Automatic digital pre-compensation in IQ modulators

We consider digital signal processor (DSP) based inphase/quadrature (IQ) modulators generating continuous phase frequency shift keying (CPFSK) signals. Departures from a flat-magnitude, linear phase frequency response in the pass bands of signal reconstruction filters in the I and Q channels cause ripple in the modulator output signal envelope. Amplitude modulation (AM) in the signal envelope function produces undesirable sidelobes in the FSK signal spectrum when the signal passes through nonlinear elements in the transmission path. A structure was developed for digitally compensating for the magnitude and phase characteristics of the signal reconstruction filters by Tuthill and Cantoni (see IEEE Trans. Comm., vol.47, no.10, p.1466-69, 1999) and by Tuthill, (see Australian PhD Dissertation, Telecommunications Research Institute, Curtin University of Technology, 2000). However, this approach results in finite impulse response (FIR) filters that have a large number of coefficients. Implementation of these filters on a DSP is computationally demanding. In this paper we have generalised the technique of Tuthill and Cantoni to include infinite impulse response (IIR) filters, which have fewer coefficients. We then investigate whether a reduction in the number of filter coefficients leads to a reduction in the computational load in implementation.

A combined source/channel coding and modulation technique is developed for the transmission of memoryless sources over an AWGN channel. Combined source/channel coding is implemented using the concepts of trellis coded quantization (TCQ) in conjunction with an Ungerboeck code defined for a continuous phase frequency shift keying (CPFSK) signaling scheme. The Viterbi algorithm is employed by the source encoder to find a minimum mean-square-error (MSE) path through the source code trellis. The resultant minimum distortion sequence is mapped to an appropriate set of CPFSK symbols for transmission. A mapping is pragmatically developed to simultaneously preserve the original branch labelings of both the source and channel code trellises. Source symbol recovery is performed by maximum likelihood sequence estimation. The total distortion is experimentally determined over a range of channel signal-to-noise ratios. Furthermore, it is demonstrated that the use of a CPFSK signaling scheme avoids the production of spectral line components in the transmitted waveform spectrum that are usually associated with joint source/channel coding schemes.

Designing communication techniques for non-Gaussian impulsive noise has been attracting considerable attention from the research community. In this paper, we focus on the continuous phase frequency shift keying (CPFSK) signal detection for both white and bursty impulsive noises. The independent and identically distributed (i.i.d.) symmetric $\alpha $ -stable ( ${S\alpha S}$ ) process and the stationary $m$ -order $\alpha $ -sub-Gaussian [ ${{\alpha \mathit {SG}}}(m)$ ] model are adopted to model the white and the bursty impulsive noises, respectively. At first, both coherent and non-coherent detections of CPFSK signals are investigated for white ${S\alpha S}$ noises. For the coherent sequence detection, the Viterbi algorithm is used by replacing the maximum likelihood (ML) measure with the myriad measure. In non-coherent detection, a multiple symbol aided scheme is proposed, for which both the ML and myriad measures are considered. Subsequently, the detection of CPFSK signals is further addressed in ${{\alpha \mathit {SG}}}(m)$ noise. For coherent detection, based on the Markov property of ${{\alpha \mathit {SG}}}(m)$ noise, we first express the likelihood function as the sum of branch measures, and the myriad measure is also provided to replace the ML measure. The multiple symbol-aided methods are also applied to the non-coherent detection in ${{\alpha \mathit {SG}}}(m)$ noise, and the corresponding ML and myriad measure-based algorithms are given as well. For the sake of comparison, the Gaussian measure is also considered. The simulation results show that the performance of the myriad measure-based algorithms can closely approach that of the ML measure-based algorithms for both coherent and non-coherent detection, and is much better than that of the Gaussian measure-based algorithms.

High-speed PAM-4 is a promising way for future low-cost short-reach fibre communications. However, for 1550 nm transmission with the lowest attenuation for standard single-mode fibre (SSMF), the fibre dispersion-induced inter-symbol interference (ISI) becomes non-linear in direct detection. Dispersion pre-compensation at the transmitter side is necessary to avoid such non-linear ISI. Different from previous methods to generate the vector signal from dispersion pre-compensation using IQ modulators, in this study a digital dispersion pre-compensation (DDPC) scheme based on an intensity modulator (IM) plus a phase modulator (PM) is studied. Taking into account the digital-to-analogue converter (DAC) bandwidth limit on the vector signal generation, anti-aliasing filtering on both the magnitude signal and the phase signal should be applied. The error rate performance of the IM-PM based DDPC scheme is analysed in detail using numerical simulations. For 50 Gb/s PAM-4 transmission at 1550 nm over SSMF, over 50 km fibre transmission can be supported if the two DACs have enough bandwidth larger than 17.5 GHz. If the DAC bandwidth is limited to 15 GHz, it is also possible to transmit over 20 km SSMF. Meanwhile, it is verified that practical 5-bit DAC resolution is enough for the IM-PM based DDPC scheme to work for a wide range of SSMF lengths.

An efficient method is proposed for the design of finite impulse response (FIR) filter with arbitrary pass band edge, stop band edge frequencies and transition width. The proposed FIR band stop filter is designed using craziness based particle swarm optimization (CRPSO) approach. Given the filter specifications to be realized, the CRPSO algorithm generates a set of optimal filter coefficients and tries to meet the ideal frequency response characteristics. In this paper, for the given problem, the realizations of the optimal FIR band pass filters of different orders have been performed. The simulation results have been compared with those obtained by the well accepted evolutionary algorithms, such as Parks and McClellan algorithm (PMA), genetic algorithm (GA) and classical particle swarm optimization (PSO). Several numerical design examples justify that the proposed optimal filter design approach using CRPSO outperforms PMA and PSO, not only in the accuracy of the designed filter but also in the convergence speed and solution quality.

This paper presents an alternative approach for the design of linear phase digital high pass FIR filter using Particle Swarm Optimization with Constriction Factor and Inertia Weight Approach (PSOCFIWA). FIR filter design is a multi-modal optimization problem. The conventional gradient based optimization techniques are not efficient for digital filter design. Given the filter specifications to be realized, PSO algorithm generates a set of optimal filter coefficients and tries to meet the ideal frequency characteristic. In this paper, for the given problem, the realization of the optimal FIR high pass filters of different orders has been performed. The simulation results have been compared to those obtained by the well accepted evolutionary algorithm such as genetic algorithm (GA). The results justify that the proposed optimal filter design approach using PSOCFIWA outperforms GA, not only in the accuracy of the designed filter but also in the convergence speed and solution quality.

A comparison of the energy operator and the Hilbert transform approach to signal and speech demodulation

This paper presents a fast and autonomous injection frequency tracking and locking technique. In the present injection-locking system, a quadrature injection-locked oscillator (QILO) is third harmonically locked to a quadrature voltage-controlled oscillator (QVCO). In the frequency tracking loop, the frequency difference between QVCO and QILO is extracted using the QILO's amplitude modulated (AM) envelope waveform. The AM frequency of the envelope signal bears frequency difference between the two oscillators. The envelope signal is further converted to pulse signal which subsequently drives digital feedback control circuitry to update the QILO's output frequency so that it can track the third harmonic of the injection QVCO. The frequency calibration process is purely autonomous, self-initiating whenever the AM modulated envelope waveform is generated. The feedback signal is primarily processed in the digital domain, resulting in a compact, fast, and power-efficient injection frequency-locked loop (IFLL). By incorporating a phase noise (PN) calibration routine after completing the frequency calibration, the presented IFLL resolves the intractable issue of PN degradation at the edge of the slave oscillator's injection locking range. This results in a large injection frequency tracking range of 26.5-29.7 GHz which is only limited by the QILO's LC tank tuning range. The IFLL realized in 0.13-μm CMOS consumes 2.4 mW with a negligible area penalty. Overall chip size including QVCO, QILO, and IFLL is 1 × 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

In the conventional unequal-split Wilkinson power divider, poor selectivity for each transmission path is usually a problem. To surmount this obstacle, the parallel coupled-line bandpass filter structure is utilized as an impedance transformer as well as a band selector for each transmission path in the proposed unequal-split Wilkinson power dividers. However, the bandpass filters in the proposed dividers require careful design because they may not be functional under certain conditions. For example, the odd-order coupled-line filters are not appropriate for impedance transformers in the proposed unequal-split dividers and high-isolation requirement. Using the even-order coupled-line filter transformers, this study proposes two types of unequal-split Wilkinson power dividers. The first type of the proposed dividers arranges two filter transformers near two output ports, respectively, and is capable of achieving a highly remarkable isolation between the two output ports and a good band selection in each transmission path. Specifically, not only the operating band but also the lower and higher stopbands can achieve highly favorable isolation for this type of divider. By arranging the load impedance of each port properly, the second type of the proposed dividers, which has only one filter transformer to be shared by each transmission path near the input port, is also proposed to provide effective isolation between two output ports and favorable selectivity in each transmission path.

In the scenario of downstream Passive Optical Networks (PON), we analyze through simulations and experiments a 100 Gbps/λ link using digital signal processing (DSP) over up to 50 km single mode fiber (SMF) in C-band. In particular, we propose chromatic dispersion digital pre-compensation (CD-DPC) and quaternary pulse amplitude modulation (PAM-4) levels optimization at the transmitter side (TX), and simple non-linear compensation (NLC) in combination with adaptive equalization (AEQ) at the receiver side (RX). Regarding NLC, we compare two approaches: a quadratic polynomial function and a square-root-like function respectively. In this paper, we analyze in detail the performances of four proposed options, namely equispaced PAM-4 levels without NLC (baseline case), equispaced PAM-4 levels in combination with NLC, optimized PAM-4 levels without NLC, and optimized PAM-4 levels in combination with NLC. We demonstrate through simulations and experiments that optimized PAM-4 levels can only offer limited enhancement when NLC is applied, and up to 3.3 dB sensitivity gain can be obtained thanks to NLC at RX when setting the optimum parameters, with respect to the baseline case.

A novel identification approach based on an input-holding scheme is proposed for a class of nonlinear system which consists of a nonlinear element followed by a linear dynamical system. By only using an observed system output where the test input signal is held during a multiple of the output sampling interval, the parameters of the transfer function of the linear part can be identified first. Then the intermediate input between the nonlinear part and the linear part is reconstructed by using the estimated parameters of the linear part. Finally, the nonlinear element is identified by using the test signal and the reconstructed intermediate input. It is clarified that the consistency of the parameter estimates of the linear part is assured under some specified assumptions. The effectiveness of the proposed algorithm is examined in identification of a Hammerstein system which includes an unknown continuous and/or discontinuous nonlinearity such as a backlash.

Some short-term fading characteristics of a type of ionospheric propagation first described by Bailey, et al. are presented and discussed. Continuous wave transmissions from Cedar Rapids, Iowa, on 49.8 mc were received at Sterling, Virginia, on either rhombic or Yagi antennas directed toward the transmitter. The second detector output of a linear receiver was used to provide a voltage proportional to the envelope of the received signal. This fluctuating voltage was recorded, edited, and then analyzed to determine the probability distribution and autocorrelation function of the signal envelope. The correlation of the envelopes of signals received on spaced antennas was similarly determined for antennas spaced both normal and parallel to the transmission path.

This paper presents the modeling and optimization of compact microwave bandpass filters whose compactness leads to complex and strong stray coupling paths, thereby making the identification of a simple and sparse coupling topology difficult and even impossible. The strong coupling coefficients needed for a broadband response can also cause an ambiguity in identifying the spatial extent of local resonances. An equivalent circuit, which is extracted directly from Maxwell's equations, is used in optimizing these filters. The filter is represented by its global resonances instead of individual resonators. The extraction of the parameters of the equivalent circuit is carried out in the physical frequency and not in the normalized frequency in order to preserve the physicality of the equivalent circuit, especially for asymmetric responses. The technique is successfully applied to the optimization of second-order suspended stripline bandpass filters with one transmission zero either below or above the passband, as well as fourth-order filters with three transmission zeros. A fourth-order filter with three transmission zeros is fabricated and measured.

A novel approach for designing a narrow dual Bandpass Filter (BPF) using microstrip Coupled-Line Resonator (CLR) is presented in this paper. The proposed CLR consists of butterfly radial stub and 45° mitred bending with new 'bell'-shaped structure. The proposed BPF consisting of two fundamental resonant modes with the resonant characteristic has been investigated using ADS software. To validate the design and analysis, the dual BPF was fabricated and measured. It is shown that the measured and simulated performances are in good agreement. A dual-band response BPF that operates at 2.4 GHz and 5.75 GHz is designed and implemented for wireless applications.

The Costas loop can be considered an extension of the phase-locked loop (PLL). The PLL, invented 1932 by French engineer Henri de Belleszice, was first used for the synchronous reception of amplitude modulated radio signals with carrier (c). If the modulation index is less than 100%, the modulated signal is always in phase with the carrier signal (c), irrespective of the polarity of the modulating signal. The PLL then reconstructs a local carrier (loc) that is in phase the carrier (c) of the AM signal. The modulating signal is recovered by multiplying the AM signal with the local carrier (c) with subsequent lowpass filtering. This method of synchronous reception fails, however, when an AM signal with suppressed carrier is transmitted. Here the modulated signal is in phase with the (suppressed) carrier when the modulating signal is positive, but it is in antiphase with the carrier when the modulating signal is negative. When this AM signal is fed to the input of a PLL, it would create a local carrier (loc) that is in phase with the carrier when the modulating signal is positive, but in antiphase with the carrier when the modulating signal is negative. Recovery of the modulating signal is therefore not possible. The situation would be the same when the transmitted signal is a Binary Phase Shift Keying (BPSK) signal, a Quadrature Phase Shift Keying (QPSK) signal, or a Quadrature Amplitude Modulation (QAM) signal. This problem has been solved by the invention of the Costas loop by American engineer James P. Costas in 1956. The Costas loop is able to lock onto a modulated signal also in cases where the carrier is suppressed. When we compare the topology of a Costas loop with that of a PLL, we recognize that the PLL is built up from a cascade of three circuits: (1) a phase detector, (2) a loop filter, and (3) a voltage-controlled oscillator (VCO). There is a feedback path from the output of the VCO to the input of the phase detector. In case of the PLL, the input signal is propagated through this cascade of three blocks. There is only one signal path in the PLL. In contrast, there are two signal paths in the Costas loop: (1) a so-called “I branch” (in phase), (2) a so-called “Q branch” (quadrature phase). It is shown that with that configuration the loop is able to create a local carrier that is correctly locked to the carrier of the received signal even in cases when the carrier is suppressed. The invention of the Costas loop had a dramatic impact on any kind of communication receivers; one or more Costas loops are found in TV, radio, satellite communications, mobile phones, modems, etc.