Category Archives: Commentary

Massive MIMO, Drone Swarms, and Some Cool Stuff

Recently, there has been a hype on the use of drones (also called unmanned aerial vehicles (UAVs)) for civilian and military applications. Especially, in the coming decades, lightweight miniature drones are expected to play a major role in the society. Nowadays, small drones are available in toy shops so that an individual could buy it for personal uses such as aerial videography. However, due to security reasons, the personal use of drones is limited to low altitudes (up to 120 m in most countries) and visible line-of-sight. On the other hand, it is most likely that, in many countries, government agencies and commercial firms will be allowed to use drones for a variety of services (See: link 1 and link 2.)

There are many foreseen applications that involve a large number of drones in a limited area such as disaster management, traffic monitoring, crowd management, and crop monitoring. The major communication requirements of most of the drone networks are: several tens of Mbps throughput for streaming high-resolution video, low latency for command and control, highly reliable connectivity in a three-dimensional coverage area, high-mobility support, and simultaneous support for a large number of drones.

The existing wireless systems are unsuitable for communicating with a large number of drones in long-range, high throughput, and high-altitude applications for the following reasons:

  • In many drone communication scenarios, the mobility and traffic patterns of drones are different from the ground users. For example, in aerial surveillance applications, the uplink traffic is much higher than the downlink traffic. Depending on the application, the drones will fly at high speed (10-50 m/s) in a 3D space.
  • The propagation environment in drone communication scenarios will be line-of-sight, even under high mobility.
  • The terrestrial wireless communication networks are optimized for indoor, short range, low mobility (e.g. WiFi), and low altitude (e.g. LTE).
  • In LTE, since the base station antennas are tilted towards the ground, coverage is possible only if the drones fly below 100 m altitude. Apart from coverage, the co-channel interference generated from the neighboring cells will be a major problem in satisfying the high throughput requirements of drones.
  • The MAC layer protocols of the existing systems have to be redesigned according to the drones’ requirements, especially regarding the re-transmission protocols which are related to latency and crucial for drone control.
  • Since the existing wireless systems are connected to the power grids, they might not be available during emergency situations such as earth-quake, massive flooding, and tsunami. Further, in mountain and sea environments, cellular networks are not widely available. This problem can be overcome by deploying flying UAV base stations over the sky.

For the above-mentioned reasons, instead of borrowing from existing wireless technologies, it would be better to develop a new technology, considering the specific drone networks’ requirements and propagation characteristics. As of now, spectrum allocation and standardization efforts for drone communication networks are in the initial stage of development. This is where Massive MIMO can play a key role. The attractive features of Massive MIMO, such as spatial multiplexing and range extension, can be exploited to design flexible and efficient drone communication systems. 5G is based on the concept of network slicing, where the network can be configured differently depending on the use case. Therefore, it is possible to deploy a variation of 5G for drone communications along with appropriately tilted antenna arrays to provide connectivity to the drones flying at high altitudes.

In our recent papers (1 and 2), we illustrated the use for Massive MIMO for drone communications. From these papers, we make the following observations:

  1. The Massive MIMO performance in rich scattering is well understood by the use of ergodic rate bounds that are available in closed form. In line-of-sight, the ergodic rate performance depends on the relative positions of the drones as they move very quickly in 3D space. Interestingly, in case of line-of-sight, the uplink ergodic rate bounds (with MRC receiver) are available in closed form for some specific cases, for example, for the uniformly distributed drone positions within a spherical volume. However, more work is needed to understand the ergodic rate performance with arbitrary drone distributions.
  2. The element-spacing in the ground station array affects the rate performance depending on the distribution of the drones. For a given distribution of the drone positions, ground station array has to be optimized to maximize the ergodic rate.
  3. The probability of outage due to polarization mismatch can be made negligible by appropriately selecting the orientation and polarization of the individual array elements. For example,  circularly polarized cross-dipole antenna elements perform much better when compared to linearly polarized dipoles. (For more details, see this paper.) This means that the use of simple antenna elements, such as cross-dipoles, reduce the concerns of
    antenna pattern designs. Further, the drones can be equipped with a single cross-dipole.
  4. The range extension due to the increased number of antennas can eliminate the need for multi-hop solutions in many drone communication scenarios.
  5. TDD based Massive MIMO can be used for simultaneously supporting several tens of drones both at μ-wave and mm-wave frequencies.
  6. TDD based Massive MIMO can support high-mobility drone communications. In some scenarios (e.g., deterministic trajectories), the channel can be extrapolated without sending pilot symbols.

Below are some examples of use cases of Massive MIMO enabled drone communication systems. The technical details of Massive MIMO based system design can be found in this paper. The Massive MIMO design parameters for some of the use cases can be found in this paper.

Drone racing: In recent years, drone racing, also called “the sport of the future”, is becoming popular around the world. In drone racing, low latency is important for drone control, because even a few tens of milliseconds delay might crash the drone when it moves at the speed of 40-50 m/s.  Interestingly, in our digital world, analog transmission is used for sending videos from racing drones to the pilots. The reason is that, unlike digital transmission, an analog transmission does not incur any processing delay and the overall latency is about only 15 ms. Currently, the 5.8 GHz band (5650 MHz to 5925 MHz) is used for drone racing. The transmitter and receiver use frequency modulation and it requires 40 MHz frequency separation to avoid cross-talks between neighboring channels. As a result, the number of simultaneous drones in a contest is limited to eight.  The video quality is also poor. By using Massive MIMO, several tens of drones can simultaneously participate in a contest and the pilots can enjoy latency-free high-quality video transmission.

Sports streaming: Utilizing drones for sports streaming will change the way we view the sports events. High resolution 4K 360-degree videos taken by multiple drones at different angles can be broadcasted to enable the viewers to have an entirely a new experience. If there are 20 drones covering a sports event, the required sum throughput will be in the order of 10 Gbps. Massive MIMO in the mm-wave frequency band can be used to achieve this high throughput. This can become reality as already there are signs towards the use of drones for covering sports events. For instance, during the 2018 Winter Olympics, drones will be extensively used.

Surveillance (or search and rescue operation) using a swarm of drones and a massive array

 

Surveillance/ Search and Rescue/Disaster management: During natural disasters, a network of drones can be quickly deployed to enable the rescue teams to assess the situation in real-time via high-resolution video streaming. Depending on the area to be covered and desired video quality, the sum throughput requirement will be in the order of Gbps. A Massive MIMO array deployed over a ground vehicle or a large aerial vehicle can be used for serving a swarm of drones.

Aerial survey: A swarm of drones can be used for high-resolution aerial imagery of several kilometers of landscape. There are many uses of aerial survey, including state governance, city planning, 3D cartography, and crop monitoring. Massive MIMO can be an enabler for such high throughput and long-range applications.

Backhaul for flying base stations: During emergency situations and heavy traffic conditions, UAVs could be used as flying base stations to provide wireless connectivity to the cellular users. A Massive MIMO base station can act as a high-capacity backhaul to a large number of flying base stations.

Massive MIMO for space exploration

 

Space exploration: Currently, it takes several hours to receive a photo taken by the Curiosity Mars rover. It is possible to use Massive MIMO to reduce the overall transmission delay. For example, by using a massive antenna array deployed in an orbiter (see the above figure), a swarm of drones and rovers roaming on the surface of another planet can send videos and images to earth. The array can be used to spatially multiplex the uplink transmission from the drones (and possibly the rovers) to the orbiter. Note that the distance between the Mars surface and the orbiter is about 400 km. If the drones fly at an altitude of a few hundred meters and spread out over the region with a few hundred kilometers of radius, the angular resolution of the array is sufficient for spatial multiplexing. The array can be used to transmit the collected images and videos to earth by exploiting the array gain. This might sound like a science fiction, but NASA is already developing a 256 element antenna array for future Mars rovers to enable direct communication with the earth.

Challenges on the Path to Deployment

Marina Bay Sands Expo and Convention Centre

I attended GLOBECOM in Singapore earlier this week. Since more and more preprints are posted online before conferences, one of the unique features of conferences is to meet other researchers and attend the invited talks and interactive panel discussions. This year I attended the panel “Massive MIMO – Challenges on the Path to Deployment”, which was organized by Ian Wong (National Instruments). The panelists were Amitava Ghosh (Nokia), Erik G. Larsson (Linköping University), Ali Yazdan (Facebook), Raghu Rao (Xilinx), and Shugong Xu (Shanghai University).

No common definition

The first discussion item was the definition of Massive MIMO. While everyone agreed that the main characteristic is that the number of controllable antenna elements is much larger than the number of spatially multiplexed users, the panelists put forward different additional requirements. The industry prefers to call everything with at least 32 antennas for Massive MIMO, irrespective of whether the beamforming is constructed from codebook-based feedback, grid-of-beams, or by exploiting uplink pilots and TDD reciprocity. This demonstrates that Massive MIMO is becoming a marketing term, rather than a well-defined technology. In contrast, academic researchers often have more restrictive definitions; Larsson suggested to specifically include the TDD reciprocity approach in the definition. This is because it is the robust and overhead-efficient way to acquire channel state information (CSI), particularly for non-line-of-sight users; see Myth 3 in our magazine paper. This narrow definition clearly rules out FDD operation, as pointed out by a member of the audience. Personally, I think that any multi-user MIMO implementation that provides performance similar to the TDD-reciprocity-based approach deserves the Massive MIMO branding, but we should not let marketing people use the name for any implementation just because it has many antennas.

Important use cases

The primary use cases for Massive MIMO in sub-6 GHz bands are to improve coverage and spectral efficiency, according to the panel. Great improvements in spectral efficiency have been demonstrated by prototyping, but the panelist agreed that these should be seen as upper bounds. We should not expect to see more than 4x improvements over LTE in the first deployments, according to Ghosh. Larger gains are expected in later releases, but there will continue to be a substantial gap between the average spectral efficiency observed in real cellular networks and the peak spectral efficiency demonstrated by prototypes. Since Massive MIMO achieves its main spectral efficiency gains by multiplexing of users, we might not need a full-blown Massive MIMO implementation today, when there are only one or two simultaneously active users in most cells. However, the networks need to evolve over time as the number of active users per cell grows.

In mmWave bands, the panel agreed that Massive MIMO is mainly for extending coverage. The first large-scale deployments of Massive MIMO will likely aim at delivering fixed wireless broadband access and this must be done in the mmWave bands; there is too little bandwidth in sub-6 GHz bands to deliver data rates that can compete with wired DSL technology.

Initial cost considerations

The deployment cost is a key factor that will limit the first generations of Massive MIMO networks. Despite all the theoretic research that has demonstrated that each antenna branch can be built using low-resolution hardware, when there are many antennas, one should not forget the higher out-of-band radiation that it can lead to. We need to comply with the spectral emission masks – spectrum is incredibly expensive so a licensee cannot accept interference from adjacent bands. For this reason, several panelists from the industry expressed the view that we need to use similar hardware components in Massive MIMO as in contemporary base stations and, therefore, the hardware cost grows linearly with the number of antennas. On the other hand, Larsson pointed out that the futuristic devices that you could see in James Bond movies 10 years ago can now be bought for $100 in any electronic store; hence, when the technology evolves and the economy of scale kicks in, the cost per antenna should not be more than in a smartphone.

A related debate is the one between analog and digital beamforming. Several panelists said that analog and hybrid approaches will be used to cut cost in the first deployments. To rely on analog technology is somewhat weird in an age when everything is becoming digital, but Yazdan pointed out that it is only a temporary solution. The long-term vision is to do fully digital beamforming, even in mmWave bands.

Another implementation challenge that was discussed is the acquisition of CSI for mobile users. This is often brought up as a showstopper since hybrid beamforming methods have such difficulties – it is like looking at a running person in a binocular and trying to follow the movement. This is a challenging issue for any radio technology, but if you rely on uplink pilots for CSI acquisition, it will not be harder than in a system of today. This has also been demonstrated by measurements.

Open problems

The panel was asked to describe the most important open problems in the Massive MIMO area, from a deployment perspective. One obvious issue, which we called the “grand question” in a previous paper, is to provide better support for Massive MIMO in FDD.

The control plane and MAC layer deserve more attention, according to Larsson. Basic functionalities such as ACK/NACK feedback is often ignored by academia, but incredibly important in practice.

The design of “cell-free” densely distributed Massive MIMO systems also deserve further attention. Connecting all existing antennas together to perform joint transmission seems to be the ultimate approach to wireless networks. Although there is no practical implementation yet, Yazdan stressed that deploying such networks might actually be more practical than it seems, given the growing interest in C-RAN technology.

10 years from now

I asked the panel what will be the status of Massive MIMO in 10 years from now. Rao predicted that we will have Massive MIMO everywhere, just as all access point supports small-scale MIMO today. Yazdan believed that the different radio technology (e.g., WiFi, LTE, NR) will converge into one interconnected system, which also allows operators to share hardware. Larsson thinks that over the next decade many more people will have understood the fundamental benefits of utilizing TDD and channel reciprocity, which will have a profound impact on the regulations and spectrum allocation.

Ten Questions and Answers About Massive MIMO

After the IEEE ComSoc Webinar that I gave this month, there was a 15 minute online Q/A session.

Unfortunately, there was not enough time for me to answer all the questions that I received, so I had to answer many of them afterwards. I have gathered ten questions and my answers below. I can also announce that I will give another Massive MIMO webinar in January 2018 and it will also be followed by a Q/A session.

1. What are the differences between 4G and 5G that will affect how Massive MIMO can be implemented?

The channel estimation must be implemented in the right way (i.e., exploiting uplink pilots and channel reciprocity) to obtain sufficiently accurate channel state information (CSI) to perform spatial multiplexing of many users, otherwise the inter-user interference will eliminate most of the gains. Accurate CSI  is hard to achieve within the 4G standard, although there are several Massive MIMO field trials for TDD LTE that show promising results. However, if 5G is designed properly, it will support Massive MIMO from scratch, while in 4G it will always be an add-on that must to adhere to the existing air interface.

2. How easy it is to deploy MIMO antennas on the current infrastructure?

Generally speaking, we can reuse the current infrastructure when deploying Massive MIMO, which is why operators show much interest in the technology. You upgrade the radio base stations but keep the same backhaul infrastructure and core network. However, since Massive MIMO supports much higher data rates, some of the backhaul connections might also need to be upgraded to deliver these rates.

3. What are the most suitable channel models for Massive MIMO?

I recommend the channel model that was developed in the MAMMOET project. It is a refinement of the COST 2100 model that takes particular phenomena of having large antenna arrays into account. Check out Deliverable D1.2 from that project.

4. For planar arrays, what is the height to width ratio that gives the highest performance?

You typically need more antennas in the horizontal direction (width) than in the vertical direction (height), because the angular variations between users is larger in the horizontal domain. For example, the array might cover a horizontal sector of 120-180 degrees, while the users’ elevation angles might only differ by a few tens of degrees. This is the reason that 8-antenna LTE base stations use linear arrays in the horizontal direction.

There is no optimal answer to the question. It depends on the deployment scenario. If you have high-rise buildings, users at different floors can have rather different elevation angles (it can differ up to 90 degrees) and you can benefit more from having many antennas in the vertical direction. If all users have almost the same elevation angle, it is preferable to have many antennas in the horizontal direction. These things are further discussed in Sections 7.3 and 7.4 in my new book.

5. What are the difficulties we face in deploying Massive MIMO in FDD systems?

The difficulty is to acquire channel state information at the base station for the frequency band used in the downlink, since it is very resource-demanding to send downlink pilots from a large array; particularly, if you want to spatially multiplex many users. This is an important but challenging problem that researchers have been working on since the 1990s. You can read more about it in Myth 3 and the grand question in the paper Massive MIMO: ten myths and one grand question.

6. Do you believe that there is a value in coordinated resource allocation schemes for Massive MIMO?

Yes, but the resource allocation in Massive MIMO is different from conventional systems. Scheduling might not be so important, since you can multiplex many users spatially, but pilot assignment and power allocation are important aspects that must be addressed. I refer to these things as spatial resource allocation. You can read more about this in Sections 7.1 and 7.2 in my new book, but as you can see from those sections, there are many open problems to be solved.

7. What is channel hardening and what implications does it have on the frequency allocation (in OFDMA networks, for example)?

Channel hardening means that the effective channel after beamforming is almost constant so that the communication link behaves as if there is no small-scale fading. A consequence is that all frequency subcarriers provide almost the same channel quality to a user. Regarding channel assignment, since you can multiplex many tens of users spatially in Massive MIMO, you can assign the entire bandwidth (all subcarriers) to every user; there is no need to use OFDMA to allocate orthogonal frequency resources to the users.

8. Is it practical to estimate the channel for each subcarrier in an OFDM system?

To limit the pilot overhead, you typically place pilots only on a small subset of the subcarriers. The distance between the pilots in the frequency domain can be selected based on how frequency-selective the channels are; if a user has L strong channel taps, it is sufficient to send pilots on L subcarriers, even if you many more subcarriers than that. Based on the received pilot signals, one can either estimate the channels on every subcarrier or estimate the channels on some of them and interpolate to get estimates on the remaining subcarriers.

9. How sensitive are the Massive MIMO spectral efficiency gains to TDD frame synchronization?

If you consider an OFDM system, then timing synchronization mismatches that are smaller than the cyclic prefix can basically be ignored. This is the case in TDD LTE systems and will not change when considering Massive MIMO systems that are implemented using OFDM. However, the synchronization across cells will not be perfect. The implications are investigated in a recent paper.

10. How does the higher computational complexity and delay in Massive MIMO processing affect the system performance?

I used to think that the computational complexity would be a bottleneck, but it turns out that it is not a big deal since all of the operations are standard (i.e., matrix multiplications and matrix inversions). For example, the circuit that was developed at Lund University shows that MIMO detection and precoding for a 20 MHz channel can be implemented very efficiently and only consumes a few mW.

MAMMOET: Massive MIMO for Efficient Transmission

MAMMOET (Massive MIMO for Efficient Transmission) was the first major research project on Massive MIMO that was funded by the European Union. The project took place 2014-2016 and you might have heard about its outcomes in terms of the first demonstrations of real-time Massive MIMO that was carried out by the LuMaMi testbed at Lund University. The other partners in the project were Ericsson, Imec, Infineon, KU Leuven, Linköping University, Technikon, and Telefonica. MAMMOET was an excellent example of a collaborative project, where the telecom industry defined the system requirements and the other partners designed and evaluated new algorithms and hardware implementations to reach the requirements.

This article is an interview with Prof. Liesbet Van der Perre who was the scientific leader of the project.

Liesbet Van der Perre while disseminating results from the MAMMOET project in September 2017.

In 2012, when you began to draft the project proposal, Massive MIMO was not a popular topic. Why did you initiate the work?

– Theoretically and conceptually it seemed so interesting that it would be a pity not to work on it. The main goal of the MAMMOET project was to make conceptual progress towards a spectrally and energy efficient system and to raise the confidence level by demonstrating a practical hardware implementation. We also wanted to make channel measurements to see if they would confirm what has been seen in theory.

It seems the project partners had a clear vision from the beginning?

– It was actually very easy to write this proposal because everyone was on the same wavelength and knew what we wanted to achieve. We were all eager to start the project and learn from each other. This is quite unique and explains why the project delivered much more than promised. The fact that the team got along very well has also laid the fundament for further research collaborations.

What were the main outcomes of the project?

– We learned a lot on how things change when going from small to large arrays. New channel models are required to capture the new behaviors. We are used to that high-precision hardware is needed, but all the sudden this is not true when drastically increasing the number of antennas. You can then use low-resolution hardware and simple processing, which is very different from conventional MIMO implementation.

Some of the big conceptual differences in massive MIMO turned out to be easier to solve than expected, while some things were more problematic than foreseen. For example, it is difficult to connect all the signals together. You need to do part of the processing distributive to avoid this problem. Synchronization also turned out to be a bottleneck. If we would have known that from the start, we could have designed the testbed differently, but we thought that the channel estimation and MIMO processing would be the challenging part.

What was the most rewarding aspect of leading this project?

– The cross-fertilization of people was unique. We brought people with different background and expertise together in a room to identify the crucial problems in massive MIMO and find new solutions. For example, we realized early that interference will be a main problem and that zero-forcing processing is needed, although matched filtering was popular at the time. By carefully analyzing the zero-forcing complexity, we could show that it was almost negligible compared to other necessary processing and we later demonstrated zero-forcing in real-time at the testbed. This was surprising for many people who thought that massive MIMO would be impossible to implement since 8×8 MIMO systems are terribly complex, but many things can be simplified in massive MIMO. Looking back, it might seem that the outcomes were obvious, but these are things you don’t know until you have gone through the process.

The real-time LuMaMi testbed at Lund University, the first one of its kind.

What are the big challenges that remains?

– An important challenge is how to integrate massive MIMO into a network. We assumed that there are many users and we can all give them the same time-frequency resources, but the channels and traffic are not always suitable for that. How should we decide which users to put together? We used an LTE-like frame structure, but it is important to design a frame structure that is well-suited for massive MIMO and real traffic.

There are many tradeoffs and degrees-of-freedom when designing massive MIMO systems. Would you use the technology to provide very good cell coverage or to boost small-cell capacity? Instead of delivering fiber to homes, we could use massive MIMO with very many antennas for spatial multiplexing of fixed wireless connections. Alternatively, in a mobile situation, we might not multiplex so many users. Optimizing massive MIMO for different scenarios is something that remains.

We made a lot of progress on the digital processing side in MAMMOET, while on the analog side we mainly came up with the specifications. We also did not work on the antenna design since, theoretically, it does not matter which antennas you use, but in practice it does.

The research team of the MAMMOET project at the final review meeting of the project in February 2017.

All the deliverables and publications in the MAMMOET project can be accessed online: https://mammoet-project.eu

The deliverables contain a lot information related to use cases, requirements, channel modeling, signal processing algorithms, algorithmic implementation, and hardware implementation. Some of the results can found in the research literature, but far from everything.

Note: The author of this article worked in the MAMMOET project, but did not take part in the drafting of the proposal.

Massive MIMO is Becoming a Marketing Term

I have been wondering for years if “MIMO” will always be a term exclusively used by engineers and a few well-informed consumers, or if it eventually becomes a word that most people are using. Will you ever hear kids saying: “I want a MIMO tablet for Christmas”?

I have been think that it can go either way – it is in the hands of marketing people. Advanced Wifi routers have been marketed with MIMO functionality for some years, but the impact is limited since most people get their routers as part of their internet subscriptions instead of buying them separately. Hence, the main question is: will handset manufactures and telecom operators start using the MIMO term when marketing products to end customers?

Maybe we have the answer because Sprint, an American telecom operator, is currently marketing their 2018 deployment of new LTE technology by talking publicly about “Massive MIMO”.  As I wrote back in March, Sprint and Ericsson were to conduct field tests in the second half of 2017.  Results from the tests conducted in Seattle, Washington and Plano, Texas, have now been described in a press release. The tests were carried at a carrier frequency in the 2.5 GHz band using TDD mode and an Ericsson base station with 64 transmit/receive antennas. It is fair to call this Massive MIMO, although 64 antennas is in the lower end of the interval that I would call “massive”.

The press release describes “peak speeds of more than 300 Mbps using a single 20 MHz channel”, which corresponds to a spectral efficiency of 15 bit/s/Hz. That is certainly higher than you can get in legacy LTE networks, but it is less than some previous field tests.

Hence, when the Sprint COO of Technology, Guenther Ottendorfer, describes their Massive MIMO deployment with the words “You ain’t seen nothing yet”, I hope that this means that we will see network deployments with substantially higher spectral efficiencies than 15 bit/s/Hz in the years to come.

Several videos about the field test in Seattle have recently appeared. The first one demonstrates that 100 people can simultaneously download a video, which is not possible in legacy networks. Since the base station has 64 antennas, the 100 users are probably served by a combination of spatial multiplexing and conventional orthogonal time-frequency multiplexing.

The second video provides some more technical details about the setup used in the field test.

Superimposed Pilots?

The concept of superimposed pilots is (at least 15 years) old, but clever and intriguing. The idea is to add pilot and data samples together, instead of separating them in time and/or frequency, before modulating with waveforms. More recently, the authors of this paper argued that in massive MIMO, based on certain simulations supported by asymptotic analysis, superimposed pilots provide superior performance and that there are strong reasons for superimposed pilots to make their way to practical use.

Until recently, a more rigorous analysis was unavailable. Some weeks ago the authors of this paper argued, that all things considered, the use of superimposed pilots does not offer any appreciable gains for practically interesting use cases. The analysis was based on a capacity-bounding approach for finite numbers of antennas and finite channel coherence, but it assumed the most basic form of signal processing for detection and decoding.

There still remains some hope of seeing improvements, by implementing more advanced signal processing, like zero-forcing, multicell MMSE decoding, or iterative decoding algorithms, perhaps involving “turbo” information exchange between the demodulator, channel estimation, and detector. It will be interesting to follow future work by these two groups of authors to understand how large improvements (if any) superimposed pilots eventually can give.

There are, at least, two general lessons to learn here. First, that performance predictions based on asymptotics can be misleading in practically relevant cases. (I have discussed this issue before.) The best way to perform analysis is to use rigorous capacity lower bounds, or possibly, in isolated cases of interest, link-level simulations with channel coding (for which, as it turns out, capacity bounds are a very good proxy). Second, more concretely, that while it may be tempting, to superimpose-squeeze multiple symbols into the same time-frequency-space resource, once all sources of impairments (channel estimation errors, interference) are accurately accounted for, the gains tend to evaporate. (It is for the same reason that NOMA offers no substantial gains in MIMO systems – a topic that I may return to at a later time.)

Six Differences Between MU-MIMO and Massive MIMO

Multi-user MIMO (MU-MIMO) is not a new technology, but the basic concept of using multi-antenna base stations (BSs) to serve a multitude of users has been around since the late 1980s.

An example of how MU-MIMO was illustrated prior to Massive MIMO.

I sometimes get the question “Isn’t Massive MIMO just MU-MIMO with more antennas?” My answer is no, because the key benefit of Massive MIMO over conventional MU-MIMO is not only about the number of antennas. Marzetta’s Massive MIMO concept is the way to deliver the theoretical gains of MU-MIMO under practical circumstances. To achieve this goal, we need to acquire accurate channel state information, which in general can only be done by exploiting uplink pilots and channel reciprocity in TDD mode. Thanks to the channel hardening and favorable propagation phenomena, one can also simplify the system operation in Massive MIMO.

This is how Massive MIMO is often illustrated for line-of-sight operation.

Six key differences between conventional MU-MIMO and Massive MIMO are provided below.

Conventional MU-MIMO Massive MIMO
Relation between number of BS antennas (M) and users (K) MK and both are small (e.g., below 10) K and both can be large (e.g., M=100 and K=20).
Duplexing mode Designed to work with both TDD and FDD operation Designed for TDD operation to exploit channel reciprocity
Channel acquisition Mainly based on codebooks with set of predefined angular beams Based on sending uplink pilots and exploiting channel reciprocity
Link quality after precoding/combining Varies over time and frequency, due to frequency-selective and small-scale fading Almost no variations over time and frequency, thanks to channel hardening
Resource allocation The allocation must change rapidly to account for channel quality variations The allocation can be planned in advance since the channel quality varies slowly
Cell-edge performance Only good if the BSs cooperate Cell-edge SNR increases proportionally to the number of antennas, without causing more inter-cell interference

Footnote: TDD stands for time-division duplex and FDD stands for frequency-division duplex.