1、Rec. ITU-R F.764-1 RECOMMENDATION ITU-R F.764- 1 147 MINIMUM REQUIREMENTS FOR HF RADIO SYSTEMS USING A PACKET TRANSMISSION PROTOCOL (Question ITU-R 158/9) ( 1992- 1994) The ITU Radiocommunication Assembly, considering 4 that there is an increasing demand to provide a virtually error-free digital dat
2、a service for HF radio systems; b) that it is desirable to specify the requirements of HF packet radio systems, recommends 1. that as a minimum requirement for HF packet radio system protocols: 1.1 the HF packet protocols should sustain the highest throughput for a wide range of channel conditions;
3、1.2 probability is better than 1 x lop8 when the channel raw bit error ratio is 1 x 1C2; the packet protocols should employ error detection and correction schemes to ensure that the undetected error 1.3 the packet protocols when operated in ARQ mode should utilize selective repeat algorithms. The re
4、dundancy of FEC should be transmitted when a repeat of the packet in error is requested. If the repeated packet is received correctly, then the information is recovered and delivered to the message destination. If the repeated packet contains one or more errors, the two versions of the packet relate
5、d to the same information are put through an FEC procedure and checked for errors. If the processed packet is deemed to be error free, then the information is delivered to the destination. The levels of FEC can be increased from 2 as described for higher performance. Also soft decision values of the
6、 received packet signal can be utilized for better FEC performance. Such a system is described in Annex 2; 1.4 the data transmission should be in synchronous mode; 1.5 the packet should contain sufficient preamble for receiver modem bit timing synchronization to take place; 1.6 the packet should hav
7、e at least 16 bit frame synchronizations in order to reduce false frame detection; 1.7 acceptance of traffic intended for other stations; the packets should contain identification of sending and receiving stations in order to prevent erroneous 1.8 the packet transmitting station should employ some m
8、ethod of carrier detection, packet detection, or synchronization with other stations sharing the same radio channel in order to reduce packet collisions and interference with the packets already in transmission in the channel; 2. reported. The study of the techniques for optimization of HF packet sy
9、stems is also recommended; that this Recommendation should be updated as new techniques are developed and relevant information is 3. that the following Notes should be considered as part of this Recommendation. Note I - Annex 1 describes a system which employs an HF packet protocol. Note 2 - Annex 2
10、 describes a system which employs an HF packet radio protocol with selective ARQ and FEC. Note 3 - Annex 3 lists features of a terminal node controller (TNC) which incorporates the AX.25 packet radio protocol. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Inform
11、ation Handling ServicesITU-R RECMNaF- 7b4-3 94 m 4855232 0522335 890 m 148 Rec. ITU-R F.764-1 Note 4 - Annex 4 describes a system which employs an HF packet protocol for data transmission, with error correction, detection of uncorrectable errors and repetition of frames not received or not corrected
12、 ANNEX 1 Transportable HF radiocommunication equipment for transmission of hard-copy messages 1. Introduction A data terminal is described which can be interfaced with HF radios for the transmission of text messages when the propagation conditions do not permit intelligible voice communications. Th
13、e system is suitable for use on ships and in remote areas where other means of communication are not available. The important characteristics of the system include: - real-time channel evaluation and selection; - - interface with the telephone network for remote control; economical deployment in rem
14、ote areas. 2. System configuration The data terminal is designed to operate with transceivers in a network of up to 128 stations sharing a common set of frequencies. 3. System description The data terminal is the size of a portable typewriter, consisting of an alphanumeric keyboard, a 20-character l
15、ight emitting diode (LED) display and a 20-column printer. The package also includes a central processor, modems and a radio interface board (Fig. 1). A message of up to 1280 characters can be entered into the terminal memory and may be checked on the LED display or printer and edited if required. W
16、hen the operator is satisfied with the content of the memory, the message can be directed to a particular destination terminal by entering the destination terminal address via the keyboard. Message transmission takes place under a packet radio protocol which not only identifies the sender and receiv
17、er but also allows the receiver to locate portions of the message corrupted by interference or fading. Optional ARQ is activated until the message is correctly received. The protocol is specifically designed to accommodate transceivers which cannot rapidly switch from receive to transmit: single fre
18、quency semi-duplex operation is used. As each packet is received, the terminal stores it and automatically prints out the entire message at the conclusion of the transmission. A confirmation signal is returned to the originating terminal to indicate that the message has been successfully received. A
19、n operator is not required at the receiving end. The system uses in-band frequency diversity to combat selective fading. Two FSK modems using 170 Hz shift operating at 100 bit/s are used to transmit identical data. The data are assembled into an 8-bit byte, including one parity bit. A one-byte cycli
20、c redundancy code is added to form a robust code which allows a character error ratio of only one in 1 Os after ARQ is applied. Frequency channel selection is under the control of the terminal. When message transmission is initiated, an automatic procedure is activated which causes the stations to s
21、earch, among the frequencies assigned to the network, for a frequency capable of supporting data transfer. In this way, the reliability of the network is maintained even when experienced operators are not available. if no suitable frequency can be found, a message is printed informing the sender of
22、this fact. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECflN*F. 764-1 94 W 4855232 052233b 727 - Mains or battery 0- 12 v Display Keyboard Power supply Printer - 1 Rec. ITU-R F.764-1 FIGURE 1 The hard-copy message terminai M
23、ain CPU memory and control madmachine interface 149 I RS 232 H Modem E- !%! access from the switched network for remote terminal operations; - - storage, editing and transmission of messages from disk media; high quality Group 3 facsimile image with 7.7 x 3.85 line/mm resolution; - - enhanced throug
24、hput through hybrid ARQ with forward error correction; - hardware built to fit into one expansion slot of a 8088 based personal computer; interfaced to HF SSB transceivers via audio ports. - 2. System description The terminal is built around an 8088 microprocessor based personal computer that has a
25、real-time multitasking disk operating system environment. The resources of the computer are utilized by the terminal software and the user has access to the file management utilities for entry or retrieval of information from the terminal. The information that is transmitted and received by the term
26、inal is stored in the disk storage of the computer. The HF modem is a 12 channel (see Recommendation ITU-R F.436) FSK modem with 142.5 Hz frequency- shift and with channel separation of 170 Hz. The modulator and demodulator are implemented in digital signal processing (DSP) devices and interfaced to
27、 the computer bus. The modem and interface hardware is contained on a plug-in card for the computer expansion bus. Data are fed to the modulators as 12 bit binary words at 10 ms intervals through a parallel data output port. The modem outputs are combined and applied to the audio input of an HF SSB
28、communication transmitter. The HF SSB receiver audio output is digitized and fed to the 12 modem inputs. The demodulator generates a i00 Hz clock synchronized to the received data signai. Each demodulator output “eye signal” is sampled in the middle of the bit timing interval and the sample is conve
29、rted into a 5 bit digital word. At every 10 ms interval 12 demodulator samples are transferred to the computer memory for processing. Although the signalling rate is 100 Bd, the presence of 12 channels results in a raw data rate of 1 200 bit/s. The terminai is designed to operate with a packet radio
30、 protocol which provides the system with a framework for exchange of control information such as station identification, message types and options. In particular, the protocol permits the implementation of a selective repeat ARQ algorithm, which ensures the message integrity. Every packet starts wit
31、h a bit synchronization sequence followed by a packet framing word transmitted on all the channels. The computer combines all the sampled data from 12 channels and performs a matched filter detection for the packet framing word. The information following the framing word is called the packet header.
32、 The data bytes of the packet header are coded with block code of (12, 8) minimum distance 3, which is transmitted in parallel from the 12 modulators. The block code is decoded with a soft decision decoding algorithm by the receiver. The validity of the packet header is verified with a high rate err
33、or detection code which is transmitted as part of the header packet. In the packet header coding diversity is utilized instead of conventional in-band frequency diversity. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R F.76
34、4-1 151 The information is assembled into 96 small packets and transmitted after the packet header. Each information packet contains data bytes plus a sequence number and error detection code. The information packets are then coded with a rate-half error correction code. The error correction code is
35、 chosen such that the information can be recovered for either half of the coded packet. The transmitter does not transmit the parity portion of the coded packets initially but saves them for future repetition requests. Each of the 12 modulators is fed with 3 information packets sequentially without
36、any duplication. In addition to information packets, a packet is sent containing the sequence numbers of packets that are contained in that transmission. The receiving terminal checks the information packets for presence of errors. If the packet is error free, the information is stored in its proper
37、 location as indicated by the sequence number. If the information packet has one or more errors, the sampled analogue values of the packet from the demodulator output are saved for future processing. The message receiving terminal requests repeat of the outstanding information packets in the acknowl
38、edgement packet. Whenever the transmitter has to repeat an information packet, the parity part of the error detection code version of the packet is sent. The transmitter alternates the repetition of the same packet between the information and parity part of the coded packet. If the parity part of th
39、e error detection code of the packet is received error free, then the information is recovered by an inversion process. If the second transmission also contains errors, a soft decision error correction process is activated using the stored samples of the same packet from first and second receptions.
40、 The output of the error correction process is verified with error detection code of the packet before accepting the data. The receiver linearly combines the stored sampled analogue values of the same packet whenever the received packet fails the error detection process in order to build signal stre
41、ngth and utilize time diversity. When the transmitter has to repeat an information packet the repetition of that packet is made through a different channel to avoid persistent channel disturbance that could be present. The terminal scans the assigned radio frequencies continually and when a message
42、transmission is initiated. the message originating terminal calls the destination terminal sequentially on all the assigned radio channels. The message session is established on a frequency that is suitable for data transmission. In this way, the reliability of the network is maintained even when ex
43、perienced operators are not available. Interface to ITU-T Group 3 facsimile apparatus is provided through a special port built into the computer interface card of the terminal. The document is scanned in 7.7 x 3.85 line/mm resolution and the image data is compressed with an error free algorithm. Thi
44、s algorithm has been found to be 65% more efficient than the ITU-T Group 4 facsimile apparatus data compression technique. The image compression algorithm when applied to eight ITU-T test images results in an average image size of 12 kbytes. The compressed image is transmitted from the disk file and
45、 the receiving terminal places the image into disk storage. The image can be expanded with the inverse of the data compression algorithm and viewed by the video display unit of the computer or printed by the facsimile machine. 3. Experimental results The system was tested on a link from Ottawa to a
46、location near Vancouver, a distance of 3 500 km. A 4096 byte test message was transmitted periodically over a 15-day trial period. The terminals were programmed to scan 3 assigned radio channels and no operator was utilized for channel selection. Figure 2 shows the distribution of transmission times
47、 for 344 experiments that were conducted during this trial. Tests were also made over a 100 km link near Ottawa; the results obtained were substantially the same as those obtained in the long range triai. The received files from the trials were checked for undetected errors and none was found in 6 M
48、bytes of data. The HF system design achieved reliable data and message transmission over HF radio channels. The memory ARQ performs as a diversity on demand technique adapting the system to changing channel conditions, thus achieving higher throughput when compared to systems utilizing in-band frequ
49、ency diversity and simple ARQ protocols where the received data is discarded in the presence of errors. The parallel modems are used in a flexible mode where the data can be coded and spread over all the channels, and the error correcting capability of soft decision decoding enhances the performance when extra protection is required for packet headers. The built-in facility for monitoring different radio COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services 1TU-R RECMNUF. 764-1 94 m 4855212 0522339 43b a 840 780 720 660 600 ,
