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Abstract-Efficient use of radio spectrum is a necessity for 3000 MHz were intended to clarify, among other aspects, future wireless systems. This can, for example, be accomplished the questions of channel occupation, how those channels are by cognitive radios. An important point for designing future sys-occupied (i.e., modulation scheme, data rate, etc.), and how tems is to gain knowledge about the occupation of the frequency .'. ' ' bands which may be used by the cognitive radio. This includes many tnsitr arept in a and andtwhre teyiae the general characterization of the traffic density at the frequency located. In [6] a concept of spectral agile software defined bands as well as a detailed analysis of the temporal frequency radios has been discussed where a method for estimating occupation. Thus, this paper describes measurements taken in underutilized spectrum bands has been presented. the GSM network at three different scenarios. Furthermore, INTRODUCTION system. Therefore, the major incentive of this paper is to T HE demand for mobile services steadily increases. Often present measurements of a typical primary system, thereupon radio resources are proclaimed to be scarce resources, but describing the statistical characteristics and discussing potenthis scarcity only arises due to an inefficient usage. By em-tial consequences for the design of cognitive radios. ploying adaptive allocation and dynamic pricing mechanismsThe remainder of the paper is organized as follows: In these resources can be exploited more efficiently. One possible section II the measurement system is described by specifying technique to realize dynamic spectrum access may be based the equipment used and the parameters chosen. The scenarios on spectrum pooling [1], [2].in which the measurements have been performed are charIn a spectrum pooling system the same spectrum is locally acterized in section III. Measurements provide information used by secondary users following proper rules to transmit about the time-frequency occupation of the frequency range wirelessly without disturbing the incumbent user. For example, under consideration. This time-frequency occupation serves spectrum pooling systems may be realized by primary systems as starting point for the statistical analysis conducted in allowing a secondary system to send in the time-frequency section IV. Possible impacts of the results on the design of gaps whenever the primary users do not transmit. Thus, the cognitive radios are discussed in section V. Finally, the paper secondary system needs to detect the primary system and it is summarized. must be able to decide about transmitting and adapting its radio access technology to the current spectrum configura uoon.rTion. eAuREment sYstAssuming that the secondary system gets no signaling inforThe measurement equipment mainly consists of an antenna, mation from the primary system, the secondary system needs a spectrum analyzer and a computer (see figure 1). The to learn about the past and act accordingly. This requirement intention ...

Abstract-Regulation will experience enormous changes in section deals with distributed resource auctioning, where a the near future resulting in seamless connectivity by bluring specific belief function is introduced that is an estimation spectrum borders. A promising approach in this context is of the cumulative distribution function. Finally, Section IV dynamic spectrum allocation which leads to a more flexible concludes the paper. access to spectral resources by employing intelligent radio devices

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SUMMARYA general expression of outage probability in a wireless relay network with an arbitrary number of relays and selection combining at the destination is derived. This formula is then adopted to independent but not identically distributed channel coefficients and lower and upper bounds are developed. We study two relay schemes: the one-relay case and the two-relay case. The latter can further be divided into a two-phase and a three-phase transmission scheme which depend on the cooperation among the relays. For all considered cases, outage probabilities in Rayleigh fading environments are derived and analysed and it is demonstrated by numerical simulations that all relay protocols achieve full order of diversity. We further elaborate the impact of relay locations on outage probability. Finally, we discuss the required signal-to-noise ratio in order to achieve a predefined outage probability. With respect to the mentioned evaluation metrics the two-phase relaying scheme achieves by far the best performance, however, with the drawback of increased receiver complexity.