Spread-spectrum receiver with progressive fourier transform

• US 7,227,884 B2
• Filed: 07/31/2002
• Issued: 06/05/2007
• Est. Priority Date: 02/28/2000
• Status: Active Grant

• Current Assignee: Comtech Mobile Datacom Corporation (Comtech Telecommunications Corporation)
• Original Assignee: AeroAstro, Inc.
• Inventors: McDermott, Scott A.
• Primary Examiner(s): PHU, PHUONG M
• Application Number:
  US10/208,882
• Publication Number:
  US 20020191632A1
• Time in Patent Office: 1,770 Days
• Field of Search: 375140-153, 370/342, 370/320, 370/335, 370/441
• US Class Current: 375/147
• CPC Class Codes: H04B 1/7075 with code phase acquisition
• CPC Class Codes: H04B 1/7087 Carrier synchronisation aspects
• CPC Class Codes: H04L 5/0023 Time-frequency-space

Abstract

A receiving system dynamically searches the communications band for transmissions of messages having the same nominal communications parameters, including the use of the same spreading code, but having potentially different specific frequencies and code-phases. The receiver, which is independent of the transmitters, samples the communications band at each code-phase of the spreading code over a span of downconverted transmission frequencies. When a message element is detected at a particular code-phase and frequency, it is forwarded to a demodulator that demodulates the message and sends it to its intended destination. In a preferred embodiment, a progressive Fourier Transform is used to incrementally determine the power level at each successive code-phase at a given frequency, thereby substantially reducing the time required to search for transmissions at each discrete code-phase. To accommodate variances in frequency and/or phase of a received signal from the same transmitter, the received signal is partitioned into subsets, and a composite measure is used to detect subsets that ‘slip’ into an adjacent frequency or phase sampling bin.

Claims

1. A communications system for receiving a plurality of messages from a plurality of transmitters that each use a common spreading code, comprising:

a receiver that is configured to receive a composite signal that comprises the plurality of messages from the plurality of transmitters, each message having a transmitter code-phase relative to a code-phase of the receiver, each transmitter code-phase being independent of the code-phase of the receiver, and

a message discriminator that is configured to;

discriminate at least one message from the plurality of messages based on the transmitter code-phase corresponding to the at least one message, and

search each frequency of a plurality of frequencies for energy content at each code-phase of a plurality of code-phases to discriminate the at least one message at the transmitter code-phase, wherein;

the message discriminator comprises;

a detector that is configured to determine a measure of the energy content at each code-phase at a given frequency of the plurality of frequencies, and to identify one or more code-phases at the given frequency, based on the measure of the energy content,

a demodulator, operably coupled to the detector, that is configured to demodulate an input signal corresponding to the composite signal, based on at least one of the identified one or more code-phases at the given frequency,

the receiver is configured to provide a downconverted composite signal at a baseband frequency of the plurality of messages, and

the detector determines the measure of the energy content at each code-phase based on a Fourier Transform that is applied to the downconverted composite signal at one or more given frequencies.

2. The communications system of claim 1, wherein

the detector is configured to determine the measure of the energy content at at least one code-phase based on the measure of the energy content of a prior code-phase.

3. The communications system of claim 2, wherein

the measure of the energy content at the at least one code phase corresponds to the measure of the energy content of the prior code-phase plus an adjustment that is based on:

addition of a new sample,

removal of an old sample, and

inversions of sample values at transition points of the spreading code.

4. A communications system for receiving a plurality of messages from a plurality of transmitters that each use a common spreading code, comprising:

a receiver that is configured to receive a composite signal that comprises the plurality of messages from the plurality of transmitters, each message having a transmitter code-phase relative to a code-phase of the receiver, each transmitter code-phase being independent of the code-phase of the receiver, and

a message discriminator that is configured to;

discriminate at least one message from the plurality of messages based on the transmitter code-phase corresponding to the at least one message, and

search each frequency of a plurality of frequencies for energy content at each code-phase of a plurality of code-phases to discriminate the at least one message at the transmitter code-phase,

wherein;

the message discriminator comprises;

a detector that is configured to determine a measure of the energy content at each code-phase at a given frequency of the plurality of frequencies, and to identify one or more code-phases at the given frequency, based on the measure of the energy content, and

a demodulator, operably coupled to the detector, that is configured to demodulate an input signal corresponding to the composite signal, based on at least one of the identified one or more code-phases at the given frequency;

the detector determines the measure of the energy content at each code-phase based on a composite of a plurality of discrete measurements of the energy content at each code-phase; and

the composite of the plurality of discrete measurements is based on a sum of magnitudes of each of a plurality of vector sums corresponding to subsets of measurements of the plurality of discrete measurements.

5. A method of communication comprising:

receiving a composite signal that comprises a plurality of messages from a plurality of transmitters,

each message being encoded with a common spreading code,

each message having a transmitter code-phase relative to a code-phase of the receiver that is independent of the code-phase of the receiver, and

each message being transmitted at a transmitter frequency that is within a frequency band about a common nominal transmit frequency,

discriminating at least one message from the plurality of messages based on the transmitter frequency and transmitter code-phase corresponding to the at least one message,

wherein;

discriminating the at least one message includes;

sampling the composite signal to provide a message sample, the message sample corresponding to a select frequency,

decoding the message sample based on the spreading code at the select code-phase, and

determining a measure of energy content of the decoded message sample at the select frequency and select code-phase, and

the method further includes selectively demodulating the composite signal at the select frequency and select code-phase based on the measure of energy content to provide the at least one message, and

discriminating the at least one message includes;

providing each of a plurality of discrete frequencies corresponding to the frequency band successively as the select frequency for sampling.

6. A method of communication comprising:

receiving a composite signal that comprises a plurality of messages from a plurality of transmitters,

each message being encoded with a common spreading code,

each message having a transmitter code-phase relative to a code-phase of the receiver that is independent of the code-phase of the receiver, and

each message being transmitted at a transmitter frequency that is within a frequency band about a common nominal transmit frequency,

discriminating at least one message from the plurality of messages based on the transmitter frequency and transmitter code-phase corresponding to the at least one message,

wherein;

discriminating the at least one message includes;

sampling the composite signal to provide a message sample, the message sample corresponding to a select frequency,

decoding the message sample based on the spreading code at the select code-phase, and

determining a measure of energy content of the decoded message sample at the select frequency and select code-phase, and

the method further includes selectively demodulating the composite signal at the select frequency and select code-phase based on the measure of energy content to provide the at least one message, and

determining the measure of energy content of the decoded message sample is based on a Fourier Transform.

7. The method of claim 6, wherein

determining the measure of energy content of the decoded message sample at the select code-phase is based on the measure of energy content of a decoded message sample at an immediately prior code-phase.

8. The method of claim 7, wherein

the measure of the energy content at the select code-phase corresponds to the measure of the energy content at the immediately prior code-phase plus an adjustment that is based on:

addition of a new sample value to the decoded message sample,

removal of an old sample value from the decoded message sample, and

inversions of sample values at transition points of the spreading code.

9. A computer program that is configured to be executed on a computer system and causes the computer system to effect the following:

determine a first measure of energy content corresponding to application of a spreading code to a first subset of a plurality of input samples, and

determining a second measure of energy content corresponding to application of the spreading code to a second subset of the plurality of input samples,

wherein;

determining the second measure of energy content is based on the first measure of energy content, and

the second subset of the plurality of input samples corresponds to an addition of a new sample to the first subset and a deletion of an old sample from the first subset, and

determining the second measure of energy content includes

adjusting the first measure of energy content for the addition of the new sample and deletion of the old sample, and

adjusting the first measure of energy content for inversions of sample values at each transition of the spreading code.