Adaptive Background Compensation of Frequency Interleaved DACs With Application to Coherent Optical Transceivers

Abstract

Digital-to-analog converters (DACs) with bandwidths larger than 70 GHz and sampling rates in excess of 170 GS/s will soon be required in ultra-high speed communication applications such as coherent optical transceivers operating at symbol rates of 140 GBd and beyond. Frequency interleaving has been proposed as a way to break the bandwidth bottleneck in such applications. Splitting the input signal into multiple frequency bands reduces the required bandwidth per interleaved DAC and therefore it enables the synthesis of greater bandwidth signals in the reconstructed output. Elaborate digital signal processing (DSP) is required to seamlessly stitch together the sub-bands and compensate the errors of the analog signal path, which would otherwise severely degrade the performance of the communication system. Adaptive DSP techniques are required to automatically compensate errors caused by process, voltage, and temperature variations in the technology (e.g., CMOS, SiGe, etc.) implementations of the data converters, and therefore ensure high manufacturing yield. These techniques must operate in background mode to avoid interfering with the normal operation of the communication system. This work introduces an adaptive background compensation scheme for frequency interleaved DACs (FI-DACs). The primary application example is a 128 GBd QAM16 coherent optical transceiver. However, the technique is applicable to other types of communication transceivers, and it can be generalized to arbitrary signals, as long as they are stationary or quasi-stationary and have a wideband continuous spectrum. The key elements of the proposed technique are a MIMO equalizer and the backpropagation algorithm. Numerical simulation results for the aforementioned application example show that the signal to noise and distortion ratio (SNDR) of the FI-DAC is boosted by more than 25 dB when the proposed compensation technique is applied in the presence of typical analog mismatches. Furthermore, the optical signal to noise ratio penalty of the optical transceiver is reduced from 6 dB to 0.1 dB. INDEX TERMS Background calibration, error backpropagation, frequency interleaving DAC, high-speed optical transmitter

Digital Background Compensation of Analog Impairments in Jointly Frequency-Time Interleaved DACs

“…Based on eqs. ( 5), (8), and (9), in Appendix A we show that the output of the TI-DAC with impairments for the q-th quadrature component of the m-th spectral band, can be expressed as a function of the input sequence x[n] with sampling rate…”

“…Some of the analog impairments existing in the implementation of FI are depicted in Fig. 6 and include [8], [9]: a) Distortions, bandwidth limitations, and mismatches in the frequency responses of the electrical paths between (i) the TI-DACs and the mixers and (ii) between the mixers and the output of the JFTI-DAC. These effects are modeled by several linear blocks with frequency responses C (m,q) (jω), and D (m,q) (jω) in Fig.…”

“…Background compensation of FI [8], [9] and TI [10] DACs with M and N SBs and sub-channels respectively, were proposed. In this work, we extend the technique of [10], based on a digital adaptive pre-filtering with a periodic time-varying impulse response to compensate for the impairments in the proposed JFTI-DAC architecture.…”

“…4 ). For more details about the FI-DAC architecture, please see [8], [9] and references therein. An emergent DAC architecture capable of meeting the aforementioned requirements consists of a 2M FI system in which each QSC is based on an N -way time-interleaved sub-DAC, conforming an 2M ×N JFTI-DAC.…”

“…voltage and temperature variations and fabrication deviations, without interrupting the communication. Previous works [8]- [10] address similar problems in FI and TI architectures, but individually. To the best of our knowledge, this is the first work dealing with all the impairments simultaneously.…”