LTE brought forth a variety of equipment and technologies. One of these new technologies is Multiple Input Multiple Output, also known as MIMO. It allows the use of use of multiple antennas in wireless communications is one of the main reasons why LTE has such high bandwidth rates.
It all started with the V-BLAST (Vertical-Bell Laboratories Layered Space-Time) project, in 1996, which is, in fact, at the basis of MIMO systems. V-BLAST was a detection algorithm of multiple signals whose main purpose was to reconstruct the multiple received signals into a single, faster stream of transmitted data. This, of course, is precisely why MIMO does.
The principal application of this technology is embodied in MIMO antennas, particularly used in LTE mobile networks. As opposed to SISO (Single Input and Single Output) – an antenna system with one transmitter and one receiver – two 2×2 MIMO antenna systems will use 2 transmitters and 2 receivers to generate 4 paths for transmitting and receiving different data at the same time. The two transmitters send different parts of the same data stream simultaneously, while the receivers have to piece them back together. MIMO increases overall performance and range and is able to send more data without additional power or added bandwidth requirements.
Typically, radio signals traveling through the air are prone to being affected by various phenomena such as: fading, interference, path loss and more. What’s special about MIMO is that it does wonders in multipath environments, increasing the data throughput and lowering the bit error rate. MIMO is able to identify one signal from another at the receiver side because they have been altered differently by multipath. The receivers can spot the ‘clues’ that multipath left behind to correctly decode the received signals into a single faster data stream. As opposed to MIMO, SISO systems perform poorly in multipath conditions. Considering that LTE has gained such momentum in urban ares, the home ground of multipath, it’s easy to understand why 4G uses MIMO antennas.
As mentioned above, a 2×2 MIMO antenna will send each data stream through two independent channels to overcome fading. This is a concept called ‘diversity’ and it ensures that at least one data stream will be less affected by fading, increasing the chances of the receiver to decode more data correctly. ‘Polarization diversity’ is a ‘diversity flavor and is also used in MIMO systems. To give a simple example, polarization diversity would translate in using antenna pairs polarized orthogonally, either in a vertical/horizontal position or slanted at ± 45º.
To sum up, the MIMO technology used in LTE antenna systems increases overall data throughput, reduces co-channel interference and multipath propagation effects, improves the signal to noise ratio and reduces the bit error rate.