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Massive MIMO (M-MIMO or MM for short), as the core technology of 5G, is carried on the AAU, and the antenna array inside the AAU is the most important carrier for realizing Massive MIMO.
This article will introduce several basic concepts: vibrator, antenna, channel, stream, MU-MIMO, SU-MIMO, and their relationship.
1. What is an oscillator?
The most basic function of an antenna is to convert the way energy is transmitted. For signals transmitted by a base station, the antenna converts the high-frequency oscillating current of the transmitter into electromagnetic waves that can propagate in free space.
The antenna emits electromagnetic waves through the internal oscillator. The capacity of a single oscillator is limited, and the emission direction is difficult to focus, so the antenna is generally composed of multiple oscillators.
As shown in the figure above, after opening the antenna cover, you can see a number of X-shaped devices arranged inside. These are oscillators, also called antenna elements. An X represents two oscillators with +45° and -45° polarization directions.
2. How do oscillators form antennas/antenna arrays?
For a general antenna, its interior is composed of multiple oscillators. Through the superposition of the transmitted energy on these oscillators, the antenna gain can reach 13 to 17dBi. When we draw an antenna schematic, we always draw some X-shaped symbols on it to represent the oscillators inside.
For 5G AAU, due to the widespread use of M-MIMO technology, the number of oscillators used in the internal integrated antenna is larger, and multiple oscillators are neatly arranged, ready for battle, so it is also called an antenna array. For AAU in the Sub6G frequency band, the industry generally uses 192 oscillators.
Although there are many antenna elements, the capability of a single element is too weak. Furthermore, if each element is connected to the power amplifier inside the AAU to send signals independently, the implementation would be too complicated. Therefore, the industry generally groups three or six elements into a group to form a logically single antenna.
Taking the M-MIMO antenna element arrangement diagram below as an example, there are 12 rows in the horizontal direction and 8 columns in the vertical direction, plus ±45° dual polarization, there are a total of 12x8x2=192 elements.
Every three oscillators form a group, called an antenna, so the M-MIMO AAU has a total of 192/3 = 64 antennas. If every 6 oscillators form an antenna, the AAU has 192/6 = 32 antennas.
Currently, the mainstream 5G AAUs in the industry all have 192 oscillators, and there are two models: 64 antennas and 32 antennas. Among them, the 64-antenna product has better performance.
In this case, why not make 64 antennas? This involves considerations related to the number of channels and cost.
3. What is a channel?
No matter how many antennas there are, they are passive devices and cannot transmit signals directly.
Therefore, the AAU needs to connect these antennas to its internal RF links, ultimately forming 64 or 32 channels for transmitting signals.
The AAU in the Sub6G band uses full digital beamforming, and it can be considered that the number of antennas, transmission channels, and power amplifiers (PAs) are the same.
Obviously, the more antennas and channels there are, the more power amplifiers there are inside the AAU, the greater the consumption of baseband resources will be, and the higher the cost of the equipment will be.
Therefore, 64-antenna equipment is mainly used in densely populated urban areas, while 32 antennas can meet the needs in ordinary urban areas and suburbs.
For more remote areas, the capacity requirement is not high, and the main issue is coverage. In this case, even Massive MIMO is not needed, and an 8-port RRU connected to the antenna can be directly used.
4. AAU uses so many oscillators, antennas and channels. What is the antenna gain?
Below we divide it into three aspects: channel gain, beamforming gain, and business gain.
As mentioned above, a single channel is generally composed of 3 or 6 oscillators, and the combined gain generated by these oscillators is called channel gain.
Beamforming gain is also called array gain. The more channels there are, the stronger the beamforming capability is, and the greater the gain is. The algorithm is: array gain = 10Log (number of channels/2). The reason for dividing by two is that the polarization directions of these channels are different, half of them are polarized at +45°, and the other half are polarized at -45°. Polarization cannot produce gain superposition, so it can only be calculated based on half the number of channels.
The service gain is actually the maximum gain of the antenna as a whole, which is obtained by adding the channel gain and the beamforming gain.
Generally speaking, the service gain of Massive MIMO is about 3dB higher than that of ordinary antennas. The multi-user multi-stream transmission capability brought by multiple transmit channels and beamforming, which will be discussed below, is the core competitiveness of M-MIMO.
5. Multiple users and empty split number
Intuitively, multi-user spatial splitting is to use different beams to send different data streams to different users in space.
For example, if a base station supports 16 streams, it can use the same resources to send 16 different data paths in space. These 16 data paths can be distributed to 8 users, with each user receiving two streams (i.e., 2x2 MIMO), or they can be distributed to 16 users, with each user receiving only one stream. In this way, although the rate experienced by each user varies greatly, the throughput of the entire cell can reach its peak value.
This technology of pairing multiple users to multiplex the same time-frequency resources to achieve multi-streaming is called MU-MIMO (Multi-User MIMO), while the multi-streaming within a user is the traditional SU-MIMO (Single-User MIMO).
Currently, mainstream 5G mobile phones can support 4-antenna reception, so they can form up to 4 independent propagation paths with the base station. That is, for a single mobile phone, SU-MIMO can support up to 4 stream transmissions.
5G AAU can implement both MU-MIMO and SU-MIMO at the same time to maximize the traffic of the entire cell.
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