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Description  |
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TECHNICAL FIELD
The present invention relates to communications systems and, more
particularly, to the use of a spread-spectrum signal for identifying an
associated information signal and classifying the information signal into
one of a plurality of communication modes.
BACKGROUND OF THE INVENTION
Communications networks propagate information signals through a complex
array of apparatus. Such information signals include voice and data, with
the latter originating from a virtually limitless number of information
sources, such as facsimile, text, video, etc. The communications media
used in such networks can be homogeneous or diversified and presently
include wire, optical fiber, radio, satellite and submarine cable.
The different types of information signals conducted through the network
combined with the myriad of dissimilar types of network apparatus give
rise to situations in which system performance is impaired. Such
situations, in the main, arise in the context of the transmission of data
as opposed to voice signals and, in particular, to the transmission of
high-speed data signals. For example, the telecommunications networks
include echo suppressors and cancellers necessary for long-haul voice
communications which are incompatible with certain types of data
communications. Or, for example, the network includes certain digital
coding and decoding devices (codecs) which do not operate harmoniously
with data transmission above a certain rate. Or, in still another example,
digital networks include apparatus which rearranges incoming groups of
digital channels carrying voice and data information into different
outgoing groups and such apparatus requires complex signal conditioning
applied to the digital channels conveying data when such channels
constitute more than a predetermined percentage of the incoming digital
channels.
One solution to the problem of incompatibility between certain information
signals and specific types of communications apparatus is to route the
troublesome information signals through segregated networks, also known as
private-line networks, which are especially reserved and "conditioned" for
such signals. Conditioning is a term which denotes that a communication
facility has been engineered to assure no more than some preselected
amount of signal impairment. While such segregation provides a
satisfactory solution, the cost of such networks, especially the cost of
conditioning, can exceed the objectives of certain system applications.
Another solution for certain forms of the referenced incompatibility
problem is to transmit a tone to identify data signals. For example, a
2100 Hertz tone, as presently defined in the CCITT V.25 standard, is
presently transmitted to identify a data signal and, depending on the
phase characteristic of this tone, to disable echo suppressors or to
disable both echo suppressors and echo cancellers. While this technique
also works satisfactorily for certain system applications, the tone must
be transmitted for a minimum time period so as to be able to distinguish
between the tone and a naturally occurring speech harmonic. This minimum
time interval makes the use of tones incompatible for use with fast
start-up modem procedures. In addition, use of the tone as an information
signal identifier with many present "standard" transmission schemes
requires a revision of such schemes to accommodate the transmission of a
tone in an already occupied time span. Such revision is difficult, if not
impossible, to obtain.
In light of the foregoing, it can be seen that the problem of
incompatibility between certain types of information signals and network
apparatus has not been optimally solved and a variety of solutions have
been developed which are not acceptable for all system applications. With
the burgeoning growth of data carried by communications networks, it would
be extremely desirable if a universally applicable technique could be
devised which could improve the present situation.
SUMMARY OF THE INVENTION
In accordance with the present invention, a spread-spectrum signature
signal is used to identify an information signal. Identification may
include distinguishing between voice and data signals as well as
distinguishing between different kinds of data signals. This
identification can then be used to assign the identified information
signal to one of a plurality of communication modes. Such an assignment
could produce many effects. For example, an assignment could be used to
route an information signal through the most compatible communication
facility or equipment available, or to selectively bypass or disable
equipment as a function of the assigned mode, or to control the operating
characteristics of such equipment, etc.
The signature signal is superimposed over the associated information signal
spectrum. Advantageously, such superposition does not alter the time
scheme of the information signal. Therefore, a spreadspectrum signature
signal can be added to any "standard" transmission scheme without
modification of the latter and without noticeable degradation in
performance.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram illustrating the principle of the present
invention;
FIG. 2 is a block diagram illustrating several possible applications of the
present invention;
FIG. 3 is a block diagram of a baseband communications system utilizing the
present invention;
FIGS. 4 and 5 are graphical depictions of illustrative waveforms useful for
understanding the operation of the present invention; and
FIG. 6 is a block diagram of a passband communications system utilizing the
present invention.
DETAILED DESCRIPTION
The present invention can best be understood with reference to the
illustrative communications system in FIG. 1. As shown, the telephone
network 100 provides a bidirectional communications path between
information signal transceiver 101 (transmitter/receiver) at a first
location 102 and information signal transceiver 103 at a second location
104. The exact structure of transceivers 101 and 103 can take any of a
variety of well-known forms and in this regard it should be noted that the
present invention is applicable to any type of information signal, i.e.,
voice or data, and any form of such a signal, i.e., analog or digital.
Moreover, there is no restriction on any particular modulation format or
on whether the signal communications system is baseband or passband,
bidirectional or unidirectional.
The particular communications facility (not shown) within network 100
through which the information signals between transceivers 101 and 103 are
routed is typically selected by the network, and a large number of
different routes is possible. Furthermore, it will be assumed that some of
the possible routes are not optimal for the transmission of the particular
information signals operated upon by transceivers 101 and 103. In
accordance with the invention, the transmission impairments which would
likely result from the selection of such non-optimal routing for these
particular information signals is avoided by transmitting a
spread-spectrum signature signal along with each information signal to
identify the same. An identification, at a minimum, encompasses
distinguishing between voice and data information signals and can also
include distinguishing between data signals based on one or more
preselected characteristics, e.g., data rate and/or full duplex vs. half
duplex, etc.
At a receiver, the transmitted spread-spectrum signature signal is
recovered and can be used to assign the associated information signal to
one of a plurality of communication modes. Such an assignment can have
many effects and some illustrative effects will be discussed hereinbelow.
At this point, however, it can be said that the assignment of the
information signal, using the associated spread-spectrum signal, makes a
communications decision about the information signal.
Each of a plurality of signature transceivers 105-108 in FIG. 1 has the
capability to transmit and receive spread-spectrum signature signals. At
location 102, signature transceiver 105 generates a spread-spectrum
signature signal associated with the information signal transmitted by
information signal transceiver 101. This spread-spectrum signal is
superimposed onto the transmitted information signal from location 102 via
an adder in node 109. The signature signal from transceiver 105 is
received by signature transceiver 107 within network 100 and/or signature
transceiver 106 at location 104, depending on the application, wherein it
is used to assign the associated information signal to one of a plurality
of communication modes. In similar fashion, signature transceiver 106
generates a spread-spectrum signal associated with the information signal
generated by information signal transceiver 103 and superimposed thereon
using an adder in node 112. This signature signal is received by signature
transceiver 108 within network 100 and/or signature transceiver 105 at
remote location 102, depending on the application, and is used to assign
the information signal from location 104 to one of a plurality of
communication modes. The signature transceiver at each of locations 102
and 104 can also receive spread-spectrum signals and provide an assignment
of an associated information signal to one of a plurality of communication
modes in response to a spread-spectrum signature signal generated by
signature transceiver 107 or 108 in network 100 and coupled through node
110 or 111, respectively. Accordingly, a spread-spectrium signal can be
viewed as providing one or more stimuli to and from network 100 and to and
from each of locations 102 and 104.
Finally, it should be noted that while a spread-spectrum signature signal
from any of signature transceivers 105-108 is added to an associated
information signal by an adder in nodes 109, 112, 110 and 111,
respectively, a signal received by these signature transceivers does not
pass through the adder in the respective node. In this regard, note that
signal lead 113 in node 110 is directly coupled through lead 114 to
signature transceiver 107 and does not pass through the adder in node 110.
Refer now to FIG. 2 which illustrates, in more detail, six exemplary
outcomes from the assignment of an information signal into a particular
communications mode. Lead 209 represents the communications facility
directly connected to node 112. In the first arrangement, the "despread"
signature signal at the output of transceiver 107 is used to control
switch 201 which couples the information signal from transceiver 101
through ADPCM codec 202 or, alternatively, around it. Such bypassing is
desirable for data signals having data rates of 12 kilobits/sec or more.
In the second arrangement, the signature signal from transceiver 107 is
used to selectively disable echo canceller 203 and/or echo suppressor 204.
This selective disablement can be used in conjunction with modems which
incorporate a fast start-up procedure or conform to some existing
communications standard and, therefore, cannot or do not incorporate the
tone defined in the CCITT V.25 standard. These echo control devices are
essential for long-distance voice connections as they remove echoes
generated in the network that are objectionable to telephone customers.
However, these devices create problems, for example, for transmission
applications using modems which internally provide such cancellation.
Another application is to use the signature signal for network traffic
studies which are routinely performed for a variety of purposes, such as
load balancing and predicting the demand for different communications
services and facilities. This application is represented by the coupling
of the signature signal to classification device 205 which would identify
the associated information signal and assign it to one of a number of
preselected categories. Device 205 would also keep a tally of the number
of information signals assigned to each category. In still another
application, the signature signal from transceiver 107 would be used to
distribute data signals to n different sets of Digital Access
Cross-connect System (DACS) equipment 207-1 through 207-n, which receive a
plurality of digital facilities and rearrange these digital channels among
a plurality of output digital facilities. Such cross-connect equipment
utilizes signalling techniques which, while not detrimental to voice
communications, can impair transmission performance if the percentage of
data channels in the mix of incoming voice and data channels exceeds some
prescribed limit and signal conditioning is not provided. Accordingly, the
signature signal could be used by a DACS controller, represented by switch
206, to distribute data signals across different DACS so that the
prescribed limit was not exceeded and the need for signal conditioning is
avoided. In still another scenario, maintenance equipment 208 which
monitors network performance could generate an inhibit transmission signal
whenever a serious communications fault was detected. This fault signal
would be fed to signature transceivers, such as 107 and 109, and coupled
back to the information signal source locations. At these locations,
signature transceivers, such as 105 and 106, would extract the inhibit
transmission signal and couple the same to their associated information
signal transceivers which, in response thereto, would inhibit transmission
until the fault was corrected. Finally, in the last arrangement,
illustrated by line 210, the signature signal would be coupled between
remote locations 102 and 104 where it would be used to identify the type
of information signal transceiver being used. One possible use for this
equipment identification is in the voiceband modem arena. In this
application, the information signal transceivers 101 and 103 would be
modems and the identification of each modem to the other via signature
transceivers 105 and 106 could permit these modems to alter their rate of
operation to some other data rate when such modems have this capability.
Another application for equipment identification is in communications
services, such as DDS (Digital Data Services) where the information signal
transceivers 101 and 103 would be Digital Communication Units (DCUs). In
the DDS application, the signature signal can be coupled between a remote
location and the network, e.g., between signature transceivers 105 and
107, to indicate a request to change the current data rate. This request
allows the network to more efficiently utilize its communication facilites
with the potential for reduced cost to the customer.
FIGS. 1 and 2, described hereinabove, reflect a somewhat simplified
representation of a communications system which incorporates the present
invention. A more detailed description of an embodiment of the present
invention for baseband communications is shown in FIG. 3. For purposes of
consistency with the prior drawing figures, components which provide the
function previously described in FIGS. 1 and 2 will bear the same
reference designations. The baseband communications system 300, shown in
FIG. 3, incorporates a spread-spectrum signature signal along with its
transmission of an associated information signal between remote locations
102 and 104 via network 100. As previously discussed, the spread-spectrum
signature signal conveys an identification of the associated information
signal. Both the spread-spectrum signature signal and the associated
information signal convey "information." To distinguish between the
content of the identification conveyed by the signature signal and that of
the associated information signal, "ancillary" information will be used
when referring to the content of the identification conveyed by the
spread-spectrum signature signal, and the "main" information will be used
when referring to the content of the associated information signal.
The spread-spectrum signature signal is formed at the transmitter 301 of
signature transceiver 105 by first generating, via ancillary information
source 302, a baseband signal that carries this ancillary information
which is ultimately conveyed by the spread-spectrum signature signal.
Source 302 supplies the ancillary information at a symbol rate of 1/T. In
the illustrative embodiment of FIG. 3, this baseband signal is a binary
signal, designated as d(t), whose amplitude fluctuates between some
preselected values of +A and -A. This signal is then multiplied, in the
time domain, by a pseudorandom sequence which "spreads" the spectrum of
the baseband signal over a significantly larger frequency band. One such
sequence, designated as p(t) and having a "chip" rate of f.sub.s, is
provided by pseudorandom sequence generator 303 and multiplication of this
sequence by d(t) is provided by multiplier 304. The resulting
spread-spectrum signal at the output of multiplier 304 is then added to
the associated information signal using adder 305 in node 109. This
associated information signal conveys main information and is generated by
information signal transceiver 101. The output of adder 305 is transmitted
through the network 100.
Within the receiver 320 of signature transceiver 106, the arriving
associated information signal is processed in conventional fashion within
information signal transceiver 103 to extract the main information. The
phrase "in conventional fashion" means using well-known techniques and is
also meant to emphasize one of the key advantages of the use of a
spread-spectrum signature signal in accordance with the invention. This
advantage is that such use does not alter the processing applied to
generate or receive the associated information signal. Accordingly,
information signal transceiver 103 incorporates whatever steps would be
performed in the absence of the use of a spread-spectrum signature signal.
To recover the ancillary information carried by the signature signal, the
incoming signal, i.e., the associated information signal and superimposed
spread-spectrum signature signal, is routed through a "despreading"
operation which boosts the spectrum of the spread-sprectrum signature
signal above that of the associated information signal. This despreading
operation is well-known and is provided in FIG. 3 by random sequence
generator 307, multiplier 308 and integrate and dump apparatus 309.
Generator 307 is synchronized to its counterpart 303 in signature
transceiver 105 using well-known techniques to produce the sequence p(t)
in the receiver. The integrate and dump apparatus accumulates the output
of multiplier 308 until some prescribed level is reached and then couples
its output to decision device 310. Device 310 recovers the ancillary
information by comparing its input against one or more preselected
threshold values. For example, in the case of the illustrative binary
signal, it is preferable to use threshold values of .+-.k, where k is an
appropriately chosen scalar quantity. This ancillary information which
identifies the associated information signal can then be used for any of
the numerous purposes herein described. Generator 307, multiplier 308 and
dump apparatus 309, collectively referred to as despreading apparatus 313,
can be replaced by a "matched" filter having an impulse response p(-t).
Use of such a filter does not require synchronization between signature
transceivers 105 and 106.
FIGS. 4 and 5 show waveforms which are useful for understanding the
operation of the present invention. Waveform 401 depicts the ancillary
information signal d(t) in the time domain which has an amplitude value of
+A during the illustrated bit period. During this period, the pseudorandom
sequence p(t) in this domain varies between amplitude values of plus or
minus 1. This sequence would be repeated in subsequent bit periods. The
duration, 1/f.sub.s, of the shortest pulse in the illustrated time domain
is called a "chip" and determines the amount of spreading in the frequency
domain. In FIG. 5, curves 501 and 502 show the frequency spectra D(f) and
D(f)*P(f) of the ancillary information signal d(t) and that of the spread
signal, d(t) multiplied by p(t), respectively. The asterisk (*) deontes a
mathematical convolution in the frequency domain between the spectrum D(f)
and the spectrum P(f) of p(t). Note the significantly lower energy level
of the spread signal and the fact that it extends over a significantly
larger frequency range than that of the ancillary information signal. Note
also that the amplitude level of the spread signal 502 is significantly
less than that of the main information signal 503. This low amplitude
level of the spread signal advantageously allows the associated
information signal receiver to operate without noticeable degradation.
As mentioned supra, the present invention can be implemented in either
baseband or passband communications systems. FIG. 6 shows one illustrative
passband communications system 600 which incorporates the present
invention. For this application, each of signature transceivers 105 and
106 comprises a transmitter 601 and receiver 610. Transmitter 601 and
receiver 610 each utilize components which are identical in structure and
function to those in the previous drawing figures and which bear the same
reference designation. In this regard, note that the transmitter 601 in
FIG. 6 is similar to transmitter 301 in FIG. 3 but for the addition of
carrier signal generator 602 which generates the sinusoidal carrier signal
cos (w.sub.c t). This carrier signal is modulated by the spread-spectrum
signature signal at the output of multiplier 304 via multiplier 603. The
modulated carrier signal is then added to the associated information
signal using adder 305 before being transmitted through network 100.
At remote location 104, the associated information signal is processed
using a conventional information signal transceiver for passband
communications. The signature signal can be recovered within the receiver
610 of signature transceiver 106 by first demodulating it using multiplier
308 and the carrier signal formed by carrier signal generator 605. Second
order harmonics in this demodulated signal are removed by low-pass filter
606 whose output is then despread using matched filter 607. Filter 607
provides the same function as multiplier 308, random sequence generator
307 and integrate and dump apparatus 308 in FIG. 3; namely, boosting the
signal energy of the ancillary information signal to a level significantly
greater than that of the associated information signal. Once this is done,
the ancillary information can be recovered using decision device 310.
It should, of course, be noted that while the present invention has been
described in terms of several illustrative embodiments, other arrangements
will be apparent to those of ordinary skill in the art. For example, while
the embodiments of the present invention have been described in reference
to discrete functional elements, the function of one or more of these
elements can be provided by one or more appropriately programmed
general-purpose processors, or special-purpose integrated circuits, or
digital signal processors, or an analog or hybrid counterpart of any of
these devices. In addition, while FIG. 6 shows the spreading of the output
of ancillary information signal source 302 prior to modulation, the order
of the spreading and modulation operations can be interchanged. This
interchanging may be desirable for certain applications or for
facilitating implementation of a particular modulation scheme for the
signature signal, such as differential phase shift keying. Interchanging
the order of spreading and modulation from that in FIG. 6 may require a
change in the position of matched filter 607 from that shown in FIG. 6 to
one preceding multiplier 308. Also, while the circuitry of the signature
transceivers in FIGS. 3 and 6 have been described with reference to remote
locations 102 and 104, identical circuitry can be used in the signature
transceivers within telephone network 100. Finally, while the embodiments
of FIGS. 3 and 6 disclose systems wherein information signals are
transmitted in two directions through a telephone network, the present
invention is not restricted to such networks or to a certain system
topology, e.g., a specific number of remote locations, or to bidirectional
communications between such locations. Indeed, the present invention may
be used within any unidirectional or bidirectional communication system,
and, in the latter case, the information rate of the transmitted
information and/or signature signals need not be the same in each
direction.
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