Analog joint source channel coding for wireless communications

Abstract

In 1948, the Shannon's work titled "A mathematical theory of communication" completely revolutionized the way to understand the problem of the reliable communications. He showed that any communications system is able to transmit with an arbitrarily low error probability as long as the transmission rate is kept below a certain limit. The separation between the source and channel coding was also shown as the optimal strategy to achieve the theoretical limits. Those ideas inspire the development of a whole digital communication theory focused on building more and more sophisticated coding schemes. It leads to most of communication systems were designed according to a digital approach and the separation principle from that moment, whereas other alternatives were set aside. However, in the last years, communication systems based on a jointly optimization of the source and channel encoder has aroused the interest of the researchers again, specially in the case of the transmission of analog signals. This strategy, referred to as analog Joint Source Channel Coding (JSCC), has been also shown to approach the theoretical limits and provides certain advantages with respect to the conventional digital systems, such as high transmission rate, low complexity, almost zero delay and the ability of the system to adapt to time-varying environments without a full redesign. Thus, these appealing properties of the JSCC approach make it specially suitable for both real-time applications with severe requirements on the complexity and/or delay and the transmission of data at high rate over wireless channels. So far, the research on this area mainly focused on the design and evaluation of different analog JSCC systems for the transmission of Gaussian sources over Additive White Gaussian Noise (AWGN) channels. However, the behavior of this strategy on more realistic environments, such as practical implementations over real wireless channels, has not been deeply studied yet. The objective of this thesis is to evaluate the feasibility of the analog JSCC technique in the context of the wireless communications. For that purpose, several analog JSCC systems are specifically designed for the transmission of discrete-time continuous-amplitude samples over wireless channels, with the aim of preserving the desirable properties described for the case of AWGN channels. The application of the JSCC strategy on wireless channels represents a series of important challenges for the design of these analog systems. On one hand, the analog JSCC scheme must be constantly adapted to the fluctuations of the wireless channel for the performance approaches the optimal distortion-cost tradeoff. Fortunately, this procedure does not involve a full redesign of the system since it is just enough to update the encoder parameters according to the actual channel conditions. On the other hand, the wireless communications undergo the undesirable effects of the delay spread caused by the multipath propagation of the transmitted signals. In the current wireless standards, different transmission techniques are usually employed in order to combat such effects and exploit the diversity gain provided by the channel, such as the use of multiples antennas both at the transmitter and the receiver as well as of a multicarrier modulation scheme. The integration of the analog JSCC strategy together with these techniques leads to make substantive modifications on the initial configuration -specially in the design of the decoding operation- to preserve the low complexity and delay of the resulting communication model. Finally, the practical implementation of the analog JSCC on real wireless environment also leads to deal with some additional problems that are not encountered in a simulation scenario.