(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0081978 A1
`
`Hou et al.
`(43) Pub. Date:
`Jun. 27, 2002
`
`US 20020081978A1
`
`(54) ANTENNA RF TRANSMISSION SAFETY
`SYSTEM AND METHOD
`
`Related US. Application Data
`
`(76)
`
`Inventors: Peter Hou, Germantown, MD (US);
`Fayez Hyjazie, Germantown, MD (US);
`Thomas Jackson, Frederick, MD (US);
`Stan Kay, Rockville, MD (US); Jack
`Lundstedt, Monrovia, MD (US); Doug
`Ricker, Clarksburg, MD (US); Ken
`Sahai, North Potomac, MD (US);
`James Zawlocki, Gaithersbnrg, MD
`(US); Walter R. Kepley, Gaithersburg,
`MD (US)
`
`(63) Non—provisional of provisional
`60/244,815, filed on Oct. 31, 2000.
`
`application No.
`
`Publication Classification
`
`Int. Cl? ..................................................... H04B 17/00
`(51)
`(52) U.S.Cl.
`.......................................... 455/67.1;455/13.4
`
`ABSTRACT
`(57)
`An RF emission hazard zone of an RF transceiver is con-
`trolled to ensure that RF energy density limits for humans is
`not exceeded when a human body part enters the RF hazard
`zone near an antenna reflector and feedhorn. In a first aspect,
`a transmitter duty cycle is reduced to effectively reduce the
`average power transmitted from the antenna Whenever a
`signal level of a received signal is reduced below a threshold
`value. The reduction in average transmitter power reduces
`the RF emission hazard zone near the antenna, and limits the
`exposure of any person who has intruded into the hazard
`zone. In a second aspect, the transmitter is disabled When-
`ever a received signal is affected so that signal quality,
`bit-energy-to-noise ratio (Eb/No), synchronized state in a
`demodulator, lock condition in a FLL, or received signal
`strength are degraded, indicating that a human has intruded
`into the RF hazard zone.
`
`ANTENNA RF HAZARD ZONE
`
`Correspondence Address:
`Hughes Electronics Corporation
`Patent Docket Administration
`P.0. Box 956
`Bldg. 1, Mail Stop A109
`El Segundo, CA 90245-0956 (US)
`
`(21) Appl. No.:
`
`09/828,733
`
`(22)
`
`Filed:
`
`Apr. 9, 2001
`
`150 ANTENNA
`
`REFLECTOR
`210
`
`
`
`
`
`FEEDHORN
`220
`
`MOUNTING
`POLE
`230
`
`
`
`
`
`
`MOUNTING
`BASE
`240
`
`
`AUTO SHUTOFF
`
`ZONE
`260
`
`
`
`Page 1 of 13
`
`SAMSUNG EXHIBIT 1035
`
`Page 1 of 13
`
`SAMSUNG EXHIBIT 1035
`
`

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`Patent Application Publication
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`Jun. 27, 2002 Sheet 2 0f 5
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`Patent Application Publication
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`US 2002/0081978 A1
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`Patent Application Publication
`
`Jun. 27, 2002 Sheet 5 0f 5
`
`US 2002/0081978 A1
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`Page 6 of 13
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`Page 6 of 13
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`

`US 2002/0081978 A1
`
`Jun. 27, 2002
`
`ANTENNA RF TRANSMISSION SAFETY SYSTEM
`AND METHOD
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`[0001] This application claims the benefit under 35 U.S.C.
`§ 119(e) of US. Provisional Application of Hou et al.
`entitled “Antenna RF Transmission Safety Mechanism”,
`serial No. 60/244,815, filed on Nov. 1, 2000,
`the entire
`contents being incorporated herein by reference.
`
`BACKGROUND OF THE INVENTION
`
`[0002]
`
`1. Field of the Invention
`
`[0003] This invention relates generally to a safety system
`and method used in connection with controlling radio-
`frequency (RF) emissions from an antenna when a portion of
`a human body moves into a hazardous emission zone of the
`antenna in which the RF power density exceeds safety limits
`established for human exposure.
`
`[0004]
`
`2. Description of the Related Art
`
`[0005] Radio-frequency electromagnetic energy emitted
`from an antenna is a safety concern when the power density
`exceeds a certain level. Federal Communication Commis-
`
`sion (FCC) rules require transmitting facilities to comply
`with RF exposure guidelines. The limits established in the
`guidelines are designed to protect the public health with a
`margin of safety. These limits have been endorsed by federal
`health and safety agencies such as the Environmental Pro-
`tection Agency (EPA) and the Food and Drug Administra-
`tion (FDA). Most electro-magnetic facilities create maxi-
`mum exposures that are only a small fraction of the limits.
`Moreover,
`the limits themselves are many times below
`levels that are generally accepted as having the potential to
`cause adverse health effects.
`
`[0006] The FCC’s limits for maximum permissible expo-
`sure (MPE) to RF emissions depend on the frequency or
`frequencies that a person is exposed to. Different frequencies
`may have different MPE levels.
`
`[0007] Exposure to RF energy has been identified by the
`FCC as a potential environmental factor that must be con-
`sidered before an emitting facility, operation of an emitter, or
`transmitter can be authorized or
`licensed. The FCC’s
`requirements dealing With RF exposure can be found in Part
`1 of its rules at 47 C.F.R. § 1.1307(b), The exposure limits
`themselves are specified in 47 C.F.R. § 1.1310 in terms of
`frequency, field strength, power density and averaging time.
`Facilities and transmitters licensed and authorized by the
`FCC must either comply with these guidelines or else an
`applicant must file an Environmental Assessment (EA) with
`the FCC. In practice, however, a potential environmental RF
`exposure problem is typically resolved before an EA would
`become necessary. Therefore, compliance with the FCC’s
`RF guidelines constitutes a de facto threshold for obtaining
`FCC approval to construct or operate a station or transmitter.
`The FCC guidelines are based on exposure criteria recom-
`mended in 1986 by the National Council on Radiation
`Protection and Measurements (NCRP) and on the 1991
`standard developed by the Institute of Electrical and Elec-
`tronics Engineers (IEEE), and later adopted as a standard by
`the American National Standards Institute (ANSI/IEEE
`C95.1—1992).
`
`Page 7 of 13
`
`
`
`[0008] The FCC’s guidelines establish separate MPE lim-
`'ts for “general population/uncontrolled exposure" and for
`‘occupational/controlled exposure.” The general popula-
`ion/uncontrolled limits set the maximum exposure to which
`most people may be subjected. People in this group include
`he general public who are not associated with the installa-
`ion and maintenance of the transmitting equipment. Higher
`exposure limits are permitted under the “occupational/con-
`rolled exposure” category, but only for persons who are
`exposed as a consequence of their employment (e.g., wire—
`ess radio engineers or technicians). To qualify for the
`occupational/controlled exposure category, exposed persons
`must be made fully aware of the potential for exposure (e.g.,
`hrough training), and they must be able to exercise control
`over their exposure. In addition, people passing through a
`ocation, who are made aware of the potential for exposure,
`may be exposed under the occupational/controlled criteria.
`The MPE limits adopted by the FCC for occupational/
`controlled and general population/uncontrolled exposure
`incorporate a substantial margin of safety and have been
`established to be well below levels generally accepted as
`having the potential to cause adverse health effects.
`
`[0009] Determing whether a potential health hazard could
`exist with respect to a given transmitting antenna is not
`always a simple matter. Several factors must be considered
`in making that determination. They include, but are not
`limited to, the frequency of the RF signal being transmitted,
`the operating power of the transmitting station, the actual
`power radiated from the antenna, how long a person is
`exposed to the RF signal at a given distance from the
`antenna, and exposure from other RF emissions located in
`the area.
`
`
`
`[0010] The MPE limits vary by frequency because of the
`different absorptive properties of the human body at differ—
`ent frequencies when exposed to whole-body RF fields. 47
`C.F.R. § 1.1310 establishes MPE limits in terms of “electric
`field strength,”“magnetic field strength,” and “far-field
`equivalent power density” (power density). For most fre-
`quencies used by wireless and satellite communication ser—
`vices, the most relevant measurement is power density. The
`VIPE limits for power density are given in terms of “milli-
`watts per square centimeter” or mW/cmz. In terms of power
`density, for a given frequency the FCC MPE limits can be
`interpreted as specifying the maximum rate that energy can
`3e transferred (i.e., the power) to a square centimeter of a
`Jerson’s body over a period of time (either 6 or 30 minutes).
`In practice, however, since it
`is unrealistic to measure
`separately the exposure of each square centimeter of the
`aody, actual compliance with the FCC limits on RF emis—
`sions should be determined by “spatially averaging” a
`erson’s exposure over the projected area of an adult human
`vody.
`
`[0011] Electric field strength (|E|) and magnetic field
`strength (IIII) are used to measure “near field” exposure. At
`requencies below 300 MHz, these are typically the more
`relevant measures of exposure, and power density values are
`given primarily for reference purposes. However, evaluation
`of far—field equivalent power density exposure may still be
`appropriate for evaluating exposure in some such cases. For
`frequencies above 300 MHz, only one field component need
`be evaluated, and exposure is usually more easily charac-
`terized in terms of power density. Transmitters and antennae
`that operate at 300 MHz or lower include radio broadcast
`
`Page 7 of 13
`
`

`

`US 2002/0081978 A1
`
`Jun. 27, 2002
`
`stations, some television broadcast stations, and certain
`personal wireless service facilities (e.g., some paging sta-
`tions). Most personal wireless services, including all cellular
`and PCS, as well as some television broadcast stations and
`microwave communications, including satellite communi-
`cations, operate at frequencies above 300 MHz.
`
`the MPE limits are specified as
`[0012] As noted above,
`time-averaged exposure limits. This means that exposure
`can be averaged over the identified time interval (30 minutes
`for general population/uncontrolled exposure or 6 minutes
`for occupational/controlled exposure). However, for the case
`of exposure of the general public, time averaging is usually
`not applied because of uncertainties over exact exposure
`conditions and difficulty in controlling time of exposure.
`Therefore, the typical conservative approach is to assume
`that any RF exposure to the general public will be continu-
`ous. The FCC’s current
`limits for exposure at different
`frequencies are shown in Table 1.
`
`[0013] Currently, for frequencies in the microwave band,
`the Federal Communications Commission (FCC) has estab-
`lished an exposure safety limit for the general public of 1
`mW/cm2, averaged over a 30-minute period.
`
`TABLE 1
`
`FCC Limits for Maximum Permissible Exposure MPE
`
`Frequency (f)
`Range (MHz)
`
`Electric
`Field
`Strength
`(E) (V/m)
`
`Magnetic Field
`Strength (II)
`(A/m)
`
`Power
`Density (S)
`(mW/cmz)
`
`Averaging
`Time [Eli
`III]2 or S
`(minutes)
`
`(A) Limits for Occupational/Controlled Exposure
`
`l:100)*
`1.63
`614
`0.3-3.0
`(900/'f2)*
`4.89/f
`1842/f
`3.0-30
`1.0
`0.163
`61.4
`30-300
`f/300
`—
`—
`300-1500
`S
`—
`—
`1500-100,000
`(B) Limits for General Population/Uncontrolled Exposure
`
`0.371.34
`1.34-30
`30-300
`300-1500
`15007100,000
`
`614
`824/f
`27.5
`—
`7
`
`1.63
`2.19/f
`0.073
`—
`7
`
`(100)*
`(MO/fl)"
`0.2
`f/1500
`1.0
`
`6
`6
`6
`6
`6
`
`30
`30
`30
`30
`30
`
`f = frequency in MHz
`*Plane-wave equivalent power density
`
`[0014] For most common reflector type of antennae used
`for satellite earth terminal transmission to a satellite, the
`transmit power density often exceeds this safety limit in the
`area between the feedhorn and reflector, and sometimes in
`the near—field a very short distance in front of the antenna.
`This poses a potential safety hazard, especially for children
`who can not read warning signs or labels, and who may
`intentionally or unintentionally place themselves in the
`emission hazard areas, and poses a safety hazard for small
`animals, such as cats, squirrels, birds, etc.
`
`[0015] Conventional approaches used to reduce RF emis-
`sion exposure hazard include placing the antenna out of
`physical reach under normal circumstances, for example, on
`a roof top or on top of a pole at least six feet off the ground,
`or using an enclosure such as a radome that limits human
`access, or access by small animals such as birds, cats, and
`squirrels.
`
`these protective measures can be
`[0016] Unfortunately,
`defeated relatively easily by a determined person who either
`climbs up the antenna mast or on the roof, or who inten-
`tionally or unintentionally breaks the protective radome and
`receives exposure to RF energy at a density level greater
`than that specified by the limits discussed above. To counter
`this possibility, additional hardware or structure around the
`antenna is necessary, or other restrictions on antenna mount-
`ing sites and methods must be imposed to render the
`installation safer, all at additional cost and inconvenience.
`
`[0017] A more reliable approach from a safety viewpoint
`would be to control the actual transmission of the RF energy
`from the feedhorn or antenna when intrusion into the haz-
`
`ardous zone is made. However, the known approaches to
`solving the problem of human exposure to RF energy in the
`antenna hazard zone does not make use of such controls, and
`instead merely relies upon physical mounting or shielding.
`
`[0018] What is needed, therefore, is a relatively low-cost,
`inexpensive, and reliable system and method for automati—
`cally reducing the RF power transmitted from an antenna
`when a human either
`intentionally or unintentionally
`intrudes into the hazardous emission zones of the antenna.
`
`What is also needed is a system and method for incremen-
`tally reducing the RF power as the blockage progressively
`worsens. What is also needed is a system and method for
`disabling the RF power transmitted when substantial block-
`age of the antenna is encountered.
`SUMMARY OF THE INVENTION
`
`[0019] The present invention solves at least one of the
`aforementioned problems of intentional or unintentional
`intrusion into a hazardous emission zone of an antenna by a
`human, including total blockage of the antenna, and miti-
`gates the associated RF emission hazard resulting from such
`intrusion or blockage.
`
`[0020] A first aspect of the invention embodies a system
`and method which controls the RF hazard zone by limiting
`the transmit duty cycle for any given maximum transmitter
`power,
`thereby limiting the average power in any give
`30-minute period. For example,
`if a 50% duty cycle is
`imposed on a 2-W maximum transmitter, then its hazard
`zone will be the same as that of a 1-W maximum transmitter
`
`operating at 100% duty cycle. As a second example, a
`reduction of duty cycle from 100% to 25% will reduce the
`size of the hazard zone by 6 dB, for the same transmitter.
`
`the average RF
`In this aspect of the invention,
`[0021]
`transmitted power from an antenna is automatically reduced
`when a portion of a human body moves into the RF hazard
`zone of the antenna (in the close vicinity of the antenna) in
`which the RF power density exceeds safety limits for human
`exposure. This is achieved by detecting changes in the
`received power level from a distant source, for example, a
`satellite, which is relatively weak and therefore, has a safe
`level of RF energy. This RF energy is present
`in the
`environment regardless of the presence of the antenna of
`interest. The detected reduction in received power level is
`used to indicate the intrusion of a foreign object into the RF
`hazard zone, for example, portions of a human body. The
`underlying physical principle is that any intrusion of human
`body of body parts in the vicinity of the antenna or the
`feedhorn will necessarily block part of the antenna aperture
`or the reflector/feedhorn pathway, thus causing a reduction
`in the received power.
`
`Page 8 of 13
`
`Page 8 of 13
`
`

`

`US 2002/0081978 A1
`
`Jun. 27, 2002
`
`[0022] A second aspect of this invention directed to a
`system and method for controlling an RF hazard zone
`similarly detects the intrusion of a human body or body parts
`into a prescribed auto-shutoff zone by continually monitor-
`ing the received power level. When the reduction in received
`power reaches a determined level, the mechanism quickly
`disables the transmitter by triggering shutoff of the transmit
`power, typically in a fraction of a second, for example, an
`output power reduction of at least 50 dB within 25 micro-
`seconds. The shutoff zone, which is determined by the
`reduction in received power level at which the shutoff is
`triggered, may completely enclose the hazard zone,
`to
`ensure absolute safety.
`
`[0023] There are several levels of hardware and software
`that may be used to fail-safe the transmitter, and to auto-
`matically disable transmission should any of the signal
`parameters or attributes used indicate a degradation of
`performance.
`
`[0024] For example, the system may determine and evalu—
`ate the following exemplary signal parameters or attributes,
`and disable the transmitter if one or more of the parameters
`indicate that
`the received signal
`is degraded, and that a
`foreign object may have intruded into the hazard zone or
`auto—shutoff zone:
`
`lock, which detects the
`(1) Receive signal
`[0025]
`presence of a received digital signal stream;
`
`(2) Demodulation lock or synchronization of
`[0026]
`the receiver demodulator, which indicates the integ-
`rity of the digital signals;
`
`(3) Intermediate Frequency Module (IFM)
`[0027]
`lock, which indicates the proper strength of the
`received signals;
`
`(4) Frequency locked loop (FLL) lock, indi—
`[0028]
`cating that the receiver front-end is properly syn-
`chronized to the received signal; or
`
`ratio Eb/NO is
`(5) The bit—energy—to—noise
`[0029]
`determined and compared to a threshold value,
`below which the transmitter is disabled.
`
`[0030] All of the above fail-safe mechanisms, or others
`not discussed but known in the art, may be set and fine tuned
`in the design to tailor the size and sensitivity of the “shutoff
`zone”. When the shutoff zone completely encloses the
`hazard zone for a prescribed minimum size of human body
`parts, fail-safe automatic shutoff of the transmitter, and
`therefore complete safety, may be achieved.
`
`[0031] The present invention has a number of features that
`distinguish it over conventional safety approaches that
`attempt to limit exposure of humans to RF emissions. For
`example, the method and system of the present invention
`uses an automated, fail-safe approach to reduce or eliminate
`the RF emission hazard associated with the RF antenna,
`whereas conventional approaches to solving this problem
`have relied only upon physical barriers or additional costly
`and potentially heavy structure to protect humans from
`deliberate or inadvertent exposure to RF emissions.
`
`[0032] These and other objects of the present application
`will become more readily apparent
`from the detailed
`description given hereinafter. However, it should be under-
`stood that the detailed description and specific examples,
`
`while indicating preferred embodiments of the invention, are
`given by way of illustration only, since various changes and
`modifications within the spirit and scope of the invention
`will become apparent from this detailed description to those
`skilled in the art.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0033] The features and advantages of the invention will
`be more readily understood upon consideration of the fol-
`lowing detailed description of the invention, taken in con-
`junction with the accompanying drawings in which:
`
`[0034] FIG. 1 depicts a typical satellite communication
`system in which the system and method of the present
`invention may be used;
`
`[0035] FIG. 2 provides a representation of the antenna RF
`hazard zone of the present invention;
`
`[0036] FIG. 3 provides a block diagram representation of
`a remote station transceiver of the present invention;
`
`[0037] FIGS. 4A and 43 provide alternative embodi-
`ments of the interface between the receiver and control
`
`processor; and
`
`[0038] FIG. 5 provides an example of a word structure of
`a multi-bit control word used in one embodiment of the
`interface between the receiver and control processor.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`[0039] Apreferred embodiment of the method and system
`of providing control of an RF emission hazard zone is
`described below. Referring to FIG. 1, a typical two-way
`satellite communication system that may employ the method
`and system of the present invention is shown.
`
`[0040] Control station 110 provides control station uplink
`120a to satellite 130 which, in turn, provides control station
`downlink 120b to one or more remote stations 140 (140a,
`140b, etc.). Control station uplink/downlink 120a/120b, for
`example, may use a time-division multiple access (TDMA)
`type signal, or other signal modulation techniques appropri-
`ate to satellite communications. Control station uplink 120a
`may be provided in a “broadcast” mode for receipt by a large
`number of users, or may be directed to one or a smaller
`number of dedicated users. Remote station 140 receives
`control station downlink 120b through antenna 150, and
`then control station downlink 120b is further provided to
`transceiver (XCVR) 160 for processing.
`
`[0041] Return channel uplink 170a represents a return
`channel path from remote station 140 back to control station
`110 through satellite 130, and may use any appropriate
`modulation technique, for example, TDMA, the preferred
`modulation technique, or other
`types of modulation
`schemes, such as code-division multiple access (CDMA) or
`frequency-division multiple access (FDMA), or other appro-
`priate modulation schemes. Preferably the transmit fre-
`quency of return channel uplink, 170a is at a different
`frequency than control station downlink 120b. Information
`contained in return channel 170 may be processed within
`control station 110, or may be further provided to or from the
`internet, an intranet, or a landline (telephone line) through
`gateway 180.
`
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`Jun. 27, 2002
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`[0042] Turning to FIG. 2, antenna 150 is depicted.
`Antenna 150 includes reflector 210 and feedhorn 220 which
`
`are appropriately arranged with respect to each other for
`directing and receiving RF energy, and which are collec-
`tively attached to mounting pole 230 and mounting base
`240. In receive mode, reflector 210 collects RF energy from
`a far-away source, for example satellite 130, and focuses it
`onto feedhorn 220. In transmit mode, feedhorn 220 spreads
`the RF energy in a prescribed manner onto reflector 210
`which in turn collimates the energy to form a narrow beam
`that is aimed at satellite 130. At distances close to feedhorn
`220, the transmit RF power density level can be relatively
`high, for example greater than I mW/cm2 when averaged
`over a 30 minute period, which defines hazard zone 250, and
`which represents an area wherein the maximum power
`density exposure limit for humans is exceeded. FIG. 2
`further shows auto-shutoff zone 260, which completely
`encompasses hazard zone 250, and which represents an area
`provided as a safety margin with respect to hazard zone 250.
`In the area between auto-shutoff zone 260 and hazard zone
`250, the power density exposure is less than the maximum
`permitted limits, nonetheless,
`intrusion into auto-shutoff
`zone 260 is also used into the present
`invention as an
`additional safety factor.
`
`[0043] With respect to FIG. 3, details of remote station
`transceiver 160 in remote station 140 are shown. Control
`
`station downlink 120b is received by antenna 150 and
`provided to coupler 310. Coupler 310 is used to separate
`transmit and receive signals to/from antenna 150 and, in a
`preferred embodiment, for example, may be implemented as
`a diplexer or waveguide filter. The signal from an output port
`of coupler 310 is provided to receiver 315, for example, an
`input of low-noise amplifier (LNA) 320, which boosts the
`received signal while reducing, to the extent possible, the
`addition of further noise during the amplification process.
`LNA 320 provides the amplified signal to down converter
`330, to translate the received signal, at an RF frequency, to
`a lower intermediate frequency (IF) which can be more
`readily processed in receiver 315. In a preferred embodi-
`ment, the signal at IF is provided to demodulator 340, which
`demodulates the received signal to provide demodulated
`data at baseband.
`
`[0044] Receiver 315 generates at least one control signal
`provided to control processor 350. Under the direction of the
`control signal from receiver 315, control processor 350
`controls transmitter 385, which includes modulator 360,
`up-converter 370 and power amplifier 380.
`
`[0045] Return channel uplink data is provided to modu-
`lator 360 at baseband, and modulator 360 formats the uplink
`data in the proper manner for the particular modulation
`scheme being used, for example, framing of data and control
`words, application of forward error correction (FEC), and
`determining the amount of bandwidth (BW), i.e. timeslots,
`to be requested from control station 110, in an exemplary
`implementation using TDMA. Modulator 360 may also
`convert
`the baseband signals to another IF for ease of
`processing, or to achieve commonality of signal formats and
`frequencies within transmitter 385. Modulator 360 provides
`the baseband or IF signal to up-converter 370, which trails-
`lates the modulator output to an RF signal (at a relatively
`low level), to a frequency which is intended to be transmit-
`ted.
`
`[0046] Power amplifier 380 boosts the signal level of the
`RF signal to a power level sufficient for transmission, and
`provides the boosted or amplified RF signal to an input port
`of coupler 310 which then, through waveguide filtering, for
`example, provides the amplified RF signal to feedhorn 220
`and reflector 210 of antenna 150. Thus, the amplified RF
`signal is propagated from antenna 150 toward satellite 130.
`
`[0047] Operation of the system and method of the present
`invention are now discussed with respect to FIGS. 3—5.
`
`the first aspect of the
`[0048] As mentioned previously,
`invention controls the RF hazard zone by limiting the
`transmit duty cycle for any given maximum transmitter
`power. The average RF transmitted power from antenna 150
`is reduced when a portion of a human body or foreign object
`moves into RF hazard zone 250 of antenna 150. This is
`achieved by detecting,
`in receiver 315, changes in the
`received power level of control station downlink 120b from
`satellite 130.
`
`If the received power level is reduced below a first
`[0049]
`threshold value, for instance, the received power level may
`still be adequate to allow receiver 315 to normally function.
`This could correlate to only a small blockage between
`feedhorn 220 and reflector 210, for example. In response to
`this first reduction in the received power level, the trans-
`mitter duty cycle may, for example, be reduced from unity
`(i.e. 100% duty cycle or duty factor) or a relatively large
`duty cycle, e.g. 80%, to a lower duty cycle, for example,
`50%. As an example, if a 50% duty cycle is imposed on a
`2-W maximum transmitter, then its hazard zone 250 will be
`the same as that of a l-W maximum power transmitter
`operating at 100% duty cycle, and the size of the hazard has
`thus effectively been reduced by 3 dB.
`
`[0050] As a second example, if the received signal power
`is reduced even further, then greater blockage or intrusion
`into hazard zone 250 may be presumed to have occurred. In
`response, an additional reduction of duty cycle from 50% to
`25% may be used to reduce the size of the hazard zone by
`an additional 3 dB, for the same transmitter 385. Such
`step-wise reductions in power such as these could be pro-
`gressively applied as the received power level lessens (but
`still remains usable) over time, indicating further levels of
`intrusion or blockage between feedhorn 220 and reflector
`210. In a preferred embodiment using TDMA techniques,
`allocation of bandwidth through slot assignment is used to
`vary the average power depending on a detected intrusion
`into hazard zone 250;
`the peak transmitter power is not
`changed to ensure that adequate signal energy reaches
`satellite 130 during the time-slots in which transmission is
`authorized. However, the present invention could alterna-
`tively reduce the peak transmitter power in an alternative
`modulation scheme, e.g. CDMA or FDMA, in a manner that
`would lower the average transmitter power, and thus reduce
`the size of hazard zone 250, consistent with ensuring
`adequate return channel uplink 170 energy was received at
`satellite 130 to effectuate a reliable communication link.
`
`[0051] The threshold that disables transmission is any one
`of the following related conditions.
`
`(a) When the input level drops below —65
`[0052]
`dBm. As an example,
`for the Shannon terminal
`antenna that is located in Maryland, this is about 5
`dB drop from the normal reception condition;
`
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`US 2002/0081978 A1
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`Jun. 27, 2002
`
`[0053]
`2E'4.
`
`(b) When the bit error rate (BER) exceeds
`
`[0054] The threshold depends on many factors, including
`the bit rates.
`
`[0055] As one way of reducing the transmit duty cycle of
`transmitter 385, receiver 315 determines a signal quality
`indicator (SQI) which may indicate that the received signal
`is above a determined threshold value by setting an SQI flag
`in the control signal, or may provide a quantitative repre-
`sentation of the received signal level to control processor
`350 via the control signal. When control processor 350
`receives an indication that the signal quality is degraded,
`control processor 350 acts to reduce the bandwidth request
`rate of transmitter 385 through use of the bandwidth request
`signal provided between control processor 350 and modu-
`lator 360, as shown in FIG. 3. In TDMA implementations,
`for example, bandwidth increases as the number of time
`slots allocated in a transmit window to a particular remote
`station 140 increases. Because available spectrum for trans-
`mission is usually limited, a single return channel uplink
`(either 170a or 170b) is often shared between multiple users.
`Thus,
`the transmit window may then be shared between
`multiple remote stations 140, wherein the available time
`slots may be apportioned between the various shared users,
`usually depending on the traffic load, or other priority
`scheme, such as a precedence indicator associated with the
`uplink traffic. Conversely, as the number of time slots used
`in a transmit window decreases, the bandwidth necessary to
`accommodate the decreased number of time slots also
`decreases. As previously discussed in connection with
`TDMA implementations, for example, a decrease in band—
`width of the transmitted signal results in a reduction in the
`average power output of transmitter 385, which is then
`subsequently realized as a reduction in the size of hazard
`zone 250.
`
`In a second aspect of the invention, detection of the
`[0056]
`intrusion of a human body or body parts into a prescribed
`auto-shutoff zone 260 is similarly accomplished by moni-
`toring the received power level of control station downlink
`120b. When the reduction in received power reaches a
`determined threshold level, the system disables transmitter
`385 relatively quickly