US 20080144701 A1 Abstract A system and method for synchronizing and selectively addressing multiple receivers in a wireless communication system includes a spread spectrum transmitter and one or more spread spectrum receivers. The transmitter transmits a signal having an observable parameter which is pseudo-randomly varied. The receiver measures the relative times between recurrences of a selected value of the observable parameter being pseudo-randomly varied, and determines an initial state of the transmitter based upon the measured relative times. The receiver then synchronizes itself to the estimated current state of the transmitter using the determined initial state as a starting reference. In a frequency hopping embodiment, the spread spectrum transmitter comprises a feedback shift register, and transmits a sequence of pseudo-randomly hopped frequencies determined by the shift register. A receiver is tuned to one of the hopping frequencies, measures the relative times of arrival between consecutive transmissions, and determines the initial code word in the transmitter feedback shift register from the measured relative times of arrival by constructing and solving a set of linear equations. The receiver then matches comprises its feedback shift register to the initial code word, adjusted by an amount of time elapsed during the synchronization process. Similar techniques may also be applied both to direct sequence spread spectrum communication systems.
Claims(43) 1. A method for wireless communication, comprising the steps of:
transmitting a series of transmissions at a predetermined frequency, said transmissions separated by one or more clock intervals pseudo-randomly determined according to an initial code word loaded into a transmitter feedback shift register; receiving said series of transmissions at a receiver; at said receiver, measuring relative times of arrival between consecutive ones of said transmissions; determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions; matching a receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions; and using said receiver feedback shift register to carry out synchronized communication with the transmitter. 2. The method of 3. The method of constructing a plurality of relationships between transition matrices, a set of unknown initial states of the transmitter feedback shift register, and information represented by each transition; deriving a plurality of linear equations from said relationships; and solving said plurality of linear equations for said set of unknown initial states when the number of linear equations exceeds the number of unknown initial states of the transmitter feedback shift register, thereby yielding the initial states of said transmitter feedback shift register. 4. The method of (a) assigning, to said q designated stages in the transmitter feedback shift register, said predetermined set of code values upon receipt of a first one of said transmissions; (b) calculating a number of intervening clock pulses until the next consecutive transmission received at the receiver; (c) obtaining a time-advanced transition matrix A ^{p }for the current received transmission, wherein an amount of time advancement is based upon a number of intervening clock pulses p between the occurrence of the first one of said transmissions and the occurrence of the current received transmission;(d) deriving a linear equation according to the form x·A ^{p}[q]=y, wherein x represents a linear vector having a number of elements equal to the number of stages n in said transmitter feedback shift register, said linear vector comprising said predetermined set of code values at locations in said linear vector corresponding to the q designated stages in said transmitter feedback shift register and a set of n-q unknowns at locations in said linear vector corresponding to all other locations in said transmitter feedback shift register, wherein A^{p }[q] represents a portion of a transition matrix A raised to a power equal to the number of intervening clock pulses p, said portion being those q columns in transition matrix A^{p }which are associated with said q designated stages in the transmitter feedback shift register, and wherein y represents a linear vector having a number of elements equal to the number of said q designated stages, said elements of y being assigned said predetermined set of code values; and(e) repeating steps (b) through (d) until a sufficient number of said linear equations are derived to allow said set of unknown initial states to be solved. 5. The method of 6. The method of initializing a modular feedback shift register corresponding to the transmitter feedback shift register; advancing the modular feedback shift register ahead by the number of clock intervals elapsed since the receipt of said first one of said consecutive transmissions, thereby obtaining a first modular feedback shift register code word from the contents of said modular feedback shift register; and incrementing the modular feedback shift register once for each additional bit needed for the transmitter feedback shift register, thereby obtaining successive modular feedback shift register code words from the contents of said modular feedback shift register at each increment. 7. The method of 8. The method of transmitting a second series of transmissions at a second predetermined frequency, said transmissions in said second series separated by one or more clock intervals pseudo-randomly determined according to said initial code word loaded into the transmitter feedback shift register; receiving said second series of transmissions at a second receiver; at said second receiver, measuring relative times of arrival between consecutive ones of said transmissions in said second series; determining, at said second receiver, said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions in said second series; matching, at said second receiver, a second receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions in said second series; and using said second receiver feedback shift register to carry out synchronized communication with the transmitter and said second receiver. 9. The method of 10. In a frequency hopping spread spectrum communication system, wherein a frequency hopping transmitter transmits in a pseudo-random manner across a plurality of frequencies including a key frequency, the transmissions over said key frequency being separated by one or more clock intervals pseudo-randomly determined according to an initial code word loaded into a transmitter feedback shift register, a method of reception comprising the steps of:
monitoring the key frequency for a series of transmissions by said frequency hopping transmitter; measuring relative times of arrival between consecutive ones of said transmissions; determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions; matching a receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions, thereby synchronizing said receiver feedback shift register to the transmitter feedback shift register; and using said receiver feedback shift register to despread the transmissions of the frequency hopping transmitter over said plurality of frequencies. 11. The method of constructing a plurality of relationships between transition matrices, a set of unknown initial states of the transmitter feedback shift register, and information represented by each transition; deriving a plurality of linear equations from said relationships; and solving said plurality of linear equations for said set of unknown initial states when the number of linear equations exceeds the number of unknown initial states of the transmitter feedback shift register, thereby yielding the initial states of said transmitter feedback shift register. 12. The method of (a) assigning, to said q designated stages in the transmitter feedback shift register, said predetermined set of code values upon receipt of a first one of said transmissions; (b) calculating a number of intervening clock pulses until the next consecutive transmission received at the receiver; (c) obtaining a time-advanced transition matrix A ^{p }for the current received transmission, wherein an amount of time advancement is based upon a number of intervening clock pulses p between the occurrence of the first one of said transmissions and the occurrence of the current received transmission;(d) deriving a linear equation according to the form x·A ^{p}[q]=y, wherein x represents a linear vector having a number of elements equal to the number of stages n in said transmitter feedback shift register, said linear vector comprising said predetermined set of code values at locations in said linear vector corresponding to the q designated stages in said transmitter feedback shift register and a set of n-q unknowns at locations in said linear vector corresponding to all other locations in said transmitter feedback shift register, wherein A^{p }[q] represents a portion of a transition matrix A raised to a power equal to the number of intervening clock pulses p, said portion being those q columns in transition matrix A^{p }which are associated with said q designated stages in the transmitter feedback shift register, and wherein y represents a linear vector having a number of elements equal to the number of said q designated stages, said elements of y being assigned said predetermined set of code values; and(e) repeating steps (b) through (d) until a sufficient number of said linear equations are derived to allow said set of unknown initial states to be solved. 13. The method of 14. The method of initializing a modular feedback shift register corresponding to the transmitter feedback shift register; advancing the modular feedback shift register ahead by the number of clock intervals elapsed since the receipt of said first one of said consecutive transmissions, thereby obtaining a first modular feedback shift register code word from the contents of said modular feedback shift register; and incrementing the modular feedback shift register once for each additional bit needed for the transmitter feedback shift register, thereby obtaining successive modular feedback shift register code words from the contents of said modular feedback shift register at each increment. 15. In a direct sequence spread spectrum communication system, wherein a direct sequence spread spectrum transmitter transmits in a pseudo-random manner over a predetermined frequency, the transmissions over said predetermined frequency being separated by one or more clock intervals pseudo-randomly determined according to an initial code word loaded into a transmitter feedback shift register, a method of reception comprising the steps of:
monitoring the predetermined frequency for a series of transmissions by said direct sequence spread spectrum transmitter; measuring relative times of arrival between consecutive ones of said transmissions; determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions; matching a receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions, thereby synchronizing said receiver feedback shift register to the transmitter feedback shift register; and using said receiver feedback shift register to despread the transmissions of the direct sequence spread spectrum transmitter over said plurality of frequencies. 16. The method of constructing a plurality of relationships between transition matrices, a set of unknown initial states of the transmitter feedback shift register, and information represented by each transition; deriving a plurality of linear equations from said relationships; and solving said plurality of linear equations for said set of unknown initial states when the number of linear equations exceeds the number of unknown initial states of the transmitter feedback shift register, thereby yielding the initial states of said transmitter feedback shift register. 17. The method of (a) assigning, to said q designated stages in the transmitter feedback shift register, said predetermined set of code values upon receipt of a first one of said transmissions; (b) calculating a number of intervening clock pulses until the next consecutive transmission received at the receiver; (c) obtaining a time-advanced transition matrix A ^{p }for the current received transmission, wherein an amount of time advancement is based upon a number of intervening clock pulses p between the occurrence of the first one of said transmissions and the occurrence of the current received transmission;(d) deriving a linear equation according to the form x·A ^{p }[q]=y, wherein x represents a linear vector having a number of elements equal to the number of stages n in said transmitter feedback shift register, said linear vector comprising said predetermined set of code values at locations in said linear vector corresponding to the q designated stages in said transmitter feedback shift register and a set of n-q unknowns at locations in said linear vector corresponding to all other locations in said transmitter feedback shift register, wherein A^{p}[q] represents a portion of a transition matrix A raised to a power equal to the number of intervening clock pulses p, said portion being those q columns in transition matrix A^{p }which are associated with said q designated stages in the transmitter feedback shift register, and wherein y represents a linear vector having a number of elements equal to the number of said q designated stages, said elements of y being assigned said predetermined set of code values; and(e) repeating steps (b) through (d) until a sufficient number of said linear equations are derived to allow said set of unknown initial states to be solved. 18. The method of 19. The method of initializing a modular feedback shift register corresponding to the transmitter feedback shift register; advancing the modular feedback shift register ahead by the number of clock intervals elapsed since the receipt of said first one of said consecutive transmissions, thereby obtaining a first modular feedback shift register code word from the contents of said modular feedback shift register; and incrementing the modular feedback shift register once for each additional bit needed for the transmitter feedback shift register, thereby obtaining successive modular feedback shift register code words from the contents of said modular feedback shift register at each increment. 20. A method for broadcasting wireless signals to selected receivers, the method comprising the steps of:
transmitting a series of frequency-hopped transmissions over a plurality of frequencies including one or more key frequencies, the transmissions on a key frequency separated by one or more clock intervals pseudo-randomly determined according to an initial code word loaded into a transmitter feedback shift register; and separately at each of a plurality of receivers:
monitoring a designated one of said key frequencies;
measuring relative times of arrival between consecutive ones of said transmissions on the receiver's designated key frequency;
determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions on the receiver's designated key frequency;
matching a receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions on the receiver's designated key frequency; and
using said receiver feedback shift register to carry out synchronized communication with the transmitter.
21. The method of 22. The method of 23. The method of deriving a plurality of linear equations from said relationships; and 24. The method of (a) assigning, to said q designated stages in the transmitter feedback shift register, said predetermined set of code values upon receipt of a first one of said transmissions on the receiver's designated key frequency; (b) calculating a number of intervening clock pulses until the next consecutive transmission on the receiver's designated key frequency that is received at the receiver; (c) obtaining a time-advanced transition matrix A ^{p }for the current received transmission on the receiver's designated key frequency, wherein an amount of time advancement is based upon a number of intervening clock pulses p between the occurrence of the first one of said transmissions on the receiver's designated key frequency and the occurrence of the current received transmission on the receiver's designated key frequency;(d) deriving a linear equation according to the form x·A ^{p}[q]=y, wherein x represents a linear vector having a number of elements equal to the number of stages n in said transmitter feedback shift register, said linear vector comprising said predetermined set of code values at locations in said linear vector corresponding to the q designated stages in said transmitter feedback shift register and a set of n-q unknowns at locations in said linear vector corresponding to all other locations in said transmitter feedback shift register, wherein A^{p }[q] represents a portion of a transition matrix A raised to a power equal to the number of intervening clock pulses p, said portion being those q columns in transition matrix A^{p }which are associated with said q designated stages in the transmitter feedback shift register, and wherein y represents a linear vector having a number of elements equal to the number of said q designated stages, said elements of y being assigned said predetermined set of code values; and25. The method of 26. The method of advancing the modular feedback shift register ahead by the number of clock intervals elapsed since the receipt of said first one of said consecutive transmissions on the receiver's designated key frequency, thereby obtaining a first modular feedback shift register code word from the contents of said modular feedback shift register; and 27. The method of 28. A wireless communication system, comprising:
a spread spectrum transmitter configured to transmit a series of transmissions at a predetermined frequency, said transmissions separated by one or more clock intervals pseudo-randomly determined according to an initial code word loaded into a transmitter feedback shift register; and a receiver comprising circuitry for receiving and synchronizing to said series of transmissions, said receiver comprising
a receiver feedback shift register;
means for measuring relative times of arrival between consecutive ones of said transmissions;
a synchronizing circuit for determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions, and matching said receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions.
29. The wireless communication system of wherein said receiver feedback shift register is used for carrying out synchronized communication with the transmitter by despreading said frequency hopping transmissions to recover a data signal. 30. The wireless communication system of deriving a plurality of linear equations from said relationships; and 31. The wireless communication system of ^{p }for the current received transmission, wherein an amount of time advancement is based upon a number of intervening clock pulses p between the occurrence of the first one of said transmissions and the occurrence of the current received transmission;^{p}[q]=y, wherein x represents a linear vector having a number of elements equal to the number of stages n in said transmitter feedback shift register, said linear vector comprising said predetermined set of code values at locations in said linear vector corresponding to the q designated stages in said transmitter feedback shift register and a set of n-q unknowns at locations in said linear vector corresponding to all other locations in said transmitter feedback shift register, wherein A^{p }[q] represents a portion of a transition matrix A raised to a power equal to the number of intervening clock pulses p, said portion being those q columns in transition matrix A^{p }which are associated with said q designated stages in the transmitter feedback shift register, and wherein y represents a linear vector having a number of elements equal to the number of said q designated stages, said elements of y being assigned said predetermined set of code values; and32. The wireless communication system of 33. The wireless communication system of initializing said modular feedback shift register with a code word of the form 100 . . . 0; 34. The wireless communication system of 35. The wireless communication system of a second receiver feedback shift register; means for measuring relative times of arrival between consecutive ones of said transmissions at said second predetermined frequency; a second synchronizing circuit for determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions at said second predetermined frequency, and for matching said receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions at said second predetermined frequency. 36. The wireless communication system of 37. In a frequency hopping spread spectrum communication system, wherein a frequency hopping transmitter transmits in a pseudo-random manner across a plurality of frequencies including a key frequency, the transmissions over said key frequency being separated by one or more clock intervals pseudo-randomly determined according to an initial code word loaded into a transmitter feedback shift register, a receiver, comprising:
a receiver feedback shift register; a receiving circuit tuned to said key frequency for detecting the transmissions over said key frequency; a clocking circuit for measuring relative times of arrival between consecutive ones of said transmissions detected by said receiving circuit; and a synchronizing circuit for determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions, and matching said receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions. 38. The receiver of deriving a plurality of linear equations from said relationships; and 39. The receiver ^{p }for the current received transmission, wherein an amount of time advancement is based upon a number of intervening clock pulses p between the occurrence of the first one of said transmissions and the occurrence of the current received transmission;^{p}[q]=y, wherein x represents a linear vector having a number of elements equal to the number of stages n in said transmitter feedback shift register, said linear vector comprising said predetermined set of code values at locations in said linear vector corresponding to the q designated stages in said transmitter feedback shift register and a set of n-q unknowns at locations in said linear vector corresponding to all other locations in said transmitter feedback shift register, wherein A^{p }[q] represents a portion of a transition matrix A raised to a power equal to the number of intervening clock pulses p, said portion being those q columns in transition matrix A^{p }which are associated with said q designated stages in the transmitter feedback shift register, and wherein y represents a linear vector having a number of elements equal to the number of said q designated stages, said elements of y being assigned said predetermined set of code values; and40. A wireless communication system, comprising:
a spread spectrum transmitter configured to transmit a series of frequency-hopped transmissions over a plurality of frequencies including one or more key frequencies, the transmissions on a key frequency separated by one or more clock intervals pseudo-randomly determined according to an initial code word loaded into a transmitter feedback shift register; and a plurality of receivers each attuned to a designated key frequency, each of said receivers comprising
a receiver feedback shift register;
means for measuring relative times of arrival between consecutive ones of said transmissions on the receiver's designated key frequency;
a synchronizing circuit for determining said initial code word in the transmitter feedback shift register from the measured relative times of arrival between the consecutive transmissions on the receiver's designated key frequency, and matching said receiver feedback shift register to the initial code word, adjusted by an amount of time since receiving the first one of said consecutive transmissions on the receiver's designated key frequency.
41. The wireless communication system of constructing a plurality of relationships between transition matrices, a set of unknown initial states of the transmitter feedback shift register, and information represented by each transition on the receiver's designated key frequency; deriving a plurality of linear equations from said relationships; and 42. The wireless communication system of (a) assigning, to said q designated stages in the transmitter feedback shift register, said predetermined set of code values upon receipt of a first one of said transmissions on the receiver's designated key frequency; (b) calculating a number of intervening clock pulses until the next consecutive transmission on the receiver's designated key frequency that is received at the receiver; (c) obtaining a time-advanced transition matrix A ^{p }for the current received transmission on the receiver's designated key frequency, wherein an amount of time advancement is based upon a number of intervening clock pulses p between the occurrence of the first one of said transmissions on the receiver's designated key frequency and the occurrence of the current received transmission on the receiver's designated key frequency;(d) deriving a linear equation according to the form x·A ^{p }[q]=y, wherein x represents a linear vector having a number of elements equal to the number of stages n in said transmitter feedback shift register, said linear vector comprising said predetermined set of code values at locations in said linear vector corresponding to the q designated stages in said transmitter feedback shift register and a set of n-q unknowns at locations in said linear vector corresponding to all other locations in said transmitter feedback shift register, wherein A^{p }[q] represents a portion of a transition matrix A raised to a power equal to the number of intervening clock pulses p, said portion being those q columns in transition matrix A^{p }which are associated with said q designated stages in the transmitter feedback shift register, and wherein y represents a linear vector having a number of elements equal to the number of said q designated stages, said elements of y being assigned said predetermined set of code values; and43. A method for synchronizing communication, comprising the steps of:
transmitting, from a transmitter, a signal having an observable parameter which is pseudo-randomly varied; receiving said signal at a receiver; measuring relative times between recurrences of a selected value of said observable parameter being pseudo-randomly varied; determining an initial state of said transmitter based upon said measured relative times; and synchronizing said receiver to an estimated current state of said transmitter using said determined initial state as a starting reference. Description Pursuant to 35 U.S.C. § 202, the United States Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract nos. F33615-01-C-1800 and F33615-00-C-1645 awarded by the United States Air Force Research Laboratory. 1) Field of the Invention The field of the present invention relates to wireless communication systems and, more particularly, to techniques for synchronizing and selectively addressing multiple receivers in a wireless, spread-spectrum communication system. 2) Background Spread spectrum communication, a technique for transmitting and receiving signal over a bandwidth wider than the data to be transmitted, has in recent years become widely for both military and commercial applications. Its advantages include, for example, resistance to interference, low power density (and hence minimal creation of interference) over the transmission frequency band, and security of communications. The two most common spread spectrum communication techniques are generally referred to as direct sequence spread spectrum (DSSS) communication and frequency hopping spread spectrum (FHSS) communication. Direct sequence spread spectrum communication involves direct sequence modulation of a carrier signal, which is a known technique for generating wide-band, low power density signals which have statistical properties similar to random noise. In a direct sequence spread spectrum communication system, the data to be transmitted is generally encoded in some fashion, in a manner which causes the signal to be “spread” over a broader frequency range and also typically causes the signal power density to decrease as the frequency bandwidth is spread. In a common method of direct-sequence spread spectrum modulation, a pseudo-random chip sequence (also called a pseudo-noise code sequence or a PN code sequence) is used to encode data which is then placed on a carrier waveform. The chipping rate of the pseudo-random sequence is usually much higher than the data rate. The resulting encoded signal is generally spread across a bandwidth exceeding the bandwidth necessary to transmit the data, hence the term “spread spectrum”. At the receiver, the signal is decoded, which causes it to be “despread” and allows the original data to be recovered. The receiver produces a correlated signal in response to the received spread spectrum signal when it is able to match the chip sequence to a sufficient degree. To do so, the receiver generates the same pseudo-random chip sequence locally, synchronizes its chip sequence to the received chip sequence, and tracks the signal by maintaining synchronization during transmission and reception of data. Frequency hopping spread spectrum communication also involves a pseudo-random (i.e., spreading) code, but the code is used to select a series of frequencies rather than as information for directly modulating a carrier, as is generally done in direct sequence spread spectrum communication. In a very broad aspect, frequency hopping spread spectrum communication may be viewed as a type of frequency shift keying, but with many more frequency choices which are selected by use of the spreading code. In what is known as “fast” frequency hopping, a number of frequency changes or “hops” are carried out during the time period of sending one or more data symbols—e.g., a set of data bits—wherein the number of frequency changes or hops is greater than the number of data symbols to be transmitted. In “slow” frequency hopping, on the other hand, one or more data symbols is transmitted during each hopping interval. In one type of frequency hopping spread spectrum communication, the frequency hopping transmitter includes a code generator and a rapid-response frequency synthesizer capable of responding to the coded output from the code generator. During each frequency hopping interval, a set of selected code bits are used to determine which frequency will be transmitted. Data may be transmitted in any way available to other communication systems, and in either analog or digital form. For example, a number of discrete data bits may be transmitted during each frequency hopping interval. Alternatively, a single data bit may be transmitted over a large number of frequency hopping intervals. A frequency hopping receiver, like the transmitter, also typically includes a code generator and rapid-response frequency synthesizer. The received frequency hopping signal is then mixed with a locally generated replica of the transmitted signal (which may be offset by a fixed intermediate frequency) such that modulation of the received signal and the locally generated replica produces a constant difference frequency when the transmitter and receiver are in synchronism. Once the spread spectrum modulation is removed, the de-hopped signal is then processed to demodulate the transmitted information. Both direct sequence and frequency hopping spread spectrum communication techniques may be used in the formation of a multiple access communication system. Distinct spreading codes can be used to distinguish transmissions, thereby allowing multiple simultaneous communication. Different users within a wireless communication system may, using distinct spreading codes, thereby transmit simultaneously over the same frequency without necessarily interfering with one another, particularly if the codes in use are selected to be orthogonal with respect to one another. A multiple-access communication system in which transmissions are distinguished according to the code used to encode the transmission is sometimes referred to as a code division multiple access (CDMA) communication system, which may be either a direct sequence or a frequency hopping spread spectrum system. In either a frequency hopping or direct sequence spread spectrum communication system, the requirement of synchronization by the receiver has generally been a problem in the art. This requirement generally increases the difficulty of initially acquiring a spread spectrum signal, especially in a noisy environment, and also can cause difficulty in tracking and/or maintaining spread spectrum communication after established. Synchronization and tracking requirements often translate into additional circuit complexity at the receiver and increased cost, and may impose operational constraints on the communication system. For example, the extra time required to achieve synchronization can degrade the efficiency of the communication system, and may be detrimental in systems requiring very rapid establishment of a communication link. In a frequency hopping system, in the absence of synchronization, the receiver must monitor all possible frequencies due to the otherwise unpredictable nature of the frequency hopped signal, which forces the receiver to employ a large number of synthesizers or even an array of distinct receivers. For example, the receiver may need to sample all of the possible frequency inputs at once, and then to select the channel with the largest signal in a given frequency hopping interval as the correct one. In order to monitor each possible frequency, the received signal is envelope detected and then band-pass filtered at each of the possible frequencies, with the largest of the filtered signals during a frequency hopping interval being deemed the transmitted frequency at that instant. This type of receiver design, however, has the drawback of requiring a potentially large number of band pass filters, one filter for each possible frequency. If proper synchronization is achieved, on the other hand, it is possible to use a single (typically relatively high speed) frequency synthesizer to demodulate the incoming frequency hopped signal. One technique for attempting to acquire synchronization of a frequency hopped signal is disclosed in U.S. Pat. No. 6,148,020. According to the technique disclosed therein, a frequency hopping receiver repeatedly mixes a partial code string which is part of the spreading code sequence for frequency hopping with the received signal. The receiver then attempts to synchronize by judging the detection level of a predetermined frequency. When the partial code string to be mixed coincides with part of the original code sequence, the detection level is presumed to become sufficiently large such that the receiver is judged to be in synchronization with the transmitter. An alternative to the above-referenced technique is to use a preamble to attempt to synchronize a frequency hopping receiver. Such a technique is disclosed, for example, in U.S. Pat. No. 6,084,905. As described therein, a frequency hopping transmitter transmits a continuous wave in a first field of a preamble field of a communication frame, then transmits a carrier which is modulated with a symbol timing signal in a second field in the preamble field, followed by transmission information. A timing recovery circuit in the receiver uses the preamble to help establish synchronization. In the particular system described in the foregoing patent, a synchronous frame is broadcast as a reference for each frequency hopping equipment in the communication system. Besides difficulties in achieving synchronization in spread spectrum systems, it can also be challenging, particularly in multiple access communication systems, for a receiver to be aware of when a transmitter is attempting to transmit information to it. This can be particularly difficult in military and other applications in which secrecy is paramount. In various situations it can be advantageous for a transmitter to be able to selectively transmit a broadcast transmission to only one or a few receivers from many possible receivers, and to exclude from reception those receivers to which the communication is not directed. However, few, if any, techniques exist for such selective broadcast. No known technique exists for selectively broadcasting a transmission only to one or a specified group of receivers from many possible receivers, without at least requiring participation of the receivers in the excluded group. It would therefore be advantageous to provide a communication technique that provides improved synchronization and the capability of selectively addressing receivers, and which overcomes the drawbacks, disadvantages or limitations of conventional techniques. The invention in one aspect provides a system and method for synchronizing and selectively addressing multiple receivers in a wireless communication system. In one aspect, a method or system for synchronizing communication includes the steps of transmitting, from a transmitter, a signal having an observable parameter which is pseudo-randomly varied, and receiving the signal at a receiver. The receiver measures the relative times between recurrences of a selected value of the observable parameter being pseudo-randomly varied, and determines an initial state of the transmitter based upon the measured relative times. The receiver then synchronizes itself to the estimated current state of the transmitter using the determined initial state as a starting reference. In one embodiment, a wireless, spread-spectrum communication system includes one or more spread spectrum transmitters and one or more spread spectrum receivers. A spread spectrum transmitter comprises a feedback shift register and transmits a sequence of pseudo-randomly hopped frequencies determined by the shift register. Each of the one or more spread spectrum receivers is tuned to one of the frequencies of the many over which the transmitter hops. Each spread spectrum receiver tuned to its unique predetermined frequency measures the relative times of arrival between consecutive transmissions, and determines, using the techniques of the a invention, the initial code word in the transmitter feedback shift register from the measured relative times of arrival. The spread spectrum receiver comprises a receiver feedback shift register configured in the same manner as the transmitter feedback shift register, and matches the receiver feedback shift register to the initial code word, adjusted by an amount of time elapsed during the synchronization process. The receiver feedback shift register is then used to carry out synchronized communication with the spread spectrum transmitter. In a preferred embodiment, a synchronization algorithm utilizing the measured relative times of arrival of transmissions on the predetermined frequency is employed by the receiver. The synchronization algorithm derives a number of equations upon receipt of each transmission on the predetermined frequency, based upon the knowledge of encoding at the transmitter, by relating a set of unknowns to the known encoding bits through a transition matrix. When a sufficient number of equations has been built up, the set of equations may be solved, yielding the initial codeword in the transmitter feedback shift register. The initial transmitter codeword is then advanced or otherwise adjusted to compensate for the lag between the receiver and the transmitter, and synchronized communication is carried out thereafter. Communication techniques as described herein may be applied both to frequency hopping spread spectrum communication systems and to direct sequence spread spectrum communication systems. Further embodiments, variations and enhancements are also disclosed herein. At the receiver This portion of the operation of the spread spectrum transmitter Further discussion of tapped shift registers, including their relationship to the generation of linear and nonlinear maximal code sequences, may be found, for example, in R. Dixon, As further illustrated in The frequency synthesizer The frequency hopping code sequence generator
The particular code sequence that will be generated depends upon the initial contents of the tapped shift register In the example illustrated in The frequency hopping code sequence generator Further details will now be explained about the initial synchronization operation of the receiver
In order to detect transmissions at the key frequency, the receiver The first detection event output from the envelope detector The synchronization algorithm The synchronization algorithm
Thus, if [z As a concrete example, the transition matrix for the ten-stage feedback shift register
As a further example illustrating how the transition matrix can be used to arrive at the contents of the feedback shift register after an arbitrary number of clock pulses, it may be noted that, in code sequence (8) defined in Table 4-1, the binary words [1101011001] and [0110011011] appear four clock pulses apart. The binary codeword [0110011011] can be derived from the initial binary codeword [1101011001] by first deriving a transition matrix based upon four clock pulses or code element intervals, and then by applying the initial binary codeword to the transition matrix. The transition matrix from a current binary word in the shift register to a binary word occurring a number of clock pulses p later is given by the expression A
Using the above-described properties of the transition matrix, a system of equations may be developed to determine the binary word present in the transmitter feedback shift register when the first detected key frequency is generated. A general methodology Next in the method In next steps Development of such a set of equations may be explained in more detail with reference once again to examples illustrated in
Taking the first two columns of the time-advanced 10×10 transition matrix results in a 10×2 matrix as follows:
and the resulting equations are given by [1,1,x The next relative time of arrival of a key frequency transmission is in time interval
and the resulting equations from the foregoing relationship are as follows: The next relative time of arrival of a key frequency transmission is in time interval
The next relative time of arrival of a key frequency transmission is in time interval
The next relative time of arrival of a key frequency transmission is in time interval
The next relative time of arrival of a key frequency transmission is in time interval
The next relative time of arrival of a key frequency transmission is in time interval
The result of the foregoing steps is a set of equations; in the instant example, the equations are as follows: In certain embodiments, it may be advantageous to store in computer memory in the receiver When a sufficient number of equations have been obtained (i.e., when the number of equations exceeds the number of unknowns), the initial binary codeword in the transmitter feedback shift register Thus, [1,1,0,0,1,1,0,1,1,1] is the state of the transmitter feedback shift register In the example of It may be noted that a missed key frequency transmission will not necessarily cause a failure of the synchronization algorithm. However, it may require waiting for an additional key frequency transmission to make up for the lost information. While the method In The receiver may be configured in a manner similar to that shown in A similar technique may be applied to a frequency hopping system in which the hopping rate is faster than the data rate (for example, 20 hops per data bit). In order to allow rapid synchronization, in one embodiment, a spread spectrum code is transmitted each frequency hop, rather than simply a brief hop. This spread spectrum code is preferably of sufficient length to provide the desired processing gain. At the receiver, a spread spectrum correlator is used to generate a correlation pulse each time a pulse signal is received. The receiver then measure the relative times of arrival of the key frequency pulse signal, as described before. It may be seen that the method As one approach to synchronizing the receiver feedback shift register Alternatively, and in a preferred embodiment, a binary operation is performed on the initial codeword to rapidly advance it to the current state of the codeword in the transmitter feedback shift register A preferred method for time advancement of the receiver feedback shift register Returning back to step In the example of
Assuming once again that 12 clock pulses separated the transmitter and receiver, the first codeword stored would be “0011101101”, the second codeword stored would be “11110000100”, and so on. The N stored codewords define a set of inner product equations by which the current contents of the transmitter feedback shift register where all operations are carried out using modulo-2 arithmetic. The final result of each dot product is a “1” or “0” binary value. However, stage-6 of what will become the current codeword for the receiver In implementation, the modular feedback shift register Alternatively, hardware registers can be used to assist in rapidly deriving a plurality of inner products. Such a configuration is illustrated in Utilizing various of the aforementioned techniques, a receiver not presently in communication with a transmitter can remain dormant until detecting a recognizable series of transmissions that appear on the key frequency that is monitored by the receiver. When the receiver is first activated, it starts monitoring the key frequency. With the first detected transmission on the key frequency, the receiver attempts to establish synchronization. The receiver accumulates information about the relative times of arrival of key frequency transmissions, and from this information determines the initial contents of the transmitter feedback shift register. The receiver then advances its own feedback shift register to match the current stage of the transmitter feedback shift register, and thereafter communicates in synchronization with the transmitter. On the chance that interference causes the receiver to misinterpret an errant signal (due to noise and/or interference) as an occurrence of the initial key frequency transmission, then synchronization may be unsuccessful in the first instance, and this fact can be discovered either by the receiver being unable to solve the set of linear equations, or else by loading an incorrect codeword in the receiver shift register and recognizing the synchronization has not occurred because the data is not properly detectable. The receiver can then attempt to synchronize again. The use of a single “selected” or key frequency as a monitoring frequency to establish initial synchronization may be advantageously employed in a multi-receiver system, to allow selective addressing of specified receivers when broadcasting signals over a region. Returning now to If a receiver While preferred embodiments of the invention have been described herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification and the drawings. For example, certain embodiments may be used to synchronize two or more systems where any observable parameter of the system is being pseudo-randomly varied. This synchronization would be achieved by measuring the relative time between recurrences of a selected value of the parameter being varied. The invention therefore is not to be restricted except within the spirit and scope of any appended claims. Referenced by
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