Multiple Input Multiple Output (MIMO)

Multiple Input Multiple Output (MIMO)


IEEE-802.11n wireless standard uses multiple antennas for high data transmission[1]. 802.11n standard was published in October 2009 by IEEE[2]. 802.11n is also known as MIMO (multiple input multiple output) pronounced as My-Mo[3].

Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple transmitters (output) and multiple receivers (input) to transfer more data at the same time.

MIMO allows single radio channel to transmit and receive more than one data signal simultaneously[4]. MIMO transmits different signals over individual antennas which are subjected to different coding, delay, amplitude and phase control during processing[5].The antennas at each end of the communication circuits are combined to minimize errors and optimize data speed.


Fig-1 shows the basic block diagram of MIMO

What led to evolution of MIMO?

The main reason for the evolution of MIMO technology is Multipath interference. Multipath becomes the boon for the evolution of MIMO. Previously, these multiple paths only served to introduce interference. By using MIMO, these additional paths can be used to increase the capacity of a link. MIMO utilizes the multipath signals for the better reception. MIMO increases receiver signal capturing power by enabling multiple antennas to combine data streams from different paths (multipath) and at different times[6].

What is Multipath?

In conventional systems/legacy devices, single transmitter and single receiver were used (SISO). In this scenario, there is no line of sight between transmitter and receiver so signal reach receiver after bouncing from obstacles. So, signal takes multipath. When transmitter sends the data; it spreads out in all directions and reach the receiver from different paths (multipath) after getting bounced from walls, ceilings and other obstacles. If the received data is out- of-sync then they will cancel out each other and null (or no signal) will be received by receiver for processing. This effect is known as Multipath interference.

To take benefit of MIMO, both the station (client device) and access point must have multiple transceivers[7] (transmitters and receivers).
Various Antenna technologies:
  1. SISO (Single input single output)
  2. SIMO (Single input multiple output)
  3. MISO (Multiple input single output)
  4. MIMO (Multiple input multiple output)
SISO - (Single input single output):

The simplest form of radio link which can be defined in MIMO term is SISO. SISO (single input, single output) refers to a wireless communications system in which one antenna is used at the source (transmitter) and one antenna is used at the destination (receiver). SISO requires no processing in terms of the various forms of diversity that may be used. However, the SISO channel is limited in its performance as interference and fading will impact the system more than a MIMO system using some form of diversity.

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Diversity refers to the method to improve the reliability of signal using various methods.
Some of the diversity schemes are[8]:
  1. Time Diversity
  2. Frequency Diversity
  3. Space Diversity/Antenna Diversity
  4. Polarization Diversity
We will focus on Space diversity or antenna diversity only.
Antenna diversity used to mitigate the multipath interference. Antenna diversity is also known as spatial diversity or space diversity or switched diversity. It is used to improve the reliability and quality of link. Antenna diversity is used in SIMO and MISO techniques. SIMO and MISO are known as smart antenna techniques or antenna arrays. Antenna array is nothing but the collection of antennas. Usually base station or transmitter have multiple antenna or antenna arrays as there is more space on the base station to deploy it[9].
SIMO - (Single Input Multiple Output):

In SIMO single transmitter and multiple or more than one receiver are used. When the multiple signals received from multiple paths at the receiver then one antenna may receive the strong signal compared to other antennas and hence signal will be processed. This technique is used to combat fading. Here only one antenna will remain active or only one antenna will process the data. This is also known as receiving diversity[10]. It has one disadvantage that processing is required at the receiver side.

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MISO - (Multiple Input Single Output):

In MISO multiple transmitters and single receiver is used. When multiple signals sent from transmitter and reached at receiver then receiver will receive the multiple copies of same data from each transmitter and process the optimum data. This is known as transmit diversity[11]. Processing of the signal is done at the transmitter side only. It is used in downlink transmission.

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MIMO - (Multiple Input Multiple Output):

MIMO explore multipath propagation using different transmission paths to receiver. MIMO is effectively a radio antenna technology as it uses multiple antennas at the transmitter and receiver to enable a variety of signal paths to carry the data, choosing separate paths for each antenna to enable multiple signal paths to be used.

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MIMO have multiple transmitters and multiple receivers.

MIMO terminology defined as (MxN:O) that can be read as - M i.e., first digit defines number of transmitters, N i.e., second digit defines number of receivers and O i.e., last digit defines the number of spatial streams (which is explained later).

The two main formats for MIMO are[12]-

A. Spatial diversity/Antenna diversity - Spatial diversity is often referred as transmit and receive diversity used in MISO and SIMO systems. Existence of multiple antennas in a system means existence of different propagation paths. Aiming at improving the reliability of the system, same data is sent across the different propagation (spatial) paths. This is called spatial diversity. It is also known as switched diversity because antenna with high SNR becomes active and processes the data.

B. Spatial multiplexing - Aiming at improving the data rate of the system, we may choose to place different portions of the data on different propagation paths. This is called spatial multiplexing. The data chunks received at receivers are multiplexed or added to increase the data capacity.

Spatial Streams plays a crucial role in MIMO.
What is spatial stream?

The source data is split into two or more independent data streams that are transmitted over multiple antennas, are called spatial streams[13]. Spatial streaming is primarily observed in wireless communications where multiple-input-multiple-output (MIMO) is being used. In a MIMO environment, the signal is being transmitted by the various antennas and are multiplexed by using different spaces within the same spectral channel. These spaces are known as spatial streams or in simple words we can explain it as it is a data that has been broken up and sent at the same time over wireless. Wi-Fi is a half-duplex connection, which basically means a device, can either send or receive but not both at the same time. To speed up throughput 802.11n uses multiple radios to send chunks of data at the same time.

In MIMO, each spatial stream is transmitted from a different radio/antenna chain in the same frequency channel. The receiver receives each stream on each of its identical antenna/radio chain.

As the receiver knows the phase offset of its own antennas, it can use the signal processing techniques to mathematically reconstruct the original stream.

Beam-forming can enhance the MIMO.

Beam forming means pointing antenna array (or group of antennas’) for shaping the beam towards a particular direction i.e. to focus the energy or beam towards particular physical location especially towards client device. Concentrating the energy/signal in one direction improves signal to noise ratio and transmit speed.

For reducing the interference and for improving the reliability of wireless network, 802.11n and 802.11ac comes with an advanced technology called beamforming.

Beamforming transmit the data in the given direction of the client instead of radiating in all directions. The best example is laser which can deliver the data towards the specific device only while old wireless standards is like a light bulb which spreads the light in whole set of area.

To give stronger signal to a specific device, a beamformer (transmitter) changes the phase and relative amplitude of the signal.

Now the question arises how to gather the location of client for beam forming?

By the mathematical procedure called channel calibration or channel sounding procedure the physical location of client can be tracked.

How Beam forming work?

It’s a channel sounding procedure in which a transmitter send a NDP (null data packet) announcement frame to the receiver and then the client device responds with a matrix indicating how well it heard the signal from each antenna. Based on this matrix data, the AP can compute the relative position of the client device, and the phase offsets on each of its antennas are required to maximize constructive interference at the client device. So, in this way the strong signal can be given in right direction and signal will be weaker in other direction, which is the way to use of energy in an efficient way.

Types of MIMO systems[14]:
  1. SU-MIMO (Single user MIMO)
  2. MU-MIMO (Multi user MIMO)

SU-MIMO – As the name suggest, Single user-MIMO i.e. multiple streams of data is sent or received between just one client at the time. Single user MIMO requires both the transmitting and receiving device support the MIMO technology, along with having multiple antennas. The multiple antennas add cost, weight, and size to the wireless devices and the processing of the MIMO signals requires more resources as well.

MU-MIMO – Multi User MIMO released in a second wave (or wave-2) of 802.11ac wireless standard. As the name suggest, multiple streams of data are sent or received between multiple clients at the same time. MU-MIMO is one step further in MIMO technology, which enables multiple independent radio terminals to access a system, which can provide significant performance gains over the original MIMO technology. MIMO provides the facility to multiple user to connect and access over the same channel simultaneously with some spatial degree of freedom and by adding more antennas/radios, it can control the phased antenna pattern to control both the areas of maximum constructive interference -- where the signal is the strongest -- and maximum destructive interference --where the signal is the weakest. With a sufficient number of antennas and knowledge about the relative positions of all associated client devices, it can actually create a phased pattern to talk to multiple clients both independently and simultaneously.

How does MIMO work -
  1. The AP broadcast a sounding frame
  2. Each MU-MIMO supportable device transmit back matrix data to access point
  3. The AP computes the relative position of client and make a group of client device to communicate simultaneously
  4. The AP computes the phase offset for each data stream for each client in group and then transmits all data stream in group
  5. The AP sends acknowledge block request to each client in group to know the confirmation to whether the client has received the data stream or not
  6. Then each client will send the block acknowledge as confirmation
  5. _WLAN80211n_from_SISO_to_MIMO.pdf