Category Archives: 5G

Cellular Multi-User MIMO: A Technology Whose Time has Come

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).

Trial with a 12-element circular array by ArrayComm, in the late nineties.


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.

How Much does Massive MIMO Improve the Spectral Efficiency?

It is often claimed in the academic literature that Massive MIMO can greatly improve the spectral efficiency. What does it mean, qualitatively and quantitatively? This is what I will try to explain.

With spectral efficiency, we usually mean the sum spectral efficiency of the transmissions in a cell of a cellular network. It is measured in bit/s/Hz. If you multiply it with the bandwidth, you will get the cell throughput measured in bit/s. Since the bandwidth is a scarce resource, particularly at the frequencies below 5 GHz that are suitable for network coverage, it is highly desirable to improve the cell throughput by increasing the spectral efficiency rather than increasing the bandwidth.

A great way to improve the spectral efficiency is to simultaneously serve many user terminals in the cell, over the same bandwidth, by means of space division multiple access. This is where Massive MIMO is king. There is no doubt that this technology can improve the spectral efficiency. The question is rather “how much?”

Earlier this year, the joint experimental effort by the universities in Bristol and Lund demonstrated an impressive spectral efficiency of 145.6 bit/s/Hz, over a 20 MHz bandwidth in the 3.5 GHz band. The experiment was carried out in a single-cell indoor environment. Their huge spectral efficiency can be compared with 3 bit/s/Hz, which is the IMT Advanced requirement for 4G. The remarkable Massive MIMO gain was achieved by spatial multiplexing of data signals to 22 users using 256-QAM. The raw spectral efficiency is 176 bit/s/Hz, but 17% was lost for practical reasons. You can read more about this measurement campaign here:

256-QAM is generally not an option in cellular networks, due to the inter-cell interference and unfavorable cell edge conditions. Numerical simulations can, however, predict the practically achievable spectral efficiency. The figure below shows the uplink spectral efficiency for a base station with 200 antennas that serves a varying number of users. Interference from many tiers of neighboring cells is considered. Zero-forcing detection, pilot-based channel estimation, and power control that gives every user 0 dB SNR are assumed. Different curves are shown for different values of τc, which is the number of symbols per channel coherence interval. The curves have several peaks, since the results are optimized over different pilot reuse factors.

Spectral efficiency
Uplink spectral efficiency in a cellular network with 200 base station antennas.

From this simulation figure we observe that the spectral efficiency grows linearly with the number of users, for the first 30-40 users. For larger user numbers, the spectral efficiency saturates due to interference and limited channel coherence. The top value of each curve is in the range from 60 to 110 bit/s/Hz, which are remarkable improvements over the 3 bit/s/Hz of IMT Advanced.

In conclusion, 20x-40x improvements in spectral efficiency over IMT Advanced are what to expect from Massive MIMO.