IRJET- BER Reduction of Distributed Spatial Modulation in Cooperative Relay Network based Cellular System

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 06 Issue: 09 | Sep 2019 www.irjet.net p-ISSN: 2395-0072
© 2019, IRJET | Impact Factor value: 7.34 | ISO 9001:2008 Certified Journal | Page 1194
BER Reduction of Distributed Spatial Modulation in Cooperative Relay
Network based Cellular System
Manisha Chouksey1, Gurpreet Singh2
1Research Scholar, Trinity Institute of Technology & Research, Bhopal
2Professor, Dept. of Electronics & Communication Engineering Trinity Institute of Technology & Research, Bhopal
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Abstract - Spatial modulation (SM) is a recently developed
multiple-input-multiple-output (MIMO) technique that uses
multiple transmit antennas in a innovative fashion. In SM, a
group of information bits is mapped into two constellations: a
signal constellation based on modulation scheme, and a
spatial constellation to encode the index of a transmit
antenna. To overcome these problems, distributed spatial
modulation (DSM) offers a promising solution. In DSM,
multiple relays form a virtual antenna array and assist a
source to transmit its information to a destinationbyapplying
SM in a distributed manner. The source broadcasts its signal,
which is independently demodulated by all the relays. Each of
the relays then divides the received data in two parts: the first
part is used to decide which one of the relays will be active,
and the other part decides what data it will transmit to the
destination. An analytical expression for symbol error
probability is derived for DSM in independent and identically
distributed (i.i.d.) Rayleigh fading channels.
Key Words: MIMO-OFDM System, Cognitive Relay
Network, Distributed Spatial Modulation
1. INTRODUCTION
The hugely popular multiple-input and multiple-output
(MIMO) technology exploits multiple antennas to achieve
different gains such as multiplexing, diversity and/or
beamforming [1]. However, these gains are often
accompanied by significant increase in computational
complexity and cost of a receiver. This is primarily due to
inter-channel interference (ICI), inter-antenna
synchronization (IAS) and the need of multiple radio
frequency (RF) chains. One approach to overcome these
issues is to use Spatial Modulation (SM) [2], [3]. In SM, a
block of any number of information bits is mapped into a
constellation point in the signal domain and a constellation
point in the spatial domain. At each time instant, only one
transmit antenna of the set will beactive.Theotherantennas
will transmit zero power. Therefore, ICI at the receiver and
the need to synchronize the transmit antennas are
completely avoided. Moreover, unlike the conventional
MIMO system, SM system does not require multiple RF
chains at the transmitter. The index of active transmit
antenna is used as an additional source of information to
boost the overall spectral efficiency. At the receiver, a low
complexity decoder such as Maximum Receive Ratio
Combining (MRRC) is used to estimate the index of active
transmit antenna, after which the transmitted symbol is
estimated. Using these two estimates, a spatial demodulator
retrieves the group of information bits. Recent studies [4],
[5] have shown that SM can outperform other state-of-the-
art MIMO schemes in terms of computational complexity if
many antennas are available at the transmitter.
Unfortunately, this makes SM useful only in the downlink of
the cellular network, where a base station can be equipped
with a large number of antennas. On the other hand, use of
multiple antennas in mobile terminals has practical
limitations. Besides constraints on complexity and cost,
decreasing terminal size restrictstheapplicationofSMin the
uplink of cellular network. To overcome these problems,
distributed spatial modulation (DSM) presents a promising
solution, where a set of neighboring mobile terminals assist
the source in conveying its information to the destination
[6]. The key idea behind DSM is that multiple cooperative
relays share their antennas to form a virtual antenna array
(VAA) and apply SM principle in a distributed manner. The
DSM system overcomes the limitations of heavy shadowing
effects while expanding network coverage. Moreover, only
one relay is selected to forward the source’s information to
the destination, thus saving overall transmit power of the
system.
2. MODULATION
Spatial modulation
Spatial modulation (SM) is a recently developed
transmission technique, which uses multiple transmit
antennas [6], [7]. In SM, an index of transmit antenna is used
asan additional source of information to improve the overall
spectral efficiency. A group ofanynumberofinformationbits
is mapped into two constellations: a signal constellation
based on modulation scheme and a spatial constellation to
encodethe index of the transmit antenna.Atanytimeinstant,
only one transmits antenna isactive; whereas othertransmit
antennas radiate zero power. This completely avoids inter
channel interference at the receiver and relaxesthestringent
requirement of synchronization among the transmit
antennas. In addition, unlike conventional MIMO system, SM
system does not require multiple RF chains at the
transmitter. At thereceiver,alowcomplexitydecodersuchas
maximum receive ratio combining (MRRC) is used to
estimatethe indexofactivetransmitantenna,andafterwhich

transmitted symbol isestimated.Usingthesetwoestimates,a
spatial demodulator retrieves the group of information bits.
Distributed Spatial Modulation
Spatial modulation (SM) is a new transmission technique for
low-complexity implementation ofMIMO systems. In SM, the
index of the active transmit antenna is employed as an
additional means of conveying information. Since, only one
transmit antenna is active at any time instant, inter-channel
interference (ICI) and inter-antenna synchronization (IAS)
are efficiently avoided. Recent studies [4]-[5] have shown
that SM can outperformotherstate-of-the-artMIMOschemes
in terms of computational complexity if many antennas are
available at the transmitter. Unfortunately, this makes SM
useful only in the downlink of the cellular network, where a
base station can be equipped with a large number of
antennas. On the other hand, use of multiple antennas in
mobile terminals has practical limitations.
3. COGNITIVE RADIO
With proliferation in smartphone devices and a growing
interest in multimedia applications, there is unprecedented
growth in mobile data traffic resulting in problem of
spectrum scarcity. Most suitable frequency bands (below
3GHz) have already been assigned under the licensed bands
for the existing wireless systems, which increases the
complexity and difficulty to find available frequency bands
for new wireless systems. Furthermore, current spectrum
allocation policy is inflexible to allow sharing of the licensed
bands, although several field measurementsperformedfrom
leading spectrum regulatory commissions, such as the
Federal Communications Commission (FCC) in the United
States [8] verified that there are licensed bands unoccupied
most of time or partially occupied.
Fig -1: Concept of opportunistic spectrum sharing:
secondary utilization of the identified spectrum holes.
Therefore, there are spectrum holes or white bands in
certain time, frequency, and positions as shown in Fig. 1,
which result in reducing the spectrum efficiency. These
challenging problems of spectrum inefficiency motivated
researchers to find efficient solutions which result in the
concept of cognitive radio. It is a new communication
paradigm that can effectively exploit the existence of
spectrum holes by enabling unlicensed users to intelligently
utilize these spectrum holes without causing harmful
interference tothelicensedusers(primaryusers).Originally,
the concept of cognitive radio is to sense the spectrum in
order to find the unoccupied bands and then utilize them
wisely at a certain time [9]. This concept has recently been
developed to be fully aware of the surrounding environment
and the primary users, and thereby further improve the
efficiency of spectrum utilization.
4. PROPOSED METHODOLOGY
Consider Nt × Nr DSM-MIMO system for point-to-point
communications. Nt is the transmitter antenna and Nr is the
receiver antenna. The basic concept of SM is to map the
information bits into two constellations: the spatial
constellation and the signal constellation. The bit stream is
divided into blocks of ns bits, where ns are the number of
bits per symbol. Each block is then split into two parts of
log2Ntand log2 M bits, where M denotes the size of the signal
constellation diagram. The first part activates one of the Nt
transmit antennas, and we denote the currently active
antenna as l. The second part selects one symbol from the
signal constellation diagram, and theactiveantenna conveys
this symbol, which is denoted by s.
The whole transmission occurs in two phases and lasts for
two time slots 1) broadcasting phase (first time slot) in
which the source broadcasts its information to all therelays.
The source applies incoming binarydata toOFDMmodulator
and broadcasts the resulting OFDM signal over multipath
Rayleigh fading channels between the sources and relays.
Each of the relays independently demodulates the received
OFDM signal using OFDM demodulator 2) relaying phase
(second time slot) in which relays use DSM-OFDM protocol
(described in the next section) to forward the sources
information to the destination. The source remains silent
during this phase to avoid collisions with data transmitted
from relays to the destination. Using DSM-OFDM protocol,
each relay encodes demodulated binary data such that in
each OFDM subcarrier, only one relay transmits the data,
while other relays radiate zero power. The number of
subcarriers required to transmit the source’s information
depends on the number of available relays and modulation
schemes used both at the source and relays. It is possible
that all the subcarriers are not required to forward the
source’s information. Therefore, the remaining subcarriers
can be utilized by the relays to transmit their own
information.
In DSM, the number of activated relays during the relaying
phase depends on the demodulation outcome at the relays.
In other words, the larger the number of activated relay is,
the higher the energy consumption is. This is in stark
contrast comparedtoconventional relayingprotocols,where
the set of activated relays is fixed and the net effect of an
incorrect demodulation at the relays results only in
forwarding erroneous data to the destination.InDSM,on the

other hand, the activation of multiple relays may lead to an
increase of the energy consumption. The objective of this
section is to show that the average (i.e., with respect to all
possible activation patterns of the relays) energy
consumption of DSM in the presence of demodulationerrors
at the relays is the same as the energy consumption of DSM
in the absence of demodulation errors at the relays, i.e., ES +
ER.
Fig -2: Block Diagram of Proposed Model
This result is important for guaranteeing a fair comparison
with conventional relaying protocols and for ensuring that
the power consumption does not increase in the presence of
unreliable source-to-relay links.
OFDM is a special form of multicarrier modulation (MCM)
with densely spaced subcarriers with overlapping spectra,
thus allowing multiple-access. MCM works on the principle
of transmitting data by dividing the stream into several bit
streams, each of which has a much lower bit rate, and by
using these sub-streams to modulate several carriers. This
technique is being investigated as the next generation
transmission scheme for mobile wireless communications
networks.
OFDM structure basically relies on three principles:
 The IFFT and FFT are used for modulating and
demodulating individual OFDM sub carriers to
transform the signal spectrum to the time domain
for transmission over the channel and then by
employing FFT on the receiving end to recover data
symbols in serial order.
 The second key principle is the cyclic prefix (CP) as
Guard Interval (GI). CP keeps the transmittedsignal
periodic. One of the reasons to apply CP is to avoid
inter-carrier interference (ICI).
Interleaving is the thirdmost importantconceptapplied. The
radio channel may affect the data symbols transmitted on
one or several sub carriers which lead to bit errors. To
encounter this issue we use efficient coding schemes [8].
5. SIMULATION RESULTS
In this section, the bit errorratio(BER)performanceofDSM-
MIMO-OFDM scheme is evaluated using Monte Carlo
simulations. It is assumed that channel state information
(CSI) is available at the destination and all the relays. For a
fair comparison, the total transmission power in each
subcarrier is set to 1W for all the schemes and it is equally
divided between the broadcasting and relaying phase.
Perfect frequency and time synchronization is assumed.
Fig -3: Total Error Rate v/s Threshold for Different System
Simulation experiments are conducted to evaluate the total
error rate VS threshold performance of the proposed
cooperative distributed spatial modulation is shown in
figure 3.
Fig -4: BER v/s SNR for Different System

Simulation experiments are conducted to evaluate the SNR
VS Bit Error Rate (BER) performance of the proposed
cooperative distributed spatial modulation is shown in
figure 4.
Fig -5: Comparative Result
Simulation experiments are conducted to evaluate the SNR
VS BER performanceoftheproposedcooperativedistributed
spatial modulation and spatial modulationisshown infigure
5.
Table -1: Comparative Result of BER
BER SNR (dB)
0 5 10 15 20
Previous
Algorithm
10-1 8×10-3 8×10-
4
10-4 3×10-5
Proposed
Algorithm
8×10-
2
5×10-3 4×10-
4
7×10-
5
2×10-5
Table 1 the tabular illustration of the performance of
different SNR discussed in this research work in term of Bit
Error Rate (BER). From the analysis of the results, it isfound
that the proposed cooperative distributed spatial
modulation gives a superior performance as compared with
the previous method.
6. CONCLUSIONS
The implementation of DSM is extended to cognitive
network scenario. A dynamic frequency allocationscheme is
proposed for the source and relaysseparatelytoallocateidle
licensed frequencies. The frequencyallocationforthesource
is formulated as unconstrained optimization problem with
an objective to maximize the probability of correct
estimation of transmitted symbols at all the relays. The
frequency allocation for the relays is modeled through a
weighted bipartite graph and the optimal frequency
allocation for the relays is formulated as minimum weight
perfect matching problem. Simulation results are provided
to verify the improvement in the SER performance of DSM
using the proposed frequency allocation schemes.
The proposed MIMO-OFDM system using cooperative
distributed spatial modulation give a lower BER 0.00002 dB
as compared with 0.00003 dB for SNR = 20 dB, previous
MIMO-OFDM system using cooperative distributed spatial
modulation. The proposed method is 33.33% BER
improvement for previous method.
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