Synchronization in a severe fading environment
The system improves RF synchronization in severe fading environments by autocorrelating and synchronously correlating RF signals with a synchronization sequence, enhancing synchronization accuracy and reducing errors.
Patent Information
- Application Number
- JP2025175398
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2021-06-04
- Filing Date
- 2025-10-17
- Publication Date
- 2026-01-14
AI Technical Summary
In severe fading environments, synchronization with RF communications from licensed transmitters is difficult due to challenging communication scenarios such as urban interference and structural limitations, making it hard to maintain good reception and fidelity.
A system utilizing a radio frequency receiver, processor, and memory to autocorrelate and synchronously correlate RF signals with a synchronization sequence comprising two pseudo-noise code sequences separated by a time delay, generating a composite synchronization signal to detect peaks for synchronization.
Enhances synchronization robustness in severe fading environments by accurately identifying synchronization times and frequencies, reducing synchronization errors to less than 1% at signal-to-noise ratios above a threshold.
Smart Images
Figure 2026004615000001_ABST
Abstract
Description
[Background technology]
[0001] RF communications are performed for a variety of purposes. Some communication channels present challenging scenarios for good reception and fidelity of RF communications. For example, in some communication sites, numerous transceivers compete for limited communication bandwidth. In some communication sites, strategies are taken to actively congest communication channels. In urban communication sites, buildings and other structures may limit the path of RF communications to various routes throughout a downtown area. In such challenging communication sites, sometimes referred to as severe fading environments, synchronization with communications from licensed transmitters can be difficult. Summary of the Invention
[0002] The apparatus and related method relate to a system for identifying a synchronization sequence of a predetermined length having a first code sequence and a second code sequence separated by a predetermined time delay, and for synchronizing to the synchronization sequence. The system includes a radio frequency (RF) receiver, a processor, and a computer-readable memory. The RF receiver receives an RF signal within a predetermined frequency band. The computer-readable memory has coded instructions that, when executed by the processor, cause the system to autocorrelate a first portion of a generated signal with a second portion of the generated signal to generate an autocorrelation signal. The first and second portions are separated in time by a predetermined time delay separating the first and second code sequences. The computer-readable memory has coded instructions that, when executed by the processor, cause the system to synchronously correlate a third portion of the generated signal with the synchronization sequence to generate a synchronous correlation signal. The third portion has the predetermined length of the synchronization sequence and includes the first and second portions of the generated signal used to generate the autocorrelation signal. The computer readable memory has coded instructions that, when executed by the processor, cause the system to multiply the autocorrelation signal and the sync correlation signal to generate a composite sync signal. The computer readable memory further has coded instructions that, when executed by the processor, cause the system to identify peaks in the composite sync signal.
[0003] Some embodiments relate to a method for identifying a synchronization sequence of a predetermined length having a first code sequence and a second code sequence separated by a predetermined time delay and synchronizing to the synchronization sequence. The method includes detecting radio frequency (RF) energy within a predetermined frequency band. The method includes generating a signal indicative of the detected RF energy. The method includes autocorrelating a first portion of the generated signal with a second portion of the generated signal to generate an autocorrelation signal. The first and second portions are separated in time by a predetermined time delay separating the first and second code sequences. The method includes synchronously correlating a third portion of the generated signal with the synchronization sequence to generate a synchronous correlation signal. The third portion has the predetermined length of the synchronization sequence and includes the first and second portions of the generated signal used to generate the autocorrelation signal. The method includes multiplying the autocorrelation signal by the synchronous correlation signal to generate a composite synchronization signal. The method also includes identifying a peak in the composite synchronization signal.
[0004] Some embodiments relate to a method for generating and communicating a synchronization sequence to indicate that a transmitted signal is from an authorized transmitter. The method includes transmitting a first code sequence. The method includes transmitting a second code sequence a predetermined time delay after the first code sequence is transmitted. The second code sequence is identical to the first code sequence. The method also includes transmitting a communication synchronized to the second code sequence following the transmission of the second code sequence. [Brief explanation of the drawings]
[0005] [Figure 1] FIG. 1 is a perspective view of radio frequency (RF) communications conducted in a severe fading environment. [Figure 2] FIG. 1 is a diagram of a synchronization sequence used for synchronization in a severe fading environment. [Figure 3] FIG. 2 is a schematic diagram of an autocorrelation between a first portion and a second portion of a received RF signal. [Figure 4] FIG. 10 is a schematic diagram of a synchronous correlation of a third portion of a received RF signal with a synchronization sequence. [Figure 5] 4 is a graph of the generated self-sequence signal, the sync-sequence signal, and the composite correlation signal. [Figure 6] FIG. 1 is a schematic diagram of signal decoding for synchronized RF communications. [Figure 7] 10 is a graph showing the signal to noise ratio for both synchronized communications using a composite correlation signal and unsynchronized communications using a composite correlation signal. DETAILED DESCRIPTION OF THE INVENTION
[0006] An apparatus and associated method provide robust synchronization of radio frequency (RF) communications in severe fading environments. A first portion of a detected RF signal is autocorrelated with a second portion of the detected RF signal. The first and second portions are separated in time by a predetermined time delay separating the first and second code sequences. A third portion of the detected RF signal is synchronously correlated with a synchronization sequence to generate a synchronous correlation signal. The third portion has a predetermined length of the synchronization sequence and includes the first and second portions of the detected RF signal used to generate the autocorrelation signal. The autocorrelation signal is multiplied by the synchronous correlation signal to generate a composite synchronization signal. A peak in the composite synchronization signal is then detected. This peak may indicate a synchronization time for authorized communications.
[0007] FIG. 1 is a perspective view of radio frequency (RF) communications conducted within a severe fading environment. In FIG. 1, a downtown area 10 includes buildings 12, streets 14, and various channel-conflicting transmitters 16. A person 18 attempts to receive a communication from a licensed transmitter some distance away via a receiver 20 configured with a severe fading resistant synchronization system 22. The severe fading resistant synchronization system 22 detects and synchronizes to a synchronization sequence of the communication, such as a synchronization sequence 24 of a predetermined structure. The synchronization sequence 24 is a signal of a predetermined length (L SYNC), which includes two pseudo-noise (PN) code sequences, PN1 and PN2, separated by a predetermined time delay (T DELAY ) are separated in time from one another. As shown in FIG. 1, a guard band may be inserted between the two PN code sequences, PN1 and PN2. In another embodiment, the two PN code sequences, PN1 and PN2, may be immediately adjacent in time to one another (i.e., the second PN code sequence, PN2, may directly follow the first PN code sequence, PN1).
[0008] A synchronization sequence such as synchronization sequence 24 may be embedded in an associated communication, for example, as a header of the associated communication. The synchronization sequence 24 may have a predetermined time relationship to the associated communication, such that when the synchronization sequence 24 is detected, the associated communication will have a predetermined time relationship to the synchronization sequence 24. For example, the associated communication may be inserted after the synchronization sequence 24, for example, a predetermined time delay (T COM ) In some embodiments, communication may begin immediately after synchronization sequence 24. In other embodiments, communication may occur before synchronization sequence 24.
[0009] The severe fading resistant synchronization system 22 detects and synchronizes to the synchronization sequence 24 using an algorithm, which will be described in more detail below with reference to subsequent figures. In short, the severe fading resistant synchronization system 22 detects and synchronizes to the expected synchronization sequence 24 for a predetermined length L SYNC At the same time, the synchronization sequence 24 is detected by windowing the synchronization correlation with the autocorrelation of the first PN code and the second PN code, PN1 and PN2, and synchronization is achieved with the synchronization sequence 24.
[0010] This autocorrelation is performed over a given length of the PN code (L PN ) and the most recently received RF signal with a predetermined time delay (T DELAY ) and received early with a time separation of a predetermined length (L PN) with the previously received RF signal. When reception of the second PN code, PN2, begins, the most recently received signal is exactly T DELAY When the second PN code, PN2, begins to be received, the signal produced by this autocorrelation begins to grow in magnitude because it is actually correlated with the signal received only a short time earlier. When the entire second PN code, PN2, has been received, the latest L of the received signal PN The time is exactly T DELAY L of the signal received just as early PN Since the signal is correlated with the time period, the magnitude of the autocorrelation continues to increase until the entire second PN code, PN2, is received. After the entire second PN code, PN2, is received, the magnitude of the autocorrelation increases over the next L until the signal correlation becomes unpredictable. PN It decreases by an amount of time.
[0011] This continuous autocorrelation essentially determines whether the RF energy is distributed over a predetermined time delay (T DELAY ) separated by a certain period. If the RF energy detected by receiver 20 does not have this required pattern, the synchronization correlation will be weighted conservatively (e.g., only low random noise energy will autocorrelate with the required pattern). Thus, if the required pattern is not detected via autocorrelation, the synchronization correlation will be weighted very low. However, if the required pattern is detected via autocorrelation, the synchronization correlation will be weighted appropriately highly. The PN code can be selected such that the synchronization correlation will only be detected if the entire synchronization sequence is received. Thus, in the time domain, the autocorrelation signal will be relatively wide (e.g., the length of the PN code, L PN The autocorrelation signal and the synchronous correlation signal are described in more detail below with reference to FIG. 5.
[0012] Figure 2 is a diagram of a synchronization sequence used for synchronization in a severe fading environment. In Figure 2, the synchronization sequence 24 is of a predetermined length L SYNCThe synchronization sequence 24 includes a first PN code and a second PN code, PN1 and PN2, which are separated by a predetermined time delay T DELAY are separated in time from each other by a predetermined time delay T DELAY is the length L of the first PN code and the second PN code, PN1 and PN2, respectively. PN longer than a given time delay T DELAY is the length of the PN code L PN Because of their longer lengths, a guard band exists between the first and second PN codes, PN1 and PN2. In some embodiments, this guard band may contain coded data transmitted by the transmitter. In other embodiments, the transmitter does not transmit any data during this guard band.
[0013] Immediately below the synchronization sequence 24 is the receive sequence RX SEQ Receive sequence RX SEQ is the given length L SYNC The receive sequence RX SEQ may be, for example, the most recent portion of a signal indicative of RF energy detected by receiver 20 (shown in FIG. 1). SEQ includes first and second portions, RX1 and RX2, separated by a predetermined time delay T separating the first and second PN code sequences, PN1 and PN2. DELAY are separated in time by a given time delay T DELAY is the length L of the first and second parts, RX1 and RX2 PN Because it is longer, there is a guard band RX between the first and second parts, RX1 and RX2. GUARD When the synchronization sequence 24 is detected by the receiver 20, the received sequence RX SEQ appears, the latest part of the received sequence RX SEQThe first and second portions, RX1 and RX2, of the received sequence RX include RF energy corresponding to the transmitted first and second PN codes, PN1 and PN2 (as well as noise from various noise sources). Similarly, in embodiments where the transmitter transmits coded data, the received sequence RX SEQ The guard bands include RF energy corresponding to such coded data.
[0014] 3 is a schematic diagram of the autocorrelation of the first and second parts of the received RF signal. SEQ The first and second parts, RX1 and RX2, of the received sequence RX are autocorrelated with each other. SEQ a conjugate version of one of the first and second parts, RX1 and RX2, of the received sequence RX SEQ RX1 and RX2 with a non-conjugated version of the other of the first and second parts, RX1 and RX2. In the embodiment of FIG. 3, the second part, RX2, is conjugated and the first part, RX1, is not conjugated. After such conjugation of one of the first and second parts, RX1 and RX2, the two sequences are multiplied together bit by bit, and then each of these resulting bit products are summed together. The received sequence RX SEQ The convolution result, such as a sum of bit products, is continually performed to detect when contains RF energy associated with such a correlation between the first and second PN codes, PN1 and PN2.
[0015] 4 is a schematic diagram of the synchronous correlation of the third part of the received RF signal with the synchronization sequence. SEQ is synchronously correlated with the synchronization sequence 24. The synchronization correlation is performed with the received sequence, RX SEQ , or a conjugated version of one of the synchronization sequences 24 as the receive sequence, RX SEQ, or a non-conjugated version of the other of the synchronization sequence 24. In the embodiment of FIG. 4, the synchronization sequence 24 is conjugated and the receive sequence RX SEQ is not conjugated. Receive sequence, RX SEQ After such conjugation of either the received sequence RX or one of the synchronization sequences 24, the two sequences are multiplied together bit by bit, and then each of these resulting bit products is summed together. SEQ The convolution result, such as a sum of bit products, is continually performed to detect when {overscore (x)} contains RF energy associated with the synchronization sequence 24 .
[0016] 5 is a graph of the generated autosequence, synchronization sequence, and composite correlation signal. In FIG. 5, graph 26 includes a horizontal axis 28, a vertical axis 30, and an autocorrelation signal 32. Horizontal axis 28 represents time. Vertical axis 30 represents the magnitude of autocorrelation signal 32. Autocorrelation signal 32 represents a signal generated by an autocorrelation algorithm, such as the autocorrelation algorithm shown in FIG. 3. In regions A and C of graph 26, the received sequence RX SEQ contains RF energy not associated with any portion of the synchronization sequence 24, or RF energy associated with a large portion of only one of the first and second PN codes, PN1 and PN2 (shown in FIG. 2). Thus, in regions A and C of graph 26, the magnitude of the autocorrelation signal 32 is relatively low. In region B of graph 26, the magnitude of the received sequence RX SEQ contains RF energy associated with portions of both the first and second PN codes, PN1 and PN2. At or near peak 34 of autocorrelation signal 32, the received sequence RX SEQ includes RF energy associated with both the first and second PN codes, PN1 and PN2.
[0017] 5, graph 36 includes a horizontal axis 38, a vertical axis 40, and a synchronization correlation signal 42. Similarly, horizontal axis 38 represents time. Vertical axis 40 represents the magnitude of synchronization correlation signal 42. Synchronization correlation signal 42 represents a signal generated by a synchronization correlation algorithm, such as the autocorrelation algorithm shown in FIG. 4. Synchronization correlation signal 42 includes peaks 44, 46, and 48. Unlike autocorrelation signal 32 in graph 26, peaks 44, 46, and 48 have a length of only one bit of synchronization sequence 24. During synchronization correlation, the received sequence RX SEQ Peak 44 occurs when the received sequence RX contains RF energy associated with the first PN code, PN1, and also aligned with the second PN code, PN2. SEQ Peak 46 occurs when the received sequence RX contains RF energy associated with both the first and second PN codes, PN1 and PN2, that are perfectly aligned with the first and second PN codes, PN1 and PN2, of the synchronization sequence 24. During synchronization correlation, SEQ Peak 48 occurs when the autocorrelation signal 32 of graph 26 contains RF energy associated with the second PN code, PN2, and also aligned with the first PN code, PN1. Thus, in the absence of noise, peaks 44 and 48 should have the same magnitude as peak 46 (for embodiments without guard bands or code data within the guard bands). However, in severe fading environments, the relative magnitudes of peaks 44, 46, and 48 are uncertain. Thus, autocorrelation signal 32 of graph 26 can be used in a windowing scheme to weight peak 46 more heavily than peaks 44 and 48.
[0018] 5, graph 50 includes a horizontal axis 52, a vertical axis 54, and a composite correlation signal 56. Similarly, horizontal axis 52 represents time. Vertical axis 54 represents the magnitude of composite correlation signal 56. Composite correlation signal 56 represents a signal generated by the product of autocorrelation signal 32 of graph 26 and synchronous correlation signal 42 of graph 36. As shown in graph 50, peaks 44 and 48 have been squelched by a windowing function resulting from the product of autocorrelation signal 32 of graph 26 and synchronous correlation signal 42 of graph 36. Thus, peak 46 has been isolated to facilitate detection and synchronization to peak 46.
[0019] Receive sequence RX SEQ may also be used to change or identify the optimum demodulation frequency to use in decoding communications associated with the synchronization sequence 24. To identify such a demodulation frequency, the receive sequence RX SEQ The first and second parts, RX1 and RX2, of the signal are multiplied by a conjugate PN code to form a series of bit products. These bit products are then summed. The sum of the bit products of the first part, RX1, and the conjugate PN code results in a first phase angle, θ1. The sum of the bit products of the second part, RX2, and the conjugate PN code results in a first phase angle, θ2. The demodulation frequency can be estimated based on the difference between the first and second phase angles, θ1 and θ2, as follows:
number
[0020] Figure 6 is a schematic diagram of signal decoding of synchronized RF communications. In Figure 6, a sine wave at the demodulation frequency identified above is used to demodulate the communications associated with the synchronization sequence 24. Coded Communications RX DATA The start of the coded communication RX is identified based on the timing of the peak 46 of the composite correlation signal 56, as shown in FIG. DATA is multiplied by a negative frequency sine wave as follows:
number
[0021] 7 is a graph showing the signal-to-noise ratio for communications synchronized using a composite correlation signal and communications not synchronized using a composite correlation signal. In FIG. 7, graph 58 includes a horizontal axis 60, a vertical axis 62, and a synchronization error rate relationship 64. Horizontal axis 60 represents the signal-to-noise ratio. Vertical axis 62 represents the synchronization error rate. The synchronization error rate relationship 64 is a function of the signal-to-noise threshold SN THRESH This indicates that the synchronization error rate is less than 1% for signal-to-noise ratios above the signal-to-noise threshold SN THRESH , communications can be accurately synchronized using the disclosed method with a low synchronization error rate. In some embodiments, such a signal-to-noise threshold SN THRESH is normalized to the energy of a signal bit and can therefore be as low as 3, 4, or 5.
[0022] Discussion of Possible Embodiments The following is a non-exclusive description of possible embodiments of the present invention.
[0023] The apparatus and related method relate to a system for identifying a synchronization sequence of a predetermined length having a first code sequence and a second code sequence separated by a predetermined time delay, and for synchronizing to the synchronization sequence. The system includes a radio frequency (RF) receiver, a processor, and a computer-readable memory. The RF receiver receives an RF signal within a predetermined frequency band. The computer-readable memory has coded instructions that, when executed by the processor, cause the system to autocorrelate a first portion of a generated signal with a second portion of the generated signal to generate an autocorrelation signal. The first and second portions are separated in time by a predetermined time delay separating the first and second code sequences. The computer-readable memory has coded instructions that, when executed by the processor, cause the system to synchronously correlate a third portion of the generated signal with the synchronization sequence to generate a synchronous correlation signal. The third portion has the predetermined length of the synchronization sequence and includes the first and second portions of the generated signal used to generate the autocorrelation signal. The computer readable memory has coded instructions that, when executed by the processor, cause the system to multiply the autocorrelation signal and the sync correlation signal to generate a composite sync signal. The computer readable memory further has coded instructions that, when executed by the processor, cause the system to identify peaks in the composite sync signal.
[0024] The system of the preceding paragraph may optionally and / or alternatively include any one or more of the following features, configurations, and / or additional components.
[0025] A further embodiment of the aforementioned system, wherein the first code sequence and the second code sequence may be identical to one another.
[0026] A further embodiment of any of the preceding systems, wherein the computer-readable memory may further have coded instructions that, when executed by the processor, cause the system to compare the identified peaks to a predetermined threshold.
[0027] A further embodiment of any of the aforementioned systems, wherein the computer readable memory may further have coded instructions which, when executed by the processor, cause the system to: determine a synchronization time of a valid transmission received at a time corresponding to the identified peak if the identified peak is greater than the predetermined threshold.
[0028] A further embodiment of any of the preceding systems, wherein the computer-readable memory may further have coded instructions that, when executed by the processor, cause the system to interpret the RF energy detected following the synchronization time as a coded communication transmitted by a licensed transmitter.
[0029] A further embodiment of any of the preceding systems, wherein the computer-readable memory may further have coded instructions that, when executed by the processor, cause the system to estimate a transmit frequency based on the first and second portions of the generated signal.
[0030] A further embodiment of any of the aforementioned systems, wherein the computer-readable memory may further have coded instructions that, when executed by the processor, cause the system to decode the coded communication transmitted by a licensed transmitter by multiplying the coded communication by a sine wave corresponding to the estimated transmission frequency.
[0031] A further embodiment of any of the aforementioned systems, wherein autocorrelating the first portion of the generated signal with the second portion of the generated signal includes convolving a conjugated version of one of the first and second portions of the generated signal, along with the second portion of the generated signal, with a non-conjugated version of the other of the first and second portions of the generated signal.
[0032] A further embodiment of any of the aforementioned systems, wherein autocorrelating the first portion of the generated signal with the second portion of the generated signal includes convolving a conjugated version of the third portion of the generated signal with a non-conjugated version of the third portion of the generated signal.
[0033] Some embodiments relate to a method for identifying a synchronization sequence of a predetermined length having a first code sequence and a second code sequence separated by a predetermined time delay and synchronizing to the synchronization sequence. The method includes detecting radio frequency (RF) energy within a predetermined frequency band. The method includes generating a signal indicative of the detected RF energy. The method includes autocorrelating a first portion of the generated signal with a second portion of the generated signal to generate an autocorrelation signal. The first and second portions are separated in time by a predetermined time delay separating the first and second code sequences. The method includes synchronously correlating a third portion of the generated signal with the synchronization sequence to generate a synchronous correlation signal. The third portion has the predetermined length of the synchronization sequence and includes the first and second portions of the generated signal used to generate the autocorrelation signal. The method includes multiplying the autocorrelation signal by the synchronous correlation signal to generate a composite synchronization signal. The method also includes identifying a peak in the composite synchronization signal.
[0034] The method of the preceding paragraph may optionally and / or alternatively include any one or more of the following features, configurations, and / or additional components.
[0035] A further embodiment of the aforementioned method, wherein the first code sequence and the second code sequence may be identical to one another.
[0036] Further embodiments of any of the preceding methods may further comprise comparing the identified peak to a predetermined threshold.
[0037] A further embodiment of any of the preceding methods, which may further include, if the identified peak is greater than the predetermined threshold, identifying a synchronization time of a valid transmission received at a time corresponding to the identified peak.
[0038] A further embodiment of any of the preceding methods, which may further include interpreting the RF energy detected following the synchronization time as a coded communication transmitted by an authorized transmitter.
[0039] A further embodiment of any of the preceding methods, which may further include estimating a transmit frequency based on the first and second portions of the generated signal.
[0040] A further embodiment of any of the aforementioned methods, wherein estimating the transmit frequency may include: i) convolving the first portion of the generated signal with a conjugate of the first code sequence to generate a first convolved signal; ii) convolving the first portion of the generated signal with the conjugate of the first code sequence to generate a second convolved signal; iii) dividing a phase difference between the first convolved signal and the second convolved signal by the predetermined time delay to generate a ratio; and iv) multiplying the ratio by a sampling frequency to generate the estimate of the transmit frequency.
[0041] A further embodiment of any of the preceding methods may further include decoding the coded communication transmitted by a licensed transmitter by multiplying the coded communication by a sine wave corresponding to the estimated transmission frequency.
[0042] A further embodiment of any of the preceding methods, wherein autocorrelating the first portion of the generated signal with the second portion of the generated signal may include convolving a conjugated version of one of the first and second portions of the generated signal, along with the second portion of the generated signal, with a non-conjugated version of the other of the first and second portions of the generated signal.
[0043] A further embodiment of any of the preceding methods, wherein synchronously correlating the third portion of the generated signal with the synchronization sequence may include convolving a conjugated version of the third portion of the generated signal with a non-conjugated version of the third portion of the generated signal.
[0044] Some embodiments relate to a method for generating and communicating a synchronization sequence to indicate that a transmitted signal is from an authorized transmitter. The method includes transmitting a first code sequence. The method includes transmitting a second code sequence a predetermined time delay after the first code sequence is transmitted. The second code sequence is identical to the first code sequence. The method also includes transmitting a communication synchronized to the second code sequence following the transmission of the second code sequence.
[0045] The method of the preceding paragraph may optionally and / or alternatively include any one or more of the following features, configurations, and / or additional components.
[0046] A further embodiment of the aforementioned method, wherein the predetermined time delay is a first predetermined time delay, and the transmitted communication is synchronized to the second code sequence by following the second code sequence with a second predetermined time delay.
[0047] While the present invention has been described with reference to exemplary embodiment(s), those skilled in the art will recognize that various changes may be made without departing from the scope of the invention and that elements of the invention may be substituted with equivalents. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is understood that the invention is not limited to the particular embodiment(s) disclosed, but that the invention is intended to include all embodiments falling within the scope of the appended claims.
Claims
1. 1. A method for identifying and synchronizing to a synchronization sequence of a predetermined length having a first code sequence and a second code sequence separated by a predetermined time delay, comprising: Detecting radio frequency (RF) energy within a predetermined frequency band; generating a signal indicative of the detected RF energy; autocorrelating a first portion of the generated signal with a second portion of the generated signal, the first portion and the second portion being separated in time by the predetermined time delay separating the first code sequence and the second code sequence, to generate an autocorrelated signal, the autocorrelating continuing to autocorrelate with most recent second portions of the generated signal to continue to generate the autocorrelated signal over time; continuously synchronously correlating a third portion of the most recent generated signal with the synchronization sequence to generate a synchronously correlated signal, the third portion having the predetermined length of the synchronization sequence and including the first and second portions of the generated signal used to generate the autocorrelation signal, the synchronous correlating continuing to synchronously correlate the third portion of the most recent generated signal to continue to generate the synchronously correlated signal over time; multiplying the autocorrelation signal by the synchronous correlation signal to generate a composite synchronous signal; identifying peaks in the composite synchronization signal; A method comprising:
2. The method of claim 1 , wherein the first code sequence and the second code sequence are identical to each other.
3. comparing the identified peaks to a predetermined threshold; The method of claim 1 further comprising:
4. if the identified peak is greater than the predetermined threshold, identifying a synchronization time of a valid transmission received at a time corresponding to the identified peak; The method of claim 3 further comprising:
5. interpreting the RF energy detected following the synchronization time as a coded communication transmitted by a transmitter; The method of claim 4 further comprising:
6. estimating a transmit frequency based on the first and second portions of the generated signal; The method of claim 5 further comprising:
7. estimating the transmit frequency convolving the first portion of the generated signal with a conjugate of the first code sequence to generate a first convolved signal; convolving the second portion of the generated signal with a conjugate of the second code sequence to generate a second convolved signal; dividing the phase difference between the first convolved signal and the second convolved signal by the predetermined time delay to generate a ratio; multiplying the ratio by a sampling frequency to generate the estimate of the transmit frequency; The method of claim 6, comprising:
8. decoding the coded communication transmitted by the transmitter by multiplying the coded communication by a sine wave corresponding to the estimated transmission frequency; The method of claim 6 further comprising:
9. Autocorrelating the first portion of the generated signal with the second portion of the generated signal comprises: convolving a conjugated version of one of the first and second portions of the generated signal with a non-conjugated version of the other of the first and second portions of the generated signal; The method of claim 1 , comprising:
10. synchronously correlating the third portion of the generated signal with the synchronization sequence; convolving a conjugated version of the third portion of the generated signal with a non-conjugated version of the synchronization sequence; The method of claim 1 , comprising:
11. 1. A system for identifying and synchronizing to a synchronization sequence of a predetermined length having a first code sequence and a second code sequence separated by a predetermined time delay, the system comprising: a radio frequency (RF) receiver for receiving RF signals within a predetermined frequency band; a processor; a computer readable memory having coded instructions; wherein the instructions, when executed by the processor, autocorrelating a first portion of the generated signal with a second portion of the generated signal, the first portion and the second portion being separated in time by the predetermined time delay separating the first code sequence and the second code sequence, to generate an autocorrelated signal, the autocorrelating continuing to autocorrelate with most recent second portions of the generated signal to continue to generate the autocorrelated signal over time; continuously synchronously correlating a third portion of the most recent generated signal with the synchronization sequence to generate a synchronously correlated signal, the third portion having the predetermined length of the synchronization sequence and including the first and second portions of the generated signal used to generate the autocorrelation signal, the synchronous correlating continuing to synchronously correlate the third portion of the most recent generated signal to continue to generate the synchronously correlated signal over time; multiplying the autocorrelation signal by the synchronous correlation signal to generate a composite synchronous signal; identifying peaks in the composite synchronization signal; The system executes the above.
12. The system of claim 11 , wherein the first code sequence and the second code sequence are identical to one another.
13. The computer-readable memory, when executed by the processor, comparing the identified peaks to a predetermined threshold; 12. The system of claim 11, further comprising coded instructions that cause the system to execute:
14. The computer-readable memory, when executed by the processor, if the identified peak is greater than the predetermined threshold, identifying a synchronization time of a valid transmission received at a time corresponding to the identified peak; 14. The system of claim 13, further comprising coded instructions that cause the system to execute:
15. The computer-readable memory, when executed by the processor, interpreting the RF signal detected following the synchronization time as a coded communication transmitted by a transmitter; 15. The system of claim 14, further comprising coded instructions that cause the system to execute:
16. The computer-readable memory, when executed by the processor, estimating a transmit frequency based on the first and second portions of the generated signal; 16. The system of claim 15, further comprising coded instructions that cause the system to perform:
17. The computer-readable memory, when executed by the processor, decoding the coded communication transmitted by the transmitter by multiplying the coded communication by a sine wave corresponding to the estimated transmission frequency; 20. The system of claim 16, further comprising coded instructions that cause the system to execute:
18. Autocorrelating the first portion of the generated signal with the second portion of the generated signal comprises: convolving a conjugated version of one of the first and second portions of the generated signal with a non-conjugated version of the other of the first and second portions of the generated signal; The system of claim 11 , comprising:
19. synchronously correlating the third portion of the generated signal with the synchronization sequence; convolving a conjugated version of the third portion of the generated signal with a non-conjugated version of the synchronization sequence; The system of claim 11 , comprising: