1 Introduction
We are witnessing an unprecedented worldwide growth of mobile data traffic that is expected to continue at an annual rate of 45 percent over the next few years, surpassing 30 exabytes per month by 2020 [1]. Traditional network capacity expansion methods such as network technology upgrades and additional spectrum acquisition are costly, time-consuming, and outpaced by the continuing traffic increase. Mobile data offloading is a promising approach to utilize certain complementary transmission technologies to deliver mobile traffic originally targeted to cellular networks. A large number of studies have investigated the potential benefits of mobile data offloading and various innovative schemes have been proposed to better manage data flows including WiFi [2], [3], [4], [5], femtocells [6], [7], [8], and opportunistic offloading [9], [10]. It is shown that in a typical urban environment, WiFi can offload about 65 percent cellular traffic and save 55 percent battery energy for mobile users (MUs) [3]. This performance gain can be further enlarged with the use of delaying transmission [11], [12]. The authors in [13], [14] investigated the network operators’ profit gains from offering dual services through both macrocells and femtocells. All of these studies have shown that mobile data offloading is a cost-effective and energy-prudent approach to resolve network congestion and improve user service quality.