Code shift keying transmitter for use in a spread spectrum communications system

• US 6,616,254 B1
• Filed: 11/30/1999
• Issued: 09/09/2003
• Est. Priority Date: 06/20/1997
• Status: Expired due to Fees

• Current Assignee: Yitran Communications Ltd. (Allalouf Capital Ltd.)
• Original Assignee: Yitran Technologies Ltd.
• Inventors: Matmor, Avner, Raphaeli, Dan
• Primary Examiner(s): Pham, Chi
• Application Number:
  US09/449,880
• Time in Patent Office: 1,379 Days
• Field of Search: 375/135, 375/146, 375/139, 375/151, 375/295, 375/296, 375/130, 370/320, 370/335, 370/347, 370/441, 370/210
• US Class Current: 375/135
• CPC Class Codes: H04B 1/69 Spread spectrum techniques
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7095 Sliding correlator type
• CPC Class Codes: H04B 2001/6912 using chirp
• CPC Class Codes: H04B 2201/70703 using multiple or variable rates
• CPC Class Codes: H04B 2203/5416 by adding signals to the wave form of the power source
• CPC Class Codes: H04B 3/54 Systems for transmission via power distribution lines
• CPC Class Codes: H04J 13/0022 PN, e.g. Kronecker
• CPC Class Codes: H04J 13/0074 Code shifting or hopping
• CPC Class Codes: H04J 13/10 Code generation
• CPC Class Codes: H04L 23/02 adapted for orthogonal signalling

Abstract

A transmitter apparatus and method for use in a spread spectrum data communications system that utilizes the Differential Code Shift Keying (DCSK) or non-differential Code Shift Keying (CSK) modulation technique. The transmitter transmits data in the form of packets to the receiver. Each packet is consists of a header, one or more data symbols followed by a CRC checksum. The preamble comprises a sequence of one or more zero rotated symbols and one or more predetermined non-zero rotated symbols. In one embodiment, a shift index is calculated based on the input data. The shift index is used to form the address in a PN sample ROM. The PN sequence is read out using a counter of length modulo the PN sequence length. Samples of the PN sequence are read out and input to a Manchester encoder whose output is amplified and filtered to obtain a particular spectrum shape. In a second embodiment, the shift index is loaded into a counter and used as a starting address to address the PN sample ROM. The counter is cycled for a symbol period causing the entire PN sequence to be read out rotated in accordance with the input data. A FIR filter is used to filter the sample stream. Optionally, notches can be placed in the FIR frequency response to filter out specific frequencies such as those corresponding to the amateur radio band. A method of constructing a notch filter having one or more notches in its frequency response is also presented.

Claims

1. A transmitter for use in a spread spectrum communications system, comprising:

an initial index calculator adapted to generate an initial index in accordance with a shift index;

a sequence memory adapted to store multiple copies of sample'"'"'s of a modulation waveform sequence, each copy having a circular rotation applied thereto corresponding to a different shift index, wherein an upper portion of an address to said sequence memory is formed from the output of said initial index calculator and a lower portion lower portion of said address to said sequence memory is formed from the output of a counter; and

a counter adapted to count the length of said modulation waveform sequence so as to cause samples of one of said copies of said modulation waveform sequence corresponding to said shift index to be output of said sequence memory.

2. The transmitter according to claim 1, wherein said modulation waveform comprises a Pseudo Noise (PN) sequence.

3. The transmitter according to claim 1, wherein said modulation waveform comprises a chirp waveform.

4. The transmitter according to claim 1, wherein said initial index is calculated in accordance with the following:

${\mathrm{initial}}_{-}<br><br>\mathrm{index}=\left(\frac{\mathrm{PN}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{sequence}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{length}}{\mathrm{number}<br><br>\text{\hspace{1em}<br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{possible}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{shifts}}\right)·<br><br>{\mathrm{shift}}_{-}<br><br>\mathrm{index}$

wherein the shift index comprises the data input from a host data source.

5. The transmitter according to claim 1, wherein said sequence sample memory comprises a Read Only Memory (ROM) storage device.

6. The transmitter according to claim 1, further comprising:

an encoder operative to encode the rotated sequence sample stream so as to shift the spectrum thereof;

an amplifier for amplifying the output of said encoder for transmission over a channel;

a filter adapted to spectrally shape the signal output of said amplifier; and

channel coupling circuitry operative to couple the signal output from said filter onto the channel.

7. The transmitter according to claim 6, wherein said amplifier is adapted to generate a differential output.

8. The transmitter according to claim 6, wherein said filter comprises a band pass filter having a pass band from 4 to 20 MHz.

9. The transmitter according to claim 6, wherein said filter comprises a band pass filter having one or more notches in its pass band.

10. The transmitter according to claim 6, wherein said filter comprises a symmetrical band pass filter.

11. The transmitter according to claim 6, wherein said channel coupling circuitry comprises a signal transformer coupled to a symmetrical high pass filter.

12. The transmitter according to claim 1, further comprising means for encoding an input bit as the sign of said rotated sequence.

13. The transmitter according to claim 6, wherein said encoder comprises a Manchester encoder.

14. The transmitter according to claim 6, wherein said amplifier comprises a non-inverting gate and an inverting gate adapted to be driven in parallel by the output of said encoder so as to generate a differential output signal.

15. The transmitter according to claim 6, wherein said amplifier comprises a single non-inverting gate adapted to be driven by the output of said encoder.

16. The transmitter according to claim 15, wherein said single non-inverting gate comprises tri-state control means.

17. The transmitter according to claim 6, wherein said amplifier comprises a single inverting gate adapted to be driven by the output of said encoder.

18. The transmitter according to claim 17, wherein said single inverting gate comprises tri-state control means.

19. A transmitter for use in a spread spectrum communications system, comprising:

an initial index calculator adapted to generate an initial index corresponding to a position in a modulation waveform sequence where transmission is to begin for a particular symbol represented by an input shift index;

a sequence memory adapted to store samples of a modulation waveform sequence; and

a counter preloaded with said initial index and adapted to count the length of said modulation waveform sequence for each symbol to be transmitted so as to cause samples of said modulation waveform sequence circularly rotated by an amount corresponding to said shift index to be output of said sequence memory.

20. The transmitter according to claim 19, wherein said modulation waveform comprises a Pseudo Noise (PN) sequence.

21. The transmitter according to claim 19, wherein said modulation waveform comprises a chirp waveform.

22. The transmitter according to claim 19, further comprising means for encoding input data as the sign of said rotated sequence.

23. The transmitter according to claim 19, wherein said initial index is calculated in accordance with the following:

${\mathrm{initial}}_{-}<br><br>\mathrm{index}=\left(\frac{\mathrm{PN}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{sequence}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{length}}{\mathrm{number}<br><br>\text{\hspace{1em}<br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{possible}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{shifts}}\right)·<br><br>{\mathrm{shift}}_{-}<br><br>\mathrm{index}$

wherein the shift index comprises input data.

24. The transmitter according to claim 19, wherein said sequence sample memory comprises a Read Only Memory (ROM) storage device.

25. The transmitter according to claim 19, further comprising:

a first filter operative to encode the rotated sequence sample stream so as to shift the spectrum thereof;

an amplifier for amplifying the output of said first filter for transmission over a channel;

a second filter adapted to spectrally shape the signal output of said amplifier; and

channel coupling circuitry operative to couple the signal output from said second filter onto the channel.

26. The transmitter according to claim 25, wherein said first filter comprises a Finite Impulse Response (FIR) filter.

27. The transmitter according to claim 25, wherein said first filter comprises a Finite Impulse Response (FIR) filter adapted to function as a band pass filter.

28. The transmitter according to claim 27, wherein said band pass filter comprises a pass band from 4 to 20 MHz.

29. The transmitter according to claim 25, wherein said first filter comprises a Finite Impulse Response (FIR) filter band pass filter having one or more notches in its pass band.

30. The transmitter according to claim 25, wherein said second filter comprises a symmetrical band pass filter.

31. The transmitter according to claim 25, wherein said channel coupling circuitry comprises a signal transformer coupled to a symmetrical high pass filter.

32. The transmitter according to claim 25, wherein said amplifier comprises a differential amplifier adapted to output a differential output signal.

33. In a spread spectrum communications system, a method for transmitting symbols onto a communication channel, said method comprising the steps of:

providing a sequence memory comprising samples of a modulation waveform sequence;

calculating an initial index into said sequence memory in accordance with a shift index;

counting a full symbol period so as to cause a sample bit stream comprising said modulation waveform sequence circularly rotated in accordance with said initial index to be output of said sequence memory;

encoding the sample bit stream output of said sequence memory; and

coupling the encoded sample bit stream onto said communication channel.

34. The method according to claim 33, wherein said modulation waveform comprises a Pseudo Noise (PN) sequence.

35. The method according to claim 33, wherein said modulation waveform comprises a chirp waveform.

36. The method according to claim 33, further comprising means for encoding input data as the sign of said rotated sequence.

37. The method according to claim 33, wherein said sequence sample memory comprises a Read Only Memory (ROM) storage device.

38. The method according to claim 33, further comprising the step of amplifying said encoded sample bit stream sufficiently to drive said communication channel.

39. The method according to claim 33, further comprising the step of spectrally shaping said encoded bit stream to generate notches at one or more frequencies.

40. A method for constructing a filter, said method comprising the steps of:

determining a first frequency response for said filter;

inverting the phase of said first frequency response to generate zero crossings at frequencies where notches are desired to generate a second frequency response;

performing an inverse Fast Fourier Transform (FFT) on said second frequency response to generate a first time domain representation;

applying a windowing function to said first time domain representation to generate a second time domain representation; and

generating filter tap coefficients from said second time domain representation.

41. A transmitter for use in a spread spectrum communications system, comprising:

an initial index calculator adapted to generate an initial index in accordance with a shift index;

a sequence memory adapted to store multiple copies of samples of a modulation waveform sequence, each copy having a circular rotation applied thereto corresponding to a different shift index, wherein an upper portion of an address to said sequence memory is formed from the output of said initial index calculator and a lower portion lower portion of said address to said sequence memory is formed from the output of a counter;

a counter adapted to count the length of said modulation waveform sequence so as to cause samples of one of said copies of said modulation waveform sequence corresponding to said shift index to be output of said sequence memory;

an encoder operative to encode the rotated sequence sample stream to shift the spectrum thereof;

a filter adapted to spectrally shape the signal output of said encoder, wherein said filter comprises one or more notches in the frequency domain whose phase alternates in sign, wherein a zero crossing in the frequency response occurs at each desired notch frequency; and

channel coupling circuitry operative to couple the signal output from said filter onto a channel.

42. The transmitter according to claim 41, wherein said channel coupling circuitry comprises:

a digital to analog (D/A) converter operative to convert the output of said filter to an analog output signal;

amplifier means operative to receive said analog output signal and to sufficiently drive said channel over which said symbols are to be transmitted; and

interface circuitry adapted to couple the output of said amplifier to said channel.

43. A transmitter, comprising:

means for generating a transmission signal in accordance with input data;

a filter adapted to spectrally shape said transmission signal, wherein said filter comprises one or more notches in its frequency domain having phase alternating in sign, wherein a zero crossing in the frequency response occurs at each desired notch frequency; and

channel coupling circuitry operative to couple the filtered signal output from said filter onto a channel.

44. The transmitter according to claim 43, wherein said channel coupling circuitry comprises:

a digital to analog (D/A) converter operative to convert the output of said filter to an analog output signal;

amplifier means adapted to receive said analog output signal and to sufficiently drive said channel; and

interface circuitry adapted to couple the output of said amplifier to said channel.

45. In a spread spectrum communications system, a method for transmitting symbols onto a communication channel, said method comprising the steps of:

providing a sequence memory for storing samples of a modulation waveform sequence;

calculating an initial index into said sequence memory in accordance with a shift index;

counting a full symbol period so as to cause a sample bit stream comprising said modulation waveform sequence circularly rotated in accordance with said initial index to be output of said sequence memory;

encoding the sample bit stream output of said sequence memory;

filtering said sample bit stream output of said sample memory utilizing a filter constructed to have one or more notches in its frequency response whereby the phase of said frequency response is inverted at each desired notch frequency such that a zero crossing is generated for each notch frequency desired in the frequency response, and whereby a windowing function is applied to a time domain representation of said response to generate windowed results therefrom and windowed results quantized to yield filter tap coefficients therefrom; and

coupling the encoded sample bit stream communication channel.