performance analysis of asynchronous distributed space time block coded system for coop comm.pptx
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Performance Evaluation of Asynchronous Distributed Space Time Block Coded
System for Cooperative Communication
Varsha Vimal
Regd. No. 901306001
With the rapid growth of multimedia services, future generations of
wireless communications require
• higher data rates and
• a more reliable transmission link
• satisfactory quality of service.
Multiple-input multiple-output (MIMO) antenna systems have been
considered as an efficient approach to address these demands by
offering significant multiplexing and diversity gains over single antenna
systems without increasing requirements on radio resources such as
bandwidth and power
• The size of the mobile devices and the requirement on the distance
between antennas (to make the channel-fading between antenna pairs
uncorrelated) may limit the multiple antennas that can be deployed.
• Also the propagation environment may not support MIMO because,
for example, there is not enough scattering.
• In the later case, even if the user has multiple antennas installed,
MIMO is not achieved because the paths between several antenna
elements are highly correlated.
• Also in cases when large –scale shadow fading contaminates the
wireless links, all the channels tend to fade together, rather than
independently, hence eroding the achievable diversity gain.
• To overcome these drawbacks, the concept of cooperative
communications has recently been proposed and gained large interest
in the research community.
WIRELESS COOPERATION
• The key idea in user-cooperation is that of resource-sharing among multiple nodes in a network.
• Due to the broadcast nature of wireless medium, as the data is transmitted to its destination in multiple hops, many nodes in the vicinity can hear these transmissions.
• In a cooperative network, two or more nodes share their information and transmit jointly as a virtual antenna array.
• Willingness to share power and computation with neighboring
nodes can lead to savings of overall network resources.
• This enables them to obtain higher data rates and diversity
than they could have individually.
COOPERATIVE DIVERSITY
• A three-terminal network is a fundamental unit in user cooperation.
Diversity obtained through multi-hop transmissions is referred to as
the cooperative diversity.
• cooperative diversity encompasses wireless nodes relaying the
signals of other nodes on to their respective destinations
• In cooperative communications, the system depends on the relay
channel to generate the independent paths between the source and
destination. The relay channel can be thought of as an auxiliary
channel to the direct channel between the source and destination.
• The relay channel is a three-terminal network, in which Terminal 1
(source) aims to transmit to Terminal 3 (destination) with the help of
Terminal 2 (relay)
Relay channel, two hop channel and direct link
Cooperative Communication Protocols
The ways in which the relays process the received signal and forwards
them to the destination are known as cooperative communication
protocols.
Amplify and Forward
• Each user overhears the noisy version of signal transmitted by its
partner and then retransmits after amplification to equalize the effect
of the channel fade between the source and the relay.
• The relay does that by simply scaling the received signal by a factor
that is inversely proportional to the received power.
• It is also known as scale and forward relaying.
Decode and Forward
• In this kind of cooperative communication the partner tries to decode
the source transmission The relay then retransmits the decoded signal
after possibly compressing or adding redundancy.
• The decode-and-forward protocol is close to optimal when the source-
relay channel is excellent, which practically happens when the source
and relay are physically near to each other.
Compress and Forward
• The key idea of Compress and Forward is that the relay quantizes and
compresses the received signal using source coding and transmits the
compressed version to the destination. Then, the destination combines
the received message from the source and its quantized/compressed
version from the relay.
• In Estimate and Forward mode, the relay forwards an analog estimate
of its received signals.
Coded Cooperation
• Coded cooperation is a method that integrates cooperation into
channel coding. Here different portions of each user’s (partners) code
word are sent via independent fading path similar to the other
cooperative schemes. Then each user tries to transmit incremental
redundancy (for eg. The parity bits) to its partner. For unsuccessful
decoding of partners second code partition, user transmits its own
second partition.
Selective Relaying
• Is a scheme where relays are selected to retransmit the source message
only if the relay path satisfies certain threshold criteria.
Incremental Relaying
• In fixed protocols category, although if the destination correctly detect
the transmitted symbols in phase 1, the channel resources divided
between the source and relay node which waste the bandwidth that
reduce the overall data rate in the system. This issue can be overcome, if
there is feedback channel from the destination to the relay nodes. This
called incremental relaying
COOPERATIVE COMMUNICATION
System structure for two-phase cooperative Communications
• A conventional cooperative communication system consists of a
source node S, n relay nodes, N1,...,NR, and a destination node D.
Each node deploys only one antenna. A two-phase communication
protocol is adopted. In the first phase, the information data is
broadcasted from the source node S to the relay nodes, N1,...,NR. In
the second phase, all the relay nodes forward the received
information data to the destination node D in a cooperative manner.
Asynchronicity in Cooperative diversity
• Synchronization means that all relays are assumed to have the
identical timing, carrier frequency and propagation delay.
• Perfect synchronization is almost impossible to be achieved because
that the relay nodes will be in random locations and their
transmissions will be affected by random different conditions.
• However, because relays are at different locations (i.e., different
propagation delays) and they have their own local oscillators with no
common timing reference; it is an asynchronous technique in nature.
• The lack of synchronization may result in inter symbol interference
(ISI) and dispersive channels .Also the lack of a common timing
reference can affect the structure of the code matrix and result in a
rank deficient space-time code.
Techniques to mitigate the effects of asynchronicity
There are three classes of techniques to mitigate the
effects of
asynchronicity:
• Equalizers
• Frequency domain
• Time domain approaches
Time Domain Approaches
Time reversal space time block codes(TR-STBC)
Every symbol of an STBC codeword is replaced by a
block of B symbols whereas the conjugate operation
refines the corresponding block to be transmitted in a
time reversed order such that the embedded
orthogonality can be explored.
Distributed Threaded algebraic space time
code (TAST)
These codes choose a suitable algebraic number
for each layer so that different layers are laid in
different algebraic space.
Linear asynchronous distributed space-
time block codes (DSTBC)
These codes employ a linear dispersive structure
using sufficient guard intervals for combating the
imperfect synchronization.
Linear Asynchronous Distributed Space-Time Block Codes
• The term Distributed-space time block codes (D-STBC)system is
referred to a wireless communication system which implements the
space time block codes over cooperative network. Such distributed
space–time codes can be based on a linear dispersion code(LD).
Linear Dispersion Codes
• The relay transmits a linear combination of the T1 symbols in s and
their complex conjugate (i)
where is the column vector containing the complex conjugates of s
and the complex T2 × T1 matrices and are called dispersion
matrices. These matrices define the space-time code. The
individual symbols , T1 are each drawn from a complex
constellation of M symbols. The family of space-time block codes
that can be represented by (i) are called linear dispersion (LD)
codes .
• This means that the relays are not required to decode. Only simple
signal processing is done at the relays. This has two main benefits.
First, the operations at the relays are considerably simplified, and
second, we can avoid imposing bottlenecks on the rate by requiring
some relays to decode.
• For communications in wireless relay networks, a two-step protocol is
used, where in the first step, the transmitter sends information and in
the other, the relays encode their received signals into a “distributed”
LD code, and then transmit the coded signals to the receive node.
Literature Survey
• Nosratinia et al.[2003] describe wireless cooperative communication, a
technique that allows single-antenna mobiles to share their antennas and
thus enjoy some of the benefits of multiple-antenna systems.
• J. N. Laneman et al.[2003]In this paper several strategies employed by
the cooperating radios are outlined, including fixed relaying schemes
such as amplify-and-forward and decode-and-forward, selection relaying
schemes that adapt based upon channel measurements between the
cooperating terminals, and incremental relaying schemes that adapt
based upon limited feedback from the destination terminal.
Sendonaris et al.[2002] The first part presents an analytical study
that demonstrates that how an increase in capacity can be traded for
an increase in cell coverage. The second part of the paper investigates
the cooperation concept further and considers practical issues related
to its implementation. The authors also illustrate the benefits of
cooperation and addresses practical issues within a CDMA
framework.
T. Hunter et al.[ ] In coded cooperation a user transmits additional
parity symbols for its partner according to some overall coding
scheme instead of repeating the symbols initially transmitted by
the partner. Bit and block error rate analysis has been performed
for coded cooperation and examples with specific coding
schemes show significant improvement over non cooperation
transmission.
S Wei et al. propose two delay diversity protocols to achieve the
cooperative diversity gain in an ad hoc wireless network without
requiring synchronization of the relayed symbols at the destination
and a novel joint DFE-MMSE equalizer was derived. Based on the
proposed outage probability criterion, simulation results demonstrate
the performance improvements of the protocols over the single hop
scheme, as well as performance comparable to protocols requiring
strict symbol synchronization.
Y. Jing and B. Hassibi [] The idea of space-time coding is devised for
multiple-antenna systems is applied a wireless relay network with
Rayleigh fading channels. A two-stage protocol is used where in one stage
the transmitter sends information and in the other, the relays encode their
received signals into a “distributed” linear dispersion (LD) code, and then
transmit the coded signals to the receive node. It is further shown that the
optimal power allocation is for the transmitter to expend half the power
and for the relays to collectively expend the other half. At low and high
SNR, the coding gain is the same as that of a multiple-antenna system
with R antennas. However, at intermediate SNR, it can be quite different,
which has implications for the design of distributed space-time codes.
• Nan Wu et al.[29] propose a family of CLDCs for cooperative
networks and demonstrated its ability to achieve full spatial diversity,
as well as its vulnerability under the situation of asynchronous
reception The linear dispersion structure is employed to accommodate
the dynamic topology of cooperative networks, as well as to achieve
higher throughput than conventional space–time codes based on
orthogonal designs. A novel time-domain delay-tolerant ACLDC
scheme is also proposed and being demonstrated that the desirable
cooperative diversity can be maintained, even if severe propagation
delay differences exist by introducing guard intervals and block
encoding/decoding techniques, the interference signals caused by
asynchronous reception can be exploited rather than discarded.
• Pierluigi Salvo Rossi [44]The performance of Distributed Linear
Block Codes(DLBC) has been presented in terms of bit error
rate(BER) and outage probability(OP). A packet-based
communication scheme using DLBC has been described and its
performance shown via numerical simulations. The effects of the
number of pilots, decoding errors, and channel estimation errors have
been studied, with a focus on a static set of users transmitting to a
mobile or static BS. The two scenarios emphasize the cases in which
the cooperative channels improve or not with the channel to the BS: in
the first case the advantage of the cooperative system saturates, as
shown by the presence of an error floor in the performance, while in
the second case the cooperative system is always effective.
• P.A. Anghel and M. Kaveh [46] propose a Distributed Space-Time Coding
(DSTC) system based on the Alamouti codes. The symbol error rate of
systems with one and two non-regenerative relays using bounds and high
signal-to-noise ratio (SNR) approximations is characterized. The asymptotic
(high SNR) symbol error probability formulas are used to optimize the power
allocation in the DSTC system. Furthermore, using the asymptotic symbol
error probability formulas it is shown that the DSTC system has at least 1.5
times the diversity achieved by point-to-point transmissions with the same
bandwidth. Simulations show not only that the DSTC outperforms the
amplify-and-forward cooperative system with orthogonal transmissions, but
also convolutional encoded one-hop transmissions with the same information
rate as the DSTC system. Numerical results show that the DSTC system with
two relays performs very close to the optimum cooperative system.
Walid Qaja et al.[ ] consider amplify-and-forward (AF) type cooperative
wireless relay networks employing single bit closed-loop extended
orthogonal space-time block coding (CL EO-STBC) over two selected
cooperating relay nodes. Selection is performed from a set of NR
available relay nodes each equipped with two antennas and outer
convolutive coding is used to improve performance. A near-optimum
detection scheme is used at the destination node for overcoming the
effects of imperfect synchronization among selected relay nodes. End-to-
end simulation results show that the employed detection scheme can
effectively eliminate the interference components induced by
asynchronism with low detection complexity. Furthermore, the one-bit
feedback scheme and relay selection technique can enhance the overall
system performance and outperform previous feedback method.
S. Ding and R. Li [49] have proposed the combination of Low Density
Parity Check -Space Time block Codes(LDPC-STBC) coder supporting
Set Partitioning in Hierarchical Trees (SPIHT) compression image
transmission in the asynchronous cooperative communication. It is found
that the increase of the compression rate doesn’t necessarily increase the
PSNR value in Rayleigh fading channels. The simulation results showed
that the LDPC-STBC with the guard intervals is better than the traditional
coding method in BER performance. The coder proposed can overcome
the ISI effectively. The encoding scheme is used to simulate the image
transmission, and the PSNR values of images using the different encoding
scheme through the fading channel in the asynchronous structure are
compared. The proposed scheme is effective in increasing the PSNR
values of the reconstructed images.
Feng-Kui Gong et al.[ ] propose a distributed orthogonal space-
time block code (STBC) by making use of the Alamouti coding
scheme and jointly processing the signals from the two antennas at
the relay node. Such a code turns out to make the equivalent
channel at each source node be a product of the two Alamouti
channels and thus, is called distributed concatenated Alamouti
STBC. In addition, the asymptotic formula of exact symbol error
probability (SEP) for a quadrature amplitude modulation (QAM)
constellation with the maximum likelihood (ML) detector is
derived. This result shows that the full diversity gain function is
achieved and proportional to ln SNR/.
Feng-Kui Gong et al.[51]consider a half-duplex amplify-and-forward
two way relaying network consisting of two sources with each having
a single antenna and N relays with each having two antennas. For such
a system with a general distributed linear dispersion code, a tight
lower bound of pair wise error probability (PEP) of the maximum
likelihood (ML) detector is derived, showing that diversity gain
function cannot decay faster than / , where SNR is signal to noise
ratio. Particularly for N = , a new distributed concatenated space-time
block code (STBC) is proposed and its asymptotic PEP formula is
attained, showing that the code presented in this paper achieves the
maximum diversity gain, i.e., meeting the lower bound of the diversity
gain function, as well as the maximum coding gain.
Gaps in Study
• Not much study has been done in the area of linear dispersive
distributed STBC in asynchronous cooperative communication
system.
• The asynchronous Distributed MIMO system is not explored much in
the light of channel coding for eg. LDPC
• The channel coded linear dispersive distributed STBC has not been
extensively studied in different fading environments.
Objectives
• To evaluate the performance of channel coded distributed space time
block codes in asynchronous cooperative MIMO system
• To evaluate the performance of the proposed concatenated DSTBC
scheme in various fading channels.
• To evaluate the performance of concatenated DSTBC in time reversal
(TR) and Linear dispersive (LD) structures respectively.
Research Methodology
1.Research and latest literature related to Distributed Space time
Block codes in asynchronous cooperative communication system
shall be explored and simulated using MATLAB software.
2.The research literature related to channel codes (for eg. LDPC) and
its implementation in asynchronous cooperative communication
shall be explored. Different parameters shall be studied and
implemented using MATLAB software.
3.Further bit error rate(BER) and outage analysis shall be done using
the MATLAB software.
References