NRL Signal Processing Method Improves Communications
To improve long-range radio frequency communications, researchers at the U.S. Naval Research Laboratory are working on a method of combining signals at the receiver level after multiple relays and a single transmitter. The solution is meant to extend line-of-sight communications. Finding an affordable alternative to beyond-line-of-sight (BLOS) communications usually provided by satellites, aircraft or drones is an important priority for the Navy and the Defense Department, says Aaron Cohen, computer engineer and electrical engineer at the Naval Research Laboratory, known as the NRL.
Engineers at the NRL have been working on improving long-range communications for several years. The technique Cohen and fellow researchers developed sends the same information on different frequencies from a single source; uses several simple, inexpensive airborne relays to send the signals; and combines it into one enhanced signal for the receiver, he explains.
The method improves “the robustness of a BLOS communications link” without relying on satellites or manned aircraft, according to their published paper, “Signal Combination Techniques to Improve Long Range Communication with Multiple Relays,” presented last year at the AFCEA/IEEE MILCOM conference in Los Angeles by Cohen and fellow NRL researchers David Heide and Thomas Moran. The NRL also published the results of their efforts in a recent NRL memorandum report earlier this year.
“Our research group has a large range of expertise in a variety of areas, ranging from voice coding technology, error control, digital signal processing, communication systems, embedded systems and standardization processing,” Cohen recently told SIGNAL. “Using all this expertise we’ve gained throughout the years, we were able to come up with what I consider is an elegant method for long-range communications. It relies on the multiple paths for long-range narrowband, radio frequency transmission created using a multiple relay approach rather than relying on a nonideal frequency-dependent physical effect for a radio frequency transmission. The multiple relays allow for more reliable communications. That’s our technique.”
Compared with other signal processing approaches, the NRL’s low-complexity method is done at the baseband layer, which makes it less expensive to implement, Cohen reports. “The low complexity doesn’t take as many resources, so it’s good for embedded systems and transitioning into embedded systems,” he says. “I like these kinds of techniques that give options … and allow us to explore different methods than what might be commonly used.”
The researchers wanted to make the communications technique versatile, so warfighters could employ various relays, Cohen explains. “There’s no specific designated relay platform,” he says. “Relays come in many sizes and shapes. Some are manned; some are unmanned. Communication researchers and even communication program managers, they’ve been very successful in the past leveraging different kinds of platforms that might not have been originally designed as relays. It could be balloon or UAV relays, almost whatever you can dream up. So there are a whole lot of options there [to use] whatever is available [and] low cost.”
The scientists focused their research on the receiver system. They wanted to enable the receipt of low-quality radio frequency signals from various unorganized relays, and then decode the signal into separate basebands before combining them. And because they did not identify a specific frequency band, the researchers were able to explore using different frequencies independent of underlying hardware restrictions or antenna restrictions, Cohen says.
“Relay signals may be degraded by fading channels, low transmission power, transmitter and receiver antenna orientation/polarization, and nonideal locations of relays,” according to the research paper published by IEEE. “Combining incoherent signals transmitted from multiple, uncoordinated relays for enhanced reception at a single receiving platform is the primary goal. While each received signal from each relay may be weak and distorted, it is the job of the receiver to combine these signals into one improved composite signal.”
To join the relay signals at the baseband or bitstream, the researchers relied on measurements of the individual bitstream bit error rates, instead of signal strength or quality, the report indicates. “In every case, results showed that the linear combination of the relay channels yielded much better performance than only using the single best relay,” the report states.
To test receiver functionality, the team simulated fading channels for an urban environment with no direct line of sight, as well as simulating a rural environment with direct line of sight to the relay. Their testing relied on three to five relays, with the use of three relays providing the best results.
“Rather than failing when the bit error rates are high, we have the diversity in these relay paths, which allows us to combine these multiple paths to improve the receiver performance,” Cohen says. “We transmit our desired signal at different frequencies using these relays, so that we get a kind of a diversity in the frequencies. And the weighting average of the signal then ensures that the best received signals are rated the highest. So the frequency diversity in the original transmission is through the relay themselves.”
Without relying on GPS or network synchronization, the researchers also had to be able to time-adjust the signals, because of time shifts in the bitstream created by the varying transmission distances from the relays.
“That’s part of what makes this different from signal processing techniques is that there’s no new challenge with regards to time alignment with this approach,” Cohen clarifies. “You can still do it at the baseband bit-level or elsewhere, which makes this solution a very viable transition to existing radios, no matter which underlying method they have used for time alignment.”
And as far as how much degradation of a signal can be tolerated before it cannot be combined, Cohen explains that, “In terms of how we’re combining the signals, any degradation will come with a sufficiently high number of what we call foot bits … and for our research we’re looking at foot bits occurring across all the redundant transmissions. So you have to know how many redundant lengths you’re weighting across and how many foot bits. And at some point, if you don’t have enough redundant lengths and there’s too many foot bits, you won’t be able to get a good reliable received transmission at the end.”
Overall, this so-called multiple, redundant relay approach offers several benefits to warfighters, according to the researchers. Most importantly, it allows radio frequency signals to be sent farther. The necessary relays—simple, low-cost airborne relays—can be in various, even suboptimal, locations. Using multiple relayed signals eliminates the single point of failure seen in traditional approaches.
In addition, the ability to combine the demodulated digital signals at the bitstream level—versus combining the analog radio frequency waveforms—allows the use of any type of receiver hardware, the researchers say.
Software-defined radios also have made a huge difference in creating solutions for radio frequency communications such as this method, says Cohen. “With a lot more software-defined components, you’re writing more software and not necessarily as much hardware,” he offers.
Also, warfighters can operate the relays using “very low transmission power levels,” according to the research paper. This enables low probability of intercept and detection communications.
“Some paths will have more interference than others, so at the far end user, he will be able to, using our technique, he will have reliability measurements from the error correction code, and he can then weight the receipt path and combine them to get a more reliable signal at the end,” Cohen explains.
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