This document summarizes an article from the International Journal of Computer Engineering and Technology that models and simulates the physical layer of the IEEE 802.22 wireless standard over a multipath fading channel. It describes the IEEE 802.22 standard and its use of OFDM and OFDMA to provide resilience against multipath propagation. It then models the physical layer in Simulink, including modulation, channel effects, demodulation and decoding. Finally, it presents simulation results for different modulation schemes and coding rates over multipath channels to analyze the performance of the physical layer under varying conditions.

1. International Journal of Computer Engineering and Technology (IJCET), ISSN 0976-6367(Print),
INTERNATIONAL JOURNAL OF COMPUTER ENGINEERING &
ISSN 0976 - 6375(Online), Volume 4, Issue 5, September - October (2013), © IAEME
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MODELING AND SIMULATION OF PHYSICAL LAYER OF IEEE 802.22
OVER A MULTIPATH FADING CHANNEL
Zarana Barot1 and Anil Kumar Sharma2
M. Tech. Scholar1, Professor & Vice Principal2,
Department of Electronics & Communication Engineering,
Institute of Engineering & Technology, Alwar-301030 (Raj.), India
ABSTRACT
IEEE 802.22, also known as Wireless Regional Area Network (WRAN), is the newest
wireless standard developed for remote and rural areas. In this work, an overview of the standard,
and more specifically its physical (PHY) layer is evaluated. For this purpose the PHY layer is
modeled using SIMULINK tool and derived the received constellation mapping of the system for
different code rates and modulation schemes with multipath noisy channel. In this paper Rayleigh
multipath channel model is used in addition of AWGN channel to analyze the 802.22 PHY layer for
likely practical conditions.
Keywords: BPSK, IFFT, OFDM, VHF and WRAN.
1. INTRODUCTION
A cognitive radio is a transceiver designed to use the best wireless channels in its vicinity [1].
Such a radio automatically detects available channels in wireless spectrum and accordingly changes
its transmission or reception parameters to allow more concurrent wireless communications in a
given spectrum band at one location. This process is a form of dynamic spectrum management. In
response to the operator's commands, the cognitive engine is capable of configuring radio-system
parameters. These parameters include "waveform, protocol, operating frequency, and networking". It
functions as an autonomous unit in the communications environment, exchanging information about
the environment with the networks it accesses and other cognitive radios (CRs). A CR "monitors its
own performance continuously", in addition to "reading the radio's outputs"; it then uses this
information to "determine the Radio Frequency (RF) environment, channel conditions, link
performance, etc.", and adjusts the "radio's settings to deliver the required quality of service subject
to an appropriate combination of user requirements, operational limitations, and regulatory
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constraints". The Wireless Regional Area Networks (WRANs) are expected to operate primarily in
low population density areas in order to provide broadband access to data networks.The WRAN
systems will use vacant channels in the Very High Frequency (VHF) and Ultra High Frequency
(UHF) bands allocated to the Television Broadcasting Service in the frequency range between 54
MHz and 862 MHz while avoiding interference to the broadcast incumbents in these bands. A typical
application can be the coverage of the rural area around a village within a radius of 10 km to 30 km
from the base station depending on its Equivalent Isotropically Radiated Power (EIRP) and antenna
height. The Media Access Control (MAC) layer can also accommodate user terminals located as far
as 100 km with proper scheduling of the traffic in the frame when exceptional RF signal propagation
conditions are present. With the PHY implemented in this standard, WRAN systems can cover up to
a radius of 30 km without special scheduling. in performance of their terrestrial television reception.
Accordingly the use of IEEE 802.22 WRAN technology should enable more efficient use of the
spectrum to be made as well as providing new services for users, especially in rural areas. A base
station (BS) complying with this standard shall be able to provide high-speed Internet service for up
to 512 fixed or portable customer premise equipment (CPE) devices or groups of devices within its
coverage area assuming different quality of service (QoS) requirements for various CPEs, while
meeting the regulatory requirements for protection of the incumbents. The standard includes
cognitive radio techniques to mitigate interference to incumbents, including geo location capability,
provision to access a database of incumbent services, and spectrum-sensing technology to detect the
presence of incumbent services, other WRAN systems, and IEEE 802.22.1 wireless beacons.This
work is based on the standard itself. In IEEE, before a standard is established proposal, for
developing systems for newly developed technology, is discussed. A working group is formed which
has many task groups working on a project related to the proposed technology. Ballot meeting are
called to get vote on highly efficient implementation of the proposed technology. Finally, the
standard is formed by taking vote of extinguished engineers, researchers, technical representatives of
leading corporate bodies. Hence it is safe to say that the standard itself is very well tested. Still there
can be many variations for testing methodologies. Under this assumption, this dissertation is done as
an attempt to analyze the PHY layer for understanding behavior of PHY layer under various SNR
values and channel variations
2. PHYSICAL LAYER OF 802.22
In order to meet the requirements for the overall 802.22 system, the physical layer maintains
a high degree of flexibility. This is built in to the basic specification of the system. This work targets
to analyze the PHY layer capabilities to survive in multipath environment [2]. One of the first
characteristics is the modulation scheme. An OFDM (Orthogonal Frequency Division Multiplex)
scheme has been adopted because the 802.22 WRAN system to provide resilience against multipath
propagation and selective fading as well as a high level of spectrum efficiency and sufficient data
throughput. To provide access for multiple users, OFDMA is used for both upstream and
downstream data links [3][4]. OFDM is a form of transmission that uses a large number of close
spaced carriers that are modulated with low rate data. Normally these signals would be expected to
interfere with each other, but by making the signals orthogonal to each another there is no mutual
interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol
period. This means that when the signals are demodulated they will have a whole number of cycles
in the symbol period and their contribution will sum to zero - in other words there is no interference
contribution. The data to be transmitted is split across all the carriers and this means that by using
error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can
be reconstructed. Additionally having data carried at a low rate across all the carriers means that the
effects of reflections and inter-symbol interference can be overcome. It also means that single
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ISSN 0976 - 6375(Online), Volume 4, Issue 5, September - October (2013), © IAEME
frequency networks, where all transmitters can transmit on the same channel can be implemented [6].
IEEE 802.22 allows a variety of modulation schemes to be used within the OFDMA signal: QPSK,
16-QAM and 64-QAM can all be selected with convolution coding rates of 1/2, 3/4, and 2/3. The
required modulation and error correction rates are chosen according to the prevailing conditions [4].
In order to meet the requirements for the individual users that may be experiencing very different
signal conditions, it is necessary to dynamically adapt the modulation, bandwidth and coding on a
per CPE basis. In order to be able to obtain the required level of performance, it has been necessary
to the IEEE 802.22 to adopt a system of what is termed "Channel Bonding." This is a scheme where
the IEEE 802.22 system is able to utilize more than one channel at a time to provide the required
throughput. Often it is possible to use adjacent channels because in many countries the regulatory
authorities and frequency planners allow two or more empty channels between stations transmitting
high power signals as this prevents interference on the TV signals. These multiple free channels
allow the use of contiguous channel bonding. In practice the maximum number of channels that are
bonded is likely to be limited to three as a result of the front-end bandwidth limitations. To provide
access for both upstream and downstream data, the form of duplex scheme that has been adopted is
TDD. This has several advantages. First it only requires one channel to be used - FDD would not be
viable because it would be more difficult to control two channels with sufficient transmit / receive
spacing. Secondly the use of TDD enables dynamic change of the upstream and downstream
capacity. The specification is for a system that uses vacant channels to provide wireless
communication over a distance of up to 100 km, the propagation time over the first 30 km range
being absorbed by the TTG at the PHY layer and the propagation time beyond 30 km being absorbed
by proper MAC packet scheduling at the BS, as well as time buffers before and after the
opportunistic bursts (ranging, BW request and UCS notification) and before and after the CBP burst
[4].The OFDM System Design Requirements are available bandwidth, required bit rate, tolerable
delay spread and Doppler values. The OFDM System design parameters are derived according to the
system requirements. The requirement of the system design must be fulfilled by the system
parameters. The various design parameters for an OFDM system are Number of subcarriers, Guard
time (CP interval) and symbol duration, Subcarrier spacing and Modulation type per subcarrier.
3. PHYSICAL LAYER MODELING
Fig 1. SIMULINK Schematic of IEEE 802.22 PHYSICAL Layer
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4. International Journal of Computer Engineering and Technology (IJCET), ISSN 0976-6367(Print),
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Variable data rate block is used to generate random data input to the WRAN transmitter. The
convolution encoder bank contains CC coders with different code rates. This block performs the
Forward error correction operation on input data stream. Interleave Bank performs interleaving
operation on the encoded bit stream. Modulator bank performs QAM modulation on the interleaved
bit stream. It will output complex numbers assigned to a group of bits. Serial to parallel block is
used to insert the pilot carriers and DC components to the bit stream and to convert the serial data
into parallel data carriers which can be given as input to the IFFT block. The IFFT block computes
the inverse fast Fourier transform (IFFT) of each channel of a P-by-N or length-P input, u. When the
Inherit FFT length from input dimensions check box is selected, the input length P must be an integer
power of two, and the FFT length M is equal to P. When the check box is not selected, P can be any
length, and the value of the FFT length parameter must be a positive integer power of two. For userspecified FFT lengths, when M is not equal to P, zero padding or modulo-M data wrapping happens
before the IFFT operation. Cyclic prefix insertion block adds the portion of input frame to its front
end. Thus inserting a guard interval. The reshape function is used to convert the parallel data streams
to serial bit stream. The wireless channel is used to insert noise in the OFDM modulated data. There
are three options i.e. no noise, non dispersive multipath noise and dispersive multipath noise. The
Multipath Rayleigh Fading Channel block implements a baseband simulation of a multipath
Rayleigh fading propagation channel. This block is useful for modeling mobile wireless
communication systems. The AWGN channel block adds white Gaussian noise to a real or complex
input signal. When the input signal is real, this block adds real Gaussian noise and produces a real
output signal. When the input signal is complex, this block adds complex Gaussian noise and
produces a complex output signal. This block inherits its sample time from the input signal. Serial to
parallel block at receiver performs inverse operation of parallel to serial block of transmitter. Cyclic
prefix removes the cyclic prefix part from the input bit stream. FFT block performs the inverse
operation of the IFFT block and will convert the time domain data to frequency domain.
Disassemble OFDM frames converts the parallel data carriers to serial carrier. Demodulator Bank
recovers digital bits from the modulation symbols. Deinterleaver Bank performs the deinterleaving
operation on the demodulated data. Viterbi decoder Bank recovers the original information data
inserted to by random data generator, by decoding the coded bitstream using viterbi decoder blocks
SNR estimation takes input from demodulator bank and estimates Signal to Noise Ratio of the
received signal. Mode Control takes input from SNR estimation block and changes the mode of the
system accordingly. When SNR is higher then mode will be increased towards 8 and when SNR is
lower the mode is decreased towards 1. Packet error rate calculation calculates the packet error rate
by comparing the bits from random data generator and the Viterbi decoded data. This parameter
shows the amount of information reached to the receiver. Manual mode control designed to
configure the IEEE 802.22 Transmitter and Receiver according to the desired bit rate. There are total
of 16 PHY modes [6]. For simplicity and compatibility check, in this work, only 8 modes are
implemented. The change in mode is dependent on level of noise in the channel. The Transmitter and
Receiver are configured to higher data rates if the channel noise is low and to lower data rates if the
channel noise is high.
4. SIMULATION RESULTS
The modes are designed to configure the IEEE 802.22 Transmitter and Receiver according to
the desired bit rate. The change in mode is dependent on level of noise in the channel. If the noise is
more in channel then the transmitter and receiver will use modes 1,2,3,4 for low data rates and if the
noise is high in channel the transmitter and receiver will use 5,6,7,8 modes for higher Bit rates. The
reason to use lower bit rates and modes for high noise in the channel is,if we send data with high bit
rates in a channel when the noise is high, more bits will be lost. When we say transmitter and
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5. International Journal of Computer Engineering and Technology (IJCET), ISSN 0976-6367(Print),
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receiver use Mode 1 we mean that the convolution encoder in Transmitter will be configured to code
rate 1/2, modulator will use BPSK Modulation scheme and the data rate will be 4.54 Mbps. Similarly
in Receiver the viterbi decoder will be configured to code rate 1/2 and the Demodulator will be
configured to BPSK Demodulation. We have to use these modes in order to provide synchronization
Transmitter and Receiver. If we don’t use any mode the transmitter may transmit data with different
data rate and the Receiver may be configured to different data rate.Thus at Receiver we will not be
able to recover any data from Transmitter. Table.1 shows the configuration of Physical blocks
according to different modes [11].
Mode
1
Table-1-Configuration of Physical Blocks
Code Rate
Modulation scheme
Bit rate (Mb/s)
1/2
BPSK
4.54
2
2/3
BPSK
6.05
3
3/4
QPSK
6.81
4
5/6
QPSK
7.56
5
2/3
16-QAM
12.1
6
3/4
16-QAM
13.61
7
2/3
64-QAM
18.15
8
5/6
64-QAM
22.69
After simulating model in 8 modes we get following results. By simulating different data
rates we get result for different modes. Modes 1 and 2 are for BPSK modulation scheme and it’s
code rate is 1/2 and 2/3 respectively. Bit rate is for Mode 1 is 4.54 and for Mode 2 are 6.05. The
simulation corresponding to mode 1and 2 is shown in Fig. 2 and 3.
Fig 2. Transmitted Constellation
Fig 3. Received Constellation
Modes 3 and 4 are for QPSK modulation scheme and it’s code rate is 3/4 and 5/6
Respectively. Bit rate is for Mode 3 is 6.81 and for Mode 2 is 7.56. The simulation corresponding to
mode 3 and 4, is shown in Fig. 4 and 5.
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Fig 4 Transmitted Constellation
Fig 5 Received Constellation
Modes 5 and 6 are for 16-QAM modulation scheme and it’s code rate is 2/3 and 3/4
Respectively. Bit rate is for Mode 5 is 12.1 and for Mode 6 is 13.61. The corresponding simulation is
as shown in Fig. 6 and 7.
Fig. 6 Transmitted Constellation
Fig 7 Received Constellation
Modes 7 and 8 are for 64-QAM modulation scheme and it’s code rate is 2/3 and 5/6
Respectively. Bit rate is for Mode 7 is 18.15 and for Mode 8 is 22.69. The corresponding simulation
is as shown in Fig. 8 and 9.
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7. International Journal of Computer Engineering and Technology (IJCET), ISSN 0976-6367(Print),
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Fig 8 Transmitted Constellation
Fig. 9 Received Constellation
5. CONCLUSION AND FUTURE SCOPE
In this paper the knowledge of WRAN technology is gained and also specifications of PHY
layer of IEEE 802.22 are understood. We have studied the OFDM based technologies. We have
developed and implemented a MATLAB SIMULINK based model of PHY layer of IEEE 802.22 to
understand the PHY layer behavior in multipath environment. The Results are taken in the form of
transmitted and received constellations. We can implement rest of 8 modes of PHY layer and also
can perform a comparative analysis of other wireless standards like IEEE 802.11, 16 and 15 with
IEEE 802.22 to exploit the future research scopes in the same direction. We can also implement the
model in FPGA using HDL and Multiple Input Multiple Output (MIMO) architecture in IEEE
802.22 PHY for experimental study on performance.
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