JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Section: Broadcast Systems, Digital Video Broadcasting (DVB), Digital Terrestrial Television Broadcasting (DTTB)


Channel Model for Digital Terrestrial Television Broadcasting

Contributed by Paul G.M. de Bot and Flavio Daffara

In the coming years, the current analog television distribution will be replaced by digital distribution. The terrestrial channel poses substantial problems to the system design. This is due to the fact that the Digital Terrestrial Television Broadcasting (DTTB) system should allow large coverage for fixed receivers (with a directional roof-top antenna), and also provide the largest possible coverage for portable receivers (indoor reception with a non-directional built-in antenna). Discussions in all three European DTTB projects focus on the use of Orthogonal Frequency Division Multiplexing (OFDM).

Channel characteristics for terrestrial broadcasting

DTTB should allow two different reception conditions, which are related to two different transmission channels. Fixed reception coverage will be mostly interference limited, where the interferer, in the DTTB introduction period, probably would be a PAL/SECAM signal. The transmission channel for portable reception however, will mainly be characterized by multipath propagation, resulting in a frequency selective, noise limited channel. Single Frequency Networks (SFNs), produce an effect similar to multipath propagation, also for fixed receivers.

The conventional network planning is based on fixed reception. It is shown that transmitters should increase their power by about 30 dB to offer the same service area for portable receivers. Therefore, in their introduction phase, the DTTB services will focus on fixed reception.

On noisy multipath channels, the received signal r can be modelled as

where alpham(f) is the complex attenuation factor of the channel, s is the transmitted signal, and n is a complex AWGN component with E[nn*] = N_0 In a Rayleigh fading channel, alpham(f) is complex Gaussian distributed, and frequency-dependent. So, we can define the frequency-dependent signal-to-noise ratio

If a channel suffers from Co-Channel Interference (CCI) that has a complex Gaussian amplitude distribution, the received signal can similarly be written as

where beta(f)n represents the combined CCI and AWGN.

Interference from Analog TV

If the CCI is caused by PAL/SECAM signals, beta will be heavily frequency dependent and show power concentrations near the luminance, chrominance and sound carriers of the PAL/SECAM signal.

As well as for the case of multipath propagation, we can define the frequency-dependent signal-to-noise ratio (where in this case the 'noise' is in fact interference) as

where alpha_I(f) = 1 / (f).


Figure above: Frequency transfer profile of multipath reception in an urban area
Figure below: profile of PAL Co-Channel Interference

Hence, we can deal with CCI from PAL/SECAM in the same way as with multipath propagation. Channels suffering from CCI from PAL/SECAM as well as channels with multipath propagation will be referred to as frequency selective channels. In the following, we will use the term signal-to-noise ratio (SNR), although we mean the signal-to-noise-plus-interference ratio S/(I+N).



JPL's Wireless Communication Reference Website © Paul G.M. de Bot, Flavio Daffara and 1993, 1995.