We studied the influence of limited bandwidth of receiver on tapping signals. The optimum sampling timing offset (STO) and filter bandwidth in 80 Gbit/s coherent optical QPSK eavesdropping system are also discussed. Different order of Bessel filters are taken into account in the presence of laser phase noise. Optimal receiver performance relies on a trade-off between additive noise and inter-symbol interference (ISI). Optimum STO and bandwidth of eavesdropping receiver are determined by signal to noise ratio (SNR) per bit as we define. Simulation results show that the ratio of optimum STO to symbol duration are 0 and 0.25 for the 1stand 5th-order Bessel frequency response of receivers, respectively.
We studied the influence of limited bandwidth of receiver on tapping signals. The optimum sampling timing offset (STO) and filter bandwidth in 80 Gbit/s coherent optical QPSK eavesdropping system are also discussed. Different order of Bessel filters are taken into account in the presence of laser phase noise. Optimal receiver performance relies on a trade-off between additive noise and inter-symbol interference (ISI). Optimum STO and bandwidth of eavesdropping receiver are determined by signal to noise ratio (SNR) per bit as we define. Simulation results show that the ratio of optimum STO to symbol duration are 0 and 0.25 for the 1stand 5th-order Bessel frequency response of receivers, respectively.
In previous research on quantum information the- ory, channel capacity of the lossy bosonic channel that is a model of free space or an ideal optical fiber was shown. However, in the optical space communication, the influence of temporal and spatial change in the propagation path on the quantum communication has not been clarified. In order to clarify them, we have studied properties of quantum communications via channels with various fluctuations. We consider phase noise since phase fluctuates by a slight drift of the receiver and it has not been considered well compared to amplitude fluctuation. In this paper, we compute the error probability of the optimum quantum receiver in the presence of phase noise and compare with that of homodyne receiver when BPSK coherent-state signals are used. Then we consider the robustness of the optimum receiver against phase fluctuations. I. INTRODUCTION In order to increase the communication rate, higher fre- quency signals should be used. Light wave has a very high frequency in particular, optical communication systems using this were realized by the invention of the laser. Currently, optical fiber communication systems which consist of laser light source, optical fiber, and photodetector have come to be generally used. Moreover, light is also a promising carrier for wireless or space communications with high capacity. However, the performance of communication systems of this scheme has been approached to the theoretical limit. The factor is due to quantum mechanical effects of light, called quantum noise. Quantum noise increases in proportion to the frequency of the electromagnetic wave. Therefore, it cannot be ignored in the high frequency band, such as optical frequency. In addition, quantum noise is essentially different from the classical noise such as thermal noise. Therefore, it is not possible to suppress quantum noise using information theory which is the basis of current communication technology. That is, in order to realize higher communication rate, a theory which can describe accurately the quantum mechanical nature of light is required. For this reason, we should study quantum information theory which had been introduced axioms of quantum mechanics into the conventional information theory (2), (3), (4). As a previous study, the channel capacity of the lossy bosonic channel which is a model of free space and optical fiber communications was shown (5). That is, performance of ideal quantum communication was clarified. Therefore, as the next step, a study aiming at the actual use is required. As an example, the study on communication model that takes account of the propagation path environment will be needed. In classical communication, such models have been studied as a theory for realizing mobile communications, etc. However, in quantum communication, there are few such results of study now. This study is intended to clarify the influence of temporal and spatial change of the propagation path environment for the quantum communication. We consider phase noise since phase fluctuates by a slight drift of the receiver and it has not been considered well compared to amplitude fluctuation.
A novel digital signal processing (DSP)-based scheme for physical layer security in coherent optical communication systems is proposed and numerically investigated. The optical layer signal encryption is accomplished by two dispersive elements and one phase modulator (PM) driven by a DSP-generated encryption key, whilst signal decryption uses similar components but with inverted dispersion values and security keys. A critical aspect of the DSP-based physical layer security is that the security keys, driving the PMs to hide/recover the data signals, must be highly unpredictable and noise-like, thus orthogonal frequency division multiplexing (OFDM) signals are employed as they possess these characteristics, they can also be easily generated and cover a suitably wide range of unique keys. Numerical simulations are conducted to determine optimum system parameters for achieving a high level of security, the key parameters requiring optimization are the dispersion of the dispersive elements and the bandwidth of the security keys. Using these determined optimum parameters, in-depth investigations are undertaken of encryption/decryption induced transmission performance penalties, sensitives to various parameter offsets and operation over various transmission distances. To observe any data signal dependencies, various performance metrics are investigated for different combinations of modulation formats (DQPSK and 16QAM) and baud rates (40 Gbaud and 100 Gbaud) for the transmitted data signals. The proposed DSP-based physical layer security scheme is shown to have the potential to achieve, in a low-cost and highly effective manner, a high level of physical layer security with acceptable performance penalties for existing coherent optical communication systems.
Quantum-noise randomized cipher (QNRC) is an anti-interception communication technology, which can realize high-speed encryption for data stream in true randomness. In this paper, the characteristics of the phase shift keying (PSK) and quadrature phase shift keying(QPSK) QNRC systems are verified by the simulation system with digital coherent demodulation. The decryption process of the QNRC coherent system is carried out in the optical domain for the first time. Specifically, at the quadrature coupler, the modulated local light is coupled with the received encrypted optical signal. The multi-symbol phase estimation(MSPE) module is employed to eliminate the influence of the phase deviation on the signal decision. Finally, error-free secure transmission is realized at 2.5Gsymbol/s over 500km optically amplified links. We reveal the new fact that the mesoscopic power of QPSK-QNRC system is higher than that of PSK-QNRC system.
In recent years, cooperative jamming (CJ) is introduced as a promising method to improve the security performance in the presence of eavesdroppers. By masking the confidential signal with the help of the cooperative jammer, the signal-to-noise ratio of the eavesdropper can be selectively reduced without prior information, whereas the authorized receiver is unaffected since the CJ can be suppressed by some means. Unfortunately, with the impacts of the clock jitter caused by non-ideal crystals, the received CJ may suffer sampling clock offset, carrier frequency offset and phase noise, which deteriorates the performance of the CJ cancellation at the authorized receiver. In this paper, the impacts of clock jitter on CJ cancellation are investigated. First, the deterioration of CJ caused by sampling clock offset is modeled as inter-symbol interference (ISI) in the time domain. Then, the impacts of the sampling clock offset, carrier frequency offset and phase noise on the spectral purity of CJ are modeled as inter-frequency interference (IFI). Furthermore, the expression of the power of residual CJ (PRCJ) is derived to analyze the impacts of clock jitter on CJ cancellation. Finally, numerical results prove the theoretical analysis, indicating that the PRCJ increases as the carrier frequency offset and the 3 dB coherence bandwidth of phase noise increases, and is more sensitive to phase noise.
Rydberg atomic quantum receivers (RAQR) exhibit exceptional sensitivity to radio-frequency (RF) signals, positioning them as promising candidates for next-generation wireless systems. Extending their capabilities to orthogonal frequency-division multiplexing (OFDM), however, requires atomic responses that go beyond broadband reception and are tailored to the structured interference of multicarrier signals. To bridge this gap, we develop an atomic-comb Rydberg receiver based on six-wave mixing (SWM) transduction. In particular, injecting a multi-tone local oscillator (LO) into the RAQR generates frequency-shifted replicas of the intrinsic SWM response. By subtly tuning the amplitude and phase of each LO tone, constructive and destructive interference can be selectively induced to preserve desired response components while attenuating harmful frequency offsets. Furthermore, we harness the response-reshaping capability of the proposed atomic comb to proactively mitigate OFDM interference. More explicitly, by exploiting the discrete difference-frequency structure of both carrier-frequency-offset (CFO)-induced and phase-noise (PN)-induced inter-carrier interference (ICI), we optimize the atomic-comb response to suppress dominant ICI components directly. We further reveal an intrinsic coupling between spectral shaping and the time-domain effective channel memory, motivating a joint optimization of both ICI and inter-symbol interference (ISI), which simultaneously suppresses inter-carrier leakage and post-cyclic-prefix (CP) tail energy. Extensive simulation results validate our atomic-comb design, unveiling that the proposed atomic comb can effectively reshape the receiver response over the OFDM, while mitigating ICI, shortening the effective channel memory, and substantially lowering CP overhead.