Spread spectrum communications system

Abstract

A spread spectrum communication system for direct sequence transmission of digital information having a modulation format which is particularly suitable for indoor communication within residential, office and industrial structures. The modulation format combines BPSK or MSK spreading with FM carrier modulation by data bits and a carrier frequency shift whose magnitude is related to both a chip rate and a spreading sequence length. The carrier, chip clock and data clock are all synchronous and the sequence length is an integral submultiple of the bit length. The system reduces the frequency error between the transmitter chip clock and the receiver chip clock to permit the elimination of a code phase tracking loop in the receiver to reduce the receiver complexity. The receiver has an extended dynamic range which makes possible the reception of very strong signal without an automatic gain control loop (AGC) as well as reducing the time needed for code phase acquisition. The transmission system is highly resistant to CW jamming and short distance multipath effects.

We claim: 
     
       1. A method of transmitting digital data, comprising the steps of: a) generating a high frequency carrier at a first carrier frequency;   b) modulating the frequency of said carrier with data bits assembled in data packets of predetermined format;   c) generating a high frequency carrier at a first carrier frequency and shifting the nominal frequency of said carrier by a frequency increment DF to obtain a second carrier frequency, where   DF=K*1/Ts+dF,   dF=approximately L/(2*Ts),   K is an integer,   L is an odd integer, and   Ts is a PN (pseudo noise) sequence period;   d) modulating the frequency of said carrier in step c) with said data bits assembled in data packets of said predetermined format;   e) spreading said carrier at said first and said second frequency, during a transmission time, by a predetermined PN sequence having a period Ts related to a bit time Tb where   Tb=N * Ts, N being an integer >1 whereby a spread carrier spectrum comprising many spectral components separated by 1/Ts is obtained, the amplitudes of said components being reduced by said spreading function, said components and the modulation imposed upon them being separable from other components by a narrow band filter.   
     
     
       2. The method according to claim 1, further comprising the steps of: f) receiving the transmitted data; and   g) multiplying said transmitted spread carrier in a receiver by a locally generated predetermined PN sequence, for collapsing the bandwidth of the received spread carrier when the local PN sequence phase is in agreement with the sequence phase imposed on the received spread carrier thereby spreading any jamming signals which are received along with the desired transmitted signal into many components separated by 1/Ts intervals and reduced in amplitude by the spreading function.   
     
     
       3. The method according to claim 1, wherein step e) generates spectral components of the spread carrier that are separated by a frequency interval greater than 2 * IFbw, where Ifbw is the receiver bandwidth after step g). 
     
     
       4. The method according to claim 1, wherein a chip clock producing a signal at a frequency of .[.1/Ts.]. .Iadd.1/Tc .Iaddend.and the carrier frequency are synchronously derived from one frequency source. 
     
     
       5. The method according to claim 4, wherein the data frequency is synchronously derived from said one frequency source. 
     
     
       6. The method according to claim 1, wherein K is greater than a sequence length for generating a frequency shift substantially equal to or greater than a .[.chip.]. .Iadd.sequence .Iaddend.frequency of 1/Ts. 
     
     
       7. The method according to claim 1, wherein said predetermined data packet format comprises a preamble period during which said carrier frequency is equal to a nominal frequency and a data period during which the carrier frequency is modulated by said data bits. 
     
     
       8. The method according to claim 1 wherein each of said components are modulated in the same manner as said carrier prior to spreading and whereby the resulting modulated component spectra do not overlap. 
     
     
       9. A transmitter for transmitting digital data comprising: a) carrier frequency generator means for generating a carrier frequency signal at a first frequency.;   b) modulator means for modulating said carrier with data bits;   c) frequency shift means for generating a second carrier frequency offset from said first carrier frequency by DF, where   DF=K*1/Ts+dF,   dF=approximately L/(2*Ts),   K is an integer,   L is an odd integer, and   Ts is a PN (pseudo noise) sequence period;   d) pseudo random sequence generator means for generating a predetermined PN sequence;   d) carrier spread means for spreading power of said first and second earlier frequencies by said pseudo noise sequence.   
     
     
       10. The transmitter according to claim 9 further including a phase lock loop means for deriving said first carrier frequency from a crystal reference oscillator. 
     
     
       11. The transmitter according to claim 10 further including divider means in said phase lock loop for dividing said first carrier frequency by M or M+1, where M is an integer; means for generating a frequency control signal, said frequency control signal controlling said divider means to divide by M or M+1. 
     
     
       12. The transmitter according to claim 11 wherein M is equal to 64. 
     
     
       13. The transmitter according to claim 11 wherein M is equal to 128. 
     
     
       14. The transmitter according to claim 9 wherein said frequency shift means generates said second carrier at a frequency shift from said first carrier frequency equal to or greater than a .[.chip.]. .Iadd.sequence .Iaddend.frequency of 1/Ts. 
     
     
       15. The transmitter according to claim 9 wherein said transmitter further incudes means for deriving chip frequency of .[.1/Ts.]. .Iadd.1/Tc .Iaddend.and said first carrier frequency from a single reference frequency source. 
     
     
       16. The apparatus according to claim 9 wherein said transmitter further includes means for deriving chip frequency of .[.1/Ts.]. .Iadd.1/Tc.Iaddend., said first carrier frequency and a data clock frequency from a single reference frequency source. 
     
     
       17. The transmitter according to claim 9 wherein said transmitter further includes means for generating said formatted data packet for modulating said carrier frequency, said data packet comprising a preamble period during which the carrier frequency is equal to a nominal value and data period during which the carrier frequency is deviated according to the corresponding data bit values. 
     
     
       18. A receiver for a spread spectrum signal comprising: a) a wideband receiving means for receiving a spread carrier radio signal for generating a spread carrier electrical signal;   b) means for generating a predetermined PN (pseudo noise) sequence;   c) multiplying means coupled to said receiving means and said generating means for multiplying said spread carrier electrical signal by said predetermined PN sequence, for collapsing a bandwidth of the received spread carrier when the local PN sequence phase is in agreement with the sequence phase imposed on the received spread carrier by a transmitter means, thereby spreading any jamming signals which are received along with the transmitted spread signal into many components separated by 1/Ts intervals and reduced in amplitude by the spreading function;   d) narrow band FM receiver means coupled to an output of said multiplying means for recovering data;   e) means coupled to said wideband receiving means for alternately receiving a radio signal on one of two preselected frequencies; and   f) frequency switch means coupled to said means for alternately receiving for switching the received frequency between a first and a second of said preselected frequencies at predetermined time intervals.   
     
     
       19. The receiver according to claim 18 wherein a bandwidth said narrow band FM receiver means is less than 1/(2*Ts) where Ts is a period of said PN sequence. 
     
     
       20. The receiver according to claim 19 wherein said means for alternately receiving is a local oscillator and said frequency switch means changes the frequency thereof. 
     
     
       21. The receiver according to claim 19 wherein said means for alternately receiving comprises an independent means for receiving operating at a second one of said preselected frequencies. 
     
     
       22. The receiver according to claim 18 wherein each of said receiver means further includes means for deriving a chip frequency of .[.1/Ts.]. .Iadd.1/Tc .Iaddend.and a carrier frequency from a single reference frequency source. 
     
     
       23. The receiver according to claim 22 wherein each of said receiver means further includes means for deriving a chip frequency of .[.1/Ts.]. .Iadd.1/Tc.Iaddend., a carrier frequency, and a data clock frequency from a single reference frequency source. 
     
     
       24. The receiver according to claim 22 wherein each of said receiver means further includes means for automatic frequency control (AFC) and means for activating said AFC upon synchronization of a phase of said PN sequence with a phase of the PN sequence modulating said received signal. 
     
     
       25. The receiver according to claim 18 wherein each of said receiver means further includes signal strength indicator means to detect power of said received signal. 
     
     
       26. A system for transmitting and receiving digital data, comprising: a) means for generating a high frequency carrier at a first carrier frequency;   b) means for modulating the frequency of said carrier with data bits assembled in data packets of predetermined format;   c) means for shifting the nominal frequency of said carrier by a frequency increment DF to obtain a second carrier frequency, where   DF=K*1/Ts+dF,   dF=approximately L/(2*Ts),   K is an integer,   L is an odd integer, and   Ts is a PN (pseudo noise) sequence period;   d) means for spreading said carrier at said first and said second frequency, during a transmission time, by a predetermined PN sequence having a period Ts related to a bit time Tb where   Tb=N * Ts, N being an integer >1 whereby a spread carrier spectrum comprising many spectral components separated by 1/Ts is obtained, the amplitudes of said components being reduced by said spreading function,   e) means for receiving the transmitted data; and   f) means for multiplying said transmitted spread carrier in said receiver means by a locally generated predetermined PN sequence, for collapsing the bandwidth of the received spread carrier when the local PN sequence phase in agreement with the sequence phase imposed on the received spread carrier.   
     
     
       27. The system according to claims 26 further comprising: g) narrow band FM receiver means coupled to an output of said multiplying means for recovering data;   h) means coupled to said receiving means for alternately receiving a radio signal on one of two preselected frequencies; and   i) frequency switch means coupled to said means for alternately receiving for switching the received frequency between a first and a second of said preselected frequencies at predetermined time intervals.   
     
     
       28. The system according to claim 27, wherein said means for spreading said carrier generates spectral components of the spread carrier that are separated by a frequency interval greater than 2 * IFbw, where IFbw is the receiver bandwidth after said multiplying means. 
     
     
       29. The system according to claim 26, wherein a chip clock producing a signal at a frequency of .[.1/Ts.]..Iadd.1/Tc .Iaddend.and the carrier frequency are synchronously derived from the frequency source. 
     
     
       30. The system according to claim 29, wherein the data frequency is synchronously derived from said one frequency source. 
     
     
       31. The system according to claim 26, wherein K is greater than a sequence length for generating a frequency shift substantially equal to or greater than a .[.chip.]. .Iadd.sequence .Iaddend.frequency of 1/Ts. 
     
     
       32. The system according to claim 26 wherein said predetermined data packet format comprises a preamble period during which said carrier frequency is equal to a nominal frequency and a data period during which the carrier frequency is modulated by said data bits. 
     
     
       33. The system according to claim 26 further comprising means for repeating the transmission of said data. 
     
     
       34. The system according to claim 26 wherein each of said components are modulated in the same manner as said carrier prior to spreading and whereby the resulting modulated component spectra do not overlap. 
     
     
       35. In a spread spectrum transmitter having a phase-lock loop for generating a carrier signal at a first frequency, said phase-lock loop including a voltage controlled oscillator having an output coupled to a frequency divider and an input coupled to an output of a phase detector, said phase detector having one input coupled to the output of said divider and a second input coupled to an output of a reference oscillator; frequency shift means for generating a second carrier frequency offset from said first carrier frequency; and means for generating a spread spectrum signal from said first and said second carrier and a spreading signal, the improvement comprising said frequency .[.driver.]. .Iadd.divider .Iaddend.consisting of a dual modulus prescaler. 
     
     
       36. In a spread spectrum .[.transmitter.]. .Iadd.receiver .Iaddend.having a phase-lock loop for generating a carrier signal at a first frequency, said phase-lock loop including a voltage controlled oscillator having an output coupled to a frequency divider and an input coupled to an output of a phase detector, said phase detector having one input coupled to the output of said divider and a second input coupled to an output of a reference oscillator; means for receiving a second carrier frequency offset from said first carrier frequency; means for generating a spread spectrum signal from said first and said second carrier and a spreading signal; and means for despreading an incoming RF signal utilizing said spread spectrum signal, the improvement comprising said frequency divider consisting of a dual modulus .[.prescale.]. .Iadd.prescaler.Iaddend..