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Modulation and demodulation
Published in Geoff Lewis, Communications Technology Handbook, 2013
256-QAM systems giving eight bits per transmitted symbol are already in limited use and 1024-QAM is being researched. Figure 22.6 indicates the way in which 16-QAM signals are demodulated. The carrier recovery circuit ensures that the local carrier oscillator is locked in frequency to the incoming signal. In-phase and quadature versions of this signal are applied to the I and Q ring demodulator stages along with the input modulated signal. After bandpass filtering, the signals at point A have an analogue form but of discrete amplitudes. Synchronism for the data rate clock circuit is also obtained from point A. The two data slicer circuits recover the original digitally formatted signal and this is then progressively decoded (points A, B, C and D) to provide the original bit stream.
Optical Coherent Detection and Processing Systems
Published in Le Nguyen Binh, Advanced Digital, 2017
As has been observed, in synchronous detection, a carrier recovery circuit is necessary and is usually implemented using a PLL, which complicates the overall receiver structure. It is possible to detect the signal by a self-homodyne process by beating the carrier of one-bit period to that of the next consecutive bit; this is called differential detection. The detection process can be modified as shown in Figure 5.7, in which the phase of the IF carrier of one bit is compared with that of the next bit, and a difference is recovered to represent the bit 1 or 0. This requires differential coding at the transmitter and an additional phase comparator for the recovery process.
Introduction to Analogue Modulation
Published in Sibley Martin, Modern Telecommunications, 2018
It is clear from Equation 4.16 that the baseband signal has been recovered and that it has had its amplitude increased by the amplitude of the local oscillator. Note also that there is a useful dc term that depends on the received carrier voltage. This can be used in an automatic gain control system, which helps when you are listening to the radio in the car and the signal fades due to obstructions. Carrier recovery can be achieved by using a phase-lock loop (PLL). There are problems associated with mixing down to the baseband: the carrier frequency and phase must be correct.
Ultrafast All Optical Parity Generator and Checker based on Quantum Dot Semiconductor Optical Amplifier
Published in IETE Journal of Research, 2023
Optical logic gates can be designed by exploiting the nonlinear characteristics of several optical devices [10]. The most commonly used nonlinear devices are semiconductor optical amplifiers and highly nonlinear fiber (HNLF). In 2005, Bogoni et al. [11] proposed reconfigurable and regenerative all-optical logic gates using nonlinear optical loop mirrors (NOLM). Olsson et al. [12] designed an all-optical AND-gate using randomly birefringent fiber in a nonlinear optical loop mirror. Agarwal et al. [13] presented XOR logic based on cross-phase modulation in SOA and demonstrated all-optical encryption-decryption circuit at 120 Gb/s. In 2016 Pallavi et al. [14] realized several combinational circuits which include an all-optical half-adder, half-subtractor and 4-bit decoder. These circuit’s truth tables were verified and the proposed circuits were based on SOA-MZI configuration at 10 Gb/s. SOA is considered to be better than other known nonlinear elements like highly nonlinear fiber due to its advantages like compact size, high non-linearity, low power consumption and ability to be integrated with other photonic devices [15]. Despite these advantages, conventional SOA has a major drawback of slow carrier recovery i.e. in the range of several hundred pico-seconds. Slow carrier recovery is an obstacle for high data rate applications as carriers are unable to recover at the pace of varying intensity-dependent pulses and lead to unequal amplification of input pulses resulting in pattern effect distortion [15]. To overcome this problem researchers have resorted to quantum dot semiconductor optical amplifiers as it has gained recovery response in the range of 300fs-10ps [16]. In the QD-SOA model carriers are inserted in the wetting layer and move into the dots. The presence of a gap between the QD levels, wetting layer and cross-section area with carrier photon interaction causes reduced carrier relaxation times and smaller gain saturation [17]. Kotb et al. [18] simulated the XNOR gate based on QD–SOA in the presence of amplified spontaneous noise at an ultrafast data rate of 250 Gb/s. They reported satisfactory quality factors at low input pulse power. Komatsu et al. [19] demonstrated all-optical digital comparators using quantum-dot semiconductor optical amplifiers at 160 Gb/s with a quality factor over 9 and an extinction ratio over 10 db. Recently Fouskidis et al. [20] used a single quantum-dot semiconductor optical amplifier (QD-SOA) and a concatenated optical filter to implement all-optically multiple logic gates AND, NOR, OR and NOT at 160 Gb/s. By proper detuning of the optical filter, specific logic gate output can be obtained without changes in its basic setup or driving conditions.