HF high data rate modem

• US 5,097,485 A
• Filed: 10/10/1989
• Issued: 03/17/1992
• Est. Priority Date: 10/10/1989
• Status: Expired due to Term

• Current Assignee: Hughes Electronics Corporation (AT&T, Inc.)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Cai, Khiem V., O'Connor, Roger J.
• Primary Examiner(s): Chin, Stephen
• Application Number:
  US07/419,166
• Time in Patent Office: 889 Days
• Field of Search: 375/1, 375/52, 375/56, 375/67, 375/83, 375/84, 375/115, 455/110, 455/205, 332/103, 329/304
• US Class Current: 375/283
• CPC Class Codes: H04B 1/713 using frequency hopping

Abstract

Methods and apparatus for achieving high frequency (HF) modem communications. A predetermined plurality of frequency hopped, digital phase shift keying modulated tones are generated at a predetermined number of pseudo-randomly generated frequencies during a successive plurality of time frames. The phase of the tone at each frequency during a particular time frame is differentially modulated to one of a predetermined number of new phases using the phase of the tone at the same frequency in the previous frame as a reference. The tones are transmitted in the high frequency (HF) band, and then received and converted into digital signals. The digital signals are processed to determine the phase of the tone at each transmitted frequency. This may be accomplished by means of a Fourier transform filter. Finally the phase of each tone is compared to the phase of the tone received at the same frequency in the previous frame to decode the received signals, and hence accomplish the desired communications. The present invention provides for communications methods and apparatus which achieves HF skywave communications at high data rates with low bit error rate performance. The invention employs the use of frequency hopped digital phase shift keying and the convolutional coding that overcomes the dispersive nature of the HF channel and achieves the high data rate without resorting to channel equalization techniques. The performance of the modem of the present invention is not limited by the multimode multipath characteristics of the HF path. This is due to selection of a frequency dwell time that is significantly less than the multipath differential times of arrival, and a frame time of the waveform that is longer than the maximum multipath differential times of arrival.

Claims

1. A high frequency modem comprising:

a transmitter comprising (a) phase generator means for processing digital words that comprise data that is to be transmitted and for converting the digital words into a series of corresponding phase encoded digital signals, each successive phase encoded digital signal having a phase relationship with respect to the preceding phase encoded signal such that the currently processed signal has its phase component incremented by the phase value of the preceding processed digital word; and

(b) digital synthesizing means coupled to the phase generator means for processing the phase encoded signals and generating a series of digital signals that have individual frequencies randomly selected from a predetermined set of frequencies, and whose respective phases correspond to the phases of the respective phase encoded signals, and wherein the frequency dwell time is significantly less than the multipath differential times of arrival, and such that further occurrences of the same frequency occur no more frequently than the maximum multipath differential times of arrival; and

a receiver comprising demodulation means for demodulating the frequency hopped, digital phase shift keying signals and providing digital words which comprise the information transmitted by the transmitter.

2. Apparatus for communicating using the high frequency (HF) band, said apparatus comprising:

digital synthesizing means for generating a predetermined plurality of frequency hopped, digital phase shift keying modulated tones at a predetermined number of psuedo-randomly generated frequencies during each frame, wherein the phase of the tone at each frequency during a particular frame is differentially modulated to one of a predetermined number of new phases using the phase of the tone at the same frequency in the previous frame as a reference, and wherein each tone has a dwell time that is significantly less than the multipath differential times of arrival, and wherein the frame time is longer than the maximum multipath differential times of arrival;

transmitting means for transmitting tones in the high frequency (HF) band;

receiving means for receiving transmitted tones;

analog to digital conversion means for converting received tones into digital signals;

filter means for processing the digital signals by means of a Fourier transform filter to determine the phase of the tone at each transmitted frequency; and

phase generation means for comparing the phase of a selected tone at a particular frequency to the phase of the tone received at the same frequency in the previous frame to decode the received signals.

3. A transmitter comprising:

phase generator means for processing digital words that comprise data that is to be transmitted and for converting the digital words into a series of corresponding phase encoded digital signals, each successive phase encoded digital signal having a phase relationship with respect to the preceding phase encoded signal such that the currently processed signal has its phase component incremented by the phase value of the preceding processed digital word; and

digital synthesizing means coupled to the phase generator means for processing the phase encoded signals and generating a series of digital signals that have individual frequencies randomly selected from a predetermined set of frequencies, and whose respective phases correspond to the phases of the respective phase encoded signals, and wherein each signal has a dwell time that is significantly less than the multipath differential times of arrival, and such that further occurrences of the same frequency occur no more frequently than the maximum multipath differential times of arrival.

4. A receiver comprising:

demodulation means for demodulating successive frames of frequency hopped, digital phase shift keying signals, wherein each signal has a dwell time that is significantly less than the multipath differential times of arrival, and wherein the frame time is longer than the maximum multipath differential times of arrival, and for providing digital words that comprise encoded information, said demodulation means comprising;

analog to digital conversion means for converting received tones into digital signals;

filter means for processing the digital signals by means of a Fourier transform filter to determine the phase of the tone at each transmitted frequency; and

phase generation means for comparing the phase of a selected tone at a particular frequency to the phase of the tone received at the same frequency in the previous frame to decode the received signals.

5. A method of transmitting and receiving data using the HF frequency bandwidth, said method comprising the steps of:

generating a plurality of digital words that comprise data which is to be transmitted;

converting the digital words into a series of signals having uniquely encoded phase information that corresponds to each digital word;

generating a plurality of digital signals which each have a frequency randomly selected from a predetermined set of frequencies, wherein each signal has a dwell time that is significantly less than the multipath differential times of arrival, and such that further occurrences of the same frequency occur no more frequently than the maximum multipath differential times of arrival;

modulating the signals having the randomly selected frequency with the uniquely encoded phase information;

transmitting the modulated signals;

receiving the transmitted signals;

converting the received signals into digital samples;

extracting the phase information from the digital samples; and

converting the phase information into digital signals corresponding to the digital words transmitted by the transmitter.

6. A method of transmitting information using the HF frequency band, said method comprising the steps of:

sequentially processing input data comprising a plurality of digital words which are to be transmitted;

sequentially generating individual phase encoded signals that correspond to the digital words;

sequentially generating HF signals having randomly selected frequencies within a predetermined frequency bandwidth that are synthesized from a predetermined set of of frequencies, wherein each signal has a dwell time that is significantly less than the multipath differential times of arrival, and such that further occurrences of the same frequency occur no more frequently than the maximum multipath differential times of arrival; and

sequentially modulating the HF signals such that the phase of a selected HF signal is comprised of the phase of the previously transmitted HF signal at that frequency plus the phase of the phase encoded signal.

7. A method of communicating using the high frequency (HF) band, said method comprising the steps of:

generating a plurality of digital words that comprise data which is to be transmitted;

converting the digital words into a series of signals having uniquely encoded phase information that corresponds to each digital word;

selecting a set of frequencies within the HF frequency band;

randomly permuting the set of frequencies on each time frame to provide modulated signals, wherein each signal has a dwell time that is significantly less than the multipath differential times of arrival, and such that further occurrences of the same frequency occur no more frequently than the maximum multipath differential times of arrival;

transmitting the modulated signals;

receiving the transmitted signals;

converting the received signals into digital samples;

extracting the phase information from the digital samples; and

converting the phase information into digital signals corresponding the digital words transmitted by the transmitter.

8. A method of communicating using the high frequency (HF) band, said method comprising the steps of:

modulating signals that are to be communicated using a predetermined plurality of successive frames of frequency hopped, digital phase shift keying tones at a predetermined number of pseudo-randomly generated frequencies during each frame, wherein each signal has a dwell time that is significantly less than the multipath differential times of arrival, and wherein the frame time is longer than the maximum multipath differential times of arrival, and wherein the phase of the tone at each frequency during a particular frame is differentially modulated to one of a predetermined number of new phases using the phase of the tone at the same frequency in the previous frame as a reference;

transmitting the modulated signals using high frequency (HF) skywave propagation;

receiving the transmitted modulated signals;

converting received modulated signals into digital signals;

processing the digital signals to determine the phase of the modulated signal at each transmitted frequency; and

comparing the phase to the phase of the modulated signal received at the same frequency in the previous frame to demodulate the received signals.

9. The method of claim 8 wherein the processing step comprises the step of:

processing the digital signals by means of a Fourier transform to determine the phase of the modulated signal at each transmitted frequency.