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Neural networks (NNs) inspired by the forward-backward algorithm (FBA) are used as equalizers for bandlimited channels with a memoryless nonlinearity. The NN-equalizers are combined with successive interference cancellation (SIC) to approach the information rates of joint detection and decoding (JDD) with considerably less complexity than JDD and other existing equalizers. Simulations for short-haul optical fiber links with square-law detection illustrate the gains., Comment: Accepted at IEEE Intern. Symp. on Inf. Theory 2024 in Athens. arXiv admin note: substantial text overlap with arXiv:2401.09217

Reliable communication over bandlimited and non-linear channels usually requires equalization to simplify receiver processing. Equalizers that perform joint detection and decoding (JDD) achieve the highest information rates but are often too complex to implement. To address this challenge, model-based neural network (NN) equalizers that perform successive interference cancellation (SIC) are shown to approach JDD information rates for bandlimited channels with a memoryless nonlinearity and additive white Gaussian noise. The NNs are chosen to have a periodically time-varying and recurrent structure that imitates the forward-backward algorithm (FBA) in every SIC stage. Simulations for short-haul fiber-optic links with square-law detection show that NN-SIC nearly doubles current spectral efficiencies, and bipolar or complex-valued modulations achieve energy gains of up to 3dB compared to state-of-the-art intensity modulation. Moreover, NN-SIC is considerably less complex than equalizers that perform JDD, mismatched FBA processing, and Gibbs sampling., Comment: Submitted to IEEE Trans. Commun. on January 11, 2024; revised June 9, 2024; revised August 9, 2024; accepted August 11, 2024

The maximum information rates for bandlimited channels with direct detection are achieved with joint detection and decoding (JDD), but JDD is often too complex to implement. Two receiver structures are studied to reduce complexity: separate detection and decoding (SDD) and successive interference cancellation (SIC). For bipolar modulation, frequency-domain raised-cosine pulse shaping, and fiber-optic channels with chromatic dispersion, SIC achieves rates close to those of JDD, thereby attaining significant energy gains over SDD and intensity modulation. Gibbs sampling further reduces the detector complexity and achieves rates close to those of the forward-backward algorithm at low to intermediate signal-to-noise ratio (SNR) but stalls at high SNR. Simulations with polar codes, higher-order modulation, and multi-level coding confirm the predicted gains., Comment: Submitted to IEEE Journal of Lightwave Technology on December 15, 2022; Resubmitted to IEEE Transactions on Communications on September 9, 2023; accepted November 17, 2023

PAM-6 transmission is considered for short-reach fiber-optic links with intensity modulation and direct detection. Experiments show that probabilistically-shaped PAM-6 and a framed-cross QAM-32 constellation outperform conventional cross QAM-32 under a peak power constraint., Comment: submitted to European Conference on Optical Communication (ECOC) 2022

Achievable information rates of bipolar 4- and 8-ary constellations are experimentally compared to those of intensity modulation (IM) when using an oversampled direct detection receiver. The bipolar constellations gain up to 1.8 dB over their IM counterparts., Comment: submitted to ECOC 2022

A multilevel coded modulation scheme is studied that uses solely binary polar codes and Honda-Yamamoto probabilistic shaping. The scheme is shown to achieve the capacity of discrete memoryless channels with input alphabets of cardinality a power of two. The performance of finite-length implementations is compared to polar-coded probabilistic amplitude shaping and constant composition distribution matching., Comment: 6 pages, 4 figures; to be presented at ISIT 2022

Spectrally efficient communication is studied for short-reach fiber-optic links with chromatic dispersion (CD) and receivers that employ direction-detection and oversampling. Achievable rates and symbol error probabilities are computed by using auxiliary channels that account for memory in the sampled symbol strings. Real-alphabet bipolar and complex-alphabet symmetric modulations are shown to achieve significant energy gains over classic intensity modulation. Moreover, frequency-domain raised-cosine (FD-RC) pulses outperform time-domain RC (TD-RC) pulses in terms of spectral efficiency for two scenarios. First, if one shares the spectrum with other users then inter-channel interference significantly reduces the TD-RC rates. Second, if there is a transmit filter to avoid interference then the detection complexity of FD-RC and TD-RC pulses is similar but FD-RC achieves higher rates., Comment: Submitted to J. Lightw. Technol. on November 15, 2021; revised January 7, 2022; accepted January 22, 2022

Coded modulation with probabilistic amplitude shaping (PAS) is considered for intensity modulation/direct detection channels with a transmitter peak-power constraint. PAS is used to map bits to a uniform PAM-6 distribution and outperforms PAM-8 for rates up to around 2.3 bits per channel use. PAM-6 with PAS also outperforms a cross-shaped QAM-32 constellation by up to 1 dB and 0.65 dB after bit-metric soft- and hard decoding, respectively. An alternative PAM-6 scheme based on a framed cross-shaped QAM-32 constellation is proposed that shows similar gains., Comment: submitted to 11th International Symposium on Topics in Coding

Polar codes are designed for parallel binary-input additive white Gaussian noise (BiAWGN) channels with an average power constraint. The two main design choices are: the mapping between codeword bits and channels of different quality, and the power allocation under the average power constraint. Information theory suggests to allocate power such that the sum of mutual information (MI) terms is maximized. However, a power allocation specific to polar codes shows significant gains., Comment: Will be presented at ISIT 2019

BMERA or convolutional polar codes are an extension of polar codes with a provably better error exponent than polar codes. A successive cancellation (SC) decoding algorithm for BMERA codes similar to SC polar decoders is introduced. A pseudocode description of the SC decoder that can be extended to SC list (SCL) decoding is provided. Simulation results with and without outer CRC codes under SC and SCL decoding are presented for QAM modulation over the AWGN channel to compare the performance of polar and BMERA codes. BMERA codes outperform polar codes by more than 0.5 dB under SCL decoding without outer CRC codes., Comment: submitted to ISTC 2018

A flexible incremental redundancy hybrid auto- mated repeat request (IR-HARQ) scheme for polar codes is proposed based on dynamically frozen bits and the quasi-uniform puncturing (QUP) algorithm. The length of each transmission is not restricted to a power of two. It is applicable for the binary input additive white Gaussian noise (biAWGN) channel as well as higher-order modulation. Simulation results show that this scheme has similar performance as directly designed polar codes with QUP and outperforms LTE-turbo and 5G-LDPC codes with IR-HARQ., Comment: 6 pages, accepted to 2018 IEEE Wireless Communications and Networking Conference Workshops (WCNCW): Polar Coding for Future Networks: Theory and Practice, presented on 15. April

A heuristic construction of polar codes for successive cancellation list (SCL) decoding with a given list size is proposed to balance the trade-off between performance measured in frame error rate (FER) and decoding complexity. Furthermore, a construction based on dynamically frozen bits with constraints among the "low weight bits" (LWB) is presented. Simulation results show that the LWB-polar codes outperform the CRC-polar codes and the eBCH-polar codes under SCL decoding., Comment: 6 pages

An efficient algorithm for the construction of polar codes for higher-order modulation is presented based on information-theoretic principles. The bit reliabilities after successive demapping are estimated using the LM-rate, an achievable rate for mismatched decoding. The successive demapper bit channels are then replaced by binary input Additive White Gaussian Noise (biAWGN) surrogate channels and polar codes are constructed using the Gaussian approximation (GA). This LM-rate Demapper GA (LM-DGA) construction is used to construct polar codes for several demapping strategies proposed in literature. For all considered demappers, the LM-DGA constructed polar codes have the same performance as polar codes constructed by Monte Carlo (MC) simulation. The proposed LM-DGA construction is much faster than the MC construction. For 64-QAM, spectral efficiency 3 bits/s/Hz, and block length 1536 bits, simulation results show that LM-DGA constructed polar codes with cyclic redundancy check and successive cancellation list decoding are 1 dB more power efficient than state-of-the-art AR4JA low-density parity-check codes., Comment: 11 figures

We describe a systematic search for X-ray counterparts of radio pulsars. The search was accomplished by cross-correlating the radio timing positions of all radio pulsars from the ATNF pulsar database (version 1.54) with archival XMM-Newton and Chandra observations publicly released by July 1st 2016. In total, 178 of the archival XMM-Newton observations and 213 of the archival Chandra datasets where found to have a radio pulsar serendipitously in the field of view. From the 288 radio pulsars covered by these datasets we identified 20 previously undetected X-ray counterparts. For 6 of them the statistics was sufficient to model the energy spectrum with one- or two-component models. For the remaining new detections and for those pulsars for which we determined an upper limit to their counting rate we computed the energy flux by assuming a Crab-like spectrum. Additionally, we derived upper limits on the neutron stars' surface temperature and on the non-thermal X-ray efficiency for those pulsars for which the spin-down energy was known. The temperature upper limits where compared with predictions from various neutron star cooling models and where found to be in agreement with the minimal cooling paradigm, Comment: 47 pages, 5 figures,7 tables, resubmitted to ApJ / version 2, revised and updated to 1st of July 2016

Aims: We present a detailed study of the supernova remnant (SNR) G296.7-0.9 in the 0.2-12 keV X-ray band. Methods: Using data from XMM-Newton we performed a spectro-imaging analysis of G296.7-0.9 in order to deduce the basic parameters of the remnant and to search for evidence of a young neutron star associated with it. Results: In X-rays the remnant is characterized by a bright arc located in the south-west direction. Its X-ray spectrum can best be described by an absorbed non-equilibrium collisional plasma model with a hydrogen density of N_H=1.24_{-0.05}^{+0.07} x 10^{22} cm^{-2} and a plasma temperature of 6.2^{+0.9}_{-0.8} million Kelvin. The analysis revealed a remnant age of 5800 to 7600 years and a distance of 9.8_{-0.7}^{+1.1} kpc. The latter suggests a spatial connection with a close-by HII region. We did not find evidence for a young neutron star associated with the remnant., Comment: accepted by A&A, 5 pages, 2 figures

Using the High Resolution Camera (HRC) aboard the Chandra X-ray Observatory, we have re-examined the proper motion of the central compact object RX J0822-4300 in the supernova remnant Puppis A. New data from 2010 August, combined with three archival data sets from as early as 1999 December, provide a baseline of 3886 days (more than 10 1/2 years) to perform the measurement. Correlating the four positions of RX J0822-4300 measured in each data set implies a projected proper motion of mu 71 \pm 12 masy. For a distance of 2 kpc this proper motion is equivalent to a recoil velocity of 672 \pm 115 km/s. The position angle is found to be 244 \pm 11 degrees. Both the magnitude and direction of the proper motion are in agreement with RX J0822-4300 originating near the optical expansion center of the supernova remnant. For a displacement of 371 \pm 31 arcsec between its birth place and today's position we deduce an age of (5.2 \pm 1.0) 10^3 yrs for RX J0822-4300. The age inferred from the neutron star proper motion and filament motions can be considered as two independent measurements of the same quantity. They average to 4450 \pm 750 yrs for the age of the supernova remnant Puppis A., Comment: Accepted for publication in the Astrophysical Journal

Aims: We present a detailed X-ray and radio wavelength study of G308.4-1.4, a candidate supernova remnant (SNR) in the ROSAT All Sky Survey and the MOST supernova remnant catalogue, in order to identify it as a SNR. Methods: The SNR candidate and its central sources were studied using observations from the Chandra X-ray Observatory, SWIFT, the Australian Telescope Compact Array (ATCA) at 1.4 and 2.5 GHz and WISE infrared observation at 24 $\mu$m. Results: We conclude that G308.4-1.4 is indeed a supernova remnant by means of its morphology matching at X-ray, radio and infrared wavelength, its spectral energy distribution in the X-ray band and its emission characteristics in the radio band. G308.4-1.4 is a shell-type SNR. X-ray, radio and infrared emission is seen only in the eastern part of the remnant. The X-ray emission can best be described by an absorbed non-equilibrium collisional plasma with a hydrogen density of $n_\mathrm{H}=(1.02\pm 0.04) \times 10^{22}$ cm$^{-2}$, a plasma temperature of $6.3^{+1.2}_{-0.7}$ million Kelvin and an under-abundance of Iron, Neon and Magnesium, as well as an overabundance in Sulfur with respect to the solar values. The SNR has a spectral index in the radio band of $\alpha=-0.7\pm0.2$. A detailed analysis revealed that the remnant is at a distance of 6 to 12 kpc and the progenitor star exploded $\sim 5000$ to 7500 years ago. Two faint X-ray point sources located near to the remnant's geometrical center are detected. Both sources have no counterpart at other wavelengths, leaving them as candidates for the compact remnant of the supernova explosion., Comment: 12 pages, 11 figures, accepted by A&A, revised draft

We discuss the possibility of an autonomous navigation system for spacecraft that is based on pulsar timing data. Pulsars are rapidly rotating neutron stars that are observable as variable celestial sources of electromagnetic radiation. Their periodic signals have timing stabilities comparable to atomic clocks and provide characteristic temporal signatures that can be used as natural navigation beacons, quite similar to the use of GPS satellites for navigation on Earth. By comparing pulse arrival times measured on-board the spacecraft with predicted pulse arrivals at some reference location, the spacecraft position can be determined autonomously with accuracies on the order of 5 kilometres. For a spacecraft at a distance of 10 astronomical units from Earth (e.g., Earth-Saturn), this means an improvement by a factor of 8 compared to conventional methods. Therefore this new technology is an alternative to standard navigation based on radio tracking by ground stations, without the disadvantages of uncertainty increasing with distance from Earth and the dependence on ground control., Comment: 4 pages, 3 figures, 1 table, to be published in the proceedings of the 2nd International Conference on Space Technology, held on 15-17 Sept. 2011, Athens, Greece

Usually, positions of spacecraft on interplanetary or deep space missions are determined by radar tracking from ground stations, a method by which uncertainty increases with distance from Earth. As an alternative, a spacecraft equipped with e.g. an X-ray telescope could determine its position autonomoulsy via onboard analysis of X-ray pulsar signals. In order to find out which pulsars are best suited for this approach and what accuracy can be achieved, we build up a database containing the temporal emission characteristics of the ~ 60 X-ray pulsars for which a pulsed radiation has been detected by mid 2010., Comment: 5 pages, 2 figures, to be published in the proceedings of High Time Resolution Astrophysics IV - The Era of Extremely Large Telescopes, held on May 5-7, 2010, Agios Nikolaos, Crete, Greece