Patent Description:
The Quasi-Zenith Satellite System (QZSS) is a Japan-based performance enhancement system for Global Positioning System (GPS) in the Asia-Pacific area. Its L6 signal, e.g., L61 or L62, carries precise GPS/QZSS positioning correction data that, for example, supports Precise Point Positioning (PPP).

The L6 signal is Binary Phase Shift Keying (BPSK) modulated by a pseudo-random noise (PRN) code of length <NUM> chips with a rate of <NUM> Mcps (Million Chips Per Second) repeating every <NUM>. The L6 signal is further modulated by code shift keying (CSK) to represent an <NUM>-bit symbol of an L6 navigation message that is <NUM> bits. As such, CSK modulation allows for the transmission of <NUM>-bits during one code period (e.g., <NUM>), and thus allows the L6 navigation message of <NUM> bits to be transmitted in <NUM> second.

Although utilization of CSK modulation is favorable in terms of data transmission rate (e.g., <NUM> kbps), it puts a computation burden on Global Navigation Satellite System (GNSS) receivers that need to demodulate CSK data (e.g., the <NUM>-bit symbol) from the L6 signal. For example, prior art techniques may perform a brute force implementation that utilizes <NUM> different correlators, where each correlator is associated with a different PRN shift from <NUM>-<NUM>. On each accumulation period of <NUM>, a single correlator corresponding to the K6 signal will have power, and the GNSS reciever determines the CSK data is the PRN shift associated with the correlator that has power. <CIT> discloses a method for demodulating and tracking of CSK-modulated signals comprising the steps of receiving a CSK-modulated signal at a plurality of correlators. "<NPL>, highlights specific features related to demodulation performance.

An alternative prior art technique for demodulating the CSK data is achieved through implementation of Fourier Transforms and Inverse Fourier Transforms. However, these implementations require great hardware, software, and/or computational resources. What is needed is a simpler and more efficient system for demodulating CSK data.

The inventive apparatus and method demodulates code shift keying (CSK) data from a satellite signal utilizing a binary search as set out in the appended set of claims.

A Global Navigation Satellite System (GNSS) receiver receives one or more satellite signals from one or more GNSS satellites. The satellite signal may be an L6 signal, e.g., L61 or L62, from the Quasi-Zenith Satellite System (QZSS) that includes a <NUM> PRN code (hereinafter "Code <NUM>") of <NUM> chips that is modulated by CSK to represent an <NUM>-bit symbol of a L6 navigation message that is <NUM> bits. As used herein, "CSK modulated symbol" refers to the <NUM>-bit symbol represented by the PRN code modulated by CSK. The L61 signal may include a <NUM> PRN code (hereinafter "Code <NUM>") of <NUM> chips (i.e., pilot channel) that is modulated by a square wave with a period of <NUM> that is utilized for synchronization purposes, while the L62 signal may include a second data channel.

The GNSS receiver maintains a plurality of receiver codes, where each receiver code represents a different shift in chips to the fundamental PRN code (i.e., the un-shifted PRN code). The total number of receiver codes may be based on the total number of bits (N) of the CSK modulated symbol. Specifically, and because the CSK modulated symbol is a binary representation, the CSK modulated symbol may be any of <NUM>N permutations. For example, and for an <NUM>-bit symbol, the CSK modulated symbol may be any of <NUM> different permutations (<NUM><NUM> = <NUM>). Thus, each of the plurality of receiver codes is the fundamental PRN code shifted a different number of chips from <NUM> to <NUM> to represent different possible CSK modulated symbols.

The GNSS receiver then generates combinational PRN codes for respective portions (e.g., halves) of the receiver codes. Specifically, the GNSS receiver performs a chip-by-chip summation (i.e., linear combination) of a first portion (e.g., first half) of the receiver codes (e.g., the codes that represent a shift in chips to the fundamental PRN code from <NUM>-<NUM>) to generate a first combinational PRN code. In addition, the GNSS receiver performs a chip-by-chip summation of a second portion (e.g., second half) of the receiver codes (e.g., the codes that represent a shift in chips to the fundamental PRN code from <NUM>-<NUM>) to generate a second combinational PRN code. Furthermore, the GNSS receiver performs a chip-by-chip summation of respective portions (e.g., halves) of receiver codes down a first hierarchy associated with the first portion of the receiver codes and a second hierarchy associated with the second portion of the receiver codes to produce respective combinational PRN codes.

In an embodiment, the GNSS receiver executes a total number of correlations equal to N times two to demodulate the CSK data. For example, if the CSK modulated symbol is <NUM>-bits, the GNSS receiver executes <NUM> correlations to demodulate the CSK data. Specifically, the GNSS receiver correlates the received signal, which includes Code <NUM> modulated by CSK, with the first combinational PRN code to produce a first correlation power level value. The GNSS receiver also correlates the received signal with the second combinational PRN code to produce a second correlation power level value.

If the first correlation power level value is greater than the second correlation power level value, the GNSS receiver correlates the received signal with the combinational PRN codes down the first hierarchy associated with the first portion of the receiver codes to produce correlation power level values that are compared to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

If the first correlation power level value is not greater than the second correlation power level value, the GNSS receiver correlates the received signal with the combinational PRN codes down the second hierarchy associated with the second portion of the receiver codes to produce correlation power level values that are compared to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

In a further embodiment, the GNSS receiver compares the correlation power level values with power detection threshold values using a total of N correlation processes to correlate the received signal with combinational PRN codes. Specifically, if the first correlation power level value is greater than the power detection threshold value based on the expected correlation power, the GNSS receiver correlates the received signal with the combinational PRN codes down the first hierarchy to produce correlation power level values that are compared to the power detection threshold values to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

If the first correlation power level value is not greater than the power detection threshold value based on the expected correlation power, the GNSS receiver correlates the received signal with the combinational PRN codes down the second hierarchy to produce correlation power level values that are compared to the power detection threshold values to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

In a further embodiment, the combinational PRN codes may be generated based on a chip-by chip summation of a first portion (e.g., first half) of the receiver codes and a chip-by-chip subtraction (i.e., linear combination) of a second portion (e.g., second half) of the receiver codes. For example, a first combinational PRN code for receiver codes <NUM>-<NUM> may be generated based on a chip-by-chip summation of receiver codes <NUM>-<NUM> and a chip-by-chip subtraction of receiver codes <NUM>-<NUM>. The GNSS receiver also performs a chip-by-chip summation and a chip-by-chip subtraction for respective portions (e.g., halves) of receiver codes down a first hierarchy associated with the first portion of the receiver code to produce respective combinational PRN codes. Further, the GNSS receiver also performs a chip-by-chip summation and a chip-by-chip subtraction for respective portions (e.g., halves) of receiver codes down a second hierarchy associated with the second portion of the receiver codes to produce respective combinational PRN codes.

The GNSS receiver correlates the received signal with the first combinational PRN code to produce a first correlation power level value. If the correlation power is positive (i.e., +), the GNSS receiver correlates the received signal with the combinational PRN codes down the first hierarchy to produce correlation power level values. The signs (e.g., positive or negative) of the produced correlation power level values based on the traversal down the first hierarchy are utilized to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

If the correlation power is negative (i.e., -), the GNSS receiver correlates the received signal with the combinational PRN codes down the second hierarchy to produce correlation power level values. The signs (e.g., positive or negative) of the produced correlation power level values based on the traversal down the second hierarchy are utilized to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

The GNSS receiver may demodulate the CSK data from <NUM> consecutive L6 <NUM> signal sample sets to determine the binary value of the entire L6 message that is <NUM> bits. The demodulated L6 message may then be utilized by the GNSS receiver for Precise Point Position (PPP) or for any of a variety of different accurate positioning techniques, as known by those skilled in the art.

Advantageously, the inventive system and method leverages the low cross correlation accumulations of PRN codes. As known by those skilled in the art, cross correlations are produced when a PRN code is correlated with a shifted version of itself. The expected value of a cross correlation of a <NUM> chip PRN code is zero with a standard deviation related to the number of chips in the PRN code. By definition, the value of each chip (e.g., +<NUM> or -<NUM>) of a PRN code is random with respect to all of its neighbors. Therefore, the PRN code's correlation noise behavior can be estimated by the Central Limits Theorem. According to the Central Limits Theorem, the variance of an accumulation can be estimated by the sum of the individual sample variances. The variance of each PRN chip by definition is <NUM>, thus the variance of a cross correlation sum of <NUM> chips is <NUM>. Therefore, the standard deviation of the correlation is approximately <NUM> (. e.g., sqrt(<NUM>)), whereas the expected value of an in-phase correlation of the PRN code is <NUM>.

A correlation accumulation of a modulated signal with a locally generated PRN code that is perfectly aligned or in-phase with the modulated signal will produce a value that is approximately <NUM> times higher than a correlation accumulation of the same received signal with a PRN code not in-phase (out-of-phase). Therefore, if a PRN code in several different phases (e.g., shifted various chips) are combined and then correlated with a signal, the correlation accumulation will be higher (e.g., approximately <NUM> times higher) if the phase of the received signal lines up with a phase of one of the PRN codes in the combination. That is, the correlation accumulation of a signal lining up in phase with one of the PRN codes in the combination will be approximately <NUM> times higher than the correlation accumulation of the signal not lining up in phase with any of the PRN codes in the combination.

Further, the base accumulation noise level will increase with the number of PRN codes combined together for the correlation. The expected increase of the base noise level will increase by sqrt(K), where K is the number of PRNs combined together. For example, if <NUM> different phases of a <NUM> PRN code arc combined together, the base noise level would be expected to increase by approximately <NUM> (sqrt(<NUM>)) to produce an expected noise floor of approximately <NUM> (sqrt(<NUM>)*sqrt(<NUM>)). This is still <NUM> times lower than the expected power level of an in-phase correlation (e.g., <NUM>) if any of the <NUM> PRN codes in the combination lines up with the phase of the signal.

The description below refers to the accompanying drawings, of which:.

Referring to <FIG>, a system <NUM> includes one or more client devices <NUM> and one or more Global Navigation Satellite System (GNSS) satellites <NUM> that transmit one or more GNSS satellite signals (not shown). The GNSS satellites <NUM> may be associated with one or more satellite navigation system such as, but not limited to, Galileo, Quasi-Zenith Satellite System (QZSS), BeiDou Navigation Satellite System (BDS), Global Positioning System (GPS), and/or GLONASS.

The client device <NUM> is typically capable of moving and includes an antenna <NUM> and a GNSS receiver <NUM>, with one or more processors <NUM> and a memory <NUM>. For example, the client device <NUM> may be a cellphone, laptop computer, portable computer, a personal digital assistant, etc. In addition, the GNSS receiver <NUM> may be a single, dual, or multi- frequency receiver.

The one or more processors <NUM> execute a code shift keying (CSK) demodulation subsystem <NUM>, which includes one or more correlators <NUM>, to demodulate CSK data from a satellite signal received at antenna <NUM> according to one or more embodiments described herein. Each of the plurality of correlators <NUM> may be a multi-bit or a single bit pseudo-random noise (PRN) correlator.

In an embodiment, the CSK demodulation subsystem <NUM> compares correlation power level values for respective portions (e.g., halves) of receiver codes to demodulate the CSK data, as will be described in further detail below. As such, and in an embodiment, a total number of correlation processes equal to twice the number of bits (N) of the CSK modulated symbol is required to demodulate the CSK data. For example, the CSK demodulated subsystem <NUM> will execute a total of <NUM> correlation processes to demodulate the <NUM>-bit symbol from the L6 signal.

In a further embodiment, the CSK demodulation subsystem <NUM> compares correlations power level values for portions (e.g., halves) of receiver codes with power detection threshold values to demodulate the CSK data, as will be described in further detail below. As such, and in this further embodiment, a total number of correlation processes is equal to N is required to correlate the received signals with the combinational PRN codes. In addition, an additional correlation process may be utilized to calculate the power detection threshold value. Thus, the CSK demodulated subsystem <NUM> utilizes a total N plus one correlation processes to demodulate the CSK data. In a further embodiment, the CSK demodulation subsystem <NUM> utilizes the signs of correlations power level values to demodulate the CSK data, as will be described in further detail below. As such, and in this embodiment, a total number of correlation processes equal to N is required to demodulate the CSK data.

The one or more processors <NUM> calculate position utilizing information from the GNSS signals (the timing of codes and carriers in the GNSS signals) received at the antenna <NUM> in conjunction with the demodulated CSK data to mitigate errors (e.g., orbit, clock, atmosphere, and/or multipath errors), resulting in the calculation of decimeter-level or better positioning accuracy. For example, the one or more processors <NUM> may demodulate CSK data (e.g., an <NUM>-bit symbol) from <NUM> consecutively received L6 <NUM> signal sample sets to construct a message, e.g., an L6 navigation message that is <NUM> bits, which may be utilized with an accurate positioning technique (e.g., Precise Point Positioning (PPP)).

The memory <NUM> may store one or more values associated with one or more embodiments described herein. For example, the memory <NUM> may store the fundamental PRN codes, where each fundamental PRN code is associated with a different GNSS satellite from which the GNSS receiver <NUM> receives satellite signals. In addition, the memory <NUM> may store receiver codes, the combinational PRN codes, correlation power level values, power detection threshold values, and one or more other values associated with the one or more embodiments described herein.

<FIG> is an exemplary flow chart for demodulating CSK data from a satellite signal utilizing a binary search by comparing correlation powers level values for respective portions of receiver codes in accordance with an illustrative embodiment of the invention. It should be understood that fewer or additional steps may be performed, and the steps may be performed in a different order.

The procedure <NUM> starts at step <NUM> and continues to step <NUM> where a CSK demodulation subsystem <NUM> generates combinational PRN codes for respective portions of the receiver codes. Specifically, the GNSS receiver <NUM> may maintain, in memory <NUM>, a plurality of receiver codes, where each receiver code represents a different shift in chips to the fundamental PRN code. In this example, the CSK modulated symbol is <NUM>-bits (N=<NUM>). As such, each of the plurality of receiver codes is the fundamental PRN code shifted a different number of chips from <NUM> to <NUM> to represent different possible CSK modulated symbols.

Specifically, the fundamental PRN code corresponds to the CSK modulated symbol of <NUM>. The fundamental PRN code shifted one chip to the left is the receiver code that corresponds to the CSK modulated symbol of <NUM>, the fundamental PRN code shifted two chips to the left is the receiver code that corresponds to the CSK modulated symbol of <NUM>, etc. The fundamental PRN code shifted <NUM> chips to the left is the receiver code that corresponds to the CSK modulated symbol of <NUM>.

The CSK demodulation subsystem <NUM> then performs a chip-by-chip summation (i.e., linear combination) of respective portions (e.g., halves) of the receiver codes to generate the combinational PRN codes. Specifically, the CSK demodulation subsystem <NUM> performs a chip-by-chip summation of receiver codes <NUM>-<NUM>, which correspond to the fundamental PRN code (e.g., the PRN code un-shifted) through the fundamental PRN code shifted <NUM> chips to the left. The chip-by-chip summation of receiver codes <NUM>-<NUM> produces a first combinational PRN code. In addition, the CSK demodulation subsystem <NUM> performs a chip-by-chip summation of receiver codes <NUM>-<NUM>, which corresponds to the fundamental PRN code shifted <NUM> chips to the left through the fundamental PRN code shifted <NUM> chips to the left. The chip-by-chip summation of receiver codes <NUM>-<NUM> produces a second combinational PRN code.

Further, the CSK demodulation subsystem <NUM> generates combinational PRN codes for respective portions (e.g., halves) of receive codes down a first hierarchy associated with receiver codes <NUM>-<NUM> and a second hierarchy associated with receiver codes <NUM>-<NUM>.

<FIG> are exemplary tables for generating combinational PRN codes for respective halves of receiver codes in accordance with an illustrative embodiment of the invention. For simplicity purposes, the symbol utilized for <FIG> is three bits and the PRN code is <NUM> chips. However, it is expressly contemplated that the technique described herein for generating combinational PRN codes may be utilized for a symbol having any number of bits and a PRN code having any number of chips. For example, the technique described herein may be utilized with an <NUM>-bit CSK modulated symbol and a PRN code that is <NUM> chips.

Specifically, the first table <NUM> includes column <NUM> entitled different possible CSK modulated symbol. Each of the eight rows in column <NUM> includes a different possible permutation of the CSK modulated symbol. Specifically, and because the symbol is <NUM> bits, there are eight different possible permutations of the CSK modulated symbol.

The first table <NUM> further includes a second column <NUM> entitled shifted PRN code. The first row of column <NUM> includes the fundamental PRN code and corresponds to CSK modulated symbol of <NUM>. The second row of column <NUM> includes a receiver code that is the fundamental PRN code shifted one chip to the left and corresponds to the CSK modulated symbol of <NUM>. The third row of column <NUM> includes a receiver code that is the fundamental PRN code shifted two chips to the left and corresponds to the CSK modulated symbol of <NUM>. Rows four through eight of column <NUM> include different receiver codes, e.g., the fundamental PRN code shifted three through seven chips to the left, that respectively correspond to CSK modulated symbols of <NUM> through <NUM> as depicted in <FIG>.

<FIG> further include tables <NUM>, <NUM>, <NUM>, and <NUM> that respectively depict the generation of the combinational PRN codes that are generated for respective halves of the receiver codes.

Table <NUM> is a subset of table <NUM> and contains the first half of the rows of table <NUM>, i.e., <NUM>, <NUM>, <NUM>, and <NUM>. As depicted in table <NUM>, the PRN codes shifted from zero to three chips to the left are summed together in a chip-by-chip manner to generate a first combinational PRN code <NUM>. Column <NUM> and column <NUM> of table <NUM> respectively include the first half of the CSK modulated symbols and the associated PRN code shifted various chips.

Table <NUM> is a subset of table <NUM> and contains the second half of the rows of table <NUM>, i.e., <NUM>, <NUM>, <NUM>, and <NUM>. As depicted in table <NUM>, the PRN codes shifted from four to seven chips to the left are summed together in a chip-by-chip manner to generate a second combinational PRN code <NUM>. Column <NUM> and column <NUM> of table <NUM> respectively include the second half of the CSK modulated symbols and the associated PRN code shifted various chips.

Table <NUM> is a subset of table <NUM> and contains a first half of the rows of table <NUM>, i.e., <NUM> and <NUM>. As depicted in table <NUM>, the PRN codes shifted zero and one chip to the left are summed together in a chip-by-chip manner to generate a third combinational PRN code <NUM>. Column <NUM> and column <NUM> of table <NUM> respectively include the first half of the CSK modulated symbols from table <NUM> and the associated PRN code shifted various chips.

Table <NUM> is a subset of table <NUM> and contains a second half of the rows of table <NUM>, i.e., <NUM> and <NUM>. As depicted in table <NUM>, the PRN codes shifted two and three chips to the left are summed together in a chip-by-chip manner to generate a fourth combinational PRN code <NUM>. Column <NUM> and column <NUM> of table <NUM> respectively include the second half of the CSK modulated symbols from table <NUM> and the associated PRN code shifted various chips. Although not shown in the tables, combinational PRN codes are generated for the second half of receiver codes, e.g., a combinational PRN code for receiver codes <NUM> and <NUM> and a combinational PRN code for receiver codes <NUM> and <NUM>.

It is noted that the chip values as depicted in <FIG> and <FIG> are either <NUM> or -<NUM> and for the combinational PRN codes described herein, the chip values depend on how many codes are being summed together. In the example of <FIG> & <FIG>, where four PRN codes are being added together (N=<NUM>), each chip of the combinational code may vary between -<NUM> and -<NUM>. Whereas if <NUM> different codes were added together, in the case of N=<NUM>, the chip values of the combinational code may range from -<NUM> to <NUM>. Although the chip values for the combinational PRN codes may be multiple bits in length and range from a value of -<NUM> to <NUM>, in a further embodiment, the combinational PRN codes may be reduced to a single bit. For example, the "sign" function (e.g., + or -) on the -<NUM> to +<NUM> values may be utilized to reduce the combinational PRN code to the single bit. This further embodiment would result in decreased performance, but simpler hardware, e.g., utilizing single bit PRN correlators.

<FIG> is an exemplary summing tree for the combinational PRN codes generated for respective halves of receiver codes in accordance with an illustrative embodiment of the invention. Specifically, and for an <NUM>-bit symbol (N=<NUM>), a first combinational PRN code is generated for receiver codes <NUM>-<NUM> by performing a chip-by-chip summation of receiver codes <NUM>-<NUM>. Similarly, a second combinational PRN code is generated for receiver codes <NUM>-<NUM> by performing a chip-by-chip summation of receiver codes <NUM>-<NUM>. In additional, and as depicted in <FIG>, combinational PRN codes are generated for respective halves of receiver codes down a first hierarchy associated with the first half of receiver codes and down a second hierarchy associated with the second half of receiver codes.

The procedure continues to step <NUM> where a GNSS receiver <NUM>, at a client device <NUM>, receives a satellite signal that is modulated by CSK. The satellite signal may be a L6 signal that includes Code <NUM> of <NUM> chips that is modulated by CSK to represent an N bit symbol. If the L6 signal is a L61 signal, it may also include Code <NUM> (e.g., pilot channel). If the L6 signal is a L62 signal, it may include an additional data channel. As such, an entire L6 navigation message of <NUM> bits can be transmitted from the GNSS satellite <NUM> to the GNSS receiver <NUM> in <NUM> second. For this example, let it be assumed that the N bit symbol where N=<NUM>, i.e., the CSK modulated symbol, is <NUM> (i.e., a binary representation of <NUM>).

The procedure continues to step <NUM> where the CSK demodulation subsystem <NUM> compares correlation power level values for respective portions (e.g., halves) of receiver codes to demodulate the CSK data. Specifically, the received signal is correlated with the first combinational PRN code stored in memory <NUM> to produce a first correlation power level value. In addition, the received signal is correlated with the second combinational PRN code stored in memory <NUM> to produce a second correlation power level value.

Specifically, and as depicted in <FIG>, the CSK demodulation subsystem <NUM> utilizes a correlator of the one or more correlators <NUM> to correlate the received signal, which includes Code <NUM> modulated by CSK, with the first combinational PRN code generated for the first half of the receiver codes (<NUM>-<NUM>) to produce a first correlation power level value. For example, the correlation may be the dot product of Code <NUM> modulated by CSK with the first combinational PRN code. The CSK demodulation subsystem <NUM> also uses a correlator of the one or more correlator <NUM> to correlate the received signal with the second combinational PRN code generated for the second half of the receiver codes (<NUM>-<NUM>) to produce a second correlation power level value.

The CSK demodulation subsystem <NUM> may compare the first correlation power level value and the second correlation power level value to a threshold value to determine if a signal is present. If the first correlation power level value and the second correlation power level value are not greater than the threshold value, the signal is determined to be lost or not present and the procedure ends at step <NUM>. It is noted that the threshold value may, for example, be set by a user or determined in any of a variety of different ways.

If the signal is determined to be present, the CSK demodulation subsystem <NUM> compares the first correlation power level value to the second correlation power level value. If the first correlation power level value is greater than the second correlation power level value, the CSK demodulation subsystem <NUM> performs additional correlations, utilizing the one or more correlators <NUM>, to correlate the received signal with the combinational PRN codes down a first hierarchy associated with the first portion (e.g., half) of the receiver codes to produce correlation power level values that are compared to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

If the first correlation power level value is not greater than the second correlation power level value, the CSK demodulation subsystem <NUM> performs additional correlations, utilizing the one or more correlators <NUM>, to correlate the received signal with the combinational PRN codes down the second hierarchy associated with the second portion (e.g., half) of the receiver codes to produce correlation power level values that are compared to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

<FIG> illustrates a binary search tree <NUM> using correlation power level values for respective portions of receiver codes to demodulate CSK data in accordance with an illustrative embodiment of the invention. Specifically, the CSK demodulation subsystem <NUM> utilizes a correlator of the one or more correlator <NUM> to correlate the received signal with the first combinational PRN code to produce the first correlation power level value. In addition, the CSK demodulation subsystem <NUM> utilizes a correlator of the one or more correlator <NUM> to correlate the received signal with the second combinational PRN code to produce the second correlation power level value as depicted in the first level of the binary search tree <NUM>.

The CSK demodulation subsystem <NUM> then determines if the first correlation power level value is greater than the second correlation power level value. In this example, the first correlation power level value is greater than the second correlation power level value.

Therefore, the CSK demodulation subsystem <NUM> traverses the binary search tree <NUM> down the first hierarchy to the second level. If the first correlation power level value was not greater than the second correlation power level value, the CSK demodulation subsystem <NUM> would traverse the binary search tree <NUM> down the second hierarchy, associated with receiver codes <NUM>-<NUM>, to the second level.

The CSK demodulation subsystem <NUM> performs a third correlation process, utilizing a correlator of the one or more correlators <NUM>, to correlate the received signal with a third combinational PRN code generated for receiver codes <NUM>-<NUM> to produce a third correlation power level value. In addition, the CSK demodulation subsystem <NUM> performs a fourth correlation process, utilizing a correlator of the one or more correlators <NUM>, to correlate the received signal with a fourth combinational PRN code generated for receiver codes <NUM>-<NUM> to produce a fourth correlation power level value. The CSK demodulation subsystem <NUM> then determines if the third correlation power level value is greater than the fourth correlation power level value. In this example, the third correlation power level value is greater than the fourth correlation power level value.

The CSK demodulation subsystem <NUM> continues to traverse the binary search tree <NUM> down the first hierarchy, based on the comparison of correlation power level values as described above and utilizing additional correlation processes, to reach the bottom level (i.e., the ninth level) to determine the CSK modulated symbol. In this example, the CSK demodulation subsystem <NUM> traverses the first hierarchy and determines that the correlation power level value produced based on the correlation of the received signal with receiver code two is not greater than the correlation power level value produced based on the correlation of the received signal with receiver code three. As such, the CSK modulated symbol is determined to be three. Therefore, the CSK demodulation subsystem <NUM> determines that the <NUM>-bit symbol represented by Code <NUM> modulated by CSK is <NUM> (i.e., a binary representation of <NUM>).

Advantageously, the inventive system and method utilizes N times two, e.g., <NUM>, correlation processes to demodulate the CSK data.

From step <NUM>, the procedure may continue to step <NUM> where the GNSS receiver <NUM> receives additional signals and demodulates the CSK data from <NUM> consecutive L6 <NUM> signal sample sets, in the manner described above, to determine the binary value of an entire L6 message that is <NUM> bits. Specifically, the GNSS receiver <NUM> may utilize the generated combinational PRN codes, stored in memory <NUM>, every <NUM> to demodulate the CSK data from <NUM> consecutive L6 signal sample sets in the manner described above.

Alternatively, from step <NUM>, the procedure may continue to step <NUM> where the one or more processors <NUM>, of the GNSS receiver <NUM>, calculate position utilizing information from the GNSS signals (e.g., the timing of codes and carriers in the GNSS signals) received at the antenna <NUM> in conjunction with the demodulated CSK data to mitigate errors (e.g., orbit, clock, atmosphere, and/or multipath errors). For example, the one or more processors <NUM> may utilize the L6 message of <NUM> bits and demodulated from the <NUM> consecutive L6 signals to implement PPP or any of a variety of different accurate positioning techniques, thus resulting in the calculation of decimeter-level or better positioning accuracy as known by those skilled in the art.

The procedure then continues to step <NUM> where the receiver <NUM> may receive additional signals, and demodulates the CSK data and determines position in the manner described above. For example, and after one second and demodulating the entirety of the L6 message, the GNSS receiver may receive additional signals and demodulate the CSK data and determine position in the manner described above.

It is noted that utilizing respective halves of the receiver codes is for exemplary purposes only, and it is expressly contemplated that the combinational PRN codes may be generated based on the receiver codes being divided in any of a variety of different ways. For example, a first combinational PRN code may be generated for <NUM>/<NUM> of the receiver codes (e.g., receiver codes <NUM>-<NUM>) and a second combinational PRN code may be generated for the other <NUM>/<NUM> of the receiver codes (e.g., receiver codes <NUM>-<NUM>). The CSK demodulation subsystem <NUM> would then correlate the received signal with the first combinational PRN code and the second combinational PRN code to produce respective first and second correlation power level value.

If the first correlation power level value is greater than the second correlation power level value, the CSK demodulation subsystem <NUM> correlates the received signal with combinational PRN codes down a first hierarchy associated with receiver codes <NUM>-<NUM>, in the manner described above, to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>).

If the first correlation power level value is not greater than the second correlation power level value, the CSK demodulation subsystem <NUM> correlates the received signal with combinational PRN codes down a second hierarchy associated with receiver codes <NUM>-<NUM>, in the manner described above, to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>).

Thus, the CSK demodulation may utilize any of a variety of partitioning algorithms to divide the receiver codes in a hierarchical manner to demodulate the CSK data in the manner described above. For example, such partitioning algorithms may include, but are not limited to, a golden-section schema or any of a variety of different schemes, as known by those skilled in the art.

<FIG> is an exemplary flow chart for demodulating CSK data from a satellite signal utilizing a binary search by comparing correlation power level values for portions of receiver codes with power detection threshold values in accordance with an illustrative embodiment of the invention. It should be understood that fewer or additional steps may be performed, and the steps may be performed in a different order.

The procedure <NUM> starts at step <NUM> and continues to step <NUM> where a CSK demodulation subsystem <NUM> generates combinational PRN codes for respective portions of the receiver codes. Specifically, and where N=<NUM>, the GNSS receiver <NUM> may maintain a plurality of different receiver codes and the CSK demodulation subsystem <NUM> may perform a chip-by-chip summation of portions (e.g., halves) of receiver codes to generate the combinational PRN codes as described above with reference to <FIG> and <FIG>.

The procedure continues to step <NUM> where the CSK demodulation subsystem <NUM> calculates a power detection threshold value. If the L6 signal is an L61 signal, the power threshold value may be based on a measured carricr-to-noisc (C/N) ratio. Specifically, and after the pilot channel has been phase locked, the CSK demodulation subsystem <NUM> may measure the C/N ratio of the pilot channel. The CSK demodulation subsystem <NUM> may then divide the measured power of the pilot channel by two to produce the power detection threshold value (e.g., original power threshold value).

If the L6 signal is an L62 signal which does not include the pilot channel, the power detection threshold value may be based on the fundamental PRN code (i.e., the un-shifted PRN code). Specifically, a correlator of the one or more correlators <NUM> may be utilized for the fundamental PRN code (i.e., the un-shifted PRN code). The CSK demodulation subsystem <NUM> may measure the power level associated with the correlator utilized for the fundamental PRN code, and the power level may be divided by two to produce the power detection threshold value. It is noted that a single multiplexed correlator may be utilized to correlate the received signal with the combinational PRN codes and to also calculate the power detection threshold value. Alternatively, a plurality of correlators, of the one or more correlators <NUM>, may be utilized to correlate the received signal with the combinational PRN codes and to also calculate the power detection threshold value.

The procedure continues to step <NUM> where a GNSS receiver <NUM>, at a client device <NUM>, receives a satellite signal that is modulated by CSK. The satellite signal may be an L6 signal that includes Code <NUM> of <NUM> chips that is modulated by CSK to represent an N bit symbol. If the L6 signal is an L61 signal, it may also include Code <NUM> (e.g., pilot channel). If the L6 signal is an L62 signal, it may include an additional data channel. As such, an entire L6 navigation message of <NUM> bits can be transmitted from the GNSS satellite <NUM> to the GNSS receiver <NUM> in <NUM> second. For this example, let it be assumed that the N bit symbol where N=<NUM>, i.e., the CSK modulated symbol, is <NUM> (i.e., a binary representation of <NUM>).

The procedure continues to step <NUM> where the CSK demodulation subsystem <NUM> compares correlation power level values for portions (e.g., halves) of receiver codes with power detection threshold values to demodulate the CSK data. Specifically, and as depicted in <FIG>, the CSK demodulation subsystem <NUM> performs a first correlation process, utilizing a correlator of the one or more correlators <NUM>, to correlate the received signal with the combinational PRN code generated for receiver codes <NUM>-<NUM> to produce a first correlation power level value. The CSK demodulation subsystem <NUM> then compares the first correlation power level value to the power detection threshold value.

If the first correlation power level value is greater than the power detection threshold value, the CSK demodulation subsystem <NUM> performs additional correlations, utilizing the one or more correlators <NUM>, to correlate the received signal with the combinational PRN codes down the first hierarchy to produce correlation power level values that are compared to power detection threshold values to determine the CSK modulated symbol. The CSK modulated symbol, determined based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> and is the demodulated CSK data.

It is noted that if the first correlation power level value is not greater than the power detection threshold value, the CSK demodulation subsystem <NUM> may test for the presence of the signal by also correlating the received signal with an additional combinational PRN code generated for a second portion (e.g., half) of the receiver codes, e.g., receiver codes <NUM>-<NUM>. If the correlation power level value from the second portion is not greater than the power detection threshold value, it may be determined that the signal is lost or not present and the procedure ends at step <NUM>.

If the first correlation power level value is not greater than the power detection threshold value and the signal is determined to be present, the CSK demodulation subsystem <NUM> performs additional correlations, utilizing the one or more correlators <NUM>, to correlate the received signal with the combinational PRN codes down the second hierarchy to produce correlation power level values that are compared to power detection threshold values to determine the CSK modulated symbol. The CSK modulated symbol, determined based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> and is the demodulated CSK data.

<FIG> illustrates a binary search tree <NUM> using correlation power level values for portions of receiver codes with power detection threshold values to demodulate CSK data in accordance with an illustrative embodiment of the invention. Specifically, the CSK demodulation subsystem <NUM> utilizes a correlator of the one or more correlators <NUM> to correlate the received signal with the first combinational PRN code generated for receiver codes <NUM>-<NUM> to produce the first correlation power level value. The CSK demodulation subsystem <NUM> then determines if the first correlation power level value is greater than the power detection threshold value. In this example, the first correlation power level value is greater than the power detection threshold.

Therefore, the CSK demodulation subsystem <NUM> determines that the signal is present and traverses the binary search tree <NUM> down the first hierarchy to the second level. If the first correlation power level value was not greater than power detection threshold value, the CSK demodulation subsystem <NUM> would test for the presence of a signal based on the correlation of the received signal with the second portion (e.g., half) of receiver codes, and then traverse the binary search tree <NUM> down the second hierarchy to the second level.

The CSK demodulation subsystem <NUM> performs a second correlation process, utilizing the one or more correlators <NUM>, to correlate the received signal with a second combinational PRN code generated for receiver codes <NUM>-<NUM> to produce a second correlation power level value. The CSK demodulation subsystem <NUM> then determines if the second correlation power level value is greater than the power detection threshold value. If soft codes (e.g., decimal values for combined codes) are being utilized, the power detection threshold is the same for all levels in the binary search tree <NUM>. If hard codes (e.g., <NUM> bit combined codes) are being utilized, the power detection threshold value may be the same for all levels or may be twice the power detection threshold value of a previous level (i.e., a level above). Therefore, and for the second level, the power detection threshold value is either the original power detection threshold value or two times the original power detection threshold value. In this example, the second correlation power level value is greater than the power detection threshold value of the second level.

The CSK demodulation subsystem <NUM> continues to traverse the binary search tree <NUM> down the first hierarchy, based on the comparison of correlation power level values with the power detection threshold values as described above and by performing additional correlation processes, to reach the bottom level (i.e., the ninth level) to determine the CSK modulated symbol. In this example, the CSK demodulation subsystem <NUM> traverses the first hierarchy and determines that the correlation power level value produced based on the correlation of the received signal with receiver code zero is not greater than the power detection threshold value. As such, the CSK modulated symbol is determined to be one. Therefore, the CSK demodulation subsystem <NUM> determines that the <NUM>-bit symbol represented by Code <NUM> modulated by CSK is <NUM> (i.e., a binary representation of <NUM>). Advantageously, the inventive system and method utilizes a total of N correlation processes to demodulate the CSK data when the L6 signal is a L61 signal.

It is noted that utilizing respective halves of the receiver codes is for exemplary purposes only, and it is expressly contemplated that the combinational PRN codes may be generated based on the receiver codes being divided in any of a variety of different ways. For example, a first combinational PRN code may be generated for <NUM>/<NUM> of the receiver codes (e.g., receiver codes <NUM>-<NUM>) and a second combinational PRN code may be generated for the other <NUM>/<NUM> of the receiver codes (e.g., receiver codes <NUM>-<NUM>). The CSK demodulation subsystem <NUM> would then correlate the received signal with the first combinational PRN code that is compared to the power detection threshold value.

If the first correlation power level value is greater than the power detection threshold value, the CSK demodulation subsystem <NUM> correlates the received signal with combinational PRN codes down a first hierarchy associated with receiver codes <NUM>-<NUM>, in the manner described above, to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>).

If the first correlation power level value is not greater than the power detection threshold value, the CSK demodulation subsystem <NUM> correlates the received signal with combinational PRN codes down a second hierarchy associated with receiver codes <NUM>-<NUM>, in the manner described above, to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>).

<FIG> an exemplary flow chart for demodulating CSK data utilizing a binary search based on the signs of correlation power level values in accordance with an illustrative embodiment of the invention. It should be understood that fewer or additional steps may be performed, and the steps may be performed in a different order.

The procedure <NUM> starts at step <NUM> and continues to step <NUM> where a CSK demodulation subsystem <NUM> generates combinational PRN codes for portions of the receiver codes utilizing chip-by-chip summation and chip-by-chip subtraction. Specifically, and where N=<NUM>, the GNSS receiver <NUM> may maintain a plurality of different receiver codes, as described above. In addition, a first combinational PRN code may be generated based on a chip-by-chip summation of a first portion (e.g., half) of receiver codes, e.g., receiver codes <NUM>-<NUM>, and a chip-by-chip subtraction of a second portion (e.g., half) of receiver codes, e.g., receiver codes <NUM>-<NUM>. The CSK demodulation subsystem <NUM> also performs a chip-by-chip summation and a chip-by-chip subtraction for respective portions (e.g., halves) of receiver codes down a first hierarchy associated with the first portion of the receiver code to produce respective combinational PRN codes. Further, CSK demodulation subsystem <NUM> performs a chip-by-chip summation and a chip-by-chip subtraction for respective portions (e.g., halves) of receiver codes down a second hierarchy associated with the second portion of the receiver codes to produce respective combinational PRN codes.

<FIG> depicts an exemplary summing tree for the combinational PRN codes generated for portions of receiver codes utilizing chip-by-chip summations and subtractions in accordance with an illustrative embodiment of the invention. Specifically, a first combinational PRN code may be generated for receiver codes <NUM>-<NUM> based on a chip-by-chip summation of receiver codes <NUM>-<NUM> and a chip-by-chip subtraction of receiver codes <NUM>-<NUM>. Furthermore, a second combinational PRN code, for receiver codes <NUM>-<NUM> and down a first hierarchy, is generated based on a chip-by-chip summation of receiver codes <NUM>-<NUM> and a chip-by-chip subtraction of receiver codes <NUM>-<NUM>. In addition, a third combinational PRN code, for receiver codes <NUM>-<NUM> and down a second hierarchy, is generated based on chip-by-chip summation of receiver codes <NUM>-<NUM> and a chip-by-chip subtraction of receiver codes <NUM>-<NUM>. Additional combinational PRN codes are generated down the first hierarchy and the second hierarchy in a similar manner and as depicted in <FIG>.

The procedure continues to step <NUM> where the CSK demodulation subsystem <NUM> utilizes the signs (e.g., positive or negative) of correlation power level values to demodulate the CSK data. Specifically, and as depicted in <FIG>, the CSK demodulation subsystem <NUM> correlates, utilizing a correlator of the one or more correlators <NUM>, the received signal with the first combinational PRN code to produce a first correlation power level value. It is noted that the CSK demodulation subsystem <NUM> may first compare an absolute value of the first correlation power level value to a threshold value to determine if a signal is present. If the absolute value of the first correlation power level value is not greater than the threshold value, the signal is determined to be lost or not present and the procedure ends at <NUM>. It is noted that the threshold value, may, for example, be set by a user or determined in any of a variety of different ways.

If the signal is determined to be present, the CSK demodulation subsystem <NUM> determines if the correlation power level value is positive (i.e., +) or negative (i.e., -). If the correlation power is positive, the CSK demodulation subsystem <NUM> correlates the received signal, utilizing a correlator of the one or more correlators <NUM>, with the combinational PRN codes down the first hierarchy to produce correlation power level values. The signs (e.g., positive or negative) of the produced correlation power level values based on the traversal down the first hierarchy are utilized to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

If the correlation power is negative, the CSK demodulation subsystem <NUM> correlates the received signal, utilizing a correlator of the one or more correlators <NUM>, with the combinational PRN codes down the second hierarchy to produce correlation power level values. The signs (e.g., positive or negative) of the produced correlation power level values based on the traversal down the second hierarchy are utilized to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>) and is the demodulated CSK data.

<FIG> depicts a binary search tree using the signs of correlation power level values to demodulate CSK data in accordance with an illustrative embodiment of the invention. Specifically, the CSK demodulation subsystem <NUM> utilizes a correlator of the one or more correlators <NUM> to correlate the received signal with the first combinational PRN code for receiver codes <NUM>-<NUM> to produce the first correlation power level value. The first combinational PRN code is based on a chip-by-chip summation of receiver codes <NUM>-<NUM> and a chip by-chip subtraction of receiver codes <NUM>-<NUM>.

The CSK demodulation subsystem <NUM> then, after confirming the presence of a signal, determines if the first correlation power level value is positive or negative. In this example, the first correlation power level value is positive.

Therefore, the CSK demodulation subsystem <NUM> traverses the binary search tree <NUM> down the first hierarchy to the second level. If the first correlation power level value was negative, the CSK demodulation subsystem <NUM> would traverse the binary search tree <NUM> down the second hierarchy, associated with receiver codes <NUM>-<NUM>, to the second level.

The CSK demodulation subsystem then performs a second correlation process, utilizing a correlator of the one or more correlators <NUM>, to correlate the received signal with a second combinational PRN code generated for receive codes <NUM>-<NUM> to produce a second correlation power level value. The second combinational PRN code is based on a chip-by-chip summation of receiver codes <NUM>-<NUM> and a chip by-chip subtraction of receiver codes <NUM>-<NUM>.

The CSK demodulation subsystem then determines if the second correlation power level value is positive or negative. In this example, the second correlation power level value is positive. The CSK demodulation subsystem <NUM> continues to traverse the binary search tree <NUM> down the first hierarchy based on the signs (e.g., positive or negative) of the produced correlation power level values utilizing additional correlation processes, to reach the bottom level (i.e., the ninth level) to determine the CSK modulated symbol. In this example, the CSK demodulation subsystem <NUM> traverses the first hierarchy and determines that the correlation of the received signal and the combinational PRN code, generated based on a chip-by-chip subtraction of receiver code one from receiver code zero, produces a positive correlation power level value. As such, the CSK modulated symbol is determined to be zero. Therefore, the CSK demodulation subsystem <NUM> determines that the <NUM>-bit symbol represented by Code <NUM> modulated by CSK is <NUM> (i.e., a binary representation of <NUM>).

Advantageously, the inventive system and method utilizes N correlation processes, e.g., eight correlation processes, to demodulate the CSK data.

<FIG> and <FIG> depict performing a chip-by-chip summation of the first half of the receiver codes and a chip-by-chip subtraction of the second half of receiver codes to generate the combinational PRN codes. However, it is expressly contemplated that the systems and methods described herein may perform a chip-by-chip subtraction of the first half of the receiver codes and a chip-by-chip summation of the second half of receiver codes to generate the combinational PRN codes. For example, the CSK demodulation subsystem <NUM> may perform a chip-by-chip subtraction of receiver codes <NUM>-<NUM> and a chip-by-chip summation of receiver codes <NUM>-<NUM> to generate the first combinational PRN Code for receiver codes <NUM>-<NUM>. The CSK demodulation subsystem <NUM> would then traverse the first hierarchy if the first correlation power value is negative and traverse the second hierarchy if the first correlation power value is positive.

In addition, it is noted that utilizing respective halves of the receiver codes is for exemplary purposes only, and it is expressly contemplated that the combinational PRN codes may be generated based on the receiver codes being divided in any of a variety of different ways. For example, a first combinational PRN code may be generated based on a chip-by-chip summation of <NUM>/<NUM> of the receiver codes (e.g., receiver codes <NUM>-<NUM>) and a chip-by-chip subtraction of the other <NUM>/<NUM> of the receiver codes (e.g., receiver codes <NUM>-<NUM>). The CSK demodulation subsystem <NUM> would then correlate the received signal with the first combinational PRN code to produce a first correlation power level value.

If the first correlation power level value is positive, the CSK demodulation subsystem <NUM> correlates the received signal with combinational PRN codes down a first hierarchy associated with receiver codes <NUM>-<NUM>, in the manner described above, to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the first hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>). If the first correlation power level value is negative, the CSK demodulation subsystem <NUM> correlates the received signal with combinational PRN codes down a second hierarchy associated with receiver codes <NUM>-<NUM>, in the manner described above, to determine the CSK modulated symbol. The CSK modulated symbol, based on the traversal down the second hierarchy, is a particular value from <NUM>-<NUM> (e.g., <NUM> - <NUM>).

Claim 1:
An apparatus (<NUM>) comprising:
a processor (<NUM>);
a demodulation subsystem (<NUM>) executed by the processor and configured to:
receive a Global Navigation Satellite System, GNSS, signal modulated by code shift keying, CSK, to represent a symbol (<NUM>);
perform a chip-by-chip linear combination of a first portion of a plurality of receiver codes (<NUM>) to generate a first combinational code (<NUM>), wherein each of the plurality of receiver codes (<NUM>) is a different shift in chips to a predetermined code;
perform a chip-by-chip linear combination of a second portion of the plurality of receiver codes (<NUM>) to generate a second combinational code (<NUM>);
correlate the modulated navigation signal with the first combinational code to produce a first correlation power level value and correlate the modulated navigation signal with the second combinational code to produce a second correlation power level value;
traverse, in response to the first correlation power level value being greater than the second correlation power level value, to a next level of a first hierarchy of a binary search tree (<NUM>) that is associated with the first portion of the plurality of receiver codes to determine the symbol, or
traverse, in response to the first correlation power level value not being greater than the second correlation power level value, to a next level of a second hierarchy of the binary search tree (<NUM>) that is associated with the second portion of the plurality of receiver codes to determine the symbol; and
continue to traverse the binary search tree (<NUM>) to reach a bottom level, to determine the symbol