课程名称︰无线多媒体系统研究 课程性质︰多媒体 课程教师︰吴晓光 开课学院：电资 开课系所︰资工 考试日期（年月日）︰ 2010.04.02 考试时限（分钟）： 180 是否需发放奖励金： 是 （如未明确表示，则不予发放） 试题 : 1. Mobile radio Propagation THe mobile radio channel places fundamental limitations on the performance of wireless communication systems. The transmission path between the transmitter and the receiver can vary from simple line-of-sight to one that is severely obstructed by buildings, mountains, and foliage. Modeling the radio channel has historically been one of the most difficult parts of mobile radio system design, and is typically done in a statistical fashion, based on measurements made specifically for an intended communication ssystem or spectrum allocation. (A) Usually the giher carrier frequency (such as 1800 MHz) or the lower carrier frequency (900MHz) will experience larger path loss? (B) What is the path loss exponent n? Usually what determine the value of n? (C) List the factors to determine the link budget (SNR) (D) Describe the two main factors to cause the small-scale fading. (E) Explain the ways (at least two schemes) to combat small-fading. (F) Explain the following three interference sources in the celllar system (G) Explain the large area interference problem for IEEE 802.11 (H) Explain how the intelligent antenna technique can improve the system capacity or performance. 2. (CSMA, MACA, CSMA/CA Protocols) In recent years, a wide variety of mobile computing devices has emerged, including portables, palmtops, and personal digital assistants. Providing adequate network connectivity for these devices will require a new generation of wireless LAN technology. (A) Describe how CSMA protocol may cause and experience "hidden terminal" and "exposed terminal " problems? (B) Explain the conditions for "collision,""caption",and "interference"? (C) Karn proposed MACA (RTS/CTS/Data without carrier sense) as an alternative to traditional CSMA scheme. Explain why MACA might solve problem (A). (D) Can you explain the problem for MACA scheme? (E) Is CSMA/CA an appropriate access protocol for Geo-Satellite system? Explain your answer. (F) Can CSMA/CA avoid hidden terminal problem completely? Explain. (G) Given the following case for 802.11 CSMA/CA: A two cell configuration where both pads are in range of their respective base stations and also in range of each other. The base stations are sending data to their respective pads. Explain the reason that S2 stream might be blocked by S1 stream. Do you have solutions to address this problem? 3 Bluetooth Spread spectrum multiple access users signals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth. There are two main types of spread spectrum mutiple access techniques: frequency hopping spread sprectrum (FH-SS) and direct sequence spread spectrum (DS-SS) (A) Bluetooth adopts FH-SS or DS-SS? (B) Explain how Bluetooth devices avoid collision with other Bluetooth devices (in the same personal area network and in the different personal area networks). (C) Can you explain how Bluetooth devices support (offer) real time traffic (such as Audio) and non-real time traffic (such as Data) services? (D) Explain why Bluetotth systems require master stations? (What major functions master station will offer?) (E) What are the advantages of Bluetooth over IR(infrared) for short range wireless communication? (F) Describe how IEEE 802.11 supports voice and data communications as well. (G) Can you explain how Bluetooth adopts a frequency hopping TDD scheme for each channel? 4. (A) SIFS (short inter frame space) is used to separate transmission belonging to a dialog (such as RTS-CTS, fragent-ACK). Usually the value will be determined by what factor? (B) Can you explain the interference problem in 802.11? Do you have any solution to account for this problem? (C) Can you explain the capture problem in 802.1? Do you have any solution to address this callenge? (D) Usually power saving could be a accomplished by reducing the transmitting power and properly coordinatin the active periods and stand-by period. Could you describe how 802.11 performs power saving mode in infrastructure mode? (E) In a shared-channel multi-cell envinronment, can you provide two scenarios that two neighboring base stations might conclude wrongly that their clients might be either turned off or in a very congested area? 5. (A) If a total of 33 Mhz of bandwidth is allocated total particular FDD cellular telephone system which uses two 25 Khz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system users (1) 3-cell reuse (2) 7-CELL reuse (3) 12-cell reuse. (B) if a signalto interference raio of 15 db is required for satisfactory foreward channel performance of a cellular system, what is the frequency reuse factor(D/R) and cluster size (N) that should be used for maximum capacity if the path loss exponent is (a) n= 4 (b) n=3? Assume that there are 6-cochannels cells in the first tier, and all of them are at the same distance from the mobile. Use suitable approximations (Candidate Cluster sizes could be 3,7,12,13) (C) How many users can be supported for 0.5% blocking probability for the following number of trunked channels in a blocked calls cleared systems (A)2, (b) 5 (c) 10 (d) 20 (e) 100 Assume each user generate 0.1 Erlangs of traffic. -- 叶子的离开, 是因为风的追求, 还是树的不挽留 --

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