Signal Processing for Cognitive Radios

14.3.2.2 Optimal Cooperative Channel Assignment at the SSDC

Since the secondary cognitive radio system is a centralized system, however, it is not the individual secondary cognitive radio’s utility that is of importance in making decisions but the overall gain to the secondary network as a whole. Hence, the SSDC is responsible for making final decisions on which secondary user should cooperate with which primary channel. A reasonable objective for the SSDC is to make bids for cooperation with primary users so as to maximize the secondary network sum rate.

At the beginning of each time frame, each secondary user computes the fractions {β_j,i}_i∈Ωj of its power that it is willing to allocate for relaying primary signal as described above, for example, in (14.42). They then informs these {β_j,i}_i∈Ωj values to the SSDC (over a control channel) as shown in Figure 14.14. The SSDC uses these {ß_j,i}_i∈Ωj, for j ∈ K_s, values and its knowledge of channel-state information (fading coefficients) to determine the cooperative channel assignment for each secondary user so as to maximize the secondary system’s sum rate, as shown in Figure 14.14. The SSDC then broadcasts these decisions back to the secondary users through a control channel [133].

Formally, we may define the scheduling function at the SSDC that assigns each secondary user to a primary channel for possible cooperation as a mapping