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Consider a wireless network that has two tiers with different priorities: aprimary tier vs. a secondary tier, which is an emerging network scenario withthe advancement of cognitive radio technologies. The primary tier consists ofrandomly distributed legacy nodes of density $n$, which have an absolutepriority to access the spectrum. The secondary tier consists of randomlydistributed cognitive nodes of density $m=n^\beta$ with $\beta\geq 2$, whichcan only access the spectrum opportunistically to limit the interference to theprimary tier. Based on the assumption that the secondary tier is allowed toroute the packets for the primary tier, we investigate the throughput and delayscaling laws of the two tiers in the following two scenarios: i) the primaryand secondary nodes are all static; ii) the primary nodes are static while thesecondary nodes are mobile. With the proposed protocols for the two tiers, weshow that the primary tier can achieve a per-node throughput scaling of$\lambda_p(n)=\Theta(1/\log n)$ in the above two scenarios. In the associateddelay analysis for the first scenario, we show that the primary tier canachieve a delay scaling of $D_p(n)=\Theta(\sqrt{n^\beta\log n}\lambda_p(n))$with $\lambda_p(n)=O(1/\log n)$. In the second scenario, with two mobilitymodels considered for the secondary nodes: an i.i.d. mobility model and arandom walk model, we show that the primary tier can achieve delay scaling lawsof $\Theta(1)$ and $\Theta(1/S)$, respectively, where $S$ is the random walkstep size. The throughput and delay scaling laws for the secondary tier arealso established, which are the same as those for a stand-alone network.

Throughput and Delay Scaling in Supportive Two-Tier Networks