Showing papers on "Modulation published in 1987"

The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation

Abstract: Frequency-selective fading caused by multipath time delay spread degrades digital communication channels by causing intersymbol interference, thus resulting in an irreducible BER and imposing a upper limit on the data symbol rate. In this paper, a frequencyselective, slowly fading channel is studied by computer simulation. The unfiltered BPSK, QPSK, OQPSK, and MSK modulations are considered first to illustrate the physical insights and the error mechanisms. Two classes of modulation with spectral-shaping filtering are studied next to assess the tradeoff between spectral occupancy and the performance under the influence of time delay spread. The simulation is very flexible so that different channel parameters can be studied and optimized either individually or collectively. The irreducible BER averaged over fading samples with a given delay profile is used to compare different modulation/detection methods, while the cumulative distribution of short-term BER is employed to show allowable data symbol rates for given values of delay spread. It is found that both GMSK and QPSK with a raised-cosine Nyquist pulse are suitable for a TDM/TDMA digital portable communications channel.

Spectral Correlation of Modulated Signals: Part II--Digital Modulation

Abstract: As a continuation of Part I, the spectral correlation function is presented for a variety of types of digitally modulated signals. These include digital pulse-amplitude, pulse-width, and pulse-position modulation, and various types of phase-shift keying and frequency-shift keying. The magnitudes of the spectral correlation functions are graphed as the heights of surfaces above a bifrequency plane, and these graphs are used as visual aids for comparison and contrast of the spectral correlation properties of different modulation types.

Modulation instability induced by cross-phase modulation.

Abstract: Modulation instability that leads to breakup of intense cw radiation into a train of ultrashort pulses during propagation in optical fibers occurs only in the presence of anomalous group-velocity dispersion. It is shown that a new kind of modulation instability can occur even in the normal-dispersion regime when two copropagating optical fields interact with each other through cross-phase modulation initiated by the nonlinearity. The quantitative aspects of this cross-phase--modulation--induced modulation instability are discussed and illustrated by use of a realistic experimental example.

Trellis Coded Modulation for 4800-9600 bits/s Transmission Over a Fading Mobile Satellite Channel

Abstract: The combination of trellis coding and MPSK signaling with the addition of asymmetry to the signal set is discussed with regard to its suitabllity as a modulation/coding scheme for the fading mobile satellite channel. For MPSK, introducing nonuniformity (asymmetry) into the spacing between signal points in the constellation buys a further improvement in performance over that achievable with trellis coded symmetric MPSK, all this without increasing the average or peak power, or changing the bandwidth constraints imposed on the system. Whereas previous contributions have considered the performance of trellis coded modulation transmitted over an additive white Gaussian noise (AWGN) channel, the emphasis in this paper is on the performance of trellis coded MPSK in the fading environment. The results will be obtained by using a combination of analysis and simulation. It will be assumed that the effect of the fading on the phase of the received signal is fully compensated for either by tracking it with some form of phase-locked loop or with pilot tone calibration techniques. Thus, our results will only reflect the degradation due to the effect of the fading on the amplitude of the received signal. Also, we shall consider only the case where interleaving/deinterleaving is employed to further combat the fading. This allows for considerable simplification of the analysis and is of great practical interest. Finally, the impact of the availability of channel state information on average bit error probability performance is assessed.

Highly Efficient Digital Mobile Communications with a Linear Modulation Method

Abstract: Although linear modulation methods can achieve high spectrum efficiency, very little attention has been directed to their use in mobile radio systems. This is mainly due to the fact that the nonlinearity of the transmitter power amplifier tends to spread the spectrum and thus eliminate any spectrum efficiency advantage gained through the use of linear modulation methods. In this paper, a linear modulation system is proposed, which solves the above difficulty and which gives higher spectrum efficiency than conventional digital FM. The modulation/demodulation method is \pi /4 shift QPSK and phase-shift detection with a limiter-discriminator and an integrate-and-dump filter. By introducing a cartesian coordinate negative feedback control, 35 percent power efficiency at 10 W output power and - 60 dB relative out-of-band radiation are simultaneously achieved with a class "AB" amplifier, owing to the 29 dB feedback gain. The receiver configuration is easy to realize and gives immunity against fast fading through the use of noncoherent detection with limiter-discriminator. By using a novel decision method, bit error rate performances under both nonfading and fading condition are comparable to those obtained by digital FM. These results make it possible for linear modulation methods to achieve higher spectrum efficiency than is possible with conventional digital FM methods in mobile radio communications.

Optically readable record carrier for recording information, method and apparatus for manufacturing such a record carrier, apparatus for recording information on such a record carrier, and apparatus for reading information recorded on such a record carrier

Abstract: An optical disc record carrier having a radiation-sensitive surface and a spiral or concentric pattern of information tracks thereon. The radial displacement of each track is modulated to produce a periodic radial wobble, the frequency of such periodic displacement being modulated by a position-information signal identifying the relative positions of the tracks. During recording or reading of information on the tracks the track modulation is read by a scanning beam. A clock signal for controlling scanning velocity is recovered from the track displacement modulation, and an FM demodulator recovers the position-information signal from the frequency modulation of such displacement.

A new modulation principle for incoherent scatter measurements

Abstract: A new method for measuring the ionosphere plasma autocorrelation function with an incoherent scatter radar is described. This method can be used in similar situations where multipulse methods have been used previously. In situations where the design of the transmitted signal is limited by a maximum allowed modulation time and a maximum radar peak power and where the signal to noise ratio is small, this method provides significantly more accurate ACF estimates than can be obtained by frequency commutated multiple pulse measurements. A simple explanation of the method is given, as well as a precise definition based on ambiguity functions. Computational algorithms are discussed, and it is shown that there are several possibilities to modify present algorithms so that this method can be implemented.

Performance Analysis of Digital Radio Links with Nonlinear Transmit Amplifiers

Abstract: This paper evaluates the effects of transmit amplifier nonlinearities on digital radio link performance. Special emphasis is given to links carrying 256-level modulations, for which the impact of even mild nonlinearities can be severe. Performance is measured in terms of the flat fade margin ( F ) normalized by its theoretical maximum (F max ). Curves of F / F_{max} versus amplifier input drive level are used to evaluate and compare numerous design strategies. A major contribution of this work is the development of a fast, accurate method of computer analysis, in contrast to the more cumbersome Monte Carlo simulations generally used for these assessments. Moreover, the new method is used to study a variety of techniques devised to combat the nonlinearity, particularly signal predistortion. It is also used to investigate various approaches to pulse shaping and filtering, to quantify the degradation in performance with increasing numbers of modulation levels, and to examine the possible benefits of linear equalization.

Mobile digital communications via tone calibration

Abstract: A new digital signaling technique that is particularly suited for channels impaired by multipath fading is presented. The proposed modulation scheme employs a continuous-wave (CW) tone to calibrate the mobile channel against the multipath-induced phase uncertainties. This technique is applicable to quaternary phase shift keying as well as to more complicated signal constellations such as M-ary phase shift keyed schemes. The advantages of tone calibration are: 1) robustness of the receiver and 2) elimination of the link dependent error floor. Furthermore, since the CW tone can also be used for coherent signal demodulation, carrier phase acquisition can be achieved within a bit time. This property is particularly attractive when a burst of data with a short burst length has to be detected. This radio technique is useful for both the terrestrial mobile and the newer satellite-aided mobile communication (SAMC) services.

Pulse oximeter sensor control system

Abstract: A pulse oximeter is provided including two light emitting diodes. Each LED is energized by a unique modulation signal. Light produced by the LEDs passes through the tissue of a subject and is detected by a photodiode. The photodiode signal is separated by tuned circuits resonant at the two modulation signal frequencies and physiological information signals are recovered from the separated signals by amplitude demodulation. The inventive arrangement provides identification of back-to-back coupled LEDs and narrow band filtering for good noise immunity.

Optical intensity modulation to 40 GHz using a waveguide electro‐optic switch

Abstract: We report the intensity modulation of an optical carrier at frequencies as high as 40 GHz using a Ti:LiNbO3 optical waveguide switch. A self-electrooptic sampling technique is used to observe the modulation.

Multichannel coherent optical communications systems

Abstract: Balanced coherent receivers perform substantially better than single-detector receivers in multichannel optical fiber FDM communications systems since the balanced approach eliminates the direct-detection and signal-cross-signal interference. The permissible channel spacing D depends on the intermediate frequency f IF , on the bit rate R b , and on the modulation/demodulation format. In particular, D increases by 2 Hz for every 1-Hz increase of the f IF . The signal-to-interference ratio SIR, as defined in the text, provides a simple measure of the amount of the interference generated by undesired channels. The criterion SIR = 30 dB is selected in this paper and leads to the following minimum channel spacings: for heterodyne systems, 3.8R_{b} for FSK, 9.5R_{b} for ASK, and 12.4R_{b} for PSK; for homodyne systems, 7.5R_{b} for ASK and 10.5R_{b} for PSK. Simultaneous transmission of several channels generates an excess shot noise studied here for the first time. If the local oscillator power is 40 dB above the received signal power and 2000 channels are transmitted without optical prefiltering, the excess shot noise power penalty is less than 1 dB.

Digital Communication Systems

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Transverse self-phase modulation and bistability in the transmission of a laser beam through a nonlinear thin film

Abstract: We present a theoretical study, with reference to experimental results, of transverse self-phase modulation effects in the transmission of a laser beam through a nonlinear thin film. The occurrence of interference rings, intensification or dimming of the on-axis beam intensity, and transverse optical bistability in the presence of a feedback can all be systematically documented in terms of geometrical/optical parameter classifications. These studies provide further insights and useful guides for experimental studies.

Time difference of arrival geolocation method, etc.

Abstract: A direction finding method and apparatus for a radio signal source, modulated by a digital information signal and existing in a heavy interference environment, is based on cyclic crosscorrelation. The direction finding technique exploits the second order periodicity of a transmitted signal having digital modulation, whereby the technique is immune to narrow-band interference. Radio frequency signals are received by two spaced antennas, and the signal of interest can be considered as a cyclostationary process. The signal time of arrival difference between the two antennas is determined using a cyclic crosscorrelation method. This is implemented by sending the electrical signals, developed by the antennas, through a variable delay device, then to a balanced mixer. The composite signal from the mixer is sent to a band-pass filter whose center frequency is equal to the baud rate of the signal of interest. The output energy of the filter is proportional to that portion of the cyclic crosscorrelation function where the cyclic frequency corresponds to the baud rate. With the use of a peak detector, which finds the peak values of the cyclic crosscorrelation function, the time difference of arrival between the two antennas is determined. This, in turn, determines a line of position to the radio source.

Baseband phase modulator apparatus employing digital techniques

Abstract: There is disclosed a Zero IF modulator apparatus for modulating carrier signals with baseband signals, the modulator includes digital logic circuitry including a sine/cosine amplitude converter which is responsive to the baseband signals for providing first and second digital quadrature baseband signals. These digital quadrature baseband signals are then employed by coupling the same to digital-to-analog converters where the outputs of each converter is indicative of a first and second quadrature signal. These signals can then be translated to a given carrier signal for transmission over a desired communication channel.

Liquid crystal video display device having pulse-width modulated "ON" signal for gradation display

Abstract: The present invention provides, in an active matrix liquid crystal display device of the type including non-linear elements, for the application of an ON pulse-width modulation signal by the data line driving circuit at the rearward end of each selected period, during which the voltage corresponding to the duty-cycle of the selected signal is applied to the liquid crystal layer. The result is accurate reproduction of gray-scale images in the display under conditions of high duty-cycle driving. Further, cross-talk between columns caused by the data signal is controlled by supplying a signal voltage which eliminates the residual charges in the liquid crystal layer at the end of each selected period.

Self-amplitude-modulation of optical pulses in nonlinear dispersive fibers

Abstract: We consider propagation of optical pulses in the anomalous dispersive region of single-mode fibers after including the effects of Kerr-type nonlinearity. It is found that the pulse develops an internal structure with deep amplitude modulation. We show that this self-amplitude-modulation of pulses is related to the modulation instability of continuous-wave optical beams in nonlinear dispersive fibers. The initiation of this phenomenon does not require an external probe or spontaneous emission. Numerical results show that self-phase-modulation broadens the power spectrum enough to encompass the frequency at which self-amplitude-modulation provides the maximum gain. The amplification of these sidebands manifests as a breaking of the optical pulse into several subpulses. For negligible loss, the pulse breakup may also be viewed as the internal structure of a high-order soliton. However, modulation instability appears to provide a simpler interpretation and explains the features of numerical simulation and experiments.

Time-division multiplexing of interferometric fiber sensors using passive phase-generated carrier interrogation.

Abstract: The multiplexing and demodulation of interferometric sensors using a time-division/phase-generated carrier approach is described. The scheme uses slightly unbalanced interferometers and laser frequency modulation to effect the interrogation of the sensors, while the time-division multiplexing allows individual sensors to be addressed. Phase sensitivities of ∼20μrad/Hz and cross-talk levels of <−47 dB are demonstrated for a three-sensor network.

Frequency chirp and spectra due to self-phase modulation and stimulated Raman scattering influenced by pulse walk-off in optical fibers

Abstract: A numerical study is presented on the frequency chirp and spectra of light pulses affected by both self-phase modulation and stimulated Raman scattering in optical fibers. The asymmetric distortion of pump and Stokes spectra is attributed to walk-off between the pulses and found to be in good agreement with recent experimental results. The stationary-phase method is employed to explain the relation between the structure of the spectra and the form of the pulse chirp. An analytic expression describing the chirp of a depletable pump and of the first Stokes pulse is given for negligible walk-off. The dominant influence of self-phase modulation over the cross phase modulation on the buildup of the chirp in the case of large walk-off and severe pump depletion is shown.

Heterodyne transmission of a 560 Mbit/s optical signal by means of polarisation shift keying

Abstract: Experimental results for a 560 Mbit/s optical heterodyne system using polarisation shift keying are presented. It is demonstrated that this modulation technique provides up to 3dB gain compared to ASK modulation.

Analysis of digital radio transmissions

Abstract: An apparatus for analysing digital radio transmissions can provide an output indicating the condition of a digital transmission link. The apparatus operates by sampling received radio signals to produce for each sampling instant a signal or signals representative of the modulation state of the transmission at the sampling instant. The apparatus includes a processor (68) which can receive and measure a given number of such signals. The processing means is arranged to process the signals according to one or more stored routines and generate one or more parameters which indicate the condition of the transmission. The apparatus includes means for representing the samples by digitally encoded numbers and the processing means arranges those samples into one of a plurality of groups according to the value of the number. The number of groups correspond to the number of modulation states. This technique allows the apparatus to carry out accurate analysis of the transmission.

Neuroendocrine Modulation of the Thymus‐Dependent Immune System. Agonists and Mechanisms

Abstract: It is becoming increasingly clear not only that the immune system is very complex in its mechanisms of intercellular molecular communication, but also that the neuroendocrine systems that impinge upon and regulate the immune system are equally complex in their molecular influences. It is the purpose of this review to summarize the major signals of the central and autonomic nervous systems, the endocrine system, and the immunologic system which regulate the thymus-dependent (T cell) system and to integrate this information with current information on the biochemical mechanisms by which these neurotransmitter and humoral influences are thought to act. At the outset, it is important to note that there are two major liabilities in undertaking such a task: The first is that a summary of the various molecules that act on T cells does not give a clear picture of the operational dynamic and relative importance of various influences in health or disease. The second is that while our knowledge of the molecular mechanisms of transmembrane signaling has grown rapidly, it is still fragmentary, making it difficult to provide an integrated view. The T-cell system has evolved to defend us against a wide variety of pathogens (bacterial, viral, fungal, and parasitic) and against spontaneous tumor development. To do so it provides evolutionarily early nonspecific cellular participants (e.g., natural killer [NK] cells and lymphokine-activated killer [LAK] cells) and specific immune effector (cytotoxic and lymphokine-producing) and regulatory (helper and suppressor) T cells. Through a variety of soluble mediators including lymphokines, prostaglandins, leukotrienes, and other substances, mature T cells regulate and interact with other effector populations, particularly the cells of the monocyte/macrophage series. The T-cell system is subject to more frequent deficiency than once thought. It is now clear that mild to severe depression of the T-cell system is seen in cancer, aging, malnutrition, and chronic infections, including AIDS. In contrast to the B-cell or antibody-producing system, the T-cell system is the major target of immunotoxic influences in terms of immunosuppressive drugs and environmental toxicants and probably also of chronic stress (see Ref. 1 for review). The results of cellular immune deficiency are acute and

Device, method and apparatus for optical modulation using ferroelectric polymer liquid crystals

Abstract: An optical modulation method comprises writing in a ferroelectric having a glass transition point at or above the glass transition point, and holding the resultant written state below the glass transition point. Preferably, the ferroelectric is formed in an optical modulation layer set to have an optical thickness capable of providing a multiple reflection state. The multiple reflection state of the optical modulation layer is locally changed at or above the glass transition point by irradiating the optical modulation layer under an electric field with a laser beam to write in the optical modulation layer.

Optical modulation device

Abstract: An optical modulation device for effecting optical modulation by controlling the diffraction phenomenon of light, which not only consumes a small amouhnt of the driving energy employed, such as a electric power, but which is also capable of improvements in reliability

Efficient electro‐optic modulator for optical pumping of Na beams

Abstract: An electro‐optic modulator using LiTaO3 is described which yields 34% of the carrier intensity in each of the first‐order sidebands with rf phase modulation frequencies fm∼10 GHz and ≲10 W input power The modulator makes use of a lumped resonator with Q∼200 to obtain efficient production of the sidebands It is shown that the device can be scaled for operation at ∼2 GHz Applications of this modulator include optical pumping of the lighter alkali atoms, FM sideband spectroscopy, and laser phase/frequency stabilization using rf modulation techniques

Response of a Fabry-Perot cavity to phase modulated light

Abstract: We present a solution to the lock‐in detection method which is valid for arbitrary values of the modulation frequency. Our solutions agree with the results of the quasistatic theory, in the limit of small modulation frequency compared to the resonance linewidth.

Sub-band speech analyzing and synthesizing device

Abstract: In a sub-band speech analyzing and synthesizing apparatus, a low-pass filter comprises a nonrecursive filter. The center frequency and bandwidth of each channel are selected so that the decimated sampling period of that channel is an integer multiple of the period of the modulating signal or the demodulating signal of the channel. Modulation or demodulation is performed simultaneously with the low-pass filtering by the nonrecursive filter.