SU-MIMO vs. MU-MIMO
In legacy LTE, the term MIMO usually refers to Single User MIMO (SU-MIMO). In Single User MIMO, both the base station and UE have multiple antenna ports and antennas, and multiple data streams are transmitted simultaneously to the UE using same time/frequency resources, doubling (2×2 MIMO), or quadrupling (4×4 MIMO) the peak throughput of a single user.
In MU-MIMO, base station sends multiple data streams, one per UE, using the same time-frequency resources. Hence, MU-MIMO increases the total cell throughput, i.e. cell capacity. The base station has multiple antenna ports, as many as there are UEs receiving data simultaneously, and one antenna port is needed in each UE.
The most commonly seen definition is that mMIMO is a system where the number of antennas exceeds the number of users. In practice, massive means there are 32 or more logical antenna ports in the base station It is expected that NEMs will start with a maximum of 64 logical antenna ports in 5G.
In MU-MIMO/mMIMO, the base station applies distinct precoding for the data stream of each UE where the location of the UE, as well as the location of all the other UEs, are taken into account to optimize the signal for target UE and at the same time minimize interference to the other UEs. To do this, the base station needs to know how the downlink radio channel looks like for each of the UEs.
Beamforming – principle of operation
Terms beamforming and mMIMO are sometimes used interchangeably. One way to put it is that beamforming is used in mMIMO, or beamforming is a subset of mMIMO. In general, beamforming uses multiple antennas to control the direction of a wave-front by appropriately weighting the magnitude and phase of individual antenna signals in an array of multiple antennas. That is, the same signal is sent from multiple antennas that have sufficient space between them (at least ½ wavelength). In any given location, the receiver will thus receive multiple copies of the same signal. Depending on the location of the receiver, the signals may be in opposite phases, destructively averaging each other out, or constructively sum up if the different copies are in the same phase, or anything in between.
Beam-based coverage measurements in 5G
The coverage is beam-based in 5G, not cell based. There is no cell-level reference channel from where the coverage of the cell could be measured. Instead, each cell has one or multiple Synchronization Signal Block Beam (SSB) beams, SSB beams are static, or semi-static, always pointing to the same direction. They form a grid of beams covering the whole cell area. The UE searches for and measure the beams, maintaining a set of candidate beams. The candidate set of beams may contain beams from multiple cells. The metrics measured are SS-RSRP, SS-RSRQ, and SS-SINR for each beam. Physical Cell ID (PCI) and beam ID are the identifications separating beams from each other. In field measurements, these metrics can be collected both with scanning receivers and test UEs. Hence, SSB beams show up as kind of new layer of mini-cells inside each cell in the field measurements.
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