We propose a concept for public safety communication realized with IMS (IP Multimedia Subsystem), the cellular standards of 3GPP and packet switched transmission. Basing the solution on mainstream cellular technology leverages the economy of scale of today's commercial networks and enables migration of technical solutions and applications.Important requirements of the Public Safety sector are group communication, low latency, high capacity, security, reliability and interoperability for voice and broadband data services. Our analysis shows that the concept has the technology potential to meet these public safety requirements.

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Abstract-HSDPA (High-Speed Downlink Packet Access), introduced in WCDMA release 5, provides a high-bandwidth shared channel with short transmission time interval (TTI). The short TTI together with appropriate scheduling enable HSDPA to support efficient multiplexing of traffic.We explain the performance of four scheduling algorithms when transmitting a traffic mix consisting of both conversational (VoIP) traffic and background (web) traffic over the high-speed downlink shared channel (HS-DSCH) of HSDPA. We consider both cell throughput and user satisfaction. The proportional fair (PF), the maximum rate (MR) scheduler and two extended versions of MR, are tested for different VoIP scheduling delay budgets and varying load.To understand the behaviour of the schedulers, we use the ns-2 simulator extended with a model of HS-DSCH to simulate a mixed traffic scenario. Our results show that a scheduler that gradually increases the VoIP priority and considers the user's current possible rate, performs well. A more drastic increase in VoIP priority is however needed when the delay budget is short. Furthermore, attempting to uphold quality for both VoIP and web traffic makes the system sensitive to overload situations.

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