The benefit of MIMO antenna technique
This post intended to briefly
describe the benefit of MIMO antenna technique using in mobile communication system like 4G/LTE and 5G.
1. Radio Channel Access Mode
a. SISO: Single Input Single Output, is the access mode of an important radio channel through the use of a single antenna at the transmitter and a single antenna at the receiver. SISO will serve as a baseline for evaluating the performance improvements that can be achieved when a larger number of antennas are used.
b. SIMO: Single Input Multiple Output, describes the receive diversity, a method that generally does not depend on the technology being used. The SIMO is appropriate when the signal-to-noise ratio (SNR) is low at the cell edge coverage or due to fading and it will give no any improvement in data rates, except for the improvement of the signal strength in situations where the SNR is low.
c. MISO: Multiple Input Single Output, is a transmitting diversity technique and requires only one receiving antenna. It has been used for some time in mobile systems, and like as SIMO, MISO it will not improve any data speeds. MISO can increase the directivity (focusing) of the transmitted signal in one direction so that the coverage could be improved.
d. MIMO: Multiple Input Multiple Output. Multiple input or multiple signal IN means multiple transmit antennas to the wireless channel, multiple output or multiple signal OUT means multiple receive antenna from the wireless channel.
2. What is MIMO
In wireless communications, MIMO refers to a wireless channel with multiple inputs and multiple outputs. A wireless communication system utilizing multiple transmit antennas and multiple receive antennas over the wireless channel is often referred to as a MIMO system.
With MIMO, multiple spatially separated users are catered for by the antenna array in the same time and frequency resource. Data is divided into independent MIMO data streams that are transmitted simultaneously.
A x B MIMO refers to the case with A transmit antennas in the base station and B receive antennas in the device. Many current devices and networks utilize 2x2 MIMO where two parallel data streams can be transmitted. Some of the latest devices and networks support 4x4 MIMO (four receive antennas) which doubles the peak rate compared to 2x2MIMO and increases the average rates typically by +50%.
3. What can gain from MIMO
MIMO technology could provide spatial multiplexing that can increase the system throughput, beamforming that can direct beam to the user, diversity that can improve the system performance etc.
a. Spatial Multiplexing
- Multiplexing gain (also referred to as spatial multiplexing gain) increases the benefit of MIMO far beyond what can be achieved by beamforming.
- The MIMO system maps independent data streams over multiple data pipes to increase throughput. Each data stream is processed with precoding before being sent over all the transmission antennas.
- Multiplexing gain does not require additional spectral bandwidth or power.
- With good channel conditions (little interference), it is more efficient to share the transmission power on multiple orthogonal (independent) data pipes. Since multiple data streams are transmitted simultaneously, the total transmission power is shared among multiple data streams. Hence, the total signal-to-noise ratio (SNR) is also shared among multiple data streams, resulting in a lower SNR on each individual data stream. If the total SNR is low, the SNR on each individual stream will be small and the throughput on each data stream will suffer. This indicates that the spatial multiplexing gain for MIMO is mostly achieved at the high SNR region, where good throughput can be achieved on each of the independent data streams.
- MU-MIMO is to increase the capacity of a user, while MU-MIMO is to increase the throughput of more than one user.
b. Beamforming
Beamforming is the use of multiple radiating elements to direct a strong beam signal to a specific desired direction by varying of the amplitude and phase that feed those elements (waves that overlapped will create constructive interference to make the signal stronger in one direction, and destructive interference to weaken the signal in the other direction). The greater the number of radiating elements that make up the antenna, the smaller the signal beam will be, and the main beam / main lobe will be more oriented (narrower, more directive). In the same time side lobes are weaken and reducing the interference between each other.
A weak wireless channel has very high interference. In this case, it is better to concentrate all (and finite) transmission power on the strongest pipe so more data can be delivered to the receiver. By doing so, the MIMO system increases the signal SNR (and increases system coverage) compared to a non-MIMO system. This increase of SNR is referred to as the (antenna) array gain.
From the spatial domain point of view, the strongest transmission pipe is selected via transmitter/receiver beamforming. With arrays of antennas at the transmitter and receiver, beamforming allows the MIMO system to increase the average received SNR by focusing signals in the correct direction at the transmitter side and coherently combining received signals at the receiver. To combine signals coherently, the system needs wireless channel knowledge at the transmitter and receiver.
