Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> A criterion for selecting a finite set of transmitter signals for a continuous communication channel is proposed. The "optimum" signal sets using this criterion are selected to maximize the minimum divergence between hypothesis pairs being tested at the receiver. The resulting signal sets have the property that the error probability using these signals is less than the error probability for any other choice of signals for some {\em a priori} message statistics. The signal selection procedure may be applied without a knowledge of the {\em a priori} message statistics and does not require an evaluation of error probabilities. Four examples of signal selection are included to illustrate the procedure. <s> BIB001 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> A radar system can be used in conjunction with a coherent transponder as a communications system. A coherent transponder ("RF tag") received a stream of radar pulses, modifies these and transmits them back to the radar. This enables the RF tag to embed information in the radar data stream. We derive capacity formulas for this radar channel and then calculate these capacities for a specific SAR radar system and the RF tag configuration. We characterize the dependence of capacity on pulse repetition frequency, standoff range, and resolution. <s> BIB002 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Far-field target detection, multi-sensor communications, and security are essential requirements of first-emergency networks. A radar-communications system is a potential opportunistic spectrum access (OSA) solution, harnessing the coexisting advantages of radio detection and ranging (RADAR), and wireless communications. A multi-functional waveform has been designed, by embedding an Orthogonal Frequency Division Multiplexing (OFDM) signal within a spectrally notched ultra-wideband (UWB) random noise waveform. Extending that development, this paper analyzes the waveform's Bit-Error-Rate (BER) and Ambiguity Function (AF) formulations to demonstrate its OSA ability, that offers reliable multi-user communications, and high range and Doppler resolution in target detection. We further conclude that up to 30% of the available UWB bandwidth can be simultaneously utilized for concealed data communications without adversely affecting radar performance or its physical layer covertness. <s> BIB003 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> The embedding of a covert communication signal amongst the ambient scattering from an incident radar pulse has previously been achieved by modulating a Doppler-like phase shift sequence over numerous pulses (i.e., on an inter-pulse basis). In contrast, this paper considers radar-embedded communications on an intrapulse basis whereby an incident radar waveform is converted into one of K communication waveforms, each of which acts as a communication symbol representing some predetermined information (e.g., a bit sequence). To preserve a low intercept probability, this manner of radar-embedded communications necessitates prudent selection of the set of communication waveforms as well as interference cancellation on receive. A general mathematical model and subsequent optimization problem is established for the design of the communication waveforms, from which three design strategies are developed. Also, receiver design issues are discussed, and an interference-canceling maximum likelihood receiver is presented. Performance results are presented in terms of the communication symbol error rate as well as a correlation-based metric from which intercept probability can be inferred. It is demonstrated that, given persistent radar illumination with a pulse repetition frequency (PRF) of 1-2 kHz, intrapulse radar-embedded communications can theoretically achieve data-rates commensurate with speech coding (for the interval of the radar dwell time) with the potential for even higher data-rates if additional diversity is appropriately incorporated. <s> BIB004 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Multifunctional radar is a trend in the future. Employing radar system for high data rate, long range, anti-intercepted communication by making use of its wide bandwidth, high power and directing property has been a research hotspot nowadays. Hereinto, radar-communication compatible waveforms design is a key technique. This paper proposes a novel compatible waveform produced by modulating LFM signal by MSK and mainly explores its ambiguity function (AF) and impacts of the AF by parameters of the waveform. Keeping the total pulse width and LFM coefficient constant, theory analysis and simulations show properties as follows: 1. AF of modulating LFM is closer to thumbtack shape than that of non-modulating LFM. 2. The larger the bit number, the closer to the thumbtack shape the AF is. 3. Whether modulating or not, resolutions of Doppler and delay are the same. 4. For zero Doppler slice, the larger bit number results in the more suppression of the side lobes near to the main lobe. From these properties, it can obtain a good radar waveform of which AF is closer to thumbtack shape by increasing bit number. <s> BIB005 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> The presence of clutter in passive radar returns imposes a minimum floor level on the ambiguity surface, masking weak returns and reducing dynamic range. When the illuminating signal is a form of OFDM, orthogonality between subcarriers can be exploited to produce a zero floor within certain delay bins. In this paper we compare and contrast several such ambiguity processing techniques for general OFDM signals. The methods are applied to simulated and real DVB-T passive radar returns having a wide range of delays. It is shown that an OFDM-specific method can outperform a standard pulse-Doppler processing method under certain conditions. An example is provided using real German DVB-T data from Fraunhofer FHR. <s> BIB006 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Previous research has shown communication waveforms have potential for passive and active radar applications. The potential of a signal is typically characterized using the ambiguity function. This paper seeks to summarize the frequency division duplex (FDD) long term evolution (LTE) downlink (DL) signal structure and illustrate its effect on the signal's self and cross ambiguity functions. In addition, performance limitations resulting from the FDD LTE DL signal structure are predicted. The variability of system performance limitations is considered for variable FDD LTE DL signal parameters. Lastly, simulated range profiles are produced using a simulated FDD LTE DL signal illustrating the derived results. <s> BIB007 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Passive radar systems offer several merits including the ease of deployment, low-cost, and non-detectability of the receivers. Traditionally, they have been employed in bistatic configuration. However, in this paper, we consider a UMTS-based passive multistatic radar with distributed antennas. We compute the ambiguity profile of this radar system under both the coherent and non-coherent processing modes in the Cartesian domain. The non-coherent processing mode improves the target detection performance by obtaining spatially diverse looks of the target. On the other hand, coherent processing enhances the resolution of target localization. We use numerical examples to demonstrate our analytical results. <s> BIB008 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Mobile communications systems are being increasingly deployed and the favorable ambiguity function properties of these signals make them useful for passive bistatic radar applications. Further, simultaneously using multiple illuminators of opportunity in a multistatic configuration will enhance the radar performance, providing spatial diversity and increased resolution. We compute modified Cramer-Rao lower bounds (MCRLB) for the target parameter estimation error using universal mobile telecommunications system (UMTS) signals as illuminators of opportunity for passive multistatic radar systems. <s> BIB009 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> This paper considers detection in passive multiple-input multiple-output (MIMO) radar sensor networks. Multiple centralized and decentralized detection architectures are surveyed and compared via Monte Carlo simulation. A recently proposed generalized likelihood ratio test (GLRT) detector, termed the reference-surveillance GLRT, is shown to have superior detection performance because it maximally exploits the correlations in the measured data. Specifically, it exploits inter-receiver reference-surveillance correlations and inter-receiver surveillance-surveillance correlations, two concepts that are explained in this paper. In this way, the reference-surveillance GLRT achieves better sensitivity under all direct-path-to-noise ratio conditions than more conventional matched filter-inspired detection approaches, which exploit only some of the correlations within the measured data. <s> BIB010 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> This paper investigates the application of a non- parametric data-dependent mismatched filtering method in GSM based passive radar systems. The Iterative Adaptive Approach for Amplitude and Phase Estimation algorithm (IAA-APES) is applied and its performance is compared to the Matched Filtering (MF) method. IAA-APES is adapted to the passive radar estimation task and a comparison in terms of range resolution and SNR loss is performed using simulated data. I. INTRODUCTION GSM based passive radar or passive coherent location (PCL) systems became very attractive due to the vast availability of potential transmitters. These illuminators of opportunity can be exploited for detection and tracking of moving targets in several applications (maritime, airborne and on ground). The experimental system GAMMA2 designed by Fraunhofer FKIE has proven the feasibility of such a technology (1), (2). The presented system comprises all the different processing blocks from the antenna to the tracker as indicated in Fig. 1. GAMMA2 was successfully tested for maritime surveillance tasks at the Baltic Sea (2). <s> BIB011 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> In this paper, we design a rake-based cellular radar receiver (CRR) scheme to detect a moving target located in a multipath environment. The modules of Doppler filter banks, threshold level test, and target detection module are newly introduced into the conventional rake receiver so that it can function as a radar system. The proposed CRR tests the Doppler-shift frequency and signal-to-noise ratio of the received signal against predefined threshold levels to determine detection and then calculates target velocities and ranges. The system performance is evaluated in terms of detection probability and the maximum detection range under a Nakagami-n channel that reflects the multipath environment. <s> BIB012 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> In this paper, we develop a new technique for dual-function radar-communications in a transmit multi-sensor array where information embedding is achieved using phase-rotational invariance. A sequence of Q bits is first mapped into a dictionary of 2Q phase rotations. Then, one pair of transmit orthogonal waveforms is used in tandem with 2Q pairs of transmit beamforming weight vectors for embedding a certain entry of the phase-rotation dictionary during each radar pulse. The same pair of waveforms is used during all pulses while the pair of transmit beamforming weight vectors changes from pulse to pulse based on which entry of the phase-rotation dictionary is embedded. During each pulse, the receiver detects the embedded phase rotation and employ it to decipher the transmitted bit sequence. The proposed information embedding technique is angle-dependant and, therefore, the communication process is inherently secure against interception from directions other than the desired communication direction. The performance of the proposed technique is investigated in terms of the bit error rate (BER). <s> BIB013 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> In this paper, we develop a new technique for dual-function radar-communications in a transmit multi-sensor array where information embedding is achieved using sidelobe control in tandem with waveform diversity. A set of Q orthogonal waveforms is exploited to embed a sequence of Q bits during each radar pulse. All waveforms are transmitted simultaneously where one bit is embedded in each waveform. We design two transmit weight vectors to achieve two distinct transmit power distribution patterns which share the same main radar beam but have different sidelobe levels towards the communication direction. The receiver interprets the bit associated with a certain waveform as binary information based on whether that waveform is radiated over the transmit beam associated with the first or the second weight vector. The proposed technique enables information delivering to a single or multiple communication directions located outside the mainlobe of the radar. The communication message has low probability of intercept from directions other than the preassigned communication directions. Additionally, the waveform diversity enables the radar to operate in multiple-input multiple-output (MIMO) mode. The performance of the proposed technique is investigated in terms of the bit error rate (BER). <s> BIB014 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> We deal with the problem of intrapulse radar-embedded communication and propose a novel waveform design procedure based on a multiobjective optimization paradigm. More specifically, under both energy and similarity constraints, we devise signals according to the following criterion: constrained maximization of the signal-to-interference ratio and constrained minimization of a suitable correlation index (which is related to the possibility of waveform interception). This is tantamount to jointly maximizing two competing quadratic forms under two quadratic constraints so that the problem can be formulated in terms of a nonconvex multiobjective optimization. In order to solve it, we resort to the scalarization technique, which reduces the vectorial problem into a scalar one using Pareto weights defining the relative importance of the two objectives. At the analysis stage, we assess the performance of the proposed waveform design scheme in terms of symbol error rate and the so-called intercept metric. <s> BIB015 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Dual-function radar-communication (DFRC) systems enable information embedding into the radar signal emission. Existing methods for non-coherent phase-modulation DFRC employ multiple pairs of orthogonal waveforms and embed one communication symbol into each pair. The total number of symbols is equal to one-half of the number of waveforms. In this paper, we propose a new signaling strategy for embedding a higher number of communication symbols. The proposed method implements non-coherent phase-shift keying (PSK) by employing one of the orthogonal waveforms as a common reference and modulating the information in terms of the phase differences between all other waveforms and the reference waveform. The number of communication symbols that can be embedded equals the total number of waveforms minus one. We introduce two schemes for achieving a desired phase constellation. The proposed approach is shown to achieve a two-fold increase in the data rate compared to existing methods for a large number of waveforms. <s> BIB016 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> We develop a new technique for a dual-function system with joint radar and communication platforms. Sidelobe control of the transmit beamforming in tandem with waveform diversity enables communication links using the same pulse radar spectrum. Multiple simultaneously transmitted orthogonal waveforms are used for embedding a sequence of LB bits during each radar pulse. Two weight vectors are designed to achieve two transmit spatial power distribution patterns, which have the same main radar beam, but differ in sidelobe levels towards the intended communication receivers. The receiver interpretation of the bit is based on its radiated beam. The proposed technique allows information delivery to single or multiple communication directions outside the mainlobe of the radar. It is shown that the communication process is inherently secure against intercept from directions other than the pre-assigned communication directions. The employed waveform diversity scheme supports a multiple-input multiple-output radar operation mode. The performance of the proposed technique is investigated in terms of the bit error rate. <s> BIB017 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> The essence of amplitude-modulation based dual-function radar-communications is to modulate the sidelobe of the transmit beampattern while keeping the main beam, where the radar function takes place, unchanged during the entire processing interval. The number of distinct sidelobe levels (SLL) required for information embedding grows exponentially with the number of bits being embedded. We propose a simple and computationally cheap method for transmit beampattern synthesis which requires designing and storing only two beamforming weight vectors. The proposed method first designs a principal transmit beamforming weight vector based on the requirements dictated by the radar function of the DFRC system. Then, a second weight vectors is obtained by enforcing a deep null towards the intended communication directions. Additional SLLs can be realized by simply taking weighted linear combinations of the two available weight vectors. The effectiveness of the proposed method for beampattern synthesis is verified using simulations examples. <s> BIB018 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Recently, dual-function radar-communications (DFRC) has been proposed as means to mitigate the spectrum congestion problem. Existing amplitude-shift keying (ASK) methods for information embedding do not take full advantage of the highest permissable sidelobe level. In this paper, a new ASK-based signaling strategy for enhancing the signal-to-noise ratio (SNR) at the communication receiver is proposed. The proposed method employs one reference waveform and simultaneously transmits a number of orthogonal waveforms equals to the number of 1's in the binary sequence being embedded. 3 dB SNR gain is achieved using the proposed method as compared to existing sidelobe ASK methods. The effectiveness of the proposed information embedding strategy is verified using simulations examples. <s> BIB019 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> Recently, dual-function radar-communication systems in which the radar platform and resources are used for communication signal embedding have emerged as means to alleviate spectrum congestion and ease competition over frequency bandwidth. In this paper, we introduce a new technique for information embedding specific to multiple-input multiple output (MIMO) radar. We exploit the fact that in a MIMO radar system, the receiver needs to know the association of the transmit waveforms to the transmit antennas. However, this association can change over different pulse repetition periods without impacting the radar functionality. We show that by shuffling the waveforms across the transmit antennas over constant pulse repetition periods, a data rate of megabits per second can be achieved for a moderate number of transmit antennas. The probability of error is analyzed and the bounds on the symbol error rate are derived. Simulation examples are provided for performance evaluation and to demonstrate the effectiveness of the proposed information embedding technique. <s> BIB020 </s> Survey of RF Communications and Sensing Convergence Research <s> H. PASSIVE & PARASITIC SYSTEMS <s> The increasing demand of spectrum resources and the need to keep the size, weight and power consumption of modern radar as low as possible, has led to the development of solutions like joint radar-communication systems. In this paper a novel Fractional Fourier Transform (FrFT) based multiplexing scheme is presented as a joint radar-communication technique. The FrFT is used to embed data into chirp sub-carriers with different time-frequency rates. Some optimisation procedures are also proposed, with the objective of improving the bandwidth occupancy and the bit rate and/or Bit Error Ratio (BER). The generated waveform is demonstrated to be robust to distortions introduced by the channel, leading to low BER, while keeping good radar characteristics compared to a widely used Linear Frequency Modulated (LFM) pulse with same duration and bandwidth. <s> BIB021
In the same line as carrier methods, many researchers sought to analyze cellular signals in a radar context should the next generation of radar systems be forced to passively exploit communications illuminations. Some researchers looked at the ambiguity function of OFDM communications transmissions BIB006 , while others analyzed LTE waveforms and how they stack up when used for radar purposes BIB007 . The ambiguity function and Cramér-Rao lower bound (CRLB) for radar estimation were researched for multistatic passive radar exploiting Universal Mobile Telecommunications System (UMTS) signals in modern cellular architectures BIB008 , BIB009 . Others have focused on detection for multistatic passive systems in both centralized and noncentralized processing scenarios BIB010 . Some researchers have investigated alternatives to matched filtering in passive radars exploiting Global System for Mobile Communications (GSM) signals using iterative least-squares methods of bounce-path estimation BIB011 , while other research groups have employed multiple orthogonal radar waveforms with embedded communications transmissions, where one waveform is the reference and the remaining waveforms exploit the differential phase from the reference to extract the parasitic data transmission BIB016 . For example, in Reference BIB017 , the authors optimize multiple spatial waveforms to modulate radar sidelobe levels while maintaining a fixed main lobe to keep radar performance constant. The modulation of the sidelobes in amplitude encode a parasitic communications data stream. A simple example of this scheme is shown in Figure 16 , where two Chebyshev windows are chosen depending on if a 0 or 1 is to be transmitted. Here, there is some difference in the main lobe width since we have only modified the Chebyshev design parameter. More sophisticated optimization schemes such as those in VOLUME 5, 2017 Reference BIB017 ensure main lobe fidelity. Similarly, another parasitic embedded sidelobe level modulation communications scheme forms a traditional radar main beam from multiple sub-beams with various sidelobe level modulation targeted at the communications receivers  . Various extensions in this family of research have looked at other methods of embedding communications, for example an orthogonal waveform for each binary 1 to be encoded for the parasitic amplitude-shift keying (ASK) communications scheme, while omitting waveforms for each binary 0 BIB018 , BIB019 . Others have embedded communications data streams in MIMO radars by shuffling which antennas transmit which orthogonal waveform at each illumination step in patterns defined by the data BIB020 . Others have looked at shared waveforms with OFDM-like schemes employing the fractional Fourier transform to modulate data onto chirped sub-carriers BIB021 . Some have looked at performing passive detection by observing the bit error patterns in Wi-Fi protocols  , while others have exploited the output of a rake receiver to detect moving targets that present delayed and Doppler-offset reflections of the communications signals BIB012 . Signal selection relative to radar in contrast to signal selection relative to communications has been a long standing trade, tracing back over five decades BIB001 . Some approaches to shared waveform outside of coding have been investigated, including multiple threads researching embedded or parasitic communications. For example, some research has investigated phase modulating communications information on top of a linear FM chirp to add channel capacity while improving the ambiguity properties of the waveform with respect to radar BIB005 . Some have looked a more deeply embedded methods, such as a radar system modulating low-rate communications on the waveform sidelobe levels BIB013 , BIB014 . Others have looked more explicitly at covert communications by embedding an OFDM waveform into a noise-radar spectrum BIB003 . Some researchers have looked at cooperative targets with known locations that re-radiate radar pulses back to the radar into one of many delay-Doppler cells to communicate within the radar physical layer BIB002 , BIB004 - BIB015 .