Massive MIMO

One of the technologies that will bolster 5G cellular is the use of massive MIMO (multiple-input, multiple-output) antenna arrays. Massive MIMO is an extension of smaller MIMO antennas that have been use for several years. For example, home WiFi routers now routinely use multiple antennas to allow for easier connections to multiple devices. Basic forms of the MIMO technology have been deployed in LTE cell sites for several years.

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.

When Customers Use Their Data

In a recent disturbing announcement ,Verizon Wireless will be disconnecting service to 8,500 rural customers this month for using too much data on their cellphones. The customers are scattered around 13 states and are a mix those with both unlimited and limited data plans.

Verizon justifies this because these customers are using data where Verizon has no direct cell towers, meaning that these customers are roaming on cellular data networks owned by somebody else. Since Verizon pays for roaming the company say that these customers are costing them more in roaming charges than what the company collects in monthly subscription fees.

Verizon may well have a good business case for discontinuing these particular data customers if they are losing money on each customer. But the act of disconnecting them opens up a lot of questions and ought to be a concern to cellular customers everywhere.

This immediately raises the question of ‘carrier of last resort’. This is a basic principle of utility regulation that says that utilities, such as traditional incumbent telephone companies, must reasonably connect to everybody within their service territory. Obviously cellular customers don’t fall under this umbrella since the industry is competitive and none of the cellular companies have assigned territories.

But the lines between cellular companies and telcos are blurring. As AT&T and Verizon take down rural copper they are offering customers a wireless alternative. But in doing so they are shifting these customers from being served by a regulated telco to a cellular company that doesn’t have any carrier of last resort obligations. And that means that once converted to cellular that Verizon or AT&T would be free to then cut these customers loose at any time and for any reason. That should scare anybody that loses their rural copper lines.

Secondly, this raises the whole issue of Title II regulation. In 2015 the FCC declared that broadband is a regulated service, and that includes cellular data. This ruling brought cable companies and wireless companies under the jurisdiction of the FCC as common carriers. And that means that customers in this situation might have grounds for fighting back against what Verizon is doing. The FCC has the jurisdiction to regulate and to intervene in these kinds of situations if they regulate the ISPs as common carriers. But the current FCC is working hard to reverse that ruling and it’s doubtful they would tackle this case even if it was brought before them.

Probably the most disturbing thing about this is that it’s scary for these folks being disconnected. Rural homes do not want to use cellular data as their only broadband connection because it’s some of the most expensive broadband in the world. But many rural homes have no choice since this is their only broadband alternative to do the things they need to do with broadband. While satellite data is available almost everywhere, the incredibly high latency on satellite data means that it can’t be used for things like maintaining a connection to a school server to do homework or to connect to a work server to work at home.

One only has to look at rural cellular networks to understand the dilemma many of these 8,500 households might face. The usable distance for a data connection from a cellular tower is only a few miles at best, much like the circles around a DSL hub. It is not hard to imagine that many of these customers actually live within range of a Verizon tower but still roam on other networks.

Cellular roaming is an interesting thing. Every time you pick up your cellphone to make a voice or data connection, your phone searches for the strongest signal available and grabs it. This means that the phones of rural customers that don’t live right next to a tower must choose between competing weaker signals. Customers in this situation might be connected to a non-Verizon tower without it being obvious to them. Most cellphones have a tiny symbol that warns when users are roaming, but since voice roaming stopped being an issue most of us ignore it. And it’s difficult or impossible on most phones to choose which tower to connect to. Many of these customers being disconnected might have always assumed they actually were using the Verizon network. But largely it’s not something that customers have much control over.

I just discussed yesterday how we are now in limbo when it comes to regulating the broadband practices of the big ISPs. This is a perfect example of that situation because it’s doubtful that the customers being disconnected have any regulatory recourse to what is happening to them. And that bodes poorly to rural broadband customers in general – just one more reason why being a rural broadband customer is scary.