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Claims  |
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What is claimed is:
1. An apparatus for maintaining received signal power levels at an average
level when a signal power estimate on average is similar to the average of
actually received signal power levels, comprising:
(a) estimating means for generating an estimate of the power of a received
signal;
(b) difference means, operatively coupled to the estimating means, for
generating a difference signal by subtracting the estimated received
signal power from a predetermined reference signal power; and
(c) adjustment means, operatively coupled to the difference means, for
adjusting a signal power control threshold as a function of the difference
signal.
2. The apparatus of claim 1 wherein the estimating means generates an
estimate of the power of a received signal by averaging a plurality of the
signal power estimates together.
3. The apparatus of claim 1 further comprising a power control means,
operatively coupled to the adjustment means, for setting a power control
indicator in response to the result of a comparison between the received
signal power estimate and the adjusted power control threshold.
4. The apparatus of claim 3 further comprising a signal transmitting means,
operatively coupled to the power control means, for transmitting the power
control indicator over a communication channel.
5. The apparatus of claim 4 wherein the signal transmitting means comprises
means for preparing the power control indicator for transmission over the
communication channel by spreading the power control indicator with a
spreading code prior to transmission over the communication channel.
6. The apparatus of claim 4 wherein the communication channel is selected
from the group consisting essentially of an electronic data bus, radio
communication link, wireline and optical fiber link.
7. The apparatus of claim 4 further comprising:
(a) signal receiving means for detecting a power control indicator within a
signal received from over the communication channel; and
(b) power adjustment means, operatively coupled to the signal receiving
means, for adjusting a particular signal transmission power of a signal
transmitter in response to the detected power control indicator.
8. The apparatus of claim 7 wherein the signal receiving means comprises
means for despreading the received signal with a spreading code to detect
the power control indicator.
9. The apparatus of claim 1 wherein the adjustment means adjusts the signal
power control threshold according to the following function:
THR(n)=THR(n-1)+.mu.[P.sub.ref -P(n)]
where,
n=a moment in time;
THR(n)=signal power control threshold at time n;
.mu.=a threshold adaption step size;
P.sub.ref =a predetermined reference signal power; and
P(n)=an estimated received signal power at time n.
10. A method for maintaining received signal power levels at an average
level when a signal power estimate on average is similar to the average of
actually received signal power levels, comprising:
(a) generating an estimate of the power of a received signal;
(b) generating a difference signal by substracting the estimated received
signal power from a predetermined reference signal power; and
(c) adjusting a signal power control threshold as a function of the
difference signal.
11. The apparatus of claim 10 wherein the estimating means generates an
estimate of the power of a received signal by averaging a plurality of the
signal power estimates together.
12. The method of claim 10 further comprising a step of setting a power
control indicator in response to the result of a comparison between the
received signal power estimate and the adjusted power control threshold.
13. The method of claim 12 further comprising a step of transmitting the
power control indicator over a communication channel.
14. The method of claim 13 wherein the step of transmitting comprises
preparing the power control indicator for transmission over the
communication channel by spreading the power control indicator with a
spreading code prior to transmission over the communication channel.
15. The method of claim 10 wherein the communication channel is selected
from the group consisting essentially of an electronic data bus, radio
communication link, wireline and optical fiber link.
16. The method of claim 13 further comprising the steps of:
(a) detecting a power control indicator within a signal received from over
the communication channel; and
(b) adjusting a particular signal transmission power of a signal
transmitter in response to the detected power control indicator.
17. The method of claim 16 wherein the step of detecting comprises
despreading the received signal with a spreading code to detect the power
control indicator.
18. The method of claim 10 wherein the signal power control threshold is
adjusted according to the following function:
THR(n)=THR(n-1)+.mu.[P.sub.ref -P(n)]
where,
n=a moment in time;
THR(n)=signal power control threshold at time n;
.mu.=a threshold adaption step size;
P.sub.ref =a predetermined reference signal power; and
P(n)=an estimated received signal power at time n.
19. An apparatus for maintaining received signal power levels at an average
level when a signal power estimate on average is similar to the average of
actually received signal power levels, comprising:
(a) estimating means for generating an estimate of the power of a received
signal;
(b) difference means, operatively coupled to the estimating means, for
generating a difference signal by subtracting the estimated received
signal power from a predetermined reference signal power;
(c) adjustment means, operatively coupled to the difference means, for
adjusting a signal power control threshold as a function of the difference
signal;
(d) power control means, operatively coupled to the adjustment means, for
setting a power control indicator in response to the result of a
comparison between the received signal power estimate and the adjusted
power control threshold; and
(e) signal transmitting means, operatively coupled to the power control
means, for transmitting the power control indicator over a communication
channel.
20. The apparatus of claim 19 further comprising:
(a) signal receiving means for detecting a power control indicator within a
signal received from over the communication channel; and
(b) power adjustment means, operatively coupled to the signal receiving
means, for adjusting a particular signal transmission power of a signal
transmitter in response to the detected power control indicator.
21. The apparatus of claim 19 wherein the adjustment means adjusts the
signal power control threshold according to the following function:
THR(n)=THR(n-1)+.mu.[P.sub.ref -P(n)]
where,
n=a moment in time;
THR(n)=signal power control threshold at time n;
.mu.=a threshold adaption step size;
P.sub.ref =a predetermined reference signal power; and
P(n)=an estimated received signal power at time n.
22. A method for maintaining received signal power levels at an average
level when a signal power estimate on average is similar to the average of
actually received signal power levels, comprising:
(a) generating an estimate of the power of a received signal;
(b) generating a difference signal by subtracting the estimated received
signal power from a predetermined reference signal power;
(c) adjusting a signal power control threshold as a function of the
difference signal
(d) setting a power control indicator in response to the result of a
comparison between the received signal power estimate and the adjusted
power control threshold; and
(e) transmitting the power control indicator over a communication channel.
23. The method of claim 22 further comprising the steps of:
(a) detecting a power control indicator within a signal received from over
the communication channel; and
(b) adjusting a particular signal transmission power of a signal
transmitter in response to the detected power control indicator.
24. The method of claim 22 wherein the signal power control threshold is
adjusted according to the following function:
THR(n)=THR(n-1)+.mu.[P.sub.ref -P(n)]
where,
n=a moment in time;
THR(n)=signal power control threshold at time n;
.mu.=a threshold adaption step size;
P.sub.ref =a predetermined reference signal power; and
P(n)=an estimated received signal power at time n. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to communication systems and, more
particularly, to a method and apparatus for adjusting a power control
threshold in a communication system.
BACKGROUND OF THE INVENTION
Communication systems take many forms. In general, the purpose of a
communication system is to transmit information-bearing signals from a
source, located at one point, to a user destination, located at another
point some distance away. A communication system generally consists of
three basic components: transmitter, channel, and receiver. The
transmitter has the function of processing the message signal into a form
suitable for transmission over the channel. This processing of the message
signal is referred to as modulation. The function of the channel is to
provide a physical connection between the transmitter output and the
receiver input. The function of the receiver is to process the received
signal so as to produce an estimate of the original message signal. This
processing of the received signal is referred to as demodulation.
Two types of two-way communication channels exist, namely, point-to-point
channels and point-to-multipoint channels. Examples of point-to-point
channels include wirelines (e.g., local telephone transmission), microwave
links, and optical fibers. In contrast, point-to-multipoint channels
provide a capability where many receiving stations may be reached
simultaneously from a single transmitter (e.g. cellular radio telephone
communication systems). These point-to-multipoint systems are also termed
Multiple Address Systems (MAS).
Analog and digital transmission methods are used to transmit a message
signal over a communication channel. The use of digital methods offers
several operational advantages over analog methods, including but not
limited to: increased immunity to channel noise and interference, flexible
operation of the system, common format for the transmission of different
kinds of message signals, improved security of communication through the
use of encryption, and increased capacity.
These advantages are attained at the cost of increased system complexity.
However, through the use of very large-scale integration (VLSI) technology
a cost-effective way of building the hardware has been developed.
To transmit a message signal (either analog or digital) over a band-pass
communication channel, the message signal must be manipulated into a form
suitable for efficient transmission over the channel. Modification of the
message signal is achieved by means of a process termed modulation. This
process involves varying some parameter of a carrier wave in accordance
with the message signal in such a way that the spectrum of the modulated
wave matches the assigned channel bandwidth. Correspondingly, the receiver
is required to recreate the original message signal from a degraded
version of the transmitted signal after propagation through the channel.
The re-creation is accomplished by using a process known as demodulation,
which is the inverse of the modulation process used in the transmitter.
In addition to providing efficient transmission, there are other reasons
for performing modulation. In particular, the use of modulation permits
multiplexing, that is, the simultaneous transmission of signals from
several message sources over a common channel. Also, modulation may be
used to convert the message signal into a form less susceptible to noise
and interference.
For multiplexed communication systems, the system typically consists of
many remote units (i.e. subscriber units) which require active service
over a communication channel for a short or discrete portion of the
communication channel resource rather than continuous use of the resources
on a communication channel. Therefore, communication systems have been
designed to incorporate the characteristic of communicating with many
remote units on the same communication channel. These systems are termed
multiple access communication systems.
One type of multiple access communication system is a spread spectrum
system. In a spread spectrum system, a modulation technique is utilized in
which a transmitted signal is spread over a wide frequency band within the
communication channel. The frequency band is much wider than the minimum
bandwidth required to transmit the information being sent. A voice signal,
for example, can be sent with amplitude modulation (AM) in a bandwidth
only twice that of the information itself. Other forms of modulation, such
as low deviation frequency modulation (FM) or single sideband AM, also
permit information to be transmitted in a bandwidth comparable to the
bandwidth of the information itself. However, in a spread spectrum system,
the modulation of a signal to be transmitted often includes taking a
baseband signal (e.g., a voice channel) with a bandwidth of only a few
kilohertz, and distributing the signal to be transmitted over a frequency
band that may be many megahertz wide. This is accomplished by modulating
the signal to be transmitted with the information to be sent and with a
wideband encoding signal.
Three general types of spread spectrum communication techniques exist,
including:
Direct Sequence
The modulation of a carrier by a digital code sequence whose bit rate is
much higher than the information signal bandwidth. Such systems are
referred to as "direct sequence" modulated systems.
Hopping
Carrier frequency shifting in discrete increments in a pattern dictated by
a code sequence. These systems are called "frequency hoppers." The
transmitter jumps from frequency to frequency within some predetermined
set; the order of frequency usage is determined by a code sequence.
Similarly "time hopping" and "time-frequency hopping" have times of
transmission which are regulated by a code sequence.
Chirp
Pulse-FM or "chirp" modulation in which a carrier is swept over a wide band
during a given pulse interval.
Information (i.e. the message signal) can be embedded in the spread
spectrum signal by several methods. One method is to add the information
to the spreading code before it is used for spreading modulation. This
technique can be used in direct sequence and frequency hopping systems. It
will be noted that the information being sent must be in a digital form
prior to adding it to the spreading code, because the combination of the
spreading code and the information, typically a binary code, involves
module-2 addition. Alternatively, the information or message signal may be
used to modulate a carrier before spreading it.
Thus, a spread spectrum system must have two properties: (1) the
transmitted bandwidth should be much greater than the bandwidth or rate of
the information being sent and (2) some function other than the
information being sent is employed to determine the resulting modulated
channel bandwidth.
As previously mentioned, spread spectrum communication systems can be
multiple access systems communication systems. One type of multiple access
spread spectrum system is a code division multiple access (CDMA) system.
In a CDMA system, communication between two communication units e.g., a
central communication site and a mobile communication Unit is accomplished
by spreading each transmitted signal over the frequency band of the
communication channel with a unique user spreading code. Due to this
spreading transmitted signals are in the same frequency band of the
communication channel and are separated only by unique user spreading
codes. These unique user spreading codes preferably are orthogonal to one
another such that the cross-correlation between the spreading codes is
approximately zero. CDMA systems may use direct sequence or frequency
hopping spreading techniques. Particular transmitted signals can be
retrieved from the communication channel by despreading a signal
representative of the sum of signals in the communication channel with a
user spreading code related to the particular transmitted signal which is
to be retrieved from the communication channel. Further, when the user
spreading codes are orthogonal to one another, the received signal can be
correlated with a particular user spreading code such that only the
desired user signal related to the particular spreading code is enhanced
while the other signals for all of the other users are not enhanced.
It will be appreciated by those skilled in the art that several different
spreading codes exist which can be used to separate data signals from one
another in a CDMA communication system. These spreading codes include but
are not limited to pseudo noise (PN) codes and Walsh codes. A Walsh code
corresponds to a single row or column of the Hadamard matrix. For example,
in a 64 channel CDMA spread spectrum system, particular mutually
orthogonal Walsh codes can be selected from the set of 64 Walsh codes
within a 64 by 64 Hadamard matrix. Also, a particular data signal can be
separated from the other data signals by using a particular Walsh code to
spread the particular data signal.
Further it will be appreciated by those skilled in the art that spreading
codes can be used to channel code data signals. The data signals are
channel coded to improve performance of the communication system by
enabling transmitted signals to better withstand the effects of various
channel impairments, such as noise, fading, and jamming. Typically,
channel coding reduces the probability of bit error, and/or reduces the
required signal to noise ratio usually expressed as bit energy per noise
density (E.sub.b /N.sub.o), to recover the signal at the cost of expending
more bandwidth than would otherwise be necessary to transmit the data
signal. For example, Walsh codes can be used to channel code a data signal
prior to modulation of the data signal for subsequent transmission.
Similarly PN spreading codes can be used to channel code a data signal.
A typical spread spectrum transmission involves expanding the bandwidth of
an information signal, transmitting the expanded signal and recovering the
desired information signal by remapping the received spread spectrum into
the original information signals bandwidth. This series of bandwidth
trades used in spread spectrum signaling techniques allow a communication
system to deliver a relatively error-free information signal in a noisy
signal environment or communication channel. The quality of recovery of
the transmitted information signal from the communication channel is
measured by the error rate (i.e., the number of errors in the recovery of
the transmitted signal over a particular time span or received bit span)
for some E.sub.b /N.sub.o. As the error rate increases the quality of the
signal received by the receiving party decreases. As a result,
communication systems typically are designed to limit the error rate to an
upper bound or maximum so that the degradation in the quality of the
received signal is limited.
In CDMA spread spectrum communication systems, the error rate is related to
the noise interference level in the communication channel which is
directly related to number of simultaneous but code divided users within
the communication channel. Thus, in order to limit the maximum error rate,
the number of simultaneous code divided users in the communication channel
is limited. However, the error rate is also affected by the received
signal power level. In some spread spectrum communication systems (e.g.,
cellular systems) a central communication site typically attempts to
detect or receive more than one signal from a particular band of the
electromagnetic frequency spectrum.
The central communication site adjusts the receiver components to optimally
receive signals at a particular received signal power threshold value.
Those received signals having a received signal power level at or near the
particular power threshold level are optimally received. In contrast those
received signals not having a received signal power level at or near the
particular power threshold level are not optimally received. A
non-optimally received signal tends to have a higher error rate or
interfere with received signals from other users. This higher error rate
can result in the communication system further limiting the number of
simultaneous users in the communication channel associated with the
central communication site. Thus, it is desirable to maintain the received
signal power level at or near the particular power threshold level. This
can be accomplished by adjusting the signal power level of transmitters
attempting to transmit to the central communication site. Therefore, by
using power control schemes to maintain the received signal power levels
at a particular power threshold level the number of simultaneous users in
a communication channel can be maximized for a particular maximum error
rate limit.
However, a need exists for a way to compensate for a mobile communication
unit's velocity (i.e., speed that a mobile cellular phone is moving). It
will be appreciated by those skilled in the art that a power control
system for a mobile unit will behave differently at different mobile unit
speed levels. Further a power control system is basically a nonlinear
feedback system. Furthermore, the power control system would, in general,
have different gain for different input frequency. Thus, the uncompensated
power control system will have different gain and result in different
average received signal power level, because the frequency contents of the
input to such a power control system, i.e., the instantaneous receiver
power, is different for each different mobile unit (vehicle) speed.
Through the use of compensation for a more accurate power control scheme,
the number of simultaneous users in a communication channel can be
increased over the number of simultaneous users in a communication channel
using a less accurate power control scheme while maintaining the same
maximum error rate limit.
SUMMARY OF THE INVENTION
A method and apparatus is provided for maintaining received signal power
levels at an average level when a signal power estimate is on average
similar to the average of actually received signal power levels. The
maintaining of the received signal power levels is accomplished by
generating an estimate of the power of a received signal. Subsequently, a
difference signal is generated by subtracting the estimated received
signal power from a predetermined reference signal power. Finally, a
signal power control threshold is adjusted as a function of the difference
signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a preferred embodiment communication
system which uses orthogonal coding and power control.
FIG. 2 is a block diagram showing a preferred embodiment power control
threshold adjustment apparatus.
FIG. 3 is flowchart detailing the power control threshold adjustment steps
performed by the preferred embodiment communication system of FIGS. 1 and
2.
DETAILED DESCRIPTION
Referring now to FIG. 1, a preferred embodiment closed-loop power control
system in a communication system is shown. The power control system is for
a reverse channel (i.e., the base communication site 100 adjusts the
transmit signal power of the mobile station 102). The base station 100
receiver estimates the received signal power transmitted by the mobile
station 102 of a particular user. In the preferred embodiment, power is
preferably estimated 104 every 1.25 ms, i.e., during the time period (T)
of 6 Walsh words. Several power estimates can be averaged together to get
a long term average power estimate. The power estimate is compared 106 to
a threshold. A control indicator (e.g., a bit or plurality of bits) is
generated 108 based on the result of the comparison. If the estimate is
larger than the threshold, the power control indicator is set to be one.
Otherwise it is set to be zero. The power control indicator is encoded 110
and transmitted 112 via the forward channel. The encoding may include
spreading the power control indicator with a spreading code prior to
transmission over the communication channel. To reduce the burden to the
forward channel, preferably only one power control indicator is
transmitted every 1.25 ms. As a result, the mobile station 102 detects the
power control indicator from within a signal received from over the
communication channel and subsequently will either increase or decrease
122 its transmitter 114 power every 1.25 ms according to the power control
indicator 120 received. The detection of the power control indicator may
involve despreading the received signal with a spreading code. The
transmitter 114 will increase the transmission power if the received
control indicator is a zero. Otherwise, it 114 will decrease the
transmission power. The step of power increase or decrease preferably is
between 0.2 to 0.8 dB, and the power change within every 12.5 ms is held
to less than 5 dB. The time delay of the mobile station 102 response after
receiving the power control indicator should be no larger than 2 ms.
From the above description, it can be seen that the power control system is
a nonlinear feedback control system with delay. The purpose of such a
control system is to track the instantaneous received signal power change,
if possible. It should also maintain the average received signal power on
a fixed level when the instantaneous power tracking cannot be achieved.
Obviously, if all the average mobile transmitter's 102 powers at the base
station 100 receiver input are equal to each other, the signal to noise
ratio of a particular mobile station 102 can be maintained above a pre
specified value by not allowing the number of mobile stations in the cell
to exceed a certain limit. The signal to noise ratio can be maintained in
this manner because the noise, or interference, for a particular received
signal is mainly due to signals from other mobile stations. If all the
average receiver signal power levels are the same, then the signal to
noise ratio at the input of any receiver is simply equal to 10Log.sub.10 N
(dB), where N is the effective number of transmitting mobile stations.
Although it is possible to perform power control based on the signal to
noise ratio for a particular receiver, a power control system solely based
on signal to noise ratio may become unstable. More precisely, because the
received signal for one mobile station causes interference for others,
then increased transmitter power from one mobile station means increased
interference for the received signals from other mobile stations. Namely,
adjustment of one mobile station's power will affect the signal to noise
ratio of other mobile stations. It will be very difficult to select a
desired signal to noise ratio value for all the mobile stations. Even if
this is possible, such a system will be unstable. For instance, assume
that mobile station A's signal power is increased for some reason. That
station's power increase will cause a decrease in the signal to noise
ratio in all of the received signals from other mobile stations. To
maintain a proper signal to noise ratio, these mobile stations must
increase their transmitter powers and this will cause mobile station A to
increase it's power again. This obviously forms an unstable positive
feedback loop.
The performance of the power control system greatly depends on the
performance of the received signal power estimator. However, once a good
received power estimator is found, the power control system should still
be enhanced. The power control system will behave differently under
different mobile station speed. Since, the power control system is
basically a nonlinear feedback system, it would, in general, have
different gain for different input frequency. Since the frequency contents
of such a system's input, i.e., the instantaneous receiver power, are
different for different mobile station speed, the power control system
will have different gain and result in different average received signal
power level. However, when the long time average of the power estimator
output P(n) is very close to the measured actual average received signal
power, the average received signal power may be maintained by adjusting
the threshold according to the long term average of the power estimator
output.
A simplified preferred embodiment implementation is shown in FIG. 2. In
this implementation, a Least Mean Squared (LMS) adaptive algorithm to
adjust the power control threshold and to perform averaging at the same
time can be used. Every time a power estimate P(n) is generated, it is
subtracted 200 from a fixed reference level P.sub.ref. The difference
signal 202 is used to update 204 the power control threshold THR(n).
Specifically, the power control threshold is updated according to the
following function:
THR(n)=THR(n-1)+.mu.[P.sub.ref -P(n)]
where,
n=a moment in time;
THR(n)=signal power control threshold at time n;
.mu.=a threshold adaption step size which controls the averaging time
constant;
P.sub.ref =a predetermined reference signal power; and
P(n)=an estimated received signal power at time n.
Preferably .mu.=0.001 to achieve a time constant .tau. of 1.25 second
(.tau.=T/.mu.). Since, such a time constant .tau. is much longer than the
response time T (i.e., the estimation time interval) of the preferred
embodiment power control feedback system, the adaptation of power control
threshold will not interfere with the normal operation of the power
control system. However, this adaption can reduce the long term average
power level variation for mobile unit's 102 traveling at different speeds.
Thus, a communication system for using adaptable signal power control
thresholds has been described above with reference to FIGS. 1 and 2. A
flowchart which summarizes the steps performed by the power control system
shown in FIGS. 1 and 2 is shown in FIG. 3. The signal power control system
begins 300 by receiving a signal 302 at the base station 100 receiver 122.
An estimate of the power of the received signal P(n) is generated 104,
304. Subsequently, the new estimate of the signal power P(n) may be
averaged together 306 with previous estimates of the signal power. A
difference signal 202 is generated 308 by subtracting the average
estimated received signal power P(n) from a predetermined reference signal
power P.sub.ref. Subsequently, a signal power control threshold THR(n) is
adjusted 310 as a function of the difference signal. Subsequently a power
control indicator 108 is set 312 in response to a comparison between the
average signal power estimate P(n) and the adjusted power control
threshold THR(n). The power control indicator 108 is spread 110, 314 with
a spreading code. The spread power control indicator 108 is transmitted
112, 316 over a communication channel. Subsequently, a mobile station 102
receives 116, 318 a signal from over the communication channel. A power
control indicator 120 is detected 118, 320 by despreading the received
signal with a spreading code. A particular signal transmission power of a
signal transmitter 114 is adjusted 122, 322 in response to the value of
the detected power control indicator 120 which completes or ends 324 one
loop of the preferred embodiment power control system.
Although the invention has been described and illustrated with a certain
degree of particularity, it is understood that the present disclosure of
embodiments has been made by way of example only and that numerous changes
in the arrangement and combination of parts as well as steps may be
resorted to by those skilled in the art without departing from the spirit
and scope of the invention as claimed. For example, the modulator,
antennas and demodulator portions of the preferred embodiment
communication system power control scheme as described were directed to
CDMA spread spectrum signals transmitted over a radio communication
channel. However, as will be understood by those skilled in the art, the
power control techniques described and claimed herein can also be adapted
for use in other types of transmission systems like those based on TDMA
and FDMA. In addition the communication channel could alternatively be an
electronic data bus, wireline, optical fiber link, or any other type of
communication channel.
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