Coherent QPSK Research Articles

A 32-Gb/s CMOS Receiver With Analog Carrier Recovery and Synchronous QPSK Demodulation

A 40-nm CMOS receiver (RX) capable of performing A-to-D-converter-less carrier recovery including frequency and phase tracking and coherent QPSK demodulation is presented. It operates in frequency-acquisition mode, followed by phase-tracking mode. It is meant as a demodulator of a 270-GHz RX operating at an intermediate frequency of 45 GHz. The highest data-rate of 32 Gbit/s was achieved dissipating 351 mW, amounting to an energy-per-bit of 11 pJ/bit.

Clock conversion for burst-mode digital coherent QPSK receivers in a PON upstream transmission with a 100-ppm clock mismatch.

This paper presents a clock conversion scheme for burst-mode digital coherent QPSK receivers in an asynchronous PON upstream and confirms its validity. The scheme converts the number of samplings of transmitted signal based on the clock at Rx. We demonstrate the proposed scheme in a realistic environment that emulates a fixed clock difference by two independent synthesizers at Tx and Rx mixed together with clock jitter induced by 20-km fiber transmission. In the real-time demonstration, a digital coherent receiver employing the proposed clock conversion successfully receives 330-µs burst QPSK frames over 20-km transmission in the presence of a 100-ppm clock mismatch.

IQ imbalance compensation based on simplified GSOP and FPGA implementation in optical coherent QPSK receiver

Abstract In this paper, a simplified Gram-Schmidt orthogonalization procedure (SG) method was proposed for in-phase/quadrature (IQ) imbalance compensation based on the Gram-Schmidt orthogonalization procedure (GSOP) algorithm. The proposed method can compensate IQ imbalance in the optical coherent system by two steps of phase orthogonalization and amplitude balance. In addition, this method avoids square root operation compared to the GSOP in the field programmable gate array (FPGA) hardware implementation, therefore it is easier to implement in terms of resources and complexity. Two steady states are achieved by accumulating scale factor to reduce resource consumption in FPGA hardware implementation of the proposed scheme. Finally, a FPGA design with 32 parallel units is performed to demonstrate the proposed method in a polarization multiplexed quadrature phase shift keying (PM-QPSK) system, the results show the proposed algorithm simplifies FPGA implementation complexity and maintains compensation performance comparable to GSOP algorithms.

Investigation on R–S Coded Coherent OFDM Free Space Optical (CO-OFDM-FSO) Communication Link Over Gamma–Gamma Channel

Atmospheric turbulence is known to significantly degrade the efficiency and reliability of free space optical communication link. Use of coded-orthogonal frequency division multiplexing (OFDM) technique to mitigate the effect of adverse atmospheric conditions on free space optical (FSO) communication link has been proposed here. With Gamma–Gamma distribution for channel modeling, the error performance of the proposed RS8 (Reed Solomon) coded, 128 sub-carrier OFDM link has been investigated using coherent BPSK and QPSK modulation scheme. The results obtained from this analysis have also been compared with intensity modulated/direction detection (IM/DD) based OOK-OFDM FSO link. In case of strong turbulence and for target BER of 10−4, it was observed that BPSK and QPSK modulated OFDM FSO link achieve a descent coding gain of 18.2 dB and 12.6 dB respectively over non coded OOK-OFDM FSO link. Also, it was observed that as the link conditions worsened from weak to strong turbulence, the effect of atmospheric impairments on FSO link becomes significantly pronounced. Additionally, in terms of BER performance, the BPSK modulated link out-performed QPSK and OOK under all the considered channel conditions.

Post-FEC performance evaluation of coherent QPSK system with an enhanced pilot-aided CPE scheme

In this paper, a simple and robust pilot-aided carrier phase estimation scheme with a modified phase unwrapping is proposed. The principle of the proposed phase unwrapping is shown with a step-by-step procedure. Further, we test the post-FEC BER performance for the pilot-aided carrier phase estimation scheme with the traditional and proposed phase unwrapping. For the proposed phase unwrapping, a cycle slip rate reduction of about 10 times is achieved than the case using the traditional phase unwrapping through simulations in the presence of Wiener phase noise. Finally, we present a 2.2 Eb/N0 gain and post-FEC error-free performance with only 0.78 % pilot overhead in a 32-Gbaud QPSK system using soft-decision forward error correction.

PERFORMANCE EVALUATION OF 1 TBPS QPSK DWDM SYSTEM OVER ISOWC

Optical wireless communications has been in latest trends of high speed communications. They enable the use of optical wireless channel in applications like inter satellite links and underwater communications etc. In this paper, we communicate an ultra high bit rate i.e. 1 Tbps (10 x 100 Gbps) QPSK WDM System over optical Wireless communication Link. The system is a Line of Sight optical wireless link incorporating Coherent QPSK modulation Scheme for10 channels each at 100 Gbps The performance is evaluated in terms of Q-Factor and Minimum Bit Error Rate which are noticed to be in acceptable standards. The Link is analyzed under various parameters such as Power, Distance etc and maximum achievable distance is noticed to be 50,000 km at power values ranging from 0 dBm to 40 dBm. Key WordsWDM, CO-QPSK, OWC ---------------------------------------------------------------------***---------------------------------------------------------------------

Phase noise analysis of injected gain switched comb source for coherent communications

We present experimentally and analytically the phase noise characterization of an externally injected gain switched comb source. The results reveal the residual high frequency FM noise in the comb lines, which stays unnoticed in the optical linewidth value but leads to an increased phase-error variance. The potential impact of the residual phase noise is investigated in a 10.7 GBaud optical DQPSK system where a 2 dB power penalty is recorded at BER of 10(-9). In a 10.7 GBaud digital coherent QPSK system no penalty is observed but with 5 GBaud 16-QAM format a 3 dBpenalty exists at the FEC limit of 4.4e-3.

Dual-polarization multi-band OFDM versus single-carrier DP-QPSK for 100 Gb/s long-haul WDM transmission over legacy infrastructure

The transmission performance of coherent dual-polarization multi-band OFDM (DP-MB-OFDM) and QPSK (DP-QPSK) are experimentally compared for 100 Gb/s long-haul transport over legacy infrastructure combining G.652 fiber and 10 Gb/s WDM system. It is shown that DP-MB-OFDM and DP-QPSK have nearly the same performance at 100 Gb/s after transmission over a 10 × 100-km fiber line. Furthermore, the origin of performance degradations and limitations of the DP-MB-OFDM is explored numerically, as well as the impact of transmission distance and sub-band spacing.

Robust 9-QAM digital recovery for spectrum shaped coherent QPSK signal

We propose 9-ary quadrature amplitude modulation (9-QAM) data recovery for polarization multiplexing-quadrature phase shift keying (PM-QPSK) signal in presence of strong filtering to approach Nyquist bandwidth. The decision-directed least radius distance (DD-LRD) algorithm for blind equalization is used for 9-QAM recovery and intersymbol interference (ISI) compression. It shows the robustness under strong filtering to recover 9-QAM signal rather than QPSK. We demonstrate 112 Gb/s spectrum shaped PM-QPSK signal by wavelength selective switch (WSS) in a 25-GHz channel spacing Nyquist wavelength division multiplexing (NWDM). The final equalized signal is detected by maximum likelihood sequence decision (MLSD) for data bit-error-ratio (BER) measurement. Optical signal-to-noise ratio (OSNR) tolerance is improved by 0.5 dB at a BER of 1x10(-3) compared to constant modulus algorithm (CMA) plus post-filter algorithm.

DGD monitoring issues in high-speed polarisation multiplexed coherent QPSK systems

Optical performance monitoring based on digital signal processing (DSP) of a polarisation diverse coherent receiver can allow the estimation of the differential group delay (DGD) of the link fibre. In this reported work, it was found that the DGD estimation can be affected by substantial errors, since the DSP can introduce or leave uncompensated any symbol-time delay between orthogonal polarisations and cannot discriminate the in-line DGD from any other link delay. These issues are investigated and a method is proposed to partially overcome the problems.

Simplified bit error rate evaluation of Nagakami‐m PSK systems with phase error recovery

AbstractAn efficient analytical and simulation techniques were developed to analyze the bit error rate of PSK Nakagami‐m fading systems with imperfect carrier recovery. We evaluate the detection loss due to the carrier recovery for different rms phase error values when coherent BPSK and QPSK systems are used in wireless channels. Our results are useful in designing practical systems and will enable designers to determine the phase precision of PSK systems in wireless environments. Copyright © 2010 John Wiley & Sons, Ltd.

Carrier phase estimation for optically coherent QPSK based on Wiener-optimal and adaptive Multi-Symbol Delay Detection (MSDD)

The MSDD carrier phase estimation technique is derived here for optically coherent QPSK transmission, introducing the principle of operation while providing intuitive insight in terms of a multi-symbol extension of naïve delay-detection. We derive here for the first time Wiener-optimized and LMS-adapted versions of MSDD, introduce simplified hardware realizations, and evaluate complexity and numerical performance tradeoffs of this highly robust and low-complexity carrier phase recovery method. A multiplier-free carrier phase recovery version of the MSDD provides nearly optimal performance for linewidths up to ~0.5 MHz, whereas for wider linewidths, the Wiener or LMS versions provide optimal performance at about 9 taps, using 1 or 2 complex multipliers per tap.

112 Tb/s superchannel coherent PM-QPSK InP transmitter photonic integrated circuit (PIC)

In this work, a 10-wavelength, polarization-multiplexed, monolithically integrated InP coherent QPSK transmitter PIC is demonstrated to operate at 112 Gb/sec per wavelength and total chip superchannel bandwidth of 1.12 Tb/s. This demonstration suggests that increasing data capacity to multi-Tb/s per chip is possible and likely in the future.

Five-channel, 114 Gbit/s per channel, dual carrier, dual polarisation, coherent QPSK, monolithic InP receiver photonic integrated circuit

The details of a monolithically integrated, five-channel, dual carrier/wavelength per channel, polarisation multiplexed, coherent QPSK receiver on InP operating at 114 Gbit/s per channel are discussed.

Inside View

Infinera Fellow Dr Radha Nagarajan manages Infinera's advanced photonic integrated circuit development for next generation telecom equipment. Here he talks about their latest PIC receiver for optical coherent QPSK applications.

Bit Error Probability Performance Of Extended Suzuki Fading Channel Simulators

This paper investigates the Bit Error Probability (BEP) performance of Deterministic Extended Suzuki fading channel simulators. The analytical and approximate expressions of the BEP for coherent QPSK over Extended Suzuki fading channel are derived. Simulation results for the BEP are presented, by using a typical stochastic procedure Method of Exact Doppler Spread (MEDS) for the computation of the parameters of simulators. Computer simulation verifies the effectiveness of the method.

Stochastic BER estimation for coherent QPSK transmission systems with digital carrier phase recovery

We propose a stochastic bit error ratio estimation approach based on a statistical analysis of the retrieved signal phase for coherent optical QPSK systems with digital carrier phase recovery. A family of the generalized exponential function is applied to fit the probability density function of the signal samples. The method provides reasonable performance estimation in presence of both linear and nonlinear transmission impairments while reduces the computational intensity greatly compared to Monte Carlo simulation.

Unveiling nonlinear effects in dense coherent optical WDM systems with Volterra series

In this paper, the effect of fiber nonlinear effects on coherent optical WDM systems is investigated through numerical simulations. The analysis of the most relevant fiber nonlinear effects is made recurring to the Volterra series transfer function method, which allows us to quantify its impact on the transmission of multi-level modulated signals employing digital coherent receiver. The performance transmission is evaluated using vector analysis of the received signal's constellation where firstly we validate against split-step simulations that VSTF is suitable for optimizing coherent optical WDM transmission; then we evaluate the different contributions of the fiber nonlinear distortions imposed on the coherent optical QPSK, 8PSK and 16QAM channels by co-propagating lower data rate intensity modulated channels.

Experimental demonstration of intra-channel nonlinearity mitigation in coherent QPSK systems with nonlinear electrical equaliser

The mitigation of intra-channel nonlinearities in 10 Gbaud coherent NRZ-QPSK systems via a nonlinear electrical equaliser based on Volterra theory is experimentally demonstrated. Without the requirement of prior knowledge of the fibre link, the equaliser significantly improves the system performance compared to digital dispersion compensation only.

Cross-Phase Modulation Induced by OOK Channels on Higher-Rate DQPSK and Coherent QPSK Channels

In this paper we show that, in hybrid wavelength division multiplexed systems, the performance of high datatrate QPSK channels impaired by cross-phase modulation (XPM) induced by the lower rate OOK channels can be simply estimated by an extension of a well-known linear model for XPM, and novel analytical expressions of the sensitivity penalty are provided. From such a model we prove that the reported QPSK penalty decrease with QPSK baud rate increase should be attributed to the action of the phase estimation process rather than to the walkoff effect. The model also simply shows how coherent QPSK is more affected by XPM than incoherent DQPSK, and allows to infer that even more impact is expected when the baudrate is further reduced through polarization multiplexing.