Cable network system with ingress noise suppressing function

Details Cable network system with ingress noise suppressing function Cable network system with ingress noise suppressing function

2000-08-18

2003-03-04

Hsia, Sherrie (Department: 2614)

Interactive video distribution systems

Video distribution system with upstream communication

Transmission network

C725S121000

Reexamination Certificate

active

06530087

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cable network system constituted in a tree or star shape like, e.g., a bidirectional CATV (Cable Access TeleVision) system and an HFC (Hybrid Fiber and Coaxial) system and, more particularly to, a cable network system having a function of suppressing ingress noise in an upward transmission path.
2. Description of the Related Art
In recent years, a trend of changing CATV systems providing mainly video broadcast services into bidirectional systems and developing various bidirectional transmission services has been activated. The bidirectional transmission services include on-line services (to be referred to as PC on-line services hereinafter) for data terminals such as personal computers, real-time communications services for telephones, videophones, and the like, and VOD (Video On Demand) services for rapidly providing desired movie video and the like to users, as needed.
When the bidirectional services are to be realized in a cable network system having a transmission path branched in a tree or star shape, like the CATV system, measures against ingress noise are required.
More specifically, if there is a connector open terminal to which, e.g., no in-home device is connected in each subscriber residence, or if a used coaxial cable is insufficient in electromagnetic shielding characteristics though the in-home device is connected, interference radio waves such as shortwave broadcasts, and electromagnetic wave noise from electrical motors, such as a vacuum cleaner, or motorcycles flow in via the connector open terminal or the coaxial cable.
FIGS. 27 and 28
show examples of data obtained by experiments on inflow noise.
FIG. 27
shows the spectrum distribution of noise flowing from the connector open terminal. The inflow noise was observed in a band of 40 MHz or less.
FIG. 28
shows the spectrum distribution of inflow noise in a state wherein a 5-cm lead line is connected to a connector terminal, on the assumption that a used coaxial cable is insufficient in electromagnetic shielding characteristics. More typical inflow noise was observed in the band of 40 MHz or less. In addition,
FIG. 29
shows actual data obtained when a connector terminal was terminated by a terminator. In this case, inflow noise was not substantially detected.
If noise flows into respective subscriber residences in this manner, lots of noise merge on an upward transmission path to increase the level and be transmitted to a headend. This noise is generally called ingress noise. The ingress noise causes degradation of the transmission quality, and in some cases causes the system to fail in transmission.
FIG. 30
shows an example of the interference percent availability of ingress noise actually observed over several days in a bidirectional cable network having 1500 subscribers. As is apparent from
FIG. 30
, the availability of satisfying the C/I (Carrier to Interference) ratio of 10 dB per 1-MHz channel bandwidth was almost 100%. The availability of satisfying the C/I ratio of 24 dB as a high-quality transmission environment was 70 to 80% on average and was below 50% depending on the band. It was found from remaining observation results that the interference percent availability became lower as the channel bandwidth was narrower and the number of subscribers was-smaller.
The ingress noise can be roughly classified into three types: narrow and coherent noise, broadband incoherent noise, and specific subscriber noise.
The narrowband coherent noise is electromagnetic waves having large transmission power, such as shortwave broadcast waves and military radio waves present in an upward frequency band (5 to 48 MHz in Japan, and 5 to 40 MHz in the United States). Although the bands are narrow, these electromagnetic waves flow from the connector open terminals and the like of almost all subscribers. If all signals which reach a headend are in phase, the noise level observed at the headend equivalently increases by 20 logs [dB] for the average inflow noise per subscriber where S is the number of accommodated subscribers. In fact, however, the noise level increases by about 14 logs [dB] because signals have a difference in propagation delay time therebetween.
The broadband incoherent noise is generated by strong electromagnetic waves radiated in the atmosphere from the sparks of an electrical motor and a gasoline engine, and discharge tubes and digital devices such as a personal computer. Although the frequency band is broad (2 kHz to 100 MHz), the noise level decreases by 1/f as a frequency f becomes higher. Noise flowing into the upward transmission path is not correlated with other noise (incoherent). The noise can be considered as Gaussian noise. For this reason, the noise level equivalently increases at the headend by 10 logs [dB] for the average inflow noise per subscriber.
The specific subscriber noise is generated when a subscriber erroneously connects an amateur radio device or a digital device such as a personal computer to a cable, or when the subscriber intentionally sends an interference signal to the cable. Since this noise directly flows into the upward transmission path, the noise level may be kept high over a long time.
In addition to the above-described three types of noise, there is harmonic noise caused by signal distortion on the transmission path. The harmonic noise is caused by a nonlinear effect generated by corrosion of a fitting connector terminal on a trunk line cable. This noise can be prevented by proper maintenance and management of cable industrial companies.
Inflow portions of the ingress noise can be classified into two portions: a portion on a trunk system and a portion inside a subscriber residence. The trunk system generally uses a coaxial cable excellent in electromagnetic shielding characteristics. The ingress noise may flow from a loose connector or an old or worn cable. However, such inflow of noise can be prevented by proper maintenance and management of cable network system industrial companies. To the contrary, no measure is provided with respect to the noise flowing from the subscriber residence. The bidirectional services must be performed in consideration of this noise.
FIG. 31
shows a comparison of the spectrum distribution of ingress noise observed on an upward transmission and the assumed level of an upward data signal. Strong noise estimated to be narrowband coherent noise was observed around 6.5 MHz, 10 MHz, and 27 MHz, and noise estimated to be broadband incoherent noise was observed at remaining frequencies. It is supposed that a very-high-quality transmission path can be realized if both the narrowband coherent noise and the broadband incoherent noise are suppressed by more than 20 dB. However, this suppression cannot be achieved, so that the various measures are proposed as follows.
(a) HFC (Hybrid Fiber and Coaxial) Architecture
The HFC architecture aims at a reduction in ingress noise level by decreasing the number S of subscribers described above. The conventional CATV system broadcasts television video from a headend to several ten thousands of subscribers via only coaxial cables by using several tens of bidirectional trunk amplifiers. This CATV system is subdivided into a maximum of 500 home paths per subsystem by combining, e.g., optical fibers and coaxial cables, as shown in FIG.
32
. Note that the home paths represent the number of homes to which cables are wired near the residences or under the eaves and services are immediately provided if the subscribers require them. The actual number of subscribers for the services is 60% on average in the United States, i.e., 300 subscribers per subsystem.
Referring to
FIG. 32
, a reception equipment for receiving television broadcasts sent via communication satellites, and an information transmission equipment for providing various bidirectional services, such as servers, routers, and switching units are installed in a headend (H/E)
1
. A plurality of distribution hubs (D,Hs)
2
, . . . are c

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