Mengqi Guo

FOFDM Based on Discrete Cosine Transform for Intensity-Modulated and Direct-Detected Systems

In this paper, three basic intensity-modulated and direct-detected (IM/DD) fast orthogonal frequency division multiplexing (FOFDM) schemes are compared, including Flip-FOFDM, asymmetrically clipped optical FOFDM (ACO-FOFDM) and DC-offset FOFDM (DCO-FOFDM). The modulation/demodulation processing of FOFDM employs the discrete cosine transform. Compared with conventional OFDM based on discrete Fourier transform, FOFDM exhibits lower computational complexity because of its simple one-dimensional modulation and real-valued operation. To the best of our knowledge, Flip-FOFDM is first reported. We analyze the key parameters of three basic IM/DD FOFDM schemes in detail, including spectral efficiency, computational complexity, bit-error-ratio performance, and power efficiency. We categorize ACO-FOFDM and Flip-FOFDM in U-FOFDM. The spectral efficiency of DCO-FOFDM is twice that of U-FOFDM. However, due to the large DC bias of DCO-FOFDM, DCO-FOFDM is less efficient than U-FOFDM in terms of optical power when the bit rate/normalized bandwidth is less than 5, but DCO-FOFDM becomes more efficient than U-FOFDM when the bit rate/normalized bandwidth is greater or equal to 5. FOFDM shows great potential for application in IM/DD systems.

Interleaved single-carrier frequency-division multiplexing for optical interconnects

In this paper, we propose a real-valued interleaved single-carrier frequency-division multiplexing (I-SC-FDM) scheme for intensity-modulation and direct-detection optical interconnects. By simplifying the encoding structure, the computational complexity can be reduced from Nlog2N complex multiplications to N complex multiplications. At the complementary cumulative distribution function of 10-2, a reduction of 10 dB and 7.5 dB for the peak-to-average power ratio (PAPR) of the I-SC-FDM is achieved than that of orthogonal frequency-division multiplexing modulated with QPSK and 16QAM, respectively, when the subcarrier number is set to 4096. We experimentally demonstrate the I-SC-FDM scheme for optical interconnects with data rates of 12 Gbit/s, 24 Gbit/s and 128 Gbit/s transmitted over 22.5-km, 22.5-km and 2.4-km standard single mode fiber, respectively. The I-SC-FDM scheme shows great potential for cost-sensitive and power-sensitive optical interconnects owing to its low computational complexity and low PAPR.

Capacity limit for faster-than-Nyquist non-orthogonal frequency-division multiplexing signaling

Faster-than-Nyquist (FTN) signal achieves higher spectral efficiency and capacity compared to Nyquist signal due to its smaller pulse interval or narrower subcarrier spacing. Shannon limit typically defines the upper-limit capacity of Nyquist signal. To the best of our knowledge, the mathematical expression for the capacity limit of FTN non-orthogonal frequency-division multiplexing (NOFDM) signal is first demonstrated in this paper. The mathematical expression shows that FTN NOFDM signal has the potential to achieve a higher capacity limit compared to Nyquist signal. In this paper, we demonstrate the principle of FTN NOFDM by taking fractional cosine transform-based NOFDM (FrCT-NOFDM) for instance. FrCT-NOFDM is first proposed and implemented by both simulation and experiment. When the bandwidth compression factor α is set to 0.8 in FrCT-NOFDM, the subcarrier spacing is equal to 40% of the symbol rate per subcarrier, thus the transmission rate is about 25% faster than Nyquist rate. FTN NOFDM with higher capacity would be promising in the future communication systems, especially in the bandwidth-limited applications.

Simplified Maximum Likelihood Detection for FTN Non-Orthogonal FDM System

In this letter, we first apply simplified maximum likelihood detection (MLD) with QR decomposition and M-algorithm (QRM-MLD) to reduce inter-carrier interference (ICI) in a faster-than-Nyquist non-orthogonal frequency division multiplexing (FTN-NOFDM) system. A QRM-MLD algorithm has lower computational complexity than an MLD algorithm owing to the decrease of searched path number. In the experiment, 10-Gb/s FTN-NOFDM with the QRM-MLD algorithm was transmitted over 25-km standard single mode fiber. For 20% bandwidth saving, FTN-NOFDM with the QRM-MLD algorithm can achieve almost the same performance as FTN-NOFDM with the MLD algorithm when the path with minimal metric is not discarded before detecting the last subcarrier, meanwhile the bit error rate of FTN-NOFDM with the QRM-MLD algorithm or the MLD algorithm can achieve 7% forward error correction (FEC) limit, but that with an iterative detection (ID) algorithm only can achieve 20% FEC limit. For reducing ICI, the QRM-MLD algorithm has lower computational complexity but can obtain almost the same performance as the MLD algorithm, and the QRM-MLD algorithm also has much better performance than the ID algorithm.

256-QAM Interleaved Single Carrier FDM for Short-Reach Optical Interconnects

In this letter, we experimentally demonstrated a 40-Gbit/s 256-quadrature amplitude modulation (256-QAM) interleaved single-carrier frequency-division multiplexing (I-SC-FDM) over 10-km standard single-mode fiber (SSMF) for optical interconnects. I-SC-FDM has lower peak-to-average power ratio and lower computational complexity of transmitter compared with orthogonal frequency division multiplex- ing (OFDM). High-level QAM can achieve high spectral efficiency; however, it is susceptible to both linear and nonlinear distortions. The frequency-domain one-tap equalizer and the time-domain decision-feedback equalizer with recursive least square and Volterra algorithm are jointly employed to eliminate the distortions for the 256-QAM I-SC-FDM. After 10-km SSMF, I-SC-FDM has better performance than localized SC-FDM and OFDM. Therefore, I-SC-FDM shows the potential for short-reach optical interconnects.

4\( \times \)25-Gb/s Duo-Binary System over 20-km SSMF Transmission with LMS Algorithm

We propose a 4\( \times \)25-Gb/s intensity-modulated direct detection (IM/DD) duo-binary system with 50-GHz channel spacing. Both of the modulator and photodetector (PD) have 10-GHz 3-dB electrical bandwidth. At receiver, least mean square (LMS) algorithm is used to compensate the signal distortion after transmission. After 20-km standard single mode fiber (SSMF) transmission, LMS algorithm improves about 2-dB receive sensitivity at forward error correction (FEC) limit (BER = \({10^{ - 3}}\)) in duo-binary system. With LMS algorithm, duo-binary system has about 5-dB receive sensitivity improvement at FEC limit compared to on-off keying (OOK) system over 20-km SSMF transmission. This paper proposes a feasible scheme for future high-speed passive optical network (PON).

Comparison of the compensation effects of fiber nonlinear impairments with mid-span optical phase conjugation between PDM CO-OFDM system and PDM QPSK system*

The compensation effects of fiber nonlinearity in 112 Gb/s polarization division multiplexing (PDM) coherent optical systems by mid-span optical phase conjugation (OPC) based on four wave mixing (FWM) effect are studied. Comparisons of the compensation results between PDM coherent optical-orthogonal frequency division multiplexing (CO-OFDM) system and the single carrier (SC) PDM quadrature phase shift keying (QPSK) system are provided as well. The results demonstrate that nonlinear compensation effect with mid-span OPC in PDM CO-OFDM system is much more obvious than that in SC PDM QPSK system.