Carrier Offsets Research Articles

A New Approach to Improve the Performance of OFDM Signal for 6G Communication

The orthogonal frequency division multiplexing is a very efficient modulation technique that can achieve very high throughput by transmitting many carriers simultaneously and it is spectrally efficient because of the proximity of the subcarriers. OFDM is used in 5G communication for higher data transmission. 6th generation communication also demands OFDM, since it is more spectrally efficient and suitable for high data transmission. The drawback of the OFDM includes peak to average power ratio and sensitivity to carrier offsets and drifts. The usage of a non-linear power amplifier causes the signal spreading and leads to inter-modulation and signal constellation distortion. These two distortions have an impact on the signalto-noise ratio and hence reduce the efficiency. The methods used to reduce PAPR are clipping and filtering, selective mapping, partial transmit sequence, tone reservation, and injection and non-linear commanding. The drawbacks of the above methods are computational complexity, spectrum inefficiency, increase in bit error rate and PAPR rate. In this work, three effective methods are discussed and compared to improve the performance parameters. These are adaptive peak window method based on harmonize clipping, harmonics kernel adaptive filter and Slepian-based flat-top window techniques are presented to reduce the BER, PAPR, and CCDF to improve the signal-to-noise of the system. This window technique averages out the noise spread out in the spectrum and thus reduces the signal loss by minimizing peak to average power ratio. The results are analyzed and compared with the existing conventional methods. Finally, the reductions in PAPR, BER and CCDF obtained are discussed in the results and comparison section. The proposed work has a higher signal-to-noise ratio than the conventional methods.

On the Frequency Carrier Offset and Symbol Timing Estimation for CCSDS 131.2-B-1 High Data-Rate Telemetry Receivers.

In recent years there have been significant developments in satellite transmitter technology to follow the rapid innovation of sensors on-board new satellites. The CCSDS 131.2-B-1 standard for telemetry downlink, released in 2012, is part of the next generation of standards that aims to support the increased data-rate caused by these improvements in resolution. As a result of its relative novelty, this standard currently lacks in-depth analysis by researchers, but it is also strongly supported by the European Space Agency (ESA) for future missions. For these reasons, it seems important to evaluate how major receiver sub-components, such as timing recovery and carrier frequency correction, can be designed and implemented in new receivers that support this standard. The timing error detectors (TED) and frequency error detectors (FED) were therefore studied on the specific peculiarities of CCSDS 131.2-B-1 in its usual environment of Low Earth Orbit (LEO). Estimators have been evaluated highlighting performances, trade-offs and peculiarities of each one with respect to corresponding architectural choices. Finally, a receiver architecture derived from the paper considerations is proposed in the aim of supporting very different mission scenarios. Specifically, the realized architecture employs a parallel feedforward estimator for the timing recovery section and a novel multi-algorithm feedback frequency correction loop to efficiently cover both low symbol rates (5 Mbaud) and high data-rates (up to 500 Mbaud). This solution represents a good trade-off to support these scenarios in a very compact footprint by pushing the clock frequency to the FPGA limit. The FPGA resources occupation on a Zynq Ultrascale+ RFSoC XCZU28DR FPGA is 5202 LUT, 4851 FF, 5 BRAM, and 21 DSP for the timing recovery part, while the frequency recovery section occupies 1723 LUT, 1511 FF, 2.5 BRAM and 32 DSP.

Experimental Investigation of Link Impairments in Pilot Tone Aided Superchannel Transmission

We investigate the performance of single-pilot-tone locked frequency comb-based superchannel transmission for distances up to 1200km. In our scheme, electro-optic transmitter and receiver combs are locked by leaving one of the transmitter carriers unmodulated and regenerating the receiver comb via optical injection locking. This approach significantly reduces carrier offsets and therefore leads to reduced digital signal processing complexity. We experimentally assess how transmission impairments such as noise added by optical amplifiers and fiber nonlinearities affect the quality of the comb regeneration. Our results show that while the operating conditions are more stringent at longer distances, the single pilot is robust to impairments. At optimal launch power, similar performance with respect to an intradyne receiver is observed, showing that the optical pilot tones can be co-transmitted with data channels even at distances spanning hundreds of kilometer. The total superchannel spectral efficiency (throughput), including the pilot tone and guardbands, is 9.6 bits/s/Hz (12 Tbit/s) after 480 km and 8.4 bits/s/Hz (10.5 Tbit/s) after 960km.

A Blind Despreading and Demodulation Method for QPSK-DSSS Signal With Unknown Carrier Offset Based on Matrix Subspace Analysis

Direct sequence spread spectrum (DSSS) signals are widely used in various military and civilian communication systems. It adopts the pseudorandom (PN) sequence to modulate baseband signal before transmission, leading to some unique and useful properties such as lower working signal to noise ratio (SNR), strong anti-jamming capability, and robustness to the effect of multi-path fading. In past few years, blind despreading for DSSS signals has been a hot topic in cognitive radio field. Previous works are mostly concentrated on BPSK-DSSS signals, and have developed theories and methods to estimate the PN sequence effectively. But unfortunately, most of them are powerless to process QPSK-DSSS signals. This work proposes a method based on matrix subspace analysis to recover the PN sequences and transmitted symbols for QPSK-DSSS signals with unknown carrier offset under non-cooperative reception occasion. To recover spreading sequences from subspace of self-covariance matrix, we derive the theoretical expressions of eigenvector by studying the complex hermit matrix decomposition and then investigate the influence of carrier residual. Finally, a structure is designed to complete blind despreading and demodulation from the received waveform to bit stream. Simulation verifies pretty good bit-error-rate (BER) performance of proposed algorithm.

12 b/s/Hz Spectral Efficiency Over the C-band Based on Comb-Based Superchannels

We present transmission of three 50 × 24 Gbaud PM-256QAM superchannels with an overall spectral efficiency of 12 b/s/Hz over the entire C-band. Each comb-based superchannel combines a single shared optical pilot tone with a low-overhead pilot-based DSP. The pilot tone is used to lock the transmitter and the receiver comb seed lasers, suppressing carrier offsets. Combined with optimized insertion ratio of the digital pilot symbols, the total overhead is minimized, avoiding any excess loss in spectral efficiency. Our results demonstrate the feasibility to exploit the unique properties of frequency combs in the design of high spectral efficiency multi-Tb/s superchannels.

A Unified Dual-frequency Constant Envelope Multiplexing Design Framework for Modernised GNSS Signals

In the field of modernised Global Navigation Satellite System (GNSS) signal design, several Dual-frequency Constant Envelope Multiplexing (DCEM) methods have been recently proposed. However, the existing DCEM methods, such as Alternative Binary Offset Carrier (AltBOC), generalised AltBOC and Asymmetric Constant Envelope Binary Offset Carrier (ACE-BOC), are only applied in some special cases. In this paper, we present a unified DCEM design framework for GNSS signals. The existing DCEM methods can be unified in this framework. First, the signal components at two carrier frequencies are combined into two single-frequency constant envelope signals. Then, the linear sum of dual-frequency signals with non-constant envelopes is obtained. Finally, the linear sum is converted into the corresponding DCEM signal by solving an optimisation problem. The proposed design framework has no strict constraints on the number, power ratio and phase relationship of the signal components. Moreover, some special design cases under this framework are also analysed in detail. The analytical results show that the proposed design method can reach higher multiplexing efficiency compared with the existing methods. Based on the proposed method, we suggest a scheme to multiplex the BeiDou regional signals and global signals at the B2 frequency. The simulation results of correlation functions and Power Spectrum Density (PSD) verify the correctness and effectiveness of the proposed design method.

10 Tb/s PM-64QAM Self-Homodyne Comb-Based Superchannel Transmission With 4% Shared Pilot Tone Overhead

We demonstrate transmission of a comb-based 10 Tb/s 50 $\times$ 20 Gbaud PM-64QAM superchannel using frequency comb regeneration to reduce carrier offsets and allow for self-homodyne detection. The regeneration is enabled by transmitting two optical pilot tones which are filtered and recovered in the receiver using optical injection locking and an electrical phase-locked loop. We show that by utilizing frequency combs together with optical pilot tones, self-homodyne detection similar to systems using one pilot tone per wavelength channel, can be achieved. Sharing the overhead for pilot tones reduces the complexity and limits the overhead to 4%. This enabled a total superchannel spectral efficiency of 7.7 b/s/Hz. To evaluate the performance, we perform both back-to-back measurements and transmission over 80 km of standard single-mode fiber. Successful self-homodyne detection of all 50 data channels in the 10-nm-wide superchannel demonstrates that the spectral coherence from frequency combs, combined with the use of optical pilots, can overcome limitations arising from frequency offset and phase noise in high-order QAM transmission while keeping the pilot overhead low.

High Spectral Efficiency PM-128QAM Comb-Based Superchannel Transmission Enabled by a Single Shared Optical Pilot Tone

We exploit the coherence of frequency combs for high spectral efficiency superchannel transmission via effective sharing of a single pilot tone. By phase-locking the receiver comb to the transmitted pilot tone, carrier offsets are suppressed while both the overhead and complexity associated with the pilot tone are reduced. We form a 55 carrier superchannel using a 25-GHz spaced electro-optic frequency comb seeded by a 100-kHz linewidth laser. At a pilot tone overhead of $ 2%, the reduction in carrier offsets is shown to facilitate blind DSP-based carrier recovery of all 54 $\times$ 24 Gbaud PM-128QAM data channels. The resulting superchannel spectral efficiency is 10.3 bits/s/Hz assuming a 28% overhead for forward error correction. Our results show the potential for optical pilot tones to reduce both overhead and complexity in systems using comb-based superchannels together with high-order modulation formats.

Multicarrier Downlink Transmission for High-speed Railway in Non-Gaussian Noise Channels

We consider a wireless wideband downlink transmission for high-speed railway. In this scenario, the wireless channel is double selective, i.e., multipath and highly time-varying, and involves non-Gaussian noise at the receiver side. Conventional receiver algorithms designed for Gaussian noise and time-invariant channels are inefficient especially for impulsive noise and time-varying multipath channels, which are the main challenges in high-speed railway. They may lead to communication outage and involve large carrier offsets, respectively. Regarding the challenges, a synthesis scheme is proposed to resolve these issues in this paper. First, in order to mitigate the impulsive noise, three impulsive noise detection algorithms are adopted to discriminate impulsive noise from the received signal. The blanking algorithm is used to suppress the discriminated noise. Then, the refined signal goes through a well-designed filtering, which transforms the received signal with varying frequency offsets into an angle domain. The advantage in the angle domain is that the frequency offset in each angle becomes nearly constant. Thus, the frequency offsets can be estimated within each angle to realize accurate estimation. In the simulation, three different impulsive noise models are considered to evaluate the communication reliability in this circumstance. The simulation results show that the proposed scheme significantly improves the reliable performance of the high-speed railway communication systems.

Baseband Communication Signal Blind Separation Algorithm Based on Complex Nonparametric Probability Density Estimation

In this paper, a blind separation algorithm based on nonparametric probability density estimation (NPDE) for baseband communication signals is proposed. The NPDE method can precisely estimate the probability density of signals, and its good data-driven property guarantees the adaptability of the proposed algorithm. The NPDE method is extended to complex field to fit the complex characteristics of baseband communication signals. The complex update procedure of demixing matrix corresponding to complex channel matrix and signals is also derived in this paper. The experimental results indicate that the proposed algorithm not only can effectively separate the mixture of baseband communication signals, which are modulated with the same or different modulation schemes, but also has better convergence properties and greater signal interference ratio gains than the contrast algorithms. Moreover, even if the local carrier of receiver is unsynchronized to the carrier of received signals due to carrier offsets, the proposed algorithm can also achieve a valid separation.

Joint Estimation of Carrier and Sampling Frequency Offsets in the Uplink of Multiuser OFDM Systems

This paper considers the joint estimation of the carrier frequency offset (CFO) and sampling frequency offset (SFO) in the uplink of multiuser orthogonal frequency division multiplexing systems, where the received signal at the base station is a mixture of all users’ signals and each user is affected by different frequency offsets. Usually, users’ separation is needed to cope with this challenging multi-parameter estimation problem. Conventional methods adopt the zero-forcing criterion, which will lead to noise enhancement. To overcome this problem, a novel joint CFO and SFO estimation algorithm is proposed. We use the QR decomposition to achieve users’ separation, and the successive interference cancellation technique is employed to estimate the frequency offsets of different users. To further improve the estimation accuracy, the minimum variance unbiased estimator is exploited. Numerical results show that the proposed algorithm offers better performance than existing methods in terms of mean squared error and bit error rate .

Indoor Localization Using 802.11 Time Differences of Arrival

This paper proposes a novel time-based method for determining the position of an IEEE 802.11g transmitter using multiple mutually synchronized 802.11g receivers. By means of baseband signal processing, the proposed algorithm obtains a high-resolution estimate of the time of arrival (TOA) of the long training sequence symbol at each receiver. An estimate of the position of the transmitter is obtained based on the estimation of the time differences of arrival (TDOA) of the symbols and the known fixed locations of the receivers. This paper investigates the effects of carrier and sampling clock offsets, in both frequency and phase, between nodes on the TOA and TDOA estimation error. In real-world experiments in a line of sight, low multipath indoor environment, the method was found to achieve mean errors of 42 cm per symbol for 1-D and 1.39 m per symbol for 2-D position estimation, for ranges of up to 25 m.

MIMO radar imaging with imperfect carrier synchronization: A point spread function analysis

Multiple-input multiple-output (MIMO) radar imaging suffers from performance degradation under carrier offsets across transmitters and receivers. This paper analyzes the impact of carrier offsets on imaging a sparse target, from the perspective of the MIMO point spread function (PSF). Conditions are established for successful support recovery using orthogonal matching pursuit (OMP), and the performance loss is characterized in terms of l2 distance. A sparse imaging algorithm taking into account the perturbation due to carrier offsets is also proposed, which improves upon the OMP algorithm. Numerical experiments corroborate this analysis.

Long-lived coherences: improved dispersion in the frequency domain using continuous-wave and reduced-power windowed sustaining irradiation.

Long-lived coherences (LLC's) are detectable magnetisation modes with favourable relaxation times that translate as sharp resonances upon Fourier transform. The frequency domain of LLC's was previously limited to the range of J-couplings within pairs of homonuclear spins. LLC evolution at high magnetic fields needs to be sustained by radio-frequency irradiation. We show that LLC-based spectral dispersion can be extended beyond the J-couplings domain using adapted carrier offsets and introduce a new reduced-power sustaining method to preserve LLC's within the required range of offsets. Spectral resolution is enhanced as the natively narrow lines of LLC's are further dispersed, making them potential probes for the study of biomolecules featuring strong resonance overlap and for media where NMR spectroscopy is commonly hindered by line broadening.

Two-way amplify-and-forward relaying with carrier offsets in the absence of CSI: differential modulation-based schemes

In this paper, differential modulation (DM) schemes, including single differential and double differential, are proposed for amplify-and-forward two-way relaying (TWR) networks with unknown channel state information (CSI) and carrier frequency offsets caused by wireless terminals in high-speed vehicles and trains. Most existing work in TWR assumes perfect channel knowledge at all nodes and no carrier offsets. However, accurate CSI can be difficult to obtain for fast varying channels, while increases computational complexity in channel estimation and commonly existing carrier offsets can greatly degrade the system performance. Therefore, we propose the two schemes to remove the effect of unknown frequency offsets for TWR networks, when neither the sources nor the relay has any knowledge of CSI. Simulation results show that the proposed differential modulation schemes are both effective in overcoming the impact of carrier offsets with linear computational complexity in the presence of high mobility.

Squared envelope-based SNR estimation

Two novel squared envelope (SQE)-based and nondata-aided (NDA) signal-to-noise ratio (SNR) estimators over slow fading channel are proposed, which are robust to carrier offsets and can be applied to both nonconstant modulus (NCM) and constant modulus (CM) constellations. First, the moment-based first-and-second moments (M1M2) estimator is proposed because of its simple computation. The NDA approach has some difficulties when it is applied to NCM constellations, mainly due to self-noise introduced by the variability in the amplitude of transmitted symbols. Therefore, an expectation-maximization (EM) estimator, to solve the problem, instead of using the maximum-likelihood estimation, which is too complex for practical implementation, is proposed. For comparison purposes, the normalized Cramér–Rao lower bound is numerically evaluated as the performance benchmark for those estimators. Simulation results have verified that the two novel SQE and NDA SNR estimators are better at estimating the corresponding SNR range for different constellations. Besides, two hybrid schemes combining M1M2 with EM estimators are suggested to reduce deficiencies caused by these methods when they are employed independently to estimate the SNR.

Compressive Link Acquisition in Multiuser Communications

An important receiver operation is to detect the presence specific preamble signals with unknown delays in the presence of scattering, Doppler effects and carrier offsets. This task, referred to as "link acquisition", is typically a sequential search over the transmitted signal space. Recently, many authors have suggested applying sparse recovery algorithms in the context of similar estimation or detection problems. These works typically focus on the benefits of sparse recovery, but not generally on the cost brought by compressive sensing. Thus, our goal is to examine the trade-off in complexity and performance that is possible when using sparse recovery. To do so, we propose a sequential sparsity-aware compressive sampling (C-SA) acquisition scheme, where a compressive multi-channel sampling (CMS) front-end is followed by a sparsity regularized likelihood ratio test (SR-LRT) module. The proposed C-SA acquisition scheme borrows insights from the models studied in the context of sub-Nyquist sampling, where a minimal amount of samples is captured to reconstruct signals with Finite Rate of Innovation (FRI). In particular, we propose an A/D conversion front-end that maximizes a well-known probability divergence measure, the average Kullback-Leibler distance, of all the hypotheses of the SR-LRT performed on the samples. We compare the proposed acquisition scheme vis-\`{a}-vis conventional alternatives with relatively low computational cost, such as the Matched Filter (MF), in terms of performance and complexity.

Particle Filters for Joint Timing and Carrier Estimation: Improved Resampling Guidelines and Weighted Bayesian Cramer-Rao Bounds

This paper proposes a framework for joint blind timing and carrier offset estimation and data detection using a Sequential Importance Sampling (SIS) particle filter in Additive White Gaussian Noise (AWGN) channels. We assume baud rate sampling and model the intractable posterior probability distribution functions for sampling timing and carrier offset particles using beta distributions. To enable the SIS approach to estimate static synchronization parameters, we propose new resampling guidelines for dealing with the degeneracy problem and fine tuning the estimated values. We derive the Weighted Bayesian Cramer Rao Bound (WBCRB) for joint timing and carrier offset estimation, which takes into account the prior distribution of the estimation parameters and is an accurate lower bound for all considered Signal to Noise Ratio (SNR) values. Simulation results are presented to corroborate that the Mean Square Error (MSE) performance of the proposed algorithm is close to optimal at higher SNR values (above 20 dB). In addition, the bit error rate performance approaches that of the perfectly synchronized case for small unknown carrier offsets and any unknown timing offset. The advantage of our particle filter algorithm, compared to existing techniques, is that it can work for the full range acquisition of carrier offsets.

Do Cooperative Radios Collide?

When cooperative terminals transmit concurrently, the receiver experiences additional distortion by the presence of radios carrier offsets and time delays. With the inevitable inaccuracy in estimating the channel at the receiving end, is it realistic to assume that relays transmissions do not collide? To address this question, we provide an analytical framework to model the effect of channel estimation methods that treat the channel as deterministic and unknown, and derive the average symbol error rate (SER) of a maximum likelihood sequence decoder (MLSD) and of a linear minimum mean squared error (LMMSE) equalizer using such estimates. We select the appropriate architecture by mapping two different channel estimation techniques onto the proposed analytical framework, and derive their diversity gain. First we analyze the well known linear least square channel estimation (LLSE), then, we show how to cast a compressed channel sensing (CCS) method onto our analytical model, by a novel approximation of the estimator noise and modeling mismatch. We observe that the latter is the key bottleneck in LLSE whereas the CCS appears sufficiently flexible to deliver the promised gains. This conclusion shows that collisions models are overly pessimistic, but it also underscores the need for an advanced receiver synchronization design.

Blind timing and carrier synchronisation in distributed multiple input multiple output communication systems

This study addresses the problem of joint blind timing and carrier synchronisation in a (distributed-M ) × N antenna system where the objective is to estimate the M carrier offsets, the M timing offsets and to recover the transmitted symbols for each of the M users given only the measured signal at the N antennas of the receiver. The authors propose a modular receiver structure that exploits blind source separation to reduce the problem into more tractable sub-problems of estimating individual timing and carrier offsets for multiple users. This leads to a robust solution of low complexity. The authors investigate the performance of the estimators analytically using modified Cramer- Rao bounds and computer simulations. The results show that the proposed receiver exhibits robust performance over a wide range of parameter values, even with worst-case Doppler of 200- 300 Hz and frame size as small as 400 symbols. This work is relevant to future wireless networks and is a complete solution to the problem of estimating multiple timing and carrier offsets in distributed multiple input multiple output (MIMO) communication systems.