The rapidly increasing power of personal mobile devices (smartphones, tablets, etc.) is providing much richer contents and social interactions to users on the move. This trend however is throttled by the limited battery lifetime of mobile devices and unstable wireless con-nectivity, making the highest possible quality of service experienced by mobile users not feasible. The recent cloud computing technology, with its rich resources to compensate for the limitations of mobile devices and

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... ons, can potentially provide an ideal platform to support the desired mobile services. Tough challenges arise on how to effectively exploit cloud resources to facilitate mobile services, especially those with stringent interaction delay requirements. In this paper, we propose the design of a Cloud-based, novel Mobile sOcialtV system (CloudMoV). The system effectively utilizes both PaaS (Platform-as-a-Service) and IaaS (Infrastructure-asa-Service) cloud services to offer the living-room experience of video watching to a group of disparate mobile users who can interact socially while sharing the video. To guarantee good streaming quality as experienced by the mobile users with timevarying wireless connec-tivity, we employ a surrogate for each user in the IaaS cloud for video downloading and social exchanges on behalf of the user. The surrogate performs efficient stream transcoding that matches the current connec-tivity quality of the mobile user. Given the battery life as a key performance bottleneck, we advocate the use of burst transmission from the surrogates to the mobile users, and carefully decide the burst size which can lead to high energy efficiency and streaming quality. Social interactions among the users, in terms of spontaneous textual exchanges, are effectively achieved by efficient designs of data storage with BigTable and dynamic handling of large volumes of concurrent messages in a typical PaaS cloud. These various designs for flexible transcoding capabilities , battery efficiency of mobile devices and spontaneous social interactivity together provide an ideal platform for mobile social TV services. We have implemented CloudMoV on Amazon EC2 and Google App Engine and verified its superior performance based on realworld experiments. Thanks to the revolutionary "reinventing the phone" campaigns initiated by Apple Inc. in 2007, smartphones nowadaysare shipped with multiple microprocessor cores and gigabyte RAMs; they possess more computation power than personalcomputers of a few years ago. On the other hand, the wide deployment of 3G broadband cellular infrastructures furtherfuels the trend. Apart from common productivity tasks like emails and web surfing, smartphones are flexing their strength-sin more challenging scenarios such as real-time video streaming and online gaming, as well as serving as a main tool for socialexchanges. Although many mobile social or media applications have emerged, truely killer ones gaining mass acceptance are stillimpeded by the limitations of the current mobile and wireless technologies , among which battery lifetime and unstablecon-nection bandwidth are the most difficult ones. It is natural to resort to cloud computing, the newly-emerged computingparadigm for low-cost, agile, scal-able resource supply, to support power-efficient mobile data communication. With virtuallyinfinite hardware and software resources, the cloud can offload the computation and other tasks involved in a mobileapplica-tion and may significantly reduce battery consumption at the mobile devices, if a proper design is in place. The bigchallenge in front of us is how to effectively exploit cloud services to facilitate mobile applications. There have been afew studies on designing mobile cloud computing systems [1][2][3], but none of them deal in particular with stringentdelay requirements for spontaneous social interactivity among mobile users.