Communications Blockset

Digital Video Broadcasting Demo

The Digital Video Broadcasting demo, dvbt_sim, models part of the ETSI (European Telecommunications Standards Institute) EN 300 744 standard for terrestrial transmission of digital television signals. The standard prescribes the transmitter design and sets minimum performance requirements for the receiver. The purpose of this demo is to:

• Model the transmitter in its "2k mode," as prescribed in the standard
• Model one possible receiver design
• Generate error statistics that will help determine whether the receiver model satisfies the performance requirements

This document highlights these aspects of the demo:

• The overall structure of the demo, which mimics the block diagram schematic in the standard
• Variables in the demo
• Design of the receiver portion of the demo
• Visible results of the demo

Structure of the Demo

Using a list and a schematic, the standard shows the major processes that the data undergoes. The top row of blocks in the demo mimics the structure of the schematic, by including subsystems that perform such major processes. The table below shows which subsystems correspond to processes from the schematic.

Process in Schematic	Subystem or Block in Demo
Outer coder	(204, 188) Shortened Reed-Solomon Encoder
Outer interleaver	Convolutional Interleaver I=12
Inner coder	Rate 3/4 Punctured Convolutional Code
Inner interleaver	DVB-T Inner Interleaver
Mapper	DVB-T 64-QAM Mapper
OFDM	OFDM Transmitter

The bottom row of icons in the demo represent subsystems that make up the receiver. The demo also includes a source of random data, a channel model, error statistic calculators, and several sinks.

Variables in the Demo

When you load the model, it creates several variables in the MATLAB workspace, using the PreLoadFcn model callback parameter.

Variable	Purpose in Demo
Ts	Sample time of random integer source
dvb_bit_int_table	Table for Bit Interleaver and Bit Deinterleaver
dvb_sym_int_table	Table for Symbol Interleaver and Symbol Deinterleaver
dvbt_qam	Signal constellation for 64-QAM mapping

To see how MATLAB computes the values of these variables, see the script dvbt_table_gen.m.

Design of the Receiver

The standard does not specify how to implement the receiver, although some inverse operations, such as deinterleaving, are clearly defined. This demo illustrates one possible receiver design by using these features:

• A 64-QAM demapper that makes soft decisions, producing a set of six real numbers for each complex number in its input. These six numbers represent soft decisions on the real and imaginary components' first bit, second bit, and third bit. The Viterbi Decoder subsystem interprets the soft-decision numbers and uses them to decode the punctured convolutional code properly.

• To examine the exact mapping more closely, see the DVB-T 64-QAM Demapper subsystem, as well as the dvbt_qam variable in the MATLAB workspace.

• A traceback depth of 136 in the Viterbi Decoder library block. This library block appears within the top-level Viterbi Decoder subsystem.

Visible Results of the Demo

To examine the performance of the demo, use the sink blocks that are included in it, listed in the table below.

Icon or Window	What it Shows
Leftmost Display icon	Error statistics for the entire system
Rightmost Display icon	Error statistics for the inner coder
Spectrum Scope window	Spectrum of the received OFDM signal
Delayed Scatter Plot window	Scatter plot of the received 64-QAM signal

Selected Bibliography

[1]  ETSI Standard EN 300 744: Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television. Valbonne, France: European Telecommunications Standards Institute, 1997.

Results and Display

HiperLAN/2 Demo