1、 STD-ITU-R RECMN M-3315-ENGL 1997 D 4855232 0533210 325 Rec. ITU-R M.1315 137 RECOMMENDATION ITU-R M. 13 15 METHODOLOGY FOR EVALUATING INTERFERENCE FROM NARROW-BAND MOBILE-SATELLITE NETWORKS TO SPREAD-SPECTRUM DIRECT-SEQUENCE MOBILE-SATELLITE NETWORKS OPERATING WITH SPACE STATIONS IN LOW-EARTH ORBIT
2、 AT FREQUENCIES BELOW 1 GHz (Question ITU-R 83/8) (1997) Summary Simplified and detailed methodologies are presented for evaluating the effect of narrow-band interfering signals on spread-spectrum communication channels. The methodologies allow a rapid comparison of the relative effects of changes t
3、o parameters of the desired and interfering signals, such as power level, bandwidth, and frequency offset, which may be investigated to facilitate sharing between narrow-band and spread-spectrum mobile-satellite networks. The ITU Radiocommunication Assembly, considering that permissible levels of in
4、terference to spread-spectrum, direct-sequence networks operating in the low-Earth a orbit mobile-satellite service (LEO MSS) should be based on performance objectives for that service; b) that LEO MSS networks proposed for operation in shared frequency bands should be designed to accommodate limite
5、d interference from systems operating in the same bands, including other LEO MSS networks (both spread-spectrum and narrow-band), recommends 1 of direct-sequence, spread-spectrum LEO MSS networks caused by narrow-band LEO MSS transmissions; 2 that the simplified methodology of Annex 2 be used to det
6、ermine whether a potential interference condition exists between transmitters in a narrow-band LEO MSS network and receivers in a direct-sequence, spread-spectrum LEO MSS network. that the detailed analysis methodology in Annex 1 be used for fully evaluating the degradation in performance 8 ANNEX 1
7、Detailed methodology for assessing interference 1 Introduction The performance of a spread-spectrum LEO MSS network can be degraded by the simultaneous operation of narrow-band transmitters in the same frequency band. Whilst it is of general interest to compute the bit-energy-to-noise density ratio,
8、 Eo, to estimate interference to a particular communication link, it is often sufficient to compute the carrier-to-noise density (CND) at the input to the receiver. In fact, without detailed knowledge of a particular receiving COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsL
9、icensed by Information Handling ServicesSTD*ITU-R RECVN HmL315-ENGL 3777 4855232 053321L 2bl 138 Rec. ITU-R M.1315 system, Eflo may be difficult or impossible to compute, whereas CND is relatively easy to compute. The effect of noise sources outside the network is easily expressed as a change to the
10、 CND, allowing easier comparison between alternative designs and frequency sharing arrangements. Interference effects expressed as changes to CND are computed both for the worst case of an interfering transmitter being within the main lobe of a tracking antenna and for the more prevalent case of the
11、 interfering transmitter being in a side lobe of the tracking antenna. 2 Step A : - - - Step B : Step C: Step D : Step E : 3. Outline of the calculation procedure Collection of network parameters: orbital parameters (altitude, constellation description), spacecraft radio parameters (power, bandwidth
12、, antenna gain pattern, polarization), Earth station radio parameters (antenna, G/T). Computation of the spread-spectrum CND network without external interference. Computation of CND due to external interference in both main beam and side lobes for each simultaneously received interfering signal, in
13、cluding the effect of frequency offset. Computation of overall CND (CI(N0 + io) for each interfering signal and of degradation to CND. Computation of the overall CND degradation due to multiple-channel simultaneous interference entries. Calculation steps (as defined in the above outline) Step A : Co
14、llection of network parameters To illustrate the calculation steps, two LEO MSS networks using the same downlink frequencies are assumed to have the parameters given in Table 1. It is assumed that the space station antennas are designed to compensate for the changing Earth-space basic transmission l
15、oss as the spacecraft rises, transits the sky, and sets. By assuming coupling into the main beam of the earth station, the computation can be made independent of receiver antenna gain. In the case of interference coming from a direction other than that of the main beam, the gain difference (or discr
16、imination) between main and side lobes must be obtained (or a typical value must be assumed). For this example, the spread-spectrum mobile earth station (user terminal) has a minimum elevation angle of 10 and an average elevation angle of 25“ to its space station, and the feeder link (gateway) earth
17、 station operates with a 5 minimum elevation angle. For the interfering transmitters in the narrow-band network, path loss values are required at the minimum elevation angle and the elevation angle which produces the maximum interfering signal. Step B: Computation of spread-spectrum CND network with
18、out external interference The overall CND of a communication network with a simple, frequency-translating transponder, is the reciprocal of the sum of the reciprocals of the CND values for the uplink and downlink. In the case of a multi-user spread-spectrum network of n users, additional noise in a
19、particular users link comes in the form of the carrier signals of the (n - 1) other users. The CND for both of the inbound links (user terminal-to-satellite and satellite-to-gateway earth station) are computed and the carrier to self-interference ratio (CND,) is computed. These will be combined to f
20、orm a CND value that can be used as a basis for comparison with values determined when out-of-network interference is present. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesSTD-ITU-R RECMN M.1315-ENGL 1797 4t855232 0533232 LTB Rec. I
21、TU-R M.1315 139 TABLE 1 Typical parameters of example non-GSO MSS networks operating below 1 GHz Network Orbital parameters - Altitude(km) - Number of satellites - Number of orbits (satellitedorbit) - Satellite separation, degree Operational parameters Inbound Link (user-satellite): - Emission desig
22、nation - Frequency (MHz) - Chip rate (kJ3z) - Maximum e.i.r.p. (dBW) - Number of users - Receiver G/T (dB(K-) - Path loss for minimum link (dB) - Path loss for average link (dB) Inbound Link (satellite-gateway): - Emission designation - Frequency (MHz) - Maximum e.i.r.p. (dBW) - - - - Channel spacin
23、g (kHz) - Antenna gain (dB) - . Side lobe gain (dB) - Polarization - Receiver G/T(dB(K-) GSO: geostationary-satellite orbit. Path loss for minimum link (dB) Path loss for average link (dB) Path loss for maximum pfd (dB) Spread-spectrum (SS) network 1 O00 24 6 (4 satellites) 90 905KG 1 D 148.4245 614
24、.4 3.5 12 -3 O 144.7 (10“ minimum elevation angle) 141.1 (25“ average elevation angle) 905KG 1 D 137.5 145.3 (5“ elevation angle) 140.7 (25“ elevation angle) 139.5 (32“ elevation angle) 16 1 Left hand circular -19.2 -14 - In the equations below, the variables are defined as follows: n: C: N: No : I:
25、 I“ : BW: X: e. i. r.p. G: Gdg: T: number of simultaneous users camer power (dBW) noise power (dBW) noise density (dB(W/Hz) interference power (dBW) interference density (dB(W/Hz) bandwidth polarization isolation (dB) effective isotropic radiated power (dBW) gain (dB) gain difference between the des
26、ired and interfering antennas (dB) noise temperature (dBK) Narrow-band (NB) network 175 32 $ (8 satellites) $5 Not applicable (the two example networks use different uplink frequencies) 5KOOG 1 D 137- I38 7 143.9 (5“ t._vation an) 140.4 (19“ elevation angle) 136 (42“ elevation angle) IO - - Right ha
27、nd circular - COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services140 GIT: CND, : CNDd : CND, : CND, : STD-ITU-R RECMN M-LLEI-ENGL 3777 m 4855232 0533233 O34 m Rec. ITU-R M.1315 10 log glt where g and fare uplink carrier-to-noise density a
28、t the antenna aperture (dB(Hz) downlink carrier-to-noise density at the antenna aperture (dB(Hz) canier-to-noise density of network self-interference at the antenna aperture (dB(Hz) overall carrier-to-noise density (dB(Hz) and 10.lT(dB(K-l) L(angie): loss (L(angie) = IO log (4 7c (angie)*) d(ang1e)
29、: slant range of a spacecraft at a specified elevation angle (km). For purposes of illustration, it is assumed that we are analysing the effect of (n-1) “average” users at 25 elevation on one “desired” user at a minimal-link elevation of 5”. For the uplink (desired user): CND, = e.i.r.p. - L(10”) +
30、G/T - (-228.6) = 3.5 - 144.7 + (-30) - (-228.6) = 57.4 dB(Hz) For the downlink: CNDd = e.i.r.p. - L(5”) + G/T - (-228.6) = -14 - 145.3 + (-19.2) - (-228.6) = 50.1 dB(Hz) For the self-interference on the uplink: CND, = e.i.r.p. - L(10”) - (e.i.r.p. + 10 log (n- 1) - L(25) - 10 log BW) (3) = 3.5 - 144
31、.7 - (3.5 + 10.4 - 141.1 - 59.6) = 45.6 dB(Hz) The overall CND for the network operating with n users and no external interference is the reciprocal of the sum of the reciprocals of these three values: Thus, when this example spread-spectrum communication network operates, it has a 44 dB(Hz) CND. If
32、 at this noise level there is a 5 dB operating margin, then any additional interference which reduces the overall CND below 39 dB(Hz) will cause the network to operate in a degraded manner. Step C: Computation of CND due to external interference in both main beam and side lobes for each/sirnultaneou
33、sly received interfering signal, including the effect offiequency offset As the desired and interfering spacecraft rise, transit the sky, and set, they will sometimes be in the same direction as viewed from the feeder link earth station antenna (i.e., both in the main beam). More often, the interfer
34、ing satellite will be radiating its signal into a side lobe of the feeder link earth station antenna. The CNDi, the interference contribution to the overall CND, is computed for both cases. For the case of cross-polarized interfering and desired networks, allowances of 13 dB for polarization isolati
35、on in the victim feeder link earth station antenna main-beam direction and 8 dB in other directions are assumed. From Table 1, we see that there is a gain difference of 15 dB between the main and side lobes of the victim antenna. COPYRIGHT International Telecommunications Union/ITU Radiocommunicatio
36、nsLicensed by Information Handling Services STD-ITU-R RECMN N.1115-ENGL 1997 4855212 0533214 T70 Rec. ITU-R M.1315 141 In addition, it is necessary to perform the entire calculation for at least two carrier frequencies of the narrow-band network, since the CND contribution of a narrow-band signal in
37、to a direct-sequence spread-spectrum network is related to the relative offset of the narrow-band camer from the spread-spectrum centre frequency. This offset relationship is given by the inverse of the “shape factor” of the spread-spectrum signal and is shown graphically for the example spread-spec
38、trum signal as Fig. 1. FIGURE 1 Inverse shape factor for minimum-shift keyed spread-spectrum modulation 120 1 O0 h m v 3 80 e 60 40 137 137.2 137.4 137.6 137.8 138 Frequency (MHz) 13 15-0 I The carrier-to-noise density contribution of a narrow-band interfering signal to a direct-sequence spread-spec
39、trum channel is proportional to the ratio of the total powers of the desired signal and the interfering signal, and inversely proportional to the “shape factor” of the spread-spectrum signal. Thus, for a minimum-shift keyed spreading code of chip rate Rc, the CNDi of an interfering signal having fre
40、quency offset (f-fo) is given by equation (4): 1 CNDi = 10 log where: CNDi : P, : Pj : Scf- fo) : camer-to-interference density at the antenna aperture (dB(Hz) power of desired signal at the antenna aperture power of the interfering signal, including polarization effects, at the antenna aperture Sha
41、pe factor for spread-spectrum signal of chip rate Rc: (4) where: f: frequency (Hz) fo : center frequency (Hz) Rc : spread-spectrum chip rate (Hz). COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services STD*ITU-R RECMN M.13LS-ENGL 1777 D 9855
42、212 0533215 707 9 142 Rec. ITU-R M.1315 Offset (kw O 50 1 O0 150 200 250 300 350 400 450 The following example table for shape factors is computed from the equation (5) and is shown in Fig. 1. The centre frequency is 137.5 MHz and the chip rate is 614.4 kHz. Inverse Shape Factor (dB) 55.79 56.00 56.
43、66 57.78 59.43 61.71 64.80 69.1 O 75.72 92.54 TABLE 2 For this example, the following CND values for a single interfering downlink signal are computed for frequency offsets of zero and 250 kHz (about 114 the spreading bandwidth). The interference CND in the spread-spectrum receiving antenna main bea
44、m is: For (f- fo) = O: CNDmaindB = SSpowerdB - NBpowerdB - 10 log s(0) = (e.i.r.p.s - L(5“) - (e.r.p.r - 45“) - X) - 10 log S(0) = (-14 - 145.3) - (7 - 143.9 - 13) + 55.8 = 46.4 dB(Hz) For (f- fo) = B W/4: - CNDmaindB - sspowerdB - NBpowerdB - 10 log S(BWl4) = (e.i.r.p.s - L(5“) - (e.i.r.p.1 - L(5“)
45、 - A) - 10 log S(250000) (7) = (-14 - 145.3) - (7 - 143.9 - 13) + 61.7 = 52.3 dB(Hz) Interference CND caused by one interfering signal impinging on a spread-spectrum receiving antenna side lobe is (assuming the interference source is producing its maximum-pfd): For (f- fo) = O: cNRSidedB For v- fo)
46、= BWI4: cNDSidedB = SSpowerdB - NBpowerdB - 10 log s(0) = (e.i.r.p.S - L(5“) - (e.z.r.p.1 - L(42“) - x - Gdirr - 10 log s() (8) = (-14 - 145.3) - (7 - 136 - 8 - 15) + 55.8 = 48.5 dB(Hz) COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services
47、STD-ITU-R RECMN M-3315-ENGL 3777 4855232 053323b 843 W Rec. ITU-R M.1315 143 Step D: Computation of overall CND (C/(N, + Io) for each interfering signal and of degradation to CND The overall impact of the frequency division multiple access (FDMA) carrier for the four cases considered above is: - Int
48、erference in main beam, zero frequency offset: CND = -10 log (O-O.I(44.0) + 1O-O-l(46.4) = 42.0dB(Hz) (44.0 - 42.0 = 2.0 dB degradation) - interference in side lobe, zero frequency offset: CND = -10 log (10-O.1(44.0) + 1O-O.l(48-5) = 42.7 dB(Hz) (3 dB degradation) - Interference in main beam, 250 kH
49、z frequency offset: CND = -10 log (10-O-1(44.0) + lo-Oel(52.3) = 43.4 dB(Hz) (0.6 dB degradation) - Interference in side lobe, 250 kHz frequency offset: CND = -10 log (1O-O.1(44.0) + 1O-O.1(54.4) = 43.7 dB(Hz) (0.3 dB degradation) In this example, for which the spread-spectrum network is assumed to have a 5 dB operating margin, the spread- spectrum network will still operate even when the interfering signal has no offset and is in the main beam of the spread-spectrum receiving antenna. Step E: Computation
