Of late, chaos based cryptography has attracted significant attraction of the researchers due to their simplicity of implementation, complex behavior and extreme sensitivity to initial conditions. In this paper an efficient technique to introduce security at physical (PHY) layer is proposed in 802.11i design to provide security in medium access control. Since it does not emphasize on availability, several denial of service (DoS) attacks are possible. The DoS attacks are easy to mount because management and control frames are unprotected and so are MAC headers. This paper proposes a security scheme at PHY layer by scrambling OFDM constellation symbols to encrypt data transmission to resist against these malicious attacks. The scrambling matrix is based on a key derived from a one dimensional chaotic nonlinear dynamical system using logistc map. The initial condition of the map acting as the derived key is dependent on the external key. The scrambling is completely reversible with the use of appropriate key. However, use of a wrong initial condition even at 4^th decimal point produces entirely different sequence resulting in erroneous constellations with equal probability.

In this paper a front-end noise power and SNR estimation method based on one OFDM preamble is proposed and compared with previously published SNR estimators - none of which are front-end estimator. The estimator makes use of two identical halves property of time synchronization preamble used in OFDM systems and relies on autocorrelation of the same. The current estimator is compared with two existing SNR estimators, the Reddy estimator and Subspace based estimator in terms of normalized mean square error (NMSE) and estimated SNR. It is observed that current estimator gives SNR estimates better than Reddy and subspace estimators. The estimator performs SNR estimation at front-end of the receiver unlike all other estimators which perform SNR estimation at back-end of the receiver.

Recently, chaos based cryptography has attracted significant attraction of the researchers due to their simplicity of implementation, complex behavior and extreme sensitivity to initial conditions. In this paper an efficient technique to introduce security at physical (PHY) layer is proposed in 802.11i design to provide security in Medium Access Control. Since it does not emphasize on availability, several denial of service (DoS) attacks are possible. The DoS attacks are easy to mount because management and control frames are unprotected and so are MAC headers. This paper proposes a security scheme at PHY layer by scrambling OFDM constellation symbols to encrypt data transmission to resist against these malicious attacks. In addition, it can also acts as random interleaver. Analysis shows that, it has good random interleaver properties and also memory efficient. The scrambling matrix is based on a key derived from a one dimensional chaotic nonlinear dynamical system using logistic map. The initial condition of the map acting as the derived key is dependent on the external key. The scrambling is completely reversible with the use of appropriate key. However, use of a wrong initial condition even at 4 ^th decimal point produces entirely different sequence resulting in erroneous constellations with equal probability.

This paper presents an adaptive modulation in a single-input-single-output (SISO)-OFDM-based cooperative system that employs a Constant Amplitude Zero Autocorrelation (CAZAC) preamble-based SNR estimator. A novel CAZAC preamble-based SNR estimator that does not incur a throughput penalty is derived by exploiting a modified CAZAC preamble structure for the OFDM system. The proposed CAZAC SNR estimator is used to estimate the channel quality by finding the instantaneous SNR of the received cooperative data, which is sent to the transmitter via feedback channel. The modulation scheme is then changed according to the estimated SNR. The performance of the CAZAC preamble-based SNR estimator exhibits less than 0.02 dB and less than 0.2 dB bias when communicating over AWGN and SUI-5 channels, respectively. The proposed adaptive modulation scheme with the CAZAC SNR estimator significantly improved the channel throughput in comparison with the corresponding benchmarker scheme dispensing with adaptive modulation that invokes M -PSK and M -QAM. More explicitly, the proposed adaptive modulation system with the CAZAC SNR estimator that invokes M -PSK exhibits significant improvement by about 5 dB at BER=10 ^-3 in comparison to its corresponding benchmarker scheme dispensing with adaptive modulation when communicating over the SUI-5 channel. On the other hand, the relay-aided cooperative transmission exhibited an approximately 6 dB SNR gain in terms of channel throughput in comparison to the corresponding non-cooperative benchmarker scheme, and benefited from spatial diversity.