U.S. patent application number 11/472797 was filed with the patent office on 2007-03-08 for automatic gain control (agc) for multichannel/wideband communications system.
Invention is credited to Manish Bhardwaj, Garret Shih.
Application Number | 20070054642 11/472797 |
Document ID | / |
Family ID | 37830625 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070054642 |
Kind Code |
A1 |
Bhardwaj; Manish ; et
al. |
March 8, 2007 |
Automatic gain control (AGC) for multichannel/wideband
communications system
Abstract
Automatic Gain Control (AGC) system for multi-channel signals
attenuates an incoming multi-channel signal by providing a gain.
The system further adjusts each individual channel, of the
multi-channel signal, by supplying a second gain if needed. The AGC
system is designed to ensure a received signal power is at an
optimal level for analog to digital conversion or any other form of
signal processing. The system also enables elimination of
mid-packet gain adjustments.
Inventors: |
Bhardwaj; Manish;
(Cambridge, MA) ; Shih; Garret; (Brookline,
MA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Family ID: |
37830625 |
Appl. No.: |
11/472797 |
Filed: |
June 22, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11357910 |
Feb 17, 2006 |
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11472797 |
Jun 22, 2006 |
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11190071 |
Jul 26, 2005 |
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11357910 |
Feb 17, 2006 |
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60591381 |
Jul 26, 2004 |
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Current U.S.
Class: |
455/234.1 |
Current CPC
Class: |
H03G 3/20 20130101; H04L
5/06 20130101; H04L 27/14 20130101; H03G 3/3036 20130101 |
Class at
Publication: |
455/234.1 |
International
Class: |
H04B 7/00 20060101
H04B007/00; H04B 1/06 20060101 H04B001/06 |
Claims
1. A multi-channel receiver comprising: an outer programmable gain
controller controlling gain of a multi-channel signal; and a
plurality of inner programmable gain controllers, each inner gain
controller controlling gain of an individual channel.
2. The multi-channel receiver of claim 1, further comprising an
analog to digital converter to digitize the gain controlled
multi-channel signal and wherein each channel further comprises a
digital filter.
3. The multi-channel receiver of claim 1, wherein the outer gain
controller receives feedback from each channel to adjust gain
values.
4. The multi-channel receiver of claim 3, wherein the feedback is
provided by a modem.
5. The multi-channel receiver of claim 3, wherein the feedback is
provided by an analyzer.
6. The multi-channel receiver of claim 1, wherein the outer gain
controller supplies a fixed nominal gain while an acquisition
threshold power level is not exceeded.
7. The multi-channel receiver of claim 6, wherein once the
acquisition threshold power level or a high threshold power level
is exceeded, the outer gain controller adjusts the fixed nominal
gain such that a total power is brought below the acquisition
threshold.
8. The multi-channel receiver of claim 1, wherein the inner gain
controller adjusts a gain when an input signal to said inner gain
controller differs from a predetermined reference level.
9. The multi-channel receiver of claim 1, wherein the outer
programmable gain controller is an analog gain controller.
10. The multi-channel receiver of claim 1, wherein the inner
programmable gain controller is a digital gain controller.
11. A method of automatic gain control for multi-channel
communications comprising: automatically controlling gain of a
multi-channel signal with an outer programmable gain controller;
and automatically controlling gain of individual channels with a
plurality of inner programmable gain controllers.
12. The method of claim 11 further comprising: digitizing the gain
controlled multi-channel signal with an analog to digital
converter, wherein each channel further comprises a digital
filter.
13. The method of claim 11, wherein providing an outer gain
controller further comprising: receiving feedback from each channel
to adjust gain values.
14. The method of claim 11 further comprising: supplying a fixed
nominal gain, with the use of the outer gain controller, to the
multi-channel signal while an acquisition threshold power level is
not exceeded; and adjusting the fixed nominal gain, if the
acquisition threshold power level or a high threshold power level
is exceeded, such that a total power is brought below the
acquisition threshold.
15. The method of claim 11 further comprising: adjusting a gain,
with the plurality of inner gain controllers, when an input signal
to said inner gain controller differs from a predetermined
reference level.
16. The method of claim 11, wherein the automatically controlling
gain of a multi-channel signal further comprises: controlling an
analog gain with the outer programmable gain controller.
17. The method of claim 11, wherein the automatically controlling
gain of a individual channels further comprises: controlling a
digital gain with the inner programmable gain controller.
18. A multi-channel receiver comprising: an attenuation gain
control means for adjusting gain for a multi-channel signal; and a
plurality of gain control means for adjusting gain of individual
channels of the multi-channel signal.
19. A multi-channel receiver comprising: an outer variable gain
amplifier that receives a multi-channel input signal; an outer
analog gain controller that controls the gain applied to the
multi-channel input signal; an analog to digital converter that
digitizes the multi-channel input signal after gain adjustment; a
plurality of band pass filters, said filters receiving the
multi-channel signal and each filter outputting an individual
channel; a plurality of inner variable gain amplifiers, each
receiving a respective individual channel; a plurality of inner
digital gain controllers, each controlling the gain applied to a
respective individual channel; and a plurality of feedback
mechanisms providing feedback from respective channels to the outer
analog gain controller to adjust gain values.
20. The multi-channel receiver of claim 19, wherein the plurality
of feedback mechanisms are modems.
21. The multi-channel receiver of claim 19, wherein the plurality
of feedback mechanisms are end point analyzers.
22. The multi-channel receiver of claim 19, wherein the outer gain
controller supplies a fixed nominal analog gain while an
acquisition threshold power level is not exceeded.
23. The multi-channel receiver of claim 22, wherein once the
acquisition threshold power level or a high threshold power level
is exceeded, the outer gain controller adjusts the analog gain such
that a total power is brought below the acquisition threshold.
24. The multi-channel receiver of claim 19, wherein the inner gain
controller adjusts a digital gain when an input signal to said
inner gain controller differs from a predetermined reference level.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 11/357,910, filed Feb. 17, 2006, which is a
continuation of U.S. application Ser. No. 11/190,071 filed Jul. 26,
2005, which claims the benefit of U.S. Provisional Application No.
60/591,381, filed on Jul. 26, 2004. The entire teachings of the
above applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] Analog to digital conversion is a commonly used technique
wherein a continuous signal is converted to a digital signal for
the purpose of signal processing. An analog to digital converter
(ADC) is often used for such a conversion. ADCs typically have a
limited number of bits available, and thus a limited conversion
range, to perform analog to digital conversions. Automatic gain
control (AGC) is therefore used to adjust the power level of an
incoming signal such that the ADC will receive signals at a fixed
level; thus, the number of bits required by the ADC to perform
conversions may be dramatically reduced. The AGC controls the gain
of a system in order to maintain an adequate performance over a
range of input signal levels.
[0003] Gain will be discussed herein in terms of decibels (dB). A
dB is typically used to describe the ratio between two measurements
of electrical power, which may be arithmetically added and
subtracted. A dBm represents an absolute unit of electrical power.
A dBm may be defined as A=10*log10(P2/(1 mW)), where A is the
absolute unit of power and P2 is a measurement of electrical power.
The ratio of power may be defined as P2/(1 mW)=10 .sup.(A/10). For
example, 1 dBm is one dB greater than 0 dBm, or about 1.259 mW
(1.259=10.sup.1/10).
[0004] A canonical form of a conventional AGC scheme in a digital
communications system 100, is illustrated in FIG. 1. The system 100
comprises a Variable Gain Amplifier (VGA) 103, that receives an
input signal 101. The AGC 105 receives a digital signal 106,
digitized via an ADC 107. The AGC 105 supplies information to the
VGA 103 via a feedback connection 104. The information supplied by
the AGC 105 is used in adjusting the gain supplied to the input
signal 101. It should be appreciated that the gain adjustment
affects the average total power of the signal and not the
instantaneous power of the signal. Thus, the gain adjusted signal
will still comprise its unique signal properties since its
instantaneous power will be intact. A modem 109 is typically used
to demodulate the signal in order to produce bits 113.
[0005] As discussed above, when designing a digital communication
system, the dynamic range must be put into consideration. The
dynamic range of the input signal may be extremely large; 802.11
modems typically support close to 90 dB of dynamic range. Area and
power requirements for an ADC typically increases by four times
every 6 dB. Hence, a large ADC dynamic range is extremely
expensive.
[0006] A solution for this problem, as previously mentioned, is to
reduce the dynamic. range seen at the ADC by performing automatic
gain control. An ideal AGC switches in the right amount of analog
gain such that the signal power at its output A, FIG. 1, is always
the same, regardless of the input signal level. Hence, an ideal AGC
completely eliminates signal dynamic range. Thus, the AGC is
essential in such a system as it controls the gain of an incoming
signal in order to bring the signal to a suitable level for
conversion or any other form of signal processing.
[0007] As an example, consider a system that must receive single
channel signals from -100 dBm to -10 dBm, 90 dB of dynamic range.
To accommodate this range, a VGA is used that must be set to 0
through 90 dB of gain. Therefore, for a signal which is (-10 -X)
dBm, X dB of gain is typically switched into the signal. Using this
technique the output always stays at -10 dBm. Otherwise, assuming 1
bit is required to convert a 6 dB analog signal to a digital
signal, a maximum of 15 bits would be needed to convert a -90 dBm
signal. A conversion requiring 15 bits is technically very
difficult. Thus, if a -40 dBm signal arrives in the system, 30 dB
of gain is added to the signal in order to obtain the optimum
value, dramatically reducing the amount of bits required for the
conversion.
[0008] One way of building such an AGC is to simply cycle through
all possible gain settings, for example in 2 dB steps, and stop
when the desired signal level is reached. One might choose to use a
binary search instead of a linear one to increase the speed of the
acquisition.
SUMMARY OF THE INVENTION
[0009] A system and method for automatically providing gain
adjustments to a multi-channel signal and gain adjustments to an
individual channel, of the multi-channel signal, is discussed. The
system comprises a multi-channel receiver, the receiver further
comprising an outer programmable gain controller, controlling gain
of a multi-channel signal, and a plurality of inner programmable
gain controllers, each inner gain controller controlling gain of a
respective individual channel. The multi-channel receiver further
comprises an analog to digital converter to digitize the gain
controlled multi-channeled signal, and each respective individual
channel further comprises a digital filter.
[0010] The outer gain controller may receive feedback from each
respective channel to adjust gain values and determine whether a
signal is being processed. The feedback may be provided by a modem
or an analyzer. The outer gain controller may supply a fixed
nominal gain while an acquisition threshold power level or a high
threshold power level is not exceeded. Once an acquisition
threshold power level or a high threshold power level is exceeded,
the outer gain controller adjusts the gain such that a total power
is brought below the acquisition threshold. The inner and outer
gain controllers may be either digital or analog.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing will be apparent from the following more
particular description of example embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating embodiments of the present invention.
[0012] FIG. 1 is a schematic of a conventional AGC scheme in a
digital communication system;
[0013] FIG. 2 is a depiction of a multi-channel signal;
[0014] FIG. 3 is a schematic of a multi-channel digital
communication system;
[0015] FIG. 4 is a graphical depiction of a dual packet arrival
example, according to the system shown in FIG. 3;
[0016] FIG. 5 is a graphical depiction of a second dual packet
arrival example according to the system shown in FIG. 3;
[0017] FIG. 6 is a schematic of a multi-channel digital
communication system, according to an aspect of the present
invention; and
[0018] FIGS. 7 and 8 are an example depicting the function of the
multi-channel digital communication system, according to an aspect
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] A description of preferred embodiments of the invention
follows.
[0020] Many problems arise when using prior art methods of
automatic gain control for multi-channel signals. Multi-channel
systems, also referred to as wideband systems, simultaneously
support multiple physical layer channels. As an example, the case
when such channels are frequency separated will be specifically
discussed, but it should be appreciated that such separation may
also be along other dimensions, for example, orthogonal signatures.
It should be understood that these channels typically overlap in
time and are separable along some other dimension.
[0021] An example of a multi-channeled system is illustrated in
FIG. 2. FIG. 2 displays a three 802.11 g channel (1, 6, and 11)
signal in a 2.4 GHz ISM band. An overall block diagram of a system,
which may support such a signal, is shown in FIG. 3. An analog
multi-channel input signal 301 is digitized with the use of an ADC
303. An AGC 304, along with a VGA 306, is used to adjust the gain
of the multi-channel signal. The multi-channel signal is then
filtered into individual channels 1, 6, and 11, with the use of
Band Pass Filters (BPF) 305, 307 and 309, respectively. Modems 0-2
demodulate the individual channels 1, 6, and 11 into bits 0-2,
respectively.
[0022] A problem in an integrated multi-channel system is that,
while multiple channels are received at different power levels and
hence have different optimal gain settings, they are forced to
share a common gain. Therefore, we provide a technique to resolve
the inevitable conflicts that result, such as mid-packet gain
adjustments.
[0023] First, a two packet arrival scenario is presented to
illustrate deficiencies of conventional AGCs in a multi-channel
system. Consider the packet arrival scenario depicted in FIG. 4,
for the communication system shown in FIG. 3. When packet 1 arrives
at -20 dBm, the AGC will attenuate the power level of packet 1 down
by 10 dB. Hence, post AGC, packet 1 will comprise a power level of
-30 dBm, as is desired. When packet 2 arrives with a power level of
-40 dBm, the optimal gain for packet 2 will be 10 dB but it instead
sees a downward attenuation of 10 dB, thus resulting in packet 2
comprising a power level of -50 dBm. This is 20 dB away from the
desired power level. With the arrival of packet 2, the AGC 304 and
VGA 306 will see a slight increase to the total power of the system
from -20 dBm to -19.96 dBm at point 302, during time 1. Once the
first packet ends, the total power of the system, at point 302, is
significantly dropped to -40 dBm at time 2; thus the amount of gain
applied to the system must be significantly increased as packet 1
leaves the system but while packet 2 is still being processed.
Thus, packet 2 will see a mid-packet gain change.
[0024] Now consider the same scenario as described above except
that the second packet arrives at -23 dBm, as shown in FIG. 5. As
was the case in the previous example, packet 1 will receive a
downward attenuation of 10 dB, resulting in its power level to be
increased to -30 dBm. The arrival of the second packet will result
in the overall total power, at point 302, being increased to about
-18 dBm at time 1. Hence, in order to keep the desired -30 dBm
total power level, the AGC would have to switch the downward
attenuation of packet 2 from 10 dB to 12 dB in the middle of packet
1 (1). This type of mid-packet gain change could be catastrophic
for packet 1. Finally, consider the AGC behavior when the first
packet ends at time 2. The power at point 302 now drops by 5 dB to
-23 dBm and the AGC switches the downward attenuation from 12 dB to
7 dB. This gain change is also catastrophic for packet 2.
[0025] A system is needed that will provide the desired gain
adjustments for multi-channel signals, while minimizing mid-packet
gain changes. A block diagram of a wideband AGC scheme, according
to one embodiment of the present invention, is shown in FIG. 6. A
multi-channel input signal 601 is adjusted in gain with the use of
an outer VGA 603. A common analog outer automatic gain controller
(OAGC) 605 provides information to the VGA 603 used to continuously
adjust the gain of the multi-channel input signal 601. The OAGC
operates on the sum of the three channel powers and cannot, for
instance, distinguish between signals traveling on different
channels.
[0026] The functionality of OAGC may be described as a two state
machine. The first state of the OAGC is called the HUNT state.
While the OAGC is in the HUNT state, a fixed, nominal analog gain
is applied. The OAGC stays in this state until the power in the
band differs from an acquisition threshold, or the desired power
level, and if the power level of the incoming signal is within the
operating range of the OAGC. When this happens, the OAGC adjusts
the analog gain such that the total power level is brought down to
the level of the acquisition threshold, and the OAGC transitions to
a second state, the LOCKED state. Thus, the OAGC attenuates the
incoming signal.
[0027] As an example, shown in FIG. 7, an acquisition level is set
to -30 dBm. Packet 1 arrives first at -10 dBm, 701, well within the
headroom. Signals that are received in the headroom range are often
clipped, therefore these signals must be brought down to the spare
range, or the range in which the signal may be successfully
decoded. Thus the OAGC will attenuate the signal downward by 20 dB
in order to bring the total power level of packet 1 to -30 dBm. The
OAGC then transitions into a LOCKED state and will therefore supply
a fixed downward attenuation of 20 dB to all incoming signals until
one of two events occur: (1) if the power exceeds a high threshold,
the OAGC adjusts the gain such that the power level is brought down
to the acquisition threshold and it continues to stay in the LOCKED
state; or (2) if the power drops below a low threshold and if none
of the modems are receiving a packet, the OAGC transitions to the
HUNT state. In FIG. 6, the `Rx in Progress` signal, one per
channel, is used to communicate whether a modem is receiving a
packet.
[0028] As seen in the example provided by FIG. 7, a second packet 2
arrives with a power level of -25 dBm, 703. With the arrival of
packet 2, the system will see an overall power level increase from
-10 dBm to -9.87 dBm. Since the increase in the total power is so
slight and does not exceed the high threshold, the system will
remain in the LOCKED state. The OAGC will therefore supply a
downward attenuation of 20 dB to both packets resulting in packet 2
comprising a total power level of -45 dBm. Packet 2 is now within
the maximum (-20 dBm) and minimum (-70 dBm) decodable level range,
but is 15 dB away from the acquisition level (-30 dBm).
[0029] Upon receiving the analog gain adjustments, the
multi-channel input signal 601 is then digitized with the use of an
ADC 607. BPFs 609-611 filter the multi-channel signal 601 into
individual channels. In order to fully utilize the word length of
the digital signal in the individual channels, the individualized
digital signal may be further adjusted in order to bring the signal
to the acquisition power level. The gain of the individual channels
are digitally adjusted, if needed, with the use of inner VGAs
613-615. Inner automatic gain controllers (IAGC) 617-619 provide
information to the inner VGAs 613-615, respectively, used to adjust
the gain of the individual channels.
[0030] Functionally, inner AGCs are similar to conventional AGCs,
with one difference being that they are entirely digital (there is
no analog gain to control). Each channel comprises its own IAGC
which operates on the output of the channelizing filter. The IAGCs
operate on a single parameter, the desired reference level. When
the input signal to the IAGC differs from the reference level, the
digital gain is adjusted to correct for that difference.
[0031] In the example provided by FIG. 7, once packet 1 is filtered
into its individual channel, the IAGC will not digitally adjust its
gain since the power level of packet 1 is already at the
acquisition level (-30 dBm). Once packet 2 is filtered into its
individual channel, the IAGC will add 15 dB of gain in order to
bring the power level of packet 2 to the acquisition level. The
digital adjustment of packet 2 is completely independent of the
processing done to packet 1. Thus, each packet may have its gain
individually adjusted, eliminating mid-packet gain adjustments.
[0032] The individual channels are then demodulated with the use of
modems 621-623. The modems 621-623 also provide feedback to the
common OAGC identifying if a packet is being processed. It should
also be appreciated that feedback may be provided with the use of
other devices, for example, an end point analyzer.
[0033] Considering the two packet scenario, depicted in FIG. 4, in
relation to the present invention, mid-packet gains are no longer
an issue. The first packet is handled in a similar manner with the
present invention, as would be with a conventional AGC. A downward
attenuation of 10 dB is applied to the input signal comprising
packet 1 (initially comprising a power level of -20 dBm);
therefore, the desired power level of -30 dBm is achieved. The OAGC
has therefore adjusted the analog gain such that the total power is
brought to the acquisition threshold (-30 dBm) and will then
transition to the LOCKED state. Since the power of packet 1 is
already at its desired level, the IAGC for path 1 will not need to
adjust its gain.
[0034] When packet 2 arrives, the total power level of the system
will be increased from -20 dBm to about -19.96 dBm at time 1, given
that this is a minimal increase in power, it will probably not be
significant enough to cross the high threshold. Hence, the OAGC
will stay in the LOCKED state and a downward attenuation of 10 dB
will also be added to packet 2. Thus, packet 2 will now comprise a
power level of -50 dBm. The IAGC of the individual channel
comprising packet 2, will adjust the gain and bring the -50 dBm
packet up to -30 dBm by adding 20 dB of gain. Of course, the ADC
must have enough spare dynamic range to support the digitization of
the -50 dBm signal.
[0035] When the -20 dBm signal ends, the OAGC will notice a 20 dB
drop in power, which may take it below the low threshold. However,
the modem on the individual channel comprising packet 2, will
indicate that a receive is in progress and the OAGC will wait for
that to finish before transitioning back to the HUNT state. Thus,
as may be seen from the above example, the OAGC acts as an
attenuator and shifts the incoming signal downward, while the IAGC
supplies a gain to the individual channels in order to raise the
signal to the acquisition level.
[0036] The values of the maximum and minimum thresholds,
acquisition, and maximum and minimum decodable levels are
determined by system requirements. The acquisition threshold may be
set as in conventional AGCs. It is simply the desired signal level
one wishes to see at the ADC input. The high threshold should be
set higher than the acquisition threshold plus the single sided
OAGC acquisition error but no higher than the tolerable saturation
limit. The low threshold should be set lower than the acquisition
threshold minus the single sided AGC acquisition error. A problem
in setting the low threshold too low is that the OAGC will not
unlock even after the packet that caused the AGC is finished.
[0037] Due to several noise sources that affect signal power
estimation and gain control, practical AGCs always have a finite
acquisition error. So an AGC with +/-1 dB of acquisition error
guarantees that the output of the variable gain stage will be
correct to within that tolerance if the input is within the
specified dynamic range.
[0038] When selecting the value of the threshold levels, it is
useful to examine statistical data to determine the range where
most of the incoming signals will fall. The solution presented is
not a perfect solution as there are occasions where a packet may be
dropped or saturated, as is shown in FIG. 8. In the example
provided by FIG. 8, a first packet 1 arrives with a power level of
0 dBm, 801, thus the OAGC will a downward attenuation of 30 dB to
packet 1 and then transition into the LOCKED state. The arrival of
packet 2, at a power level of -25 dBm, will increase the total
power of the system by barely 0.01 dB, thus the high threshold will
not be exceeded, keeping the OAGC in the LOCKED state. Therefore,
packet 2 will also receive a downward attenuation of 30 dB,
resulting in a power level of -55 dBm. The power level of packet 2
is now below the minimum decodable level and will therefore need a
further adjustment in the individual channel with use of the IAGC.
Although packet 2 is below the minimum decodable level, the IAGC
will still be able to boast the signal up the to acquisition level.
Signals coming in below the minimum decodable level will be
adjusted in gain, or boasted up into the spare range, while the
noise associated with the signal will also be boosted. A modem in
such a case may not have the signal to noise (SNR) capabilities to
decode the signal.
[0039] The amount of headroom budgeted for the system must also be
put into consideration. For example, consider if 15 packets arrived
at the same time, all at the acquisition level (-30 dBm). Although
no gain would be needed, the system would see an overall power
level of -18.24 dBm. Thus, the headroom must be set at a level
greater than -18.24 dBm in order to accommodate the incoming
signal. For the above mentioned reasons it is also useful to
statistically examine the range incoming signals are likely to
fall.
[0040] This invention is applicable to any communications systems
that supports two or more concurrent physical layer channels and
minimizes the need for mid-packet gain adjustments. Although the
channels discussed in this application are separated in frequency
to illustrate the key concepts of the invention, it should be
appreciated that the channels may be separable along other
dimensions.
[0041] Furthermore, although an analog OAGC was considered, it is
conceivable that the OAGC could be fully digital in those
communications systems that utilize this invention to minimize the
digital word-length (which is analogous to ADC dynamic range).
Therefore, all controllers described herein may be analog or
digital controllers and use proportional, integral, and
differential (PID) controllers, state-space controllers, or other
forms of control known in the art.
[0042] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *