Performance Study of Spread Spectrum Systems with Hard Limiters

Published Online October 2020 in MECS DOI: 10.5815/ijcnis.2020.05.01

Wideband signals find ample application in telecommunication networks to improve their performance in many ways. One of the main approaches to enlarge occupied signal spectrum is to use spreading coding sequences. This technique can provide higher interference immunity and transmission concealment characteristics in wireless telecommunication networks. Performance of this method is largely determined by the autocorrelation properties of the deployed spreading coding sequences. For this reason, research and development of the sequences with the required parameters is a very important task in itself.

Primitive polynomials are well-known to be used for synthesizing spreading coding sequences. Those sequences are the backbone of the third generation mobile networks, as the crucial part of CDMA technology. In this case, however, the spreading sequences are usually very long and used only to organize multi-access to the network in combination with the other coding sequences, such as Walsh codes.

In this paper, an attempt is made to study properties of much shorter spreading coding sequences based on primitive polynomials to improve specifically interference immunity and information concealment properties in telecommunication channels. Such properties are vital, for example, when it comes to control of unmanned aerial vehicles (UAV). Currently, to improve reliability of the UAV control channel, frequency hopping, transmission power adjustment and antenna pattern changes are used.

Additionally, it is well-known that interfering signals can exceed dynamic range of the receiver during wireless information transfer. This can lead to undesired and unpredictable consequences. One of the approaches to elimination of this problem is to use limiters that cut off the signal above a certain level.

Up until now, this problem has not been sufficiently addressed; therefore, this work aims to fill this gap and find out whether utilization of spreading coding sequences based on the primitive polynomials with additional application of the limiters can be used in practice to improve performance of telecommunication networks.

To find a solution to the problem, the research presented here involves a search of a primitive polynomial of a required degree, synthesis of the spreading coding sequence based on the polynomial and study of the characteristics of the telecommunication channel with interferences that includes the transmitted desired bits, spread by the coding sequence, in both the presence and the absence of the limiter.

2.    Literature review

Design of control channels for UAV is considered in [1]. This book provides good coverage for many ways of practical implementation of these channels, but, unfortunately, it does not address the use of pseudo noise coding sequences for improving noise-immunity of the control channels.

Detailed description of spreading coding sequences that are used in third generation mobile networks is provided in [2-4]. In these books, one can find many spreading coding sequences, as well as their areas of implementation. Still, the main focus is made on securing multiple access to the networks and effective separation of the different subscribers. So, issues of using spreading coding sequences for designing of noise immune telecommunication channels are not entertained.

Papers [5-7] address the problems of using smart antennas to improve noise immunity of telecommunication channels and provide good insights into workings of the adaptive antennas. However, they do not offer to combine smart antennas with spread spectrum techniques to secure good performance of a telecommunication channel.