Minimization of multipath and doppler effects in radiant energy ...

1. In a system for communicating information from a transmitter to a receiver, through a medium in which transmitted energy representative of the information is subject to multipath delay spread effects and doppler frequency-shift effects, where the information is transmitted in the form of a series of modulated information pulses of differing nominal frequencies, the nominal frequencies of successive pulses being repeatedly shifted through a predetermined sequence of F frequencies repeated every transmitter time frame of period T=F.tau., where .tau. equals the duration of each information pulse and where the modulation represents the information, each of said information pulses being received in the form of a plurality of doppler-shifted arrival pulses which travelled different paths of propagation,

an improved apparatus for reproducing the information at the receiver, said apparatus comprising:

(a) a plurality of detection units, each adapted to detect arrival pulses received at a distinct frequency, the range of frequencies detected being broad enough to include all shifted frequencies which will be received at predetermined doppler shifts occurring over an expected range of relative velocities of the transmitter and the receiver, each of said detection units being further adapted to produce, from each arrival pulse detected thereby, a demodulated arrival pulse simultaneous therewith, said demodulated arrival pulse having a polarity representing the information communicated by the arrival pulse and having a magnitude representing the magnitude of the arrival pulse;

(b) means for comparing the energy received at the distinct frequencies to determine the F doppler-shifted frequencies at which the arrival pulses are received and for producing a doppler select signal representative of the doppler shift;

(c) means for producing a pulse coexistent with the reception of each arrival pulse detected at a predetermined one of said doppler-shifted frequencies;

(d) a delay unit for receiving the coexistent pulses and producing F replicas of each pulse after delays of 0, .tau., 2.tau., . . . (F-1).tau., the replicas of each coexistent pulse being produced simultaneously with the reception of F arrival pulses detected at said F doppler-shifted frequencies;

(e) a plurality of accumulators each adapted to algebraically accumulate the magnitudes of the demodulated arrival pulses produced by a respective one of the detection units;

(f) a plurality of switches, each connected between one of the accumulators and its respective detection unit;

(g) a frequency select unit for receiving the doppler select signal and the F replicas of each coexistent pulse, said doppler select signal causing the F replicas to be applied as gating pulses to respective ones of the F switches connected to the detection units adapted to detect the doppler-shifted frequencies received, the specific switch to which each of the F gating pulses is applied being selected such that the switch closes each time a demodulated arrival pulse is produced by the detection unit connected thereto and passes said pulse to the connected accumulator;

(h) means for sampling the accumulation in each accumulator after the magnitudes of all demodulated arrival pulses representing the information communicated by a single information pulse have been accumulated thereby, the sequential samples thus formed representing the information transmitted; and

(i) means for resetting each accumulation to a predetermined magnitude after it has been sampled.

2. In a system for communicating information from a transmitter to a receiver, through a medium in which transmitted energy representative of the information is subject to multipath delay spread effects and doppler frequency-shift effects, where the information is transmitted in the form of a series of modulated information pulses of differing nominal frequencies, the nominal frequencies of successive pulses being repeatedly shifted through a predetermined sequence of F frequencies repeated every transmitter time frame of period T=F.tau., where .tau. equals the duration of each information pulse and where the modulation represents the information, each of said information pulses being received in the form of a plurality of doppler-shifted arrival pulses which travelled different paths of propagation,

an improved apparatus for reproducing the information at the receiver, said apparatus comprising:

(a) a plurality of detection units, each adapted to detect arrival pulses received at a distinct frequency, the range of frequencies detected being broad enough to include all doppler shifted frequencies which will be received at predetermined doppler shifts occurring over an expected range of relative velocities of the transmitter and the receiver, each of said detection units being further adapted to produce, from each arrival pulse detected thereby, a demodulated arrival pulse simultaneous therewith, said demodulated arrival pulse having a polarity representing the information communicated by the arrival pulse and having a magnitude representing the magnitude of the arrival pulse;

(b) a plurality of delay units each utilized to test for the existence of one of the predetermined doppler shifts, each unit including F delay elements connected to respective ones of the detection units adapted to detect the doppler-shifted frequencies at which the arrival pulses will be received when one of the predetermined doppler shifts exists, each delay element delaying the detected arrival pulses produced by the detection unit connected thereto by the number of time intervals .tau. by which the corresponding transmitted frequency precedes a selected one of the transmitted frequencies in the sequence;

(c) a plurality of analog summers utilized in conjunction with the delay units to test for the existence of one of the predetermined doppler shifts, each summer being connected to a respective one of the delay units to continuously sum the magnitudes of the detected arrival pulses delayed thereby and to produce a series of composite pulses therefrom;

(d) comparator means for determining which analog summer is producing the composite pulses of greatest magnitude and for producing a doppler select signal representing the doppler shift tested for by the summer, said doppler select signal representing the existing doppler shift and said composite pulses representing the propagation delays at which each arrival pulse is received;

(e) switching means for receiving the composite pulses and the doppler select signal, and for passing to an output thereof the composite pulses produced by the summer testing for the doppler shift represented by said doppler select signal;

(f) a delay unit connected to the output of the switching means for receiving the composite pulses passed thereto and producing F replicas of each pulse after delays of 0, .tau., 2.tau., . . . (F-1).tau., the replicas of each composite pulse passed being produced simultaneously with the reception of F arrival pulses detected at the F doppler-shifted frequencies received, and at the propagation delay represented by the composite pulse;

(g) a plurality of accumulators each adapted to algebraically accumulate the magnitudes of the demodulated arrival pulses produced by a respective one of the detection units;

(h) a plurality of switches, each connected between one of the accumulators and its respective detection unit;

(i) a frequency select unit for receiving the doppler select signal and the F replicas of each composite pulse passed, said doppler select signal causing the F replicas to be applied as gating pulses to respective ones of the F switches connected to the detection units adapted to detect the doppler-shifted frequencies received, the specific switch to which each of the F gating pulses is applied being selected such that the switch closes each time a demodulated arrival pulse is produced by the detection unit connected thereto and passes said pulse to the connected accumulator;

(j) means for sampling the accumulation in each accumulator after the magnitudes of all demodulated arrival pulses representing the information communicated by a single information pulse have been accumulated thereby, the sequential samples thus formed representing the information transmitted; and

(k) means for resetting each accumulation to a predetermined magnitude after it is has been sampled.

3. In a system for communicating information from a transmitter to a receiver, through a medium in which transmitted energy representative of the information is subject to multipath delay spread effects and doppler frequency-shift effects, where the information is transmitted in the form of a series of modulated information pulses of differing nominal frequencies, the nominal frequencies of successive pulses being repeatedly shifted through a predetermined sequence of F frequencies repeated every transmitter time frame of period T=F.tau., where .tau. equals the duration of each information pulse and where the modulation represents the information, each of said information pulses being received in the form of a plurality of doppler-shifted arrival pulses which travelled different paths of propagation,

an improved apparatus for reproducing the information at the receiver, said apparatus comprising:

(a) a plurality of detection units, each adapted to detect arrival pulses received at a distinct frequency, the range of frequencies detected being broad enough to include all doppler shifted frequencies which will be received at predetermined doppler shifts occurring over an expected range of relative velocities of the transmitter and the receiver, each of said detection units being further adapted to produce, from each arrival pulse detected thereby, a demodulated arrival pulse simultaneous therewith, said demodulated arrival pulse having a polarity representing the information communicated by the arrival pulse and having a magnitude representing the magnitude of the arrival pulse;

(b) means for producing a series of frame interval pulses during each successive receiver time frame of period T, said pulses being produced at predefined delays relative to the beginning of the time frame in which they are produced;

(c) a switching device having a plurality of inputs and outputs, each input being connected to a respective one of the detection units to receive arrival pulses detected at one of the distinct frequencies, each output being provided to enable testing for a different one of the possible doppler shift/propagation delay combinations that can be formed from said predetermined doppler shifts and the predefined delays at which the frame interval pulses are produced, the switching device performing said testing during each receiver time frame by connecting each output to the inputs at which detected arrival pulses will be received if the doppler shift tested for at the output exists, the inputs being connected to the output in the same sequence as detected arrival pulses will be received thereby if the propagation delay tested for at the output exists;

(d) a plurality of storage elements utilized in conjunction with the switching device to test for the possible doppler shift/propagation delay combinations, each storage element being connected to a respective one of the switching device outputs to receive the detected arrival pulses passed to said output by the switching device, each storage element producing a test signal having a magnitude representative of the repetitiveness with which detected arrival pulses are received thereby;

(e) comparator means connected to the storage elements for determining which test signal has the greatest magnitude and for producing a doppler select signal representing the doppler shift tested for by the storage element producing the test signal of greatest magnitude, said doppler select signal representing the existing doppler shift;

(f) comparator means connected to the storage elements to determine which of the test signals produced by the storage elements testing for the existing doppler shift and the possible propagation delays have magnitudes indicative of an existing propagation delay, and for each of such signals producing a delay select signal representing the existing delay;

(g) switching means for receiving the frame interval pulses and the delay select signals, and for passing to an output thereof the frame interval pulses corresponding to the existing delays represented by said delay select signals;

(h) a delay unit connected to the output of the switching means for receiving the frame interval pulses passed thereto and producing F replicas of each pulse after delays of 0, .tau., 2.tau., . . . (F-1).tau., the replicas of each frame interval pulse passed being produced simultaneously with the reception of F arrival pulses detected at the F doppler-shifted frequencies received, and at the propagation delay represented by the frame interval pulse;

(i) a plurality of accumulators each adapted to algebraically accumulate the magnitudes of the demodulated arrival pulses produced by a respective one of the detection units;

(j) a plurality of switches, each connected between one of the accumulators and its respective detection unit;

(k) a frequency select unit for receiving the doppler select signal and the F replicas of each frame interval pulse passed, said doppler select signal causing the F replicas to be applied as gating pulses to respective ones of the F switches connected to the detection units adapted to detect the doppler-shifted frequencies received, the specific switch to which each of the F gating pulses is applied being selected such that the switch closes each time a demodulated arrival pulse is produced by the detection unit connected thereto and passes said pulse to the connected accumulator;

(l) means for sampling the accumulation in each accumulator after the magnitudes of all demodulated arrival pulses representing the information communicated by a single information pulse have been accumulated thereby, the sequential samples thus formed representing the information transmitted; and

(m) means for resetting each accumulation to a predetermined magnitude after it has been sampled.