Wimax IEEE 802.16e Physical Layer Introduction

1. OFDM and IEEE 802.16e
Physical Layer
Deepak Kumar Rathore

2. Outline
 Overview
 OFDM
 WiMAX and IEEE 802.16 std
 IEEE 802.16e Physical layer
 Conlusion

3. OFDM Basic Concept
 Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier
modulation scheme
 First break the data into small portions
 Then use a number of parallel orthogonal sub-carriers to transmit the data
 Conventional transmission uses a single carrier, which is modulated with all the
data to be sent

4. OFDM Basic Concept
 OFDM is a special case of Frequency
Division Multiplexing (FDM)
 For FDM
 No special relationship between the carrier
frequencies
 Guard bands have to be inserted to avoid
Adjacent Channel Interference (ACI)
 For OFDM
 Strict relation between carriers: fk = k·Df
where Df = 1/TU
(TU - symbol period)
 Carriers are orthogonal to each other and
can be packed tight

5. OFDM Transmission model
Channel, h(t)
Modulator
and transmitter
Wireless channel
Receiver and demodulator

6. Orthogonality – the essential property
 Example: Receiver branch k
 Ideal channel: No noise and no multipath
Tu = 1/Df gives subcarrier orthogonality over one Tu
=> possible to separate subcarriers in receiver
 
















  
 





D




D

q
k
,
0
q
k
,
a
dt
e
T
a
dt
e
e
a
T
1 k
1
N
0
q
T
0
t
T
1
k
q
2
j
U
q
T
0
ft
k
2
j
1
N
0
q
ft
q
2
j
q
U
c U
U
U c
Received signal, r(t)

7. 7
OFDM – Signal properties
Time domain
Frequency domain
Power Spectrum for OFDM symbol
frequency

8. Multipath channel
]
,
[ 0
0 

]
,
[ 1
1 

Diffracted and Scattered Paths
Reflected Path
LOS Path
]
,
[ k
k 


9. Multipath channel (cyclic prefix)
Time
[]
Amplitude
[]
Example multipath profile
0 1 2
The prefix is made cyclic to avoid inter-carrier-interference (ICI)
(maintain orthogonality)
Multipath introduces inter-symbol-interference (ISI)
TU
Prefix is added to avoid ISI
TU
TCP

10. Multipath channel (cyclic prefix)
 Tcp should cover the maximum length of the time
dispersion
 Increasing Tcp implies increased overhead in
power and bandwidth (Tcp/ TS)
 For large transmission distances there is a trade-
off between power loss and time dispersion
CP Useful symbol CP Useful symbol
CP Useful symbol
TU
Tcp
TS

11. Multipath channel (frequency diversity)
=
• The OFDM symbol can be exposed to a frequency selective channel
• The attenuation for each subcarrier can be viewed as “flat”
– Due to the cyclic prefix there is no need for a complex equalizer
• Possible transmission techniques
– Forward error correction (FEC) over the frequency band
– Adaptive coding and modulation per carrier

12. Frequency/subcarrier
Pilot carriers /reference signals
Data carriers
Multipath channel (pilot symbols)
 The channel parameters can be estimated based on known symbols (pilot
symbols)
 The pilot symbols should have sufficient density to provide estimates with good
quality (tradeoff with efficiency)
 Different estimation methods exist
 Averaging combined with interpolation
 Minimum-mean square error (MMSE)
Pilot symbol
Time
Frequency

13. The Peak to Average Power Problem
 A OFDM signal consists of a number of independently modulated symbols
 The sum of independently modulated subcarriers can have large amplitude variations
 Results in a large peak-to-average-power ratio (PAPR)



D



1
N
0
k
t
f
k
2
j
k
c
e
a
)
t
(
x
PA

14. Choosing the OFDM parameters
 Symbol time (TU) and subcarrier
spacing (Df) are inverse
 TU = 1/Df
 Consequences of increasing the
subcarrier spacing
 Increase cyclic prefix overhead
 Consequences of decreasing the
subcarrier spacing
 Increase sensitivity to frequency
inaccuracy
 Increasing number of subcarriers
increases Tx and Rx complexity
Increasing
subcarrier spacing
Decreasing
subcarrier spacing
Increase sensitivity to
frequency accuracy
TU
Increase CP
overhead

15. 4G Overview
 Background and Motivation
 Broadband Wireless Access
 Promising solution for last mile access
 High speed internet access
 Advantages of BWA
 Ease of deployment and installation
 Much higher data rates can be supported
 Capacity can be increased by installing more base stations
 Challenges for BWA
 Price
 Performance
 Interoperability issues

16. Advantages of OFDMA cont..
 Efficient use of Spectrum
4/3 Hz per symbol
6/5 Hz per symbol

17. WiMAX and IEEE 802.16 Standards
 WiMAX is an acronym of Worldwide Interoperability for Microwave
Access.
 IEEE 802.16 Standard is a IEEE802 family of specifications for PHY
and MAC layer.
 It is a metropolitan area network standard also called WirelessMAN.
 WiMAX define a High Speed wireless access (up to 70 Mbps) with
larger cell radius (up to 50 km) for fixed and mobile (180 kmph)
applications.
 IEEE 802.16-2004 include P2P, P2MP and mesh access networks with
2-11GHz for NLOS and 10-66 GHz for LOS.
 IEEE 802.16-2005 includes mobility. with the access of IP based
wireless network at broadband data rate . Making global roaming a
reality.

18. IEEE 802.16 Standards
IEEE 802.16 IEEE 802.16d IEEE 802.16e
Completed Dec – 2001 Jan – 2004 Mid – 2005
Spectrum 10-66 GHz 2-11 GHz 2-6 GHz
Channel condition LOS only NLOS NLOS
Mobility Fixed
Fixed as well as
portable
Nomadic Portable
Typical cell
Radius
2-5 km
7-10 km max. range
50 km
2-5 km
Bit rate
32-134 Mbps in 28 MHz
channel BW
Up to 25 Mbps in 20
MHz channel BW
Up to 15 Mbps in 5
MHz channel Bw
Modulation
QPSK, 16QAM &
64QAM
OFDM 256 OFDMA
2048 QPSK, 16QAM,
64QAM
Same as 802.16d and
Scalable OFDMA
Channel BW 20, 25 and 28 MHz
Scalable 1.5 to 20
MHz
Same as 802.16d with
uplink Sub channel
Application Backhaul
Wireless DSL and
Backhaul
Mobile Internet

19. Others Competing Technologies
 WiMAX Vs HSDPA
 WiMAX Vs WiBro
 WiMAX Vs LTE

20. Speed Vs Mobility
WI-Fi
WiMAX
HSPA
UMTS
GSM
Mobility
Speed

21. WiMAX
802.16e
802.11d
Wimax
4G
WI-Fi
HSDPA
3G
UMTS
CDMA2000
EDGE
GSM
GPRS
Bluetooth
Mbits/s 0.1 1 10 60 100
0-75Mph
Mobile
Fixed A Comparison chart of Mobility Vs Data
rate
0-75Mph
Mobile
0-75Mph
Fixed
Mobile
0-75Mph
Mbits/s
Fixed
Mobile
0-75Mph
0.1
Mbits/s
Fixed
Mobile
0-75Mph
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
Wimax
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
802.11d
Wimax
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
WI-Fi
802.11d
Wimax
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
HSDPA
WI-Fi
802.11d
Wimax
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
3G
UMTS
CDMA2000
HSDPA
WI-Fi
802.11d
Wimax
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
EDGE
3G
UMTS
CDMA2000
HSDPA
WI-Fi
802.11d
Wimax
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph
GSM
GPRS EDGE
3G
UMTS
CDMA2000
HSDPA
WI-Fi
802.11d
Wimax
WiMAX
802.16e
4G
100
60
10
1
0.1
Mbits/s
Fixed
Mobile
0-75Mph

23. WiMAX NETWOKS ARCHITECTURE
ASN-Access service network, CSN- connectivity service network
NSP-Network service provider, NAP-Network access provider

24. Applications
The bandwidth and range of WiMAX make it suitable for the
following potential applications:
 Providing high speed mobile and fixed Internet connectivity.
 Connecting Wi-Fi hotspots to the Internet.
 Providing a wireless alternative to cable and DSL for "last mile"
broadband access.
 Providing data and telecommunications services.
 Providing a source of Internet connectivity as part of a business
continuity plan. That is, if a business require a fixed and a wireless
Internet connection,
 Easy and cost effective supplements for rural penetration in
broadband communication (data, voice, video, multimeadia
applications)

25. IEEE 802.16e Physical layer

26. Physical Layer in IEEE 802.16e Standards
 WirelessMAN SC
 WirelessMAN SCa
 WirelessMAN OFDM
 WirelessMAN OFDMA
 WirelessHUMAN

27.  WirelessMAN SC - A single-carrier PHY layer intended for
frequencies beyond 11GHz requiring a LOS condition. This PHY layer is
part of the original 802.16 specifications.
 WirelessMAN SCa - A single-carrier PHY for frequencies
between 2GHz and 11GHz for point-to-multipoint operations.
 WirelessMAN OFDM - A 256-point FFT-based OFDM PHY
layer for point-to-multipoint operations in non-LOS conditions at
frequencies between 2GHz and 11GHz. This PHY layer, finalized in the
IEEE 802.16-2004 specifications, has been accepted by WiMAX for fixed
operations and is often referred to as fixed WiMAX.

28.  WirelessMAN OFDMA - A 2,048-point FFT-based OFDMA
PHY for point-to-multipoint operations in NLOS conditions at frequencies
between 2GHz and 6GHz. In the IEEE 802.16e-2005 specifications, this
PHY layer has been modified to SOFDMA (scalable OFDMA),
where the FFT size is variable and can take any one of the following
values: 128, 512, 1,024, and 2,048. The variable FFT size allows for
optimum operation/ implementation of the system over a wide range
of channel bandwidths and radio conditions. his PHY layer has been
accepted by WiMAX for mobile and portable operations and is also
referred to as mobile WiMAX.
 WirelessHUMAN - Wireless high speed unlicensed metropolitan
area network is the name used to explicitly indicate 802.16 air
interface used for unlicensed bands.

29. OFDM …As multi-carrier Transmission

30. OFDM …As multi-carrier Transmission...
Time Time
S1 S2 S3 S4 S5 S6 S7 S8
S1
S6
S2
S5
S3
S4
S7
S8
Ts Ts
frequency

31. OFDM …As multi-carrier Transmission...
 Advantages of OFDM
 High spectral efficiency
 Efficient implementation using FFT
 Robust against narrow-band co-channel interference
 Robust against Intersymbol interference (ISI) and fading caused by multipath
propagation
 Low sensitivity to time synchronization errors
 Disdvantages of OFDM
 Sensitive to Doppler shift.
 Sensitive to frequency synchronization problems.
 High peak-to-average-power ratio (PAPR).
 Loss of efficiency caused by Cyclic prefix/Guard interval.

32. System Model
M
A
C
/P
H
Y
Modulator
(PSK/QAM)
N-Point
IFFT
Append
Cyclic
Prefix
Randomization
FEC Encoder
Interleaving
A/D
Conversion
Demodulator
(PSK/QAM)
N-Point
FFT
Remove
Cyclic
Prefix
FEC Decoder
Deinterleaving
D/A
Conversion
Derandomization
AWGN
Wireless Channel
Subcarrier
Allocation
Pilot
Symbols
Subcarrier
Allocation
Pilot
Symbols
M
A
C/
P
H
Y
int
erf
ac
e
Modulator
(PSK/QAM)
N-Point
IFFT
Append
Cyclic
Prefix
Randomization
FEC Encoder
Interleaving
A/D
Conversion
Demodulator
(PSK/QAM)
N-Point
FFT
Remove
Cyclic
Prefix
FEC Decoder
Deinterleaving
D/A
Conversion
Derandomization
Wireless Channel
Subcarrier
Allocation
Pilot
Symbols
Pilot
Extraction
Channel
Estimation

33. Simulation Model…
 Channel coding
 Modulator (QPSK/QAM)
 Subcarrier Allocation
 IFFT
 Cyclic Prefix
 A/D Conversion

34. Channel coding
 Randomization
 FEC
 Interleaving

35. Randomization
 The scrambler performs randomization of input data on each burst for each
allocation to avoid long sequence of continuous ones and zeros. This is
implemented with a Pseudo Random Binary Sequence (PRBS) generator
which uses a 15 stage shift register with a generator polynomial of
1+x14+x15 with XOR gates in feedback configuration as shown in figure.
On the downlink the randomizer shall be re-initialized at the start of each
frame with the sequence 1000101010000000. The implemented scrambler
complies with the initialization process as specified in section 8.3.3.1 of the
standard [2].
1 2 3 4 5 6 7 8 9 10 11 15
14
13
12
LSB MSB
Data in
Data out
Fig : PRBS for data randomization

36. FEC encoder
 Concatenated reed-solomon-convolutional code
(RS-CC)
 Outer RS encoder
 Inner convolutional encoder

37. RS-CC
 The randomized data are arranged in block format before
passing through the encoder and a single 0x00 tail byte is
appended to the end of each burst. The implemented RS
encoder is derived from a systematic RS (N=255, K=239,
T=8) code using GF (28). The following polynomials are used
for code generator and field generator:
 g(x)=(x+λ0)(x+ λ1)… (x+ λ2T-1), λ = 02HEX (1)
 p(x)=x8 + x4 + x3 + x2 + 1 (2)

38. Convolutional Encoder
 The outer RS encoded block is fed to inner binary
convolutional encoder. The implemented encoder has native
rate of 1/2, a constraint length of 7 and the generator
polynomial given by following Equation to produce its two
code bits. The generator is shown in next slide.
 G1 = 171OCT For X
 G2 = 133OCT For Y

39. Convolutional encoder of rate 1/2
1 bit
delay
1 bit
delay
1 bit
delay
1 bit
delay
1 bit
delay
1 bit
delay
+
X output
+
Y output
Data in
Data in

40. The inner convolutional code with
puncturing configuration
 In order to achieve variable code rate a puncturing operation is
performed on the output of the convolutional encoder in accordance to
Table 1. In this Table “1” denotes that the corresponding convolutional
encoder output is used, while “0” denotes that the corresponding output
is not used. At the receiver Viterbi decoder is used to decode the
convolutional codes.
Code Rate
Rate 1/2 2/3 3/4 5/6
dfree 10 6 5 4
X 1 10 101 10101
Y 1 11 110 11010
XY X1Y1 X1Y1Y2 X1Y1Y2X3 X1Y1Y2X3Y4X5

41. Mandatory channel coding per modulation
Modulation
Uncoded block
size (bytes)
Coded block
size (bytes)
Overall
coding rate
RS
code
CC code
rate
BPSK 12 24 1/2 (12,12,0) 1/2
QPSK 24 48 1/2 (32,24,4 2/3
QPSK 36 48 3/4 (40,36,2) 5/6
16-QAM 48 96 1/2 (64,48,8) 2/3
16-QAM 72 96 3/4 (80,72,4) 5/6
64-QAM 96 144 2/3 (108,96,6) 3/4
64-QAM 108 144 3/4 (120,108,6) 5/6

42. Interleaving
 The encoded data are interleaved by a block interleaver. The
size of the block is depended on the numbers of coded bit per
subchannel in one OFDM symbol, Ncbps. In IEEE 802.16e,
the interleaver is defined by two step permutation. The first
ensures that adjacent coded bits are mapped onto nonadjacent
subcarriers. The second permutation ensures that adjacent
coded bits are mapped alternately onto less or more significant
bits of the constellation, thus avoiding long runs of unreliable
bits [2].

43. Block size of the bit interleaver
Default
(16
subchanne)
8
subchannel
4
subchannel
2
subchannel
1
subchannel
BPSK 192 96 48 24 12
QPSK 384 192 96 48 24
16-QAM 786 384 192 96 48
64-QAM 1152 576 288 144 72

44. Constellation Mapper
 The bit interleaved data are then entered serially to the
constellation mapper. The Matlab implemented constellation
mapper support BPSK, greymapped QPSK, 16QAM, and
64QAM as specified in IEEE 802.16e. The complex
constellation points are normalized with the specified
multiplying factor for different modulation scheme so that
equal average power is achieved for the symbols. The
constellation mapped data are assigned to all allocated data
subcarriers of the OFDM symbol in order of increasing
frequency offset index.

45. Sub-carrier Allocation

46. IFFT
 The grey mapped data are then sent to IFFT for time domain
mapping. Mapping to time domain needs the application of
Inverse Fast Fourier Transform (IFFT). In our case we have
incorporated the MATLAB ´ifft´ function to do so. This block
delivers a vector of 256 elements, where each complex number
clement represents one sample of the OFDM symbol.

47. Cyclic prefix
 A cyclic prefix is added to the time domain samples to combat the effect of
multi-path. Four different duration of cyclic prefix are available in the
standard. Being G the ratio of CP time to OFDM symbol time, this ratio
can be equal to 1/32, 1/6, 1/8 and 1/4, can be selected accordingly Channel
Model. In order to evaluate the performance of the developed
communication system, an accurate description of the wireless channel is
required to address its propagation environment. [5]. How ever in our
simulation we used G=1/32 as we considering AWGN channel model.
Ts
Tb
Tg
CP

48. Channel Model
The wireless channel is characterized by:
 Path loss (including shadowing)
 Multipath delay spread
 Fading characteristics
 Doppler spread
 Co-channel and adjacent channel interference

49. Stanford University interim
Channel
type
Terrain
type
Delay (µs) Power (dB) Doppler (Hz)
Tap1 Tap2 Tap3 Tap1 Tap2 Tap3 Tap1 Tap2 Tap3
SUI-1 C 0 0.4 0.9 0 -15 -20 0.4 0.3 0.5
SUI-2 C 0 0.4 1.1 0 -12 -15 0.2 0.15 0.25
SUI-3 B 0 0.4 0.9 0 -5 -10 0.4 0.3 0.5
SUI-4 B 0 1.5 4 0 -4 -8 0.2 0.15 0.25
SUI-5 A 0 4 10 0 -5 -10 2 1.5 2.5
SUI-6 A 0 14 20 0 -10 -14 0.4 0.3 0.5

50. Signal Model
1. Input to time domain
2. Guard Interval
3. Channel
   
  1
,...,
2
,
1
,
0 

 N
n
k
X
IDFT
n
x
 
 
 












1
,...,
1
,
0
,
1
,...,
1
,
,
N
n
n
x
N
N
n
n
N
x
n
x g
g
f
     
n
w
n
h
n
x
y f
f 



51. Signal Model…
 Guard Removal
 Output to frequency domain
 Output
    1
,...,
1
,
0 

 N
n
n
y
n
y f
   
  1
,...,
2
,
1
,
0 

 N
k
n
y
DFT
k
Y
         
1
,...,
1
,
0 




N
k
k
W
k
I
k
H
k
X
k
Y

52. Signal Model…
1. Estimated data
Where He(k)= estimated channel
   
 
1
,...,
1
,
0 

 N
k
k
H
k
Y
k
X
e
e

53. Pilot based Channel Estimation
Time
Carrier
Time
Carrier
 Comb Type:
 Part of the sub-carriers are
always reserved as pilot
for each symbol
 Block Type:
 All sub-carriers is used as
pilot in a specific period

54. Used Comb Type Channel Estimation
   
   
   
L
l
m
H
L
l
m
H
m
H
l
mL
H
k
H
p
p
p
e
e








0
1
 LS: Least Square Estimation
 Linear Interpolation
 
 
 
1
,...,
1
,
0 

 p
p
p
p N
k
k
X
k
Y
k
H
 Low-Pass Interpolation (‘interp’ in MATLAB)
Insert zeros into the original sequence
Low-pass filter while passing original data unchanged

55. Parameters
Type Parameters Value
Primitive
Nominal Channel Bandwidth, BW 3.5 MHz
Number of Used Subcarrier, Nused 200
Sampling Factor, n 8/7
Ratio of Guard time to G useful symbol time, 1/4 ,1/8, 1/16, 1/32
Derived
NFFT (smallest power of 2 greater than Nused) 256
Sampling Frequency, Fs Floor(n.BW/8000) X 8000
Subcarrier Spacing, ∆f Fs/NFFT
Useful Symbol Time, Tb 1/ ∆f
CP Time, Tg G.Tb
OFDM Symbol Time, Ts Tb+Tg
Sampling Time Tb/NFFT

56. Conclusion

57. References
1. IEEE Std 802.16-2004. Part 16: Air Interface for Fixed Broadband
Wireless Access Systems", Oct. 2004.
2. IEEE Std 802.16e-2005. Part 16: Air Interface for Fixed and Mobile
Broadband Wireless Access Systems - Amendment2: Physical and
Medium Access Control Layers for Combined Fixed and Mobile
Operation in Licensed Bands. February 2006.
3. Da Fan, Q. Wang, Y. Lin and Z. Zhu, “Design and simulation of the BS
Transceiver for IEEE 802.16e OFDMA Mode” IEEE, ICASSP 2008 pp.
1513-1516.
4. Hassan Yagoobi, “Scalable OFDMA Physical Layer in IEEE 802.16
WirelessMAN,” Intel Technology Journal, vol.8, Aug 2004, pp. 1-14.

58. References…
5. V. Erceg, K.V.S. Hari, M.S. Smith, et al, “Channel Models for Fixed
Wireless Applications,” Contribution IEEE 802.16a-03/01, Jun. 2003.
6. C. Mehlfifuihrer, S. Caban and M. Rupp, “Experimental evaluation
of adaptive modulation and coding in MIMO WiMAX with limited
feedback ” 2007
7. Simon Plass and Stefan Kaiser “MC-CDMA versus OFDMA in cellular
environments.” 2003.
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61. Thank You

62. Queries ???