U.S. patent application number 11/006403 was filed with the patent office on 2005-06-23 for system and method for adjusting a power level of a transmission signal.
This patent application is currently assigned to Intel Corporation. Invention is credited to Talwar, Shilpa, Tellado, Jose.
Application Number | 20050135503 11/006403 |
Document ID | / |
Family ID | 29999715 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050135503 |
Kind Code |
A1 |
Talwar, Shilpa ; et
al. |
June 23, 2005 |
System and method for adjusting a power level of a transmission
signal
Abstract
The invention includes an apparatus and a method for adjusting a
power level of a transmission signal for minimal distortion. The
method includes modulating the transmission signal. The modulated
transmission signal is processed to reduce a peak to average ratio
of the modulated transmission signal based upon modulation
parameters of the modulated transmission signal. The power level of
the modulated transmission signal is adjusted according to the peak
to average ratio of the modulated transmission signal. The
transmission signal is amplified and transmitted.
Inventors: |
Talwar, Shilpa; (Palo Alto,
CA) ; Tellado, Jose; (Sunnyvale, CA) |
Correspondence
Address: |
INTEL CORPORATION
P.O. BOX 5326
SANTA CLARA
CA
95056-5326
US
|
Assignee: |
Intel Corporation
|
Family ID: |
29999715 |
Appl. No.: |
11/006403 |
Filed: |
December 6, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11006403 |
Dec 6, 2004 |
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10189755 |
Jul 2, 2002 |
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6891902 |
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Current U.S.
Class: |
375/297 |
Current CPC
Class: |
H04L 27/3411 20130101;
H04L 27/2624 20130101; H04W 52/52 20130101 |
Class at
Publication: |
375/297 |
International
Class: |
H04K 001/02 |
Claims
What is claimed:
1. A method for adjusting a power level of a transmission signal
for minimal distortion, comprising: modulating the transmission
signal; processing the modulated transmission signal to reduce a
peak to average ratio of the modulated transmission signal based
upon modulation parameters of the modulated transmission signal;
adjusting the power level of the modulated transmission signal
according to the peak to average ratio of the modulated
transmission signal; amplifying the transmission signal; and
transmitting the transmission signal.
2. The method for adjusting the power level of a transmission
signal of claim 1, wherein adjusting the power level of the
modulated transmission signal is dependent upon the peak to average
ratio of the modulated transmission signal.
3. The method for adjusting the power level of a transmission
signal of claim 1, wherein the processing the modulated
transmission signal comprises windowing time samples of the
modulation transmission signal.
4. The method for adjusting the power level of a transmission
signal of claim 3, wherein windowing time samples is dependent upon
a bandwidth of the modulated transmission signal.
5. The method for adjusting the power level of a transmission
signal of claim 3, wherein windowing the time samples comprises
centering the windowing dependent upon characteristics of the time
samples.
6. The method for adjusting the power level of a transmission
signal of claim 3, wherein an amplitude of the windowing is
dependent upon characteristics of the time samples.
7. The method for adjusting the power level of a transmission
signal of claim 1, wherein the modulation parameters comprise
modulation rate.
8. The method for adjusting the power level of a transmission
signal of claim 1, wherein the modulation parameters comprise
modulation order.
9. The method for adjusting the power level of a transmission
signal of claim 1, wherein adjusting the power level of the
modulated transmission signal based upon modulation parameters is
adaptive to changes in the modulation parameters of the modulated
transmission signal.
10. The method for adjusting the power level of a transmission
signal of claim 1, wherein processing the modulated transmission
signal to reduce a peak to average ratio of the modulated
transmission signal is adaptive to changes in the modulation
parameters of the modulated transmission signal.
11. The method for adjusting the power level of a transmission
signal of claim 1, wherein a transmission channel that the
modulated transmission signal is transmitted through is
adaptive.
12. The method for adjusting the power level of a transmission
signal of claim 3, wherein the windowing comprises at least one of
Hamming, Hanning, Chebychev and Gaussian windows.
13. The method for adjusting the power level of a transmission
signal of claim 3, wherein the windowing comprises a window length
that is determined through the use of a look-up-table (LUT).
14. The method for adjusting the power level of a transmission
signal of claim 13, wherein the look-up-table (LUT) provides a
window length based upon a desired out-of-band distortion of the
transmission signal.
15. The method for adjusting the power level of a transmission
signal of claim 3, wherein output PAR level is determined through
the use of a look-up-table (LUT).
16. The method for adjusting the power level of a transmission
signal of claim 15, wherein the look-up-table (LUT) provides an
output PAR level based upon a modulation order of the transmission
signal.
17. The method for adjusting the power level of a transmission
signal of claim 13, wherein the look-up-table (LUT) provides an
output PAR level based upon a modulation order and coding rate of
the transmission signal.
18. The method for adjusting the power level of a transmission
signal of claim 1, further comprising: linearizing a transmitter
amplifier that amplifies the modulated transmission signal.
19. The method for adjusting the power level of a transmission
signal of claim 17, wherein linearizing a transmitter amplifier
comprises: pre-distorting the modulated transmission signal to
pre-correct for non-linearities of the transmitter amplifier.
20. The method for adjusting the power level of a transmission
signal of claim 17, wherein the pre-distorting is adaptive to
changes in non-linearities of the transmitter amplifier over
time.
21. The method for adjusting the power level of a transmission
signal of claim 1, wherein the transmission signal includes signals
from a plurality of transmission chains.
22. A method for adjusting a power level of a transmission signal
for minimal distortion, comprising: modulating the transmission
signal; processing the modulated transmission signal to reduce a
peak to average ratio of the modulated transmission signal based
upon modulation parameters of the modulated transmission signal,
wherein the processing the modulated transmission signal comprises
windowing time samples of the modulation transmission signal.
adjusting the power level of the modulated transmission signal
according to the peak to average ratio of the modulated
transmission signal; amplifying the transmission signal; and
transmitting the transmission signal.
23. An apparatus for adjusting a power level of a transmission
signal for minimal distortion, comprising: means for modulating the
transmission signal; means for processing the modulated
transmission signal to reduce a peak to average ratio of the
modulated transmission signal based upon modulation parameters of
the modulated transmission signal, wherein the processing the
modulated transmission signal comprises windowing time samples of
the modulation transmission signal. means for adjusting the power
level of the modulated transmission signal according to the peak to
average ratio of the modulated transmission signal; means for
amplifying the transmission signal; and means for transmitting the
transmission signal.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to wireless communications.
More particularly, the invention relates a method and system for
adjusting a power level of a transmission signal to minimize
distortion of the transmission signal.
BACKGROUND OF THE INVENTION
[0002] Wireless communication systems commonly include information
carrying modulated carrier signals that are wirelessly transmitted
from a transmission source (for example, a base transceiver
station) to one or more receivers (for example, subscriber units)
within an area or region.
[0003] FIG. 1 shows a portion of a single cell of a cellular
wireless network system. A base transceiver station 110 provides a
wireless connection to a plurality of subscriber units 120, 130,
140. The base transceiver station is generally connected to a
network that provides access to the Internet. The cell of FIG. 1 is
generally repeated forming a cellular network. The base transceiver
station 110 and the subscriber units 120, 130, 140 include one or
more antennas allowing two-way communication between the base
transceiver station 110 and the subscriber units 120, 130, 140.
[0004] FIG. 2 shows a typical transmission chain 200 of a wireless
transmitter or transceiver. The transmitter receives a stream of
data (Data In) to be transmitted. A transmission signal is
modulated by the stream of data by a modulator 210. The modulated
transmission signal is typically frequency up converted by mixing
the modulated carrier signal with a local oscillator (LO) signal
through a frequency mixer 220. The frequency up converted signal is
generally amplified by a power amplifier 230 before transmission
through a transmission antenna T.
[0005] Distortion of the modulated transmission signal by the
transmission chain 200 can reduce the effectiveness of signal
transmission from the transmitter to a receiver. The power
amplifier 230 can cause distortion to modulated transmission signal
if the amplitude of the modulated transmission signal is too large,
and therefore, reduce the effectiveness of the signal
transmission.
[0006] FIG. 3 shows a example of a typical modulated transmission
signal in which the amplitude of the modulated transmission signal
varies with time. The modulated transmission signal typically
include peaks 310, 320 which indicate the maximum modulated
transmission signal amplitude over the time period of interest. A
dashed line 330 indicates an average signal amplitude of the
modulated transmission signal. A peak to average ratio (PAR) is
defined as the ratio of the peaks of the amplitude of the signal,
to the average amplitude of the signal.
[0007] Generally, it is desirable to maintain a particular average
power level. For example, an average power level can insure a
desired signal to noise ratio of the received transmission signal.
Therefore, it is generally at the peaks 310, 320 in which the power
amplifier 230 causes distortion of the modulated transmission
signal.
[0008] FIG. 4 shows a typical curve 400 representing a relationship
between the input signal amplitude versus output signal amplitude
of the power amplifier 230. The curve 400 is typically linear until
the output signal power become large enough that the power
amplifier 230 begins to saturate. Dashed line 410 roughly
designates the point in which the power amplifier transitions from
an essentially linear range to a non-linear range. As the input
signal amplitude increase past the dashed line 410, the output
signal amplitude compresses and no longer linearly increases.
Operation of the power amplifier in the non-linear range distorts
the modulated transmission signal.
[0009] It is clear from FIG. 3 and FIG. 4, that it is desirable to
minimize the PAR of the modulated transmission signal. It is also
clear that the power level of a modulated transmission signal
having a given PAR should be adjusted so that the peaks of the
modulated transmission signal do not cause a power amplifier of
wireless transmitter to saturate, and therefore, distort modulated
transmission signal of the transmitter.
[0010] Reduction of the PAR of the modulated transmission signal
provides two advantageous features. First, a reduced PAR allows
transmission of a greater average transmission signal power level.
This provides the advantage of an enhanced signal to noise ratio.
Second, a reduced PAR allows for the use of a typically less
expensive power amplifier for a given average transmission signal
power level. That is, the non-linear region of a power amplifier is
typically reached at a lower output power level than for a less
expensive power amplifier. Therefore, the reducing the amplitudes
of peaks of the transmission signal generally allows for the use of
a less expensive power amplifier for a given transmission signal
power level.
[0011] It is desirable to have an apparatus and method that
provides reduction of a PAR of a transmission signal. It is
additionally desirable to provide adjustment of the average power
level of the transmission signal so that a power amplifier of a
transmitter of the transmission signal does not excessively distort
the transmission signal.
SUMMARY OF THE INVENTION
[0012] The invention includes an apparatus and a method for
reducing the PAR of a transmission signal to minimize distortion of
the transmission signal. The invention further includes adjusting
the power level of the transmission signal.
[0013] An embodiment of the invention includes a method for
adjusting a power level of a transmission signal for minimal
distortion. The method includes modulating the transmission signal.
The modulated transmission signal is processed to reduce a peak to
average ratio of the modulated transmission signal based upon
modulation parameters of the modulated transmission signal. The
power level of the modulated transmission signal is adjusted
according to the peak to average ratio of the modulated
transmission signal. The transmission signal is amplified and
transmitted.
[0014] Other aspects and advantages of the present invention will
become apparent from the following detailed description, taken in
conjunction with the accompanying drawings, illustrating by way of
example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows a prior art wireless system that includes a
transceiver and multiple subscriber units.
[0016] FIG. 2 shows a typical transmission chain of a wireless
transmitter.
[0017] FIG. 3 shows a waveform representing an amplitude of a
typical transmission signal.
[0018] FIG. 4 shows a curve representing an input signal amplitude
versus output signal amplitude of a typical power amplifier of a
transmitter.
[0019] FIG. 5 shows an embodiment of the invention.
[0020] FIG. 6 shows time samples of a modulated transmission
signal.
[0021] FIG. 7 shows a sample modulated transmission signal waveform
after being windowed, and before being windowed.
[0022] FIG. 8 shows another embodiment of the invention.
[0023] FIG. 9 shows a flow chart of steps included within an
embodiment of the invention.
DETAILED DESCRIPTION
[0024] As shown in the drawings for purposes of illustration, the
invention is embodied in a method and system for reducing the PAR
of a transmission signal to minimize distortion of the transmission
signal. The invention further includes adjusting the power level of
the transmission signal.
[0025] Particular embodiments of the present invention will now be
described in detail with reference to the drawing figures. The
techniques of the present invention may be implemented in various
different types of wireless communication systems. Of particular
relevance are cellular wireless communication systems. A base
station transmits downlink signals over wireless channels to
multiple subscribers. In addition, the subscribers transmit uplink
signals over the wireless channels to the base station. Thus, for
downlink communication the base station is a transmitter and the
subscribers are receivers, while for uplink communication the base
station is a receiver and the subscribers are transmitters.
Subscribers may be mobile or fixed. Exemplary subscribers include
devices such as portable telephones, car phones, and stationary
receivers such as a wireless modem at a fixed location.
[0026] The techniques of the present invention apply to
point-to-multipoint systems, they are not limited to such systems,
but apply to any wireless communication system having at least two
devices in wireless communication. Accordingly, for simplicity, the
following description will focus on the invention as applied to a
single transmitter-receiver pair, even though it is understood that
it applies to systems with any number of such pairs.
[0027] Point-to-multipoint applications of the invention can
include various types of multiple access schemes. Such schemes
include, but are not limited to, time division multiple access
(TDMA), frequency division multiple access (FDMA), code division
multiple access (CDMA), orthogonal frequency division multiple
access (OFDMA) and wavelet division multiple access.
[0028] The transmission can be time division duplex (TDD). That is,
the downlink transmission can occupy the same channel (same
transmission frequency) as the uplink transmission, but occur at
different times. Alternatively, the transmission can be frequency
division duplex (FDD). That is, the downlink transmission can be at
a different frequency than the uplink transmission. FDD allows
downlink transmission and uplink transmission to occur
simultaneously.
[0029] Typically, variations of the wireless channels cause uplink
and downlink signals to experience fluctuating levels of
attenuation, interference, multi-path fading and other deleterious
effects. In addition, the presence of multiple signal paths (due to
reflections off buildings and other obstacles in the propagation
environment) causes variations of channel response over the
frequency bandwidth, and these variations may change with time as
well. As a result, there are temporal changes in channel
communication parameters such as data capacity, spectral
efficiency, throughput, and signal quality parameters, e.g.,
signal-to-interference and noise ratio (SINR), and signal-to-noise
ratio (SNR).
[0030] Information is transmitted over the wireless channel using
one of various possible transmission modes. For the purposes of the
present application, a transmission mode is defined to be a
particular modulation type and rate, a particular code type and
rate, and may also include other controlled aspects of transmission
such as the use of antenna diversity or spatial multiplexing. Using
a particular transmission mode, data intended for communication
over the wireless channel is coded, modulated, and transmitted.
Examples of typical coding modes are convolution and block codes,
and more particularly, codes known in the art such as Hamming
Codes, Cyclic Codes and Reed-Solomon Codes. Examples of typical
modulation modes are circular constellations such as BPSK, QPSK,
and other m-ary PSK, square constellations such as 4 QAM, 16 QAM,
and other m-ary QAM. Additional popular modulation techniques
include GMSK and m-ary FSK. The implementation and use of these
various transmission modes in communication systems is well known
in the art.
[0031] FIG. 5 shows an embodiment of the invention. This embodiment
includes a transmitter chain 500.
[0032] The transmitter chain 500 includes a modulator 510 that
receives a stream of data. The modulator modulates a carrier signal
with the stream of data according to any of the previously
mentioned typical modulation modes.
[0033] The modulated transmission signal is processed by a peak to
average ratio (PAR) processing block 520. The PAR processing block
520 generally reduces the PAR of the modulated transmission signal.
The processing to some extent is dependent upon the modulation
parameters, such as the order of modulation or modulation rate.
Therefore, the PAR processing block 520 also receives modulation
order information from the modulator 510.
[0034] A power level adjuster 530 varies the amplitude of the PAR
processed modulated transmission signal. The power level adjuster
530 can be a variable attenuator. As will be described later, the
PAR of the processed modulated transmission signal is to some
extent dependent upon the modulation parameters. Therefore, the
optimal setting of the power level adjuster 530 is to some extent
dependent upon the modulation parameters.
[0035] An amplitude controller 525 controls the power level
adjuster 530. As previously mentioned, the modulation parameters of
the modulated transmission signal effects the PAR of the modulated
transmission signal. Therefore, the amplitude controller 525
receives the modulation information (order and rate) to help
control the power lever adjuster 530.
[0036] A power amplifier 540 amplifies the modulated transmission
signal before transmission from a transmission antenna T. As
previously described, the power amplifier 540 distorts the
modulated transmission signal when the amplitude of peaks of the
modulated transmission signal are too large.
[0037] PAR Processing
[0038] The PAR processing block 520 receives the modulated
transmission signal and reduces the PAR of the modulated
transmission signal. Reducing the PAR provides the previously
described benefits of allowing a higher average transmission signal
power level, or the use of a less expensive power amplifier.
[0039] An embodiment of the invention includes windowing samples of
the modulated transmission signal to reduce the PAR. For example,
FIG. 6 shows time samples of a modulated transmission signal. These
time samples can be windowed to reduce the PAR. The windowing
introduces limited or controlled distortion of the modulated
transmission signal, and prevents the transmission chain from
substantially distorting the modulated transmission signal.
[0040] The sample spacing Ts of the modulated transmission signal
is determined by the maximum frequency component of the modulated
transmission signal. Generally, the sampling period Ts is chosen to
provide one to four times over-sampling of the modulated
transmission signal. The greater sampling rates can be advantageous
because peaks of the modulated transmission signal are less likely
to be missed.
[0041] FIG. 7 shows a sample modulated transmission signal waveform
after being windowed 710, and before being windowed 720. As shown,
the windowing reduces the PAR of the modulated transmission
signal.
[0042] The length of the windowing W is generally dependent upon
the data bandwidth of the modulated transmission signal. Typically,
the greater the data bandwidth, the greater the length of
window.
[0043] The windowing based PAR reduction scheme of the invention
can adaptively adjust the window length W base upon requirements of
the wireless system of the transmitter. As previously mentioned,
the windowing introduces a controlled distortion of the modulated
transmission signal. The introduced signal distortion can be both
in-band and out-of-band.
[0044] In-band distortion generally refers to distortion of the
transmission signal within the transmission frequency band of the
transmission signal. In-band distortion must generally meet a
criterion required to ensure a particular bit error rate (BER) of
the transmission signal. Smaller window length results in less
in-band distortion but more out-of-band distortion, and vice
versa.
[0045] Out-of-band distortion generally refers to distortion of the
transmission signal outside of the transmission frequency band of
the transmission signal. Out-of-band distortion can interfere with
other transmission signals. The out-of-band distortion must
generally meet a criterion as established by, for example, the FCC.
Typically, longer window lengths are used to minimize out-of-band
distortion.
[0046] The windowing can include any one of several possible
windows. For example, some well known windows include Hamming,
Hanning, Chebychev and Gaussian windows.
[0047] An embodiment of the invention includes a look-up-table
(LUT) that determines the window length for each signal bandwidth
available to the transmitter. Generally, this embodiment is
implemented in hardware. The length of the window is selected so
that the out-of-band distortion criterion is met for each
transmission bandwidth. Therefore, as the transmission is assigned
more or less bandwidth over time, a longer or shorter window may be
used to reduce the transmit signal PAR. An embodiment includes
storing only the longest window, and deriving smaller windows by
sub-sampling the longest window.
[0048] PAR reduction also causes in-band distortion in the
transmitted signal. The distortion is function of several
parameters, such as post-processing (output) PAR, window type,
window length, and probability distribution of input signal
samples. The most influential parameter, however, is the output
PAR. The lower the required output PAR, the greater the distortion.
The amount of distortion that can be tolerated without
significantly degrading the receiver SINR or the system's BER
performance is a function of order of modulation. Higher order
modulations such as 16 and 64 QAM typically require higher receiver
SINR's for proper operation, and can therefore, tolerate less
distortion. Lower order constellations such as 4 QAM require lower
SINR's and can tolerate higher distortion.
[0049] An embodiment of the invention includes adaptive modulation
and coding in which the PAR processing should generally only
introduce a small amount of PAR distortion. Therefore, the
resultant signal-to-interference-and-noise-and-distortion (SINDR)
at a receiver is only slightly lower than it would be without PAR
processing, for example 0.1 to 1 dB. Similarly, the PAR processing
can be such that there is a minimal increase in receiver BER after
decoding. An implementation includes a look-up-table that can be
used to determine output PAR as a function of modulation order or
modulation order and code rate. In general, higher order
modulations include higher output PAR, while lower order
modulations include lower output PAR. As the wireless channel
changes over time, the modulation order and possibly the code rate
can be adapted to optimize receiver performance. Consequently, the
PAR processing can be adapted based on output PAR specified in the
LUT.
[0050] The invention includes finding peaks in a transmission
signal, centering a window, and determining an amplitude for the
window. A peak is defined as a signal sample with amplitude that is
greater than a threshold. The threshold is a direct function of
desired output PAR. That is, Threshold=AX.sub.rms, where
A=10.sup.PAR.sup..sub.OUT/.sub.20 and X.sub.rms is the
root-mean-square value of the transmission signal. Peaks can occur
in clusters, especially when the signal is over-sampled. A
technique has been developed that determines a local maximum of a
set (cluster) of peaks, and an attenuating window is centered at
the local maximum. This technique is not always successful at
reducing the output PAR to desired level because centering a window
at the local maximum may not reduce peaks that are further away
from the local maximum, since the peaks are not attenuated as much.
Moreover, identifying a peak cluster is difficult to implement in
hardware.
[0051] An embodiment of the invention includes a "generalized peak
cluster", and includes a method for choosing the window center and
the window amplitude such that all peaks within the generalized
cluster are simultaneously reduced to below the PAR threshold.
[0052] A generalized peak cluster is defined to be a set of P
samples, in which the first sample of the P samples exceeds a PAR
threshold. The selection of P is predetermined based on peak
statistics and desired hardware complexity. A value of P=1
corresponds to a cluster of length one, and requires each peak to
be windowed independently, which includes significant hardware
complexity. If the desired output PAR is low, there can be
significant number of peaks that exceed the PAR threshold. It is
desirable to simultaneously reduce a set of peaks to simplify
hardware implementation. This can be achieved by choosing P to be
larger than a maximum length of a peak cluster or multiple
neighboring peak clusters (namely generalized peak cluster). The
length of a generalized peak cluster typically depends on how much
the transmission signal is over-sampled and a statistical
distribution of signal samples.
[0053] The larger the value of P, the fewer windows required to be
implemented in hardware. However, as P increases, the window
amplitude increases, leading to more attenuation of input samples
and thereby causing more distortion. Typically, P is chosen to be a
fraction of window length (1/8, 1/4, etc). However, P may be as
large as the window length to simplify hardware complexity.
[0054] Mathematically, the problem of generalized peak reduction
can be expressed as follows. Given a window fixed g(n), a window
amplitude .alpha. and window center .beta. can be determined such
that when the resulting PAR reduction window
w(n)=1-.alpha.g(n-.beta.)
[0055] is multiplied with the transmission signal x(n), no peaks
occur within a peak cluster of length P. That is;
y(n)=x(n)[1-.alpha.g(n-.beta.)]
[0056] includes no amplitude values greater than the PAR threshold,
A.sub.x.ident.AX.sub.rms, for n=p, p+1, . . . , p+P-1.
[0057] Assuming there is only 1 peak within cluster of length P,
then by definition, the peak must be at the first sample n=p. The
optimal choice for .alpha. and .beta. are 1 = 1 - A X x ( p ) , = p
.
[0058] Assuming there are 2 peaks with a peak cluster at samples
n=p and n=m. Then the optimal choice for .alpha. and .beta. is
determined by minimizing the following expression over values of
.beta. in the range p.ltoreq..beta..ltoreq.m: 2 max [ a p g ( p - )
, a m g ( m - ) ] ,
[0059] in which the variable .alpha..sub.k is defined as 3 a k = 1
- A X x ( k )
[0060] for a peak at sample k. The value of .beta. that minimizes
the expression is the window center, and the corresponding minimum
value of the expression is the window amplitude .alpha..
[0061] In general, assuming there are multiple peaks within a peak
cluster of length P, for example at samples n=p, n=m, . . . , and
n=1. Then the optimal choice for .alpha. and .beta. is obtained by
optimizing the following min-max criterion 4 = min p l [ max [ a p
g ( p - ) , a m g ( m - ) , , a 1 g ( m - l ) ] ]
[0062] and .beta. is the value that yields the minimum of min-max
criterion.
[0063] The hardware implementation of min-max criterion can be
complex for large values of P. An embodiment includes the
expression min-max criterion being evaluated at only the peaks
rather than over the whole range of .beta.. Another embodiment
includes the min-max criterion being evaluated at a select set of
peaks where the peak selection may be based on some desired
criterion. In an embodiment, the min-max criterion is only
evaluated at two peaks n=r and n=s, where n=r corresponds to the
largest peak in first-half of peak cluster and n=s corresponds to
largest peak in second half of the peak cluster. This approach is
sub-optimal, and the desired output PAR is not exactly achieved,
but is very close.
[0064] An embodiment includes the PAR reduction scheme described
being combined with a linearization scheme that linearizes the
amplitude characteristic of transmitter amplifier. An example of
such a scheme is a predistorter that pre-corrects for the
nonlinearity in amplifier response before amplification.
Predistortion can be implemented digitally by pre-characterizing
the amplifier response. The response can change over time, so the
response must be measured and compensated for adaptively. The
combination of PAR reduction with amplifier linearization results
in efficient use of the power amplifier and causes no additional
distortion since the amplifier is being operated only in a
linearized region.
[0065] FIG. 8 shows another embodiment of the invention. This
embodiment includes a first transmitter chain and a second
transmitter chain. Additional transmitter chains can be
included.
[0066] The first transmitter chain includes an encoder 810, a
modulator 820 and a frequency up converter 850.
[0067] The second transmitter chain includes an encoder 830, a
modulator 840 and a frequency up converter 860.
[0068] A summer 870 receives and combines modulated transmission
signals from the first transmitter chain and the second transmitter
chain.
[0069] A PAR processor 875 according to the invention provides the
previously described PAR processing. A digital to analog (D/A)
converter 880 converts digital signal into analog signals before
transmission.
[0070] An amplitude adjuster 885 is controlled by an amplitude
controller 895 to provide adjustment of an amplitude of the
transmission signals before being transmitted through a transmitter
antenna T.
[0071] The inclusion of multiple transmitter chains and the
corresponding transmission signals generally increases the PAR of
the combined transmitted signal. The transmission signals
corresponding to different transmitter chains can each include
different modulation rates, and/or different codes. The LO for each
transmission chain can be a different frequency for a multiple
carrier transmission system. For a multi-code code division
multiplexed access (CDMA) system, the transmitter signals can be
generated by including a different code for each transmitter chain.
For such a system, the LO of each chain can be the same, and
therefore, located between the summer 870 and the PAR processor
875. Multi-code CDMA can be used to increase the data rate
associated with the transmission signal.
[0072] The previously described processing techniques can be used
to reduce the PAR of the transmission signal. The PAR processing
should be based upon the modulation parameters of the signal of the
transmitter chain that requires the minimum distortion.
[0073] FIG. 9 shows a flow chart of steps included within an
embodiment of the invention. A first step 910 includes modulating
the transmission signal. A second step 920 includes processing the
modulated transmission signal to reduce a peak to average ratio of
the modulated transmission signal based upon modulation parameters
of the modulated transmission signal. A third step 930 includes
adjusting the power level of the modulated transmission signal
according to the peak to average ratio of the modulated
transmission signal. A fourth step 940 includes amplifying the
transmission signal. A fifth step 950 includes transmitting the
transmission signal.
[0074] Although specific embodiments of the invention have been
described and illustrated, the invention is not to be limited to
the specific forms or arrangements of parts so described and
illustrated. The invention is limited only by the appended
claims.
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