Understanding MIMO Technology

Introduction

MIMO is an abbreviation for Multiple Input Multiple Output. It is an Antenna Technology used in Wireless Communication which deploys multiple antennas at the receiving and transmitting node for better transmission and reception of signal.

MIMO Technology is an evolution of existing SISO (Single Input Single Output) Technology. Between SISO and MIMO technologies, other antenna technology like MISO (Multiple Input and Single Output) and SIMO (Single Input Multiple Output) were also used but were phased out due to their inefficiency.

Wireless Communication uses Radio Wave propagation for signal transmission. A Radio Frequency Carrier signal is modulated by a low frequency message signal. Now, the RF Signal is required to be converted into EM Wave as it can travel in space without any medium. It is done by the Antenna, which is nothing but a special transducer. So, the Antenna converts RF Signal into Electromagnetic Wave of equivalent frequency. Next, the EM Wave is transmitted to Receiver Node by the Transmitter. Upon reception, the EM Wave is converted into RF Signal and this RF Signal undergoes Demodulation to retrieve the Message Signal.

Fading

Once the signal is transmitted by the transmitter, the signal propagates through various environment and over varied distance to reach the receiver. In the process of propagation, the signal may across multiple hindrances due to which there is a drop in the quality and strength of the transmitted signal. This is called fading. Due to fading, there is a chance of information loss at the receiver and the receiver might not be able to correctly reconstruct the message signal.

MIMO Technology works as a solution to the problem of Fading in Wireless Communication.

With respect to MIMO Technology, Communication Systems are broadly categorized into four categories based on the number of antennas in the transmitter and the receiver.

● SISO — Single Input Single Output system — 1 Tx antenna , 1 Rx antenna.

● SIMO — Single Input Multiple Output system — 1 Tx antenna, Multiple Rx antenna.

● MISO — Multiple Input Single Output system — Multiple Tx antennas, 1 Rx antenna.

● MIMO — Multiple Input Multiple Output system — Multiple Tx and Rx antennas.

Spatial Diversity

In spatial diversity same information is sent across independent fading channels in order to reduce fading. As the fading channels are independent, the amount of fade suffered by each copy of the data will be different. Due to this, there is an assurance that one of the copies will suffer less fading than other copy. Thereby improving the chance of properly receiving the transmitted data. In effect, this also improves the overall reliability of the system.

Considering a data stream of [1, 0, 1, 1, 1] is being transmitted through a channel with deep fades. Due to the variations in the channel quality, the data stream may get lost or severely corrupted that the receiver cannot recover. The solution to the problem is to add independent fading channel and increase the number of transmitter antennas or receiver antennas or the both.

The SISO System with single antenna on each node will not provide any diversity as there is no parallel link, thus there is a chance of data loss whereas a 2x2 MIMO System will provide a diversity of 4. Also, if one of the links fails to deliver the data, the chances of proper delivery of the data across the other link will be there. Thus, Spatial Diversity increase the reliability of the overall system.

Spatial Multiplexing

In Spatial Multiplexing, each spatial channel carries independent information i.e., parts of data are distributed over different antennas which transmit them parallelly, thereby increasing the data rate of the system. This is similar to the Orthogonal Frequency Division Multiplexing (OFDM) technique, wherein different frequency subchannels carry different parts of the data. But in Spatial Multiplexing, several independent subchannels are created in the same allocated bandwidth. Thus, the multiplexing comes at no additional cost on bandwidth or power.

Massive MIMO

The 4G LTE(Long Term Evolution) System used the Legacy MIMO Technology but the 5G Systems use Massive MIMO Technology. The advantage of the Massive MIMO System over Legacy MIMO is that it can multiply the capacity of a wireless connection without requiring more spectrum. Massive MIMO includes Beamforming along with Spatial Multiplexing and Diversity. Existing LTE Networks allot a pool of spectrum between all users in an area resulting in performance dip in densely populated areas. With Beamforming Technology, this process is handled much more efficiently, improving Latency and Data speeds.

In the implementation of the new age Massive MIMO Systems which is associated with THz frequency band, scientists have developed Line of Sight Spatial Multiplexing Technique. Line of Sight is the direct path from the transmitter to the receiver and the obstructions that may fall in that path. A clear line of sight is important to have efficient communication. In THz frequency band, multipath propagation is very weak due and the roughness of most surfaces is comparable to wavelength of signal leading to major signal loss. So, in the new generation of communication systems, problems seem to arise with the existing MIMO Technology for which LoS MIMO Systems can act as a potential solution.

Spatial Multiplexing is one of the important features of Spatial Multiplexing as it increases Data Rate of Communication.

Benefits & Challenges

The MIMO Technology is instrumental in modern day wireless communication systems. It has a plethora of advantages like improved data rates which is achieved by transmitting multiple data streams simultaneously. Moreover, MIMO mitigates interference and fading, enhancing communication reliability and offers Improved Stability of the system. The MIMO System was successfully implemented in LTE Systems, then made its way forward into 5G Systems as Massive MIMO and now it is being tested for 6G Systems working in THz frequency range.

Though it is extensively beneficial, it has certain challenges like the Complex Design and implementation of MIMO systems as it requires careful design and implementation due to the cost, physical size of multiple antennas, and increased processing power needed for signal processing.