U.S. military researchers are asking industry to develop secure radio frequency (RF) transmitter and receiver technologies to enable the next generation of secure military tactical radio systems. Tactical communications is critical requirement of military communications in which information of any kind, especially orders and military intelligence, are conveyed from one command, person, or place to another upon a battlefield, particularly during the conduct of combat. Secure tactical radios are a critical capability for DoD systems. Current DoD tactical radios attempt to achieve security over a wireless channel by spreading transmitted content over time and operating frequency. Temporal and spectral spreading reduces transmitted power density, so as to potentially operate below the adversary’s receiver detection limit.
The basis of spread spectrum technique is Shannon’s formula for channel capacity provides a relationship between achievable bit rate, signal bandwidth and signal to noise ratio. Channel capacity is proportional to bandwidth and the logarithm to the base of two of one plus the signal to noise ratio. What this means is that the more bandwidth and the better the signal to noise ratio, the more bits per second you can push through a channel. This is indeed common sense. However, let us consider a situation where the signal is weaker than the noise which is trashing it. Under these conditions this relationship becomes much simpler, and can be approximated by a ratio of Capacity/Bandwidth = 1.44* SNR. What this says is that we can trade signal to noise ratio for bandwidth, or vice versa. If we can find a way of encoding our data into a large signal bandwidth, then we can get error free transmission under conditions where the noise is much more powerful than the signal we are using. This very simple idea is the secret behind spread spectrum techniques.
Direct Sequence Spread Spectrum Radios (DSSS) radio starts with a data packet that it is required to send. But instead of sending that data as a “narrow band” signal, the DSSS generates a 15, 63 or 127-bit pseudo-random code word for every bit in that packet and then combines these code words with the packet by multiplying them together. These code words spread the “narrow” data being sent across a much wider bandwidth than would normally be required. The result is a signal with lower power density, stretched across a wide bandwidth waiting for a receiver to find it.
At the DSSS receiver, the same spreading code is reapplied to the spread received power signal once it is found and the wideband signal is narrowed. The data is retrieved intact because the spreading process is independent of the data and when the spreading is canceled; the data is left in its original state. The data has moved from a narrow state to a wide one, and back to narrow. The nature of this process enables data to be moved through the DSSS radio very quickly, and in terms of security, the radio signal essentially hides amidst the “background noise,” undetectable.
But what happens when a narrowband interfering signal enters the wide low power signal’s bandwidth? If the original data signal is stronger than the interference, the spreading of the interfering signal by the receiver’s code generator is rejected in favor of the more powerful narrowed data signal and 100% of the data gets through. The DSSS has overcome its interference. But when the strength of the interfering signal exceeds that of the original data signal by some margin (depending on the radio’s process gain or upper jamming margin), errors occur repeatedly and data throughput of the DSSS radio ceases.
Today’s tactical radios use spread spectrum techniques to reduce the spectral power density of transmitted signals. Signal spreading is mainly used to accommodate spectrum sharing by multiple users and to reduce transmission power below the sensitivity of the adversary’s detection threshold. The limitations of current spread spectrum approaches include:
- Narrowband signals that are only spread in the time and frequency domains and that contain cyclic features. The RF waveform is generally a narrow band with prescribed fixed and limited dynamic range (< 30 dB), leading to the inability to remain undetectable while providing persistent communications. Even worse, the transmitted signal can be detected by adversarial receivers with highly sensitive detectors and cyclostationary feature detection. More recent waveforms, such as chaotic waveforms reducing cyclic features, only provide marginal reduction of detectability, require higher signal-to-noise ratios to synchronize and operate, and are not sufficiently featureless to evade detection.
- Highly directional beams and reconstruction of coherent scattered signals are impractical to implement using today’s tactical radios. While detection could be mitigated by the use of highly directional antennas, at low-frequency such antennas are too large to be portable. Spatial-temporal spreading is not possible for current tactical radios whose narrow bandwidth of 10 to 100’s MHz produce low spatial resolution or require vast antenna size. Reconstruction of scattered signal using digital rake receivers can only recover a few multipath signals, limited by high processing power and very poor recovered signal strength.
- Loss of connectivity or data rate due to environmental impairments. While spread spectrum techniques minimize the signal strength in order to avoid being detected, today’s tactical radios face additional operational challenges due to channel impairments that reduce the link margin of the radio. With fixed operational frequency and bandwidth, existing tactical radios provide limited options and margins to sustain persistent transceiver operations under varying and unpredictable natural and man-made channel impairments.
In addition, current spread spectrum techniques lack sufficient complexity to evade detection by modern signal intelligence (SIGINT) receivers or interception by compromised devices. Specific vulnerabilities of current secure tactical radios include:
a) Hypersensitive receivers. Modern SIGINT receivers utilize cryogenic cooled energy detectors and cyclostationary processing over prolonged observation time to increase detection sensitivity by significantly reducing uncorrelated noise. This technique reveals salient cyclic features, such as chip rate, modulation format, etc., that are used to establish spread spectrum transmissions.
b) Collaborative receivers. Current tactical RF communication systems are vulnerable to interception by remote adversary multi-receiver networks that coherently recombine power to detect the transmitter.
The WiSPER program seeks to develop a fundamentally disruptive wireless air interface transceiver technology to enable and sustain secure and robust high-bandwidth RF communication links. The WiSPER wideband adaptive air interface will also mitigate impairment due to dynamic harsh and contested environments to maintain a stable communication link. It is envisioned that the capabilities developed under the WiSPER program will provide future U.S. warfighters with a dominant technology advantage over their adversaries. DARPA is soliciting solutions that are implemented in a tactical radio size, weight, and power suitable for portable or ground installations.
WiSPER is a 48-month, three-phase program with an 18-month Phase 1, 18-month Phase 2, and 12-month Phase 3. WiSPER consists of a single highly focused technical area.
Phase 1 performers will carry the WiSPER system architecture through a conceptual design supported by modeling and simulation, as well as critical experimentation, culminating in a benchtop implementation and lab test.
Phase 2 performers will enhance the design through to improve the performance, culminating in a transportable implementation and field test.
Phase 3 performers will further optimize the air interface to demonstrate adaptation to weather and other impairments in a portable prototype implementation
Officials of the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., awarded contracts in March 2021 to CACI International Inc. in Florham Park, N.J.; and to Perspecta Labs Inc. in Basking Ridge, N.J., for the Wideband Secure and Protected Emitter and Receiver (WiSPER) project.
CACI won a $30.7 million contract on 25 March, and Perspecta Labs won a $19.2 million on 24 March. The companies are participating in the project’s first phase, by carrying the WiSPER system architecture through a conceptual design supported by modeling and simulation, culminating in a benchtop implementation and lab test. More WiSPER phase-one contracts may be awarded.
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