16-09-2014, 10:12 AM
Cooperative MIMO System for WiMAX Technology
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Abstract
Multipath fading is one of the primary factors that degrade the performance of wireless
networks. One of the most powerful techniques to combat the effect of fading is by using
multiple antennas that provides space diversity and spatial multiplexing; this technique is
called Multiple-Input Multiple-Output (MIMO).
Cooperative diversity, an alternative form of realizing MIMO, has been recently
proposed to realize the diversity advantage in a distributed manner. Cooperative diversity
exploits the broadcast nature of wireless transmission and creates a virtual antenna array
through cooperating nodes. Although, prior research in cooperative diversity considers
users equipped with single antenna, in practical scenarios users may be able to
accommodate multiple antennas due to the recent advances in semiconductor industry.
Hence, the primary purpose of this thesis is to model, and by using simulation,
investigates the end-to-end performance of a cooperative diversity system employing
multi-antenna at cooperating nodes; the intention is to simultaneously exploit the
diversity gain offered by the cooperative diversity and multiple antennas
Introduction
Next generation wireless communication system demand both high transmission rates
and a quality of service guarantee. These demands are hardly achievable due to properties
of the wireless medium. As a result of scatterers in the environment and mobile terminals,
signal components received over different propagation paths may add destructively or
constructively and cause random fluctuations in the received signal strength. This
phenomenon, which is called fading, degrades the system performance [1].
Diversity combats multipath fading by providing the receiver with redundant signal
information through uncorrelated channels thereby allowing the receiver to average
individual channel effects. The most common forms of diversity are space, time and
frequency; however, space and time diversity are the most widely used.
Spatial diversity is achieved by transmission and/or reception of multiple copies of a
signal from physically different points in space (e.g., multiple transmit and/or receive
antennas). If an appropriate distance separates the points from where the signals are
transmitted or received, then the characteristics of the channel are probabilistically
uncorrelated and full diversity can be obtained [2].
WiMAX Technology
The rapidly growing demand for flexible, high speed broadband services requires
advanced communication technologies. The more conventional family of high rate
broadband access techniques has relied on wired access, such as Digital Subscriber Line
(DSL), cable modems, Ethernet and optical fibers. However, the extension of the
coverage area results in a significantly increased cost imposed by building and
maintaining wired networks. This is particularly true for less densely populated zones, for
example suburban and rural areas [8
. Problem statement
MIMO is an advanced technology that can effectively exploit the spatial domain of
wireless fading channels to bring significant performance improvements. Conventional
MIMO systems require both the transmitter and receiver of a communication link to be
equipped with multiple antennas. In a case that, wireless devices do not support multiple
antennas due to size, cost and/or hardware limitations, cooperative diversity can be used.
Cooperation aims to utilize distributed antennas on multiple radio devices to achieve
some benefits similar to those provided by conventional MIMO systems.
However, in case implementing multiple antennas in a given node is possible,
cooperative diversity in addition to MIMO strategy can further improve the performance
of the communication system by providing substantial additional diversity benefits [4].
The problem is how to combine MIMO techniques with cooperative diversity to improve
the system performance further. Also, how does the performance of the resulting system
looks like? Moreover, there are different types of detection techniques so that the
performance of each technique should be investigated so as to find the optimal one. Most
of the previous studies in cooperative communication either use single antenna on either
each node or one of their nodes. This research analyzes cooperative diversity with
multiple antennas at each node and taking various detection and combining strategies
Conclusion
6.1. Summary of Results and Conclusions
In Chapter 2 we initially describe the MIMO system and performance of various antenna
array architectures in terms of capacity for quasi-static flat fading environments.
Additionally, we introduce the practical space-time codes such as the Alamouti scheme
and VBLAST architecture and its detection techniques.
In chapter 3, we describe a single antenna three terminal cooperative diversity using
different relaying and combining techniques.
In Chapter 4 we describe the simple three terminal relay network employing multiple
antennas at all nodes of a wireless network.
Simulation results of a three terminal multiantenna relay network using spatial
multiplexing architecture, VBLAST, are described with the following conclusions:
Conventional MIMO detection schemes such as ZF, MMSE and ML are extensively
studied for both MIMO and cooperative MIMO systems in this work. As it shown in
Figures 5.3 and 5.5, ML is obtained to have optimal performance while ZF and MMSE
are suboptimal in performance when compared to ML.
Cooperative MIMO systems achieve significantly better performance in symbol error rate
(SER) than conventional cooperative diversity and MIMO systems. For instance;
Comparing Figure 5.3 and Figure 5.5 for BER of 10-3
, cooperative MIMO system have
13 dB, 11 dB and 5 dB SNR gain over MIMO system for ZF, MMSE and ML detection
schemes respectively.