Performance Evaluation of WiMAX / IEEE 802.16
OFDM Physical Layer
Mohammad Azizul Hasan
Master’s thesis presentation, 5th June, Espoo
Supervisor: Prof. Riku Jäntti
Instructor: Lic. Tech. Boris Makarevitch
HELSINKI UNIVERSITY OF TECHNOLOGY
Communications Laboratory
Agenda
 Introduction
 IEEE 802.16 and Wireless Broandband Access
 IEEE 802.16 Physical Layer
 Simulation Model
 Simulation Results
 Conlusion and Futurework
2
Introduction

Background and Motivation

Broadband Wireless Access


Promising solution for last mile access
High speed internet access in residential as well as small and medium sized enterprise sector

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

Broadband access is currently dominated by DSL and cable modem technologies
Limitations:
•
•
•



dsl can reach only three miles from central office switch
Lack of return channel in older cable network
Commercial areas are often not covered by cable networks
IEEE 802.16 is the first industry based standard for BWA
Objective
Evaluate the effect of various modulation and coding schemes as well as interleving on PHY layer performance
Methodology
PHY layer simulation is used to investigate the performance
3
IEEE 802.16 and Broadband Wireless Access (BWA) (1/5)
•
Evolution of IEEE family of standard for BWA
-EEE 802.16 Working group on BWA is responsible for development of the standards
-The standard provides secification for PHY and MAC layer

IEEE 802.16-2001
-First issue of the family intend to provide fixed BWA access in a point-to-point (PTP) topology.
-Single carrier modulation
-10-66 GHz frequency range
-QPSK, 16-QAM (optional in UL) and 64-QAM (optional) modulation scheme

IEEE 802.16a
-Added physical layer support for 2-11 GHz
-Non Line of Sight (NLOS) operation becomes possible
-Advanced power management technique and adaptive antenna arrays were included
-OFDM was included as an alternative to single carrier modulation
-BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM (optional)

IEEE 802.16-2004
-2-11 GHZ frequency range
-256 subcarriers OFDM Technique
-BPSK, QPSK, 16-QAM, 64-QAM
-Fixed and Nomadic access

IEEE 802.16e
-Scalable OFDMA
-Mobile BWA
4
IEEE 802.16 and BWA (2/5)
Scope of standard
 IEEE 802.16 Protocol Stack


MAC Layer

Service-Specific
Convergence Sublayer
(CS)
M
A
C
Service Specific CS
MAC SAP
MAC Common Part
Sublayer (MAC CPS)
Management Entity
MAC CPS
MAC Common Part Sublayer (CPS)
- System access, bandwidth allocation, connection
management
-QoS provisioning
Privacy Sublayer
-Authentication, secure key exchange, encryption
Security Sublayer
P
H
Y
PHY
SAP
Physical Layer (PHY)
Data /Control Plane

Management Entity
Service specific convergence Sublayer(CS)
-MAC CS receives higher level data
-provides transformation and mapping into MAC SDU
-ATM CS and packet CS

CS
SAP
Security Sublayer
Management Entity
PHY
Management Plane
PHY Layer
-Four different physical layer specifications
-SC, SCa, OFDM, OFDMA
5
IEEE 802.16 and BWA (3/5)
 Network Architecture and Deployment Topology

Architecture


Resembled to cellular networks
Each cell consists of a BS and one or
more SS
BS provides connectivity to core network

SSs
SSs

Topology



Point to point (PTP)
Point to multi point (PTM)
Mesh
BS
Core Network
BS
SSs
BS
6
IEEE 802.16 and BWA (4/5)
 Application
-Supports ATM, IPv4, IPv6, Ethernet and VLAN

Cellular Backhaul
- hotspots, PTP back haul

Residential Broadband
-fill the gaps in cable and dsl coverage

Underserved Areas
-rural areas

Always Best Connected
- roaming
7
IEEE 802.16 and BWA (5/5)
 WiMAX Forum and IEEE 802.16




Worldwide Interoperability for Microwave Access (WiMAX)
An allince of telecommunication equipment and component manufacturers and service
providers
Promotes and certify the compatibility and interoperability of BWA products
Adopted two version of the IEEE 802.16 standard

Fixed/nomadic access: IEEE 802.16-2004 OFDM PHY layer

Portable/Mobile access: IEEE 802.16e
8
IEEE 802.16 Physical Layer (1/4)
 PHY Layer attributes:

Defines duplexing techniques (TDD, FDD)

Supports multiple RF bands



Flexible bandwidths



Up to 134 MHz in 10-66 GHz band
Up to 20 MHz in < 11GHz band
Defines multiple PHYs for different Applications




10-66 GHz for LOS
below 11GHz for NLOS
SC for point-to-point long range application
OFDM for efficient Point-to-Multi-Point high data rate applications
OFDMA more optimized for mobility, using sub-channelizationon on Downlink and Uplink
Specifies Modulation and channel coding schemes
9
IEEE 802.16 Physical Layer (2/4)
IEEE 802.16 Airinterface nomenclature and description
Desgnation
Band of operation
Duplexing Technique
Notes
WirelessMAN-SC™
10-66 GHz
TDD,
FDD
Single Carrier
WirelessMAN-SCa™
2-11 GHz
Licensed band
TDD,
FDD
Single Carrier technique for
NLOS
WirelessMAN-OFDM™
2-11 GHz
Licensed band
TDD,
FDD
OFDM for NLOS operation
WirelessMAN-OFDMA™
2-11 GHz
Licensed band
TDD,
FDD
OFDM Broken into subgroups
to provide multiple access in a
single frequency band
2-11 GHz
Licensed Exempt Band
TDD
May be SC, OFDM, OFDMA.
Must include Dynamic
Frequency Selection to
mitigate interfarence
WirelessHUMAN™
10
IEEE 802.16 Physical Layer (3/4)
 WirelessMANTM OFDM PHY Layer

Flexible Channel Bandwidth


Robust Error Control Mechanism



8 different scheme
Adaptive Antenna System


outer Reed-Solomon (RS) code and inner Convolutional code (CC).
Turbo Coding (optional)
Adaptive Modulation and Coding


integer multiple of (1.25 1.5, 1.75, 2 or 2.75) MHz with a maximum of 20 MHz
Transmission of DL and UL burst using
directed beams
Transmit Diversity
11
IEEE 802.16 Physical Layer (4/4)
 OFDM


Special form of MCM technique
Dividing the total bandwidth into a number of sub-carriers



Densely spaced and orthogonal sub-carriers
Orthogonality is acheived by FFT
ISI is mitigated
Comparison between conventional FDM and OFDM
12
Simulation Model (1/5)
PHY Layer Setup
Transmitter
Random data
generation
Output Data
Channel
Encoding
Channel
decoding
Mapping
De-mapping
IFFT
FFT
Cyclic Prefix
insertion
Cyclic Prefix
removal
Receiver
13
Simulation Model (2/5)
 Channel coding
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
14
Simulation Model (3/5)
 Channel Coding (contd.)

Data randomization
•
•
•
Implemented with PRBS generator
15-stage shift register
XOR gates in feedback

RS-encoding
•
Derived from RS(N=255, K=239, T=8)
•
Shortend and punctured

CC Encoder
•
•
Native code rate ½
Supports punctureing to acheive variable code rate
Data
Randomization
Reed-Solomon
Encoding
Convolutional
Encoding
Interleaving
FEC
 Interleaver
•
•
•
Two step permutation
First step:adjacent coded bits are mapped onto non-adjacent subcarriers
Second step: adjacent coded bits are mapped alternately onto less or more significant bits of the constellation
15
Simulation Model (4/5)
 Simulator Description

Each block of the transmitter, receiver and channel is written in separate ’m’ file

The main procedure call each of the block in the manner a communication system works

initialization parameters: number of simulated OFDM symbols, CP length, modulation and coding rate, range of
SNR values and SUI channel model for simulation.

The input data stream is randomly generated

Output variables are available in Matlab™ workspace

BER and BLER values for different SNR are stored in text files
16
Simulation Model (5/5)
 Channel model

wireless channel is characterized by:





Path loss
Multipath delay spread
Fading characteristics
Doppler spread
Co-channel and adjacent channel interference

Stanford University Interim (SUI) channel models




-empirical model
-six channel model to address three different terrain types
-3 taps used to model multipath
-tap delay: 0-20 µs
17
Simulation results (1/10)
 Scatter plots
•
•
'+' transmitted data
'*' received data.

Sppead reduction is taking place with
the increaseing values of SNR

Validates the implementation
of channel model
Scatter Plots for 16-QAM modulation (RS-CC 1/2) in SUI-1 channel model
18
Simulation results (2/10)
 BER Performance
BER vs. SNR plot for different coding profiles on SUI-2 channel
19
Simulation results (3/10)
SNR required at BER level 10-3 for different modulation and coding profile
BPSK ½
QPSK ½
QPSK ¾
16-QAM ½
16-QAM ¾
64-QAM 2/3
64-QAM 3/4
SNR (dB) at BER level 10-3
Channel
SUI-1
4.3
6.6
10
12.3
15.7
19.4
21.3
SUI-2
7.5
10.4
14.1
16.25
19.5
23.3
25.4
SUI-3
12.7
17.2
22.7
22.7
28.3
30
32.7
20
Simulation results (4/10)

BER performance:variations with the change in channel conditions


Severity of corruption is highest on SUI-3
Lowest in SUI-1

Tap power dominates in determining
the order of severity of corruption
BER vs. SNR plot for 16-QAM 1/2 on different SUI channel
21
Simulation results (5/10)

BLER performance
BLER vs. SNR plot for different modulation and coding profile on SUI-1
22
Simulation results (6/10)

BLER Performance
SNR required at BLER level 10-2 for different modulation and coding profile
BPSK ½
QPSK ½
QPSK ¾
16-QAM ½
16-QAM ¾
64-QAM
2/3
64-QAM
3/4
SNR (dB) at BLER level 10-2
Channel
SUI-1
7.3
7
11
12.6
15.6
19.6
21.3
SUI-2
10.7
12.7
15.4
16.5
20.8
23.8
26.1
SUI-3
15
17.7
22.7
24.4
28.8
31.2
33.8
23
Simulation results (7/10)

BLER performance:variations with the change in channel conditions
•
Results are consistant with
the BER performance
BLER vs. SNR plot for 64-QAM 2/3 modulation and coding profile on different SUI channel
24
Simulation results (8/10)


Effect of Forward Error Correction
FEC gains 4.5 dB improvement
at BER level of 10-3
Effect of FEC in 64-QAM 2/3 on SUI-3 channel model
25
Simulation results (9/10)

Effect of Reed-Solomon Encoding
Performance improvement due to RS Coding
QPSK ½
16-QAM ½
64-QAM 2/3
SNR(dB) at
BER
10-3
1
1.2
1.4
SNR(dB) at
BLER
10-2
3
4.5
5
Effect of Reed Solomon encoding in QPSK ½ on SUI-3 channel model
26
Simulation results (10/10)

Effect of Bit interleaver
Performance improvement due to bit interleaving
SNR(dB) at
BER 10-
BPSK 1/2
QPSK ½
16-QAM ½
64-QAM 2/3
2.2
0.8
1.4
2.2
1
1.2
1.7
2.5
3
SNR(dB) at
BLER
10-2
Effect of Block interleaver in 64-QAM 2/3 on SUI-2 channel model
27
Conclusion and Future Work

Conclusion
•
•
•
•
•
•
Lower modulation and coding scheme provides better performance with less SNR
The results are ovious from constallation mapping point of view
Results obtain from the simulation can be used to set threshold SNR to implement adaptive modulation scheme to
attatin highest transmission speed with a target BER
FEC improves the BER performance by 6 dB to 4.5 dB at BER level 10 -3
RS encoding improves the BER performance by 1dB to 1.4 dB at BER level 10 -3
RS encoder provides tremendous performance when it is concatenated with CC

Future Works

The implemented PHY layer model still needs some improvement. The channel estimator can be implemented to
obtain a depiction of the channel state to combat the effects of the channel using an equalizer.

The IEEE 802.16 standard comes with many optional PHY layer features, which can be implemented to further
improve the performance. The optional Block Turbo Coding (BTC) can be implemented to enhance the
performance of FEC. Space Time Block Code (STBC) can be employed in DL to provide transmit diversity.
28
Thank You !
29