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Communications Blockset 4.3
Digital Video Broadcasting - Terrestrial
This demo 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
Contents
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 major processes.
The table below shows which subsystems correspond to processes from the schematic.
The bottom row of icons in the demo represents 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
The model uses variables as listed below.
To see how MATLAB® computes the values of these variables, see the script commdvbt_tablegen.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.
- A traceback depth of 136 in the Viterbi Decoder library block. This library block appears within the top-level Viterbi Decoder subsystem.
Results and Displays
To examine the performance of the demo, use the sink blocks that are included in it, listed in the table below.
Spectrum Scope
Digital Video Broadcasting-Terrestrial, Alternate Form
The model commdvbt_alt illustrates an alternative way to model the 64-QAM Demapper in the receiver.
To see how the alternative version implements the 64-QAM Demapper, compare the alternative DVB-T 64-QAM Demapper subsystem in commdvbt_alt demo with the original DVB-T 64-QAM Demapper subsystem in the commdvbt demo.
Original: In the original form, soft decisions are computed using a subsystem-based implementation. In-phase and quadrature phase signal components are extracted after appropriately scaling the received signal, and then they are shifted to obtain soft decisions for various bits.
Alternative: In the alternative form, the built-in Rectangular QAM Demodulator block is configured to compute exact bitwise log-likelihood ratios (LLRs). Noise variance needs to be provided and it is computed using the received signal and the signal generated by the DVB-T 64-QAM Mapper. This approach makes derivation of soft decisions easy for any signal constellation through the use of the built-in block.
Selected Bibliography
ETSI Standard EN 300 744: Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television, European Telecommunications Standards Institute, Valbonne, France, 1997.
