 |
Claims  |
|
|
What is claimed is:
1. A data transmission apparatus for use in transmitting data spread across
at least a portion of a spectrum provided for a cable television channel
of a cable television system comprising a cable television signal
distribution plant susceptible to interference noise from proximate power
lines, telephone lines, defective cable television signal distribution
apparatus of the distribution plant, ineffective grounding of cable
sheaths, radio frequency interferences and other sources of interference
noise, the data transmission apparatus comprising:
a controller, responsive to an incoming data signal, for providing a
stepwise digital control signal for controlling the power level of a
signal input to the cable television signal distribution plant, for
providing the incoming data signal at a data bit rate to a spread spectrum
data signal generator and for controlling the operation of a carrier
signal oscillator;
the carrier signal oscillator, responsive to the controller, for providing
a carrier signal at a frequency consistent with the frequency of the cable
television channel;
a frequency divider, responsive to the carrier signal oscillator, for
providing a sequencing signal;
a pseudorandom sequence generator, responsive to the sequencing signal, for
generating a predetermined pseudorandom chip sequence at a chip rate an
order of magnitude greater than the data bit rate for transmission on the
cable television channel within at least a portion of the spectrum
corresponding to the cable television channel;
the spread spectrum data signal generator, responsive to the pseudorandom
sequence generator and the data signal output of the controller, for
generating a spread spectrum data signal;
a carrier signal modulator, responsive to the carrier signal oscillator and
the spread spectrum data signal generator, for modulating the spread
spectrum data signal for transmission over the cable television channel;
and
a programmable power amplifier for amplifying the modulated spread spectrum
data signal output of the carrier signal modulator in accordance with said
stepwise digital control signal from the controller.
2. The data transmission apparatus of claim 1, the cable television channel
being a channel selected within a reverse path frequency band.
3. The data transmission apparatus of claim 1, the spread spectrum data
signal generator comprising an Exclusive-OR gate.
4. The data transmission apparatus of claim 1, the carrier signal modulator
comprising an Exclusive-OR gate coupled to the programmable power
amplifier.
5. The data transmission apparatus of claim 2, the cable television channel
being channel T7.
6. The data transmission apparatus of claim 1 comprising means for
simultaneously transmitting a video signal and the modulated spread
spectrum data signal, the power level of the video signal being
substantially above the power level of the modulated spread spectrum data
signal so as to not interfere with television reception.
7. A data reception apparatus for use in receiving data spread across at
least a portion of a spectrum provided by a cable television channel of a
cable television system comprising a cable distribution plant susceptible
to interference noise, the data reception apparatus comprising:
a quadrature demodulation circuit for removing a spread spectrum data
signal from a carrier of the cable television channel;
a Costa loop, responsive to the quadrature demodulation circuit, for
synchronization of the quadrature demodulation circuit with a carrier
signal modulator of a cable television transmitter;
a pseudorandom sequence generator for generating a pseudorandom chip
sequence identical to a pseudorandom sequence generated at said cable
television transmitter, the chip rate of said chip sequence being much
greater than the data bit rate;
a digital correlation loop, responsive to the quadrature demodulation
circuit, for correlating and synchronizing the pseudorandom chip sequence
generated at the cable television receiver with the sequence generated at
the cable television transmitter; and
a quadrature mixer, responsive to the pseudorandom sequence generator and
the quadrature demodulation circuit, for recovering the data signal from
the spread spectrum data signal.
8. The data reception apparatus of claim 7, wherein the Costa loop
synchronizes the quadrature demodulator by mixing with the spread spectrum
data signal a generated frequency and by mixing with the spread spectrum
data signal a frequency 90.degree. out of phase with said generated
frequency.
9. The data reception apparatus of claim 8, wherein the Costa loop further
comprises a voltage controlled oscillator for generating said generated
frequency.
10. A cable television data transmission system for transmitting data
spread across at least a portion of a spectrum provided by a cable
television channel of a cable television system, comprising a cable
television signal distribution plant susceptible to interference noise
from proximate power lines, telephone lines, defective cable television
signal distribution apparatus of the distribution plant, ineffective
grounding of cable sheaths, radio frequency interferences and other
sources of interference noise, the data transmission system comprising:
a controller, responsive to an incoming data signal, for providing a
stepwise digital control signal for controlling the power level of a
signal input to the cable television signal distribution plant, for
providing the incoming data signal at a data bit rate to a spread spectrum
data signal generator and for controlling the operation of a carrier
signal oscillator;
the carrier signal oscillator, responsive to the controller, for providing
a carrier signal at a frequency consistent with the frequency of the cable
television channel;
a frequency divider, responsive to the carrier signal oscillator, for
providing a sequencing signal;
a first pseudorandom sequence generator, responsive to the sequencing
signal, for generating a predetermined pseudorandom chip sequence at a
chip rate for transmission on the cable television channel within at least
a portion of the spectrum corresponding to the cable television channel;
the spread spectrum data signal generator, responsive to the first
pseudorandom sequence generator and the data signal output of the
controller, for generating a spread spectrum data signal;
a carrier signal modulator, responsive to the carrier signal oscillator and
the spread spectrum data signal generator, for modulating the spread
spectrum data signal for transmission over the cable television channel;
a programmable power amplifier for amplifying the modulated spread spectrum
data signal output of the carrier signal modulator in accordance with said
stepwise digital control signal from the controller;
a quadrature demodulation circuit for removing said spread spectrum data
signal from a carrier of the cable television channel;
a Costa loop, responsive to the quadrature demodulation circuit, for
synchronization of the quadrature demodulation circuit with the carrier
signal modulator;
a second pseudorandom sequence generator for generating a pseudorandom
sequence identical to the predetermined pseudorandom chip sequence
generated by the first pseudorandom sequence generator;
a digital correlation loop, responsive to the quadrature demodulation
circuit, for correlating and synchronizing the pseudorandom chip sequence
generated by the second pseudorandom sequence generator with the sequence
generated by the first pseudorandom sequence generator; and
a quadrature mixer, responsive to the second pseudorandom sequence
generator and the quadrature demodulation circuit, for recovering the data
signal from the spread spectrum data signal.
11. The cable television data transmission system of claim 10, the cable
television channel being a channel selected within a reverse path
frequency band.
12. The cable television data transmission system of claim 10, the spread
spectrum data signal generator comprising an Exclusive-OR gate.
13. The cable television data transmission system of claim 10, the carrier
signal modulator comprising an Exclusive-OR gate coupled to the
programmable power amplifier.
14. The cable television data transmission system of claim 10, the chip
rate being at least an order of magnitude greater than the data rate.
15. The cable television data transmission system of claim 11, the cable
television channel being channel T7.
16. The cable television claim transmission system of claim 10, wherein the
Costa loop synchronizes the quadrature demodulator by mixing with the
spread spectrum data signal a generated frequency and by mixing with the
spread spectrum data signal a frequency 90.degree. out of phase with said
generated frequency.
17. The cable television data transmission system of claim 16, wherein the
Costa loop further comprises a voltage controlled oscillator for
generating said generated frequency. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to cable television systems and, more particularly,
to apparatus for transmitting data over a cable television channel
susceptible to interference noise, the transmitted data being spread over
at least a portion of the spectrum of the cable television channel.
2. Description of the Prior Art
The development of cable television systems has reached the stage where not
only is the provision of two way information flow desirable but is
practically required by the implementation of new services. For example,
in the implementation of impulse pay-per-view service where the subscriber
may impulsively select an event for viewing and assume a charge, at least
one data channel is required in a direction from a cable television
subscriber to a cable television headend to report service usage data.
Other uses for a return path include power meter reading, alarm services,
subscriber polling and voting and home shopping. While not every cable
television system operator provides for two way transmission,
manufacturers of cable television equipment have tended to provide for
so-called upstream transmission in a direction from a subscriber toward a
headend. Practically all such manufacturers provide so-called split or two
way systems having a spectrum of frequencies for upstream transmission
which at least includes a band from 0 and 30 megahertz. This band of
interest comprises cable television channel T7 (5.75-11.75 megahertz), T8
(11.75-17.75 megahertz), T9 (17.75-23.75 megahertz) and T10 (23.75-29.75
megahertz). Whether a so-called "sub-split", "midsplit" or "high-split"
system is applied for two way transmission by a headend operator, all
three types of split transmission systems typically involve an upstream
transmission in the 0-30 megahertz band of interest.
An article entitled "Two-Way Cable Plant Characteristics" by Richard Citta
and Dennis Mutzbaugh published in the 1984 National Cable Television
Association conference papers demonstrates the results of an examination
of typical cable television (CATV) return plants. Five major
characteristics in the 0-30 megahertz upstream band were analyzed. These
include white noise and the funneling effect; ingress or unwanted external
signals; common mode distortion resulting from defective distribution
apparatus; impulse noise from power line interference and other
influences; and amplifier non-linearities.
White noise and Gaussian noise are terms often used to describe random
noise characteristics. White noise describes a uniform distribution of
noise power versus frequency, i.e., a constant power spectral density in
the band of interest, here, 0-30 megahertz. Components of random noise
include thermal noise related to temperature, shot noise created by active
devices, and 1/f or low frequency noise which decreases with increased
frequency. The term noise floor is used to describe the constant power
level of such white noise across the band of interest.
This noise is carried through each return distribution amplifier which adds
its own noise and is bridged to the noise from all branches to a line to
the headend. This addition of noise from each branch of a distribution
tree in a direction toward a headend is known as noise funnelling or the
funnelling effect. The constant noise floor power level defines a noise
level which a data carrier power level should exceed.
The present invention is especially concerned with interference noise which
causes peaks in the noise spectral density distribution in the band of
interest. Interference noise destroys effective data transmission when
known data transmission coding techniques such as frequency or phase shift
keying are practiced. In particular, interference noise especially relates
to four characteristics of return plant introduced above: ingress, common
mode distortion, impulse noise and amplifier non-linearities.
Ingress is unwanted external signals entering the cable plant at weak
points in the cable such as shield discontinuities, improper grounding and
bonding of cable sheaths, and faulty connectors. At these weak points,
radio frequency carriers may enter caused by broadcasts in, for example,
the local AM band, citizen's band, ham operator band, or local or
international shortwave band. Consequently, interference noise peaks at
particular carrier frequencies may be seen in noise spectral density
measurements taken on plant susceptible to ingress.
Common mode distortion is the result of nonlinearities in the cable plant
caused by connector corrosion creating point contact diodes. The effect of
these diodes in the return plant is that difference products of driving
signals consistently appear as noise power peaks at multiples of 6
megahertz, i.e. 6, 12, 18, 24 and 30 megahertz in the band of interest.
Impulse noise is defined as noise consisting of impulses of high power
level and short duration. Corona and gap impulse noise are created by
power line discharge. Temperature and humidity are especially influential
in determining the degree of corona noise, while gap noise is a direct
result of a power system fault, for example, a bad or cracked insulator.
The resultant impulse noise spectrum can extend into the tens of megahertz
with a sin x/x distribution.
Amplifier nonlinearities or oscillations relate to pulse regenerative
oscillations caused by marginally stable or improperly terminated
amplifiers. The result is a comb of frequency peaks within the return
plant band whose spacing is related to the distance between the
mistermination and the amplifier.
All of these phenomena define interference noise as used in the
specification and claims. The present invention provides an approximately
20 dB advantage in interference noise rejection over known modulated
frequency or phase shift keying data transmission techniques.
From examining typical cable distribution plants, Citta et al. concluded
that "holes" exist in valleys between peaks in the noise spectrum they
plotted between 0 and 30 megahertz. They proposed that these valleys may
be used to advantage by carefully choosing return carriers "slotted" in
these valleys.
In follow-up articles published at the 1987 National Cable Television
Conference, Citta et al. conclude that a 45 kilobit data signal may be
transmitted by a coherent phase shift keying (CPSK) technique over
carriers at 5.5 megahertz and 11.0 megahertz or in the vicinity of the T7
and T8 cable television channels respectively. While the choice of these
carrier frequencies is claimed to avoid the noise distribution peaks
caused by interference noise, there is considerable concern that such a
modulated phase shift keyed data stream will run into noise peaks in a
cable television distribution network outside of the investigations of
Citta et al.
Other return path or upstream data transmission schemes have been tried.
These schemes include, for example, the telephone system, described as
"ubiguitous" by Citta et al. In other words, the return data path to a
cable television headend is not provided over the cable television
distribution plant at all. The serving cable is intentionally avoided
either because of the interference noise problem in a split system or
because the system is a one way downstream system. Instead, the
subscriber's telephone line is used for data transmission. In this
distance, however, there is concern that local telephone data tariffs may
require the payment of line conditioning surcharges if the telephone line
to a subscriber's home is used for data transmission in addition to normal
"plain old" telephone service. Furthermore, the telephone line is only
available when the subscriber is not using it, requiring an unscheduled or
periodic data flow.
Another known return data transmission scheme involves the application of a
separate data channel at a carrier frequency that avoids the troublesome
0-30 megahertz band. This scheme, of avoiding the noisy 0-30 megahertz
band, is only possible in midsplit and high split systems.
So-called spread spectrum transmission of data is a technology which
evolved for military requirements from the need to communicate with
underwater submarines in a secure manner. Spread spectrum derives its name
from spreading a data signal having a comparatively narrow bandwidth over
a much larger spectrum than would be normally required for transmitting
the narrow band data signal.
More recently the security advantages provided by spread spectrum
transmission have been disregarded in favor of its capability of
application in an environment of interference. For example, communications
systems operating over a power line where impulse noise levels due to the
power line are high have been attempted in the past but found to be only
marginally acceptable, for example, power line plug-in intercom systems
commercially available from Tandy Radio Shack. The Japanese N.E.C Home
Electronics Group, however, has demonstrated a spread spectrum home bus
operating at 9600 baud over an AC line in a home that is practical up to
distances of 200 meters of power line. The NEC system has been
characterized as the missing link between a coaxial cable (for example, a
cable television cable) and an AC power line common to the majority of
homes.
To understand spread spectrum and how it operates to eliminate the effects
of interference, it is first important to define terminology surrounding
pseudorandom noise or chip sequence generation. A binary pulse value of
one or zero in a pseudorandom sequence is known as a chip. The speed of
chip sequence generation is known as a chip rate. Rather than calling a
sequence of bits a bit sequence, a pseudorandom sequence generated by a
pseudorandom sequence generator is known as a chip sequence.
The process of spread spectrum involves the spreading of a comparatively
narrowband binary data signal over a relatively broad frequency spectrum
such as by mixing the binary data signal with a pseudorandom chip sequence
at a much higher chip rate. The effectiveness of improving signal to
interference ratio is related to the ratio of the bandwidth of the spread
spectrum to the bandwidth of the data signal. The larger the ratio, the
greater the effectiveness. When the pseudorandom chip sequence used to
spread the data at the transmitter is correlated with an identical chip
sequence generated at a receiver, the original data stream will be
recovered. For broader band noise interference, the correlation process
picks out the broadband wanted signal while keeping the interference
broadband. For narrowband interference, the interfering signal is spread
over the spread spectrum bandwidth. In both cases the correlation process
provides a narrowband output for the wanted signal. The net effect of the
entire process is a much enhanced signal-to-interference ratio.
Despite the development of the spread spectrum arts, the requirement
remains in the cable television art for an upstream data transmission from
a subscriber premises to a cable television headend that is impervious to
interference noise.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a data
channel for upstream data transmission from a cable television subscriber
to a cable television headend over a CATV distribution plant which is
impervious to interference noise.
It is a further object of the present invention to provide an upstream data
channel in the noisy 0-30 megahertz upstream data transmission band of the
cable television system spectrum.
It is a still further object of the present invention to reduce the cost of
a data transmitter as much as possible.
It is a still further object to provide a data transmission impervious to
the effects of interference noise regardless of the choice of data carrier
frequency.
These and other objects are achieved by the principles surrounding the
development of the present invention, apparatus for spread spectrum data
transmission over a cable television channel, in particular, one in a
noisy 0-30 megahertz cable television upstream transmission band. In order
to conserve costs at a subscriber location associated with providing a
data transmitter, the present data transmitter involves a microprocessor
normally present in a subscriber cable television terminal for data
formatting and transmission control. An oscillator may provide a carrier
channel frequency output and sequence the operation of a pseudorandom
noise sequence generator. The oscillator may be the same oscillator as is
normally applied to clock the operation of the microprocessor. Aside from
these components, two Exclusive-OR gates are employed for the respective
purposes of providing a spread spectrum data signal and a modulated data
carrier signal. A gain control amplifier under the control of the
microprocessor may provide a plurality of stepped power level outputs to
the serving cable. By means of a diplex filter, a coherent phase shift
keyed (CPSK) signal may be alternatively provided to the serving cable and
the higher frequency band of downstream television or data channels
received simultaneously.
A data receiver provided at the headend receives spread spectrum data
transmissions which the headend has addressably ordered a particular
subscriber terminal to provide. In the alternative a so-called global
command issued by the headend may initiate a data response by a group of
subscribers over a corresponding group of code division multiplexed spread
spectrum channels in the 0-30 megahertz band.
The data receiver particularly comprises a pseudorandom noise sequence
generator having the same predetermined chip sequence as the transmitter.
After the spread spectrum signal is demodulated from the carrier
frequency, the locally generated pseudorandom chip sequence is subtracted
so that only the data remains. To synchronize the carrier oscillators of
the transmitter and receiver, a Costas loop is employed at the receiver
which derives an error signal for controlling the carrier frequency
oscillator by comparing the data outputs provided from an in-phase signal
and from a quadrature signal (or 90.degree. out-of-phase signal). The
pseudorandom noise sequence generator at the receiver is synchronized and
correlated to be in phase with one at the transmitter by means of less
expensive but slower early/late gating techniques or by more expensive but
faster correlation circuitry. The faster correlation circuitry, although
more expensive, provides improved responsiveness and, hence, greater data
throughput at the headend when a considerable number of subscribers are
involved.
These and other features of the present invention will be readily
understood by one skilled in the art from the following detailed
description of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overview block diagram depicting a CATV distribution plant
with distribution amplifiers and splitters enabling connection of a CATV
subscriber terminal (spread spectrum transmitter) to a headend (spread
spectrum receiver);
FIG. 2 is a plot of noise level versus frequency over the upstream 0-30
megahertz band of one typical CATV distribution plant;
FIGS. 3(a), 3(b), and 3(c) are timing diagrams depicting the mixing of a
pseudorandom noise signal with binary data;
FIGS. 4(a) through 4(d) are frequency plots for narrowband and broadband
interference showing the desired signal separated from the interference
when the transmitted signal is correlated at the receiver;
FIG. 5 is a schematic block diagram showing the spread spectrum transmitter
for a CATV terminal connected to the CATV distribution plant;
FIG. 6 is a schematic block diagram showing the spread spectrum receiver at
the headend of a CATV plant;
FIG. 7 is a detailed schematic block diagram of the spread spectrum
receiver of FIG. 6 using early/late gating; and
FIG. 8 is a detailed schematic block diagram of the spread spectrum
receiver of FIG. 6 using a custom correlator circuit.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a typical cable TV distribution plant 100 for distributing
cable television signals to a subscriber and for receiving upstream
messages from a subscriber terminal 120. The CATV plant 100 connects a
headend 110 to a plurality of subscriber's televisions 130 through CATV
terminal 120. CATV plant 100 is connected in a "tree" configuration with
branches 148 and 150 using splitters 143. In this configuration, a
plurality of subscribers receive the same signal sent from the headend
110. Distribution amplifiers 142 are also regularly distributed along
cable plant 100 to boost or repeat a transmitted signal. A transmission
from headend 110 to the subscriber at CATV terminal 120 is susceptible to
noise introduced along the trunk line 141 and branch lines 148, 147, 146,
145 and drop 144.
Spread spectrum transmitter 200 may be included in or associated with CATV
terminal 120 and allows a subscriber to communicate with headend 110 by
transmitting messages upstream in the CATV plant. Headend 110 includes
spread spectrum receiver 300 for receiving messages transmitted by spread
spectrum transmitter 200 in CATV terminal 120 located at any or all of the
plurality of subscribers. Many CATV plants are so-called split systems
equipped for two-way transmission, that is, transmission from headend to
subscriber and from subscriber to headend. In these CATV plants,
amplifiers 142 are equipped for bidirectional transmission including
reverse path amplification.
Two-way transmission in CATV plants heretofore has been avoided by cable
television companies in part because upstream transmission from the
subscriber to the headend is significantly more susceptible to
interference noise. Upstream communication is more susceptible to
interference noise because a CATV plant is configured in a "tree"
configuration allowing interference noise from every point in the CATV
plant to be propagated and amplified in the upstream direction. This may
be referred to as the funneling effect. For instance, interference noise
161 and 160 on lines 154 and 144 will combine at splitter 143 connected to
drop 144 and branch 154. As the signals travel toward headend 110, the
noise will combine with noise on branch lines 153, 152, 151, 150 and every
other line in the entire CATV plant. In the upstream direction, it is more
difficult to discriminate a transmitted data signal at headend 110 from
the noise induced in every branch of the CATV plant.
Interference noise can include impulse noise, common mode distortion,
ingress and amplifier nonlinearities. Lighting 10, radio broadcasts 11,
and power lines 12 are exemplary sources of interference noise. CATV
plants may contain old and faulty connectors, corroded connectors, shield
discontinuities, improperly grounded and bonded cable sheaths or the like,
which allow noise to enter anywhere in the CATV plant. Aging splitters 143
or old, non-linear amps 142 may also cause interference noise. Because
interference noise from all branches of the CATV plant affects upstream
transmission while interference noise along only a single downstream line
(for example, 141, 148, 147, 146, 145, 144) affects downstream
transmission, an upstream CATV plant as it ages will require costly
maintenance sooner than a downstream CATV plant. The present invention
allows transmission of upstream communication signals on an "imperfect"
CATV plant where upstream transmission was heretofore difficult without
costly routine maintenance of the CATV plant. The present invention allows
bidirectional transmission of messages in a CATV plant much noisier than
heretofore possible.
Referring now to FIG. 2, there is shown a graph of noise power level versus
frequency for a typical cable television plant. The measurements were
taken at prime time viewing (evening) on a relatively new installation.
The effects of ingress are seen to be especially severe in the measured
plant from a local AM station at 1500 kilohertz, the British World
Service, the Voice of America and a ham operator broadcasting at 21
megahertz. It can be quickly seen that transmission by known techniques on
channel T7 (5.75-11.75 megahertz) would be practically impossible.
Furthermore, it may be generally seen from the distribution that the
higher the frequency, the less troublesome the interference noise.
The effects of common mode distortion were not particularly severe at the
time of the measurements. However, the plant was again examined
approximately one year later and peaks due to common mode distortion were
predictably seen at 6, 12, 18, and 24 megahertz.
FIG. 3(a) shows a binary data signal at, for example, 9600 baud for
transmission using spread spectrum techniques over a CATV plant. In the
transmitter, the binary data having a pulse width 1/D shown in FIG. 3(a)
is mixed with a pseudorandom noise sequence shown in FIG. 3(b). A
pseudorandom noise sequence is a repeating predetermined sequence of
pulses, for example, at approximately 1.2192 megachips per second, each
pulse having a chip width t.sub.w. The ratio of pulse width to chip width
or alternatively chip rate to bit rate determines the effectiveness of
interference noise rejection. Since the chip rate is less than the
bandwidth of a television channel, spread spectrum data transmission may
comprise at least a portion of the channel spectrum and may comprise
multiplexed plural transmissions at different carrier frequencies within
the spectrum. Identical pseudorandom noise sequences are contained in both
the transmitter and receiver so a transmitted binary data signal can be
encoded by spreading using the pseudorandom noise sequence at the
transmitter and decoded by de-spreading using the identical pseudorandom
noise sequence at the receiver. FIG. 3(c) shows the encoded spread
spectrum signal formed by adding the binary data of FIG. 3(a) and the
pseudorandom noise sequence of FIG. 3(b). At the receiver, when the
transmitted spread spectrum signal is decoded, the pseudorandom noise
sequence from the transmitter must be synchronized with the pseudorandom
noise sequence in the receiver. This is performed by synchronization and
correlation of the sequences in the receiver. Schematic block diagrams of
synchronization and correlation circuitry in the transmitter and receiver
will later be discussed in reference to FIGS. 5 and 6.
Spread spectrum techniques allow an overall signal power gain of 1/Dt.sub.w
to be achieved, wherein the data elements are of duration 1/D (FIG. 3(a))
and the chip width of the pseudorandom sequence is t.sub.w (FIG. 3(b)).
FIGS. 4(a) through 4(d) illustrate spread spectrum's effective reduction
of noise effects for both narrowband and broadband interference. FIG. 4(a)
shows a frequency plot of encoded (spread) data signal 21 and narrowband
interference 20 as received at the receiver. FIG. 4(b) shows a frequency
plot of data signal 21 and narrowband interference 20 after correlation
and decoding (despreading). FIG. 4(c) shows a frequency plot of data
signal 23 and broadband interference 22 as received at the receiver after
correlation and decoding (despreading). FIG. 4(d) shows a frequency plot
of correlated and decoded, (de-spread) data signal 23 and broadband
interference 22. As can be seen from the frequency plots of FIGS.
4(a)-4(d), the net effect of encoding (spreading) and decoding
(de-spreading) the desired data signal for either type of narrowband or
broadband interference is therefore a much enhanced signal-to-interference
ratio.
A brief overview of spread spectrum and how it relates to other
communication systems can be found in HF Communications: A Systems
Approach, by Nicholas N. Maslin, published in 1987 by Pittman Publishing,
London, England, pp. 196, 197, 222 and 223.
FIG. 5 shows a spread spectrum transmitter 200 of a CATV terminal 120 (FIG.
1) connected to a television 130 and CATV plant 100 via drop 144. CATV
terminal 120 comprises transmitter 200 and other circuitry 121.
Microprocessor 201 preferably controls all the operations of the CATV
terminal 120 responsive to commands received from headend 110 or from a
subscriber via, for example, a remote control keyboard (not shown).
However, a separate microprocessor can be used to control the CATV
terminal circuitry 121 and microprocessor 201 can be used to control the
spread spectrum transmitter 200.
Responsive to an addressed or global command, microprocessor 201 retrieves
data from memory (for example, impulse pay-per-view service usage data)
and formats the data for transmission. A clock signal operating processor
201 is divided down to clock the data stream output DATA at the
appropriate rate, for example, 9600 baud and the microprocessor provides
the data in a predetermined format and in phase with a pseudo-random
sequence generation (FIGS. 3a and 3b).
The pseudorandom noise sequence is generated by pseudorandom noise sequence
generator 202. Microprocessor 201 controls a 7.3152 megahertz carrier
frequency oscillator 203 over RUN/OFF line 204. Oscillator 203 provides
both a carrier frequency output for example, within the T7 band at 7.3152
megahertz and a sequencing clock for the pseudorandom sequence generator.
Additionally, oscillator 203 may provide clock signal for clocking the
operation of microprocessor 201. Of course, other carrier frequencies may
be appropriately selected for application in sub-split, mid-split or high
split systems which may, for example, be multiples of 7.3152 megahertz.
The output of carrier frequency oscillator 203 is divided down by six in
counter 205. Alternatively a separate oscillator may be provided for
generating a sequencing signal. The 0.219 megahertz output of counter 205
clocks the sequencing in pseudorandom noise generator 202. Microprocessor
201 controls the operation of pseudorandom noise generator 202 over
INIT/RUN line 206. The pseudorandom noise sequence, for example, of a
length of 127 chips generated at the output of the pseudorandom noise
generator 202 is mixed in mixer 207 with binary data signal 209. Mixer 207
preferably comprises an Exclusive-OR gate. The output 208 of mixer 207 is
the encoded spread spectrum data signal shown in FIG. 3(c). Binary data
signal 209 is depicted in FIG. 3(a) and the pseudorandom noise sequence is
depicted in FIG. 3(b). The encoded spread spectrum data signal output 208
of mixer 207 is mixed with the 7.3152 megahertz carrier frequency output
from oscillator 203. Mixer 210 is also preferably an Exclusive-OR gate.
The output 211 of mixer 210 is the modulated spread spectrum data signal
to be transmitted. The output 211 of mixer 210 is provided to programmable
amplifier 212.
Divide by six circuit 205 can be constructed using a four bit counter, for
example, Motorola 74HC161. The pseudorandom noise sequence generator can
be constructed using a shift register. Outputs selected from the shift
register predetermine the desired sequence to be generated. Outputs of the
shift register are logically summed together and can be fed back to the
input creating a sequence longer than the length of the shift register.
The pseudorandom noise generator can be constructed using a Motorola MC
74HC164 and 74HC86. Those of skill may also construct the pseudorandom
noise sequence generator using a clocked, sequential address EPROM or
other memory device.
Modulated spread spectrum data signal 211 is amplified by programmable
power amplifier 212. Programmable power amplifier 212 is illustrated with
eight levels of gain. The eight levels of gain are programmable by three
binary control lines 213, 214 and 215, connected to microprocessor 201. A
desired level may be downloaded to microprocessor 201 by means of an
addressed command from headend 110 or preset at installation. The output
of programmable power amplifier 212 connects to a bandpass filter 216
having an approximately six megahertz or less passband, the bandwidth of
any one of the T7, T8, T9 or T10 channels in the 0-30 megahertz band of
interest. The center frequency of bandpass filter 216 is the carrier
frequency selected for the transmitter.
Bidirectional coupler 217 allows the spread spectrum signal output from
bandpass filter 216 to connect through to the CATV plant at subscriber
drop 144. Bidirectional coupler 217 allows bidirectional transmission to
television 130 and other CATV terminal circuitry 121. In this arrangement,
it is possible for other CATV terminal circuitry 121 and/or televisions
130 to receive and transmit video, audio, data, and other communications
from headend 110 via bidirectional coupler 217 independently of the spread
spectrum transmitter 208 possibly using other data encoding and modulation
techniques known in the art such as coherent phase shift keying (CPSK).
To reduce costs, transmitter 200 is designed to use a minimum number of
components since there are typically hundreds or thousands of transmitters
200 in a CATV system. For example, the microprocessor is used to every
extent possible for data formatting and transmission control limiting the
hardware requirements to an oscillator, two Exclusive OR gates, a counter,
a sequence generator, and an amplifier. Because there is only one headend
110 in a CATV system, the cost of additional components at the location of
the spread spectrum receiver 300 can be tolerated.
FIG. 6 shows spread spectrum receiver 300 connected at trunk 141 to the
CATV distribution plant 100 and connected at terminal 325 to other headend
equipment and circuitry 111. The received spread spectrum signal at trunk
141 is input through bidirectional coupler 310. Bidirectional coupler 310,
like bidirectional coupler 217, allows bidirectional communication of
video, audio, data, and other communications to and from other headend
equipment and circuitry 111. Bandpass filter 311, like bandpass filter
216, is connected to the output of bidirectional coupler 310 and has a
bandwidth less than or equal to six megahertz (or a portion of the channel
spectrum) and a center frequency at the modulated spread spectrum data
channel frequency to be received. Automatic gain control 312 adjusts the
output of bandpass filter 311 to a predetermined level.
Costas loop 313 synchronizes the carrier frequencies of the transmitter and
receiver using carrier synchronizing circuitry 314 and voltage controlled
oscillator 315. An error signal is derived from received data output 324
in the Costas loop through carrier synchronizing circuitry 314 for
adjusting voltage controlled oscillator 315. At mixer 316 the carrier
freq | | |
