6.2 Wireless Transmission for Dynamic Spectrum Access 151 Spectral White Spaces Occupied by Secondary Users Frequency Spectral White Spaces Occupied by Primary Users FIGURE 6.2 The utilization of noncontiguous regions of spectrum for wireless transmission. unused portions can be used by secondary users for high-speed wireless com- munications while simultaneously ensuring that the primary user's rights are not violated. This idea of using multiple noncontiguous portions of spectrum is referred to as spectrum pooling.