Abstract: In the emerging paradigm of open spectrum access, cognitive radios dynamically sense the radio-spectrum environment and must rapidly tune their transmitter parameters to efficiently utilize the available spectrum. The unprecedented radio agility envisioned, calls for fast and accurate spectrum sensing over a wide bandwidth, which challenges traditional spectral estimation methods typically operating at or above Nyquist rates. Capitalizing on the sparseness of the signal spectrum in open-access networks, this paper develops compressed sensing techniques tailored for the coarse sensing task of spectrum hole identification. Sub-Nyquist rate samples are utilized to detect and classify frequency bands via a wavelet-based edge detector. Because spectrum location estimation takes priority over fine-scale signal reconstruction, the proposed novel sensing algorithms are robust to noise and can afford reduced sampling rates.

Abstract: Ultrawideband (UWB) that is regulated by the Federal Communications Commission (FCC) for short-range high-throughput wireless communication and sensor networks with advantageous features, such as immunity to multipath fading, extremely short time duration, being carrier free, and having low duty cycle, wide bandwidth, and low power spectral density, has been a topic of interest recently. By wireless transmission, UWB communications systems can only operate in a short distance of a few meters to tens of meters. The convergence of UWB and optical fiber distribution techniques, or UWB over fiber, offers the availability of undisrupted service across different networks and eventually achieves high-data-rate access at any time and from any place. To distribute the UWB signals over the optical fiber, it is also desirable that the UWB signals can be generated in the optical domain without having extra electrical-to-optical conversion. In addition, UWB signals that are generated in the optical domain can be easily tailored to have a spectrum that meets the FCC-specified spectral mask. In this paper, techniques to generate UWB signals in the optical domain will be discussed. These techniques are divided into three categories, with the generation of UWB signals based on the following: 1) phase-modulation-to-intensity-modulation conversion; 2) a photonic microwave delay-line filter; and 3) optical spectral shaping and dispersion-induced frequency-to-time mapping. The areas for future development and the challenge of implementation of these techniques for practical applications will also be discussed.

Abstract: The FMCW-Radar-Principle is widely used for automotive radar systems. The basic idea for FMCW-Radars is to generate a linear frequency ramp as transmit signal. The difference frequency between the transmitted and received signal is determined after downconversion. In order to detect range and velocity together, the information, extracted from one frequency ramp, is not enough, because it is ambiguous. Several subsequent ramps have to be generated to remove the ambiguity between the frequency portion produced by range and the doppler frequency. In this paper two methods are presented to perform this basic signal processing step. The two methods have the Fast Fourier Transform as basic calculation step in common, but the number of FFTs and their length are different. The requirements on bandwidth for the IF-Hardware and A/D-Converters are also determined by the algorithm for Range-Doppler-Detection. The CFAR (Constant False Alarm Rate)-Algorithm for target detection must also be adapted to the chosen method. Both methods have been verified with a 24 GHz radar prototype. The Radar-Frontend has been built with a newly developed SiGe-Radar-Chipset on Infineon's B7HF200-Process with a transition frequency of 200 GHz.

Abstract: A wideband E-shaped microstrip patch antenna has been designed for high-speed wireless local area networks (IEEE 802.11a standard) and other wireless communication systems covering the 5.15-5.825 GHz frequency band. Two parallel slots are incorporated to perturb the surface current path, introducing local inductive effect that is responsible for the excitation of the second resonant mode. The length of the center arm can be trimmed to tune the frequency of the second resonant mode without affecting the fundamental resonant mode. A comprehensive parametric study has been carried out to understand the effects of various dimensional parameters and to optimize the performance of the antenna. A substrate of low dielectric constant is selected to obtain a compact radiating structure that meets the demanding bandwidth specification. The reflection coefficient at the input of the optimized E-shaped microstrip patch antenna is below -10 dB over the entire frequency band. The measurement results are in excellent agreement with the HFSS simulation results.

Abstract: A compact ultra-wideband (UWB) bandpass filter (BPF) with narrow notched (rejection) band in the UWB passband realized on a microstrip line is implemented and presented in this letter for use in wireless communication applications within the unlicensed UWB range set by the Federal Communications Commission (FCC). The filter consists of five short-circuited stubs separated by nonredundant connecting lines in order to exhibit a high selectivity filtering characteristic. The narrow notched (rejection) band was introduced by using a new technique which involves embedding open stubs in the first and last connecting lines in order to reject any undesired existing radio signal which may interfere with the determined UWB passband. The bandwidth of the notched filter can be controlled by adjusting the width of the stubs or the gaps or both. The length of the stubs can be tuned to select a specific frequency for the notched band. The embedded stubs can be used to excite single or double band-reject response. Two UWB BPFs with narrow notched band having a fractional bandwidth (FBW) of about 4.6% and 6.5% were realized theoretically and verified by full-wave EM simulation and the experiment. Excellent agreement between the predicted and measured results was obtained and is demonstrated

Abstract: The wireless medium contains domain-specific information that can be used to complement and enhance traditional security mechanisms. In this paper we propose ways to exploit the fact that, in a typically rich scattering environment, the radio channel response decorrelates quite rapidly in space. Specifically, we describe a physical-layer algorithm that combines channel probing (M complex frequency response samples over a bandwidth W) with hypothesis testing to determine whether current and prior communication attempts are made by the same user (same channel response). In this way, legitimate users can be reliably authenticated and false users can be reliably detected. To evaluate the feasibility of our algorithm, we simulate spatially variable channel responses in real environments using the WiSE ray-tracing tool; and we analyze the ability of a receiver to discriminate between transmitters (users) based on their channel frequency responses in a given office environment. For several rooms in the extremities of the building we considered, we have confirmed the efficacy of our approach under static channel conditions. For example, measuring five frequency response samples over a bandwidth of 100 MHz and using a transmit power of 100 mW, valid users can be verified with 99% confidence while rejecting false users with greater than 95% confidence.

Abstract: We study the performance of multi-input multi-output (MIMO) channels with coarsely quantized outputs in the low signal-to-noise ratio (SNR) regime, where the channel is perfectly known at the receiver. This analysis is of interest in the context of ultra-wideband (UWB) communications from two aspects. First the available power is spread over such a large frequency band, that the power spectral density is extremely low and thus the SNR is low. Second the analog-to-digital converters (ADCs) for such high bandwidth signals should be low-resolution, in order to reduce their cost and power consumption. In this paper we consider the extreme case of only 1-bit ADC for each receive signal component. We compute the mutual information up to second order in the SNR and study the impact of quantization. We show that, up to first order in SNR, the mutual information of the 1-bit quantized system degrades only by a factor of 2/pi compared to the system with infinite resolution independent of the actual MIMO channel realization. With channel state information (CSI) only at receiver, we show that QPSK is, up to the second order, the best among all distributions with independent components. We also elaborate on the ergodic capacity under this scheme in a Rayleigh flat-fading environment.

Abstract: An active filtering technique to remove the out-of-band blockers in wireless receivers is presented. The circuit employs a feed-forward filtering path to produce an arbitrarily narrow frequency response in the low-noise amplifier (LNA), eliminating the need for an external surface acoustic wave (SAW) filter at the receiver front-end. The required notch filtering in the feed-forward path is realized through a receiver translational loop, driven by the same local oscillator (LO) signals used in the main receiver. For the proof of concept, a prototype amplifier in 65 nm standard CMOS, intended for Global System for Mobile Communication (GSM) applications, is implemented. When the filtering is enabled, the amplifier 3-dB bandwidth reduces from 220 MHz to about 4.5 MHz, and a stop-band rejection of over 21 dB is achieved.

Abstract: The present invention relates to a communication system comprising a plurality of terminals each having an uplink transmission unit (1) for transmitting radio frequency OFDM signals at a radio frequency and an access point having an uplink receiving unit (4) for concurrently receiving said radio frequency OFDM signals from at least two terminals, said OFDM signals being Orthogonal Frequency Division Multiplex (OFDM) modulated, wherein the bandwidth of said uplink transmission units and of the transmitted radio frequency OFDM signals is smaller than the bandwidth of said uplink receiving unit, that the bandwidth of at least two uplink transmission units and of their transmitted radio frequency OFDM signals is different and that the uplink transmission unit is adapted to assign different connections for concurrently transmitting radio frequency OFDM signals to different sub-carriers in the same time slots or to the same or different sub-carriers in different time slots.

Abstract: Using interferometric couplers and thermal tuning, we demonstrate a novel design of compact microring resonators on silicon-on-insulator platform with tunable bandwidth from 0.1 to 0.7 nm. The structures present an extinction ratio higher than 23 dB and a footprint of less than 0.001 mm(2), which are suitable for integrated optical signal processing such as reconfigurable filtering and routing.

Abstract: Using the idea of a transformation medium, a cloak can be designed to make a domain invisible to one target frequency. We examine the possibility of extending the bandwidth of such a cloak. We find that causality requirements impose severe constraints on the system parameters of the transformation medium, and we show that a specific form of ``reduction'' can help us to create a cloak that offers a reduced cross section in a finite frequency range. We also give a simple inequality that limits the bandwidth of operation.

Abstract: A new method for designing the microstrip branch- line couplers with predetermined compact size and bandwidth is proposed in this paper. With the proposed approach, the size of the quarter-wavelength transmission line in the branch-line coupler can be reduced greatly. In addition, the proposed couplers can be easily fabricated on the printed circuit board without any lumped element. A chart concludes the relationship between bandwidth and size reduction rate. It shows that open stubs with low impedance perform better than those with high impedance; moreover, the more open stubs with low impedance utilized, the broader the bandwidth will be. Furthermore, the measured frequency responses show good agreement with the theoretical results.

Abstract: A simple technique is developed to improve the axial ratio (AR)-bandwidth and quality of circularly polarized stacked microstrip antennas (CPSMAs) using a new C-type single feed. The proposed antenna has been optimized and fabricated, and the computed results agree very well with measurements. The antenna has a 3 dB AR bandwidth of 13.5%, gain is more than 7.5 dBi over the 3 dB AR bandwidth and the 10 dB return-loss bandwidth is 21%. The proposed feed optimization technique is useful for rapid design of circular polarized stacked microstrip antennas.

Abstract: In this letter, a novel microstrip parallel-coupled line structure with two asymmetric loading stubs is proposed for notched band implementation in ultra-wideband (UWB) bandpass filter (BPF). The rejection band is introduced by adding asymmetric loading stubs to the two outer arms of three parallel-coupled lines. The lengths and the widths of the stubs can control the bandwidth of the notched filter and can set the notched band at a desired frequency. This structure has been applied to a single-stage UWB BPF in order to produce a narrow notched band inside its passband. The design is successfully realized in theory and verified by full-wave electromagnetic simulation and the experiment.

Abstract: An exact and general expression for the analytic wavelet transform of a real-valued signal is constructed, resolving the time-dependent effects of non-negligible amplitude and frequency modulation. The analytic signal is first locally represented as a modulated oscillation, demodulated by its own instantaneous frequency, and then Taylor-expanded at each point in time. The terms in this expansion, called the instantaneous modulation functions, are time-varying functions which quantify, at increasingly higher orders, the local departures of the signal from a uniform sinusoidal oscillation. Closed-form expressions for these functions are found in terms of Bell polynomials and derivatives of the signal's instantaneous frequency and bandwidth. The analytic wavelet transform is shown to depend upon the interaction between the signal's instantaneous modulation functions and frequency-domain derivatives of the wavelet, inducing a hierarchy of departures of the transform away from a perfect representation of the signal. The form of these deviation terms suggests a set of conditions for matching the wavelet properties to suit the variability of the signal, in which case our expressions simplify considerably. One may then quantify the time-varying bias associated with signal estimation via wavelet ridge analysis, and choose wavelets to minimize this bias.

Abstract: Synthetic Aperture Radar (SAR) is active and coherent microwave high resolution imaging system, which has the capability to image in all weather and day-night conditions. SAR transmits chirp signals and the received echoes are sampled into In-phase (I) and Quadrature (Q) components, generally referred to as raw SAR data. The various modes of SAR coupled with the high resolution and wide swath requirements result in a huge amount of data, which will easily exceed the on-board storage and downlink bandwidth of a satellite. This paper addresses the compression of the raw SAR data by sampling the signal below Nyquist rate using ideas from Compressed Sensing (CS). Due to the low computational resources available onboard satellite, the idea is to use a simple encoder, with a 2D FFT and a random sampler. Decoding is then based on convex optimization or uses greedy algorithms such as Orthogonal Matching Pursuit (OMP).

Abstract: In the context of array sensors such as radar, sonar, and communications receiver arrays, the present invention exploits the geometry phase components of radiated wavefronts associated with the signals of interest in order to reduce the bandwidth requirements for DOA and beamforming processing. Additionally, geometry phase is exploited in order to effectively increase the resolution of an array without changing the size of its physical footprint or the number of array elements. Other embodiments of the invention include the use of virtual array elements for increase in effective array size.

Abstract: This letter presents the design of a planar antenna for ultrawideband (UWB) applications with a bandwidth of 5.5 GHz over 3.7 to 9.2 GHz and return loss values lower than -10 dB. The antenna geometry is described in terms of a spline-based representation whose control parameters, together with other geometrical descriptive quantities, are determined through a suitable particle swarm optimizer (PSO) in order to fit the UWB requirements. Representative results of both numerical and experimental validations are reported in order to assess the performance of the prototype as well as to give some preliminary indications on the reliability and effectiveness of the whole synthesis approach

Abstract: Novel fourth-order balanced coupled-resonator bandpass filters are proposed using suitably designed half-wavelength (lambda/2) multisection resonators for common-mode suppression. By properly designing the input/output (I/O) resonators associated with the filter composed of four bi-section resonators, a balanced filter with good common-mode suppression is realized, but its rejection bandwidth is rather limited. To widen the rejection bandwidth, the I/O bi-section resonators are replaced by the tri-section ones so that a balanced filter with good common-mode suppression and wide rejection bandwidth may be realized by suitably arranging the composed bi-/tri-section resonators. Specifically, a stopband-extended balanced filter with good common-mode suppression (>50 dB) within the differential-mode passband is implemented and its stopbands are also extended up to 5f0 d with a rejection level of 30 dB, where f0 d is the center frequency in differential-mode operation.

Abstract: This paper presents a novel approach to design a digitally programmable low pass filter (LPF) and variable gain amplifier (VGA) intended for a software-defined radio (SDR) front-end. These flexible analog circuits are driven by a network-on-chip (NoC) that is able to set performance parameters like cut-off frequency, selectivity, noise, and gain guaranteeing at any time a near-optimal power/perfomance trade-off. A design approach is proposed to tackle the challenges imposed by flexibility in analog design. A silicon prototype is realized in 0.13-μm CMOS technology with 1.2-V supply voltage to prove the validity of the proposed solution. The LPF provides a frequency tuning range between 0.35 MHz and 23.5 MHz with an adaptive integrated noise level between 85 μVrms and 163 μVrms whereby the power consumption conveniently varies from 0.72 mW to 21.6 mW according to the required performance. The VGA is made up of two cascaded gain stages and provides a gain range from about 0 dB to 39 dB with a reconfigurable power/bandwidth.

Abstract: In the present work, a widely tunable high-Q air filled evanescent cavity bandpass filter is created in an LTCC substrate. A low loss Rogers Duroidreg flexible substrate forms the top of the filter, acting as a membrane for a tunable parasitic capacitor that allows variable frequency loading. A commercially available piezoelectric actuator is mounted on the Duroidreg substrate for precise electrical tuning of the filter center frequency. The filter is tuned from 2.71 to 4.03 GHz, with insertion losses ranging from 1.3 to 2.4 dB across the range for a 2.5% bandwidth filter. Secondarily, an exceptionally narrow band filter is fabricated to show the potential for using the actuators to fine tune the response to compensate for fabrication tolerances. While most traditional machining techniques would not allow for such narrow band filtering, the high-Q and the sensitive tuning combine to allow for near channel selection for a front-end receiver. For further analysis, a widely tunable resonator is also created with a 100% tunable frequency range, from 2.3 to 4.6 GHz. The resonator analysis gives unloaded quality factors ranging from 360 to 700 with a maximum frequency loading of 89%. This technique shows a lot of promise for tunable RF filtering applications.

Abstract: This paper presents a novel approach to design a digitally programmable low pass filter (LPF) and variable gain amplifier (VGA) intended for a software-defined radio (SDR) front-end. These flexible analog circuits are driven by a network-on-chip (NoC) that is able to set performance parameters like cut-off frequency, selectivity, noise, and gain guaranteeing at any time a near-optimal power/performance trade-off. A design approach is proposed to tackle the challenges imposed by flexibility in analog design. A silicon prototype is realized in 0.13-mum CMOS technology with 1.2-V supply voltage to prove the validity of the proposed solution. The LPF provides a frequency tuning range between 0.35 MHz and 23.5 MHz with an adaptive integrated noise level between 85 muVrms and 163 muVrms whereby the power consumption conveniently varies from 0.72 mW to 21.6 mW according to the required performance. The VGA is made up of two cascaded gain stages and provides a gain range from about 0 dB to 39 dB with a reconfigurable power/bandwidth.

Abstract: We present results of two indoor ultrawideband channel measurement campaigns in the 2-5 GHz frequency band. In measurement campaign I (MC I), the channel is static and we sample it spatially, while in MCII the transmitting and receiving antennas are fixed and channel variation is induced by people moving in the environment. Transmitter and receiver are separated by up to 27 m in MC I and up to 20 m in MC II. To determine suitable small-scale fading distributions for the tap amplitudes of the discrete-time baseband-equivalent channel impulse response, we use Akaike's information criterion (AIC). Despite the large bandwidth, AIC supports the Rayleigh (MCI) or the Rice distribution (MC II). For data from MC II, we estimate the covariance matrix of the random channel impulse response and demonstrate that the number of corresponding significant eigenvalues, and hence the diversity order of the channel, scales approximately linearly with bandwidth. Contrary to the uncorrected scattering assumption, we find that the channel taps are weakly correlated. The ergodic capacity predicted by the Ricean channel model with parameters estimated from MC II shows good agreement with the ergodic capacity obtained by direct evaluation of the measurement results, while the corresponding outage capacities show a worse fit for low outage probabilities because of shadowing.

Abstract: A new media access control (MAC) protocol for cognitive wireless networks is described. The new MAC protocol allows each of multiple nodes, such as cell phones and computers with wireless, to determine utilization of a communication spectrum, such as the television broadcast band. The nodes collaborate to achieve a combined view of spectrum utilization in their local vicinity, in which scheduled users and empty time segments are mapped across a wide range of frequencies. Nodes negotiate with each other to reserve idle segments of the spectrum for packet exchange on negotiated frequencies. Control packet structure allows nodes to become prescient of the local spectrum utilization during handshaking. A cognitive device operating under the new MAC has a first radio that both scans the spectrum and monitors a control channel; and a second reconfigurable radio with adjustable parameters, including frequency and bandwidth, for packet transmission.

Abstract: Traditional design of the current loop controller in a single-phase power factor correction boost converter is not suitable for applications with higher line frequencies (up to 800 Hz) because of the zero-crossing distortion and high harmonic content due to the current phase lead effect. Increasing the control bandwidth and switching frequency in order to avoid this effect would reduce converter efficiency and is objectionable. The paper presents the leading-phase admittance cancellation (LPAC) technique, which improves the current-shaping control structure and eliminates the current phase lead without increasing the bandwidth requirement. The LPAC method extends the allowable line frequency range from 1/150 to 1/5 of the current loop bandwidth. The LPAC method is load-invariant and superior to other previously proposed methods. The LPAC network can be added to existing designs, which would require only two passive components in the simplest case

Abstract: We apply the optical nanocircuit concepts to design and analyze in detail a three-dimensional (3D) plasmonic nanotransmission line network that may act as a relatively broadband negative-refraction metamaterial at infrared and optical frequencies. After discussing the heuristic concepts in our theory, we show full-wave analytical results of the expected behavior of such materials, which show increased bandwidth and relative robustness to losses. The possibility and constraints of getting a 3D fully isotropic response are also explored and conditions for minimal losses and increased bandwidth are discussed. Full-wave analytical results for some design examples employing realistic plasmonic materials at infrared and optical frequencies are also presented, and the case of a subwavelength imaging system using a slab of this material is numerically investigated.

Abstract: The pioneers of radio science made their first trials of wireless information transmission and demonstrated localization of a steel vessel by radio waves more than 100 years ago. Back then, the world of radio frequencies was organized in a very simple way. The researchers could use any frequency band. There was no interference by others and no controlling government bodies. Since then, governmental authorities have established tight regulations that have split up the available frequency band into small partitions for exclusive use. This article discusses the architecture of a baseband, pseudo-noise UWB radar and gives some examples of applications

Abstract: Intercell interference co-ordination is considered within 3GPP for evolved UTRA (E-UTRA). The objective is to improve coverage and increase cell-edge bitrate. A number of static and dynamic schemes have been suggested. In this study some basic schemes have been evaluated by means of simulations. Also, the impact of link performance and carried services has been investigated. Of the static schemes the simple 1-reuse performs best for wideband services. The bandwidth reduction (in Hz) with other schemes cannot be regained by the link improvement (in bps/Hz) achieved by the interference reduction. The link performance has a significant impact on the co-ordination gain. Static schemes improve the cell-edge bitrate with a single receiver antenna but not with the two receiving antennas expected in E-UTRA. Interference co-ordination will be more efficient for narrowband services since the frequency bandwidth allocated to each cell is then better utilized. For wideband packet data services a dynamic scheme is required to improve compared to a simple 1-reuse.

Abstract: Underwater acoustic channels are wideband in nature due to the fact that the signal bandwidth is not negligible with respect to the center frequency. OFDM transmissions over UWA channels encounter frequency-dependent Doppler drifts that destroy the orthogonality among OFDM subcarriers. In this paper, we propose a two-step approach to mitigating the frequency-dependent Doppler drifts in zero-padded OFDM transmissions over fast-varying channels: (1) non-uniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problem; and (2) high-resolution uniform compensation on the residual Doppler. Based on block-by-block processing, our receiver does not rely on channel dependence across OFDM blocks, and is thus desirable for fast-varying UWA channels. We test our receiver with data from a shallow water experiment at Buzzards Bay, Massachusetts. Our receiver achieves excellent performance even when the transmitter and the receivers have a relative speed up to 10 knots, where the Doppler drifts are several times larger than the OFDM subcarrier spacing.