Patent Publication Number: US-6701264-B2

Title: Method of and apparatus for calibrating receive path gain

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to calibration of signals in a wireless communication system. In particular, the present invention relates to calibrating receive path gain in a satellite communication system. 
     In many communication systems, one method of communicating involves time-division multiplexing (“TDM”). With TDM, data is ordered into regularly repeating time slots according to a data transfer standard, such as, for example, the asynchronous transfer mode (“ATM”) standard. Time division multiple access (“TDMA”) systems divide the radio spectrum into time slots, and in each slot only one terminal of many is allowed to transmit. Each terminal is assigned a cyclically repeating time slot, and a channel may be thought of as a particular time slot that reoccurs every N-time slot frame in a particular frequency band. The number N depends on several factors specific to each communication system and may vary widely. 
     TDMA systems typically transmit data in a buffer-and-burst method, thus the transmission for any terminal is noncontinuous. Therefore, unlike frequency division multiple access (“FDMA”) systems (which can accommodate analog signals), digital data and digital modulation techniques are almost exclusively used with TDMA. 
     In a time division communication system, there are generally two types of slots, data slots and non-data slots. In this context, data slots refer to time slots in which terminals generally transmit “useful” user information. The non-data slots in this context refer to time slots containing overhead information, for example, framing information, routing information, timing information, and requests for service. A frame may include a preamble, information message (a sequence of a predetermined number of time slots), and trail bits. 
     When signals transmitted in an uplink arrive at a satellite, present systems use a demodulator, including an analog to digital converter (“ADC”), to process the signals. In the past, the ADCs used with such systems were unduly complicated, expensive, and power consuming, however, and had as many as nine bits of output or more. Past systems required complicated ADCs in part because a portion of the signal input to the ADC was variable gain in the receive path electronics of the satellite. 
     The receive path gain varies widely and is generally unknown, thereby requiring an increase in the ADC&#39;s dynamic range (and number of output bits). Thus, ADCs with as many as nine bits of output were needed to cover the total possible dynamic range of input signal levels in the face of unpredictable receive path gain. 
     Unfortunately, the use of an ADC with a large dynamic range results in a more complex and power consuming ADC and also adds to the number of bits that must be carried into the digital processing system downstream of the ADC. Therefore, the use of a more complex ADC increases the cost of both the ADC and the downstream digital processing system. 
     In a multiple access system such as a TDMA system, the dynamic range of the uplink depends primarily upon the number of transmitting terminals and the gain variation in the RF receive path (which typically includes one or more amplifiers and additional receive path electronics). The only controllable factor of these two is the receive path gain variation. Unfortunately, past wireless communication systems did not provide a method of adequately controlling the gain variation in the receive path. 
     Some past systems use Automatic Gain Calibration (“AGC”) in attempting to alleviate some of the problems noted above. AGC does not, however, operate correctly in TDMA systems because of the dynamic loading of the input signal. In other words, the received signal power at the receiver (e.g., a satellite antenna) varies with the number of active terminals. Because the number of terminal signals simultaneously present at the input is not constant, the received signal power at the satellite antenna varies over time. As a result, the AGC circuit cannot differentiate between gain due to receive path electronics and power due to a large number of active terminals. Therefore, AGC techniques were unable to solve the problems noted above. 
     A need has long existed in the industry for a method of and apparatus for controlling gain variation in the receive path. Additionally, a need has long existed for a method of and apparatus for calibrating receive path gain, thereby permitting a less complex ADC to be used. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce the required dynamic range of an analog to digital converter in a wireless communication system demodulator. 
     It is another object of the present invention to provide an accurate gain measurement for the receive path of a satellite uplink. 
     It is an additional object of the present invention to provide a gain measurement for a satellite uplink that would allow for calibration and testing of any hardware component on the satellite. 
     It is a further object of the present invention to provide an improved analog to digital converter for use with demodulators in wireless communication systems. 
     It is yet another object of the present invention to enable a reduced complexity analog to digital converter to be used with demodulators in satellite communication systems. 
     It is a further object of the present invention to reduce the number of bits carried in the digital processing system downstream of an ADC. 
     It is a still further object of the present invention to calibrate receive path gain using an uplink signal without loss in bandwidth. 
     One or more of the preceding objects, or one or more other objects which will become plain upon consideration of the present specification, are satisfied, at least in part, by the invention described herein. 
     One aspect of the invention, which satisfies one or more of the above objects, is an apparatus for calibrating gain in satellite uplink receiver electronics. The apparatus includes an uplink receiver, a measurement processor, and an attenuator. The uplink receiver receives a satellite uplink and outputs an uplink signal to the measurement processor. 
     The measurement processor receives the uplink signal and includes circuitry to sample the uplink signal during a blanking interval (i.e., during a period of no transmission over the uplink, such as an empty slot or an empty time probe slot  306   a  in a TDMA uplink structure). 
     The measurement processor may connect to a snapshot buffer and include logic to sample the uplink signals periodically (e.g., approximately every thirty minutes) and may sample the uplink signals responsive to extreme changes in conditions). The measurement processor may also include circuitry that samples the uplink signals multiple times during a blanking interval to produce a multiple measurement sample (e.g., a sample average). 
     The measurement processor outputs a gain calibration (e.g., a sample average) which the attenuator uses to attenuate or calibrate the uplink signal to produce a calibrated signal. The attenuator may be, for example, a variable attenuator responsive to a gain calibration signal. 
     The apparatus may thereby use a lower complexity ADC coupled to the calibrated signal. The converter may have approximately 42 dB of dynamic range, for example, corresponding to 7 bits or less of output. 
     Another aspect of the invention is a method for calibrating gain in satellite uplink receiver electronics to reduce the dynamic range of the received uplink signals. The method includes the steps of receiving an uplink signal during a scheduled blanking interval, measuring gain associated with the blanking interval, and generating a calibrated signal by responsively controlling a variable attenuator in accordance with the measured gain. The method may sample the uplink signals periodically (e.g., every thirty minutes) and may sample the uplink signals responsive to extreme changes in conditions. The method may also include taking multiple samples during a blanking interval and averaging the multiple samples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of an apparatus for calibrating gain in satellite uplink receiver electronics according to a particular embodiment of the present invention. 
     FIG. 2 presents a method of calibrating gain in satellite uplink receiver electronics according to a particular embodiment of the present invention. 
     FIG. 3 is a data chart showing the TDMA structure in a satellite uplink. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus  100  illustrated in FIG. 1, method shown in FIG. 2, and data chart  300  of FIG. 3 correspond to one another at several points. Accordingly, FIGS. 1,  2  and  3  will be discussed together after being introduced immediately below. 
     Turning now to FIG. 1, that figure presents an apparatus  100  for calibrating gain in uplink signals received at a satellite receiver. The apparatus  100  includes a receive antenna  102 , receive path electronics  104 , and a variable attenuator  106 . A calibrated signal output  108  is also shown. The apparatus  100  also includes an RF amplifier  110 , an analog to digital converter  112 , and an X channelizer  114 . Also shown are a sub-channel processor  116  and corresponding sampling circuitry  118  and a snapshot buffer  119 . A demodulator controller  120 , a calibration software routine  122 , and averaging circuitry  124  are also shown. 
     Turning now to FIG. 2, that figure shows a flow chart  200  of a method for calibrating gain in satellite uplink receiver electronics. The method includes a receive uplink signal step  202 , a measure gain step  204 , and a generate calibrated signal step  206 . Also shown is a conversion step  208 . The measure gain step  204  may further include the sampling step  210  and averaging step  212 . 
     Turning to FIG. 3, that figure is a data chart  300  showing time slots in a satellite uplink. The masterframe  302  shown includes eight frames  304 . Frame one  304  is expanded to show time probe slots  306 . The time probe slots  306  are further expanded and empty  306   a  and allocated  306   b  time slots are shown. Traffic slots  308  are also shown. 
     Returning to FIG. 1, at the satellite, the receive antenna  102  receives the satellite uplink and transmits the uplink signal to the receive path electronics  104 . The receive path electronics  104  may include, for example, one or more amplifiers and may also include tuners, mixers, and filters. The receive path electronics  104  may perform operations on the uplink signals including, for example, amplification. The receipt of the uplink signal by the receive antenna  102  and the receive path electronics  104  corresponds to the receive uplink signal step  202  of FIG.  2 . 
     In the illustrated embodiment, the receive path electronics  104  provide the input signal to the variable attenuator  106 , which, in turn, provides a calibrated signal output  108  through the RF amplifier  110  to the ADC  112 . 
     The ADC  112  converts the signal from analog to digital format through methods including, for example, flash conversion or successive approximation conversion. After passing through the ADC  112 , the signal is in digital format and may be operated on by a variety of subsequent digital processing components such as, for example, CPUs, error correction coders, and the like. For example, in the illustrated embodiment, the digital signal is sent through the X channelizer  114 , which may be implemented as a bank of filters that separate the signal into frequency sub-channels. 
     Regardless of whether an X channelizer  114  or other digital components are present, the sampling circuitry  118  (which may be one of a series of sampling circuits for a series of corresponding sub-channels) then takes a “snapshot” of the data coming out of the ADC  112  or other component and stores the snapshot in the snapshot buffer  119 . Preferably, the snapshot is taken during periods where there is no transmission over the uplink, i.e., during blanking intervals, for reasons that will be discussed below. Although not required, in the preferred embodiment there is one sampling circuit  118  associated with the sub-channel processor  116  for each sub-channel (i.e., each frequency channel). The sampling circuitry  118  in the illustrated embodiment is located in the sub-channel processor  116 , but this is not required. The sampling circuitry  118  controls the storage of snapshots in the snapshot buffer  119 . 
     The demodulator controller  120  then reads the snapshot and submits it to the calibration routine  122 . The calibration routine  122  runs on the demodulator controller  120  and may perform, for example, an averaging operation on the snapshot. Alternatively, averaging circuitry  124  may be used to perform an averaging operation on the snapshot. The operations performed by the sampling circuitry  118 , demodulator controller  120 , and calibration routine  122  correspond to the measure gain step  204  of FIG.  2 . 
     Based on the snapshot data, the demodulator controller  120  and the calibration routine  122  determine how much unwanted gain is present in the receive path electronics  104 . The amount of unwanted gain may be determined, for example, by determining the measured power level or by comparing the measured power level with a reference level. The reference level may be, for example, a predetermined threshold or a prior power measurement. The demodulator controller  120  and calibration routine  122  may determine the amount of unwanted gain using one snapshot or several snapshots. If several snapshots are used, the snapshots may be averaged as noted above to provide a more representative gain measurement. 
     The proper gain calibration setting, i.e., the one that will result in removal of the unwanted gain, is then sent to the variable attenuator  106  by either the demodulator controller  120 , calibration routine  122 , or both. With the gain calibration setting, the variable attenuator  106  removes the proper amount of gain from subsequent input signals (e.g., the traffic slots  308 ) received by the receive path electronics  104 . 
     As a result, a gain measurement and gain calibration of any hardware component on the satellite, including components in the receive path electronics  104 , may be made. Preferably, the gain measurement and calibration are performed for gain attributable to components disposed between the receive antenna  102  and the ADC  112 . Additionally, in this manner, a gain calibration is performed for any gain attributable to the receive antenna  102 . The gain calibration described above corresponds to the generate calibrated signal step  206  of FIG.  2 . 
     As a result of the calibration, the dynamic range of the uplink signals for which the gain is calibrated is reduced before the uplink signals are processed by the ADC  112 . Accordingly, the required dynamic range of the ADC  112  (i.e., the output range required of the ADC  112 ) is reduced. Because the dynamic range of the ADC  112  is reduced, the ADC  112  may use fewer output bits. In other words, recognizing that each bit in an ADC  112  typically covers a finite number of dB of dynamic range (e.g., 6 dB), when the calibration reduces the required dynamic range the ADC  112  requires fewer output bits. As an example, the present invention reduces the dynamic range by approximately 6-17 dB, thereby eliminating as many as two ADC output bits. 
     Note, however, that because the calibration results in a smaller required dynamic range, the ADC  112  may still achieve the same resolution as if it had the greater number of output bits. On the other hand, if an ADC having a smaller number of output bits were used without the present gain calibration the ADC would not sufficiently cover the dynamic range of the uplink. In such a case, clipping (distortion) of the uplink signal or overloading of the ADC might result. 
     Furthermore, by reducing the number of bits in the ADC  112 , the cost of producing the ADC  112 , the complexity of the ADC  112 , and the power consumption of the ADC  112  are reduced. Additionally, the number of bits that must be processed by and therefore the cost of the digital processing system downstream of the ADC  112 , for example the X channelizer  114 , is also reduced. A lower complexity, more cost effective, and more power efficient system may thereby result. 
     Gain variations are introduced into the uplink signal in the receive path electronics  104 , for example in one or more amplifiers. Gain variations may also be introduced as a result of the receive path electronics  104  passing into and out of the earth shadow and being heated or cooled by solar rays or the lack thereof. Additionally, gain variations may be introduced if the angle at which the sun&#39;s rays contact the receive path electronics  104  changes or if the receive path electronics  104  are shielded from the sun by another part of the satellite or other object. Because the gain variation may be a result of slowly occurring phenomena, for example the receive path electronics  104  passing into the earth&#39;s shadow, it is anticipated that a gain measurement made once every thirty minutes will produce representative gain variation measurements. One skilled in the art will readily appreciate, however, that gain measurements may be taken at any interval and may further depend on how quickly the gain varies in any particular environment. For example, if an extreme change in conditions occurs, i.e., any phenomena that causes a large or sudden change in the receive path gain, additional gain measurements may be taken. 
     The apparatus  100  may be used with a wide variety of demodulators including, for example QPSK, BPSK, FSK and N 28  Quam demodulators. Additionally, the apparatus  100  may be implemented with any signaling standard that may introduce a blanking interval. Such standards include, for example, TDM, TDMA, CSMA (carrier sense multiple access), and the like. 
     As stated above, the benefits of measuring the receive path gain during a blanking interval will now be discussed. In general, when the sampling circuitry  118  or other gain measuring device measures the gain from the receive path of the satellite, it is important that the gain measuring device know what is transmitted over the uplink in order to produce accurate results. 
     If, for example, the gain measuring device attempts to measure power in a data time slot, the gain measuring device cannot know what it is measuring; in other words, the device does not know what portion of the power is attributable to receive path gain and what portion is attributable to the data transmission(s). The power attributable to the data transmission(s) is unknown because the device does not know how many terminals are transmitting over the uplink at a given time. As a result, any gain measurement is likely to be an inaccurate measure of the true gain. 
     If, however, the gain measurement is made during a blanking interval, such as, for example an empty time probe slot  306   a  in a TDMA system, the gain measuring device can measure the gain with much greater accuracy. A time probe slot  306  is a type of “non-data” slot used with a TDM or TDMA system that typically contains framing information, routing information, timing information, and/or requests for service. The receive path gain measurement of an empty time probe slot  306   a  is accurate because no data or other information is transmitted at that time. Specifically, the power detected for the blanking interval is primarily attributable to noise at the receive antenna  102  and gain in the satellite receive path. As a result, a more accurate receive path gain measurement may be made. Additionally, to account for variations in gain over time, a new gain calibration setting may be computed periodically. 
     Which time probe slots are kept empty may be set, for example, by a network control center on the ground. Through timing information, the timing of the blanking intervals is transmitted to the satellite so that the sampling circuitry  118  will know when to measure the receive path gain. 
     Preferably, in a FDM/TDMA system, for example, no data is transmitted over any frequency channel during the blanking intervals, so that interference between multiple sub-channels (i.e., frequency channels) is kept to a minimum. If the sub-channels have time slots that vary in length, more blanking intervals may be used in some frequencies than are used in other frequencies. If, for example in a FDM/TDMA system, one time probe slot of the frequency channel with the longest time slot is kept empty, that time period may correspond to five or twenty-five time slots of another frequency channel, for example. 
     Again referring to TDMA systems in which time probe slots  306  are used, although the length of the longest frequency channel time slot may be the empty time probe slot  306   a , the measurement of the gain may occur in a shorter period of time than the duration of the empty time probe slot  306   a.    
     Referring generally to FIG. 1, the attenuator  106 , ADC  112 , sampling circuitry  118 , and calibration routine  122  may be implemented using combinatorial logic, an ASIC, through software implemented by a CPU, a DSP chip, or the like. Additionally, the foregoing hardware elements may be part of hardware that is used to perform other operational functions at the receiving satellite. The gain measurements, gain calibration settings, and timing information may be stored in registers, RAM, ROM, or the like, and may be generated through software, through a data structure located in a memory device such as RAM or ROM, and so forth. 
     In an oversubscribed uplink there may be a slight reduction in the total number of terminals that may be in the system. This is because, in an oversubscribed uplink, there are more subscribed terminals than can transmit over the uplink at one time. The ratio of the number of terminals to the number of channels is based on statistical multiplexing (i.e., based on the fact that not every terminal will place a call at the exact same time), and may be for example, four-to-one. Even when terminals are not transmitting, however, each terminal is assigned a time probe slot  306 . Thus, there may be, for example, four times as many time probe slots as there are data slots. Because time probe slots  306  can be allocated to be empty with the present apparatus  100  and method, there may be a slight overall loss in the number of terminals that can subscribe. There is no corresponding reduction in the number of terminals that can transmit over the uplink at one time, however, since the number of data slots remains the same. 
     Referring again to FIG. 3, representative numbers that may be used with the present embodiment, but are not required, will now be discussed. An uplink bandwidth of 200 MegaHertz (MHz) may be used. The masterframe  302  may be approximately 0.75 seconds long and each frame  304  within the masterframe  302  may last approximately 92.4 milliseconds. The portion of the frame  304  allotted to time probe slots  306  may be approximately 3.24 milliseconds and traffic slots  308  may be approximately 89.01 milliseconds. If these numbers are used, then each time probe slot  306  may be about 131 microseconds long. In this example, there are 26 time probe slots  306  per frame  304  or  208  time probe slots  306  per masterframe  302 . 
     Based on these exemplary numbers, in the present embodiment, less than 0.5% (1/208) of the time probe slots  306  are allocated to be empty. Because the time probe slots  306  are only a portion of the uplink, it is seen that the overall “loss” in bandwidth due to allocating blanking intervals is only about 0.018% of the total bandwidth (131 microseconds/0.75 seconds). 
     While particular elements, embodiments and applications of the present invention have been shown and described, it is understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teaching. It is therefore contemplated by the appended claims to cover such modifications and incorporate those features which come within the spirit and scope of the invention.