A Novel Seed Based Random Interleaving for OFDM System and Its PHY Layer Security Implications

• Main Author: Khan, Muhammad Asif
• Format: Thesis
• Language: English
• Published: 2009
• Online Access: http://utpedia.utp.edu.my/3009/1/my_thesis.pdf; http://utpedia.utp.edu.my/3009/

Wireless channels are characterized by multipath and fading that can often cause long burst of errors. Even though, to date, many very sophisticated error correcting codes have been designed, yet none can handle long burst of errors efficiently. An interleaver, a device that distributes a burst of errors, possibly caused by a deep fade, and makes them appear as simple random errors, therefore, proves to a very useful technique when used in conjunction with an efficient error correcting code. In this work, a novel near optimal seed based random interleaver is designed. An optimal interleaver scatters a given burst of errors uniformly over a fixed block of data - a property that is measured by so called ‘spread’. The design makes use of a unique seed based pseudo-random sequence generator or logistic map based chaotic sequence generator to scramble the given block of data. Since the proposed design is based on a seed based scrambler, the nature of input is irrelevant. Therefore, the proposed interleaver can interleave either the bits or the symbols or the packets or even the frames. Accordingly, in this work, we analyze the suitability of interleaver when introduced before or after the modulation in single carrier communication systems and show that interleaving the bits before modulation or interleaving the symbols after modulation has same advantage. We further show that, in an orthogonal frequency division multiplexing (OFDM) systems, the position of interleaver, whether before or after constellation mapper, has no significance, and is interchangeable. However, scrambling symbols is computationally less expensive than scrambling bits. For the purpose of analyzing the performance of the proposed seed based random interleaver, simulations are carried out in MATLAB®. Results show that our proposed seed based random interleaver has near optimal properties of ‘spread’ and ‘dispersion’. Furthermore, the proposed interleaver is evaluated in terms of bit error rate (BER) versus length of burst error in a single carrier system both before and after modulation. The proposed interleaver out-performs the built in RANDINTLV in MATLAB® when used in the same system. It shows that proposed interleaver can convert greater amount of burst errors into simple random errors than that of MATLAB® interleaver. The proposed interleaver is also tested in IEEE 802.16e based WiMAX system with Stanford University Interim (SUI) channels to compare the performance of average BER versus SNR for both pre modulation and post modulation interleaver. Results show that pre modulation interleaver and post modulation has same performance. There is also a side advantage of this seed based interleaver, in that it generates a variety of unique random-looking interleaving sequences. Only a receiver that has the knowledge of the input seed can generate this sequence and no one else. If the interleaving patterns are kept secure then it can possibly be used to introduce an extra layer of security at physical (PHY) layer. In that way, at PHY layer, one builds an additional entry barrier to break through and it comes with no extra cost. This property has been investigated by carrying out key sensitivity analysis to show that the attacks to guess key can be very futile, as difference at 4th decimal place in the initial condition can lead to entirely different scrambling.