5G cellular networks are supposed to deal with many challenging communication scenarios where today’s cellular networks fall short.  In this post, we have a look at one such scenario, where Massive MIMO is key to overcome the challenges.

The METIS research project has identified twelve test cases for 5G connectivity. One of these is the “Dense urban information society”, which is

“…concerned with the connectivity required at any place and at any time by humans in dense urban environments. We here consider both the traffic between humans and the cloud, and also direct information exchange between humans or with their environment. The particular challenge lies in the fact that users expect the same quality of experience no matter whether they are at their workplace, enjoying leisure activities such as shopping, or being on the move on foot or in a vehicle.”

Source: METIS, deliverable D1.1 “Scenarios, requirements and KPIs for 5G mobile and wireless system”

Hence, the challenge is to provide ubiquitous connectivity in urban areas, where there will be massive user loads in the future: up to  200,000 devices per km² is predicted by METIS. In their test case, each device requests one data packet per minute, which should be transferred within one second. Hence, there is on average up to 200,000/60 = 3,333 users active per km² at any given time.

This large number of users is a challenge that Massive MIMO is particularly well-suited for. One of the key benefits of the Massive MIMO technology is the high spectral efficiency that it achieves by spatial multiplexing of tens of user per cell. Suppose, for example, that the cells are deployed in a hexagonal pattern with a base station in each cell center, as illustrated in the figure. How many simultaneously active users will there be per cell in the dense urban information society? That depends on the area of a cell. An inter-site distance (ISD) of 0.25 km is common in contemporary urban deployments. In this case, one can show that the area covered by each cell is √3×ISD²/2 = 0.05 km².

The number of active users per cell is then obtained by multiplying the cell area with the user density. Three examples are provided in the table below:

	103 users/km2	104 users/km2	105 users/km2
Total number of users per cell	54	540	5400
Average active users per cell	0.9	9	90

Recall that 1/60 of the total number of users are active simultaneously, in the urban information society test case. This gives the numbers in the second row of the table.

From this table, notice that there will be tens of simultaneously active users per cell, when the user density is above 10,000 per km². This is a number substantially smaller than the 200,000 per km² predicted by the METIS project. Hence, there will likely be many future urban deployment scenarios with sufficiently many users to benefit from Massive MIMO.

A fraction of these users can (and probably will) be offloaded to WiFi-like networks, maybe operating at mmWave frequencies. But since local-area networks provide only patchy coverage, it is inevitable that many users and devices will rely on the cellular networks to achieve ubiquitous connectivity, with the uniform quality-of-service everywhere.

In summary, Massive MIMO is what we need to realize the dream of ubiquitous connectivity in the dense urban information society.

Both the number of devices with wireless connection and the traffic that they generate have steadily grown since the early days of cellular communications. This continuously calls for improvements in the area capacity [bit/s/km²] of the networks. The use of adaptive antenna arrays was identified as a potential capacity-improving technology in the mid-eighties. An early uplink paper was “Optimum combining for indoor radio systems with multiple users” from 1987 by J. Winters at Bell Labs. An early downlink paper was “The performance enhancement of multibeam adaptive base-station antennas for cellular land mobile radio systems” by S. C. Swales et al. from 1990.

The multi-user MIMO concept, then called space-division multiple access (SDMA), was picked up by the industry in the nineties. For example, Ericsson made field-trials with antenna arrays in GSM systems, which were reported in “Adaptive antennas for GSM and TDMA systems” from 1999. ArrayComm filed an SDMA patent in 1991 and made trials in the nineties. In cooperation with the manufacturer Kyocera, this resulted in commercial deployment of SDMA as an overlay to the TDD-based Personal Handy-phone System (PHS).

Given this history, why isn’t multi-user MIMO a key ingredient in current cellular networks? I think there are several answers to this question:

1. Most cellular networks use FDD spectrum. To acquire the downlink channels, the SDMA research first focused on angle-of-arrival estimation and later on beamforming codebooks. The cellular propagation environments turned out to be far more complicated than such system concepts easily can handle.
2. The breakthroughs in information theory for multi-user MIMO happened in the early 2000s, thus there was no theoretical framework that the industry could use in the nineties to evaluate and optimize their multiple antenna concepts.
3. In practice, it has been far easier to increase the area capacity by deploying more base stations and using more spectrum, rather than developing more advanced base station hardware. In current networks, there is typically zero, one or two users per cell active at a time, and then there is little need for multi-user MIMO.

Why is multi-user MIMO considered a key 5G technology? Basically because the three issues described above have now changed substantially. There is a renewed interest in TDD, with successful cellular deployments in Asia and WiFi being used everywhere. Massive MIMO is the refined form of multi-user MIMO, where the TDD operation enables channel estimation in any propagation environment, the many antennas allow for low-complexity signal processing, and the scalable protocols are suitable for large-scale deployments. The technology can nowadays be implemented using power-efficient off-the-shelf radio-frequency transceivers, as demonstrated by testbeds. Massive MIMO builds upon a solid ground of information theory, which shows how to communicate efficiently under practical impairments such as interference and imperfect channel knowledge.

Maybe most importantly, spatial multiplexing is needed to manage the future data traffic growth. This is because deploying many more base stations or obtaining much more spectrum are not viable options if we want to maintain network coverage—small cells at the street-level are easily shadowed by buildings and mm-wave frequency signals do not propagate well though walls. In 5G networks, a typical cellular base station might have tens of active users at a time, which is a sufficient number to benefit from the great spectral efficiency offered by Massive MIMO.