Tahir Abbas Khan

Earthquake onset detection using spectro-ratio on multi-threshold time--frequency sub-band

Automatic onset detection and picking algorithm has been proposed by applying the spectro-ratio on time-frequency sub-band. The proposed algorithm does not need any parameter settings as it will work on data generated by either short or very broad band seismometers. Our algorithm is applied on local events from Cairo region recorded by three stations of the Egyptian National Seismic Network (ENSN). Maximum standard deviation is observed to be 0.113 s of the corresponding manual picks made by analysts.

An a posteriori probability threshold rule for multiuser code acquisition in DS/CDMA systems: theoretical and approximate methods

Exact and approximate methods, based on an a posteriori probability threshold rule for multiuser code acquisition, utilizing interference cancellation, in the base station reverse link of a DS/CDMA system are proposed. Expectation and variance of the correlator output corresponding to a synchronization chip and otherwise are derived, including effects of data and timing estimation errors. By applying a central limit theorem, density functions for the two cases are obtained and this leads to formulation of a joint density function of correlator output data. After receiving an output, the conditional probability of the corresponding chip being the synchronization chip is updated and compared with a threshold value. If the updated value of a posteriori probability is greater than or equal to a threshold value, acquisition is declared; otherwise, the next correlator output is examined and the process carries on until acquisition is declared. Hence, an upper bound on acquisition error probability is guaranteed beforehand. Accumulated data is used for decision making, unlike conventional threshold rules that use only current data. Computer simulations show significant gains in mean acquisition time and correct acquisition probability compared to a similar multiuser system with a conventional threshold rule. An approximate method with substantially less complexity than the exact method is derived. Instead of the exact value of correlator output variance, an approximation is used. Consequently, the joint density function of the correlator output is considerably simplified; however, performance is only slightly degraded.

Multiuser code acquisition in DS/CDMA systems

A multiuser code acquisition system based on interference cancellation (IC) is proposed for the reverse link of the base station of direct sequence code division multiple access (DS/CDMA) systems. Explicit expressions for the variance and density function of the correlator output are derived incorporating data and timing estimation errors. Consequently, a threshold rule based on a posteriori probability can be applied which guarantees beforehand an upper bound on acquisition error probability, a very important system performance criterion. Computer simulation results show that acquisition error probability is below the upper limit; system capacity and mean acquisition time performance are also promising.

A novel code acquisition algorithm and its application to Markov spreading codes

In terms of bit error rate (BER) and synchronization performances, Markovian codes are superior to i.i.d. codes in asynchronous mode, for the single-user case. Their BER performance is better than i.i.d. codes also in the case of multiuser detection (MUD). The paper shows that a simple code acquisition scheme based on the statistical properties of spread spectrum (SS) codes efficiently makes the performance of Markov codes superior to i.i.d. codes in asynchronous MUD systems. Hence, Markov codes are a strong candidate for being SS codes in the MUD case as well. Furthermore, the proposed code acquisition scheme significantly outperforms conventional schemes.

Bit error rate in digital watermarking systems using spread spectrum techniques

In spread spectrum (SS)-based digital watermarking systems, watermark information is spread by a pseudorandom code and added to the original information signal. On the other hand, we have proposed another watermarking system having one watermark channel and several image transmission channels. Information data is modulated by SS codes with variable code length according to its significance. This paper inspects bit error rate of such a watermarking system for both watermark channel and image transmission channels.