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 ...
This paper considers opportunistic gray-space primary-secondary spectrum sharing when the primary is a rotating radar. We assume that a secondary device is allowed to transmit when its interference does not exceed the radar's tolerable level, probably because the radar's directional antenna is pointing elsewhere, in contrast to current approaches that prohibit secondary transmissions if radar signals are detected at any time. The secondary system is a cellular system using the shared spectrum in some but not all of its cells; this may occur when the cellular system needs the shared spectrum to supplement its dedicated spectrum, or for a broadband hotspot service. It is shown that, even fairly close to the radar, extensive secondary transmissions are possible, although subject to interruptions as the radar rotates. For example, with 20% of the cells transmitting in the shared spectrum, on average the cellular system can achieve a data rate close to the one obtained in dedicated spectrum, even at less than 9% of the distance that secondary transmissions will not cause harmful interference in the radar's main beam. By evaluating quality of service, it is shown that spectrum shared with radar is attractive for applications that generate much of the traffic on the Internet, including video streaming, peer-topeer file sharing, downloads of large files, and web browsing, but not for an application sensitive to interruptions, like VoIP.
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