Large scale MIMO antenna technology

1、 What is large-scale MIMO antenna technology

Understanding large-scale antennas requires an understanding of beamforming technology. The traditional mode of communication is the electromagnetic wave propagation from a single antenna to a single antenna between the base station and the mobile phone. In beam forming technology, the base station has multiple antennas, which can automatically adjust the phase of the transmitted signals from each antenna, so that they can form the superposition of electromagnetic waves at the receiving point of the mobile phone, so as to improve the strength of the received signals.

From the perspective of the base station, the superposition effect generated by digital signal processing is like completing the construction of the virtual antenna pattern at the base station end, hence it is called "beamforming". Through this technology, the transmitted energy can be concentrated at the user's location without spreading in other directions, and the base station can monitor the user's signal and track it in real time, making the optimal transmission direction follow the user's movement, ensuring that the electromagnetic wave signal at the mobile phone reception point is in a superimposed state at any time.

The ordinary omnidirectional antenna covers the antenna with wave velocity shaping in all areas and concentrates energy in one direction. In practical applications, multi antenna base stations can also target multiple users simultaneously, construct different beams towards multiple target customers, and effectively reduce interference between each beam. This multi user beamforming effectively separates electromagnetic waves between different users in space, which is the foundation of large-scale antennas.

2、 Advantages of large-scale MIMO antennas

(1) Improve spectral efficiency. According to the law of large numbers, when the number of base station antennas continues to increase to infinity, the channels of different users show asymptotic orthogonality, which is called favorable channel conditions. In theory, interference between users can be completely eliminated, and noise tends to disappear as the antenna increases to infinity. At the same time, the spatial resolution of large-scale MIMO is significantly improved, and the extremely high degree of spatial freedom can meet the requirements for multiple users to communicate simultaneously on the same time-frequency resource. The above factors can significantly improve the system spectral efficiency;

(2) Improve energy efficiency. The use of large-scale antenna arrays improves the array benefits. Both uplink and downlink transmission can use smaller transmission power to achieve better communication quality, thus improving the system energy efficiency by several orders of magnitude.

(3) Simplify upper level user scheduling. Random matrix theory indicates that as the number of base station antennas increases sharply, some random channel characteristics begin to become deterministic, such as the singular value distribution of the channel matrix tending to be deterministic and the channel matrix tending to be benign. This phenomenon is called the channel hardening effect of large-scale MIMO. 

(4) Reduce system complexity. Large scale MIMO adopts simple linear pre encoders, such as maximum ratio transmission and zero forcing, to achieve performance similar to the optimal nonlinear pre encoding scheme (such as dirty paper encoding), greatly simplifying system complexity;

(5) Reduce system deployment costs. Large scale MIMO has extremely high spatial freedom and can be used for beamforming. If a constant envelope modulation signal is used, the peak to average ratio of the transmitted signal can be significantly reduced, thus enabling the use of low linear complexity, low cost, and low power hardware devices in the RF front-end, greatly reducing deployment costs; III. Basic Principles of Large Scale MIMO

Large scale MIMO improves data rate and link reliability by using a large number of antennas on the base station side. In the mMIMO system with large antenna array, the signal can be dynamically adjusted in the horizontal and vertical directions, so the energy can be more accurately focused on specific UEs, thus reducing inter cell interference and supporting spatial multiplexing between multiple UEs. The use of a large number of transceivers (TRX) and multiple antenna arrays can combine beam shaping with spatial multiplexing between users, greatly improving regional spectral efficiency. In short, a large-scale MIMO system is defined as:

1. Large number of transceivers (TRX)

2. Spatial reuse characteristics

3. Multi user scheduling (MU-MIMO)

4. A large number of high gain antenna arrays in the uplink and downlink directions

4、 System Architecture for Large Scale MIMO

The active antenna base station architecture supporting Massive MIMO is represented by three main functional modules: RF transceiver unit array, RF distribution network and multi antenna array. The RF transceiver unit array includes multiple transmitting and receiving units. The transmitting unit obtains baseband input and provides RF transmission output. The RF transmission output will be distributed to the antenna array through the RF distribution network, and the receiving unit will perform the work opposite to the operation of the transmitting unit operation.

RDN distributes the output signal to the corresponding antenna path and antenna unit, and distributes the input signal of the antenna to the opposite direction. The antenna array can include various implementations and configurations, such as polarization, spatial separation, etc. The physical location of the RF transceiver unit array, RF distribution network and antenna array may be different from the logical representation in the figure below, depending on the implementation.

5、 Application Scenario

The main application scenarios of Massive MIMO with centralized antenna configuration include urban coverage, wireless return, suburban coverage, and local hotspots. The urban coverage is divided into macro coverage and micro coverage (such as high-rise office buildings). Wireless backhaul mainly solves the data transmission problem between base stations, especially between macro stations and Small Cells. Suburban coverage mainly solves the wireless transmission problem in remote areas, and local hotspots mainly target areas with high user density such as large-scale events, concerts, shopping malls, outdoor gatherings, transportation hubs, etc.

Considering practical issues such as antenna size and installation, distributed antennas also have potential applications, with a focus on the collaboration mechanism and signaling transmission issues between antennas. The main application scenarios of large-scale antennas in the future can be divided into three main scenarios: outdoor macro coverage, high-rise coverage, and indoor coverage.