Thông tin thiết kế mạch P11 ppsx

11 PERSONAL WIRELESS COMMUNICATION SYSTEMS 11.1 INTRODUCTION The idea that a person could carry around with him a telephone booth of his own is a very attractive one. However, the technology required to make the telephone booth small and light enough for this to be possible, and furthermore convenient to carry, has been available only in the last 30 years. Mobile radio has been available in North America since the 1930s but they were exclusive in the hands of the police. Later taxicab operators installed radios in their vehicles. These mobile units were large, heavy and electrical power hungry. They used amplitude modulation which is notorious for poor performance in the presence of electrical noise and there was more than enough noise generated by the ignition systems of the vehicles in which they were installed. Moreover, they were operational in either the transmit or the receive mode at any given time; they were of the ‘‘push-to-talk’’ type. The invention of the transistor and its progression to integrated circuits made it possible to reduce the weight and size of circuits and, at the same time, increase their capability and flexibility. These advances were accompanied by an enormous reduction in the amount of power required to operate transistor circuits. The stage was set for the introduction of the ‘‘personal telephone booth’’. There are many applications in which radio plays a vital part. These go from ‘‘remote keys’’ for the automobile, garage door openers, pagers, walkie-talkies, cordless telephones to cellular telephones with access to the Internet. In this chapter, we limit ourselves to a discussion of paging systems, cordless and cellular telephones. The paging system was designed to send information from a base station to a mobile terminal. The mobile terminal has no capability to transmit information in the opposite direction. A serviceman for home heating furnaces, for example, only needs to know the address of his next assignment; in general, he does not have to contact his base office. A communication system in which information travels in only one direction is described as simplex and the pager is an example. 325 Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije Copyright # 2002 John Wiley & Sons, Inc. ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic) Radio systems with a push-to-talk button use the same channel in both the forward and reverse directions. It is therefore necessary for each person to indicate when they have finished talking with the familiar word ‘‘ roger’’. They are described as half- duplex.Afull-duplex system uses two channels simultaneously, the first for transmission and the second for reception. The cellular telephone is an example of a full-duplex system. 11.2 MODULATION AND DEMODULATION REVISITED In Section 9.2.1 we discussed the generation of a single-sideband-suppressed carrier (SSB-SC) signal using a balanced modulator and a bandpass filter. Fig. 11.1(a) shows the circuit configuration of the SSB-SC as well as the frequency spectrum of the input and output. The output shows that the baseband signal has experienced an upward frequency shift equal to the carrier frequency, o c and an ‘‘inverted’’ version of it appears at a lower frequency and the two are symmetrically spaced about the position of the carrier. A bandpass filter is used to select the upper sideband. Clearly, the upper and lower sidebands contain the same information and only one of them should be required for the recovery of the original signal. Figure 11.1(b) shows a circuit in which upper sideband is multiplied (balance modulated) with the carrier signal, o c . The corresponding spectrum shows that the Figure 11.1. (a) The structure of the modulator and the spectrum of the corresponding frequency shift. (b) The structure of the demodulator and the spectrum of the corresponding frequency shift. 326 PERSONAL WIRELESS COMMUNICATION SYSTEMS output has two ‘‘sidebands’’. The first is at a frequency 2o c and the second occupies the position of the original baseband signal in the spectrum. The original signal is recovered by using a lowpass filter. Equations (9.2.1 to 9.2.5) are the relevant equations. This modulation and demodulation technique can be used with amplitude, frequency and phase or angle modulation schemes. When demodulation is carried out using the carrier signal in a balanced modulator as shown in Figure 11.1(b), it is referred to as coherent demodulation or coherent detection. The use of an envelope detector to demodulate an AM signal is known as non-coherent demodulation. 11.3 ACCESS TECHNIQUES 11.3.1 Multiplex and Demultiplex Revisited When modulation is used to accommodate a number of signals on a single channel we refer to it as multiplexing. Figure 11.2 shows five baseband signals, each of which occupies the frequency band 300 Hz to 3 kHz. By choosing suitable carrier frequencies for each one, they may be transmitted over the same cable or the airwaves by radio and subsequently demodulated with no interference between them. When different carrier frequencies are used to multiplex the baseband signals, it is referred to as frequency-division multiplex (FDM). Other methods of multiplexing are described below. The success of personal wireless communication systems is in part due to the development of techniques which allowed a large number of signals to share a limited spectrum. One of the boundaries of the spectrum available for personal wireless communication is dictated by the size of the antenna for the radio interface. Efficient transmission of radio signal at low frequency requires antennas several thousand meters tall. Clearly, this is not possible as portability of the device is essential. The boundary on the other end of the spectrum is set by the character of high-frequency transmission which increasingly takes on the properties of visible light which requires line-of-sight. Clearly, the modern environment (cities) in which most of the potential subscribers live and work make line-of-sight communication Figure 11.2. Five baseband signals occupying the same bandwidth can be separated by using frequency-division multiplex. 11.3 ACCESS TECHNIQUES 327 devices inadmissible. Between these two boundaries we have other systems in competition for the spectrum, such as air and sea navigation, satellite communica- tion, radio and television broadcasting. It so happens that national governments have arrogated to themselves the power to assign portions of the spectrum for specific purposes within their territories and to negotiate international treaties which govern their use. This, in short, brings us to the assigned frequency bands of 824–849 MHz and 869–894 MHz for personal wireless communication. For the large number of anticipated subscribers to be accommo- dated in such a restricted bandwidth it is necessary to develop techniques which reduce the possibility of interference with each other. One major factor working in our favor is that, provided we keep the radiated power below a given level, and we are separated sufficiently by distance, we can reuse the spectrum over and over again. We shall now discuss the techniques which enable us to share the spectrum available. 11.3.2 Frequency-Division Multiple Access (FDMA) Frequency-division multiple access is a fancy name for what is commonly done with AM and FM radio broadcasting and TV stations; they are assigned different carrier frequencies with suitable separation between them to ensure minimal interference with each other. They are required by law to keep their carrier frequencies constant. They also have a limited bandwidth and radiated power. The division of the spectrum according to frequency was discussed in Section 9.2 under the heading ‘‘Frequency- Division Multiplex’’ (FDM). Figure 11.3 shows a representation of the channels spaced by their assigned carrier frequencies and separated by limited bandwidth and appropriate guard bands. 11.3.3 Time-Division Multiple Access (TDMA) In FDMA, a frequency band is dedicated to a particular channel for as long as it is required. In TDMA, several channels share the same bandwidth but each channel has the use of that bandwidth for a fraction of the time. TDMA was discussed in Section 9.3 under the other name used to describe this technique: ‘‘Time-Division Figure 11.3. In frequency-division multiple access (FDMA), channels are spaced by their assigned carrier frequencies and separated by limitation on bandwidth and appropriate guard bands. 328 PERSONAL WIRELESS COMMUNICATION SYSTEMS Multiplex’’ (TDM). The basis of this technique is the ability to reconstruct a signal from samples taken from it. Figure 11.4 shows how each channel is structured in time to form frames and the sequences of the content of each channel. In TDMA, it is necessary to synchronize the transmitter to the receiver so that bits from one channel do not end up in another channel, hence the synchronizing bits. 11.3.4 Spread Spectrum Techniques In spread spectrum communication systems the radio-frequency carrier is changed very rapidly in a pseudo-random fashion over a bandwidth which is much wider than the minimum required to transmit the signal. Potentially it should cause interference with other users of the airwaves but, in fact, because the carrier operates for such a short time at any given frequency, its effect is almost imperceptible. The average perceived power on any given channel is very low and it therefore behaves like a low-power noise source spread across the bandwidth it uses. Many communication channels can operate in this fashion without interfering with each other. Spread spectrum technology has been of particular interest to the military because it is almost impossible to predict the next frequency of the transmission; they like to stay away from eavesdroppers and to avoid the jamming of their communication systems by the enemy. The real challenge in spread spectrum communication is to keep the receiver synchronized to the transmitter. We shall return to the problem of synchronization later. There are two major types of spread spectrum techniques. They are frequency hopped and direct sequence spread spectrum technologies. 11.3.4.1 Frequency Hopped Multiple Access (FHMA). In FHMA trans- mission, the information is first digitized and then broken up into short passages. Each passage is transmitted on a different carrier frequency determined by a pseudo- random number generator. Because the modulation used is either narrow band FM or frequency-shift keying, at any instant, a frequency hopped signal occupies a single narrow channel. However, because the carrier frequency hops around, it makes use of a much wider bandwidth. Figure 11.5 shows a representation of a system that uses FHMA. Clearly, in an FHMA the receiver has to have prior access to the sequence of the carrier frequencies transmitted as well as the timing to be able to follow the hops (synchronize). It is quite likely that two or more transmitters will at some time try to use the same frequency. 11.3.4.2 Code Division Multiple Access (CDMA). In CDMA transmission, the information is first digitized and then multiplied by a binary pseudo-random sequence of bits (called chips) with a bit rate much higher than that of the digitized information [1]. Figure 11.6 shows the binary message signal, bits of a pseudo- random code, and the spread spectrum (coded) signal. Note that, because the bit rate of the pseudo-random sequence is much higher than that of the message signal, it requires a much larger bandwidth for its transmission. The spread signal is used to modulate a carrier (usually FM or PM) 11.3 ACCESS TECHNIQUES 329 Figure 11.4. In time-division multiple access (TDMA), each channel is structured in time to form frames and the sequence of the content of each channel. 330 and then transmitted. At the receiving end the spread signal is demodulated then decoded using a locally generated pseudo-random bit sequence in a process called correlation. Because the number of chips representing a message bit (1 or 0) is large, the correlation does not have to be perfect; it has to correctly recognize the majority of the chips as representing that message bit (1 or 0). In a system which is subject to multipath fading, this is an advantage. It is clear that the receiver has to have prior knowledge of the pseudo-random code to be able to decode the message. To other receivers not using the identical code, the message appears to be just noise. The attraction of this technique is that it can be used to accommodate a large number of subscribers with different codes and they will not even know that they are sharing the same bandwidth. A by-product of CDMA is improved security of the message. One disadvantage of CDMA is that the power of individual transmitters has to be controlled very carefully. A strong signal from one of the transmitters within the wideband can overwhelm the sensitive front-end of the system and prevent the reception of other signals. The transmit power control system for all the mobiles adds complexity and costs. 11.4 DIGITAL CARRIER SYSTEMS So far, we have discussed carrier systems in which the message signal is in analog form. Increasingly, electronic systems are using a digital format. For example, it has Figure 11.5. A representation of a frequency hopped multiple access (FHMA) system. Although no instances of two or more transmission on the same frequency and at the same time are shown, there is a clear possibility that this can happen. Note that for simplicity, all channels have equal bandwidth and occupy that bandwidth for the same length of time. Neither of these conditions apply in practice. 11.4 DIGITAL CARRIER SYSTEMS 331 Figure 11.6. The binary data and its equivalent bipolar form, the pseudo-random sequence (chips) and the resulting spread spectrum data. 332 taken the music recording industry less than 15 years to replace the analog vinyl record with the digital compact disc. There are technical as well as economical advantages to be gained from this move. Moreover, the advent of integrated circuit technology with its ability to fabricate extremely large numbers of circuits on minuscule pieces of semiconductor has made the move to digital systems seem inevitable. To transmit a baseband (message) signal over a radio channel, it is necessary to change some property of the radio-frequency signal using the baseband signal. We can change its amplitude, its frequency, or its phase angle. In Chapter 2 we discussed the modulation of a radio-frequency signal by a message signal in which the amplitude of the RF signal varied according to the amplitude of the message signal (amplitude modulation; AM). In Chapter 4 we discussed how to change the frequency of the RF about a fixed value using the message signal (frequency modulation; FM). It is now time to discuss the modulation scheme in which we vary the phase of the RF signal according to the message signal (phase modulation; PM). It must be pointed out that frequency and phase modulation are, in fact, the same. The only difference is that in PM the phase of the modulated waveform is proportional to the amplitude of the modulating waveform, while in FM it is proportional to the integral. Both schemes are sometimes referred to as angle modulation. Phase modulation, when the message signal is a continuous (analog or tone) function, does not appear to have any practical applications. When the message signal is digital, it has distinct advantages such as improved immunity to noise. 11.4.1 Binary Phase Shift Keying (BPSK) When the modulating (message) signal is in binary form we refer to it as keying. This is a left-over from the days when telegraph operators opened and closed a circuit (presumably, using a ‘‘Morse key’’ ) to generate Morse code. Figure 11.7 shows a comparison of the waveforms of the three modulating schemes. It should be noted that, for clarity, the RF has been chosen to be only four times the data rate. In practice, the RF is much higher than the data rate. If we represent the digit 1 by the binary pulse pðtÞ¼1, the digit 0 by pðtÞ¼À1 and the carrier by cos o c t, then after modulation we have sðtÞ¼pðtÞ cos o c t ð11:1Þ for the digit 1 and sðtÞ¼ÀpðtÞ cos o c t ¼ pðtÞ cosðo c t þ pÞð11:2Þ for the digit 0. Demodulation of a BPSK signal requires a balanced mixer and an exact replica of the carrier. 11.4 DIGITAL CARRIER SYSTEMS 333 11.4.2 Quadrature Phase Shift Keying (QPSK) In quadrature phase shift keying, the message signal is separated into in-phase (I) and quadrature-phase (Q) components and are then modulated separately by two carriers of the same frequency but with a phase difference of 90  . QPSK is used because twice the information can be carried in the same bandwidth as when BPSK is applied [2]. Figure 11.8 shows the structure of the QPSK modulator. It has been assumed that the pulses used for modulating the carrier are rectangular. In fact, rectangular pulses are quite undesirable since, in a limited bandwidth channel, they tend to smear into the time intervals of other pulses [3]. The pulse shaping filter is used at the baseband or at the IF stage to limit adjacent channel interference. Figure 11.9 shows the waveforms of the original data, the I and Q components, the I cos ot and Q sin ot as well as the QPSK signal. Note that the QPSK signal is a combination of the waveforms of the I cos ot and Q sin ot components. The demodulation of the QPSK signal is done coherently as shown in Figure 11.10. After down-conversion the received signal is split into two parts and each part is demodulated using a carrier signal derived from the received signal by a carrier Figure 11.7. (a) Binary data, (b) its bipolar equivalent, (c) the amplitude-modulated waveform, (d) the frequency-modulated waveform, and (e) the phase-modulated waveform. 334 PERSONAL WIRELESS COMMUNICATION SYSTEMS [...]... short tune The subscriber answers the telephone and the conversation can start The MSTO continues to monitor the call and will initiate a handoff if and when the quality of the channel falls below an acceptable level A handoff may be necessary because the mobile has moved away from the original base station resulting in a weak signal When the conservation is over the subscriber presses a button which... number programmed into the unit during manufacture), (d) the destination telephone number through a reverse control channel The base station receives the information and passes it onto the MSTO for validation If everything is in order, the MSTO instructs the base station and the mobile to move to an available pair of voice channels, dials the destination telephone number, and connects the system to the PSTN... filter following the limiter is responsible for attenuating the higher harmonics of the signal which appear at the output of the clipper The design of filters is outside the scope of this book The interested reader is encouraged to refer to a specialized text on the subject The supervisory audio tone (SAT) is a sinusoidal audio frequency oscillator operating at approximately 6 kHz The design of sinusoidal... outside the 200 meter radius with minimal or no interference The real disadvantage of the system is that, because it uses a radio link with no attempts at encryption, anyone with an FM radio capable of operating in the frequency bands assigned to cordless telephones can tune in and listen to the conversation As mentioned earlier a new frequency band at 2.4 GHz has been assigned to cordless telephones in North... channels must be avoided (3) Due to the variability of the path between the base station and the mobile, fragments of the signal will find their way to the receiving antenna over different paths, resulting in different amplitudes and phases The vector sum of the fragments may result in an augmentation or, with equal probability, a total cancellation of the signal This process is called fading It is... centers, the cells are made smaller and there are more of them The radiated power levels are much lower In more sparsely populated areas, the cells are larger and the radiated power is higher 11.7.2.2 Setting up the Call The steps described below have been simplified considerably in the interest of brevity and clarity When a cellular telephone is turned off, the system has no way to reach the subscriber... described very briefly below (1) ERMES (European Radio Message System) This system was introduced in the early 1990s by the European Community The data rate is fixed at 6250 bps and it is capable of operating over multiple radio-frequency channels The pager can scan all the channels when the subscriber is away from his home base (2) FLEXTM (Flexible wide-area paging protocol) This system was introduced... of RF duplexer technologies used in cordless telephones They are ceramic, Surface Acoustic Wave (SAW), and Film Bulk Acoustic Resonator (FBAR) Ceramic RF duplexers are considered to have superior operating characteristics compared to SAW devices but they are, in general, bulkier FBAR is a new technology that is supposed to match the performance while occupying only about 10% of the space of ceramic... and connects the system to the PSTN When the call is terminated, the mobile sends an ‘‘end-of-call’’ code and goes into the idle state The MSTO dismantles the connection and goes into standby mode waiting for the next call 11.7.2.3 Handoff Process Handoff occurs when the signal reaching the base station from the mobile gets too weak for adequate voice quality Very often this 11.7 THE CELLULAR TELEPHONE... base stations in adjacent cells for various parameters such as overload conditions It then tests to determine whether the signal strength of the mobile in the intended new cell is adequate before initiating the handoff There is a built-in hysteresis so that at the cell boundary the tendency to switch back and forth between the two base stations is eliminated One has to remember that there is no frequency . number of chips representing a message bit (1 or 0) is large, the correlation does not have to be perfect; it has to correctly recognize the majority of the chips as representing that message bit. the modulating waveform, while in FM it is proportional to the integral. Both schemes are sometimes referred to as angle modulation. Phase modulation, when the message signal is a continuous (analog. the message signal is digital, it has distinct advantages such as improved immunity to noise. 11.4.1 Binary Phase Shift Keying (BPSK) When the modulating (message) signal is in binary form we