Abstract: A method for determining the best time to sample data bits of a data bit stream of a received information signal in a digital communication system includes the steps of determining the current derivative of a respective sample of the information signal, weighting the sample by a predetermined amount, determining an accumulated derivative value for the same sample number of a previous symbol of the information signal, weighting the previous accumulated derivative value by another predetermined amount, summing the two weighted values together and assigning this accumulated derivative value to the respective sample. The method further includes the step of comparing the accumulated derivative values of a predetermined number of consecutive samples to determine which sample has the greatest accumulated derivative value. The sample having the greatest accumulated derivative value substantially corresponds to the beginning of a symbol.

Abstract: Apparatus for generating the family of PSK (phase shift keyed) modulations, which include BPSK (binary PSK), QPSK (quaternary PSK), MSK (minimum shift keying) and the like. The carrier is generated with the desired digital information already phase-modulated onto it by directly introducing a phase shift or delay onto the error path of a phase-locked loop. causing the phase-locked loop to create the phase modulation. The [proposed scheme differs from common practice approaches, which are usually implemented by linear synthesis (an AM technique); rather, it] invention employs direct nonlinear synthesis (an FM technique). The invention [yields good phase precision with arbitrary spectral shaping under the constraint of constant envelope signaling. It] permits the connection of the output of a simple, inexpensive VCO (voltage controlled oscillator) directly to a system's antenna without the need for intervening circuit elements such as phase splitters, mixers, and the like which is applicable to [.

Abstract: A technique for modulating and demodulating CPM spread spectrum signals and variations of CPM spread spectrum signals. A transmitter divides a signal data stream into a plurality of data streams (such as I and Q data streams), independently modulates the I and Q data streams using CPM or a related technique, and superposes the plurality of resultants for transmission. A receiver receives the superposed spread spectrum signal and simultaneously attempts to correlate for a plurality of chip sequences (such as I and Q chip sequences), and interleaves the correlated I and Q data streams into a unified signal data stream. In one embodiment, the receiver separates the received spread spectrum signal into real and imaginary parts, attempts to correlate both real and imaginary parts for a plurality of chip sequences, and combines the real I, real Q, imaginary I, and imaginary Q signals into a unified signal data stream.

Abstract: A technique for modulating and demodulating CPM spread spectrum signals and variations of CPM spread spectrum signals. A transmitter divides a signal data stream into a plurality of data streams (such as I and Q data streams), independently modulates the I and Q data streams using CPM or a related technique, and superposes the plurality of resultants for transmission. A receiver receives the superposed spread spectrum signal and simultaneously attempts to correlate for a plurality of chip sequences (such as I and Q chip sequences), and interleaves the correlated I and Q data streams into a unified signal data stream. In one embodiment, the receiver separates the received spread spectrum signal into real and imaginary parts, attempts to correlate both real and imaginary parts for a plurality of chip sequences, and combines the real I, real Q, imaginary I, and imaginary Q signals into a unified signal data stream.

Abstract: A system and method are provided in which a phase modulation generator is used to modulate the output signal of a DDS with a modulation index, h/N. The phase-modulated. DDS output drives a PLL, which is an upconversion stage for a radio transmitter, with the modulated DDS signal as the PLL's reference. The output of the PLL will have a modulation index of h. Consequently, the frequency band of the DDS output is significantly narrower than the frequency band of upconversion stages used in conventional transmitters, and there are significantly less spurious emissions in the transmitted signal. However, a transmitter employing the present phase-modulated DDS-driven PLL topology can still transmit over a relatively wide frequency band.

Abstract: A method for generating minimum shift keying (MSK) modulation signals with relatively reduced energy sidelobes relative to known methods. In one embodiment, a second derivative of the phase waveform is taken and convolved with a selectable smoothing function and integrated twice to generate a smooth phase waveform S2D.sub.-- MSK (i)=where .o slashed. (i)=.pi. i d(i)/2T. In the alternate embodiment, the sidelobe energy can be varied by simply changing the smoothing function, which can be stored in a lookup table. In the embodiment, the MSK modulation signal has a constant amplitude envelope allowing it to be transmitted using class C amplifiers. Since the embodiment provides continuous phase derivatives, the frequency and thus the sidelobe energy is sufficiently reduced to minimize interference with cosite equipment and nearby communication channels bands.

Abstract: A Gaussian Minimum Shift Keying modulator that provides direct modulation of a carrier signal, produced by a single microwave high power voltage controlled oscillator. A continuous phase frequency shift keyed modulated signal with a modulation index of 0.5 is produced at the desired output frequency using a full 360 degree linear continuous phase modulator, controlled by a linear baseband signal that is the integral of the binary baseband information signal. This modulated signal is used as the reference signal for a phase locked high power voltage controlled oscillator. The phase locked loop provides frequency tracking and Gaussian spectral shaping to the modulated output signal.

Abstract: Coherent modulation schemes for CPM signals. The disclosed schemes avoid the error probability increase due to the differential encoding by using a coherent encoder. A simple demodulator can be implemented by using a Nyquist-3 CPM scheme with coherent encoding. Such a scheme can be demodulated in substantially the same manner as signals modulated using M-PSK.

Abstract: A wireless radio modem that may be incorporated into a host system or connected through a PCMCIA or similar port to a host system includes radio frequency modulation/demodulation circuitry employing electronic device elements that operate in a frequency range that minimizes the RF interference between the radio modem and the host system. Radio modem power conservation is maximized by 1) simplifying signal modulation processing by use of a two-point waveform transition table, thereby reducing processing requirements; and 2)incorporating a "sleep mode" feature in which all non-timer circuitry is powered-down when not in use.

Abstract: A technique for modulating and demodulating CPM spread spectrum signals and variations of CPM spread spectrum signals. A spread spectrum transmitter includes a chip sequence generator for generating a chip sequence from a data stream, a switch for dividing said chip sequence into an odd chip sequence and an even chip sequence, and a modulator for generating and transmitting a continuous phase modulated signal from said odd chip sequence and said even chip sequence. A spread spectrum receiver comprises a plurality of non-coherent serial CPM correlators, each generating a correlation signal. In a preferred embodiment, the chip sequence generator of the transmitter comprises a table of symbol codes, each symbol code comprising a series of chips corresponding to a unique series of bits in said data stream, and each non-coherent serial CPM correlator is configured to detect one of the symbol codes.

Abstract: Device for continuous phase modulation, produced from a frequency synthesizer including a variable oscillator and a phase-locked loop with, in series, a frequency division circuit, a phase comparator and a low-pass filter. In order to reduce modulation in the loop, the modulation is applied not only to the input of the synthesizer but also, in compensation, in the loop. In order to eliminate the modulation residue in the loop, a control signal for the oscillator is derived by a circuit which performs a correlation between the residue of the modulation and the output signal from a filter producing the same filtering effect as the loop, and which receives the modulation signal from the input of the synthesizer.

Abstract: A method and apparatus for digitally phase modulating and frequency upconverting communication signals to an intermediate frequency, producing the IF output with only digital hardware and without the use of digital to analog converters, analog multipliers (mixers) or power combiners. A digital phase modulator provides an in-phase and a quadrature output, each of which is coupled to one input of a relatively simple multi-bit to single-bit delta-sigma data converter. The output from the converter is a pair of single-bit digital output signals. Each such single-bit output is inverted and both the inverted single-bit output and the non-inverted single-bit output of both the in-phase and the quadrature outputs are coupled to a 4:1 multiplexer. One of these four inputs is then selected by a modulo-4 counter. The modulo-4 counter is incremented at a rate that is selected based upon the desired IF frequency.

Abstract: A technique for modulating and demodulating continuous phase modulation (CPM) spread spectrum signals and variations thereof. A transmitter encodes M data bits using a selected spread spectrum code, divides the spread spectrum code into a plurality of chip codes (such as even chips and odd chips), independently modulates the even and odd chips with orthogonal carrier signals using CPM or a related technique, and superposes the plurality of resultants for transmission. A receiver receives the superposed spread spectrum signal and divides it into a real signal and an imaginary signal. The real signal is connected to a real CPM correlator which demodulates its input into a real I signal and a real Q signal, separately correlates the real I signal with the odd chips of a chip code and the real Q signal with the even chips of the chip code, and combines the correlation signals into a real correlation signal.

Abstract: A method and corresponding apparatus for error correction of MSK-coded data receives a signal (101) partitionable into adjacent frames (111,113) each subdividable into three tridents (103, 105, 107). A first sample quality metric (225) applicable to a p (123) of the signal (101) associated with a last trident (107) of the tridents of a first frame (111) of the adjacent frames (111, 113) is synthesized, and a second sample quality metric (227) applicable to a portion (125) of the signal (101) associated with a first trident (103') of the tridents of a second frame (113) following the first frame (111) is synthesized. Each of the first and second sample quality metrics (225, 227) have a magnitude dependent on a time that the associated portion (123) of the signal (101) has a magnitude exceeding an average magnitude (127) of the signal (101).

Abstract: An AFC system and method of detecting and compensating frequency drift in the carrier frequency of a Gaussian filtered Minimum Shift Keying (GMSK) signal by demodulated frequency compensated samples of the received signal and calculating a frequency compensation angle based upon the demodulated frequency compensation samples. The compensation angle is fed back to the frequency compensator and error detector and used to calculate the error signal.

Abstract: A decision feedback multiple symbol differential demodulator for Gaussian filtered Minimum Shift Keying (GMSK) has a performance very close to theoretical limit even under a fading channel. The demodulator implements a non-coherent, maximum likelihood (ML) search over multiple bit-time intervals. The past bit decisions are fed back to the ML decision logic. The improved performance of the demodulator is achieved by using old decisions in the ML decision logic.

Abstract: A surface acoustic wave (SAW) device is described that comprises a piezoelectric medium using input and output transducers for launching and detecting a surface acoustic wave on the piezoelectric medium. In particular, a device is described that uses a special combination of transducers for the direct generation of a coded minimum shift keyed (MSK) waveform from an input impulse waveform. The same device is used for the direct detection of an input MSK waveform in order to generate an impulse waveform with sidelobe characteristics similar to those expected from a specific code. The SAW device provides the advantages of MSK encoded waveforms for spread spectrum applications and the advantage of implementing such entirely on a SAW substrate.

Abstract: A variable multi-threshold detection circuit for a sampled baseband signal in the receiver of a communication system using GMSK modulation is disclosed. The circuit comprises an integrate/dump circuit for accumulating a predetermined number of samples of the baseband signal to generate a sum for a present bit, a level selector for selecting a threshold from a plurality of groups of multi-thresholds based on an estimated phase offset and the binary value of two previous bits, a compare circuit for comparing the sum from the integrate/dump circuit with the threshold selected from the level selector to generate a binary value of either one of "0" or "1" for the present bit, and a delay circuit for delaying the present bit to be used by the level selector to determine a next threshold for a next bit.

Abstract: In the modulation and demodulation of a plurality of frequency separated channels on a radio frequency carrier by digitally coded speech or data, the speech or data is modulated on a digitally generated sub-carrier by quadrature phase shift keying and after conversion to analogue form the modulated sub-carrier is mixed with an RF carrier of fixed frequency to produce the signal for transmission. Reception and demodulation of the transmitted signal are effected by the reverse processes. Frequency multiplication is effected after the digital to analogue conversion by producing analogue samples of very short duration and applying them to a suitable filter. Frequency division during the analogue to digital conversion is effected by sub-sampling.

Abstract: An MSK receiver includes a downconverter for downconverting the received MSK signals. The phase drift or phase error per bit of the message preamble is determined. The reference carriers are generated, and the phase of the reference signals is adjusted during message demodulation under the control of the estimated rate of phase change across the preamble portion. In accordance with another aspect of the invention, the downconverted MSK signal burst is digitized and the digitized burst signal is stored in memory, following which it is read repeatedly so that bit timing, start-of-message timing, carrier phase, and carrier drift processing may be performed in a desired sequence, using all the available information for more accurate demodulation. According to a yet further aspect of the invention, a second order tracking loop is used to adjust the phase of the demodulation reference signals.