1、STD-ITU-R RECMN S-1327-ENGL 1777 M 4855212 0530477 244 Rec. ITU-R S.1329 1 RECOMMENDATION ITU-R S.1329* FREQUENCY SHARING OF THE BANDS 19.7-20.2 GHz AND 29.5-30.0 GHz BETWEEN SYSTEMS IN THE MOBILE-SATELLITE SERVICE AND SYSTEMS IN THE FIXED-SATELLITE SERVICE (Question ITU-R 81/4) ( 1997) The IT Radio
2、communication Assembly, considering a) that the World Administrative Radio Conference for Dealing with Frequency Allocations in Certain Parts of the Spectrum (Malaga-Torremolinos, 1992) (WARC-92) made allocations, on a primary basis, to the mobile-satellite service (MSS) in the bands 19.7-20.1 GHz a
3、nd 29.5-29.9 GHz in Region 2 and 20.1-20.2 GHz and 29.9-30.0 GHz in all three Regions; b) that earlier WARCs allocated these bands to the fixed-satellite service (FSS) on a primary basis; cl that WARC-92 adopted Recommendation No. 719 which requests the ITU-R “to study as a matter of urgency technic
4、al characteristics, including pointing techniques, of multiservice*-satellite networks using the geostationary- satellite networks encompassing mobile-satellite and fixed-satellite applications, and the sharing criteria necessary for compatibility with the fmed-satellite service in the frequency ban
5、ds referred to above“; d) that the Radiocommunication Assembly in 1993 approved Question ITU-R 81/4 and accorded it priority; e) that separately owned and operated networks may include FSS only satellites, MSS only satellites and dual-service (i.e. FSS and MSS) satellites, and that each of these thr
6、ee types of network may have to share the bands with either or both of the other two types; 0 8) uplink antenna beam widths equal to or smaller than lo; h) that is co-frequency co-coverage to provide necessary protection to the MSS, that in 1997 more than 180 20/30 GHz geostationary-satellite orbit
7、(GSO) FSS networks are in coordination; that the technology of some currently planned GSO FSS networks might support 2“ spacing of the arc with that GSO MSS networks as originally studied would require more than 2“ spacing from the nearest GSO FSS recommendr 1 that, in the planning and development o
8、f FSS, MSS and dual-service systems to utilize the geostationary orbit and to operate in the above frequency bands, the information on the technical characteristics and interference protection criteria of such systems contained in Annex 1 may be taken into consideration; 2 that new planned GSO FSS a
9、nd MSS networks in the 29.5-30.0 and 19.7-20.2 GHz bands should take into account the technical characteristics of the FSS as described in Annex 2 and the characteristics of the MSS as being studied by Radiocommunication Study Group 8 under Question IT-R 10418. NOTE 1 - Administrations are urged to
10、submit further contributions on the subject matter contained in Annexes 1 and 2, particularly with regard to parameters of planned or future systems intended to operate in the above bands. * This Recommendation should be brought to the attention of Radiocommunication Study Group 8. This term is used
11、 here to imply satellites equipped to operate in more than one service category. * - STD-ITU-R RECMN S-1327-ENGL 1777 4855222 0530478 180 H 2 Rec. ITU-R S.1329 ANNEX 1 Frequency sharing between FSS and MSS networks in the 30120 GHz bands 1 Introduction WARC-92 produced Recommendation No. 719, which
12、states that studies should be carried out on the technical characteristics of 30/20 GHz multiservice satellite networks and the sharing criteria necessary for their compatibility with the FSS. A considerable amount of work has been carried out on this subject, with several administrations submitting
13、 input papers to the annual WP 4A meetings. At the fourth meeting of WP 4A, in November 1993, a number of contributions were submitted, including papers suggesting that the use of code division multiple access (CDMA), as the multiple access technique, might help alleviate the sharing situation. Some
14、 opposition to this suggestion was expressed, on the grounds that the use of CDMA can have certain disadvantages from the point of view of the mobile operator. For example, one particular problem that was highlighted was the effect of power control on CDMA systems and their capacity. The material in
15、 this Annex is based on a broad range of contributions and includes some detailed consideration of various aspects of CDMA, power control and other 30/20 GHz system parameters. 2 Some aspects of current MSS technology This section outlines some key aspects of MSS technology which need to be carefull
16、y considered before any reliable analysis of orbit efficiency and sharing potential can be developed. Following a brief treatment of some fundamental theory relating to CDMA systems, some practical aspects are considered, based on available information on some planned future 30/20 GHz band systems.
17、It should be emphasized that this is in no way a fll, definitive treatment. However, results are produced which allow useful conclusions to be drawn. 2.1 CDMA satellite systems Two forms of CDMA exist, frequency hopping CDMA (FH-CDMA) and direct sequence CDMA (DS-CDMA). For economic reasons the use
18、of FH-CDMA is usually limited to military systems hence only DS-CDMA is considered here. CDMA can be transmittedreceived using two basic methods (although the boundary between these methods is blurred by the fact that quasi-synchronous CDMA exists). The two basic methods are asynchronous and synchro
19、nous CDMA. In an asynchronous CDMA system a user can transmit information with no particular regard to the state of his unique chip sequence. In synchronous CDMA, a master code is transmitted that is received by every station in the system. This master code, amongst other things, allows every transm
20、itting station to synchronize their chip codes and helps receiving stations to acquire the wanted incoming chip codes. Since all transmitting stations are synchronized, the cross-correlation products of their codes are kept to a minimum (especially if low cross-correlation codes are used, e.g. Gold
21、codes), and so system self noise is minimized implying that the systems capacity is maximized. However, in a synchronous system, achieving initial synchronization to the wanted code is harder due to the reduced signal-to-noise ratio which arises because of the lack of synchronizm. 2.1.1 Theoretical
22、maximum number of accesses Consider a direct sequence CDMA (DS-CDMA) system, with m received carriers all of equal power C. The useful carrier power at the receiver input is therefore C and if Eb is the energy per information bit and i?b is the information bit rate: The total noise power at the rece
23、iver input (in receiver bandwidth B) is given by the sum of the system self-noise power generated by (rn - 1) users, the thermal noise power and any other interfering noise power: STD-ITU-R RECMN S.3329-ENGL 1797 H 4855232 0530479 O17 = Rec. ITU-R S.1329 So, using these two equations: Eb Rb NOB - (m
24、 - 1)C - NOTH B c/z = (-)-(*-l)-( NOB NB O? ) Eb Rb Now, for a carrier of bit rate R, the modulation spectral efficiency is given by: The processing gain of a CDMA system is defined as: Rc FA- R, Combining these three equations, and rearranging, gives: 3 This equation shows how an increase in extern
25、al interference can be readily accommodated in a CDMA system by reducing system capacity. For a given required all the carrier powers, at the receiver input, are equal, also as stated above; the receiver is locked onto, and tracks perfectly, the incoming signal frequency; the receiver is perfectly s
26、ynchronized to the incoming chips. 2.1.2 Many texts show that CDMA has a poor throughput compared with frequency division multiple access (FDMA) and, especially, time division multiple access (TDMA). Despite this, CDMA may have mitigating advantages relative to both FDMA and TDMA in some particular
27、applications. Some of the common reasons given for advocating the use of CDMA, even with the reduced capacity that CDMA offers, are summarized below: a) Under overload conditions, CDMA system quality degrades gracefully, i.e. allowing a temporary increase in bit error ratio (BER) makes more capacity
28、 temporarily available. Conversely if the system load, at any time, is actually lower than the peak load then link quality improves (in terms of BER) compared with a fully loaded system. Some advantages of using CDMA in particular applications STD-ITU-R RECMN S-1327-ENGL 1997 4855232 0530480 839 9 R
29、ec. ITU-R S.1329 4 b) Voice activation can easily be applied to systems using CDMA as their multiple access method. Voice activated CDMA provides simultaneous demand assignment multiple access (DAMA) of both a transponders bandwidth and its power. CDMA reduces each individual terminals transmitted p
30、ower spectral density (PSD) so reducing its interference potential into narrower bandwidth carriers. A DS-CDMA system spreads the terminals baseband spectrum over a large bandwidth, reducing its PSD by an amount equal to the processing gain of the CDMA system. This is a very useful property when a C
31、DMA system is sharing with terrestrial systems, particularly if the CDMA system has mobile terminals with wide beamwidth, small diameter antennas. c) d) A CDMA signal, because of its lack of a sharp threshold, will perform better in a fadinglshadowing environment due to the simultaneous shadowing of
32、 both the wanted and the interfering signals (compared with FDMA) and due to its lower sensitivity to multipath. Multipath signals will either add constructively or destructively to the wanted signal. If they add destructively they appear simply as more system noise which is correlated out of the wa
33、nted signal in the receiver. 2.1.3 Acquisition and synchronization One of CDMAs advantages in frequency tracking is its relative immunity to Doppler shift. However, three fundamental problems for a CDMA system are the initial acquisition, synchronization and tracking of both the wanted chip code and
34、 the transmission frequency. These problems have been well studied and can be now fully overcome using digital signal processing. 2.1.4 The effect of imperfect power control A further problem that must be addressed when considering the use of CDMA, at 30/20 GHz frequencies, is the effect of power co
35、ntrol. In an ideal CDMA system, every transmitted signal will have the same carrier power at the receiver input. Indeed this was one of the basic assumptions that was made when the CDh4A capacity equation was developed earlier. From this equation it can be seen that system capacity is approximately
36、inversely proportional to - rain fading; - - - geographical effects due to satellite antenna discrimination and different free space path attenuations; shadowing and blockage (for mobile terminals); antenna mispointing (this is not particularly important when wide beamwidth mobile antennas are consi
37、dered, but is a problem for narrow beamwidth vehicle mounted antennas); - satellite movement (this is not too much of a problem with geostationary satellites). To overcome these factors and to equalize, as far as possible, all signal carrier powers at the receiver, some form of up- path power contro
38、l at each transmitter must be implemented. This implies that every earth terminal, in a CDMA system, requires: - - some method(s) of monitoring its power flux-density (pfd) at the satellite; a transmitter with adjustable output power over some pre-determined range. Note that however good the earth t
39、erminal power control system, there will always be errors resulting in different received signal strengths if only due to the time delays inherent when using geostationary satellites and differences in up-path and down-path propagation fades. The term “imperfect power control” is used to describe th
40、ese errors. There have been many studies into the effect of imperfect power control (sometimes called the “near-far effect”) in cellular terrestrial systems. However, due to the fundamental differences between the channel characteristics of terrestrial and satellite systems (including different time
41、 delays and different propagation environments), these studies provide limited insight into the effects of imperfect power control on satellite CDMA systems. The derivation given below examines the effects of imperfect power control on a satellite CDMA systems capacity. - - - - STD-ITU-R RECMN S-132
42、9-ENGL 1777 W 4855232 0530483 775 m Rec. ITU-R 5.1329 5 Consider a single user, referred to as user 1, in an usynchronous DS-CDMA system (which is a worst case assumption). The total system self-noise interference power spectral density, ZOSN, as seen by user 1 is: where: ISN = Now, the total noise
43、PSD, NO, is given by the sum of the 2 ci i=2 (9) system self-noise interference PSD, ZOSN, the thermal noise PSD, Nom, and any other external interference PSD, lo. So the as expected, minimizing the power control error maximizes the system capacity. - Using equation (16), if the maximum system capac
44、ity is known (assuming the power is perfectly controlled), the system capacity at any level of power control can be found by calculating the constant A and assuming that the system parameters do not change. If the maximum system capacity is relatively large, equation (16) can be approximated by m =
45、A/D. This approximation can be used to plot a graph (see Fig. 1) of power control versus percentage of capacity lost for a large number of carriers. From this graph, for example, if D = 1.25 (= 1 dB) then 20% of the maximum capacity is lost. FIGURE 1 Percentage of CDMA capacity lost (for a large max
46、imum capacity) versus power control error, D 100 O O 1 2 3 4 5 6 7 8 9 10 Power control error, D (dB) It is very important to note that these equations have been developed assuming fully asynchronous CDMA. This represents a worst case assumption since a fully synchronous CDMA system will be less aff
47、ected by the magnitude of the imperfect power control (particularly if low cross-correlation code sequences are used, e.g. Gold codes). This is the case because the maximum system capacity will be increased, compared with that achieved with asynchronous CDMA, since each transmitters chip sequence wi
48、ll be synchronized carefully minimizing all cross-correlation products and hence the system self-noise interference. However, the use of fully synchronous CDMA in mobile systems may not be viable for economic reasons. Despite the problem of imperfect power control, CDMA possesses certain features wh
49、ich may make it an attractive multiple access technique for systems using mobile terminals. Frequency sharing can be eased by the ability to readily accommodate interference by a reduction in system capacity. Many texts state that CDMA throughput is low; this STD-ITU-R RECMN S.1329-ENGL 3997 m 4855232 0530483 548 m 7 Rec. ITU-R S.1329 implies that CDMA spectral efficiency is also low. Now, although the single satellite efficiency is low, the efficiency around the orbit arc might be larger than with other access techniques due to the closer
