Preface

We are delighted to edit this new book, Signal Processing for Joint Radar and Communications, published by the prestigious Wiley-IEEE Press. The book comprises 14 chapters that are written by exceptionally well-qualified experts from academia and research laboratories across the globe. The focus of this book is on signal processing aspects of joint radar and communications (JRC)-one of the most actively researched problems across as many as 15 IEEE societies/communities. The chapters were selected by the editors, who have a combined experience of 120 years in prestigious research laboratories.

The future of spectrum access will be increasingly shared, dynamic, and secure. The International Telecommunication Union (ITU) began allocating radio bands at least as early as 1937. The IEEE Radar Band Standard 521, which has been maintained since 1976, follows the spectrum designation practices that originated during World War II. Over the interceding multiple decades of bandwidth expansion for sensing, navigation, wireless communications, timing, and positioning, policy planners and technologists have faced pressures of inefficient and excessively cautious use of the spectrum. Conventional spectrum sharing rules have been framed more for worst-case scenarios than for an optimal utilization of available frequencies. This approach inevitably leads to conflicts because the electromagnetic spectrum is a scarce resource. In 2021, the US Federal Communications Commission (FCC) was sued by AT&T over FCC's allocation of 6-7 GHz for dynamic allocation of allowed channels to Wi-Fi access points for indoor Wi-Fi 7 protocol. The US Federal Aviation Administration (FAA) was recently embroiled in a highly public battle with the airline industry over the use of the C-band for fifth-generation (5G) wireless services near airports. At lower bands, the FCC has been challenged to update their propagation models for their TV-band rulings so as to allow the reuse of TV bands for other services.

Consequently, sensing systems (radar, lidar, or sonar) that share the spectrum with wireless communications (radio-frequency/RF, optical, or acoustical) and still operate without any significant performance losses have captured significant research interest. Although a large fraction of these bands remains underutilized, radars need to maintain constant access to these bands for target sensing and detection as well as to increase the spectrum to accomplish missions such as secondary surveillance, multi-function integrated RF operations, communications-enabled autonomous driving, and cognitive capabilities. On the other hand, the wireless industry's demand for spectrum for providing new services and accommodating a massive number of users with high data rate requirement continues to increase. The present spectrum is used very inefficiently due to its highly fragmented allocation. Emerging wireless systems such as commercial Long-Term Evolution (LTE) communications technology, fifth-generation (5G), WiFi, Internet-of-Things (IoT), and Citizens Broadband Radio Services (CBRS) are already causing spectral interference to legacy military, weather, astronomy, and aircraft surveillance radars. Similarly, radar signals in adjacent bands leak to spectrum allocated for communications and deteriorate the service quality. Therefore, it is essential and beneficial for radar and communications to develop strategies to simultaneously and opportunistically operate in the same spectral bands in a mutually beneficial manner.

The interference from other emitters and its mitigation has been of interest within IEEE for decades. Until the early 1960s, IEEE used to publish IEEE Transactions on Radio Frequency Interference led by IRE Professional Technical Group on Radio Frequency Interference. The journal ceased publication in 1963. The periodical Frontiers of Technology: Trends in Electronics Research by Mattraw and Moyer focused on radar-communications interference in its Volume 63 published in 1958. Scientific literature on JRC systems remained largely scattered until the 2000s. However, the spectral overlap of centimeter-wave radars with a number of wireless systems at the 3.5 GHz frequency band led to the 2012 U.S. President's Council of Advisors on Science and Technology (PCAST) report on spectrum sharing. Thereafter, changes in regulation for this band became a driver for spectrum-sharing research programs of multiple agencies including the Defense Advanced Research Projects-Agency (DARPA) and National Science Foundation (NSF). Today, it is the higher end of the RF spectrum-millimeter-wave and terahertz band-that requires concerted efforts for spectrum management. Some recent studies also mention joint visible light communications (VLC) and visible light positioning (VLP). Conceptual articles have also been reported on joint quantum communications and quantum sensing.

At the time of the publication of this book, journals/conferences sponsored by the following 15 IEEE societies have published JRC studies: IEEE Aerospace and Electronic Systems Society, IEEE Antennas & Propagation Society, IEEE Circuits and Systems Society, IEEE Communications Society, IEEE Computer Society, IEEE Control Systems Society, IEEE Engineering in Medicine and Biology Society, IEEE Geoscience and Remote Sensing Society, IEEE Information Theory Society, IEEE Instrumentation & Measurement Society, IEEE Microwave Theory and Techniques Society, IEEE Photonics Society, IEEE Signal Processing Society, IEEE Solid-State Circuits Society, and IEEE Vehicular Technology Society. In addition, spectrum sharing and/or JRC working groups and task forces have been reported from various IEEE societies, ITU, International Union of Radio Science (URSI), and American Meteorological Society (AMS).

In the context of overwhelmingly fast-paced developments in JRC, the goal of this book is twofold:

• - to provide a list of references to JRC researchers and engineers, helping them to find information required for their current research, and
• - to serve as a reference text in an advanced graduate level course on JRC.

The book consists of three parts: fundamental limits and background; physical layer signal processing; networking and hardware implementation.

Part I: Fundamental Limits and Background

We begin with a historical context, nomenclature, classification, and the principles of JRC systems through the first five chapters.

Chapter 1: A Signal Processing Outlook Toward Joint Radar-Communications

This chapter provides an overview of the JRC origins and subsequent developments. It details various signal processing (SP) techniques employed to achieve JRC systems by exploiting different degrees of freedom at transmitter and receiver as well as information on the channel state and target scenarios. The chapter summarizes various current JRC topologies and state-of-the-art solutions. It describes in detail the generic technical issues related to both transmit and receive signal processing, with examples of each prevailing design topology.

Chapter 2: Principles of Radar-Centric Dual-Function Radar-Communication Systems

This chapter provides an overview of existing radar-centric DFRC techniques and discusses their potential and future directions. It focuses on radar as an incumbent service defining the system resources while the communications functionality is seen as an added service carried out using the same system resources under the conditions of minimal overhead and changes to radar parameters. This dual functionality is enabled through multiple signal and system design strategies, each of which is clearly elucidated. The techniques presented leverage the developments in concepts, algorithms, design, and implementation of radar-centric waveforms, and the chapter offers an ideal platform for the reader on the technical details of DFRC.

Chapter 3: Interference, Clutter, and Jamming Suppression in Joint Radar-Communications Systems - Coordinated and Uncoordinated Designs

Expanding the scope of the DFRC design, this chapter introduces interference due to the coexistence of radar and communications systems and explores different interference suppression methods. It starts with the joint design of transmission and reception strategies in MIMO radar and MIMO communications systems wherein information exchange is enabled for coordinated interference suppression. Subsequently, an uncoordinated approach is pursued whereby interference suppression is integrated into the processing method used for the system's primary function. The chapter leverages constrained maximization techniques including iterative optimization, sparse modeling, and model-based deep learning architecture.

Chapter 4: Beamforming and Interference Management in Joint Radar-Communications Systems

This chapter continues the discussion of interference management and focuses on beamforming, multicarrier waveform design, and interference management using precoder and decoder designs in joint radar and communications systems. Cooperative settings where information about the state of the radio spectrum and awareness of the environment can be exchanged among the users and subsystems for mutual benefit are considered. A variety of precoding and decoding methods that allow for managing interference to avoid significant deterioration of sensing...