Are There Any Massive MIMO Books?

9781107175570_200x_fundamentals-of-massive-mimoI regularly get the question “are there any Massive MIMO books?”. So far my answer has always been “no”, but now I can finally give a positive answer.

My colleagues Erik G. Larsson and Hien Quoc Ngo have written a book entitled “Fundamentals of Massive MIMO” together with Thomas L. Marzetta and Hong Yang at Bell Labs, Nokia. The book is published this October/November by Cambridge University Press.

I have read the book and I think it serves as an excellent introduction to the topic. The text is suitable for graduate students, practicing engineers, professors, and doctoral students who would like to learn the basic Massive MIMO concept, results and properties. It also provides a clean introduction to the theoretical tools that are suitable for analyzing the Massive MIMO performance.

I personally intend to use this book as course material for a Master level course on Multiple-antenna communications next year. I recommend other teachers to also consider this possibility!

A preview of the book can be found on Google Books:

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:

http://www.bristol.ac.uk/news/2016/may/5g-wireless-spectrum-efficiency.html

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.

Are 1-bit ADCs Sufficient?

A series of recent papers,

suggest the use of 1-bit ADCs in Massive MIMO base station receivers. Important studies of a concept, that offers great potential for cost saving and simplification of transceiver hardware.

One-bit ADCs
One-bit ADCs quantize the sign of the real and imaginary part of the complex baseband signal.

 

Granted, much lower resolution will be sufficient in Massive MIMO than in conventional MIMO, but will one bit be sufficient? These papers indicate that the price to pay is not insignificant: the number of antennas may have to be doubled in some cases. Also, while the use of symbol-sampled models as in these studies may give correct order-of-magnitude estimates of capacity, much future work appears to remain to understand the effects of digital channelization/prefiltering and sampling rate conversion if 1-bit frontends are going to be used.

Massive MIMO in Mobile Environments

An impressive experiment, recently reported by colleagues at Univ. of Lund and Univ. of Bristol, shows TDD (reciprocity-based) Massive MIMO multiplexing to mobile terminals:

The interesting part starts at 2:48, with the terminals onboard cars. While it has been contested whether Massive MIMO can work in mobility (because of channel aging) this experiment confirms that it does — as predicted by theory for a long time. In fact, at 3.7 GHz carrier and with a slot length of 0.5 ms, the maximum permitted mobility (assuming a two-ray model with Nyquist sampling, and a factor-of-two design margin) is over 140 km/h. So the experiment is probably still not close to the physical limits.