Patent Grant
8160527
The present invention relates generally to multi-carrier wireless telecommunication systems and relates in particular to methods and apparatus for automatic gain control in multi-carrier wireless receivers.
Automatic gain control (AGC) is widely used for handling varying received signal strengths in wireless receivers. A typical AGC control system measures the signal strength of the received signal, e.g., by analyzing a digitized data stream, and compares the measured signal level to a desired signal strength, a so-called set point. The difference between the measured signal strength and the desired signal level is used to control one or several amplifiers, to keep the output signal as constant as possible with respect to signal strength, or within a desired range, at a given reference point in the receive path. In many systems, the signal strength is processed in logarithmic units, so that the gain control system has a linear response with respect to decibels.
In a digital wireless system, the received signal is initially an analog signal that must be converted into digital form to extract the encoded digital information. The analog-to-digital (A/D) converter used for this conversion has limits on its dynamic range, and often has a dynamic range that is far less than the desired system dynamic range for the receiver. This means that the analog gain of the receiver path must be regulated, based on the received signal strength, before the signal is converted to digital form. However, conventional techniques for regulating the analog gain of the system can give rise to undesired transient effects. For instance, each analog gain adjustment typically results in a change in DC offset in the digitized signal, in addition to the gain change. This unwanted effect can distort the signal and cause degraded performance. Although these DC offset changes can be mitigated with high pass filtering, frequent analog gain changes will still cause degradation of the signal, since it takes a while to remove the offset of each gain change by this filtering.
One way to minimize such transient effects is to make as much use of the dynamic range at the A/D converter as possible, thus adjusting the analog gain prior to the converter only when necessary, e.g., only when the received signal strength passes outside some pre-defined boundaries. Remaining gain adjustments, e.g., to provide a level signal for digital demodulation, may then be performed in the digital domain, e.g., using a digital gain amplifier (DGA). These digital gain changes do not give rise to distorting transient effects.
In a multi-carrier system, two or more distinct radio-frequency carriers are placed close to one another in frequency, with both carrying information targeted to a given receiver. As specified by the 3rd-Generation Partnership Project (3GPP) for Wideband Code-Division Multiple Access (W-CDMA) systems, a W-CDMA multi-carrier system in its simplest form uses two adjacent carriers of width 3.84 MHz, with approximately 5 MHz spacing between the carrier center frequencies. However, the 3GPP standards also support use of different spacing and different numbers of carriers.
Regardless of the details of carrier spacing and bandwidth, each carrier will be subject to different variations in received signal strength as received through the antenna or antennas of a wireless receiver, even if the carriers are transmitted with identical signal levels. Thus, one approach to automatic gain control in a multi-carrier receiver is to use two independent receiver chains, each with a separate and independent AGC system. The independent AGC systems provide each carrier with the gain changes needed to keep that signal constant in signal strength. However, this approach can be very expensive in terms of receiver components and integrated circuit chip area, which translate directly to production cost for the receiver. Furthermore, to the extent that independent receiver chains are used for each carrier, the power consumption of the receiver will tend to increase linearly with the number of carriers. Since market pressures make it very important to keep the number of radio parts and the power consumption as low as possible, multi-carrier receiver designs that re-use several components of the receiver are highly desirable.
In several embodiments of the present invention, a receiver circuit configured for processing multi-carrier signals comprises a common analog signal path, for conditioning the received multi-carrier signal and converting the received multi-carrier signal into digital form, and at least first and second carrier-specific paths for extracting first and second distinct carriers, respectively, from the digitized signal and for processing the carrier-specific signals to obtain the transmitted information. The common analog signal path includes an analog variable-gain circuit, such as an analog variable-gain amplifier, prior to an analog-to-digital converter. The first and second carrier-specific paths include first and second carrier-specific, digital variable-gain circuits, respectively. The receiver circuit further comprises a gain control circuit configured to control the analog variable-gain circuit and the digital variable-gain circuits and to allot gain adjustments to the analog variable-gain circuit based on a difference between carrier signal levels for the first and second carriers.
In some embodiments, the gain control circuit is further configured to selectively operate, based on the difference between the first and second carrier signal levels, in an all-carrier mode, in which gain adjustments for the analog variable-gain circuit are calculated from both of the first and second carrier signal levels, or in one of one or more unequal-priority modes, in which gain adjustments for the analog variable-gain circuit are calculated from only one of the first and second carrier signal levels. In some of these embodiments, the gain control circuit is further configured to calculate gain adjustments for each of the digital variable-gain circuits based on the corresponding first or second carrier signal levels and previously calculated gain adjustments for the analog variable-gain circuit.
Exemplary embodiments of the gain control circuit are configured to operate in the all-carrier mode when the difference between the first and second carrier signal levels is less than a first pre-determined threshold and to calculate gain adjustments for the analog variable-gain circuit, when in the all-carrier mode, based on an average of the first and second carrier signal and a pre-determined optimal input level for the analog-to-digital converter. In some embodiments, the one or more unequal-priority modes include a signal-to-interference-ratio mode (SIR mode), in which the gain control circuit is configured to calculate gain adjustments for the analog variable-gain circuit based on the carrier signal level for the one of the first and second carriers having the higher signal-to-interference ratio. In some of these embodiments, the gain control circuit is configured to operate in the SIR mode when the difference between the first and second carrier signal levels is between first and second pre-determined thresholds, and to calculate gain adjustments for the analog variable-gain circuit, when in the SIR mode, based on the carrier signal level for the one of the first and second carriers having the higher signal-to-interference ratio (SIR) and a selected one of two pre-determined target input levels for the analog-to-digital converter. The gain control circuit may be further configured to select the upper one of the two pre-determined target input levels for calculating the adjustments when the carrier having the higher signal-to-interference ratio also has the higher carrier signal level and to select the lower one of the two pre-determined target input levels for calculating the adjustments when the carrier having the higher signal-to-interference ratio has the lower carrier signal level, in these embodiments.
Some of the above embodiments are further configured to selectively operate in a dropped-carrier mode, in which the gain control circuit is configured to calculate gain adjustments for the analog variable-gain circuit based on the higher of the first and second carrier signal levels. The gain control circuit in these embodiments may be configured to operate in the dropped-carrier mode when the difference between the first and second carrier signal levels is greater than a pre-determined maximum threshold, and to calculate gain adjustments for the analog variable-gain circuit, when in the dropped-carrier mode, based on the higher of the first and second carrier signal levels and a selected one of a pre-determined optimal input level for the analog-to-digital converter and a pre-determined maximum input level for the analog-to-digital converter. In some of these embodiments, the gain control circuit is further configured, when in the dropped-carrier mode, to determine whether the carrier having the lower signal level is deemed to have higher priority, and to base the calculated gain adjustments for the analog variable-gain circuit on the pre-determined maximum input level and the higher of the carrier signal levels when the carrier having the lower signal level is deemed to have higher priority, otherwise basing the calculated gain adjustments for the analog variable-gain circuit on the pre-determined optimum input level and the higher of the carrier signal levels.
Other embodiments of the invention include methods for processing a received multi-carrier signal, including methods that generally correspond to the apparatus discussed above. These methods may be implemented using a suitable wireless receiver or gain control circuit. An exemplary method for processing a received multi-carrier signal thus comprises conditioning the received multi-carrier signal and converting the received multi-carrier signal into a digital signal, using a common analog signal path comprising an analog variable-gain circuit and an analog-to-digital converter, extracting first and second carrier-specific signals from the digital signal, adjusting the first and second carrier-specific signals using first and second carrier-specific, digital variable-gain circuits, respectively, and allotting gain adjustments to the analog variable-gain circuit based on a difference between carrier signal levels for the first and second carriers.
In particular, some embodiments comprise selecting, based on a difference between the first and second carrier signal levels, an all-carrier mode, in which the gain adjustments for the analog variable-gain circuit are calculated from both of the first and second carrier signal levels, or one of one or more unequal-priority modes, in which gain adjustments for the analog variable-gain circuit are calculated from only one of the first and second carrier signal levels.
Of course, those skilled in the art will appreciate that the present invention is not limited to the above features, advantages, contexts or examples, and will recognize additional features and advantages upon reading the following detailed description and upon viewing the accompanying drawings.
Although embodiments of the present invention are described herein with respect to multi-carrier operation in W-CDMA systems, those skilled in the art will recognize that the inventive techniques disclosed and claimed herein are not so limited and may be advantageously applied to a wide array of multi-carrier wireless systems, such as a multi-carrier Long-Term Evolution (LTE) system, a multi-carrier Enhanced Data rates for GSM Evolution (EDGE) system, or the like. Furthermore, the use of the term “exemplary” is used herein to mean “illustrative,” or “serving as an example,” and is not intended to imply that a particular embodiment is preferred over another or that a particular feature is essential to the present invention. Likewise, the terms “first” and “second,” and similar terms, are used simply to distinguish one particular instance of an item or feature from another, and do not indicate a particular order or arrangement, unless the context clearly indicates otherwise.
Those skilled in the art will readily appreciate that the simple solution illustrated in
In several embodiments of the present invention, as pictured in
In the second portion of the receiver, i.e., after the A/D converter 220, the individual carriers are extracted and processed individually, using carrier extraction circuits 225 and 226 and digital variable-gain circuits 230 and 231. A single automatic gain control (AGC) unit 235 has access to the variable-gain circuits (i.e. VGA 215 and digital variable-gain circuits 230 and 231) in each of these two different receiver portions. The digital variable-gain circuits 230 and 231 are followed by RAKE receiver 240, channel & signal measurements circuit 245, combiner 250, and decoder 250; as with the corresponding circuits in
Although the common analog variable-gain circuit 215 in the pictured embodiment comprises a single amplifier, referred to in the following discussion as a variable-gain amplifier (VGA), those skilled in the art will appreciate that an analog variable-gain circuit may comprise two or more amplifiers, in some embodiments, and may even comprise one or more variable-attenuation circuits, in other embodiments. In any case, the analog variable-gain circuit in various embodiments is common for two or more carriers of a received multi-carrier signal, and is situated before A/D conversion and carrier extraction occurs.
The carrier-specific, digital variable-gain circuits 230 and 231, sometimes referred to herein as a digital variable-gain amplifiers (DVGAs), are situated after A/D conversion and carrier extraction. Because this portion of the receiver processes the individual carriers of the multi-carrier signal separately, there is one such digital variable-gain circuit for each carrier. Generally speaking, many embodiments of these DVGAs may operate by multiplying samples of the extracted carrier by a scale factor. However, those skilled in the art will appreciate that a number of techniques for adjusting the level of a digitized signal are possible, and that not all of these techniques necessarily include “amplification” per se.
AGC circuit 235 coordinates the gain adjustments needed for the two carriers, controlling the analog variable-gain circuit and the digital variable-gain circuits and allotting gain adjustments to the variable-gain circuits based on the carrier signal levels and signal-to-interference ratios (SIRs). In particular, as will be described in further detail below, the gain control circuit 235 allots adjustments to the analog variable-gain circuit by selectively operating in one of two or more operating modes based on the difference in carrier signal levels for the carriers of the multi-carrier signal. These operating modes include at least a dual-carrier mode, in which gain adjustments for the analog variable-gain circuit are calculated from both of the first and second carrier signal levels, and one or more unequal-priority modes, in which gain adjustments for the analog variable-gain circuit are calculated from only one of the first and second carrier signal levels.
Those skilled in the art will appreciate that AGC unit 235 may be implemented using digital hardware, one or more appropriately programmed programmable circuits, or a combination thereof, in various embodiments. In some embodiments, AGC unit 235 may be implemented as part of a more general baseband processing circuit, such as the baseband and processing and control circuit 400 pictured in
Although pictured as a single, monolithic block in
In any case, since there are at least two receiver chains in the receiver, one for each carrier, but only a single common analog variable-gain circuit, each receiver path will “request” or require separate individual gain changes to best control the power of the receiver path's specific carrier. Thus, a centralized control circuit decides how to make the best use of the available gain-adjustment resources, and thus how to best accommodate the differing and sometimes conflicting gain control needs from the two or more paths. In extreme cases, e.g., where there are strong fading effects which cause large differences in received signal levels between the two or more carriers, the control circuit must decide how to prioritize between the two carriers.
In some embodiments of the present invention, the receiver is thus configured to selectively operate in one of three distinct AGC “modes” at any given time, depending on the relative power levels of the carriers involved. If the power levels are close enough to one another, the analog gain changes to be used can be calculated as averages of the individual requested analog gain changes from the AGC. Specifically, in some embodiments, the gain control circuit is configured to operate in this first mode when the difference between carrier signal levels is less than a first pre-determined threshold, and to calculate gain adjustments for the analog variable-gain circuit, when in this first mode, so that the average of the carrier signal levels at the input to the A/D converter falls at or near a pre-determined optimal input level for the A/D converter. Although this first operating mode is referred to herein as “dual-carrier mode”, those skilled in the art will appreciate that the techniques described here may be generalized to receivers handling two or more carriers, in which case the mode might instead be referred to as an “all-carrier mode.” In this mode, as with the others described below, the gain control circuit calculates gain adjustments for each of the digital variable-gain circuits based on the corresponding carrier levels and the previously calculated adjustments for the analog variable-gain circuit. In other words, any remaining gain adjustments necessary to bring the carrier-specific signals to a desired signal level for demodulation and other processing are allotted to the carrier-specific digital variable-gain circuits.
In a second operating mode, the power levels of the two (or more) carriers differ enough that they cannot be adjusted with the common analog gain adjustments to all fit within pre-determined upper and lower limits at the input to the A/D converter, but can nonetheless be adjusted to fall within pre-determined absolute maximum and minimum limits. In this operating mode, referred to hereinafter as “SIR mode,” the gain control circuit calculates gain adjustments for the analog variable-gain circuit based on the carrier signal level for the one of the first and second carriers having the higher signal-to-interference ratio. Specifically, in some embodiments, the gain control circuit is configured to control the analog variable-gain circuit so that the carrier having the higher SIR is at or near an upper pre-determined target input level if that carrier also has the highest carrier signal level. If the carrier having the higher SIR has the lower carrier signal level, the gain control circuit instead adjusts the analog gain so that the carrier with the higher SIR falls at or near a lower pre-determined target input level. In this manner, the carrier with the best SIR is given priority by the AGC circuit, but in a manner that tends to minimize any harm to the other carrier(s).
Finally, a third operating mode applies to situations where the carrier signal levels are so different that the two (or more) carriers cannot simultaneously be adjusted to fit within pre-determined maximum limits for the A/D converter input level. In this mode, called “dropped-carrier mode” in the discussion that follows, the gain control circuit is configured to calculate gain adjustments for the analog variable-gain circuit based on the higher of the carrier signal levels. Specifically, in some embodiments, the gain control circuit is configured to operate in the dropped-carrier mode when the difference between the first and second carrier signal levels is greater than a pre-determined maximum threshold, and to calculate gain adjustments for the analog variable-gain circuit, when in the dropped-carrier mode, based on the higher of the of carrier levels and either a pre-determined optimal input level for the A/D converter or a pre-determined maximum input level for the A/D converter.
This choice depends on whether one of the carriers is deemed to have priority over the others. If one of the carriers is a so-called “anchor” carrier and has a lower signal level than the other(s), then the gain control circuit is configured to set the higher-level carrier to the maximum input level for the A/D converter. On the other hand, if the anchor carrier has the highest signal level, the AGC circuit instead adjusts the analog gain so that the anchor carrier is at or near an optimal input level for the A/D converter. If none of the carriers has priority over the others, the higher-level carrier is adjusted to fall at or near the optimal input level for the A/D converter.
Following is a more detailed discussion of the above-described operating modes, with reference to the receiver circuit 200 of
Referring to the receiver 200 of
These measures of the carrier signal levels into the A/D converter 220 may be compared to pre-determined upper and lower limits for these signal strengths denoted (in dB) by PMAX and PMIN, respectively. If the implemented analog gain adjustments force either carrier into a lower power level than its defined PMIN, then that carrier is effectively dropped from consideration by the AGC circuit. The AGC circuit then operates in a so-called single-carrier mode (or a reduced-carrier mode, if more than two carriers), centering on the remaining carrier (or carriers). The “dropped” carrier may still be demodulated, but may provide significantly reduced throughput, due to the lower signal level. Although the limits PMAX and PMIN might be individualized for each carrier, the following description assumes that they are the same, to make the description easier to read. However, those skilled in the art will appreciate that the extension of the techniques described below to the more general case is straightforward; this generally holds for all such limits defined in the detailed description below.
In some embodiments, an additional set of “stricter” upper and lower limits PUPPER and PLOWER may be introduced. These limits are stricter in the sense that the range from PUPPER and PLOWER falls entirely within the range PMAX to PMIN In other words, PLOWER≧PMIN and PUPPER≦PMAX for both carriers. As will be illustrated in more detail below, if the difference between the carrier signal levels is greater than the difference between PUPPER and PLOWER, so that both carriers cannot be maintained within those limits, but the difference is not so great that the carriers cannot be maintained between the PMAX and PMIN, then the AGC circuit can still take both carriers into account, but will use the measured signal-to-interference ratio (SIR) for each carrier, together with the PMAX and PMIN limits, to determine how to allot gain adjustments between the analog variable-gain circuit and the digital paths. In the discussion that follows, this mode is called “SIR mode.”
Those skilled in the art will appreciate that if the pre-determined thresholds are selected so that PLOWER=PMIN and PUPPER=PMAX, then a simpler circuit results, in which either all carriers are considered, and maintained within the limits, or the weaker carrier is dropped. In this case, the intermediate situation where the SIR of each carrier is compared is not considered. This model follows automatically as a limiting case of the general model.
In any event, given the three operating modes discussed above, two difference thresholds may be computed from the pre-determined signal level limits. In particular:
Δ1=PUPPER−PLOWER, (1)
and
Δ2=PMAX−PMIN, (2)
If it is assumed that the center (in dB) of the converter's operating range is optimal in some sense, then the optimal A/D converter input level (in dB) can be written as
For embodiments in which separately implemented carrier-specific AGC algorithms each separately request an analog VGA gain change and a digital gain change, the requested analog gain changes from each AGC algorithm may be denoted (in dB), with respect to Pn, as δg1, and δg2 respectively. Furthermore, the requested digital gain changes (in dB) from each AGC algorithm may be denoted δd1 and δd2, respectively. In these embodiments, if PLOWER≦Pn≦PUPPER for carrier n, then the corresponding carrier-specific algorithm should request a gain change only for the DGA, as changes to the analog gain should only be made when necessary. On the other hand, if Pn is outside that range, an analog gain change that aims at setting Pn=POPT will be requested, as the individual algorithms will choose their respective requested gain changes independently of each other, i.e., without consideration of the signal levels for the other carrier(s).
In operating modes where both carriers are used, the gain control circuit can have two (or more) analog gain adjustment requests, and must therefore determine how to convert these requests into a single analog gain adjustment. Of course, if the resulting analog gain change differs from the analog gain change requested for a specific carrier, then the digital gain adjustment for carrier may be easily compensated to account for the difference between the actually applied analog gain and the requested analog gain for that carrier. In this manner, each receiver path will get the overall total gain change it requests, although not necessarily distributed between the analog and digital adjustments in the manner originally requested.
Before each gain change, the difference in amplitudes between the two carriers are calculated as:
Δ=|P1−P2|, (4)
where P1 and P2 are the latest measured carrier-specific signal strengths, referred to the input of the A/D converter 220. The value of Δ may be used to determine which operating mode should be used.
For instance, if Δ≦Δ, (where Δ1 is defined in Equation (1)), then both carriers' signal strengths fit within the inner pre-defined dynamic window. This is the case illustrated by carriers C1 and C2 in
In the second case, at least one of the requested analog gain changes is not zero, i.e., δg1≠δg2 or δg1=δg2≠0. In this case, the analog variable-gain circuit should be adjusted so that the amplitudes of the two carriers are centered symmetrically around the optimal amplitude POPT. Expressing this in terms of the requested gain changes from the AGC's and the optimal signal level POPT:
where δ{tilde over (g)} is the adjustment to the analog variable-gain circuit, relative to the previous setting. Once the analog gain adjustment is calculated, then gain adjustments for each of the digital variable-gain circuits may be calculated, based on the corresponding carrier signal levels and δ{tilde over (g)}:
δ{tilde over (g)}1=δd1−(δ{tilde over (g)}−δg1), and (6)
δ{tilde over (d)}2=δd2−(δ{tilde over (g)}−δg2), (7)
where δdi is the requested digital gain adjustment and δ{tilde over (d)}i the calculated digital gain adjustment for carrier i, relative to the previous gain setting for the digital variable-gain circuit.
After implementing these gain changes, the new analog gain for the common analog signal path is:
ga=ga,old+δ{tilde over (g)}, (8)
and the new digital gain for each carrier-specific receiver path is:
dn=dn,old+δ{tilde over (d)}n, (9)
where dn,old and ga,old are the corresponding values before update.
If Δ1≦Δ≦Δ2, the carriers cannot be adjusted to both fit between PUPPER and PLOWER, but can be adjusted to fit within the extended dynamic window, i.e., between PMAX and PMIN. An example of this scenario is illustrated in
For instance, if it is assumed that carrier 2 of two carriers has the best SIR, and if the signal strength of carrier 2, P2, is the weaker of the two, then the AGC circuit will calculate an analog gain adjustment that puts carrier 2 at the pre-determined threshold level PLOWER. This may be achieved by setting:
δ{tilde over (g)}=PLOWER−P2, (10)
δ{tilde over (d)}1=δd1−(δ{tilde over (g)}−δg1), (11)
and
δ{tilde over (d)}2=δd2−(δ{tilde over (g)}−δg2), (12)
On the other hand, if carrier 2 has the best SIR and also has the stronger signal of the two, then the AGC circuit will calculate an analog gain adjustment that puts carrier 2 at the threshold level PUPPER. This is achieved by setting:
δ{tilde over (g)}=PUPPER−P2, (13)
where δ{tilde over (g)}1 and δ{tilde over (d)}2 are calculated in the same way as above. If carrier 1 has the best SIR, then the indices 1 and 2 in the two formulas above are simply interchanged.
Those skilled in the art will appreciate that since the above approach overrides the adjustments requested by the individual AGC algorithms, and instead puts the carrier with worse SIR outside the “allowed” analog gain range, then the AGC algorithm associated with that carrier will request an analog gain change whenever a new gain change for that carrier is requested. If a new analog adjustment is re-calculated for each cycle, the resulting transients due to DC offsets will happen for every cycle. To avoid this problem, hysteresis may be incorporated into the control cycle. For instance, assuming that the weaker carrier has the best SIR, a range from PLOWER to PLOWER+x(dB) may be defined to take care of small fading variations. In these embodiments, the weaker carrier is initially centered within that region. (In some embodiments, the size of the region, x, may dynamically depend on the strength of the stronger carrier.) As long as the signal strength for the weaker carrier remains within that region, all analog gain change requests are translated into corresponding digital gain changes, and the analog gain is kept unchanged. A similar approach may be adapted to the upper threshold PUPPER for situations in which the stronger carrier has the better SIR. In either case, if hysteresis is applied to the threshold, then a distinction must be made between how to treat the initial gain change when entering the SIR mode, and the subsequent gain changes when only DGA gain changes are made. These subsequent gain changes are then made until a new recalibration of the SIR mode is needed, e.g., when both signals fall outside the desired window, or when the AGC circuit leaves the SIR mode altogether.
In a third operating mode, denoted “single-carrier” or “reduced-carer” mode, the difference in signal strength between the two (or more) carriers is so large that both carriers cannot simultaneously be adjusted to fall within the outer A/D operating limits PMAX and PMIN, i.e., Δ>Δ2. In this case there are two possibilities. Either the two carriers are considered equally important from a power controlling perspective, or one carrier is considered to be a so called anchor carrier. If the two carriers can be considered equally important, then the algorithm centers on the strongest carrier. As mentioned above, the “dropped” carrier is still demodulated, but will yield reduced throughput due to the low signal strength. Analog gain adjustment requests for the weaker carrier may be ignored while in single-carrier mode. Instead, the digital gains corresponding to the weakest carrier are updated so that the output from the weaker carrier will still give an output signal at the requested set point for that carrier-specific circuit. In this sub-mode of the single-carrier mode, the AGC circuit may simply adopt the gain change requests corresponding to the strongest carrier, without any changes. This means that this carrier will have its analog signal strength Pn centered in the dynamic range of the input to the A/D converter, i.e., at POPT, as shown in
If one carrier is an anchor carrier, then that carrier must get priority. If the anchor carrier is the stronger carrier, then the single-carrier algorithm described above is unchanged. If the anchor carrier is the weaker carrier, on the other hand, then the analog gain changes should instead be such that:
PS=PMAX, (14)
where PS is the carrier signal level for the stronger carrier, with the necessary adjustments to the digital gain changes resulting from this setting.
As above, in order to avoid analog transients, an anchor carrier interval width of x dB can be defined. The stronger carrier should then be centered such that:
PS=PMAX−x/2. (15)
(Typically, x must be less than Δ2 for this interval to make any sense at all.) While in single-carrier mode the AGC circuit would regulate the power with only digital gain changes so long as the signal level PS remains inside of that anchor carrier interval (assuming the anchor carrier remains the weaker carrier).
One possible concern with the above-described approach is that that the circuit may in theory, when in SIR mode, toggle between the two different carriers if they have nearly equal SIR. In practice this is not likely to happen, because the first gain change will increase the SIR of the prioritized carrier further, and worsen the SIR of the not prioritized carrier. In any event, if slowly filtered SIR values are used, the updates to the values will be less frequent, e.g., once every frame. Those skilled in the art will also appreciate that limitations in gain ranges for the analog and digital variable-gain circuits have not been expressly considered in the discussions above. If these limitations are such that the above adjustments cannot be implemented, the formulas above must be complemented with max and min values corresponding to the maximum and minimum variable-gain adjustments, as applicable. Furthermore, those skilled in the art will appreciate that the above discussion implicitly assumes that the gain range for the digital variable-gain circuit is significantly larger than the effective upper/lower and max/min limits due to the upper/lower and max/min bounds of the input levels to the A/D converter. However, since the digital nature of this circuit provides a great deal of flexibility, this is a reasonable assumption.
Those skilled in the art will also appreciate that there may be more than one analog variable-gain amplifier in the receiver chain. The procedures described herein can readily be modified to account for boundary conditions that dictate how the analog gains should be distributed between these components; the details of these modifications will depend on the specific circuits involved and will be apparent to those skilled in the art, and are thus not discussed further herein.
Finally, the discussion above generally assumed a two carrier multi-carrier system. Those skilled in the art will appreciate that the specific techniques described above can be modified in a straightforward manner to operate on more than two carriers. For example, the generalization of the difference in amplitude, Δ, to the case with more than two carriers, becomes:
Δ=max(P1, . . . , PN)−min(P1, . . . , PN), (16)
where N is the number of carriers.
The above detailed discussion generally assumed that separate gain control loops for each of the carrier-specific receiver chains generated gain adjustment requests having two parts, i.e., an analog gain adjustment request and a digital gain adjustment. This approach may be preferred in some embodiments, so that the receiver can more easily be switched to operate with just a single received carrier, as the carrier-specific gain control loop can then operate without modification (and without assistance from a “master” gain control algorithm). On the other hand, if the individual single carrier AGC's only give their requested gains as a single value, in dB, without any information on how to distribute between different amplifiers, then an analogous implementation to the one just described can be easily developed. The difference in this case is that the “master” gain control circuit will decide how to distribute the gain changes between the various analog and digital gains based on gain adjustment requests from the carrier-specific control circuits that specify a total needed adjustment.
Assume the individual AGC's give as their output, single gain requests in dB, denoted ak1 and ak2 respectively, for an illustrative dual-carrier situation. The master control circuit in these embodiments is configured to store the previous gain requests as ak1old and ak2old respectively. Then, the gain changes in dB become:
δa{tilde over (k)}1=ak1−ak1old, and (17)
δa{tilde over (k)}2=ak2−ak2old, (18)
For this embodiment, the same modes of operation apply as in the previous description, i.e., dual-carrier mode, SIR mode and single-carrier mode. As above, if Δ≦Δ1 then both carriers fit within the defined dynamic window for the dual-carrier operating mode. This mode of operation is similar to the previous version, though somewhat simpler. The condition for the gain changes to be accommodated entirely by changing the DGA gain can be expressed as the requirement that min(P1,P2)>PLOWER and max(P1,P2)<PUPPER must both be satisfied. In that case, the applied gain changes are simply δ{tilde over (g)}=0, δ{tilde over (d)}1, and δ{tilde over (d)}2=δa{tilde over (k)}2.
If the needed gain changes cannot be accommodated by the digital variable-gain circuits alone, then the gain control circuit adjusts the analog variable-gain circuit to center the average signal strength of the two carriers within the dynamic window for the input to the A/D converter. The digital variable-gain circuit may then be controlled to take care of any additional gain change that may be needed to reach the set point for each individual carrier. The gain changes that achieve this are given by
δ{tilde over (d)}1=δa{tilde over (k)}1−δ{tilde over (g)}, and δ{tilde over (d)}2=δa{tilde over (k)}2−δ{tilde over (g)}.
If the difference between the carrier signal levels is such that the carriers cannot be adjusted to fall between PUPPER and PLOWER at the A/D converter input, but can be simultaneously adjusted to fall between PMAX and PMIN, then the gain control circuit enters SIR mode. As in the earlier description, if we assume that carrier 2 has the best SIR, then if the signal strength of carrier 2, P2, is the weaker signal of the two, the gain control circuit will adjust the analog variable-gain circuit so that carrier 2 is placed at or near the lower end of the pre-determined input window for the A/D converter, i.e., at PLOWER. This is achieved by setting: δ{tilde over (g)}=PLOWER−P2, δ{tilde over (d)}1=δa{tilde over (k)}1−δ{tilde over (g)}, and δ{tilde over (d)}2=δa{tilde over (k)}2−δ{tilde over (g)}. On the other hand, if the signal strength of carrier 2, P2, is the stronger signal of the two, then the gain control circuit will adjust the analog variable-gain circuit so that carrier 2 falls at or near the other end of the pre-determined input window for the A/D converter, i.e., at PUPPER. This is achieved by setting: δ{tilde over (g)}=PUPPER−P2, δ{tilde over (d)}1=δa{tilde over (k)}1−δ{tilde over (g)}, and δ{tilde over (g)}2=δa{tilde over (k)}2−δ{tilde over (g)}. If carrier 1 has the best SIR, then the indices 1 and 2 may simply be interchanged in the above equations.
In the third operating mode, single-carrier mode, the gain changes can be executed analogously with the original proposal for single carrier mode. Thus, if no carrier has priority over the other, or if the priority carrier is also the strongest carrier, then the gain control circuit simply adjusts the analog variable-gain circuit so that the stronger carrier is centered on the A/D converter's input operating window. If one carrier is prioritized, and is weaker than the other, then the stronger carrier is instead adjusted to fall at or near the top end of the A/D converter's absolute operating limit.
With the above-described automatic gain control techniques in mind, those skilled in the art will appreciate that
As shown at block 510, the received multi-carrier signal is amplified, downconverted, as applicable, and digitized. Based on signal level measurements for each of the carriers of the multi-carrier signal, a common analog gain adjustment and carrier-specific digital gain adjustments are calculated, as shown at block 520. The common analog gain adjustment is applied to the received multi-carrier signal using an analog variable-gain circuit, as shown at block 530. First and second carrier-specific signals are extracted from the multi-carrier signal, as shown at block 540, and the carrier-specific digital gain adjustments, are applied separately to the first and second carrier-specific signals, as shown at block 550. Of course, those skilled in the art will appreciate that the general technique illustrated in
As described above, an automatic gain control circuit according to some embodiments of the invention may be configured to selectively operate in one of several modes, based on the difference between carrier signal levels for first and second carriers of a multi-carrier signal.
In any case, the process illustrated in
As shown at block 620, the absolute difference between the carrier signal levels is calculated; this difference is used to select one of the three operating modes described above. In particular, as shown at block 630, if the difference is less than a first threshold value, here defined by the preferred A/D operating window PUPPER to PLOWER, then mode “A” is selected. As will be seen in the discussion of
If the difference between the carrier signal levels is greater than the first threshold value, but is less than a second threshold value, here defined by the operating limits of the A/D converter PMAX to PMIN, then mode “B” is selected. Otherwise, mode “C” is selected. As will be seen in the discussions of
Referring now to
Although not shown in
Referring now to
If the SIR for carrier 2 is lower than that of carrier 1, the process follows the right-hand side of
As with the dual-carrier mode described above, carrier-specific digital gain adjustments may then be calculated based on the calculated analog gain adjustment and the carrier signal levels. Subsequent variations in the signal level may be compensated by adjusting the digital gain adjustments alone, until the SIR values change so that the other carrier should have priority, or until the higher-priority carrier slips outside of the A/D converter's preferred operating window, or until the difference between the carriers changes so that either dual-carrier mode or dropped-carrier mode becomes appropriate.
If one of the carriers is an anchor carrier, then processing proceeds according to the right-hand side of
Once again, carrier-specific digital gain adjustments may then be calculated based on the calculated analog gain adjustment and the carrier signal levels. Subsequent variations in the signal level may be compensated by adjusting the digital gain adjustments alone, until the relative priorities of the carriers change, until the difference between the carriers changes so that SIR mode becomes appropriate, etc.
With the above variations and examples in mind, those skilled in the art will appreciate that the preceding descriptions of various embodiments of methods and apparatus for processing a received multi-carrier signal are given for purposes of illustration and example. As suggested above, one or more of the specific processes discussed above, including the process flows illustrated in
Those skilled in the art will recognize, of course, that the present invention may be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. For instance, although some of the embodiments illustrated and described herein are configured to process only a two-carrier multi-carrier signal, the inventive techniques disclosed and claimed are not so limited, and can be readily extended to apply to multi-carrier signals having more than two component carriers. The present embodiments are thus to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
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