Transmitter-side Digital Signal Processing Research Articles

Decision-directed CPR-assisted IQ imbalance-multiplexing for coherent PON downlink system with access-span length difference

In the 10 GSymbol/s/λ coherent PON downlink system with I- and Q-axis mapping for different optical node units (ONUs), an extra power transmission happens because of the transmission difference between the ONUs. An adaptive IQ imbalance modulation with optimum IQ power loading for two ONUs is introduced at the transmitter-side digital signal processing (DSP). For scaling of the IQ power ratio setting range, DD-CPR with PADE scheme as an accurate frequency offset compensation is used at the receiver-side DSP. We numerically and experimentally show 2.8 dB power saving compared to a conventional balanced QPSK signal using the adaptive IQ imbalanced QPSK signal (θ=π/12) in the case of the 57 km downlink transmission difference between two ONUs.

Real-Time 2.2-Gb/s Water-Air OFDM-OWC System With Low-Complexity Transmitter-Side DSP

Underwater optical wireless communication (UOWC) is of great interest to the academic and the industry community. In this article, we propose a low-complexity and effective joint transmitter-side digital signal processing (DSP) including geometric shaping, time-domain tone reservation (TR), and clipping. The peak-to-average power ratio reduction performance and implementation complexity of the proposed time-domain TR are extensively analyzed. We then develop a real-time 2.2-Gb/s system, which is optimized for its shaping ratio and clipping ratio. With the help of the efficient DSPs, an 8-dB received optical power enhancement is realized under the bit-error-rate threshold of 3.8 × 10−3. We successfully demonstrate a time-multiplexed four 4K video transmission in real-time using the proposed scheme over a 3.6-m underwater and 8-m free-space channel. The implementation details, as well as the analyses of system stability, resource utilization, and latency, are presented. The results validate the feasibility and effectiveness of the proposed scheme and a 2.2-Gbit/s real-time OWC system is demonstrated for a water-air communication link.

86-GBaud subcarrier multiplexed 16QAM signal generation using an electrical 90 degree hybrid and IQ mixers.

We experimentally demonstrate an aggregate 86-GBaud (over three sub-bands and one polarization) signal generation based on subcarrier multiplexing technique using IQ mixers, an electrical 90 degree hybrid, and diplexers. The electrical hybrid allows transmitter-side digital signal processing to be simplified to pulse shaping and digital pre-emphasis. We verified the configuration by testing the performance of an 86-GBaud Nyquist-shaped 16 quadrature amplitude modulation signal with differential bit encoding. The implementation penalty assuming 7% hard-decision forward error correction is reduced to 2 dB by utilizing a 31-tap decision-directed least mean square based multiple-input multiple-output equalizer for sideband crosstalk mitigation.

Electrical spectrum synthesis technique using digital pre‐processing and ultra‐broadband electrical bandwidth doubler for high‐speed optical transmitter

Proposed is an electrical spectrum synthesis technique that converts low-speed signals to a high-speed signal using transmitter-side digital signal processing (DSP) and an ultra-broadband electrical bandwidth doubler. The transmitter-side DSP converts a high-speed signal to low-speed upper and lower sideband signals, down- and up-converts the low-speed signals into baseband signals, and adds and subtracts the in-phase and the quadrature components of the baseband signals. The digital pre-processed low-speed signals output from digital-to-analogue converters are multiplexed to the high-speed signal by the bandwidth doubler, which consists of analogue multiplexers designed and fabricated using in-house indium phosphide heterojunction bipolar transistor technologies. A 120 GBaud quadrature phase shift keying signal has been successfully generated by using the proposed technique as a demonstration of a high-speed optical transmitter.

Enhancement of Signal Performance in bidirectional RSE-based CVLC system employing NPM modulation scheme

Abstract Camera-based visible light communication (CVLC) exploiting rolling shutter effect (RSE) is a cost-effective technique that ensures secure data transmission. In this paper, a bidirectional CVLC system is experimentally demonstrated using smartphone/laptop as downlink/uplink transceiver in an experimental prototype testbed. Meanwhile, a Nyquist pre-distorted Manchester (NPM) modulation scheme is proposed and validated in the experimental testbed system. The proposed scheme is based on low-complexity transmitter-side digital signal processing (DSP) techniques, Nyquist shaping and linear pre-distortion, which can be employed for mitigation of the impact of extinction ratio (ER) fluctuation and exposure overlapping in the RSE-based CVLC system. The experimental results show that by using the proposed NPM modulation, the uplink and the downlink can both achieve about 4.8 Kb/s transmission with bit error rate (BER) of below than 3.8e−3. It verifies the effectiveness of the proposed NPM scheme and the corresponding transmitter-side DSP techniques.

100 Gb/s PAM4 transmission system for datacenter interconnects using a SiP ME-MZM based DAC-less transmitter and a VSB self-coherent receiver.

We experimentally demonstrate a digital-to-analog-converter-less (DAC-less) vestigial sideband (VSB) 4-level pulse amplitude modulation (PAM4) transmission system for data center interconnects (DCIs) using a silicon photonic (SiP) multi-electrode Mach-Zehnder modulator (ME-MZM) based DAC-less transmitter and a VSB self-coherent receiver. The impacts of linear and nonlinear impairments on the proposed system and their mitigation methods are comprehensively studied. By using Kramer-Kronig (KK) detection, frequency domain chromatic dispersion compensation, and short-memory time domain Volterra equalization at the receiver, we report a 112 Gb/s PAM4 transmission over 40 km standard single mode fiber (SSMF) with a bit error rate (BER) below the 7% overhead (OH) hard-decision forward error correction threshold of 3.8 × 10-3, and a 120 Gb/s PAM4 transmission over 80 km SSMF with a BER below the 20% OH soft-decision forward error correction threshold of 2 × 10-2, without any transmitter side digital signal processing such as pre-emphasis and pulse shaping.

Spectral Shaping in Long Haul Optical Coherent Systems With High Spectral Efficiency

One of the main obstacles faced when decreasing channel spacing in optical wavelength division multiplexing (WDM) systems is the linear crosstalk resulting from spectral overlap between neighboring channels. Methods of reducing channel bandwidth and corresponding crosstalk using optical pre-filtering are reviewed along with the limitations of these techniques. High speed transmitter side digital signal processing (DSP) and digital-to-analog converters enable the use of more powerful techniques that precisely shape and limit the bandwidth of the transmitted optical signal. These techniques allow channel spacing to be reduced to nearly the theoretical minimum Nyquist spacing with only negligible crosstalk penalty. We review well known principles of linear modulation theory and apply them to coherent optical transmission systems to describe the desired channel transmit spectra along with the associated transmit and receive DSP. The implementation issues of a typical optical transmitter, which uses these techniques, are discussed. Experimental results of a laboratory long-haul WDM optical system using spectrally shaped PDM-16-QAM type modulation are presented demonstrating near Nyquist channel spacing on a trans-pacific length system with more than 44 terabits of capacity. We also present an additional application of these spectral shaping techniques, which can carefully expand the bandwidth of the transmitted channels to reduce non-linear transmission penalties. Experimental results show the advantages of these spectral broadening techniques and their ability to trade spectral efficiency for increased reach.

Design and Analysis of a CO-OFDM Transmitter With Limited Modulator Extinction Ratio

In this paper, we consider the design and analysis of a coherent optical OFDM (CO-OFDM) transmitter using an optical in-phase/quadrature (I/Q) modulator with limited extinction ratio (ER) under four modulation formats. The transmitter-side digital signal processing (DSP), including hard clipping and predistortion, is introduced to combat the penalty induced by the limited ER of the modulator and the quantization noise of the digital-to-analog converter (DAC). The DAC resolution needed for transmitters using modulators with various ER values are examined. The peak-to-peak driving voltage (Vpp) and the clipping ratio (CR) of the transmitted signals are also optimized with respect to minimizing the Q-factor penalty for the modulator with different ER values. The simulation results show that, unlike previous work using modulators with an infinite ER, the optimum Vpp and CR values vary with different ERs and modulation levels.

Optimization of DSP to Generate Spectrally Efficient 16QAM Nyquist-WDM Signals

We investigate transmitter-side digital signal processing to generate 16-ary quadrature-amplitude-modulation (16QAM) Nyquist wavelength-division-multiplexing (WDM) signals by simulations. We first study required digital finite-impulse response (FIR) filter tap numbers and optimum roll-off factors to generate Nyquist 16QAM WDM signals. We show that a root-raised cosine spectrum with a roll-off factor of 0.1 is optimal for Nyquist 16QAM signals considering the tradeoff between the system performance and complexities, and a digital FIR filter with 37 taps is sufficient to generate such signals. Then we look into the effects of digital-to-analog converter (DAC) sampling speeds on the performance of Nyquist-WDM systems. We show that the electrical filter shape and spectral pre-emphasis have a big impact on the system performance. With a fourth-order super-Gaussian electrical filter and spectral pre-emphasis, DACs with 1.2 samples/symbol sampling speed can generate 16QAM Nyquist-WDM signals at 1.05-symbol-rate channel spacing, with an OSNR penalty below 0.2 dB compared with 2 samples/symbol DACs.

Digital signal processing for coherent multi-level modulation formats (Invited Paper)

This paper presents a review on the recent progress of digital signal processing (DSP) in the high-speed optical transmission system using single-carrier based multi-level modulation formats. The principle of several digital phase and polarization tracking algorithms recently proposed and demonstrated for future spectrally-efficient optical transmission system are discussed in detail. A novel DSP-based interference mitigation algorithm for the single-ended coherent receiver has been included. Recent technology advance on transmitter-side DSP such as digital pulse-shaping, pre-equalization and digital nonlinear compensation has also been discussed.