CA1108301A - Surface acoustic wave unique word detector and coherent demodulator - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A surface acoustic wave device is used as a unique word detector and coherent demodulator for a digital code differential phase-shift signal. The device improves code word detection by providing a coherent reference with a correlation gain of 10 log M dB. M is the additional number of fingers introduced into the surface acoustic wave device, which determine the phase of the unique word. To achieve this result with higher reliability and higher correlation coefficient, use is made also of a few bits of the clock recovery which precede the unique word in a preamble as used in Time Division Multiple Access (TDMA) systems. The device requires two interdigital output transducers, one of which provides maximum output in response to surface acoustic waves representing the unique word, and the other of which provides a maximum output in response to surface acoustic waves corresponding to a preassigned pattern of the unique word, such as a sequence of all zeros combined with a partial sequence of the clock recovery bits. The outputs of the two interdigital output transducers are maximized at the same instant to detect the auto-correlation peak of the unique word with performance equivalent to coherent demodulation. Also, clock and data are recovered in the second set of interdigital transducers.

Description

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SURFACE ACOUSTIC WAVE IJNIQUE
WO~D DETE:CTOR AND COHERENT DEMODULATOR

BACKGROUND OF THE I NVENT I ON
The present invention generally relates to Surface Acoustic Wave (S~W) devices, and, more particularly, to the application of such de~ices to a unique word detector and coherent demodulator for a digital code differential phase-shlft signal. Such a detector and demodulator has particular application in communications 10 systems, and~he use of the present invention results in an improvement in the unigue word error rate (UWER3, with a simpler structure.
It is common in TDMA communications systems to transmit a frame of data preceded by a preamble. The 15 preamble typically includes a clock recovery se~uence, a - unigue word and various housekeeping signals. ~ "unique word" is a digital word having a minimum of ten symbols and exhibiting high auto-correla~i~n proper~ies.
Detection of the unique word is used for frame 20 synchronization.
SAW devices have been usPd for performing numerous functions, such as ban~pass filters, correlators, coders, decoders, modulators and the like, at radio frequencies between 10 M bit and 1 G bit. A unique word ~5 detector can be implemented in a straightforward manne~
with SAW technology. The fingers of the interdigital output ~ transducer are arranged in a pattern corresponding to the unigue word to be detected. In other words, the interdigital output transducer is 30 phase-coded to have an impulse response equal to the time inverse of the unique word differential phase-shift signal waveform. A sharp correlation peak occurs when the unique word signal just fills the phase-coded output transducer. When implemented in this manner, the SAW

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device is a differential non-coherent decoding element -- that is, it has 0 and 180 phase shifts at each finger point, depending on its location, without being able to identify the absolute phase. In other words, the SAW device, used as a unique word detector, will detect not only the uni~ue word but its inverse as well without discriminating between the ~wo. This is, of course, undesirable in a TDMA communication system because of data decoding errors resulting ~rom failure 10 to properly detect the unigue word and establish frame synchronization. In addition, the inverse word is often used for "super-frame" synchronization.

SUMMARY OF THE INVENTION
The SAW device of the present in~ention improves 15 unique word detection by providing a coherent reference with an improvement in the correlation gain of 10 log M
~B. M is the additional number of fingers intxoduced into the SAW device. More particularly, the SAW device, according to the present invention, includes at least 20 two interdigital output transducers, hoth positloned to intercept surface acoustic waves launched by an input tr~nsducer. The first interdigital output transducer is formed with a first group of fingers arranged to provide a maximum output ~in response to surface acoustic waves 25 represen~ing the unique word or its in~erse. In addition, the first output transducer is formed with a second group of fingers arranged and summed in a predetermined pattern of several bits of the clock recovery sequence. The second output transducer 30 produces a maximum output at the same instant in time when the ~irst output transducer produces a maximum output in response to surface acoustic waves representing the u~ique word. The signal at the output of the second transducer provides a coherent re~erence .

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which is used to d~tect the auto-correlation peak of the unique word, wth performance equivalent to coherent demodulation. For large M, the performance improvement approaches 3 dB. In addition, the present invention may be used to provide both clock recovery and differentially decoded data outputs. A threshold detector with positive reference provides ~he true unique word detection, while a threshold device with negative reference provides the i~verse unique word 10 detection.

RIEF DE:SCRIPTION OF T~; DRAWINGS
The present invention will be better understood from the following detailed description with reference to the accompanying drawings, in which:
15Figure 1 is a perspective view o~ a prior art SAW
device designed to detect a 5-bit uni~ue word;
Figure 2 is a schematic plan view illustrating the ., geometry and relative dimensions of the input and output transducers of the SAW device shown in Figure l;
20Figure 3 is a~ graph illustrating the output of a conventional 16~bit SAW devi~e versus the number of matched bit conditions;
Figure 4 is a pl~n view and block diagram of an exemplary preerred embodiment of the SAW device 25 according to the present invention; and Figure 5 is a graph illustrating the output of a 16-bit SAW device according.to the present invention versus the number of matched bit conditions.

DETAILED DESCRIPTION OF THE INV~NT _ 30The SAW device illustrated in Figure 1 comprises a piezoelPctric substrate 10 having a polished, planar surface, The most widely used piezoelectric materials for SAW de~ices include quartz, LiNbO~, Bil~GeO~ and ~. , .

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LiTaO3. These materials are suitable for the device according to the present invention; however, it should be understood that the practice of the invention is not limited to these particular piezoelectric materials. An input signal is applied to the input terminals 11 which are connected to an interdigital input transducer 12 on the planar surface of the substrate 10. The transducer 12 comprises a pair of metal film electrodes, each comprising a set of fingers extending from ~ metallized 10 area co~nected to a respective one of the input terminals ll. The fingers of the two electrodes are interdigitated in parallel-spaced relationship in a well -kno~n manner. The metal electrodes of the input transducer 12 may be, for example, gold plated on the 15 planar surface of the substrate 12 using standard photolithographic techniques. When a sinusoidal electrical voltage is applied to the input terminals 11 of the device, the electrical field between adjacent fingers of the linput transducer 12 fringes into the 20 substrate, producing an alternating strain field and, conseguently, an acoustic wave. Acoustic waves propagate away from the transducer along the surface in both direc-tions perpendicular to the fingers. The forward propagating wave which is generated by the input 25 transducer is partially converted to an electrical signal at the output -transducer 13 by the inverse piezoelectric effect. Waves propagat.ing ln the reverse direction from the input transducer 12 or propagating past -the output transducer 13 are damped by acoustic 30 absorbing material (not shown) as is conventional. The output transducer 13 is similar in construction to the input transducer 12, and comprises a pair of metal film electrodes, each comprising a set of fingers extending from metallized areas which are respectively connected 35 to output terminals 14. The fingers of output 3r~

transducer 13 are interdig1tated in parallel-spaced relation, but the pattern of interdigitation is modified to produce a phase-coded array corresponding to the uni~ue word which is to be detected.
As is shown in Figure 2, the spacing between adjacent fingers in each of the input and ou~put transducers is A/2 where A is equal to the surface wavelength of the input phase-coded signal. Figure 2 also shows the code word corresponding to each finger 10 element of the output transducer 13. The data OlQ01 corresponds to a 5-bit unique word sequence, and the data 10110 is the complement of the unique word sequence and is detected as an equally correlated data wth opposite phase.
As will be appreciated from Figure 2, the output transducer ~3 has two finger pairs for each bit in the unique word. The geometry o~ khe finger pairs is such that the output transducer provides a maximum envelope output in response to a predetermined pattern of phase 20 changes rather than a predetermined pattern of absolute phase coding.
To better illustrate the problem of the prior art SAW unique word detector, assume, for example, that a 16-bit unique word is to be detected. This reguires an 25 output transducer having 16 pairs of fingers, or 32 fingers, as the decoding element. ~hen a matched condition for all 16 bits prevails, the output will be maximum. For purposes of illustration, this amplitude will be assumed to be 16 units high. When a 15-bit 30 matched condition prevails, the output drops by 2 units ~-that is, the output becomes 14 units high-- and this trend ~ontinues until the output becomes 0 at the 8-bit matched condition. At the 7-bit matched condition, the output envelope starts to increase again so that at the 3S 0 matched condition, which corresponds to the inverse of ..:. .
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the unique word to be detected, the output becomes 26 units high again. The output of a 16-bit SAW uni~ue word detector versus the number of matched bit conditions is graphically shown in Figure 3.
The S~W device according to the invention is shown in Figure 4, and comprises input terminals 21 connected to an input transducer 22. The input transducer 22 is similar to ~he inpu~ transducer 12 of the device as shown in Figures 1 and 2, except that it is extended 10 transversely of the direction of acoustic wave propaga~ion to cover not only the output transducer 23 for detecting the unique word but also an output transducer 25 for providin~ an estimation of absolute phase. The output transducer 23 is like the output 15 transducer 13 in the device of Figures 1 and 2 but with the addition of two finger pairs in an alternating ~equence. In other words, the output transducer 23 illus~,rated in Figure 4 is comprises of a first group of intexdigitated fingers arranged in a pattern 20 corresponding to the unique word to be detected, and a 'second group of interdigitated fingers in an alternating , sequence corresponding to a clock recovery pattern.
Tho,se skilled in the art will understand that the clock recovery sequence usually precedes the unique~word in : ~5 the preamble of a communications data frame. In the specific example ~given, :the unique word is defined as 01001 so that the preceding clock sequence will be 01, reading from left to right. Trans~ucer 2S comprises three pairs of fingers, two of which are responsive to 30 the alternating clock sequence of 0]. and one is responsive to the first bit of data.which is 1. Thus, the second group of~ fingers maximizes to 101. In operation, the output of both transducers 23 and 25 are maximized at the same lnstant. Txansducer 23 has a 35 maximum output in response to a 5AW corresponding to the , - .

unique word, whereas transducer 25 has a ma~imum output when the SAW according to the M-phase estimate symbols, part of which precede the unique word, passes the fingers of transducer 25. The delays in the SAW device are controlled by appropriate placement of the fingers of each of the transducers 23 and 25 such that these maximums occur at the same instant of time.
The signal a~ the output of transducer ~5 provides a coherent reference which is used to detect the 10 auto-correlation peak of the unique word at the output of transducer 23. More particularly, the lower one of the output terminals 24 of the transducer 23 is connected to the lower one of the output terminals 26 of transducer 25 to a common reerence potential or ground.
15 The upper one of each of the output terminals 24 and ~6 is connected to a mixer 27, which provides an output having an amplitude egual to the produc~ of the amplitudes at each of the output terminals 24 and Z5 and an algebraic sign dependen-t on the relative phase of the 20 two signals. The output of mixer 27 is connected to the positive input of an operational amplifier 28~ which has its negative input terminal connected to a source of positive reference voltage ~ V. Thus, the operational amplifier 28 does not provide a positive output unless 25 the output of mixer 27 excee~s the reference volta~e +
V. Diode clamping clrcuits can be provided in a manner well known in the art so that the operational amplifier 28 provides an output only when this reference voltage +
V is exceeded by the output of mixer 27 so that the 30 combination of the mixer 27 and the operational amplifier 28 per~orms the function o~ a threshold detector having a coherent reference. Should it also be desired to detect the inverted unique word, a second operational amplifier 29 can be provided with its 35 negative input connected to the ou~put of mixer 27 and its positive input connected to a source of negative reference potential - V.
The output of the present coherent detector can be described as in Figure 5, where the detection line of Figure 3 extends now intQ the negative direction, identifying also the inverse unique word. A SAW unique word detector with N number of~ finger pairs has a correlation gain of 10 log N dB. In the example illustrated in Figure 4~ N = 5, and this is the least 10 number of bits which will define a unique word. The estimation of absolute phase should ha~e a similar correlation gain so that the correlation advantage obtained from the estimation of the unique word is similarly reflected to the estimation of phase.
15 Practical limitations may prevent ~he realization of N
phase pairs. Thus, it may be necessary to confine to the possible estimation pairs where M ~ N. In such a case, the absolute phase correlation gain will be limited to lQ log M dB, and the overall correlation 20 advantage will be a unction of M. In the example illustrated in Figure 4,~M is equal to 3, two of which ~re supplied by the clock recovery sequence and one by the unique word. The choice of N and M will depend on the application to which the invention is put as well as 25 physical limitations of the SAW device itself. It should be noted that for sufficiently large M, the uni~ue word error rate approaches coherent detection performance which is 3 dB better than prior art non-coherent detection.
According to another feature of the invention, additional fingers 31, 32, 33 and 34 are provided. The distance betweeIl these fingers is A/~ as in the output transducers 23 and 25, but they are not connected together. Fingers 31, 32, 33 and 3~ are totally 35 independent. By means of these addi-tional fingers, in 3~1 associat1on wi-th external circuitry, it is possible to both recover the clock and differentially decode the data. More specifically, fingers 33 and 34, which may be considered to be a separ~te output transducer, are connected to the input of mixer 35 to provide ~ clo k recovery output. In addition, fingers 31 and 33, which toge~er with finger 32 may be considered yet another output transducer, are connected to the inputs of a mixer 36 which provides a dif~eren~ially-deco~ed data 10 output.
A ~r~me of information as repres~nted by a signal applied to the input terminals 21 of the input transducer 22 typically comprises a preamble followed by a data block, and the pxeamble is composed to a clock 15 recovery sequence followed by the unique word.
Therefore, the output transducex comprising the fingers 33 and 34 initially provides inputs to the mixer 35 for purposes of clock reco~ery. In other words, the local clock is synchronized using the ini.tial clock recovery 20 sequ~nce in the transmitted frame. The SAWs launched by the input transducer continue to propagate on the substrate, and, at' some~ point in time, the SAWs corresponding to the uni~ue word are under the decoder section of output transducer 23. At the same ins~ant of 25 time, ~he SAWs coxresponding to th clock recovery sequence have progressed to the point that they are under the output transducer 25. As a result, maximum outputs are provided at output terminals 24 and 26. The sum of these outputs provided at the output of mixer 27 30 exceeds the threshold established at the operational amplifier 28 to provide a detection of the unique word.
This identifies the beginning of the data block, and, at this point in time, the output of mixer 36 can be accepted as the differentially-decoded data. This can 35 be easily accomplished by using the output of 3~

operational amplifier 28 to enable a gate circuit (not shown) to pass the ou-tput of mixer 36.
It will be understood by those skilled in the art that the invention has been described in terms Gf a simple, preferred embodiment, and that modifications can be made within the scope of the invention. Obviously, ~he number M of the finger pairs used in the phase estimate decoder of the output transducer 25 and the number N of the finger pairs used in the unigue word 10 decoder section of output ~ransducer 23 can be changed from ~he specific example given, depending on the particular application and physical limitations.
Moreover, depending on the specific application, clock recovery andjor differential decoded data outputs may 15 not be required.

Claims (3)

CLAIMS:

1. A coherent unique word detector and differential demodulator for a digital code differential phase-shift signal comprising:
a piezoelectric crystal substrate having a planar surface;
an interdigital input transducer on the planar surface of said substrate for launching onto said substrate surface acoustic waves, a first interdigital output transducer on the planar surface of said substrate positioned to intercept surface acoustic waves launched by said input transducer, and having a first group of fingers arranged to provide a maximum output in response to surface acoustic waves representing a predetermined unique word or its inverse, and a second group of fingers arranged in a predetermined alternating sequence, said second group of fingers being further from said input transducer than said first group of fingers;
a second interdigital output transducer on the planar surface of said substrate positioned to intercept surface acoustic waves launched by said input transducer, and having a group of fingers arranged in said predetermined alternating sequence and positioned with respect to said input transducer to provide a maximum output in response to surface acoustic waves representing a coherent reference at the same instant in time as a maximum output is provided by said first interdigital output transducer in response to surface acoustic waves representing said predetermined unique word; and threshold means connected to said first and second interdigital output transducers for detecting the autocorrelation peak of said predetermined unique word as represented by a maximum output from said first interdigital output transducer using the maximum output of said second interdigital output transducer as a coherent reference.

2. A coherent unique word detector and differential demodulator as recited in Claim 1 further comprising:
a third interdigital output transducer on the planar surface of said substrate positioned to intercept surface acoustic waves launched by said input transducer, and having two fingers separated by one-half wavelength of said surface acoustic waves, said third interdigital output transducer being closer to said input transducer than said second interdigital output transducer; and clock detector means connected to said third interdigital output transducer for providing a recovered clock signal derived from said surface acoustic waves representing a coherent reference.

3. A coherent unique word detector and differential demodulator as recited in Claim 2 further comprising:
a fourth interdigital output transducer on the planar surface of said substrate positioned to intercept surface acoustic waves launched by said input transducer, and having two fingers separated by one wavelength of said surface acoustic waves; and data detector means connected to said fourth interdigital output transducer for providing a decoded data signal derived from surface acoustic waves occurring subsequent in time to said surface acoustic waves representing a predetermined unique word.