A method for triggering spectrum sensing, said method comprising: receiving, by a central node, channel quality conditions notified by cognitive nodes, wherein the channel quality conditions are determined by the cognitive nodes from their monitored channel quality information respectively; attributing, by the central node, respective cognitive nodes operating at a same frequency to a same set; judging, by the central node, from the channel quality conditions notified by the cognitive nodes whether or not a channel quality corresponding to each set has declined to a requirement for performing in-band spectrum sensing; and triggering, by the central node, for each set, corresponding cognitive nodes to perform in-band spectrum sensing only upon determining that a channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing; wherein judging, by the central node, whether or not the channel quality corresponding to each set has declined to the requirement for performing in-band spectrum sensing comprises: determining, by the central node, for each set that the channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing if the number of declines of channel quality among channel quality conditions from cognitive nodes in the set is above a threshold; or determining, by the central node, for each set, a weight of each cognitive node from a distance between each cognitive node and a base station serving an authorized system in use and judging from weights of cognitive nodes in the set and received channel quality conditions whether or not the channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing; wherein the channel quality information comprises at least one of following information: an interference value, a received power value of a useful signal, feedback information, a synchronization status and Cyclic Redundancy Check, CRC, information; the performing in-band spectrum sensing refers to detecting a spectrum use status. receiving, by a central node, channel quality conditions notified by cognitive nodes, wherein the channel quality conditions are determined by the cognitive nodes from their monitored channel quality information respectively; attributing, by the central node, respective cognitive nodes operating at a same frequency to a same set; judging, by the central node, from the channel quality conditions notified by the cognitive nodes whether or not a channel quality corresponding to each set has declined to a requirement for performing in-band spectrum sensing; and triggering, by the central node, for each set, corresponding cognitive nodes to perform in-band spectrum sensing only upon determining that a channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing; wherein judging, by the central node, whether or not the channel quality corresponding to each set has declined to the requirement for performing in-band spectrum sensing comprises: determining, by the central node, for each set that the channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing if the number of declines of channel quality among channel quality conditions from cognitive nodes in the set is above a threshold; or determining, by the central node, for each set, a weight of each cognitive node from a distance between each cognitive node and a base station serving an authorized system in use and judging from weights of cognitive nodes in the set and received channel quality conditions whether or not the channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing; wherein the channel quality information comprises at least one of following information: an interference value, a received power value of a useful signal, feedback information, a synchronization status and Cyclic Redundancy Check, CRC, information; the performing in-band spectrum sensing refers to detecting a spectrum use status. determining, by the central node, for each set that the channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing if the number of declines of channel quality among channel quality conditions from cognitive nodes in the set is above a threshold; or determining, by the central node, for each set, a weight of each cognitive node from a distance between each cognitive node and a base station serving an authorized system in use and judging from weights of cognitive nodes in the set and received channel quality conditions whether or not the channel quality corresponding to the set has declined to the requirement for performing in-band spectrum sensing; wherein the channel quality information comprises at least one of following information: an interference value, a received power value of a useful signal, feedback information, a synchronization status and Cyclic Redundancy Check, CRC, information; the performing in-band spectrum sensing refers to detecting a spectrum use status. an interference value, a received power value of a useful signal, feedback information, a synchronization status and Cyclic Redundancy Check, CRC, information; the performing in-band spectrum sensing refers to detecting a spectrum use status.