Massive MIMO differs from current technology by the use of big arrays of antennas. For example, Sprint, along with Nokia demonstrated a massive MIMO transmitter in 2017 that used 128 antennas, with 64 for receive and 64 for transmit. Sprint is in the process of deploying a much smaller array in cell sites using the 2.5 GHz spectrum.
Massive MIMO can be used in two different ways. First, multiple transmitter antennas can be focused together to reach a single customer (who also needs to have multiple receivers) to increase throughput. In the Sprint trial mentioned above Sprint and Nokia were able to achieve a 300 Mbps connection to a beefed-up cellphone. That’s a lot more bandwidth than can be achieved from one transmitter, which at the most could deliver whatever bandwidth is possible on the channel of spectrum being used.
The extra bandwidth is achieved in two ways. First, using multiple transmitters means that multiple channels of the same frequency can be sent simultaneously to the same receiving device. Both the transmitter and receiver must have the sophisticated and powerful computing power to coordinate and combine the multiple signals.
The bandwidth is also boosted by what’s called precoding or beamforming. This technology coordinates the signals from multiple transmitters to maximize the received signal gain and to reduce what is called the multipath fading effect. In simple terms the beamforming technology sets the power level and gain for each separate antenna to maximize the data throughput. Every frequency and its channel operates a little differently and beamforming favors the channels and frequency with the best operating capabilities in a given environment. Beamforming also allows for the cellular signal to be concentrated in a portion of the receiving area – to create a ‘beam’. This is not the same kind of highly concentrated beam that is used in microwave transmitters, but the concentration of the radio signals into the general area of the customer means a more efficient delivery of data packets.
The cellular companies, though, are focused on the second use of MIMO – the ability to connect to more devices simultaneously. One of the key parameters of the 5G cellular specifications is the ability of a cell site to make up to 100,000 simultaneous connections. The carriers envision 5G is the platform for the Internet of Things and want to use cellular bandwidth to connect to the many sensors envisioned in our near-future world. This first generation of massive MIMO won’t bump cell sites to 100,000 connections, but it’s a first step at increasing the number of connections.
Massive MIMO is also going to facilitate the coordination of signals from multiple cell sites. Today’s cellular networks are based upon a roaming architecture. That means that a cellphone or any other device that wants a cellular connection will grab the strongest available cellular signal. That’s normally the closest cell site but could be a more distant one if the nearest site is busy. With roaming a cellular connection is handed from one cell site to the next for a customer that is moving through cellular coverage areas.
One of the key aspects of 5G is that it will allow multiple cell sites to connect to a single customer when necessary. That might mean combining the signal from a MIMO antenna in two neighboring cell sites. In most places today this is not particularly useful since cell sites today tend to be fairly far apart. But as we migrate to smaller cells the chances of a customer being in range of multiple cell sites increases. The combining of cell sites could be useful when a customer wants a big burst of data, and coordinating the MIMO signals between neighboring cell sites can temporarily give a customer the extra needed bandwidth. That kind of coordination will require sophisticated operating systems at cell sites and is certainly an area that the cellular manufacturers are now working on in their labs.