1、 Rec. ITU-R F.763-5 1 RECOMMENDATION ITU-R F.763-5*Data transmission over HF circuits using phase shift keying or quadrature amplitude modulation (Question ITU-R 145/9) (1992-1994-1995-1997-1999-2005) Scope This Recommendation provides data transmission systems using phase-shift keying (PSK) and qua
2、drature amplitude modulation (QAM) over HF channels. Information is contained in Annex 6 for data rates from 3 200 to 12 800 bit/s. The ITU Radiocommunication Assembly, considering a) that there is an increasing demand for high-rate data transmission; b) that to meet this need, two types of phase-sh
3、ift keying (PSK) modems may be used, namely parallel transmission modems using multi-channel voice frequency telegraphy and serial transmission modems using a single sub-carrier; c) that to compensate for the unfavourable nature of the transmission medium, the following techniques are available for
4、the two types of modems: various forms of dual diversity operation including separate single sideband (SSB) emissions or a single independent sideband (ISB) emission; error detection and error correction coding combined with time interleaving; variable data rate to adapt the system to the channel ca
5、pacity; and, for parallel transmission modems only: several levels of in-band frequency diversity; introduction of guard times between frames to combat multipath propagation and group-delay distortion, recommends 1 that for data transmission at binary data rates up to 2 400 bit/s using frequency-div
6、ision multiplex (FDM) and PSK systems, the system described in Annex 1 is preferred; 2 that for data transmission at binary data rates up to 3 600 bit/s using serial transmission modems, the system described in Annex 2 is preferred; 3 that reference should be made to Annex 3 for additional informati
7、on concerning generalized PSK; *This Recommendation should be brought to the attention of Radiocommunication Study Group 8. 2 Rec. ITU-R F.763-5 4 Annex 4 describes mode/polarization diversity systems to improve the performance of HF PSK systems; 5 that for data transmission at binary rates up to 4
8、800 bit/s using serial transmission modems, the system is described in Annex 5; 6 that for data transmissions at binary rates from 3 200 to 12 800 bit/s using serial transmission modems, the preferred system characteristics are described in Annex 6. Annex 1 Data transmission at 2 400/1 200/600/300/1
9、50/75 bit/s over HF circuits using multi-channel voice-frequency telegraphy and PSK 1 System description 1.1 A receiving/transmitting terminal of the system consists of: a sender and receiver of digital information (e.g. computer); a modem, the primary function of which is the conversion of informat
10、ion from digital to analogue form compatible with the input to a radio transmitter and conversion of the analogue information at a radio receiver output into digital data compatible with the digital receiver input. This modem also performs various coding functions and effects diversity combination;
11、RF receiving and transmitting equipment connected to antennas. 1.2 At the transmit side, the 2 400 bit/s incoming data stream is fed to a serial-to-parallel converter. At 32-bit intervals (i.e. 13.33 ms intervals) the content of this converter is transferred in parallel to a 32-bit memory device, th
12、e output of which is connected to a QPSK modulator. The modem generates in transmission a composite audio signal consisting of a set of 18 tones in the band 300 to 3 000 Hz. Of these tones, 16 have a spacing of 110 Hz (935 to 2 585 Hz) and are modulated in differentially encoded quaternary phase shi
13、ft keying (DE-QPSK) mode, each at 75 Bd, thus permitting a data rate of 16 75 2 = 2 400 bit/s. The tone at 605 Hz is used for the correction of end-to-end frequency errors, including any Doppler effect. The tone at 2 915 Hz (or 825 Hz) is used for system synchronization. The dual diversity combiner
14、can accept inputs either from two receivers operating in space, frequency or polarization diversity mode or from one receiver operating in ISB mode. Rec. ITU-R F.763-5 3 When the data rate is a sub-multiple of the transmission speed, various in-band diversity arrangements can be implemented. As an e
15、xample, a data rate of 1 200 bit/s provides a dual diversity (1 200 2), a data rate of 600 bit/s, a quadruple diversity (600 4) and so forth, all with a transmission speed of 2 400 bit/s. Utilization of the maximum possible diversity, both in-band and between independent channels, can thus be made a
16、ccording to the data rate selected. Provision is made for 75/150/300/600/1 200 bit/s. In addition to a choice of coded/uncoded operation, with selectable data rate and diversity mode, this modem also allows setting of the interleaving interval thus providing a flexible communication system as summar
17、ized in Table 1. The transmission signal consists of frames whose duration is 13.33 ms. This includes a time guard (4.2 ms) which is introduced to offset the effects of multipath propagation. The modem uses two techniques to reduce signal impairments, particularly those caused by impulsive noise and
18、 flat fading: error correction code; time interleaving. A form of BCH cyclic block code (16,8) is used. The BCH codewords are stored in a memory to be extracted during the interleaving process. Interleaving is obtained by considering: the first bit of the last stored word; the second bit of the “(m)
19、 word stored before”; the third bit of the “(2 m) word stored before” .; the 16th bit of the “(15 m) word stored before”. TABLE 1 Data rates/modes (independently selectable for transmission and reception) Uncoded modes Coded modes Data rate (bit/s) Diversity modes Time interleaving Available time sp
20、read Additional diversity modes In-band Channel Total (transmitter and receiver) (s) In-band Channel Total 2 400 2 2 1 200 2 2 4 0-12.8 2 2 600 4 2 8 0-25.6 2 2 4 300 8 2 16 0-51.2 4 2 8 150 16 2 32 0-102.5 8 2 16 75 0-205 16 2 32 4 Rec. ITU-R F.763-5 The interleaving level (m codewords) can be chos
21、en according to the propagation conditions of the radio path from 0 (no interleaving), 1, 2, 4, 8, 16, 32, or 64, corresponding to a data reception delay ranging from a few milliseconds to tens of seconds. As the wrong bits do not belong to the same coded word, a better protection against burst erro
22、rs is achieved. In Fig. 1, the performance of the modem with Gaussian distributed noise is given in terms of bit error probability, Pe, as a function of signal-to-noise ratio, S/N, for both with coding and without coding modes, in a 250-3 000 Hz bandwidth. The effects of coding become prominent at t
23、he higher values of S/N. The curves were obtained with an experimental test set-up in which the modem was fed with a test pattern to produce the audio frequency tones. The output of the modem was summed with Gaussian noise, filtered and applied to the receiving input of another modem from which the
24、test pattern was retrieved at the output. The test pattern was then fed to a data error analyzer to enable the bit-error ratio (BER) to be determined. Figure 2 indicates the results of a computer simulation of the modem performance in a fading channel. A fading channel was simulated in which two equ
25、i-amplitude paths carry signals separated by a multipath delay of 1 ms and differing in frequency by 1 Hz, in order to obtain fades which ran through the passband rather than remaining at certain fixed frequencies. From Fig. 2, it can be seen that the performance is improved by using a combination o
26、f the various types of diversity (in-band and out-of-band), error correcting codes and interleaving techniques for 600, 1 200 and 2 400 bit/s rates. The modem is currently in experimental use as part of an HF link between two radio stations located in central and southern Italy, and separated by app
27、roximately 800 km (500 miles). 1.3 The RF equipment performs, in transmission, operations relative to channel modulation and produces an emission having suitable radio frequency and power characteristics. Reverse operations, relative to frequency conversion, are carried out in reception so as to obt
28、ain the composite audio signal to be conveyed to the modem. The RF equipment has the following specific characteristics: phase jitter: less than 5 for 10 ms time interval (100 samples); group delay distortion: 500 s in transmission, 500 s in reception; intermodulation: 36 dB below peak envelope powe
29、r. Rec. ITU-R F.763-5 5 FIGURE 1 Bit error probability versus S/N for various data rates using with coding or without coding modes with in-band diversity for a non-fading channel with Gaussian noise 6 Rec. ITU-R F.763-5 FIGURE 2 Bit error probability versus S/N for a selective fading channel using d
30、ata rates of 600, 1 200 and 2 400 bit/s in the following cases Rec. ITU-R F.763-5 7 Annex 2 Data transmission at rates up to 3 600 bit/s over HF circuits using a serial transmission modem 1 General The modem permits data transmission in a 3 kHz HF channel. It receives and reconstitutes digital data
31、at a rate of 3 600 bit/s and generates an analogue AF signal within the 300-3 300 Hz audio band. It incorporates protection against multipaths, Doppler effect and fading. 2 Modem operating modes There are three possible operating modes. 2.1 Semi-duplex forward error correction (FEC) mode 2.1.1 This
32、mode uses an MPSK (M = 2, 4, 8) modulation at 2 400 Bd, with a user bit rate of 75, 150, 300, 600, 1 200, 2 400 or 3 600 bit/s (not all of the bit rates are available with all of the waveforms), and with frames of 256 modulated symbols (of which 128 are user symbols), i.e. 106.6 ms. 2.1.2 A data exc
33、hange comprises three phases, namely preamble, traffic and end of transmission: The preamble phase enables the called modem to detect the call and to receive the technical parameters (encoding, interleaving, data rate, modulation) that it needs for the rest of the transmission. The traffic phase con
34、tains the data to be transmitted. The end of transmission phase enables the called modem to detect an end of message word in order to terminate the link and return to traffic standby. The end of transmission is effected when the calling modem transmits on-hook frames. These frames are similar to pre
35、amble frames, but include a bit containing the on-hook information. 2.1.3 The functions provided are as follows: Emission: data encoding and interleaving; framing and modulation; transmission of AF signal. Reception: reception of AF signal; 8 Rec. ITU-R F.763-5 detection of synchronization; demodula
36、tion of received signal; data de-interleaving and decoding. 2.2 Full-duplex FEC mode This mode amounts to the same thing as two independent FEC-type semi-duplex links. In each direction a preamble followed by data and an end of message word are sent and recognized by the called modem. As in the semi
37、-duplex FEC mode, this preamble specifies the technical parameters that are to follow. 2.3 Automatic repeat request (ARQ) mode 2.3.1 This mode uses an MPSK (M = 2, 4, 8) modulation at 2 400 Bd, with a user bit rate of 600, 1 200, 1 800 or 2 400 bit/s (not all of the bit rates are available with all
38、of the waveforms), with frames of 256 modulated symbols (of which 128 are user symbols), i.e. 106.6 ms. 2.3.2 The ARQ mode is a data transmission mode involving selective repetition by block. The data for transmission are divided up into blocks corresponding to a modem frame. The calling modem sends
39、 a superframe of N blocks (N is nominally equal to 64, but may be lower than this during transmission of the last data) and waits for the called modem to acknowledge its receipt. If any blocks have not been correctly received, they are re-transmitted in the following superframe, which is made up wit
40、h new blocks. The phases contained in this mode are call set-up (connection), data transmission and end of transmission (disconnection). In addition, the ARQ mode allows for momentary disconnection, caller/called party switching, flow control, and adaptive power, data rate and frequency control. The
41、 ARQ mode thus comprises two distinct phases, namely a transmission phase (transmission of a superframe at the calling end, and of an acknowledgement at the called end), and a reception phase (reception of an acknowledgement at the calling end, and of a superframe at the called end). 2.3.3 Adaptive
42、control 2.3.3.1 The ARQ mode allows adaptive power, data rate and frequency control. Of these, only the adaptive data rate control is entirely managed by the modem. In the case of power control, the Rec. ITU-R F.763-5 9 modem indicates to the system the adaptation to be effected and continues the tr
43、ansmission, while in the case of frequency control, the modem momentarily disconnects itself after indicating to the system the need to find a new frequency. 2.3.3.2 The adaptive power control procedure is based on statistical measurements of the link quality. Adaptive power increase is achieved ver
44、y rapidly, while power decrease involves a large time constant. 2.3.3.3 Adaptive data rate control is effected on three of the data rates chosen from among the four that are available, namely 2 400, 1 800, 1 200 and 600 bit/s. Adaptive increases in data rates are based on statistical measurements of
45、 the link quality, while decreases are based either on statistical measurement of the link quality, or on the non-reception of data or acknowledgements during the transmission. 2.3.3.4 If the adaptive data rate decrease control is not sufficient to continue the transmission, a request is made to the
46、 system to implement adaptive frequency control. In order that a new frequency may be sought, the modem momentarily disconnects itself and stands by to resume the transmission, storing the data which have not yet been transmitted. 2.3.3.5 It is possible to set up the modem in ARQ mode in such a way
47、that it does not implement adaptive data rate control. In this case, only the frequency and power control are effected. 2.3.4 The functions provided are as follows: Send, at the calling end: data segmenting, data encoding, framing and modulation, transmission of AF signal. Send, at the called end: e
48、ncoding of acknowledgements, framing and modulation, transmission of AF signal. Receive, at the calling end: reception of AF signal, detection of synchronization, received signal demodulation, decoding of acknowledgements. Receive, at the called end: reception of AF signal, detection of synchronizat
49、ion, received signal demodulation, decoding of data, data reassembling. 10 Rec. ITU-R F.763-5 3 Technical characteristics of the modem 3.1 Modulation 3.1.1 The modulation technique involves phase shift of a sub-carrier with a frequency of 1 800 Hz. The modulation rate is 2 400 Bd, with a minimum accuracy of 105. 3.1.2 The clock stability associated with the generation of the 1 800 Hz is 105. 3.1.3 The phase shift of the modul
