Analog-to-Digital Conversion Using Sub-Nyquist Sampling Rate in Flexible Delay-Division Multiplexing OFDMA PONs

This paper details a delay-division multiplexing (DDM) scheme for orthogonal frequency-division multiple access (OFDMA) passive optical networks (PONs). Unlike detection in a conventional OFDMA PON, the detection of OFDM signals in the proposed scheme involves an analog-to-digital converter (ADC) operating at a sampling rate below the Nyquist rate for each optical network unit (ONU). Based on preallocated relative time delays among the ONUs, preprocessed signals sent from an optical line terminal can be detected as different downstream signals following spectral aliasing caused by ADCs operating at a sub-Nyquist sampling rate. It should be noted that the proposed scheme reduces the requirement of sampling rate, but not the analog bandwidth of an ADC, such that all sent subcarriers can be retained before spectral aliasing. Thus, the proposed DDM scheme can reduce computational complexity and power consumption in the demodulation process, leading to an improvement in cost efficiency. In experiments, we demonstrate that each ONU with an ADC operating at 1/2-1/32 of the Nyquist sampling rate is able to receive 1/2-1/32 of the downstream data, with an insignificant performance penalty. Furthermore, in the DDM scheme, the sampling instant of an ADC is a key parameter in the selection of required data. This paper also proposes a method for estimating the adjustments required in the sampling instant at ONUs. The proposed method is based on the use of deviations in the channel response, which provides the ability to identify with a high degree of precision the adjustments required for an ADC with sub-Nyquist sampling rate. Estimation error associated with the necessary adjustments has only negligible effects on performance, thereby confirming the reliability of the proposed method.