Survey of RF Communications and Sensing Convergence Research <s> I. INTRODUCTION <s> We investigate cooperative radar and communications signaling. While each system typically considers the other system a source of interference, by considering the radar and communications operations to be a single joint system, the performance of both systems can, under certain conditions, be improved by the existence of the other. As an initial demonstration, we focus on the radar as relay scenario and present an approach denoted multiuser detection radar (MUDR). A novel joint estimation and information theoretic bound formulation is constructed for a receiver that observes communications and radar return in the same frequency allocation. The joint performance bound is presented in terms of the communication rate and the estimation rate of the system. <s> BIB001 </s> Survey of RF Communications and Sensing Convergence Research <s> I. INTRODUCTION <s> Abstract Due to the explosive growth of wireless devices and wireless traffic, the spectrum scarcity problem is becoming more urgent in numerous Radio Frequency (RF) systems. At the same time, many studies have shown that spectrum resources allocated to various existing RF systems are largely underutilized. As a potential solution to this spectrum scarcity problem, spectrum sharing among multiple, potentially dissimilar RF systems has been proposed. However, such spectrum sharing solutions are challenging to develop due to the lack of efficient coordination schemes and potentially different PHY/MAC properties. In this paper, we investigate existing spectrum sharing methods facilitating coexistence of various RF systems. The cognitive radio technique, which has been the subject of various surveys, constitutes a subset of our wider scope. We study more general coexistence scenarios and methods such as coexistence of communication systems with similar priorities, utilizing similar or different protocols or standards, as well as the coexistence of communication and non-communication systems using the same spectral resources. Finally, we explore open research issues on the spectrum sharing methods as well as potential approaches to resolving these issues. <s> BIB002
The problem of spectral congestion is forcing legacy radar band users to investigate methods of cooperation and codesign with a growing number of communications applications. This problem has motivated government entities like the Defense Advanced Research Projects Agency (DARPA) to begin funding and investigating these methods to not only ensure military radar coverage is maintained as spectral allocation is renegotiated, but to potentially improve both military radar and military communications by co-designing the systems from the ground up  . However, these issues extend far beyond just commercial communications and military radar, and include a wide variety of applications such as next generation automobiles, medical devices, and 5G wireless backhaul. As a result, researchers have begun investigating not just methods of military radar and communications coexistence, but more fundamentally methods of joint remote sensing and communications. These two functions, at their core, tend to be at odds with one another. For example, sensing typically sends a known waveform or stimulus and measures a response from the environment, often referred to as the channel. In the case of the radar system, the sent signal is known and the target channel is unknown and is desired to be sensed (estimated). However, a communications system typically sends an unknown signal with the assumption that the propagation channel is known or previously estimated. We can also consider the near inverse of this situation: passive radar. In this case, we must estimate the data as a nuisance parameter to obtain the information we care about (channel estimation). A non-adaptive communications channel, where the channel is stationary or controlled, is the dual of the traditional radar system. Therefore, when considering the general task of jointly sensing and communicating, it becomes immediately apparent that the solution is non-trivial. A typical two-user topology and the problem of spectral congestion is illustrated in Figure 1 . With opposing requirements, sensing and communications systems are often designed in isolation. The only consideration for the other user in legacy systems has been in the form of regulatory constraints, such as those imposed by the FCC in the United States. However, governmental regulation does nothing to incentivize either user to minimize interference beyond the required limits or assist each other to mutual benefit. As future systems vie for spectral resources, RF convergence and cooperation is the solution to an increasingly crowded FIGURE 1. Current RF spectrum challenges illustrated in a simplified topology. Only showing two users, a communications and radar user, and some external interference, the challenges facing heterogeneous two-user legacy systems is readily apparent. The users can be operating in the same band and adjacent in space, or co-located and operating in adjacent spectral bands. Regardless, both users present interference to the other and require mitigation to function optimally. wireless domain. We define the ultimate solution, RF convergence, to be the operating point at which a given bandwidth allocation is used jointly for radar and communications to mutual benefit. This includes but is not limited to multifunction transceivers, in-band full-duplex (IBFD) operation, shared waveforms, and dynamic time allocation. FIGURE 2. Future RF spectrum topology. Users are dynamic and adapt to the environment and one another. There are no dedicated radar or communications resources, but dynamic elements cooperate and are co-designed to meet the instantaneous mission. External interference is reduced inherently, as two-user systems are more easily extensible to multi-user, multi-function nodes capable of adapting and communicating to enable the joint, distributed mission. Figure 2 is where the authors see the future of channel topologies heading. Rather than dedicated radar or communications elements, universal dynamic users are designed to adapt to meet instantaneous mission needs. Bandwidth, data rate, and estimation rate BIB001 are modulated depending on communications need, targets present, and target dynamics. While one may note both cognitive radio and cognitive radar are both active fields of development, cognitive radio has typically been developed in the context of resource sharing  , while cognitive radar has traditionally focused more on intelligent radar systems to improve radar performance  . Previous surveys have looked at the spectral congestion problem from a dynamic access perspective with a focus on regulatory issues and signal processing  . However, the focus in that work is still on dynamic communications users, not necessarily including remote sensing users. Recent work surveyed spectrum sharing methods and underutilization of RF resources BIB002 , focusing mainly on communications with some mention of sharing with noncommunications users. This work focused more on existing spectrum sharing regulation, as opposed to future architectures and limits of coexistence.