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 radio frequency carrier;   b) modulating the frequency of said carrier by data bits assembled in data packets of predetermined format;   c) spreading said carrier during a transmission time, which a predetermined PN sequence having a period Ts related to a bit time Tb where   Tb=N*Ts,        N being an integer >1 Ts is a pseudo noise (PN) sequence period; 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 modulated imposed upon said component being separable from other components by a narrow band filter having a bandwidth less than 1/(2*Ts).       
     
     
       2. The method according to claim 1, further comprising the steps of: d) receiving the transmitted data; and   e) 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 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 c) generates spectral components of the spread carrier that are separated by a frequency interval greater than 1/Ts. 
     
     
       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 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. 
     
     
       7. A transmitter for transmitting digital data comprising: a) carrier frequency generator means for generating a carrier frequency signal;   b) modulator means for modulating said carrier by data bits;   c) pseudo random sequence generator means for generating a predetermined PN sequence;   d) carrier spread means for spreading power of said carrier frequency by said pseudo noise sequence, said spread carrier generating spectral components separated by a frequency interval greater than 1/Ts where Ts is a pseudo noise (PN) sequence period.   
     
     
       8. The transmitter according to claim 7 wherein Ts is related to the bit time Tb where   Tb=N*Ts,     N being an integer>1.   
     
     
       9. The transmitter according to claim 7 further including a phase lock loop means for deriving said first carrier frequency from a crystal reference oscillator. 
     
     
       10. The transmitter according to claim 7 wherein said transmitter further includes means for deriving chip frequency of . .1/Ts.!. .Iadd.1/Tc .Iaddend.and said carrier frequency from a single reference frequency source. 
     
     
       11. The transmitter according to claim 7 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. 
     
     
       12. The transmitter according to claim 7 wherein said transmitter further includes means for generating said formated 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. 
     
     
       13. A receiver for a spread spectrum signal comprising: a) a wide band receiving means for receiving a spread carrier radio signal for generating a spread carrier electric 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 and having a bandwidth less than 1/(2*Ts) where Ts is at period of said PN sequence.   
     
     
       14. The receiver according to claim 13 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. 
     
     
       15. The receiver according to claim 14 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. 
     
     
       16. The receiver according to claim 14 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 receiver signal. 
     
     
       17. The receiver according to claim 13 wherein each of said receiver means further includes signal strength indicator means to detect power of said received signal. 
     
     
       18. A system for transmitting and receiving digital data, comprising: a) means for generating a radio frequency carrier;   b) means for modulating the frequency of said carrier by data bits assembled in data packets of predetermined format;   c) 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 Ts is a PN (pseudo noise) sequence period; 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;     d) means for receiving the transmitted data; and   e) 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;   f) narrow band FM receiver means coupled to an output of said multiplying means for recovering data and having a bandwidth less than 1/(2*Ts).   
     
     
       19. The system according to claim 18, wherein said means for spreading said carrier generates spectral components of the spread carrier that are separated by a frequency interval greater than 1/Ts. 
     
     
       20. The system according to claim 18, 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. 
     
     
       21. The system according to claim 20, wherein the data frequency is synchronously derived from said one frequency source. 
     
     
       22. The system according to claim 18, 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. 
     
     
       23. The system according to claim 18, 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.