Postdetection Filtering Research Articles

Design of realisable optimum post‐detection filters for high data‐rate m ‐PAM systems with clock jitter tolerance

The continual increase in demand for extremely high data-rate systems spans the full range of network systems from local area networks to long-haul systems. While coherent optical systems dominate the long-haul market, there is significant interest in using simpler direct detection systems for application areas, where simplicity and lower-cost is a key driver. Pulse-amplitude modulation (PAM) is appropriate to consider as it enables the reduction of the channel symbol rate for a given data rate, without unduly increasing the complexity of the system. A route to the design of optimum post-detection filter for PAM systems is presented that offers significantly improved robustness to timing jitter, which is an impairment of increasing import as symbol rates are increased.

Evaluation of Ultrasound-Based Sensor to Monitor Respiratory and Nonrespiratory Movement and Timing in Infants.

To describe and validate a noncontacting sensor that used reflected ultrasound to separately monitor respiratory, nonrespiratory, and caretaker movements of infants. An in-phase and quadrature (I & Q) detection scheme provided adequate bandwidth, in conjunction with postdetection filtering, to separate the three types of movement. The respiratory output was validated by comparing it to the electrical activity of the diaphragm (Edi) obtained from an infant ventilator in 11 infants. The nonrespiratory movement output was compared to movement detected by miniature accelerometers attached to the wrists, ankles, and heads of seven additional infants. Caretaker movement was compared to visual observations annotated in the recordings. The respiratory rate determined by the sensor was equivalent to that from the Edi signal. The sensor could detect the onset of inspiration significantly earlier than the Edi signal (23+/-69 ms). Nonrespiratory movement was identified with an agreement of 0.9 with the accelerometers. It potentially interfered with the respiratory output an average of 4.7+/-4.5% and 14.9+/15% of the time in infants not requiring or on ventilatory support, respectively. Caretaker movements were identified with 98% sensitivity and specificity. The sensor outputs were independent of body coverings or position. This single, noncontacting sensor can independently quantify these three types of movement. It is feasible to use the sensor as trigger for synchronizing mechanical ventilators to spontaneous breathing, to quantify overall movement, to determine sleep state, to detect seizures, and to document the amount and effects of caretaker activity in infants.

A Fast-Converging Equalizer for Upstream DOCSIS Channels

This paper proposes a method of increasing the efficiency of the CATV plant in the upstream direction. The general approach is to reduce the convergence time of the equalizer to allow a shorter training sequence. Specifically, the upstream channel is modeled as a postdetection filter. A short unique word is inserted near the beginning of the preamble to estimate the coefficients of the postdetection filter. The postdetection filter is then crudely inverted to estimate a few key tap weights of the equalizer. The equalizer is initialized with these estimated tap weights before adaptive updating is turned on. The method is evaluated for the echo-laden DOCSIS CATV upstream channel with a signal-to-noise ratio (SNR) of 25 dB. Plots of the probability of the modulation error ratio exceeding 19- and 22-dB values versus the length of the training sequence show that the performance is comparable to an unseeded equalizer with a training sequence that is about 50 symbols longer.

An iterative filter to reconstruct planetary transit signals in the presence of stellar variability

The detrending algorithms which are widely used to reduce the impact of stellar variability on space-based transit surveys are ill-suited for estimating the parameters of confirmed planets, as they unavoidably alter the transit signal. We present a post-detection detrending algorithm, which filters out signal on other timescales than the period of the transit while preserving the transit signal. We compare the performance of this new filter to a well-established pre-detection detrending algorithm, by applying both to a set of 20 simulated light curves containing planetary transits, stellar variability, and instrumental noise as expected for the CoRoT space mission, and performing analytic fits to the transits. Compared to the pre-detection benchmark, the new post-detection filter systematically yields significantly reduced errors (median reduction in relative error over our sample of about 40%) on the planet-to-star radius ratio, system scale and impact parameter. This is particularly important for active stars, where errors induced by variability can otherwise dominate the final error budget on the planet parameters. Aside from improving planet parameter estimates, the new filter preserves all signal at the orbital period of the planet, and thus could also be used to search for light reflected by the planet.

Analytical Performance Evaluation of Optical DQPSK Systems With Postdetection Filtering

A novel analytical method for evaluating the performance of optical transmission systems using differential quadrature phase-shift keying is presented. It is shown that the bit-error probability (BEP) can be evaluated through the statistics of the received signal phase difference directly filtered by the postdetection filter. The accuracy of the method is checked against the results obtained through a simulator based on the multicanonical Monte Carlo algorithm. Some results in terms of BEP evaluation and nonlinear phase noise issues are also provided.

Fast digital processing of images of artificial Earth satellites

Experiments on the improvement of images of artificial Earth satellites are performed via the methods of digital postdetection filtering in the case when a telescope with an aperture diameter of approximately 1 m is employed to observe extended objects. The resolution obtained in white-light observations is three times higher than the atmosphere-limited resolution. The results of calibration of the processing method with the use of speckle images of a single star are presented. Photographs of recorded and recovered images of the International Space Station and Saturn are presented.

Limiter discriminator detection of narrow‐band duobinary FSK in a land mobile channel

AbstractError rates (BER) for narrow‐band duobinary FM with discriminator detection and integrate and dump (LDI) post‐detection filtering in land mobile channel characterized by fading and Doppler frequency shift are obtained. The fading process is assumed to be much slower than data rate, allowing the fading process to be analysed in a quasistatic manner as it passes through the narrow‐band filter. A simulation model was constructed using the signal processing simulator, and the simulated error rate results are shown to agree well with the calculated error rate results. Copyright © 2004 John Wiley & Sons, Ltd.

Evaluating the error probability in lightwave systems with chromatic dispersion, arbitrary pulse shape and pre- and postdetection filtering

A novel approach to analytically evaluate the bit error probability in optically preamplified direct-detection systems is presented, which can take into account the effects of pulse shaping, chirping, filtering at the transmitter and the receiver, both pre- and postdetection, chromatic dispersion, and ASE noise. The method is computationally very fast in that the saddle point integration method for solving the resulting line integral of a particular moment generating function is adopted. A closed-form approximation for the bit error probability is also provided, which is within 0.01 dB from the exact numerical results.

Comment: Optically preamplified receiver with low quantum limit

Summary form only given. We clarify that the sensitivity of direct detection optically preamplified receivers using on/off keying can never be lower than the well-known quantum limit of 38.4 photons/bit, achievable with a matched optical filter and no post-detection filtering only. In this Comment, we (i) clarify that the (standard) quantum limit of 38.4 photons/bit cannot be beaten by any receiver of this type, and (ii) give the correct results for the receiver considered by Tafur in ibid., vol. 35, p. 1182, 1999. The following reasoning can be applied to prove that no direct detection optically preamplified receiver can perform better than 38.4 photons/bit: the quantum limit - the ideal performance for a given class of optical receivers and a given modulation format is defined as the minimum number of photons per bit required on average at the receiver input to obtain a certain bit error probability (BEP).

Evaluating the error probability in lightwave systems with chromatic dispersion, arbitrary pulse shape and pre- and postdetection filtering

A novel approach to analytically evaluate the bit error probability in optically preamplified direct-detection systems is presented, which can take into account the effects of pulse shaping, chirping, filtering at the transmitter and the receiver, both pre- and postdetection, chromatic dispersion, and ASE noise. The method is computationally very fast in that the saddle point integration method for solving the resulting line integral of a particular moment generating function is adopted. A closed-form approximation for the bit error probability is also provided, which is within 0.01 dB from the exact numerical results.

System Performance of Optical Heterodyne CPFSK Receivers with AFCs

We analyze the system performance of optical heterodyne CPFSK receivers with AFC loops. The effects of closed loop gain and postdetection filters are taken into account. The GQR (Gaussian quadrature rule) method is used to deal with the laser phase noise characteristics in computing the bit-error rates. The impact of closed loop gain on the system performance is investigated. The optimum design of postdetection filters is outlined in showing the improvement of system performance compared with the system without AFC.

Two-dimensional equalization in coherent and incoherent page-oriented optical memory

The low-pass nature of the optical systems (both coherent and incoherent) used for volume optical storage results in the presence of intersymbol interference (ISI) at the output of these systems. Since ISI can seriously degrade retrieved data fidelity, we consider the design of linear, minimum-mean-square-error equalizers for two-dimensional finite-contrast optical ISI channels. Signal models are developed and filter design is conducted for various operating environments associated with particular implementations of page-oriented optical memories (POM’s). Specifically, we consider optically incoherent systems dominated by either postdetection thermal or photon-shot noise, and coherent systems are treated subject to either postdetection thermal or coherent speckle noise. Simple locally connected postdetection filters (equalizers) are designed to reduce the impact of ISI and finite contrast on retrieved data. It is demonstrated how these simple ISI mitigation algorithms may be used to improve the fidelity (i.e., bit error rate) of retrieved data and also to enhance the space–bandwidth-product (SBP), the storage density, and the memory capacity of POM systems. The notion of a fidelity-based SBP is quantified and shown to depend strongly on the receiver processing. The fidelity-based SBP of thermal-noise-dominated incoherent imaging systems operating at the Rayleigh resolution is shown to improve by 28% through the use of equalization, and a 48% SBP increase is found in the shot-noise-dominated case. More dramatic gains are found for thermal-noise-dominated coherent systems operating at the Rayleigh resolution, with 116% SBP gains typical in the infinite-contrast case and 30% gains possible for low-contrast (C=4) cases. Equalization is also shown to facilitate a capacity increase for holographic POM systems, providing a 47% increase in the number of stored pages and the storage density for a system operating at the Rayleigh resolution. The maximum storage density in holographic POM is increased by 20% through the use of equalization.

Error Rate Analysis of Optical Receivers with Fabry-Perot Optical Filter and Equalizing Postdetection Filtering

A complete analytic solution in form of the moment-generating function is presented for the statistical distribution of the decision variable of an on-off system with a Fabry-Perot optical filter. The results include ASE noise, photodetector shot noise and dark current together with thermal noise in the decision circuit. The influence of intersymbol interference is incorporated into the analysis and the optimal bandwidth of the optical filter is determined. It is shown that an equalizing electrical postdetection filter may make a significant improvement in performance. The analysis is extended to cover receivers with an arbitrary optical filter and arbitrary postdetection filtering by making use of a Gaussian approximation.

Evaluation of optimal bandwidth in optical FSK system influenced by laser phase noise

In this paper, an optical frequency shift keying (FSK) system with TFS (two-filter structure), also known as dual-filter FSK, is observed in the presence of laser phase noise. Sensitivity of the receiver is shown as a function of intermediate frequency (IF) bandwidth and a bandwidth of the low-pass postdetection filter. It is shown that there is an optimal pair of bandwidths which minimizes required sensitivity; a few figures for different amounts of phase noise are given. The results in this paper are based on truncated Taylor expansion method and a method based on complex analysis is used to evaluate the integrals involved.

Performance of phase-locked loop receiver in DECT system

The performance of the noncoherent phase-locked loop (PLL) receiver is compared to the performance of the limiter-discriminator detector and differential detector in the digital enhanced cordless telephone system. The results indicate that a well-designed PLL receiver based on the optimisation of PLL parameters and post-detection filtering has a performance advantage.

Improved receivers for coherent FSK systems

There has been considerable interest over the last decade in the detection of digital coherent lightwave signals corrupted by phase noise. However, the majority of the work has been in the performance analysis of various modulation schemes. In addition, the receivers studied have, in general, been zero-phase noise optimal or ad hoc modifications of these. In this paper, we consider the design problem of constructing optimal receivers for systems using frequency shift keying (FSK). Using the innovations approach, we show the format of the optimal receiver and use a small phase noise approximation to derive suboptimal receivers. Also, receivers based on optimum linear filters are also derived. Both receivers are analyzed by simulation and performance improvements over the standard receivers are shown. Furthermore, the receivers developed can equally well be used to further improve the performance of other detection schemes such as postdetection filtering and time diversity. The methodology is also used to derive equivalent receivers for systems using on-off keying (OOK).

Error floors of digital FM in simulcast and Rayleigh fading

A digital FM system in the presence of simulcast interference and/or Rayleigh fading exhibits performance floors, where increased signal strength does not reduce the error rate. Results obtained both in the lab, using a radio channel simulator, and in the field show this performance. Observation of signal waveforms indicates that the bit-error rate (BER) floor is a result of FM clicks during deep fades. We present a mathematical model that can be used to calculate the BER floors. The model is based on the calculation of the click shape as a function of amplitude imbalance, frequency offset, and postdetection filter impulse response. The Rayleigh fading is approximated with a two-signal simulcast by substituting the RMS Doppler spread for the simulcast frequency offset. A good match between calculated and measured results is obtained. It is found that the symbol errors caused by simulcast and Rayleigh fading can be treated as independent, and their effect is cumulative.

The EISCAT Svalbard radar: A case study in modern incoherent scatter radar system design

The EISCAT (European incoherent scatter) Svalbard radar (ESR) was officially inaugurated on August 22, 1996. This event marked the successful completion on schedule of the first phase of the EISCAT Svalbard radar project. In contrast to previous incoherent scatter radars, the ESR system design was adapted to make use of commercial off‐the‐shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum. Commercial hardware is also used in the digital signal processing system. Control and monitoring are performed by distributed, networked VME systems. Thanks to modern reflector antenna design methods and extreme efforts to reduce the receiver noise contribution, the system noise temperature is only 70 K, thus making the ESR about 30% faster than the much more powerful EISCAT UHF radar in F region experiments! Once the transmitter power is increased to 1 MW, it will become about 2–3 times faster than the UHF radar. State‐of‐the‐art exciter and receiver hardware has been developed in‐house to accommodate the special requirements introduced by operating the radar at the exceptionally high duty cycle of 25%. The RF waveform is generated by a system based on four switchable direct digital synthesizers. Continuous monitoring of the transmitted RF waveform by the receiver system allows removal of klystron‐induced spurious Doppler effects from the data. Intermediate‐frequency sampling at 7.5 MHz is employed, followed by fully digital channel separation, signal detection, and postdetection filtering in six parallel receiver channels. Radar codes for both E and F layer observation have been designed and perfected. So far, more than 40 hours of good quality ionospheric data have been collected and analyzed in terms of plasma parameters. While the tragic loss of the Cluster mission suddenly changed the plans and dispositions of a majority of the ESR user community, the radar has still been in high demand since its inauguration. It is now being operated by EISCAT staff on a campaign basis, to provide ground‐based support data for a number of other magnetospheric satellites, notably Polar and FAST, and will be opened to the EISCAT user community for special program operations later in 1997.

New method of analyzing BER performance of GFSK with postdetection filtering

This paper describes the theoretical performance of a frequency-hopping system using Gaussian-filtered FSK (GFSK), which is a promising candidate for 2.4-GHz wireless local area networks (WLAN). We have improved previous methods to calculate the bit-error rate (BER) performance of the GFSK system by considering the effect of a postdetection filter in the first-order approximation. The improved method is applicable when the single-sided bandwidth of the postdetection filter is wider than the bandwidth containing 99% of the GFSK signal energy. It is used to calculate the BER performance as well as the dependence of the BER on the level of interchannel interference. The calculated performance of the GFSK system suggests that the postdetection filter is very effective in reducing interchannel interference. The feasibility of a GFSK system with a 1-Mbd symbol rate and 0.32 modulation index is confirmed by the combination of a predetection filter with a 0.85-MHz bandwidth and a postdetection filter with a 0.8-MHz single-sided bandwidth.

Electrical filtering strategies to compensate waveform distortion in optically amplified IM/DD systems

Postdetection filtering strategies are investigated as a means of compensating the waveform distortion caused by the combined effect of chromatic dispersion and self-phase-modulation in intensity modulated, direct detection (IM/DD) systems. A design lechnique based on realisable filter models and a rigorous performance evaluation is introduced. In addition, a method of finding the theoretical optimum filter is given, providing a measure of the achievable system performance and the efficacy of the various filtering strategies. It is shown that the optimised postdetection filtering is effective in reducing the system power penalty, and the relative merits of different filters are quantified. The optimised filters are also shown to be tolerant to various system impairments, such as noise, timing jitter and perturbation of filter parameters.