14-12-2012, 12:34 PM
Performance Measurement of Various Symbol Detection Techniques in Multiple-Input Multiple-Output
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Abstract:
Wireless communication using multiple-input multiple-output (MIMO) systems enables increased spectral efficiency for a given total transmit power. Demand for high data rate and increasing applications offered by a wireless device calls for an effective method for communication. Due to constrain on the available bandwidth, there is a need for exploiting the available bandwidth in a way so that we get maximum advantage. MIMO system does exactly this thing. They multiply the data rate without any expansion in the bandwidth. MIMO system utilizes the spatial diversity property of the multi channel system V-BLAST detection technique is employed for this purpose. This paper depicted the advantages of using multiple antennas by exploiting signal diversity offered by multipath effect.
INTRODUCTION
Multiple-input and multiple-output, or MIMO is the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO technology has attracted attention in wireless communications, because it offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this by spreading the same total transmit power over the antennas to achieve an array gain that improves the spectral efficiency (more bits per second per hertz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading). Because of properties, MIMO is an important part of modem wireless communication.
Correlated channel: If the probability of a given output does not depend upon the several preceding input symbols then the channels are known as correlated channels.
SYMBOL DETECTION TECHNIQUES
The detection strategy is one of the prime criteria to determine the effectiveness of a communication system. The best performance is obtained by using ML estimate. But it has a flaw .The computational complexity is very high. It increases with the increase in number of transmitting and receiving antennas and the constellation size. Other alternative were then thought of which reduces complexity and gives performance close to ML estimation scheme.
Maximum Likelihood Detector
The maximum likelihood detector (MLD)[14] outperforms all the existing detection methods but its computational complexity increases exponentially with the increase in number of transmitting antennas. In ML detection method minimization is performed over all possible codeword vectors.
One of the methods to reduce computational complexity of ML detector is sphere decoding. The complexity coefficient can also become very large when the Signal-to-Noise Ratio is low or when the problem dimension is high, e.g., at high spectral efficiencies.
Sphere Decoder
Sphere decoder [15] restricts the range of search in ML estimation. The principle of the sphere decoding algorithm is to search the closest lattice point to the received signal within a sphere radius, where each codeword is represented by a lattice point in a lattice field. The computational coding is reduced in sphere decoder by searching over only those points that lie within a hemisphere of radius √C around the received signal y, rather than searching over the entire lattice.
CONCLUSION
V-BLAST is a very efficient technique for symbol detection which reduces computational complexity manifold. To achieve high data rate diversity is exploited in which several replicas of the signal are made available to the receiver in the hope that at least some of them are not attenuated severely. As available bandwidth is finite, the space diversity is promising, since it does not involve any loss of bandwidth. V-BLAST is an example of space diversity scheme. Space time block code and space time trellis code combines both space and time diversity which provides better performance.
It is applicable for all type of channels (slow fading, fast fading, flat fading and frequency selective fading) and Rayleigh, Nakagami etc. The various methods for increasing diversity of the data stream are available like STBC, STTC, and LDPC etc. These methods when applied to MIMO OFDM or CDMA system provides high data rate along with good performance. So, V-BLAST technique offers a wide area to work on different systems using various coding scheme under diverse channel condition so that the resultant system has least complexity along with high effectiveness.