Pulse Position Modulation Symbols Research Articles

Real-Time and High-Speed Underwater Photon-Counting Communication Based on SPAD and PPM Symbol Synchronization

Photon-counting detectors can increase the range of underwater optical wireless communication (UWOC) by enhancing the detection sensitivity and have thus become an active research area in recent years. However, system performance still needs to be improved in the reported underwater photon-counting communication (UPCC) experiments. In this study, a real-time, high-speed UPCC system was designed and experimentally validated based on a single-photon avalanche diode (SPAD). A reliable symbol synchronization method based on pulse-position modulation (PPM) was developed. This method achieved slot synchronization by using a narrow-pulse-width laser and a simple matched filter and realizes frame synchronization through an improved synchronization sequence. In addition, channel errors were corrected by serially concatenated convolutionally coded PPM (SCPPM), and digital signals were processed in real time based on field-programmable gate arrays (FPGAs). Finally, desktop communication experiments were completed at a slot frequency of 25 MHz and a communication rate of 6.21 Mbps. The system exhibited a bit error rate (BER) of less than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–7</sup> , a received optical power of only −84.3 dBm, and an efficiency of 1.35 photons per bit.

Mark ratio modulation over pulse position modulation

• Mark ratio modulation over inverse PPM modulation is proposed for optical label switching. • Two signals are transmitted by one ASK modulation. • The mark ratio difference of PPM and inverse PPM is utilized to deliver an overlay signal. • The mark ratio modulation doesn’t degrade the PPM signal. Orthogonal modulation superimposes non-amplitude-modulated signals on Manchester coded or pulse position modulated amplitude shift keying (ASK) signals, allowing two traffic flows with different bit rates to be modulated on the same wavelength channel, and hence improving spectrum efficiency. Inspired by the orthogonal modulation, this paper proposes a novel modulation format, i.e., mark ratio modulation over pulse position modulation (PPM), which utilizes the mark ratio difference between the PPM symbols and the inverse PPM symbols to deliver an overlaid signal. Better than traditional orthogonal modulation, in the mark ratio modulation over PPM, both low-speed and high-speed traffic flows are modulated by ASK with no need to sacrifice the extinction ratio, while keeping the reception simple and easy. According to theoretical analysis and test, we found 4PPM is a good option, which can balance the trade-off between the PPM signal’s effective bit rate and the mark ratio modulated signal’s quality.

Time Synchronization in Photon-Limited Deep Space Optical Communications

Random jitter or offset between the transmitter/receiver clocks is an important parameter that has to be accurately estimated for optimal detection of pulse position modulation (PPM) symbols for high-data-rate optical communications. This parameter, in general, is modeled as an unknown random quantity that depends on the clock drift between the transmitter/receiver clocks and the random motion between the transmitter and receiver stations. In this paper, we have modeled the time jitter for two scenarios—phase modulation jitter and frequency modulation jitter. The phase modulation jitter is modeled as a Gaussian random variable which is estimated with the help of a maximum a posteriori probability (MAP) estimator. The frequency modulation jitter is characterized as a random walk, and this leads to the modeling of the jitter as a state space variable in the context of a dynamical system. Since the observations are the photon counts in each slot of a PPM symbol (for both MAP estimation and tracking), the resulting dynamical model is highly nonlinear, and particle filters are employed for tracking the frequency modulation jitter. We evaluate the performance of both the maximum a posteriori estimators and the particle filters in terms of the relative mean-square error and probability of error. We conclude that with MAP estimation and particle filters that estimate/track the time offset, we achieve a significant performance gain in terms of probability of error as compared to systems that do not have a time synchronization system in place.

The Impact of Optical Beam Position Estimation on the Probability of Error in Free-Space Optical Communications

Optical beam position on a detector array is an important parameter that is needed to optimally detect a pulse position modulation (PPM) symbol in free-space optical communications. Since this parameter is generally unknown, it is essential to estimate the beam position as accurately as possible. In this paper, we examine that an accurate estimate of the beam position is required in order to minimize the probability of PPM symbol detection error. Furthermore, we employ different estimators/trackers of the beam position (which could be stationary or time-varying), and compare the probability of error performance of the detectors using those estimators. The probability of error is calculated with the help of Monte Carlo simulations for the uncoded 8-PPM and 16-PPM systems, each of which employ photon counting maximum likelihood receivers. It is found out that the probability of error is minimum for the maximum likelihood estimate of the stationary beam position. Moreover, for the dynamically varying beam position, a particle filter with a sufficiently large number of particles provides a close-to-optimal probability of error performance.

Dephasing in coherent communication with weak signal states

We analyse the ultimate quantum limit on the accessible information for an optical communication scheme when time bins carry coherent light pulses prepared in one of several orthogonal modes and the phase undergoes diffusion after each channel use. This scheme, an example of a quantum memory channel, can be viewed as noisy pulse position modulation (PPM) keying with phase fluctuations occurring between consecutive PPM symbols. We derive a general expression for the output states in the Fock basis and implement a numerical procedure to calculate the Holevo quantity. Using asymptotic properties of Toeplitz matrices, we also present an analytic expression for the Holevo quantity valid for very weak signals and sufficiently strong dephasing when the dominant contribution comes from the single-photon sector in the Hilbert space of signal states. Based on numerical results we conjecture an inequality for contributions to the Holevo quantity from multiphoton sectors which implies that in the asymptotic limit of weak signals, for arbitrarily small dephasing the accessible information scales linearly with the average number of photons contained in the pulse. Such behaviour presents a qualitative departure from the fully coherent case.

Free space laser communication experiments from Earth to the Lunar Reconnaissance Orbiter in lunar orbit

Laser communication and ranging experiments were successfully conducted from the satellite laser ranging (SLR) station at NASA Goddard Space Flight Center (GSFC) to the Lunar Reconnaissance Orbiter (LRO) in lunar orbit. The experiments used 4096-ary pulse position modulation (PPM) for the laser pulses during one-way LRO Laser Ranging (LR) operations. Reed-Solomon forward error correction codes were used to correct the PPM symbol errors due to atmosphere turbulence and pointing jitter. The signal fading was measured and the results were compared to the model.

Joint Modulation and Coding Scheme Toward Low-Complexity WBAN System

This letter proposes a new pulse position modulation (PPM) combined with coding scheme for wireless body area network (WBAN) system. By utilizing redundant time slots in grouped PPM symbols, we incorporate a low-complexity block code to achieve coding gain without bit-rate reduction. Simulations and FPGA elaboration verify the proposed joint modulation and coding scheme.

Reed—solomon coded homodyne digital pulse position modulation

Digital pulse-position modulation (PPM) is a scheme whereby the abundance of bandwidth available in monomode fibres may be exchanged for improved receiver sensitivity. As this format relies on the position of the pulse to convey information, it is subject to a positional error as well as to the familiar false-alarm and erasure errors. A performance and optimisation analysis for both uncoded homodyne digital PPM and digital PPM employing Reed-Solomon errorcorrection codes is presented. System performance for a range of fibre bandwidths and PPM symbol sizes is analysed, and it is shown how the predetection filter may be configured in order to minimise the three error sources and achieve maximum transmission efficiency (nats/photon). Results are presented at a bit rate of 565 Mbit/s and a wavelength of 1.5 μm, comparing both uncoded and coded homodyne digital PPM with shot-noise-limited coherent PCM. It is shown that there are optimum PPM symbol sizes, fibre bandwidths and Reed-Solomon code rates at which to operate. The conclusion is that uncoded digital PPM offers an improvement of 5 dB over homodyne PSK PCM, and that the Reed-Solomon error-correction coded system offers a 4 dB improvement over uncoded PPM, when operating at the optimum 3/4 code rate.

On the synchronizability and detectability of random PPM sequences

The problem of synchronization and detection of random pulse-position modulation (PPM) sequences is investigated under the assumption of perfect slot synchronization. Maximum-likelihood PPM symbol synchronization and receiver algorithms are derived that make decisions based on both soft and hard data; these algorithms are seen to be easily implementable. The author derives bounds on the symbol error probability and the probability of false synchronization that indicate the existence of a severe performance floor, which can be the limiting factor in the overall system performance. The performance floor is inherent in the PPM format and random data and becomes more serious as the PPM alphabet size Q is increased. A way to eliminate the performance floor is suggested by inserting 'special' PPM symbols in the random data stream. >