Note: Descriptions are shown in the official language in which they were submitted.
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CDMA SIGNAL TRANSMISSION USING
RATIOS OF IN-BAND AND OUT-OF-BAND SIGNALS
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to Code Division Multiple Access
(CDMA) systems. More particularly, the present invention includes, but is not
limited to, a novel and improved CDMA base station that generates a ratio of
in-band to out-of-band signal strength for use in base station transmission
control.
II. Description of the Related Art
Code Division Multiple Access (CDMA) technology is commonly used
in communications systems. In a typical CDMA system, a CDMA base station
transmits a CDMA signal to numerous CDMA communications devices, such
as wireless telephones. The CDMA signal is comprised of numerous individual
user signals. The CDMA base station generates the CDMA signal by encoding
each individual user signal with a unique spreading sequence, such as a pseudo
random sequence. The CDMA base station then adds the encoded user signals
together to form the CDMA signal.
In a CDMA system, individual user signals are not separated based on
frequency or time, but are spread across the entire frequency band. Each
CDMA communications device derives its particular user signal based on the
unique spreading sequence. Due to this combination of multiple signals
encoded with random sequences, the CDMA signal has random signal
characteristics that create special power control concerns.
CDMA base stations generate undesirable noise in the form of signal
power outside of the frequency band of the CDMA signal. This undesirable
power is referred to as out-of-band signal power. Out-of-band signal power is
a problem because it interferes with other signals in the neighboring
frequency
bands. These other signals are disrupted by the interference. Government
agencies, such as the Federal Communications Commission in the United
States, strictly regulate the interference caused by out-of-band signal power.
An existing solution to the problem is implemented during base station
testing. Test equipment is used to calculate a ratio for a test CDMA signal
transmitted by the base station. The ratio represents the in-band signal power
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versus the out-of-band signal power. The base station transmit power is
adjusted during the testing so the ratio is below a maximum value with a
margin for some ratio increase under the maximum value. This usuallu
Unfortunately, the ratio is not calculated and is not used during normal base
station operation in the field. Test equipment is used to calculate the ratio,
and
base stations are not equipped to calculate the ratio in the field. Thus, the
ratio
is not automatically generated and used to control operation in the field
where
changes in temperature and load alter base station operation.
Another existing solution to this problem is to operate the CDMA base
station so a ratio of the power out to the pilot signal does not exceed a
value,
such as five. This solution is lacking because a maximum power level based on
the pilot signal is not an optimal estimate of the point where out-of-band
signal
power becomes a problem. As a result, the range and capacity of the base
station is not optimized.
CDMA systems would be improved through transmission at a power
level just below the point where out-of-band signal power becomes a problem.
Transmission at this power level would optimize the range and capacity of the
base station.
SUMMARY OF THE INVENTION
The above-described problem is solved with CDMA transmission
control technology. The CDMA transmission control technology allows a
CDMA base station to operate at an optimized power level without generating
improper amounts of out-of-band noise. The optimized power level extends
the range and capacity of the base station. The increased range and capacity
is
passed on to the end-user in the form of decreased costs and increased
functionality.
A transmitted CDMA signal has in-band components and out-of-band
components. Transform logic automatically generates a ratio of the signal
strength of the in-band components versus the out-of-band components. In
some examples of the invention, control logic uses the ratios to generate
metric
signals that indicate if transmit power should be limited and that indicate
excess forward link capacity. The invention eliminates the need to leave a
margin during testing for future ratio increases. The elimination of this
margin
allows operation at higher power levels.
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BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the present invention will
become more apparent from the detailed description set forth below when
taken in conjunction with the drawings in which like reference characters
identify correspondingly throughout and wherein:
FIG. 1 is a block diagram of a CDMA system with transmission control
logic;
FIG. 2 is a graph illustrating the frequency spectrum of a CDMA signal;
FIG. 3 is a block diagram of a CDMA system with transmission control
logic;
FIG. 4 is a block diagram of a CDMA base station with transmission
control logic;
FIG. 5 is a is a graph illustrating the frequency spectrum of portions of a
CDMA signal used for transmission control; and
FIG. 6 depicts a logical table used for transmission control.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
CDMA Transmission Control Technolo~y - FIGS. 1-2
FIG. 1 depicts a CDMA signal 100, a CDMA transmitter 101, an RF
CDMA signal 102, and a CDMA receiver 103. CDMA is a spread-spectrum
communications technology. Some versions of CDMA are specified by
standards, such as IS-95 approved by the Telecommunications Industry
Association. The CDMA signal 100 could be any CDMA signal, such as the
signal produced by a cell site modem in a CDMA base station. The CDMA
receiver 103 could be any CDMA device capable of receiving a CDMA signal,
such as a wireless CDMA telephone.
The CDMA transmitter 101 transmits the CDMA signal 102 to the
CDMA receiver 103. The CDMA transmitter 101 could be any CDMA
transmission device that includes transmit control logic 116. One example of
the CDMA transmitter 101 is a CDMA base station.
The transmit control logic 116 in the CDMA transmitter 101 generates a
ratio based on the signal strength of in-band versus out-of-band portions of
the
CDMA signal 102. Signal strength can be measured in various ways with
examples being power, voltage, or energy. In some examples of the invention,
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the transmit control logic 116 generates metric signals that indicate if
transmit
power should be limited and that indicate excess forward link capacity.
FIG. 2 illustrates the frequency spectrum of a CDMA signal. The vertical
axis represents signal power, and the horizontal axis represents frequency.
The
desired in-band signal power is contained within the bandwidth defined by
corner frequencies around a center frequency. A typical example is a 1.25 MHz
bandwidth centered about a 1.96 GHz center frequency with corner frequencies
at (1.96 GHz - 625 KHz) and (1.96 GHz + 625 KHz). The signal power drops
significantly outside of the bandwidth, but some undesired out-of-band signal
power is still present and is shaded on FIG. 2. Out-of band signal power is
undesirable because it represents wasted power that interferes with other
signals in neighboring frequency bands.
CDMA System with Transmission Control Technology - FIGS 3-5
FIGS. 3-5 depict a specific example of a CDMA system that uses the
transmission control technology of the present invention, but those skilled in
the art will recognize numerous other types of CDMA systems that are
applicable to the invention described above.
FIG. 3 depicts a communications system 304 that is connected to the
CDMA communications system 306. The CDMA communications system 306
communicates with CDMA communications devices 308. The CDMA
communications system 306 is comprised of a switching center 310 and a base
station 312. The communications system 304 exchanges communications
signals 305 with the switching center 310. The switching center 310 exchanges
communications signals 311 with the base station 312. The base station 312
exchanges wireless CDMA communications signals 307 over the air interface
with the CDMA communications devices 308. Although the invention is
depicted using an air interface, other transmission media could also be used,
such as RF cable, power lines, or telephone lines.
The communications system 304 could be any communications system
capable of exchanging communications signals 305 with the CDMA
communications system 306. The communications system 304 is typically a
conventional public telephone network, but could also be many other
networks, such as a local area network, wide area network, or Internet.
The switching center 310 could be any device that provides an interface
between the base station 312 and the communications system 304. Typically,
numerous base stations are connected to the communications system 304
through the switching center 310, but the number of base stations has been
restricted for the purpose of clarity.
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The base station 312 exchanges wireless CDMA signals 307 with the
CDMA communications devices 308. The base station 312 includes transmit
control logic 316 that generates power and capacity metric signals based on
ratios of the in-band versus out-of-band power in the CDMA signals 307.
5 Those skilled in the art could adapt the base station 312 from known
systems,
such as the base stations provided by Qualcomm, Inc. of San Diego,
California.
The CDMA communications devices 308 exchange wireless CDMA
signals 307 with the base station 312. Typically, numerous CDMA
communications devices exchange signals with the base station 312, but the
number of communications devices has been restricted for the purpose of
clarity. The typical CDMA communications device is a mobile telephone, but
other CDMA communications devices are also possible, such as fixed wireless
devices, data terminals, set-top boxes, or computers.
In operation, the CDMA communications devices 308 communicate
through the CDMA communications system 306 with the communications
system 304 or with each other. On the forward link communications path from
the communications system 304 to the CDMA communications devices 308, the
transmit control logic 316 generates various ratios based on the in-band
versus
out-of-band power in portions of the CDMA signal 307. The transmit control
logic 316 compares the ratios to pre-determined values that represent the
point
where out-of-band signal power becomes improper.
The transmit control logic 316 generates a power metric signal and a
capacity metric signal based on the comparison. If one of the calculated
ratios
exceeds its associated pre-determined value, then the power metric signal
indicates that the transmit power of the base station 312 should be limited.
The
capacity metric signal indicates an estimate of the excess forward link
capacity
of the base station 312. The estimate is typically given in a number of
additional simultaneous calls that can be handled by the base station 312
without one of the calculated ratios exceeding its associated pre-determined
value.
FIG. 4 depicts the base station 312 of FIG. 3 receiving the
communications signals 311 and transmitting the CDMA communications
signals 307. The base station 312 is comprised of the following elements
connected in series: cell site modems 421, digital-to-analog conversion and
filter 422, up-converter 423, gain limiter 424, power amplifier 425, and
antenna
426. The transmit control logic 316 is coupled to the path between the power
amplifier 425 and the antenna 426 to monitor the transmitted CDMA signal 307.
The transmit control logic 316 is comprised of down-converter 432, transform
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logic 434, and control logic 436. Aside from the transmit control logic 316,
those
skilled in the art are familiar with these elements and their operation.
The cell site modems 421 produce a baseband CDMA signal and provide
it to the digital-to-analog conversion and filter 422. The digital-to-analog
conversion and filter 422 converts the CDMA signal to analog and filters out
components outside of the desired bandwidth. The digital-to-analog
conversion and filter 422 provides the CDMA signal to the up-converter 423.
The up-converter 423 modulates the CDMA signal with intermediate and radio
frequencies to form a Radio Frequency (RF) CDMA signal, and typically
generates undesirable out-of-band signal power. The up-converter 423 provides
the RF CDMA signal to gain limiter 424. The gain limiter 424 limits the power
level of the RF CDMA signal based on a power metric signal 437. The gain
limiter 424 provides the RF CDMA signal to the power amplifier 425. The
power amplifier 425 amplifies the RF CDMA signal, and typically generates
undesirable out-of-band signal power. The power amplifier 425 provides the
amplified RF CDMA signal to the antenna 426 for transmission the RF CDMA
signal 307.
The down-converter 432 of the transmit control logic 316 monitors the
CDMA signal 307. The down-converter 432 de-modulates the RF CDMA signal
307 to form a baseband CDMA signal. The down-converter 432 provides the
baseband CDMA signal to the transform logic 434.
FIG. 5 illustrates the frequency spectrum of the baseband CDMA signal
received by the transform logic 434. Those skilled in the art recognize that
FIG.
5 is an ideal representation of the signal. The vertical axis represents
signal
power, and the horizontal axis represents frequency. The desired in-band
signal power is contained within the bandwidth defined by corner frequencies
around a center frequency. Bandwidth segments 551-557 are shown.
Bandwidth segment 551 is in-band, and bandwidth segments 552-557 are out-
of-band. The power in each bandwidth segment 551-557 is shaded on FIG 5.
The segments depicted on FIG. 5 are shown for illustrative purposes, and the
actual segments used could vary in number or bandwidth. The segments could
also be determined based on industry standards
The transform logic 434 performs Fast-Fourier transforms to generate
values representing the power in each bandwidth segment 551-557. Those
skilled in the art are familiar with the logic required to perform such Fast
Fourier transforms. The transform logic 434 then generates ratio values that
will depend on the segments used. In this example, the following ratio values
are generated:
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ratio 1- bandwidth segment 551 power/bandwidth segment 552 power;
ratio 2 - bandwidth segment 551 power/bandwidth segment 553 power;
ratio 3 - bandwidth segment 551 power/bandwidth segment 554 power;
ratio 4 - bandwidth segment 551 power/bandwidth segment 555 power;
ratio 5 - bandwidth segment 551 power/bandwidth segment 556 power;
ratio 6 - bandwidth segment 551 power/bandwidth segment 557 power.
The transform logic 434 generates a ratio signal 435 that indicates the
ratio values and transfers the ratio signal 435 to the control logic 436. The
control logic 436 compares each of the ratio values to an associated pre-
determined maximum value for the particular ratio. The control logic 436
determines if any of the calculated ratios exceed their respective maximum
values.
FIG. 6 depicts a logical table containing the ratio values 1-6, the
respective maximum values, an indication if the ratios exceed the maximum
values, and the difference between the ratios and the maximum values. Those
skilled in the art are aware that the table is a logical representation that
is
capable of numerous implementations using conventional technology. The
letters A-G that are listed in the table for the power ratio entries represent
actual power measurements. The letters H-M that are listed in the table for
the
maximum value entries represent actual maximum values that can be readily
obtained in standard industry publications, such as IS-97 by the
Telecommunication Industry Association. The Federal Communications
Commission also publishes maximum ratio values.
The control logic 436 of the base station 312 generates a power metric
signal 437 and transfers it to gain limiter 424. The power metric signal 437
sets
a flag in the gain limiter 424 if one of the ratios exceeds its maximum value.
The flag causes the gain limner 424 to limit the transmit power of the base
station 312. The power metric signal 437 clears the flag when none of the
ratios
exceed their maximum values. In this fashion, the transmit power of the base
station 312 is optimized to the point set by the maximum ratio values.
The control logic 436 of the base station 312 generates a capacity metric
signal 438 and transfers it to a base station control system (not shown). The
capacity metric signal 438 indicates an estimate of the excess forward link
capacity of the base station 312. To generate the estimate, the control logic
436
determines the average difference between the measured ratios and the
maximum values and translates the difference into a number of additional
simultaneous calls that the base station 312 can handle without one of the
calculated ratios exceeding its associated pre-determined value. The base
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station control system can determine whether or not to block call hand-offs or
new calls based on the capacity metric signal 438. In this fashion, the number
of
simultaneous calls handled by the base station 312 is optimized to the point
set
by the maximum ratio values.
Other applications of the ratio are described in United States patent
applications assigned to the same entity as this application. The use of the
ratio
to control power amplifier pre-distortion is described in "Predistortion
Technique For High Power Amplifiers", filed on June 26, 1998, and hereby
incorporated by reference into this application. The use of the ratio to
control
decresting is described in "CDMA Signal Transmission Control", filed on the
same date as this application, and hereby incorporated by reference into this
application.
The previous description of the preferred embodiments is provided to
enable any person skilled in the art to make or use the present invention. The
various modifications to these embodiments will be readily apparent to those
skilled in the art, and the generic principles defined herein may be applied
to
other embodiments without the use of the inventive faculty. Thus, the present
invention is not intended to be limited to the embodiments shown herein but is
to be accorded the widest scope consistent with the principles and novel
features disclosed herein.
WE CLAIM:
