Design of Antennas for Handheld DVB-H ... - Lunds tekniska högskola

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2.1.2 OFDM choose DVB. But DVB is not the only standard for digital broadcast in the world, e.g. Japan has developed Integrated Services Digital Broadcasting - Terrestrial (ISDB-T) and in US they have implemented Advanced Television System Committee (ATSC) [3]. One of the best things with the digital system compared to the analogue is the spectrum efficiency. It is possible to send four to six digital channels by the multiplex instead of one analogue channel on the same frequency. The DVBstandard uses the video format MPEG-2 while the sound is coded with MPEG-1, layer II. Normally 80 percent of the capacity is reserved for the video stream and 10 percent each for the sound and teletext [4]. Orthogonal Frequency Division Multiplexing (OFDM) was developed in 1960, but it has not been used much until lately. It has become very popular because it is now possible to build the integrated circuits that are needed to perform the highspeed digital operations necessary for OFDM. It is based on Frequency Division Multiplexing (FDM), which is a technology that uses multiple frequencies to simultaneously transmit multiple signals in parallel [5]. It is different to transmit a terrestrial broadcasting channel than to transmit a satellite link or a cable transmission channel. You will get reflections from buildings and mountains, which will result in a multi path propagation of the transmitted signal (See figure 2.1). These reflected signals will be time-delayed at the receiver and can cause harmful interference. If the delay time of the echo signals is in the range of the symbol duration of the transmitted signal it will result in a selective behaviour of the frequency response [6]. The individual echoes which successively arrive at the receiver, vary in amplitude and delay time. By superimposing themselves on the main signal they cause fluctuations in the complex channel transfer function. A characteristic value of such fading channels is given by the ratio of the directly received signal power to the total of the power of all echo signals [7]. It is possible to compensate for the distortions in the frequency domain by using suitable equalizers. Usually the time delay of the echo exceeds the symbol duration. This means that the adjacent symbols affect each other. A filter for inter symbol interference must therefore be of a high order, which is hard and expensive to implement. To minimize the number of symbols affecting each other you can just make the duration of the transmitted symbol longer. This can be done by the parallel transmission of several symbols. If, for instance, the information to be transmitted 4
is simultaneously modulated into 1000 symbols of different carrier frequencies, then for each individual symbol there is a time slot available, which before changing to parallel transmission, was allotted to all the sequentially transmitted symbols together. The value of the bandwidth and the transmission time of some information can vary as a function of each other. If OFDM is used to send a symbol the frequency range required for the transmission of an individual subsymbol is reduced [6]. In the OFDM system all the subcarrier frequencies should be orthogonal to each other. To get the subcarriers to be orthogonal to each other the Inverse Discrete Fourier Transform (IDFT) must be calculated. It is necessary for a predetermined number of subsymbols to be available simultaneously at the input of the IDFT unit. The data that are to be transmitted are temporarily stored until the required number of subsymbols for parallel transmission are ready to be sent, and are then read out in parallel. One of the major problems of OFDM is that the peak amplitude of the emitted signal can be considerably higher than the average amplitude. This Peak to Average Ratio (PAR) problem comes from the fact that an OFDM signal is the superposition of all the sinusoidal signals of the different subcarriers. On average the emitted power is linearly proportional to the number of carriers. Sometimes the signals on the subcarriers ad up constructively and then the amplitude of the signal is proportional to the number of carriers. There are three ways of dealing with this: 1. Use a power amplifier in the transmitter that can amplify linearly up to the possible peak value. This solution requires expensive and power consuming class A amplifiers. 2. Use a nonlinear amplifier, and accept that the amplifier characteristic will lead to distortions of the output signal. Those nonlinear distortions destroy the orthogonality between the subcarriers, and also leads to out of band emissions. 3. Use PAR reduction techniques. There are several different approaches these will not be described in this rapport. 5
Page 1 and 2: Design of Antennas for Handheld DVB
Page 3 and 4: Acknowledgements This master thesis
Page 5 and 6: 4 DESIGN AND TESTING OF ANTENNA SOL
Page 7 and 8: Figure 5.2. Reference dipole antenn
Page 9 and 10: 1 Introduction The passed decade th
Page 11: 2 Background This chapter provides
Page 15 and 16: 2.1.3 Reed-Solomon Codes 2.1.4 DVB-
Page 17 and 18: 2.1.5 DVB-H DVB-H is the latest dev
Page 19 and 20: 2.1.5.2 Time slicing A large proble
Page 21 and 22: figure 2.5). All three elements tog
Page 23 and 24: symbols or four 2K OFDM symbols. Th
Page 25 and 26: 2.2 Competing standards The DVB-H s
Page 27 and 28: 2.3 Antenna theory This section des
Page 29 and 30: Figure 2.9. Impedance bandwidth def
Page 31 and 32: unloaded quality factor (Q0) and th
Page 33 and 34: losses should be avoided in handhel
Page 35 and 36: 3.3 Loop Figure 3.1. Yagi-Uda anten
Page 37 and 38: PIFA Figure 3.2. Four different con
Page 39 and 40: The size of the antenna is another
Page 41 and 42: 4.4 PIFA To verify the balun a resi
Page 43 and 44: 4.5 Folded-Patch With the PIFA-ante
Page 45 and 46: Figure 4.9a. Folded-patch 2, front
Page 47 and 48: Figure 4.11. Loop antenna schematic
Page 49 and 50: Figure 4.15. Wire loop antenna retu
Page 51 and 52: using an H-field antenna together w
Page 53 and 54: antenna. The dimension of the anten
Page 55 and 56: The result of the return loss measu
Page 57 and 58: Figure 4.24. Return loss graph for
Page 59 and 60: The result of the measurement of th
Page 61 and 62: Figure 4.31. Return loss graph for
Page 63 and 64:
frequency band, the first from 470
Page 65 and 66:
Figure 5.6. Radiation pattern for R
Page 67 and 68:
5.1.3 Measurement result In this ch
Page 69 and 70:
Figure 5.10. Radiation pattern for
Page 71 and 72:
Page 73 and 74:
Table 5.4. Switched monopole 1, gai
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Power gain / dB 2 0 -2 -4 -6 -8 -10
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Page 79 and 80:
was received in all five channels.
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6.3 Control signal 6.4 Cost The swi
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not be possible, there will always
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There are many advantages with tuna
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8 References [1] Schiller, Jochen -
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[29] Gardiol, Fred - Microstrip cir
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Design of Antennas for Handheld DVB-H ... - Lunds tekniska högskola

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