JPS60153243A - Two-way catv communication system - Google Patents

Abstract

PURPOSE:To obtain a two-way CATV communication system having high resistance against interference and noise and a simple system constitution by using spread spectrum processing based upon a pseudo noise code string. CONSTITUTION:Transmission data are supplied from an input terminal 11 of a subscriber's transmission system 10 to a multiplier 12. A code string provided with pseudo noise PN features is supplied from a code generator 13 to the multiplier 12 and an exclusive OR output between the PN code string and said transmission data is inputted to a balance modulator 14. The modulator 14 supplies a signal obtained by PSK-modulating a carrier from an oscillator 15 by the output of the multiplier 12 and applying spread spectrum SS processing to a transmission line TL through a BPF16. In a receiving system 20 on the information center side, the SS signal is frequency-converted 21 and supplied to a multiplier 23 through a BPF22. The multiplier 23 multiplies a PN code string generated from a slide correlator 24 and the same as that of the generator 13 by the SS signal, separates the PN code string and inputs the separated output to a demodulator 25, which PSK-demodulates the input data and outputs the demodulated data.

Description

[Detailed description of the invention] Industrial applications The present invention relates to improvements in bidirectional CATV communication systems.

BACKGROUND TECHNOLOGY AND CONSIDERATIONS The CATV (Cable Te'1Evision) system is a broadband transmission system that uses coaxial cables as transmission lines and is mainly used for rebroadcasting television broadcasts, and also provides regional information services centered on video. However, in the case of a two-way CATV communication system, in addition to this, each subscriber sends home information such as requests for service information to the information center, and each subscriber's electricity and gas information is sent by computer control of the information center. - Business information such as automatic water meter reading data and safety monitoring data for fire detectors, emergency buttons, etc. is collected through this coaxial cable. This collection of business information is called polling.

While referring to Figures 1 and 2, the conventional bidirectional CAT
An example of the V communication method will be explained.

In the bidirectional CATV communication system, the same coaxial cable is used bidirectionally as a down line from the center to the subscriber and as an up line from the subscriber to the center by frequency division. That is, as shown in Fig. 1, the frequency band for CATV downlink is 54 to 4, including the VHFTV broadcast band.
50MHz is allocated. For the above-mentioned polling, the downlink frequency band is further divided into the downlink polling band Pd, and the polling signal from the center is F.
Sent using SK (frequency shift keying). Also,
5 to 35 MHz is allocated as the uplink frequency band U, and in one CATV communication network, a 5 MHz wide frequency band Pu of the uplink frequency band U is used for uplink polling. PSK (phase shift keying) is used for polling responses from subscribers to the center.

However, in the uplink frequency band U mentioned above, radio waves such as shortwave broadcasting, amateur radio, citizen radio, etc. are mixed, so the polling response signal is interfered with by these radio waves, etc., and the center side is not satisfied. There were many cases where it was not possible to receive messages.

On the other hand, as schematically shown in Figure 2, the coaxial cable network of the normal two-way CATV communication system has a main line TR originating from the information center C and a large number of distributions 4if3+, Bz...Bn...
. . . etc. are branched into multiple stages in the form of a tree, and subscriber terminals are connected to the ends of the branches. In the case of such a dendritic network, trunk amplifiers and distribution amplifiers (not shown) are inserted at key points in the downlink to ensure the necessary signal level to the end subscriber terminals. It is done like this. However, in the uplink, thermal noise power equivalent to 75Ω generated from each subscriber's terminal device is added together and concentrated at the information center, deteriorating the C/N at the center and making reliable data collection difficult. Become. Therefore, conventionally, bridges were installed at important points + remote locations.
B82 --- BSn... is installed to connect the entire cable network to an appropriately sized subscriber group (3t * 02 +... On...
The switch (52ij in the figure) of the group to which the subscriber (52ij in the figure) belongs is polled by the control signal from the center.
In the figure, only G1 . Measures were taken to separate Ga...Gn... from the main line and to keep the thermal noise from terminal equipment concentrated in information center C to the lowest possible level.

Ideally, the shoe noise countermeasure using the above-mentioned blizzard switch should be applied to all terminals, but from the economical point of view, it is limited to an appropriate stage. Therefore, as the scale of the system increases, the noise level at the center increases, impairing safe reception, and the use of a large number of bridger switches complicates the system configuration.

OBJECTS OF THE INVENTION In view of the above points, an object of the present invention is to provide a two-way CATV communication system that is resistant to interference waves and noise and has a simple system configuration.

Summary of the Invention The present invention provides a subscriber-side code sequence generation means for generating a predetermined pseudo-noise code sequence, and a subscriber-side code sequence generation means for generating a predetermined pseudo-noise code sequence; a subscriber side multiplier, a carrier wave generation means, a phase shift key ink modulation means to which the output of the subscriber side multiplier and the carrier wave from the carrier wave generation means are supplied; and a subscriber side code sequence. A center-side code sequence generating means for generating a pseudo shoe sound code sequence that matches the pseudo-noise code sequence generated by the generating means; A center-side receiving system comprising a center-side multiplier to which an output is supplied and multiplied, and a phase shift keying duplicator to which the output of the center-side multiplier is supplied and demodulates predetermined data from a subscriber side. According to the present invention, there is provided a two-way CATV communication system which is resistant to interference and noise and has a simple system configuration. An embodiment of the bidirectional CATV communication system according to the present invention will be described.

FIG. 3 shows a main part of an embodiment of the present invention. In FIG. 3, α represents the transmission system as a whole on the terminal side of an arbitrary adder among a plurality of subscribers, and input terminal 0 of the transmission system 00).
1), the transmission data (half cycle Td) as shown in FIG. 4B is supplied to a multiplier (exclusive OR circuit) (12). (13) is a code generator, vessel(
Rather than a code system in which 1 turbidity occurs, pseudo-shoe sounds
This is the longest code sequence with characteristics (hereinafter referred to as PN), and a predetermined and different code sequence is assigned to each subscriber. As shown in FIG. 4A, its half period is T'n (('I'd). This PN code sequence is also supplied to the multiplier 02 and multiplied by the data. The output is the exclusive OR of the PN code series and the data, as shown in FIG. 4C, and the data is embedded in the PN code series. 041 is a balanced modulator; Vessel a
′;! PSK @ key, more specifically, carrier suppression two-phase balanced modulation is performed by the output of l. The output of the modulator (143) is supplied to the transmission line (coaxial cable) TL via the bandpass filter (16).

Here, the frequency spectra of the data, the PN code, and the output of the multiplier 021 are as shown in FIG. 5A, B, and C, respectively. That is, in the data and PN code spectra, the main lobe band is the reciprocal of the half period, 1/
Td, equal to 1/Tn. Also, multiplier lI';
! In the output of I, the center of the main lobe is equal to the carrier frequency fc, and the band is twice the reciprocal of the half period of the PN code, 1/Tn. Therefore, the center frequency of the bandpass filter (16) is f
c, and the passband is set sufficiently wide.

In addition, in the above-mentioned spread spectrum (hereinafter referred to as SS) processing, the spreading gain Gs is defined by equation (1).

Gs = 10 Qog Td/Tn”・(11(
2υ shows the reception system on the information center side as a whole, and the SS signal from the transmission system (QO) passes through the transmission line TL to the reception system (20
), which is frequency-converted in the frequency converter (21) to become an intermediate frequency signal with a center frequency of 10.7 MHz, for example, and then passed through a bandpass filter to a multiplier (exclusive OR circuit) (c). supplied to (24) is a slide correlator, which generates a receiving side PN code sequence that is the same as the PN code sequence generated by the code generator (I3) of the transmitting system α0), and synchronizes this with the received SS signal. and supplies it to the multiplier (23). Therefore, in the multiplier e, (), an exclusive OR operation is performed, which is the reverse process to that shown in FIG. The data is separated from the PN code sequence, and an intermediate frequency signal that is two-phase modulated only with the data is supplied to a demodulator (251).

Demodulator (I-Q loop or Costas loop (Co5tasloop) is used because it operates well even at low S/N and can perform two-phase PSK demodulation.
) is used. (&
The output data of the 11 units (25) is outputted from the output terminal - and processed in a predetermined manner at the information center.

Now, the virtual adder TLaK in the middle of the transmission line TL! From a sound source N, a white noise N as shown in FIG. 6A, B, and C is generated.
Consider w, an addition boxed with a single continuous wave (disturbance) noise Nc or noise Np due to other PN sequences.

In this case, at the output of the demodulator (2!'9) of the receiving system (2111), the received SS signal is reduced to a high level by the aforementioned despreading, as shown in FIG. 7A, B, and C. The main lobe is being reproduced.On the other hand, as shown in FIG. 7A, the level of white noise Nw does not change depending on the PN code sequence on the receiving side, but as shown in FIG. 7B,
The single continuous wave noise Nc is spread by the PN code sequence on the receiving side to become a low-level noise Ncs. Furthermore, the seventh
As shown in Figure C, the spectrum of key tone Np due to other PN code sequences is not affected by the PN code sequence on the receiving side.
remains at a low level.

Here, the received signal power is the energy per bit Eb
and the bit rate R, (= 1/'I'd), and each throttling power is approximately equal to the product of the noise spectrum)/the density No. and the reception bandwidth B. Therefore, the coverage ratio 8/I between the signal on the receiving side and the interference wave (noise) is as shown in equation (2).

S / I = Eb R / No B ... f
In the case of 21SS, since B/R can be large, Eb/No can be made large even if S/■ is small.

That is, in either case, the level of noise or interference is extremely low, and the transmission system from each subscriber terminal to the information center is extremely resistant to noise or interference. Therefore, there is no need to use a bridger switch, which was required in the past.

Effects of the Invention As detailed above, according to the present invention, since spectrum spread using a PN code sequence is used, it is resistant to interference and noise, and does not require a pre-switch, and has a simple bidirectional system configuration. A CATV communication system is obtained.

[Brief explanation of the drawing]

Fig. 1 is a diagram for explaining the present invention, Fig. 2 is a wiring diagram showing a configuration example of a conventional two-way CATV communication system, and Fig. 3 shows an embodiment of the two-way CATV communication system according to the present invention. The block diagram, FIG. 4 is a time chart for explaining the present invention, and FIGS. 5 to 7 are frequency spectrum diagrams for explaining the present invention. (+21 and (c) are multipliers, Q3) is a code generator, +
141 is a balanced modulator, (24) is a slide correlator, (2
51 is a wandering device.

Claims (1)

[Claims] subscriber-side code sequence generation means for generating a predetermined pseudo-acoustic code sequence; and a subscriber-side multiplier to which predetermined data from the subscriber side and the output of the subscriber-side code sequence generation means are supplied and multiplied. a subscriber-side transmission system comprising: a carrier wave generating means; and a phase shift keying variable means to which the output of the subscriber-side multiplier and the carrier wave from the carrier wave generating means are supplied; and the subscriber-side code sequence. center side code sequence generation means for generating a pseudo sound code sequence that matches the pseudo rudder sound code sequence generated by the generation means; an output of the phase shift keying modulation means of the subscriber side transmission system; and the center side a center side multiplier to which the output of the code sequence generation means is supplied and multiplied; and a phase shift keying demodulation means to which the output of the center side multiplier is supplied and demodulates predetermined data from the subscriber side. A two-way CA'I'V communication system characterized by having a center-side receiving system.