Radar and Communication Spectrum Sharing
Radar and Communication Spectrum Sharing addresses the growing conflict over use of the radio-frequency spectrum by different systems, such as civil and security applications of radar and consumer use for wireless communications. The increasing demand for this finite resource is driving innovation into new ways in which these diverse systems can cohabit the spectrum. The book provides a broad survey of recent and ongoing work on the topic of spectrum sharing, with an emphasis on identifying the technology gaps for practical realization and the regulatory and measurement compliance aspects of this problem space. The introductory section sets the scene, making the case for spectrum access and reviewing spectrum use, congestion, lessons learned, ways forward and research areas. The book then covers system engineering perspectives, the issues involved with addressing interference, and radar/communication co-design strategies. With contributions from an international panel of experts, this book is essential reading for researchers, engineers and advanced students in radar, communications, navigation, and electronic warfare whose work is impacted by spectrum engineering requirements.
Inspec keywords: cellular radio; radio spectrum management; radar applications; systems engineering; radar interference
Other keywords: systems engineering; radar/communication co-design; interference; communication spectrum sharing; radar spectrum sharing
Subjects: Radar and radionavigation; Radio links and equipment; General electrical engineering topics; Electromagnetic compatibility and interference
- Book DOI: 10.1049/SBRA515E
- Chapter DOI: 10.1049/SBRA515E
- ISBN: 9781785613579
- e-ISBN: 9781785613586
- Page count: 863
- Format: PDF
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Front Matter
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Part I The big picture
1 The case for spectrum access
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Fundamentally, this book seeks to capture the salient aspects of the exceedingly complex topic of radar and communication spectrum sharing, which is itself a menagerie of different problem spaces that depend on the particular goals one is attempting to achieve. The most prominent problem, and consequently the one most often first considered, is that of sharing between commercial communications and monolithic, stationary radars (e.g. for weather monitoring or air traffic control). Due to the sheer breadth of this topic, we do not attempt to codify all the many ways in which spectrum sharing could be performed. In fact, with the notion of radar spectrum sharing only rather recently becoming manifest due to the confluence of exponentially growing spectrum demand and emerging software-driven radio capabilities, one could well contend that we are now only glimpsing what will later be considered the earliest stages of spectrum sharing innovations.
2 The spectrum crunch – a radar perspective
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This chapter has provided an introduction to the spectrum crunch problem from a radar perspective. At one level it can be said that the problem is already severe. There is ever-greater competition for a resource that is strictly finite, and radar is only one voice among many pressing their case. All users have a need for greater bandwidth, and the only thing that can be said with certainty is that the problem is only going to get worse.Yet if spectrum usage were measured at a given point as a function of frequency, time, space and polarisation, it would certainly be found that the spectrum is currently not being used that efficiently. There is therefore great potential for approaches aimed at using the spectrum in a more efficient and dynamically controlled manner. The regulatory framework has thus far taken a relatively conservative approach. However, it is important to have a proper quantitative understanding of the effect of interference of one service upon another in order to adopt appropriate regulation measures, rather than taking the view that no service should ever occupy the same part of the spectrum as any other. To date, a number of novel radar technology approaches have been introduced, including improvements to the spectral purity of transmitters, intelligent, cognitive approaches to dynamic frequency allocation, passive sensing based on the emissions of other RF applications, and even through learning to mimic the behaviour of echolocating animals. These topics and others are developed in the chapters that follow, and give some cause for optimism that a solution can be found.
3 Spectrum sharing between radar and small cells
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The escalating interest on the topic of coexistence between radar systems and broadband communication devices is a direct consequence of the significant portion of the international radio spectrum currently allocated to radar systems. However, studies show that their spectrum occupancy is low in the spatial, temporal, and frequency domains. The most promising radar bands for shared use are the L, S, and C bands located in the 960-1400 MHz, 2.7-3.6 GHz, and 5.0-5.85 GHz frequency ranges, respectively. These frequencies are sufficiently low to avoid high power consumption and the usage of highly directional antennas, and sufficiently high to offer considerable bandwidth to commercial services. Furthermore, they are close to the cellular and ISM bands used for 2G/3G/4G and wireless local area networks (WLAN), respectively, facilitating the production of low-cost devices capable of using all these frequencies.
4 Radar spectrum sharing: history, lessons learned, and ways forward
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The development and deployment of DFS-based 5 GHz spectrum-sharing technology has arguably been largely successful, though achieving this success has required considerable use of government resources on an ongoing basis. As spectrum sharing between radars and non-radar systems are considered in other bands, the lessons described above ought to be heeded. Fundamentally, substantial resources will likely be needed to support sharing in new bands. These resources will be needed on both the front end (in design and development phases of sharing technologies) and at the back end, on an ongoing basis after such technologies have been deployed. It is unlikely that complex spectrum-sharing schemes between radars and non-radar systems will ever be fire-and-forget in the sense that they can be deployed without the need for ongoing work by engineers and spectrum managers to keep them functional.
5 Spectrum use, congestion, issues, and research areas at radio-frequencies
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The objective of this chapter is to provide a high-level perspective on the growing conflict over use of the radio-frequency (RF) spectrum, a precious and highly sought resource extending from below 1 MHz to above 100 GHz, caused by the accelerating demand for consumer use via 4G and soon-to-be 5G wireless communications. The world at large now faces serious spectrum-compatibility problems that require new and innovative solutions-increased spectral congestion and crowding are especially challenging. However, anticipated improvements in electromagnetic (EM) systems up to 300 GHz are beginning to be realized. Less restrictive constraints on communication systems, inherent in one-way propagation paths and much less expensive components, have allowed that community to design and develop more diverse waveforms and systems. Consequently, commercial cellular systems are proliferating at incredible rates, resulting in extremely spectrally dense environments and fierce competition for spectrum that traditionally has been the almost exclusive province of radars as primary legal users. For radar applications, however, the promise is being realized much more slowly, and the inundation of communication devices from the commercial sector has caused significant radar-communication interference problems. In addition, radar and communication systems are important components of military operations, and advances in waveform-diversity signal and data-processing techniques that are likewise relevant to spectrum sharing offer the promise of significantly improved performance.
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Part II Systems engineering perspectives
6 Spectrally efficient communications and radar
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In this chapter we focus on the attributes of spectral efficiency for the communication and radar modes.
7 Passive bistatic radar for spectrum sharing
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This chapter has provided a brief introduction to the properties of PBR, and considered how PBR may be able to help address spectral congestion. The problem itself is already severe, and is likely to get worse. But there are some grounds for optimism, and PBR certainly has a part to play in providing solutions.
8 Symbiosis for communications, broadcasting and sensor systems in the white space TV band
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This chapter is not about providing complete mathematical analysis of some systems but rather to show a new generation of sensor engineers that there are an enormous number of exciting possibilities, provided we are prepared to break from the somewhat rigid suite of sensors of the past. These ideas are summarised at the end of the chapter.
9 Fusion of radar sensing, data communications, and GPS interoperability via software-defined OFDM architecture
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This chapter considers a variety of functionalities that are typically assigned to distinctly different devices, each designed separately and each requiring its own power supply, analog front end (AFE), physical space, spectral allocation, and computational processing power. Specifically, these devices comprise a radar-based sensor, a data communication transceiver, and a radio frequency (RF) position/navigation system. The rationale behind this “division of labor” is well understood. Certain types of waveforms are simply better suited to particular tasks. For example, linear frequency modulation (LFM) chirp signals are a veritable radar staple, while a variety of keying modulation formats are used for communications. There is an obvious redundancy in this approach; a redundancy that often leads to a critical increase in power consumption, spectral overcrowding, instrument weight and size, and demands upon computing resources. When combined, these increases may represent a formidable challenge especially when these systems are designed for autonomous, long-endurance platforms such as unmanned (aerial, ground, underwater) vehicles. Consequently, instead of choosing system parameters that are best suited to each function, it is worth investigating a prospective compromise solution that allows for a combination of these functions via judicious signal design. A uniting factor throughout this chapter is thus the signal and system architecture format that enables the fusion of the aforementioned functionalities. We explore this potential via the orthogonal frequency division multiplexing (OFDM) software-defined radar system (SDRS) concept.
10 Adaptive RF multi-interference suppression for wideband radar/communication receivers
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In this chapter, a comprehensive review of recent contributions in the field of fully reconfigurable RF/microwave multi-notch filters is presented. These circuits are understood to be key components in future RF front-ends for wideband receivers to efficiently mitigate intentional and unintentional narrowband interference [18]. Specifically, tunable implementations for planar technologies are described in which the adaptive multi-notch filtering action is co-integrated with the wideband signal preselection process in a single dual-function RF filter. These filtering devices exploit different operating principles such as frequency-controllable circuit networks and coupled-resonator arrangements, whose advantages and disadvantages are discussed. Further, state-of-the-art mobile form factor high-Q filters for ultra-narrowband interference suppression based on mixed-technology acoustic-wave lumped-element resonators are also reported. A brief summary of future trends in this area is also provided at the end of the chapter.
11 Transmitter architectures for radar/communication spectral coexistence
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The scope of this chapter is limited to the transmitter portion of a generic radar or communication system, mostly focusing on descriptions of current approaches for transmitter design to provide a departure point from which to propose architectures for future implementations. The first section briefly discusses the EM and operational environments from a radar perspective. The EM environment includes the portion of the spectrum in which sensing is performed along with the related propagation effects, while the operational environment includes practical issues such as how the transmitter interacts with other proximate receivers and emitters, and the regulatory framework that defines and constrains what the transmitter is permitted to do.
12 Adaptively reconfigurable radar: real-time optimization of the transmitter amplifier and waveforms
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Future radar systems will need to be adaptive and reconfigurable to allow for adequate sharing of the spectrum with wireless communication systems. This chapter has described several algorithms that facilitate the optimization of transmitter circuitry andradarwaveforms. The goals of these approaches include (1) ensuring desiredradar range/Doppler resolution characteristics, (2) achieving high powerPAE, and (3) spectral compliance to minimize the interference to other spectrum users. Algorithms for the fast tuning of the power amplifier load impedance, as well as simultaneous tuning of load impedance with input power or bias voltage, have been demonstrated to locate the optimal solution within a feasible number of required measurements.
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Part III Addressing interference
13 Radar/Wi-Fi spectrum sharing: evaluation of radar protection regions
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In this chapter, an aspect of spectrum sharing between a monostatic radar and 802.11 wireless local area networks (WLANs) (Wi-Fi) is explored. Specifically, it focuses on the determination of protection regions around the radar to keep the aggregate interference caused by activeWi-Fi (secondary) sources to within acceptable limits. Of particular interest is the impact of the deployment geometry of the Wi-Fi networks- modeled via suitable random spatial processes-on the statistics of the aggregate interference. The impact of various stochastic geometries on the resulting protection region is explored through both (Monte Carlo) simulation and analytical approaches.
14 Spectrum sharing via interference tolerant transform domain waveform design
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This chapter focuses on the application of spectral diversity via transform domain (TD) waveform design instead of the traditional time domain formulation. The basic idea behind TD waveform design is to address interference at the transmitter and thereby minimize signalprocessing complexities at the receiver [3]. This approach is generalized within the spectrally-modulated-spectrally-encoded (SMSE) framework and includes all multicarrier waveform designs such as orthogonal frequency division multiplexing (OFDM) and multicarrier code-division multiple access (MC-CDMA) [4,5] as special cases. These multicarrier SMSE waveforms are utilized in the design of overlay and underlay cognitive radio waveforms. The vast majority of existing research on physical layer spectrum coexistence addresses cognitive radio systems, where the focus is mainly on the sharing of spectrum between multiple communication systems [2,6,7]. In this chapter, we introduce the foundational work for a transform domain communication system (TDCS) and SMSE framework, and then discuss the design of multicarrier communication and radar waveforms based on SMSE.
15 Radar bandwidth optimization for interference mitigation
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This chapter investigates techniques that modify radar waveforms in order to share the band with communication systems while maximizing radar performance. These techniques are based on a coexistence strategy involving a two-step approach that characterizes the electromagnetic environment via spectrum sensing and then accesses the spectrum dynamically. Spectrum sensing can detect communication systems that are attempting to access the same band as the radar. For the paradigm considered here, the frequency locations of these communication systems are estimated using energy detection. Access to the spectrum is then achieved by identifying a channel, or sub-band, devoid of activity so as to minimize radio frequency interference (RFI). This approach avoids potentially harmful RFI while simultaneously reducing the radar spectral footprint. Proper sub-band selection is key to maximizing radar performance in the presence of RFI.
16 Compressed sensing and interference occupancy monitoring for spectrum sharing in spectrally dense environments
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In this chapter, we analyze the general problem of a wideband radar that shares the same channel with a communication system, assuming that the communication band is divided into several frequency channels used for dynamic spectrum access (DSA). We specifically consider the case in which the radar is the secondary user (SU) and the communication system is the primary user (PU). In cognitive radio terminology, PUs are those who have higher priority or legacy rights to the usage of a specific part of the spectrum. SUs have lower priority and exploit the spectrum in such a way as to not cause interference to PUs. Therefore, SUs must possess some cognitive radio capabilities, such as an ability to sense the spectrum reliably to determine whether or not it is currently occupied by a PU. If the given channel becomes occupied by a PU, the SU must then move to an unused part of the spectrum, often referred to as white spaces or spectrum opportunities.
17 Radar waveform design for spectral coexistence
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In this chapter we discuss design techniques and constraints that facilitate a waveform that is useful for radar. While signal-to-noise ratio (SNR) and signal-to-interference-plus-noise ratio (SINR) have historically been the primary drivers of waveform design and are certainly discussed here, the growing need for spectral coexistence is eliciting new metrics and design approaches that address a broader set of considerations. This chapter also examines the use of performance prediction models as a means to establish relationships between waveform constraints and SINR performance, which are critical for detection in spectrally crowded environments. An empirical approach is discussed along with models that provide an intuitive way to understand the complex relationships between constraints and SINR performance in dynamic situations.
18 Space–time transmit nulling for RF spectrum interoperability
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This chapter begins with a formulation of the RUWO algorithm in Section 18.2 and its application to producing spatial, frequency, and space-frequency transmit nulls for radio frequency interference avoidance, followed by a description of the Naval Research Laboratory's Space Time Adaptive Nulling (STAN) radar test bed, an eight-channel X-band coherent MIMO radar test bed used to experimentally verify the performance of the RUWO algorithm in Section 18.3. Section 18.4 then provides a description of narrowband loop-back and open-air experimental applications of RUWO that were originally presented in [14,15], respectively. Following the narrowband results is an explanation in Section 18.5 of how the RUWO algorithm is incorporated into a method for producing wideband FM waveforms with large space-frequency nulls as discussed in [11]. The chapter concludes with a summary of presented work and a discussion of future work and open problems on the topic of space-time transmit nulling with the goal of RF spectrum interoperability.
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Part IV Radar/communication co-design
19 Communication and radar co-design
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In this chapter, we develop a set of tools to study the co-design ofjoint communication and radar systems. For the sake of discussion, given a fixed set of resources, one can determine an n-dimensional space of performance metrics such that the best performance of the system can be described by a manifold in this metric space. By changing the parameters the overall system employs, one can therefore move along this surface to different operating points as indicated in Figure 19.1. Thus, an intelligent system can dynamically modify the parameters of a network of joint radar and communication nodes in response to multiple goals that, along with the environment, can likewise be dynamic. Further, in highly dynamic scenarios, distributed (as opposed to centralized) optimization may be required. In fact, dynamic joint radar and communication systems may actually provide a more appropriate example for use of the adjective “cognitive” than other systems that often employ this monicker.
20 Real-time radar/communication spectrum sharing based on information exchange
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Analysis has shown that information exchange between radar and communications greatly enhances sharing performance. Once the current/future system state information is exchanged, radar and communication systems may use coordinated interference avoidance, mitigation, and amalgamation techniques that are otherwise impossible when the systems do not share information and are forced to reactively sense-andavoid in the spectrum. In addition, information exchange enables these systems to engage in joint co-designed and cooperating operating modes that open the door to amalgamated RC waveforms where the RF equipment can synergistically support each other's operation.
21 Embedding communication symbols in radar clutter on an intrapulse basis
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The concept of RCEC has been proposed, with the prospect of various different communication symbol and receive filter design strategies. Several of these symbol/filter designs were examined from the perspective of spectral containment, processing gain, probability of detection, and SER. Of the three proposed symbol designs, it was shown that the DP method of embedding communication symbols into clutter is the most effective from the perspectives of detectability, SER, and interference at the radar receiver. This symbol design offers significant improvements over a traditional DSSS design strategy. Finally, simulation results show that the DLDF provides the most robust performance for all symbol designs, independent of the incident clutter power.
22 Dual-function radar–communications using sidelobe control
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This chapter provides an overview of recent advances in radar-embedded communication signals. Several information-embedding techniques have recently been successful in establishing dual-function systems that simultaneously perform both radar and communication functions. Similarto single-function communication platforms, dual function systems should provide secure communications to protect user signals from being intercepted by other users or unintended receivers. Information embedding into the emission of multiple-input (MIMO) multiple-output radar provides a means for broadcast communications to be secondary to the primary radar function of the dual system.
23 Embedding communications into radar emissions by transmit waveform diversity
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The challenge posed by the ever-increasing RF spectrum demand for communication applications is in part met with radar and communication spectrum sharing by codesign of dual-function systems. Co-design of joint radar/communication systems requires the use of some form of waveform diversity via exploitation of the available time, frequency, coding, spatial, or polarization degrees-of-freedom. In this chapter, we have focused on methods for coding, spatial, and spectral forms of waveform diversity, presenting particular dual-function design strategies for each along with practical implementation issues.
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Back Matter
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