課程名稱︰無線多媒體系統研究 課程性質︰多媒體 課程教師︰吳曉光 開課學院：電資 開課系所︰資工 考試日期（年月日）︰ 2009.11.05 考試時限（分鐘）： 180 是否需發放獎勵金： 是 （如未明確表示，則不予發放） 試題 : 1. Fairness in 802.11 In this questio, we study the problem of maintaining fairness for upstream and downstram connections in wireless local area networks (WLANs) based upon the IEEE 802.11 standard. Current implementations of 802.11 use the so-caleed Distributed Coordination Function (DCF), which provides similar medium access priority to all stations.Although this mode of operation ensures fair access to medium at the MAC lavel, it does not provide any provisions for ensuring fairness among the upstream and downstream connections. Connection unfairness may result in significant degradation of performance leading to users perceiving unsatisfactory quality of service. (A) Can you explain why cyurrent 802.11 might produce this so-called "critical unfairness" between upstream and downstream connection as the attached Figure 2. (B) Can you revise IEEE 802.11 to solve this problem? Can you ensure fairness among the upstream and downstream connections? 2. Cellular wireless technology Fixed cellular wireless technology proviudes a means to offer broadband access to multimedia services without requiring extensive copper or fiber subscriber plant. Relatively wide unused spectrum at frequencies above 20GHz has led to the development of fixed broadband wireless access system operating in these bands, such as LMDS (Local multipoint distribution system) (A) Can yo uexplain why LMDS does not turn out to be one of the major broadband solutions as ADSL? (B) Can you explain what is software radio and cognitive radio (C) Can you explain why the first generation cellular wireless mobile system was once projected by some that the cellular industry would only see limited growth? (D) Given cellular phone with 1W transmitted power Unity gain antenna, 900MHz carrier frequency, SNR must be at least 25db for proper reception, Receiver BW is B = 30KHz, noise figure F=10db. Determine the maximum distance (free space n=2 and shadowed urban n=4)? 3 Packet switched To deliver an Internet connection wirelessly, good packet-switched networks are desired. This raises the question as to whether voice over IP will be feasible and an all-IP wireless network possible for efficient delivery of botrh voice and data once data traffic is comparable to or more than voice. (A) Provide the major callenges for an all-IP wireless network for both voice and data? (B) Explain the possible challenges to move from traditional circuit switched system to packet switched programmable system in mobile network area. (C) There are two types of geo-location techniques, one based on network and the other on the handset. Describe the difference of these two solutions. (D) Can you offer major reasons (boosts) for the success of the second generation cellular wireless mobile system? 4 Mobile station (A) When a mobile station moves into a different cell while a conversation is in Progress, the MSC automatically transfer the call to a new channel belonging to the new base station. Can you compare two methods to prioritize handoff: the guard channel method and queueing of handoff requestions (B) A given radio spectrum (or bandwidth) can be divided into a set of disjoint or non- interfering radio channel. All sych channels can be used simultaneously while maintaining an acceptable received radio signal. Channel allocation schemes can be divided into a number of different categories depending on the comparisionbasis. Describe FCA (Fixed Channel Allocation), DCA (Dyanmic Channel Allocation) and HCA (Hybrid Channel Allocation) (C) Describe what is the meaning of Handoffs, intra-cell handoff, inter-cell handoff? In order to determine handoff initiations, several stragies have bedd proposed: Relative signal strength, Relative signal strength with threshold, Relative signal strength with hysteresis (plus optinally dwell timer). Explain the approaches. (D) Discuss the advantages and disadvantages of Geo Satellitre Communication System and LEO (low earth orbit) Satellite Communication System in terms of propagation, ahdnoff and power issues. (E) Cellualr systems are designed to operate with groups of low-power radios spread out over the geographical service area. As the traffic grows and the blocking probability is high, usually what are your possbile solutions to decrease the blocking probability? -- 沒有不可能的事, 只有不願做的事 --

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