(12)Un1ted States Patent
`(10) Patent N0.:
`US 6,212,360 B1
`
`Fleming, 111 et al.
`(45) Date of Patent:
`Apr. 3, 2001
`
`US006212360B1
`
`(54) METHODS AND APPARATUS FOR
`CONTROLLING EARTH-STATION
`TRANSMITTED POWER IN AVSAT
`
`(75)
`
`NETWORK
`Inventors: Robert F. Fleming, III, Derwood, MD
`,
`,
`(Us); Wllham A- Che“? Great Fa115>
`VA(US); JOSePh A- 01151101111,
`Manassas, VA (US); Brlan J.
`Glinsman, Herndon, VA (US); David
`B. Kim, Great Falls, VA (US); Ronald
`L. Kronz, Fairfax, VA (US)
`(73) Assignee: GE Capital Spacenet Services, Inc.,
`McLean, VA (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/050,972
`.
`Frled:
`
`(22)
`
`M313 31: 1998
`
`(60)
`
`_
`_
`Related U'S' Appllcatlon Data
`PTOViSiOHal application NO- 60/042335: filed on APL 9:
`1997'
`
`Int. Cl.7 ..................................................... H04B 7/185
`(51)
`(52) US. Cl.
`..................
`. 455/134, 455/427, 455/2321
`(58) Field of Search ..................................... 342/358, 352,
`455/10, 121’ 13.4, 13.2, 427, 466, 68,
`2321; 370/320; 375/202
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`1/1986 Oshima etal.
`...................... 343/358
`4,567,485 *
`
`9/1987 Ohno et al.
`..... 342/358
`4,697,187 *
`3/1988 Muratani et a1.
`......... 455/9
`4,731,866 *
`
`6/1988 Bustamante et al.
`455/12
`4,752,967 *
`
`1/1990 Adams, Jr. et al.
`.
`455/12
`4,896,369 *
`
`2,3239%:
`: 13/1990 Saam """"""""
`455/10
`
`,
`,
`/1991 Ayukawa et al.
`455/52
`
`5,257,029 * 10/1993 Miyo ...................
`.. 342/352
`
`2/1997 Kartalopoulos .......... 370/406
`5,606,551 *
`..
`..... 375/200
`5,619,525 *
`4/1997 Wiedeman et al.
`
`..
`5,708,966 *
`1/1998 Al—Dhahir et a1.
`455/13.4
`................ 455/13.4
`5,787,336 *
`7/1998 Hirschfield et al.
`* cited by examiner
`
`Primary Examiner—William G. Trost
`Assistant Examiner—Charles Craver
`
`(74) Attorney, Agent, or Firm—Jones Volentine, L’L’C‘
`(57)
`ABSTRACT
`
`A method is shown for providing stable communication
`between a hub earth-station and a VSAT earth-station by
`regulatmg the power of Slgnals transmltted v1a a satelhte. In
`this method,
`the hub receives a beacon signal from the
`satellite and a previous outbound signal. The hub then
`regulates the power of an outbound signal transmitted by the
`hub to VSAT via the satellite, based on either the beacon
`signal or the previous outbound signal. The hub then sends
`a current outbound signal, including signal data and param-
`eter data to the VSAT via the satellite. The VSAT receives
`the current outbound signal and determines a number of
`signal properties pertaining to the current outbound signal.
`The VSAT then regulates the power of an inbound signal
`transmitted by the VSAT to the hub via the satellite, based
`on the signal properties, the parameter data, and a set of
`reference data.
`
`4,004,224 *
`
`1/1977 Arens et al.
`
`............................. 325/2
`
`10 Claims, 6 Drawing Sheets
`
`
`
`
`
`
`COMPUTER
`
`
`
`
`
`
`
`
`
`
`325
`
`
`
`2337
`
`
`
`
`
`
`320
`350
`€315
`BB‘xCON
`SIGNAL
`POWER
`DOWN CONVERTER
`DOWN CONVERTER
`AMPLIFIER
`
`
`331
`
`
`
`
`
`UP CONVERTER
`335
`S
`HUB
`SIGNAL
`RECENER
`i {340
`€333
`BEACON
`DEMODULATOR
`RECEIVER
`
`
`
`
`
`
`
`
`
`
`
`
`
`8I'I‘ICI
`
`
`1.545
`
`305
`$370
`
`
`
`
`
`{375
`4,
`MODULATOR
`
`
`
`1
`
`APPLE 1010
`
`APPLE 1010
`
`1
`
`

`

`US. Patent
`
`Apr. 3, 2001
`
`Sheet 1 0f 6
`
`US 6,212,360 B1
`
`30
`
`
`
`FIG.1
`
`2
`
`

`

`US. Patent
`
`Apr. 3, 2001
`
`Sheet 2 0f 6
`
`US 6,212,360 B1
`
`20
`
`20
`
`1 O
`
`20
`
`FIGZA
`
`30
`
`1‘ \‘ %
`
`
`
`FIGZB
`
`3
`
`

`

`US. Patent
`
`Apr. 3, 2001
`
`Sheet 3 0f 6
`
`US 6,212,360 B1
`
`\
`
`1O
`
`20
`
`/
`
`FIG.2C
`
`20
`
`20
`
`1 0
`
`20
`
`FIG.2D
`
`4
`
`

`

`US. Patent
`
`Apr. 3, 2001
`
`Sheet 4 0f 6
`
`US 6,212,360 B1
`
`BEACON
`DOWN CONVERTER
`
`SIGNAL
`DOWN CONVERTER
`
`POWER
`AMPUFIER
`
`330
`
`315
`
`360
`MODULATOR
`
`BEACON
`RECEIVER
`
`HUB
`
`DEMODULATOR l
`
`335
`
`340
`
`up CONVE TER
`R
`
`310
`
`375
`
`305 -
`
`COMPUTER
`
`370
`
`5
`
`

`

`US. Patent
`
`Apr. 3, 2001
`
`Sheet 5 0f 6
`
`US 6,212,360 B1
`
`OUTBOUND LEVEL
` 410
`
`IS THERE
`YES
`A BEACON
`
`
`SIGNAL.) REC’D
`
`SAMPLE & FILTER
`
`BEACON RECEIVER
`N0
`SIGNAL
`
`SAMPLE 8c FILTER
`LOCAL RECEIVER
`
`405
`
`
`
`“5
`
`
`
`
`4 5
`2
`SOUND
`NO
`'5
`
`
`
`?
`
`
`
`
`S FADE
`INDICATED BY
`
`BR<FADE
`
`INDICATED BY
`
`COMPUTE
`LR DIFF
`
`COMPUTE
`A2 = I(LR)
`
`LR?
`
`
`445
`‘
`
`450
`
`47D
`
`
`
`430
`
`COMPUTE
`BRDIFF
`
`44°
`
`COMPUTE
`
`At = f(BR)
`
`TO
`
`SET A
`
`
`CLOSEST VALUE
`
`IN RANGE BROADCAST ERROR E
`
`
`VALUE BEIW/ At
`
`AND ACTUAL SETTING
`
`FIG.4
`
`6
`
`

`

`US. Patent
`
`Apr. 3, 2001
`
`Sheet 6 0f 6
`
`US 6,212,360 B1
`
`BEGIN WITH
`NOMINAL VALUES
`
`COMPARE RECEIVED
`Eb/N 0 WITH RESERVE
`Eb/N oAND ADJUST
`ACCORDINGLY
`
`ADJUST
`
`RECEIVE FEEDBACK
`INFORMATION AND
`
`RECENE HUB
`CONCESIIDN LEVEL
`INDICATION
`
`COMPARE INTERNAL
`VSAT CONGESTION
`
`LEVEL
`
`505
`
`510
`
`515
`
`520
`
`525
`
`~550
`
`~535
`
`COMPARE HUB
`CONGESITON LEVEL
`WITH INTERNAL VSAT
`CONGESTION LEVEL
`AND ADJUST
`
`, AND ADJUST
`
`RECEIVE CHANGE
`IN OUTBOUND
`POWER LEVEL
`
`RECEIVE INDICATION
`OF WHETHER HUB
`POWER IS OUT OF
`
`RANGE AND ADJUST
`
`
`
`RECEIVE INFORMATION
`AS TO WHETHER THERE
`
`
`IS A MODE CHANGE
`
`
`AND ADJUST
`
`
`~545
`
`RECEIVE INFORMATION
`AS TO WHETHER THERE
`IS ANY CHANGE IN TRAFFIC
`
`M550
`
`TYPE AND ADJUST
`555 IS A CHANGE
`
`AND SPREADING FACTOR
`
`YES
`
`
`REQUIRED?
`
`
`
`
`
`IN DATA RATE
`
`560
`
`ADJUST DATA RATE
`
`AND SPREADING
`
`FACTOR
`
`NO
`
`570
`
`
`
`
`IS DESIRED POWER
`SET DESIRED
`GREATER THAN
`POWER TO
`
`MINIMUM ALLOWED
`MAXIMUM ALLOWED
`
`
`POWER?
`POWER
`
`
`
`
`
`DESIRED POWER
`
`TRANSMIT INBOUND
`SIGNAL TO THE
`SATELLITE AT
`
`575
`
`FIG.5
`
`7
`
`

`

`US 6,212,360 B1
`
`1
`METHODS AND APPARATUS FOR
`CONTROLLING EARTH-STATION
`TRANSMITTED POWER IN A VSAT
`NETWORK
`
`This application claims the benefit of US. Provisional
`Application No. 60/042,835 filed Apr. 9, 1997.
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention pertains to the field of satellite
`communications. More particularly,
`the present invention
`relates to satellite-communication networks comprising a
`master earth-station and a number of remote earth-stations.
`In particular, this invention pertains to the control of the
`signal power level transmitted by the earth-stations in such
`networks.
`
`2. Description of the Related Art
`FIG. 1 shows a conventional very small aperture terminal
`(VSAT) satellite-communication network. The VSAT net-
`work comprises a master earth-station, referred to herein as
`a “hub” 10, a number of remote earth-stations, referred to
`herein as “VSATs” 20, and a geostationary communication
`satellite transponder, referred to herein as a “satellite” 30.
`The hub 10 communicates with the VSATs and the VSATs
`20 communicate with the hub 10 by sending transmission
`signals 40 through the satellite 30.
`FIGS. 2A through 2D illustrates the operation of the
`conventional VSAT satellite communication network of
`FIG. 1. As these figures show, the communication between
`the hub 10 and the VSATs 20 is accomplished through the
`use of an outbound transmission signal from the hub 10 to
`the VSATs 20, and an inbound transmission signal from the
`VSATs 20 to the hub 10.
`FIGS. 2A and 2B illustrate an outbound transmission
`signal from the hub 10 to a VSAT 20. As shown in FIG. 2A,
`the outbound transmission signal first includes an outbound
`uplink portion 210, passing from the hub 10 to the satellite
`30. As shown in FIG. 2B, the outbound transmission signal
`also includes an outbound downlink portion 220, passing
`from the satellite 30 to the hub 10 and all VSATs 20.
`FIGS. 2C and 2D illustrate an inbound transmission
`signal from a VSAT 20 to the hub 10. As shown in FIG. 2C,
`the inbound transmission signal first includes an inbound
`uplink portion 230, passing from a VSAT 20 to the satellite
`30. As shown in FIG. 2D, the inbound transmission signal
`also includes an inbound downlink portion 240, passing
`from the satellite 30 to the hub 10.
`
`The outbound transmission signal 210 and 220 is a
`continuous signal sent from the hub 10. In contrast,
`the
`inbound transmission signal 230 and 240 is sent in bursts as
`needed by the various VSATs 20.
`Satellite transponder resources are sold and leased in units
`of power and bandwidth. A VSAT network operator must
`carefully control both resources in order to achieve eco-
`nomical operation.
`Code-division multiple-access (CDMA) is a multiple-
`access technique that forms the basis for the 18-95 digital
`cellular telephony standard, and has some important advan-
`tages for use in VSAT networks, particularly when it is
`important to be able to use small antennas at the remote
`terminals. The first widely-deployed VSAT networks using
`CDMA used the C200 product developed by Equatorial
`Communications Company (ECC) of Mountain View, Calif.
`It is widely recognized that accurate power control is
`required to equalize the received power levels of signals
`multiplexed on a channel using the CDMA technique to
`maximize the operational efficiency of the network.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`Qualcomm, Inc. has developed a number of power control
`techniques for use in the CDMA cellular telephony networks
`that
`they have developed based on the 18-95 standard.
`Qualcomm’s techniques are designed for terrestrial net-
`works that operate without a satellite relay, and are designed
`to cope with the rapid fading that occurs in a mobile
`terrestrial microwave propagation environment.
`Operational experience with the ECC C200 networks also
`showed the need for accurate power control of the VSAT
`transmitters. The C200 product was limited by the fact that
`the control of the VSATs” inbound (VSAT-to-hub) transmit-
`ted power levels had to be accomplished by manual inter-
`vention of an operator at the hub. Since the C200 system was
`primarily designed for C-Band operation, the rapid fading
`that occurs at higher frequencies due to rain was not a
`serious problem for the system. However, VSAT networks
`based on this product typically required periodic expert
`rebalancing of the inbound power levels across the network
`to compensate for gradual changes in the equipment or
`haphazard adjustments by inexperienced operators.
`For VSAT networks operating at Ku-Band and higher
`frequencies, rain fade is a serious problem. Rain fade results
`from the absorption and scattering of the transmission
`signals 40 between the hub 10 and satellite 30 and between
`the VSATs 20 and the satellite 30 by water droplets or ice
`crystals in the atmosphere. During rain fade, changes in
`attenuation and hence the received signal level can occur
`within a few seconds. At Ku-Band and higher frequencies,
`rapid and automatic uplink power control becomes very
`important.
`Uplink power control has typically been implemented
`only on the outbound (hub-to-VSAT) link, where the addi-
`tional cost of the equipment at the hub is of minor conse-
`quence. A rain fade affecting the outbound uplink (a rain
`fade between the hub 10 and the satellite 30) affects the
`entire network, while a rain fade on the inbound uplink (a
`rain fade between a VSAT 20 and the satellite 30) only
`affects that VSAT 20. Standard practice has been to operate
`the VSATs 20 with enough inbound (VSAT-to-hub) power to
`overcome most rain fades.
`
`For CDMA VSAT operation at Ku-Band and higher
`frequencies, however, uplink power control on the inbound
`signal becomes a necessity. Uplink power control can also
`benefit TDMA and other modes of satellite access operation
`by providing the network operator the ability to control and
`thus reduce his transponder power requirement, by only
`operating the VSAT transmitter at high power levels when
`required to overcome rain fades.
`SUMMARY OF THE INVENTION
`
`It is an object of this invention to provide methods and
`apparatus for precisely and accurately controlling the power
`levels of both the hub earth station and the VSAT transmit-
`ters in a VSAT network.
`
`It is another object of this invention to provide power
`control methods and apparatus that take into account the
`specific effects of the satellite transponder relay between the
`hub and the VSATs.
`
`It is yet another object of this invention to provide power
`control mechanisms for both the outbound and the inbound
`links in a VSAT network that respond rapidly to changes in
`the atmospheric attenuation between the earth stations and
`the satellites.
`
`It is still another object of this invention to provide power
`control mechanisms that include checks against long-term
`creep in the inbound power level settings of the VSATs in a
`VSAT network.
`
`It is a further object of this invention to provide multiple
`independent means of determining whether and when the
`8
`
`8
`
`

`

`US 6,212,360 B1
`
`3
`power level of an individual VSAT transmitter should be
`adjusted to maintain its link performance close to a setpoint.
`It is a still further object of this invention to provide VSAT
`power control means that facilitate adjustment of the VSAT
`inbound link performance to take into account the require-
`ments of different types of traffic.
`It is also an object of this invention to provide VSAT
`power control techniques that include the ability to change
`the link rate to effect a change in the link performance when
`this cannot be accomplished by adjustments in transmitter
`power alone.
`It
`is an additional object of this invention to provide
`power control techniques that take into account the capa-
`bility of the VSAT transmitter to control its output spectrum.
`Therefore, a method is presented for providing stable
`communication between a hub earth-station and a VSAT
`earth-station by regulating the power of signals transmitted
`via a satellite, the method comprising the steps of regulating
`the power an outbound signal transmitted by the hub to
`VSAT via the satellite, based on one of a beacon signal
`received from the satellite and a previous outbound signal,
`sending signal information from the hub to the VSAT in the
`outbound signal, and regulating the power of an inbound
`signal transmitted by the VSAT to the hub via the satellite,
`based on properties of the outbound signal and the signal
`information.
`
`A method is also presented for adjusting the power in an
`uplink transmission from a hub earth-station to a satellite,
`the method comprising the steps of receiving an outbound
`signal from a local receiver, and determining an outbound
`signal power level, conditionally receiving a beacon signal
`from a satellite local receiver, and determining a beacon
`signal power level, computing a first difference between the
`received beacon signal power level and a nominal beacon
`signal power level, when the beacon signal
`is received,
`computing a second difference between the received out-
`bound signal power level and a nominal outbound signal
`power level, when the beacon signal is not received, com-
`puting a desired amount of attenuation based on the first
`difference if the beacon signal is received and the second
`difference if the beacon signal is not received.
`A method is also provided for regulating the power of an
`inbound signal sent from a VSAT earth-station to a hub
`earth-station, the method comprising the steps of receiving
`an outbound signal sent from the hub earth-station to the
`VSAT earth-station, the outbound signal including signal
`data and parameter data, determining signal properties of the
`received outbound signal, and modifying the inbound trans-
`mission signal based on the parameter data,
`the signal
`properties, and reference data.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The above and other objects and advantages of the present
`invention will become readily apparent from the description
`that follows, with reference to the accompanying drawings,
`in which:
`1 shows a conventional VSAT
`FIG.
`communication network.
`
`satellite-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`FIGS. 2A through 2D illustrate the operation of the
`conventional VSAT satellite communication network of
`FIG. 1.
`
`60
`
`FIG. 3 is a more detailed block diagram of the hub and
`satellite shown in FIG. 1, according to a preferred embodi-
`ment of the present invention.
`FIG. 4 is a flow chart showing the operation of a sampling
`interval in an outbound transmission signal power level
`setting method according to a preferred embodiment of the
`present invention.
`
`65
`
`4
`FIG. 5 is a flow chart showing a VSAT inbound uplink
`power control method according to a preferred embodiment
`of the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`In the present invention, it is necessary to regulate the
`power of both the outbound signal initially transmitted by
`the hub 10, and the inbound signal, initially transmitted by
`the VSAT 20. Since both power outputs are continually
`regulated, the hub 10 provides additional information to the
`VSAT 20 to increase the accuracy of its power regulation.
`The disclosure of the preferred embodiment of the present
`invention will address these two power regulation schemes
`separately, first addressing the regulation of power at the hub
`10, and then addressing the regulation of power at the VSAT
`20.
`
`According to the preferred embodiment, the hub 10 in a
`VSAT satellite-communication network using Ku-Bands or
`higher must control the power level of the outbound uplink
`210 for three reasons. First,
`it must overcome hub-to-
`satellite rain attenuation in order to maintain a sufficient
`level of the outbound signal for the VSATs 20 to properly
`receive. Second, it must provide a stable Eb/NO level of the
`outbound signal as transmitted by the satellite 30 for the
`VSATs 20 to use as a reference for inbound uplink power
`control purposes in determining the satellite-to-VSAT rain
`fade, where Eb/NO is the ratio of the received energy per bit
`to the noise density received by each demodulator. And
`third, it must avoid exceeding the transponder output power
`level leased from the satellite operator.
`FIG. 3 is a more detailed block diagram of the hub 10 and
`satellite 30 shown in FIG. 1, according to a preferred
`embodiment of the present invention. As shown in FIG. 3,
`the hub 10 comprises a hub modulator 305, an upconverter
`310, a high-power amplifier 315, a beacon downconverter
`320, a beacon receiver 325, a signal downconverter 330, first
`through nth local receivers 335, 340, 345, a hub antenna 360,
`a computer 370, and first through nth hub demodulators 381,
`383, 387. The hub modulator 305 further comprises a hub
`attenuator 375. The antenna 360 further comprises a low-
`noise amplifier 365. In the preferred embodiment, there is at
`least one local receiver at the hub 10 for each outbound
`signal in the system. In alternate embodiments, using 1-for-1
`redundant local receivers, two local receivers are supplied at
`the hub 10 for each outbound signal in the system. The
`satellite 30 further comprises a beacon transmitter 380, a
`signal transponder 385, and a satellite antenna 390.
`The hub modulator 305 generates an outbound uplink
`immediate frequency signal that is converted to Ku-Band by
`the upconverter 310 and is amplified by the high-power
`amplifier 315 before being transmitted as a radio-frequency
`signal by the hub antenna 360 as outbound uplink signal
`210. The satellite antenna 390 receives the outbound uplink
`signal 210 and send it to the signal transponder 385, which
`performs frequency conversion and amplification of the
`signal and sends it back to the satellite antenna 390 for
`transmission as a radio-frequency outbound downlink signal
`220.
`The hub antenna 360 receives the outbound downlink
`signal 220 and sends it to the low-noise amplifier 365, which
`amplifies the outbound downlink signal and sends it to the
`signal downconverter 330. The signal downconverter 330
`converts the outbound downlink signal from a radio-
`frequency signal to an intermediate-frequency signal that is
`processed by local receivers 335, 340, 345.
`Abeacon signal 395 is generated by the beacon transmit-
`ter 380 and is transmitted by the satellite antenna 390. The
`beacon signal 395 is received by the hub antenna 360, is
`9
`
`9
`
`

`

`US 6,212,360 B1
`
`5
`amplified by the low-noise amplifier 365, and is sent to the
`beacon downconverter 320. The beacon downconverter con-
`verts the beacon signal from a radio-frequency signal to an
`intermediate-frequency signal that is processed by the bea-
`con receiver 325.
`
`It is important to note that signal downconverter 330,
`beacon downconverter 320, low noise amplifier 365, and
`beacon transmitter 380 have stable gains and therefore do
`not contribute significant errors to the power control pro-
`cess.
`
`The hub 10 typically uses the hub attenuator 375 built into
`the hub modulator 305 to control the outbound power level.
`The hub modulator 305 will normally be operated in clear
`sky conditions with at least as much attenuation as the
`maximum desired rain fade compensation. In other words, if
`operational parameters require the ability to increase out-
`bound uplink power by up to 6 dB, the hub modulator must
`operate in clear sky conditions with at
`least 6 dB of
`attenuation. This allows for 6 dB of attenuation that can be
`removed to increase the output power by 6 dB when the
`skies are not clear.
`
`The hub uplink power control method uses measurements
`from the beacon receiver 325 of satellite-to-hub rain fade as
`the primary input to determine how to control the outbound
`uplink power to overcome a hub-to-satellite rain fade. The
`beacon receiver 325 provides a measurement of the received
`signal strength of a signal from the beacon transmitter 385
`located on the satellite 30.
`
`The relationship between the rain fade experienced by the
`beacon signal 395 and the rain fade that is experienced by
`the outbound uplink signal 210 is a non-linear function of
`the relative frequencies of the two signals. The relative
`attenuation A, measured in dB, of two Ku-Band RF signals
`due to rain in the atmosphere is roughly
`
`10
`
`15
`
`20
`
`25
`
`30
`
`—[ji—](:i—J
`
`(1)
`
`35
`
`where AH is the attenuation at the higher frequency, where
`AL is the attenuation at the lower frequency, where k is a
`constant between 1.25 and 1.5, where fH is the higher of the
`two frequencies, and where fL is the lower of the two
`frequencies. This relationship is described in greater detail in
`the CCIR XIIIth Plenary Assembly, Vol. V, Report 233-3,
`Geneva, 1974.
`In the preferred embodiment, the beacon signal 395 will
`experience somewhat less attenuation than the outbound
`uplink signal 210 will, since the outbound uplink signal is
`typically higher in frequency given the standard Ku-Band
`geostationary communication satellite frequency plan.
`The frequency of the beacon signal 395 is preferably in
`the range of 10.95 GHZ through 12.75 GHZ for a Ku-Band
`system, and the frequency of the outbound uplink signal 210
`is preferably in the range of 14 to 14.5 GHZ. The square of
`the ratio of the two frequencies can thus vary considerably,
`from about 1.21 to about 1.75. The amount of the outbound
`uplink attenuation is calculated as a function of the actual
`beacon signal frequency, and the actual outbound uplink
`frequency, and the beacon fade.
`In the preferred embodiment, each of the first through nth
`local receivers 335 through 345 have the ability to measure
`and report the outbound power level of the outbound down-
`link signal 220 received at the hub 10. Measurement by the
`local receivers 335 through 345 of the outbound signal level
`provides two functions. One is to enable monitoring of the
`effect of the outbound power control when a rain fade
`occurs: each local receiver 335 through 345 measures the
`same fade on the outbound uplink signal 210 (ignoring
`frequency differences for
`the moment) as the beacon
`
`40
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`45
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`50
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`55
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`60
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`65
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`6
`receiver 325 measures on the beacon signal 395, as long as
`uplink power control is functioning correctly. The second
`function is to serve as an alternative to the beacon receiver
`beacon measurement, in case the beacon signal 395 goes
`away.
`If the local receiver’s measurement of the outbound
`uplink signal 210 is the only hub rain fade measurement
`available, the amount of increase in uplink power required
`can be approximated by recognizing that the total rain fade
`is equal to the uplink rain fade plus the downlink rain fade,
`and that the uplink rain fade is higher than the downlink rain
`fade by the amount given by Eq.
`(1) above, since the
`outbound uplink frequency is higher than the outbound
`downlink frequency. In this case, the total outbound rain
`fade is given by the equation:
`
`FT =FD+k(f0_U)2FD =[1+k(f0u]2]FD
`for)
`fo—D
`
`(2)
`
`where FT is the total fade measured by the local receiver,
`where FD is the amount of the outbound downlink fade,
`where k is the constant previously discussed, where fOU is
`the outbound uplink frequency, and where fOD is the fre-
`quency of the outbound downlink.
`Similarly, the total fade as a function of the uplink fade
`similarly given by the equation:
`
`[—JH—J]
`
`(3)
`
`where FU is the amount of the outbound uplink fade, and the
`rest of the variables are the same as in equation (2).
`Thus, the amount of the uplink fade as a function of the
`total fade, and therefore the increase in the uplink power
`level required to overcome the fade, AU, is simply
`
`Au—
`
`FT
`
`1+ Hg—ZJZ
`
`(4)
`
`where Au is the amount of outbound attenuation to be
`
`removed to compensate for the rain fade.
`It is important that the hub 10 not increase its outbound
`power by more than the amount required to overcome a
`hub-to-satellite rain fade. This is because the VSATs 20 use
`the received outbound downlink Eb/NO level as an input to
`their own inbound uplink power control method. Otherwise,
`when the VSATs 20 see the received Eb/NO increase, they
`will decrease their power in response,
`resulting in an
`increase the block error rates of the inbound signals at the
`hub demodulators 381, 383, 387. Furthermore, the hub 10
`must not transmit at a level higher than that required to
`achieve the transponder output power level leased from the
`satellite operator.
`A hub-to-satellite rain fade will affect the inbound down-
`link signals 240 as well as the outbound uplink signal 210.
`For this reason, in a hub-to-satellite rain fade situation, one
`might think that it is advantageous to increase the inbound
`transmitted power level. However, the inbound downlink
`signal margin is typically 9 dB or more, mitigating the
`hub-to-satellite rain fade effect on the inbound downlink
`signal 240. Given these considerations,
`the hub uplink
`power control system is designed to not cause the outbound
`downlink signal transmitted by the satellite 30 to increase
`during a hub-to-satellite rain fade, and a small amount of
`fading is permissible, or perhaps even desirable.
`It is also important that the downlink outbound signal 220
`originating from the satellite 20 not fade by more than one
`10
`
`10
`
`

`

`US 6,212,360 B1
`
`7
`dB or so during a hub-to-satellite rain fade. This, too, is due
`to the fact that the VSATs 20 use the measured outbound
`Eb/NO to control their inbound uplink power. If the VSATs
`20 all see an outbound downlink fade, they will raise their
`inbound power level to compensate. This will reduce their
`margin to cope with a satellite-to-VSAT rain fade.
`In
`addition, it will raise the inbound power spectral density,
`which is limited by the FCC for antennas that do not meet
`the beamwidth requirements of Part 25.209 of the FCC
`Rules.
`These two considerations mean that the outbound power
`control method at the hub 10 needs to maintain better control
`than has typically been required in the past of VSAT
`systems. The downlink outbound signal 220 transmitted by
`the satellite 30 should be maintained within a range of +0,
`—1 dB of its nominal clear-sky level. This can be accom-
`plished by reducing die factor of k in the uplink power
`control equation from the range of 1.25 to 1.5 down
`somewhat, to, say, 1.2, sampling the beacon receiver beacon
`level measurement and local receiver’s outbound level mea-
`surements frequently, and updating the hub modulator
`power level frequently.
`The power level of the outbound signal is thus controlled
`by an outbound transmission signal power level setting
`method. This method begins by establishing a nominal
`outbound power level, Pnom, a nominal beacon receiver level
`nom)
`BR
`and a nominal local receiver outbound power level
`LRnom, for clear-sky conditions. The level of Pnom in turn
`determines a hub IF nominal attenuator setting, Anom.
`The system then adjusts the power level over each of a set
`of constant sampling intervals tmmp, where tmm is a system
`parameter that is preferably set between 1 andP10 seconds.
`FIG. 4 is a flow chart showing the operation of a sampling
`interval in the outbound transmission signal power level
`setting method, according to a preferred embodiment of the
`present invention. As shown in FIG. 4, the hub 10 begins by
`sampling the local receiver’s outbound power level LR, and
`filtering it with an exponential smoothing filter to get a
`filtered local receiver’s outbound power level LRt' (Step
`405).
`Then, the hub 10 determines if it is receiving a beacon
`receiver signal (Step 410). If a beacon receiver signal is
`received,
`then the system computes the beacon receiver
`signal power BR and filters it with an exponential smooth-
`ing filter to get a filtered beacon receivers signal power level
`BRt' (Step 415).
`The hub 10 then compares the current sample beacon
`receiver’s measurement of fade with the current sample
`local receiver’s measurement of fade (Step 420). This is a
`calculation of the current measurement of the drop, in dB, of
`the beacon signal power, minus the current measurement of
`the drop, in dB, of the outbound downlink signal power. At
`this point, it is also necessary to correct for the frequency
`difference between the outbound downlink signal and the
`beacon receiver signal to account for the frequency differ-
`ence (Step 430). If the two differ by more than 1 dB, the
`system raises an alarm (Step 425) to call the operator’s
`attention to the problem.
`After this step, the hub 10 computes the beacon receiver
`difference BRdifl between the nominal beacon receiver level
`BRnom, and the filtered beacon receiver level BRt',
`i.e.,
`BRdlfBRnom—BR't (Step 435). The system then computes
`the desired attenuation A, for the outbound uplink signal 210
`as a function of the beacon receiver difference BRdiff (Step
`440).
`the system determines in step 410 that a
`If, however,
`beacon receiver signal is not being received, then the system
`computes the local receiver outbound power difference
`LRdl— between the nominal local receiver outbound power
`levelfiz LRnom, and the filtered local receiver’s outbound
`power level LRt', i.e., LRdlfLRnom—LR't (Step 445). In this
`
`10
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`15
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`30
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`8
`case, the system computes the desired attenuation A, for the
`outbound uplink signal 210 as a function of the local
`receiver outbound power difference LRdl— (Step 450).
`Regardless of the path taken, the hub 0 then determines
`whether the desired attenuation At (however calculated) is
`within a permissible attenuator range for the hub power
`control attenuator (Step 460). If the desired attenuation A, is
`outside of the permissible range, the hub 10 sets the attenu-
`ator to the attenuator limit closest to A, (Step 465).
`Finally,
`the hub 10 broadcasts the value of the error
`between the desired A, and the actual attenuator setting, if
`any (Step 470). The VSATs 20 then use this error value in
`their inbound power control method.
`In step 405, the exponential smoothing filter preferably
`has the form:
`
`LR’;=15*LRm+(1-15)*LRQ.1,
`
`(5)
`
`is a
`where LR't is the filter output at sample t, where [3
`system parameter that may be set between 0 and 1, where
`LRm is the current sample measured value, and where LR't_1
`is the previous sample filter output.
`In step 415 the exponential smoothing filter preferably has
`the form:
`
`BR’,=a*BRm+(1—a)*BR’,,1,
`
`(6)
`
`where BR't is the filter output at sample t, where (X is a
`system parameter that may be set between 0 and 1, where
`BRm is the current sample measured value, and where BR't_1
`is the previous sample filter output; similarly.
`is preferably
`In step 440,
`the desired attenuation A,
`computed according to the following equation:
`
`At=Anom—[BRdzfir* 12* (fDU/fDD)2]
`
`(7)
`
`where A, is the transmit attenuator setting, using the filtered
`beacon level measurement, if it is available, where fOU is the
`outbound uplink frequency and where fOD is the beacon
`downlink frequency.
`is preferably
`In step 450,
`the desired attenuation At
`computed according to the following equation:
`
`At=Anom_[LRdifl/(1+0'83 *(foD/foU)2)]
`
`(8)
`
`where fOU is the outbound uplink frequency and where fOD
`is the outbound downlink frequency.
`Once the outbound power level is regulated, it is then
`necessary to regulate the inbound power level. The VSATs
`20 are able to do this efficiently using information provided
`by the hub 10 in the outbound signal
`For efficient CDMA-mode operation, the VSAT 20 must
`implement an inbound power control method in order to
`keep its inbound Eb/NO, as received by the hub 10, balanced
`with respect to the received signals from other VSATs 20 in
`the network. Preferably, the received inbound Eb/NO (in the
`absence of multiple-access i