SATELLITE RECEIVER SOFTWARE ALGORITHM FOR SETTING AN

INPUT ATTENUATOR

Priority Claim

This application claims the benefit of United States Provisional Patent Application No. 61/005,756 filed December 7, 2007 entitled "SATELLITE RECEIVER SOFTWARE ALGORITHM TO PREVENT RECEIVER OVERLOAD BY STRONG MULTICARRIER INPUT SIGNALS" which is incorporated herein by reference.

Field of the Invention

The present invention relates to determining conditions indicative of signal fade conditions and loss of signal conditions, and circuitry adjustment to compensate for such situations. Specifically, the present invention teaches an apparatus and method of coupling and decoupling an attenuator from a signal path in response to at least one of signal to noise ratio (SNR), signal received power, and frame error rate.

Background of the Invention

In any type of RF transmission, differing signal strengths present a problem for tuning amplifiers. For example, strong signals can cause overload or saturation at the input of a tuner, resulting in distortion of the tuned signal. Weak signals must be sufficiently amplified before processing to facilitate extraction of the transmitted data. Noise floor, adjacent signals and other interference complication this operation. Furthermore, signal strength can change over time in response to external factors, such as position of the receiver or weather conditions.

Fading is a distortion that results from multipath interference created by the presence of reflectors in the environment surrounding the transmitter and

receiver which create multiple transmission paths, resulting in multiple, superimposed, received signals that are time, phase and/or amplitude shifted due to the differing propagation distances and reflection effects. These superimposed signals result in either constructive or destructive interference. Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel SNR.

In radio frequency (RF) receivers, it is common to implement an attenuator before the amplifier to attenuate signals that would overload the receiver input. Typically, these attenuators would be switched into the signal path when an overload condition was determined. The overload condition is determined through an estimate of receiver input power through a receiver automatic gain control (AGC) measurement. The AGC measurement is either a voltage or the digital accumulator of an AGC control loop. This AGC can be calibrated for each input power and used to estimate the input power of the desired received signal. Once the estimate of AGC reading is obtained, it is compared with a predetermined threshold. If the signal power is greater than the threshold, then the attenuator is inserted as it is assumed that the large input powers would create a receiver overload.

A problem with this technique is that it does not estimate the total multicarrier power at the receiver input but rather the power of the signal that is selected for demodulation. Some assumptions on the multicarrier signal distribution have to be made in order to estimate the total power input to the receiver. A problem that is experienced with this method, particularly with the newer integrated tuner demodulators, is that the AGC reading for a constant received power versus input frequency is extremely dependant on frequency. This complicates the receiver calibration because the receiver frequency range has to be subdivided into several subranges to get accurate power estimates. To make matters worse, the receiver input electronics is frequently changed during the product development phase making recalibration necessary. A further disadvantage is that the software code that implements the AGC calibration versus frequency requires many "if then, else" statements

(one per frequency subrange), thus slowing down the reception. It is assumed that the received signal frequency is known so that the proper threshold corresponding to the frequency subrange can be established.

In satellite signal transmission, different transponders frequently do not all transmit at the same power and the translation schemes ahead of the receiver can create level differences in the power distribution versus input frequency. The choice of threshold point is valid for some distributions of input power and it is possible to create some distributions that incorrectly switch in or out the attenuator. These power distributions are first a function of the satellite operators use of transponders and the gain of the satellite transmit antenna and second of abnormal propagation effects.

In the first case, a particularly damaging power distribution versus frequency is the one where for example most of the multicarrier energy is concentrated over the lower receive portion of the band (950-1500 MHz) while the desired signal is in the top of the higher end of the receive band (1500-2150 MHz). In this case, the preamplifiers located at the receiver input will get a considerable amount of intermodulation distortion in the desired signal reception range. One practical frequency domain manifestation of this intermodulation will be an increase in the noise floor in the upper end of the receive band. Another view of this effect, but in the time domain, will be a modulation transfer occurring from the composite wideband signals onto the desired signal. This can also be viewed as a weak signal suppression effect that occurs when multiple modulated carrier signals must share an amplifier that has a physically limited amount of output power available. The multicarrier signals collectively have a larger power than the single modulated signal that is currently demodulated. The power sharing of the large multicarrier signal with the desired weak signal does not occur in a uniform proportion when going through non linear amplification and takes on the average a larger proportion of the total power output of the amplifier thus effectively suppressing the desired signal.

A method based on a desired input amplitude is optimal only for areas where the received signal matches the threshold selected at design time in the software code that switches in or out the attenuator because the absolute value of the desired receive signal power will change as a function of the location in the satellite footprint. It would be desirable to have a system and method that does not rely on the absolute input power but on a quantity that indicated more closely the actual damage done to the signal detection by the overload manifestations described in the previous paragraph. Furthermore, a desirable system would address signal attenuation when there is a new request to tune while the fade is already in progress and when the receiver is already tuned to a signal and a frequency selective fade occurs.

Summary of the Invention

In accordance with an aspect of the present invention, a method of setting an input attenuator is disclosed. According to an exemplary embodiment, the method of setting an input attenuator comprises the steps of receiving a signal, estimating a first SNR, attenuating the signal, estimating a second SNR, and discontinuing said attenuation step if said second SNR is less than said first SNR.

In accordance with another aspect of the present invention, a method of setting an input attenuator is disclosed. According to an exemplary embodiment, the method comprises the steps of determining at least one of a SNR, signal received power; and frame error rate of a tuned signal, and adjusting the attenuation of said tuned signal in response to said determination.

Description of the Drawings

FIG. 1 is a block diagram of an embodiment of a satellite signal processing device for reception of electromagnetic signals according to the present invention;

FIG. 2 is a first state diagram of an exemplary embodiment of the operation of circuitry according to the present invention;

FIG. 3 is a second state diagram of an exemplary embodiment of the operation of circuitry according to the present invention.

The examples set out herein illustrate presently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

Description of the Preferred Embodiment

As described herein, the present invention provides a method and apparatus for receiving and processing satellite, terrestrial, wireless and/or cellular electromagnetic signals, and more specifically of detecting when there is a condition of overload caused by strong signals and coupling an input attenuator at the input of a receiver in response to the situation. While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims. For instance, the described technique is applicable to transmission systems designed for other types of data or that use different coding, error-correction, redundancy, interleaving, or modulation schemes.

Referring now to the drawings, and more particularly to FIG. 1, a block diagram of an embodiment of a satellite signal processing device 100 for reception of electromagnetic signals according to the present disclosure is shown. The satellite signal processing device comprises an input attenuator 110, an amplifier 120, a tuner 130, a demodulator 140, and a controller 150.

The input attenuator 110 is a switchable or adjustable RF attenuator used to attenuate incoming RF signals received from the input. If a switchable attenuator is used, the RF signal can either pass through the attenuator or an alternate path unattenuated in a first attenuator state, or the RF signal can be attenuated by the attenuator in a second attenuator state. The attenuator state is controlled by a control signal from the controller 150. If an adjustable RF attenuator is used, the RF signal passes through the attenuator and is attenuated in response to a control signal from the controller 150. The input signal is generally attenuated by the attenuator 110 when there is a condition of overload caused by strong input signals. The attenuator is removed from the signal path when the signals are weak and do not risk overloading the receiver.

The amplifier 120 is operative to receive the RF signal from the attenuator, in either an attenuated or unattenuated state. The amplifier 120 amplifies the RF signal to an level suitable for coupling into the tuner 130. The amplifier may be controlled by an automatic gain control system which is used to adjust the amplification level of the attenuator in response to the RF signal characteristics, such as amplitude, SNR, or frame error rate. Other characteristics can be used to control the automatic gain control system.

The tuner 130 is operative to receive the attenuated and/or amplified RF signal at a first frequency and downconvert that signal to a intermediate frequency for coupling to the demodulator 140. According to an exemplary embodiment, the tuner 130 receives an RF signal from amplifier 120, and performs the signal tuning function by band pass filtering and frequency down converting (i.e., single or multiple stage down conversion) the RF signal to thereby generate an intermediate frequency (IF) signal. The RF and IF signals may include audio, video and/or data content (e.g., television signals, etc.), and may be of an analog signal standard (e.g., NTSC, PAL, SECAM, etc.) and/or a digital signal standard (e.g., ATSC, QAM, QPSK, etc.).

The demodulator 140 is operative to recover the information content from the IF signals coupled from the tuner 130. The demodulator removes the

modulation from the IF signal to recover the original baseband signal. The baseband signal is then coupled to the output for further signal processing operations such as equalization and display on a video display device (not shown). A demodulator is further operative to perform other calculations and techniques when extracting the information content, such as carrier recovery, clock recovery, bit slip, frame synchronization, rake receiver, pulse compression, Received Signal Strength Indication, error detection and correction, noise figure, bit error rate, and frame error rate. The demodulator is further operative to use these computed values, such as received signal strength indication, noise figure, and frame error rate to couple control signals to the controller 150 for use in controlling the attenuator 110 and possibly the amplifier 120.

The controller 150 is operative to receive control data signals from the demodulator and generate control signals for the attenuator 110 and the amplifier 120 in response to that control data. The control signals generated by the controller 150 are operative to switch the input attenuator 110 in or out of the signal path or to adjust the attenuation level of an adjustable attenuator.

The satellite signal processing device 100 according to the exemplary embodiment as depicted in Fig. 1 is operative to address a number of problems. It is desirable to set the input attenuator so that the receiver will not be overloaded when there is a new request to tune a signal. In a first situation when the selective fade affects both desired and undesired signals, the optimal setting of the input attenuator 110 is the one that provides for the best signal to noise at the detection point of the demodulator 140. If the fade depth is such that the receiver signal is near the minimum input power threshold, any other signal present will be also attenuated and will produce negligible amounts of noise floor increase. The main contributor to the signal to noise will then be the combined effect of the front end low noise converter and the receiver preamplifier noise figure. The attenuator setting will be set to a lower input attenuation and thus get the best receiver noise figure. An improved decision based on both input power and signal to noise will provide improved results over one parameter only.

In a second situation where the selective fade affects only the desired signal but does not affect the other signals present at the wideband receiver input, a decision to switch in the input attenuator 110 based on input power will provide an improved signal to noise for the demodulator 140 as long as the noise floor increase caused by the amplifier overload is small. There will be an input power range where it will be advantageous to switch in the attenuator 110 and thus increase the receiver noise floor but reduce the effect of the noise floor created by the amplifier overload. For that input power range, a decision based on input power may be erroneous. A decision based on signal to noise of the demodulator will provide improved results over the input power decision for this situation.

In a third situation, the selective fade affects only the other signals present but not the desired signal. In this case the increase of noise floor caused by the undesired signals present at the receiver input will cause negligible effects. A decision based on power input should coincide with the decision based on signal to noise. For marginal cases, an improved decision based on both input power and signal to noise will provide improved results over one parameter only.

In each of the above three situations, the measurement of SNR with and without the input attenuator 110 can provide an accurate method to find the optimum setting at the instant the measurements are done. The SNR measurement may be available by reading a register from the demodulator 140. It may derived by measuring the dispersion of data points for the received signal constellation. This method has the disadvantage of requiring two distinct measurements of signal to noise in the receiver baseband: one with the attenuator 110 and one without, thus taking more time to tune the broadcast channel. The demodulator may be operative to compute the signal constellation and the dispersion that is proportional to received signal to noise. Thus the time to make this type of measurement is reduced to reading hardware registers in a demodulator 140 integrated circuit. The signal to noise

measurement is done on noisy received signals. The measurement has to have a small enough variance to get reliable decisions. This can be achieved by averaging or taking the most significant bits of the register read. The same problem may exist when taking decisions based on a noisy AGC measurement.

Referring now to Fig. 2 a first state diagram 200 of an exemplary embodiment of the operation of circuitry according to the present invention is shown. The method as depicted in Fig. 2 addresses the problem of setting the input attenuator when there is a request to tune a new signal or channel. The exemplary embodiment according to the present invention provides for a simplified path for weak signals that would be unlikely to overload the demodulator's input amplifier. A crude measurement of the desired signals input power is done early and the tuning time is reduced when a desired signal is actually present. Reduced tuning time is also created by providing a bypass path for signals that are known to be very weak or very strong. In those cases, the position of the input attenuator is straightforward, and signal to noise measurements are not required.

The method 200 starts with a request to tune a new signal or channel 205. The method then removes any attenuation from the signal path 210 in response to receiving the request. The demodulator then attempts a signal lock 215. If the signal lock is achieved 220, the system then compares the received signal strength to a first threshold 225. If the signal strength is less than the first threshold, the signal parameters are considered acceptable and the system returns a locked indication 290.

If a signal lock is not achieved 220, the system attenuates the input signal 275 and attempts a lock of the demodulator 280. If a lock is made in less than a predetermined time, the signal parameters are considered acceptable and the system returns a locked indication 290. If no lock is made in the predetermined time, the signal parameters are considered unacceptable, or a signal is not present, and the system returns an unlocked indication 285.

If a signal lock is achieved 220, but the signal strength is greater than a second threshold 230, the system then attenuates the signal 270 on the signal path and the system returns a locked indication 290. If the received signal strength is greater than the first threshold and less than the second threshold, the system estimates the signal to noise ratio a first time 235, and attenuates the signal on the signal path 240. If no signal lock is achieved 245 after the attenuation, the system removes the attenuation 265 and returns a locked indication 290. If the signal lock 245 is achieved, the system estimates the signal to noise ratio 250 a second time with the attenuation in place. If the second signal to noise ratio is greater than the first signal to noise ratio 255, the system returns a locked indication 290. If the first signal to noise ratio is greater than the second signal to noise ratio 255, the attenuation is removed 260 and the system then returns a locked indication 290.

This method, according to an exemplary embodiment of the present invention, provides for a simplified way to determine if there is no desired signal present in the minimum time possible. This situation occurs when there is no actual signal transmitted, there is no connection from the receiver to the antenna, the block downconverter on the antenna is not powered or is defective, or many other cases that prevent a desired signal to be present at the receiver input. It is desirable to know as soon as possible if the desired signal is not present. Given the modulation and bit rate expected for the input frequency of the tuned signal requested, a calibration of the demodulator acquisition time for the worse case reception conditions of noise and adjacent channel interference is done in lab conditions. This worse case acquisition time is then used in the production demodulator software to determine the maximum time to wait while trying to lock to a desired signal. When this worse case time is reached, it can be assumed that a valid signal is not present or that the noise floor caused by receiver overload is so large that the receiver would not lock correctly. Thus prompting the introduction of an attenuator ahead of the receiver to try to reduce the overload situation and thus reduce the noise floor. If a second conditional test also fails, it can be conclude that not only that the receiver is not locked but also that the signal was not present at the receiver input (Fig. 2, 285) This last conclusion would be valid assuming the antenna

installation was already set to receive the signal correctly. The conclusion is obtained waiting for the minimum time possible: the time for the receiver to lock onto the input signal in presence of the worse possible channel impairment.

Referring now to Fig. 3 a second state diagram of an exemplary embodiment of the operation of circuitry according to the present invention is shown. The method shown addresses the problem of when frequency selective fade occurs when the receiver is already tuned to the desired channel and the audiovisual broadcast is in progress. This problem occurs when it is desired to find the optimum setting of the input attenuator after the signal received was successfully demodulated. The decision to set the attenuator can occur while a fade was in effect, or the fade can occur after the receiver was successfully tuned with the position of the attenuator selected at the time of the request to tune.

Generally, it is not desirable to change the attenuator setting while the broadcast is in progress as it can change the instantaneous phase and amplitude of the received signal. This in turn causes bit errors in phase coherent demodulators. However, when bit or frame errors are detected by the receiver, the reception conditions can be improved by selecting an alternate position of the input attenuator. It is desirable to monitor the input power of the desired signal, this signal's carrier to noise ratio and if there are some errors reported in the demodulated data packet as those measurements can be implemented with relative simplicity. The implementation often calls for a program to read an AGC value that can be calibrated to approximate the desired channel input power. The carrier to noise ratio can be estimated by software that computes the dispersion of the signals on a constellation diagram. Finally, the bit error rate of the channel can be monitored by observing the number of bit corrections done by the forward error decoder part of the receiver or by monitoring the number of cyclic redundancy check (CRC) errors reported in outgoing packets from a demodulator to a video decoder device.

The satellite signal processing device (Fig. 1 , 100) is operative to address four distinct cases of changing selective fade conditions that affect the broadcast reception: selective fade for undesired signal stops, selective fade for undesired signal starts, selective fade for desired signal starts, and selective fade for desired signals stops. In these exemplary embodiments, SNR is used in the conventional context wherein a larger value indicates a better noise margin. The following examples are provided for a switchable attenuator. For a variable attenuator, the attenuation would be increased or decreased accordingly. When the SNR is increasing, desired signal power stays constant, and BER or PER decreasing, indicating selective fade for undesired signal starting, it is desirable to leave the attenuator off or remove the attenuator. When SNR is decreasing, desired signal power stays constant, and BER or Packet errors rate is increasing, indicating selective fade for undesired signal stopping, it is desirable to insert the attenuator and check if the SNR is increased, or to leave the attenuator on. When SNR is decreasing, desired signal power decreases, and BER or PER is increasing, indicating selective fade for desired signal starting, it is desirable to leave the attenuator off or remove the attenuator. Finally, when SNR is increasing, desired signal power is increasing, and BER or PER is decreasing, indicating selective fade for desired signals stopping, it is desirable to leave the attenuator off or remove the attenuator from the signal path.

The implementation of the solution to the second problem would typically involve a software task that makes 3 type of measurements by acquiring the values of registers inside the demodulator chip. The monitoring would be to find the trend over a time interval of the signal to noise, the signal received power, the reported CRC errors on a frame. Given those measurements, it is possible to extract information about the type of selective fade activity that is occurring. It is assumed that the receiver operates in a multicarrier environment. This multicarrier signal can overload the receiver input thus causing an increase in noise floor. This overload condition will be improved when the selective fade affects the multicarrier signals that cause the increase in the noise floor around the desired signal. In order to adapt optimally to the dynamic conditions observed , demodulator measurements are used as an

input to an algorithm that controls the attenuator setting. Similarly, the desired signal may be affected by the selective fade without the damaging multicarrier being affected. This condition is also detected by the algorithm and a course of action is recommended to get optimal reception.

The method 300 starts in a situation where a tuned signal is being received 305. Periodically, the SNR trend 310, AGC trend 315 and the MPEG frame error rates are determined 320. The frequency of these determinations is a matter of design choice, with more frequent determinations possibly resulting in degraded system performance and less frequency determinations possibly resulting in degraded signal reception. After these determinations are made, the system compares the frequency of MPEG frame error rate to a threshold indicating unacceptable performance 325. If the MPEG frame error rate does not exceed the threshold, the system keeps acquiring data and reports if an unlocked condition arises 360.

If the MPEG frame error rate exceeds the threshold, 325, indicating unacceptable performance, the system evaluates the SNR trend to determine if it is decreasing 330. If the SNR trend is decreasing, the system then determines if the desired signal power is constant 340. If the desired signal power is constant 340, the system makes a determination that the selective fade for an undesired signal is decreasing or stopping 385. If the desired signal power is not constant 340, the system determines whether the desired signal power is decreasing 345. If the desired signal power is decreasing, the system determines that the selective fade for a desired signal is starting or increasing 375. If the desired signal power is not decreasing 345, and not constant 340, the system determines that the desired signal power is increasing and then keeps acquiring data and reports if an unlocked condition arises 380.

If the MPEG frame error rate exceeds the threshold, 325, indicating unacceptable performance, the system evaluates the SNR trend to determine if it is decreasing 330. If the SNR is not decreasing, the system determines whether the SNR is remaining constant 335. If the SNR is remaining

constant, the system keeps acquiring data and reports if an unlocked condition arises 360. If the SNR is not remaining constant 335, the system determines whether the desired signal power is constant 350. If the desired signal power is constant 350, the system determines that the selective fade for undesired signals is starting or increasing 370. If the desired signal power is not staying constant 350, the system determines whether the desired signal power is increasing 355. If the desired signal power is not increasing, the system keeps acquiring data and reports if an unlocked condition arises 360. If the desired signal power is increasing 355, the system determines that the selective fade for the desired signal is decreasing or ending 365.

While the present invention has been described in terms of a specific embodiment, it will be appreciated that modifications may be made which will fall within the scope of the invention. For example, various processing steps may be implemented separately or combined, and may be implemented in general purpose or dedicated data processing hardware. Furthermore, various encoding, modulation or compression methods may be employed for video, audio, image, text, or other types of data.