Simulation of a Radio Communication Channel - ee578

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Nov 21, 2013 (3 years and 6 months ago)

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SIMULATION OF THE
W
i
MAX

(IEEE 802.16
e
) PHYSICAL LAYER


(PHASE 4)

Presented by:

Ahmad Salim


2

INTRODUCTION


The acronym
WiMAX

stands for “Worldwide
Interoperability for Microwave Access”. It
is based
on IEEE 802.16
standard for
Wireless Metropolitan
Area Network (Wireless MAN).



It
specifies the air interface for fixed, portable, and
mobile broadband wireless access (BWA) systems
supporting multimedia services
.


W
i
MAX

Block Diagram (Physical Layer)

3

FEC Encoding

1.Reed
-
Solomon

2.
Convolutional

3. Optional:


Turbo, LDPC, ..

OFDM

IFFT, + CP..

Channel

+

Randomization

Interleaving

Data

Digital

Modulation

(Symbol Mapping)

AWGN

FEC

Decoding

1.Reed
-
Solomon

2.
Convolutional

3. Optional:


Turbo, LDPC, ..



OFDM

FFT,
-

CP..




De
-
Randomization

De
-
Interleaving

Estimated

Data

Digital

De
-
Modulation

(Symbol De
-
Mapping)

RANDOMIZER


Uncorrelates

long sequence of 1s or 0s by
XORing

with the synchronization frame data.


The purpose of randomization is to maintain better data integrity. Also the output of the
randomizer has equal number of 0’s and 1’s for given binary FEC block input.


The random sequence generator is a 2
15

− 1 Pseudo
-
Noise (PN) sequence generator with the
initial sequence set as
-

1 0 0 1 0 1 0 1 0 0 0 0 0 0 0


The initial sequence is reloaded for each FEC frame.


The random sequence generation is synchronized with the receiver which descrambles the
data.


From IEEE Std 802.16
-
2004 [
1]

FEC ENCODER


The 802.16* standards propose the following can be
used



Reed Solomon concatenated Convolution Coder (Mandatory)


Convolutional Turbo Codes (mandatory for Mobile Wimax)


Block Turbo Codes (Optional)


Low Density Parity Check Codes (Optional)


ENCODER


WiMAX modulation and coding schemes

AMC

Modulation


RS code

CC code rate

Overall

code
rate

1

BPSK

(12,12,0)

1/2

1/2

2

QPSK

(32,24,4)

2/3

1/2

3

QPSK

(40,36,2)

5/6

3/4

4

16
-
QAM

(64,48,4)

2/3


1/2

5

16
-
QAM

(80,72,4)

5/6

3/4

6

64
-
AQM

(108,96,6)

3/4


2/3


7

64
-
QAM

(120,108,6)

5/6

3/4


REED
-
SOLOMON ENCODER


A Reed
-
Solomon code is specified by RS(n, k, t).


The encoder takes k data symbols of l bits each and adds 2t
parity symbols to construct an n
-
symbol codeword.


n: number of bytes after encoding,


k: number of data bytes before encoding,


t: number of data bytes that can be corrected.


As
specified

in the standard, the Reed
-
Solomon encoding shall
be derived from a systematic RS( 255, 239, 8)

CONVOLUTIONAL

ENCODER


The generator polynomials used to derive its two output code bits,
denoted X and Y, are specified in the following expressions:

1
2
171 for X,
133 for Y
OCT
OCT
G
G


INTERLEAVER


Distribute the coded bits over subcarriers. A first
permutation ensures that adjacent coded bits are
mapped on to nonadjacent subcarriers.


The second permutation insures that adjacent coded
bits are mapped alternately on to less or more
significant bits of the constellation, thus avoiding long
runs of bits of low reliability.


MODULATION
MAPPER


BPSK, 4
-
QAM and 16
-
QAM constellation maps. (using Gray
mapping)

OFDM DEFINITION


OFDM = Orthogonal FDM


Carrier centers are put on orthogonal frequencies


ORTHOGONALITY
-

The peak of each signal coincides
with
trough

of other signals


Subcarriers are spaced by 1/
Ts



BASIC IDEA :
Channel bandwidth is divided into multiple
subchannels

to reduce ISI and frequency
-
selective fading.


FDM
VERSUS OFDM





Frequency

Division
Multiplexing





OFDM
frequency

dividing








Increase

In spectral
efficiency

OFDM

IN
WIMAX


WiMAX
specifications

for the 256
-
point FFT OFDM PHY layer
define

three types of subcarriers; data, pilot and null.



200 of the total 256 subcarriers are used for data and pilot
subcarriers, eight of which are pilots permanently spaced
throughout the OFDM spectrum.


The rest of the potential carriers are
nulled

and set aside for
guard bands.

OFDM frequency description.


The remaining 55 carriers, that are zero subcarriers appended at the
end of the cited structure, act as guard bands with the purpose to
enable the naturally decay of the signal.


These guard bands are used to decrease emissions in adjacent
frequency channels.

the structure of the subcarriers before and after appending the guard bands.

INVERSE FAST FOURIER TRANSFORM ALGORITHM


The IFFT is used to produce a time domain
signal.


each of the discrete samples before applying
the IFFT algorithm corresponds to an individual
subcarrier.



Besides ensuring the
orthogonality

of the
OFDM subcarriers, the IFFT represents also a
rapid way for modulating these subcarriers in
parallel.

THE CYCLIC PREFIX


The robustness of any OFDM transmission
against multipath delay spread is achieved by
having a long symbol period with the purpose
of minimizing the inter
-
symbol interference.

T
sym

: OFDM symbol time

T
b

: useful symbol time

T
g

: CP time.

g
b
T
G
T


Each OFDM symbol is preceded by a periodic
extension of the signal itself.


CP is a copy of the last portion of the data
symbol.


When eliminating ISI, it has to be taken into
account that the CP must be longer than the
dispersion of the channel.

SIMULATING SAMPLE SPACED RAYLEIGH
FADING CHANNEL



By sample spaced channel taps, we mean that the
difference in delays between different waves is either
some sampling interval T
s

or a multiple of it.







This channel can easily be implemented using a 3
-
tap
FIR filter as the sampling frequency is fixed.


CHANNEL

Propagation model

Tap number
i

Tap amplitude
C
i

Tap delay T
i
(ns)

Clear LOS (Type 0)

1

1.0 0

0

Multipath (Type 1)

1

0.995

0

2

0.0995 exp(
-
j0.75)

400/R

Multipath (Type 2)

1

0.286 exp(
-
j0.75)

0

2

0.953

400/R

3

-
0.095

800/R


R is the channel symbol rate in
MBd

Propagation path parameters are valid for R from 15 to 25
MBd
.

Propagation models for 802.16e

Multipath (Type 1) Channel
Specifications


No. of Taps = 2


Ex: R= 20MBd


Tap Weights and Delays


First Tap = 0 dB with
delay of 0 nanoseconds


Second Tap =
-
10 dB with
delay of 20 nanoseconds



we will make 2 correlated
Rayleigh faded channel
taps, each will be fed
samples taken from Jakes
filter.




0
2
4
6
8
10
12
14
16
18
20
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Power delay profile
Arrival time for each multipath (ns)
Mean power for each multipath normalized by direct wave
RECEIVER


OFDM: Fast Fourier Transform, CP removal


Removing the guard bands


Demapping


Deinterleaving


Decoding


Derandomization


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 for simulation.



The input data stream is randomly generated


NUMERICAL RESULTS


AWGN

1
1.5
2
2.5
3
3.5
4
4.5
5
10
-2
10
-1
10
0
E
b
/N
0
(dB)
BER


AWGN

Multipath (Type 1) with QPSK, R=1/2


0
5
10
15
10
-0.9
10
-0.8
10
-0.7
10
-0.6
10
-0.5
10
-0.4
E
b
/N
0
(dB)
BER


0
2
4
6
8
10
12
10
-2
10
-1
10
0
Error rate
E
b
/N
0
(dB)
QPSK (R=1/2) --- Multipath Type 1


BER
ADAPTIVE MODULATION AND CODING (AMC)


Basic Idea:

1.
„ Measure the channel at the receiver

2.
„ Feed the measurement back to the transmitter

3.
„ Adapt the transmission scheme relative to the
channel estimate to
maximize the data rate,
minimize transmit power, or minimize BER


„ What to adapt?

1.
„ Constellation size/power

2.
„ Symbol rate

3.
„ Coding rate/scheme

ADAPTIVE MODULATION AND CODING (AMC)


Bit rate shifting is achieved using adaptive
modulation


„ When the MS is close to the BS, it is offered high
bit rate (higher speed)


When the MS is far from the BS, the reliability
decreases and it is offered a lower bit rate


ADAPTIVE MODULATION AND CODING (AMC)


5
10
15
20
25
30
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
E
b
/N
0
(dB)
BER


BPSK 1/2
QPSK 1/2
QPSK 3/4
16-QAM 1/2
16-QAM 1/2
64-QAM 2/3
64-QAM 3/4
Target BER=10
-
3

DETAILED

RESULTS


(Configuration 1, Channel 1)

CONCLUSIONS AND FUTURE WORK


Conclusion



Lower modulation and coding scheme provides better performance at lower
SNR



Results obtained from the simulation can be used to set threshold SNR to
implement adaptive modulation scheme to attatin highest transmission
speed with a target BER




Future Work



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. Also, the use of the optional LDPC codes can provide
an improvement in the performance provided that the word length is long
enough.


REFERENCES


IEEE Standard for Local and metropolitan area networks Part16: Air Interface for Broadband Wireless Access Systems
(
http://standards.ieee.org/about/get/802/802.16.html
)



http://www.wimaxforum.org/



http://grouper.ieee.org/groups/802/16/



http://en.wikipedia.org/wiki/IEEE_802.16m#802.16e
-
2005_Technology



http://ecee.colorado.edu/~ecen4242/WiMax/WiMAX_802_16e.htm#_edn1



http://www.scribd.com/doc/2945438/PHY
-
Layer
-
of
-
WiMAX



http://www.google.com.sa/search?q=channel+wimax&ie=utf
-
8&oe=utf
-
8&aq=t&rls=org.mozilla:en
-
US:official&client=firefox
-
a&safe=on



http://www.wimax360.com/forum/topics/610217:Topic:61844?groupUrl=wimaxradioengineering&id=610217%3ATo
pic%3A61844&groupId=610217%3AGroup%3A18095&page=2#comments



http://dspdotcomm.blogspot.com/2008/11/simulating
-
sample
-
spaced
-
rayleigh.html



http://www.mathworks.com/matlabcentral/fx_files/18869/1/ChannelModelingWhitePaper.pdf