1、 STD-ITU-R RECMN M-20LO-L-ENGL 1997 4855212 0536063 S7T D Rep. ITU-R M.2010-1 1 REPORT ITU-R M.20 10- 1 IMPROVED EFFICIENCY IN THE USE OF THE BAND 156-174 MHz BY STATIONS IN THE MARITIME MOBILE SERVICE (Question ITU-R 9618) ( 1993- 1997) 1 Introduction 1.1 Recommendation 3 18 (Mob-87) of the World A
2、dministrative Radio Conference for the Mobile Services (Geneva 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
3、 spectrum conserving technologies and systems, used in or proposed 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 de
4、termine the likely improvement in spectrum utilization and to identify related issues, both technical and operational, and areas requiring further study. 2 Survey of technologies and systems The maritime service must at ail times provide an effective communication channel for distress and safety cal
5、ls, for search and rescue operations, and for navigational information. 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 o
6、f the alternative technologies and systems. It is particularly 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 wil
7、l have to provide enough capacity to satisfy the growth expected 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 ban
8、d; have minimal impact on the existing services, particularly 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 t
9、echnologies and systems reviewed in this study are outlined below. 2.1 Narrowband 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 pract
10、ice adjacent and co-channel performance is usually reduced with the result that reuse distances are increased and the full potential pain is not always realized. The following narrow-band technologies have been considered: - - 12.5 kHz channel spacing using analogue FM. Potentially this could provid
11、e up to twice as many channels; 6.25 kHz channel spacing using 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
12、 as many channels. - STD.ITU-R RECMN MJOLO-L-ENGL 1997 m 4212 053bob4 401, m 2 Rep. ITU-R M.2010-1 A11 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
13、 in $ 3. 2.2 25 kHz 4-time division multiple access (CTDMA) 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
14、 mobile 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 represen
15、ts a raw gain in channelskHz 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
16、in 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 1
17、200 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,
18、 but 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 Automat
19、ic 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
20、 applicable 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
21、equipment 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 Spectrumkapacity gain Halving the channel bandwidth would provide double the number of ch
22、annels. 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. 3.1.2 Operational iss
23、ues and migration Operationally there would need to be no changes and the new equipment would be interoperable with old equipment. Migration would be straightforward. Initially new channels could be interleaved (with suitable planning e.g. with sufficient geographical or frequency separation), and t
24、hen 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 land 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
25、kHz linear modulation Linear modulation based on amplitude compandored SSB (ACSSB) with transparent tone in band (TTIB) and feed forward signai regeneration (FFSR) has 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 chann
26、els. 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 limited availability of commercial equipment at the present time, although some use is being made of 5 kHz and 6.25 kHz equipment for th
27、e land mobile service in the United States and is therefore likely to become more readily available in the future. 3.2.1 Spectrurn/capacity gain 5 kHz channelling would provide five times as many channels as are presently available. As with 12.5 kHz analogue FM the susceptibility to Co-channel inter
28、ference, 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 19791, suggests that a factor of 2.5 is likely, although later (unpublished) studies indicate the higher reuse factor can be expec
29、ted. 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 be required. Migration would be by interleaving (possibly with two SSB channels between each old channel). Thereafter FM channels
30、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 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 digita
31、l 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 built-in advantage of this system is that of inherent privacy and security, thus alleviating growing problems of this nature. 3.3
32、.1 Spectrum/capacity 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, however, but in areas of intense frequency reuse the gain achievable may be less. 3.3.2 Operational issues and migration Operatio
33、nally 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 5 kHz ACSSB. 3.3.3 Equipment There is no known prototype equipment. Initially costs would be expected to be higher than current
34、25 kHz equipment but would fall with volume production. STD-ITU-R RECMN Mm20LO-L-ENGL 1997 9 4855232 053bObb 289 m 4 Rep. ITU-R M.2010-1 Option 12.5 kHz analogue FM 5 kHz or 6.25 kHz linear modulation 4 Re-allocation of duplex channels to simplex Gain in Operational implications capacity x 1.5 to x
35、2 None, interoperable with existing equipment Extra or dual mode equipment required x 2.5 to x 5 4.1 Spectrumlcapacity gain 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 correspondence channe
36、ls, 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 equivalent
37、 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 requires very hig
38、h levels of filtering and considerably increases the engineering problems and cost of the installation. Interleaving (with careful planning) 4.2 Operational issues and migration The introduction of additional simplex channels would not require any operational changes. Duplex channels could be change
39、d over individually or in groups. New equipment would be required only where existing equipment was not re-programmable. In limited use in land mobile radio service 4.3 Equipment There are no technical problems or risks associated with this change. Interleaving (with careful planning) 5 Summary and
40、conclusions Table 1 summarizes the main characteristics of the selected options. No commercial equipment available TABLE 1 Comparison of the selected options Reallocation of duplex channels to simplex x 1.3 Irx4 6.25 kHz channelling with digital modulation “Over night” changeover but simple or dual
41、mode equipment required Minor changes to current equipment Long transition period None Maritime versions of land mobile radio equipments New equipment required 1x4 I 25 kHz 4-TDMA approach Migration Equipment service Changing to 12.5 kHz analogue FM or re-allocating duplex channels to simplex operat
42、ion would be the simplest approach to improving spectrum utilization. Both would have minimal impact on current operations and a straightfor- ward migration path. Spectrum utilization would increase by a factor of between 1.5 and 2 with 12.5 kHz analogue FM, and by a factor of 1.3 with re-allocation
43、 of duplex channels. By combining both changes, the number of duplex channels could be maintained at their present levels and the spectrum utilization gain increased to a factor of approximately 2.5. STDmITU-R RECMN M-2030-3-ENGL 1997 4855232 053bOb7 115 W Rep. ITU-R M.2010-1 5 Larger gains in spect
44、rum utilization would be achieved with either 5 kHz linear modulation or with 6.35 kHz channels and digital voice. The former would increase utilization by a factor of between 2.5 and 5, the latter by a factor of up to 4 plus inherent security and privacy. The penalty would be the need for dual mode
45、 equipment during changeover and the increase in equipment costs. Neither technology is yet in widespread use but 5 kHz linear modulation (ACSSB) is in limited commercial use in the United States. However, it should be borne in mind that any change to the channelling arrangement of Appendix Si 8 to
46、the Radio Regulations (RR) will require a decision by a future competent world radio conference which could not occur before 1997 at the earliest, by which time use of these technologies by the land mobile service is likely to have reduced the cost significantly. The estimates of spectrum gain prese
47、nted in this Report are based on studies of land mobile radio and as such provide only a guide to the performance likely in the VHF maritime band. Before firm conclusions can be drawn further work is required to verify the estimates. In particular the adjacent and co-channel performance and its impl
48、ications for frequency reuse require further study. It can be seen that 5 kHz linear modulation or 6.25 kHz channelling with digital voice or data provide the greatest potential for a significant increase in efficiency in the use of the maritime VHF band and should be the prime candidates for furthe
49、r study. Annex 1 (Rationale for implementing the interim step of 12.5 kHz channel spacing) provides an example and description of a system which may be implemented within the United States for use with vessel traffic service (VTS) systems. Annex 2 provides a description of how a 25 kHz TDMA system could be employed in the maritime VHF working environment. REFERENCES BADEN, C. C. E. and JENKINS. A. P. August 19901 Linear modulation trials. Final report, University of Bradford. FRENCH, R. C. 3 August 19791 The effect of fading and shadowing on channel reuse in mobile
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