Showing papers on "Demodulation published in 1981"

GMSK Modulation for Digital Mobile Radio Telephony

Abstract: This paper is concerned with digital modulation for future mobile radio telephone services. First, the specific requirements on the digital modulation for mobile radio use are described. Then, premodulation Gaussian filtered minimum shift keying (GMSK) with coherent detection is proposed as an effective digital modulation for the present purpose, and its fundamental properties are clarified with the aid of machine computation. The constitution of modulator and demodulator is then discussed from the viewpoints of mobile radio applications. The superiority of this modulation is supported by some experimental test results.

Amplitude modulation using digitally selected carrier amplifiers

Abstract: An amplitude modulator (10) is disclosed which generates an amplitude modulated carrier signal by selectively combining varying numbers of other carrier signals. A circuit (20) is included which provides plural carrier signals of like frequency and phase where the number of carrier signals being provided is dependent upon the level of an input signal. A combiner circuit (22) combines the plural carrier signals to thereby provide a combined signal which is the desired amplitude modulated carrier signal. In one embodiment (FIG. 3) the carrier signals which are combined are all equal in amplitude. In another embodiment (FIG. 4) the amplitudes of some of the carrier signals are weighted in a binary progression. In yet another embodiment (FIG. 9) the carrier signals are all frequency modulated whereby the output signal has both a varying frequency and amplitude.

Switched-capacitor building blocks for adaptive systems

Abstract: A number of switched-capacitor (SC) building blocks useful in adaptive systems are presented. These include a phase-lock loop, a tracking filter, a second-order equalizer with programmable gain, zeros' frequency and zeros' Q factor, a quadrature sine-wave generator, and a synchronous demodulator. It is shown, how a number of these signal processing blocks can be combined together to produce an adaptive channel equalizer. The operation of most of the circuits presented has been verified using discrete prototypes.

TDMA/PSK Carrier synchronization without preamble

Abstract: An apparatus for providing coherent detection of a series of data bursts without a preamble by providing a reference waveform which has the same frequency as the carrier for a received TDMA/MPSK transmission, but having an arbitrary phase with respect to such signals The reference waveform is used for pseudo-coherent demodulation Symbol timing is available at all times by using symbol synchronization obtained from the previous bursts of the same transmission The symbol timing is used to sample two quadrature outputs of the pseudo-coherent demodulator once per MPSK symbol interval Storage of a sufficient number of demodulator samples allows a carrier phase estimate to be obtained from the message burst itself The carrier phase estimate is then used for coherent detection on a delayed sequence of the demodulator samples

A Voiceband Codec with Digital Filtering

Abstract: Oversampling and digital filtering have been used to design a per-channel voiceband codec with resolution that exceeds the typical transmission system requirement by more than 15 dB. This extended dynamic range will allow for the use of digital processing in the management of signal levels and system characteristics in many telecommunication applications. Digital filtering contained in the codec provides rejection of out-of-band inputs and smoothing of the analog output that is sufficient to eliminate the need for analog filtering in most telephone applications. Some analog filtering may be required only to maintain the expanded dynamic range in cases where there is a danger of large amounts of out-of-band energy on the analog input impairing the dynamic range of the modulator. The encoder portion of the oversampled codec comprises an interpolating modulator that samples at 256 kHz followed by digital filtering that produces a 16-bit PCM code at a sample rate of 8 kHz. In the decoder, digital processing is used to raise the sampling rate to 1 MHz prior to demodulation in a 17-level interpolating demodulator. The circuits in the codec are designed to be suitable for large-scale integration. Component matching tolerances required in the analog circuits are of the order of only ± 1 percent, While the digital circuits can be implemented with fewer than 5000 gates with delays on the order of 0.1 μs. In this paper the response of the codec is described mathematically and the results are confirmed by measurements of experimental breadboard models.

Method and apparatus for pulse angle modulation

Abstract: A method and apparatus for phase modulating a carrier signal to convey an information signal (12) such that the carrier signal has a constant amplitude envelope. A Hilbert transform signal (14) of the information signal (12) is produced. The signals (12, 14) are sampled to produce signals (16, 18), which represent cartesian coordinate values. The cartesian coordinate values are then converted into equivalent polar vectors (20-36) which have both an amplitude (R) and an angle (θ). The polar vector quantity (R,θ) is converted into two unity amplitude vectors (A, B). The unity amplitude vectors (A, B) are offset from the polar vector quantity by an angle the cosine of which is proportional to the amplitude of the polar vector (R). The carrier signal is sequentially phase modulated by each of the angles of the unity amplitude vectors (A, B) for each sample period of the information signal. This modulation procedure maintains a constant amplitude envelope for the carrier signal and makes possible simultaneous demodulation of a plurality of carrier signals in a single demodulation channel.

FSK Demodulator employing a switched capacitor filter and period counters

Abstract: An FSK demodulation circuit especially suited for integrated construction is provided. The demodulating circuit uses N counters and a counter selector circuit for sequentially switching the counters at every zero-cross point in the received FSK signal. Sensitivity of demodulation is improved by N-time zero-cross detection rather than counting the time between two adjacent zero-cross points. The FSK demodulation circuit uses a Switched Capacitor Bandpass Filter whose characteristics are changed by changing the frequency of an internal clock using simple dividing circuitry.

Heterodyne-type optical fiber communications

Abstract: Standard optical fiber communication systems employ intensity-modulation/direct-detection schemes. This is noise-carrier communication and in a sense is more primitive than the radio engineering in Marconi’s age. However, it has the practical advantage of system simplicity and low cost. On the other hand, some applications of optical fiber communication exist in which a long repeater separation is the primary concern; an example is submarine optical cable communication between islands, in this case, the improvement of the bit-error rate (BER) by a coherent modulation/demodulation scheme such as PCM-PSK or PCM-FSK may be advantageous even at the sacrifice of simplicity and low cost.

System for extracting timing information from a digital waveform

Abstract: System for extracting timing information from a digital waveform supplied to the system in which the waveform is modulated by a modulator, passed through a surface acoustic wave transversal filter and demodulated by a demodulator. Zero crossings in the resultant transformed waveform are detected by a detector and used to determine the times at which another version of the original waveform is sampled by a sampling circuit. The characteristics of the surface acoustic wave transversal filter are chosen to be such that each elementary pulse in the original waveform is transformed into an oscillatory waveform, the zero crossings of the different oscillatory waveforms coinciding with one another and defining the sampling times.

Digital FSK demodulator

Abstract: The disclosed FSK demodulator includes a digital PLL for converting an FSK modulated input signal with two different frequencies to rectangular waveforms having durations as determined by those frequencies, two reversible N bit counters for counting clock pulses to determine instantaneous duty factors in the form of N parallel bits of the waveforms for each pulse repetition period, and a read only memory for multiplying alternate outputs, except for the sign bit, from the counters by a ratio of the lower to the higher frequency of the input signal. Then, a digital first-order low-pass filter removes a high frequency component from the output of the read only memory, resulting in a demodulated binary signal. The binary signal may be produced by setting or resetting a FLIP-FLOP when the sign bit from each of the counters is respectively a binary ONE or ZERO. Alternatively, another reversible M bit counter (when M>N) may count the clock pulses to equal the output from the read only memory.

Modem to be coupled to a directional transmission line of an SS multiplex communication network

Abstract: In an SS (spread spectrum) modem, a high-pass filter (81) for rejecting a low frequency component including an information signal received by demodulation, produces an SS signal component modified by a PN (pseudo noise) code sequence used for the demodulation. A multiplier (82) substantially regenerates the SS signal component. Band division may be resorted to, in which case an input filter (161) selects a partial band SS signal for the demodulation. A transit filter (164) rejects the partial band SS signal. An SS signal is sent to a destination modem through one of output filters (162) selected for the destination modem. Each generator (67, 71) may generate a PN code sequence given by a product of PN codes, one having a frame period equal to an integral multiple (unity allowed) of the PN clock period of another. Each partial band may be about 1/5 of the entire frequency band of the PN code sequence. A center portion of the band may be used in transmitting a narrow-band synchronizing signal. In a band division network, the highest partial band may be used for an SS (multiplex) signal and other partial bands, collectively for a baseband TDM signal.

Symbol Error Probability Bounds for Coherently Viterbi Detected Continuous Phase Modulated Signals

Abstract: Recently the minimum Euclidean distance and bandwidth properties of continuous phase modulated (CPM) signals have been considered. It has also been shown that, for rational modulation indexes, a state description of these signals is possible and that the Viterbi algorithm (VA) can be used for demodulation. In this paper the performance analysis of CPM systems is extended to obtain bounds on the symbol error probability when the VA is used. The calculation of these bounds is based on the transfer function technique, which has been generalized. From numerical comparisons of the upper and lower bounds, it is concluded that the minimum Euclidean distance is a good performance measure for a broad class of CPM signals, even when the symbol error probability is as large as 10-2.

Satellite block transmission using wideband fiber optic links

Abstract: A system for transmitting one or more blocks of information signals from the output of a low noise amplifier associated with a satellite antenna to a remote headend or terminal is disclosed Block conversion is used to translate the signal spectrum from the satellite transmission band, typically 37-42 GHz, to a frequency range of 06-11 GHz A wideband fiber optic link is used to extend the link length from a maximum of 300 feet when coaxial cable is used to approximately 1 km (3281 feet) The output of the optical receiver in the fiber optic link is converted to the original satellite transmission band so that a standard satellite receiver can be used to demodulate the signal Alternatively, a satellite receiver which interfaces directly at the 06-11 GHz can be used The system also may include means to synchronize frequency conversion at each end of the fiber optic link The high quality of the system lends itself to application in television receive-only as well as date communications

Demodulation and Carrier Synchronization of Multi-h Phase Codes

Abstract: Multi- h phase coding is a bandwidth-efficient modulation which offers substantial performance improvement over conventional digital modulations with little or no bandwidth expansion. This paper examines the demodulation of the multi- h signals, and in particular examines the behavior of a decision-directed carrier loop which utilizes the Viterbi algorithm decoder in the calculation of the loop error signal. Both analytic and simulation results are presented and the agreement between the two is shown to be very close.

Method and apparatus for generating an equipment reply signal for the automatic identification of objects and/or living beings

Abstract: Method and apparatus for generating an equipment reply signal for the automatic identification of objects and/or living beings, including a stationary interrogator having an energy transmitter, a receiver, an evaluator connected to the receiver, an opening code generator, and a first modulator connected to the energy transmitter, and a transponder fastened to the object or living being having a circuit unit with an energy receiver and converter, a code memory, a clock generator, a code transmitter, at least one antenna, an opening code receiver, an opening code memory, an opening code comparator connected to the opening code memory and the opening code receiver, a demodulator connected to the energy receiver and converter, and a second modulator connected to the demodulator, code transmitter, opening code transmitter and code memory, which includes radiating an opening code in the interrogator, storing the opening code in the transponder, radiating a code stored in the code memory by the code transmitter if the stored and radiated opening codes coincide, modulating a reply carrier signal on an energy wave with the first modulator, separating the reply carrier signal from the energy wave with the demodulator, modulating the reply signal carrier with the code with the second modulator, and radiating the modulated reply carrier signal as the reply signal with the code transmitter.

Phase locked loop FM demodulator with variable bandwidth loop filter

Abstract: In an FM demodulation circuit which demodulates an FM signal by the use of a phase-lock loop constructed of a phase comparator, a loop filter and a voltage-controlled oscillator, the loop filter is a variable loop filter whose loop band width is varied in correspondence with the modulation band width and the carrier/noise ratio of the input signal by a base band processing circuit.

Adaptive signal quality measurement circuit for PSK and FSK demodulators

Abstract: In a demodulator for an antipodal data stream, such as a PSK or FSK demodulator, a signal quality measuring circuit having a high dynamic range. Any signal quality parameter can be measured, such as BER and E b /N o . The number of zero crossings in a certain error gate is compared with the total scaled number of zero crossings in the sampling interval. These values are translated into a quantitative measure of signal quality by means of a pre-established table of values obtained during calibration of the instrument. The wide dynamic range of the circuit is achieved by using several gates of various widths. The gates are derived from a bit synchronizer; the system functions without the necessity for superimposing upon the data stream any coding information as is common with BER measurement devices of the prior art.

Receiver for a distribution network power line carrier communication system

Abstract: A receiver for a distribution network power line carrier communication system is magnetically coupled to a distribution power line. A carrier signal, phase-shift keyed modulated by an information signal and carried by the distribution power line, is thereby coupled to the receiver circuitry. A first receiver amplification circuit includes an automatic gain control circuit, to prevent saturation of the receiver electronics, and a feedback circuit to determine the gain. The automatic gain control circuit utilizes a field effect transistor which cooperates with the amplifier's feedback circuit to modify the gain. The drain source resistance of the field effect transistor is varied in accord with the magnitude of the received noise or modulated carrier signal to lower the amplifier's gain when excessive positive or negative noise or modulated carrier signal peaks are encountered. The modulated carrier signal is then processed through several filter stages and further amplified. Finally, the information signal is removed from the modulated carrier by a phase shift keyed demodulator. The information is available for input to a remote device which is responsive thereto for performing any of a variety of automated functions.

Receiver for AM stereo signals having a circuit for reducing distortion due to overmodulation

Abstract: A receiver for AM signals, in which the carrier is amplitude modulated with a first signal (L+R) and phase modulated with a second signal (L-R), includes a circuit for reducing distortion in the output signal due to amplitude overmodulation. This circuit includes an overmodulation detector for controlling a switch in a phase demodulation signal channel, which switch, coupled between a demodulator and an integrator in this signal channel, effectively short-circuits the input to the integrator causing the output thereof to remain constant for the duration of the overmodulation.

NRZ Digital data recovery

Abstract: The clock signal required for demodulation of a received NRZ digital data stream is generated by detecting each transition across a reference axis made by the received NRZ digital data stream. This transition data is then utilized to change the voltage applied to a voltage controlled oscillator. The change in the voltage applied to the voltage controlled oscillator causes the frequency of the output from the voltage controlled oscillator to change. The change in the frequency of the output of the voltage controlled oscillator adjusts the period of the clock signal until the clock signal is synchronized with the transition data at which time it may be used to demodulate the recieved NRZ digital data.

Side lock avoidance network for PSK demodulator

Abstract: A sidelock avoidance scheme for preventing sidelock in a PSK demodulator's carrier recovery loop contains augmenting sweep control circuitry, including a frequency discriminator and an associated window comparator. The output of the frequency discriminator, which is low pass filtered to remove noise, is applied to the window comparator which compares any differential between the true carrier and the output of a carrier recovery loop to a preset reference threshold representative of a frequency error condition that may approach sidelock. When the output of the frequency discriminator is greater that this preset reference threshold, an augmented frequency control voltage is applied to the voltage control oscillator of the loop to drive the oscillator away from a possible sidelock condition and toward the true carrier. The augmented frequency control voltage may be derived from a frequency sweep generator or from the output of the frequency discriminator, depending upon a selected strapping option.

Optical modulation/demodulation system

Abstract: An optical modulation/demodulation system comprising a transmitting side circuit which is capable of detecting the leading or the falling edge of a timing pulse, converting the leading and falling detection pulse signal into the pulse of either positive or negative polarity in response to the level of the electric-input signal data, in the case of synchronous systems. The transmitting side circuit in asynchronous systems is capable of generating pulses in response to the leading and falling edge of an electric input signal data, by converting the leading and falling pulse into the one of either positive or negative polarity, overlapping these positive or negative polarity signals on a ready-for-sending signal of a predetermined to be used as an input signal for a light-emitting element.

On the performance of phase-shift-keying systems

Abstract: Coherent phase-shift keying (CPSK) and differential phase-shift keying (DPSK) are widely used modulation methods in digital communications Bandwidth efficiency, good noise immunity, constant envelope, and simplicity of implementation make these schemes particularly attractive for use over the satellite, terrestrial radio and voiceband telephone channels While system analyses abound in the literature, treatment is usually restricted to the additive Gaussian channel Important issues determining ultimate performance, such as the joint effect of intersymbol interference and the acquisition of carrier phase have not been adequately addressed The main purpose of this paper is to develop analytical tools that can be used to assess system performance under practical operating conditions Pure coherent demodulation schemes such as CPSK are ideals which are rarely achieved in practice, and carrier phase must be estimated prior to and/or during data transmission This requires start-up time, as well as added equipment, and the fidelity of the phase estimate ultimately determines performance In contrast, DPSK is independent of carrier phase, since decisions are made on phase differences However, this comes at a price, and it is known that ideal multiphase DPSK suffers an asymptotic performance penalty of 3 dB in signal-to-noise ratio (s/n) over ideal CPSK We develop a new rigorous method for calculating the error rates of both CPSK and DPSK, under a variety of operating conditions In particular, we find that the intersymbol interference penalty for quaternary DPSK is about 1 dB worse in s/n than for CPSK We demonstrate that the detection efficiency of CPSK approaches the ideal, provided that the s/n of the phase-recovery circuit is about 10 dB more than that at the receiver input Alternatively, for the same s/n, a 10-baud phase-locked loop integration time is required to achieve near-ideal performance

Automatic threshold squelch

Abstract: An automatic threshold squelch circuit, responsive to noise signals from the receiver demodulator, having automatic means for setting the squelch level. A noise detector is coupled to the demodulator and to a comparator and sample and hold circuit such that a predetermined portion of the detected noise signal level is stored in the sample and hold circuit when no signal is being received. The stored noise level is then compared to the detected noise signal from the noise detector and a mute signal is generated in response to the detected noise level exceeding the stored signal level.

Microwave Phase Detectors for PSK Demodulators

Abstract: The simplest circuits for microwave phase detectors and their operation are described. Approximate analytical expressions for the output characteristic of the various circuits are given. Accurate prediction of detector performance is achieved with a large-signal nonlinear analysis using simultaneously the time- and frequency-domain approach. Applying the theory developed, the effects which cause deformation of the detector characteristic are investigated. Results of practical circuits operating in the 14-GHz range are given and compared with regard to phase-demodulator applications. A low-level phase detector is presented which permits 20-dB level variation with less than 2° phase error.

Color demodulation device for use in color television receivers

Abstract: A composite video signal is separated into luminance signal component and carrier chrominance signal component by means of a signal separation circuit using a comb filter. The carrier chrominance signal component is multiplexed in frequency-interleaved relation with the luminance signal component, and formed by quadrature modulation of a color subcarrier with a wideband I signal and a narrowband Q signal. A line correlation detector for detecting the correlation between video signals on adjacent horizontal scanning lines is connected with both outputs of the signal separation circuit. The detector detects the absence of line correlation when color subcarrier frequency appears simultaneously at both outputs of the signal separation circuit. The I signal is demodulated in wideband when the line correlation exists and in narrowband when no line correlation exists.

MSK and OK-QPSK signal demodulator

Abstract: An MSK or OK-QPSK data signal demodulator includes a filter and limiter for filtering and amplitude limiting the data signal, a timer for generating a timing signal indicating elapsed time and a microcomputer coupled to the limited data signal from the limiter and the timing signal from the timer. The microcomputer is interrupted by each transition of the limited data signal and stores the timing signal from the timer. An interrupt routine of the microcomputer extracts the clock offset, which is an estimate of the relative location of the bit boundaries, and the phase intercept, which is an estimate of the phase of the data signal at the bit boundaries, from the timing signals.

Radio-frequency synthesizer for duplex radios

Abstract: An improved frequency synthesizer for duplex radios is described that includes a phase-locked loop coupled to a reference oscillator for developing a transmit carrier signal. The phase-locked loop includes a phase comparator, loop filter and voltage controlled oscillator in a forward path for generating a transmit and receive injection signal. A first mixer mixes the injection signal and an offset signal from an offset oscillator to generate the transmit carrier signal. A divider divides the transmit carrier signal by the pre-selected number to generate a feedback signal which is also coupled to the phase comparator. A second mixer mixes the injection signal with a receive carrier signal to generate an intermediate frequency signal, which is coupled to a receiver and demodulated. A duplexer couples the transmit carrier signal to an antenna and also couples the receive carrier signal from the antenna. Two techniques are described for modulating the transmit carrier signal with information signals, such as voice, tone, or data signals. According to one technique, both the reference oscillator and the offset oscillator are modulated. Since the modulations are cancelled by the phase detector, the injection signal is not modulated. According to the other technique, only the offset oscillator is modulated. Modulation of the error signal is cancelled by combining it in a summer with the integrated information signal provided by an integrator. The second modulation technique may be utilized when it is desired to lock the phase-locked loop to an external reference signal or when the reference oscillator cannot be modulated.

Line signaling apparatus and techniques for digital data links

Abstract: Statistical properties of a signal are used to distinguish between a voice signal and an "on hook" indication on a low bandwidth digital data link. Noise is intentionally added to the voice signal to guarantee a minimum power level, while not disrupting normal demodulation. A digital signal, having a known maximum power level upon demodulation, is used to represent the "on hook" condition. Upon demodulation, the minimum power level of the noise-colored voice signal is greater than the maximum power level of the "on-hook" signal, and the two signals can be distinguished by a power level threshold detector. This apparatus and technique is particularly well-suited to under-sampled, such as CVSD (continuously variable slope delta), data links, which are usually noisy and distorted. It alleviates the need for direct access to the digital link and simplifies the required hardware and retrofit modifications.

The Impact of Pulsed RFI on the Coded BER Performance of the Nonlinear Satellite Communication Channel

Abstract: The coded bit error rate (BER) performance of a satellite communications system, wherein the satellite repeater contains an arbitrary nonlinearity and the system operates in the presence of pulsed radio frequency interference (RFI), is examined. A major result is an analytic method for determining soft decision statistics of the receiver demodulator output in which pulsed RFI effects are accounted for. It is further demonstrated how this result can be analytically applied to the approximate determination of the BER at the output of the Viterbi decoder when convolutional coding is employed. Computed results specialize the nonlinearity to either a hard limiter or clipper, in conjunction with an arbitrarily specified AM/PM characteristic. Performance curves examine BER sensitivity to RFI duty cycle, form of RFI (CW or noise), and various coding/decoding conditions.