roviding in power by the spreading gain, and thus appear as noise a look-through capability by duty-cycling transmit and to the CR, resulting in reduced link margin. The ability of monitor functions. The five methods shown in Fig- each of these communication systems to tolerate this ure 2.4b-9 avoid the legacy signal in various ways. reduced link margin is link specific and therefore a sub- The first method assumes a fixed bandwidth wave- ject of great concern to the legacy system operators. form: the center frequency may be adapted. The second An OFDM waveform, the third method in Fig- method assumes a variable bandwidth signal, such as ure 2.4b-9, has several benefits including flat fading a direct-sequence spread-spectrum waveform, where the subchannels, elimination of equalizer due to long symbol chip rate is adapted and the center frequency is adapted. time and guard intervals, the ability to occupy a variety of The third method uses a water-filling method to populate bandwidths that "fit to the available opportunity," and a subset of the carriers in an OFDM waveform. These the ability to null subcarriers to mitigate interference. three methods impact legacy signals very little if appro- Variable bit loading enables pre-nulling and dynamic priate guard bands are observed. The fourth method is nulling. Table 2.4b-5 compares several methods for dy- a direct-sequence spread-spectrum waveform that un- namic bit loading an OFDM waveform. Not loading derlies the legacy signals. The interference from this subcarriers occupied by legacy signals with guard bands underlay must be very small SO that legacy systems do not around them minimizes interference between CR and experience noise; thus the processing gain of the spread non-CR systems [37-39]. spectrum underlay must be very high. The last method Because spectrum utilization is a spatially and tem- shown also avoids the legacy signals by frequency hop- porally variant phenomenon, it requires repeated moni- ping into only unoccupied channels. toring and needs coope