A general overview of the optical double-sideband with suppressed carrier (DSB-SC) modulation technique and the digital signal processing (DSP)-based optical transmission systems is given in this chapter.
DSB-SC is a modulation technique which could be implemented simply by using a standard Mach-Zhender modulator (MZM) biased at a minimum transmission point of its transfer curve and driven with an electrical tone signal. Various applications has been published in using this modulation technique. In microwave distribution networks, the photonic generation, distribution and transmission of microwave signals has been proposed [1], as shown in Fig. 1.1. In this network, A microwave signal at frequency 2fc is generated and transmitted through optical fiber with low propagation loss using the DSB-SC scheme; moreover, the same scheme could also be used to simultaneously upconvert wavelength division multiplexing (WDM) signals [2], as shown in Fig. 1.2. These WDM signals are modulated with basedband data and the received signals are ASK modulated at microwave frequency 2fc. In the full duplex radio-over-fiber systems [3], wavelength reuse could also be achieved by separating these two sideband through optical filter, modulating one of them, and leaving the unmodulated carrier as the upstream carrier. In all-optical subcarrier labeling based networks [4], the payload carrier is suppressed and the label is modulated over the two sideband. It should be noted that a limited extinction ratio of the payload is required in order to correctly detect the label, as shown in Fig. 1.3.
With the rapid development of complementary metal–oxide–semiconductor (CMOS) digital technology, high speed digital signal processing (DSP) has been recently proposed to be used at both the transmitter and receiver side of optical transmission systems. A typical coherent and DSP-based optical receiver is shown in Fig. 1.4. It can be seen that this typical configuration consists of a local laser diode, a hybrid coupler, multiple photo-detectors, multiple high-speed analog-to-digital converters (ADCs), and a high-speed DSP to carry out carrier recovery, timing recovery and equalization [5]. Various DSP-based data recovery and equalization algorithm have been studied in the literature for both wireless and wireline radio-frequency (RF) transmission systems [33], [34]. Depending on the utilization of the demodulated data sequence in the synchronization process, the synchronization
schemes could be categorized into decision-directed or data-aided (DD/DA) and non-data-aided (NDA). The structure of the synchronizer could be further classified into feedforward (FF) and feedback (FB), according to how the timing/phase estimates are extracted from the received signal. Other applications of DSP in optical transmission systems include optical orthogonal frequency division multiplexing (OFDM) transmission systems [8], equalization based on maximum likelihood sequence estimation (MLSE) [6], and electronic predistortion [7].
A critical block in DSP-based optical transmission systems is the high speed ADC.
To achieve a practical solution, an interleaved array of ADCs sampling at lower rate can be employed. By breaking a high speed ADC into parallel low speed ADCs, and independently adjusting the timing phase, gain, offset and frequency response of each low speed ADC, the same resolution is achieved without loss of performance at high speed using the digital calibration techniques [6]. Generally speaking, DSP have been used to relax the component requirement, and extend the dispersion-limited transmission distance.
For traditional coherent receivers, homodyne or heterodyne demodulation scheme uses an optical or electrical phase-locked loop (PLL) that synchronizes the phase and frequency of the local oscillator with the transmitter laser. However, there are limitations on the PLL loop bandwidth, delay and the tracking error, and it has been shown that the PLL-based receivers have a stringent requirement on laser linewidth.
With high speed ADCs, the carrier phase can be estimated and tracked by DSP in a feedforward architecture. Recent theoretical and experimental results have shown that the DSP-based coherent receivers with feedforward phase estimation algorithms are more tolerant to laser phase noise than PLL-based receivers [9]. A real-time experiment for 800 Mb/s QPSK transmission based on field-programmable gate array (FPGA) has been demonstrated [10].
Optical orthogonal frequency division multiplexing (OFDM) transmission systems, which have become very popular to achieve a high dispersion-tolerant performance in recent years, rely heavily on DSP technology [8]. OFDM splits a high-speed data stream into a number of low-speed data streams that are transmitted simultaneously over a number of harmonically related narrowband subcarriers. To efficiently implement the multi-carrier modulation, DSP is employed in the transmitter and receiver for the purpose of achieving digital Fourier transformation. Both coherent and direct detection schemes have been proposed and analyzed to implement the optical OFDM transmission systems[8][13]. It should be mentioned that the major
drawbacks of the OFDM systems are (1) the requirement on system linearity, due to the high peak-to-average power ratio (PAPR) of the multi-carrier signals and (2) sensitivity to synchronization parameter, including timing, phase, and frequency errors[44]; therefore, pilot-aided synchronization strategies are usually employed[8], [13].
To compensate the effect of chromatic dispersion (CD) and polarization mode dispersion (PMD), electronic equalization schemes are considered because they are more flexible and cost-efficient than optical equalization schemes. An MLSE receiver provides robust performance in the presence of inter-symbol interference (ISI) by choosing the bit sequence that maximizes the logarithm of the likelihood function [6].
With the efficient realization of Viterbi algorithm, the technical implementation of MLSE receivers at 10 Gb/s is currently commercially available [11].
Another efficient electronic dispersion compensation scheme is electronic predistortion. With the use of a dual-drive MZM or a Cartesian (triple) MZM, real and imaginary part of the predistorted field samples are calculated from the bit pattern in a DSP according to the channel response, and after photodetection, the original electrical signal waveform is recovered [7]. The performance of the electronic predistortion is better than receiver-side equalization schemes with direct detection, because the field signal is processed. An FPGA-based optical transmitter design has been demonstrated [12].
Network protection at the optical layer is generally required to increase the reliability and decrease the downtime of an optical network, especially for transparent optical networks. Conventionally, in the metro/access area, a ring topology with two fibers is usually considered for protection against fiber failure. The common optical fiber protection scheme in the dual-fiber ring networks is optical unidirectional path-switched ring (O-UPSR) [14]. To reduce the number of optical fibers and optical components, single-fiber self-healing ring with bidirectional optical add/drop multiplexers (BD-OADM) has recently been proposed. However, the number of the transmitters and receivers are doubled in these proposed schemes [38].
The dissertation is organized as follows. In chapter 2, we discuss the application of the DSP technology in a remote heterodyne detection system based on DSB-SC modulation technique. In chapter 3, we propose a single-fiber ring architecture to further reduce the number of optical transponders with the application of DSB-SC modulation technique. Chapter 4 concludes the dissertation.
Fig. 1.1 Block diagram of photonic generation, distribution and transmission of microwave signals. Microwave signal at frequency twice of the modulating signal is generated.
Fig. 1.2 Block diagram of simultaneously upconversion of wavelength division multiplexing (WDM) signals. These WDM signals are on-off keying (OOK) modulated and the received signals are amplitude-shift keying (ASK) modulated at microwave frequency 2fc.
Fig. 1.3 Block diagram of label swapping function, the payload carrier is separated, branched, and suppressed. The label is modulated over the two sideband. It should be noted that a limited extinction ratio of the payload is required in order to correctly detect the label.
Fig. 1.4 A typical coherent and DSP-based optical receiver. LO: local oscillator, BPD:
balanced photo-diodes, ADC: analog-to-digital converter, DSP: digital signal processor. This typical configuration consists of a local laser diode, a hybrid coupler, multiple photo-detectors, multiple high-speed analog-to-digital converters (ADCs), and a high-speed DSP to carry out carrier recovery, timing recovery and equalization.