| With the knowledge that a majority of licensed spectrum is underutilized in both time and frequency, the concept of dynamic spectrum access (DSA) has been proposed to alleviate the spectrum scarcity problem that wireless communications face today. In DSA networks (DSAN), as being standardized in IEEE 802.22, Cognitive Radio (CR) has been employed as an enabling technology to allow unlicensed radio transceivers to operate in the licensed bands at locations where that spectrum is temporally not in use.;One of the key challenges of the CR-based DSANs is to address two conflicting requirements: QoS assurance for DSANs and reliable protection for licensed user. This problem is overcome by a technique proposed in IEEE 802.22 called Dynamic Frequency Hopping (DFH) where data transmission of a DSAN is performed in parallel with spectrum sensing. We present the principle of DFH and the coordination mechanisms that allow multiple DSANs operating in DFH mode to achieve efficient spectrum usage and reliable spectrum sensing. Seemingly, dynamic sharing of the scarce spectrum among the collocated networks is another challenge of DSANs. We describe a distributed spectrum sharing protocol called On-demand Spectrum Contention (ODSC) that uses interactive MAC messaging to enable efficient, scalable, and fair inter-network spectrum sharing. Additionally, in order to support coordinated DFH, ODSC, and other inter-network coordination functions, we introduce a beacon-based inter-network communication protocol called Beacon Period Framing (BPF) that realizes reliable, efficient, and scalable over-the-air inter-network communications. |
