1、Rep. ITU-R M.2010-1 1 REPORT ITU-R M.2010-I IMPROVED EFFICIENCY IN THE USE OF THE BAND 156-174 MHz BY STATIONS IN THE MARITIME MOBILE SERVICE (Question ITU-R 96/8) (1993-1997) 1 Introduction 1.1 Recommendation 3 18 (Mob-87) of the World Administrative Radio Conference for the Mobile Services (Geneva
2、 1987) (WARC Mob-87) invites the ITU-R urgently to undertake studies to determine the most appropriate means of promoting a more efficient use of the frequency spectrum in the VHF maritime mobile band. 1.2 This Report includes a survey of spectrum conserving technologies and systems, used in or prop
3、osed for the private land mobile services, and examines various options for their suitability to the VHF maritime mobile service. A small number were selected as having the greatest potential. These have been examined in more detail to determine the likely improvement in spectrum utilization and to
4、identify related issues, both technical and operational, and areas requiring further study. 2 Survey of technologies and systems The maritime service must at all times provide an effective communication channel for distress and safety calls, for search and rescue operations, and for navigational inf
5、ormation. In addition the service supports public correspondence, the broadcast of weather bulletins, port and harbour control communications and intership communications. These factors have to be considered in assessing the suitability of the alternative technologies and systems. It is particularly
6、 important that any changes to the current system: - be implementable within the maritime VHF band as additional spectrum cannot be expected in the foreseeable future; - provide a significant increase in spectrum capacity; the changes will have to provide enough capacity to satisfy the growth expect
7、ed over the next ten or more years. However it should be noted that existing terrestrial cellular systems already cover some coastal waters and are relieving some of the pressure from public correspondence channels in the maritime VHF band; have minimal impact on the existing services, particularly
8、the operation of distress and safety channels; take advantage of new technologies available including data transmission (see Annex 1) to provide new features, such as encryption to provide added security and privacy. - - The alternative technologies and systems reviewed in this study are outlined be
9、low. 2.1 Narrow-band modulation Replacing the current 25 kHz channels with channels of a narrower bandwidth would be a straightforward way of obtaining more channels. In principle halving the bandwidth would provide twice as many. In practice adjacent and co-channel performance is usually reduced wi
10、th the result that reuse distances are increased and the full potential gain is not always realized. The following narrow-band technologies have been considered: - - 12.5 kHz channel spacing using analogue FM. Potentially this could provide up to twice as many channels; 6.25 kHz channel spacing usin
11、g digital speech and modulation. Potentially this could provide up to four times as many channels; 5 kHz channel spacing using linear modulation (a form of single-side band (SSB) modulation). Potentially this could provide up to five times as many channels. - COPYRIGHT International Telecommunicatio
12、ns Union/ITU RadiocommunicationsLicensed by Information Handling Services2 Rep. ITU-R M.2010-1 All three approaches could provide significant capacity gains, are applicable to the VHF band, and would not entail any major changes in the way that current services operate. All are evaluated further in
13、Q 3. 2.2 25 kHz 4-time division multiple access (4-TDMA) approach 25 kHz 4-TDMA is likely to be used for land mobile applications in some parts of the world and is therefore likely to benefit from economies of scale. A 25 kHz 4-TDMA system called TETRA is the most likely candidate for future land mo
14、bile applications in Europe. TETRA is an open European Standard and is a spectrally efficient, feature-rich system which could readily be adapted for operation in the maritime environment. In terms of spectral efficiency, TETRA compares well with the other systems under consideration and represents
15、a raw gain in channelskaz of 4:l over 25 kHz FM with a high data rate capability, particularly if multiple time slots are used. TETRA is being considered for use in the United Kingdom for maritime applications limited to national maritime applications. A description of this approach can be found in
16、Annex 2. 2.3 Replacement of speech by data In applications where standard messages are often used, or the message is one way, the transmission of text instead of voice can save a significant amount of channel time. For example, a 10 s voice message can be sent as text using data transmission at 1200
17、 bids in 2 s, and in less at higher bit rates. This offers a 5 to 1 improvement in channel capacity or better. However the extent to which this can be realized in practice depends on the extent to which text can replace voice. On the optimistic assumption that half of all port operations traffic, bu
18、t not public correspondence or ship-to-ship communications, can be replaced by data transmission the increase in capacity on the international frequencies would be equivalent to an additional six duplex and four simplex channels. Overall capacity would be increased by a factor of 1.2. 2.4 Automatic
19、call Set-up The introduction of automatic call Set-up systems provides a small increase in capacity, e.g. 20% assuming an existing manual call Set-up time of 0.5 min for an average 2.5 min call. 3 Selected narrow-band modulation options All the narrow-band technologies considered here are equally ap
20、plicable to duplex and simplex channels. 3.1 12.5 kHz analogue FM 12.5 kHz FM modulation is already widely used in land mobile radio and could be adopted to give a halving of the channel spacing. The main advantage of this approach is that the technology is available and proven, and that the new equ
21、ipment would be inter-operable with existing sets (with some reduction in performance). The major disadvantage is the limited gain in capacity relative to alternative narrow-band modulation techniques. 3.1.1 Spectrudcapacity gain Halving the channel bandwidth would provide double the number of chann
22、els. There is, however, an increase in susceptibility to Co-channel interference and therefore the minimum reuse distance would be increased. In areas where the reuse distance is anyway greater than this minimum the full gain in capacity of a factor of two would be obtained. COPYRIGHT International
23、Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRep. ITU-R M.2010-1 3 3.1.2 Operational issues and migration Operationally there would need to be no changes and the new equipment would be interoperable with old equipment. Migration would be straightforward. I
24、nitially new channels could be interleaved (with suitable planning e.g. with sufficient geographical or frequency separation), and then progressively changed over to 12.5 kHz. Thus extra channels can be provided first where needed most. 3.1.3 Equipment Equipment is available and in use for private l
25、and mobile today in the VHF bands. Costs would be expected to be about the same as for existing 25 kHz equipment. 3.2 5 kHz or 6.25 kHz linear modulation Linear modulation based on amplitude compandored SSB (ACSSB) with transparent tone in band (TTIB) and feed forward signal regeneration (FFSR) has
26、been shown to be suitable for land mobile radio use in 6.25 kHz McGeehan and Bateman, 19831 and 5 kHz Baden and Jenkins, 19901 channels. The major advantage of this technology is the large gain in spectrum capacity with little or no change to operational procedures. Its main disadvantage is the limi
27、ted availability of commercial equipment at the present time, although some use is being made of 5 kHz and 6.25 kHz equipment for the land mobile service in the United States and is therefore likely to become more readily available in the future. 3.2.1 Spectrumhapacity gain 5 kHz channelling would p
28、rovide five times as many channels as are presently available. As with 12.5 kHz analogue FM the susceptibility to co-channel interference, and therefore the minimum reuse distance, is increased. In areas of intense frequency reuse the overall gain in capacity will be less than a factor of 5. French
29、1979, suggests that a factor of 2.5 is likely, although later (unpublished) studies indicate the higher reuse factor can be expected. 3.2.2 Operational issues and migration Operationally there need be no changes. During the changeover phase, however, extra equipment or dual mode transceivers would b
30、e required. Migration would be by interleaving (possibly with two SSB channels between each old channel). Thereafter FM channels have to be taken out and replaced by narrow-band channels. 3.2.3 Equipment ACSSB equipment is not at present in widespread use. However equipment has been developed and is
31、 being used on a limited basis at 220 MHz in the United States of America. 3.3 6.25 kHz channels with digital modulation A digital speech codec and digital modulation could be used to provide a single speech channel in a 6.25 kHz channel. Such a system could flexibly support both speech and data. A
32、built-in advantage of this system is that of inherent privacy and security, thus alleviating growing problems of this nature. 3.3.1 Spectrumhapacity gain This approach would increase the number of channels by a factor of 4. The adjacent and co-channel performance of this format is not established, h
33、owever, but in areas of intense frequency reuse the gain achievable may be less. 3.3.2 Operational issues and migration Operationally there need be no changes but extra equipment or dual mode transceivers would be required during the changeover phase. Migration to the new system would be similar to
34、5 kHz ACSSB. 3.3.3 Equipment There is no known prototype equipment. Initially costs would be expected to be higher than current 25 kHz equipment but would fall with volume production. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services4 R
35、ep. ITU-R M.2010-1 Gain in capacity 4 Re-allocation of duplex channels to simplex Operational implications 4.1 Spectrdcapacity gain x4 The capacity of each pair of duplex frequencies re-allocated as simplex channels is doubled. However, not all duplex channels could be re-assigned. Public correspond
36、ence channels, for example, would not be suited to simplex working. Making the assumption that all duplex channels exclusive to port operations and half those shared with public correspondence could be re-allocated as two single frequency channels the number of extra channels obtained is 16. This is
37、 equivalent to a gain in capacity of a factor of 1.3. It should be noted that single frequency operation is normally to be avoided at radio stations required to operate on more than one channel at a time. Receiving on one antenna while transmitting on a nearby frequency on an adjacent antenna requir
38、es very high levels of filtering and considerably increases the engineering problems and cost of the installation. New equipment required 4.2 Operational issues and migration The introduction of additional simplex channels would not require any operational changes. Duplex channels could be changed o
39、ver individually or in groups. New equipment would be required only where existing equipment was not re-programmable. 4.3 Equipment There are no technical problems or risks associated with this change. 5 Summary and conclusions Table 1 summarizes the main characteristics of the selected options. TAB
40、LE 1 Comparison of the selected options Option 12.5 kHz analogue FM 5 kHz or 6.25 kHz linear modulation 6.25 kHz channelling with digital modulation Reallocation of duplex channels to simplex 25 kHz 4-TDMA approach x 1.5 to x 2 None, interoperable with existing equipment Extra or dual mode equipment
41、 x 2.5 to x 5 t required scheme with Gaussian-shaped bandwidth- limiting filters at baseband to accomplish the ultimate FM-compatible BER performance at moderately low S/N ratios (10 dB S/N and above). GMSK is widely used on 12.5 kHz channel spacing in land mobile applications at 8 O00 bids. One iss
42、ue with this scheme is that implementation may be encumbered with royalty considerations due to proprietary hardware and software since no public-domain use is in effect at present. Another consideration is that actual use of high data rate operation will normally be in higher S/N environments (12 d
43、B S/N and higher), where the emphasis may shift from (S/N versus BER) to (data rate versus occupied bandwidth). This is especially me if FEC is used to improve BER in moderate SIN conditions. FEC is needed in order to protect radio data systems from signal fades and impulse noise interference. For t
44、hese reasons, GMSK was not considered to be the best candidate for implementation in the near term. See Figs. 1,2 and 3 for measurement results of 8 O00 bids GMSK performance. FIGURE 1 GMSK 8 O00 bit/s modulation. x RF frequency spectrum resulting from a random data input O - 20 h F4 v z -40 A - 60
45、- 80 I Unmodulated carrier level f, - 20 f,- 10 f, f,+ 10 f, + 20 Frequency (kHz) Rap 2010-0 I COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services10 Rep. ITU-R M.2010-1 O 50 h F4 v z 3 A 1 O0 150 O O .3 Y e W O 10-1 10-2 1 104 1 1 o-6 FIG
46、URE 2 GMSK 8 O00 bitls modulation. Typical “x out” frequency spectrum for a random data input I O dB = 1.0 V r.m.s I MOBITEX settings Data rate = 8 O00 bit/s II II. 5 10 15 Frequency (kHz) 20 Rap 20 10-02 FIGURE 3 GMSK 8 O00 bit/s modulation. Typical error ratios I I 4 5 6 7 8 9 10 11 12 SIN ratio (
47、dB) Rap 2010-03 3.3.3 The 4-level FSK (C4FM) method uses a four-level frequency offset (+A +f/3, -f3, -ffor equal “eye” openings) scheme (see Fig. 4) with bandwidth-limiting filters at baseband to accomplish the ultimate FM-compatible data rate performance in a minimum occupied bandwidth at moderate
48、 SYN ratios (12 dB SYN and above). See Fig. 5 for filter response. 4-level FSK (C4FM) is used on 12.5 kHz channel spacing in land-mobile applications at 9 600 bit/s. FEC is very 4-Level FSK (C4FM) with baseband filtering COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicense
49、d by Information Handling ServicesRep. ITU-R M.2010-1 11 effective in this method to reduce BER to levels that approach GMSK performance at SYN ratios between 10-12 dB. Occupied bandwidth is less than GMSK, and 9 600 bids performance is achieved with absolutely no encroachment on 25 kHz wide-band channel occupied bandwidth. Another prime positive factor is the availability of hardware and software with usage experience in the public domain. This is due to widespread adoption in the United States of America which has been encouraged by the influence of an international group of governme
