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| Traditionally, interference protection is guaranteed through a policy of spectrum licensing, whereby wireless systems get exclusive access to spectrum. This is an effective way to prevent interference, but it leads to highly inefficient use of spectrum. Cognitive radio along with software radio, spectrum sensors, mesh networks, and other emerging technologies can facilitate new forms of spectrum sharing that greatly improve spectral efficiency and alleviate scarcity, if policies are in place that support these forms of sharing. On the other hand, new technology that is inconsistent with spectrum policy will have little impact. This paper discusses policies that can enable or facilitate use of many spectrum-sharing arrangements, where the arrangements are categorized as being based on coexistence or cooperation and as sharing among equals or primary-secondary sharing. A shared spectrum band may be managed directly by the regulator, or this responsibility may be delegated in large part to a license-holder. The type of sharing arrangement and the entity that manages it have a great impact on which technical approaches are viable and effective. The most efficient and cost-effective form of spectrum sharing will depend on the type of systems involved, where systems under current consideration are as diverse as television broadcasters, cellular carriers, public safety systems, point-to-point links, and personal and local-area networks. In addition, while cognitive radio offers policy-makers the opportunity to improve spectral efficiency, cognitive radio also provides new challenges for policy enforcement. A responsible regulator will not allow a device into the marketplace that might harm other systems. Thus, designers must seek innovative ways to assure regulators that new devices will comply with policy requirements and will not cause harmful interference.

Abstract-This paper considers opportunistic primarysecondary spectrum sharing when the primary is a rotating radar. A secondary device is allowed to transmit when its resulting interference will not exceed the radar's tolerable level, in contrast to current approaches that prohibit secondary transmissions if radar signals are detected at any time. We consider the case where an OFDMA based secondary system operates in non-contiguous cells, as might occur with a broadband hotspot service, or a cellular system that uses spectrum shared with radar to supplement its dedicated spectrum. It is shown that even fairly close to a radar, extensive secondary transmissions are possible, although with some interruptions and fluctuations as the radar rotates. For example, at 27% of the distance at which secondary transmissions will not affect the radar, on average, the achievable secondary data rates in down-and upstream are around 100% and 63% of the one that will be achieved in dedicated spectrum, respectively. Moreover, extensive secondary transmissions are still possible even at different values of key system parameters, including cell radius, transmit power, tolerable interference level, and radar rotating period. By evaluating quality of service, it is found that spectrum shared with radar could be used efficiently for applications such as non-interactive video on demand, peer-topeer file sharing, file transfers, and web browsing, but not for applications such as real-time transfers of small files and VoIP.Index Terms -Coexistent, Cooperative, Opportunistic, OFDMA, Primary-secondary spectrum sharing, Radar I. INTRODUCTIONRIMARY-SECONDARY spectrum sharing can substantially alleviate spectrum scarcity [1]. Radars could be a good candidate for the primary systems in spectrum sharing because they operate in a large amount of spectrum. For example, in the US over 1.7 GHz of spectrum from 225 MHz to 3.7 GHz "involves radar and/or radionavigation infrastructure," [2] and around 1.1 GHz of this 1.7 GHz is used by fixed land-based radars in non-military applications [3].This paper considers opportunistic sharing where a secondary device can transmit only when its transmissions will not cause harmful interference, i.e., interference that causes noticeable service disruption to the primary [4].Currently, there are some bands in which radars are protected from harmful interference by granting them exclusive rights to operate in a given area and frequency band, and other bands (e.g., at 5 GHz) in which radars share spectrum with unlicensed devices that can transmit only if they are so far from any radar that the radar is undetectable [5]. Both of these are white space approaches to primarysecondary sharing, where secondary wireless systems are allowed to operate in frequency bands and geographic regions that are found to be entirely "unused" by any primary system. In contrast, with the gray space sharing [6] considered in this paper, a secondary device is allowed to transmit near a radar, but only when and with a transmit power that ...

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