Elvino S. Sousa

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Traditionally, the frequency spectrum is licensed to users by government agencies in a rigid manner where the licensee has the exclusive right to access the allocated band. Therefore, licensees are protected from any interference all the time. From a practical standpoint, however, an unlicensed (secondary) user may share a frequency band with its licensed(More)
ln this paper, we obtain the optimum transmission ranges to maximize throughput for a direct-sequence spread-spectrum multihop packet radio network. In the analysis, we model the network selfinterference as a random variahle which is equal to the sum of the interference power of all other terminals plus hackground noise. The model is applicable to other(More)
Spectrum scarcity is becoming a major issue for service providers interested in either deploying new services or enhancing the capacity for existing applications. On the other hand, recent measurements suggest that many portions of the licensed (primary) spectrum remain unused for significant periods of time. This has led the regulatory bodies to consider(More)
The increasing demand for the radio spectrum along with the inefficient usage of the licensed bands has led the regulatory bodies to consider opening up the under-utilized licensed frequency bands for dynamic access by unlicensed users. Such dynamic spectrum access is envisioned to resolve the spectrum scarcity by allowing unlicensed users to(More)
Opportunistic unlicensed access to the (temporarily) unused frequency bands across the licensed radio spectrum is currently being investigated as a means to increase the efficiency of spectrum usage. Such opportunistic access calls for implementation of safeguards so that ongoing licensed operations are not compromised. Among different candidates,(More)
Motivated by the low utilization of the licensed spectrum across many frequency bands, sensing-based opportunistic spectrum access has recently emerged as an alternative to the outdated exclusive spectrum access policy. Under this new paradigm, a secondary (unlicensed) user monitors a primary (licensed) frequency band for a given time and opportunistically(More)
We propose a multi-carrier orthogonal CDMA signaling scheme for a multiple-access communication system, such as the reverse channel of a cellular network, as an alternative to the multiuser interference cancellation approach. The average variance of cross-correlations between sequences is used as a measure for sequence design. We search for sets of(More)
Collaborative spectrum sensing enables opportunistic unlicensed access to the unused portions of the licensed spectrum. We characterize the performance degradation of collaborative sensing due to correlated shadowing by deriving a lower-bound on the probability of missing the opportunities for unlicensed access. Moreover, we evaluate the effective number of(More)