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In-depth exploration of the coupled and coordinated relationship among regional digital economy (DE), technological innovation (TI) and ecological environment (EE) is a vivid embodiment of implementing sustainable development. In order to show the spatiotemporal features of the coupling coordination degree (CCD) from 2011 to 2020, this study builds an evaluation index system from three target levels, namely, the digital economy DE, science and technology innovation TE, and ecological environment EE. Based on this, global and local spatial econometric models, namely the global Moran's I index and the spatio-temporal geographically weighted regression (GTWR) model, are used to identify the spatio-temporal heterogeneity features of each explanatory variable on the CCD. The study results include: (1) The comprehensive evaluation index shows a rising trend, but the development is uneven among systems. (2) The CCD continues to rise steadily, and the regional disparity is widening; the transformation from the near-disorder level to the primary coordination level is realized over the research period. Spatially, the coupling coordination is higher in the eastern and southern regions, while the western and northern regions are relatively low. (3) The GTWR model demonstrates that human capital, urbanization rate, and openness to the outside world promote the CCD. In contrast, the social unemployment rate inhibits CCD, among which human capital is the main force behind coupled and coordinated development.

We propose and demonstrate via both simulation and experiment a scheme for a low-bit resolution digital-to-analog convertor (DAC) to improve the signal-to-quantification noise ratio (SQNR) of an intensity-modulated direct-detection (IM-DD) orthogonal frequency division multiplexing (OFDM) signal. In this scheme, the discrete output levels of a DAC are nonuniform and properly assigned based on the distribution of the OFDM signal. Such nonuniformity allows a better exploitation of output levels for the OFDM signal so that low-level waveforms that appear much more frequently experience mitigated quantization noise. To calculate these optimized output levels with the distribution of an OFDM signal, a numerical algorithm is proposed and verified. Our proposed scheme is proven effective in the simulations for Gaussian-distributed signals and brings over 3.6-dB and ~2.5-dB improvements on SQNR and receiver sensitivity, respectively. The feasibility is also verified by a proof-of-concept experiment with ~7.4-Gbps quadrature phase shift keying (QPSK)-modulated OFDM signals output by a 3-bit DAC. Our scheme enables transmission of such signals with no further optimization with ~-25-dBm receiver sensitivity, while it is impossible using a conventional way. Both simulation and experimental results validate our scheme as a promising way to be implemented in low-bit resolution DACs for IM-DD OFDM applications.