Direct sequence spread spectrum method computer-based product apparatus and system tolerant to frequency reference offset

• US 6,639,939 B1
• Filed: 07/22/1999
• Issued: 10/28/2003
• Est. Priority Date: 05/20/1997
• Status: Expired due to Term

• Current Assignee: Globalstar Incorporated
• Original Assignee: Axonn LLC (Globalstar Incorporated)
• Inventors: Naden, Gary A., Reiser, Dale E., Rouquette, Robert E.
• Primary Examiner(s): VO, DON NGUYEN
• Application Number:
  US09/359,441
• Time in Patent Office: 1,559 Days
• Field of Search: 375/140, 375/142, 375/143, 375/145, 375/149, 375/150, 375/152, 375/316, 375/340, 375/355, 375/344, 375/346, 375/350
• US Class Current: 375/140
• CPC Class Codes: H04B 1/707 using direct sequence modulation

Abstract

A method, apparatus, computer-based product and system employs a digital receiver (or transceiver) to receive, digitize and process a direct sequence spread spectrum signal using efficient, low-cost digital signal processing components. A radio front end portion of the receiver receives and digitizes the signal, and a digital signal processing portion downconverts and despreads the signal by applying a pseudorandom noise (PN) code, used at a transmitter to spread a data signal contained in the direct sequence spread spectrum signal, to the received signal. In order to initially align, and maintain alignment of, the PN code with the direct sequence spread spectrum signal, a timing and state control mechanism is included that provides time reference correction information to the signal processing components of the receiver. This time reference correction information allows the receiver to be compatible with transmitters using inaccurate frequency references which impart a significant frequency ambiguity in the received signal. Additional features include computer-based synchronization methods and mechanisms suitable for use for low performance digital signal processors and employ power management mechanisms that enable long-term operation using battery power. The power management mechanisms enable the receiver to operate in a network setting, over the course of multiple years off battery power, with similar receivers, transmitters and transceiver that communicate with one another using direct sequence spread spectrum signals.

Claims

1. A method for reducing signal loss caused by decimating a digital signal in a receiver with a decimation operator having a predetermined bandwidth, a response characteristic of said decimation operator imparting a greater amount of attenuation in a first part of said predetermined bandwidth than in a second part of said predetermined bandwidth, comprising the steps of:

receiving said digital signal with said receiver, said receiver employing a receiver frequency reference that operates within a predetermined accuracy;

decimating said digital signal with said decimation operator by decimating a sample rate of said digital signal, a position of said digital signal in said predetermined bandwidth being influenced by a frequency offset between a transmitter frequency reference and said receiver frequency reference;

determining an amount by which said digital signal is removed in frequency from said second part of said predetermined bandwidth and whether said amount is greater than a predetermined standard; and

adjusting a signal downconversion operator by an amount and a direction sufficient to reposition said digital signal within said second part of said bandwidth when it is determined in said step that the amount by which the signal is from the second part of said bandwidth is greater than said standard.

2. A receiver comprising:

means for receiving a digital signal with a receiver frequency reference that operates within a predetermined accuracy;

means for decimating said digital signal to reduce a sample rate of said digital signal, a position of said signal in said predetermined bandwidth being influenced by a frequency offset between a transmitter frequency reference and said receiver frequency reference;

means for determining an amount by which said signal is removed in frequency from said second part of said predetermined bandwidth and whether said amount is greater than a predetermined standard; and

means for adjusting an amount and a direction by which said digital signal is downconverted sufficiently to reposition said digital signal within said second part of said predetermined bandwidth, when said means for determining determines that the amount by which the digital signal is offset from the second part of said bandwidth is greater than said predetermined standard.

3. A receiver comprising:

a receiver frequency reference having a predetermined accuracy;

a processor being programmably configured to

digitize and decimate a received signal so as to reduce a sample rate of said received signal, a position of said received signal in said predetermined bandwidth being influenced by a frequency offset between a transmitter frequency reference and said receiver frequency reference,

determine an amount by which said received signal is removed in frequency from a predetermined part of said bandwidth and whether said amount is greater than a predetermined standard, and

adjust a signal downconversion operation by an amount and a direction sufficient to reposition said received signal within said predetermined part of said bandwidth, when said processor determines that the amount by which the received signal is from the predetermined part of said bandwidth is greater than said predetermined standard.

4. A receiver that suppresses signal loss from decimating a digital signal with a decimation operator having a predetermined bandwidth, where a response characteristic of said decimation operator imparts a greater amount of attenuation in a first part of said predetermined bandwidth than in a second part of said predetermined bandwidth, comprising:

means for receiving said digital signal using a receiver frequency reference that operates within a predetermined accuracy;

means for decimating said digital signal with said decimation operator to reduce a sample rate of said signal, where a position of said digital signal in said predetermined bandwidth being influenced by a frequency offset between a transmitter frequency reference and said receiver frequency reference;

means for determining an amount by which said digital signal is removed in frequency from said second part of said predetermined bandwidth and whether said amount is greater than a predetermined standard; and

means for adjusting a signal downconversion operation by an amount and a direction sufficient to reposition said digital signal within said second part of said predetermined bandwidth, when said means for determining determines that the amount by which the signal is removed from the second part of said bandwidth is greater than said predetermined standard.

5. A method for suppressing signal loss caused by decimation of a direct sequence spread spectrum signal transmitted from a transmitter, said decimation having associated therewith a predetermined bandwidth and response characteristic that imparts a greater amount of attenuation in a first part of said predetermined bandwidth than in a second part of said predetermined bandwidth, comprising the steps of:

receiving said direct sequence spread spectrum signal with said receiver, said receiver employing a receiver frequency reference and said transmitter employing a transmitter frequency reference where tolerances of the respective references combine to create a predetermined signal position uncertainty range in which said direct sequence spread spectrum signal is positioned prior to signal demodulation;

digitizing said direct sequence spread spectrum to form a digital signal;

decimating said digital signal with said decimation operator to reduce a sample rate of said digital signal;

subdividing said predetermined signal position uncertainty range bandwidth into a plurality of candidate downconversion frequency bands;

changing an amount by which a downconversion operator translates the signal toward the second part of said bandwidth, said amount being set under a hypothesis that the digital signal is in one of the plurality of candidate downconversion frequency bands;

attempting to correlate said digital signal with a spreading code used to spread the direct sequence spread spectrum signal at the transmitter and determining whether correlation is achieved; and

repeating sequentially said changing step for the other channels and stopping said repeating step when said attempting step determines that correlation is achieved.

6. A receiver in a communication system that employs at least one transmitter and at least one receiver, the receiver comprising:

means for receiving a signal by employing a receiver frequency reference and said transmitter employing a transmitter frequency reference, combined tolerances of the receiver frequency reference and the transmitter frequency reference creating a predetermined signal position uncertainty range bandwidth in which said signal is positioned prior to signal demodulation;

means for decimating said signal with said decimation operator to reduce a sample rate of said signal;

means for subdividing said predetermined signal position uncertainty range bandwidth into a plurality of candidate downconversion frequency bands;

means for controllably downconverting the signal using means for changing an amount by which the signal is translated toward a predetermined part of said bandwidth, said amount being set under a hypothesis that the signal is placed in one of the plurality of candidate downconversion frequency bands;

means for attempting to correlate said signal with a spreading code used to spread the signal at the transmitter and means for determining whether correlation is achieved; and

means for returning process control to said means for attempting so that said means for attempting attempts to correlate said signal for other candidate downconversion frequency bands until said means for attempting determines that correlation is achieved.

7. A receiver in a communication system that employs at least one transmitter with a transmitter frequency reference and at least one receiver, the at least one receiver comprising:

means for receiving said signal using a receiver frequency reference, combined tolerances of the receiver frequency reference and the transmitter frequency reference creating a predetermined signal position uncertainty range bandwidth in which said signal is positioned prior to signal demodulation;

a processor configured to

decimate said signal to reduce a sample rate of said signal,

subdivide said predetermined signal position uncertainty range bandwidth into a plurality of candidate downconversion frequency bands,

change an amount by which a downconversion operator translates the signal toward a predetermined part of said bandwidth, said amount being set under a hypothesis that the signal is in one of the plurality of candidate downconversion frequency bands, and

attempt to correlate said signal with a spreading code used to spread the signal at the transmitter and determine whether correlation is achieved, and

continue to attempt to correlate said signal with the spreading code for the other candidate downconversion frequency bands until correlation is achieved for one of the candidate downconversion frequency bands.