impulse response
64
C
L
Channel impulse
response
C
L L
In order to determine the starting point, compute the energy on a sliding window and
choose the point of maximum energy
]
[
n
r
yp
1
xcorr
]
[
n
y
]
[
n
p
n
1
0
[ ] [ ]
L
yp
k
c n r n k
]
[
n
c
]
[
n
r
yp
Maximum
energy
]
[
n
c
L
=max
length of
channel = length of CP
1
n L
]
[
n
r
yp
xcorr
]
[
n
y
]
[
n
p
]
[
n
c
Impulse response
of channel
Example
]
[
n
c
Auto
correlation
Cross
correlation
]
[
n
p
]
[
n
y
max
]
[
n
h
]
[
n
w
OFDM
TX
OFDM
RX
]
0
[
m
X
]
[
k
X
m
]
1
[
N
X
m
]
0
[
m
Y
]
[
k
Y
m
]
1
[
N
Y
m
]
[
k
X
m
]
[
]
[
]
[
]
[
k
W
k
X
k
H
k
Y
m
m
]
[
k
H
]
[
k
W
m

th data block
Channel Estimation
Recall that, at the receiver, we need the frequency response of the channel:
Transmitted:
Received:
channel freq.
response
From the Preamble
: at the beginning of the received packet. The transmitted signal in
the preamble is known at the receiver: after time synchronization, we take the FFT of
the received preamble
]
0
[
Y
64
128
128
Received Preamble:
Estimated initial time
256 samples
FFT
]
255
[
Y
]
[
k
Y
255
,...,
0
],
[
]
[
]
[
]
[
k
k
W
k
X
k
H
k
Y
p
0
n
255
,...,
0
],
[
]
[
]
[
]
[
k
k
W
k
X
k
H
k
Y
p
Solve for using a Wiener Filter (due to noise):
*
2 2
[ ] [ ]
ˆ
[ ]
 [ ] 
P
p w
Y k X k
H k
X k
noise covariance
]
[
k
H
Problem
: when we cannot compute the corresponding
frequency response
0
]
[
k
X
p
]
[
k
H
Fact: by definition,
j
k
X
p
1
]
[
254
,...,
158
,
156
100
,...,
4
,
2
k
k
if
0
]
[
k
X
p
otherwise (ie DC, odd values,
frequency guards)
Two solutions:
1.
Compute the channel estimate
2
2
*

]
[

]
[
]
[
]
[
ˆ
w
p
k
X
k
X
k
Y
k
H
preamble
only for the frequencies
k
such that
0
]
[
k
X
p
and interpolate for the other frequencies. This might not yield good results and the
channel estimate might be unreliable;
k
known
interpolate
2.
Recall the FFT and use the fact that we know the maximum length
L
of the
channel impulse response
* *
2
1
0
[ ] [ ]
[ ] [ ] [ ]
2 2
L
jk n
p p
N
n
X k X k
Y k h n e W k
]
[
]
[
]
[
]
[
k
W
k
X
k
H
k
Y
p
Since the preamble is such that either or
0

]
[

k
X
p
2

]
[

k
X
p
for the indices where we can write:
254
,...,
158
,
156
100
,...,
4
,
2
k
k
for
so that we have
100 equations and
L=
64 unknowns.
2

]
[

k
X
p
This can be written in matrix form:
* *
[ ] [ ]
[ ] [ ],
2 2
p p
k
X k X k
Y k v h W k
254
,...,
158
,
156
100
,...,
4
,
2
k
k
where
2 2
( 1)
256 256
1,
jk jk L
k
v e e
]
1
[
]
1
[
]
0
[
L
h
h
h
h
Write it in matrix form:
z Vh e
* *
1 1
2
2 2
* *
1 1
200
2 2
[2] [2] [0] [2] [2]
[200] [200] [63] [200] [200]
p p
p p
Y X v h W X
Y X v h W X
1
100
100 64
64 1
100 1
Least Squares solution
1
* *
ˆ
T T
h V V V z
this is ill conditioned.
0
10
20
30
40
50
60
70
10
15
10
10
10
5
10
0
10
5
eigenvalues
128
1
* *
ˆ
,
T T
h V V I V z
3
10
1. Generate matrix
kF=[2,4,6,…,100, 156, …, 254]’; non

null frequencies (data and pilots)
n=[0,…,63]; time index for channel impulse response
V=exp(

j*(2*pi/256)*kF*n);
M=inv(V’*V+0.001*eye(64))*V’;
Channel Frequency Response Estimation:
1
* *
T T
M V V I V
2. Generate vector
z
from received data y[n]:
Let n0 be the estimated beginning of the data, from time synchronization.
Then
y0=y(n0

256:n0

1); received preamble
Y0=
fft
(y0); decoded preamble
z=Y0(kF+1).*
conj(Xp256(kF+1
))/2; multiply by transmitted preamble
h=M*z; channel impulse response
3. Channel Frequency Response: H=fft(h, 256);
Data in
Trigger when preamble is detected
Channel
Estimate out
Simulink Implementation
]
[
n
y
]
[
k
Y
]
[
*
k
X
p
]
[
n
h
]
[
k
H
Example
:
Spectrum of
Received Signal
Estimated
Frequency
Response of
Channel
NOT TO
SCALE
As expected, it
does not match in
the Frequency
Guards
WiMax

2004 Demodulator
WiMax256.mdl
data
Ch.
Start after processing
preamble
Standard OFDM
Demod (256 carriers)
Error Correction
Decoding
Channel Tracking
In mobile applications, the channel changes and we need to track it.
IEEE802.16

2005 tracks the channel by embedding pilots within the data.
In the FUSC (Full Use of Sub Carriers) scheme, the pilots subcarriers are chosen
within the non

null subcarriers as
1
3
9
m
k
with
2
,
1
,
0
3
mod
ex
symbol_ind
m
128
for
11
,...,
0
512
for
47
,...,
0
1024
for
95
,...,
0
2048
for
191
,...,
0
FFT
FFT
FFT
FFT
N
N
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