Is Two Better Than One? Multi-beam Signals Target 5G Reliability and Throughput
Multipath signals are generally considered an undesirable noise source in RF links. Flipping conventional thinking on its head, UC San Diego Researchers are capitalizing on two beams being better than one.
As you might be aware, 5G has been struggling with creating a balance between fast download speeds and coverage, which is usually a trade-off. One solution plausible solution for this issue comes from looking into multipath signals.
Multipath signals can create destructive summation at receivers, which degrades signal power and noise relationships. However, there is a new approach to multipath mitigation; using it.
Utilizing multipath as a feature of the radio unit is a novel idea, and researchers at UC San Diego have written a paper on how “two beams are better than one.” This research takes advantage of the beamforming capabilities of modern phased array antennas to split RF power into multiple beams to optimize the radio channel algorithmically.
The outdoor DUT setup for multi-beam mmWave testing. Image used courtesy of UC San Diego
Link establishment, throughput, coverage, and reliability are four critical areas of concern for RF communications. The researchers at UC San Diego are building upon existing research into the first three areas; however, their focus is on reliability while maintaining high throughput.
For this article, let's delve into what challenge these researchers are attempting to solve, what their solution is, and finally, what the benefits and limitations are for this solution.
Link Budgets & Blockages for mmWave Radio
All radio systems suffer from attenuation due to distance and atmospheric effects. Signals attenuate at different rates, dependent on the frequency of the carrier signal and the distance from the transmitter to receiver. Conservatively, the loss can exceed 120 dB/km for 5G mmWave (~28 GHz) frequency bands.
Linear attenuation in dB/km by frequency (GHz) and atmospheric gases. Screenshot used courtesy of Rohde & Schwarz
Phased arrays, a common antenna type for mmWave technology, must also contend with issues such as phase coherency.
RF phase-controlled signals must have well-matched frequency synthesis, which generally means using the same local oscillator. The electrical length of each signal chain can also be negatively affected by temperature differentials.
Highly directive antenna patterns are required to achieve a reasonable link budget for mmWave technology; however, the reliability suffers as the link is prone to beam misalignments and blockages.
The multibeam system utilizes multipath to its advantage. Image used courtesy of Jain et al
As mentioned earlier, hoping to mitigate these two issues is the key focus of the research conducted by the UC San Diego team.
Two Beams Technology Introduces Multipath
The research test platform used by these UC San Diego researchers operates at 28 GHz with a 400 MHz bandwidth transmitted on a 64-element phased array.
The system is based on consumer off-the-shelf (COATS) subsystems, is 5G NR compatible, and is said to provide a 2.3x improvement in a 'throughput-reliability' product compared to single-beam systems.
Signal-to-noise ratio improvements with multibeam. Screenshot used courtesy of Jain et al
Developing a multi-beam system requires two major hardware features, phase control and power control, in addition to novel algorithm development. Doing just that, these researchers have developed an algorithm set that completes two main tasks: beam training and beam maintenance.
The building blocks of "mmReliable" from UC San Diego researchers. Screenshot used courtesy of Jain et al
The beam training process is used to determine the optimal path for the maximal signal-to-noise ratio and throughput, either via a reflected beam or line of sight.
Then, utilizing phase and power control [video] over the phased array enables the multi-beam technology to direct more power towards the least obstructed angle and constructively add phases. This process is said to provide the stated 2.3x improvement over conventional single-beam systems.
What exactly are the benefits and limitations of this solution?
Benefits and Limitations of Multibeam Technology
All in all, correcting user mobility antenna misalignment and the mitigation of blocked paths are the principal goals of this technology. The research data shows a substantial improvement in reliability along with an improvement in overall throughput concerning standard mmWave 5G single-beam technologies.
Four evaluations for multi-beam mmWave.Screenshot used courtesy of Jain et al
As for benefits, this technology is analyzed in four categories: maximum throughput, reliability, throughput-reliability product, and beam maintenance probing overhead. In each category, the research indicates overall improvement over single-beam technologies.
Although promising, there are several critical limitations that the researchers acknowledge needing further work.
First, the presence of low loss reflective surfaces is a required feature for this technology to function.
Further, there is performance overhead where possible tracking error in the beam-maintenance phase of the operation could require additional beam training sessions.
Finally, the current system utilizes a single RF link for one user. The researchers are currently examining the methods to achieve a multi-user scenario.
Algorithms development is the natural next step since the advent of electronically steered antenna technology. Dynamically tracking signal-to-noise ratio (SNR), and optimizing the RF link as environmental conditions change, will likely become necessary for the commercialization of mmWave technology.
The multibeam technology developed by UC San Diego makes a solid argument for the feasibility of using multipath effects as a beneficial design criterion.
It will be interesting to see what else comes out of this project with further testing, as well as what could come from outside of research institutions to solve these 5G mmWave challenges.
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