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Claims  |
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What is claimed and desired to be secured by United States Letters Patent
is:
1. A telephone communications system, the system comprising:
(a) a base unit comprising:
(i) a first transmitter subsystem,
(ii) a first receiver subsystem, and
(iii) means for connecting said first transmitter subsystem and said first
receiver subsystem to a telephone line;
(b) an extension unit comprising:
(i) a second transmitter subsystem, and
(ii) a second receiver subsystem; each of said first and second transmitter
subsystems comprising:
(a) means or converting analog telephone communications signals to digital
data signals;
(b) means for generating a transmission carrier controlled by at least one
multiple access means, said at least one multiple access means being
selected form the group consisting of:
(i) time division multiple access multiplexing means for controlling the
times at which the digital data signals are transmitted; and
(ii) frequency division multiple access means for changing said
transmission carrier's frequency;
(c) means for generating a pseudonoise code;
(d) means for modulating said transmission carrier by said digital data
signals and said pseudonoise code to produce a direct spread code division
multiple access signal; and
(e) means for coupling said direct spread code division multiple access
signal to a communications medium;
and each of said first and second receiver subsystems comprising:
(a) means for receiving said direct spread code division multiple access
signal from said communications medium;
(b) means for recovering said modulated transmission carrier from said
received direct spread code division multiple access signal; and
(c) means for demodulating said recovered modulated transmission carrier to
produce digital data corresponding to said analog telephone communications
signals.
2. A telephone communications system as in claim 1 further comprising off
hook circuitry for detecting off hook status in said extension unit and
placing said telephone line connecting means of said base unit into an off
hook condition.
3. A telephone communications system as in claim 1, said base unit further
comprising ring detect circuitry for detecting ring signals on said
telephone line and transmitting a ring command to said extension unit; and
said extension unit further comprising ring generator circuitry for
generating a ring signal in said extension unit upon receipt of said ring
command.
4. A telephone communications system as in claim 1, wherein said means for
coupling said direct spread code division multiple access signal to a
communications medium comprises an RF antenna.
5. A telephone communications system as in claim 1, wherein said means for
coupling said direct spread code division multiple access signal to a
communications medium comprises a connection to power lines of a building.
6. A telephone communications system as in claim 1, further comprising one
or more additional extension units.
7. A telephone communications system as in claim 1, wherein said telephone
line connecting means of said base unit connects a plurality of telephone
lines to said transmitter subsystem and said receiver subsystem.
8. A telephone communications base unit comprising:
(a) a transmitter subsystem,
(b) a receiver subsystem, and
(c) means for connecting said transmitter subsystem and said receiver
subsystem to a telephone line;
said transmitter subsystem comprising:
(a) means for converting first analog telephone communications signals to
first digital data signals;
(b) means for generating a first transmission carrier controlled by at
least one multiple access means, said at least one multiple access means
being selected from the group consisting of:
(i) time division multiple access multiplexing means for controlling the
times at which said first digital data signals are transmitted; and
(ii) frequency division multiple access means for changing said
transmission carrier's frequency;
(c) means for generating a pseudonoise code;
(d) means for modulating said first modulated transmission carrier by said
first digital data signals and said pseudonoise code to produce a first
direct spread code division multiple access signal; and
(e) means for coupling said first direct spread code division multiple
access signal to a communications medium;
and said receiver subsystem comprising:
(a) means for receiving a second direct spread code division multiple
access signal from said communications medium;
(b) means for recovering a second modulated transmission carrier from said
second direct spread code division multiple access signal; and
(c) means for demodulating said second modulated transmission carrier to
produce second digital data corresponding to second analog telephone
communications signals.
9. A telephone communications base unit as in claim 8 further comprising
off hook circuitry for placing said telephone line connecting means of
said base unit into an off hook condition.
10. A telephone communications base unit as in claim 8, said base unit
further comprising ring detect circuitry for detecting ring signals on
said telephone line and transmitting a ring command to an extension unit.
11. A telephone communications base unit as in claim 8, wherein said means
for coupling said direct spread code division multiple access signal to a
communications medium comprises an RF antenna.
12. A telephone communications base unit as in claim 8, wherein said means
for coupling said direct spread code division multiple access signal to a
communications medium comprises a connection to power lines of a building.
13. A telephone communications base unit as in claim 8, wherein said
telephone line connecting means of said base unit connects a plurality of
telephone lines to said transmitter subsystem and said receiver subsystem.
14. A telephone communications extension unit comprising:
(a) a transmitter subsystem, and
(b) a receiver subsystem;
said transmitter subsystem comprising:
(a) means for converting first analog telephone communications signals to
first digital data signals;
(b) means for generating a first transmission carrier controlled by at
least one multiple access means, said at least one multiple access means
being selected form the group consisting of;
(i) time division multiple access multiplexing means for controlling the
times at which said first digital data signals are transmitted; and
(ii) frequency division multiple access means for changing said
transmission carrier's frequency;
(c) means for generating a pseudonoise code;
(d) means for modulating said first modulated transmission carrier by said
first digital data signals and said pseudonoise code to produce a first
direct spread code division multiple access signal; and
(e) means for coupling said first direct spread code division multiple
access signal to a communications medium;
and said receiver subsystem comprising:
(a) means for receiving a second direct spread code division multiple
access signal from said communications medium;
(b) means for recovering a second modulated transmission carrier from said
second direct spread code division multiple access signal; and
(c) means for demodulating said second modulated transmission carrier to
produce second digital data corresponding to second analog telephone
communications signals.
15. A telephone communications extension unit as in claim 14, said
extension unit further comprising ring generator circuitry for generating
a ring signal in said extension unit upon receipt of a ring command from a
base unit.
16. A telephone communications extension unit as in claim 14, wherein said
means for coupling said direct spread code division multiple access signal
a communications medium comprises an RF antenna.
17. A telephone communications extension unit as in claim 14, wherein said
means for coupling said direct spread code division multiple access signal
to a communications medium comprises a connection to power lines of a
building.
18. A method for telephone communications between a first unit and a second
unit, the method comprising the steps of:
(a) said first unit converting analog telephone communications signals to
digital data signals;
(b) said first unit generating a transmission carrier controlled by at
least one multiple access technique selected form the group consisting of:
(i) time division multiple access for controlling the times at which the
transmission carrier is transmitted; and
(ii) frequency division multiple access for changing said transmission
carrier's frequency;
(c) said first unit generating pseudonoise code;
(d) said first unit modulating said transmission carrier with said digital
data signals and said pseudonoise code to produce a direct spread code
division multiple access signal;
(e) said first unit transmitting said direct spread code division multiple
access signal to said second unit;
(f) said second unit recovering said modulated transmission carrier from
said direct spread code division multiple access signal; and
(g) said second unit demodulating said modulated transmission carrier to
produce digital data corresponding to said analog telephone communications
signals.
19. A method for telephone communications as in claim 18, further
comprising:
(h) connecting one or more telephone lines to said first unit with
corresponding ring, off hook and duplex audio signals to provide said
first unit with a base unit capability. |
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Claims |
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Description |
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BACKGROUND
1. The Field of the Invention
This invention relates generally to digital communication systems and,
particularly, to such systems which provide for multiple access to a
plurality of signals carried on a single communications medium. More
specifically, this invention relates to telephone extension systems, by
which signals are transferred simultaneously between a plurality of
telephone lines and telephone extensions by means of the AC power lines of
a building or an RF (radio frequency) transmission medium.
2. The Background Art
When conventional telephone systems are installed in a building, a
significant expense is frequently associated with running the necessary
telephone wires for all desired telephone extensions. In an existing
building, the telephone installation process may also significantly
disrupt the building's normal use. Moreover, due to the time and expense
involved, the installation is very often not susceptible to convenient
modification, despite changes in the needs of the telephone system user.
In an effort to overcome the foregoing disadvantages, various types of
wireless telephone systems have been developed. Wireless telephone systems
typically include a base unit which receives the telephone signal from a
conventional telephone line. The signal is then transmitted between the
base unit and one or more extension locations in some manner. Most
commonly, the telephone signal is transmitted between the base unit and
the extensions using conventional radio frequency (RF) transmission
signals and techniques. More recently, however, attempts have been made to
transmit the telephone signal using the existing power lines of the
building. These prior efforts have had varying degrees of success.
For example, one of the major deterrents in transmitting telephone signals
over existing power lines is the nature of the power line medium itself,
which presents a low and variable impedance to carrier signals as well as
an extremely noisy communications environment. Studies have demonstrated
that the optimum carrier frequency range lies between 3 and 15 MHz. Most
prior art attempts to operate below 2 MHz have failed commercially because
of noise or interference problems from other equipment operating on the
electrical system.
Numerous prior art signal modulation techniques have also been attempted,
primarily employing FM modulation of the carrier by audio (speech) signals
(U.S. Pat. Nos. 3,949,172 and 4,701,945 being examples, the disclosures of
which are incorporated herein by this reference). The problem with FM
modulation is that no security is afforded the users; that is, other users
with the same devices can make calls on another user's line and eavesdrop
on conversations. The impact of these problems has already been
demonstrated in the cordless telephone industry, which shares the same
limitations as the line carrier industry. Additionally, commercial AM and
FM broadcast stations are often heterodyned and demodulated in the RF
range utilized by these systems, thereby interfering with the reception of
telephone conversations. The transmissions from one of these systems will
often also radiate and interfere with other types of FCC licensed and
unlicensed commercial and residential equipment. Even the use of two FM
modulation stages, as described in U.S. Pat. No. 4,701,945, is not able to
solve these problems.
In prior art systems, full duplex voice communication is usually attempted
by using two carrier frequencies, one for each direction. Usually a
transmitter and receiver are included in each station which are operating
simultaneously. This leads to mutual interference as well as increasing
the normal interference drift problems and does not eliminate the security
problems.
Recently, attempts have been made to transmit relatively low frequency
digital data (<2 Kbs) via a line carrier and employing a multiple access
technique known as direct spread. (See, for example, U.S. Pat. Nos.
4,641,322 and 4,864,589, the disclosures of which are incorporated herein
by this reference.) Generally, the carrier frequencies (200-500 KHz) and
corresponding data rates (20-1000 bs) are too low to provide sufficient
processing gain to permit real time full duplex voice communication which
generally requires about 100 Kbs. The systems using direct spread
techniques also typically employ line carrier remote data collection and
control applications for which high speed multiple channel data
transmission is not required. Such systems likewise do not typically
accommodate more than one system using the same power lines in the same
building.
In summary, therefore, no prior art line carrier telephone extension system
is known which permits private, multiple line, high quality duplex voice
communications which does not interfere with other electronics systems.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
In view of the foregoing, it is a primary object of the present invention
to provide an effective method of multiple access communication which
provides for multiple access of a plurality of signals on a single
communications medium.
It is also an object of the present invention to provide a method and
system of line carrier communications utilizing both TDMA (time division
multiple access) and CDMA (code division multiple access) to permit high
data rates and multiple access by two or more telephone lines.
Further, it is an object of the present invention to provide a method and
system of line carrier telephone communications which utilizes CDMA (code
division multiple access) to provide a high degree of security for
preventing unauthorized access to the subscriber's line, and which
provides privacy with respect to the conversation from third parties.
It is a still further object of the present invention to provide a method
and system of code synchronization to provide multiple extensions for the
same subscriber line which do not interfere with each other.
An additional object of the present invention is to provide a method and
system of line carrier telephone communications which utilizes FDMA
(frequency division multiple access) in combination with CDMA (code
division multiple access) to prevent interference between relatively close
neighboring transmission systems or partner transmissions in the same
system and to provide for multiple access (simultaneous transmission) of
duplex signals for at least one telephone line.
Also, it is an object of the present invention to provide a method and
system of multiple access cordless telephone extension communications
which applies the same techniques to obtain the same advantages as for the
line carrier telephone extension systems and methods.
Consistent with the foregoing objects, and in accordance with the invention
as embodied and broadly described herein, a telephone communications
system and method is disclosed in one embodiment of the present invention
for conducting multiple access simultaneous telephone communications in
full duplex either over the power lines of a building or over a common RF
transmission means. The method employs a combination of multiple access
techniques selected from among the following: time division, code
division, and frequency division. The following features result: a)
security coding to prevent unauthorized access and eavesdropping, b)
multiple simultaneous conversations through identical and closely coupled
transmission media, c) non-interference to other communications systems
and users, and d) processing gain for operating in noisy environments. The
method also relates to improvements in cordless telephone communication.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects and features of the present invention will
become more fully apparent from the following description and appended
claims, taken in conjunction with the accompanying drawings. Understanding
that these drawings depict only typical embodiments of the invention and
are, therefore, not to be considered limiting of its scope, the invention
will be described with additional specificity and detail through use of
the accompanying drawings in which:
FIG. 1 is a block diagram of a line carrier telephone extension system in
accordance with one presently preferred embodiment of the present
invention, the system servicing a plurality of subscriber lines and
corresponding extension phone sets;
FIG. 2 is a block diagram of a line carrier PABX telephone extension system
in accordance with one presently preferred embodiment of the present
invention, the system servicing a plurality of line carrier extension
phones and conventional extension phones;
FIG. 3 a is multiple access signal coverage diagram illustrating the
deployment and combination of multiple access techniques for solving
near-far problems and simultaneous use of a transmission medium in
accordance with one presently preferred embodiment of the present
invention;
FIG. 4 is an electrical block diagram of a base unit of a multiple access
line carrier telephone extension system in accordance with one presently
preferred embodiment of the present invention, said base unit interfacing
a plurality of subscriber lines to the power line distribution system of a
building;
FIG. 5 is an electrical block diagram of an extension unit of a multiple
access line carrier telephone extension system in accordance with one
presently preferred embodiment of the present invention, the extension
unit interfacing an extension phone to the power line distribution system
of a building; and
FIG. 6 is a schematic and block diagram illustrating one presently
preferred embodiment of the base unit system diagram of FIG. 4, including
the associated transmitter and receiver subsystems.
FIG. 7 (i.e. FIGS. 7A, 7B, 7C, 7D and 7E) is a schematic diagram of base
unit subsystems described in FIG. 6.
FIG. 8 is a block diagram of an extension unit system controller and
digital data multiplexer, including the associated transmitter and
receiver subsystems.
FIG. 9A and 9B are a complete schematic diagram of the PN generator
employed in FIGS. 6 through 8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
It will be readily understood that the components of the present invention,
as generally described and illustrated in the Figures herein, could be
arranged and designed in a wide variety of different configurations. Thus,
the following more detailed description of the embodiments of the system
and method of the present invention, as represented in FIGS. 1 through 9,
is not intended to limit the scope of the invention, as claimed, but it is
merely representative of the presently preferred embodiments of the
invention.
The presently preferred embodiments of the invention will be best
understood by reference to the drawings, wherein like parts are designated
by like numerals throughout.
It will be readily apparent from the discussion which follows that the
present invention is adapted for use in a wide variety of specific
communications applications, including telephone communications. The
present invention may likewise be used with virtually any communications
medium, such as, for example, RF signals or line carriers such as the
power lines in a building. Nevertheless, in order to simplify the
following discussion, the presently preferred embodiments of the present
invention will be described below with specific reference to a telephone
communications system which employs line carriers as the communications
medium.
An increasing number of people live in apartments and condominiums which
share power lines. It is, therefore, important for a telephone extension
system to utilize multiple access techniques in a unique and skillful
manner so as to permit acceptable operation. Many prior art limitations
can be overcome by proper application of multiple access techniques as
employed in the instant invention.
The following specific multiple access (MA) techniques are employed in the
presently preferred embodiments of the present invention:
A) Time Division Multiple Access (TDMA), and two spread spectrum
techniques, namely:
B1) Code Division Multiple Access (CDMA), often called direct spread, which
can include subcarrier CDMA, and
B2) Frequency Division Multiple Access (FDMA), which includes frequency
hopping techniques and deployment of multiple simultaneous transmission
frequencies.
These techniques are more fully described in a work by Robert C. Dixon,
"Spread Spectrum Systems," 2nd Ed., John Wiley & Sons, (U.S.A., 1984),
which is made a part hereof by reference.
TDMA (time division multiple access) is employed in the instant invention
to permit the bidirectional transmission of duplex voice data for one or
more subscriber lines which are "partners" in the same multiline
installation. A single base unit interfaces with the subscriber lines and
transmission medium (AC power line) and controls the transmission of
bidirection 1 voice data by breaking the transmissions into time frames
and windows, in which each communications channel is assigned a specific
transmit and receive time window. By this means the system controller can
guarantee that only one transmission will occupy the medium at one time.
The frequency with which each frame of windows is repeated (20-40 KHz) is
high enough to transfer high speed voice data.
CDMA (code division multiple access) provides four benefits: security,
privacy, low interference with other FCC licensed systems and antijamming
margin from interference and competing users. The fact that neighboring
users may wish to make simultaneous use of the transmission medium
requires that the transmission be coded so that a casual neighbor cannot
access (transmit to) another's phone line to make long distant calls
("security") or intercept (eavesdrop/receive) their conversation
("privacy"). CDMA also spreads the radiation spectrum of the transmission
energy over a very wide bandwidth (2-10 MHz) so that the energy content of
any licensed or unlicensed narrow band is too small to interfere with FCC
licensed users. CDMA also rejects many types of power line interference
due to inherent "processing gain"; and the multiple access feature of
using mutually orthogonal codes prevents jamming interference between
users of similar telephone extension systems if their signals are below
the "jamming margin". A discussion of these terms follows.
Processing gain (Pg) is acquired by using more bandwidth than the data
requires. Processing gain is defined by the following equation:
##EQU1##
Thus, if the RF bandwidth is 5 MHz and the data bandwidth is 100 KHz then
the processing gain would be 50 times or 17 dB. Since approximately 6 dB
of gain is required for correlation and demodulation, the resultant 11 dB
is the jamming margin (Mj) (i.e., Mj=Pg-6 dB). A 10 dB Mj would provide
that ten jamming sources of equal strength or distance to the "friendly"
signals could be rejected, or one "unfriendly" source of equal strength
ten times closer, or one unfriendly source ten times greater in strength
and equal in distance. For this reason, CDMA cannot be relied upon to
carry the burden of jamming margin for near jamming sources. The CDMA
jamming margin is also related to code length which is the number of code
generator clock cycles ("chips") a code generator will cycle through
before the code pattern repeats; but as long as the code length exceeds
the processing gain, the main advantage gained by using long codes is
privacy and security related.
Because of the limitations of CDMA (code division multiple access) to
provide a high antijamming margin for near neighbors, FDMA (frequency
division multiple access) is employed to the extent that it is limited by
the bandwidth of the media. Thus, if the center frequency of near
neighbors were offset by 1 to 3 MHz, the correlation and recovery of the
required IF (intermediate frequencies) is diminished substantially.
Prior art line carrier and cordless telephone extension systems do not
combine the advantages of the forms of multiple access technology
described above. Consequently, they suffer from significant operational
disadvantages which are believed to be overcome by system and method of
the present invention, a presently preferred embodiment of which will now
be described in greater detail.
Reference is first made to FIG. 1 which illustrates one presently preferred
embodiment of a line carrier telephone extension system in accordance with
the present invention. As shown, the line carrier telephone extension
system in FIG. 1 services a plurality of subscriber lines 1-2 and
corresponding extension phone sets 7, 10 and 13. Subscriber lines 1 and 2
plug into base unit 3 which provides the interface circuitry and protocols
for the subscriber line signals (including ring, off hook and duplex audio
signals), and for multiple access power line carrier signals, which are
carried through a | | |
