Abstract: Multiple-Input Multiple-Output (MIMO) technologies has recently emerged as one of the most significant technical breakthroughs in modern digital communications due to its promise of very high data rates at no cost of extra spectrum and transmit power. MIMO-based systems take advantage of what is probably the last unexploited frontier in wireless communications, the spatial domain. No doubt the MIMO gain is achieved only when the signal reaches at the receiver via two or more independent paths resulting in multipath fading, which otherwise is an impediment to communication using Non Line of Sight (NLOS) mode of communication. MIMO technology offers many advantages and degrees-of-freedom, such as: (a) space and multiuser diversity gain, (b) spatial multiplexing gain, (c) array and coding gain, and (d) interference reduction. MIMO is of less or of no advantage in LOS (Line of Sight) communication. Mobile satellite communication channel can be modelled as a multipath fading channel in the case when the receiver with multiple antennas is moving on the road side with long trees on both sides of the road in urban areas and moving through vegetation in rural areas. Since there is limitation of increased volume and weight on board satellite due to which having two independent antennas is a difficult proposition, it is suggested to use Dual Circular Polarised Antennas on board satellite and two or more antennas on moving platform/s on ground. It has been found in simulates results that at high signal to noise ratio a multiplexing gain of nearly 1.6 has been achieved by using single antenna on board satellite but transmitting and receiving signal with dual polarised (Right Hand Circular Polarised (RHCP) & Left Hand Circular Polarised (LHCP) wave. The phenomenon of multipath fading is more pronounced at higher frequencies like Ku and Ka band compared to L or S bands, but for mobile satellite mobile communication the band L & S band (having frequency well below 10 GHz) is used. The only disadvantages of MIMO systems are basically its designing, multichannel synchronization, DSP engineers are required to implement more sophisticated baseband processing algorithm to better interpret the channel model. (1), (2)

Keywords: Multiple Input Multiple Output (MIMO), Dual Polarisation Per Beam (DPPB), Multiplexing Gain, Array Gain. Multipath Fading, Line of Sight (LOS), Low Elevation Angel, Channel Capacity, Non Line of Sight (NLOS).