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： 电资学院 开课系所︰ 资工系 考试日期（年月日）︰ 2011.05.13 考试时限（分钟）： 180 是否需发放奖励金： YES （如未明确表示，则不予发放） 试题 : 1 (CARA, Collison Aware Rate Adaptation Protocol) Today's IEEE 802.11 WLANs (Wireless LANs) provide multiple transmission rates so that different rates can be exploited in an adaptive manner depending on the underlying channel condition in order to maximize the system performance. Many rate adaptation schemes have been proposed so far while most (if not all) of the commercial devices implement a simple open-loop rate adaptation scheme (i.e., withouy feedback from the receiver),call ARF (Automatic Rate Fallback) due to its simplicity. The following figure shows IEEE 802.11 DCF throughtput as a afunction of the number of stations from an indoor table top experiment for different data rates of 802.11b-11Mbps, 5.5Mbps, 2Mbps, and 1Mbps-and dynamic rate shifting ("auto rate") implemented in Enterasys WLAN cards. (A) Explain the throughput degradation in the figure. (B) Explain clearly how CARA could address this problem. Describe (i) the way to distinguish packet errors. (ii) the way to enhance IEEE 802.11 protocol. 2. (Fairness in 802.11) I. In this question, we study the problem of maintaining fairness for upstream and downstream connections in wireless local area networks (WLANs) based upon the IEEE 802.11 standard. Current implementations of 802.11 usee the so-called priority to all stations. Although this mode of operation ensures fair access to the medium at the MAC level, 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 current 802.11 might produ ce this so-called "critical unfairness" between upstream and downstream connection as the attached Figure 3. (B) Can you revise IEEE 802.11 to solve this problem? Can you ensure fairness among the upstream and downstream connections? II. Given you following case for 802.11 CSMA/CA in figure 4: 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 re 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) It is widely recognized that as time progressed, the number of short wires connecting computer peripherals has been increasing day be day. Low-cost, low-power, radio-based wireless links eliminate the need for short cables. An infrared (IR) link can easily provide a link with speeds up tp 10 Mbps at very low case and ease of installation. (A) What are the advantages of Bluetooth over IR (infrared) for short range wireless communication? (B) Bluetooth utilizes the unlicensed ISM band at 2.4 GHz. Explain how Bluetooth could avoud collision or interference (such as hidden terminal or exposed terminal problem). (C) Describe how Bluetooth can support viuce and data communications with limited QoS support. (D) Describe how Bluetooth device performs "peer dicovery" to identifu devices in range that wish to paticipate in the piconet as FIgure 5. (E) Explain why the slave performs a random backoff and then sends an inquiry response after receiving an inquiry packet. 4. (Cellular Concept) (A) If a signal to interference ratio of 10 db is required for satisfactory forward 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 (1)n=4, (2)n=2? Assume that there are 6-co-channels 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 1,3,4 ,7 9, 12, 13, 16, 19, 21, 28...etc) (B) Assumes you have 5100 subscribers ineach service cell and each subscriber generates 0.01 Erlangs of traffice. For 0.2% blocking probablity, how much total bandwidth should be allocated for a FDD cellular telephone systems which usestwo 25KHz simplex channels to provide full duplex voice and control channels for problems A. 5. (DS-CDMA) (A) Direct sequence cellular system adopts universal frequency reuse policy. Explain what is universal frequency reuse policy? What is the advantage and disadvantage of this policy? (B) CDMA also provides a natural way to exploit the burstly nature of a source for added capacity. Can you explain how CDMA improves the capacity in the case of a two-way telephone conversation? (C) Explain why DS (Direct Sequence) waveforms can be used to either reject multipath returns that fall outside of the correlation internal of the spreading waveform, or enhance overall performance by diversity combining multipath returns in a RAKE receiver. (D) Explain why reverse link is vulnerable to the "near far" problem, and rhat is solution to maximize the system capacity? (E) how WCDMA could provide flexible services (flexible bit rates) for different find of users? -.-.-.-.-.-.-.-.-.-.--.-.-.-.-.- -.-.-..-.-.-.-.-.-.-.-.-.--.-..- -- 没有不可能的事, 只有不愿做的事 --

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