|Publication number||US20080144701 A1|
|Application number||US 09/832,067|
|Publication date||Jun 19, 2008|
|Filing date||Apr 9, 2001|
|Priority date||Apr 9, 2001|
|Also published as||US7386026|
|Publication number||09832067, 832067, US 2008/0144701 A1, US 2008/144701 A1, US 20080144701 A1, US 20080144701A1, US 2008144701 A1, US 2008144701A1, US-A1-20080144701, US-A1-2008144701, US2008/0144701A1, US2008/144701A1, US20080144701 A1, US20080144701A1, US2008144701 A1, US2008144701A1|
|Original Assignee||Robert Gold Communication System, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (21), Classifications (12), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
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.
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 130, a receiver antenna 132 receives the spread spectrum transmission signal and conveys it to front-end processing circuitry 135 and to a demodulator 153. In a frequency-hopping spread-spectrum system, the front-end processing circuitry 135 preferably comprises a band pass filter tuned to a selected frequency hopping “key” frequency (as further described herein) followed by an envelope detector or the equivalent. In a direct-sequence spread-spectrum system, the front-end processing circuitry 135 preferably comprises a despreading circuit, such as a spread-spectrum correlator for detecting spread-spectrum encoded pulses, as further described herein. The demodulator 153 outputs the signal to a synchronizer 140, which preferably operates according to techniques as described herein for detecting the proper synchronization between the receiver 130 and the transmitter 105. A timing shift circuit 145 is preferably employed to properly align the receiver spreading code with the transmitter spreading code, so as to allow proper demodulation of the received spread spectrum transmission signal. When the receiver feedback shift register 150 is properly aligned with the received code the output data signal 155 is obtained. The receiver feedback shift register 150 is preferably configured the same as the transmitter feedback shift register 110 (e.g., same number of delay stages, same number and location of taps, etc.), so that when both the transmitter feedback shift register 110 and receiver feedback shift register 150 have the same contents, the transmitter 105 and receiver 130 can communicate in synchronization. The receiver feedback shift register 150 outputs a receiver spreading code to the demodulator 153, which outputs a data signal 155 for further processing.
This portion of the operation of the spread spectrum transmitter 200 may be explained in more detail with reference to
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, Spread Spectrum Systems with Commercial Applications (J. Wiley & Sons, 3d ed. 1994), hereby incorporated by reference as if set forth fully herein.
As further illustrated in
The frequency synthesizer 415 receives the frequency select signal 412 from the selected delay stages 408, and outputs a sequence of frequencies in a random-like pattern based upon the progression of the frequency select signal 412 from clock cycle to clock cycle.
The frequency hopping code sequence generator 400 shown in
The particular code sequence that will be generated depends upon the initial contents of the tapped shift register 407. In the present example, code (8) from Table 4-1 corresponds to the initial contents of the tapped shift register 407, so the output of the frequency hopping code sequence generator 400 will be “11010110011011111111 . . . ” By selecting a different initial code sequence, any other of the code sequences from Table 4-1 may be generated by the feedback shift register 410 of
In the example illustrated in
The frequency hopping code sequence generator 400 of
Further details will now be explained about the initial synchronization operation of the receiver 300 shown in
In order to detect transmissions at the key frequency, the receiver 300 preferably passes the received frequency hopping transmission signal 305 into a band pass filter 310 which is centered at the key frequency. The band-pass filtered signal is input to an envelope detector 314, which may be constructed according to any of a variety of techniques well known in the art. The envelope detector 314 includes a threshold circuit (not shown) or the equivalent decision circuitry, and when the envelope of the signal at frequency “d” exceeds the threshold, a detection event is declared. The detection events are conveyed to the synchronizer 380, which uses the time-of-arrival information to achieve initial synchronization with the transmitter.
The first detection event output from the envelope detector 314 enables a counter 316 utilized in the synchronizer 380. Each incidence of the key frequency in the received frequency hopping transmission signal 305 causes the counter 316 to output its value to the synchronization algorithm 320 (which may be programmed, for example, into a processor, not shown in
The synchronization algorithm 320 computes the binary word that was in the transmitter feedback shift register 410 at the time the transmitter generated the first occurrence of the key frequency observed by the receiver 300. The computed binary word is then temporarily loaded into a “lag” feedback shift register 325, and advanced in time using a phase shift algorithm 330, which may be implemented by a phase shift circuit (preferably comprising digital logic or a programmed processor). The time-advanced binary word output from the phase shift algorithm 330 is loaded into the receiver feedback shift register 340, and used for controlling the frequency synthesizer 345 to effectuate demodulation of the received frequency hopping transmission signal.
The synchronization algorithm 320 makes use of a transition matrix of the feedback shift register 410 in order to facilitate determination of the initial binary word that was in the transmitter feedback shift register 410 at the time it first transmitted an instance of the key frequency that was detected by the receiver 300. The transition matrix of an N-stage feedback shift register may generally be defined as the N×N binary matrix whose product with a binary word in the shift register is that binary word which occurs in the shift register at the next clock pulse. For example, for a 10 stage shift register with feedback taps given by [a0, a1, . . . a9] where ai=0 or 1 depending upon whether or not there is a feedback tap to the ith stage, the transition matrix from the current binary word in the shift register to the next binary word is given by:
Thus, if [z0,z1, . . . z9] is the binary word in the transmitter feedback shift register 410 at a given clock pulse, then the binary word [z1,z2 . . . z10] of the register after the next clock pulse is given by the matrix multiplication [z0,z1 . . . z9]×A=[z1,z2 . . . z10]. By extrapolating further, the binary word that will be present in the transmitter feedback shift register 410 after any arbitrary number of clock pulses p is generally given by the matrix multiplication:
[z 0 ,z 1 . . . z 9 ]×A p =[z p ,z p+1 . . . z p+9].
As a concrete example, the transition matrix for the ten-stage feedback shift register 410 of the example shown in
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  and  appear four clock pulses apart. The binary codeword  can be derived from the initial binary codeword  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 AP (i.e., A×A× . . . ×A, or A to the pth power). Derivation of the binary word in the feedback shift register 410 after, e.g., four clock pulses or code element intervals, may be made using the transition matrix as follows:
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 600 for such a determination is illustrated in
Next in the method 600, a loop 610 is entered, in which relative arrival times of the key frequency transmissions are used in connection with further processing to eventually build up a set of equations that will allow determination of the initial contents of the binary codeword in the transmitter feedback shift register 410. For each received key frequency transmission (step 613), the number of intervening clock pulses (i.e., code element intervals) from the initially received key frequency transmission is determined, as indicated by step 615. In the example illustrated in
In next steps 618 and 622 in the loop 610, for each received key frequency transmission, the transition matrix for the feedback shift register is advanced by the number of clock pulses for the particular key frequency transmission, and an equation is developed from each received key frequency transmission based upon the knowledge that the initial contents of the transmitter feedback shift register, advanced by the number of clock pulses to generate the next occurrence of the key frequency, will result in a set of code elements corresponding to the binary code for the key frequency. The result over several received key frequency transmissions is a set of equations that can be used to solve for the initial contents of the transmitter shift register.
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,x2,x3,x4,x5,x6,x7,x8,x9]A4=[1,1]. Expanding out the equations represented by this relationship, two equations will remain:
The next relative time of arrival of a key frequency transmission is in time interval 7. Using the same approach, another two equations are derived by calculating a transition matrix A7 based upon advancement by 7 clock pulses, and utilizing the first two columns of the transition matrix as a 10×2 matrix to set up the appropriate relationships. These steps are illustrated mathematically as follows:
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 8, leading to the following transition matrix A8 and two further equations:
The next relative time of arrival of a key frequency transmission is in time interval 9, leading to the following transition matrix A9 and two more equations:
The next relative time of arrival of a key frequency transmission is in time interval 10, leading to the following transition matrix A10 and two more equations:
The next relative time of arrival of a key frequency transmission is in time interval 11, leading to the following transition matrix A11 and two more equations:
The next relative time of arrival of a key frequency transmission is in time interval 12, leading to the following transition matrix A12 and two further equations:
The result of the foregoing steps is a set of equations; in the instant example, the equations are as follows:
x 3 +x 5 +x 6 +x 8=1
x 3 +x 4 +x 6 +x 7 +x 9=0
x 3 +x 4 +x 5 +x 7 +x 8 +x 10=1
x 3 +x 4 +x 6 +x 7 +x 9=0
x 3 +x 4 +x 9=1
In certain embodiments, it may be advantageous to store in computer memory in the receiver 300 the transition matrices (or the needed columns/portions thereof) that are to be used in developing the equations in accordance with steps 618 and 622 of the method 600 of
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 410 may be determined. Thus, in a next step 630 in the method 600 shown in
Thus, [1,1,0,0,1,1,0,1,1,1] is the state of the transmitter feedback shift register 410 when the first occurrence of the key frequency (i.e., frequency “d”) is being generated—i.e., the “initial state” of the transmitter feedback shift register as perceived by the receiver.
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 600 of
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 600 set forth in
As one approach to synchronizing the receiver feedback shift register 340 with the transmitter feedback shift register 410, the receiver 300 may simply load the initial codeword that was determined by the synchronization algorithm 320 into the receiver feedback shift register 340, and then apply clocking signals to the receiver feedback shift register 340 at an accelerated rate based upon the number of counts in the counter 316, until the receiver feedback shift register 340 is caught up with the transmitter feedback shift register 410. Preferably, the clocking signals are applied to the receiver feedback shift register 340 at such a rate that the advancement of the receiver feedback shift register 340 is completed within a single code clock interval.
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 410, and to thereby synchronize the receiver feedback shift register 340 with the transmitter feedback shift register 410. This rapid time advancement is accomplished by modulo-two adding selected code elements of the initial binary codeword in various combinations to arrive at a new (i.e., the current) binary codeword. The specifics of the binary operation depend upon the particular configuration (i.e., tap locations) of the tapped shift register and the amount of time advancement that is needed.
A preferred method for time advancement of the receiver feedback shift register 340 may be explained with reference to the flow diagram of
Returning back to step 703 of the method 700 of
In the example of
0 1000000000 1 0100000000 2 0010000000 3 0001000000 4 0000100000 5 0000010000 6 0000001000 7 0000000100 8 0000000010 9 0000000001 10 1110110100 11 0111011010 12 0011101101 13 1111000010 14 0111100001 etc. . . .
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 210 can be derived. Thus, in a next step 730, the initial codeword that had been determined by the receiver 300 for the transmitter feedback shift register 410 is multiplied (using inner product multiplication) to arrive at the individual bits (i.e., code elements) of the current codeword of the transmitter feedback shift register 210. This series of operations is shown graphically in
[a 0 a 1 a 2 . . . ]·[b 0 b 1 b 2 . . . ]=a 0 b 0 +a 1 b 1 +a 2 b 2+ . . .
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 300 is derived from the contents of stage-0, stage-1, stage-2, stage-4, stage-5 and stage-7 of the initial transmitter codeword, according to the 10th codeword “1110110100” of the sequentially advanced modular shift register 800. Likewise, stage-7, stage-8 and stage-9 of what will become the current codeword in the receiver 300 are each derived from the contents of multiple stages in the initial transmitter codeword, according to codewords 11, 12, 13 and 14 of the sequentially advanced modular shift register 800.
In implementation, the modular feedback shift register 800 can take the form of a register or set of memory locations of a processor or processor-based circuitry that resides in the receiver 300. For example, the modular feedback shift register 800 may be implemented as a pre-defined variable in a software program that executes, among other things, the synchronization algorithm 320. In such a case, the processor may be programmed through software code to perform steps to time-advance the modular feedback shift register 800 by, e.g., initializing the modular feedback shift register variable, and then consecutively performing a binary operation on the modular feedback shift register a number of times necessary to obtain the ten codewords following the kth codeword (assuming ten delay stages in the receiver feedback shift register—more generally, the algorithm obtains the N codewords following the kth codeword). The processor then performs N inner product multiplications, as previously described with respect to the method 700 of
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 1220 fails to receive a transmission on its key frequency, then the receiver 1220 has no way to synchronize, and it will be effectively excluded from receiving the transmitter's signal. Accordingly, the foregoing provides a technique for selectively excluding receivers from receiving a broadcast transmission from a transmitter 1210.
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.
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|U.S. Classification||375/132, 375/E01.005, 375/E01.037, 375/140, 375/E01.035, 375/E01.003|
|Cooperative Classification||H04B1/7143, H04B1/7156, H04B2001/71563|
|European Classification||H04B1/7143, H04B1/7156|
|Jul 6, 2001||AS||Assignment|
Owner name: ROBERT GOLD COMMUNICATION SYSTEMS, INC., CALIFORNI
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:GOLD, ROBERT;REEL/FRAME:011994/0199
Effective date: 20010409
|Jan 4, 2002||AS||Assignment|
Owner name: UNITED STATES AIR FORCE, OHIO
Free format text: CONFIRMATORY LICENSE;ASSIGNOR:ROBERT GOLD ASSOCIATES;REEL/FRAME:012457/0629
Effective date: 20011105
|Dec 2, 2011||FPAY||Fee payment|
Year of fee payment: 4