1、-1- Rap. UIT-R BS.2004 Rep. ITU-R BS.2004 DIGITAL BROADCASTING SYSTEMS INTENDED FOR AM BANDS (1995) 1 Introduction In the last decades, very few innovations have been brought to radiobroadcasting techniques in AM bands (150 kHz - 30 MHz). The simplicity of the receiver has always been a great asset
2、to amplitude modulation, and because of their long range, these AM waves sill stand as best suited to national and international broadcasting. However, under typical conditions of propagation such as ionospheric instability, a classical analogue system may provide poor quality reception. The technic
3、al developments in the other frequency bands of sound-programme broadcasting and also political changes have resulted in the situation that the AM bands have evidently lost their practical and strategic significance to a large degree. The poor transmission quality inherent in AM transmission is main
4、ly characteristic of the modulation procedure rather than of the frequency band. If amplitude modulation is replaced by a digital modulation procedure, we can achieve a very good transmission quality and at the same time retain the long range of the transmission. However, The digital transmission ha
5、s to fit into the existing channel pattern. Digital transmission is suitable not only for sound-programme broadcasting but also for the transmission of additional information and for data transmission in general (value-added services). 2 AM channel characteristics AM bands include long waves (LF, 15
6、0 to 285 kHz), medium waves (MF, 525 to 1605 kHz), short waves (HF, 3.3 to 26 MHz). The characteristics of AM channels vary considerably depending on the frequency bands: LF: MF: Propagation during night hours : ground- and sky-wave propagation and, as a consequence, strong interference. HF: The ion
7、osphere is a dispersive propagation medium which is characterized by the presence of multi-modes and multi- paths, each mode (or path) presenting a particular group delay, amplitude, polarization and Doppler frequency shift. Table 1 gives the typical order of magnitudes of the main parameters of the
8、 ionospheric propagation. 150 - 285 kHz, channel width 9 kHz; ground-wave propagation, little interference by sky-waves. 525 - 1 605 kHz, channel width 9 kHz or 10 kHz; propagation during daylight hours: as LF, but shorter range. 3.3 - 26 MHz, channel width 10 kHz (DSB); 5 kHz (SSB), sky-wave propag
9、ation. -2- Rap. UIT-R BS.2004 Parameter Modes and paths number Total delay spread TABLE 1 Order of magnitude of the main ionospheric propagation parameters Average channel behaviour Depending on the length of the radio link I 8 for sky waves with a O to - 40 dB level Ground wave for short distances
10、I5 ms I8 ms Extreme channel behaviour Delay spread on each path Mean Doppler shift for each path Doppler spread for each path A few 10 ps A few 1/10 Hz A few Hz fd I 2.5 Hz A few 1/10 Hz A few Hz A Afd I 2 Hz fd I 10 Hz Afd I 5 Hz 3 Theoretically, single-carrier procedures and (orthogonal) multi-car
11、rier procedures are almost equivalent if transmission is performed via time-variable and frequency-selective channels and if coding and/or equalization is used to compensate for the error patterns typical of the modulation procedure. So both types can be used for Single Frequency Network (SFN) opera
12、tion. The number of modulation states is the same in both cases. It depends on the ratio between the needed data rate ad the symbol rate. The symbol rate depends on the existing channel spacing. Finally, mention must be made of the 9 kHz (or 10 kHz) bandwidth of AM channels which will only afford a
13、few tens of kbitls data rate. The different channel spacing of LF and MF (9 or 10 kHz) and HF (10 or 5 kHz) lead to differences in the digital modulation procedures, if we require that the digital procedure be compatible with the existing channel spacing. From these limitations the digital sound enc
14、oder should provide a data rate of approximately 20 kbitls. With a usable RF bandwidth of 7 kHz for both the LF and the MF bands, a spectral efficiency of the modulation procedure of approximately 3 bits per Hz of bandwidth is required. For the HF bands, with a usable RF bandwidth of 4 kHz, a necess
15、ary spectral efficiency of 5 bits per Hz of bandwidth is required if the same sound quality as in the LF and MF bands is desired. Depending upon the parameters selected for the roll-off, code rate, frame structure ratio and guard period, both procedures require 32 to 64 modulation states. The decisi
16、on in favour of a multi-carrier or single-carrier procedure is mainly influenced by the length of the channel pulse response, which is specified in terms of the symbol length. In consideration of the technical outlay needed in the receiver the procedure is normally selected according to the followin
17、g rule: Single-carrier procedure: Length of the channel pulse response is less than or equal to the length of 16 symbols. Multi-carrier procedure: Length of the channel pulse response is more than or equal to the length of 64 symbols. If the length of the channel pulse response is less than the leng
18、th of 64 symbols, the reduction in complexity expected from the multi-carrier procedure is no longer ensured. On the other hand, the efficiency of an equalizer required in the single-carrier procedure declines if the channel pulse responses exceed the length of 16 symbols. Selection criteria for a d
19、igital modulation procedure -3- Rap. UIT-R BS.2004 3.1 Transmitter hardware The decision in favour of a multi-carrier or single-carrier procedure is also influenced by the added complexity on the transmitter side. If we wish to maintain the coverage zones of AM broadcasting, digital transmission wou
20、ld allow us to reduce the transmitter power by some dl3 compared with todays analogue transmission. In general, however, the required PF power level will still be large enough to rule out a linear transmitter output stage because of its poor efficiency. Therefore, it must be possible to continue the
21、 operation of existing AM transmitters (class C). For this purpose the transmitter will be complemented by a phase modulator, which is inserted behind the master oscillator. The amplitude modulator should be capable of transmitting a DC component. This requirement is fulfilled by PDM modulators, pul
22、se step modulators etc., i.e. by all modern modulator types. Usually, digital modulation is represented by Cartesian coordinates based on real and imaginary parts (I and Q signals). For this reason modulation procedures with a large number of states, e.g. the 64 QAM procedure, often have square phas
23、e stirs (symbol constellations). For a digital complex modulation of a retrofitted conventional AM transmitter, however, the modulation signal must be converted into an amplitude signal and a phase signal. This is the polar representation by means of A and cp signals. The amplitude signal is applied
24、 to the amplitude modulator, while the phase signal is applied to the phase modulator. Therefore, it is expedient that modulation procedures including a large number of states show a certain rotational symmetry in the phase star. Such procedures should be called APSK (amplitude and phase shift keyin
25、g) procedures. The state points are arranged on concentric rings. Figure I shows in example of APSK. Other forms of APSK ire possible. 1.5 1 O. 5 C -0.5 -1 -1.5 FIGURE i 1/7/12/12 AYSK = 32 APSK of a a) non-compact b) compact form I 1 I I -1 -0.5 o 0.5 1 1.5 1.5 1 0.5 C -0.5 -1 -1.5 I I -1 -0.5 o 0.
26、5 1 1.5 This modulation procedure, which is adjusted to the characteristics of the transmitter with high efficiency, has the following advantages: e Non-linearities in the amplitude modulator affect only the diameters of the concentric rings. Amplitude-to-phase conversions in the transmitter output
27、stage will result in a slight rotation of the concentric e rings only. This may he compensated for by differential coding on the rings. However, the procedure also has its drawbacks, which must be accepted if transmitter efficiency is emphasized : e Despite the band limitation applied to I and Q sig
28、nals, the A and cp signals are normally not bind-limited. In practice, however, the A and (cp signals can be band-limited to about 2.5 times the symbol frequency. -4- Rap. UIT-R BS.2004 e Due to the delays in the transmitter a time offset occurs between the A and the (p signals, which must compensat
29、ed for. In the case of a modulation procedure of 32 states the time offset should not exceed 2% of the symbol length. These considerations are independent of the modulation system chosen. 4 Already implemented in the EU 147-DAB ( Digital Audio Broadcasting) project, associated with ISO-MPEG audio en
30、coding to meet the requirements of high-quality digital audio broadcasting to mobile, portable and fixed receivers, the COFDM (Coded Orthogonal Frequency Division Multiplex) system could also be the solution to digital broadcasting in AMbands. The COFDM system could successfully perform despite sele
31、ctive fadings of AM bands, because it has been designed to make use of the presence of multipath rather than he restricted by it. The COFDM system relies on two principles: The first principle consists of splitting the information to be transmitted into a given timber of modulated carriers with indi
32、vidual low bit rates, so that each carrier is affected by a flat or non-selective fading only. The second principle systematically exploits multipath between the transmitter and the receiver, by using the fact that signals sufficiently separated in frequency and time cannot be identically affected b
33、y the propagation conditions. This is exactly what happens in a 9 kHz channel, in case of delays within a range of a few milliseconds and in the presence of Doppler shift. Therefore, the COFDM system incorporates the linking of elementary signals transmitted at distant locations of the time-frequenc
34、y domain. This is achieved by convolutional coding associated with soft decision Viterbi decoding, in conjunction with frequency and time interleaving: the more diversity there will be, the more robust the system. In addition, due to its ability to handle strong multipath, including man-made echos,
35、COFDM provides the opportunity to design more spectrum efficient broadcasting networks. The capability of COFDM to allow operation with a O dB echo would ensure that non-directional antennas could be used to receive a program broadcast, in a cellular fashion - at least on a country scale - from two
36、or more transmitters operating on the same frequency (Single Frequency Network, SFN). Such a network needs multiple frequencies in a conventional analogue or digital broadcast. This cannot be overlooked when one considers the lack of spectrum resources. Multi-carrier modulation intended for AM bands
37、 (COF1)M) 4. I A demonstration prototype After simulation studies, the CCETT (France) has developed a laboratory prototype of a COFDM system dedicated to AMbands. A major goal of the work was to design a system with a sufficient useful data-rate in a 9 kHz channel, to support services such as audio
38、broadcasting of quite good quality. For that purpose, the CCETT has implemented advanced digital techniques, such as: trellis-coded modulation of high spectra efficiency and also robust against channel distortion, continuous channel estimation, by inserting reference carriers among the carriers used
39、 by COFDM. The key parameters of the prototype are listed in Table 2. This prototype is designed for channel delay spread within a value of 2 ins (typically a Single Frequency Network of the size of France) and for a useful data rate of 24 kbitls. It is connected to a digital sound codec, adapted fr
40、om MPEG2 version (ISOBIPEG2 Audio Layer II at reduced sampling frequency), also developed at the CCETT. The whole system test set-up also includes a hardware channel simulator, which has already verified the advantages of the COFDM technique over classical analogue modulation on typical AM channels.
41、 Further work will now include on-air experiments. -5- Rap. UIT-R BS.2004 TABLE 2 Performance parameters of the laboratory prototype Channel bandwidth Used bandwidth Modulation of each carrier Coding rate FFT size Useful symbol duration Guard interval Total symbol duration Total number of carriers N
42、umber of useful carriers Number of reference carriers for continuous channel estimation Number of carriers for automatic frequency control (not used at the moment) Useful data rate Net spectral efficiency CLV for operating on Gaussian channel C/N for operating on Rayleigh channel with a O dB echo 9
43、kHz 8.7 kHz 64-QAM 213 256 points 21.333 ms 2.666 ms 24 ms 184 144 24 16 24 kbits 2.8 bitMHz 17 dB 25 dB 4.2 In a COFDM receiver, digital signal processing must be implemented allowing, for automatic, data-aided tuning. With respect to the transmitting infrastructures, the first tests on new generat
44、ion solid state AM transmitters have given good results: COFDM on AM bands should operate well on the same amplifier stage as analogue broadcasting (with Single Side Band). Transmitting infrastructures and receiving equipment required 5. A single carrier modulation system has been constructed in Ger
45、many and is currently under test. Owing to the transmitter characteristics the multi-carrier modulation approach has the following disadvantages compared with the single-carrier approach: Single-carrier modulation intended for AM bands e The time-offset between amplitude and phase signal results in
46、the loss of orthogonality. e Multi-carrier modulation has a high crest factor so that the requirements with respect to transmitter linearity are far more stringent than in the case of single-carrier modulation. As a consequence, the radiated power of the transmitter is reduced. There are further asp
47、ects mentioned in the following which finally led to the decision in favour of a single-carrier procedure: Due to the relatively short length of the channel impulse response the statistical variations of the channel are not sufficient in the frequency direction. Therefore, the averaging effects of t
48、he multi-carrier procedure exploited for DAB cannot be achieved to the required extent, where the length of the channel impulse response is some hundred symbols. In the case of LF, MF and HF there are less than 16 symbols. -6- Rap. UIT-R BS.2004 This requires an extensive and continuous channel esti
49、mate in the case of the multi-carrier procedure. In the case of a large number of modulation stages it is not easy to apply differential modulation or demodulation to the multi-carrier procedure. For DAB, however, only differential 4 PSK is used. The single-carrier modulation allows a simple or a very complex structure of the receiver depending on the requirements. This complies with the need for portable and fixed receivers. Multi-carrier modulation -Wows complex receiver structure only. 5.1 Frame structure The use of a frame structure has proved successful for various applications, inclu
