Hfc Network Design Of Cable Amplifier Computer Science Essay

Published:

Chapter 3

In this chapter the HFC network and the design of one of the components of the network, namely the cable amplifier will be will be discussed.

A hybrid fiber coaxial (HFC) network is a telecommunication technology in which optical fiber cable and coaxial cable are used in different portions of a network to carry broadband content (such as video, data, and voice). This technology is used by telecommunication industries, both TV cable and telephone companies, in new and upgraded networks to provide better multimedia interactive services, such as video-on demand, high quality videophone, and high-speed Internet access.

In the HFC network, fiber optic cables are installed at the headend (distribution center) to serve nodes close to business and residential users. The signals are then distributed within the serving area of the node (individual businesses and homes) by amplified coaxial cables.

A headend is the facility at a local cable TV which delivers cable TV services and cable modem services to subscribers. In distributing cable TV services, the headend includes a satellite dish antenna for receiving incoming programming. For provision of Internet access to subscribers, the headend includes the computer system and databases needed to deliver the services.

Lady using a tablet
Lady using a tablet

Professional

Essay Writers

Lady Using Tablet

Get your grade
or your money back

using our Essay Writing Service!

Essay Writing Service

The most important component located at the head-end is the cable modem termination system (CMTS), which sends and receives digital cable modem signals on a cable network and is necessary for providing Internet services to cable subscribers.

Reasons why cable TV and telephone companies are upgrading facilities to HFC:

The use of fiber optic cable for the backbone paths allows more data to be carried than coaxial cable alone.

Users can make connections into the network without replacing exciting coaxial cable.

That portion of the infrastructure with fiber optic cable is more reliable than coaxial cable. Reliability is perceived as more important in an interactive environment.

The higher bandwidth supports reverse paths for interactive data flowing back from the user.

Low noise and interference susceptibility.

Figure 3.1: Simplified HFC network with its -components.

Transport over HFC Network

An HFC network uses frequency division multiplexing (FDM) to deliver a variety of services, namely analog TV, digital TV (SDTV or HDTV), high-speed data, VOD and telephony. These services are carried on RF signals in the 5 MHz to 1000 MHz frequency band.

The HFC network can be operated bi-directionally, meaning that signals are carried in both directions on the same network from the headend office to the home (forward-path or downstream), and from the home to the headend office (return-path or upstream). The forward-path carries information such as video content, voice and internet data. Whereas the return-path carries information such as control signals to order a movie or internet data to send an email. The forward-path and the return-path are actually carried over the same coaxial cable in both directions.

HFC channel model with additional components

The cable network is modelled as a network of coaxial cables connecting components. Any of these connectors may cause reflections, thus creating an unwanted signal in the opposite direction. A subsequent second reflection then redirects part of the signal in the channel. Dependent on the length of the cable between the two connectors, a signal with a delay ranging from 10 ns up to 1000 ns and longer can be generated. As a result of the attenuation of the coaxial cable, the signal strength of the delayed signal will decrease with the delay time. In the frequency domain, echo pops up as a periodic ripple; for long delays (> 100 ns) with a periodicity equal or less than the 8 MHz channel width, for short delays (< 100 ns) with a periodicity equal to one or a number of 8 MHz channels. As such, up to a certain extend echo and ripple describe the same phenomenon of reflections.

Figure : Block diagram of DVB-C2 simulation platform.

Figure : Configuration of channel parameters in dependence of network load.

The channel model is composed of a basic model in combination with additional components.

Echo and group delay variation

Gaussian Noise

Gaussian noise consists of thermal noise and intermodulation products. In case of a cable network, the absolute signal level of this Gaussian noise is a network design parameter.

Lady using a tablet
Lady using a tablet

Comprehensive

Writing Services

Lady Using Tablet

Plagiarism-free
Always on Time

Marked to Standard

Order Now

Impulse Noise

The occurrence of impulse can be avoided by an appropriate design of the signal levels along the cascade. Because of this we consider impulse noise as an additional component of the cable channel model.

Narrow band beats

Narrowband interferences

Burst Noise

Adjacent channel Noise

Narrowband Ingress

Phase noise

HUM

Performance investigations of DVB-C2 transmission through cable networks were carried out by means of computer simulations based on comprehensive simulation platform.

The platform consists of a transmitter block delivering the DVB-C2 load for investigation and a receiver block.

A cable channel model was developed to investigate the transmission performance of the new technology. The model considers transmission disturbances typically occurring in cable networks.

AWGN (Thermal Noise), narrow-band interference broadband distortions (IM) and impulse noise

Cable Amplifier

In order to overcome the loss of the distribution cables and to provide power to drive end terminals, signal boosting amplifiers are used throughout the coaxial distribution system. These must be designed to add as little signal degradation as possible, particularly noise and distortion, consistent with providing the required gain and total power output.

Design Cable Amplifier

Distortion

Amplifiers are not perfectly linear. There are 2 causes of this nonlinearity: the inevitable minor small-signal nonlinearities of the semiconductor devices used and the compression that takes place as the amplifier nears its saturation voltage. Though the small variations define a distortion "floor", most of the distortion is due to saturation effects. They can take of three related forms: even-order distortion, odd-order distortion, and cross-modulation.

Even-Order Distortion (Composite Second Order)

Odd-Order Distortion (Composite Triple Beat)

Thermal Noise

Because of the random motion of electrons in conductors, all electronic systems generate an irreducible amount of noise power. This noise is a function of the absolute temperature and the bandwidth in which the noise is measure. The minimum thermal noise power ( the noise floor) can be calculated using

where

= the noise power in watts

= Boltzmann's constant

= the absolute temperature in

= the bandwidth of the measurement in

Amplifier Noise

Amplifiers generate added noise at various points in their circuitry. For convenience, however, the added noise is treated as if it were coming from an independent generator and summed into the input port.

The ratio of total effective input noise power to the thermal nose floor (dB), is known as the noise figure of the amplifier. Thus, an amplifier with a noise figure of dB will have a total equivalent input noise power of , where

And the output noise power will be the input, increased by the gain, , of the amplifier in decibels:

[…] J. Steel, A. Parker, D. Skellern: "Characterization of Cable Amplifiers for Broadband Network Applications", Electronics Department, Macquarie University, NSW 2109, Australia, 1997.

The characterization of cable network elements required for development of detailed models for design of digital networks. A unique approach has been adopted to provide an efficient model for the nonlinear and frequency dependent behavior of cable network elements.

Cable television networks

Cable amplifier elements

A cable amplifier model describes frequency response (amplitude and phase), thermal noise and nonlinearity

The signal quality of the digital signals may degrade, as well the maximum load will be limited due the non-linear behavior of the amplifiers of the HFC network. The transfer function of these components can be expressed as a mathematical expansion:

Previous studies have explained only the contribution of the 2nd and the 3rd order distortion products. In [5] it is explained that these orders cannot explain the degradation of the digital carriers but that instead higher (like 4th, 5th, 6th and 7th) order terms have to be included.

The data used for the calculation of the distortion products have been gained by CENELEC measurements, which are performed with a load of 42 uncorrelated carriers for several amplifiers. The CENELEC data file contained the data e.g.: - carrier frequencies; - reference output level for each of the carrier frequency; - even order intermodulation frequencies, are at -0,75 MHz relative to the carrier frequency; - odd order intermodulation frequencies, are at the carrier frequency.

Lady using a tablet
Lady using a tablet

This Essay is

a Student's Work

Lady Using Tablet

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Examples of our work

The transfer function of the component (non-linear power amplifier) can be written as a Taylor-expansion up to the seventh order:

The linear gain of the network component is defined by the following transfer function:

The variables and will be treated as having the dimension μV. In practice, the gain is specified as being 'G [dB]'. The linear coefficient now follows from:

The coefficients which in fact determine the non-linear-behavior of the component is usually not part of the component specification. Manufacturers specify the non-linear-behavior of the component by the result of a measurement where a defined input-signal is fed to the component and the levels of the intermodulation components is measured. In its general form, the input-signal consists of several carriers ():

where and is the carrier frequency . Due to the nature of it may also be called a 'measurement-raster'.

The design of the non-linear power amplifier:

Measurement raster:

To design the measurement raster, a load of 42 uncorrelated carriers and a reference output signal level are used. To minimize the correlation between the several carriers, each carrier has been assigned a random phase and a frequency-offset .

Non-linear power amplifier:

The intermodulation products are calculated on basis of the output signal of the end-amplifiers. To avoid the existence of receiving the same output level for two different input levels, the non-linear part needs to be normalized by multiplying the non-linear part with the gain.

Obtaining the distortion products:

Although the time-domain transfer function provides an elegant description, it is not appropriate for the purpose of numerical system modeling. Instead, a frequency domain description appears more appropriate. Considering that following the Central Limit theory, the composite signal will approach a Gaussian signal distribution, the time-domain response function can be restated into a frequency-domain description using the Price Theorem:

With and the Fourier transforms of the composite in and output signals and and and the pre-weighting and post-weighting functions of the non-linear distortion product of order i. The frequency domain response function in fact refers to a generalization of the time-domain response function.

www.vector.com !!!

[3] One of the key elements in coaxial cable networks is a broadband amplifier. Its main task is the amplification of RF signals to obtain a signal level necessary for signal distribution. Modern broadband amplifiers can transmit signals in the forward (to subscribers) and return (from subscribers to a headend) paths. Thanks to this feature it is possible to provide subscribers with Triple Play services (TV, Internet and telephony).

Cable amplifier: non-linearity

The power amplifier, Vector Lambda Pro L2, is used for the simulations with intermodulation constants of = 1.0e+000; = 2.5e-009; = 6.1e-016; = 1.1e-022; = 8.4e-029;

= 1.8e-036; = 1.3e-041

Thermal Noise: noise caused at the input of the non-linear amplifier

3.579e-07 [V] = -128.925 dB

= 1.38e-23, the Boltzman's constant (J/K)

, Room Temperature 17

, Source Impedance

, Channel Bandwidth

, Noise Figure

, is the Gain of the amplifier

, The noise caused by the amplifier

In order to overcome the loss of the distribution cables and to provide power to drive end terminals, signal boosting amplifiers are used throughout the coaxial distribution system. These must be designed to add as little signal degradation as possible, particularly noise and distortion, consistent with providing the required gain and total power output.

Thermal Noise

Because of the random motion of electrons in conductors, all electronic systems generate an irreducible amount of noise power. This noise is a function of the absolute temperature and the bandwidth in which the noise is measure. The minimum thermal noise power ( the noise floor) can be calculated using

where

= the noise power in watts

= Boltzmann's constant

= the absolute temperature in

= the bandwidth of the measurement in

Amplifier Noise

Amplifiers generate added noise at various points in their circuitry. For convenience, however, the added noise is treated as if it were coming from an independent generator and summed into the input port.

The ratio of total effective input noise power to the thermal nose floor (dB), is known as the noise figure of the amplifier. Thus, an amplifier with a noise figure of dB will have a total equivalent input noise power of , where

And the output noise power will be the input, increased by the gain, , of the amplifier in decibels:

Distortion