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In this paper, we propose a feature-based method for spectrum sensing of OFDM signals from sub-Nyquist samples over a single band. We exploit the structure of the covariance matrix of OFDM signals to convert an underdetermined set of covariance-based equations to an overdetermined one. The statistical properties of sample covariance matrix are analyzed and then based on that an approximate Generalized Likelihood Ratio Test (GLRT) for detection of OFDM signals from sub-Nyquist samples is derived. The method is also extended to the frequency-selective channels. [ABSTRACT FROM AUTHOR]

Orthogonal frequency-division multiplexing (OFDM) has been selected as a baseline waveform for long-term evolution (LTE) and fifth-generation new radio (5G NR). Fast-convolution (FC)-based frequency-domain signal processing has been recently considered as an effective tool for transmitter and receiver side subband filtering of OFDM-based waveforms. However, for the original continuous FC-based model, the filtering can, in general, be configured in time-direction only with the granularity of half subframe, corresponding to 7, 14, or 28 symbols with 15kHz, 30kHz, or 60kHz subcarrier spacing, respectively. In this paper, we present a symbol-synchronous FC-processing scheme flexibly allowing filter re-configuration with the time resolution equal to one OFDM symbol while supporting tight carrier-wise filtering for 5G NR in mixed-numerology scenarios with adjustable subcarrier spacings, center frequencies, and subband bandwidths, as well as providing co-existence with LTE. Proposed approach segments each stream of time-domain OFDM symbols into overlapping processing blocks of fixed size. Symbol synchronous processing is achieved by dynamically adjusting the overlap between the processing blocks while aligning the payload part of the processing blocks with the boundaries of the OFDM symbols. The proposed scheme is demonstrated to support the envisioned use cases of 5G NR and provide a flexible starting point for sixth generation (6G) development. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the sensitivity of orthogonal frequency-division multiplexing waveforms to the joint distortion effects of coexisting nonidealities in the implementation of transceiver radio frequency (RF) electronics, combined also with time variations of the multipath radio channel. To cope with the challenge in evaluation of error rates under non-Gaussian joint interference, we propose a semianalytical method for the evaluation of the uncoded symbol-error rate for an arbitrary rectangular quadrature amplitude modulation as subcarrier modulation. This allows deriving maximum tolerable impairment levels for RF circuits such that the link detection error rate is kept at a target level. [ABSTRACT FROM PUBLISHER]

Integrating efficient connectivity, positioning and sensing functionalities into 5G New Radio (NR) and beyond mobile cellular systems is one timely research paradigm, especially at mm-wave and sub-THz bands. In this article, we address the radio-based sensing and environment mapping prospects with specific emphasis on the user equipment (UE) side. We first describe an efficient $\ell _1$ -regularized least-squares (LS) approach to obtain sparse range–angle charts at individual measurement or sensing locations. For the subsequent environment mapping, we then introduce a novel state model for mapping diffuse and specular scattering, which allows efficient tracking of individual scatterers over time using interacting multiple model (IMM) extended Kalman filter and smoother. Also the related measurement selection and data association problems are addressed. We provide extensive numerical indoor mapping results at the 28 GHz band deploying OFDM-based 5G NR uplink waveform with 400 MHz channel bandwidth, covering both accurate ray-tracing based as well as actual RF measurement results. The results illustrate the superiority of the dynamic tracking-based solutions, compared to static reference methods, while overall demonstrate the excellent prospects of radio-based mobile environment sensing and mapping in future mm-wave networks. [ABSTRACT FROM AUTHOR]

Balancing between power amplifier (PA) linearity and power efficiency is one of the biggest implementation challenges in radio communication transmitters. Among various linearization methods, the feedforward linearization technique is a fairly established principle offering a good tradeoff between linearity and power-efficiency even under wideband operation. Moreover, adaptive techniques for such linearizer have been proposed in literature to track parameter changes in the main PA and other circuitry. Among those, least mean squares (LMS) method for adapting signal cancellation loop (SCL) and error cancellation loop (ECL) coefficients is an attractive low-complexity alternative. In this paper, we carry out extensive closed-form performance analysis of the achievable intermodulation distortion (IMD) reduction of the overall LMS-adaptive feedforward linearizer, as a function of the used step-sizes and essential waveform statistics. Such analysis is currently missing from the state-of-the-art literature. Both memoryless nonlinearities and Wiener–Hammerstein type PA memory models are studied for which IMD suppression expressions are derived. Comprehensive computer simulations are also provided to illustrate the accuracy of the analysis when practical OFDM waveforms are used. Design examples are given as well where the analysis results are used to choose proper linearizer step-sizes to meet given transmitter spectral mask specifications. [ABSTRACT FROM PUBLISHER]

This article studies the processing principles, implementation challenges, and performance of orthogonal frequency-division multiplexing (OFDM)-based radars, with particular focus on the fourth-generation Long-Term Evolution (LTE) and fifth-generation (5G) New Radio (NR) mobile networks’ base stations and their utilization for radar/sensing purposes. First, we address the problem stemming from the unused subcarriers within the LTE and NR transmit signal passbands and their impact on frequency-domain radar processing. In particular, we formulate and adopt a computationally efficient interpolation approach to mitigate the effects of such empty subcarriers in the radar processing. We evaluate the target detection and the corresponding range and velocity estimation performance through computer simulations and show that high-quality target detection as well as high-precision range and velocity estimation can be achieved. In particular, 5G NR waveforms, through their impressive channel bandwidths and configurable subcarrier spacing, are shown to provide very good radar/sensing performance. Then, a fundamental implementation challenge of transmitter–receiver (TX–RX) isolation in OFDM radars is addressed, with specific emphasis on shared-antenna cases, where the TX–RX isolation challenges are the largest. It is confirmed that from the OFDM radar processing perspective, limited TX–RX isolation is primarily a concern in the detection of static targets, while moving targets are inherently more robust to transmitter self-interference (SI). Properly tailored analog/RF and digital SI cancellation solutions for OFDM radars are also described and implemented and shown through RF measurements to be key technical ingredients for practical deployments, particularly from static and slowly moving targets’ point of view. [ABSTRACT FROM AUTHOR]

Direct-conversion architectures can offer highly integrated low-cost hardware solutions to communication transceivers. However, it has been demonstrated that radio frequency (RF) impairments such as amplifier nonlinearities, phase noise, and in-phase/quadrature-phase imbalances (IQI) can lead to severe degradation of the performance of such systems. Motivated by this, the present work is devoted to the quantification and evaluation of the effects of RF IQI on wireless communications in the context of cascaded fading channels for both single-carrier and multicarrier systems. To this end, closed-form expressions are first derived for the outage probability (OP) over N^\ast Nakagami-$m$ channels for the cases of ideal transmitter (TX) and receiver (RX), ideal TX and IQI RX, IQI TX and ideal RX, and joint TX/RX IQI. The offered expressions, along with several deduced corresponding special cases, are subsequently employed in vehicle-to-vehicle (V2V) communications to justify their importance and practical usefulness in the context of emerging communication systems. The offered analytic results are corroborated by extensive comparisons with respective results from computer simulations. It is shown that considering non-ideal RF front-ends at the TX and/or RX introduces nonnegligible errors in the OP performance that can exceed 20% under several communication scenarios. It is further demonstrated that the effects by cascaded multipath fading conditions are particularly detrimental as they typically result in considerable performance losses of around or over an order of magnitude. [ABSTRACT FROM PUBLISHER]

This paper focuses on evaluating the gains obtained through cooperative spectrum sensing in the real world while using cyclostationary-based mobile sensors. In cooperative sensing (CS), different secondary users (SUs) in a geographical neighborhood cooperate to detect the presence of a primary user (PU). Compared with single-user sensing, cooperation provides diversity gains in the face of multipath fading and shadowing. The effectiveness of CS is demonstrated by analyzing data acquired in two extensive field measurement campaigns. The first measurement campaign (MC-I) focuses on measurements at fixed locations, whereas the second measurement campaign (MC-II) focuses on a scenario where measurements are taken inside a moving car. These measurements are carried out for DVB-T channels in the Capital Region of Finland, which consists of urban and suburban environments. Hard decision rules such as or, and, and majority and a soft decision rule such as sum of cyclostationary test statistics (sum) are employed, and their detection performances are compared with a cyclostationary-based single-user detector. A performance parameter of relative increase in probability of detection (RIPD) is used to efficiently demonstrate the cooperation gain obtained relative to local sensing. It is shown that cooperation can significantly improve the performance of a sensor severely affected by fading and shadowing effects. Furthermore, it is shown that increasing the number of collaborating users beyond few users (five to eight) does not, in practice, bring significant improvement in terms of the expected RIPD. The performances of CS schemes evaluated from MC-I are also compared with the corresponding simulated CS results using empirical channel models and terrain data for the same experimental parameters. It is shown that the use of empirical or theoretical models may result in detection errors in practical conditions, and measurements should be used to improve the accuracy in such scenarios. [ABSTRACT FROM AUTHOR]

Single-carrier frequency-division multiple access (SC-FDMA), which is a modified form of orthogonal frequency-division multiple access (OFDMA), has been adopted as the uplink physical-layer radio access technique for the Third-Generation Partnership Project Long-Term Evolution (3GPP-LTE) and LTE-Advanced. Radio transceiver implementations for such OFDM-based systems with the direct-conversion architecture are desirable to enable small-size, low-cost, and low-power-consumption terminals. However, the associated circuit impairments stemming from the processing of analog radio frequency (RF) signals, such as in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset errors, can severely degrade the obtainable link performance. In this paper, we analyze the effects of these radio impairments in a multiuser SC-FDMA uplink system and present digital-signal-processing-based methods for the joint estimation and equalization of impairments and channel distortions on the receiver side with an arbitrary number of receiver antennas. For the equalization, linear equalizers such as the zero-forcing (ZF) and the minimum mean square error (MMSE) equalizers that utilize pairs of mirror subcarriers are formulated, and the MMSE equalizer is developed to effectively handle mirror subband users with different power levels. Furthermore, for reduced computational complexity, the joint channel and impairment filter responses are efficiently approximated with polynomial-based basis function models. The parameters of the basis functions are then estimated by exploiting the time-multiplexed reference symbols in the LTE uplink subframe structure. The performance of the proposed estimation and equalization methods is assessed with extensive multiuser link simulations, with both single-antenna and dual-antenna base-station receivers, and the results show that the proposed algorithms are able to significantly reduce the impact of channel distortions and radio impairments. The resulting receiver implementation with the proposed techniques enables improved uplink link performance, even when the mobile terminals fulfill their emission requirements, in terms of I/Q images, with no changes in the LTE standard's frame and pilot structures. [ABSTRACT FROM PUBLISHER]

We propose a discrete-time model for phase noise processes occurring in phase locked loop (PLL) based oscillators. By using linear time-invariant analysis in the phase domain, we arrive at a parallel auto-regressive moving-average phase noise model that captures the major noise sources and corresponding loop transfer functions of the PLL circuitry. The acquired model allows us to derive analytically the distribution of the inter-carrier interference (ICI) power in the receiver, shown to be a sum of correlated gamma random variables with a model-specific covariance matrix. Finally, we utilize the derived ICI distribution and proceed to analyze the effects of phase noise and multipath fading on the performance of OFDM radio links. For example, we evaluate the rate loss without ICI compensation or knowledge at the receiver, and illustrate the trade-off introduced by phase noise between the OFDM symbol length and the overhead caused by the cyclic prefix. Throughout the paper, simulations confirm the accuracy of the analytical expressions. [ABSTRACT FROM PUBLISHER]