Path Delay Profile. Echoes without demodulation.

BSA331. Broadcasting Signal Analyzer. Digital Processing specially designed for broadcasters.

The deployment of digital television is a global reality. The characteristics of this new system can be analysed from different perspectives and you will find many advantages over traditional analogue television.

From the network planning point of view, the Single Frequency Network (SFN) is one of the most important concept. A single frecuency network means that all transmitters emit the same signal at the same frequency. Figure 1 shows a typical network architecture, in which the network operator sends a signal to all transmitters simultaneously (usually via satellite). The delay in the delivery of that signal can vary from one location to another and that's why in order to ensure the proper functioning of the system, all transmitters must be synchronized.

To achieve this, the DVB-T transmitters use a SFN system, defined in the ETSI EN 101 109 standard.

The communication channel estabilished from the transmitters to the receiver can be mathematically modelled by the equation:

This equation includes a set of possible signals, all of them received in the same frequency but with different amplitude values β(k) and different time delay σ(t-τ_k).

This concept is called multipah, and may be caused by several factors:  nature of the transmission channel, obstacles, effects of diffraction, reflection, etc. In the SFN network area this prenomemon is accentuated by the fact that all transmitters are synchronized on the same frequency but at different distances from the receiver.

This multipath effect may cause a degradation in the system that may even preclude the reception. The delay in the reception causes an N symbol to be destroyed by a replica of the same symbol that reaches the receiver with a delay. This phenomenon is know as interference between symbols.

To reduce such interference, the OFDM system has a protection mechanism called guard interval.

In a OFDM system, the signal is segmented into symbols of time T_u - useful information, at the end of each of these symbols part of the useful information of the own symbol is repeated with a time frame T_g. This is what we call guard interval, and the therefore the final duration of the symbol is T_s gt; = T_u + T_g

The following figure allows us to analyse this mechanism in detail:

The DVB-T standard clearly defines the length of each one these paremeters and can be symmarized as follows:

Knowing the channel is therefore a decisive factor in the DVB-T system. There are several system parameters that can be modified (delays, antenna orientation, etc.) that will allow this multipath protection mechanism to be effective or not.

To develop an appropiate strategy for network planning, we need a tool that allows us to parameterise the reception channel. Most of the TV signal analysers have this tool and it is nothing else than the calculation of the temporal response of the communications channel.

All of them are based on demodulating the received signal and  analysing the 'drivers' or synchronization signals. These signal are inserted by the transmitter and already known by the receiver. From these signals we can obtain information about the channel and the used type of modulation.

What happens in case of a signal with poot C/N values that does not allow demodulation? What happens in case of a signal with Echoes outside guard interval that precludes the reception?

Until now, in those circunstances, the response could not be visualized and therefore we missed the information needed to solve a serious problem.

The traditional way of analysing the problem did not allow another type of solution and therefore we needed a new concept in the analysis of the channel's response.

The new approach is based on treating the DVB-T signal from a radio frecuency point of view, without analysing it's internat structure and therefore without the need to demodulate the channel.

Based on the signal samples processed in intermediate frequency and using different mathematical algorithms, which are under patent, you can get to obtain the following mathematical expression:

This expression underlies the Path Delay Profile fuction, a function designed by GSertel for the BSA331, and equipment specifically meant for broadcasters.

In the figure we can analyse the information shown by this fuction.

The main advantages over the traditional system of echoes representation are:

• Temporal span to 1.148µs: Possibility to view echoes that inhibit demodulation outside the guard interval. This scenario is very commonn in real situations, where the distances from the receiver to the multiple transmitters are usually bigger than the guard interval. The figure belows shows a scenario with an 260ms echo with an amplitude of 10dB that precludes demodulation.

• Possibility to display echoes in case of very low quality signals that preclude demodulation. This picture shows an example of a DVB-T channel that does not allow demodulation due to a problem with the echoes. Impossible to observe with the traditional functions.