1、 Rec. ITU-R M.1476 1 RECOMMENDATION ITU-R M.1476 PERFORMANCE OBJECTIVES FOR NARROW-BAND DIGITAL CHANNELS USING GEOSTATIONARY SATELLITES TO SERVE TRANSPORTABLE AND MOBILE EARTH STATIONS IN THE 1-3 GHz RANGE FORMING PART OF THE INTEGRATED SERVICES DIGITAL NETWORK (Question ITU-R 112/8) (2000) Rec. ITU
2、-R M.1476 The ITU Radiocommunication Assembly, considering a) that performance objectives for digital mobile-satellite service (MSS) channels using narrow-band modulation and geostationary satellites should include an allowance for inter-system and intra-system interference levels for systems operat
3、ing in the 1-3 GHz range, as well as comply with the user performance requirements; b) that the bit-error ratio (BER) will vary with time due to the effects of varying propagation conditions including the effects of multipath fading; c) that reception of signals transmitted to and from mobile earth
4、stations at most locations may be substantially degraded by location and orientation dependent on signal propagation impairments which determine spatial performance; d) that digital MSS channels may be used in global or spot coverage beams as part of a worldwide MSS, in which case stringent satellit
5、e power constraints are expected; e) that the performance objectives of the MSS for services which form part of the integrated services digital network (ISDN) should be defined considering the ITU-T Recommendations concerning ISDN performance objectives (Recommendations ITU-R G.821 and G.826) with a
6、n appropriate consideration for the effect of propagation conditions in the MSS; f) that performance objectives for MSS which are not connected via the ISDN will be the subject of other ITU-R Recommendations, recommends 1 that in both forward and return directions of transmission, through the servic
7、e and feeder links, the combined radio-link performance objectives for MSS digital channels forming part of the ISDN are as follows: 1.1 the combined MSS radio link unavailability due to propagation should provisionally be not more than 0.1% of the time, where the definition of available time and un
8、available time is given in ITU-T Recommendation G.821, and provided for clarity in 2, Annex 1 of this Recommendation; 1.2 a BER better than 9 107after error correction for more than 99% of the available time; 2 that the method given in Annex 2 may be used as a guideline for the system designer, for
9、apportioning the overall unavailability between the feeder link and service link parts; 3 that the following Notes are an integral part of this Recommendation. NOTE 1 An analysis linking the available and unavailable time defined by ITU-T Recommendation G.821 to unavailability due to propagation of
10、MSS channels is provided in Annex 1. NOTE 2 For data communications, error control techniques are generally provided as an integral part of the modem. Additional end-to-end error control measures may be implemented to enhance performance for specific user applications. However, for the purpose of th
11、is Recommendation, the effects of these user application techniques are not included in the performance objectives. NOTE 3 The effects of obstruction and shadowing of the direct propagation path are not considered in the performance objectives defined above. Digital mobile-satellite channels used by
12、 vehicular land mobile earth stations are more susceptible to reduced spacial availability due to shadowing and obstruction. 2 Rec. ITU-R M.1476 NOTE 4 Spatial availability is defined as the cumulative probability of link availability arising from the joint distribution of location and orientation d
13、ependent random losses in the link. NOTE 5 For maritime and aeronautical earth stations at low elevation angles operating through the digital mobile-satellite channel, the BER performance objectives given in recommends 1 might be achieved for a lower percentage of the time. NOTE 6 In the case of bac
14、kup using a previous generation satellite the BER performance objectives given in recommends 1 might be achieved for a lower percentage of the time. NOTE 7 The effects of aggregate interference from other systems and services should be taken into account in ensuring that the overall performance obje
15、ctives of the digital MSS channel are met. NOTE 8 A BER objective better than 1 106for more than 99% of the available time (reflecting propagation conditions) is apportioned between the MSS link and the other components forming part of the end-to-end ISDN link. With reference to the hypothetical ref
16、erence connection (HRX) given in Fig. 1 of ITU-T Recommendation G.821, 90% of the performance objectives (errored seconds (ES), sererely errored seconds (SES) are apportioned to the MSS radio link since it is the most significant contributor to BER degradation. Therefore, the BER performance apporti
17、oned to the MSS radio link is 9 107, during the available time. NOTE 9 Further study is required for the necessity of short-term performance criteria to define the associated BER thresholds and percentages of time. ANNEX 1 Methodology for calculating unavailability of the MSS radio link based on the
18、 unavailability defined for the ISDN in ITU-T Recommendation G.821 1 Introduction For fixed services (FS) and fixed-satellite services (FSS), quality objectives for the ISDN are defined by ITU-T Recommendation G.821. For MSS, on the other hand, availability is the main quality objective so far and i
19、t is assessed by the statistics of C/N versus a threshold C/N which corresponds to a specified BER in probability. In the mobile ISDN era where mobile-satellite communication systems will be incorporated in the global ISDN system, however, circuit quality of MSS should be defined so as to keep consi
20、stencies with the FS and FSS. This Annex discusses various propagation impairments for mobile-satellite systems (such as maritime mobile-satellite service (MMSS), aeronautical mobile-satellite service (AMSS) and land mobile-satellite service (LMSS) from a viewpoint of the available and unavailable t
21、ime defined by ITU-T Recommendation G.821, and stresses the importance of the concept that unavailability and degradation during available time are two different measures for evaluation of mobile-satellite communication in the mobile ISDN era. 2 Definition of parameters used in ITU-T Recommendation
22、G.821 To clarify the discussion in this Annex, parameters used in ITU-T Recommendation G.821 are listed below. Available time and unavailable time A period of unavailable time begins when the BER in each second is worse than 1 103for a period of ten consecutive seconds. These ten seconds are conside
23、red as the unavailable time. A new period of available time begins with the first second of a period of ten consecutive seconds each of which has a BER better than 1 103. SES One-second period which has a BER worse than 1 103. ES One-second period in which one or more bits are in error. In ITU-T Rec
24、ommendation G.821 for ISDN performance objectives, availability is not discussed and only percentages of SES and ES are defined. Rec. ITU-R M.1476 3 3 Relation between availability defined by traditional analysis and ITU-T Recommen-dation G.821 Table 1 summarizes the characteristics of fading phenom
25、ena for various MSS. From this Table, it can be recognized that: LMSS propagation environments are most complex due to the variety of fading phenomena, and a consideration of availability for MMSS is needed because the fading period of MMSS is comparable with 10 s which is a criteria for available a
26、nd unavailable time in the ITU-T definition. TABLE 1 Characteristics of propagation impairments in several MSS communication environments Figure 1 shows an example of flat fading in which the C/N value changes with time. Since noise level N can be regarded as constant in the ordinary case, the varia
27、tion of C/N is equivalent to that of the received level. Also, C/N and BER correspond to each other, the variation of C/N in Fig. 1 can be regarded as a variation of BER, (here, BER is defined as a probability, not the BER in one second mentioned above. We will therefore write the former BERproband
28、the latter BERsecto distinguish them). We denote the median (nominal) value of C/N as C/Nmed, and the threshold C/N value at which BERprobbecomes the threshold BER as C/Nthres, for example 1 103. Service Characteristics MMSS AMSS LMSS-V (vehicular) LMSS-P (handheld terminal) Communication environmen
29、t Line-of-sight Line-of-sight Line-of-sight Out-of-sight Line-of-sight (out-of-sight communication is difficult) Primary factor of fading Sea reflection fading Sea reflection fading Ground reflection fading Shadowing by terrain, buildings and trees Multipath fading Same as LMSS Shadowing by human bo
30、dy Typical fade depth(1)Within 10 dB Within 8 dB Varies with environment (more than 20 dB when maximum) Within 10 dB C/M (2)More than 6 dB More than 7 dB 0-20 dB (various) More than 6 dB Dominant frequency component of variation 0.5 to several hundred Hz 20 to several hundred Hz 10 to several tens o
31、f Hz (multipath fading) Below 1 Hz (shadowing by terrain) Several Hz or below (1)Fade depth: 50-99 percentile fade level. (2)C/M ratio of direct wave power, C, to average multipath power, M. 4 Rec. ITU-R M.1476 1476-01TDTI10 sC/NmedLC/NtC/NthresHighLowTraditionalanalysisOccurenceof SESAvailable/unav
32、ailabletime (ITU-T)UnavailabletimeAvailabletimeSESs (10 s) SESs ( UAITU-T), and unavailability defined by ITU-T becomes smaller than that defined by the traditional analysis. This is only a matter of definition, and there is no change in the quality of actual MMSS communication. 4 Unavailability in
33、MMSS fading environments In this section MMSS unavailability is discussed as a case study of the previous section. In MMSS, sea reflection fading is the primary factor in propagation impairments. Many studies have been carried out on this phenomenon, and have revealed its characteristics. In previou
34、s studies, the following characteristics have been identified. 4.1 Fading spectrum The fading spectrum (spectrum of intensity variation) can be approximated by the following second order Butterworth characteristic: 1410491)(Gf7Gf7Gf8Gf6Ge7Ge7Ge8Ge6+fffW (5) Here, f10represents the 10 dB spectral ban
35、dwidth. This bandwidth depends on frequency, elevation angle, sea surface condition, ship velocity, etc., and its value varies over a wide range. Its characteristics are summarized in Fig. 2. 4.2 Mean fade duration and mean fade occurrence interval The mean values of TDand TIwhen the threshold value
36、 is set to a p percentile fade level, Lp, can be expressed by equation (6): 2/)(exp3)(210pmfpTI=Gf1Ge1 (6a)Gf7Gf8Gf6Ge7Ge8Ge6Gf1Ge1=Gf1Ge11001)()(ppTpTID(6b) Here, the relation between p and m is expressed by equation (7): Gf2=mttpp2%d2/exp2100)( (7) 6 Rec. ITU-R M.1476 1476-0210125110255 10 15FIGUR
37、E 2Spectral bandwidth of multipath fading due to sea surfacereflection in MMSS environmentsElevation angleSpectralbandwidth, f10(Hz)Frequency = 1.5 GHzWave height: 1 mShip velocity: 20 knotsRolling: 30Wave height: 1 mShip velocity: 0 knotsRolling: 0FIGURE 2/M.176.D01 = 3 CM 4.3 Probability distribut
38、ion functions (PDFs) of TDand TIThe PDFs of TDand TIwhen threshold value is set to a p (p = 50 to 99) percentile fade level Lpcan be expressed as exponential distributions of the means and , respectively. PDF of TDis expressed by equation (8): )/(exp1);( =f (8) Substituting equation (8) in (4), the
39、relation between the two unavailabilities in MMSS environments is expressed by equation (9): tradDpTT-ITUUApTUADGf7Gf7Gf8Gf6Ge7Ge7Ge8Ge6+)(1e0)(/0 (9) Since unavailable time becomes likely to occur as the period of variation becomes longer, unavailability for f10= 0.5 Hz (fairly slow within the obse
40、rved cases), f10= 0.3 Hz (rare case within the observed cases) and f10= 0.2 Hz (may not be observed in the actual communication) are estimated. Figure 3 shows the estimation results for p = 99% and p = 99.9% (BERprob= 1 103). In this Figure, 0 = 10 s corresponds to the unavailability according to th
41、e ITU-T definition. From the result, it can be concluded that the probability of occurrence of unavailable time is negligible for all MMSS communication environments. Rec. ITU-R M.1476 7 1476-03101011021031041051151050a) p (= 100 UAtrad) = 99%1011021031041051151050Amount of time when TD 0Total time(
42、%)Amount of time when TD 0Total time(%)Threshold fadeduration,0(s)Threshold fadeduration,0(s)b) p (= 100 UAtrad) = 99.9%UAITU-TFIGURE 3Relations between two unavailabilities based on the traditional analysisand ITU-T Recommendation G.821 in MMSS environmentsf10= 0.2 Hzf10= 0.3 Hzf10= 0.5 HzFIGURE 3/
43、M.1476.D01 = 3 CM 8 Rec. ITU-R M.1476 Although the above discussion is a rough estimation, it may be concluded that duration of sea reflection fading in an MMSS environment does not exceed 10 s and availability according to the ITU-T definition is 100%. Needless to say, the quality of MMSS communica
44、tion circuits is not high even if availability is 100%. The quality of communication circuits should be defined by another definition such as occurrence probability of SES during the available time. 5 Conclusion This Annex has discussed various propagation impairments for mobile-satellite systems fr
45、om a viewpoint of the available and unavailable time defined by ITU-T Recommendation G.821. The unavailability of MMSS due to sea reflection fading was examined. As a result, it was concluded that unavailability and degradation during available time should be considered separately as the different m
46、easures in the evaluation of mobile-satellite communication. ANNEX 2 A method for allocating the overall percentage of time for which the link is available between the service link and feeder link Generally individual uplinks and downlinks are so designed that the overall percentage of time for whic
47、h the link is available is to a large extent determined by the service link. This means that the service link is subject to a much tighter power constraint. The percentage of time for which the end-to-end link is available may be achieved by allocating percentages of available time to the correspond
48、ing uplink and downlink using Table 2. TABLE 2 For example, an end-to-end link availability requirement of 90% could be achieved with a service link availability requirement of 90.9%, and a feeder-link availability requirement of 99%. In the described allocation method the feeder link is ascribed 10
49、% of overall link unavailability. Ascribing a larger unavailability to the feeder link will result in a significant increase in the service link availability requirement. Conversely, reducing the unavailability ascribed to the feeder link will result in little or no change to the service link requirement, i.e. the overall link availability is dominated by the service link. Direction of transmission Radio path Percentage of time for which the link is available Forward Feeder uplin
