Abstract
In optical communication systems, coherent detection is a standard method. The received signal enters the digital domain after passing through a time-interleaved analog-to-digital converter (TI-ADC). However, the time delay of the ADC brings noise into the signal, which decreases the signal quality; therefore, ADC calibration is essential. At present, there are many calibration methods for time delay, but their performances are not satisfactory at a high sampling frequency. This paper presents a method of time delay estimation and calibration in a coherent optical communication system. First, the expected maximum (EM) method is used to roughly estimate the time delay and then transfer the estimated value into the trained back propagation (BP) neural network to generate more accurate results. Second, the sampled signal is reconstructed, and then a finite impulse response (FIR) filter is designed to compensate for the time delay. There are several advantages of the proposed method compared with previous works: the convergence with a BP network is faster, the estimation accuracy is higher, and the calibration does not affect the sample operation of the ADC working in the background mode. In addition, the proposed calibration method does not need additional circuits and its low power consumption provides more sources for dispersion compensation, error correction, and other subsequent operations in the coherent optical communication system. Based on the quadrature phase shift keying (QPSK) system, the proposed method was implemented in a 16-channel/8-bit, 40-GS/s ADC. After estimation and calibration, the relative error of estimation was below 1%, the signal noise distortion rate (SNDR) reached 55.9 dB, the spurious free dynamic range (SFDR) improved to 61.2 dB, and the effective number of bits (ENOB) was 6.7 bits. The results demonstrate that the proposed method has a better calibration performance than other methods.
Highlights
In many optical communication infrastructures, in order to meet the broader bandwidth requirements, the sampling frequency and the bandwidth of ADC need to be continuously improved
The relative error of time delay estimation was below 1% with fast convergence, which is smaller than that of the least squares (LS) method; after calibration, the signal noise distortion rate (SNDR) reached 55.9 dB, the spurious free dynamic range (SFDR) improved to 61.2 dB, and the effective number of bits (ENOB) reached 6.7 bits, which are better results than those obtained in previous works
The EMBP method showed higher accuracy and faster convergence compared with those of previous works
Summary
In many optical communication infrastructures, in order to meet the broader bandwidth requirements, the sampling frequency and the bandwidth of ADC need to be continuously improved. Compared with static error calibration, time delay calibration is more complicated and researchers have conducted a lot of research on the subject: The authors of [7,10,11,12] divided the TI structure into two topological structures, which improves the SNDR in time delay calibration, and the convergence is fast, but the SNDR is unavailable in coherent communication due to the low sampling frequency, and the digital-mixing structure causes crosstalk among channels. The method is efficient and straightforward, there is no iteration to improve the accuracy, the type and bandwidth of signal are limited, and it may be unsuitable in the QPSK coherent optical communication system at a high sampling frequency.
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