To get the most use out of scarce spectrum, technologies have emerged that permit single systems to accommodate both radar and communications functions. Dual-function radar communication (DFRC) systems, where the two systems use the same platform and share the same hardware and spectral resources, form a specific class of radio-frequency (RF) technology. These systems support applications where communication data, whether as target and waveform parameter information or as information independent of the radar operation, are efficiently transmitted using the same radar aperture and frequency bandwidth. This is achieved by embedding communication signals into radar pulses. In this article, we review the principles of DFRC systems and describe the progress made to date in devising different forms of signal embedding. Various approaches to DFRC system design, including downlink and uplink signaling schemes, are discussed along with their respective benefits and limitations. We present tangible applications of DFRC systems and delineate their design requirements and challenges. Future trends and open research problems are also highlighted.

Dual-Function Radar Communication Systems: A solution to the spectrum congestion problem

Frequency-hopping (FH) MIMO radar-based dual-function radar communication (FH-MIMO DFRC) enables communication symbol rate to exceed radar pulse repetition frequency, which requires accurate estimations of timing offset and channel parameters. The estimations, however, are challenging due to unknown, fast-changing hopping frequencies and the multiplicative coupling between timing offset and channel parameters. In this article, we develop accurate methods for a single-antenna communication receiver to estimate timing offset and channel for FH-MIMO DFRC. First , we design a novel FH-MIMO radar waveform, which enables a communication receiver to estimate the hopping frequency sequence (HFS) used by radar, instead of acquiring it from radar. Importantly, the novel waveform incurs no degradation to radar ranging performance. Then , via capturing distinct HFS features, we develop two estimators for timing offset and derive mean squared error lower bound of each estimator. Using the bounds, we design an HFS that renders both estimators applicable. Furthermore , we develop an accurate channel estimation method, reusing the single hop for timing offset estimation. Validated by simulations, the accurate channel estimates attained by the proposed methods enable the communication performance of DFRC to approach that achieved based on perfect timing and ideal knowledge of channel.

Waveform Design and Accurate Channel Estimation for Frequency-Hopping MIMO Radar-Based Communications

A view on radar and communication systems coexistence and dual functionality in the era of spectrum sensing

In this paper, a new technique which enables communication data to be embedded into multiple-input multiple-output (MIMO) radar waveforms is presented. A linear frequency modulated (LFM) signal is used for radar sensing, and multiple phase-shift keying (PSK) symbols or bit sequences are embedded into the LFM signal for communications. Such waveforms are subsequently used for radar sensing with MIMO beamforming. Orthogonality between transmitters is ensured using either a time-division multiplexing (TDM) or code-division multiplexing (CDM) approach. The performance of these novel techniques is demonstrated through both simulation and experimentation.

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Joint MIMO Radar and Communication System Using a PSK-LFM Waveform With TDM and CDM Approaches

In this paper, we propose a method for information embedding into the emission of frequency hopping (FH) multiple-input multiple-output (MIMO) radar. The differential phase shift keying (DPSK) modulated communication symbols are embedded into each pulse of the FH radar waveforms. We examine the effect of DPSK symbol embedding on the radar operation by analyzing the range sidelobes performance, power spectral density (PSD) and data rate of the system. The proposed system shows significant reduction in range sidelobes, good spectral containment and achieves high communication data rates. The latter is enabled by synthesizing a large number of orthogonal waveforms.

Dual Function FH MIMO Radar System with DPSK Signal Embedding

Intensifying competition over the frequency spectrum has driven the research effort into strategies for coexistence between radar and communications. Strategies for achieving this range from spectrum sharing using cognitive radio techniques to co-design where the radar and communications systems are re-imagined to ensure they do not interfere with each other. In this paper we propose a new signaling scheme for Dual-Function Radar Communications (DFRC) that enables frequency-hopped multiple-input-multiple-output (MIMO) orthogonal radar wave-forms to carry communication symbols. The Frequency-hopping (FH) code is changed from subpulse to another such that the index of the selected code carries the desired symbol. Contrary to recent phase-shift keying (PSK)-based schemes which embed one PSK symbol within each subpulse, the proposed scheme does not suffer from phase-discontinuity and is shown to have better spectral efficiency. We show that the data rate that can be achieved using the proposed scheme is proportional to the size of the FH code, the number of transmit antennas, number of subpulses within a pulse repetition interval, and pulse repetition frequency. Simulations examples are provided to evaluate the performance of the proposed method and demonstrate the effectiveness of this information embedding scheme.

Dual-Function MIMO Radar-Communications Via Frequency-Hopping Code Selection

Contention in the frequency spectrum has seen the emergence of Dual-Function Radar Communications (DFRC) systems, enabling frequency-hopped (FH) multiple-input-multiple-output (MIMO) waveforms to carry communication symbols. While a variety of novel signaling strategies have been developed to facilitate the communication function, their implementation in slow-time results in the achievable data rate being limited by the radar pulse repetition interval. We develop a generalized framework for performing information embedding in DFRC systems by exploiting the fast-time structure of the transmitted radar waveform. By defining a unified formulation, we show that a variety of existing signaling strategies can be accommodated, including FH index modulation, FH permutation, quadrature amplitude modulation (QAM), M-arry PSK (MPSK) modulation and/or frequency carrier index modulation. In addition, we use this framework to propose hybrid modulation strategies constructed using combinations of the aforementioned schemes to produce significant improvements in the achievable data rate over the individual signaling schemes. Simulation results demonstrate that the hybrid schemes can deliver significantly higher bit rates with only small increases in the required $E_b/N_0$. They also show that, in terms of the impact on the radar operation, the frequency hopping code selection has the highest range sidelobes whereas the PSK schemes suffer from significant spectral leakage. Finally, we also give a discussion of the issues and open problems that remain unaddressed.

Joint Radar and Communications for Frequency-Hopped MIMO Systems

We present a fast direction-of-arrival (DOA) estimation algorithm for coprime array structures. In many practical applications of radar signals processing, reducing the redundancy inherent in linear arrays is advantageous. Arrays employing a coprime-pair configuration offer an increased number of degrees of freedom and permit the detection of more sources than sensors. Traditionally, this is achieved using conventional subspace-based methods. While these methods, such as MUltiple Signal Classification (MUSIC), are capable of high-resolution, the computational complexity of these algorithms is a significant hindrance to their practical implementation as the number of sensors increases. Furthermore, they must resort to spatial smoothing in order to calculate the covariance matrix. We, on the other hand, propose a fast iterative interpolated beamforming algorithm that has a computational complexity of the same order as the fast Fourier transform (FFT). The proposed approach is capable of delivering high fidelity DOA estimates that outperform the traditional high resolution methods. Simulation results demonstrate the DOA estimation performance of the technique.

Fast Direction-of-Arrival Estimation in Coprime Arrays

In radar, the minimisation of redundancies within the sensing array can lead to significant hardware computational savings. Arrays with a coprime-pair configuration enjoy increased degrees of freedom and can detect more sources than sensors. To achieve this, a virtual array (VA) consisting of the full complement of lags is usually constructed and subspace-based algorithms are then employed to obtain the direction-of-arrival (DOA) or frequency estimates. However, the application of the subspace techniques to the VA incurs a significant computational cost and requires spatial smoothing. The authors propose and analyse the application of the fast iterative interpolated beamformer (FIIB) to the coprime DOA estimation problem. The FIIB enjoys a computational complexity of the same order as the fast Fourier transform and does not require spatial smoothing. They consider two implementations that construct the VA differently with the first selecting a single estimate for each lag and the other employing averaged values of the lag estimates. They present a comprehensive study of the estimation performance as a function of signal-to-noise ratio, number of snapshots and source separation. The results clearly show that the FIIB delivers high-fidelity frequency estimates that consistently outperform the high-resolution subspace-based methods.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rsn.2018.5647

Coprime beamforming: fast estimation of more sources than sensors

Increasing pressure on the available spectrum, particularly from wireless communications services, has led to significant research into strategies for coexistence between radar and communications. In this context, Dual-Function Radar-Communications (DFRC) systems have emerged as a potential solution to the spectrum congestion problem. DFRC schemes treat the radar as the primary modality and aim to embed communications symbols into the transmitted waveforms. One such approach is to use the frequency hops in a Frequency-Hopped (FH) Multiple-Input Multiple-Output (MIMO) radar to encode communications symbols. While this scheme allows for higher data rates, embedding the information in the fast time impacts the radar performance. Therefore, the code book selection is an important design consideration that we focus on in this work. We discuss the impact of the selection of the code book on the ambiguity functions of the radar waveforms and elucidate its relation to the probability of having degenerate waveforms. We illustrate these ideas using two code book selections, namely an arbitrary selection and a balanced code book. The results show that balancing the codebook leads to improved radar performance.

William Baxter

Papers

Dual-Function MIMO Radar-Communications Via Frequency-Hopping Code Selection

Joint Radar and Communications for Frequency-Hopped MIMO Systems

Fast Direction-of-Arrival Estimation in Coprime Arrays

Coprime beamforming: fast estimation of more sources than sensors

A Study on the Performance of Symbol Dictionary Selection for the Frequency Hopped DFRC Scheme