CO-DIRECTIONAL Ka-BAND FREQUENCY SHARING BETWEEN NON-GSO SATELLITE NETWORKS AND GSO SATELLITE NETWORKS

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1 CO-DIRECTIONAL Ka-BAND FREQUENCY SHARING BETWEEN NON-GSO SATELLITE NETWORKS AND GSO SATELLITE NETWORKS F. Ghazvinian - Teledesic Corporation M. A. Sturza - Teledesic Corporation 1. INTRODUCTION This paper presents extensive analysis and simulation results related to the feasibility of codirectional frequency sharing between geo-stationary orbit (GSO) satellite networks operating in the fixed-satellite service (FSS), and non-gso (NGSO) satellite networks operating service and feeder links in the FSS and in the mobile-satellite service (MSS). The feasibility of co-directional frequency sharing between networks and NGSO satellite networks is necessary in determining the optimum choice of frequency bands for these systems. The SPACEWAY system is used as an example of a network operating in the Kaband. The TELEDESIC system is used as a representative example of a NGSO satellite network operating in the Ka-band. It has both FSS and MSS service links, and FSS and MSS feeder links. Extensive computer simulations are used to calculate the statistics of the interference between the links of the two networks. The two representative networks are proposing to operate at different Ka-band frequencies in some parts of the world and thus, the analysis presented in this paper only examines the possibility of co-frequency sharing. 2. TECHNICAL CHARACTERISTICS OF THE SATELLITE SYSTEMS Table 2.1 shows the orbital parameters of the NGSO Satellite network. The technical characteristics of the communications systems of both the GSO and NGSO networks are shown in Table 2.2. The antenna patterns are shown in Figures 2.2 through The NGSO network uses a constellation of 84 operational interlinked low-earth orbit (LEO) satellites. The 's surface is mapped into a fixed grid of approximately 2, supercells. Each supercell is a square with sides of 16 km in length. Each supercell is further divided into nine square cells 53.3 km on a side. The network uses a combination of space, time and frequency division multiple access techniques. At any instant in time, each fixed supercell is served by 1 of the 64 transmit and 1 of the 64 receive beams on one of the satellites. The scanning beam scans the 9 cells within the supercell with a msec scan cycle, resulting in time division multiple access (TDMA) among the cells in a supercell. Physical separation and a checkerboard pattern of left and right circular polarization eliminate interference among cells scanned at the same time in adjacent supercells. Within each cell's time slot, terminals use frequency division multiple access (FDMA) on the uplink and asynchronous time division multiple access (ATDMA) on the downlink. The network consists of seventeen GSO satellites to cover the entire world, however in this paper, only the two GSO satellites serving North America are considered. These satellites are located at longitudes of 11 W and 99 W and can each provide a total of fortyeight (24 in each polarization), 12 MHz spot beams for both the uplinks and the downlinks. 1

2 Table 2.1 Orbital Parameters of the NGSO Satellite System No. of Planes 21 No. of Satellites Per Plane spares Altitude 7 km Inclination 98.2 Table 2.2 Communications Parameters NGSO Satellite Network Links Standard Mobile High Rate Uplink Polarization LHC/RHC LHC/RHC LHC/RHC LHC/RHC Downlink Polarization LHC/RHC LHC/RHC LHC/RHC LHC/RHC Satellite Transmit Power (dbw) to Station Transmit Power (dbw) -2.3 to to to to -3. Satellite Receive Bandwidth (MHz) Station Receive Bandwidth (MHz) Required C/I Protection Ratio (db) The NGSO Satellite Service network uses a wide variety of earth stations to support different kinds of services and users. The Standard Stations and High Rate Stations support FSS service and feeder links. The Mobile Stations support MSS applications, with the Standard and High Rate Stations providing the required MSS feeder links. The three types of earth stations are described below. Figure 2.1 depicts the data flow between them. Standard Mobile High Rate Station Station Station Transmit Receive Standard Mobile High Rate Station Station Station Figure 2.1 NGSO Satellite Service Network Data Flow a. NGSO Standard Stations 2

3 The NGSO Standard Stations are designed to accommodate data rates from 16 Kbps (Basic Channel) up to 2.48 Mbps (E1). Hundreds of these earth stations can be located within a single cell, but at any instant of time, a maximum of 144 Basic Channel earth stations are allowed to operate. Equivalently, 15 T1-rate (1.544 Mbps) earth stations, or a combination of different data rate earth stations, may operate simultaneously within a single cell. Each cell within a supercell is scanned by the satellite in a time division multiplexed manner, resulting in a bursty transmission of approximately 1% duty cycle. For uplinks within a cell, signals from different terminals are frequency multiplexed within the 5 MHz bandwidth available. The frequency band assigned to a certain terminal depends on the data rate the terminal requested. The downlinks operate in ATDMA manner. The satellite transmits a series of packets addressed to terminals within a cell. Each terminal selects only those packets addressed to it. Figures 2.8 and 2.9 show the antenna patterns of the Standard Station and Figure 2.6 shows the contour of the satellite antenna footprint for the Satellite Standard Link. b. NGSO Mobile Stations The NGSO Satellite Service network also supports Mobile Stations. The Mobile Stations operate at multiple rates of the 16 Kbps basic rate up to 2.48 Mbps (E1). The operation of the Mobile Stations are similar to the Standard Stations with the following exceptions: - Mobile Stations can use a smaller antenna. - The bandwidth for MSS is 1 MHz which can accommodate up to 36 basic channels in each cell. The antenna patterns for the Mobile Station Terminal and the Satellite Mobile Links are the same as the Satellite Standard Links as shown in Figures 2.8, 2.9 and 2.6. c. NGSO High Rate Station The NGSO Satellite Service network also supports a smaller number of fixed-site High Rate Stations that operate at the OC-3 rate ( Mbps) and multiples of this rate, up to OC- 24 ( Gbps). Each satellite can support up to sixteen High Rate Stations within its service area. Figures 2.1 and 2.11 show the antenna patterns of the High Rate Station and Figure 2.7 shows the contour of the satellite antenna footprint for the Satellite High Rate Link. 3. INTERFERENCE ANALYSIS In order to calculate the complex, time-varying interference statistics between the and the NGSO Satellite Service links, a sophisticated computer simulation program has been developed. The simulation program is based on software modules that have been previously developed and tested. The results of the simulation runs were validated by simple analysis and by comparison to another, independently developed, simplified simulation program. The simulation program includes the satellite orbital ephemeris and visibility characteristics as viewed from any given location on the earth. It allows for a wide range of choices in specifying the NGSO orbital parameters, link parameters, and simulation duration and step size. The output of the simulation program at each step is the Carrier-to-Interference power ratio at the interfered with receiver as given by: 3

4 C C C C C / I = P T + GT () - PL C + G R () -1 log 1 P T I I +G T (θ I/C )-PL I/C +G R (θ C/I )-BWF I/C where PT C is the desired signal transmit power ( dbw ) for all I GT C () is desired signal transmit antenna peak gain ( db ) PL C is the path loss from the desired transmitter to the receiver ( db ) GR C () is the receiver antenna peak gain ( db ) PT I is the interference signal transmit power ( dbw ) GT C (θ I/C) is the interference signal transmit antenna gain in the direction of the receiver ( db ) PL I/C is the path loss from the interfering transmitter to the receiver ( db ) GR C (θc/i) is the receiver antenna gain in the direction of the interfering transmitter ( db ) BWF I/C is a bandwidth factor equal to db if the interference signal transmit bandwidth is less than or equal to the desired receive bandwidth and it is equal to 1 Log1 ( BWtransmit / BWreceive ) if the interference signal transmit bandwidth is greater than the desired signal receive bandwidth. Studies were performed with each system s space segment and ground segment independently identified as the source of interference. For each of these four cases, the interference statistics between the and the three service links of the NGSO Satellite Service network were calculated. Figure 3.1 depicts the ground segment model used in the simulations involving the NGSO Standard and Mobile links. A 5-by-5 grid of 25 Stations, spaced at 64 km, is centered at latitude 4ºN and longitude 1ºW. Centered over this grid is a 21-by-21 grid of 441 NGSO Standard (or Mobile) Stations with 16 km spacing. Thus, each Station is collocated with a NGSO Station to provide worst case interference conditions. Figure 3.2 depicts the ground segment model used in the simulations involving the NGSO High Rate links. This has been determined to require a distribution of only nine NGSO High Rate Stations, one of them is collocated with a Station. In what follows, each of the interference cases are described and the method used in calculating the interference statistics is outlined. Case 1 : NGSO Satellites interfering with a Station Case 1 is shown in Figure 3.3. At each simulation step, the carrier-to-interference (C/I) power ratio at a Station that is collocated with a NGSO Satellite Standard or Mobile Station is calculated. The collocated earth stations are assumed to be at latitude 4ºN and longitude 1ºW. Additional simulations were also performed at latitude 25ºN to evaluate the sensitivity of the interference statistics to the location of the collocated earth stations. The system does not employ power control for downlink. The interference power at the Station is calculated by summing the interference power received from all NGSO satellites communicating with their corresponding earth stations. For each of the 441 NGSO Stations, the closest corresponding NGSO satellite at an elevation angle of at least 4º, is determined. The interference power from that satellite into the Station is calculated. The GSO Station s antenna discrimination from the NGSO satellite transmit beam is computed as a function of its distance from the NGSO Station communicating with that satellite. 4

5 Case 2: NGSO Stations interfering with a Satellite Case 2 is shown in Figure 3.4. The simulation procedure for this case is the same as that of Case 1 with the exception that this case deals with the interference into a Satellite. The sum of the interference power from the 441 NGSO Stations is calculated in this case. The system employs power control for its uplink to compensate for slant range and to achieve a desired carrier-to-noise ratio at the satellite of 1.6 db. Case 3: Stations Interfering with a NGSO Satellite Case 3 is shown in Figure 3.5. At each simulation step, the carrier-to-interference power ratio at a NGSO Satellite is calculated. This is based on the aggregate interference power received from all 25 Stations. It is assumed that the NGSO Station communicating with this NGSO Satellite is collocated with a Station. The NGSO Satellite System employs power control for uplink to maintain a carrier-to-noise power ratio of 8.6 db for the Standard and Mobile Links and 17.3 db for the High Rate links. Case 4: Satellites Interfering with a NGSO Station Case 4 is shown in Figure 3.6. At each simulation step the carrier-to-interference ratio at the NGSO Satellite Station is calculated. The aggregate interference power received from the GSO Satellites communicating with all 25 Stations is considered. The NGSO Satellite System does not employ power control on the downlinks. 4. SIMULATION RESULTS The simulation program described in the previous section has been used to calculate the interference statistics between the System, and the FSS and MSS service and feeder links of a NGSO Satellite System. Simulation runs were performed for each of the four cases described above, with the Station grid centered at longitude 1ºW and latitudes of both 4ºN and 25ºN. The output of the simulation program is the time history of the C/I power ratio at each simulation step. In order to gain additional insight and understanding of the simulation results, a post-processing program has been developed that calculates and plots the following statistics: a) C/I time history b) C/I probability density function c) C/I probability distribution function d) Interference event duration statistics e) Interval between interference event statistics The interference between the and three different NGSO Satellite System links are considered. a) NGSO Standard Links - Probability of interference for Case 1 (NGSO Satellites interfering into Station) increases from 5.4% for the case with collocated earth stations at latitude 4 N, to 11.9% for collocated earth stations at latitude 25 N. The interference for Case 2 (NGSO Stations interfering into Satellite) is not very severe. The interference probability for Case 3 ( Stations interfering into NGSO Satellite) is very severe and its value changes between 15.5% to 24.9% depending on the latitude of the earth stations. b) NGSO Mobile Links - For Case 1, the interference for Mobile Links is more severe than for the Standard Links. This is due to the higher power flux density transmitted by the NGSO Satellites. The interference statistics of Mobile Links for Cases 2 and 3 are the same as for the Standard Links. Due to the higher power flux density of the NGSO satellites for Mobile Links, the interference statistics of Case 4 for Mobile Links are less severe than for the Standard Links. 5

6 c) NGSO High Rate Links - The interference between the NGSO High Rate Links and the GSO FSS links are not very severe, and can further be reduced by separating the earth stations of the two systems. Table 4.1 summarizes the results of the simulation runs. 5. CONCLUSION This paper presents statistical results that are useful in evaluating the potential for frequency sharing. However, the exact criteria for frequency sharing between the System, and the FSS and MSS, Service and Feeder links of a NGSO Satellite System have not been defined. The simulation results indicate a positive potential for sharing between the and the NGSO Satellite System High Rate Links (FSS Service Links and FSS and MSS Feeder Links) with site separation between the respective Stations. Additional work is required to determine the minimum separation requirement based on the acceptable interference levels. The interference levels between the System and the NGSO Satellite System Standard and Mobile Links are significantly higher. The interference levels change as a function of earth station latitudes. Additional work is required to determine the effect of mitigation techniques in improving the sharing potential. 6. ACKNOWLEDGMENTS The authors wish to thank Ali Zahid and Mike Yang of LinCom Corporation, and Janet King, Tom Hayden and the other members of the Teledesic team for their contributions to this paper. 6

7 Peak Gain = 46.5 dbi 4 3 Peak Gain = 46.5 dbi to 18 o to 18 o Angle Off Boresight (degrees) Angle Off Boresight (degrees) Figure 2.2 Satellite Transmit Antenna Pattern Figure 2.3 Satellite Receive Antenna Pattern Peak Gain = 44.4 dbi Peak Gain = 43.1 dbi 1 1 to 18 o to 18 o Angle Off Boresight (degrees) Angle Off Boresight (degrees) Figure 2.4 Station Transmit Antenna Pattern Figure 2.5 Station Receive Antenna Pattern 7

8 Kilometers d B i -2 db -4 db -6 db -8 db -1 db -15 db -2 db -3 db -35 db Kilometers dbi -2 db -4 db -6 db -8 db -1 db -15 db -2 db -35 db -5 db Kilometers Kilometers Figure 2.6 NGSO System Satellite Standard Link and Mobile Link Transmit and Receive Gain Contours Figure 2.7 NGSO System Satellite High Rate Link Transmit and Receive Gain Pattern 8

9 Peak Gain = 36 dbi 25 Peak Gain = 33 dbi 2 2 Gain (dbi) 15 Gain (dbi) to 18 o Angle off boresight (Degree) 5 to 18 o Angle off boresight (Degree) Figure 2.8 NGSO Satellite System Standard Station and Mobile Station Transmit Antenna Pattern Figure 2.9 NGSO Satellite System Standard Station and Mobile Station Receive Antenna Pattern Peak Gain = 5 dbi 4 Peak Gain = 47 dbi 3 3 Gain (dbi) 2 1 Gain (dbi) to 18 o -1 to 18 o Angle Off Boresight (Degree) Angle Off Boresight (Degree) Figure 2.1 NGSO Satellite System High Rate Station Transmit Antenna Pattern 9 Figure 2.11 NGSO Satellite System High Rate Station Receive Antenna Pattern

10 Collocated Station 4 35 Station Longitude (degrees) NGSO Standard Station 3 25 NGSO High-rate Station Longitude (degrees) Figure 3.1: Ground Segment Distribution of NGSO Standard Terminals Figure 3.2: Ground Segment Distribution of NGSO High-rate Terminals Satellite C NGSO Satellite I i I j I k NGSO Co-located GSO-FSS Station and NGSO Station NGSO Station Figure 3.3 Case 1: NGSO Satellites Interfering with a Station 1

11 GSO-FSS Satellite I n I i I j I k NGSO Satellite C NGSO Station (n) Co-located GSO-FSS Station and NGSO Station (i) NGSO Station (j) NGSO Station (k) Figure 3.4 Case 2: NGSO Stations Interfering with a Satellite Satellite NGSO Satellite C I i I j I k Station (k) Co-located Station (j) and NGSO Station Station (i) 11

12 Figure 3.5 Case 3: Stations Interfering with a NGSO Satellite Satellite I j NGSO Satellite I i I k C Station (j) Co-located Station (k) and NGSO Station Station (i) Figure 3.6 Case 4: Satellites Interfering with a NGSO Station 12

13 NGSO Station Collocated with the GSO Station at Latitude 4 Mean Maximum Interference Interference Event Event Duration Duration (sec) (sec) Probability of Interference (%) Mean Time Between Events (minutes) Probability of Interference (%) NGSO Station Collocated with the GSO Station at Latitude 25 Mean Maximum Interference Interference Event Event Duration Duration (sec) (sec) Mean Time Between Events (minutes) Standard Case Case 2 <.1 [B] [B] [B] Station Case Collocated Case 4 <.1 [B] [B] [B] Mobile Case Case 2 <.1 [B] [B] [B] Station Case Collocated Case 4 <.1 [B] [B] [B] <.1 [B] [B] [B] High rate Case 1 <.1 [B] [B] [B] Case <.1 [B] [B] [B] Station Case 3 <.1 [B] [B] [B] Collocated Case 4 <.1 [B] [B] [B] High rate Case 1... NA... NA Case 2... NA... NA Station Case 3... NA... NA at 16 km Case 4 <.1 [B] [B] [B].6 [B] [B] [B] Table 4.1 Note [B] : The number of interference events that occurred during the simulated period was too small to obtain reliable statistics. 13