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The sharing waveform based integrated radar and communication (IRC) has great potential to apply in Internet of Things (IoT) devices due to its equipment miniaturizing and high efficiency of spectrum and energy, etc. However, the sharing waveform scheme demands a trade-off between radar and communication performances. In this paper, we focus on the design of multiple phase shift keying (MPSK) based linear frequency modulation (LFM) sharing waveform by exploring the trade-off among energy leakage, peak-to-side lobe ratio (PSLR) and symbol rate. First, we derive the energy leakage and PSLR with respect to the symbol rate. Next, a novel LFM-MPSK waveform design is carried out by formulating a communication frame based optimization, where the symbol rate is maximized with the constraints of the energy leakage and PSLR. Then, a branch and reduce algorithm is proposed to solve the optimization. Compared with the conventional one, the proposed LFM-MPSK waveform has a higher symbol rate with the same energy leakage and PSLR. Besides, the proposed waveform design alleviates the difficulty of the trade-off among the energy leakage, PSLR and symbol rate due to more Pareto solutions. Finally, all the analytical results are validated by simulations.

Symbol rate is one of the most important parameters in signal demodulation process. In real-time signal processing, traditional symbol rate estimation algorithms for the Multiple Phase Shift Keying (M-PSK) and the Multiple Quadrature Amplitude Modulation (M-QAM) are based on the Fourier transform of signal’s complex envelope. At the low signal-to-noise ratio (SNR), the accuracy of symbol rate estimation can be improved by increasing the number of symbols as much as possible. However, this improvement is infeasible in many applications such as the energy-limited Internet of Things devices and sporadic noncooperative transmissions. In this paper, we propose a data-driven bandpass filter (BPF) design scheme for accurate estimation of symbol rate under low SNR with only a small number of symbols available. The proposed scheme considerably improves the estimation performance by optimizing the BPF design using the equivalent dynamic linearization model with time-varying pseudo-partial derivatives. Specifically, the proposed scheme iteratively optimizes the upper and lower cut-off frequencies of the BPF based on the measured complex envelope spectrum until achieving the optimal BPF. Therefore, the peaks of the complex envelope spectrum are extracted as the estimate of the symbol rate by applying the optimal BPF. Experimental results indicate the promise of the proposed scheme as an efficient symbol rate estimator for sporadic signal at low SNR and with a small number of symbols.

Commercial coherent receivers utilize balanced photodetectors (PDs) with high single-port rejection ratio (SPRR) to mitigate the signal-signal beat interference (SSBI) due to the square-law detection process. As the symbol rates of coherent transponders are increased to 100 Gbaud and beyond, maintaining a high SPRR in a cost-effective manner becomes more and more challenging. One potential approach for solving this problem is to leverage the concept of single-ended coherent receiver (SER) where single-ended PDs are used instead of the balanced PDs. In this case, the resulting SSBI should be mitigated in the digital domain. In this paper, we show that SSBI can be effectively mitigated using various low-complexity techniques, such as the direct filed reconstruction (DFR), clipped iterative SSBI cancellation (CIC) and gradient decent (GD). In addition, we present a self-calibration technique for SERs which can be extended for characterizing the optical-to-electrical (O/E) response of a conventional balanced coherent receiver (BR). Using the developed techniques, we then experimentally demonstrate a 90 Gbaud probabilistically constellation shaped 64-QAM (PCS-64QAM) transmission using a SER, achieving a net data rate of 882 Gb/s over 100 km of standard single mode fiber (SSMF). The sensitivity penalty compared to the BR is below 0.5 dB. We expect that when the symbol rate is increased further, a SER can potentially outperform a BR, especially when applied to cost-sensitive commercial pluggable coherent transceivers

In this paper, we propose a pre-coded multi-carrier M-ary chaotic vector cyclic shift keying (PC-MC-M-CVCSK) transceiver for reliable information transmissions. In this design, we exploit the principle of diagonalization to combat noises and interferences. At the PC-MC-M-CVCSK transmitter, we propose to precode the M-ary modulated symbols based on the discrete Fourier transform (DFT) and the reverse matrix of an eigenvalue matrix. Meanwhile, the chaotic generator outputs chaotic sequences, which are then cyclicly shifted and used to modulate precoded symbols. At the receiver, reverse operations are conducted to recover the data. Moreover, we prove that a circulant matrix can be constructed by selecting the cyclic shift length, while the DFT aided processing will diagonalize the circulant matrix. Thanks to the diagonalization design, the inner product of information-bearing chaotic vectors becomes zero, thus signals can be orthogonalized to suppress interferences. Furthermore, theoretical performances of the proposed transceiver, including the bit error rate (BER), the symbol rate, the energy efficiency and the spectrum efficiency, and the computational complexity are analyzed. Simulation results validate the exact analysis. Furthermore, the results demonstrate that the proposed PC-MC-M-CVCSK system outperforms the counterpart schemes, which can provide M-ary high symbol rate transmission services with enhanced reliability performances.

Optical wireless communication (OWC) systems have been widely studied to provide high-speed connections in indoor environments. In this paper, we propose a filter-enhanced indoor OWC system with hybrid spatial modulation (SM) and spatial multiplexing (SMUX) to further increase the data rate. The filter-enhanced SM enables extra information be transmitted via the spatial indices of OWC transmitters, and the SMUX further increases the data rate via the parallel transmission of multiple signals even under highly-similar channels. The proposed scheme is experimentally demonstrated. Up to 35 Gb/s data rate can be achieved with 2.5 GBaud/s symbol rate and four transmitters. Compared with the filter-enhanced SM scheme, the achievable data rate can be increased by 100%. Compared with the conventional SM and SMUX, the bit-error-rate (BER) is improved by over two orders-of-magnitude.

In the next-generation flexible point-to-multipoint optical networks such as optical access systems, it is desirable for one transceiver to handle various Internet-of-Things traffic streams. A symbol rate-adaptive multiband-coherent-transceiver, that can transmit and receive wavelength division multiplexing (WDM) signals arranged at the Nyquist spacing after Nyquist spectrum shaping, achieves flexibility by changing the number of subbands (SB) and the symbol rate per SB. Band-asymmetric coherent access systems with wide bandwidth optical transceivers on the optical-line-terminal side and narrowband optical transceivers on the optical-networking-unit side have been reported for uplink and downlink transmissions. We also examined the multiband-coherent-transceiver for ultra-dense WDM signals with a narrow analog-device bandwidth and narrow SB spacing to improve cost and spectral efficiency. To the best knowledge of the authors, there is no other research on a Nyquist spectrally shaped WDM-passive optical network system that applies these two functions and flexibly changes the symbol rate per SB with a SB spacing less than the Nyquist spacing.

Digital subcarrier multiplexing (SCM) has recently emerged as a promising solution for next-generation ultra-high-baudrate coherent optical communication systems. Among its distinctive advantages over traditional single-carrier modulation, SCM enables the exploitation of symbol-rate optimization (SRO), which has been shown to enable the passive mitigation of the nonlinear interference noise (NLIN) that is generated during propagation over dispersion-unmanaged optical fiber systems. However, the full exploitation of SRO-based NLIN mitigation is severely hindered by the uncompensated distortion caused by laser phase noise (LPN) and non-linear phase noise (NLPN), whose impact is magnified by the use of low-baudrate subcarriers. Resorting to low-complexity carrier phase estimation (CPE) algorithms, in this paper we experimentally demonstrate that it is possible to overcome the hurdles posed by LPN and NLPN in SCM systems, provided that adequate joint-subcarrier CPE processing is employed. A dual-stage joint-processing approach composed of a pilot-based CPE optionally followed by a blind phase search (BPS)-based estimator is implemented and experimentally assessed, enabling to effectively optimize the symbol-rate per subcarrier down to 3 GBaud, in accordance with the theoretical SRO predictions for the system under test. In addition, we demonstrate that signal-to-noise ratio (SNR) gains of more than 1 dB can be achieved through joint-subcarrier CPE processing in shorter-reach links, while this gain tends to progressively reduce with increasing propagation distance, down to about 0.5 dB gain after 3000 km propagation.

We experimentally demonstrated a 10-channel 50-GHz-grid wavelength division multiplexing (WDM) signal transmission over 400-km standard single-mode fiber (SSMF) with the net bit rate of 614.4 Gbps/carrier and the spectral efficiency (SE) of 12.288 b/s/Hz, which is the highest SE for 256QAM to the best of our knowledge. Thanks to the joint pre-equalization and Volterra nonlinear equalization (VNLE), 49-Gbaud polarization division multiplexing (PDM) 256-ary quadrature amplitude modulation (256QAM) signal transmission with a novel constellation using probabilistic shaping (PS) and geometric shaping (GS) techniques is realized, considering the 0.828-normalized generalized mutual information (NGMI) low-density parity-check (LDPC) threshold with 25% overhead. Compared with the regular 256QAM at the same net bit rate, the designed hybrid PS/GS 256QAM at the expense of 1-Gbaud increase in the symbol rate obtains 1.7-dB and 1.5-dB shaping gain in single-channel case and WDM case, respectively. Meanwhile, an improvement of at least 0.6-dB in optical signal to noise ratio (OSNR) over the hybrid PS/GS 256QAM is achieved in comparison with PS-256QAM for both single-channel and WDM transmission.

Aiming to shed light on the operation of fully-digital transmitters, we present the first symbol-rate baseband model that makes explicit the inter-symbol interference introduced by the RF pulse-width modulation (RF-PWM). The proposed model expresses the transmitter output as symbols plus interference, paving the way for the derivation of in-band performance metrics of the communication system. Then, we find an approximation for the error vector magnitude that allows us to evaluate the impact of the oversampling ratio, the statistics of the transmitted symbols and the shaping pulse in the transmitter performance. Another advantage of the model is that simulation times are much shorter since it does not require interpolation to appreciate the PWM distortion. To obtain the model, we first assume a generic expression of the continuous-time signal to be transmitted, in terms of the information symbols and the reconstruction filter. This allows us to express the nonlinear differential equation that models the intrinsic distortion also in terms of the sent symbols. Then, we elaborate on the formulation of the continuous-time RF-PWM distortion to procure an expression of the received signal sampled at symbol rate. The article concludes with a numerical validation of the model, and a simulation of the bit error rate of the system.

The recent emergence of orthogonal time frequency space (OTFS) modulation as a novel PHY-layer mechanism is more suitable in high-mobility wireless communication scenarios than traditional orthogonal frequency division multiplexing (OFDM). Although multiple studies have analyzed OTFS performance using theoretical and ideal baseband pulseshapes, a challenging and open problem is the development of effective receivers for practical OTFS systems that must rely on non-ideal pulseshapes for transmission. This work focuses on the design of practical receivers for OTFS. We consider a fractionally spaced sampling (FSS) receiver in which the sampling rate is an integer multiple of the symbol rate. For rectangular pulses used in OTFS transmission, we derive a general channel input-output relationship of OTFS in delay-Doppler domain without the common reliance on impractical assumptions such as ideal bi-orthogonal pulses and on-the-grid delay/Doppler shifts. We propose two equalization algorithms: iterative combining message passing (ICMP) and turbo message passing (TMP) for symbol detection by exploiting delay-Doppler channel sparsity and the channel diversity gain via FSS. We analyze the convergence performance of TMP receiver and propose simplified message passing (MP) receivers to further reduce complexity. Our FSS receivers demonstrate stronger performance than traditional receivers and robustness to the imperfect channel state information knowledge.

Silicon photonics can be used to increase the versatility of wavelength division multiplexing (WDM). Ultra-dense wavelength division multiplexing (uDWDM) shrinks channel spacing between WDM channels to decrease guard bands and increase spectral efficiency. Microring modulators (MRMs) provide a simple solution for uDWDM transmitters - they require no multiplexer/demultiplexer. When used with comb lasers, MRMs provide extremely compact and low power solutions. The spectral efficiency is limited by interchannel interference (ICI) from incomplete isolation of channels. We report an MRM-uDWDM transmitter achieving channel spacing comparable to the symbol rate in each WDM channel, reaching a spectrum occupation of 80%. By choosing the correct operating point, we demonstrate experimentally our fabricated MRM chip design causes no discernable ICI power penalty. To the best of our knowledge, this is the first demonstration of MRM-uDWDM to support such dense channel spacing.

In this paper, the influence of noise induced by classical signals on independent core transmitting quantum key distribution (QKD) is studied over multicore fiber (MCF). We propose a theoretical model of the cascaded noise of spontaneous Raman scattering and inter-core crosstalk (SpRS-ICXT), and the cascaded noise of four-wave mixing and inter-core crosstalk (FWM-ICXT) considering modulation frequency over MCF. To analyze the effect of modulation frequency on QKD, we define and simulate the average power difference and power fluctuation difference of SpRS-ICXT and FWM-ICXT. Then, SpRS-ICXT and FWM-ICXT noises are simulated in two cases: (1) 1 or 3 classical channels and 1 km MCF; (2) 8 or 16 classical channels and 45 km MCF. For the latter case, SpRS-ICXT and FWM-ICXT noises have a serious impact on QKD. Therefore, the impact of noise on QKD is studied. A higher modulation frequency can improve QKD performance, and the secure transmission distance can be further extended by 15.2% when the modulation frequency is 10 GHz relative to ignoring the modulation frequency. However, inter-core crosstalk (ICXT), SpRS-ICXT and FWM-ICXT noises will shorten 52.2% secure transmission distance compared to only transmitting QKD. Finally, the noise of quantum channel is experimentally measured at different symbol rates and modulation formats of classical signals, and it is shown that increasing the symbol rate of data signals at the same optical power degrades the secure key rate of QKD system.