Direct sequence frequency ambiguity resolving receiver

In a direct sequence spread spectrum system for receiving a direct sequence spread spectrum signal with both chip code phase uncertainty and frequency uncertainty between a carrier frequency of a transmitter transmitting said direct sequence spread spectrum signal and a local reference frequency utilized by a receiver, the receiver comprising:

• a despreader configured to despread a received direct sequence spread spectrum signal, comprising,
• a memory configured to store tables of coefficients, each said table of said coefficients being a product of a respective chip code sequence and at least one respective bandpass filter characteristic and a lowpass filter characteristic, each bandpass filter characteristic being one of a plurality of adjacent frequency filter characteristics,
• a multiplier configured to multiply in parallel the received direct sequence spread spectrum signal and the coefficients of a plurality of said coefficient tables thereby to implement plural parallel filters producing plural parallel despread output signals each corresponding to a filter output corresponding to a respective one of said tables of coefficients;
• an operator mechanism configured to detect at said filter outputs at least one parameter selected from the group consisting of a) signal strength, b) quieting, c) phase lock, and d) signal quality and based on the at least one detected parameter to select an output of a filter for demodulation;
• and
• a demodulator configured to demodulate data having a data bandwidth at the selected filter output.

In a direct sequence spread spectrum receiver, a method for receiving a transmitted direct sequence spread spectrum signal, comprising the steps of:

• despreading a received direct sequence spread spectrum signal by mixing the received signal with a chip code reference signal;
• applying the despread signal simultaneously to a plurality of parallel filters, each filter of said plurality of filters having a bandwidth based on a reference chip code rate and chip code length of said receiver, where said bandwidth is less than a frequency uncertainty between the transmitted and received direct sequence spread spectrum signals;
• detecting at an output of each of the plurality of filters at least one parameter of a) signal strength, b) quieting, c) phase lock, and d) signal quality;
• selecting for data demodulation an output of a filter based upon the at least one parameter detected at the output of each of said plurality of filters;
• and
• demodulating one of data having a data bandwidth and voice information from the selected filter output.

A receiving method employed in a direct sequence spread spectrum receiver for receiving a direct sequence spread spectrum signal with time uncertainty in a chip code alignment and frequency uncertainty between a carrier frequency of a transmitter and a local reference frequency of the receiver, the method comprising the steps of:

• despreading a received direct sequence spread spectrum signal to produce a despread signal;
• applying the despread signal simultaneously to a plurality of filters each having a bandwidth based on a reference chip code rate and chip code length of said receiver, where said bandwidth is less than said frequency uncertainty;
• applying a trip level algorithm to outputs of the plurality of filters, in order to achieve initial spread spectrum synchronization and resolve the time uncertainty of the chip code alignment;
• selecting the output of at least one filter for subsequent demodulation based on the applying said trip level algorithm;
• and
• demodulating the selected filter output of at least one of data having a data bandwidth and voice.

In a receiving method employed in a direct sequence spread spectrum receiver for receiving a direct sequence spread spectrum signal with time uncertainty in a chip code alignment and frequency uncertainty between a carrier frequency of a transmitter transmitting said direct sequence spread spectrum signal and a local reference frequency of the direct sequence spread spectrum receiver, the improvement comprising the steps of:

• applying a trip level algorithm to the outputs of a plurality of filters in order to achieve initial spread spectrum synchronization, including,
• determining at each chip code phase alignment a previous history average of outputs of said filters from at least the previous chip code phase alignment,
• comparing, for each successive chip code phase alignment, the signal strength of the outputs of each of the plurality of filters to said previous history,
• determining at least one filter output that is higher than said previous history, by at least a predetermined level;
• selecting for demodulation an output of one of said filters which has a signal strength higher than said previous history, by at least said predetermined level based on the determined at least one filter;
• and
• demodulating data having a data bandwidth at the output of the selected filter.

A method for receiving a direct sequence spread spectrum signal with time uncertainty in a chip code alignment and frequency uncertainty between a carrier frequency of a transmitter transmitting said direct sequence spread spectrum signal and a local reference frequency of a receiver, comprising the steps of:

• despreading a received direct sequence spread spectrum signal to produce a despread signal;
• applying the despread signal simultaneously to a plurality of filters each having a bandwidth based on a reference chip code rate and chip code length of said receiver, where said bandwidth is less than said frequency uncertainty;
• and
• applying a trip level algorithm to outputs of the plurality of filters, comprising,
• computing, at a first chip code alignment, a signal strength of an output of each of the filters relative to an output of a filter having a lowest signal strength,
• summing the signal strengths, computed at the first chip code alignment, of all filter outputs into a first combined signal strength, computing at a second chip code alignment a signal strength of an output of each of the filters relative to an output of a filter having a lowest signal strength,
• summing the signal strengths computed at the second chip code alignment, of all filter outputs into a second combined signal strength, and
• comparing the second combined signal strength to the first combined signal strength to determine if the second combined signal strength exceeds the first combined signal strength by a trip level.

A method of receiving a direct sequence spread spectrum signal having both chip code phase uncertainty and frequency uncertainty, comprising:

• despreading a received direct sequence spread spectrum signal by means of a chip code sequence to produce a despread signal;
• applying the despread signal to plural filters;
• selecting an output of a candidate filter from said plural filters for demodulation;
• measuring a frequency offset from the candidate filter to a filter representing a zero offset frequency error in the absence of frequency uncertainty;
• calculating a chip code sequence phase correction factor for compensation of code drift;
• and
• using the chip code sequence phase correction factor to correct a phase in the chip code sequence.

A method of receiving a direct sequence spread spectrum signal having both chip code phase uncertainty and frequency uncertainty, comprising:

• despreading the received direct sequence spread spectrum signal to produce a despread signal;
• applying the despread signal to a filter bank comprising plural filters;
• measuring signal strengths at outputs of each of the plural filters;
• applying a trip level algorithm to determine if the measured signal strengths exceeds a predetermined value;
• selecting an output of a candidate filter from said plural filters for demodulation;
• measuring a frequency offset from the candidate filter output to a center frequency filter output;
• and
• selecting, based on the frequency offset measured in said measuring step, an alternative filter bank having plural filters and a center frequency offset by a portion of a filter bandwidth.

A direct sequence spread spectrum system including a transmitter which transmits a spread spectrum signal comprising a direct sequence chip code and a carrier frequency, and a receiver which receives the transmitted signal and resolves carrier frequency uncertainty between the carrier frequency of the transmitter and a local reference frequency of the receiver and chip code phase uncertainty between the chip code of the transmitter and a chip code of the receiver, the system comprising:

• a despreader configured to despread the received signals, comprising a chip code sequence phase shifter which shifts a phase of the receiver chip code sequence while searching to resolve said chip code phase uncertainty;
• a plurality of filters each having an output of the despreader applied thereto and each having a bandwidth less than said carrier frequency uncertainty;
• and
• an operator mechanism configured to detect outputs of said filters at consecutive code chips and to use concurrently detected outputs of the plurality of said filters to determine existence of chip code synchronization at each code chip, and to cause said chip code phase shifter to cease shifting the phase of the receiver chip code when chip code synchronization is determined to exist.