WO2002073848A1

WO2002073848A1 - Frame synchronization method and system

Info

Publication number: WO2002073848A1
Application number: PCT/CA2001/000362
Authority: WO
Inventors: Michel FORTÉ
Original assignee: Cmc Electronics Military Communications Inc.
Priority date: 2001-02-28
Filing date: 2001-03-19
Publication date: 2002-09-19
Also published as: CA2439287C; WO2002073848B1; CA2439287A1

Abstract

A method and system for frame synchronization in a digital data transmission. A synchronization word is selected that is suitable for good correlation in a receiver detector. Either traffic bits or control bits are interleaved within the synchronization word in a predetermined pattern that is fixed for consecutive frames. The position of the sync word is varied from frame-to-frame. Also, the interleaved bits that are either traffic or control bits varies substantially between consecutive frames.

Description

FRAME SYNCHRONIZATION METHOD AND SYSTEM

BACKGROUND OF THE INVENTION

The present invention relates to a method of frame synchronization in a

digital data transmission. The present invention also relates to a frame

synchronization system that embodies the method of this invention.

In frame synchronization systems, particularly for critical applications,

such as in military applications, it is becoming more important to use a scheme

wherein the transmitted data (traffic) can not be easily deciphered or decoded

by an eavesdropper. In this regard, a typical data transmission sequence is

composed of consecutive frames of traffic data (bits) and an overhead word

(bits). It is furthermore typical to provide, usually as part of the overhead

word, a frame synchronization signal, usually of multiple bits disposed at the

same position of each frame, usually at the start position of a frame. With such

a fixed position synchronization word, there is not the level of security that one

would like to see. Also, existing techniques may be susceptible to a loss of

synchronization, and are, furthermore, not well adapted to provide rapid re-

synchronization.

Accordingly, it is an object of the present invention to provide a method

and system of frame synchronization in a digital data transmission and in which

there is practiced a scheme that enhances the security of the transmission. SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a method of

frame synchronization in a digital data transmission. This method comprises

the steps of selecting a synchronization word suitable for good correlation in a

receiver detector and interleaving bits of at least one of traffic bits and control

bits within the synchronization word in a predetermined pattern and fixed for

consecutive frames. A feature of the present invention is that the interleaved at

least one of traffic bits and control bits varies substantially between

consecutive frames. A further step of the method comprises varying a position

of the interleaved synchronization word and at least one of traffic bits and

control bits within consecutive frames in the transmission and in a predictable

manner. In a preferred embodiment disclosed herein, the step of interleaving

comprises interleaving the control bits, with the control bits defining the

position, said position varying according to a pseudo-random function. The

step of interleaving may also be comprised of interleaving only control bits

within the synchronization word. Additional overhead bits may comprise an

orderwire word and a radio information link word. The additional overhead

bits are positioned adjacent the interleaved bits.

Furthermore, in accordance with the present invention there may be the

additional steps of detecting, at a receiver, both the synchronization word and

the control bits. Next is detecting an error in the synchronization word. Then is the step of confirming that the synchronization is valid when the error is

detected and the control bits detected match and expected next value from the

pseudo-random function. The synchronization word may have an

autocorrelation characterized by a number of bits that are alike being not larger

than n/2+2, where n is the number of synchronization word bits compared. In

the disclosed embodiment there are 15 control bits and a synchronization word

comprises 49 bits for a total of 64 bits. A further step of the present invention

may comprise scrambling or encrypting the traffic bits using the position

determined according to the pseudo-random function.

Also, in accordance with the present invention there is provided a frame

synchronization system for a digital data transmission. The system comprises a

synchronization store for providing a synchronization word, a pseudo-random

number generator for providing a control word including control bits, and an

interleaver coupled from the synchronization store and the pseudo-random

generator for interleaving the control bits within the synchronization word in a

predetermined pattern that is fixed for consecutive frames, and in which the

control bits vary substantially between consecutive frames. The value of the

control word defines the position of the synchronization word along a frame.

The position of the synchronization word along the frame varies between

consecutive frames.

DESCRIPTION OF THE DRAWINGS Other features of the present invention should now be apparent from a

reading of the following detailed description taken in conjunction with the

accompanying drawings;

FIG. 1 is a diagram of consecutive frames, illustrating the

synchronization scheme of the present invention;

FIG. 2 illustrates somewhat more detail of a single frame of the

consecutive frames of FIG. 1;

FIG. 3 is a diagram illustrative of the random interleaving of control bits

within the synchronization word;

FIG. 4 is a flow chart illustrating the frame synchronization algorithm as

in accordance with one embodiment of the present invention; and

FIG. 5 is a block diagram of the system of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present invention describes a frame synchronization technique that

is used in digital data transmission. This technique involves the selection of a

synchronization word that is suitable for good correlation in a receiver detector.

Data or bits are interleaved within the synchronization word. These interleaved

bits may be either traffic bits or control bits and they are interleaved within the

synchronization word in a predetermined pattern, where this pattern is fixed for

consecutive frames. Also, the interleaved traffic or control bits vary

substantially between consecutive frames. Reference is now made to FIG. 1 which illustrates a stream of digital

data. More particularly, there is illustrated in FIG. 1 three frames Fl, F2 and

F3. Each frame is defined as a 2 msec time interval which contains the traffic

bits, shown in FIG. 1 in separate traffic bit fields 10. Also depicted in FIG. 1 is

a word of 128 adjacent overhead bits, located pseudo-randomly in the frame.

This is identified in FIG. 1 by the overhead word 12.

In FIG. 1 the overhead word in separated into a sync word 14 and an

extra overhead word 16. In this regard also refer to FIG. 2 that shows in

somewhat further detail a single one of the frames of FIG. 1. FIG. 2 also

depicts the extra overhead word 16 as broken down into an orderwire word

16A and a radio information word 16B.

Depending upon the traffic rate, there will be a certain number of traffic

bits. For example, in frame Fl of FIG. 1 these traffic bits appear in the field 10

on either side of the overhead field 12. For operation at say 256 Kbs, there are

512 traffic bits in a frame and 128 overhead bits in that frame.

The aforementioned 128 overhead bits distributed as follows in the order

in which they appear in the data stream. In this regard refer to FIGS 1 and 2.

The 128 bits are separated as follows:

64 bits form the framing word and they in turn are composed of a 15 bit

control word (C0-C14). The control word provides the pseudo¬

random sequence seed. This determines the position within a

frame that the synchronization word appears. The 64 bit framing word also includes a 49 bit sync word (S0-S48) for timing

recovery. These bits are interleaved for security purposes, such

as in the manner illustrated in FIG. 3 and discussed in further

detail hereinafter.

32 bit orderwire word (W0-W31) corresponding to 16 Kbs (word 16 A)

32 bit radio information word (R0-R31) corresponding to 16 Kbs (word

16B).

Now, with further reference to FIG. 1 , it is noted that the overhead word

12 is disposed in the different frames illustrated in FIG. 1, at different positions

along each frame. For example, the synchronization word in frame Fl is more

toward the right of the frame while in frame F2 the synchronization word is

more to the left of the frame, while in frame F3 the synchronization word is

approximately in the middle of the frame. The position of the synchronization

word along the frame is determined by the pseudo-random sequence

represented by the control word (C0-C14).

As indicated previously, the sync word and the control word are

interleaved so as to present a data stream with as little as possible repetition

from frame to frame. A fixed interleaving scheme may be employed, however,

it is preferred to have a random interleaving such as partially illustrated in the

drawing of FIG. 3. It is noted that the 64 bits include a 15 bit control word

(C0-C14) and a 49 bit sync word (S0-S48). The bit SO represents the first bit of the sync word and appears at the beginning of the framing word. The bit

S48 represents the end of the sync word and appears as the last bit of the

framing word.

Two frame examples may be considered, one for a traffic rate of 256 and

the other for a traffic rate of 1544. At 265 Kbs, there are 512 traffic bits and

128 overhead bits (20%) per frame for a total of 640, corresponding to 320

Kbs. At 1544 Kbs, there are 3088 traffic bits and 128 overhead bits (4.0%) per

frame for a total of 3216, corresponding to 1608 Kbs.

In the disclosed embodiment there is one and only one overhead word in

each frame. The first overhead word bit (SO) position varies pseudo-randomly

from one frame to the next. There preferably is a forbidden zone at the end of a

frame where bit SO is not allowed to hop so as to prevent a collision with the

next frame overhead word which could possibly be located at the start of the

frame. This forbidden or dead zone is preferably at least 128 bits wide. Since

the natural length of a pseudo-random (PN) sequence is a power of 2 to

simplify the design, the largest possible power of 2 for the hopping time

interval is used. There is also a small dead zone (preferably 1 bit) located at

the beginning of the frame. This stems from the fact that the pseudo-random

number generator that determines the frame beginning should not be 0. The

following parameters apply:

a) The start of the frame is defined as the mux elk edge on which the

frame counted goes back to 0. b) The beginning of the overhead burst (SO) occurs at the soonest when

the counter reaches 1.

c) The value found in the control word C0-C14 is always the actual

position of the overhead burst in the frame (1,2,3...; never 0).

d) When the overhead burst hopping is disabled, the beginning of the

overhead burst (SO) occurs when the counter reaches 0.

e) The PN generator which determines the position of the overhead

burst is 15 bits long. It generates 32767 different states. Only as the

highest rate are all these numbers required. At the lowest rates, the

appropriate number of msb bits are masked to provide the required

number of states.

Now, reference is made to FIG. 4 which is a frame synchronization

algorithm described by way of the flow chart of FIG. 4. In FIG. 4

the reset occurs at 20. From there the algorithm continues to box 22

where the "DEMOD LOCK" flag is false. Box 24 indicates a wait

for the first sync signal. Box 26 indicates the "DEMOD LOCK" flag

true. Valid sync words are first used by the demodulator to phase

lock itself through the "DEMOD LOCK" status bit. This bit is set

when the first "sync word" is detected, and reset after 250 frames

without sync detect of a valid PN. Refer in particular to box 46 in

FIG. 4. The first sync word triggers the PN acquisition. Refer in FIG. 4 to

boxes 26, 28, and 30. The PN is used to initialize the local frame

counter as well as to load the comparator buffer used to check the

frame synchronization on every sync word received. Once the frame

counter has been initialized, the "IN SYNC" status bit is set; this bit

is used to start the demux. The bit is reset at the same time as the

"DEMOD LOCK" status bit, after 250 frames without either a sync

detect or a valid PN.

With further reference to FIG. 4, it is noted that when the output of box

32 indicates that the sync is received first, the algorithm loops back to the input

of box 28. If the output of box 32 indicates the end of the frame is received

first, then the algorithm proceeds to box 34. If the sync signal is received first

then PN is wrong and the algorithm loops back to the input of box 28. The

output of box 34 indicates the next expected time slot and then the decision box

36 inquires as to whether there is a sync. If the answer is "YES" then the "IN

SYNC" status bit is set. Refer to box 42 in FIG. 4. If the output of the decision

box 36 is "NO" then the algorithm proceeds to box 38 for an increment and to

decision box 40 to enquire as to whether the detected pseudo-random number is

the same as the local one. If the answer is "YES" this also sets the status bit at

box 42. If the decision from box 40 is "NO" this causes an increment at box 44

and initiates the count at decision box 46. A "NO" output from box 46 causes a loop of the algorithm back to the input of box 32. A "YES" decision from

box 46 causes a resetting.

Regarding the sync word itself, which is composed of 49 sync bits S0-

S48, this can be any predetermined series of binary numbers. A number has

been chosen by trial and error on the basis of its weak autocorrelation. There

are 49 possible positions which can be considered when the data stream enters

the correlator and gets compared to the data stream. The sync word has been

chosen so that in each of the 49 different possible positions when it is entering

the correlator, it looks like random data. More precisely, it is such that for any

of the 49 positions, the number of bits that are alike is not larger that n/2+2,

where n is the number of sync word bits compared. In one illustration, for the

data stream halfway through the correlator, we have the number of bits alike

equal to 11, which is not larger than 25/2+2=14.5

Reference is now made to the block diagram of FIG. 5 which is a

representation of hardware employed in accordance with the present invention

for carrying out the frame synchronization. In FIG. 5 the traffic data is stored

in the traffic data source 50. The control word bits are generated by the

pseudo-random number generator 52. The sync word store 54 holds the sync

word bits. It is noted that the source 50, the generator 52 and the store 54 all

couple to an interleaver 56. Interleaver 56 provides the interleaving of bits,

either traffic or control bits within the synchronization word. The traffic data source 50, as noted in FIG. 5, couples to the frame

builder 58. The output from pseudo-random number generator 52 also couples

to the frame builder 58. Also, the output of the interleaver 56 couples to the

frame builder 58. The circuit 58 builds the frame, on a frame-by- frame basis

for coupling by way of a scrambler 60 to the transmitter 62.

As indicated previously, the bits that are interleaved in the

synchronization word may be either traffic bits or control bits. This is why

there is shown a connection to the interleaver 56 from both the traffic data

source 50 as well as the generator 52.

Regarding the scrambler 60, the mux data (OW, RIL and traffic) of each

frame is encrypted (scrambler) preferably by an XOR operation of a pseudo¬

random sequence having as its seed value the control word value.

FIG. 5 also shows the receiver portion of the system. This includes a

receiver 64 that received the data transmitted from transmitter 62. The output

of the receiver 64 couples to a synchronization recovery circuit 66 as well as to

a traffic recovery circuit 70. The output of the recovery circuit 66 couples to a

validator 68. The output of the validator couples to both the traffic recovery

circuit 70 and a descrambler 76 with the data output being taken at the output

of the descrambler 76. Also illustrated in the receiver section is an

initialization unit 72 and a pseudo-random number generator 74.

The receiver unit of FIG. 5 operates as in accordance with the algorithm

described by way of the flow chart of FIG. 4. Thus, if there is a sync detected (refer to decision box 34 in FIG. 4), the traffic data is recovered by way of

traffic recovery circuit 70 to the output data line. The validator 68 receiving

the control word, determines the position of traffic bits versus sync bits.

Also, in accordance with the flow chart of FIG. 4, if there is no sync,

then by way of the decision box 40, if the pseudo-random number (control bits)

is the same as the number from the generator 74, the validator 68 indicates a

"IN SYNC" flag as per the box 42 in FIG. 4.

In FIG. 5 it is also noted that the control word from the validator 68 also

couples to the descrambler 76 for the control thereof. This control word

controls the descrambling operation.

Having described a limited number of embodiments of the present invention, it

should now be apparent to those skilled in the art that other embodiments and

modifications thereof are contemplated as falling within the scope of the

present invention. For example, the control bits in a particular frame may be

used to determine the position of the sync word in that frame or, alternatively,

the control bits in a particular frame can be used to determine the position of

the sync word in a next frame. It could even determine the position in

subsequent frames.

Claims

CLAIMS:

1. A method of frame synchronization in a digital data transmission, the

method comprising the steps of:

selecting a synchronization word suitable for good correlation in a

receiver detector;

interleaving bits of at least one of traffic bits and control bits within said

synchronization word in a predetermined fixed for consecutive frames,

said interleaving at least one of fraffic bits and control bits varying

substantially between consecutive frames; and

varying a position of said interleaved synchronization word and

at least one of traffic bits and control bits within consecutive frames in

said transmission in a predictable manner.

2. The method as claimed in claim 1, wherein said interleaving comprises

interleaving said control bits, said control bits defining said position, said

position varying according to pseudo-random function.

3. The method as claimed in claim 2, wherein said step of interleaving

comprises interleaving only said control bits within said synchronization word.

4. The method as claimed in claim 3, wherein additional overhead bits are

positioned adjacent said interleaved bits.

5. The method as claimed in claim 4, wherein said additional overhead bits

comprise an orderwire word and a radio information link word.

6. The method as claimed in claim 2, further comprising of: detecting an error in said synchronization word;

confirming that said frame synchronization is valid when said

error is detected and said control bits detected match and expected next

value from said pseudo-random function.

7. The method as claimed in claim 3, wherein said synchronization word has

an autocorrelation characterized by a number of bits that are alike being not

larger than n/2+2, where n is the number of synchronization word bits

compared.

8. The method as claimed in claim 7, wherein there are 15 said control bits

and said synchronization word comprises 49 bits.

9. The method as claimed in claim 2, further comprising a step of scrambling

or encrypting said traffic bits using said position determined according to said

pseudo-random function.

10. A method of frame synchronization in a digital data transmission, the

method comprising the steps of:

selecting a synchronization word suitable for good correlation in a

receiver detector;

interleaving bits of at least one of traffic bits and control bits

within said synchronization word in a predetermined pattern;

said predetermined pattern being the same for consecutive

frames; said interleaved at least one of traffic bits and control bits being

different for consecutive frames; and

varying the position of said sync word from frame to frame.

11. A method as claimed in claim 10 wherein said interleaving comprises

interleaving said control bits, said control bits defining said position, said

position varying according to pseudo-random function.

12. A method as claimed in claim 11 wherein said step of interleaving

comprises interleaving only said control bits within said synchronization word.

13. A method as claimed in claim 12 wherein additional overhead bits are

positioned adjacent said interleaved bits.

14. A method as claimed in claim 13 wherein said additional overhead bits

comprise an orderwire word and a radio information link word.

15. A method as claimed in claim 11 further comprising of:

detecting an error in said synchronization word;

confirming that said frame synchronization is valid when said

error is detected and said control bits detected match and expected next

value from said pseudo-random function.

16. A method as claimed in claim 12 wherein said synchronization word has an

than n/2+2, where n is the number of synchronization word bits compared.

17. A method as claimed in claim 16 wherein there are 15 said control bits and

said synchronization word comprises 49 bits.

18. A method as claimed in claim 11 further comprising a step of scrambling or

encrypting said traffic bits using said position determined according to said

pseudo-random function.

19. A method as claimed in claim 10 wherein said interleaving comprises

interleaving said control bits, said control bits defining said position in the

current frame.

20. A method as claimed in claim 10 wherein said interleaving comprises

interleaving said control bits, said control bits defining said position in the next

frame.

21. A frame synchronization system for a digital transmission, said system

comprising:

a synchronization store for providing a synchronization word;

a pseudo-random number generator for providing a control word

including control bits;

an interleaver coupled from said synchronization store and said

pseudo-random generator and for interleaving said control bits within

said synchronization word in a predetermined pattern that is fixed for

consecutive frames and in which the control bits vary substantially

between consecutive frames;

the value of said confrol word defining the position of said

synchronization word along a frame; and wherein the position of said synchronization word along said

frame varies between consecutive frames.

22. A frame synchronization system as claimed in claim 21 wherein the value

of the control word defines the position of the synchronization word along the

current frame.

23. A frame synchronization system as claimed in claim 21 wherein the value

of the control word defines the position of the synchronization word along the

next frame.

24. A frame synchronization system as claimed in claim 21 further including a

traffic data source and a frame builder with the output of the traffic data source,

as well as the output from the interleaver coupling to the frame builder.

25. A frame synchronization system as claimed in claim 24 further including a

scrambler coupled from the output of the frame builder and a transmitter

coupled from the output of the scrambler.

26. A frame synchronization system as claimed in claim 25 wherein said

scrambler is controlled in accordance with the output from said pseudo-random

number generator.

27. A frame synchronization system as claimed in claim 25 further including a

receiver section including a sync recovery unit, a traffic recovery unit and a

validator.

28. A frame synchronization system as claimed in claim 27 further including a

receiver for coupling signals to the sync recovery unit and the traffic recovery

unit, the output of the sync recovery unit coupling to the validator.

29. A frame synchronization system as claimed in claim 28 further including an

initialization unit coupled from the output of the sync recovery unit and a

pseudo-random number generator unit coupled from the initialization unit and

having its output coupled to the validator.

30. A frame synchronization system as claimed in claim 29 further including a

descrambler coupled from the output of the validator and the traffic recovery

unit, with the date output taken from the output of the descrambler.