With the rapid development and broad applications of unmanned aerial vehicle (UAV)-based wireless stations in the sky, fundamental understanding and characterization of the realistic air-to-ground ...(A2G) communication link properties are crucial. In this article, a UAV-aided channel sounder with a real-time processing hardware system is developed for highly dynamic and nonstationary A2G channel measurements. In the hardware system, a global positioning system (GPS)-based triggering signal is designed, the equivalent antenna pattern affected by the UAV airframe is considered, and an appropriate sounding signal is selected, to improve the accuracy of measured channel impulse response (CIR). Moreover, real-time hardware processing algorithms for raw channel data, that is, CIR extraction, system response elimination (SRE), power loss recovery (PLR), and adaptive multipath component (MPC) recognition are developed and implemented on a single field-programmable gate array (FPGA) chip. In this way, the required storage size of channel data and the processing time for one slice of CIR is greatly decreased, which can meet the requirement of nonstationary A2G channel measurement with a high sampling rate and long-time measurement. A commercial channel emulator is used to reproduce controllable channels and verify the performance of the developed channel sounder. Finally, the developed channel sounder is applied to carry out A2G measurement campaigns at 3.5 GHz in a campus scenario. The channel characteristics, that is, path loss (PL), Formula Omitted-factor, and path angle are analyzed. The measured channel characteristics are consistent with existing measurements under a similar scenario. The estimated path angles are also validated by the theoretical results. Thus, the channel sounder can be used to capture the nonstationary A2G channel characteristics for the system design and algorithm optimization of A2G communications.
Voice Activity Detection (VAD) is a technique used to identify the presence of human voice in an audio signal. It is implemented as an always-on component in most speech processing applications. As ...speech is absent most of the time, this component typically dominates the overall average power consumption of the system (excluding microphone). The widespread usage in speech applications and the need for ultra low power VAD have led to a plethora of algorithms and implementations in the hardware domain, necessitating a comprehensive study and analysis to understand (real-time) requirements, different design parameters, testing strategies, but also to identify design trends, challenges and guidelines for future implementations and testing of VAD devices. A scoping review was conducted to identify the articles for hardware implementations of VAD from January 2010 -December 2021, the results of which are presented in this article. The results highlight a big design space being used for VAD along with a lack of standard testing methodology and usage of application-dependent performance metrics. An increased usage of filter-based feature extractors along with neural-network-based classifiers is observed. Due to lack of standardisation, no other trends can be established from the results. A set of rules and guidelines are therefore provided to facilitate the future development and benchmarking of VADs.
In this paper, a methodology for developing a human walking model adapted to the individual measured by radar was proposed. The fundamental parameters used in the model are the gait parameters and ...physical dimensions. Empirical validation of the proposed methodology is undertaken, involving the acquisition of data using a 77-GHz FMCW radar. The data is collected from three distinct individuals walking five different trajectories with respect to the radar. Moreover, the gait parameter estimation accuracy is evaluated for the different walking trajectories of the target. The studied gait parameters are speed, step length, and step frequency. These could be estimated with mean errors up to 0.077 m/s, 9.3 cm, and 0.128 Hz for all trajectories, respectively. Nevertheless, these errors diminish to 0.022 m/s, 2.2 cm and 0.03 Hz, respectively when the targets walk in a straight trajectory aligned with the radar beam. Moreover, the feasibility of estimating body part dimensions directly from the radar data is investigated. It was found that only the total human height could be directly estimated using the employed hardware. Except for the tallest participant of 2.01 m, the height could be estimated with a mean absolute error up to 10.9 cm. Enhanced hardware configurations or the integration of machine learning techniques may improve the accuracy of body part dimension estimations.
Over-the-air (OTA) testing and electromagnetic compatibility (EMC) measurements are widely performed in classical reverberation chambers. A less-known reverberation chamber, yet a considerably ...efficient environment for generating multipath conditions, is the vibrating intrinsic reverberation chamber (VIRC). This article thoroughly investigates the radio propagation channel inside the VIRC. The specific channels studied are narrowband single-input single-output (SISO) systems operating in the frequency range from 670-2740 MHz. A relevant application is OTA testing wireless baseband algorithms or modems for wireless sensor networks, specifically low-power wide-area networks such as Narrowband-IoT (NB-IoT), Long Range (LoRa) and Ultra-Narrowband (UNB). The focus is on the first- and second-order temporal and spectral channel characteristics: coherence time, Doppler spectrum, Doppler spread, frequency autocovariance function, coherence bandwidth, rejection rate of chi-squared goodness-of-fit test for Rician distribution, Rician <inline-formula> <tex-math notation="LaTeX">K </tex-math></inline-formula>-factor, and channel gain. Besides the impact of the frequency of operation, the investigation considers the effect of the rotational speed of the VIRC motors, two loading conditions, on the channel characteristics. An analysis of the measurement results shows that the stirring efficiency degrades while the coherence time increases at slow rotational speeds, with loading, and/or at low frequencies. Moreover, empirical models closely fitting the behavior of various investigated characteristics are proposed, for which we provide foundational physical interpretations. A further investigation is carried out to demonstrate the generality and briefly illustrate the potential usability of these models for both EMC and OTA testing.
In recent years, promising results have been reported with the vibrating intrinsic reverberation chamber (VIRC) combining performance and cost-efficiency for electromagnetic compatibility (EMC) ...measurements. This also makes it a potentially attractive solution for over-the-air (OTA) testing, which is yet to be investigated. Therefore, this article proposes the first systematic and thorough methodology to characterize the VIRC for use in EMC and OTA testing of wireless baseband algorithms of narrow-band single-input single-output channels. This methodology has been developed to measure and estimate the channel first- and second-order temporal and spectral characteristics taking into consideration the effect of different carrier frequencies, rotational speeds of VIRC motors, and loading conditions. It is then applied to a channel setup inside a VIRC for a preliminary investigation before the VIRC itself is fully characterized. It is shown that mounting an absorber in a specific location on the hatch significantly improves the rejection rate of the chi-squared goodness-of-fit test for Rician distribution without increasing the <inline-formula><tex-math notation="LaTeX">K</tex-math></inline-formula>-factor above <inline-formula><tex-math notation="LaTeX">-</tex-math></inline-formula>10 dB over the frequency range 755-2740 MHz in the VIRC under investigation. However, the proposed methodology has been devised to be universal to any reverberation chamber, and the obtained results can be used to improve EMC testing due to the better understanding of the unique VIRC environment.
Spectrum sensing is a key enabler of cognitive radio but generally suffers from what is called a signal-to-noise ratio (SNR) wall, i.e., a minimum SNR below which it is impossible to reliably detect ...a signal. For energy detection, which has the advantage of not requiring knowledge of the signal, the SNR wall is caused by uncertainty in the noise level. Cross-correlation has been suggested as a possible means to obtain higher sensitivity but has received little attention in the context of noise uncertainty. The idea of cross-correlation is to have two receive paths, where each path independently processes the signal before they are combined, such that the noise added to the input signal at the individual paths is largely uncorrelated. In this paper, we mathematically quantify the SNR wall for cross-correlation, showing that it linearly scales with the amount of noise correlation. This lower noise correlation results in higher sensitivity, which is significantly better than that for autocorrelation. Equations that can be used to estimate the benefit over autocorrelation and the measurement time for a required probability of detection and false alarm are derived.
The DFT-based demodulator for BFSK has been introduced for applications where the received signal experiences a carrier frequency offset (CFO) much larger than the symbol rate. The Ultra-Narrowband ...(UNB) communication techniques have been introduced for implementing the emerging Low Power Wide Area Networks (LPWAN). Since UNB communication is prone to CFO, a DFT-based BFSK demodulator is an interesting option for this type of communication. However, for proper operation in a large frequency offset, the DFT-based demodulator requires a complex window synchronization which is not desirable for low power nodes. The main source of complexity, is calculating the DFT of a window which slides over the preamble. In this work, the complexity is alleviated by considering the window synchronization algorithm and its implementation together. First, a new window synchronization algorithm is proposed which is designed such that an efficient class of implementations of the sliding DFT (SDFT), called Single Bin SDFT (SB-SDFT) in this work, can be used. Moreover, a new stable implementation of SB-SDFT is designed to enable zero-padding which is required by the demodulator. The complexity of the proposed algorithm implemented using the SB-SDFT, scales more efficiently compared to the conventional algorithm when the range of tolerable CFO increases. Using the proposed method, for a CFO tolerance in the order of 14.5 times the symbol rate (±14.5 kHz for a symbol rate equal to 100 Hz), the number of complex operations is reduced by more than 85% (and memory by 90%) compared to the conventional method.
We develop a systematic methodology to experimentally investigate the impact of interference from a non-listen-before-talk ultra-narrowband (UNB) signaling technique on Wi-Fi links. The methodology ...is based on a worst-case interference scenario, and consists of three investigating steps. This methodology is then applied to a measurement setup to practically study the case of 100 bps UNB signals interfering with an IEEE 802.11n transmission in the 2.4GHz band. Five different Wi-Fi devices are tested. The UNB signal is generated in two modulation schemes, the on-off-keying (OOK) and the Gaussian minimum-shift-keying. Both single and multiple simultaneous UNB interferers are considered. An analysis of the measurement results shows that three of the tested Wi-Fi devices cannot coexist with the considered non-listen-before-talk UNB communication system. The throughput performance analysis of the other tested devices shows that the OOK-modulated UNB signal has the least interfering impact, and the Wi-Fi pilot subcarriers are the most vulnerable to UNB interference. However, if a single UNB interferer avoids these subcarriers and employs the OOK-modulation scheme, then wireless coexistence is possible as long as the signal-to-interference ratio of Wi-Fi to UNB is greater than 30dB, given that a drop to 75% of the maximum Wi-Fi throughput is acceptable.
Exposed to strong cochannel/adjacent-channel interference, digital multiple-input-multiple-output (MIMO) receivers require high-dynamic-range analog-to-digital converters (ADCs). Hybrid beamforming ...featuring spatial filtering before the ADCs can adaptively mitigate interference in both the analog and digital domains; hence relaxing the required dynamic ranges of the ADCs. This article demonstrates the effectiveness of hybrid beamforming in this mitigation utilizing adaptive minimum mean square error (MMSE) and error vector magnitude (EVM) as an optimization criterion. Extensive EVM measurements are carried out with a conductive setup using a four-element 22-nm FD-SOI CMOS prototype MIMO receiver chip to verify the performance of the adaptive MMSE algorithm. Over-the-air (OTA) measurements with a linear four-element dipole antenna array with half-wavelength spacing in the 2.4-GHz industrial scientific and medical (ISM) band quantify the improvement for real-world scenarios, e.g., having a multipath propagation channel and mutual coupling between the antenna array elements. OTA results show that a rejection of 22.5 and 24.5 dB can be achieved on average in an in-door laboratory environment utilizing vector modulator (VM) constellations with 16 and 64 points, respectively.
In traditional multiple-input multiple-output (MIMO) receivers, radio frequency (RF) front-ends are exposed to interference as no analog spatial filtering is employed before the digital beamforming ...stage. Therefore the RF front-end is power-hungry, and analog to digital converters require a high dynamic range. In this paper, we consider an analog beamforming system in case of narrowband signals to cancel interference early in the analog domain, thus reducing the required ADC resolution. In contrast to existing analog beamformers with only phase shifts, our proposed design employs vector modulators where the coefficients can be selected from a set of weights with variable phases and amplitudes. We also propose an efficient and fast Euclidean distance algorithm to determine the analog beamformer coefficients while being suitable for realistic scenarios. Finally, an expression is introduced to estimate the interference rejection achieved by employing the proposed algorithm and a vector modulator in the RF domain. The introduced algorithm leads to considerable improvement in computational complexity by slightly sacrificing interference rejection.
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