科技英语课件 Exercises
即使在子载波间存在频谱部分重叠，符号仍能无损失地恢复。为了实现最大的频谱效率，子载波之间的间隔通常选取为符号周期的倒数。因此基带子载波频率为fn = n/T, (n = 0, 1, ... , N?1)，第n个调制符号记为X(n)，在一个符号周期内的OFDM波形可表示：
n??x(t)??X(n)exp?j2?t?,0?t?T
T??n?0在t = T/N 时刻采样这个波形得到：
N?1(1)
kT?N?1nk???x(k)??X(n)exp?j2????X(n)exp?j2??,k?1,2,?,N?1
N?n?0N???n?0从上面的表示中可以看到，x(k) 和X(n)构成了一对离散傅里叶变换，因此基带OFDM波形
m
可根据N个调制符号的离散傅里叶变换来生成。当N=2，其中m为整数，IDFT的快速算法很容易实现。(2) Choose the word or phrase that is closest in meaning to the underlined part.
1. The process gain of a system indicates the gain or signal to noise improvement exhibited by a spread spectrum system by the nature of the spreading and despreading process. A. by virtue of the spreading/dispreading process B. by processing the spreading/despreading naturally
C. according to the spreading/despreading characteristics D. using the natural spreading/dispreading process
2. Due to the unavoidable timing errors between the users, there is little point in using Walsh codes as they will no longer be orthogonal. A. using Walsh codes will have some point due to the absence of orthogonality B. using Walsh codes is meaningless since orthogonality will not exist any more C. Walsh code cannot be used since they are not orthogonal
D. it is unnecessary to use Walsh codes because orthogonality is lost
3. For example, when a part of the channel bandwidth is faded, frequency interleaving ensures that the bit errors that would result from those subcarriers in the faded part of the bandwidth are spread out in the bit-stream rather than being concentrated. A. that would be produced in those subcarriers suffering from frequency selective fading B. that would cause some subcarriers being faded in some part of the bandwidth C. that would came from the fading subcarriers falling in part of the bandwidth D. that would result in those faded subcarrier of the part of the bandwidth
4. The basic idea of applying the beamformer technique to this approach is very promising and might give a significant improvement for source localization. A. might obtain formidable gain in local sources
B. provide possibilities to substantially enhance source locations C. might present a major advance in localizing sources
D. is likely to considerably refine techniques to locate sources
5. Early we introduced the concept that the identity of an amplitude modulated message signal may under certain circumstances be communicated by transmitting regular samples of the message, rather than the continuous signal. A. values of the message signal taken at regular intervals B. formally obtained message samples C. samples of the message produced in a specific manner D. samples of the regularly obtained message
6. Having defined these functions, we will proceed to show how they can be used to evaluate certain integrals characteristic of system error performance. A. evaluate characteristics of certain integrals in system error performance
B. assess certain integrals that are typical in describing error performance of the systems C. decide the value of some characteristic integrals of error performance of the systems D. carry out evaluations on the integrated characteristics of system error performance 7. Modulation is the systematic variation of some attribute of a carrier waveform such as the
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N?1(2)
科技英语课件 Exercises
amplitude, phase, or frequency in accordance with a function of the message signal. A. in terms of a message carried by the signal B. according to the behavior of the signal C. due to the performance of the signal D. in relation with a quantity derived from the signal 8. Over recent years this potential has largely been realized in the costs of the optical fiber transmission medium that for bulk purchases is now becoming competitive with copper wires. A. that is specifically for purchases, and is becoming superior to copper wires B. that is bulkily comparable in price with copper wires
C. that is competing with copper wires in terms of the scale of purchases
D. that is becoming a challenge to copper wires when bought in a large quantityUnit
6 Exercises (1) Translate the following sentences into Chinese.
1. Communication is the transmission of information from one point to another. This transmission requires the ability to vary signals with time in a manner which is unpredictable to the receiver. 通信就是信息从一点传送到另一点。这种传送要求信号随时间发生的变化不能在接收端预测。
2. It is true of any communication system that the shape and amplitude of the transmitted signal will be continuously degraded by the introduction of noise, and the attenuation along the transmission path. 传输的信号会不断地被噪声的引入和线路上的衰减所损伤，对任何通信系统都是如此。
3. Since the very beginning, communications systems have consisted of three major parts, namely, the equipment, the medium, and the protocol. It is still the case today. However, the close link of telecommunications with the computer technology has brought about tremendous changes in communications, from the concepts, to the contents and the methods. 从一开始通信系统就由设备、媒介和协议三个要素组成。至今依然如此。然而电信与计算机技术的紧密联系使得通信从概念到内容和手段都发生了重大的变化。 4. The availability of such a large collection of system performance curves in a single compilation allows the researcher or system designer to perform trade-off studies among the various communication type/fading channel/diversity combinations so as to determine the optimum choice in the face of his or her available constraints. 在一本书中提供如此大量的系统性能曲线，使研究者或系统设计者能在各种通信类型、衰落信道、分集组合之间进行比较研究，从而在他或她所面临的限制之下做出最佳的选择。
5. The channel provides the electrical connection between the information source and the user. The channel can have many different forms such as a microwave radio link over free space, a pair of wires, or an optical fiber. 信道提供信息源和用户之间的电路联接。信道可以有许多不同的形式，比如自由空间里的微波联接，一对电线，或者一根光纤。
6. The degradation of the transmitted signal is a result of signal distortion due to imperfect response of the channel and due to undesirable electrical signals (noise) and interference. Noise and signal distortion are two basic problems of electrical communication. 传输信号的退化是信号失真的结果，而这种失真又是由信道的非理想响应及不想要的
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科技英语课件 Exercises
电信号（噪声）和干扰引起的。噪声和信号畸变是电子通信的两个基本问题。 (2) Choose the phrase that is closest in meaning to the underlined part.
1. This is possible because stations that are physically far enough from each other can use the same carrier frequency (frequency reuse) with negligible mutual interference, except at some point in the middle where the stations are received with comparable signal levels. A. where the signals received from the stations are relatively strong B. where the signals from different stations are similar in level
C. where the receiving stations can be compared with the signal levels D. where the stations can receive comparatively high level signals
2. Space diversity or antenna diversity employs two or more antennas spaced apart by a significant fraction of the wavelength so as to achieve a higher probability of receiving a non-faded signal.
A. placed sufficiently apart measured by the wavelength
B. far enough to make the wavelength only a fraction of the spacing C. separated by a distance less than a wavelength but not too small D. divided by a few wavelengths
3. Wireless services have been growing at a rate greater than 50% per year, with the current second-generation European digital systems (GSM) being expected to be filled to capacity by the early 2000s. A. going to become saturated by the early 2000s B. going to fill the gap by the early 2000s C. presumably being expanded to the capacity by the early 2000s D. expectedly being exceeding the filled capacity by the early 2000s
4. As long as 750Mbps lines are not likely to be a part of the telecommunication system generally accessible to the public in the near future, the latter strategy appears to be more feasible.
A. In case probability is slim to incorporate 750Mbps lines into the telecommunication
system
B. Provided 750Mbps lines do not like to become a part of the telecommunication system C. So far as 750Mbps lines do not take part of the telecommunication system D. With long lines of 750Mbps not included in the telecommunication system
5. While the subject of coding often carries an air of secrecy, a more important motive in many modern coding systems is the improved efficiency in conveying information. A. always brings secret information in the air
B. is generally transmitted through air and kept secret C. often uses secret carriers D. is considered mysterious and often used in secret communication 6. Our reliance on computer technology has left us vulnerable to attack, and the vulnerability creates difficult political dilemmas that must be dealt with should we wish to continue following the currents of the Third Wave. A. has left us to launch attack
B. has made us immune against hostile actions C. has made us susceptible to assault D. has left us voluntary in offense
7. Our reliance on computer technology and our quick transition into a knowledge-based economy have left us vulnerable to attack, and that vulnerability creates difficult political dilemmas that must be dealt with should we wish to continue following the current of the Third Wave.
A. we should wish to go on after the following current B. so that we should carry on the current
C. only if we want to continue following the current D. if we wish to go ahead to follow the current
8. We shall use this parameter as a measure of degradation during the evaluation of different quantization systems later in this chapter. A. an action of processing
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科技英语课件 Exercises
B. a degree of enhancement C. an amount of impairment
D. an effect of interferenceUnit 7 Exercises
(1) One of the underlined part in each sentence is incorrect. Try to find and correct it.
1. In PCM each amplitude value is approximate (close) to the nearest permitted level, which is represented by a set of pulses. 2. If the study shows the temperature rising, Munk said, it could provide clear evidence of global warming and help governments planning (plan) how to reduce atmospheric carbon gases believed responsible. 3. It was designed to withstand nuclear war, could (should) one node be taken out in, say, Salt Lake City, whatever was left of the network was designed to carry on, routed around the dead zone.
4. Microwaves are electromagnetic waves that have a frequency range from around 0.3 GHz (there is no actual specified lower frequency limit) to 300 GHz with corresponding wavelengths range (ranging) from 1m to 1mm. 5. One problem that is common with all satellite communication systems is the time delay for (in) a signal to pass from earth to the satellite, and return to earth again. 6. The amplitude of the signal is linear (linearly) proportional to the time interval between the mean sampling instant and the instant when the signal intersects the sloping edge of the ramp.
7. There are good technical reasons for the choice that was made, and careful consideration has been given to the interface requirement that bridges the differences between the approaches of different industries and make (makes) the system interoperable. 8. The widespread introduction of digital transmission equipment within the telephone networks have (has) not been rapid since the original French patent was issued in 1939. (2) Choose the word or phrase that is closest in meaning to the underlined part.
1. Television became the main market, its demand for simultaneous delivery of few signals of large bandwidth to many receivers being a more precise match for the capabilities of geosynchronous comsats.
A. requirement for synchronized transmission of some large bandwidth signals to many
users
B. need for sending a small variety of wideband signals to a large number of users
simultaneously
C. demand for simulation of signal delivery with large bandwidth to many users D. need for providing simultaneous signals with large bandwidth to many receivers
2. By utilizing a 9600 bit/s connection that is offered in standard GPRS, the upload volume is
very low, and since this service is not per-time charged, but charged by volume uploaded, users are able to surf and download in broadband speeds.
A. this service takes charge of volume uploading instead of average time
B. payment of this service is not made each time, but when the volume is uploaded
C. cost of this service is based on the quantity of uploaded data rather than on the
connection time
D. no time is charged for this service, but the volume is charged while being uploaded 3. Remote sites use the proxy server at the earth station, which is configured to route all
outbound traffic to the QoS server, which makes sure no user exceeds the allotted bandwidth or monthly traffic limits. A. no user allotted bandwidth and monthly traffic limits excessively
B. no user gets more bandwidth than assigned and more data flow each month than
allowed
C. no user can execute the bandwidth allocation and monthly traffic limits D. no user takes surplus bandwidth or limits monthly data traffic
4. The development of optical fiber over the last twenty years has resulted in the production
of optical fiber cables that exhibit very low attenuation and transmission loss in comparison with the best copper conductors.
A. cause very small reduction in the power of signals traveling over them B. make very severe enhancement resulting in loss of transmission C. show very weak coupling and infrequent transmission failure
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