10-IEEE802.16 and WiMax

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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
N
N
k
nulls

DC
(null)

pilots

data

nulls

OFDM
Symbol

m

subcarrier

k

0
1
4
7