Patent Application
20060222116
This invention relates in general to communication receivers and more specifically to an automatic gain control (AGC) system including wideband interferer signal detection.
Automatic gain control or AGC systems are known and widely used. However present communications systems being developed and proposed, such as UMTS (Universal Mobile Telephone Systems) also known as WCDMA (Wideband Code Division Multiple Access) when referring to the air interface, are considering relatively high data rates with complex modulation schemes and channel coding schemes each of which is expected to place very stringent demands on the overall AGC system that is used for receivers that will be deployed in these systems. Known AGC systems typically assess on channel signal levels and make gain adjustments accordingly.
Receiver architectures are evolving and due in part to economic pressures, less selectivity is being incorporated in receiver front ends and intermediate frequency (IF) stages. Practitioners are opting instead to incorporate the selectivity in later digital processing stages. Unfortunately that means a broader band of signals may be present in the front ends, IF stages, or analog to digital converter(s) (ADC) (used to convert from analog received signals to the digital domain). The chances that the front ends and particularly later receiver stages such as IF stages or ADCs may be overloaded by a large near band or out of band (off-channel) signal has increased with the reduction in selectivity. This may cause serious overloading conditions (exceeding dynamic range) for cost effective ADCs.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
In overview, the present disclosure concerns communications systems and equipment that provide service to communications units or more specifically users thereof operating therein. More particularly various inventive concepts and principles embodied in apparatus and methods for providing automatic gain control (AGC) systems for receivers in communication units, where the AGC systems operate to protect various functions in the receiver from off-channel or out of band signals as well as normal on channel AGC are discussed and described. The systems and receivers of particular interest are those being developed and deployed such as UMTS (Universal Mobile Telecommunication System)/WCDMA (Wideband Code Division Multiple Access) systems and the like as well as extensions, evolutions and so forth for such systems and equipment operating therein, particularly where such systems and equipment co-exist and operate with other systems such as legacy GSM (Global System for Mobile) systems. Note that the concepts and principles according to the present invention, while described in the context of a receiver or UMTS/W-CDMA system, are believed to be applicable in many systems in the digital signal processing field where overload situations may exist due to noise or other undesirable artifacts.
As further discussed below various inventive principles and combinations thereof are advantageously employed to detect off-channel or out-of-band signals that may result in overloading one or more functions, e.g., analog to digital converters (ADCs), and as needed effect a controlled gain reduction in such situations and thus avoid any detrimental impact that may otherwise occur. This inventive AGC system and techniques can be particularly advantageously utilized within an exemplary WCDMA receiver, thereby alleviating various problems associated with known AGC systems and facilitating lower cost higher performance receivers while still providing an autonomous and low power version of an AGC system, provided these principles or equivalents thereof are utilized.
The instant disclosure is provided to further explain in an enabling fashion the best modes of making and using various embodiments in accordance with the present invention. The disclosure is further offered to enhance an understanding and appreciation for the inventive principles and advantages thereof, rather than to limit in any manner the invention. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
It is further understood that the use of relational terms, if any, such as first and second, top and bottom, and the like are used solely to distinguish one from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.
Much of the inventive functionality and many of the inventive principles are best implemented with or in integrated circuits (ICs) and software or firmware instructions, such as custom or semi-custom ICs, e.g., application specific ICs. It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such instructions and ICs with minimal experimentation. Therefore, in the interest of brevity and minimization of any risk of obscuring the principles and concepts according to the invention, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts of the preferred embodiments
Referring to
From the mixer stage 107 the down converted receiver signal is filtered by a low pass filter 109. Specifically the I channel and Q channel signals are filtered by respective low pass filters 109, such as a resistor capacitor filter with a corner frequency set to accommodate the bandwidth of any signal of interest. The outputs or I and Q channels from the low pass filter 109 are coupled to respective variable gain amplifiers (PMA—post mixer amplifier) 111. The PMA is a variable amplifier that is operable to amplify a received signal in accordance with a gain control signal. The PMA has variable or controllable gain, via a gain control input 113, and can be utilized as a variable attenuator, responsive to a variable control signal, to provide a variable amount of attenuation or gain for the received signal (I and Q channel). The PMA 111 in some embodiments is arranged to provide approximately −30 to +15 db (45 dB range) of attenuation or gain control in nominally 3 dB steps for the I and the Q channel signal. The outputs of the PMAs 111 are coupled to a further low pass filter 115, with one each for the I and Q channel. In one or more embodiments these low pass filters are realized as a 6 dB inverting amplifier with a real pole, two active bi-quad filtering stages with two complex poles and unity gain, and an output buffer. In one exemplary embodiment an analog based integrated circuit includes the mixer stage 107 through the low pass filter 115.
The outputs of the low pass filter 115 are coupled, in one or more embodiments, to a digital integrated circuit that includes Analog to Digital converters, various digital circuitry, and digital signal processing based functionality. Specifically in an exemplary embodiment, the low pass filter 115 outputs are coupled or applied to a analog to digital converter (ADC) 117, with one ADC each for the I and Q channel signal. In one exemplary embodiment suitable for WCDMA receivers, the ADC is a second order sigma delta based ADC that is 12× over sampled (the chip rate for WCDMA signals is 3.84 M chips/second) and provides 6 bit output words at a rate of 46.08 M w/s (million words per second). The output from the ADC 117 is coupled to a normal receive data path 19 and an off-channel processing path 121.
The receive data path 119 includes an on-channel detector 123 that can be arranged and configured to provide an on-channel automatic gain control (AGC) indication, for example, at terminal 125. This on-channel detector 123 can be referred to as an on-channel AGC detector or a narrow band AGC detector. The receive data path 119 includes a high order decimation filter 127 that decimates the I and Q channel signals from the ADC 117. The high order decimation filter in one embodiment decimates the I and Q signals by 3, using known 3 stage cascaded comb filters to provide output signals comprising 14 bit words at a 15.36 MHz rate. The decimation filter 127 provides an I and Q input signal to a channel filter 129, specifically to an I and Q channel filter. The channel filter in one or more embodiments includes known half band type of filters that further decimate the respective input signals by 2. The half band filters are followed by I and Q matched selectivity filters that provide compensation for other filters, etc. in the system and that are running at 7.68 MHz to provide an output signal comprising 13 bit words at 7.68 MHz. The output signal from the channel filter 129 is coupled to additional receive processing 131 (demodulation, error correcting, etc., etc. not relevant to this disclosure) as well as on-channel AGC processes. The composite response of the filtering lineup through the receiver is arranged and configured to provide a square root raised cosine response with a bandwidth suitable for receiving the signal(s) of interest, e.g., 1.92-2.0 MHz for a W-CDMA signal. Note that some or all of the digital filters or other digital signal processing may perform double duty, i.e., can be multiplexed between the I and Q channel signals thus saving some silicon area in an integrated circuit embodiment.
The on-channel AGC processes include in various embodiments an on-channel signal level detector that as shown in exemplary form as an on-channel power detector path. The On-channel Power Detector path samples, filters and integrates incoming I and Q channel signals over a programmable bandwidth and converts the accumulated/normalized value to dB, and then computes a Received Signal Strength Indication (RSSI) value. This information is used to control the gain or attenuation of the PMA 111 and switch in or out the LNA(s) 103 for the given band under “normal” circumstances. The on-channel power detection includes a Sum-of-Squares function 133 that computes the sum of squares (I2+Q2) that is used to compute the detected power or on-channel power level. The Sum-of-Squares function 133 also filters and accumulates the in-band I and Q channel signals or data streams that enter the on-channel AGC. An IIR HP Filter (not specifically shown) is placed before the Sum-of-Squares function 133 and performs programmable high-pass filtering (corner frequency approximately 100 KHz) of the incoming data streams in order to remove any DC offsets that may result from the mixer stage 107 or the like (this insures loop stability).
The Sum-of-Squares function 133 is coupled to a loop filter 135 that is a known Accumulate-and-dump filter that provides an output signal corresponding to an on-channel signal level, e.g., power level. The loop filter 135 in one or more embodiments is set to accumulate approximately 1000 chips (260 micro-seconds). The accumulate time together with feedback delays and data conversion delays allows an update approximately every 270 microseconds. The loop filter to a large extent establishes the on-channel AGC control loop dynamics as will be appreciated by one of ordinary skill. The loop filter output and corresponding signal is coupled to a RSSI calculator 137 that provides an RSSI value to a Compare/Hysteresis function 139 as well as other receive processing functions at 141. Note that the RSSI calculator includes AGC control inputs 143 (further discussed below) that are used to adjust the RSSI value and thus account for any attenuation that is provided by the various gain control stages, e.g., LNA(s) 103 or PMA 111, such that the RSSI value represents the power input at the antenna. The Compare/Hysteresis function 139 compares the on-channel signal level, e.g., on-channel power level, to an on-channel threshold that is programmable. The Compare/Hysteresis function 139 thus checks whether the detected on-channel power satisfies, (above or below) a programmable on-channel threshold to determine if more or less attenuation of, for example, the PMA or LNA is needed. This is reflected in the on-channel AGC indication at terminal 125. It will be appreciated that the on-channel AGC system is required to have some precision. Accordingly the AGC indication at terminal 125 can include both a magnitude and a sign, where the magnitude is indicative of the amount of difference between the RSSI value and the threshold and thus the amount of change in attenuation that may be required. Note that the Hysteresis operates to apply two different thresholds, e.g., a high threshold that when exceeded by the on-channel RSSI value indicates more attenuation is required/appropriate and a lower threshold that when the RSSI is below the lower threshold indicates that less attenuation is needed. By appropriately choosing the high and low thresholds hunting or toggling between different gain or attenuation settings by the on-channel AGC loop can be reduced. The on-channel AGC indication is coupled to an on-channel state machine or controller 143 that will be described below.
The off-channel processing path 121 includes an off-channel signal detector 145 that is arranged and configured to provide one or more off-channel AGC indications, for example, at terminals 146, 147 or via OR gate 148 at terminal 149. The off-channel signal detector can be referred to as a wideband AGC detector that in one or more embodiments is a digital wideband AGC detector. The wideband detector path samples, filters and integrates the I and Q channel signals from the ADC 117 to provide an assessment of off-channel signal levels, such as signal levels for varying types of interferer signals, e.g., other W-CDMA signals or GSM signals from adjacent channels or other carriers or other interfering signals that may fall within the bandwidth of the receiver system as will be briefly further discussed below with reference to
The off-channel signal detector comprises a wideband decimation filter that in one embodiment is a low order decimation filter 151, e.g., a filter for each of the I and Q digital signals from the ADC 117. The decimation filter 151, in one embodiment, provides one stage of comb filtering. Note various embodiments can use the interleaved RX_IQ signal after one stage of comb filtering in the high order decimation filter 127. The single stage of decimation filtering allows the off channel signal detector to measure off-channel interferers over a wide bandwidth and thus measure an interferer at 2.7 MHz, 3.5 MHz, or adjacent W-CDMA interferers in the adjacent 5 MHz channel. The outputs from the low order decimation filter 151 are coupled to a high pass filter (HPF) 153. The HPF 153 is an infinite impulse response filter with a corner frequency around 2.5-2.6 MHz. The HPF 153 can be programmable (corner frequency or bandwidths, gains, etc) to high-pass filter the incoming data stream, e.g. I and Q channel signal. The HPF 153 is arranged and configured to suppress or attenuate on-channel signals and thus insure that whatever signal is detected by the off channel detector is an off-channel signal or interferer.
The outputs from the HPF 153 are coupled to a wideband power detector 155 and a wideband peak detector 157. The wideband power detector in one embodiment comprises a Sum-of-Squares function 155. The sum of squares function includes an output coupled to a loop filter 159. These collectively operate in a fashion similar to the on-channel path as discussed above. Note that in various embodiments, the wideband power detector can be advantageously utilized to detect power levels of signals such as adjacent channel W-CDMA signals. The output of the loop filter 159 is coupled to a comparator, e.g., Compare/Hysteresis block 161 that compares the detected power level, e.g., interferer power level, to one or more power or wideband thresholds available at 163 and thus determines based on the wideband interferer thresholds if more or less attenuation of, for example, the PMA is indicated. The Compare/Hysteresis block 161 provides the off-channel AGC indication or off-channel wideband AGC indication corresponding to an off-channel power level at terminal 146.
The wideband peak detector 157 comprises a known peak detector for assessing signal envelope magnitude. An indication of the peak level as detected by the peak detector is coupled to a loop filter 165. The loop filter 165 is similar to and performs similar functions to the loop filter 135. However in one or more embodiments the integrate time is shorter, e.g., 100-500 chips or approximately 25-130 micro-seconds, yielding an update period of approximately 30-135 microseconds. Note that in various embodiments, the wideband peak detector can be advantageously utilized to detect peak levels of signals such as adjacent channel narrow band interferers, for example GSM signals. The output of the loop filter 165 is coupled to a comparator, e.g., another Compare/Hysteresis block 167 that compares the detected peak level, e.g., interferer peak level, to one or more peak or narrow band interferer thresholds available at 171 and thus determines based on the narrow band interferer thresholds whether more or less attenuation of, for example, the PMA 111 is indicated. It should be noted that the peak thresholds can vary from or be independently selected relative to the power thresholds noted above, e.g., in one embodiment the peak thresholds are selected to be approximately 6-12 dB larger than the power thresholds. Note that the loop filters 159, 165 can also have their respective accumulate times independently adjusted or programmed. The Compare/Hysteresis block 167 provides the off-channel wideband AGC indication corresponding to an off-channel peak level at terminal 147. The power and peak based AGC indications are OR'd at OR gate 148 and coupled to an off-channel state machine or controller 173. Note that the controller 173 is inter coupled to the controller 143.
The on-channel controller 143 and off-channel controller 173 are coupled respectively to the on-channel AGC indication and the off-channel AGC indication and collectively function as a controller configured to provide a gain control signal corresponding to the on-channel AGC indication or the off-channel AGC indication. The controller thus comprises an on-channel controller coupled to the on-channel AGC indication and an off-channel controller coupled to the off-channel AGC indication, where the on-channel controller and the off-channel controller are cooperatively operable to provide the gain control signal, specifically a digital gain control signal at 175 that comprises a plurality of control lines (PMA[M:0], LNA[0], . . . LNA[N]. The PMA[M:0] carries a 4 bit control signal to a digital to analog converter (DAC) 177. This control signal is converted to an analog gain control signal by the DAC 177 and this signal is used as a gain control signal 113 to adjust the gain or attenuation of the PMA 111. As will be further discussed below with reference to
Referring to
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Suppose that the off-channel AGC data or indication exceeds the HI threshold and thus a strong off-channel signal, e.g., an interferer, is present, thereby suggesting a decrease in gain in, for example, the PMA, is appropriate. At 305, under these circumstances, optionally the method first checks to see whether the on-channel data or AGC indication satisfies, e.g., exceeds, a strong signal threshold (on-channel signal is large enough to over load the ADC or other functionality). If the on-channel data is less than or equal to a strong signal threshold, one or more embodiments initiate or enter a shutdown process and will normally disable on-channel AGC control of the PMA 309. In any event, 311 indicates that the PMA (or other controllable gain) is decremented (under off-channel AGC control) and the method repeats from 301. Thus the providing the on-channel AGC indication and the off-channel AGC indication further comprises, in one or more embodiments, providing an off-channel AGC indication corresponding to an interferer satisfying an off-channel or HI threshold and optionally providing an on-channel indication corresponding to an on-channel signal not satisfying a strong signal threshold. In this event, the selecting further comprises selecting the off-channel AGC indication; and the providing the gain control signal further comprises providing the gain control signal responsive to the off-channel AGC indication.
If the on-channel AGC data or indication satisfies the strong signal threshold, optionally 313 shows the on-channel control of the AGC system continuing. Thus, the providing the on-channel AGC indication and the off-channel AGC indication further comprises providing an on-channel indication corresponding to an on-channel signal satisfying an on-channel strong signal threshold. In these embodiments, the selecting further comprises selecting the on-channel AGC indication and the providing the gain control signal further comprises providing the gain control signal responsive to the strong signal AGC indication (strong signal RSSI less normal threshold). Note that when the off-channel AGC indication corresponds to an interferer satisfying one of the off-channel thresholds, the off-channel AGC system will ordinarily control and provide the gain control signal, regardless of the on-channel AGC indicator. The exception is when, optionally, the on-channel AGC indicator satisfies a large signal threshold.
Note that off-channel AGC control and disabling on-channel AGC control requires interaction and cooperative functionality between the process of
In
Suppose the off-channel data or AGC indication does not exceed the HI threshold (peak or power threshold) and the AGC on-channel PMA control system is not in shutdown, then the on-channel AGC system is deferred to for AGC control 307 and the method continues from 301. If the AGC method is already in shutdown (see 309), then at 315, the off-channel data is compared to a LOW threshold 315 (4 dB-6 dB less than HI threshold in some embodiments). As noted above the HI and LOW thresholds are part of the Hysteresis functions 161, 167. If the off-channel AGC indication or data is not less than the LOW threshold then continue the shutdown processes 317, e.g. continue disabling of the on-channel AGC PMA control and decrement PMA. If the off-channel AGC data is less than the LOW threshold then put the on-channel AGC processes in a recovery mode, i.e. enable the on-channel AGC control but only allow small gain changes, possibly at faster rate than would be normal. Thus, the method of
Referring to
The above discussion, referencing
Referring to
A recovery flag 509 or signal is set to “1” and the shutdown flag or signal 501 is set to “0” when a recovery process 510, 512 is initiated 511, 513. Thus the first recovery begins and a small max gain change (3 dB) is used to slowly increase the gain of PMA from −3 to 6 dB. Note that in recovery the steps may be positive or negative. Note also that the recovery count 515 during the first recovery process 510 is reduced from 5 to 2. This recovery is interrupted by presumably another off-channel interferer signal at 505 and the second shutdown process begins 505. The second recovery 512 starts at 513, however in this case the PMA gain is further reduced from −9 to −27, the counter is reduced from 5 to 0, the recovery flag is set to “0” and normal on-channel AGC control is started at 517, where normal control allows for larger gain change steps.
Referring to
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An automatic gain control system for a receiver that is arranged and constructed to provide gain control to mitigate the impact of off-channel interferers in a receiver with limited protection against such interferers has been discussed and described. Generally the automatic gain control advantageously uses a conventional on-channel AGC detector as well as an off-channel AGC detector and controllers that cooperatively operate to provide an appropriate gain control signal when an interferer is present and when such an interferer is not present.
This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
