using this spectrum have to cover their costs through licence fee or advertising income. Cellular operators using this spectrum have to recover their costs by charging an access fee. The assumption that unlicensed spectrum is inherently more cost economic than licensed spectrum is only partly true and dependent on certain preconditions. The 2.4-GHz and 5-GHz ISM spectrum is 'free' in as far as there are no license fees that need to be paid, but there are still access costs that need to be recovered. In domestic and SME applications, costs are limited to buying a wireless router and paying for an ADSL connection. Hardware costs are low because the ISM band is, more or less, a global allocation and this has allowed vendors to realise substantial economies of scale. However, in other applications, for example public WiFi in hotels, airports, trains and hospitals, significant hardware installation and real-estate costs can be incurred that can in turn prompt aggressive access pricing. Similar constraints would apply to white-space devices. For white-space devices to be successful they must have similar scale economies to existing WiFi and WiMax and cellular handsets. The devices must support equivalent or faster data rates than existing cellular products and/or cellular handsets and when used as portable devices should use similar amounts of DC power. However, the receive and transmit functions have to work across operational bandwidths that are far wider (when expressed as a percentage of the centre frequency of the band) than all other existing mainstream radio systems. Additionally, the receiver dynamic range needed from these devices is substantially more than cellular or WiFi handsets. This introduces a number of practical performance and cost issues. 15.9.1 Spectrum Sensing White-space devices need to be able to detect when signals are present and not present. The signals to be detected are either high-power TV or low-power wireless microphones One of the points when discussing the detection of TV signals is ho