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SUMMARYSuper-orthogonal space-time trellis codes (SOSTTC) are full rate, full diversity space-time codes with high coding gains for quasi-static fading channels. In this paper, this design approach is extended to fast fading channels and new SOSTTC are proposed for BPSK and QPSK modulations based on the design criteria valid for this type of channels. The new full rate codes have 4-and 16-state trellises for BPSK and QPSK cases, respectively, to avoid parallel transitions which restrict the error performance. The frame error performances are evaluated through computer simulations and it is shown that the proposed codes have superior performance in the fast fading case compared to their counterparts previously given in the literature for fast and quasi-static fading channels.

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In this study, orthogonal frequency division multiple access (OFDMA) based primary and secondary usage in a cognitive radio system is considered and conventional power spectral density (PSD) based technique is used to detect the opportunities in the spectrum. Primary user employs blockbased or scattered subcarrier allocation scheme and the overlay system aims to utilize the spectral gaps. Comparison of the allocation schemes and effect of windowing are presented in terms oftrue detection andfalse alarm rates. It is observed that when block-based allocation is employed, the unlicensed user will be able to detect more opportunities in the spectrum compared to the case in which the scattered allocation is employed. Simulation results show that threshold selection is crucial since it is the main factor in determining the false alarm and true detection rates. It has been shown that, for either ofthe schemes all possible spectral gaps cannot be found when PSD is used. Questioning the suitability ofPSD method results in that a new definition of opportunity is needed, forming a basis for the secondpart ofthis study.

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