Extended time tracking and peak energy in-window demodulation for use in a direct sequence spread spectrum system

• US 5,778,022 A
• Filed: 12/06/1995
• Issued: 07/07/1998
• Est. Priority Date: 12/06/1995
• Status: Expired due to Term

• Current Assignee: Skyworks Solutions Incorporated
• Original Assignee: Rockwell International Corporation
• Inventors: Walley, John S.
• Primary Examiner(s): TSE, YOUNG TOI
• Application Number:
  US08/568,053
• Time in Patent Office: 944 Days
• Field of Search: 375/202, 375/206-208, 375/343, 375/344, 375/355, 370/503
• US Class Current: 375/152
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7093 Matched filter type
• CPC Class Codes: H04L 7/046 using a dotting sequence

Abstract

A direct sequence spread spectrum communication system in a digital cordless telephone having a basestation and a handset that each implement an extended .linevert split.EE.linevert split.+.linevert split.E.linevert split.-.linevert split.L.linevert split.-.linevert split.LL.linevert split. time tracking system and peak matched filter demodulation within a related window to permit one sample per chip analog-to-digital conversion and demodulation for longer code sequences (providing savings in hardware and complexity) and to permit enhanced two sample per chip analog-to-digital conversion and demodulation for shorter code sequences (providing more robust fading immunity).

Claims

1. A direct sequence spread spectrum communication system for receiving an analog spread spectrum baseband signal that was formed by directly modulating a carrier with the chips of a code sequence at a chip rate, the system comprising:

analog-to-digital converting means for converting the analog spread spectrum baseband signal to corresponding digital spread spectrum baseband data at a sampling rate;

matched filter means for removing the code sequence by correlating the digital spread spectrum baseband data with the code sequence, said matched filter means outputting correlation values at a sampling rate of the analog-to-digital converting means;

large adjust means for coarsely positioning a demodulation window that contains a plurality of correlation values that are in the vicinity of an ideal peak correlation value, the demodulation window having a nominal period of one bit interval to generally remain centered on the peak correlation value output by the matched filter means;

small adjust means for making small positioning adjustments to the demodulation window by comparing the correlation values that are on one side of a center value to the correlation values that are on the other side of the center value;

peak value determining means for determining a peak correlation value from among the plurality of correlation values in the demodulation window; and

demodulation means for demodulating the peak correlation value.

2. The direct sequence spread spectrum communication system of claim 1 wherein there are five correlation values that are in the demodulation window, the five correlation values comprising EE, E, Center, L, and LL samples.

3. The direct sequence spread spectrum communication system of claim 2 wherein the sampling rate is two times the chip rate to provide two sample per chip analog-to-digital conversion and matched filtering that is suitable for relatively short code sequences.

4. The direct sequence spread spectrum communication system of claim 3 wherein the system is a digital cordless telephone.

5. The direct sequence spread spectrum communication system of claim 4 wherein the code sequence is an even code sequence having an equal number of "1'"'"'s" and "0'"'"'s".

6. The direct sequence spread spectrum communication system of claim 5 wherein the even code sequence is a bipolar code sequence wherein the "1'"'"'s" and "0'"'"'s" represent voltages of +1 and -1 volt, respectively.

7. The direct sequence spread spectrum communication system of claim 5 wherein the even code sequence is an even 12-Chip code sequence.

8. The direct sequence spread spectrum communication system of claim 4 wherein the code sequence is a 12-Chip augmented Barker Code.

9. The direct sequence spread spectrum communication system of claim 2 wherein the sampling rate is equal to the chip rate to provide one sample per chip analog-to-digital conversion and matched filtering that is suitable for relative long code sequences.

10. The direct sequence spread spectrum communication system of claim 1 wherein the analog spread spectrum baseband signal is demodulated into in-phase and quadrature components and further comprising analog-to-digital converting means and matched filter means for each of the in-phase and quadrature components.

11. In a digital cordless telephone having a basestation connectable to a switched telephone network and a handset, said basestation and handset communicating over an RF link using direct sequence spread spectrum modulation, said basestation and said handset each connected to receive and demodulate digital in-phase and quadrature baseband signals using analog-to-digital converters, matched filters, a demodulation circuit, and a window time tracking circuit, the window time tracking circuit comprising:

energy detector means for receiving in-phase and quadrature correlation data from the matched filters at a sampling rate and outputting an energy signal at the sampling rate;

window NCO means for periodically generating window timing signals on bit intervals to define an extended demodulation window spanning five sample intervals, a first two of the five sample intervals being designated EE and E and a last two of the five sample intervals being designated L and LL, said window NCO means adjusting the position of the demodulation window relative to a bit interval, if necessary, upon receiving a large adjust signal;

large adjust timing circuit means for receiving the energy signal from the energy detector means and, based on the window timing signals from the window NCO means, comparing a peak inside energy defined as the peak energy inside of the demodulation window to a peak outside energy defined as the peak energy outside of the demodulation window, said large adjust timing circuit means outputting the large adjust signal if the peak outside energy is larger than the peak inside energy; and

small adjust timing circuit means for receiving the energy signal from the energy detector means and, based on the window timing signals from the window NCO means, determining if the demodulation window is moving relative to an energy centroid by comparing the EE and E correlation values to the L and LL correlation values, and if the demodulation window is moving, outputting a small adjust signal to advance or retard a main receiver clock.

12. The digital cordless telephone of claim 11 wherein the energy signal output by the energy detector means is a low-compute estimate of the energy contained in the in-phase I and quadrature Q correlation data.

13. The digital cordless telephone of claim 12 wherein the low-compute estimate of the energy contain in the in-phase I and quadrature Q correlation data is characterized by the formula:

space="preserve" listing-type="equation">Energy Estimate=Max(.linevert split.I.linevert split., .linevert split.Q.linevert split.)+1/4*Min(.linevert split.I.linevert split., .linevert split.Q.linevert split.).

14. The digital cordless telephone of claim 11 wherein the in-phase I and quadrature Q correlation data are coded into 8-bit, 2'"'"'s complement values and the energy detector means comprises:

means for obtaining the absolute value of the I and Q values by exclusively-OR'"'"'ing the sign bit (7) with the value bits (6;

0);

means for comparing the absolute value of the I and Q values to determine which is larger and which is smaller; and

means for adding the larger value (bits 6;

0) to 1/4^th of the smaller value (bits 6;

2) to obtain the Energy Estimate.

15. The digital cordless telephone of claim 11 further comprising a reference oscillator for generating reference pulses and a clock generation circuit having an adjustable divider stage for generating the main receiver clock, the small adjust signal advancing or retarding the main receiver clock by temporarily causing the adjustable divider stage to require one less or one more reference pulse.

16. In a digital cordless telephone having a basestation connectable to a switched telephone network and a handset, said basestation and handset communicating over an RF link using direct sequence spread spectrum modulation, said basestation and said handset each connected to receive and demodulate digital in-phase and quadrature baseband signals using analog-to-digital converters and matched filters operating at a sample rate to produce complex in-phase and quadrature digital data, and a demodulation circuit, an improvement comprising:

extended time tracking means for defining and maintaining a multiple sample demodulation window around a centroid of correlation energy for each bit interval; and

means for demodulating the in-phase and quadrature digital data that has the highest energy content within at least a portion of the demodulation window, the portion including a sample at a predicted center of the multiple sample demodulation window and at least two adjacent samples; and

means for selecting one of the demodulated samples as best representing valid data.

17. The digital cordless telephone of claim 16 wherein the multiple sample demodulation window is a five-sample wide demodulation window.

18. The digital cordless telephone of claim 17 wherein the demodulating means demodulates the in-phase and quadrature digital data within a 3-sample wide portion of the demodulation window.