  WiFi offloading is regarded as one of the most promising techniques to deal
with the explosive data increase in cellular networks due to its high data
transmission rate and low requirement on devices. In this paper, we investigate
the mobile data offloading problem through a third-party WiFi access point (AP)
for a cellular mobile system. From the cellular operator's perspective, by
assuming a usage-based charging model, we formulate the problem as a utility
maximization problem. In particular, we consider three scenarios: (i)
successive interference cancellation (SIC) available at both the base station
(BS) and the AP; (ii) SIC available at neither the BS nor the AP; (iii) SIC
available at only the BS. For (i), we show that the utility maximization
problem can be solved by considering its relaxation problem, and we prove that
our proposed data offloading scheme is near-optimal when the number of users is
large. For (ii), we prove that with high probability the optimal solution is
One-One-Association, i.e., one user connects to the BS and one user connects to
the AP. For (iii), we show that with high probability there is at most one user
connecting to the AP, and all the other users connect to the BS. By comparing
these three scenarios, we prove that SIC decoders help the cellular operator
maximize its utility. To relieve the computational burden of the BS, we propose
a threshold-based distributed data offloading scheme. We show that the proposed
distributed scheme performs well if the threshold is properly chosen.
