December Spectrum Management and Telecommunications Policy

Transcription

1 December 2003 Spectrum Management and Telecommunications Policy A Staff Study on the Potential Impact of Satellite Digital Audio Radio Services Terrestrial Repeaters on Wireless Communications Service Receivers Operating in the Adjacent Band at 2.3 GHz Aussi disponible en français

2 1. Disclaimer This staff analysis is intended to highlight issues that should be considered when examining the potential for interference from Satellite Digital Audio Radio Services (S-DARS) terrestrial repeaters on Wireless Communications Service (WCS) systems operating in the adjacent band. The results of this analysis depend on a set of parameters and assumptions and do not purport to be findings or conclusions of the Department as to the potential levels of interference that might be experienced in actual deployment. The actual level of interference that may be experienced depends on a number of factors, including equipment characteristics, radio propagation conditions, the relative strength of the wanted signal, the power level of the S-DARS terrestrial repeater stations, and the amount of filtering employed by the WCS receiver. Considering that some parameters of WCS equipment are not available, this analysis generally assumes a worst-case situation. Readers are encouraged to make their own assessment as more accurate parameters become available. 2. Executive Summary and Introduction In the Policy and Licensing Procedures for the Auction of Spectrum Licences in the 2300 MHz and 3500 MHz Bands (the Policy), the Department established, among other things, a set of emission and power parameters for the operation of WCS systems in the bands and MHz [1]. In this policy, the Department also indicated that it would consider making spectrum available for satellite radio applicants wishing to pursue a broadcasting licence from the CRTC using U.S. S-DARS satellites in the band MHz. Such an application is currently before the CRTC. The U.S. S-DARS spectrum is divided between two licensees, each of whose spectrum is further sub-divided into satellite-based broadcast and terrestrial based re-broadcast channels (see Figure 1). It is likely that a similar band structure would be used in Canada. Figure 1: U.S. WCS and S-DARS Spectrum (not to scale) WCS S-DARS S-DARS WCS Sat. Ter. Sat. Sat. Ter. Sat As indicated in the Policy, it is anticipated that Canadian S-DARS offerings would require the use of Canadian terrestrial repeaters to provide service in urban areas where the visibility of S-DARS satellites may be poor. These S-DARS terrestrial repeaters could operate with power levels up to 12.5 kw e.i.r.p. In the U.S., both WCS and S-DARS have been licensed for some time, however the potential impact of S-DARS terrestrial repeaters on WCS receivers and the appropriate mitigation measures to be taken by affected parties remains unresolved. Pending the development of adjacent band sharing conditions between these two services, the U.S. Federal Communications Commission has issued Special Temporary Authorities to allow for the operation of S-DARS terrestrial repeaters with power levels up to 40 kw e.i.r.p.[3] subject to certain conditions. 1

3 The analysis in this paper looks at the potential for adjacent band interference from S-DARS terrestrial repeaters into WCS receivers in terms of a separation distance around the terrestrial repeater stations, beyond which interference to WCS receivers is unlikely to occur. Within these separation distances the level of interference that may actually be experienced will depend on a number of factors including actual propagation conditions and the relative strength of the wanted WCS signal. One significant finding of this study is that the major mechanism of interference to WCS receivers is likely to be receiver desensitization at the WCS receiver input stage, which occurs when a strong adjacent-band signal impairs a receiver s ability to detect its desired signal. Separation distances required to mitigate this form of interference depend strongly on the power level of the interfering S-DARS terrestrial repeater and the level of preselect filtering at the WCS receiver input stage. These separation distances can be as little as a few hundred metres (for relatively low-power S-DARS terrestrial repeaters and WCS receivers with significant preselect filtering at the receiver input stage) or as much as several kilometres (for higher-power terrestrial repeaters and/or WCS receivers with little preselect filtering). The rest of this report expands on these results and how they were determined. 3. System Specifications The technical characteristics used in this study for WCS and S-DARS terrestrial systems are shown in Tables 1 and 2 below. These parameters are based on U.S. systems since neither WCS nor S-DARS repeaters have been deployed in Canada. Sources include the relevant FCC technical rules, WCS vendor data sheets, and input to the FCC proceedings on sharing between WCS and S-DARS (also see Section 6). In some cases where data was not available, assumptions were made regarding appropriate values for parameters. 3.1 S-DARS Systems Table 1: S-DARS Terrestrial Repeater Characteristics S-DARS 1 S-DARS 2 Channel Width 2.53 MHz 4.2 MHz EIRP DARS Up to 12.5 kw Up to 12.5 kw Out-of-Band Attenuation (A OOB(DARS) ) 75+10log(EIRP) in 1 MHz [3] 75+10log(EIRP) in 1 MHz [3] 2

4 3.2 WCS Systems Table 2: WCS Receiver Characteristics Hub Stations Remote Stations Fixed Mobile Channel Width 2 MHz assumed 2 MHz assumed 2 MHz assumed Antenna Gain (G R(WCS) ) 15 dbi assumed 17 dbi assumed 6 dbi assumed Noise Figure 4 db assumed 4 db assumed 4 db assumed Noise Floor (N) -140 db(w/mhz) -140 db(w/mhz) -140 db(w/mhz) Interference Objective 1 (I WCS = N 6) -146 db(w/mhz) -146 db(w/mhz) -146 db(w/mhz) Receiver Desensitization Threshold (T d ) -25 to -34 dbm [4] -25 to -34 dbm [4] -25 to -34 dbm [4] Receiver Feeder Losses 2 db assumed 2 db assumed 2 db assumed (L r(wcs) ) Preselect Filtering at the Receiver Input 2 (F p ) 0 to 10 db 0 to 10 db 0 to 10 db 4. Analysis The following analyses are all based on the free-space propagation model. The potential for adjacent band interference is expressed as a separation distance around S-DARS terrestrial repeater stations, beyond which interference to WCS receivers is unlikely to occur. Within these separation distances the level of interference that may actually be experienced will depend on a number of factors including actual propagation conditions and the relative strength of the wanted WCS signal. 4.1 Adjacent Band Interference Adjacent band interference from S-DARS terrestrial repeaters into WCS receivers is one of two interference scenarios considered in this report. This form of interference would typically manifest as degradation in the bit error rate of the WCS system. The level of adjacent band interference depends on the output power of the S-DARS terrestrial repeater as well as the amount of adjacent band filtering in both systems. It is assumed that the S-DARS terrestrial repeater station and the WCS receiver are both using the closest channel to their respective band edges. Additional frequency separation, for example moving the WCS channel away from the S-DARS band edge, would improve the ability of the WCS receiver to withstand interference from S-DARS terrestrial repeaters and would likely result in a significant reduction in the calculated separation distances. 1 2 I/N = -6 db for a 1 db degradation in the fade margin. This range of attenuations is illustrative only; the exact amount of preselect filtering will depend on the particular WCS system. 3

5 Minimum propagation losses to preclude this form of interference are calculated according to the following formula: L = EIRP DARS A OOB(DARS) L r(wcs) + G R(WCS) - I WCS Where: EIRP DARS A OOB(DARS) = -75 db(w/mhz) for any value of EIRP DARS Using the propagation losses determined above and the free-space propagation model, it is possible to calculate separation distances sufficient to avoid the potential for this form of interference between these two services. These values are given in Table 3. Table 3: Separation Distances to Avoid Interference from S-DARS Terrestrial Repeaters into WCS Receivers Victim Required Path Loss (L) Separation Distance Hub 84 db 160 m Fixed Sub 86 db 210 m (main beam), negligible in other directions Mobile Sub 75 db 60 m Note that these distances do not vary with the power level of the S-DARS terrestrial repeater since the required out-of-band attenuation for the terrestrial repeater increases proportionately with its power. 4.2 WCS Receiver Desensitization The second interference scenario considered is the potential for S-DARS terrestrial repeater emissions to cause desensitization of a WCS receiver. Desensitization occurs when a strong interference signal at the first stage of a receiver, even though this signal may be filtered out by the receiver at later stages, impairs the receiver s ability to amplify its desired signal by driving the first stage of the receiver into saturation. This interference mechanism is also known as front-end or brute force overload. It is assumed that the S-DARS terrestrial repeater station and the WCS receiver are both using the closest channel to their respective band edges. Minimum propagation losses to preclude this form of interference are calculated according to the following formula: L = EIRP DARS + G R(WCS) L r(wcs) T d F p Using the propagation losses determined above and the free-space propagation model, it is possible to calculate separation distances sufficient to avoid the potential for this form of interference between these two services. Since exact values are not known for all of the parameters used in this equation, the following figures show separation distances for a variety of parameter values. 4

6 Figures 2 and 3 show the separations for WCS hub stations and both fixed and mobile WCS remote stations calculated using two different values for the sensitivity of WCS receivers to this form of interference (T d = -34 dbm and -25 dbm) and with no receiver preselect filtering (F p = 0 db). Figure 2: Separation Distance for Td = -34 dbm and Fp = 0 db Minimum Separation (km) Hub Fixed Remote Mobile Remote S-DARS Terrestrial Repeater EIRP (kw) Figure 3: Separation Distance for Td = -25 dbm and Fp = 0 db Minimum Separation (km) Hub Fixed Remote Mobile Remote S-DARS Terrestrial Repeater EIRP (kw) 5

7 Figures 4 and 5 show the separations for WCS hub stations and both fixed and mobile WCS remote stations with T d = -34 dbm and -25 dbm and with F p = 10 db. Figure 4: Separation Distance for Td = -34 dbm and Fp = 10 db Minimum Separation (km) Hub Fixed Remote Mobile Remote S-DARS Terrestrial Repeater EIRP (kw) Figure 5: Separation Distance for Td = -25 dbm and Fp = 10 db Minimum Separation (km) Hub Fixed Remote Mobile Remote S-DARS Terrestrial Repeater EIRP (kw) 6

8 As shown in these figures, the ability of WCS receivers to coexist with S-DARS terrestrial transmitters is strongly dependent on the WCS receiver desensitization threshold, amount of preselect filtering present in the WCS receiver, and the power level of the S-DARS terrestrial repeater. 5. Summary of Results This study considered two possible mechanisms by which S-DARS terrestrial repeaters could cause interference to WCS receivers operating in the adjacent band. The potential for these types of interference are expressed as separation distances around S-DARS terrestrial repeater stations, beyond which interference to WCS receivers is unlikely to occur. Within these separation distances the level of interference that may actually be experienced will depend on a number of factors including actual propagation conditions and the relative strength of the wanted WCS signal. The first potential interference mechanism is adjacent band interference (See Table 3). This form of interference can be mitigated by separations of 60 to 210 metres between the two systems, depending on the type of WCS receiver being considered (hub station or fixed or mobile subscriber station). These distances are independent of the power level of the S-DARS terrestrial repeater, since the greater the power of the repeater station, the more adjacent band filtering it is required to have. The second mechanism that is considered in this report is potential interference due to WCS receiver desensitization (See Figures 2 through 5). Receiver desensitization occurs when a strong adjacent-band signal impairs a receiver s ability to detect its desired signal. This form of interference can potentially occur at much larger separations from S-DARS terrestrial repeaters. Separation distances required to mitigate this form of interference depend strongly on the power level of the interfering S-DARS terrestrial repeater and the level of preselect filtering at the WCS receiver input stage. As illustrated in Figures 2 through 5, a separation of 130 metres to 1.3 kilometres may be required to avoid desensitization interference to a WCS receiver with 10 db of preselect filtering from an S-DARS terrestrial repeater with a power level of 2 kw e.i.r.p. For an S-DARS terrestrial repeater with a power level of 12.5 kw e.i.r.p and a WCS receiver without any preselect filtering, as much as 8 to 10 kilometres of separation may be required. Within these calculated separation distances the actual level of interference that may be experienced depends on a number of factors, including equipment characteristics, radio propagation conditions, the relative strength of the wanted signal, the power level of the S-DARS terrestrial repeater stations, and the amount of filtering employed by the WCS receiver. Given the large number of assumptions and variables upon which the actual level of interference will depend, readers are encouraged to make their own analysis taking into account the particulars of their own situation. 7

9 6. References [1] Section 5.3.2, Policy and Licensing Procedures for the Auction of Spectrum Licences in the 2300 MHz and 3500 MHz Bands, Industry Canada, September [2] Amendments and Supplements and Clarification Questions to the Policy and Licensing Procedures for the Auction of Spectrum Licences in the 2300 MHz and 3500 MHz Bands, Industry Canada, November [3] FCC, Special Temporary Authorities to Operate Satellite Digital Audio Radio Service Complementary Terrestrial Repeaters, File Nos. SAT-STA and SAT-STA , September 17, [4] Comments submitted in response to FCC Proceeding No [5] Part 27 of the FCC Rules (CFR Title 47, Part 27). 8