Reverse noise of CATV broadband integrated service network

Because the cable part of HFC network adopts tree topology structure and the reverse channel noise is serious, the reverse channel noise of HFC network can be divided into two categories.

(1) structural noise

Structural noise is generated by the device itself, mainly thermal noise. The structural noise of the network is mainly generated as follows:

① Basic thermal noise?

It's about bandwidth. Expressed by the formula 15-: PNO = K T B.

Where: [ZK (] k is Boltzmann constant K= 1.38× 10.23.

T is the absolute temperature (t = 239K at room temperature) and b is the noise bandwidth.

② External noise

When amplifiers are connected in series or in parallel and multiple branches are connected in parallel, the noise power will be superimposed at the confluence point, which is called funnel effect.

③ Noise of optical transceiver.

(2) Invasion noise

Intrusion noise is a random RF interference. There are mainly the following forms:

(1) Impact interference, such as random sharp pulse interference caused by starting of antenna, engine and industrial appliances.

② Narrow-band continuous wave interference. The interference mainly introduced by various short-wave broadcasting and radio communication shows the interference of a certain frequency. (1) adopts a modulation method with strong anti-noise ability. E.g. S-CDMA of QPSK. Generally speaking, on the constellation diagram, the greater the distance between adjacent signal points, the larger the decision-making space and the stronger the anti-interference ability. QPSK； in C/N >; At 13dB, the bit error rate can reach? And 64QAM should be in c/n >; At 20dB, the bit error rate can reach? .

(2) Using switching method to reduce noise accumulation. For example, a filter for eliminating interference and noise is installed on a coaxial user terminal. When the user terminal cable modem sends an uplink signal, the uplink channel is opened to let the uplink data signal pass, and once the transmission is completed, the uplink channel is closed.

(3) Using frequency division to reduce the influence of noise.

The uplink signal is transmitted by a separate optical fiber, and each optical node can be modulated in a different frequency band. This can avoid noise superposition of different optical nodes.

(4) Strengthen the shielding function of cables and connectors, improve the construction quality and reduce intrusion interference. The quality of network construction, especially the quality of wired network, will be a very difficult problem in the future construction of broadband integrated network in China, which should be highly valued.