Transmissão digital em banda passante

Passband Modulation

Este modelo mostra uma maneira simples para executar a modulação de banda passante, multiplicando um sinal complexo modulado com uma onda senoidal para transladar o sinal na frequência. Mais informações [1].

Primeiramente baixe o arquivo a seguir Sistema.zip (que é uma versão parametrizável daquele encontrado na pasta em /opt/MATLAB/R20xxx/toolbox/comm/commdemos). Descompacte e certifique-se que no Matlab você esteja no diretório onde descompactou o arquivo. Digite no terminal do Matlab:

open_system('bandpass_modulation')

ou

'bandpass_modulation'

Toolbox e blocos necessários

Para realização da simulação, é necessário o Communications System Toolbox™ html, pdf fornecido pelo próprio Simulink. O exemplo é realizado de acordo com o seguinte diagrama de blocos:

• Random Integer Generator;
• QPSK Modulator Baseband;
• Goto;
• Raised Cosine Transmit Filter;
• Upconverter (Subsystem);
• Sum;
• Interference (Subsystem);
• Upconverted Spectra (Subsystem);
• AWGN Channel;
• Downconverter (Subsystem);
• QPSK Demodulator Baseband;
• Compute BER (Subsystem);
• Display;
• Constellation Diagram;
• Calculate RMS EVM (Subsystem);
• Spectrum Analyzer;
• Eye Diagram.

Para uma melhor visualização dos blocos e subsistemas usar [Tools>Model Explorer]

Subsistemas

Alguns dos blocos do diagrama citado acima, correspondem a um conjunto de partes inter-relacionadas integrante de um sistema mais amplo, em outras palavras um subsistema. Os blocos utilizados em cada subsistema são especificados abaixo.

Upconverter

• Inport;
• Sine Wave;
• Product;
• Outport;

Interference

• Sine Wave;
• Constant;
• Math;
• Gain;
• Outport;

Upconverted Spectra

• Inport;
• Concatenate;
• Spectrum Analyzer;

Downconverter

• Inport;
• Sine Wave;
• Product;
• Math (conj);
• Outport;

Compute BER

• Inport;
• Error Rate Calculation;
• Integer to Bit Converter;
• Constant;
• Outport;

Calculate RMS EVM

• From;
• Delay;
• Inport;
• Outport;
• EVM Measurement;

Modelo e parâmetros

O modelo realiza a transmissão em banda passante de sinais modulados digitalmente por um canal ruidoso ou com desvanecimento por multipercurso (Rayleigh ou Rician). Ambos são combinados com uma interferência fora da faixa causadas por um tom processado com uma não-linearidade cúbica. A transmissão dos símbolso é realizado em frames. Neste modelo é possível controlar através das variáveis do bloco "Parâmetros do modelo", os parâmetros para realização do mesmo bem como eventual testes. Os parâmetros fornecidos por este bloco são:

The communications link in this model includes these components:

• A Random Integer Generator block, used as source of random data
• A modulator and a pulse shaping filter that perform QPSK modulation and root raised cosine pulse shaping.
• An Upconverter block that multiplies the modulated signal by a carrier frequency.
• A source of tone interference. The interference has a cubic nonlinearity which may be toggled on or off. When the nonlinearity is off, the interference falls completely out of band, but when on, the third harmonic of the tone is introduced into the desired band, causing co-channel interference.
• An AWGN Channel block, set to Eb/No mode. It specifies two bits per symbol because the modulation format is QPSK. The signal power is 1/(2*8) watts. This is because the original signal power at the modulator is 1 watt. The root-raised cosine filter upsamples the signal by a factor of 8, which reduces the power by that factor. The frequency upconversion block output takes only the real part of the signal, thereby reducing the power again, this time by a factor of 2. Finally, the symbol period is 1e-6 seconds, to match the original sample time on the Random Integer Generator source.
• A Downconverter block that converts the signal from real passband to complex baseband.
• A root raised cosine pulse shaping filter that decimates back to one sample per symbol, and a QPSK demodulator block.
• BER and RMS EVM metric calculation blocks.

Testes que podem ser feitos

Os testes foram realizados nas versões 2014a e 2015a do software Matlab, funcionando perfeitamente nas mesmas. A seguir alguns testes que podem ser feitos com este modelo.

When the simulation runs, two spectrum analyzers and one scatter plot open. The first spectrum analyzer shows the signal and the interference signal at passband. With the nonlinearity turned off, the spectrum of the tone interferer falls outside the bandwidth of the desired signal. With the cubic nonlinearity on, the third harmonic of the interference falls into the band of the desired signal. The second scope illustrates the signal after it has been downconverted back to baseband at the receiver, prior to the root raised cosine filtering. Note that with the nonlinearity on, you can see the interfering tone present with the baseband signal. The third scope shows the scatter plot of the received signal, and by toggling the nonlinearity on and off, you can view the effect the interference has on the scatter plot. With the nonlinearity on, the signal constellation is more diffuse than when the nonlinearity is not present.

The model also contains two numerical displays. The first one displays the BER of the link. The BER calculation resets each time the nonlinearity is toggled on or off. The second numerical display is the RMS Error Vector Magnitude (EVM).

Experiências com o Exemplo

School between on/off the nonlinearity on the interference signal. Observe the changes this has on the received spectrum, constellation, BER and EVM. By varying the Eb/No parameter, you can produce BER curves, and compare the results of the model with theoretical results. Note that the model achieves expected theoretical results[ 1 ] for QPSK with the nonlinearity off. Furthermore, you can see the effects the nonlinearity has on overall BER. For further experimentation, try changing the value of the Eb/No parameter on the AWGN channel block, or changing the power of the interference signal. To change the power of the interference signal, open the Interference with Nonlinearity subsystem, and modify the gain value.

The Raised Cosine Transmit Filter and Raised Cosine Receive Filter blocks use a simple filtering method to perform pulse shaping. You can find an example showing more efficient pulse shaping implementations at: Pulse Shaping Filter Design The Downconverter block uses a simple complex multiplication method to perform downconversion. You can find an example showing more efficient downconversion using IF subsampling at: IF Subsampling with Complex Multirate Filters