Spatial Multiplexing vs Transmit Beamforming

Multiple-input/several-output (MIMO) wireless technology configurations are used in spatial multiplexing, where the transmit stations use multiple broadcast and receiver antennas to provide sophisticated signal outcomes. Numerous radios are used by a wireless communications access point to allow multiple data streams to pass between the various transmitter antennas and receiver. This improves throughput and is a frequent way to experiment with wireless installations.


Using Multiple Radios to Scale Performance

Radio Chain

Prior to 802.11n, most wireless systems had a single transmitter and receiver. In other words, the components combined to form one radio, yielding a single radio chain. A single radio and all of its supporting architecture, including mixers, amplifiers, and analog/digital converters, are referred to as a radio chain.

Single in–Single (SISO)

SISO is a wireless communication technology that uses one transmit and receive antennas each at the source (transmitter) and one antenna at the destination (receiver). The simplest antenna technique is SISO.

Multiple Input–Multiple Output (MIMO systems)

The MIMO system is made up of numerous radio chains and multiple streams, one with its own antenna system. The utilization of several radio components, creating multiple antennas, is one key to the improved performance channel capacity of 802.11n, 802.11ac, and 802.11ax radio networks.


Spatial Multiplexing

Spatial multiplexing, also known as space-division or spatial multiplexing techniques, is a multiplexing technique used in MIMO wireless communication, fiber-optic communication, and other communication technologies to transmit independent channels separated in space.

Spatial Multiplexing is a MIMO wireless technology that distributes different data signals or streams across antennas to improve the performance or usefulness of wireless transmissions. It’s a form of “spatial variety” or spatial diversity and an engineering gimmick that allows for more end-to-end transmission options.  Data stream can be multiplexed or dispersed across two or more radio chains to boost the data rate or throughput—all running on the same channel but separated by spatial diversity. Spatial diversity is referred to as spatial multiplexing.

The number of radio chains possible on 802.11n, 802.11ac, and 802.11ax devices is defined. This is expressed as TR, where T denotes the number of transmitters and R denotes the number of receivers. A device with 2 x 2 MIMO has two transmitters and two receivers, whereas a device with 2 x 3 MIMO has two transmitters and three receivers. Figure 1 depicts a comparison of number of transmit antennas on the typical 1 x 1 SISO device with the 2 x 2 and 2 x 3 MIMO devices.

Examples of SISO and MIMO devices

Several radio chains can be used in a number of ways. Extra radios, for example, can be used to increase received data rates and signal quality, improve transmission to specific client locations, and transmit data from many clients at the same time.

The objective is to maintain each signal separate or distinguishable from the others. Each radio chain has its own antenna; if the antennas are too far apart, the signal reaching the receiver’s antennae (which are also correctly spaced) will be out of sync with each other or at varying amplitudes. This is particularly true if the signals reflect off obstacles along the route, causing each antenna’s signal to take a slightly different path from the receiver to reach the intended recipient.


Spatial Multiplexing Streams

When many antennas send or receive wireless signals concurrently in a MIMO setting, the signals from receiving antennas are multiplexed by using different regions within the same spectral channel, resulting in spatial streams. These areas are referred to as spatial streams.

Furthermore, data may be delivered in a predictable manner throughout the transmitter’s radio chains. In reality, various independent data stream can be handled as spatial streams multiplexed and transmitted over radio chains. The receiver must be able to comprehend the arriving signals and reconstruct the original data streams by reversing the multiplexing function of the transmitter.

Spatial multiplexing necessitates a significant amount of digital signal processing at both the sending and receiving ends. This pays off by improving the channel’s throughput; The greater the number of spatial streams accessible, the more data that can be delivered through the channel. Spatial multiplexing between two 3 x 3:2 MIMO devices is seen in Figure 2low. A 3 x 3:3 devices are similar, except it supports three spatial streams.

Spatial streams are disseminated over various radio chains after being processed. The number channel quality of available spatial streams is determined by the device’s processing capabilities and transmitter feature set, not by the number of radios.

Spatial Multiplexing Between Two 3x3:2 MIMO Devices

Transmit Beamforming

A flexible approach for signal transmission from an array of N antennas to one or more consumers is transmit beamforming. The purpose of wireless communication is to boost signal power to the intended user while reducing interference to non-intended users.

The 802.11n, 802.11ac, and 802.11ax amendments allow you to tailor the broadcast signal to favor one receiver over another. The same signal may be sent via several antennas using MIMO to reach particular client locations more efficiently.


Multiple signals often travel over somewhat different courses to approach a receiver, causing them to emerge delayed and out of sync with one another. Normally, this is detrimental, resulting in a reduced SNR and a distorted signal. When transmit beamforming (TxBF) is used, the phase of the as the signal is sent into each transmitting antenna, is changed such that the resultant signals arrive in phase at a specified receiver. This has a positive effect since it improves signal quality and SNR.

Figure 3 depicts a device on the left transmitting beamforming to target device B on the right. Each duplicate of the broadcast signal’s phase is altered such that all three signals arrive at device B in about the same phase. The identical three signal copies arrive at device A, which TxBF does not target. As a result, the signals arrive in their current state and are out of phase.

Using Transmit Beamforming to Target a Specific Device

Maximal-ratio Combining

The signals from each channel are put together, and the gain of each channel is proportional to the RMS signal level and inversely proportional to the mean square noise level in that channel.

To receive the many sent copies of the signal, the receiving equipment might employ multiple antennas and radio chains. One copy may be better than the others, or one copy may be better for a period of time before becoming worse than the others. In any case, maximal-ratio combining (MRC) can combine the copies to get a single signal representing the best version at any given moment. As a result, the reconstructed signal has a higher SNR and Sensitivity than the receiver.


Classification of MIMO based on the antenna configuration

Communication systems are roughly classified into four groups in MIMO language, as shown below, based on the number of antennas in the transmitter and the antenna elements per receiver.

SISO stands for Single Input Single Output.

SIMO stands for Single Input Multiple Output.

MISO stands for Multiple Input Single Output.

MIMO stands for Multiple Input Multiple Output.

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