DVB -C

Introduction

• In many countries, good radio and TV coverage is provided via broadband cable, especially in densely populated areas

• Cable exhibits a much better signal/noise ratio than in satellite transmission and there are not many problems with reflections which permits digital modulation methods of higher quality to be used, from 64QAM (coax) to 256QAM (optical fiber).

• A broadband cable network consists of the cable head end, of the cable distribution links consisting of coaxial cables and cable amplifiers

The DVB-C Standard

• in the DVB-C modulator, the MPEG-2 transport stream passes through almost the same stages of conditioning as in the DVB-S satellite standard

• the last stage of convolutional coding which is missing • it is simply not needed because the medium of

propagation is so much more robust. • This is followed by the 16, 32, 64, 128 or 256QAM

quadrature amplitude modulation. • In coax cable systems, 64QAM is used • In optical fibre networks 256QAM is used

• A conventional coax system with a channel spacing of

• 8 MHz normally uses a 64QAM-modulated carrier signal with a symbol rate of 6.9 MS/s.

• The symbol rate must be lower than the system bandwidth of 8 MHz in the present case.

• Given 6.9 MS/s and 64 QAM (6 bits/symbol), a gross data rate of 41.4 Mbit/s

• In DVB-C, only Reed-Solomon error protection is used which is the same as in DVB-S, RS(188,204).

• Thus, an MPEG-2 transport stream packet of 188 bytes length is provided with 16 bytes of error protection, resulting in a total packet length of 204 bytes during the transmission.

• The resultant net data rate is 38.15 Mbit/s; • the DVB-C channel has a much better signal/noise

ratio (S/N) with about >30 dB compared with about 10 dB in the case of DVB-S.

DVB-C Modulator

Interference Effects on the DVB-C

• An ideal, completely undistorted constellation diagram would show only a single constellation point per decision field in the exact center of the fields

• However, such a constellation diagram can only be generated in a simulation.

• If the correct signal is present at the test receiver and all settings at the receiver have been selected so that it can correctly lock to the QAM signal, a constellation diagram with constellation points of varying size and the appearance of noise clouds is obtained.

• The size of the constellation points depends on the magnitude of the interference effects. The smaller the constellation points, the better the signal quality.

• If there is simply no signal in the selected RF channel, the constellation analyzer of the test receiver will display a completely noisy constellation diagram which exhibits no regular features whatever.

• It appears like a giant constellation point in the center of the display, but without sharp contours.

• If accidentally an analogue channel has been selected instead of DVB-C channel, constellation diagrams like Lissajou figures are produced which change continuously depending on the current content of the analogue TV channel.

• If, however, there is a QAM signal in the selected channel but some of the receiver parameters have been selected wrongly (RF not exactly right, maybe the wrong symbol rate, wrong QAM level etc), a giant constellation point with much sharper contours appears.

• If all parameters have been selected correctly and only the carrier frequency is still divergent, the constellation diagram will rotate. It is then possible to see concentric circles.

Additive White Gaussian Noise (AWGN)

• Additive white Gaussian noise (AWGN) is a basic noise model used in Information theory to mimic the effect of many random processes that occur in nature. The modifiers denote specific characteristics:

• 'Additive' because it is added to any noise that might be intrinsic to the information system.

• 'White' refers to idea that it has uniform power across the frequency band for the information system. It is an analogy to the color white which has uniform emissions at all frequencies in the visible spectrum.

• 'Gaussian' because it has a normal distribution in the time domain with an average time domain value of zero.

If these hits or counts within a constellation field were to be displayed multi-dimensionally, a two-dimensional bell-shaped Gaussian curve would be obtained

This two-dimensional distribution will then be found similarly in every constellation field

Phase Jitter

• Phase jitter or phase noise in the QAM signal is caused by converters in the transmission path or by the I/Q modulator itself.

• In the constellation diagram, phase jitter produces smear distortion of greater or lesser magnitude

• The constellation diagram ‘totters’ in rotation around the centre point.

An ideal oscillator would generate a pure sine wave. In the frequency domain, this would be represented as a single pair of Dirac delta functions (positive and negative conjugates) at the oscillator's frequency, i.e., all the signal's power is at a single frequency..

All real oscillators have phase modulated noise components. The phase noise components spread the power of a signal to adjacent frequencies, resulting in noise sidebands

Sinusoidal Interferer

• A sinusoidal interferer produces circular distortions of the constellation points.

• These circles are the result of the interference vector rotating around the centre of the constellation point.

• The diameter of the circles corresponds to the amplitude of the sinusoidal interferer.