oordination of multiple radios within a single device, such as the Wi-Fi plus Bluetooth coexistence measures described in the Section "Bluetooth Coexistence with 802.11 WLANs, p. 247", or the coordination of radios via a separate common spectrum coordination channel (CSCC) that connects devices using beacon broadcasts in an edge-of-band channel, or by some other means, including potentially via the Internet. The 802.22 approach is based on vertical sharing, and the key challenge here is spectrum sensing - reliable dynamic detection of the presence of a primary (licensed) user operating in or returning to a channel. Three possible approaches to spectrum sensing are summarised in Table 14-5. Table 14-5: Spectrum Sensing Approaches for Vertical Spectrum Sharing Spectrum sensing Characteristics approach Matched receiver A receiver matched to the signal characteristics of the primary user (coding, modulation and synchronisation) and with knowledge of other signal characteristics and channel usage (spreading codes, pilots and training sequences, etc.). This provides very reliable detection but is relatively inflexible, although this can be partly overcome using a software defined radio. Energy detection Similar to a spectrum analyser, the receiver determines the presence or absence of a primary user by aggregating energy detected in the target channel. This can be effective for narrow band signals, but DSSS, FHSS or other wideband signals may not be detected. Feature detection Periodic features of modulated signals are detected by computing a spectral correlation, which detects periodicity from pulse trains, hopping cycles, cyclic prefixes, etc. Chapter Fourteen Using these sensing approaches, a spectrum agile radio would build up a local spectrum map, identifying the detected power level, signal type (e.g. FM, FHSS, OFDM) and bandwidth in use in each channel, as well as estimating the quality of empty channels in terms of their potential As an alternative to each spectrum agile radio making decisions based on its loca