Good day all!
This article is about MIMO in wireless communication. Have a look below for more information on the topic.
Wi-Fi signals are transmitted and received through antennas. What good is that if the Wi-Fi is too slow? Add more antennas. This method of using multiple antennas to improve wireless transmission quality is called multiple-input multiple-output (MIMO). It has become an essential element of wireless communication standards including IEEE 802.11n (first Wi-Fi standards that introduced MIMO), substantially increasing the theoretical Wi-Fi rate.
DEFINITION OF MIMO
A MIMO antenna system includes m transmit antennas and n receive antennas. Signals are transmitted and received by multiple antennas at the transmit end and the receiving end, enhancing communication quality. MIMO multiplies the capacity of a system channel by using multiple transmit antennas and receiving antennas under the same spectrum resources and antenna transmit power. With this obvious advantage, MIMO is regarded as the core technology for next-generation mobile communication.
The below table lists the theoretical air interface rates supported by 802.11n/ac with different numbers of spatial streams.
| MIMO (m×n) | IEEE 802.11n （64QAM，40MHz）（Mbit/s） | IEEE 802.11ac （256QAM，80MHz）（Mbit/s） |
|---|---|---|
| 1×1 | 150 | 433 |
| 2×2 | 300 | 867 |
| 3×3 | 450 | 1300 |
| 4×4 | 600 | 1733 |
MIMO FORMS
Based on the numbers of transmit antennas or receive antennas, MIMO can be classified into SISO, SIMO, MISO, and MIMO.
FUNCTIONS OF MIMO
MIMO can be sub-divided into three main categories: precoding, spatial multiplexing (or SM) and diversity coding.
Precoding is multi-stream beamforming, in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. In (single-layer) beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the received signal gain, by making signals emitted from different antennas add up constructively and to reduce the multipath fading effect. In the absence of scattering, beamforming results in a well-defined directional pattern, but in typical cellular conventional beams are not a good analogy. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas and precoding with multiple streams is used. Note that precoding requires knowledge of channel state information (CSI) at the transmitter.
Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower-rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams into (almost) parallel channels. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios (SNR). The maximum number of spatial streams is limited by the lesser number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers, known as space-division multiple access. By scheduling receivers with different spatial signatures, good separability can be assured.
Diversity coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods, a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain from diversity coding.
Spatial multiplexing can also be combined with precoding when the channel is known at the transmitter or combined with diversity coding when decoding reliability is in the trade-off.
SUMMARY
In summary, MIMO refers to the incorporation of multiple antennas into a single wireless device for the end purpose of increasing data transfer rates. This is achieved by aggregating signals from each of its antennas to clean up a noisy signal, leading to faster signal processing and faster data transmission overall.
