-
A Physical Layer Simulation for WiMAX
MIMO-OFDM System Throughput Comparison Between 2x2 STBC and 2x2
V-BLAST
in Rayleigh Fading Channel
Hadj Zerrouki*, Mohammed Feham STTC Laboratory ,
Department of Electronics and Electrical Engineering, Faculty of
Technology, Tlemcen University, Algeria.
zerrouki. hadj@gmail. com*, feham _ m@yahoo. fr
Abstract- WiMAX is a broadband wireless technology based on
IEEE802.16 standards family, which defines the physical (PHY)
and medium access control (MAC) layers and makes several
possible configurations available along with non-mandatory
options. WiMAX is a new OFDM based technology and promises
to combine high data rate services with wide area coverage.
In
this paper, the performance of Wi MAX PHY layer is
investigated
for two MIMO (Multiple-Input Multiple-Output) PHY layer
modifications, (Space-Time Block Codes, STBC) and (Spatial
multiplexing, SM) to provide high suppression against
multipath
fading, provide high bandwidth efficiency and high
throughput
with high data rates. This work incorporates the model
building
of the WiMAX Physical layer using MA TLAB simulation. The
results obtained for these modifications show that these
mechanisms enhance the performance of the WiMAX PHY layer
in fixed environments with high spectral efficiency.
Keywords-component; WiMAX; IEEE 802.16; OFDM; MIMO; STBC;
V-BLAST; SNR; Physical Layer (PHy);
I. INTRODUCTION
Worldwide Interoperability for Microwave Access (WiMAX) is
introduced by the Institute of Electrical and Electronic Engineers
(IEEE) which is standard designated 802.16 d-2004 [1 ] (used in
fixed wireless applications) and 802.16 e-2005 [2] (mobile
wireless) to provide a worldwide interoperability for microwave
access. The IEEE 802. 16 d-2004 air interface standard is basically
based on technology namely Orthogonal Frequency Division
Multiplexing (OFDM), that has been regarded as an efficient way to
combat the InterSymbol Interference (lSI) for its performance over
frequency selective channels for the broadband wireless
networks.
In an OFDM system, the data is divided into multiple parallel
sub-streams at a reduced data rate, and each is modulated and
transmitted on a separate orthogonal subcarrier. This increases
symbol duration and improves system robustness. OFDM is achieved by
providing multiplexing on users' data streams on both Uplink and
Downlink transmissions. A valuable solution consists of the
introduction
of advanced digital signal processing techniques based on
Multiple Input Multiple Output (MIMO) concept.
The key feature of MIMO is the capability to increase channel
capacity without increasing transmitted power and RF bandwidth [3
]. Nowadays, MIMO techniques present some well-promising
applications in wireless standards like IEEE 802.11 n and IEEE
802.16 x (WiMAX). Different space-time processing techniques have
been proposed in literature in order to fully exploit
potentialities of MIMO systems. The most popular one is Space-Time
Coding [4], in which the time dimension is complemented with the
spatial dimension inherent to the use of multiple
spatially-distributed antennas
Commonly used Space Time coding schemes are SpaceTime-trellis
codes and Space-Time Block Codes (STBC). A well-known example of
conceptually simple, computationally efficient and mathematically
elegant STBC scheme has been proposed by Alamouti in [5].
Substantially Alamouti's coding is an orthogonal space-time block
code, where two successive symbols are encoded in an orthogonal 2x2
matrix. The columns of the matrix are transmitted in successive
symbol periods, but the upper and the lower symbols in a given
column are sent simultaneously through the first and the second
transmit antenna respectively.
The alternative solution to space-time coding is represented by
Spatial Multiplexing (SM) [6 ]. Spatial multiplexing is a
space-time modulation technique whose core idea is to send
independent data stream from each transmit antenna. This is
motivated by the spatially white property of the distribution which
achieves capacity in MIMO i. i. d. Rayleigh matrix channels [7 ].
SM is addressed to push up link capacity rather than to exploit
spatial diversity . Although various implementtation architectures
for MIMO systems have been introduced since the BLAST (Bell
Laboratories Layered Space-Time) system was proposed in [6 ] and
[8], a variation of such system, V-BLAST still emerges as a
promising architecture due to lower receiver complexity (V-BLAST
receiver algorithm) and higher data rates in the case of large
number of antennas.
978-1-4799-3824-7/14/$31.00 2014 IEEE
-
The paper is organized as follows: Model of WiMAX PRY layer is
explained in section 2. Explanation of the results obtained via
simulation is done in Section 3 . At the end conclusion is given in
section 4.
II. MODEL OF WIMAX PRY LAYER
Physical layer set up the connection between the communicating
devices and is responsible for transmitting the bit sequence. It
also defines the ty pe of modulation and demodulation as well as
transmission power. WiMAX physical layer is based on the orthogonal
frequency division multiplexing (OFDM). OFDM is a good choice of
high speed data transmission, multimedia communication and digital
video services. [t even can maintain very fast data rate in a non
line of sight condition and multipath environment. [n the following
subsection we provide a detailed description of the OFDM.
The role of the PRY -layer is to encode the binary digits that
represent MAC frames into signals and to transmit and receive these
signals across the communication media. The proposed block diagram
of WiMAX-MIMO-OFDM PRY system is given in Fig. I.
, , : m' - ' < , m' '
,-'
' .
--------- ---------
MIMO En coder STB C / SM
..... _ ... -- - -------- .......
, ... ' : z ' "" s: : :3: ga:
Figure 1. Block diagram of WiMAX-MIMO-OFDM PHY system
The random binary signal information is first generated and
grouped in symbols, then coded for error correction. Digital
modulation system is used because different modulation scheme is
needed for different data rates. Then multiple antennas are used,
the M[MO space-time diversity encoder is implemented. After that
guard band is inserted and the Inverse Fast Fourier Transform
(IFFT) block transforms the data sequence into time domain. Then a
cyclic prefix is used which is chosen larger than the expected
delay spread to avoid intersymbol and inter-carrier interferences
(lSI and ICI). The channel is considered to be a multipath fading
channel followed by addition of white Gaussian noise.
At the receiver, the FFT is used to transform the data back to
frequency domain. Adaptive filtering technique is used for channel
estimation. The FFT is taken in each of receives antenna. Each
antenna receives a different noisy superimposetion of the faded
versions of the transmitted signals. Lastly, the binary information
data is obtained back after the demodulation and channel decoding.
In following sections, we will take each block one by one in
detail.
A. Channel Encoding
The radio linle is a quickly vary ing linle, often suffering
from great interference. Channel coding, whose main tasks are to
prevent and to correct the transmission errors of wireless systems,
must have a very good performance in order to maintain high data
rates. The 802.16 channel coding chain is composed of three steps:
Randomizer, Forward Error Correction (FEC) and [nterleaving. They
are applied in this order at transmission.
1) Randomization: The Randomizer performs randomization of input
data on each burst on 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 1 5 stage shift
register with a generator polynomial of 1 + Xl4 + XiS with XOR
gates in feedback configuration as shown in Fig. 2.
D Data O u t
D _"t_"_l n ____ ---I -------
Figure 2. Randomization generator.
Forward Error Correction (FEC) codes: The bits issued from the
randomizer are then applied to the FEC encoder. FEC techniques ty
pically use error-correcting codes that can detect with high
probability the error location. These channel codes improve the bit
error rate performance by adding redundant bits in the transmitted
bit stream that are employed by the receiver to correct errors
introduced by the channel.
-
The FEC is achieved using Convolutional Codes (correct
independent bit errors) and Reed Solomon codes (correct burst
errors at byte level) to provide the additional coding gain which
measures the amount of additional SNR that would be required to
provide the same BER performance for an uncoded message signal in
the same channel conditions.
The Reed Solomon (RS) codes are mandatory codes on both sides i.
e. Uplink and Downlink. These are non-binary cyclic codes that add
redundancy to the data that improves the probability of block
errors.
The outer RS encoded block is fed to inner binary convolutional
encoder (see Fig. 3 ). The implemented encoder has native rate of
112, a constraint length of 7 (m = 7 ). The generator polynomials
used to derive its two output code bits, denoted X and Y , are
specified in the following expressions: G 1 = 1330cT for X and G2 =
17 10cT for Y .
X=1330CT
Figure 3. Convolutional encoder (rate = 112, m = 7).
Coding rate is defined as the ratio of the input bits to the
output bits. Higher rates like 2/3 and 3/4, are derived from it by
employing "puncturing. " Puncturing is a procedure that involves
omitting of some of the encoded bits in the transmitter thus
reducing the number of transmitted bits and hence increasing the
coding rate of the convolutional code and inserting a dumm y "zero"
metric into the convolution Viterbi decoder on the receive side of
WiMAX Physical layer in place of the omitted bits. For decoding the
Viterbi algorithm is used at the receiver side of the PHY layer. To
describe a convolution code, one need to characterize the encoding
function (m), so that given an input sequence m, one can readily
compute the output sequence U.
2) Interleaving: Interleaving is used to protect the
transmission against long sequences of consecutive errors, which
are very difficult to correct. These long sequences of error may
affect a lot of bits in a row and can then cause many transmitted
burst losses. Interleaving, by including some diversity , can
facilitate error correction. The encoded data bits are interleaved
by a block inter-leaver with a block size corresponding to the
number of coded bits per allocated sub-channels per OFDM symbol [1
]. The interleaver is made of two steps:
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 alternatively on to less or more significant bits of the
constellation, thus avoiding long runs of bits of low
reliability.
B. Digital Modulation
After channel coding, data bits are mapped and modulated onto
the allocated subcarriers. We passed the random values through the
adaptive modulation schemes according to the constellation mapped.
The data was modulated depending their size and on the basis of
different modulation schemes like BPSK, Gray-mapped BPSK, QPSK, 16
-QAM and 6 4-QAM modulation. Inverse process, called demodulation,
is done by the receiver to recover the transmitted digital
information.
C. MIMO Encoder
The proposed system consists of 2 transmit and 2 receive
antennae. Our MIMO codes use two ty pes of encoders, STBC and SM.
The MIMO STBC encoder system is implemented using Alamouti 2x2
STBC. In case of spatial multiplexing
(SM) technique, MIMO encoder includes V-BLAST technology that is
used to improve the spectral efficiency of the system with two
transmits and two receive antennas.
1) Space-Time Block Codes: Space-Time Block Codes (STBCs) are
the simplest ty pes of spatial temporal codes that exploit the
diversity offered in systems with several transmit antennas. The
transmit diversity technique proposed by Alamouti was the first
STBC [9]. The encoding and decoding operation is carried out in
sets of two modulated symbols. Therefore, let us denote by S] and
S2 the two modulated symbols that enter the space-time encoder. In
the Alamouti scheme, during the first time instance t], the symbols
S] and S2 are transmitted by the first and the second antenna
element, respectively. During the second time instance t2, the
negative of the conjugate of the second symbol, i. e. , -S2
' is sent to the first
antenna while the conjugate of the first constellation point. i.
e. , S/, is transmitted from the second antenna. The transmission
rate is equal to the transmission rate of a SISO system. The
space-time encoding mapping of Alamouti 2x2 can be represented by
the coding matrix:
-S; ] S' 1
(1)
The received signals at the time t and t + T can then be
expressed as:
r1 = r1(t) = hl1S1 + hZ1SZ + n1
rz = r1(t + T) = -hl1S; + hZ1S; + nz
r3 = rz(t) = h12S1 + hzzSz + n3
r4 = rz(t + T) = -h12S; + hzzS; + n4
(2)
where r], r3 are the received signals at time t and r], r4 are
the received signals at time t + T, n], n2, n3 and n4 are complex
random variables representing receiver noise and interference. This
can be written in matrix form as:
r = HS +n (3)
-
where H is the complex channel vector and n is the noise vector
at the receiver.
Figure 4. MIMO channel model (2x2).
The estimate of the transmitted symbol using Zero Forcing (ZF)
decoder is:
(4)
2) Spatial Multiplexing: In spatial multiplexing, a signal is
divided into different streams and each stream is transmitted from
a different transmit antenna in the same frequency channel. If
these signals arrive at the receiver antenna array with
sufficiently different spatial signatures, the receiver can
separate these streams, creating parallel channels for free.
Spatial multiplexing is very powerful technique for increasing
channel capacity at higher Signal to Noise Ratio (SNR). It can be
used with or without transmit channel knowledge. This technique
includes V-BLAST technology.
Consider that we have a transmission sequence, for example S],
S2,00., Sn. For 2 transmit antennas, we group the symbols into
groups of two. In the first time slot, send S] and
S2 from the first and second antenna. In second time slot, send
S3 and S4 from the first and second antenna. Notice that as we are
grouping two symbols and sending them in one time slot, we need
only n/2 time slots to complete the transmission, so data is
doubled. The V -BLAST transmission for 2 x 2 MIMO system can be
represented in matrix notation as follows:
(5)
Where, r], r2 are the received symbol on the first and second
antenna respectively, hii is the channel from /h transmit antenna
to /h receive antenna, S], S2 are the transmitted symbols that use
first and second constellation mapped respectively and n], n2 is
the noise on \ st, 2nd receive antennas.
The decoding is done using ZF technique which generates an
estimate of the transmitted matrix as:
(6)
D. OFDM System
OFDM technique is a bandwidth efficient multicarrier technique,
which splits the system bandwidth into orthogonal
sub channels, each of which occupies only a narrow bandwidth and
a separate sub carrier is assigned to each. By means of guard
interval and cyclic prefix, an OFDM system also achieves good
resistance against multipath fading.
1) Inverse Fast Fourier Transform (IFFT): This block implements
the OFDM modulation and is preceded by a serial to parallel
converter, and, at its output, another block converts the data back
to its serial format. The two converters are practically
incorporated by the IFFT transform, as it is implemented in Matlab.
Hence, An IFFT converts the input data stream from frequency domain
to time domain representing OFDM Subcarrier as the channel is
basically in time domain. IFFT is useful for OFDM system as it
generates samples of a waveform with frequency components satisfy
ing the orthogonality condition such that no interference occurs in
the subcarriers. The mathematical model of OFDM symbol defined by
IFFT which would be transmitted during our simulation as given
bellow:
N-l 1 "'\' j2rrnk xci, n) = IFFTN[X(i, k)] = N L XCi, k)
e-N-
k=O
(7)
where X(i,k) is the transmitted data symbol at the kth
subcarrier of the ith OFDM symbol, N is the FFT size.
Similarly FFT converts the time domain to frequency domain as
basically we have to work in frequency domain [\ 0]. By calculating
the outputs simultaneously and taking advantage of the cyclic
properties of the multipliers FFT techniques reduce the number of
computations to the order of
N log(N). The FFT is most efficient when N is a power of
two.
2) Cyclic Prefix Insertion: After performing Inverse Fast
Fourier Transform (IFFT) the cyclic prefix (CP) will be add with
each OFDM symbol. The CP consists in a copy of the last samples
composing the OFDM symbol added in front of it
(see Fig. 5). This function is built according to IEEE 802.\ 6
specifications, which define 4 possible values for the ratio
between the duration of the cyclic prefix and the duration of the
useful OFDM symbol. This ratio can be equal to 1/4, 1/8,
\/16 and 1/32.
Figure 5. Cyclic Prefix insertion.
E. Communication Channel
Communication channels are kind of medium of communication
between transmitter and receiver. The channel adds a white noise n
of a certain variance to a flat faded variant of the useful signal
S:
r = Ray.S + n (8)
-
where Ray represents a Rayleigh random variable and n is the
Additive White Gaussian Noise (A WGN). We selected the Rayleigh
model for the channel to simulate a Non Line Of Sight (NLOS)
communication.
III. SIMULATION RESULTS
The performance of MIMO-OFDM PHY layer of WiMAX system for
different MIMO configurations (STBC, V-BLAST) is analyzed for
different values of FFT size, Bandwidth, cyclic prefix factor and
modulation techniques. A comparative performance analysis with FEC
encoder based WiMAXMIMO-OFDM PHY layer has also been approved.
The simulated parameters used in the present study are shown in
Table I. The WiMAX simulator presented in this paper allows a
better understanding of the processes involved at the PHY layer
level. Furthermore, quantitative results may be provided by
computing of WiMAX Downlink throughput in fading Rayleigh
channel.
TABLE I. SIMULA nON PARAMETERS
Parameter Value
Carrier Frequency 3.5 GHz
Channel Model Non-LOS
MIMO 2x2 STBC, 2x2 SM
Modulation BPSK, QPSK, 16QAM, 64QAM
OFDM subcarriers (IFFT/FFT) 128,256,512,1024
Channel Bandwidth 1.25, 2.5, 5, 10 MHz
Frame Duration 5 ms
Number of Frames (per sec) 200
Guard Interval 1/4,1/8,1/16,1/32
EncoderlDecoder CCNiterbi
Fading Channel Rayleigh Fading Channel
Noise AWGN
A. MIMO Techniques
In this section, we demonstrate the enhanced performance of the
proposed detection scheme through simulation results.
We consider a MIMO-OFDM system with two modes, 2x2 STBC
(Alamouti) and 2x2 SM (V-BLAST) compared to a traditional
communications SISO. Moreover, the number of subcarrier equals 512,
convolutional encoder with constraint length of 7 and code rate 112
has been used with 16 QAM modulation and CP factor of 114. We
assume that the channel is a frequency flat fading during two OFDM
symbol periods. Moreover, we suppose that Channel State Information
(CSI) is known to receiver perfectly.
Fig. 6 shows the comparison of WiMAX Downlink throughputs of
various modes. It is seen the interest of spatial diversity.
Throughput gains are highly significant for 2x2 V-BLAST. Thus, at
SNR of 7 dB, 2x2 MIMO STBC system
improves WiMAX throughput by 6 . 5 Mbps compared to SISO system.
In contrast, 2x2 V-BLAST mode is poorer than the SISO scheme.
However, for a SNR up to 13 dB, 2x2 V-BLAST system performs better
then SISO and 2x2 STBC systems, it can be observed that V-BLAST
system has a throughput factor of 2 at SNR of3 5 dB compared to the
others systems.
We also note that when SNR great than 20 dB, both SISO and STBC
systems remain almost same throughput. So STBC system produces the
best performance at low and medium values of SNR (from 1 to 13 dB),
due to their robustness in poor channel conditions. On the other
hand, at high SNR (up to 13 dB) the increased error-free data rate
makes V-BLAST the best choice.
18--------------------------
SISO 16 - 2x2 MIMOSlBC
- 2x2 MIMO-V-BLAST 14
12 "' "-;[ 10-
4
2-
O('l __ ("1------{'>_ ... 1_;} __ -4-;0"--= __
=__--------____:_ -5 . . 0 . 5 10 15 20 25 30 35
SNR (dB)
Figure 6. Performance comparison for 2x2 STBC, 2x2 V-BLAST and
SISO systems.
B. Effect a/FEe Encoder
In this section of our research work, we represent various
throughput vs. SNR plots to evaluate the effect of FEC encoding
technique. The tow WiMAX MIMO systems (STBC and V-BLAST)
performances are evaluated by convolutional encoding with three
code rate cases (1/2, 3/4 and 1= without FEC), the FFT size is
fixed to 512 with CP factor of 1/4 and 16 QAM modulation is
employed.
On comparison, From Fig 7 , it is observable that the two
systems throughput shows comparatively much better performance
without FEC encoding at high SNR compared to coded data. On the
other hand, the systems performance with FEC is satisfactory at low
SNR, although the use of FEC causes redundant bits in the
transmitted bit stream. When FEC is applied error reduces to a
considerable label.
For a typical SNR values respectively of 10 dB and 1 5 dB, the
STBC and V-BLAST systems performances are improved by 4.6 5 Mbps
and 4.7 6 Mbps for the case of 112 rated FEC encoded and 6 .27 Mbps
and 13 Mbps for FEC = 3/4.
We can also see the benefit of using a Viterbi decoding to
correct errors introduced by the channel at low SNR. The
-
counterpart is a loss throughput (a factor of code rate) and an
additional time due to the interleaver.
35
'2x2 STBC, 1/2
2x2 STBC, 3/4 30 -
__ 2x2 STBC, 1
2x2 V-BLAST, 1/2
25 - - - '2x2 V-BLAST, 3/4
2x2 V-BLAST, 1
10
5 -
Ot c.' ':;' I '::J ( 5 -5 0
'c
15 20
SNR (dB)
, _ __ L-- -, r
,
25 30
Figure 7. Effect of FEC rate on throughput for 2x2 STBC and 2x2
V-BLAST systems.
C. Effect a/Constellation Size
35
To verify the STBC and V-BLAST systems throughput performances,
an OFDM system with 512 subcarriers is simulated with 3/4 FEC
convolutional code rate and CP factor of 1/4. Four different
digital modulation schemes, namely BPSK, QPSK, 16 QAM and 6 4QAM,
are used in our simulation system, based on the observed channel
conditions.
In modulation, bits are transmitted in symbols form, not as they
are. The number of bits included in each symbol denotes the
constellation size. Therefore, more this size will be large;
throughput will be high and vice versa.
Fig. 8 shows the effect of constellation size on the throughput
as function of the SNR for 2x2 STBC and 2x2 V-BLAST systems. It is
evident that the throughput increase when constellation size
increase.
40
'STBC, BPSK
35 _ ' STBC, QPSK
,STBC, 16QAM
- J_ 'STBC, 64QAM 30 -
_ _ V-BLAST, BPSK
25 .c
a. 20 .c OJ :J E'
.c 15-f-
10
5
, V-BLAST, QPSK
V-BLAST, 16QAM
V-BLAST, 64QAM
SNR (dB)
Figure 8. Effect of constellation size on throughput for 2x2
STBC and 2x2 V-BLAST systems.
Hence, the higher throughput is obtained by 6 4QAM by transmit
more bits per symbol and the lower throughput when using BPSK.
Since increased constellation size implies shorter distance between
neighboring symbols, the received data is more susceptible to
errors at higher rates when the channel is weak. In this way there
is a balance between obtaining the higher throughput and
maintaining an acceptable bit error rate for any radio
communications system.
As the SNR decrease, so it required switching from higher
modulation level to lower modulation level. The transmitter will
choose the appropriate modulation scheme depend upon the SNR
threshold value.
D. Effect 0/ FFT Size
This parameter specifies the FFT size used in our simulation.
Four FFT sizes are supported here: 128, 256 , 512 and 1024. The FFT
size determines the number of available subcarriers and OFOM symbol
duration. In general, for a given bandwidth, a larger FFT size
results in a greater number of available subcarriers and a longer
OFOM symbol duration.
Fig. 9 shows the throughput comparison of the two modes STBC and
V-BLAST for 128, 256 , 512 and 1024 FFT size with 1/2 FEC CC rate,
16 QAM modulation and 114 CP factor. It is clear from below figure
that 2x2 STBC system throughput remains constant for any FFT
size.
ST6C,128
-- - ST6C, 256
- - ST6C,512
. - ST6C, 1024
V-BLAST, 128
- V-BLAST, 256
15 20
SNR (dB)
25 30
Figure 9. Effect of FFT size on throughput for 2x2 STBC and 2x2
V-BLAST systems.
35
The 2x2 V-BLAST system throughput is gradually enhanced by
increasing the FFT size; this rate remains insufficient given the
high complexity of the system due to this increase. An exception
for the passage of a FFT size from 256 to 512 subcarriers, the gain
in throughput is significant. Thus, for an SNR of 1 5dB, we gain
about 5.7 Mbps.
E. Effect a/Cyclic Prefix Factor
This parameter specifies the ratio of useful symbol time to
cyclic prefix time. Four ratios are supported here: 1/4, 1/8, 1/16
and 1/32. We have compared the effect on throughput of
-
2x2 STBC and 2x2 V-BLAST systems. Let's check effect on the
throughput.
From Fig. 10 , it can be seen that throughput increases
appreciably by decries the values of cyclic prefix factor. At the
lower SNR, fading is more and signal strength is going low as the
distance increases.
To defeat this problem higher value of CP need to be chosen.
Large value of the CP means large time gap between two frames which
mean extra time to receive signal from mu1tipath signals. Although
the large values of the CP reduces throughput however, it increases
coverage up to large distance. Thus chosen the appropriate value of
the CP gives the desired distance that need to cover by the signal
[11 ].
Hence, the throughput is reduced by a factor of 4/5, 8/9, 16 /17
and 32/33 depending on the cyclic prefix configuration (1/4, 1/8,
1116 and 1/32) to extract the real useful bits. It was found that
more the cyclic prefix duration is higher; more the resistance to
OFDM Inter-Carriers Interference is effective. However, throughput
is then lower.
35=======----------------------
"' Q.
30
25 -
20 "
g 15 -e .c f-
10
5 -
STBC, 1/ 16
STBC, 1/32
VBLAST, 1/4
--"- VBLAST, 1/8
- VBLAST, 1/ 16
VBLAST, 1/32
o 0 o 25 30
Figure 10. Eflect of CP factor on throughput for 2x2 STBC and
2x2 V-BLAST systems.
F. Effect of System Bandwidth
35
WiMAX has a scalable physical-layer architecture that allows for
the throughput to scale easily with available channel bandwidth.
This scalability is supported in the OFDMA (Orthogonal Frequency
Division Multiple Access) mode, where the FFT size may be scaled
based on the available channel bandwidth.
In our simulation, the WiMAX system may use 128, 256 , 512 or
1024 FFTs corresponding to the transmission channel bandwidths of
l.25MHz, 2. 5MHz, 5MHz or lOMHz, respectively. This scaling may be
done dynamically to support user roaming across different networks
that may have different bandwidth allocations [12].
Fig. 11 shows the effect of variation of bandwidth on throughput
of 2x2 STBC and 2x2 V-BLAST systems, 3/4 rate binary convolutional
code is used with 16 QAM modulation
and CP factor of 1/4. We can see that the throughput of the two
systems is affected by change in bandwidth. It achieves maximum
values for 1024 FFT size and 10 MHz bandwidth of both systems at
SNR of25 dB; these values are 13 .3 Mbps and 3 4. 5 Mbps
respectively for 2x2 STBC and 2x2 V-BLAST.
Hence, there is a fixed relationship between the occupied
bandwidth and the OFDM symbol sample rate. The implementation of a
bandwidth-scalable air interface makes the subcarrier separation
and symbol duration remain invariant as the deployment bandwidth
changes.
35.---------------------= 8TBC, 128/1.25MHz
8TBC, 256/2.5MHz
30 8TBC, 512/5MHz
8TBC, 1024/10MHz
VBLAST, 128/1.25MHz
25 -c-- VBLAST, 256/2.5MHz
VBLAST, 512/5MHz
1 -- VBLAST,1024/10MHz 20 " a.
.c '" 15 f-
10
O. :::l r: - I -r:' 5 0
,
10
SNR (dB)
,
15
- _-_-=D=-
,
20
Figure 11. Effect of system bandwidth on throughput for 2x2 STBC
and 2x2 V-BLAST systems.
25
The ability to scale system bandwidth while maintaining constant
symbol duration provides greater commonality in equipment
components and offers the operator the advantage of being able to
deploy today and grow their future system bandwidth at lower cost
and reduced network impact.
IV. CONCLUSION
In this paper, we proposed a downlink physical layer simulator
for WiMAX-MIMO-OFDM in flat Rayleigh fading channel. The WiMAX
simulator allows a better understanding of the signal processing
steps taking place at the PHY layer corresponding to the IEEE
802.16 specifications.
A comparison for the Throughput performance at the downlink of
2x2 STBC and 2x2 V-BLAST systems are done using simulation. It is
shown that the V-BLAST system has a better performance than STBC
and SISO at high SNR range.
On the other hand, at low to medium values of SNR, STBC produces
the best performance, due to its robustness in poor channel
conditions and increase the diversity.
The simulation results also indicate the effects of each bloc of
MIMO-OFDM physical layer in WiMAX system, namely Forward Error
Correction (FEC) rate, modulation constellation size, OFDM IFF
TIFFT size, Cyclic Prefix (CP) factor and finally system
bandwidth.
-
ACKNOWLEDGMENT
The research reported in this paper was developed in the
framework of research activities of ICS (Information and
Communication Systems) team in Systems and Technologies of
Information and Communication (STlC) laboratory at Tlemcen
University, Algeria.
REFERENCES
[I] IEEE Standard for Local and Metropolitan area networks Part
16, The Institute of Electrical and Electronics Engineering, Inc.
Std. IEEE 802. 16d-2004.
[2] IEEE Standard for Local and Metropolitan area networks Part
16, The Institute of Electrical and Electronics Engineering, Inc.
Std. IEEE 802. I 6e-2005.
[3] A1. Paulraj, and C.B. Papadias, "Space-Time Processing for
Wireless Communications'", IEEE Sig.Process. Mag., pp. 49-83, Nov.
1997.
[4] D. Gesbert, M. Shafi, et. aI., "From Theory to Practice: An
Overview of MIMO Space-Time Coded Wireless Systems'", IEEE 1. Sel.
Areas in Comm., vol. 21, no. 3, pp. 281-301, Apr. 2003.
[5] S.M. Alamouti, "A simple transmit diversity tecnique for
wireless communications", IEEE J. Sel. Areas in Comm., vol.16, no.
8, pp. 1451-1458, Oct. 1998.
[6] G.J. Foschini, "Layered Space-Time Architecture for Wireless
Communication in a Fading Environment when using Multi-Element
Antennas'", Bell Labs Tech. Jour., vol. I, no. 2, pp.
41-59,1996.
[7] R. V. Heath, and AJ. Paulraj, "Switching between Diversity
and Multiplexing in MIMO Systems'", IEEE Trans. on Comm., vol.53,
no.6, pp. 962-968, June 2005.
[8] P. W. Wolniansky et aI, "V -BLAST: An architecture for
realizing very high data rates over the rich-scattering wireless
channel", Proc. ISSSE Conference, Pisa, Italy, September 1998.
[9] A Slaney and Y. Sun "Space-time coding for wireless
communications: an overview", lEE Proc.-Commun., vol. 153, no. 4,
August 2006.
[10] M. Wang, "Wi MAX Physical Layer: Specifications Overview
and Performance Evaluation"', 2nd IEEE CCNC Research Student
Workshop, pp 10-12,2011.
[II] L. Yang, S. Cheng, H. Wang, "Effects of cyclic prefix on
OFDM systems over time-varying channels.... in IEEE 16th
International Symposium on Personal Indoor and Mobile Radio
Communications, vol. 2,no. 11-14, pp. 750-753, Sept. 2005.
[12] J. G. Andrews, A Ghosh, and R. Muhamed. "Fundamentals of
WiMAX Understanding Broadband Wireless Networking," Pearson
Education, Inc., 2007.