Обучение чтению литературы на английском языке по специальности «Автономные информационные и управляющие системы» (96

sense. If the resonant frequency of the supporting structure is within the signal band or not much higher than the roll-off, the vibrations of the PC board and its mounting system will show up in sensor output.

The best way to minimize these effects is to make the mounting scheme as stiff as possible, thereby transmitting the system acceleration more faithfully to the sensor and increasing the resonant frequency. Since a PC board is much stiffer in its plane than perpendicular to its surface, the accelerometers sensitive axis (both axes, if dual) should be in the plane of the board. Because the ADXL 150 and ADXL 250 have their sensitive axes in the plane of the chip, and the surface of the chip

The sensitive axis were perpendicular to the plane of the chip (as is the case for some bulk-micromachined sensors), soldering the package to the board. Thereby it makes the measurement most susceptible to PC-board flexibility. A right-angle mounting system could be used to orient the sensitive axis parallel to the PC board, but the mounting system itself can deform, producing false acceleration readings. The mounting system, and PC board stiffness, add cost to the acceleration measurement. Also the additional mass of the mounting system lowers its resonant frequency causing larger false acceleration signals.

14. Translate texts IID and IIE in the written form without a dictoinary.

Text IID. The New Product

The TMP 17 is a monolithic temperature-to-current transducer that provides an output current proportional to absolute temperature (PTAT), 1 microampere per Kelvin, for temperatures from – 40 to +105 °C (233 to 378 K). Current is independent of applied voltage, for supplies from +4 to +30 V.

The device is inherently linear (within 0.5 °C) and stable (repeatability within 0.2 °C), and initial calibration error (25 °C) is ± 2.5 °C max (f grade). Initial calibration error can be trimmed out by adjusting the measuring resistance that converts current to voltage.

Because the TMP 17 is a current source, its output is immune to voltage noise pickup and IR drops in the signal leads in remote applica-

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tions. The TMP 17’s small size, low cost, stability and simplicity make it useful in applications such as cold-junction compensation, thermal compensation of instrumentation, and over-temperature warning system in automotive, HVAC1, and industrial temperature control.

The TMP 17 is housed in a tiny SO-8 package and available in F and G grades.

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1 HVAC (Heating Ventilation and Air Conditioning) – нагревание, вентиляция и кондиционирование воздуха.

Text IIE. Ultrafast and Fast 14-bit A/D Converters

The AD 9243 and AD 7851converters are available today. They are smaller, less costly and run much cooler than hybrid devices of comparable speed and resolution.

With its 3-MPS sampling rate and 40-MHz full-power signal bandwidth the AD 9243 creates a new IC speed class, especially useful in communications and imaging, but of interest in many other fields. Its low dissipation, single 5-V supply, differential inputs and internal reference as well as its low price and small package make possible new applications including providing an ideal upgrade for 12-bit devices.

Besides its 333-ksps sampling rate, 20-MHz full-power signal bandwidth and serial interface, the AD 7851 has a number of useful features. For example, its already low maximum dissipation of <90 mW can be further reduced to 52.5 μW max in sleep mode; with the external clock on, dissipation in that mode is increased to only 105μW. The device contains selfand system calibration options to ensure accurate operation over time and temperature. It powers up with a set of default conditions and can be operated as a read-only ADC.

Both devices are available for – 40 to + 85 °C operation. The AD 7851 has a choice of 24-pin DIP1 and SOIС2.

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1DIP (Dual in line package) – корпус с двухрядным расположением выводов.

2SOIC (Silicon on integral circuit) – интегральная схема в корпусе типа SO.

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UNIT III

New Words and Word Combinations

arbitrary modulation – модуляция произвольного вида arbitrary modulation technique – произвольная техника

модуляции

bandwidth n – ширина полосы частот, ширина спектра сигнала dispersive channel – канал с дисперсией

discard v – раскладывать eliminate v – устранять employ v – использовать

impairement n – неполадка; недостаток

inter-symbol interference – межсимвольное искажение внутри сигнала

invoke v – вызывать, инициировать map v – распределять

multiplicity n – множество multiplexing n – частотное уплотнение recover v – восстанавливать

robust a – надежный sample n – выборка scheme n – метод передачи split v – разделять subcarrier n – поднесущая

symbol n – условное графическое изображение символа

1. Read and translate the text with a dictonary.

Text IIIA. OFDM. General Structure

High-data-rate communications are limited not only by noise but espeacially with increasing symbol rates and often more significantly by the inter-symbol interference due to the memory of the dispersive wireless communications channels.

Orthogonal Frequecy Division Multiplexing (OFDM) is well known as a highly spectral efficient transmission scheme capable of

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dealing with severe channel impairements encountered in mobile environment.

OFDM can largely eliminate the effects of inter-symbol interference for high-speed transmission in higly dispersive channels with a relatevely low implementation cost. It can be done by separating a single high speed bit stream into a multiplicity of much lower speed bit streams, each modulating a different subcarrier.

The principle of any FDM1 system is to split the information to be trasmitted into N parallel streams, each of which modulates a carrier using an arbitrary modulation technique.

The frequency spacing between adjacent carriers is ∆f, resulting in a total signal bandwidth of N∆f. The resulting N modulated and multiplexed signals are transmitted over the channel, and at the reciever N parallel receiver branches recover the information. A multiplexer then recombines the N parallel information streams into a high-rate serial stream.

The digital implementation of FDM subcarrier modulators/demodulators was suggested to be based on the DFT2. The DFT is employed for the base-band OFDM modulation/demodulation process. The associated harmonically related frequencies can hence been used as the set of subchannel carriers required by the OFDM system.

The serial data stream is mapped to data symbols with a symbol rate of 1/Ts, employing a general phase and amplitude modulation scheme. The resulting symbol stream is demultiplexed into a vector of N data symbols So to Sn-1. The parallel data symbol stream is demultiplexed into a vector of N data symbols So to Sn-1. The parallel data symbol rate is 1/NTs, i.e., the parallel symbol duration is N times longer than the serial symbol duration Ts. Hence the effects of the despersive channels which are imposed on the transmitted signal with the CIR3 become less damaging.

Since the harmonically related and modulated individual OFDM subcarriers can be conveniently visualized as the spectrum of the signal to be transmitted, it is the IFFT4 – rather than the FFT5 – which is invoked, in order to transform the signal’s spectrum to the time domain for transmission over the channel.

In the context of OFDM N frequency-domain samples produce N time-domain samples. Both signals are assumed to be periodically repeated over an infinite time-domain and frequency-domain interval, respectively. In order to ensure that the received time-domain OFDM

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symbol is demodulated from the channel’s steady-state – rather than from its transient – response, each time-domain OFDM symbol is extended by the so-called cyclic extention or guard interval of Ng samples duration, in order to overcome the inter-OFDM symbol interference due to the channel’s memory. The signal samples recieved during the guard interval are discarded at the receiver, and the N – sample received time-domain OFDM symbol is deemed to follow the guard interval of Ng samples duration. The demodulated OFDM symbol is then generated from the remaining N samples upon invoking the IFFT.

OFDM is of great interest to researchers and research laboratories all over the world. It has been accepted for the new wireless local area network standards IEEE 802.11a, High Performance LAN type 2 (HIPERLAN/2) and MOBILE MULTIMEDIA ACCESS COMMUNICATION SYSTEMS. It is expected to be used for wireless broadband multimededia communications.

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1FDM (Frequency Division Multiplexing) – частотное уплотнение.

2DFT (Descrete Fourier Transformation) – дискретное преобразование Фурье.

3CIR (Committed Information Rate) – согласованная скорость передачи ин-

формации.

4IFFT (Inverse Fast Fourier Transformation) – обратное быстрое преобразова-

ние Фурье.

5FFT (Fast Fourier Transformation) – быстрое преобразование Фурье.

2. Find English equivalents in text IIIA for the following Russian words and word combinations:

–значительно;

–эффективный метод высокоспектральной передачи;

–значительные недостатки;

–соседний канал;

–межчастотный промежуток;

–ширина полосы частот;

–последовательный поток с высокими значениями;

–временная область;

–техника произвольной модуляции;

–принимать.

3. Translate from English into Russian the word combinations:

–high data rate communications;

–inter-symbol interference;

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–highly spectral efficient transmission;

–frequency spacing;

–adjacent carriers;

–arbitrary modulation teqnique;

–serial data stream;

–amplitude modulation scheme;

–time domain;

–frequency domain samples;

–cyclic extention;

–guard interval;

–wireless broadband multimedia communication.

4. Answer the questions to text IIIA.

1.What limits high data communication?

2.How can the main principles of OFDM performance be implemented?

3.Why do the effects of despersive channels imposed on the transmitted channel become less damaging?

4.What is the scheme of OFDM sample producing?

5.Why is OFDM approach of great interest all over the world nowadays?

5. Complete the following sentences using the information from text IIIA.

1.OFDM approach is capable to solve __________ in high-data rate communication.

2._________________ is the objective of the OFDM.

3.The information can be recovered by ________________ as

__________________________ .

4.Mapping of the data stream results in _____________________ .

5.In OFDM concept ________________ periodically repeated, the recieved time domain sygnal being demodulated ____________ .

6.Write a brief summary of text IIIA using the phrases:

the article is headlined ...; to be reviewed;

to consider briefly;

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the discussion deepens when ...; it is noted that ...;

it is expected that…

7. Translate the sentences paying attention to the Complex Subject and the Complex Object.

1.Multiplexing is believed to be a process of transmitting more than one signal over a signal link, route or channel.

2.OFDM modems are assumed to employ a set of subcarriers in order to transmit information symbols in parallel – in so called sub-channels.

3.OFDM application proved to be already adopted as the new European digital audio broadcasting as well as for the terrestrial digital video broadcasting systems.

4.The digital implementation of FDM subcarrier modulators\demodulators are suggested to be based on the DFT.

5.N frequency-domain samples and N time-domain samples are assumed to be periodically repeated over an infinite time-domain and frequency domain interval.

6.However we expect OFDM to be vulnarable to syncronization errors due to the narrow spacing between sub-carriers.

7.The designers wanted the system to eliminate the effects of intersymbol interference and to have a sufficiently low implementation cost for high-speed mobile data communication.

8.OFDM proves to be sensitive to the phase noise in the oscillators.

9.We watched the expert explaining the architecture of the reciever based on OFDM concept.

8. Translate the words paying attention to the parts of speech.

(1)module – modulated – modulate – modulation;

(2)receieve – recived – receiving – receiver;

(3)syncronize – syncronized – syncronization – syncronizated;

(4)implement – implementing – implemented – implementation;

(5)exhibit – exhibited – exhibition – exhibiting;

(6)apply – applied – application – applying;

(7)carrier – carried – carry – carring;

(8)symbol – symbolize – symbolic – symbolizing;

(9)relate – relating – related – relation;

(10)employment – employ – employed – employing.

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9. Match words and word combinations from two columns.

10. Read and translate the text.

Text IIIB. OFDM Implementation

In practice, the OFDM signal for the standard IEEE 802.11a is generated as follows: in the transmitter, binary input data is incoded by a rate 1/2 convolutional encoder1. The rate can be increased to 2/3 and 3/4. After interleaving, the binary values are converted to QUAM2 values. Four pilot values are added each 48 data values, resulting in a total of 52 QUAM values per OFDM symbol. The symbol is modulated onto 52 subcarriers by applying the values per OFDM symbol. The symbol is modulated onto 52 subcarries by applying the Inverse Fast Fourier Transform (IFFT). The output is converted to serial and a cyclic extention is added to make the system robust to multipath propagation. Windowing is applied after to get a narrow output spectrum. Using an IQ modulator3, the signal is converted to analog, which is upconverted to the 5Ghz band, amplified, and transmitted through the antenna.

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Basically, the receiver performs the reverse operation of the transmitter, with additional training tasks. In the first step, the receiver has to estimate frequency offset and symbol timing, using speacial training symbols in the preamble. After removing the cyclic extention, the sygnal can be applied to a Fast Fourier Transform to recover the 52 QUAM values of all subcarriers. The training symbols and the pilot subcarriers are used to correct for the channel response as well as remaining phase drift. The QUAM values are then demmapped into binary values, and finally a Viterbi decoder decodes the information bits.

The IFFT modulates a block of input QUAM values onto a number of subcarriers. In the receiver, the subcarriers are demodulated by the FFT. Which is the reverse operation of the IFFT. These two operations are almost identical. In fact, the IFFT can be made using an FFT by conjugation input and output of the FFT and deviding the output by the FFT size. This makes it possible to use the same hardware for both the transmitter and the receiver. Of course, this saving in complexity is only possible when the modem does’t have to transmit and receive simultaneously, which is the case for the standard.

Orthogonal frequency division multiplexing OFDM is a special case of multicarrier transmission, where a single data stream is transmitted over a number of lower rate subcarriers. In 1998 OFDM was decided to be selected as the basis for the new 5-GHz standard, targeting a range of data stream from 6 up to 54 Mbps. This new standard is the first one to use OFDM in packet-based communications, while the use of OFDM until now was limited to continuous transmission systems.

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1convolutional encoder – конвольвер.

2QUAM (Quadrate Amplitude Modulation) – квадратурная амплитудная моду-

ляция.

3(IQ) (Inphase Quadrature) modulator – фазовый квадратурный модулятор.

11. Answer the questions to text IIIB.

1.How is the OFDM signal generated practically?

2.What operations does the receiver perform?

3.What modulating and demodulating processes take place in the OFDM receiver? Why?

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12. Compose a short summary of text IIIB using the words and phrases:

to propose; to be considered in details; basically; to be of great interest.

13. Discuss three topics.

1.The main principles of OFDM performance.

2.The scheme of OFDM sample producing.

3.OFDM implementation.

14. Translate the text in the written form.

Text IIIC. The OFDM Receiver Architecture

The final architecture of the OFDM receiver exhibits robust performance in various mobile channels with high Doppler frequency. The IF signal is sampled at 26 MHz, then digitally downconverted1 and decimated to baseband through the I/Q demodulator. Both the pre-FFT syncronization and post-FFT syncronization units are adopted to provide fast coarse acquisition as well as fine tracking. Channel estimation based on the 1-D FFT/IFFT plus linear time interpolation is efficiently implemented by reusing the FFT/IFFT engine.

The complete OFDM receiver architecture has been simulated with the syncronization (timing, frequency, AGG) loop in dynamic and the proposed 1-D FF/ IFFT based channel estimation and correlation algorithm.

Various mobile channels (TV and HT) profiles have been applied to test the robustness of the receiver. Through the simulation a frequensy offset is assumed to be ∆f Tu = 0.1 and the transmit timing is unknown at the receiver. The performance achived with perfect channel estimation and syncronization is also plotted as a comparison.

The performance loss of the proposed receiver is 2/2.5 dB with respect to the ideal receiver. The receiver should be notted to be robust to Doppler frequency. The degradation from low (40Hz) to high (200 Hz) Doppler frequency is negligible.

Such a receiver will be used to demonstrate the feasibility of highspeed mobile data access based on OFDM for the next generation cellular system.

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1 to downconvert – преобразовывать с понижением частоты.

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