As electromagnetic spectrum availability shrinks, there is growing interest in combining multiple functions, such as radar and communications signals, into a single multipurpose waveform. Historically mixed-modulation has used orthogonal separation of different message signals in different dimensions such as time or frequency. This research explores an alternative approach of implementing an in-band, mixed-modulated waveform that combines surveillance radar and communication functions into a single signal. The contribution of this research is the use of reduced phase-angle binary phase shift keying (BPSK) along with overlapped (channelized) spread-spectrum phase discretes based on pseudorandom noise sequences to encode multiple messages in a single pulse. The resulting mixed-modualted signal provides a low data rate communications message while minimizing the effect on radar performance. For the purpose of this research, radar performance will be evaluated in terms of power spectral density, matched filter auto-correlation for target detection, and the ambiguity function.

Co-designed radar-communication using linear frequency modulation waveform

This paper investigates the combination of chirp modulation and pseudonoise phase shift keying for spread-spectrum transmission of digital data. Data encoding is accomplished by assignment of positive and negative dispersive slope chirps for binary ones and zeros. Each chirp is then modulated by one period of a PN signature sequence. The performance in additive white Gaussian noise is analyzed, measures of performance being mean acquisition time, mean hold-in time, and bit error probability. Calculated and experimental results are presented for a 105 kbit/s modem. In the test system, SAW dispersive delay lines with a bandwidth of 16.4 MHz and 9.5 μs dispersion time serve for the key operations of chirp generation and pulse compression, and the signature codes are 127 bit m -sequences.

A Spread-Spectrum Concept Combining Chirp Modulation and Pseudonoise Coding

Increasing congestion of the electromagnetic spectrum has spurred investigation into approaches that provide more efficient spectrum use. While many approaches focus on some type of orthogonality in time, frequency, or coding, another less explored option is mixed modulation of two complementary signals through intentional modulation of pulse. This study discusses intentional modulation of a linear frequency modulated (LFM) chirp radar pulse with a communications signal in order to conduct complementary activities with a combined signal. In order to reduce cross-interference between the signals, the communications message employs spread-spectrum encoding (binary M-sequences) and new modulation scheme with reduced phase change to modulate the LFM waveform. Preferred pair of M-sequences exhibit excellent auto-correlation and low cross-correlation properties that lend themselves to simultaneous transmission of channelised communication signals. The proposed approach is to modulate a radar signal with preferred pair M-sequences using binary reduced phase shift keying modulation that provides a low throughput, communications signal that could be used for administrative or navigation purposes. To measure effect on radar performance, comparison of the radar ambiguity function for the LFM signal is contrasted with combined radar-communication signal.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-rsn.2016.0249

Mixed-modulated linear frequency modulated radar-communications

The chirp signal is one of the first bio-inspired signals commonly used in RF applications where the term chirp is a reference to the chirping sound made by birds. It has since been recognized that birds communicate through such chirping sounds to attract other birds of the same species, to transmit an alarm for specific threats, and so on. However, birds of a different species, or sometime even birds in a different social group within a species, are unable to connect a specific meaning to certain calls - they will simply hear a bird chirping. Inspired by such, this article provides a tutorial on a novel RF steganography scheme to conceal digital communication in linear chirp radar signals. We first provide a review of the linear chirp signal and existing communication systems using chirp waveforms. Next we discuss how to implement the RF steganography and hide digitally modulated communication information inside a linear chirp radar signal to prevent an enemy from detecting the existence of such hidden information. A new modulation called reduced phase shift keying is employed to make the modulated chirp waveform almost identical to the unmodulated chirp signal. Furthermore, variable symbol durations are employed to eliminate cyclostationary features that might otherwise be exploited by an enemy to detect the existence of the hidden information.

Bio-inspired RF steganography via linear chirp radar signals

A photonics-based wideband distributed coherent aperture radar (DCAR) system is proposed and experimentally demonstrated. In the proposed system, the central controlling system and several spatially dispersed remote transceivers are connected by the optical fiber-based time synchronization network. In the central controlling system, the optical-carried orthogonal/coherent linear frequency modulated waveforms (LFMWs) are generated by a reconfigurable multi-channel optical arbitrary waveform generator (RMOAWG), and the signal processing for the echo waves is also implemented there. While in the remote transceivers, only the optical/RF and RF/optical conversions are carried out. Benefitting from the use of photonics-based methods, bandwidths of the generated radar signals can be large, improving the detection resolution of the system. Due to the centralized signal generation and processing, the remote transceivers can be simplified, reducing the system complexity. Moreover, the fiber-based distribution ensures low loss, good transportability, and great flexibility. Experimentally, a two-unit DCAR system operating in X-band with a bandwidth of 3 GHz is presented. When full coherence is achieved, signal-to-noise ratio (SNR) gains of 8.3 dB and 8.33 dB are obtained over a single radar for radar 1 and radar 2, respectively. Results are in good agreement with theoretical prediction. Theoretically, with such SNR gain, the range detection precision can be improved to about 2.6 times that of a single radar.

Photonics-based wideband distributed coherent aperture radar system.

The combination of phase shift keying (PSK) with different chirp waveforms is investigated in respect to its signal form and spectrum shape. A reasonable compromise for low probability of exploitation and jam sensitivity is found for a system where the chirp bandwidth is in the vicinity of the bitrate of the pseudonoise code.

Johann Lafferl

Papers

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