Modal propagation of information through a defined transmission medium

A method and system are provided for transmission of m information signal combinations over a known transmission medium at a high rate of speed without crosstalk between data transferred through the transmission medium. The technique employed is to convert each of the m information signal combinations to a unique line signal corresponding to one of n intrinsic orthogonal modes of the defined transmission medium through which the information is to be transmitted (wherein m>n). The encoded line signals, propagated via the orthogonal modes, are received and decoded at a remote location along the transmission medium to restore the original information data. The problem of assigning m unique line signals to each of the m possible information signal combinations is addressed by amplitude modulation of the transmission medium's n intrinsic orthogonal modes.

TECHNICAL FIELD 
The present invention relates in general to data communications and, more 
particularly, to the transmission of analog or digital data through a 
transmission medium having known characteristics at a high rate of speed 
without crosstalk between information signals transferred in parallel 
through the transmission medium. 
BACKGROUND ART 
A well documented and inherent difficulty in the parallel transmission of 
analog/digital information over a transmission medium, such as a plurality 
of parallel transmission lines, is signal crosstalk resulting from 
capacitive and inductive coupling within the transmission medium. The 
error rate due to crosstalk signal degradation typically increases with 
increasing speed of data transmission. This is because higher transmission 
speeds involve a greater number of possible values which make it 
increasingly difficult to distinguish between signals in the presence of 
an electrical impairment. The open literature contains a vast number of 
references which in general attempt to address the problem of signal noise 
on transmission lines, and in particular, to minimize crosstalk between 
information transmitted in parallel through a transmission medium having 
defined characteristics. 
One interesting approach is discussed by T. H. Nguyen and T. R. Scott in an 
IBM Technical Disclosure Bulletin entitled "Propagation Over Multiple 
Parallel Transmission Lines Via Modes,+ Vol. 32, No. 11, April, 1990. As 
discussed therein, it is known that electromagnetic waves propagating over 
two conductors and a ground line have two orthogonal modes referred to as 
the "even mode" and the "odd mode." In general, n conductors and a ground 
line have n such orthogonal modes. Because of its orthogonal properties, 
each mode propagates independent of all other modes. The resolution of 
line voltages into orthogonal modes is analogous to the concept of 
resolving a pulse waveform into its orthogonal Fourier components. Just as 
the Fourier components can be studied separately because they are 
orthogonal, and hence independent of each other, so the intrinsic 
orthogonal modes of a plurality of transmission lines can be studied 
separately since they are also independent of each other. 
The above-referenced IBM Technical Disclosure Bulletin describes the 
simplicity of orthogonal mode determination for those transmission lines 
having 2, 4, 8, . . . , 2.sup.n conductors symmetrically arranged on a 
cylindrical surface with respect to a coaxial ground conductor. The 
calculation of orthogonal modes for certain such transmission line 
configurations is presented. Although the mathematical groundwork for 
transmission of intrinsic orthogonal modes over a plurality of parallel 
transmission lines is discussed in this Technical Disclosure Bulletin, a 
practical implementation of the concept is lacking. The difficultly 
encountered is that the propagation of binary signals (for example) via 
modes is often not possible with only digital (i.e., on/off) signals since 
the number of modes is equal to the number of conductors (which can be 
doubled by using plus/minus designations), but the number of binary 
combinations is exponential, i.e., two raised to the number of conductors 
n. Therefore, a need exists in the art for a practical approach (such as 
described herein) to allow the encoding of, for example, each possible 
binary signal combination as a unique orthogonal mode signal for 
transmission through a transmission medium having defined characteristics. 
DISCLOSURE OF INVENTION 
Briefly summarized, the present invention comprises in one aspect a method 
for transferring m information signal combinations through a defined 
transmission medium having n orthogonal modes, wherein m&gt;n. The method 
includes the steps of: encoding the m information signal combinations for 
propagation through the transmission medium as line signals, each of which 
corresponds to one of the transmission medium's orthogonal modes (this 
encoding step includes deriving line signals for at least some of the m 
information signal combinations by amplitude modulating at least some of 
the transmission medium's n orthogonal modes); propagating the line 
signals through the transmission medium; and receiving the line signals 
propagated through the transmission medium and decoding the received line 
signals to restore the encoded m information signal combinations. 
Propagation of an information signal as a line signal pursuant to the 
method steps outlined restricts crosstalk within the transmission medium 
during the propagating step. This is because signals are transferred only 
in the orthogonal modes of the transmission medium. 
Also disclosed is a method for encoding one of m information signal 
combinations for transfer over a transmission medium having known 
characteristics and n orthogonal modes, wherein m&gt;n. The information 
encoding method includes the steps of: assigning a unique modal signal to 
each of the m information signal combinations such that each unique modal 
signal is identified with one of the transmission medium's n orthogonal 
modes; converting the one of m information signal combinations into its 
assigned unique modal signal; and driving the transmission medium in the 
orthogonal mode corresponding to the assigned unique modal signal. Thus, 
when propagating the information signal through the transmission medium as 
a signal corresponding to one of the transmission medium's orthogonal 
modes, crosstalk within the transmission medium is restricted. A decoding 
method, comprising the inverse to the outlined encoding technique, is also 
described and claimed. 
In another aspect of the present invention, a system is disclosed for 
transferring one of m information signal combinations through a 
transmission medium having known characteristics and n orthogonal modes, 
wherein m&gt;n. The system includes an encoder for encoding the selected one 
of m information signal combinations as a unique line signal corresponding 
to one of the transmission medium's orthogonal modes for propagation 
through the transmission medium in the corresponding orthogonal mode. The 
encoder includes a transformation mechanism for converting the selected 
information signal into a unique preassigned line signal. (At least some 
of the unique line signals are defined by amplitude modulating at least 
some of the transmission medium's n orthogonal modes.) Drivers then 
propagate the unique line signal through the transmission medium in the 
corresponding orthogonal mode. A receiver receives the propagated line 
signal and decodes the line signal to restore the one of m information 
signal combinations. Thus, propagation of the information signal as a line 
signal corresponding to one of the transmission medium's n orthogonal 
modes restricts crosstalk within the transmission medium during signal 
propagation through the transmission medium. Unique to the invention is 
the concept for defining a line signal for each of m information signals 
to be propagated over the transmission medium wherein the medium has n 
orthogonal modes and wherein m&gt;n. 
To summarize, presented herein is a novel and practical approach to 
implementation of orthogonal mode signal transmission of m information 
signals over a predefined transmission medium having n orthogonal modes, 
wherein m&gt;n. Specific encoding and decoding circuits and techniques are 
presented. The important benefit to the practical approach described is 
that orthogonal modes propagate independently of each other with no 
crosstalk. Therefore, accurate propagation of information over longer 
transmission lines is feasible and practical, along with an increase in 
data transmission rates without increased error. Further, parity checking 
is less important for propagation of signals via modes since they do not 
interact. In fact, parity information is intrinsic to the transmitted 
signals.

BEST MODE FOR CARRYING OUT THE INVENTION 
Disclosed herein are encoding/decoding methods and systems for converting 
each of m possible information signal combinations into a unique line 
signal identified with one of n orthogonal modes (including .+-. 
polarities) of a transmission medium having known characteristics through 
which the information signals are to be transmitted, wherein m&gt;n. The 
encoded line signals (or line voltages), propagated via the orthogonal 
modes, are received and decoded at a remote location along the 
transmission medium to restore the original data, i.e., information 
signals. Again, the benefit to such a transmission approach is that 
crosstalk between signals transmitted in parallel is eliminated. The 
inherent problem associated with modal transmission of information, along 
with the inventive solution presented herein, can be better explained with 
reference to a specific example. 
FIG. 1 depicts a transmission medium 10 having four parallel transmission 
lines, labeled A, B, C & D, arranged substantially symmetrically about a 
coaxial ground line 12. Selection of transmission medium 10 to have a 
symmetrical configuration such as that depicted simplifies decomposition 
of voltages into orthogonal modes. However, symmetry is not essential to 
the present invention. Those skilled in the art will be able to determine 
orthogonal modes for almost any transmission medium (including a single 
strand optical fiber) through which information is to be transferred and 
crosstalk between lines minimized, but the invention assumes that the 
characteristics of the medium are known. 
With four parallel transmission lines, sixteen possible parallel signal 
combinations of binary information (2.sup.4 =16) can be transmitted. Eight 
of the sixteen combinations can be readily converted to mode signals 
having symmetrical plus and minus polarities, as shown by the following 
matrix transformation. 
##EQU1## 
As can be seen from matrix [1], eight bolded columns (i.e., signal 
combinations) are converted to orthogonal modes with amplitudes of -4, 0, 
and +4. The problem addressed by the present invention is how to also 
place the other eight columns into unique orthogonal modes for transfer 
over the transmission medium. (In matrix [1] those bolded columns having 
amplitudes of -4, 0, and/or +4 are in orthogonal modes since the parallel 
transmission of those signal combinations will generate minimal crosstalk 
(i.e., capacitive and inductive coupling) within the transmission medium.) 
The preferred approach described and claimed herein is to change the 
amplitude of the intrinsic orthogonal mode signals (i.e., the bolded 
columns) to define unique modal signals for each of the remaining 
information signals. Again, in this example the goal is to convert the 
remaining eight non-modal combinations (see matrix [1]) into a unique 
modal signal identified with an orthogonal mode of the transmission 
medium. Pursuant to the present invention, this is accomplished by 
changing the amplitudes of these intrinsic orthogonal modes (i.e., the 
bolded columns) by a constant factor. For example, .times.2 amplitude 
modulation can be imposed on the above-noted intrinsic orthogonal modes to 
produce unique modal signals as set forth in matrix [2]. The eight 
intrinsic orthogonal modes as multiplied by two are then substituted for 
the eight non-modal combinations. 
##EQU2## 
Thus, in the example presented all sixteen possible binary combinations 
for parallel transmission over the four transmission lines depicted in 
FIG. 1 are resolved into unique modal signals each of which is identified 
with an orthogonal mode. Again, this result is made possible by selective 
use of amplitude modulation (e.g., as shown in matrix [2]). 
To summarize, in a set of n transmission lines and a ground line, there 
exists 2.times.n modes of propagation (i.e., assuming that .+-. symmetry 
is employed). Each mode is a particular combination of all the line 
voltages to be transmitted in parallel through the transmission medium. 
Even though the transmission of binary information via orthogonal modes 
pursuant to the present invention requires additional hardware for 
encoding and decoding, the real benefit is that signals in these 
orthogonal modes propagate completely independently of each other, i.e., 
without crosstalk. Therefore, signal noise is reduced and propagation over 
longer distances (than otherwise feasible) becomes practical, along with 
the possibility of increasing transmission rates without increasing error. 
Specific embodiments of encoder/decoder circuitry pursuant to the present 
invention are next discussed with reference to FIGS. 2-7, as well as 
related encoding/decoding methods. In these figures the same reference 
numbers/characters are used throughout multiple figures to designate the 
same or similar components. 
In FIG. 2 an encoding circuit, denoted 20, is depicted for converting four 
signal voltages Ein1, Ein2, Ein3, Ein4 (again assuming binary signals) to 
appropriate line voltages EA, EB, EC, ED for transmission on four parallel 
conductors 22 which are assumed to be arranged as shown in FIG. 1. Circuit 
20 includes a selector (front) circuit 24 which selects between (i.e., 
enables one of) a first signal voltage converter 26 and a second signal 
voltage converter 28. Each converter 26 & 28 outputs mode signals (see 
matrix [2]) to modal line drivers 30 which are connected to transmission 
lines 22. Drivers 30 present the appropriate line voltages EA, EB, EC, ED 
to lines 22 for transmission of the information in orthogonal modes. 
One embodiment of selector (front) 24 is depicted in FIG. 3. In this 
embodiment, the four bits of binary information Ein1, Ein2, Ein3 & Ein4, 
are fed in parallel to each of eight AND circuits 40, which are 
respectively configured with inverters as shown. The outputs from AND 
circuits 40 are fed in parallel to a NOR circuit 42, the output of which 
comprises the control signal fed to signal voltage converter 26 and signal 
voltage converter 28 (FIG. 2). By way of example, if a "0" (or "-1") is 
output from selector 24 then modal converter 26 is assumed to be selected 
(i.e., enabled) through a gate control (not shown). Alternatively, if a 
"1" is output, then the amplitude-modulated (AM) converter 28 is driven. 
Converters 26 & 28 transform binary signal voltages Ein1, Ein2, Ein3, Ein4 
to intermediate modal signals V.sub.1, V.sub.2, V.sub.3, V.sub.4 and 
V.sub.5, V.sub.6, V.sub.7, V.sub.8, respectively (see matrix [2]). These 
modal signals are attained by matrix multiplication of the incoming signal 
voltages with the Eigenvectors matrix employed above and separately set 
out below as matrix [3]. Each row/column of the matrix represents a coding 
for a single mode. As shown in FIG. 4 (which depicts one embodiment of 
converter 26), for all four modes (i.e., EVEN, ODD1, ODD2, ODD3) of the 
four conductor transmission medium of FIG. 1, the gains of the matrix of 
amplifiers must be configured as set forth in matrix [3]. 
##EQU3## 
The upper row 50 of converter 26 is configured to generate an even modal 
signal (EVEN), while the next three rows 52, 54, & 56 respectively 
generate one of three odd signal modes (ODD1, ODD2, ODD3). (As already 
noted, for a set of four symmetrical transmission lines and a center 
ground line, there exist four intrinsic orthogonal modes for signal 
propagation through the medium (each of which can also have a plus or 
minus polarity).) Each amplifier in rows 50, 52, 54 & 56 of converter 26 
comprises a unity-gain, voltage-to-voltage amplifier. As shown, the 
amplifiers are arranged in a four-by-four matrix (which, as noted, 
corresponds to matrix [3]). The amplifiers in each column of the matrix 
have their outputs connected in series so that there are four distinct 
sets with four amplifiers in each set and the output of each set comprises 
one output (V.sub.1, V.sub.2, V.sub.3, V.sub.4) of converter 26. In 
converter 26, the gain of each amplifier may be either +1 or -1, therefore 
the four inputs (which correspond to bit positions: units, twos, fours, & 
eights) can be coded into four outputs which correspond to (i.e., 
identify) the intrinsic orthogonal modes of the set of transmission lines. 
Signal voltage converter 28 (for the amplitude-modulated modal (AM) 
signals) (FIG. 2) is essentially identical to the four-by-four matrix of 
voltage-to-voltage amplifiers depicted in FIG. 4. The principle difference 
would be in the use of 100 percent gain voltage-to-voltage amplifiers so 
that the modal signals V.sub.5, V.sub.6, V.sub.7, V.sub.8 output from 
converter 28 (FIG. 2) are twice (2.times.) the value of the modal signals 
V.sub.1, V.sub.2, V.sub.3, V.sub.4 output from converter 26. 
Modal line drivers 30 receive an identified non-amplified or amplified mode 
signal from converter 26 or 28, respectively, and linearly combine the 
identified mode signal to drive the parallel transmission lines with line 
voltages EA, EB, EC, ED in the corresponding orthogonal mode of the 
transmission medium (i.e., EVEN, ODD1, ODD2, ODD3). More particularly, for 
a non-amplified mode signal, line voltages EA, EB, EC & ED are produced by 
implementing equations (1) within modal line drivers 30. 
EQU EA=1.multidot.(V.sub.1)+1.multidot.(V.sub.2)+1.multidot.(V.sub.3)+1.multido 
t.(V.sub.4) 
EQU EB=1.multidot.(V.sub.1)+1.multidot.(V.sub.2)-1.multidot.(V.sub.3)-1.multido 
t.(V.sub.4) 
EQU EC=1.multidot.(V.sub.1)-1.multidot.(V.sub.2)+1.multidot.(V.sub.3)-1.multido 
t.(V.sub.4) 
EQU ED=1.multidot.(V.sub.1)-1.multidot.(V.sub.2)-1.multidot.(V.sub.3)+1.multido 
t.(V.sub.4) (1) 
These linear equations could also be expressed in matrix form as matrix 
[4]: 
##EQU4## 
Wherein: 
##EQU5## 
Such that: 
##EQU6## 
Similarly, when amplitude-modulated (AM) mode signal are input to drivers 
30, equations (2) are implemented. 
EQU EA=1.multidot.(V.sub.5)+1.multidot.(V.sub.6)+1.multidot.(V.sub.7)+1.multido 
t.(V.sub.8) 
EQU EB=1.multidot.(V.sub.5)+1.multidot.(V.sub.6)-1.multidot.(V.sub.7)-1.multido 
t.(V.sub.8) 
EQU EC=1.multidot.(V.sub.5)-1.multidot.(V.sub.6)+1.multidot.(V.sub.7)-1.multido 
t.(V.sub.8) 
EQU ED=1.multidot.(V.sub.5)-1.multidot.(V.sub.6)-1.multidot.(V.sub.7)+1.multido 
t.(V.sub.8) (2) 
Equations (2) can also be expressed in matrix form as matrix [5]: 
##EQU7## 
Wherein: 
##EQU8## 
Such that: 
##EQU9## 
To summarize, the function of modal line drivers 30 is to ensure that 
transmission lines 22 are driven in orthogonal modes, with the desired 
orthogonal mode being indicated by the output of enabled converter 26 or 
enabled converter 28 as determined by selector (front) circuit 24 in 
response to the inputted binary signals Ein1, Ein2, Ein3, Ein4. At a 
receiving end of transmission lines 22, which are perfectly terminated for 
all modes of propagation, decoding circuitry is provided which is 
essentially a mirror image of the encoding circuitry presented in FIG. 2. 
One embodiment of a decoding circuit, labeled 70, pursuant to the present 
invention is depicted in FIG. 5. 
Decoding circuit 70 includes modal line receivers 72 which accept 
transmitted line voltages EA, EB, EC, ED and convert a received signal to 
a corresponding orthogonal mode signal. (This is essentially the inverse 
operation of that performed by modal line drivers 30 of FIG. 2.) These 
mode signals, which comprise linear combinations of the line voltages, are 
either non-amplified orthogonal modes of the transmission medium (e.g., 
+4, -4) or amplitude-modulated (AM) orthogonal modes (+8, -8) as described 
above. If non-amplified orthogonal modes have been received, the signals 
are fed to inputs O.sub.1, O.sub.2, O.sub.3, O.sub.4 of a first signal 
voltage reconverter 74, while if amplitude-modulated (AM) orthogonal modes 
are identified, the signals are passed to inputs O.sub.5, O.sub.6, 
O.sub.7, O.sub.8 of a second signal voltage reconverter 76. Reconverter 74 
or reconverter 76 is selectively enabled by a selector (end) circuit 78 
which has as inputs the four line voltages EA, EB, EC, ED on lines 22 of 
the transmission system. The outputs of reconverters 74 & 76 are connected 
as lines OUP1, OUP2, OUP3, OUP4. These lines contain the originally 
encoded signal voltage, which in this embodiment, comprises the identical 
binary signal input at encoder circuit 20 (FIG. 2) at the front end of the 
transmission system as signal voltages Ein1, Ein2, Ein3, Ein4. 
As noted, modal line receivers 72 implement the inverse operation of that 
performed by modal line drivers 30 of FIG. 2, except a divide by four 
logic 73 is interposed to identify the corresponding modal signals. The 
column outputs to the respective reconverters 74, 76 are defined by 
equations (3): 
EQU O.sub.1 
(O.sub.5)=+1.multidot.(EA)+1.multidot.(EB)+1.multidot.(EC)+1.multidot.(ED) 
EQU O.sub.2 
(O.sub.6)=+1.multidot.(EA)+1.multidot.(EB)-1.multidot.(EC)-1.multidot.(ED) 
EQU O.sub.3 
(O.sub.7)=+1.multidot.(EA)-1.multidot.(EB)+1.multidot.(EC)-1.multidot.(ED) 
EQU O.sub.4 
(O.sub.8)=+1.multidot.(EA)-1.multidot.(EB)-1.multidot.(EC)+1.multidot.(ED) 
(3) 
The identified mode signals are then input to the appropriate reconverter 
74,76. One embodiment of a signal voltage reconverter 74 for the 
non-amplified modal signals is depicted in FIG. 6. Again, this embodiment 
is essentially the mirror image of converter 26 of FIG. 4. 
In FIG. 6, the reconverter comprises a four-by-four array of unity-gain, 
current-to-voltage amplifiers 80. A network of resistors 82 terminates all 
four modes properly so that there are no reflections. The values of these 
resistors can be derived by one of ordinary skill in the art directly from 
the impedance matrix for the set of transmission lines. The voltage output 
from each row of amplifiers OUP1, OUP2, OUP3, OUP4, respectively, 
corresponds directly to the original binary information or signal voltage 
(Ein1, Ein2, Ein3, Ein4) of FIG. 2, only delayed by the length of time 
equal to the propagation delay through the set of transmission lines. 
One embodiment of selector (end) circuit 78 is depicted in FIG. 7. This 
circuit essentially comprises a voltage comparator and determines whether 
the received orthogonal mode signals (EA, EB, EC, ED) are plus/minus four 
(corresponding to a non-amplified orthogonal mode signal) or plus/minus 
eight (corresponding to an amplitude-modulated (AM) orthogonal mode 
signal). If plus/minus 4, then reconverter 74 is enabled, while if 
plus/minus eight, reconverter 76 is enabled. This is accomplished by four 
parallel connected amplifiers 90 whose outputs are fed to a AND circuit 
92. Other possible circuit configurations will be readily apparent to one 
of ordinary skill in the art. 
To summarize, presented herein is a novel and practical approach to 
implementation of orthogonal mode signal transmission of m information 
signals over a predefined transmission medium having n orthogonal modes, 
where m&gt;n. Specific encoding and decoding circuits and techniques are 
presented. The important benefit to the practical approach described is 
that orthogonal modes propagate independently of each other with no 
crosstalk. Therefore, accurate propagation of information over longer 
transmission lines is feasible and practical, as well as increasing data 
transmission rate without increasing error. 
While the invention has been described in detail herein in accordance with 
certain preferred embodiments thereof, many modifications and changes 
therein may be affected by those skilled in the art. Accordingly, it is 
intended by the appended claims to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.