Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> MIMO radar systems have been proposed elsewhere which utilise OFDM waveforms as the scene illuminator. This suggests an opportunity exists to code the OFDM radar waveform in such a way as to provide a communication link to broadcast the radar data to remote users. The benefit of this would arise from the dual use of the microwave band, addressing the problem where demand for bandwidth is exceeding capacity. This paper explores the technical issues associated with this idea and outlines some of the key features of such a system. We describe how OFDM waveforms can be applied to MIMO radar; and what constraints must be placed on the waveform to ensure robust operation for both radar and communication functions. A candidate system design is presented, along with basic analysis of the expected performance of both radar and communications functionality. <s> BIB001 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> In this paper the possibility of designing an OFDM system for simultaneous radar and communications operations is discussed. A novel approach to OFDM radar processing is introduced that overcomes the typical drawbacks of correlation based processing. A suitable OFDM system parameterization for operation at 24 GHz is derived that fulfills the requirements for both applications. The operability of the proposed system concept is verified with MatLab simulations. Keywords-OFDM; Radar, Communications <s> BIB002 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> The design process of a joint radar and communication system employing OFDM is analyzed. In particular, the question on how to choose OFDM modulation parameters such as carrier distance, guard interval length, frame length or pilot design is studied. Such systems use the same OFDM frames to transmit data and image the surroundings, so both radar and communication are affected by the choice of these parameters. The main design criteria for the radar component are accuracy of distance and speed measurements, whereas the communication subsystem needs to be designed such that it transmits robustly through a mobile communication channel. As these design criteria often are antipodal, arguments for choosing a particular set of modulation parameters are discussed. The presented design process is applied to the case of a vehicular (car-to-car) communication, in which moving participants communicate in the 24 GHz range. <s> BIB003 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> This paper offers a tutorial treatment of constant envelope OFDM, covering power efficiency and spectral efficiency tradeoffs and discusses recent advancements in applying CE-OFDM to radar. A dual-use architecture is proposed offering both radar and communications capabilities on a common platform. <s> BIB004 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> The performance of the multi-carrier radar signal is investigated using the ambiguity function. The multi-carrier waveform under consideration is modulated by random phase coding, with the intention to combine radar and communications and at the same time solve the Doppler ambiguity. The connection between the instantaneous power of the waveform and the Doppler sidelobes of the ambiguity function are explored. The variance of the Doppler sidelobes, which arises from the random phase coding of the carriers, is connected to the peak to average power ratio of the waveform. <s> BIB005 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> In this paper an extension of the OFDM joint radar and communication system concept to cope in a multipath and multiuser environment is explored. A realistic multipath propagation channel model is built with an established ray-tracing algorithm and incorporated into the radar system simulation and one additional user/interferer is then introduced into the scenario for the observation of the radar's performance. From a theoretical point of view as well as from measurement results, the interferer causes a severe degradation of the radar's dynamic range, thus requiring an interference cancellation scheme to remove the traces of the interferer. Inherent to the interference cancellation scheme is the phase and frequency offset estimation algorithm for the high fidelity reconstruction of the interferer signal for subtraction from the radar signal which are explored in this paper. The result of the end-system and the improvement to the radar's dynamic range are then presented. <s> BIB006 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> In this paper, we consider the theory and implementation of a joint radar-communication system. A radar-software library compatible with Texas Instruments (TI's) small form factor software defined radio (SDR) platform is developed which allows users to implement real-time adaptive radar algorithms in MATLAB without requiring detailed knowledge of the inner workings of the SDR. As an example design and demonstration of the capabilities of the radar-software library, a joint radar-communication system has been implemented. The system uses wideband digital communication signals to simultaneously interrogate a scene as a radar while communicating range-Doppler maps resulting from previous interrogations. This combined system eliminates the need for separate hardware for communicating recorded radar returns to a base station. We study digital communication waveforms under the performance criteria of a radar waveform. <s> BIB007 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> Since traditional radar signals are “unintelligent,” regarding the amount of information they convey on the bandwidth they occupy, a joint radar and wireless communication system would constitute a unique platform for future intelligent transportation networks effecting the essential tasks of environmental sensing and the allocation of ad-hoc communication links, in terms of both spectrum efficiency and cost-effectiveness. In this paper, approaches to the design of intelligent waveforms, that are suitable for simultaneously performing both data transmission and radar sensing, are proposed. The approach is based on classical phase-coded waveforms utilized in wireless communications. In particular, requirements that allow for employing such signals for radar measurements with high dynamic range are investigated. Also, a variety of possible radar processing algorithms are discussed. Moreover, the applicability of multiple antenna techniques for direction-of-arrival estimation is considered. In addition to theoretical considerations, the paper presents system simulations and measurement results of complete “RadCom” systems, demonstrating the practical feasibility of integrated communications and radar applications. <s> BIB008 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> Due to limitations in the bandwidth resources, sharing the spectrum across several modalities has become increasingly important. In this paper, we will consider OFDM spectral sharing between radar and wireless communication systems. We will define and compute the importance of each sub-carrier to both the modalities by considering the radar detection performance and channel capacity. We will design the OFDM waveforms for communications and radar by appropriately allocating the sub-carriers using these importance functions. Using numerical simulations, we will demonstrate the performance of our proposed design mechanism. <s> BIB009 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> Motivated by dual use of OFDM signal format for communications and radar in ever-worsening Electromagnetic (EM) coexistence environments, this paper deals with transmit waveform design problem considering multiple design objectives. Spectral nulling is a typical way for friendly coexistence with narrow band systems. However, a Non-Contiguous Orthogonal Frequency-Division Multiplexing (NC-OFDM) waveform generated by turning off the interfering sub-carriers does not lead to satisfactory results. In this paper a convex optimization based waveform design framework is used to achieve deep spectral nulling while retaining low waveform autocorrelation side lobes and good range resolution. Because of dual use of the waveform, the data blocks to transmit are either unknown or chosen from a known dataset. Optimal sub-carrier weights are obtained for given transmission data blocks. In addition, waveform design for unknown data blocks are discussed and examined. <s> BIB010 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> In this paper, we address the problem of spectrum sharing and co-existence between radar and communications systems by using a reconfigurable dual-use radar/comm system on a single RF platform. We develop a nonlinear least-squares method for range and velocity estimation of moving targets using weighted orthogonal division multiplexing (WOFDM) waveform modulation scheme. The corresponding Cramer-Rao low bounds for the target parameter estimator are derived. Weighted OFDM waveform is derived subject to a low peak to average power ratio constraint. The proposed WOFDM modulation scheme shows improved accuracy for delay estimation compared with the classic constant envelope OFDM modulation method while achieving a lower peak to average power ratio. <s> BIB011 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> Orthogonal frequency division multiplexing (OFDM) radar is highly appropriate in applications where communication and radar functionalities are intended simultaneously. The same waveform and the same infrastructure are used, reducing hardware complexity. In this paper, we focus on the radar processing of a pulsed waveform, where the pulses are composed of several OFDM symbols. We derive a novel signal processing scheme that retrieves an estimate of the range and the radial velocity when the target is a point scatterer. Rather than a processing based on correlation functions we exploit the phases of the subcarriers and our knowledge of the phase codes. Then, we discuss the impact of the Doppler modulation within the individual pulses and we show how the use of identical sequences (IS) for the phase codes can mitigate the degradation of the retrieval in the range Doppler image. Lastly, we propose two alternative methods that retrieve an unambiguous estimate of the Doppler frequency. <s> BIB012 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> We investigate a joint radar and communications performance bound for a single pair of bistatic transmit and receive antennas that uses orthogonal frequency-division multiplexing (OFDM) transmission. The OFDM transmission signal is designed to simultaneously meet radar target detection and communications signal recovery objectives using training and information symbols on different subcarriers. Given a frequency-selective Rayleigh-distributed fading channel, equally powered and equally spaced training symbols across the occupied bandwidth minimize the mean-square error of the channel estimate, which is tied to a lower bound on ergodic channel capacity. We perform a hypothesis test on the resulting channel estimate to detect and locate targets. We examine the performance region of capacity versus probability of detection in an example scenario in which a single stationary target is present. <s> BIB013 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> A novel concept of orthogonal frequency division multiplexing (OFDM) multiple input multiple output (MIMO) radar for automotive applications is presented in this paper. Multiple transmit antennas are multiplexed via OFDM subcarrier interleaving. Such a multiplexing enables a simultaneous operation of transmit antennas, while the complete bandwidth is utilized by each of them. Whereas the known OFDM-MIMO radar concepts perform multiplexing via equidistant subcarrier interleaving, we propose the use of non-equidistant subcarrier interleaving for multiplexing of transmit antennas. The advantage of this approach over the equidistant interleaving is the full unambiguously measurable distance range, not reduced through the subcarrier interleaving. To cope with the increased sidelobes in distance estimation due to the non-equidistant subcarrier interleaving, we propose a novel two-dimensional distance-velocity estimation algorithm based on compressed sensing that incorporates the radar images from all antennas. The proposed algorithm is able to detect weak targets, originally masked by the sidelobes of more dominant ones. <s> BIB014 </s> Survey of RF Communications and Sensing Convergence Research <s> C. OFDM WAVEFORM DESIGN <s> It is well known that OFDM-based systems are susceptible and weak towards subcarrier misalignment, which can lead to erroneous demodulation of the receive signal. In the usage of OFDM Radar-Communication systems, whether within cooperative or non-cooperative networks, non-overlapping subcarriers are assigned to all RadCom nodes in the vicinity. Even so OFDM subcarrier misalignment can still arise from Doppler and hardware carrier frequency offset, which lead to inter-system interference. Particularly for radar applications, the goal of interference cancellation is not only to recover the payload data correctly, but also to enable a reliable radar estimation. This paper presents a measurement verification of an inter-system OFDM-based MIMO RadCom system interference cancellation concept for use in a multiple-user access environment at a scaled-down carrier frequency using universal software radio peripherals (USRPs). <s> BIB015
Almost immediately since the resurrected interest in joint radar-communications, multiple threads looked at OFDM as a viable option. Specifically, V2V applications were explored BIB002 , BIB003 . This work was extended to include multiple targets  , and multipath BIB006 . Others have worked the joint system into existing software-defined radio (SDR) architectures, using a given illumination to also simultaneously communicate the previous radar image BIB007 . However, often times results showed conflicting cyclic prefix requirements, data-dependent ambiguities, and trouble mitigating PAPR for typical radar power requirements. As these various problems arose, research shifted to designing joint systems to suppress side-lobes BIB010 , maintain a constant envelope BIB004 , or reduce PAPR BIB005 . Some methods attempted to remove the dependency of the data from the radar processing BIB008 but still suffered from conflicting cyclic prefix requirements. Others tried to minimize the effects by only allocating some sub-carriers to the radar operation BIB009 . In some research, combined OFDM-multiple-input multiple-output (MIMO) radar and communications is accomplished by nonuniformly spacing subcarriers to combat Doppler and range aliasing issues associated with other OFDM-based joint systems BIB014 . Improvements in range estimation were also found by weighting various sub-carriers to improve the joint waveform root mean square (RMS) bandwidth with constraints to control the PAPR BIB011 . Some have experimentally demonstrated OFDM-MIMO joint radar-communications systems employing adaptive interference cancellation BIB015 . Other researchers have looked at selection of OFDM phase codes in a tracking context, where prior knowledge of the target state is used to minimize Doppler ambiguity at the expense of increased PAPR BIB012 . Finally, work to combine MIMO radar, which itself requires orthogonal waveforms, with OFDM to create a joint system that senses radar imagery and then communicates this to users has been investigated BIB001 . However, Barker sequences were required to be overlaid onto the data for radar performance, greatly reducing the available communications rate. Limits to joint radar detection and data rate have been explored for OFDM systems with variable data/radar allocation in prior work as well BIB013 .