Cell-free massive MIMO might be one of the core 6G physical layer technologies. One of my students, Giovanni Interdonato, defended his Ph.D. thesis on this topic earlier this week. In this video, he speaks with me about his thesis work and his time as a doctoral student.
In the news: Nokia delivers record 5G capacity gains through a software upgrade. No surprise! We expected, years ago, this would happen.
What does this software upgrade consist of? I can only speculate. It is, in all likelihood, more than the usual (and endless) operating system bugfixes we habitually think of as “software upgrades”. Could it be even something that goes to the core of what massive MIMO is? Replacing eigen-beamforming with true reciprocity-based beamforming?! Who knows. Replacing maximum-ratio processing with zero-forcing combining?! Or even more mind-boggling, implementing more sophisticated processing of the sort that has been stuffing the academic journals in the last years? We don’t know! But it will certainly be interesting to find out at some point, and it seems safe to assume that this race will continue.
A lot of improvement could be achieved over the baseline canonical massive MIMO processing. One could, for example, exploit fading correlation, develop improved power control algorithms or implement algorithms that learn the propagation environment, autonomously adapt, and predict the channels.
It might seem that research already squeezed every drop out of the physical layer, but I do not think so. Huge gains likely remain to be harvested when resources are tight, and especially we are limited by coherence: high carriers means short coherence, and high mobility might mean almost no coherence at all. When the system is starved of coherence, then even winning a couple of samples on the pilot channel means a lot. Room for new elegant theory in “closed form”? Good question. Could sound heartbreaking, but maybe we have to give up on that. Room for useful algorithms and innovation? Certainly yes. A lot. The race will continue.
Emerging intelligent reflecting surface (IRS) technology, also known under the names “reconfigurable intelligent surface” and “software-controlled metasurface”, is sometimes marketed as an enabling technology for 6G. How do they work, what are their use cases and how much will they improve wireless access performance at large?
The physical principle of an IRS is that the surface is composed of atoms, each of which acts as an “intelligent” scatterer: a small antenna that receives and re-radiates without amplification, but with a controllable phase-shift. Typically, an atom is implemented as a small patch antenna terminated with an adjustable impedance. Assuming the phase shifts are properly adjusted, the scattered wavefronts can be made to add up constructively at the receiver. If coupling between the atoms is neglected, the analysis of an IRS essentially entails (i) finding the Green’s function of the source (a sum of spherical waves if close, or a plane wave if far away), (ii) computing the impinging field at each atom, (iii) integrating this field over the surface of each atom to find a current density, (iv) computing the radiated field from each atom using physical-optics approximation, and (v) applying the superposition principle to find the field at the receiver. If the atoms are electrically small, one can approximate the re-radiated field by pretending the atoms are point sources and then the received “signal” is basically a superposition of phase-shifted (as ), amplitude-scaled (as ) source signals.
A point worth re-iterating is that an atom is a scatterer, not a “mirror”. A more subtle point is that the entire IRS as such, consisting of a collection of scatterers, is itself also a scatterer, not a mirror. “Mirrors” exist only in textbooks, when a plane wave is impinging onto an infinitely large conducting plate (none of which exist in practice). Irrespective of how the IRS is constructed, if it is viewed from far enough away, its radiated field will have a beamwidth that is inversely proportional to its size measured in wavelengths.
Two different operating regimes of IRSs can be distinguished:
1. Both transmitter and receiver are in the far-field of the surface. Then the waves seen at the surface can be approximated as planar; the phase differential from the surface center to its edge is less than a few degrees, say. In this case the phase shifts applied to each atom should be linear in the surface coordinate. The foreseen use case would be to improve coverage, or provide an extra path to improve the rank of a point-to-point MIMO channel. Unfortunately in this case the transmitter-IRS-path loss scales very unfavorably, as where is the number of meta-atoms in the surface, and the reason is that again, the IRS itself acts as a (large) scatterer, not a “mirror”. Therefore the IRS has to be quite large before it becomes competitive with a standard single-antenna decode-and-forward relay, a simple, well understood technology that can be implemented using already widely available components, at small power consumption and with a small form factor. (In addition, full-duplex technology is emerging and may eventually be combined with relaying, or even massive MIMO relaying.)
2. At least one of the transmitter and the receiver is in the surface near-field. Here the plane-wave approximation is no longer valid. The IRS can then either be sub-optimally configured to act as a “mirror”, in which case the phase shifts vary linearly as function of the surface coordinate. Alternatively, it can be configured to act as a “lens”, with optimized phase-shifts, which is typically better. As shown for example in this paper, in the near-field case the path loss scales more favorably than in the far-field case. The use cases for the near-field case are less obvious, but one can think of perhaps indoor environments with users close to the walls and every wall covered by an IRS. Another potential use case that I learned about recently is to use the IRS as a MIMO transmitter: a single-antenna transmitter near an IRS can be jointly configured to act as a MIMO beamforming array.
So how useful will IRS technology be in 6G? The question seems open. Indoor coverage in niche scenarios, but isn’t this an already solved problem? Outdoor coverage improvement, but then (cell-free) massive MIMO seems to be a much better option? Building MIMO transmitters from a single-antenna seems exciting, but is it better than using conventional RF? Perhaps it is for the Terahertz bands, where implementation of coherent MIMO may prove truly challenging, that IRS technology will be most beneficial.
A final point is that nothing requires the atoms in an IRS to be located adjacently to one another, or even to form a surface! But they are probably easier to coordinate if they are in more or less the same place.
Cell-free massive MIMO is likely one of the technologies that will form the backbone of any xG with x>5. What distinguishes cell-free massive MIMO from distributed MIMO, network MIMO or cooperative multi-point (CoMP)? The short answer is that cell-free massive MIMO works, it can deliver uniformly good service throughout the coverage area, and it requires no prior knowledge of short-term CSI (just like regular cellular massive MIMO). A longer answer is here. The price to pay for this superiority, no shock, is the lack of scalability: for canonical cell-free massive MIMO there is a practical limit on how large the system can be, and this scalability concerns both the power control, the signal processing, and the organization of the backhaul.
At ICC this year we presented this approach towards scalable cell-free massive MIMO. A key insight is that power control is extremely vital for performance, and a scalable cell-free massive MIMO solution requires a scalable power control policy. No surprise, some performance must be sacrificed relative to canonical cell-free massive MIMO. Coincidentally, another paper in the same session (WC-26) also devised a power control policy with similar qualities!
Take-away point? There are only three things that matter for the design of cell-free massive MIMO signal processing algorithms and power control policies: scalability, scalability and scalability…
Check out this video, produced by the IEEE Signal Processing Society’s Signal Processing for Communications and Networking (SPCOM) technical committee. The video explains to the layman what 5G is for, and how massive MIMO comes in…
I had an interesting conversation with a respected colleague who expressed some significant reservations against massive MIMO. Let’s dissect the arguments.
The first argument against massive MIMO was that most traffic is indoors, and that deployment of large arrays indoors is impractical and that outdoor-to-indoor coverage through massive MIMO is undesirable (or technically infeasible). I think the obvious counterargument here is that before anything else, the main selling argument for massive MIMO is not indoor service provision but outdoor macrocell coverage: the ability of TDD/reciprocity based beamforming to handle high mobility, and efficiently suppress interference thus provide cell-edge coverage. (The notion of a “cell-edge” user should be broadly interpreted: anyone having poor nominal signal-to-interference-and-noise ratio, before the MIMO processing kicks in.) But nothing prevents massive MIMO from being installed indoors, if capacity requirements are so high that conventional small cell or WiFi technology cannot handle the load. Antennas could be integrated into walls, ceilings, window panes, furniture or even pieces of art. For construction of new buildings, prefabricated building blocks are often used and antennas could be integrated into these already at their production. Nothing prevents the integration of thousands of antennas into natural objects in a large room.
Outdoor-to-indoor coverage doesn’t work? Importantly, current systems provide outdoor-to-indoor coverage already, and there is no reason Massive MIMO would not do the same (to the contrary, adding base station antennas is always beneficial for performance!). But yet the ideal deployment scenario of massive MIMO is probably not outdoor-to-indoor so this seems like a valid point, partly. The arguments against the outdoor-to-indoor are that modern energy-saving windows have a coating that takes out 20 dB, at least, of the link budget. In addition, small angular spreads when all signals have to pass through windows (maybe except for in wooden buildings) may reduce the rank of the channel so much that not much multiplexing to indoor users is possible. This is mostly speculation and not sure whether experiments are available to confirm, or refute it.
Let’s move on to the second argument. Here the theme is that as systems use larger and larger bandwidths, but can’t increase radiated power, the maximum transmission distance shrinks (because the SNR is inversely proportional to the bandwidth). Hence, the cells have to get smaller and smaller, and eventually, there will be so few users per cell that the aggressive spatial multiplexing on which massive MIMO relies becomes useless – as there is only a single user to multiplex. This argument may be partly valid at least given the traffic situation in current networks. But we do not know what future requirements will be. New applications may change the demand for traffic entirely: augmented or virtual reality, large-scale communication with drones and robots, or other use cases that we cannot even envision today.
It is also not too obvious that with massive MIMO, the larger bandwidths are really required. Spatial multiplexing to 20 terminals improves the bandwidth efficiency 20 times compared to conventional technology. So instead of 20 times more bandwidth, one may use 20 times more antennas. Significant multiplexing gains are not only proven theoretically but have been demonstrated in commercial field trials. It is argued sometimes that traffic is bursty so that these multiplexing gains cannot materialize in practice, but this is partly a misconception and partly a consequence of defect higher-layer designs (most importantly TCP/IP) and vested interests in these flawed designs. For example, for the services that constitute most of the raw bits, especially video streaming, there is no good reason to use TCP/IP at all. Hopefully, once the enormous potential of massive MIMO physical layer technology becomes more widely known and understood, the market forces will push a re-design of higher-layer and application protocols so that they can maximally benefit from the massive MIMO physical layer. Does this entail a complete re-design of the Internet? No, probably not, but buffers have to be installed and parts of the link layer should be revamped to maximally use the “wires in the air”, ideally suited for aggressive multiplexing of circuit-switched data, that massive MIMO offers.
From the results:
“We observed up to a 3.4x increase in downlink sector throughput and up to an 8.9x increase in the uplink sector throughput versus 8T8R (obviously the gain is substantially higher relative to 2T2R). Results varied based on the test conditions that we identified. Link budget tests revealed close to a triple-digit improvement in uplink data speeds. Preliminary results for the downlink also showed strong gains. Future improvements in 64T64R are forthcoming based on likely vendor product roadmaps.”