Smart antennas help make 5G available (part 2)

Original author: Theodore S. Rappaport, Wonil Roh & Kyungwhoon Cheun
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In the previous part of the translated article, we talked about the prospects of using the millimeter range for creating new generation communication networks and the current state of affairs in this area. The second part describes various experiments for checking the range and stability of communication in the millimeter range. We will also finally find out whether these frequencies can form the basis of next-generation networks.

Experiment Results


How true are the fears regarding poor coverage and low penetration of millimeter waves? In 2011, experiments were conducted at the University of Texas at finding out how millimeter waves are scattered and reflected by various objects in their way, how quickly the signal loses energy. Reception horn antennas were used, which were the development of the Boche design used more than 100 years ago . Such an antenna forms a directional concentrated beam without increasing the power of the transmitter or receiver. Four antennas were placed on rotating robotic platforms to direct the signal in any direction.

Such beam control may be a key feature of future millimeter-wave cellular systems. Moreover, both at base stations and at end devices. But for this you have to introduce arrays of electronically controlled antennas into smartphones and tablets.

In total, more than 700 combinations of the relative positions of the transmitters and receivers were tested. The signal frequency was about 38 GHz. This part of the range is a good candidate for use in mobile communications, because it has already been allocated for commercial use in many countries.

In the course of the experiments, it was found that millimeter waves provide a very good level of coverage. In particular, there was no need to maintain direct line of sight between the receiver and the transmitter, the high ability to reflect was an advantage, not a disadvantage, of waves in this range.

Of course, as with any wireless system, the probability of signal loss increased as the receiver moved away from the transmitter. In the case of a low power signal, interruptions began at a distance of about 200 meters. For early generations of cellular communications, this would be a problem, but in recent years, operators have been forced to reduce cell sizes in order to increase throughput. In the most densely populated areas, for example, in the center of Seoul, they began to create ultra-small cells based on compact base stations that are placed on a lamp post or a kiosk at a bus stop. Such cells operate at a distance of not more than 100 meters.

Another argument in favor of small cells is the scattering effect of rain on millimeter waves, they lose energy faster than decimeter waves. However, studies have shown that at a distance of several hundred meters this effect is negligible. Although there are a few exceptions.

Then the same experiment, with the same equipment, was carried out in New York, one of the most radio-saturated cities in the world. During 2012-2013, the behavior of waves at frequencies of 28 and 73 GHz, also allocated for commercial use, was studied. The result was almost the same . Even on the streets of Manhattan at a distance of 200 meters, communication was maintained for 85% of the time. More accurate antenna arrays will be able to increase the distance of stable communication over 300 meters.



Comparison of the current decimeter technology and the future millimeter.

It was also found that the waves of the indicated frequencies pass through drywall and glass with small energy losses. Brick, concrete and highly tinted glass block them almost completely. Therefore, depending on the construction of buildings, operators may need to install repeaters to provide indoor communications.

New research


Inspired by the results described above, researchers from the University of Texas at the collaboration with Samsung experts began to create a prototype of the communication system . Instead of bulky motorized horn antennas, arrays of rectangular metal plates called patch antennas were used. Their big advantage is their small size, which must be half the wavelength. The prototype was created for a signal at 28 GHz (about 1 cm), so each patch antenna was no more than 5 mm.

Such arrays of antennas (phased array antennas) have long been successfully used in radar and space communications, and many chip manufacturers, including Intel, Qualcomm and Samsung, are now integrating them into WiGig chipsets. Thanks to the electronic control of the signal of each antenna, such arrays allow you to quickly rebuild the beam and direct it to a specific device.

An array that can hold the beam on a moving object is called adaptive, or "smart." The larger the array, the narrower the beam. To rebuild the beam, the array changes the amplitude or phase (or both) of the signal from each antenna. In relation to the cellular network, the base station and the end device establish communication, "feeling for" each other's rays, determining in which direction the signal is strongest. Then the communication channel is established.

Such beam shaping and control can be implemented in two ways.

The first way : the analog signal is immediately processed (or immediately after reception) by digital phase controllers or amplifiers.

The second way : processing occurs in digital form before conversion to analog (or after digitization).

Each approach has its pros and cons.

Digital beamforming gives higher accuracy. But it is more complicated, and therefore more expensive, because it requires separate computing modules and “gluttonous” digital-to-analog (or analog-to-digital) converters for each patch antenna.

Analog beamforming, on the other hand, is simpler and cheaper, since fixed components are used for this. But this method is less flexible.



To take all the best from both methods, a hybrid architecture was used in the prototype . In particular, phase controllers on the analog front end were used to form narrow directional beams, which made it possible to increase the communication range. In the back end, digital processing was used to separately control different sections of the array. Digital input made it possible to send several beams simultaneously to several end devices, or to concentrate all beams on one device. That is, the MIMO method was applied.

An array of 64 antennas was the size of a note sheet. It was divided into two MIMO channels, each of which used a 500 MHz band and could form a beam 10 degrees wide. In laboratory conditions, these two beams made it possible to ensure almost error-free data transmission with a speed of more than 500 Mbps to two mobile stations simultaneously. Both beams directed to the same station made it possible to transmit data at a speed of more than 1 Gbit / s. For comparison, in New York, the average speed in LTE networks is about 10 Mbit / s, and in theory it can reach 50 Mbit / s.

The prototype provided stable communication when moving mobile stations in random directions at a speed of up to 8 km / h. Communication was maintained at a distance of almost 300 m out of line of sight. And in the absence of obstacles between the receiver and the transmitter, the communication range increased to 2 km.

Please note that this prototype was created just to prove the concept. If you use a wider band, narrower beams and increase the number of MIMO channels, then you can achieve much higher results in transmission speed and communication range. Computer simulations have shown that under real-life conditions, it is possible to provide a transfer rate of several gigabits per second.

But there is one important limitation - where to get free space for placing the antenna array in smartphones and tablets? The Samsung Galaxy Note II managed to shove 32 antennas at the top and bottom of the device, which provided 360-degree coverage.

Conclusion


All these numerous experiments give every reason to believe that cellular communication based on the millimeter range is not just possible, it will be a breakthrough. Of course, the work is still in its infancy. To create a full-sized engineering model of the network, it will be necessary to develop statistical models of millimeter channels, beamforming algorithms, new energy-efficient standards, and much more. Government organizations should also be involved in this process.

Now various industrial groups around the world are already in search of candidates for the role of “5G technologies”, including interference control schemes and dense architectures based on small cells. And the understanding is already coming that the millimeter range is the key component that can combine all kinds of ingredients.

- Of course, it is too early to talk about timing, but even about some specific technologies that will form the basis of 5G. The millimeter range cannot be called completely suitable for the organization of cellular communications. But it is possible that its advantages will outweigh the shortcomings, and the developers will figure out how to compensate for the most important "inconveniences." We, in turn, would like to know the opinion of Russian experts regarding the prospects for using the millimeter range. Do you consider him the main candidate for the role of 5G technology, and why? Perhaps you have your own experience with millimeter equipment? It would be great if you talked about him in the comments.

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