Hello everyone！
Today I'm going to share with you about MIMO technology, which is a major breakthrough in the history of communications.
What is MIMO?
MIMO is an antenna technology for wireless communication in which multiple antennas are used at both the source (transmitter) and the destination (receiver) to minimize bit error rates and achieve optimized transmission speeds.
Background of Multi-Antenna Technology
The effective way to increase capacity is to use multi-antenna technology when the SINR is high. Multi-antenna technology can make better use of spatial resource, it can increase transmission capacity of a wireless communication System without increasing the transmit power and bandwidth.
Classification of Multi-antenna Technology
According to different purposes, different multi-antenna techniques can be used:
1. Diversity mode. Multiple antennas can be used to provide diversity to resist fading of radio channels. In this case, fading on different antennas should have a low correlation.
Transmit the same data on different antennas.
Diversity mode can indirectly increase the capacity.
Number of receiving antennas may be less than the number of transmitting antennas.
2. Spatial multiplexing. Transmitter and receiver using two or more antennas, Spatial multiplexing can be performed.
Transmit the different data on different antennas.
Diversity mode can directly increase the capacity.
Number of receiving antennas is not less than the number of transmitting antennas.
3. Beamforming. Multi-antenna can perform beamforming, such as the maximizing objective of the antenna gain in the specified direction or suppress specific main interference signal. According to the correlation between different antennas, system can adopt different beamforming methods.
The data are weighted and sent by transmitter forming narrow transmit beamforming energy and aim at the target user thereby improving demodulation SINR of the target user and reducing the interference of
non-target USers.
MIMO Overview
1. MIMO is the key technology of LTE system, it relates to the use of multiple antennas at both the transmitter (Multiple Input) and receiver (Multiple Output).
2. Theoretically, the channel capacity can be increased linearly by the number of transmitter and receiver, so the channel capacity in MIMO mode is larger than in single antenna mode.
3. MIMO can better use space resource and improves spectrum efficiency, so the signal obtain array gain, diversity gain, multiplexing gain and interference cancellation gain, which can improves system capacity, Coverage and user throughput.
Figure 1 MIMO Overview
Radio Channel Access Mode
SISO (Single input single output)
SISO is the basic radio channel access mode, it is the default method in radio communication system at the beginning.
MlSO (Diversity transmitting mode)
MISO, multiple input single output, in other words, it means two or multi-transmitter but one receiver. Because the same information transmitted by different antenna, which can enhance the robustness against fading and improve the transmission performance in low SNR scenarios. It can be used at the same rate with lower transmission power but cannot improves the transmission rate.
SIMO (Diversity receiving mode)
SIMO is opposition with MISO. SIMO means single input multiple output, namely two or more receivers but one transmitters, usually refers to the receive diversity. SIMO is similar to transmit diversity which is particularly suitable for low SNR scenarios. In this scenario, if you use dual receiver, in theory, the gain will be 3dB. Compared with transmit diversity, the cell edge coverage has improved because the SNR has improved when using receive diversity.
MIMO (Multiple input multiple output)
MIMO, two or more receivers and two or more transmitters. This model is not only the simple sum of SIMO and MISO, because multiple data streams are simultaneously transmitted in frequency domain and time domain to fully utilize the radio channel in different paths. For MIMO system, the number of multiple receivers must be at least equal to transmit data stream. (Notes: Number of transmit data stream may be unequal to number of transmit antennal).
SU-MIMO/MU-MIMO Introduction
In the picture above, the transmitter and receiver are not specified to belong to one base station/one UE or more, so we have different scenarios on this page.
The purpose of the pre-coding is based on the optimal transmission characteristics of the channel, so that when the signal is received, it can more easily be separated and then restored to the original data stream.
The intention of SU-MIMO is to increase the rate of a single user which would also enhance the capacity of the cell. The figure shows that two different data streams are assigned to the same UE. SU-MIMO can be used for uplink or downlink.
The second scenario is multi-user MIMO(MU-MIMO), which is just implemented in uplink nowadays. MU-MIMO can't increase the rate of a single user but can enhance cell capacity gain, it is similar to SU-MIMO or better. The figure shows that two data streams come from different UEs, and there is not physical connection between two transmitters, which means no chance to mix two data flows to optimize channel coding. However the spatial separation method improves the opportunity for eNodeB to collect irrelevance channel of UE. Thus, compared to the pre-encoded SU-MIMO, it maximizes the possible capacity gain. Another advantage of MU-MIMO is the UE does not raise the power of the transmitter, but the cell capacity stills can improve.
The third scenarios is Co-MIMO, the essence of this mode is the end point of two transmitters have been associated. The figure shows that in DL scenario, through sharing data stream and pre-coding spatial separate antenna cooperation, two eNodeBs can better communicate with at least one UE. This is the best methods in the cell edge, where the SINR will suffer but the radio channel is non-related, so it will potential enhance the system coverage. Co-MIMO can also be used in the uplink by theory, but it is difficult to be applied because there is no physical connection between UE. That's why Co-MIMO pre-coding is not used in MU-MIMO uplink. Uplink Co-MIMO also names virtual MIMO. There is no Co-MIMO defined in 3GPP R8 protocol, it will be studied and achieved in 3GPP R9/R10, to satisfy the requirement of IMT-Advance 4G.
Advantages of MIMO
1. Array gain: MIMO system Uses the signal correlation and noise non-correlation on each antenna, improves the average SINR of the combined signal, and obtains the performance gain.
2. Diversity gain: MIMO system against the channel fading of the effect of performance, reduce the fading range of combined signal and obtain performance gain base on the signal deep fading non-correlation on each antenna.
3. Multiplexing gain: On the premise of same bandwidth and same total transmission power, increase the number of spatial channel (Increase the number of antennas) and obtain throughput gain.
4. Interference cancellation gain: Make use of IRC(Interference Rejection Combining) and the other multiple antenna interference rejection algorithms, obtain the gain on interference scenarios.
That's the message I'm sharing, and you're welcome to read and comment.
