Apparatus for and method of adaptive synchronization in a spread spectrum communications receiver

• US 6,614,864 B1
• Filed: 10/12/1999
• Issued: 09/02/2003
• Est. Priority Date: 10/12/1999
• Status: Expired due to Fees

• Current Assignee: Yitran Communications Ltd. (Allalouf Capital Ltd.)
• Original Assignee: Yitran Technologies Ltd.
• Inventors: Raphaeli, Dan, Matmor, Avner
• Primary Examiner(s): Pham, Chi
• Assistant Examiner(s): Nguyen, Dung X
• Application Number:
  US09/415,723
• Time in Patent Office: 1,421 Days
• Field of Search: 375/371, 375/139, 375/143, 375/145, 375/149, 375/152, 375/130, 375/367, 375/365, 375/366, 370/503, 370/515, 370/509, 370/510, 370/511, 370/512, 370/513, 370/514, 370/516
• US Class Current: 375/371
• CPC Class Codes: H04B 1/70718 with asynchronous demodulation, i.e. not requiring code synchronisation
• CPC Class Codes: H04B 1/70753 Partial phase search
• CPC Class Codes: H04B 1/7077 Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
• CPC Class Codes: H04L 7/041 using special codes as synchronising signal

Abstract

An apparatus for and method for acquiring synchronization for use in communications systems, i.e., acquiring the presence of a packet of data and associated timing information. A sequence of symbols having known rotation and phase is transmitted to the receiver. The receiver attempts to match the received vectors in a predefined manner in order to determine whether a signal or noise is being received. CSK modulation is used for the synchronization acquisition stage and, any desired data carrying modulation may be switched to once synchronization is obtained, which may or may not be CSK. The transmitter transmits data in the form of packets to the receiver, wherein each packet is preceded by a preamble comprising a number of symbols. The length of the preamble can be any suitable number of symbols such that the receiver is able to synchronize with the transmitter. The preamble comprises a sequence of rotated or non-rotated symbols, inverted or non-inverted (or generally phase-rotated by some amount). In some embodiments, the preamble comprises a sequence of non-rotated symbols (or symbols with a constant fixed rotation) followed by one or more symbols with a known random shift, the rotation applied to each rotated symbol being independent of the rotation applied to other symbols.

Claims

1. In a communications system, a method of acquiring synchronization, comprising the steps of:

transmitting a preamble over a communication channel, said preamble comprising a plurality of symbols having known rotation and phase;

receiving a sequence of input samples from said communication channel;

dividing said received sequence into two or more vectors;

de-rotating and phase correcting said vectors back to their original rotation and phase; and

applying a matching function between said vectors so as to generate a metric indicative of the degree of synchronization.

2. The method according to claim 1, wherein said symbols comprise spreading waveforms characterized by high, sharp autocorrelation.

3. The method according to claim 1, wherein said symbols comprise pseudo noise sequences.

4. The method according to claim 1, wherein said step of applying a matching function utilizes a correlator to compare said one or more vectors and a template formed from some of the vectors in a recursive fashion.

5. The method according to claim 1, wherein said step of applying a matching function utilizes a correlator to compare said one or more vectors and a template formed from some of the vectors in a non-recursive fashion.

6. In a communications system, a method of acquiring synchronization, comprising the steps of:

transmitting a preamble over a communication channel, said preamble comprising a plurality of symbols having known rotation and phase;

receiving a sequence of input samples from said communication channel over a plurality of channels, each channel corresponding to a different frequency band;

dividing said received sequence into two or more vectors within each channel;

de-rotating and phase correcting said vectors back to their original rotation and phase;

applying a matching function between said vectors so as to generate a metric for each channel indicative of the degree of synchronization; and

combining the metric generated for each said channel so as to produce a combined output metric indicative of the degree of synchronization for all channels.

7. The method according to claim 6, wherein said step of combining comprises generating said combined output metric utilizing a single synchronization circuit.

8. The method according to claim 6, wherein said step of combining comprises generating said combined output metric utilizing separate synchronization circuits, each synchronization circuit associated with a single channel, wherein the output of one of said synchronization circuits is selected according to a predetermined criterion.

9. The method according to claim 6, wherein said step of combining comprises the steps of:

passing the output of each channel through a nonlinear function; and

summing the outputs of the nonlinear function for each channel.

10. The method according to claim 6, further comprising the step of dividing each channel into separate I and Q channels.

11. The method according to claim 6, wherein said symbols comprise spreading waveforms characterized by high, sharp autocorrelation.

12. In a communications system, a method of acquiring synchronization, comprising the steps of:

transmitting a preamble comprising a plurality of symbols having a first rotation and phase followed by one or more rotated symbols each having a predefined rotation and phase, said preamble transmitted over a communications channel;

generating a template adapted in accordance with the contents of said plurality of symbols during the reception thereof;

generating vectors from a signal received from said communications channel, said vectors generated in accordance with said rotated symbols;

de-rotating and phase correcting said vectors back to their original rotation and phase; and

matching said vectors with said template so as to generate a metric indicative of the degree of synchronization.

13. The method according to claim 12, wherein the step of generating vectors comprises the step of inputting said received signal into a shift register at a tap position a distance from the first tap corresponding to the amount of a second rotation.

14. The method according to claim 12, wherein the step of matching said vectors comprises the step of correlating the contents of said template with the contents of a shift register so as to generate a correlation sum, and detecting synchronization in response to a maximum correlation sum.

15. The method according to claim 12, wherein the amount of said first rotation is equal to zero.

16. The method according to claim 12, wherein the amount of said first rotation is equal to a fixed value wherein each symbol in said plurality of symbols is rotated similarly.

17. The method according to claim 12, wherein the template comprises a plurality of shift registers corresponding to the number of symbols in said preamble.

18. The method according to claim 12, wherein the template comprises a plurality of shift registers independent of the number of symbols in said preamble.

19. The method according to claim 12, wherein said step of generating a template comprises adapting the template as a function of the previous contents of said template and the received signal.

20. The method according to claim 12, wherein said step of generating a template comprises adapting the template as a function of said symbols de-rotated.

21. The method according to claim 12, wherein said step of generating a template comprises adapting the template as a function of said symbols phase corrected.

22. An apparatus for acquiring synchronization in a communications system, the communications system including a preamble consisting of a plurality of symbols having a first rotation followed by a shifted symbol having a second rotation, said preamble transmitted over a communications channel, said apparatus comprising:

a template including means for adapting said template to the characteristics of said communication channel during the reception of said plurality of symbols;

a shift register adapted to input a received signal at a tap position a distance from the first tap corresponding to the amount of said second rotation; and

a correlator operative to correlate the contents of said template with the contents of said shift register so as to generate a correlation sum, synchronization being detected in response to a maximum correlation sum.

23. The apparatus according to claim 22, wherein the amount of said first rotation is equal to zero.

24. The apparatus according to claim 22, wherein the amount of said first rotation is equal to a fixed value wherein each symbol in said plurality of symbols is rotated similarly.

25. The apparatus according to claim 22, wherein said template comprises a plurality of shift registers corresponding to the number of symbols in said preamble.

26. The apparatus according to claim 22, wherein said template comprises a plurality of shift registers independent of the number of symbols in said preamble.

27. The apparatus according to claim 22, wherein said means for adapting said template comprises means for adapting the template as a function of the previous contents of said template and the received signal.

28. The apparatus according to claim 22, wherein said function comprises an averaging function.

29. The apparatus according to claim 22, further comprising a maximum correlation detector adapted to find a maximum correlation sum.

30. The apparatus according to claim 22, further comprising a synchronization circuit adapted to generate a synchronization output signal in response to a maximum correlation sum.

31. The apparatus according to claim 22, wherein each symbol in said plurality of symbols may be phase rotated or phase inverted.

32. In a communications system, a method of acquiring synchronization, comprising the steps of:

transmitting a preamble comprising a plurality of symbols having a first rotation followed by M shifted symbols, wherein each shifted symbol is shifted by an amount independent of the shifts of other symbols, said preamble transmitted over a communications channel;

generating a template that is adapted to the characteristics of said communication channel during the reception of said plurality of symbols;

inputting a received signal into an I^th shift register at a tap position a distance from the first tap corresponding to the amount of rotation of the I^th shifted symbol;

correlating the contents of said template with the contents of said M shift registers so as to generate a plurality of correlations; and

summing said plurality of correlations to generate a correlation sum and detecting synchronization in response to a maximum correlation sum.

33. An apparatus for acquiring synchronization in a communications system, the communications system including a preamble consisting of a plurality of symbols having a first rotation followed by M shifted symbols, wherein each shifted symbol is shifted by an amount independent of the shifts of other symbols, said preamble transmitted over a communications channel, said apparatus comprising:

a template including means for adapting said template to the characteristics of said communication channel during the reception of said plurality of symbols;

M shift registers, the I^th shift register adapted to input a received signal at a tap position a distance from the first tap corresponding to the amount of rotation of the I^th shifted symbol;

M correlators operative to correlate the contents of said template with the contents of said M shift registers so as to generate M correlation outputs; and

a summer for summing said M correlation outputs so as to generate a correlation sum, synchronization being detected in response to a maximum correlation sum.

34. An apparatus for acquiring synchronization in a communication system, the communication system including a preamble sequence consisting of a plurality of symbols wherein each symbol has either a zero shift or a non-zero shift rotation associated therewith, said apparatus comprising:

N shift registers wherein N is equal to the number of symbols in said preamble sequence;

a first set of M functions wherein each function comprises N inputs, one input from each shift register, said first set of M functions for processing samples input to the left most tap of said N shift registers and for processing samples input to the tap position corresponding to those symbols having non-zero shift rotations;

a second set of M functions wherein each function comprises N inputs, one input from each shift register, said second set of M functions for processing samples output from the right most tap of said N shift registers and for processing samples output from tap positions one before the tap positions corresponding to those symbols having non-zero shift rotations;

an accumulator coupled to the output of each said function in said first set of M functions and said second set of M functions, wherein said output of said first set of M functions is added to said accumulator and the output of said second set of M functions is subtracted from said accumulator; and

maximum detector circuitry operative to declare synchronization in response to a maximum peak output of said accumulator during a symbol time period.

35. The apparatus according to claim 34, wherein each said function comprises summing all N inputs and squaring the result.

36. The apparatus according to claim 34, further comprising means for phase correcting said symbols in accordance with a phase rotation or phase inversion d_j previously applied to said symbols in said preamble.

37. In a communication system including a preamble sequence consisting of a plurality of symbols wherein each symbol has either a zero shift or a non-zero shift rotation associated therewith, a method of acquiring synchronization, said method comprising the steps of:

transmitting a preamble comprising a plurality of symbols, each symbol having either a zero rotation shift or a non-zero rotation shift, wherein each non-zero shifted symbol is shifted by an amount independent of the shifts of other symbols, said preamble transmitted over a communications channel;

inputting a received signal into a set of N shift registers where N is equal to the number of symbols in said preamble sequence;

processing selected taps of said N shift registers via a first set of M functions wherein each function comprises N inputs, one input from each shift register, said first set of M functions for processing samples input to the left most tap of said N shift registers and for processing samples input to the tap position corresponding to those symbols having non-zero shift rotations;

processing selected taps of said N shift registers via a second set of M functions wherein each function comprises N inputs, one input from each shift register, said second set of M functions for processing samples output from the right most tap of said N shift registers and for processing samples output from the tap positions one before the tap positions corresponding to those symbols having non-zero shift rotations;

accumulating an accumulator value by adding the output of said first set of M functions to said accumulator value and subtracting the output of said second set of M functions from said accumulator value; and

declaring synchronization in response to the occurrence of a maximum peak of said accumulator value during a symbol time period.

38. The method according to claim 37, wherein each said function comprises summing all N inputs and squaring the result.

39. The apparatus according to claim 37, further comprising means for phase correcting said symbols in accordance with a phase rotation or phase inversion d_j previously applied to said symbols in said preamble.

40. In a communications system, a method of acquiring synchronization, comprising the steps of:

transmitting a preamble over a communication channel, said preamble comprising a plurality of symbols having known time shift and phase;

receiving a sequence of input samples from said communication channel;

dividing said received sequence into two or more vectors;

correcting said vectors in time and phase back to their original time shift and phase; and

applying a matching function between said vectors so as to generate a metric indicative of the degree of synchronization.

41. In a communications system, a method of acquiring synchronization, comprising the steps of:

transmitting a preamble over a communication channel, said preamble comprising a plurality of symbols having known rotation, time shift and phase;

receiving a sequence of input samples from said communication channel;

dividing said received sequence into two or more vectors;

de-rotating, de-shifting and phase correcting said vectors back to their original rotation, time shift and phase; and

applying a matching function between said vectors so as to generyate a metric indicative of the degree of synchronization.