Consider a wireless network of two tiers with different priorities: a primary tier and a secondary tier, which is an emerging network scenario with the advancement of cognitive radio technologies. The primary tier is constructed over static nodes of density n, which are randomly distributed and have an absolute priority to access the spectrum. The secondary tier contains mobile nodes of density m = n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ß</sup> with ß ¿ 2, which can only access the spectrum opportunistically to limit the interference to the primary tier. By allowing the secondary tier to relay the packets for the primary tier, we show that the achievable per-node throughput scaling for the primary tier can be improved to ¿ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> (n) = ¿(1/log n). In the associated delay analysis, two mobility models are considered for the secondary nodes: an i.i.d. mobility model and a random walk model. We show that the primary tier can achieve delay scaling laws of ¿(1) and ¿(1/S) with the two mobility models, respectively, where S is the random walk step size. Furthermore, we show that the primary tier can achieve a delay-throughput tradeoff of D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> (n) = O (n¿ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> (n)) with ¿ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> (n) = O(1/log n) for the random walk model. The throughput and delay scaling laws for the secondary tier are also established, which are the same as those for a stand-alone mobile network.
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