Differential Phase-shift Keying Demodulator Research Articles

Chirp-spreaded OFDM-MFSK with differentially encoded phase for applications in low-power wide-area networks

For the unlicensed frequency bands, long-range wide-area networks (LoRaWAN) is a widespread solution for low-power wide-area networks. The physical layer of LoRaWAN is long range (LoRa) which uses a combination of chirp spread spectrum (CSS) and M-ary frequency-shift keying (M-FSK) for transmission. LoRa is known to be highly robust and well-suited for low-complexity implementation, but the weak aspect is its low spectral efficiency. Orthogonal frequency division multiplexing M-FSK (OFDM-MFSK) is a technique which combines M-FSK with OFDM. The bandwidth is divided into several parallel M-FSK transmissions, i.e., more than one sub-carrier is active at a time. This leads to an increased peak-to-average power ratio of the transmit signal. On the other hand, it allows to combine OFDM-MFSK with differential phase-shift keying (DPSK) to increase the data rate and thus the spectral efficiency. The reception of OFDM-MFSK as well as the DPSK demodulation can be done non-coherently. Therefore, they hold many similar design principles as LoRa. We propose a combination of OFDM-MFSK with differentially encoded phase and CSS. Simulation results show, that LoRa and OFDM-MFSK with CSS show similar performance with respect to power and bandwidth efficiency. Adding DPSK leads to an improved spectral as well as power efficiency.

Intelligent Matching of the Control Voltage of Delay Line Interferometers for Differential Phase Shift Keying-Modulated Optical Signals

In optical communications, differential phase shift keying (DPSK) provides a desired modulation format that offers high tolerance to nonlinear effects in high-speed transmissions. A DPSK demodulator converts the phase-coded signal into an intensity-coded signal at receivers. One demodulation scheme is called balanced detection and is based on a tunable delay line interferometer (DLI). Demodulation performances are determined by the phase delay generated by the DLI, while the phase delay is controlled by a tunable driving voltage on the DLI device. However, a problem in the dynamic adjustment of the control voltage prevents the application of DPSK demodulators. The receivers need to scan the whole control voltage range of the DLI and find the control voltage that maximizes the demodulation performance, but the scan-based method needs to undergo a very long searching time. In our work, we found that the relation between DLI control voltages and demodulation performance can be predicted rapidly by a feedforward neural network (FNN). In this paper, we propose a new method to quickly locate the best DLI control voltage based on an FNN. We also verify the proposed method in simulations and telecommunication systems, and the results show that the proposed method can significantly improve the efficiency of resolving the best demodulation voltages.

DPSK modulation and demodulation system based on a novel Mach-Zehnder interference structure

As the conventional fiber Mach-Zehnder (M−Z) interference structure being sensitive to temperature variation and mismatching of arm length in Differential Phase Shift Keying (DPSK) system, we proposed a modulation and demodulation system of DPSK based on novel M−Z interference structure. This novel M−Z interference structure has some advantages of higher temperature stability, better robustness and lower restriction of laser linewidth. A detailed discussion of the working principle is presented and demonstrated, the influences of the delay induced by arm mismatching and temperature variation on the Q factor of DPSK system are analyzed. Experimental results as well as simulations show that the delay length mismatching corresponding to descent point of 3dB increases from [0.925,1.124] to [0.536,1.31], which reduces the requirement for the length accuracy of the interference arm, and the demodulation effect of the novel structure is better under the same deviation. The Q factor of the system keeps fluctuation within a narrow range when the temperature variation in the range of 20°C–120°C, and the demodulation effect of the system is almost unaffected by the variation of ambient temperature.

On Multi-User Deep-Learning-Based Non-Coherent DPSK Multiple-Symbol Differential Detection in Massive MIMO Systems

In view of reducing the complexity of signal detection in massive multiple-input multiple-output (MIMO) receivers, the use of non-coherent detection is favored over usual coherent techniques that require complex channel estimation. In this paper, a deep-learning approach to implement non-coherent multiple-symbol detection for differential phase-shift keying (DPSK) multi-user massive MIMO systems is proposed. The proposed deep-learning implementation reduces the high computational complexity required in conventional DPSK detection techniques. Two deep-learning-based multiple-symbol differential detection receiver designs are proposed and compared with decision-feedback differential detection (DFDD), and conventional multiple-symbol differential detection (MSDD) for the same system parameters. Multiple-symbol differential sphere detection (MSDSD) is used to implement conventional MSDD. The results show that the proposed deep-learning-based multiple-symbol differential detection implementation outperforms decision-feedback differential detection and achieves an optimal performance compared to conventional multiple-symbol differential detection implemented by multiple-symbol differential sphere detection for single-user and multi-user scenarios.

A DPSK demodulator for biomedical application

SummaryIn this study, a differential phase‐shift‐keying (DPSK) demodulator, which is implemented by digital circuit, is proposed. A differential voltage clipper is used to generate narrow pulses at the extremum of the received modulated signal. Regarding the DPSK signal, the outputs of the clipper have double frequency. In order to halve the frequency, a frequency divider is applied to get synchronous clock. Two binary counters are used to detect phase variation; the data are recovered after two times sampling. This technique removes the need for voltage controlled oscillator (VCO). The proposed demodulator is postlayout simulated in a 0.18‐μm CMOS process, with a power of 64 μW and active area of 68 × 70 μm2. The demodulator recovers 10 Mb/s data rate at 10‐MHz carrier frequency using a 1.8‐V power supply.

Differential phase-shift-keying demodulation by coherent perfect absorption in silicon photonics

We demonstrate a novel differential phase-shift-keying (DPSK) demodulator based on coherent perfect absorption (CPA). Our DPSK demodulator chip device, which incorporates a silicon ring resonator, two bus waveguide inputs, and monolithically integrated detectors, operates passively at a bit rate of 10Gbps at telecommunication wavelengths, and fits within a mm-scale footprint. Critical coupling is used to achieve efficient CPA by tuning the gap between the ring and bus waveguides. The device has a vertical eye opening of 12.47mV and a quality factor exceeding 3×104. The fundamental principle behind this photonic circuit can be extended to other formats of integrated demodulators.

Linear all-optical signal processing using silicon micro-ring resonators

Silicon micro-ring resonators (MRRs) are compact and versatile devices whose periodic frequency response can be exploited for a wide range of applications. In this paper, we review our recent work on linear all-optical signal processing applications using silicon MRRs as passive filters. We focus on applications such as modulation format conversion, differential phase-shift keying (DPSK) demodulation, modulation speed enhancement of directly modulated lasers (DMLs), and monocycle pulse generation. The possibility to implement polarization diversity circuits, which reduce the polarization dependence of standard silicon MRRs, is illustrated on the particular example of DPSK demodulation.

Monolithic <inline-formula> <tex-math notation="LaTeX">$1\times 2$ </tex-math></inline-formula> MMI-Based 25-Gb/s SOI DPSK Demodulator Integrated With SiGe Photodetector

In this letter, the performance of a 25-Gb/s 1 × 2 multimode interference (MMI) coupler-based monolithic differential phase shift keying (DPSK) receiver integrated with Germanium (Ge) on silicon-on-insulator (SOI) photodetector (PD) is presented. Theoretical analysis suggests that the device exhibits more uniform extinction ratio over the C-band compared with a DPSK demodulator based on 2×2 MMI coupler. Electrical scattering parameters (S-parameters) investigated through component level simulation comparing the fabrication tolerance shows power penalty of 1.2 and 5 dB for unbalanced and balanced detection DPSK receiver, respectively. The SOI 1×2 MMI coupler-based unbalanced DPSK receiver has a sensitivity of 0.41 dBm at a bit error rate of 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> without postelectrical amplification. The PD integrated single-end detectionbased receiver has a total footprint area of 0.39 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

High-speed wavelength tunable DPSK demodulation using a phase modulator based loop mirror filter.

A high-speed wavelength tunable differential phase-shift-keying (DPSK) signal demodulator is presented using a phase modulator (PM) based fiber loop mirror filter. By controlling the birefringence of the PM inside the loop through electro-optic effect, wavelength tuning speed of tens of GHz and tuning range of over two free spectral ranges are achieved. Stable filter spectra with extinction ratio over 30dB are obtained through the tuning process. By combining the birefringence of polarization maintaining fiber and PM, error-free demodulation performance is experimentally achieved throughout the wavelength tuning range for DPSK signals with flexible bit-rate range from 2.5 to 10 Gb/s. This design significantly improves the wavelength tuning speed and is potentially valuable for high-speed switching and tuning applications.

Using Downstream DPSK and Upstream Wavelength-Shifted ASK for Rayleigh Backscattering Mitigation in TDM-PON to WDM-PON Migration Scheme

A migration scheme from time-division-multiplex passive optical network (TDM-PON) to wavelength-division-multiplex PON (WDM-PON) using differential phase-shift keying (DPSK) for the downstream signal and wavelength-shifted amplitude-shift keying (WS-ASK) for the upstream signal is demonstrated. The migration scheme does not change the existing fiber infrastructure. An optical filter is preinstalled at the optical networking unit (ONU) to select the desirable downstream wavelength for the WDM-PON and simultaneously demodulate the downstream DPSK signal. Signal remodulation is used to generate the upstream signal by reusing the downstream wavelength. In the ONU, by wavelength shifting the upstream optical spectrum with respect to the downstream optical spectrum, the Rayleigh backscattering (RB) interference beat noise affecting the upstream signal can be significantly mitigated. The optimum bandwidth for the downstream DPSK demodulation is analyzed. The downstream and upstream transmission performances at different split ratios are also discussed.

Bit-rate transparent DPSK demodulation scheme based on injection locking FP-LD

We propose and demonstrate a bit-rate transparent differential phase shift-keying (DPSK) demodulation scheme based on injection locking multiple-quantum-well (MQW) strained InGaAsP FP-LD. By utilizing frequency deviation generated by phase modulation and unstable injection locking state with Fabry–Perot laser diode (FP-LD), DPSK to polarization shift-keying (PolSK) and PolSK to intensity modulation (IM) format conversions are realized. We analyze bit error rate (BER) performance of this demodulation scheme. Experimental results show that different longitude modes, bit rates and seeding power have influences on demodulation performance. We achieve error free DPSK signal demodulation under various bit rates of 10Gbit/s, 5Gbit/s, 2.5Gbit/s and 1.25Gbit/s with the same demodulation setting.

Multifunctional Current-Controlled InP Photonic Integrated Delay Interferometer

We demonstrate a novel and flexible InP monolithically integrated optical circuit with multiple applications. The circuit is an interferometric delay loop and its particular configuration allows to perform a number of different functions, namely, optical buffering, differential phase-shift keying (DPSK) demodulation, intensity modulation, and differential XOR logic operation. By properly controlling the current supplied to the active elements in the circuit loop (i.e., a semiconductor optical amplifier and a variable optical attenuator), the relative phase of the propagating signals can be adjusted, changing the interference condition between the input signal and its delayed copies. In this way the four different functions are enabled. Experimental results are reported for all the different functions of the device. When the circuit is operated in buffer configuration, up to 13 circulations (corresponding to a 1.62 ns delay) of an input bit at 12.5-Gb/s data rate are shown before the output signal degrades due to excessive ring losses. Its behavior as a current-controlled DPSK demodulator is demonstrated for 8-Gb/s signals, and the resulting bit error rate measurements are compared with those of a thermally tuned commercial demodulator, showing no significant power penalty. A proof-of-concept demonstration as an intensity modulator is reported for relatively low-frequency signals at 10 and 20 MHz, being limited by electrical components of the setup. Finally, error-free operation for a 1-, 2-, and 4-b differential XOR logic gate has been demonstrated at 8, 16, and 32 Gb/s, respectively.

Crosstalk tolerance of direct detection differential phase-shift keying optical systems in the presence of receiver imperfections

An analytical method based on the knowledge of the moment generating function of the decision variable, which permits to assess the in-band crosstalk tolerance of direct detection differential phase-shift keying (DPSK) optical systems with receiver imperfections relatively to conventional on–off keying systems, is presented. It is shown that the crosstalk tolerance of DPSK systems can be greatly reduced or even disappear when the receiver imperfections are taken into account.

Structured FBG filters for 10-Gb/s DPSK signal demodulation in single ended applications

Differential phase-shift keying (DPSK) demodulations operated by a structured fiber Bragg grating (FBG) filter and by a Mach–Zehnder delay interferometer (MZDI) in a single-ended configuration are compared. Experimental measurements at 10 Gb/s demonstrate that a specially designed FBG outperforms an integrated-optic MZDI of ∼4 dB and ∼3.5 dB in back-to-back and after 25-km propagation, respectively. Both demodulators show low polarization sensitivity and signal frequency detuning dependence, but only MZDI operating point requires a thermal control. FBG filter proves an interesting solution for DPSK demodulation in low-cost applications and, moreover, can be designed to match colorless requirements of wave division multiplexed passive optical network (WDM-PON) applications.

Four-State Data Encoding for Enhanced Security Against Upstream Eavesdropping in SPECTS O-CDMA

This paper presents an implementation of a modulation technique which is effective in obscuring spectral phase encoded time-spreading (SPECTS) optical code division multiple access (O-CDMA) data streams from eavesdroppers tapping into single-user uplinks. This data modulation technique employs a finite-state Markov chain following a four-state trellis to encode user data in the electronic domain. The encoding redistributes the SPECTS O-CDMA user data bits across four different waveforms to defeat the eavesdropper attacks on upstream links via power detectors or differential phase-shift keying (DPSK) receivers. The four-state encoder-decoder is implemented in a field-programmable gate array (FPGA) with high-speed serial transceivers. A SPECTS O-CDMA testbed with four-state encoded data modulation at up to 2.5 Gb/s/user is demonstrated and its single user link security is tested using a DPSK demodulator to emulate the eavesdropping detection. The security test verifies that this modulation technique effectively prevented interception by DPSK detection. The four-state coding can be extended to be time variable through switching among several trellis state definitions to achieve more rigorous security.

Theoretical Insights Into the Impact of Coherent and Incoherent Crosstalk on Optical DPSK Signals

This paper provides new theoretical insights into the properties of direct detection differential phase-shift keying (DPSK) signals impaired by coherent and incoherent crosstalk. Coherent crosstalk is due to multiple replicas originated while a data signal is routed through an optical network, whereas the source of incoherent crosstalk resides on the interference from other DPSK signals. A special emphasize has been put on modeling the multipath coherent crosstalk, with analytical expressions being derived and presented for both the moment generating function of the decision variable and the average error probability. A rigorous analysis, capable of dealing with arbitrary filtering, is also presented, which is used afterwards to assess the accuracy of the analytical formulas. A detailed comparison with incoherent crosstalk is also performed. Using also an exact treatment for this type of crosstalk it is shown that for low OSNR penalties the coherent crosstalk leads, in some circumstances, to slightly worse results than the incoherent one, but this situation is reversed when the total crosstalk level and the number of interferers increase.

Signed and Accurate Measurement of Phase Offset in Optical DPSK Demodulator

Abstract-A signed and accurate phase offset estimation method of the delay interferometer (DI) for the direct detection differential phase-shift keying system based on a delay-tap sampling technique has been demonstrated. A chromatic dispersion offset module was introduced to help the phase offset measurement. The proposed method can not only measure the value but also distinguish the polarity of phase offset of DI. The measurement sensitivity can reach ±4° and measurement range can reach ±75°. The simulation and experimental measurement results are in close agreement with each other.

A Proposal and Analytical Investigation of an All-Optical T-type Flip-Flop Using Semiconductor Optical Amplifier Mach–Zehnder Interferometer Configuration for Differential Phase-Shift Keying Encoding Operation

An all-optical differential phase-shift keying (DPSK) regenerator with a DPSK demodulator and a conventional on–off keying (OOK) regenerator requires a DPSK encoding process that is realized by T-type flip-flop (T-FF) operation. To solve this problem, we propose an all-optical T-FF circuit consisting of counter-faced semiconductor optical amplifier (SOA)-based exclusive OR (XOR) gates for the first time. Numerical investigation with rate equation reveals the possibility of its stable operation in 10 Gbps with 27-1 pseudorandom binary sequence (PRBS) signal. It also reveals that it is necessary to decrease the equivalent turn-off time for high bit-rate operation using push–pull configuration.

Simultaneous demodulation and demultiplexing of multi-rate WDM DPSK signals using a programmable wavelength-selective switch

We show demodulation and demultiplexing of multi-channel WDM differential phase-shift-keying (DPSK) signals operating simultaneously at 10 Gb/s and 40 Gb/s using a 1x4 wavelength selective switch. We achieve error-free performance for four channels equally spaced by 100 GHz propagated over an 85 km dispersion-compensated link. We study the advantages and limitations of this new DPSK demodulator compared with the conventional delay-line interferometer.

Phase-erased wavelength/format conversion and demodulation of 40 Gbit/s DPSK assisted by periodically poled lithium niobate

We report a novel phase-erased demodulation of differential phase-shift keying (DPSK) by exploiting cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) in a periodically poled lithium niobate (PPLN) waveguide. Analytical solutions are derived to clearly describe the operation principle. The binary optical phase information carried by the conventional DPSK demodulation outputs is removed thanks to the cSHG/DFG in a PPLN waveguide. PPLN-assisted phase-erased wavelength conversion and demodulation of 40 Gbit/s non-return-to-zero DPSK (NRZ-DPSK), return-to-zero DPSK (RZ-DPSK), and carrier-suppressed return-to-zero DPSK (CSRZ-DPSK) are demonstrated in the experiment. Moreover, the accompanying all-optical format conversions from optical duobinary (ODB) to NRZ and from ODB/alternate-mark inversion (AMI) to RZ are also substantiated in the experiment. In addition, the calculated theoretical results including optical spectra, temporal waveforms, and phase diagrams also confirm the successful implementation of PPLN-assisted 40 Gbit/s NRZ-DPSK/RZ-DPSK/CSRZ-DPSK phase-erased wavelength conversion, demodulation, and ODB-to-NRZ and ODB/AMI-to-RZ format conversions.