A robust timing and frequency synchronization for OFDM systems

Abstract—A robust symbol-timing and carrier-frequency synchronization scheme applicable to orthogonal frequency-division-multiplexing systems is presented. The proposed method is based on a training symbol specifically designed to have a steep rolloff timing metric. The proposed timing metric also provides a robust sync detection capability. Both time domain training and frequency domain (FD) training are investigated. For FD training, maintaining a low peak-to-average power ratio of the training symbol was taken into consideration. The channel estimation scheme based on the designed training symbol was also incorporated in the system in order to give both fine-timing and frequency-offset estimates. For fine frequency estimation, two approaches are presented. The first one is based on the suppression of the interference introduced in the frequency estimation process by the training symbol pattern in the context of multipath dispersive channels. The second one is based on the maximum likelihood principle and does not suffer from any interference. A new performance measure is introduced for timing estimation, which is based on the plot of signal to timing-error-induced average interference power ratio against the timing estimate shift. A simple approach for finding the optimal setting of the timing estimator is presented. Finally, the sync detection, timing estimation, frequency estimation, and bit-error-rate performance of the proposed method are presented in a multipath Rayleigh fading channel. Index Terms—Frequency-offset estimation, orthogonal frequency-division multiplexing (OFDM), symbol-timing estimation, synchronization, training symbol. I

2nd IEEE International Mediterranean Conference on Communications and Networking (IEEE MeditCom 2022) 5-8 September 2022, Athens, Greece

International audienc

Improved spectrum sensing for OFDM cognitive radio in the presence of timing offset

Spectrum sensing is an important aspect of an (interweave) cognitive radio network. In the particular case of orthogonal frequency division multiplexing (OFDM) transmission, many previous spectrum sensing algorithms have utilized the unique correlation properties provided by the cyclic prefix (CP). However, they have also had to both estimate and compensate for the inherent timing offset of a practical system. This is because the timing offset will affect both the test statistic and the threshold, and the inaccurate estimation of timing offset will lead to poor performance. So in this paper, we propose an improved CP detector by constructing a likelihood ratio test (LRT) based on the multivariate probability density functions (pdf) of a particular auto-correlation vector that is chosen to exploit the existence of the CP. This leads to ‘probability of detection’ (Pd) and ‘probability of false alarm’ (Pf) terms that are actually independent of timing offset, and we can get an accurate threshold without estimating timing offset. Simulation results illustrate that the proposed algorithm outperforms existing methods, even for low SNR values. Finally, we show how the algorithm’s parameters must be carefully chosen in a trade-off between spectrum sensing success and overall system performance

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Performance and simulation of communications systems with applications to TCM over fading channels

Over the past decade there has been considerable interest in using Monte Carlo (MC) simulation techniques for performance analysis of communications systems. Unfortunately, this approach often becomes intractable because of the prohibitive amount of simulation time required to achieve accurate estimates. Importance sampling is a modified MC technique which can results in a significant reduction in the computational cost required to obtain accurate MC estimates. In this paper, we will illustrate the use of importance sampling for simulating coded communications systems by presenting an efficient importance sampling methodology for simulating trellis code modulation (TCM) over Rayleigh fading channels. In particular, it is demonstrated that the proposed method can estimate the performance of these systems with a high degree of accuracy to warrant its possible use as a powerful tool for performance analysis and design of practical TCM systems

Symbol-based space diversity for coded OFDM systems

The authors present a coded orthogonal frequency division multiplexing (COMM) system with multiple-input/multiple-output (multiple transmit and/or multiple receive) antennas for high-rate wireless data transmission. A symbol-based space diversity technique, which can take advantage of the inherent space diversity, is proposed. In contrast to conventional subcarrier-based space diversity, it is shown that the proposed technique can be implemented using only one discrete Fourier transform block and the same weighting coefficients for the whole OFDM symbol. This significantly reduces the system complexity while achieving almost the same diversity order as that of the traditional space diversity approach. They also propose an iterative algorithm to obtain the antenna weighting coefficients. Simulation results show that the proposed algorithm converges fast and approaches the global optimal solution for most channel realizations. It is also. shown that, when the proposed technique is employed in a time division duplex scenario, where the uplink and downlink channels are reciprocal, the system complexity can be further reduced

Bit-error rate computation of optical CDMA communication systems by large deviations theory

Recently, there has been a lot of interest in using code-division multiple access (CDMA) for optical fiber communication systems. In this letter, we present a technique for estimating the bit error rate of optical CDMA communication systems employing avalanche photodiode receivers. The proposed technique, which is based on large deviations theory, is featured as a simple, accurate, and flexible method for performance evaluation of such systems

An interference-cancellation scheme for carrier frequency offsets correction in OFDMA systems

Recently, orthogonal frequency-division multiplexing (OFDM), with clusters of subcarriers allocated to different subscribers (often referred to as OFDMA), has gained much attention for its ability in enabling multiple-access wireless multimedia communications. In such systems, carrier frequency offsets (CFOs) can destroy the orthogonality among subcarriers. As a result, multiuser interference (MUI) along with significant performance degradation can be induced. In this paper, we present a scheme to compensate for the CFOs at the base station of an OFDMA system. In the proposed scheme, circular convolutions are employed to generate the interference after the discrete Fourier transform processing, which is then removed from the original received signal to increase the signal-to-interference power ratio (SIR). Both SIR analysis and simulation results will show that the proposed scheme can significantly improve system performance