Cognitive waveform design for spectral coexistence
3.1 Introduction
Cognitive waveform design for
electromagnetic considerations, such as good foliage penetration [3], low path loss attenuation and reduced size of the devices push some systems to coexist in the same frequency band [4] (for instance very high frequency and ultra high fre- quency). As a result, the RF spectrum congestion problem has been attracting the interest of many scientists and engineers during the last few years and is currently becoming one among the hot topics in both regulation and research field [5,6].
RF spectrum assignment and regulation is coordinated worldwide through the International Telecommunications Union (ITU) and it is reviewed every three to four years at the World Radiocommunication Conference (WRC) [6,7]. Specifically, the scope of the WRC is to review and, if necessary, revise the radio regulations that form the international treaty ruling the use of the RF spectrum, as well as geosta- tionary-satellite and non-geostationary-satellite orbits. Revisions are performed on the basis of an agenda determined by the ITU Council, which accounts for recom- mendations made by previous WRCs. Significantly, the trend of the actual regula- tions is to relax the original conservative, static and possibly inefficient strategy in which ‘‘nothing should ever interfere with anything else’’ allowing some levels of
‘‘acceptable interference’’ among coexisting systems. This is because experimental evidences have revealed the substantial underutilization of the instantaneous spec- trum at a given spatial location, as a function of direction, frequency, time and polarization, which should be reduced as much as possible, via a more flexible, dynamic and intelligent spectrum usage [6,8]. Of course, this process will require a quantitative and rigorous study of what can be accepted as tolerable level of disturbance so as to suitably formulate the regulations.
Starting from the above considerations, during the last year’s several approa- ches have been proposed in both radar and communication research field to deal with the spectrum congestion problem and allow a more efficient spectrum use.
In this context, passive bistatic radar (PBR) represents a reasonable strategy to handle this critical issue [6,9] for low/medium range applications. In fact, this sensing system, also known as ‘‘Green Radar’’, is able to detect, track and classify objects of interest without additional RF emissions but exploiting non-cooperative high-power illuminators of opportunity, i.e., radio FM, GSM, Universal Mobile Telecommunications System, GPS, Digital Video Broadcasting, other radar, satellite transmission, etc., as sources [10–12]. In this context, the main research activity concerns the design of advanced receiver structures able to pro- cess the direct path signal (from the selected emitter) and the received echoes from the surveillance area [13–16]. Evidently, this technology avoids the need of trans- mit licences and, as a by-product, offers a high level of covertness to the monitoring system, which may be advantageous for defence applications. Nevertheless, this approach may suffer significant performance degradation, in terms of detection and tracking capabilities, with respect to conventional monostatic radars as the employed waveforms are not devised to fulfil radar requirements and also exhibit time-varying features due to their information content sensitivity [6,17,18].
A possible means to overcome the above PBR shortcomings is represented by the so-called commensal radar (literally ‘‘from the same table’’) [19,20]. The key idea behind this approach is to synthesize the waveforms associated with broadcast, communications, or navigation services so that they not only satisfy their primary
goals but also share some features making them appealing as radar sources [21].
In this respect, in [21], it is highlighted that there are several parameters of the long term evolution (LTE) modulation and signal format that can be optimized.
A dual perspective to PBR and commensal radar approaches is represented by radar-embedded communication strategies [22,23]. Therein, covert communica- tions are established embedding the information in the environmental reverberation induced by the radar probing waveforms [24,25]. Otherwise stated, the commu- nications’ signals are devised so that the information signal looks like a clutter return. By doing so, such communication link does not require additional spectrum resources and indirectly provides spectrum coexistence. Nevertheless, the achiev- able data rate is usually low.
Last but absolutely not least, an important solution to the spectrum congestion problem is provided by the waveform design and diversity (WDD) paradigm, pioneered by Dr. Wicks [26–29]. It refers to the radar waveform adaptation aimed at dynamically optimizing the radar performance for the particular scenario and tasks. This amazing and powerful feature is enabled by the new computing architectures, high-speed and off-the-shelf processors, arbitrary digital waveform generators, solid-state transmitters, active phased arrays, etc. and represents a viable tool to improve spectrum-usage efficiency. Relying on real-time spectrum occupancy awareness, it is possible to dynamically [30–32] select the probing waveforms in response to changing conditions so as to enhance radar performance while controlling its impact on the other surrounding RF systems. Specifically, WDD can enable an intelligent and agile spectrum management. Significantly, the underlying optimization process can also benefit from multiple degrees of freedom, including spatial, temporal and polarization’s domains, to further improve the achievable performance.
The research in this field has been quite fertile. A plethora of papers have addressed the problem of designing radar waveforms with a smart frequency allocation [33], so as to control the interference brought on overlaid wireless networks (communication and navigation systems), while enhancing radar perfor- mance requirements in terms of range-Doppler resolution, low range and Doppler sidelobes, detection and tracking capabilities. In [34], a waveform design techni- que is introduced to confer some desired spectral nulls to the radar signal. The idea is to perturb a stepped frequency modulated pulse forcing an additional fast time polyphase code. The approach is extended in [35] to the case of continuous phase waveforms that place nulls at specific frequencies. The effectiveness of both the aforementioned methods is considered in [36], via an experimental analysis. An alternate projection algorithm for the construction of chirp-like constant-modulus signals with a single spectral null is proposed in [37], whereas in [38], its extension, addressing the production of multiple notches, is established. Some iterative algo- rithms are introduced in [39] for the joint design of the transmit signal and the receive filter achieving frequency stop-band suppression and range sidelobes minimization. A genetic algorithm to design sparse waveforms for high-frequency surface wave radar systems is investigated in [40]. In [41], a fast coding technique based on alternate projections and successive fast Fourier transforms is developed to obtain sparse waveforms with a controlled peak sidelobe level (PSL). In [42] and
[43], sparse frequency constant modulus radar signals with a low integrated sidelobe level (ISL) are built optimizing a suitable combination between the ISL metric and a penalty function accounting for the waveform frequency allocation. In [44], a spectrum-centric signal design is developed based on the minimization of the transmitted energy on a set of disjoint stop-band frequencies under a unim- odularity constraint and autocorrelation function (ACF) masking. In [45], a friendly spectral shaped radar waveform design is considered to allow the coexistence of the radar with one or more communication systems. Finally, in [46], the design of sparse frequency waveform with low ISL values is addressed.
In this chapter, following the WDD paradigm, we provide a unifying and systematic approach that summarizes some recent results on waveform optimiza- tion with spectral compatibility requirements [47–49]. Specifically, we present an optimization theory-based waveform design framework that attempts to enhance the target detection probability while controlling both the amount of interfering energy produced in the licensed bands and some desirable features of the trans- mitted waveform. It is supposed that the radar system has the ability to predict the behaviour of surrounding licensed RF systems, for instance using a radio envir- onmental map (REM) [50], containing geographical features, available wireless services and their spectral regulations and locations and activities of the trans- mitters. This is the key to an efficient adaptation as the aforementioned information allows an intelligent spectrum utilization in a spectrally crowded environment.
More in details, the described approach considers as figure of merit the signal to interference plus noise ratio (SINR) and jointly optimizes the radar code and the receive filter constraining the amount of interference energy on crowded/reserved frequency bands. Both signal-dependent and signal-independent interference scenarios are addressed. To manage some relevant features of the probing signal, other than an energy constraint, a similarity constraint is enforced on the transmit sequence, so as to control significant characteristics of the waveform, such as range-Doppler resolution, variations in the signal modulus, and PSL.
Significantly, the similarity and the spectral compatibility constraints are generally competing requirements that may lead to an unfeasible design problem.
Hence, the feasibility of the resulting waveform design is analyzed by means of the interference/similarity (I/S) achievable region, namely the set of the admissible interference and similarity levels. Then, solution techniques leading to optimized waveform both in the presence of signal-independent and signal-dependent interference are presented. Finally, some interesting case studies are reported highlighting the trade-off among the achievable SINR, spectral shape, and ACF features of the synthesized signals. The results illustrate the effectiveness of the considered waveform design framework and show that high SINR values and enhanced interference suppression capabilities can be traded off with a partial degradation in terms of autocorrelation properties, in both signal-dependent and signal-independent interference scenario.
The remainder of the chapter is organized as follows. In Section 3.2, the model for the radar transmitted signal, the description of coexisting wireless systems, and the formulation of the waveform design problem are reported. In Section 3.3, the
joint design of the transmit code and receive filter in the presence of signal- independent disturbance is addressed, and the performance of the described algo- rithm analyzed. In Section 3.4, the signal-dependent interference environment is considered, and the effectiveness of the described procedure assessed. Finally, Section 3.5 is devoted to conclusions and proposals for possible future research tracks.