【正文】 ortedly under development [4]. The mobile FM receiver and transmitter are intended to allow users to listen to the broadcasted programs, and stream music to shortrange home and automobile stereos, respectively. However, we go beyond such intended uses of the mobile FM radio and enable data broadcasting applications based on existing hardware.We reveal two challenges toward enabling practical and deployable data broadcasting applications. First, simultaneous broadcasters must not collide. Therefore, they should coordinate with each other to use the available FM channels while remaining quickly identifiable by interested listeners. Unlike radios in white space, FM radio has a much shorter range (~5 meters) and multiple orthogonal channels so that the solution requires special treatment toward the hiddennode problem and channel allocation. Second, the solution must not modify the FM radio hardware, but has to rely on the device’s audio interface to realize data broadcasting. Therefore, binary data must be properly converted into audio, and the broadcaster and listener must be symbolsynchronized using software.We address these challenges with μStation, a software solution that operates without modification to device Paper Session: DIY and Design hardware, operating system, and the FM radio driver. μStation has two key modules: Channel Selector and Data Codec. First, Channel Selector enables automatic and efficient FM channel allocation, so that nearby broadcasters use orthogonal channels with high probability. Meanwhile, it ensures listeners to find the channel with broadcasting of interest timely, by letting broadcasters periodically announce their presence. Second, Data Codec performs conversion between binary data and audio to patibly work with the FM radio hardware. More importantly, it effectively achieves symbol synchronization in software by leveraging different sampling frequencies in the FM transmitter digitaltoanalog converter (DAC) and FM receiver analogtodigital converter (ADC).We have implemented μStation on the Nokia N900 smartphone and provided both lowlevel APIs and services to support a large set of mobile applications. We have also developed two representative applications using μStation to demonstrate its effectiveness. First, FacebookFM broadcasts a user’s Facebook ID so that nearby users can load the corresponding Facebook profile to facilitate social interaction. Second, SyncFlash realizes a synchronized flashing pattern using many smartphone screens from users physically close to each other, ., the audience of a concert or a sports game.As the first publicly reported system that aims to enable ubiquitous data broadcasting applications using existing mobile FM radio hardware, μStation has offered the FM radio, which possibly obtains little attention before, significant potential to facilitate the interaction between physically adjacent mobile devices. We hope our initial endeavor with μStation can enlighten more innovative ways to leverage the FM radio as an important hardware feature on current and emerging mobile devices.BACKGROUND OF MOBILE FM RADIOWe next provide background of the mobile FM radio.Spectrum UsageThe spectrum usage of FM radio falls into the UHF band and spans from MHz to MHz in the . In this case, the spectrum is equally divided into 103 orthogonal channels, each of which is 200 KHz apart from its neighbors. A FM channel can be identified by its center frequency, or by a unique number (., Channel 1 for the channel at MHz). In each FM channel, FM radio uses 15 KHz and 30 KHz bandwidth for broadcasting monaural and stereo audios respectively. The remaining bandwidth is used for sideband applications such as the Radio Broadcast Data System (RBDS otherwise known as RDS in Europe) [5], and the currently obsolete Microsoft DirectBand [6].Both licensed and unlicensed FM broadcasting use the same set of channels. Licensed broadcasting usually es from radio stations owned by mercial organizations or educational institutions, with a fixed coverage and schedule.Figure 1: FM radio hardware architecture on mobile devicesAs a result, given time and location, licensed broadcastings are often highly predictable, as indicated by [7].Regulations for Unlicensed BroadcastingMost countries allow unlicensed FM broadcasting. The US FCC regulates the output power and mandates that the field strength of any unlicensed FM emissions must not exceed 250 microvolt/meter at 3 meters, leading to a maximum output power of approximately 15 nW. The EU regulation is similarly enacted but the maximum allowed output power is 50 nW. Australia allows output power as high as 10 μW, but only if the unlicensed transmission does not interfere with present licensed broadcastings. We note that a few countries such as China do not yet allow unlicensed FM broadcasting。 mobile devices used in these countries usually have the FM transmitter disabled. Due to our implementation and experimental constraints, we work under the US FCC regulations.We define available FM channels in the following way: a FM channel is available if the channel RSSI under licensed broadcasting is below certain threshold (., 110dBm in our implementation). We note that a FM channel can be available even with present licensed broadcastings, as long as the licensed broadcasting is sufficiently weak. As a result, checking the availability of a FM channel can be simply done by measuring the channel RSSI, which is supported by mobile FM radio. Our RSSI based definition ensures the quality of unlicensed broadcastings, yet there can be alternative ways to define available FM channels such as using the human perceivable quality of licensed broadcasting. Nonetheless, μStation is agnostic of the definition of channel availability, as we will show later.Hardware ArchitectureWe show the hardware architecture of mobile FM radio in Figure 1. We note that the FM transmitte