Technical insights

Pilot Contamination in a Nutshell

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One word that is tightly connected with Massive MIMO is pilot contamination. This is a phenomenon that can appear in any communication system that operates under interference, but in this post, I will describe its basic properties in Massive MIMO.

The base station wants to know the channel responses of its user terminals and these are estimated in the uplink by sending pilot signals. Each pilot signal is corrupted by inter-cell interference and noise when received at the base station. For example, consider the scenario illustrated below where two terminals are transmitting simultaneously, so that the base station receives a superposition of their signals—that is, the desired pilot signal is contaminated.

When estimating the channel from the desired terminal, the base station cannot easily separate the signals from the two terminals. This has two key implications:

First, the interfering signal acts as colored noise that reduces the channel estimation accuracy.

Second, the base station unintentionally estimates a superposition of the channel from the desired terminal and from the interferer. Later, the desired terminal sends payload data and the base station wishes to coherently combine the received signal, using the channel estimate. It will then unintentionally and coherently combine part of the interfering signal as well. This is particularly poisonous when the base station has M antennas, since the array gain from the receive combining increases both the signal power and the interference power proportionally to M. Similarly, when the base station transmits a beamformed downlink signal towards its terminal, it will unintentionally direct some of the signal towards to interferer. This is illustrated below.

In the academic literature, pilot contamination is often studied under the assumption that the interfering terminal sends the same pilot signal as the desired terminal, but in practice any non-orthogonal interfering signal will cause the two effects described above.

Education

Teaching a Course on Massive MIMO?

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Our Cambridge book, Fundamentals of Massive MIMO, ships now from all major retailers.

Problem set: We have developed an extensive set of problems to go with the book. This problem set can be downloaded from the Cambridge resource page, www.cambridge.org/Marzetta, or from this direct link.

The difficulty level of the problem varies widely, rendering the material suitable for instruction at all levels. The problem set is very much a living document and may be extended or improved in the future. Many, though not all, of the problems have been tested on my students when I taught the subject last year. We appreciate, as always, comments or suggestions on the material.

A detailed solution manual is available to instructors who adopt the book.

 

 

List of errata: There is also a list of errata to the book – available via this direct link, or from the Cambridge resource page.

Have no fear of perfection — you’ll never reach it. — Salvador Dali

5G, Commentary, News

Which Technology Can Give Greater Value?

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The IEEE GLOBECOM conference, held in Washington D.C. this week, featured many good presentations and exhibitions. One well-attended event was the industry panel “Millimeter Wave vs. Below 5 GHz Massive MIMO: Which Technology Can Give Greater Value?“, organized by Thomas Marzetta and Robert Heath. They invited one team of Millimeter Wave proponents (Theodore Rappaport, Kei Sakaguchi, Charlie Zhang) and one team of Massive MIMO proponents (Chih-Lin I, Erik G. Larsson, Liesbet Van der Perre) to debate the pros and cons of the two 5G technologies.

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For millimeter wave, the huge bandwidth was identified as the key benefit. Rappaport predicted that 30 GHz of bandwidth would be available in 5 years time, while other panelists made a more conservative prediction of 15-20 GHz in 10 years time. With such a huge bandwidth, a spectral efficiency of 1 bit/s/Hz is sufficient for an access point to deliver tens of Gbit/s to a single user. The panelists agreed that much work remains on millimeter wave channel modeling and the design of circuits for that can deliver the theoretical performance without huge losses. The lack of robustness towards blockage and similar propagation phenomena is also a major challenge.

For Massive MIMO, the straightforward support of user mobility, multiplexing of many users, and wide-area coverage were mentioned as key benefits. A 10x-20x gain in per-cell spectral efficiency, with performance guarantees for every user, was another major factor. Since these gains come from spatial multiplexing of users, rather than increasing the spectral efficiency per user, a large number of users are required to achieve these gains in practice. With a small number of users, the Massive MIMO gains are modest, so it might not be a technology to deploy everywhere. Another drawback is the limited amount of spectrum in the range below 5 GHz, which limits the peak data rates that can be achieved per user. The technology can deliver tens of Mbit/s, but maybe not any Gbit/s per user.

Although the purpose of the panel was to debate the two 5G candidate technologies, I believe that the panelists agree that these technologies have complementary benefits. Today, you connect to WiFi when it is available and switch to cellular when the WiFi network cannot support you. Similarly, I imagine a future where you will enjoy the great data rates offered by millimeter wave, when you are covered by such an access point. Your device will then switch seamlessly to a Massive MIMO network, operating below 5 GHz, to guarantee ubiquitous connectivity when you are in motion or not covered by any millimeter wave access points.

Beyond 5G, Commentary

Extreme Massive MIMO

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Suppose extra antennas and RF chains came at no material cost. How large an array could eventually be useful, and would power consumption eventually render “extreme Massive MIMO” infeasible?

I have argued before that in a mobile access environment, no more than a few hundred of antennas per base station will be useful. In an environment without significant mobility, however, the answer is different. In [1, Sec. 6.1], one case study establishes the feasibility of providing (fixed) wireless broadband service to 3000 homes, using a single isolated base station with 3200 antennas (zero-forcing processing and max-min power control). The power consumption of the associated digital signal processing is estimated in [1, homework #6.6] to less than 500 Watt. The service of this many terminals is enabled by the long channel coherence (50 ms in the example).

Is this as massive as MIMO could ever get? Perhaps not. Conceivably, there will be environments with even larger channel coherence. Consider, for example, an outdoor city square with no cars or other traffic – hence no significant mobility. Eventually only measurements can determine the channel coherence, but assuming for the sake of argument 200 ms by 400 kHz, gives room for training of 40,000 terminals (assuming no more than 50% of resources are spent on training). Multiplexing these terminals would require at least 40,000 antennas, which would, at 3 GHz and half wavelength-spacing, occupy an area of 10 x 10 meters (say with a rectangular array for the sake of argument) – easily integrated onto the face of a skyscraper.

  • What gross rate would the base station offer? Assuming, conservatively, 1 bit/s/Hz spectral efficiency (with the usual uniform-service-for-all design), the gross rate in a 25 MHz bandwidth would amount to 1 Tbit/s.
  • How much power would the digital processing require? A back-of-the envelope calculation along the lines of the homework cited above suggests some 15 kW – the equivalent of a few domestic space heaters (I will return to the “energy efficiency” hype later on this blog).
  • How much transmit power is required? The exact value will depend on the coverage area, but to appreciate the order of magnitude, observe that when doubling the number of antennas, the array gain is doubled. If, simultaneously, the number of terminals is doubled, then the total radiated power will be independent of the array size. Hence, transmit power is small compared to the power required for processing.

Is this science fiction or will we be seeing this application in the future? The application is fully feasible, with today’s circuit technology, and does not violate known physical or information theoretic constraints. Machine-to-machine, IoT, or perhaps virtual-reality-type applications may eventually create the desirability, or need, to build extreme Massive MIMO.

[1] T. Marzetta, E. G. Larsson, H. Yang, H. Q. Ngo, Fundamentals of Massive MIMO, Cambridge University Press, 2016.

extreme-mimo

5G, Commentary

The Dense Urban Information Society

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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.

Hexagonal cellular network, with adjacent cells having different colors for clarity.

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.

intersite-distance

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.

Commentary, News

Macrocell Massive MIMO at 4.5 GHz: Field Trials in Japan

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This impressive experiment serves 23 terminals with 64 base station antennas, at 4.5 GHz carrier, with a reported total spectral efficiency in the cell of nearly 80 bps/Hz. Several of the terminals are mobile, though it is not clear how fast.

Merouane Debbah, Vice-President of the Huawei France R&D center, confirms to the Massive MIMO blog that this spectral efficiency was achieved in the downlink, using TDD and exploiting channel reciprocity. This comes as no surprise, as it is not plausible that this performance could be sustained with FDD-style CSI feedback.

Another piece of evidence, that the theoretical predictions of Massive MIMO performance are for real.

Beyond 5G, Technical insights

Cell-Free Massive MIMO: New Concept

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Conventional mobile networks (a.k.a. cellular wireless networks) are based on cellular topologies. With cellular topologies, a land area is divided into cells. Each cell is served by one base station. An interesting question is: shall the future mobile networks continue to have cells? My quick answer is no, cell-free networks should be the way to do in the future!

Future wireless networks have to manage at the same time billions of devices; each needs a high throughput to support many applications such as voice, real-time video, high quality movies, etc. Cellular networks could not handle such huge connections since user terminals at the cell boundary suffer from very high interference, and hence, perform badly. Furthermore, conventional cellular systems are designed mainly for human users. In future wireless networks, machine-type communications such as the Internet of Things, Internet of Everything, Smart X, etc. are expected to play an important role. The main challenge of machine-type communications is scalable and efficient connectivity for billions of devices. Centralized technology with cellular topologies does not seem to be working for such scenarios since each cell can cover a limited number of user terminals. So why not cell-free structures with decentralized technology? Of course, to serve many user terminals and to simplify the signal processing in a distributed manner, massive MIMO technology should be included. The combination between cell-free structure and massive MIMO technology yields the new concept: Cell-Free Massive MIMO.

What is Cell-Free Massive MIMO? Cell-Free Massive MIMO is a system where a massive number access points distributed over a large area coherently serve a massive number of user terminals in the same time/frequency band. Cell-Free Massive MIMO focuses on cellular frequencies. However, millimeter wave bands can be used as a combination with the cellular frequency bands. There are no concepts of cells or cell boundaries here. Of course, specific signal processing is used, see [1] for more details. Cell-Free Massive MIMO is a new concept. It is a new practical, useful, and scalable version of network MIMO (or cooperative multipoint joint processing) [2, 3]. To some extent, Massive MIMO technology based on the favorable propagation and channel hardening properties is used in Cell-Free Massive MIMO.

Cell-Free Massive MIMO is different from distributed Massive MIMO [4]. Both systems use many service antennas in a distributed way to serve many user terminals, but they are not entirely the same. With distributed Massive MIMO, the base station antennas are distributed within each cell, and these antennas only serve user terminals within that cell. By contrast, in Cell-Free Massive MIMO there are no cells. All service antennas coherently serve all user terminals. The figure below compares the structures of Cell-Free Massive MIMO and distributed Massive MIMO.

comami cellfree
Distributed Massive MIMO Cell-Free Massive MIMO

[1] H. Q. Ngo, A. Ashikhmin, H. Yang, E. G. Larsson, and T. L. Marzetta, “Cell-Free Massive MIMO versus Small Cells,” IEEE Trans. Wireless Commun., 2016 submitted for publication. Available: https://arxiv.org/abs/1602.08232

[2] G. Foschini, K. Karakayali, and R. A. Valenzuela, “Coordinating multiple antenna cellular networks to achieve enormous spectral efficiency,” IEE Proc. Commun. , vol. 152, pp. 548–555, Aug. 2006.

[3] E. Björnson, R. Zakhour, D. Gesbert, B. Ottersten, “Cooperative Multicell Precoding: Rate Region Characterization and Distributed Strategies with Instantaneous and Statistical CSI,” IEEE Trans. Signal Process., vol. 58, no. 8, pp. 4298-4310, Aug. 2010.

[4] K. T. Truong and R.W. Heath Jr., “The viability of distributed antennas for massive MIMO systems,” in Proc. Asilomar CSSC, 2013, pp. 1318–1323.

News – commentary – mythbusting