Dynamic Radio Spectrum Research Articles

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J Burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined, as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7~text{mdyn},text{cm}^{-2}, and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

Compensation method of signal flatness for a broadband solar millimeter radio observation system

<p indent=0mm>In a solar radio observation system, the receiver, as an essential instrument, processes the solar radio signal received by the antenna with amplification, filtering, digitization, and so on, and yields the solar radio dynamic spectrum. However, the amplitude–frequency inconsistencies of devices inside the receiver may lead to distortion of the output. In particular, the distortion increases when a broadband signal is processed by the receiver, and high uncertainty may appear. To solve this problem, in this paper we present a compensation method for the signal flatness for a solar microwave observing system. According to the analysis, the fluctuation of the signal flatness is caused by fluctuation of the gain with frequency and the random noise introduced by the system. First, we compensate the unflatness of the gain by subtracting the background signal from the observed signal. Quantile–quantile plots are then used to assess the compensation effect, and a new round of compensation is reprocessed if the plots do not follow a normal distribution. The integration times of the signals are finally deduced to reduce the random noise on account of the relationship between the integration times and noise depression. A simulation experiment can provide a good resolution for real-time data processing. This method has been used in the <sc>35–40 GHz</sc> broadband solar radio observation system in the Chashan Solar Radio Observatory of Shandong University and demonstrates good stability and application potential.

ICBCA – Improved Cluster Based Channel Allocation in Cognitive Radio Sensor Networks

Wireless sensor networks (WSNs) operate in an overcrowded electromagnetic environment, as the spectrum is shared by various wireless communication technologies. This gives rise to various challenges related to optimized and efficient spectrum utilization. Cognitive radio (CR) has emerged as a solution satisfying this requirement, as it is capable of adapting to the dynamic radio spectrum. Thanks to the deployment of cognitive radio in WSNs, the spectrum may be utilized in a more efficient manner. CR may identify the vacant channels dynamically, allowing the sensor nodes to effectively communicate with each other. In this paper a clustering algorithm known as improved cluster-based channel assignment (ICBCA) is implemented, forming clusters of CR sensor nodes and then selecting vacant channels for data transmission purposes. Simulation results show that ICBCA outperforms existing clustering algorithms in CR sensor networks

Observational study on the fine structures of solar type III radio bursts

Solar type III radio bursts are transient radio radiation enhancement phenomena, which appear in the solar radio dynamic spectrum as signals that rapidly drift from high to low frequencies. The bursts are believed to be the plasma emissions excited by energetic electrons that propagate outward from the corona along the magnetic field lines, and the emission frequency is close to the local plasma frequency. Certain type III radio bursts have special morphologies or fine structures, such as the types J, U, and N and the inverted/bi-directional drift types III, IIIb, and III-L radio bursts. These fine structures can be used to diagnose the coronal parameters, such as the electron densities, atmospheric turbulences, and energetic electron velocities. In addition, these structures can forecast the space weather. The burst was regarded as the key point in the study of solar eruptions and solar radio bursts. The reasons for the appearance of fine structures are still unclear, and further studies are needed. This article investigates the progress made by the observational studies of the fine structures of the solar type III radio burst.

Locating the source field lines of Jovian decametric radio emissions

Decametric (DAM) radio emissions are one of the main windows through which one can reveal and understand the Jovian magnetospheric dynamics and its interaction with the moons. DAMs are generated by energetic electrons through cyclotron-maser instability. For Io (the most active moon) related DAMs, the energetic electrons are sourced from Io volcanic activities, and quickly trapped by neighboring Jovian magnetic field. To properly interpret the physical processes behind DAMs, it is important to precisely locate the source field lines from which DAMs are emitted. Following the work by Hess et al. [2008, 2010], we develop a method to locate the source region as well as the associated field lines for any given DAM emission recorded in a radio dynamic spectrum by, e.g., Wind/WAVES or STEREO/WAVES. The field lines are calculated by the state-of-art analytical model, called JRM09 [Connerney et al., 2018]. By using this method, we may also derive the emission cone angle and the energy of associated electrons. If multiple radio instruments at different perspectives saw the same DAM event, the evolution of its source region and associated field lines is able to be revealed. We apply the method to an Io-DAM event, and find that the method is valid and reliable. Some physical processes behind the DAM event are also discussed.

Diode Based Reconfigurable Microwave Filters for Cognitive Radio Applications: A Review

The cognitive radio paradigm for developing next-century wireless communication systems is rapidly entering the mainstream, and various aspects of it are currently being applied in 5G technology, aeronautical engineering, military communications, emergency, and public safety applications, satellite communication, and healthcare. Cognitive radio focuses on the existence of software defined radio architectures that allow dynamic reconfiguration. Many researchers have taken initiatives in the last decade to achieve the reconfiguration ability in cognitive radio systems to support the concept of dynamic spectrum access. As cognitive radio adapts dynamic spectrum allocation for its users, the physical implementation requires reconfigurable filters that can alter the carrier frequencies and bandwidth. Although there are many ways to reconfigure filter operation, diode based reconfiguration has received utmost attention among researchers because of its shorter response delay and easy implementation. In the last decade, researchers have reported several diode-based reconfigurable filters, including their characteristics such as filter function, filter combination, tuning range, variation in bandwidth, isolation, and resonance. However, to examine the potential of these filters in the application of cognitive radio, a comprehensive review needs to be pursued. In this review article, the descriptions of several diode based reconfigurable filters are illustrated with their exhibiting characteristics. The detailed information provided in this article has disclosed that primarily three different types diode based reconfigurable filters have been reported by researchers: Tunable, Switchable, and Hybrid (Both Tunable and Switchable). It is also found that each type of reconfiguration can further be segregated in terms of filter function, centre frequency variation, and bandwidth variation. The detailed categorization of the reconfiguration presented in this paper provides a systematic approach to select the correct reconfigurable filter for the desired frequency reconfiguration in cognitive radio.

Research and Application of Recurrent Neural Network in Solar Radio Interference Filtering

The various interfering signals present in the space result in the inability to obtain a clean and clear solar radio dynamic spectrum, which affects the effective observation of solar radio burst events. At the data processing level, we propose a method for predicting and processing radio interference signals in solar radio burst events using recurrent neural network in deep learning. Firstly, the radio signal that satisfies the condition is selected by the amount of solar radio flux of all frequency channels at some moment, and then the position of the initial time of the burst event is located by using the variation curve of the solar radio flux with time under the single frequency channel. After that, the constructed recurrent neural network is used to predict the signal value of the radio in the burst area. Finally, according to the linear additivity of the signal, the value of the clean pure burst event is obtained by subtracting the predicted radio value from the original value of the burst area. The experimental results show that the proposed method can effectively remove the interference in the solar radio dynamic spectrum and preserve the effective information of the burst event to the greatest extent. This provides a new idea and research direction for deep learning in the anti-interference processing of astronomical big data.

A Major Geoeffective CME from NOAA 12371: Initiation, CME–CME Interactions, and Interplanetary Consequences

In this article, we present a multi-wavelength and multi-instrument investigation of a halo coronal mass ejection (CME) from active region NOAA 12371 on 21 June 2015 that led to a major geomagnetic storm of minimum $\mathrm{Dst} = -204$ nT. The observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory in the hot EUV channel of 94 A confirm the CME to be associated with a coronal sigmoid that displayed an intense emission ( $T \sim6$ MK) from its core before the onset of the eruption. Multi-wavelength observations of the source active region suggest tether-cutting reconnection to be the primary triggering mechanism of the flux rope eruption. Interestingly, the flux rope eruption exhibited a two-phase evolution during which the “standard” large-scale flare reconnection process originated two composite M-class flares. The eruption of the flux rope is followed by the coronagraphic observation of a fast, halo CME with linear projected speed of 1366 km s−1. The dynamic radio spectrum in the decameter-hectometer frequency range reveals multiple continuum-like enhancements in type II radio emission which imply the interaction of the CME with other preceding slow speed CMEs in the corona within $\approx10$ – $90~\mbox{R} _{\odot}$ . The scenario of CME–CME interaction in the corona and interplanetary medium is further confirmed by the height–time plots of the CMEs occurring during 19 – 21 June. In situ measurements of solar wind magnetic field and plasma parameters at 1 AU exhibit two distinct magnetic clouds, separated by a magnetic hole. Synthesis of near-Sun observations, interplanetary radio emissions, and in situ measurements at 1 AU reveal complex processes of CME–CME interactions right from the source active region to the corona and interplanetary medium that have played a crucial role towards the large enhancement of the geoeffectiveness of the halo CME on 21 June 2015.

Radio signatures of shock-accelerated electron beams in the solar corona

Context.The Sun’s activity can appear in terms of radio bursts. In the frequency range 8−33 MHz the radio telescope URAN-2 observed special fine structures appearing as a chain of stripes of enhanced radio emission in the dynamic radio spectrum. The chain drifts slowly from 26 to 23 MHz within 4 min. The individual structures consist of a “head” at the high-frequency edge and a “tail” rapidly drifting from the “head” to lower frequencies over an extent of ≈10 MHz within 8 s. Since they resemble the well-known “herring bones” in type II radio bursts, they are interpreted as shock accelerated electron beams.Aims.The electron beams generating these fine structures are considered to be produced by shock drift acceleration (SDA). The beam electrons excite Langmuir waves which are converted into radio waves by nonlinear wave-plasma processes. That is called plasma emission. The aim of this paper is to link the radio spectral data of these fine structures to the theoretical results in order to gain a better understanding of the generation of energetic electrons by shocks in the solar corona.Methods.Adopting SDA for generating energetic electrons, the accelerated electrons establish a beam-like velocity distribution. Plasma emission requires the excitation of Langmuir waves, which is efficient if the velocity of the beam electrons exceeds a few times thermal electron speed. That is the case if the angle between the shock normal and the upstream magnetic field is nearly perpendicular. Hence, the Rankine-Hugoniot relationships, which describe the shock transition in the framework of magnetohydrodynamics, are evaluated for the special case of nearly perpendicular shocks under coronal circumstances.Results.The radio data deduced from the dynamic radio spectrum can be related in the best way to the theoretical results, if the electron beams, which generate these fine structures, are generated via SDA at an almost perpendicular shock, which is traveling nearly horizontally to the surface of the Sun.

Solar radio emission as a disturbance of aeronautical radionavigation

On November 4th, 2015 secondary air traffic control radar was strongly disturbed in Sweden and some other European countries. The disturbances occurred when the radar antennas were pointing at the Sun. In this paper, we show that the disturbances coincided with the time of peaks of an exceptionally strong (∼105 Solar Flux Units) solar radio burst in a relatively narrow frequency range around 1 GHz. This indicates that this radio burst is the most probable space weather candidate for explaining the radar disturbances. The dynamic radio spectrum shows that the high flux densities are not due to synchrotron emission of energetic electrons, but to coherent emission processes, which produce a large variety of rapidly varying short bursts (such as pulsations, fiber bursts, and zebra patterns). The radio burst occurs outside the impulsive phase of the associated flare, about 30 min after the soft X-ray peak, and it is temporarily associated with fast evolving activity occurring in strong solar magnetic fields. While the relationship with strong magnetic fields and the coherent spectral nature of the radio burst provide hints towards the physical processes which generate such disturbances, we have so far no means to forecast them. Well-calibrated monitoring instruments of whole Sun radio fluxes covering the UHF band could at least provide a real-time identification of the origin of such disturbances, which reports in the literature show to also affect GPS signal reception.

OBSERVATION OF A METRIC TYPE N SOLAR RADIO BURST

ABSTRACT Type III and type-III-like radio bursts are produced by energetic electron beams guided along coronal magnetic fields. As a variant of type III bursts, Type N bursts appear as the letter “N” in the radio dynamic spectrum and reveal a magnetic mirror effect in coronal loops. Here, we report a well-observed N-shaped burst consisting of three successive branches at metric wavelength with both fundamental and harmonic components and a high brightness temperature (&gt;109 K). We verify the burst as a true type N burst generated by the same electron beam from three aspects of the data. First, durations of the three branches at a given frequency increase gradually and may be due to the dispersion of the beam along its path. Second, the flare site, as the only possible source of non-thermal electrons, is near the western feet of large-scale closed loops. Third, the first branch and the following two branches are localized at different legs of the loops with opposite senses of polarization. We also find that the sense of polarization of the radio burst is in contradiction to the O-mode and there exists a fairly large time delay (∼3–5 s) between the fundamental and harmonic components. Possible explanations accounting for these observations are presented. Assuming the classical plasma emission mechanism, we can infer coronal parameters such as electron density and magnetic field near the radio source and make diagnostics on the magnetic mirror process.

DYNAMIC SPECTRAL MAPPING OF INTERSTELLAR PLASMA LENSES

ABSTRACT Compact radio sources sometimes exhibit intervals of large, rapid changes in their flux density, due to lensing by interstellar plasma crossing the line of sight. A novel survey program has made it possible to discover these “Extreme Scattering Events” (ESEs) in real time, resulting in a high-quality dynamic spectrum of an ESE observed in PKS 1939–315. Here we present a method for determining the column-density profile of a plasma lens, given only the dynamic radio spectrum of the lensed source, under the assumption that the lens is either axisymmetric or totally anisotropic. Our technique relies on the known, strong frequency dependence of the plasma refractive index in order to determine how points in the dynamic spectrum map to positions on the lens. We apply our method to high-frequency (4.2–10.8 GHz) data from the Australia Telescope Compact Array of the PKS 1939–315 ESE. The derived electron column-density profiles are very similar for the two geometries we consider, and both yield a good visual match to the data. However, the fit residuals are substantially above the noise level, and deficiencies are evident when we compare the predictions of our model to lower-frequency (1.6–3.1 GHz) data on the same ESE, thus motivating future development of more sophisticated inversion techniques.

Secondary System Initialization Protocol Using FFT-based Correlation Matching for Cognitive Radio Ad-hoc Networks

Due to the increasing demand for spectrum resources, cognitive radio networks and dynamic spectrum access draw a lot of research into efficiently utilizing limited spectrum resources. To set up cluster-based CR ad-hoc common channels, conventional methods require a relatively long time to successfully exchange the initialization messages. In this paper, we propose a fast and reliable common channel initialization protocol for CR ad-hoc networks. In the proposed method, the cluster head sequentially broadcasts a system activation signal through its available channels with a predetermined correlation pattern. To detect the cluster head`s broadcasting channels and to join the cluster, each member node implements fast Fourier transform (FFT) and computes autocorrelation of an FFT bin sequence for each available channel of the member node. This is compared to the predetermined reference pattern. The join request and channel decision procedures are also presented in this paper. In a simulation study, the performance of the proposed method is evaluated.

Energy-aware resource allocation for cooperative cognitive radio dynamic spectrum access networks

The resource allocation problem is investigated for cooperative cognitive radio networks, considering energy efficiency of the primary users (PUs) and spectrum efficiency of the secondary users (SUs). The cooperation framework involves a PU selecting a SU as the relay and allocating the spectrum access intervals for the selected SU as a reward. The above sequential decision procedure is formulated as a Stackelberg game. Based on the knowledge of the effects of PUs decision on the behavior of a SU, the leader (PU) determines a fraction of the time slot duration to be used for the primary. The follower (SU) is allowed to transmit its own data to the corresponding receiver in the remaining duration and determines its optimal transmission power. The outcomes of the proposed cooperative resource allocation scheme, including primary user and secondary user power control, as well as dynamic access spectrum, are analyzed. Numerical results demonstrate that, with the proposed resource allocation scheme, the PU can achieve high energy saving by cooperation with the SU.

Interaction Between Two CMEs During 14 – 15 February 2011 and Their Unusual Radio Signature

We report a detailed analysis of an interaction between two coronal mass ejections (CMEs) that were observed on 14 – 15 February 2011 and the corresponding radio enhancement, which was similar to the “CME cannibalism” reported by Gopalswamy et al. (Astrophys. J. 548, L91, 2001). A primary CME, with a mean field-of-view velocity of 669 km s−1 in the Solar and Heliospheric Observatory (SOHO)/Large Angle Spectrometric Coronagraph (LASCO), was more than as twice as fast as the slow CME preceding it (326 km s−1), which indicates that the two CMEs interacted. A radio-enhancement signature (in the frequency range 1 MHz – 400 kHz) due to the CME interaction was analyzed and interpreted using the CME data from LASCO and from the Solar Terrestrial Relations Observatory (STEREO) HI-1, radio data from Wind/Radio and Plasma Wave Experiment (WAVES), and employing known electron-density models and kinematic modeling. The following results are obtained: i) The CME interaction occurred around 05:00 – 10:00 UT in a height range 20 – 25 R⊙. An unusual radio signature is observed during the time of interaction in the Wind/WAVES dynamic radio spectrum. ii) The enhancement duration shows that the interaction segment might be wider than 5 R⊙. iii) The shock height estimated using density models for the radio enhancement region is 10 – 30 R⊙. iv) Using kinematic modeling and assuming a completely inelastic collision, the decrease of kinetic energy based on speeds from LASCO data is determined to be 0.77×1023 J, and 3.67×1023 J if speeds from STEREO data are considered. vi) The acceleration, momentum, and force are found to be a=−168 m s−2, I=6.1×1018 kg m s−1, and F=1.7×1015 N, respectively, using STEREO data.

A Survey on Dynamic Spectrum Access via Cognitive Radio: Taxonomy, Requirements, and Benefits

Conventionally, interference protection among radio spectrum users is guaranteed through a policy of spectrum licensing that provides exclusive access to licensed users. However, as it is becoming extremely difficult to satisfy the growing demand for radio spectrum with this conventional policy, a flexible policy that can improve radio spectrum access and utilization without causing interference amongst users has recently gained increasing interest. In this context, this paper presents a comprehensive survey on flexible radio spectrum management with full focus on a cognitive radio technology application for dynamic spectrum access. The study approach involves analytical evaluation of the performance of the current fixed radio spectrum allocation policy examining both technical and economic perspectives. The result of which shows that the current fixed radio spectrum management policy is too rigid, inefficient and out-dated and needed to be replaced with a flexible radio spectrum management policy. Based on this analytical result, an in-depth survey on innovative technology and standards for a dynamic spectrum management model that can enhance an increase in spectral utilization and coexistence of multiple radios with adequate interference free communication was carried out. The paper also discusses the interrelationship between technologies, business and policies for cognitive radio standardization for seamless wireless communication. In addition, the paper presents both the technical and economic benefits of the dynamic radio spectrum management model over the current fixed radio spectrum allocation policy. The paper was finally concluded recommending the commercial adoption of dynamic spectrum access worldwide keeping in mind all the benefits of the dynamic spectrum access model and standards in place to ensure interference free communication in a cognitive radio environment.

Quality of service‐aware and security‐aware dynamic spectrum management in cyber‐physical surveillance systems for transportation

AbstractCyber‐physical system has been widely used in various areas as the integration of computing and physical system. As a typical application of cyber‐physical system, cyber‐physical surveillance system for transportation (CPSST) allows real‐time video monitoring to facilitate the deploying of smart transportation systems. For video streaming in CPSST, dynamic radio spectrum management is a key technology dealing with the current situation that the spectrum resource is almost used up. In this paper, taking into account the application layer quality of service and wireless link security, a novel dynamic spectrum management scheme has been proposed to minimize the system cost of CPSST. Video distortion is considered as the application layer quality of service metric, and the system cost is defined as a combination of distortion and security cost. We use intra‐refreshing rate in video coding to minimize the distortion. Furthermore, the problem is formulated as a restless bandit system, which uses current and historical information to optimize the action, with the objective of maximizing the total discounted system reward. We also describe the two spectrum management processes. Extensive simulation results are presented to demonstrate the significant performance improvement of the proposed scheme compared with the existing one that ignores video distortion and subband security optimization. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Simultaneous EUV and radio observations of bidirectional plasmoids ejection during magnetic reconnection

We present a multiwavelength study of the X-class flare, which occurred in active region (AR) NOAA 11339 on 3 November 2011. The EUV images recorded by SDO/AIA show the activation of a remote filament (located north of the AR) with footpoint brightenings about 50 min prior to the flare occurrence. The kinked filament rises-up slowly and after reaching a projected height of ~49 Mm, it bends and falls freely near the AR, where the X-class flare was triggered. Dynamic radio spectrum from the Green Bank Solar Radio Burst Spectrometer (GBSRBS) shows simultaneous detection of both positive and negative drifting pulsating structures (DPSs) in the decimetric radio frequencies (500-1200 MHz) during the impulsive phase of the flare. The global negative DPSs in solar flares are generally interpreted as a signature of electron acceleration related to the upward moving plasmoids in the solar corona. The EUV images from AIA 94 \AA reveal the ejection of multiple plasmoids, which move simultaneously upward and downward in the corona during the magnetic reconnection. The estimated speeds of the upward and downward moving plasmoids are ~152-362 and ~83-254 km/s, respectively. These observations strongly support the recent numerical simulations of the formation and interaction of multiple plasmoids due to tearing of the current-sheet structure. On the basis of our analysis, we suggest that the simultaneous detection of both the negative and positive DPSs is most likely generated by the interaction/coalescence of the multiple plasmoids moving upward and downward along the current-sheet structure during the magnetic reconnection process. Moreover, the differential emission measure (DEM) analysis of the active region reveals presence of a hot flux-rope structure (visible in AIA 131 and 94 \AA) prior to the flare initiation and ejection of the multi-temperature plasmoids during the flare impulsive phase.

Radio evidence for breakout reconnection in solar eruptive events

Context. Magnetic reconnection is understood to be fundamental to energy release in solar eruptive events (SEEs). In these events reconnection produces a magnetic flux rope above an arcade of hot flare loops. Breakout reconnection, a secondary reconnection high in the corona between this flux rope and the overlying magnetic field, has been hypothesized. Direct observational evidence for breakout reconnection has been elusive, however.Aims. The aim of this study is to establish a plausible interpretation of the combined radio and hard X-ray (HXR) emissions observed during the impulsive phase of the near-limb X3.9-class SEE on 2003 November 03.Methods. We study radio spectra (AIP), simultaneous radio images (Nancay Multi-frequency Radio Heliograph, NRH), and single-frequency polarimeter data (OAT). The radio emission is nonthermal plasma radiation with a complex structure in frequency and time. Emphasis is on the time interval when the HXR flare loop height was observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to be at its minimum and an X-ray source was observed above the top of the arcade loops.Results. Two stationary, meter-wavelength sources are observed radially aligned at 0.18 and 0.41 R ⊙ above the active region and HXR sources. The lower source is apparently associated with the upper reconnection jet of the flare current sheet (CS), and the upper source is apparently associated with breakout reconnection. Sources observed at lower radio frequencies surround the upper source at the expected locations of the breakout reconnection jets.Conclusions. We believe the upper radio source is the most compelling evidence to date for the onset of breakout reconnection during a SEE. The height stationarity of the breakout sources and their dynamic radio spectrum discriminate them from propagating disturbances. Timing and location arguments reveal for the first time that both the earlier described above the flare loop top HXR source and the lower radio source are emission from the upper reconnection jet above the vertical flare CS.

Spectrum Occupancy Measurements and Analysis in Beijing

A 24-hour spectrum measurement campaign was performed in Beijing in 2012 August in the frequency bandwidth which is 450-2700MHz in order to find out the radio spectrum allocation situation in Beijing, and identify frequency bandwidth which can be used by dynamic spectrum access devices. The results from the spectrum measurements indicate that a lot of spectrum in Beijing has very low signal occupancy in both time and spectrum domains. The measurement results show that the actual spectral occupancy in Beijing is about 13.5%. The low spectral occupancy rate even in a metropolis such as Beijing suggests a bright future for aggressive expansion of the cognitive radio and other dynamic spectrum access.