OPEN-LOOP UP-LINK POWER CONTROL USING NOISE INJECTION RADIOMETRY

Abstract: A noise injection radiometer (410) operating at the down-link frequency is implemented to estimate fading present on the up-link signal. The up-link fade is then compensated for by increasing the power transmitted (250) by the earth station based on the up-link fade estimation (230). In addition, most of the radiometer circuitry is contained within a carefully controlled temperature environment (40) to compensate for temperature variation at the earth station receiver that can affect the accuracy of the determination of the fade estimation. According to an exemplary embodiment of the invention the noise injection radiometer (410) shares most of the RF electronics present at the earth station receiver and therefore can be implemented without significantly increasing the cost or complexity of the overall system.

Description: OPEN-LOOP UP-LINK POWER CONTROL USING NOISE INJECTION RADIOMETRY FIELD OF THE INVENTION The present invention generally relates to satellite communications and in particular to improved communication resistant to signal fading thereby improving satellite availability. BACKGROUND OF THE INVENTION Signal fading can seriously impair satellite communications. Fading is the attenuation of the signal due to physical phenomena, such as rain, clouds, and gasses in the troposphere that hinder satellite communications by diminishing the power of signals transmitted between ground stations and satellites. The amount of these various phenomena serves to limit the availability of satellite links operating at frequencies above 10 GHz. Conventional systems attempt to compensate for fading through the use of power control of the power amplifier at transmitting earth stations. The communication path between a transmitting earth terminal and a satellite is referred to as an up-link and the reverse path is know as the down-link. Under certain weather conditions, a rainstorm for example, a communications signal can become severely attenuated causing loss of the link or loss of information. Individuals having a home satellite dish for receiving video/audio signals have on occasion experienced this problem on the down-link from a satellite, where ordinarily signal fading is simply tolerated. On the up-link, however, something can be done to overcome the problem. Transmitting earth stations often attempt to support communication links under faded conditions by providing a high power amplifier (HP A) at the earth station that has sufficient excess power available to support clear up-link communications even under faded conditions. By adjusting the HPA output, the power of the transmitted signal may be boosted to ensure that a sufficient amount of the signal is received at the satellite to maintain a communication link between the satellite and the earth station. However, implementation of power control of the amplifier requires an estimation of fade experienced on the up-link in order to determine how much power is necessary for the prevailing condition on the communications link. In turn, the accuracy of the fade estimation has a direct bearing on the satellite network operation. As a result, if communications are to be sustained, then the accuracy of the estimated signal fade must be maintained within certain limits. For example, if the fade is overestimated interference with respect to other carriers operating on the same frequency may result. In extreme cases overestimation may result in a power level that will cause harm to the satellite transponder. On the other hand, if the fade is underestimated, the correction is not effective and loss of the satellite link may result. Two techniques, closed-loop and open-loop, are conventionally used to derive fade estimation. Closed-loop estimation is inherently superior to open loop estimation because it relies on a direct feedback of the result of a control activity. In a closed loop system, feedback from the receiving end on the level being transmitted is available, and therefore corrective action can be taken to stay within a pre-assigned range of error. However, closed loop implementation requires either that the transmitting station can receive its own transponded signal or that the transmitting earth station and receiving earth station must be in the same satellite antenna beam, which often is not the case with international satellite links. Moreover, for the receiving earth station to send a feedback signal, the satellite network must be individually modified or designed to support this feature. Accordingly, many existing networks will not have this feature due to the portion of available satellite capacity that must be assigned to send the feedback signal. A further consideration related to the cost and complexity of a closed loop feedback system is its dependence on the satellite network architecture. For example, due to differences in geography, climate and equipment, it is not generally possible to construct a universal closed-loop implementation that will work with every possible satellite network architecture. As a result, a particular closed loop system must be individually designed or at least customized for a particular network. This effort can be costly and is not efficient. On the other hand, open-loop systems can be used to implement power control without regard to network architecture; however, they suffer from reduced accuracy. In open-loop systems, no feedback is available and the system must rely on the accuracy of its own estimate of the fade level. On the other hand, while open-loop systems can be used to implement power control without regard to network architecture, they suffer from reduced control accuracy. According to the open-loop control method, measurement of a parameter that is not related to the up-link signal transmitted from the ground station is used to estimate the up-link fade. Parameters that may be used for power control estimation of the up-link fade include the carrier to noise ratio (CNR) of the down-link signal, bit error ratio (BER) of the down-link signal, amplitude of a narrow band beacon signal, and sky noise measured by a radiometer. Each of these parameters has its own limitation in terms of the measurement accuracy and its ability to provide an unambiguous measure of the uplink fade. In applications where the power control range is limited to values of less than lOdB, measurement of the sky noise at the down-link frequency can be used to determine an estimation of the signal attenuation on the up-link. Every object emits noise known as black body radiation. Sky noise is the radiation emitted by gases and precipitation. Sky noise can be measured through the use of a radiometer. After measuring the sky noise on the down-link, the measurement is converted to an estimation of path attenuation that is suffered by a radio signal on the up link, using the black body radiation principle. However, the accuracy of the conversion to a path attenuation degrades with an increase in the amount of attenuation on the signal path, although good accuracy can be obtained for the attenuation levels up to about 6dB for frequencies that are less than 20 GHz. For example, in Ku and Ka-band systems, an attenuation level of 6dB on the down-link translates to approximately 10 dB on the up-link, satisfying the limited power control range mentioned above. Earth stations operating with a relatively small power control range tend to be lower in cost. As a result, the addition of a radiometer to improve power control accuracy must not significantly add to the overall system cost. Most conventional open-loop power control systems fall into two categories: slow reacting simple control systems and fast reacting controllers that are very expensive. The slow reacting control systems tend to be less accurate by virtue of the fact that they are not able to adequately track changing fade conditions. On the other hand, while fast reacting systems have better accuracy, such controllers are normally based on the measurement of a pilot or beacon signal transmitted from the satellite, which requires the use of a dedicated receiver, and in some cases a separate down- converter. These extra requirements lead to higher overall system cost and increased complexity. SUMMARY It is therefore an object of the invention to provide improved satellite communication, particularly under conditions that cause signal fade. It is another object of the invention to provide a system for increasing availability of satellite links that suffer from attenuation phenomena, such as rain fades and other absorptive impairments present in the troposphere. It is a further object of the invention to provide improved estimation of signal fading without significantly increasing system cost or complexity. According to an exemplary embodiment of the present invention, the foregoing and other objects are accomplished through the use of a noise injection radiometer operating at the down-link frequency to estimate fading present on the uplink signal. Increasing the power transmitted by the earth station based on the up-link fade estimation then compensates the up-link fade. In addition, the radiometer circuitry is contained within a carefully controlled temperature environment to compensate for temperature variation at the earth station receiver that can affect the accuracy of the fade estimation. According to an exemplary embodiment of the invention the signal injection radiometer shares most of the RF electronics present at the earth station receiver and, therefore, can be implemented without significantly increasing the cost or complexity of the overall system. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other features, objects, and advantages of the invention will be better understood by reading the following description in conjunction with the drawings, in which: FIG. 1 shows an exemplary block diagram of a noise injection radiometer according to one embodiment of the invention; and FIG. 2 shows a flow chart for implementing noise injection and power adjustment according to an exemplary method of the present invention. DETAILED DESCRIPTION OF THE INVENTION The various features of the invention will now be described with respect to the figures, in which like parts are identified with the same reference characters. Turning to Fig. 1, an exemplary embodiment of the invention showing operation of an open-loop power control system based on noise injection radiometry will be described in detail. An antenna 10 is provided at the earth station for receiving and transmitting signals between the earth station to a satellite (not shown). Connected to the antenna is a solid state power amplifier 20. The power amplifier receives signals on communications line 21 that are to be transmitted to the satellite via the antenna 10 and amplifies the signals and outputs the transmission signal 22 at sufficient power to be received by the satellite on the up-link communication path. The power amplifier 20 is adjustable and can be used to vary the power level of the signal 22 that is transmitted to the satellite. As pointed out above in the background of the invention, precipitation and various gases found in the troposphere can absorb or attenuate signals transmitted from an earth station to the satellite, and vice versa, i.e., causes fading. A goal of the present invention is to maintain the power level received at the satellite at a constant level irrespective of what interference or fading is encountered along the path to the satellite. In order to provide a sufficient power transmission level, and therefore maintain the satellite link, the power amplifier must be adjusted so that the power level of the transmitted signal compensates for the conditions encountered along the communication path. Fading along the communication path may be compensated for by increasing the power level of the power amplifier 20 during conditions causing fading and decreasing the power level during clearer conditions. However, the power level used should only be high enough to maintain the communications link with the satellite in order to avoid excessive interference with respect to other communications channels passing through the satellite and to protect against damage to the satellite transponder. According to the present invention, a radiometer controller 50 controls the transmission power level of the power amplifier 20. The radiometer controller 50 issues commands to the power amplifier 20 to adjust the transmission power level via connection 51. The radiometer controller 50 may be implemented with a microprocessor, microcomputer, DSP, or ASIC, for example. A low noise amplifier (LNA) 30 is connected to the antenna 10. In addition, a radiometer 410, comprising an electronic subsystem, is provided within temperature controlled enclosure 40. The radiometer 410 measures the noise level coming from the direction of the satellite. The radiometer 410 can be implemented using a stable noise source 455 to compensate for the gain variations in the LNA 30, in the preferred embodiment. The radiometer 410 is coupled to the LNA 30 via a pair of internal switches 401 and 402, and external couplers 31 and 32. As a result of this arrangement, the radiometer 410 shares the antenna 10 and LNA 30 of the earth station and does not require a dedicated radio frequency receiver that is required by some conventional radioactive systems. Consequently, the cost and complexity of the system according to the present invention are minimized. The radiometer 410 monitors the sky noise that affects the performance of the communication down-link. Because sky noise varies with the amount of fading on the down link, and the down link fading is highly correlated with the up link fading, the sky noise measurement can be used to estimate the level of the fading or attenuation encountered on the up-link communication path. According to the exemplary embodiments of the present invention, the accuracy of the sky noise measured by the radiometer 410 is affected by the gain and noise temperature variations found in the LNA 30. For example, when temperature is constant the amount of black body radiation emanating from an object is constant and thus noise introduced is constant. However, when the temperature changes, the noise due to the change in temperature may increase or decrease; similarly the LNA gain. Therefore, in order to provide an accurate estimate of the sky noise on the communications down-link, it is important, according to the present invention, to eliminate gain and noise temperature variations of the LNA 30 so that an accurate estimate of fading on the up-link can be derived. In order to eliminate these variations, an RF noise signal produced from the noise source 455 is periodically injected at the input of the LNA 30 through use of an RF switch 402 and waveguide coupler 32 along a first signal path. The waveguide coupler 32 is used to inject the signal into the LNA. Since the injected noise signal is highly stable, any variation in the level detected by the power detector 445 can be attributed to gain changes in the receiving chain. A second waveguide coupler 31 detects the RF output of the LNA 30 and the signal is connected to the radiometer components along a second path by switch 401. An RF source 460 in the radiometer 410 is used to down convert the detected radio frequency signals using the mixer 430. According to a preferred embodiment, the RF source 460 may be implemented using a low cost Gunn oscillator, however, one skilled in the art will appreciate that other sources could be used without departing from the invention. The noise signal is input to a band pass filter 450 before being injected into the LNA 30 via switch 402, attenuator 420, and coupler 32. A signal outside the frequency band of the down-link is used and adequate filtering 450 is provided to minimize any potential interference with the communication signal received on the down-link. One skilled in the art will appreciate that the desired accuracy of the signal will determine the switch rate for switches 401 and 402, which work in tandem, and the rate at which the injected signal is used. Higher rate means greater accuracy. The radiometer 410 includes a variable attenuator 415, which is connected between switches 401 and 402, and is responsive to an output of controller 50 to calibrate the opening point of the radiometer. For this purpose, the output of the RF noise source 455 is periodically provided to the variable attenuator 415, and its output switched to a second path comprising a mixer 430, low pass filter 440 and detector 445. The sky noise temperature, T S (K), may be obtained by sampling the output voltage of the power detector 445. The output voltage from the detector 445 is processed by the radiometer controller 50 to remove receiver gain and the system noise due to changes in temperature of the LNA 30. Since the output of the noise source 455 is maintained at a constant level, the detected noise level along the LNA 30 path and the variable attenuator 415 path can be used to eliminate gain variations in the LNA 30. The detector 445 output is expressed as: V = G (T sys + T s ) where T sys =system temperature including antenna noise (K) G = receiver gain In a direct noise power measurement, the receiver comprising the antenna 10, LNA 30, and a power detector 445 are used to monitor the noise level in the direction of the antenna 10. The minimum incremental change in noise due to variation in temperature, that may be measured by a direct or total power radiometer, can be expressed by the following relationship: ΔT=(T S + T sys ) sqrt(l/Bτ + (ΔG/G) 2 ) where: AT = the minimum detectable change in input temperature (K) B = the detection bandwidth (Hz) τ = the detector's integration time (sec) ΔG = the operational gain fluctuations in the receiver. Typically, the bandwidth of the radiometer 410 is of the order of 40 MHz and is set by the low pass filter 440, based on the down converted output of the LNA 30. An integration period on the order of several seconds can provide adequate measurement accuracy; however, one skilled in the art having read this disclosure will appreciate that the integration time can be selected to provide the desired accuracy in any particular implementation. In addition to noise caused by temperature variations of the LNA 30, the effect of the gain variation of the LNA 30 also has a significant effect on measurement accuracy. However, the effect of the gain variation of the LNA 30 is drastically reduced by the exemplary switching arrangement shown in Fig. 1, based on the previous discussion on gain correction. Furthermore, according to another feature of the present invention, all of the radiometer electronics, except the radiometer controller 50, are housed inside a temperature controlled enclosure 40 in order to further reduce measurement errors. A temperature gauge outside the enclosure, (not shown in Fig. 1 ), is used for measuring the ambient temperature that is required for the estimation of the effective medium temperature involved in the conversion of sky noise to attenuation and the estimation of gaseous absorption as explained in further detail below. The enclosure 40 can be constructed in aluminum or copper and the temperature stabilization carried out using Peltier-effect devices 465 mounted outside the enclosure. The Peltier-effect devices can be used to both generate and remove heat using DC power. The DC current through the device is controlled via line 54 by the radiometer controller 50. The power control scheme is further illustrated in the flow chart shown in FIG. 2. Turning to FIG. 2, following a start at step 200, the sky noise on the down-link that is measured by the radiometer is determined as explained above in step 202. In addition, the ambient temperature of the temperature gauge(s) is also determined in step 201. At step 210, the sky noise measured by the radiometer is converted to an estimated attenuation level suffered by the down-link signal using the relationship: Ad = 10 1og (T (T m -T s ))(dB) where T m (K) is the physical medium temperature set equal to T a (l - α) (K), where T a is the ambient temperature, and α is a correction factor at step 210. The estimated attenuation level is compared to a power control threshold, according to this exemplary embodiment, such that the power controller reacts more efficiently to rainy conditions. The threshold magnitude may be selected on the basis of a signal frequency band and may be within a range extending from values below 1 dB to several dB. For example, a threshold of IdB at Ku-band or 2 dB at Ka-band can be used according to this exemplary embodiment. The threshold level is determined by the fading level due to atmospheric gases and light to moderate cloud cover, and the values cited above are typical for the two frequency bands. It is then determined if the estimated fade exceeds the threshold at step 224. If the estimated fade remains below the threshold, then the process of monitoring the down-link sky noise is repeated and ambient temperature determination is again started at step 200. On the other hand, if the estimated fade exceeds the threshold, the estimation is then performed on the basis of a separate gaseous absorption component at step 220 and a rain attenuation component at 222. The gaseous absorption is calculated using the ambient temperature for an atmosphere saturated with water vapor (e.g., 100% relative humidity). The gaseous absorption at the up and down-link frequencies, A ug and A dg , are estimated using well known models for oxygen and water vapor absorption (see ITU Recommendation RPN.618, ITU, Geneva, Switzerland, 1999). The down-link rain attenuation may be expressed as: A_ r — A - A The up-link rain attenuation can be estimated using the following frequency scaling law: A ur = A d , (f u /fd) 2 where f u and f_ are the up- and down-link frequencies, respectively. After determining the rain attenuation and gaseous absorption, the up-link attenuation is determined at step 230. Rain attenuation scales approximately according to the frequency squared; as an example a 2 dB attenuation at 11 GHz scales to 2 (14.0/11.0) — 3.2 dB at 14 GHz. Scaling of gaseous attenuation must be carried out using the formulas given in ITU Recommendation RPN.618, ITU, Geneva, Switzerland, 1999. After the up-link attenuation is determined the power adjustment to be fed to the SSPA is set at step 250. The process is then repeated. According to the exemplary embodiment shown in FIG. 1 of the invention, the radiometer controller 50 of the power control system carries out the above-described computations in order to determine an estimate for the up-link fade. It is noted, for example, that sky noise can result from the positioning of the sun and moon. For example, when the sun is directly behind a satellite (i.e., along the communication path) interference with the communication signal on the up-link may result. However, these events can be predicted with great accuracy, unlike bad weather, and compensated for. Therefore, the calculations described above make allowance for predictable interference from the sun and the moon. The estimated up-link fade is then used to control the output power of the HPA 20 by setting a power level through an attenuator (not shown) at the input or output stage of the up-converter at step 250. The attenuator is adjusted in 1:1 proportion to the estimated fade; e.g. if the estimated fade is 3 dB, then the value of the attenuator is reduced by 3 dB from its nominal setting. The present invention has been described by way of example, and modifications and variations of the exemplary embodiments will suggest themselves to skilled artisans in this field without departing from the spirit of the invention. The preferred embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is to be measured by the appended claims, rather than the preceding description, and all variations and equivalents that fall within the range of the claims are intended to be embraced therein.

Claims: WHAT IS CLAIMED IS: 1. A method of improving RF communications comprising the steps of: determining sky noise on a down-link communication path; determining down-link attenuation based on said sky noise; determining if said down-link attenuation is above a threshold; estimating up-link attenuation on said communication path if said threshold is exceeded; and determining a power adjustment for a power amplifier based on said up-link attenuation. 2. The method of claim 1 further comprising the step of adjusting a power amplifier according to said determined power adjustment. 3. The method of claim 1 further comprising the steps of: measuring ambient temperature of an earth station receiving said down-link, wherein said step of determining down-link attenuation is also based on said measured ambient temperature. 4. The method of claim 1, wherein said step of determining up-link attenuation further comprises the steps of: determining a rain attenuation on said down-link; and determining said up-link attenuation based on said determined rain attenuation. 5. The method of claim 3 further comprising the step of adjusting a power amplifier according to said determined power adjustment. 6. The method of claim 1, wherein said step of determining up-link attenuation further comprises the steps of: determining a gaseous absorption on said down-link; and determining said up-link attenuation based on said determined gaseous absorption. 7. The method of claim 1, wherein said step of determining up-link attenuation further comprises the steps of: determining a rain attenuation on said down-link; determining a gaseous absorption on said down-link; and determining said up-link attenuation based on said determined rain attenuation and said determined gaseous absorption. 8. The method of claim 6 further comprising the step of adjusting a power amplifier according to said determined power adjustment. 9. The method of claim 3, wherein said step of determining up-link attenuation further comprises the steps of: determining a rain attenuation on said down-link; and determining said up-link attenuation based on said determined rain attenuation. 10. The method of claim 5, wherein said step of determining up-link attenuation further comprises the steps of: determining a gaseous absorption on said down-link; and determining said up-link attenuation based on said determined gaseous absorption. 11. The method of claim 5, wherein said step of determining up-link attenuation further comprises the steps of: determining a rain attenuation on said down-link; determining a gaseous absorption on said down-link; and determining said up-link attenuation based on said determined rain attenuation and said determined gaseous absorption. 12. The method of claim 10 further comprising the step of adjusting a power amplifier according to said determined power adjustment. 13. A terminal for use at an earth station in a satellite communication system including a low noise amplifier (LNA), power amplifier, and antenna for receiving and broadcasting data on an up-link and down-link communication path between the earth station and a satellite, said system comprising: a noise injection radiometer coupled to the LNA and operative to measure sky noise on the down-link; and a radiometer controller operative to monitor the radiometer and adjust the power amplifier based on said measured sky noise. 14. The system of claim 13 further comprising a temperature controlled enclosure housing said noise injection radiometer. 15. The system of claim 13, wherein said radiometer controller is responsive to an output of the radiometer to control receiver gain and the system noise temperature. 16. The system of claim 13, wherein said signal injection radiometer comprises a noise source for periodically providing a stable noise signal along a first signal path as an input to said LNA and a power detector for periodically detecting the output signal of the LNA along a second signal path, the output of said power detector being provided to said radiometer controller to determine an estimation of sky noise on the down-link. 17. The system of claim 16, wherein said first path comprises a filter and a first switch for periodically injecting a filtered noise signal into said LNA and said second path further comprises a second switch and a multiplier, said second path periodically receiving the output of said LNA and an RF source. 18. The system of claim 17, wherein said filter is a bandpass filter which is operative to minimize potential interference with a desired down link communication signal. 19. The system of claim 17, wherein said first and second switches operate in tandem in response to said radiometer controller. 20. The system of claim 17 further comprising a variable attenuator responsive to said controller and operative to provide said noise signal to said second path.

Inventors: DISSANAYAKE ASOKA W, LIN KUAN-TING, DISSANAYAKE, ASOKA, W., LIN, KUAN-TING
Applicants: COMSAT CORP, COMSAT CORPORATION
Application year: 2000
Main classification: H04B7
IPC codes: H04B7