Modulation format identification is an important part of optical performance monitoring in elastic optical networks. We propose a modulation format identification scheme based on the information entropy analysis of received optical communication signal. In our method, the information entropy is calculated based on the amplitude distribution of received signal and employed to identify 4 commonly used modulation formats like QPSK, 16QAM, 32QAM, and 64QAM. The results demonstrate that the modulation format can be identified with the required OSNR not higher than the FEC threshold. The influence of the residual chromatic dispersion on the proposed scheme is also discussed. It is insensitive to phase noise caused by laser linewidth and frequency offset, and can tolerate a range of fiber nonlinearity, which is also an advantage of our method. Compared to other blind modulation format identification methods, the proposed method needs no additional hardware or training samples, is more simple and cost-effective.

Modulation format identification (MFI) is necessary for future elastic optical networks. We propose an MFI scheme based on the amplitude deviation analysis of the received optical communication signal. In our method, for a received optical signal, two values of normalized average amplitude deviation with reference to amplitude of ideal polarization-division multiplexing (PDM) QPSK and 16 quadratic-amplitude modulation (QAM) are computed, respectively, and the ratio of the two values is used to identify the commonly used formats like PDM-QPSK, PDM-8 QAM, PDM-16 QAM, PDM-32 QAM, and PDM-64 QAM. The results demonstrate that the modulation format can be identified with the required optical signal-to-noise ratio not higher than the forward error correction (FEC) threshold. The advantage of our method is that it is not sensitive to phase noise, frequency offset, and nonlinearity in optical fiber communication systems. The proposed method needs no training samples or additional hardware, so it is simple and cost-effective. Meanwhile, we also discuss the influence of the residual chromatic dispersion on our proposed scheme.

Conventional fast Fourier transform based frequency offset estimation (FFT-FOE) algorithm is suitable for QPSK and 8/16/64QAM signals. However, due to the non-rectangular distribution of constellation points, the conventional FFT-FOE is not suitable for 32-QAM signal. In this paper, we report a factor which can indicate whether the FFT-FOE algorithm can be used for frequency offset estimation for different modulation formats. The FFT-FOE algorithm is hard to apply for frequency offset estimation if the value of is low. Fortunately, the value of can be increased by digital amplification and phase rotation. Based on the digital amplification and phase rotation, a modified FFT-FOE algorithm is proposed and can realize the frequency offset estimation of 32-QAM signal with only 512 symbols. The bit error ratio (BER) of a 28 Gbaud 32-QAM signal is lower than the soft-decision forward-error correction (SD-FEC) limit with the optical signal-to-noise ratio (OSNR) of 22dB. The proposed method has better robustness to phase noise. If BER at SD-FEC limit is considered for 28Gbaud 32-QAM signal, the required OSNR is relaxed by 2dB when the laser linewidth is greater than 5 MHz. This will make sense when the wide-linewidth laser is used in short reach optical communication system.

The phase calibration of distributed multi-node receiving system is required considering the phase synthesis working mode of distributed radar system for echo signals. Whereas, in respect to the traditional calibration tower and unmanned aerial vehicle scheme, there are many disadvantages. The wide range distributed multi-node receive chain calibration method based on GNSS software receiver was proposed in this paper, taking advantage of the software receiver to calculate and obtain the actual phase difference among the receiving nodes as well as the coordinate information of satellites in transmitting signal. Therefore, the phase difference resulted from different transmit path can be deleted, and the phase inconsistencies among the receiving nodes can be obtained. Besides, the fingerprint maps database regarding inconsistencies among nodes and two-dimensional angle of relative receiving node for satellites was constructed in this paper, the correspondence between the omnidirectional two-dimensional angle and the inconsistencies among different nodes was established by the actual results and two-dimensional fitting, and the phase inconsistency results among nodes can be obtained only by solving the satellite coordinate information. There are many advantages for the method proposed in this paper, such as slight influence of multipath effect, capability of all-weather all-day calibration, low costs.