Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> A novel approach to spectrum and power allocation is proposed for joint cognitive communication-radar networks, which aim at integrating cognitive radio (CR-o) and cognitive radar (CR-r) paradigms to achieve intelligent utilization of spectrum resources in wireless networks. This approach exploits the location information offered by CR-r combined with spectrum sensing capability of CR-o to aid spectrum and power allocation by minimizing harmful interference among neighboring devices. Such systems require both coexistence and sharing of perception of radio environment and radar scene. To offer better spectrum resource utilization, entropy of the received signal is employed in order to detect spectrum holes over the network topology. This entropy-based technique also demonstrates superior performance as compared to the conventional method based on energy detection. Simulation results indicate both throughput improvement and interference reduction among neighboring devices. <s> BIB001 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> In this paper, the problem of adaptively selecting radar waveforms from a pre-dened library of waveforms is ::: addressed from an information theoretic perspective. Typically, radars transmit specic waveforms periodically, ::: to obtain for example, the range and Doppler of a target. Although modern radars are capable of transmitting ::: dierent waveforms during each consecutive period of transmission, it is hitherto unclear as to how these ::: waveforms must be scheduled to best understand the dynamic radar scene. In this paper, a new information ::: theoretic metric - directed information - is employed for waveform scheduling, and is shown to incorporate ::: the past radar returns to eectively schedule waveforms. We formulate this waveform scheduling problem in a ::: Gaussian framework, derive the corresponding maximization problem, and illustrate several special cases. <s> BIB002 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> A novel approach to combining communication and radar functionalities in a single waveform design for cognitive radar radio (CRR) networks is proposed. This approach aims at extracting the target parameters from the radar scene, as well as facilitating high-data-rate communications between CRR nodes, by adopting a single waveform optimization solution. The system design technique addresses the coexisting communication and radar detection problems in mission-critical services, where there is a need of integrating the knowledge about the target scene gained from distinct radar entities functioning in tandem with each other. High spatial resolution and immunity to multipath fading make ultrawideband (UWB) signals an appropriate choice for such applications. The proposed solution is achieved by applying the mutual-information-based strategy to design the sequence of UWB transmission pulses and embed into them the communication data with the pulse position modulation scheme. With subsequent iterations of the algorithm, simulation results demonstrate an improvement in extraction of the parameters from the radar scene, such as target position and impulse response, while still maintaining high-throughput radio links with low bit error rates between CRR nodes. <s> BIB003 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> Cognitive radar (CR) is a paradigm shift from a traditional radar system in that previous knowledge and current measurements obtained from the radar channel are used to form a probabilistic understanding of its environment. Moreover, CR incorporates this probabilistic knowledge into its task priorities to form illumination and probing strategies, thereby rendering it a closed-loop system. Depending on the hardware's capabilities and limitations, there are various degrees of freedom that a CR may utilize. Here we concentrate on spatial illumination as a resource, where adaptive beamsteering is used for search-and-track functions. We propose a multiplatform cognitive radar network (CRN) for integrated search-and-track application. Specifically, two radars cooperate in forming a dynamic spatial illumination strategy, where beamsteering is matched to the channel uncertainty to perform the search function. Once a target is detected and a track is initiated, track information is integrated into the beamsteering strategy as part of CR's task prioritization. <s> BIB004 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> In this paper, we deal with cognitive design of the transmit signal and receive filter optimizing the radar detection performance without affecting spectral compatibility with some licensed overlaid electromagnetic radiators. We assume that the radar is embedded in a highly reverberating environment and exploit cognition provided by Radio Environmental Map (REM), to induce spectral constraints on the radar waveform, by a dynamic environmental database, to predict the actual scattering scenario, and by an Electronic Support Measurement (ESM) system, to acquire information about hostile active jammers. At the design stage, we develop an optimization procedure which sequentially improves the Signal to Interference plus Noise Ratio (SINR). Moreover, we enforce a spectral energy constraint and a similarity constraint between the transmitted signal and a known radar waveform. At the analysis stage, we assess the effectiveness of the proposed technique to optimizing SINR while providing spectral coexistence. <s> BIB005 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> This paper deals with the problem of a cognitive radar system that shares the same frequency band with a communication system, supposed to be the primary user of channel. A cognitive algorithm is proposed to estimate the channel parameters that describe the behaviour of the primary user and how to exploit these estimates to minimize the interference between the radar and the communication system. The performance of the proposed algorithm are assessed in terms of probability of collision, that is the probability that the radar transmits when the primary user already occupies the channel, and probability to lose a spectrum opportunity, that is the probability that the radar does not transmit when the channel is free. <s> BIB006 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> The idea of spectrum sharing between radar and wireless communication systems arise because of the need for extra bandwidth for wireless communication systems. Due to strong and ever growing economic, political, social, and technological driving factors, realization of this vision is inevitable in near future. Therefore, novel solutions for the efficient and fair sharing of the spectrum is needed. In this paper, we present a concise review of the state of the art on radar and communication system coexistence and bandwidth sharing. <s> BIB007 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> The radio-frequency (RF) electromagnetic spectrum, extending from below 1 MHz to above 100 GHz, represents a precious resource. It is used for a wide range of purposes, including communications, radio and television broadcasting, radionavigation, and sensing. Radar represents a fundamentally important use of the electromagnetic (EM) spectrum, in applications which include air traffic control, geophysical monitoring of Earth resources from space, automotive safety, severe weather tracking, and surveillance for defense and security. Nearly all services have a need for greater bandwidth, which means that there will be ever-greater competition for this finite resource. The paper explains the nature of the spectrum congestion problem from a radar perspective, and describes a number of possible approaches to its solution both from technical and regulatory points of view. These include improved transmitter spectral purity, passive radar, and intelligent, cognitive approaches that dynamically optimize spectrum use. <s> BIB008 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> The proliferation of mobile devices of late has been exponential in numbers as well as heterogeneity, which gives rise to a great increase in the demand of spectral resources. Researchers have suggested that in order to solve the spectrum crisis, future spectrum users will be required to be frequency-flexible and cognitive, and spectrum sharing protocols on communication system have thus been studied. Radars, in the present form, are spectrum users which have fixed operating frequencies, high power and tendency to interfere with other users. Because of these characteristics, conventional radar will have difficulty in cooperating with spectrum sharing communication systems. Though the idea of combining radar and communication has been around for a long time, few have considered operating a radar in the communication band until recently. In order to make radar and communication systems coexist in the same band, one must maintain the quality of communication links. Additionally, radar should also be cognitive to benefit from the knowledge provided by the communication systems. In this paper, we investigate how future radars respond to spectrum sharing environment and derive optimal waveforms satisfying the spectral requirement. <s> BIB009 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> A non-repeating FMCW waveform was recently developed and experimentally demonstrated to provide a feasible instantiation of FM noise radar. This emission scheme was subsequently examined in terms of the impact of both stationary and hopped spectral gaps with the prospect of enabling in-band interference avoidance for cognitive sensing and possibly tandem hopped radar/communications. Here this gap-hopped spectrum framework is further explored with regard to the relation between the shaping of spectral gaps and the associated time sidelobe response. Experimental loopback measurements are shown that provide a sense of how this form of emission would operate on a real system. <s> BIB010 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> A joint radar/communication system is considered, where the radar adaptively designs the transmitted waveform such that the interference caused to the cellular systems is strictly controlled. In this paper, different Mutual Information based criteria for radar waveform optimization are proposed and the corresponding waveform optimization problems are formulated and solved analytically. Radar performance trade-offs for the considered Mutual Information based criteria are presented and, using simulation results, it is shown that a larger maximized Mutual Information does not guarantee an optimal detection performance. It is also emphasized the importance of exploiting the communication signals scattered off the target for the detection task when dealing with weak radar returns. <s> BIB011 </s> Survey of RF Communications and Sensing Convergence Research <s> I. COGNITIVE APPROACHES <s> The spectrum sensing, multi-optimization (SS-MO) technique was recently investigated to enhance radar performance when the radar operates in the presence of radio frequency interference (RFI). The SS-MO technique passively monitors the operating band of the radar using spectrum sensing to identify a frequency sub-band with minimal RFI, thus allowing the radar to maintain a high signal to inference plus noise ratio (SINR). Prior results have indicated significant improvement of SINR and PSLR (peak to average sidelobe level ratio) at the cost of a high computational complexity. In this paper, the non-dominated sorting genetic algorithm II (NSGA-II) is used to lower the computational complexity while maintaining performance. <s> BIB012
Advancements in cognitive radios and radar have been proposed as a natural solution to spectrum congestion problems. Traditionally, communications phenomenology has advanced beyond radar in terms of cognitive, bandwidth sharing techniques  . This is because radar has enjoyed access to excellent spectral resources and remained unchallenged for many decades BIB008 . Therefore cognitive techniques in a radar context were primarily for enhanced dynamic behavior in complex environments  . Researchers have begun to look at radar scheduling as an application for cognitive systems as the spectral scarcity problem has sparked interest in this area BIB002 . Some have looked at adapting waveforms to signal-dependent interference from communications users BIB005 . Others have employed cognitive FIGURE 17. State diagram for joint radar-communications cognitive scheme. Both users need to perform the spectrum sensing task. However, in the joint case, this task is enhanced due to the dual-function circuitry which is optimized for both communications and radar sensing (instead of one or the other). Further, accomplishing both tasks in the same RF band provides improved overall sensitivity and environmental feedback into the reconfiguration task. techniques to estimate communications channel parameters to reduce the mutual interference between a primary communications user and the secondary radar user BIB006 . This is analogous to the previously mentioned ELINT assisted techniques where the radar was the primary user. Researchers have looked to develop cognitive radar much more closely resembling cognitive radio by employing similar spectrum sensing techniques, emitter localization, and power allocation to avoid interference with cognitive radio users BIB001 . Others have developed cognitive techniques to extend prior work in information-theoretic waveform design for radar cross section estimation and detection to include a communications user sharing the same waveform BIB003 , BIB009 . Information based techniques have been looked at for intelligent target scheduling in a cognitive framework BIB004 , though similar to classical cognitive radios in that the resource is optimized within the radar phenomenology only. Preliminary work has compared cognitive operation of both users independently and jointly as well BIB007 . Nonrecurrent, nonlinear frequency-modulated continuous-wave (FMCW) waveforms with hopping spectral gaps with shaping to support range sidelobe roll-off were developed to support access from dynamic communications users and minimize interference BIB010 . Some researchers have noted that spectrum crowding is an issue within remote sensing allocations itself, and identified cognitive radar as a natural solution to the many user problem  . Other researchers have looked at various joint radar-communications mutual information criteria and means to maximize them, and noted that maximizing the mutual information did not always maximize radar probability of detection BIB011 . Finally, there have been examples of systems employing genetic algorithms to modulate radar spectral access opportunistically, similar to cognitive radio users BIB012 . A jointly cognitive system state diagram example is shown in Figure 17 . Both users have to accomplish the spectrum sensing task, and so a joint receiver is used to sense both types of user in gray spectrum allocation. Once active, the dual function of the transmitter and receiver enable enhanced environmental sensing and feedback to adapt system configuration and maximize the joint mission.