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Cognitive radios sense the radio spectrum in order coding scheme, training or pilot signals, guard periods, and to find unused frequency bands and use them in an agile manner. the power level or correlation properties of the signal, just Transmission by the primary user must be detected reliably even to mention a few. These properties may be used to design a in the low signal-to-noise ratio (SNR) regime and in the face of shadowing and fading. Communication signals are typically deteto at worseinavy low SNRer an d hasl cyclostationary, and have many periodic statistical properties complexity and consequently low power consumpton. These related to the symbol rate, the coding and modulation schemes are very desirable properties especially for cognitive radios as well as the guard periods, for example. These properties in mobile applications. In the absence of any knowledge of can be exploited in designing a detector, and for distinguishing the signal, one may have to resort to classical techniques between the primary and secondary users' signals. In this paper, such as energy detection [1]. An energy detector may need to a generalized likelihood ratio test (GLRT) for detecting the suct a over lon [1]. ofene o detect the to presence of cyclostationarity using multiple cyclic frequencies is collect data over a long period of time to detect the primary proposed. Distributed decision making is employed by combining users reliably. Moreover, controlling the false alarm rates in the quantized local test statistics from many secondary users. mobile applications is difficult because the statistics of the User cooperation allows for mitigating the effects of shadowing signals, noise and interference may be time-varying. Another and provides a larger footprint for the cognitive radio system. . . 'Simulation examples demonstrate the resulting performance snficant drawback iS that energy detecton has no capability gains in the low SNR regime and the benefits of cooperative to distinguish among different types of transmissions or to detection. dichotomize between primary and secondary users of the

We investigate dual concentric core and microstructure fiber geometries for dispersion compensation. Dispersion values as large as -59 000 ps/(nm km) are achieved, over a broad wavelength range with full width at half maximum exceeding 100 nm. The trade-off between large dispersion and mode area is studied. Geometries with an effective mode area of 30 microm2 and dispersion -19 000 ps/(nm km) and 80 microm2 with -1600 ps/(nm km) are proposed.

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Abstract-Cooperative spectrum sensing among multiple cognitive radios mitigates the effects of shadowing and fading. However, it also generates overhead traffic which consumes more power in battery operated mobile terminals. In this paper a censoring scheme for spectrum sensing is proposed. Only informative test statistics are transmitted to the fusion center or shared with other secondary users. Two cooperative censoring test statistics based on cyclostationarity are proposed. Constant false alarm rate tests are derived and asymptotic distributions of test statistics established. The asymptotic distributions are approximated using characteristic functions. Limits for the censoring (no-send) region are derived. The performance of the proposed censoring scheme is illustrated through simulations in a multipath radio environment. Only a minor performance loss is experienced in comparison to uncensored cooperative detection even under very strict constaints on communication rates for the secondary users.