While parts of the world obsess over whether Huawei is safe to deploy within 5G infrastructures, others quietly get on with deploying its products. This is certainly the case for Huawei’s Distributed Massive MIMO (DM-MIMO) package aimed at indoor deployments, which is being rolled out by China Unicom at numerous public and enterprise sites, including stadiums and railway stations.
Quiet seems to be the operative word since Huawei has said little about the technology after a brief flurry of publicity almost a year ago in April 2020 when the company revealed the fruits of collaboration with Unicom by launching their joint DM-MIMO package. This was based on Huawei’s existing Lampsite Digital Indoor System (DIS), incorporating MIMO antenna arrays of up to 64T64R (64 transmit 64 receive) in size on top. That meant it had 64×64 antenna arrays supported by 64 transceiver channels on the chip.
Reporting on the launch we suggested then that Huawei was keen to underline the technological leadership that was at the root of the dilemma over its involvement in 5G deployments, with the DM-MIMO package reminding countries what they would be missing.
We noted that DM-MIMO represented a significant leap forward from established legacy distributed antenna systems (DAS) and that while it presented significant challenges, Huawei had appeared to be at least well on the way to overcoming them. As we observed, high bandwidth indoor wireless communication is work in progress whatever technology is adopted and a key point is that China has decided to push 5G rather than WiFi as the wireless technology most likely to meet objectives or performance, coverage and robustness.
China’s focus, or Huawei’s at any rate, can be traced back at least to 2013 and that year’s Mobile World Congress in Barcelona. Huawei then first unveiled its LampSite indoor coverage portfolio, at this stage unaided by any form of M-MIMO. It used time division duplex (TD)-LTE technology, with the innovation being introduction of baseband sharing, allowing a single fiber to serve several indoor cells. This saved up to 87% of the fiber typically used for indoor deployments, but the main benefit was the ability to split cells using SFN (single frequency network) technology to balance loads and mitigate interference.
It was not until 2018 though that Huawei proposed the transition to its current DIS approach featuring DM-MIMO to replace the traditional more static DAS (distributed antenna system) technology. Under DAS, antenna nodes are split into smaller units that are distributed in space with the effect of reducing power consumption and increasing reliability within a given coverage area. This is possible because a group of distributed low power antennas replacing a single high power one wastes less power overcoming fade and other losses, because on average users are closer to at least one transmitter. Because each device is within line of sight of an antenna more often, average fade is reduced.
However, DAS had some significant disadvantages, especially indoors, including higher cost compared with single antenna systems, complexity of backhaul and lack of scalability to very high data rates or user numbers. This prompted Huawei to develop and promote DM-MIMO.
To achieve the promised performance and capacity, DM-MIMO builds on the gains already achieved by centralized massive MIMO. These include beamforming gain, resulting from the coherent transmission and reception, as signals from all sources are constructively superposed, rather than destructively interfering. This is achieved by focusing the radio waves into beams in a controlled way so that multiple users can be served in given time and frequency slots. It has become possible by use of higher frequencies and therefore shorter wavelengths at which signals can be transmitted and received over correspondingly smaller antenna.
The initial architecture proposed for M-MIMO was colocated M-MIMO, or CM-MIMO, where base station antenna are confined in a single large array, up to hundreds or potentially a few thousand at higher frequencies. Then DM-MIMO came along where the antenna are distributed physically in small clusters within the coverage area and interconnected over a high speed backhaul.
DM-MIMO has several advantages, including easier network planning and also enhanced coverage, especially indoors, sharing some of the benefits of DAS but with far greater scalability and efficiency.
It also increases capacity further over CM-MIMO by allowing greater scope for spatial multiplexing gain in addition to the beamforming. As the name implies, this ‘multiplexes’ data into multiple streams known as layers, each one sent via a different antenna. It exploits the spatial differences between beam trajectories to enable the information to be recovered from multiple beams and is similar in principle to frequency reuse in SFN. In effect, each signal occupies a space in a cell at a given time. As in all multiplexing schemes, the data received is the sum of the data sent so that the whole stream can then be reconstructed.
Spatial multiplexing has been part of the 3GPP standard from the start of 4G but with growing sophistication. Support for multiantenna configurations came in with LTE Release 10, with support for more elaborate coordination. Improved support for massive antenna arrays than came with Release 13 and been refined since, allowing scope for the full 3D configuration of arrays so that individual antenna can be placed relative to others at different elevations and horizontal angles, or azimuths, sometimes referred to as ‘full dimension MIMO’.
This opened the door to beamforming, which requires such individual orientation of antenna. The spatial degrees of freedom also ushered in another level of MIMO, Multiuser (MU) MIMO, where multiple devices, each of which can have multiple antenna, can communicate with each other independently within a cell.
Huawei has exploited these principles in DM-MIMO to create a distributed approach with DIS that has also been called ‘cell-free’ wireless, although it could be said to be just one large cell extended indoors through MIMO in given allocated spectrum. This is now being rolled out across China in stadiums and stations in particular, with a few deployments elsewhere, such as one by Sunrise in Switzerland.