Abstract:
A method is shown for providing stable communication between a hub earth-station and a VSAT earth-station by regulating the power of signals transmitted via a satellite. 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 parameter 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.

Description:
This application claims the benefit of U.S. 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 network 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 economical operation. 
     Code-division multiple-access (CDMA) is a multiple-access technique that forms the basis for the IS-95 digital cellular telephony standard, and has some important advantages 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. 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 IS-95 standard. Qualcomm&#39;s techniques are designed for terrestrial networks 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&#39; inbound (VSAT-to-hub) transmitted power levels had to be accomplished by manual intervention 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 additional cost of the equipment at the hub is of minor consequence. 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 transmitters 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 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 requirements 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 capability 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 outbound signal power level and a nominal outbound signal power level, when the beacon signal is not received, computing 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 transmission 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: 
     FIG. 1 shows a conventional VSAT satellite-communication network. 
     FIGS. 2A through 2D illustrate the operation of the conventional VSAT satellite communication network of FIG.  1 . 
     FIG. 3 is a more detailed block diagram of the hub and satellite shown in FIG. 1, according to a preferred embodiment 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. 
     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 E b /N 0  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 E b /N 0  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 n th  local receivers  335 ,  340 ,  345 , a hub antenna  360 , a computer  370 , and first through n th  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 . 
     A beacon signal  395  is generated by the beacon transmitter  380  and is transmitted by the satellite antenna  390 . The beacon signal  395  is received by the hub antenna  360 , is amplified by the low-noise amplifier  365 , and is sent to the beacon downconverter  320 . The beacon downconverter converts the beacon signal from a radio-frequency signal to an intermediate-frequency signal that is processed by the beacon 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 process. 
     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 outbound 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                  A   H       A   L       =       k        (       f   H   2       f   L   2       )       =       k        (       f   H       f   L       )       2               (   1   )                                
     where A H  is the attenuation at the higher frequency, where A L  is the attenuation at the lower frequency, where k is a constant between 1.25 and 1.5, where f H  is the higher of the two frequencies, and where f L  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 n th  local receivers  335  through  345  have the ability to measure and report the outbound power level of the outbound downlink 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 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&#39;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:                F   T     =         F   D     +         k        (       f   OU       f   OD       )       2                     F   D         =       (     1   +       k        (       f   OU       f   OD       )       2       )          F   D                 (   2   )                                
     where F T  is the total fade measured by the local receiver, where F D  is the amount of the outbound downlink fade, where k is the constant previously discussed, where f OU  is the outbound uplink frequency, and where f OD  is the frequency of the outbound downlink. 
     Similarly, the total fade as a function of the uplink fade similarly given by the equation:                F   T     =         F   U     +       1   k            (       f   OD       f   OU       )     2                     F   U         =       (     1   +       1   k            (       f   OD       f   OU       )     2         )          F   U                 (   3   )                                
     where F U  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, A U , is simply                A   U     =       F   T       1   +       1   k            (       f   OD       f   OU       )     2                   (   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 E b /N 0  level as an input to their own inbound uplink power control method. Otherwise, when the VSATs  20  see the received E b /N 0  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 downlink 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 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 E b /N 0  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 accomplished 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&#39;s outbound level measurements 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, P nom , a nominal beacon receiver level BR nom , and a nominal local receiver outbound power level LR nom , for clear-sky conditions. The level of P nom  in turn determines a hub IF nominal attenuator setting, A nom . 
     The system then adjusts the power level over each of a set of constant sampling intervals t samp , where t samp  is a system parameter that is preferably set between 1 and 10 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&#39;s outbound power level LR t  and filtering it with an exponential smoothing filter to get a filtered local receiver&#39;s outbound power level LR t ′ (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 t  and filters it with an exponential smoothing filter to get a filtered beacon receivers signal power level BR t ′ (Step  415 ). 
     The hub  10  then compares the current sample beacon receiver&#39;s measurement of fade with the current sample local receiver&#39;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 difference (Step  430 ). If the two differ by more than 1 dB, the system raises an alarm (Step  425 ) to call the operator&#39;s attention to the problem. 
     After this step, the hub  10  computes the beacon receiver difference BR diff  between the nominal beacon receiver level BR nom , and the filtered beacon receiver level BR t ′, i.e., BR diff =BR nom −BR′ t  (Step  435 ). The system then computes the desired attenuation A t  for the outbound uplink signal  210  as a function of the beacon receiver difference BR diff  (Step  440 ). 
     If, however, the system determines in step  410  that a beacon receiver signal is not being received, then the system computes the local receiver outbound power difference LR diff  between the nominal local receiver outbound power level LR nom , and the filtered local receiver&#39;s outbound power level LR t ′, i.e., LR diff =LR nom −LR′ t  (Step  445 ). In this case, the system computes the desired attenuation A t  for the outbound uplink signal  210  as a function of the local receiver outbound power difference LR diff  (Step  450 ). 
     Regardless of the path taken, the hub  10  then determines whether the desired attenuation A t  (however calculated) is within a permissible attenuator range for the hub power control attenuator (Step  460 ). If the desired attenuation A t  is outside of the permissible range, the hub  10  sets the attenuator to the attenuator limit closest to A t  (Step  465 ). 
     Finally, the hub  10  broadcasts the value of the error between the desired A t  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′   t   =β*LR   m +(1−β)* LR′   t-1,   (5) 
       
     
     where LR′ t  is the filter output at sample t, where β is a system parameter that may be set between 0 and 1, where LR m  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′   t   =α*BR   m +(1−α)* BR′   t-1 ,  (6) 
       
     
     where BR′ t  is the filter output at sample t, where α is a system parameter that may be set between 0 and 1, where BR m  is the current sample measured value, and where BR′ t-1  is the previous sample filter output; similarly. 
     In step  440 , the desired attenuation A t  is preferably computed according to the following equation: 
     
       
           A   t   =A   nom   −[BR   diff * 1.2* ( f   OU   /f   OD ) 2 ]  (7) 
       
     
     where A t  is the transmit attenuator setting, using the filtered beacon level measurement, if it is available, where f OU  is the outbound uplink frequency and where f OD  is the beacon downlink frequency. 
     In step  450 , the desired attenuation A t  is preferably computed according to the following equation: 
     
       
           A   t   =A   nom   −[LR   diff /(1+0.83*( f   OD   /f   OU ) 2 )]  (8) 
       
     
     where f OU  is the outbound uplink frequency and where f OD  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 E b /N 0 , as received by the hub  10 , balanced with respect to the received signals from other VSATs  20  in the network. Preferably, the received inbound E b /N 0  (in the absence of multiple-access interference, or MAI) of all the VSATs  20  is kept as close to a target value as possible. This is required in order to keep the inbound block error rates for the VSATs  20  within a nominal operating range. This means that some VSATs  20  may be required to transmit significantly less power (6 dB or so) than they are capable of during clear sky conditions, while others may be required to transmit all the power they can, especially during a rain fade. 
     For TDMA-mode (non-spread) operation, inbound power control is not as critical as it is for CDMA-mode operation, since only one VSAT  20  transmits on a given frequency at a given time (ignoring collisions), and so there is no need to balance the received E b N 0  levels from multiple VSATs  20 . VSATs  20  operating in TDMA-mode networks normally transmit at or near their maximum power level (i.e. at or slightly below P1 dB, the 1 dB compression point of the SSPA) in order to achieve good E b /N 0  margins. However, inbound power control may provide the opportunity to reduce space segment cost in a situation where the inbound link is power-limited (vs. the usual bandwidth-limited situation). Therefore, inbound power control is also preferably implemented in the VSAT network in the TDMA mode. 
     FIG. 5 is a flow chart showing a VSAT inbound uplink power control method according to a preferred embodiment of the present invention. As shown in FIG. 5, the nominal inbound level is initially set during the commissioning of the VSAT  20 , this nominal inbound power level is used as a starting point for determining the actual inbound power level used (Step  505 ). 
     The VSAT then computes the difference between a current measurement of the outbound received E b /N 0  level and a reference E b /N0 level determined during commissioning and adjusts the desired inbound power level accordingly (Step  510 ). If the current outbound E b /N 0  level is lower than the reference E b /N 0  level, then the VSAT  20  must increase the desired inbound power level in accordance with the equation relating the fade at two frequencies (according to equation 1). If the current outbound E b /N 0  level is higher than the reference E b /N 0  level, then the VSAT  20  must decrease the desired inbound power level in accordance with the equation relating the fade at two frequencies. 
     Feedback of the measured inbound received power level of previous successful inbound bursts is then received from the hub  10  and the desired inbound power level is adjusted accordingly (Step  515 ). Since the hub demodulators measurement of power level is most accurate when the desired inbound power level is higher than desired, feedback from the hub  10  that the desired inbound power level must be reduced is preferably acted upon immediately. Feedback from the hub  10  that the desired inbound power level should be increased, however, is preferably filtered so as to be acted upon at a slower rate. 
     The hub  10 , through a satellite-modem processor, will keep track of the aggregate retransmission rate from the VSATs  20  and will periodically broadcast a hub congestion level indication signal (Step  520 ). The VSATs  20  will also determine their own internal VSAT congestion levels by determining their own retransmission rates (Step  525 ). Upon receiving the broadcast hub congestion level signal, a VSAT  20  will compare its internal VSAT congestion level with the hub congestion level (Step  530 ), and based on these two measurements, and any difference between them, will adjust its desired inbound power level accordingly. 
     The VSAT  20  also receives an indication from the hub  10  of any change in the actual outbound power level used as compared to the nominal (clear sky) outbound power level and adjusts the desired inbound power level accordingly (Step  535 ). The VSAT  20  can thus correct the power level if necessary, based on indication by hub  10  of a change in the nominal (clear sky) outbound power level. Any change so indicated is used to adjust the reference outbound E b /N 0  level determined during commissioning so that the VSAT can determine a proper actual outbound E b /N 0  level for the outbound transmission signal. 
     The VSAT  20  also receives an indication from the hub  10  as to whether the outbound uplink power control is out of range and adjusts the desired inbound power level accordingly (Step  540 ). If the hub  10  indicates the outbound uplink power control is out of range, the VSAT  20  will decrease the desired inbound power level by the amount by which the hub indicates the outbound has faded, again, adjusted for frequency. 
     The VSAT  20  then determines whether there is an operating mode change in the inbound data rate (Step  545 ). If so, it adjusts the data rate and signal power level accordingly. 
     The VSAT then determines whether there is a change in the inbound traffic type (Step  550 ). For example, traffic type may change to voice traffic from data traffic. Some changes in traffic type will bring with them a corresponding need to change the data rate and an associated adjustment to the desired power level 
     The VSAT  20  then determines whether the result of steps  545  and  550  will cause the desired level of the inbound transmission signal to rise above the level at which the VSAT  20  can transmit without exceeding its P1 dB level (Step  555 ). If so, the VSAT  20  acts to reduce the data rate of the inbound signal by a factor of two and, if operating in CDMA mode, to increase the spreading factor by a factor of two to get 3 dB more E b /N 0  (Step  560 ). This reduction of the data rate can only be done a limited amount of times, depending upon the design of the VSAT satellite-communication network system. 
     The VSAT  20  then compares the desired inbound power level with a maximum inbound power level at which the VSAT  20  can transmit (Step  565 ). If the desired inbound power level exceeds the maximum inbound power level by more than a predetermined amount, then the desired inbound power level is reset to the maximum power level (Step  570 ). 
     Finally, the VSAT  20  transmits the inbound uplink signal to the satellite  30  using the desired inbound power level as an actual inbound power level (Step  575 ). 
     A more detailed description of these individual steps for the preferred embodiment of the present invention is shown as follows. 
     As shown above in step  505 , the nominal inbound power level that the VSAT  20  will transmit is initially set during an installation or commissioning procedure. During this procedure, the hub  10  will command the VSAT  20  to transmit a continuous wave (CW) signal. The hub  10  will then command the VSAT  20  to increase its power level in small steps from a minimum until the hub determines that the VSAT solid-state power amplifier (SSPA) has gone one dB into compression, i.e, until it reaches the P1 dB level. The P1 dB level is the point at which the output power level of the SSPA falls one decibel below the expected output power level based on the gain of the SSPA in a linear operation mode. The P1 dB level is determined by increasing the input power to the SSPA in 1 dB steps from a point at which the SSPA is known to operate in a linear mode, and noting the point at which the output power level reaches a level one decibel below the expected output level. This final output level is called the P1 dB level. 
     The hub measures the inbound carrier level using a spectrum analyzer (or other suitable receiver). Upon completion of this measurement process, the hub  10  will tell the VSAT  20  what nominal inbound power level to use and at what level the SSPA went into compression. This process can be performed manually or automatically. 
     The inbound link power adjustment process should be avoided if the hub  10  is experiencing a rain fade that exceeds the ability of the hub uplink power control mechanism to overcome. This situation should be relatively brief in duration. The VSAT  20  also stores the received downlink outbound E b /N 0  level that it measures during the inbound power adjustment process, as well as the hubs indication of its outbound power level (Step  530 ). This will allow the VSAT  20  to compensate for a satellite-to-VSAT rain fade that may have been in progress during the commissioning process. 
     As shown in step  510 , the VSAT  20  receives outbound E b /N 0  values, compares a current measurement of the E b /N 0  value of the outbound received signal with a reference E b /N 0  value determined during commissioning. The VSAT  20  then adjusts the desired inbound power level accordingly. In this procedure, changes in the received outbound E b /N 0  level are measured by the VSAT RF modem demodulator circuits, and are available immediately and on a continuous basis (assuming the VSAT is locked to the downlink outbound signal). The VSAT RF modem&#39;s measurement of the outbound E b /N 0  will have an accuracy that is dependent on the actual E b /N 0  level—the higher the actual E b /N 0  , the more accurate the measurement will be. 
     In the preferred embodiment, the outbound E b /N 0  measurements will be updated by the VSAT RF modem at about a 1 Hz rate. These measurements should preferably be sampled (read) at constant intervals, preferably in the range of one to ten seconds (set as a system parameter) and then filtered by a exponential smoothing filter, preferably of the form: 
     
       
         ( E   b   /N   0 )′ t =κ*( E   b   /N   0 ) m+( 1−κ)*( E   b   /N   0 )′ t-1   (9) 
       
     
     where (E b /N 0 )′ t  is the filter output at sample t, κ is a system parameter that may be set between 0 and 1, (E b /N 0 ) m  is the current sample measured value, and (E b /N 0 )′ t-1  is the previous sample filter output. 
     As shown in step  515 , changes in the VSAT&#39;s received inbound power level are measured by the hub demodulator  305 , and are only available following a successful burst transmission from the VSAT  20 , and are made available to the VSAT  20  only after hub processing and queuing delays and the propagation delay from the hub  10  back to the VSAT  20 . The accuracy of the hub demodulator&#39;s measurement of the inbound burst power is a function of the actual power during the burst, the number of interfering signals, the spreading ratio if CDMA is used, and other factors. The higher the actual inbound burst power, the lower the number of interfering signals, and the higher the spreading ratio, the more accurate the hub demodulators measurement will be. 
     The inbound burst power has to be measured by each hub demodulator  305  in the system. The IF signal distribution mechanism may introduce small (±1 dB) differences in the IF signal levels reaching each hub demodulator  305  in the system. Therefore, there is a mechanism for calibrating the hub demodulators  305  so that when they are given an IF signal, they will all measure the same power level for that signal. One possible mechanism is to periodically transmit a burst from a local test transmitter in a reserved time slot, and to configure all the hub demodulators  305  that can receive that burst to measure and report the power level of the burst. 
     Also, since the system measures inbound power level, and since this power level will be a function of the hub-to-satellite rain fade, the inbound power level measured by the hub demodulators needs to be corrected to remove errors introduced by this rain fade. The measurement of inbound rain fade is normally provided by the beacon receiver  310  or the hub local receiver unit  315  through  325 . Correction for the frequency difference between the inbound frequency and the beacon frequency or outbound frequency can be accomplished as previously described. 
     Each hub satellite-modem processor preferably maintains estimates of inbound power both for the whole population of VSATs managed by it and for each individual VSAT  20 . These estimates will be based on each and every measurement. Exponential smoothing is preferably used to filter the results. Note that different smoothing parameters are appropriate for an individual VSAT  20  versus all of them together. That is, there are two different smoothing parameters involved: one for use in filtering the measurements for individual VSATs,  20  and one for all of the VSATs  20  combined. 
     For each packet received, the hub satellite-modem processor updates the aggregate estimate for inbound power level using the aggregate smoothing parameter, and it updates the inbound power level estimate for the VSAT  20  involved using the smoothing parameter for filtering individual VSAT&#39;s  20  inbound power level. When the estimated inbound power level for a VSAT  20  rises or drops below the estimate for all the VSATs  20  by a configurable amount (about one dB), the hub satellite-modem processor sends a message to the VSAT  20  involved instructing it to decrease or increase its power level by a configurable amount. 
     If the feedback from the hub  10  indicates that the VSAT&#39;s power level should be increased, it should be filtered so that it takes effect at a slower rate. An appropriate and preferred filter is of the form: 
       P′   t   =λ*P   m +(1−λ)* P′   t-1   (10) 
     where P′ t  is the next power level to transmit, P m  is unfiltered estimate of the correct power to transmit, λ is a TBD constant (0&lt;λ&lt;1.0), and P′ t-1  is the previous filter output. 
     As shown in steps  520  and  525 , the inbound link is designed to experience some block error rate, during peak traffic conditions, assuming the E b /N 0  is at the nominal level. If the E b /N 0  drops, the block error rate will increase. If an inbound burst suffers an error, the VSAT  20  will have to retransmit the burst, or a portion thereof. The VSAT  20  will be able to determine its own retransmission rate very easily. However, an inbound burst can suffer an error due to a collision as well as due to a low signal level. Therefore, the VSAT  20  must keep two retransmission statistics—one for random Aloha (contention) transmissions, and one for reservation mode (non-contention) transmissions. The hub will transmit a contention level indication. If, in Aloha mode, the hub  10  is indicating a congestion level exists when the VSAT  20  is having to retransmit frequently, the rate at which the VSAT  20  increases its power should be reduced. If the VSAT  20  is having to retransmit frequently for bursts sent during reservation slices or when the hub  10  is indicating very low levels of congestion, the rate at which the VSAT  20  increases its power should be increased. 
     There will be several (typically eight, and possibly as many as 16) different “congestion levels” that designate bands of Aloha retransmission rates (as measured by the hub satellite-modem processor) that are specified by an array of retransmission rate boundaries between 0.0 and 1.0. Normally, these boundaries will be roughly evenly spaced. 
     The actual retransmission rate measurements should be done both by the VSATs  20  and by the hub satellite-modem processor. In the preferred embodiment, the hub satellite-modem processor only measures the aggregate retransmission rate, not individual retransmission rates for each VSAT  20 . This is because the VSATs  20  can measure their retransmission rates more accurately than the hub satellite-modem processor can. 
     Instead of measuring retransmission rate directly, the preferred embodiment of the present invention measures the average number of transmission attempts per successful transmission. Then it uses exponential smoothing (with different configurable parameters for the VSAT  20  measurements and the hub satellite-modem processor measurements) to filter the series of measured values. 
     Basically, the hub satellite-modem processor sends a broadcast message to all its VSATs  20  declaring the new congestion level whenever its filtered estimate of the retransmission rate crosses a boundary from one level to the next. Such broadcasts should be suppressed if they occur within a configurable minimum interval since the most recent congestion level broadcast messages. A maximum congestion level change should also be imposed to prevent, for example, jumping from congestion level two to level seven. 
     As shown in step  530 , the VSAT&#39;s  20  measurements of their own “congestion levels” are compared to the congestion levels broadcast by the hub satellite-modem processor to serve as the basis for adjustments to the VSAT uplink power level. Whenever a VSAT  20  estimates that its retransmission rate is in a band that is more than one congestion level different from the latest broadcast congestion level, the VSAT  20  should increase or decrease its power by a configurable amount. The amount of these power adjustments would normally be appropriate for changing the VSATs retransmission rate to the middle of the congestion level that&#39;s the nearest neighbor of the broadcast congestion level. That is, we do not want to fully compensate for the VSAT&#39;s retransmission rate variance from the aggregate retransmission rate, only to adjust its power to get close to the average retransmission rate for all of the VSATs  20 . Also, VSATs  20  should not adjust their power levels by more than a configurable amount, and they should not adjust their power levels more often than a configurable time interval. 
     In step  530 , exponential smoothing is preferably employed both by the hub satellite-modem processor on the aggregate congestion level and by the VSAT on its own retransmission rate. 
     If practical, the VSAT power level should be increased above the level for initial attempts for all retransmissions. These power adjustments should be a small configurable amount, possibly about 0.5 dB. 
     As shown in step  535 , the hub  10  will broadcast the nominal power level at which it is transmitting the outbound carrier on a periodic (and frequent) basis. If the nominal hub outbound power level is increased or decreased either for availability reasons or due to an outbound link rate change, this allows the VSAT  20  to determine whether a change in the outbound E b /N 0  is due to a rain fade or due to a change in the nominal level the hub  10  is transmitting. For example, if the hub outbound power level is intentionally decreased, the VSATs  20  will see a reduction in the outbound E b /N 0 , which would otherwise indicate a rain fade is in process. This would cause all of the VSATs  20  to increase their inbound levels to compensate for the perceived rain fade. But if the hub  10  broadcasts the power level setting, the VSATs can determine that the change in E b /N 0  is due to an intentional outbound level change, and not due to a rain fade. 
     As shown in Step  540 , when the hub  10  is experiencing a hub-to-satellite rain fade, it will increase its power output to compensate for the rain fade. If the rain fade is very severe, the power increase required to overcome it may exceed the capability of the hub  10  to increase its power. In this case, the outbound E b /N 0  level received by the VSAT  20  will drop even when there is no rain fade between the satellite  30  and the VSAT  20 . The hub shall therefore broadcast an indication of the estimated amount of reduction of downlink outbound signal power as transmitted by the satellite  30 . This will allow the VSATs  20  to determine that the resultant reduction in received outbound E b /N 0  is due to an outbound power control error, and not due to a rain fade at the VSAT  20 . 
     As shown in steps  545  through  560 , there are two operating mode changes that may require the VSAT  20  to change its inbound power level: an inbound data rate change, and an inbound traffic type change. 
     Normally, if the VSAT decreases or increases its data rate by a factor of two or four, it should decrease or increase its power level by a factor of 3 or 6 dB, respectively. However, in CDMA mode, during a severe satellite-to-VSAT rain fade (for example), the VSAT could reduce its inbound data rate by a factor of two, while increasing its spreading factor by two and maintaining its inbound power level the same, to effect a 3 dB increase in its transmitted E b /N 0 , while also increasing its signals processing gain by two and maintaining a constant inbound bandwidth. In the preferred embodiment, this can be done two times for a maximum E b /N 0  increase of 6 dB in the transmitted E b /N 0  and a maximum increase in the signal processing gain of 4. 
     The inbound block error rate that corresponds to the received inbound E b /N 0  operational setpoint may be at a level that is acceptable for routine data traffic, but is unacceptable for voice traffic, which typically requires a low block error rate without retransmissions for useful communications. In a network carrying mainly data traffic with occasional voice traffic, the inbound data rate for data traffic might be 32, 64, or 128 kbps (nominally). However, for voice traffic alone, 16 kbps will suffice. This makes it possible to reduce the data rate for a VSAT  20  handling voice-type traffic by a factor of two or four. If the VSAT inbound channel is operating in CDMA mode, the VSAT  20  can increase its spreading factor in inverse proportion to keep the chipping rate and hence the bandwidth of its inbound signal the same. If the inbound power level stays the same, the E b /N 0  of the VSAT  20  with the reduced data rate will increase according to the equation:              10                   log        (       R   D       R   V       )               (   11   )                                
     where R D  is the inbound data rate of the data traffic VSATs  20 , and R V  is the inbound data rate of the voice traffic VSAT(s)  20 . 
     As shown in Steps  565  and  570 , the VSAT system has a maximum allowed inbound power level. This maximum level is measured by the hub  10  and recorded by the VSAT  20  during installation and commissioning. If it is exceeded, the VSAT inbound spectrum will begin to exhibit unacceptable sidelobes.