Carrier detection of PSK signals

This paper reports on a theoretical study of the detection of the 2f and 4f carrier components of phase-shift keying (PSK) signals produced by passing signal and noise through a nonlinear device. Unbalanced quadrature PSK (QPSK) signals, i.e., QPSK signals with unequal power in the two channels, are studied. The complete range of channel power ratio is covered, with equal emphasis on the general unbalanced case and the two limiting cases of binary PSK and (balanced) QPSK. Analytic expressions are derived for the detected SNR of carrier harmonics 2f and 4f as a function of SNR, channel power ratio, and normalized input bandwidth. The results apply equally well to PSK data signals and direct-sequence spread-spectrum signals. Measurements confirm every aspect of the theory. The least detectable signal type is balanced QPSK, which is detectable (at 4f) at a threshold SNR ranging from -2 to -13 dB as the detection process gain (chip or data rate/detection bandwidth) is varied from 40 to 80 dB     

Operation of diode laser pumped Tm3+ ZBLAN upconversionfiber laser at 482 nm

Lasing at 482 nm is observed in Tm³⁺-doped ZBLAN glass fiber pumped with single-mode InP semiconductor diode lasers. Up to 5 mW of 482 nm light is obtained with <40 mW of absorbed pump power from a single 1135 nm pump diode laser. The optimum pump wavelength is measured to be 1135-1340 nm. More efficient laser operation is observed in fiber with 2500 ppm Tm³⁺ compared to 1000 ppm Tm³⁺ because of the reduced length of the fiber laser cavity possible with increased doping. Improved slope efficiencies are also demonstrated when the fiber laser is co-pumped with up to 5 mW from a 1220 nm diode laser. The relative intensity noise (RIN) of the fiber laser displays a maximum of -90 dB/Hz at relaxation oscillation frequencies of a few tens of kHz. The measurement of RIN is limited by shot-noise of -152 dB/Hz above 2 MHz. At higher frequencies, self mode-locking was observed in the fiber laser, which may indicate the existence of saturable absorbers in the fiber core. The presence of such bleachable absorbers is indicated by the observed increase in threshold after upconversion lasing at 482 mm     

External optical feedback effects on intensity noise ofvertical-cavity surface-emitting lasers

The authors have experimentally evaluated the effect of external optical feedback on the relative intensity noise (RIN) of a vertical-cavity surface-emitting laser (VCSEL) diode. In the absence of optical feedback, the smallest RIN is found to be -135 dB/Hz. For optical feedback levels approaching -25 dB, the RIN is degraded by about 20 dB. The authors have measured feedback-induced power penalties in a 500-Mb/s intensity-modulation/direct-detection (IM/DD) link and determined that the penalty exceeds 1 dB if the optical feedback ratio is larger than -30 dB     

The effects of constellation density on trellis-coded modulation infading channels

Dense constellations such as 16-QAM (quadrature amplitude modulation) have not seen much use in mobile communication because of their greater peak-to-average power ratio and their seemingly greater sensitivity to noise and channel interference. It is demonstrated that dense constellations can actually improve performance. Using a completely analytical method, three constellations are compared with the same net throughput of 2 bits/symbol: uncoded QPSK (quadrature phase-shift keying), rate 2/3 TCM (trellis-coded modulation) 8-PSK and rate 1/2 TCM 16-QAM. Comparison on the basis of average power puts TCM 16-QAM 5-dB ahead of TCM 8-PSK (phase-shift keying). Even comparison on the basis of peak power gives TCM 16-QAM a 2.44-dB advantage over TCM 8-PSK. QPSK is much poorer than either     

CD3-OFDM: a novel demodulation scheme for fixed and mobilereceivers

This paper describes a novel channel estimation scheme identified as coded decision directed demodulation (CD3) for coherent demodulation of orthogonal frequency division multiplex (OFDM) signals making use of any constellation format [e.g., quaternary phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM]. The structure of the CD3-OFDM demodulator is described, based on a new channel estimation loop exploiting the error correction capability of a forward error correction (FEC) decoder and frequency and time domain filtering to mitigate the effects of noise and residual errors. In contrast to the conventional coherent OFDM demodulation schemes, CD3-OFDM does not require the transmission of a comb of pilot tones for channel estimation and equalization, therefore yielding a significant improvement in spectrum efficiency (typically between 5-15%). The performance of the system with QPSK modulation is analyzed by computer simulations, on additive white Gaussian noise (AWGN) and frequency selective channels, under static and mobile reception conditions. For convolutional coding rate 1/2, the results indicate that CD3-OFDM allows one to achieve a very fast adaptation to the channel characteristics in a mobile environment (maximum tolerable Doppler shift of about 80 Hz for an OFDM symbol duration of 1 ms, as differential demodulation) and an E_b /N₀ performance similar to coherent demodulation (e.g., E_b/N₀=4.3 dB at bit-error rate (BER)=2·10 ^-4 on the AWGN channel). Therefore, CD3-OFDM can be suitable for digital sound and television broadcasting services over selective radio channels, addressed to fixed and vehicular receivers     

Capacity of QAM SCM systems utilising optically linearisedMach-Zehnder modulator as transmitter

The limitation imposed by nonlinear distortion and Gaussian noise on the capacity of a subcarrier multiplexed system with quadrature amplitude modulated channels and an optically linearised Mach-Zehnder externally modulated laser transmitter is calculated. A typical system can support 140 6 MHz quadrature phaseshift keying channels (an aggregate bit rate of 1.4 Gbit/s) at 10^-9 bit error rate with a received power of -25 dBm or 300 64-QAM channels (8.9 Gbit/s) with a received power of -15 dBm     

An adaptive receiver for the time- and frequency-selective fadingchannel

An adaptive receiver is presented in this paper for the reception of linearly modulated signals transmitted over a time- and frequency-selective fading channel. The channel is modeled as a truncated power series which represents the dispersive fading channel as a sum of three elementary flat-fading channels. The proposed receiver consists of a sequence estimator with a parallel channel estimator. The channel estimator recovers the instantaneous fading processes associated with each elementary channel and filters them to generate one-step predictions of each fading process. Some implementation difficulties and solutions are also discussed. Computer simulations using quadrature phase-shift keying (QPSK) and channels with moderate delay spreads and fade rates have been used to evaluate the performance of the receiver. The results show that our technique has potential in channels with delay spread of about 20%, signal-to-noise ratio (SNR) greater than 15 dB, and applications requiring bit-error rates (BER's) less than 10^-2     

Joint noncoherent demodulation and decoding for the block fading channel: a practical framework for approaching Shannon capacity

The paper contains a systematic investigation of practical coding strategies for noncoherent communication over fading channels, guided by explicit comparisons with information-theoretic benchmarks. Noncoherent reception is interpreted as joint data and channel estimation, assuming that the channel is time varying and a priori unknown. We consider iterative decoding for a serial concatenation of a standard binary outer channel code with an inner modulation code amenable to noncoherent detection. For an information rate of about 1/2 bit per channel use, the proposed scheme, using a quaternary phase-shift keying (QPSK) alphabet, provides performance within 1.6-1.7 dB of Shannon capacity for the block fading channel, and is about 2.5-3 dB superior to standard differential demodulation in conjunction with an outer channel code. We also provide capacity computations for noncoherent communication using standard phase-shift keying (PSK) and quadrature amplitude modulation (QAM) alphabets; comparing these with the capacity with unconstrained input provides guidance as to the choice of constellation as a function of the signal-to-noise ratio. These results imply that QPSK suffices to approach the unconstrained capacity for the relatively low information and fading rates considered in our performance evaluations, but that QAM is superior to PSK for higher information or fading rates, motivating further research into efficient noncoherent coded modulation with QAM alphabets.     

A 1024-QAM Analog Front-End for Broadband Powerline Communication Up to 60 MHz

A high performance analog front-end (AFE) for broadband powerline communication between 1.6 and 60 MHz is presented. The frequency division multiplexing AFE supports optimum channel selection, avoids disturbing RF signals and allows co-existence with other users of the spectrum. The direct-conversion receiver operates linearly up to a + 18 dBm input level. Tunable low-pass filters, integrated into the receive path, support a wide class of service requirements by channel bandwidth selection. The dynamic range is 99.5 dB for 2 MHz channels, and 90.5 dB for 16 MHz channels. Error vector magnitude measurements are presented for a single-carrier 1024-QAM and a 1024-carrier 64-QAM signal as function of frequency and channel attenuation. For 1024-QAM, the error vector magnitude (EVM) is below or equal to 1.2% rms up to 60 dB of attenuation, whereas the 1024-carrier 64-QAM performs well up to 80 dB of attenuation. The presented chip was fabricated in a 0.25 mum SiGe BiCMOS process, and the measured power consumption from a single 2.5 V supply is 668 mW.     

Further results in the unified analysis of digital communicationsystems

Stein's (1964) method can be extended to the analysis of the bit error probability (BEP) of quadrature phase shift keying (QPSK), staggered QPSK, and minimum shift keying (MSK) communication systems. The resultant noisy reference BEP waterfall curves are presented. The numerical advantages of this technique and some practical results are discussed. In a parallel manner, the BEP for quadrature amplitude modulation (QAM) communication systems can be analyzed. This technique is numerically more intensive but is used to generate noisy reference BEP waterfall curves for 16-QAM and 64-QAM modulations. Unfortunately, few carrier synchronizers produce a complex Gaussian reference signal, but pragmatically many reference signals can be accurately approximated by a complex Gaussian at moderate to high SNR. Actual BEP performance and the approximate results are compared     

10 kHz-linewidth diode-pumped Er:Yb:glass laser

A single-mode Er:Yb:phosphate glass laser pumped at 980 nm by an InGaAs diode laser is reported. The laser emits over 15 mW output power with 6% slope efficiency. The measured linewidth is 10.6 kHz and the RIN power is less than 140 dB/Hz.<>     

A new generation of 90 Mb/s systems: bandwidth efficient, fieldtested 16-QAM

16-QAM (quadrature amplitude modulation) digital satellite broadcast equipment and satellite communications (SATCOM) systems that double the spectral efficiency of currently operational satellite links is described. It is shown that with this field-proven system, data transmission at a rate of 90 Mb/s (two multiplexed DS-3 signals) is feasible in a transmission bandwidth of 30 MHz. Extensive operational satellite tests performed over the T-303 satellite of AT&T demonstrated a BER <10^-10 and no errors for several days. The practical bandwidth efficiency of these SATCOM systems is 3 b/s/Hz, i.e. double the currently used 1.5 b/s/Hz QPSK (quadrature phase-shift keyed) systems. The doubling of the spectral efficiency is attained with advance modem (modulation-demodulation), adaptive equalization robust synchronization, high-power amplifier (HPA) linearization (predistortion), and low redundancy powerful forward-error-correction (FEC) subsystems. The systems may make possible the conversion of currently operational analog FM links into bandwidth efficient digital systems. In one 30 MHz satellite transponder three to four digitized high quality TV signals could be broadcast, or two standard rate DS-3 signals time-division multiplexed with a DS-1 rate signal and additional auxiliary data streams     

Line broadening and intensity noise due to polarization switchingin external cavity diode lasers

The authors report on polarization switching as a new source of linewidth broadening and intensity noise in external grating diode laser setups that emit in a sequence of alternating wavelength intervals having TE and TM polarization, respectively. At the borders between two consecutive intervals, the continuous random hopping of the system between the two polarizations results in line broadening and in RIN values of -100 dB/Hz at frequencies below 100 MHz. In the middle part of each wavelength interval, linewidth of less than 300 kHz and low noise (<-145 dB/Hz) have been measured     

Significant performance advantage of electroabsorption modulatorintegrated distributed feedback laser (EML) transmitter in transportingmulticarrier QAM signals

Data are presented that show that, for transporting quadrature amplitude modulated (QAM) radiofrequency (RF) subcarriers in suboctave frequency range, electroabsorption modulator integrated distributed feedback lasers (EMLs) can he modulated with significantly higher (2.5 times) modulation index without any in-band signal distortion as compared to the directly or externally modulated (using LiNbO₃ MZ modulator) distributed feedback (DFB) laser transmitters in the 1.55-μm band. This occurs when the selection of frequency range and biasing the modulator section of an EML is such that the third-order intermodulation distortion is suppressed and the second-order distortion is outside the in-band frequency range. We have used an equivalent of 64- and 256-QAM 40 carriers in 550-800 MHz with the test carrier modulated with 5.063 M symbols per second. For a given bit error rate (BER), the receiver sensitivity was as much as 4 dB (optical) higher with an EML-based transmitter compared to all other transmitters. The results are presented for QAM signals but they are equally applicable for other formats of digital modulation of RF carriers, such as quadrature phase shift keying (QPSK) in suboctave frequency range     

非限幅QPSK类正弦调制大气激光通信系统的性能研究

提出了一种非限幅四相相移键控(QPSK)类正弦调制技术,并将其运用到大气激光通信系统中。介绍了大气激光通信信道模型,给出了非限幅QPSK类正弦调制的大气激光通信系统模型。在此基础上,对采用非限幅QPSK类正弦调制和采用直流偏置副载波强度调制的大气激光通信系统在无湍流和弱湍流信道下的功率利用率、误码率、中断概率和信道容量进行了分析和比较。结果表明,非限幅QPSK类正弦调制具有更好的抗噪声性能、更高的功率利用率、较低的中断概率及较高的信道容量,提高了大气激光通信系统的性能,可满足大气激光通信系统的需要。     

Performance of predistorted APK Modulation for one- and two-link nonlinear power amplifier Satellite communication channels

Digital satellite communications has been restricted to date to digital modulations having an efficiency of 2 bps/Hz; that is, filtered-quadrature phase-shift keying (QPSK) with close to a constant envelope. The main limitation to higher order digital modulations is the need for linearity of a high power amplifier both in the satellite and a terminal. Many years ago, Thomas et al. conducted a complete study of the use of higher order linear modulations on nonlinear satellite channels. However, pulse-shaping and predistortion were not treated. We consider these items for 16-point amplitude phase-shift keying (APK) modulation; that is, signal points assigned on concentric circles. A simple predistortion algorithm is given for APK modulations and symbol error-rate performance is determined for both one- and two-link nonlinear channels. Our best result is for an one-link channel and (5, 11) APK-5 points on an inner circle and 11 on the other, which, for predistortion on the nonlinear channel, loses only about 1 dB to 16-point quadrature amplitude modulation transmission on the linear channel.     

Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation

This paper describes a coherent optical receiver for demodulating optical quadrature phase-shift keying (QPSK) signals. At the receiver, a phase-diversity homodyne detection scheme is employed without locking the phase of the local oscillator (LO). To handle the carrier phase drift, the carrier phase is estimated with digital signal processing (DSP) on the homodyne-detected signal. Such a scheme presents the following major advantages over the conventional optical differential detection. First, its bit error rate (BER) performance is better than that of differential detection. This higher sensitivity can extend the reach of unrepeated transmission systems and reduce crosstalk between multiwavelength channels. Second, the optoelectronic conversion process is linear, so that the whole optical signal information can be postprocessed in the electrical domain. Third, this scheme is applicable to multilevel modulation formats such as M-array PSK and quadrature amplitude modulation (QAM). The performance of the receiver is evaluated through various simulations and experiments. As a result, an unrepeated transmission over 210 km with a 20-Gb/s optical QPSK signal is achieved. Moreover, in wavelength-division multiplexing (WDM) environment, coherent detection allows the filtering of a desired wavelength channel to reside entirely in the electrical domain, taking advantage of the sharp cutoff characteristics of electrical filters. The experiments show the feasibility to transmit polarization-multiplexed 40-Gb/s QPSK signals over 200 km with channel spacing of 16 GHz, leading to a spectral efficiency as high as 2.5 b/s/Hz.     

Intensity noise of semiconductor laser diodes in fiber optic analog video transmission

This paper presents an investigation of the effects of laser diode noise on analog video transmission in the HF and VHF bands, which resulted in the development of several designs of graded-index multimode fiber systems that can ignore reflection induced laser noise. The contents of the investigation include: 1) The evaluation of intrinsic laser noise of various laser structures and the evaluation of modulation effects on laser diode noise characteristics. It was found that the relative intensity noise (RIN) is less than -145 ∼ -150 [dB/Hz] when the modulation factor is less than 0.7 for index-guide mode stabilized lasers; 2) The quantitative evaluation of reflected laser beam effects on laser noise characteristics. The maximum laser-coupled reflected optical power that does not increase laser noise was determined as-65 ∼ -73 dB or less depending on the kind of laser structure; and 3) The evaluation of optical power reflected back into the laser in graded-index multimode fiber systems.     

100 Gbit/s Nyquist-WDM PDM 16-QAM Transmission over 1200 km SMF-28 with Ultrahigh Spectrum Efficiency

Nyquist wavelength-division multiplexing (N-WDM) allows high spectral efficiency (SE) in long-haul transmission systems.Compared to polarization-division multiplexing quadrature phase-shift keying (PDM-QPSK),multilevel modulation,such as PDM 16 quadrature-amplitude modulation (16-QAM),is much more sensitive to intrachannel noise and interchannel linear crosstalk caused by N-WDM.We experimentally generate and transmit a 6 × 128 Gbit/s N-WDM PDM 16-QAM signal over 1200 km single-mode fiber (SMF)-28 with amplification provided by an erbium-doped fiber amplifier (EDFA) only.The net SE is 7.47 bit/s/Hz,which to the best of our knowledge is the highest SE for a signal with a bit rate beyond 100 Gbit/s using the PDM 16-QAM.Such SE was achieved by DSP pre-equalization of transmitter-side impairments and DSP post-equalization of channel and receiver-side impairments.Nyquist-band can be used in pre-equalization to enhance the tolerance of PDM 16-QAM to aggressive spectral shaping.The bit-error ratio (BER) for each of the 6 channels is smaller than the forward error correction (FEC) limit of 3.8 × 10-3 after 1200 km SMF-28 transmission.