SOURCEFORGE » RALF » 2016 Channel emulation for satellite communications

Software architecture

To order to control the vector signal analyzer (VSA) and the vector signal generator (VSG) of the NI PXIe-5644R and perform the signal processing a source code is developed in LabView which is installed in the NI PXIe-8135.

The LabView development is mainly based on the VST Streaming project and the previous QPSK implementation functions such as the noise generation, the delay effect and the visualization tools.  

The front panel is illustrated in Figure wwww, the following features are provided:

•	Eb/No level [dB]

•	Time delay [ms]

•	Input/Output frequency carrier [GHz]

•	Sampling frequency [MHz]

•	Power spectral density

•	Eye diagram

•	Symbol constellation

Front Panel.png

On the other hand, the whole block diagram implementation consist of five major parts which will be explained below: 

•	PXIe-5644R initialization and configuration

•	Acquisition of the QPSK signal

•	Noise and delay generation

•	Streaming of the channel-applied signal

•	Visualization and monitoring

PXIe-5644R initialization and configuration

The initialization and configuration of the VST is shown in Figure lllll

Block diagram Configuration.png

Acquisition of the QPSK signal

In Figure qqq, it can be seen how the acquisition loop reads the samples stored in a Direct Memory Access (DMA) buffer. A DMA channel is composed of two FIFO buffers: one on the computer side and another one on the FPGA side. Therefore, in order to recover the QPSK samples, a function block writes data from the FPGA to the DMA buffer and another function block on the computer side reads the data from the target buffer.

Block diagram Input.png

Since the unsigned 16-bit integer format is used to represent the samples, a function converts these samples into complex symbols in order to reuse the functions developed in the previous implementation. 

Once the data is formatted, the samples are added to a FIFO queue. As in the previous LabView development, the queue approach is implemented to connect the main function loops. 

Noise and delay generation

After removing the complex symbols from the queue, the signal power is calculated in order to determine the required noise power for a given Eb/No value that is selected by the user. So that, a noise signal is generated and added to the complex one as shown in Figure www. 

Block diagram Channel.png

In order to create a time delay t_d in the transmission, the noisy samples must wait in the loop for a period of t_w before being delivered to the DMA buffer. As explained later, it must be noticed that the total delay generated by the emulator comprises the required delay as well as the processing time performed in the VST.

Therefore, the waiting time t_w is calculated as follows

t_w = t_d - (t_n - t_a) 

where t_a represents the time when the samples are acquired from the DMA buffer and t_n represents the time when the noisy samples are received in the output loop. The term (t_n - t_a) corresponds to the processing time that must be removed from the required delay. 

Considering the mentioned aspects, the data transmission to the DMA buffer is delayed by t_w once when the first cluster of symbols is received in the output loop queue. The block diagram implementation can be observed in Figure cccc.

Block diagram Noise.png

Streaming of the channel-applied signal

Finally, the channel-applied signal samples are dequeued and written to the DMA buffer in order to be converted from a digital format to an analog signal as shown in Figure yyyy. These samples are interpolated via a DAC, subsequently, the analog signal is up-converted to the required frequency carrier.

Block diagram Output.png

Visualization and monitoring

In order to analyze the transmitted signal, the symbol constellation, the eye diagram and the power spectral density of the QPSK signal are plotted. These loops are executed in parallel to exploit the multiprocessor computing power. As can be observed in Figure rrr, there are two visualization and monitoring loops: one before and another after the channel.

Block diagram Monitor.png