7. Conclusion and Recommendations

7.1 Conclusions

LabVIEW Communications is indeed a tool designed for studying RF communications, better equipped with special features for this purpose. Nonetheless, considering that this is still new in comparison with other versions of NI software, the best support to have while working on LabVIEW Communications would be from the NI forum (opened less than a year ago). It is still recommended to have LabVIEW System Design Software, as a lot of examples found on the internet with this software could be used as guides for LabVIEW Communications Design Suite.

USRPs are a good tool for protoyping RF communication systems, with programmable elements that can improve signal transmission or reception. Using USRP with the software mentioned above creates a good environment for research, simulating a given communication chain with different stages such as modulation, filtering, coding, interleaving, etc. However, when communicating with USRPs synchronization is of high importance, but can be difficult to achieve as war our case.

Adding coding to a transmission can greatly improve the performance, as was seen when applying BCH coding. This will allow the system to require less $E_b/N_0$, but be able to achieve the same desired BER than when no coding is used, impacting cost in a positive way as well as power requirements. It also depends on the type of coding applied, BCH is a block code, but the parameters to implement will depend on the length of bit stream. In satellite communications, there is a large amount of data being dealt with, so the parameters are very high, in the order of thousands (depending on the size of the frame).

LDPC is implemented in the project in order to show how BCH-LDPC concatenated codes, which is what the new standard follows, can greatly improve communications. This was demonstrated in another VI before applying it to the QPSK chain. However, this was a complicated thing to do, due to the complexity of LDPC decoding, the formula required for bit likelihood calculation on a communication link (which was difficult to employ), that this is not a familiar topic to us at the moment and we have not seen practical examples and the software tool used is still somewhat new and does not have as much support as other very commonly used tools. The calculations for the BER with LDPC coding were not as expected and were similar to a BER when no coding is applied, we can infer from the results the our LDPC implementation was not accurate. This is something that could be improved in the future.
Even though LDPC did not work as anticipated when applied to our existing transmission system, it was proven on a previous example that concatenated codes of BCH and LDPC gives a better performance, since LDPC offers powerful coding and decoding, and given current computational power, it can be widely used compared to years ago, when it was first designed.

There was a learning curve for training on functionalities of USRPs and LabVIEW Communications. It was of high interest to understand the different aspects of
each requirement and observe how varying different parameters could add complexity, or affect the signal quality, constellation, coding and decoding.

7.2 Futher Improvements

7.2.1 Synchronization

In the context of evaluating a satellite communication chain over USRP this project has to provide synchronization between the Transmitter and the Receiver. For a very promising designing software tool like LabVIEW Communications we managed to implement the Transmission providing modulation, filtering, coding and the implementation of an AWGN channel and accordingly for the Receiver, demodulation, matched filtering and decoding. Although synchronization is not an easy task we would have liked to accomplish that. However this project will be shared among the LabVIEW Community and uploaded in the official NI LabVIEW Communications System Design Suite Support http://www.ni.com/examples/ for further academic research, improvements and hopefully a synchronized version over the USRPs.

7.2.2 DVB-S2 compliant

The DVB-S2 standard includes many features which were not implemented in the context of this simulation. The use of interleavers would be a complement to our work done, and one feature that given the time, would be interesting to implement and observe how it further improves the communication chain, by reducing even more the Eb/No required for a specific BER.