ISSN (Print) : 2320 – 3765 ISSN (Online) : 2278 – 8875 International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering Vol. 2, Issue 4, April 2013 Copyright to IJAREEIE www.ijareeie.com 1272 Performance Analysis of MIMO Wireless Communications over Fading Channels - A Review Juhi Garg 1 , Kapil Gupta 2 , P. K. Ghosh 3 Student, Dept. of FET, Mody Institute of Technology and Science, Lakshmangarh (Sikar), Rajasthan, India 1 Assistant Professor, Dept. of FET, Mody Institute of Technology and Science, Lakshmangarh (Sikar), Rajasthan, India 2 Professor, Dept. of FET, Mody Institute of Technology and Science, Lakshmangarh (Sikar), Rajasthan, India 3 Abstract: Multiple Input Multiple Output (MIMO) technology uses multiple antennas at the both link ends and does not need to increase additional transmit power and spectrum, leading to promising link capacity gains of several-fold increase in spectrum efficiency. Increased capacity can be achieved by introducing additional spatial channels that are exploited by using coding such as space-time coding (STC). The spatial diversity improves the link reliability by reducing the adverse effects of link fading and shadowing. In this article, we survey environmental factors that affect MIMO capacity. These include channel complexity, external interferences, and channel estimation errors. With the use of MIMO communication techniques, multipath need not be a hindrance and can be exploited to increase potential data rates and simultaneously improve robustness of the wireless links. This review article provides a detailed explanation of this MIMO technology and explains how such benefits can be achieved using this technological breakthrough. Keywords: Diversity, Fading channels, Combining Techniques, Wireless Communications, MIMO, STC, spatial multiplexing, Capacity, Amplify and Forward, Decode and Forward, Cooperation I. CHALLENGES IN WIRELESS SIGNAL TRANSMISSION Wireless Communication has made a tremendous impact on the lifestyle of a human being. Wireless Network provides high speed mobility for voice as well as data traffic from variety of sources. The fundamental phenomenon which makes transmission unreliable is time varying fading [1]. The phenomenon is described as the constructive and / or destructive interference between signals arriving at the same antenna via different paths, and hence with different delays and phases, resulting in random fluctuations of the signal strength at the receiver. When destructive interference occurs, the signal power can be significantly reduced and the phenomenon is called as fading. Deep fades that may occur at particular time or frequency or in space result in severe degradation of the quality of the signal at the receiver making it sometimes impossible to decode or detect. Multipath fading arises due to the non-coherent combination of signals arriving at the receiver antenna. There are many kind of interference in real wireless communications. One important cause is the multi-path propagation. In analog type of TV broadcasting, ghost phenomena are observed due to multipath effects. The delayed signal is generated by the multi-path. This causes the overlapping between the current symbol and the previous symbol. This overlap causes the inter symbol interference (ISI) which destroys the sub-carrier orthogonality in Orthogonal Frequency Division Multiplexing (OFDM) system. The multipath fading can often be relatively deep, i.e. the signals fade away completely, whereas at other times the fading may not cause the signal to fall below a certain useable strength [2]. Interference is caused by deep-fades that occur at a particular point in space, or at a particular time or frequency, and results in severe degradation of the quality of signals at the receiver making it impossible to detect or decode. Several mathematical models have been developed to describe such channels. The model takes into account the phenomenon of multipath fading and correlation between sub-channels. Common models employ Rayleigh, Ricean and Nakagami-m distributions to approximate actual channel conditions [3]. The performance of the systems (in terms of error rate) can be severely degraded by fading. Communication through these channels is difficult and special techniques may be required to achieve satisfactory performance [4]. Figure 1 shows the concept of interference caused by reflection of signals.