Multiple Input Multiple Output (MIMO) technology is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. It is one of the key technologies in modern wireless communication standards such as IEEE 802.11n (Wi-Fi), 3GPP Long Term Evolution (LTE), and 5G. With the increasing demand for higher data rates in wireless communication, MIMO technology plays an important role in enhancing wireless network capacity and performance.
What is MIMO Technology?
MIMO uses multiple antennas at both the source (transmitter) and the destination (receiver) to improve either throughput, reliability, range or reduce required transmit power of wireless links. The idea is that by using multiple antennas and leveraging multipath propagation, more information can be transmitted in parallel within the same band resulting in increased spectral efficiency. The key aspects of MIMO include:
– Multiple input streams: Information bits are divided into multiple input streams that are transmitted from different transmit antennas in parallel.
– Multiple output streams: At the receiver, the multiple input streams get combined due to multipath propagation and appears as multiple, and possibly different, signals at each receiver antenna.
– Multiple paths: Multipath propagation enables each transmit-receive antenna pair to have an independent fading path or channel. This creates independent channels between each antenna element.
– Spatial diversity: MIMO leverages spatial diversity created by these independent fading paths to improve performance by combining signals from different paths. This reduces effects of deep fading.
– Diversity gain and spatial multiplexing gain: MIMO can provide diversity gain by sending duplicates of the same signal over different channels or spatial multiplexing gain by sending independent data streams resulting in higher throughput.
MIMO Operating Modes
The two primary modes of MIMO operation are:
1. Spatial diversity: Uses multiple transmit and receive antennas to implement a form of error control coding known as space-time coding. It is used to provide array gain by diversity combining of signals or paths and improves reception by increased SNR. This provides diversity gain to combat multipath fading but does not increase capacity.
2. Spatial multiplexing: Utilizes multiple antennas to simultaneously transmit independent and separately encoded data streams (layers) from each transmit antenna to increase spectral efficiency by multiplying available data rates using the same bandwidth. It also relies on the propagation environment to separate signals via independent channels between each transmit-receive antenna pair. This provides multiplexing gain to enhance throughput.
MIMO Techniques
There are different techniques employed in MIMO systems to achieve its benefits:
– Beamforming: Uses phase shifts between antenna elements to direct radiation in a given direction to achieve improved and focused signal reception. It increases SNR by concentrating power on intended directions and nullifying noise/interference coming from other directions.
– Spatial Modulation: A low complexity MIMO scheme that encodes information bits not just in amplitude/phase but also in the indices of transmit antennas. Transmits a single RF chain at a time resulting in low hardware complexity.
– Space-Time Coding: Combines coding theory with MIMO spatial diversity to provide high performance wireless transmission. Alamouti code is a popular STBC that transmits signals over multiple antennas in space and time domains.
– Pre-coding: Performs linear transforms on transmitted symbol streams at the transmitter side before transmission at different antennas to achieve spatial multiplexing, interference suppression and other gains. Examples are Block diagonalization, Tomlinson-Harashima.
Applications and Advantages of MIMO
MIMO has brought revolutionary changes to modern wireless systems and has become an integral part of various wireless standards. Some key applications and advantages include:
– Mobile networks: MIMO-OFDM is used in 4G LTE and 5G networks to achieve multiple-fold increase in data rates through spatial multiplexing and improved link reliability.
– Wi-Fi: IEEE 802.11n and later versions use MIMO to dramatically increase Wi-Fi speeds and range. MIMO-OFDM aggregates subcarriers making Wi-Fi robust against interference.
– Satellite communications: MIMO helps improve link budgets and increase throughputs for satellite broadband services operating in challenging environments.
– Radar systems: MIMO enables developing advanced radars with high resolution, range/Doppler estimation capabilities useful for automotive radar, imaging and other applications.
– Internet of things: MIMO assists in meeting connectivity and throughput needs of emerging IoT deployments with massive number of devices operating in unlicensed bands.
– Improved spectral efficiency: MIMO increases link capacity without requiring additional spectrum or transmit power resulting in higher bandwidth efficiency.
– Robust to fading: Spatial diversity reduces fading through constructive combination of multipath signals making links more reliable in environments with scattering/obstructions.
Challenges in Multiple Input Multiple Output or MIMO technology
While MIMO provides significant gains, there are also some challenges that need to be addressed:
– Increased complexity: MIMO architectures require more RF chains, ADCs/DACs and DSP implementations resulting in higher device costs compared to SISO systems.
– Pilot contamination: In multi-cell MIMO systems, reuse of common pilots causes interference limiting performance of cell-edge users. requires careful pilot design.
– Channels estimation: Acquiring accurate channel state information (CSI) at the transmitter and receiver is crucial for MIMO to realize its potential but channel varies rapidly requiring frequent re-estimation.
– Inter-layer interference: Signals transmitted through different antennas can interfere with each other if not properly separated at the receiver end reducing spectral efficiency. Precoding/detection are employed to tackle this challenge.
– Cross-polarization interference: Signals transmitted through co-located antennas with orthogonal polarizations can still couple and cause interference under certain reflection scenarios. Polarization diversity is studied to handle this.
– Sensitivity to hardware impairments: Implantation non-idealities in RF/analog front-ends like I/Q imbalance impacts performance significantly requiring robust designs.
Future Outlook
Looking ahead, MIMO will continue being a pivotal technology for future wireless networks. Research is ongoing to address challenges and further improve MIMO architectures and algorithms:
– Massive MIMO: Using very large number (100s) of inexpensive antennas at base stations to achieve unprecedented array and multiplexing gains through simple processing. Promising for 5G networks.
– Full-dimension MIMO (FD-MIMO): Extension of massive MIMO concept to 3D using antenna arrays with vertically stacked elements to realize extra degree of freedom.
– Millimeter wave MIMO: Key
Note:
1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it
