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This paper is envisaged to provide a first quantitative study on how much indoor deployed Wi-Fi can offload the operator's 3G HSPA macro cellular networks in a real large-scale dense-urban scenario. Wi-Fi has been perceived as a cost-effective mean of adding wireless capacity by leveraging low-cost access points and unlicensed spectrum. However, the quantitative offloading gain that Wi-Fi can achieve is still unknown. We studied the Wi-Fi offloading gain as a function of access point density, where it is shown that 10 access points/km2 can already boost average user throughput by 300% and the gain increases linearly proportional to the access point density. Indoor Wi-Fi deployment also significantly reduces the number of users in outage, especially for indoor area. A user is considered to be in outage if they have a user throughput less than 512 kbps. We also propose three Wi-Fi deployment algorithms: Traffic-centric, Outage-centric, Uniform Random. Simulation results show that Traffic-centric performs best in boosting average user throughput while Outage-centric performs best in reducing user outage. Finally, Wi-Fi offloading solution is compared with another offloading solution - HSPA Femto cell. We show that Wi-Fi provides both much higher average user throughput and network outage reduction than HSPA Femto cells by exploring 20 MHz unlicensed ISM band.