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FIELD OF THE INVENTION
This invention relates generally to noise code type of communication
systems and more particularly to a multichannel time division multiplexed
trunk transmission link utilizing code mate pairs having autocorrelation
functions which upon detection provide an impulse autocorrelation
function.
BACKGROUND OF THE INVENTION
Correlation techniques used in conjunction with noise codes have been
utilized in the past in signal processing systems. These noise coded
systems include, for example, over the horizon systems employing various
types of scatter techniques, satellite communications systems, and the
like, and multiple access systems employing address codes to enable
utilization of the system. One such system is shown and described in U.S.
Pat. No. 3,908,088, entitled, "Time Division Multiple Access
Communications System", issued to Frank S. Gutleber, the present inventor,
on Sept. 23, 1975.
The correlation and coding techniques employed in this type of
communications system results in increased signal-to-noise ratios without
any increase of transmitter power. It not only operates to minimize
multipath effects, but also obviates interference or cross-talk between
the channels while operating with overlapping noise coded signals.
Typically, the technique employed utilizes a passive matched filter which
pulse compresses a wide pulse to a narrow pulse whose peak amplitude is
increased by the number of code bits present in the processed code.
Accordingly, the output comprises a single peak of relatively high
amplitude having a pulsewidth substantially narrower than the pulsewidth
of the received signal without spurious peaks of lower amplitude elsewhere
in the waveform.
Furthermore, one class of noise codes are known wherein pairs of coded
signals termed "code mates" have autocorrelation functions which upon
detection with a matched filter provide a peak output at a given time and
a zero output or outputs having the same amplitude with the opposite
polarity at all other times. When the code mates are detected and the
resultant detected outputs are linearly added, there is provided an
impulse output of high amplitude at a given time and a zero output at all
other times. Typical examples of means for generating such codes and the
utilization thereof in communications systems is typically shown in the
following patents issued to the present inventor: U.S. Pat. No. 3,461,451,
entitled, "Code Generator To Produce Permutations Of Code Mates", Aug. 12,
1969; U.S. Pat. No. 3,519,746, entitled, "Means And Method To Obtain An
Impulse Autocorrelation Function", July 7, 1970; and U.S. Pat. No.
3,634,765, entitled, "System To Provide An Impulse Autocorrelation
Function . . . ", Jan. 11, 1972.
Accordingly, it is an object of the present invention to provide an
improvement in pulse code communications systems.
Another object of the present invention is to provide an improvement in
multichannel time division multiplexed trunk transmission links.
Still another object of the present invention is to provide an improvement
in a multichannel time division multiplexed trunk transmission link which
provides a large degree of interference or jamming rejection by employing
code mates for selected channels which when detected in a matched filter
compress to a single impulse containing substantially no side lobes for
each selected channel.
SUMMARY
These and other objects are achieved by means of a multichannel time
division multiplexed trunk transmission link, the bit stream of which has
specific channels of a composite group of channels selectively gated out
in the respective time slot and which are then spread spectrum coded as
multiplexed noise code mates having a predetermined code length up to the
original number of multiplexed channels. The selected noise coded channels
are then multiplexed and used to modulate an RF carrier which is
transmitted over a transmission link. The modulated RF carrier is received
whereupon the code mates are demultiplexed, matched filter detected, and
linearly added whereby the codes are compressed to individual channel
single bit output signals which are totally non-interfering with one
another.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram of transmitter apparatus utilized in a
single channel communications link according to the known prior art which
employs multiplexed code mate pairs;
FIG. 2 is a functional block diagram of receiver apparatus utilized in the
communications link of FIG. 1;
FIG. 3 is a functional block diagram of transmitter apparatus included in a
noise modulated communications system in accordance with the principles of
this invention;
FIG. 4 is a functional block diagram further illustrative of the two code
generators shown in FIG. 3;
FIG. 5 is a functional block diagram of receiver apparatus utilized in
connection with the transmitter apparatus of FIG. 3; and
FIG. 6 is a functional block diagram further illustrative of the matched
filter detectors shown in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is directed to a spread spectrum pulse code
modulation communications system employing a type of multibit digital
noise code referred to as code mates, meaning that the information is
coded with a code that is "noise like" in that it will compress to an
impulse when detected with a matched filter. As noted above, one class of
noise codes are known wherein pairs of coded signals termed "code mates"
have autocorrelation functions which provide a peak output at a given time
and a zero output or outputs having the same magnitude but opposite
polarity, at all other times. When these code mate signals, for example,
are demultiplexed, matched filter detected and linearly added, there is
provided a lobeless impulse output of a relatively high amplitude at one
given time (.tau.=0) and a zero output at all other times (.tau..noteq.0).
For a pair of code mates a and b, this may be stated mathematically as,
.phi..sub.a (.tau.)=-.phi..sub.b (.tau.) (1)
for all .tau..noteq.0, where .phi..sub.a (.tau.) is the autocorrelation
function of code a, .phi..sub.b (.tau.) is the autocorrelation function of
code b, and where .tau.=0 is the location of the main lobe. This can be
illustrated by the following example.
Consider code mate a and b where a=1 0 0 0 and b=0 0 1 0. The
autocorrelation function .phi..sub.a (.tau.) of code a can be obtained in
a well known fashion by detection in a matched filter. As is well known, a
matched filter detector can be implemented by a combination of time delay
circuitry, phase control circuits and a linear adder which operates to
generate a digital autocorrelation sequence .phi..sub.a (.tau.) in the
following manner:
##EQU1##
where 0 denotes a pulse of unit amplitude and positive polarity and 1
denotes a pulse of unit amplitude and negative polarity, the . denotes the
absence of a pulse, and wherein the exponent signifies the amplitude of
the respective pulses. As shown in equation (2), the main lobe (.tau.=0)
comprises a positive pulse having an amplitude of four times the unit
amplitude.
In a similar manner, the autocorrelation function .phi..sub.b (.tau.) of
code b is generated in its corresponding matched filter as:
##EQU2##
From equations (2) and (3) it can be seen that .phi..sub.a
(.tau.)=-.phi..sub.b (.tau.) for all .tau..noteq.0, and furthermore, when
added together, compress to a lobeless impulse at .tau.=0 when linearly
added together. This is shown below as:
##EQU3##
A functional block diagram of a single channel communications link
employing multiplexed coding of code mate pairs and illustrative of the
known prior art is illustrated in FIGS. 1 and 2. As shown in FIG. 1, a
binary modulator 10 and a coder-multiplexer 12 comprise clock,
synchronizing generator, code generator and mixing apparatus to provide
output code signals of code mate pairs a and b, for example, multiplexed
in time and which are amplified and modulated on an RF carrier in a block
designated power amplifier 14 and thereafter propagated by an antenna 16.
Further, as shown in FIG. 2, the RF signals radiated from the antenna 16
are received by an antenna 18 which is coupled to a receiver 20. The
receiver 20 outputs an IF signal comprised of the code mate pair signals
which are fed to a demultiplexer 22 whereupon the code mates a and b are
separated and fed to their respective matched filters 24 and 26. The
outputs of the filters 24 and 26 comprise autocorrelation function output
signals .phi..sub.a (.tau.) and .phi..sub.b (.tau.) which are combined in
a linear adder 28 to provide a single lobeless impulse output signal
.phi..sub.T (.tau.). The specific type of multiplexing employed in the
communication system of FIGS. 1 and 2 may be of any type by which the code
mate signals may be later separated and made orthogonal to each other so
as to be non-interfering. The demultiplexer 22 accordingly is consistent
with the type of multiplexing employed at the transmitter which, for
example, may include time division multiplexing, frequency division
multiplexing, quadrature phase modulation, or horizontal and vertical
antenna polarization. Thus the preferred approach depends upon the
specific application and the user requirements accompanying this use.
Turning now to the present invention, the principles outlined above are
applied to a multichannel time division multiplexed (TDM) trunk
transmission link coupling a plurality of diversely located transceivers
together. As shown in FIG. 3, reference numeral 30 designates a
multichannel TDM input apparatus which couples a multibit stream of n
channels in a corresponding number of n sequential time slots covering a
predetermined time frame period. The multichannel TDM bit stream is
coupled to a gate 32 which is controlled by a system control channel
selector 34 which is operable to enable the gate 32 to selectively output
certain channel bits 1 through n in each time frame period. As shown, two
channels (Ch. #1 and Ch. #3) are selected in time slots 1 and 3 while the
remaining channels in alloted time slots 2, 4, 5 . . . n are inhibited.
Considering the selected code bits in time slots 1 and 3 as a pulse of unit
amplitude and positive polarity, i.e. a 0 signal, the two signals gated
out in time slots 1 and 3 are concurrently fed to two code mate generators
36 and 38, the details of which are shown in FIG. 4, whereupon two
composite code mates .SIGMA.(a) and .SIGMA.(b) are applied to multiplexer
40. The multiplexed code mates outputted by the multiplexer 40 are next
coupled to a bi-phase modulator 41 whose output is coupled to an RF output
amplifier 42. The output of the RF amplifier 42 in turn is coupled to a
transmitting antenna 44 which radiates an RF carrier containing the two
composite multiplexed code mates .SIGMA.(a) and .SIGMA.(b).
Referring now briefly to FIG. 4, there is disclosed embodiments of the a
and b code mate generators 36 and 38. Both the code generators are
comprised of a plurality of time delay circuits and a plurality of phase
control circuits coupled to a linear adder. More specifically, the code
mate generator for generating code a is comprised of three series
connected time delay circuits 45, 46 and 47, each having a time delay
.tau. equal to a pulse width, four phase control circuits 48, 49, 50 and
51 providing a signal feedthrough of either 0.degree. or 180.degree. phase
shift (indicated by 0 and 1, respectively) of the specific signal applied
thereto, and a linear adder 52. Accordingly, for the selected two channel
sequence of 0 . 0 . . . in time slots 1 and 3, the code generator will
output the composite binary code .SIGMA.(a)=1 0 . 0.sup.2 0 0 . . . .
The code mate generator for generating code b as shown in FIG. 4 is
comprised of three time delay circuits 53, 54 and 55, each having a time
delay .tau. equal to a pulse width, four phase shifters 56, 57, 58 and 59,
phased as shown, and a linear adder 60. In this instance, the same
selected two channel sequence of 0 . 0 . . . applied to the input of code
generator 38 will output the composite code .SIGMA.(b)=0 0 . 0.sup.2 1 0 .
. . .
This code generation can be illustrated as shown below.
The two selected channels in time slots 1 and 3 can be illustrated as:
##EQU4##
With the phase shifters set as shown in FIG. 4, codes will be generated
such that for each channel sequence applied a=1000 and b=0010.
Accordingly, a composite output .SIGMA.(a) is generated by code generator
36 as:
##EQU5##
In the same manner a composite output .SIGMA.(b) is generated by by code
generator 38 as:
##EQU6##
The codes .SIGMA.(a) and .SIGMA.(b) are orthogonally multiplexed and
transmitted from the antenna 44 as two overlapped 8 bit noise codes. In
general, the total noise code length per channel can be any length up to
the number of channels n in the TDM bit stream.
Considering now the receiver, it is shown in FIG. 5 and includes an antenna
62 coupled to a receiver 64 which in turn is coupled to the demultiplexer
66 in the same manner as shown in FIG. 2. The output of the demultiplexer
comprises the noise codes .SIGMA.(a) and .SIGMA.(b) which are fed to two
matched filter detectors 68 and 70. The details of the matched filters 68
and 70 are shown in FIG. 6 and are identical to the corresponding code
generators 36 and 38 of FIG. 4 with the exception that the sequence of the
phase shifters 48, 49, 50, 51 and 56, 57, 58 and 59 are reversed and
operate, as will be shown below, to provide outputs of .SIGMA..phi..sub.a
(.tau.)=1 . . . 0.sup.4 0.sup.2 0.sup.4 . . . 1 and .SIGMA..phi..sub.b
(.tau.)=0 . . . 0.sup.4 1.sup.2 0.sup.4 . . . 0. These codes are applied
to a linear adder 72 which will provide two lobeless output pulses
.SIGMA..phi..sub.T (.tau.)=0.sup.8 . 0.sup.8 . . . which have an amplitude
eight times greater than the input bits applied to the code generators 36
and 38 of FIG. 3.
The manner in which pulse compression and the autocorrelation functions
.SIGMA..phi..sub.a (.tau.) and .SIGMA..phi..sub.b (.tau.) are generated is
shown below. The matched filter detector 68 develops a digital
autocorrelation function sequence .SIGMA..phi..sub.a (.tau.) in the
following manner:
##EQU7##
In a similar manner, the autocorrelation function .SIGMA..phi..sub.b
(.tau.) is developed in matched filter detector 70 as:
##EQU8##
When .SIGMA..phi..sub.a (.tau.) and .SIGMA..phi..sub.b (.tau.) are added
together, there is provided the signal .SIGMA..phi..sub.T (.tau.) which is
illustrated below as:
##EQU9##
The two major factors to note in the output .SIGMA..phi..sub.T (.tau.) are
that the compressed information bits (0.sup.8) for the two selected
channels are totally non-interfering and the received signal voltage is
eight times greater than the uncoded TDM system. This amplification factor
of eight is simply the time bandwidth product or equivalently twice the
number of noise code bits included in each mate code.
In the foregoing example, since four code bits were used to implement code
generators 36 and 38 (FIG. 4), the resulting gain achieved was eight. The
signal to noise power ratio (P/N) or the signal to jammer power (P/J)
ratio in a hostile environment is thus increased by the time-bandwidth
product n. The improvement reflected in the output (P/N) or (P/J) ratio is
readily demonstrated. The signal voltage is coherently summed in the
matched filter so that an input voltage V becomes nV at the output. The
input noise voltage or jammer voltage interference, however, is totally
uncorrelated at the various tap points being summed and therefore
increases as a root-mean-square summation. An input interference voltage
.sqroot.N then becomes .sqroot.nN at the output of the matched filter and
the resultant output signal to interference voltage ratio is then
.sqroot.nV/.sqroot.N. The output signal to interference power ratio is
simply the square of the output signal to interference voltage ratio or
nV.sup.2 /N=nP/N. Thus if 100 separate users were accessing the system,
then the interference power of a jammer would be reduced by 100/1 or 20
db. Simultaneously, the output signal to noise power ratio would be
enhanced by 20 db over a TDM system using no coding and the same peak
power.
Having thus shown and described what is at present considered to be the
preferred embodiments of the invention, it is noted that the same has been
made by way of illustration and not limitation. Accordingly, all
modifications, alterations and substitutions may be made without departing
from the spirit and scope of the invention as set forth in the appended
claims.
* * * * *
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