Power control system for satellite communications

A system for compensating for varying attenuation of an uplink signal from a local node to a satellite. The system monitors two beacon signals and the local downlink signal to determine fade. An error signal, indicating the uplink fade, is generated and utilized to adjust the gain of the uplink transmitter to compensate for the fade.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to satellite communication systems 
and, more particularly, relates to an apparatus and method of power 
control to compensate for the effects of varying uplink signal 
attenuation, whatever their source. In the following description the 
invention and its operation are presented on the basis of varying uplink 
attenuation due to rain fall. 
2. Description of the Relevant Art 
A satellite communication system generally includes a satellite and several 
ground stations or nodes. The system is a frequency division multiplexed 
system for providing signal paths between various nodes via the satellite. 
A signal path includes an uplink, which is a signal transmitted from one 
node to the satellite at a given frequency in the frequency multiplexed 
communication system, and a downlink, which is the uplink signal frequency 
translated, power amplified and retransmitted by the satellite to the 
nodes in the communication system. The satellite has a transponder on 
board for receiving the uplink signals and transforming these signals into 
the downlink signals. Given a nominal operating point of the satellite 
transponder, variation in output, or downlink, power of a signal are very 
nearly equal to the variation to the input, or uplink, power of that 
signal. Once the nominal operating point has been selected, the downlink 
power of a particular signal is equal to the uplink power of that 
particular signal multiplied by a constant G.sub.s. Thus, the downlink 
power is controlled by the uplink power. The transponder in the satellite 
is characterized by a fixed total power limitation, P.sub.s, on the power 
that may be transmitted into the various downlink signals in the frequency 
multiplexed system. 
A design problem common to the above-described satellite communication 
system is the allocation of the limited power, P.sub.s, of the satellite 
transponder between the various downlink signals. The design goal of the 
satellite communication system is to transmit the maximum possible number 
of downlink signals at any given time. One particular problem in such a 
satellite communication system is the attenuation of the uplink and 
downlink signals due to rain or other atmospheric conditions. A rain 
condition at a given node attenuates the uplink signal transmitted by the 
node and the downlink signals received by the node. 
The attenuation of the downlink signal may cause the power of the downlink 
signal to be decreased to a level where extraction of the information 
contained in the downlink signal is precluded. Thus, the information 
contained within the downlink signal may be lost to the nodes, thereby 
causing a serious disruption in the communication system. To prevent this 
disruption, the nominal operating input/output power conditions at the 
satellite are established to guarantee operations of the downlink up to 
some maximum attenuation level including rain fade. 
On the other hand, the attenuation of the uplink signal from the node 
during a rain condition may also cause the power of the downlink signal to 
the nodes in the system to be decreased, below the minimal downlink power 
level described above. This decrease is due to the nature of the 
transponder on board the satellite. Since the power of the downlink signal 
is a multiple of the power of the uplink signal, the attenuation of the 
uplink signal will decrease the power of the downlink signal. This 
decrease in the power of the downlink signal can result in the inability 
of the various nodes to extract the information carried by the signal from 
the transmitting local node and relayed by the satellite to a receiving 
node. 
Presently, a rain condition at a node frequently shuts down the node since 
the uplink signal cannot penetrate the rain. If the uplink signal power 
was fixed at a level sufficient to burn through the rain then, in the 
absence of rain, the resulting downlink power would consume a large 
fraction of the total transponder transmit power, P.sub.s. 
Accordingly, a system that compensates for atmospheric attenuation while 
allocating power among the various signals in the frequency multiplexed 
satellite communication system to maximize the information handling 
capacity of the system is greatly needed. 
SUMMARY OF THE INVENTION 
The present invention is a system and apparatus for compensating for uplink 
fade at a local node, L1, due to atmospheric attenuation so that the 
limited power of a satellite transponder may be distributed across the 
various signals of a frequency multiplexed satellite communication system 
to optimize the information handling capacity of the system. 
In the present system the power of the uplink signal at a local node, L1, 
is set so that, under clear sky conditions, the power of the retransmitted 
downlink, TP(L12), is sufficient to overcome rain attenuation at nodes 
selected to receive the downlink signal. However, during a rain condition 
at L1, the power level of the uplink signal measured at the satellite 
input will be attenuated by the rain. The downlink power is equal to the 
gain, G.sub.s, of the transponder on board the satellite times the 
received power of the uplink signal. Since the power of the uplink signal 
is decreased, the power of the downlink signal is also decreased and the 
downlink signal will be lost at nodes experiencing excessive rain 
conditions. 
In the present invention, node L1, receives its own downlink signal and it 
also receives a beacon downlink signal. The beacon downlink signal may be 
the downlink signal in a beacon signal path from a beacon node, located at 
a geographic site with a low probability of rainfall, to the local node. 
The received power of uplink signal is essentially constant because of this 
lack of rainfall and, thus, the transmitted power of the beacon downlink 
signal is essentially constant and is used as a reference to measure 
downlink attenuation. The node L1 includes apparatus to measure the 
difference between the long term and short term average power of the 
received beacon downlink signal and of its own downlink signal. 
Logical circuitry at L1 calculates an error signal based on these 
difference measurements. The error signal is a measure of the attenuation 
of the uplink from L1 to the satellite. 
In one embodiment, a control unit is included that utilizes the error 
signal to adjust the gain of the transmitter at L1 to compensate for 
uplink fade. This compensation maintains the received power level of the 
uplink at the satellite at about a constant level thereby maintaining the 
downlink power at a constant level during a rain condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is a system for allocation of available satellite 
downlink power among transmitted signals, in real time, as required to 
compensate for variations in atmospheric attenuation at nodes in the 
satellite communication system. 
FIG. 1 is a depiction of a satellite communication system. Referring now to 
FIG. 1, nodes L1, B1, and B2, are depicted. The satellite communication 
system may have a larger number of nodes than depicted, but the three 
nodes depicted are sufficient to illustrate the operation of the 
invention. Compensation for rain attenuation will be described with 
respect to a local node L1. However, the compensation system described may 
be utilized in all nodes of the system. 
The communication system also includes a satellite 12 with a transponder, 
having a nominal gain, G.sub.s, on board. The signal paths are defined as 
follows. Signal path PB(1) includes uplink signal path PB11 from node B1 
to the satellite 12 and downlink signal path, PB12, from the satellite to 
the local node L1. It is to be understood that the retransmitted downlink 
signals described below will be received at all nodes in the system. 
However, a given node may not have a receiver turned to the particular 
frequency band of the downlink signal. Beacon signal path PB(2) includes 
the uplink signal path PB21 from node B2 to the satellite 12, and the 
downlink signal path, PB22, from the satellite 12, to local node L1. The 
local signal path PL(1) includes the uplink signal path, PL11 from the 
local node L1 to the satellite and the downlink signal path, PL12, from 
the satellite to the local node. The signal in path PB(1) includes a 
signal broadcast in the frequency band F(B1), the signal in path PB(2) is 
broadcast at F(B2), and the signal in path PL(1) is broadcast at F(L1). 
During clear sky conditions the received power, RP(L11), of the local 
uplink signal at the satellite 12 is equal to C11. The nominal gain, 
G.sub.s, of the transponder on board the satellite is set so that the 
downlink power, G.sub.s .multidot.C11, is sufficient to overcome rain 
attenuation at selected ground stations most of the time. However, if 
RP(L11) is decreased due to rain at L1, then the downlink signal power 
will also be decreased. This decrease in downlink signal power can cause 
loss of the downlink signal at nodes experiencing rain. The present 
invention prevents this decrease in downlink power due to rain at L1 by 
increasing the power of L11, TR(L11), to compensate for rain attenuation 
and to maintain RP(L11) near C11. 
Nodes B1 and B2 are located at geographical sites having low probability of 
rainfall and are utilized as beacon sites. The signals from B1 and B2 are 
utilized as beacons. 
The received power of the beacon uplink signal is generally constant 
because the beacon nodes are located in regions of low rainfall. 
Accordingly, the transmitted power of the beacon downlink is constant and 
may be utilized as a reference to determine downlink attenuation. 
The following signal path attenuation conditions are defined for a case 
where rain attenuation is present at the local node L1 but there is no 
rain present at nodes B1 and B2. 
EQU L(1)=L11+dL11+L12+dL12 
EQU B(1)=B11+B12+dL12 
EQU B(2)=B12+B22+dL12 
In the above equations, the term dL11 represents the magnitude of the rain 
produced change in attenuation of the uplink signal attenuation, L11, from 
the local node L1 to the satellite. The quantity dL12 represents the rain 
produced change in attenuation of the downlink signal attenuation, L12, 
from the satellite to the local node L1. Note that the path PB(1) includes 
no uplink attenuation change term because there is no rain at site B1. 
However, the down link attenuation term is set equal to dL12. Within the 
accuracy of the present system, the signal level received in the downlink 
signal path PB12 is set equal to the signal level received in the downlink 
signal path PL12. The equation for the beacon signal path PB(2) is derived 
similarly. 
The power allocation system of the present invention utilizes measurements 
which are indicative of changes in the power of the signals transmitted 
from B1, B2 and L1 and received at the local node L1. In the preferred 
embodiment, these signal power changes are determined by the difference 
between a long term average representing the received powers of the 
signals during non-rain conditions and a short term average, calculated in 
real time, of the power of the signal received at the local site. These 
signal power changes are then utilized to calculate a compensation factor. 
The compensation factor is utilized by a power control loop in the 
transmitter at the local node L1 to boost the power of the transmitted 
uplink signal so that the power, RP(L11), of the uplink signal received at 
the satellite 12 is maintained at a constant level. A constant uplink 
received power level at the satellite maintains a constant transmitted 
power level on the satellite downlink thereby insuring the required 
downlink performance characteristics at all receiving nodes. 
The magnitude of the uplink signal fade, dL11, must be determined to 
compensate for uplink signal fade and maintain the received power, 
RP(L11), of the received uplink signal at the satellite at a constant 
level. The quantity dL11 is calculated as follows, where a quantity having 
a (t) represents the short term or real time value of the quantity and a 
quantity with a (T) indicates the long term average of the quantity. 
EQU L(t)=Lr+Le+dL11+dL12 and 
EQU L(t)=Lr+Le+La 
where 
L(t)=short term signal level measured at the demodulator at L1. 
L(T)=long term signal level measured at the local demodulator. 
Lr=received signal level under clear sky conditions. 
Le=measurement error introduced by local demod. 
dL11=uplink signal attenuation due to local rain. 
dL12=downlink signal attenuation due to local rain. 
La=error in long term signal level due to local rain. 
Note that La is a time varying function dependent on the intensity and 
duration of local rain. 
The signal power change estimate becomes: 
EQU dL(t)=L(t)-L(T)=dL11+dL12-La 
The advantage of using the local long term estimate is made clear here in 
that it eliminates the local demodulator measurement error. It must be 
noted that there is also generated another error, La, as a result of an 
imperfect local estimate. This is discussed further below. 
Using similar notation for the beacon measurements: 
EQU B1(t)=B1r+B1e+dL12 and 
EQU B1(T)=B1r+B1e+B1a 
The beacon signal power change estimate for the first beacon becomes: 
EQU dB1(t)-B1(t)-B1(T)=dL12-B1a 
For the second beacon: 
EQU dB2(t)=B2(t)-B2(T)=dL12-B2a 
Combining the two beacon signals results in: 
EQU dB'(t)=[dB1(t)+dB2(t)]/2=dL12-[B1a+B2a]/2 
EQU dB'(t)=dL12-B'a 
The local signal correction function becomes, 
##EQU1## 
where dB'(t) is the average signal power change estimate for both signals 
and B'a is the average long term signal level of both beacon signals. 
Clearly, dB1(t) may also be utilized to determine the signal correction 
value, dG(t). 
FIG. 2 is a block diagram of an uplink power compensation system at local 
node L1. 
For the sake of simplicity and explanation, standard satellite receiving 
components well known in the art, such as the LNA and down converter are 
not illustrated. The system is divided into a receiver section 20 and a 
transmitter section 22. Turning first to the receiving section 20, a 
receiving antenna 24 receives first, second, and third downlink signals 
traversing downlink paths PL12, PB12, and PB22, respectively. The received 
powers at L1 of these signals are RP(L12), RP(B12), and RP(B22), 
respectively. These downlink signals are routed to the input port of 
frequency demultiplexer 26. The frequency demultiplexer 26 includes output 
ports for the first signal at frequency F(L1), for the second signal 
frequency F(B1), and third signal at frequency F(B2). The first signal is 
directed to the input port of the local fade measuring unit 28, the second 
signal is routed to the input port of the B(1) fade measuring unit 30, and 
the third signal is routed to the input port of the B(2) fade measuring 
unit 32. The output ports of the B(1) fade measuring unit 30 and B(2) fade 
measuring unit 32 are routed to sum inputs of first summing element 34. 
The output of first summing element 34 is coupled to the input port of 
multiplier 36. A difference input port of second summing element 38 is 
coupled to the output of the multiplier 36. The sum input of second 
summing element 38 is coupled to the output port of the L(1) fade 
measuring unit 28. 
Referring now to the transmitter side 22, a control loop 40 has its error 
input coupled to the output of the second summing element 38. The control 
output of the control loop 40 is coupled to a gain adjustment for the 
local uplink (L11) transmitter 42. The output of the transmitter 42 is 
coupled to the transmit antenna 44. 
The circuitry depicted in FIG. 2 performs the mathematical operations 
described above. The output of the L(1) fade measuring unit 28 is equal to 
dL(t), the output of the B(1) signal fade measuring device is equal to 
dB1(t), and the output of the B(2) signal fade measuring unit 32 is equal 
to dB2(t). The output of first summing element 34 is equal to dB1+dB2. The 
multiplier is set to multiply the input by one half so that the output of 
the multiplier is equal to dB'(t). The second summing element 38 forms the 
difference between dL and dB'(t) so that its output is equal to dG. The 
quantity dG is utilized by the control loop 40 to adjust the amplitude of 
the uplink signal, L11, to compensate for attenuation due to rain. 
FIG. 3 is a block diagram depicting exemplary circuitry for the fade 
measuring units 28, 30, or 32. Each fade measuring unit 28, 30, 32 samples 
the automatic gain control (AGC) output of a demodulator 50. The AGC 
values indicate the strength of the signal received at the local node L1. 
The sampled AGC values are directed to the input of a short term average 
calculating unit 52. The quantity calculated by this unit, S(t), is an 
estimate of real time average value of the power of the incoming signal. 
The number of samples required to generate a statistically reliable short 
term average term is discussed below. The output port of the short term 
average calculating unit 52 is coupled to the sum input of summing element 
54 and to the input port of a long term average calculating unit 56. The 
output of the long term average, S(T), calculating unit 56 is coupled to 
the difference input port of the summing element 54. The signal at the 
output port of summing element 54 is an estimate of the attenuation of the 
received signal. 
The operation of the control loop 40 (FIG. 2) will now be described with 
reference to FIGS. 4-7. In FIG. 4, a block diagram of the control loop is 
presented. The error correction signal dG is received by a quantization 
and hysteresis circuit 60. The output of the quantization and hysteresis 
circuit 60 is directed to gain control signal generator 61 which generates 
a quantized gain control signal to adjust the gain of the transmitter 42 
(FIG. 2) to a value equal to a multiple of a fixed power increment Q. 
The function of the quantization and hysteresis circuit will now be 
described with reference to FIG. 5. FIG. 5 is a graph depicting the gain 
change of the transmitter amplifier on the vertical axis and the amplitude 
of the error correction signal, dG, on the horizontal axis. 
The error correction signal is quantized utilizing the function 
EQU INT[dG]=INT[dL(t)-(dB1(t)+dB2(t))/2] 
where the function INT[x] is the integer I where I is less than or equal to 
x and where x is less than or equal to I+1. The function dG is directly 
proportional to the estimated uplink rain attenuation. The function 
INT[dG] is a quantized measure of the estimated rain attenuation and, 
after further processing, is used as the gain control signal. This further 
processing causes the signal INT[dG] to have a hysteresis characteristic. 
FIG. 5 illustrates this hysteresis characteristic. Gain change decision 
boundaries are identified by the numbers 0, 1Q, 2Q, and 3Q on the vertical 
scale where Q is the the increment step size by which the gain control 
will be varied, e.g., Q is equal to 1dB. As the variable INT[dG] increases 
it will follow the path identified by the solid lines joining the 
successive points A, B, C, D, E, F, etc. As it decreases, it will follow 
the path illustrated in part by the successive points E, G, C, H, and A. A 
typical software implementation of the hysteresis function is illustrated 
in FIG. 6. 
This quantization and hysteresis prevents correction of the gain for the 
transmitter in the presence of small correction signal variations. For 
example, consider the point E of FIG. 5. The gain will remain set at 2Q 
for fluctuations of dG between 1dB and 3dB. 
FIG. 7 is a time plot of several power control system functions. The solid 
curved line 70 represents the signal level variation, dG, in real time. 
The corrected transmitted gain is represented by the step function 72. 
Finally, the residual error, i.e., the difference between the actual 
signal level variation and the transmitter gain set is depicted by the 
dotted line 74. The residual error function 74 is bounded by the 
quantization interval Q and -Q. 
The calculation of the short and long term averages illustrated in FIG. 3 
will now be described. An adjustment to the gain of the transmitter is to 
take place at time T.sub.A. Subsequent adjustments are to take place at 
T.sub.A +iT, where the interval T is determined by the characteristics of 
the atmospheric attenuation and the index. In the present embodiment T is 
about ten seconds, which represents an interval over which the uplink fade 
due to rain attenuation should not exceed one or two dB. The short term 
averages of the first, second, and third received signals, RP(L12), 
RP(B12), and RP(B22), are a real time measure of the local received signal 
levels. These short term averages are utilized to identify rain at the 
local site and to quantize the uplink attenuation, dL11. The short term 
averages must be developed over a time interval long enough to provide a 
useful average of reasonable accuracy and short enough to provide 
efficiently fast control loop response to effect power compensation before 
the attenuation increases to a level that causes failure of the node to 
receive the downlink signal. The short term averages, S(t), are generated 
over a thirty second to one minute time interval. The different downlink 
signals are sampled every ten seconds and thus there are six sample 
periods that are generated over a "sliding window" containing the last six 
most recent sample periods. The short term average, S(t), may be 
calculated using a tapped delay line and summation element. 
FIG. 8 is a schematic diagram for implementing either the short term or 
long average calculating units 52 to 56. In FIG. 8 a shift register 80 
includes delay elements 82 D1 through D6. The shift register 80 input 
receives the sampled AGC signals S.sub.i. The delay implemented by each 
delay element is set so that if, at time T.sub.m, the AGC value S.sub.m is 
input to the shift register, then the output of D1 is S.sub.m-1, of D2 is 
S.sub.m-2, of D3 is S.sub.m-3, of D4 is S.sub.m-4, and of D5 is S.sub.m-5. 
The output of each delay element is routed to a summing element 84 through 
a weighting multiplier 86. The output of the summing element 82 is 
directed the input of a normalizing multiplier 88. The output of the 
normalizing multiplier 59 is the time average, S. If the weighting values 
b.sub.i and a.sub.i are all unity, then the output S is a pure average. In 
the present embodiment, the contribution of the most recently received 
signals to the average is increased by adjusting the values of the 
weighting multipliers so that b&gt;a,&gt;a.sub.2, . . . &gt;a.sub.5. The actual 
values of the weighting multipliers and number of delay elements 82 in the 
shift register 80 are design choices and not part of the present 
invention. 
The long term average is the average of a series of short term averages 
taken over a specified time duration. Since the difference between the 
short term average at time T.sub.A and the long term average is utilized 
to correct the uplink fade in real time, the long term average must 
reflect a steady value of the signal power level about which fluctuation 
can be measured. The time period utilized to calculate the long term 
average will depend on the system utilized and weather conditions at the 
local site. For example, the long term errors in local and beacon signal 
levels due to local rain, La and Ba, may be minimized by excising rain 
periods from the local average calculation period. An alternative to 
excising rain data is to make the long term average interval long enough 
so that the degradation of the long term estimate by rain attenuation is 
negligible. 
In a preferred embodiment, the beacons, B1 and B2, are also communication 
nodes in the satellite communication system. Accordingly, no dedicated 
transmitters and receivers are required to establish the reference 
beacons. These beacons are termed pseudobeacons in contrast to a true 
reference beacon broadcast from a special transmitter on the satellite. 
This pseudo-beacon technique allows existing, data-carrying, links to be 
utilized as compensation references. Accordingly, no extra transmission or 
reception equipment is required. The present system processes the AGC 
signal from the local demodulator to control the gain of the local 
transmitter. 
It should be understood, that the power control system of the present 
invention may be implemented using a single earth satellite beacon from a 
nearly ideal climatic location, such as the desert, which would assure 
very infrequent attenuation variations. Clearly, either dB1(t) or dB2(t) 
may be utilized to determine the local signal correction function, dG(t). 
The short term signal averages may be determined every 30 to 60 seconds. 
Thus, the cycle time of the control loop is on the order of a minute. This 
fast cycle time allows compensation to take place before the downlink 
signal power gets too close to the signal loss threshold. This is an 
important advantage. If compensation is attempted too close to threshold, 
the signal may be lost before compensation can be implemented. 
The compensation system of the present invention also functions to 
eliminate system errors due to equipment parameter variations. 
For example, all hardware component characteristics will change in time due 
to aging and temperature. The gain of the uplink equipment, downlink 
equipment and the satellite equipment will change and effect the measured 
signal changes. Use of the pseudo-beacon technique is very advantageous in 
that any changes seen by both the local signal and the beacon signal will 
cancel out in the mathematics of combining the signals in the control 
loop. However, parameter changes which are not shared by the two paths are 
not cancelled. The uncancelled changes are those occurring in both the 
local and beacon link uplink equipment. It is expected that these changes 
can significantly exceed 1 dB. Note, however, that these uplink equipment 
changes will be indistinguishable from rain fade effects and will be 
compensated for by the power control algorithm within its control 
capability and range. 
It will be appreciated that the power control system of the present 
invention does not require a central control, as each earth station has 
its own transmit control. Therefore, in the unfortunate event of the 
failure of the control at any one earth station, the remainder of the 
system would not be adversely affected. 
The present invention has now been described with reference to preferred 
embodiments. Variations and substitutions for elements in these 
embodiments will now be apparent to persons of skill in the art. For 
example, the details of the short term and long term average calculations 
may be varied; the invention can be applied to other systems utilizing 
resource sharing (multiplexing) schemes other than frequency multiplexing, 
and the invention is not limited to satellite communications but can be 
utilized in any system operating in a broadcast mode. Accordingly, the 
scope of the invention is defined by the appended claims.