"Optimization of Protection Techniques Based on FEC Codes for the Transmission of Multimedia Packetized Streams" 

Filippo Casu

E.T.S. Ing. Telecomunicación, Universidad Politécnica de Madrid, May 2017, "Cum Laude".

Ph.D. thesis Director: Julián Cabrera Quesada.

This thesis presents two enhanced FEC-based schemes to protect real-time packetized multimedia streams in bursty channels. The objective of these novel architectures has been the optimization of existing FEC codes, that is, Reed-Solomon codes and LDPC codes. On the one hand, the optimization is focused on the achievement of a lower computational cost for Reed-Solomon codes, since their well known robust recovery capability against any type of losses needs a high complexity. On the other hand, in the case of LDPC codes, the optimization is addressed to increase the recovery capabilities for a bursty channel, since they are not specifically designed for the scenario considered in this thesis.

The scheme based on Reed-Solomon codes is called inter-packet symbol approach, and it consists in an alternative bit structure that allocates each symbol of a Reed-Solomon code in several media packets. This characteristic permits to exploit better the recovery capability of Reed-Solomon codes against bursty packet losses. The performance of this scheme has been studied in terms of encoding/decoding time versus recovery capability, and compared with other proposed schemes in the literature. The theoretical analysis has shown that the proposed approach allows the use of a lower size of the Galois Fields compared to other solutions. This lower size results in a decrease of the required encoding/decoding time while keeping a comparable recovery capability.

Although the use of LDPC codes is typically addressed for channels where losses are uniformly distributed (memoryless channels) and for large information blocks, this thesis suggests the use of this type of FEC codes at the application layer, in bursty channels and for real-time scenario, where low transmission latency is requested. To fulfill these constraints, the appropriate configuration parameters of an LDPC scheme have been determined using small blocks of information and adapting the FEC code to be capable of recovering packet losses in bursty environments. This purpose is achieved in two steps. The first step is performed by an algorithm that estimates the recovery capability of a given LDPC code in a burst packet loss network. The second step is the optimization of the code: an algorithm optimizes the code structure in terms of recovery capability against the specific behavior of the channel with memory, generating a burst oriented version of the considered LDPC code.

Finally, for both proposed FEC schemes, experimental results have been carried out in a simulated transmission channel to assess the performances of the schemes and compared to several other schemes. Download here  

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