U.S. patent application number 12/415676 was filed with the patent office on 2009-11-19 for efficient peak cancellation method for reducing the peak-to-average power ratio in wideband communication systems.
This patent application is currently assigned to DALI SYSTEMS CO. LTD.. Invention is credited to Kyoung Joon Cho, Jong Heon Kim, Wan Jong Kim, Shawn Patrick Stapleton.
Application Number | 20090285194 12/415676 |
Document ID | / |
Family ID | 41135991 |
Filed Date | 2009-11-19 |
United States Patent
Application |
20090285194 |
Kind Code |
A1 |
Kim; Wan Jong ; et
al. |
November 19, 2009 |
Efficient Peak Cancellation Method for Reducing the Peak-To-Average
Power Ratio in Wideband Communication Systems
Abstract
An efficient peak cancellation method for reducing the
peak-to-average power ratio in wideband communication systems uses
repeated clipping and frequency domain filtering to achieve a
desired peak-to-average power ratio for wideband code division
multiple access and orthogonal frequency division multiplexing
signals. The maximum magnitude of the filtered pulse is determined
by a scaling factor which permits eliminating several iterations
while still achieving convergence to the targeted peak-to-average
power ratio, thereby reducing computational load and saving
hardware resources. This results in improved performance in terms
of error vector magnitude, adjacent channel leakage ratio and
peak-to-average power ratio.
Inventors: |
Kim; Wan Jong; (Coquitlam,
CA) ; Cho; Kyoung Joon; (Coquitlam, CA) ; Kim;
Jong Heon; (Yongsan-Gu, KR) ; Stapleton; Shawn
Patrick; (Burnaby, CA) |
Correspondence
Address: |
Law Offices of James E. Eakin
P.O. Box 1250
Menlo Park
CA
94026
US
|
Assignee: |
DALI SYSTEMS CO. LTD.
Grand Cayman
KY
|
Family ID: |
41135991 |
Appl. No.: |
12/415676 |
Filed: |
March 31, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61041164 |
Mar 31, 2008 |
|
|
|
Current U.S.
Class: |
370/342 ;
370/310 |
Current CPC
Class: |
H03F 2201/3227 20130101;
H03F 3/19 20130101; H03F 3/20 20130101; H03F 2200/129 20130101;
H03F 1/3258 20130101; H03F 2201/3233 20130101; H03F 2200/451
20130101; H03F 2201/3224 20130101; H03F 1/3247 20130101; H03F 3/24
20130101 |
Class at
Publication: |
370/342 ;
370/310 |
International
Class: |
H04B 7/216 20060101
H04B007/216; H04B 7/00 20060101 H04B007/00 |
Claims
1. A method for reducing peak-to-average power ratio in wideband
communication systems using multiplexing modulation techniques
comprising the steps of: (a) clipping a baseband input signal; (b)
subtracting the baseband input signal from the result of said step
(a); (c) noise shaping the result of step (b); (d) scaling the
result of the said step (c); and (e) subtracting from the result of
step (d) the delayed baseband input signal.
2. The method of claim 1 wherein steps (a) to (e) are iterated
until a desired clipping level is achieved.
3. The method of claim 1 wherein the clipping step includes using
at least one of a group comprising an amplitude calculator, a
comparator, a lookup table, a multiplier, a constant and a
multiplexer.
4. The method of claim 1 wherein the noise shaping step is
performed by converting the digitally clipped signal to a frequency
domain signal, filtering by at least one finite impulse response
filter, reconverting the filtered frequency domain signal to the
baseband signal, and combining to yield an output signal.
5. The method of claim 1 wherein said step (c) is performed by
converting the digitally clipped signals for multi-carrier such as
WCDMA to frequency domain signals by (.omega..sub.n), filtering by
finite impulse response filters, reconverting the filtered
frequency domain signals to the baseband signals, and combining the
signals.
6. The method of claim 1 wherein said step (d) is performed by
scaling in accordance with the following equation: .alpha. ( i ) =
max ( p m ( i ) ) max ( pf n ( i ) ) ##EQU00005## wherein
.alpha..sup.(i) is a scaling factor at i-th iteration, p.sub.n the
clipped signal or peak cancellation signal, and pf.sub.n the output
signal of the noise shaper.
7. The method of claim 1 further comprising iterating steps (a) to
(e) until a desired clipping level is achieved, and wherein the
number of iterations needed to converge to the desired PAPR level
is reduced by applying a scaling factor.
8. The method of claim 6 wherein applying a scaling factor reduces
computational load for reducing PAPR.
9. The method of claim 6 wherein applying a scaling factor reduces
hardware implementation complexity arising from the number of
iterations.
10. The method of claim 6 wherein error vector magnitude is
significantly improved.
11. The method of claim 6 wherein adjacent channel leakage ratio is
significantly improved.
12. The method of claim 6 wherein peak-to-average power ratio is
significantly improved.
13. The method of claim 1, further comprising the step of
compensating for errors by combining power amplifier output with
the signal resulting from step (d) through an additional
digital-to-analog converter and an upconverter.
Description
RELATED APPLICATION
[0001] This application incorporates by reference and claims the
benefit of U.S. Provisional Patent Application Ser. No. 61/041,164,
filed Mar. 31, 2008, and having the same inventors and title as the
present application
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to wideband
communication systems using multiplexing modulation techniques.
More specifically, the present invention relates to methods for
reducing the peak-to-average power ratio for wideband code division
multiple access and orthogonal frequency division multiplexing
signals.
[0004] 2. The Prior Art
[0005] As a result of the increasing importance of spectral
efficiency in mobile communications, effective modulation
techniques, such as wideband code division multiple access (WCDMA)
and orthogonal frequency division multiplexing (OFDM), have been
used. These modulations have large envelope fluctuations, since the
transmitted signal is generated by adding a large number of
statistically independent signals. The high peak-to-average power
ratio (PAPR) sets strict requirements for the linearity of the
power amplifier (PA) leading to low power efficiency, since it is
desirable for the PA to operate in its linear region. The use of
deliberate envelope clipping to digitally distort the signal while
maintaining the signal quality at a sufficient level is a simple
and practical way to decrease PAPR. Moreover, the reduced PAPR via
clipping gives rise to the possibility of utilizing the dynamic
range of the digital-to-analog-converter (DAC) more efficiently.
The various PAPR techniques can be categorized into two groups
depending on whether they use linear techniques
(modulation-and-coding-dependent) or nonlinear techniques
(modulation-and-coding-independent). Methods that use linear
techniques for OFDM systems do not distort the signal in the time
domain so that the spectral properties are not altered.
[0006] On the other hand, nonlinear techniques modify the envelope
of the time domain signal and are mainly based on
clipping-filtering (CF) and peak windowing (PW) clipping. The idea
of the PW clipping method is to filter the clipped output signal
using the window function with the coefficient weights. The
windowed output signal must satisfy the inequality so as to achieve
the desired clipping level. To minimize the resultant error in the
time domain, the inequality must be as close to equality as
possible. This is dependent on the type and length of the window.
The resultant function is then multiplied by the delayed input
signal [O. Vaananen, J. Vankka, and K. Halonen, "Effect of Clipping
in Wideband CDMA System and Simple Algorithm for Peak Windowing,"
World Wireless Congress, San Francisco, pp. 614-619, May 2002].
[0007] To suppress peak re-growth when filtering the out-of-band
distortion of the clipped signal, iterative clipping and filtering
for OFDM systems have been used. This approach has suggested
iterative clipping and filtering of the clipped pulses, so as to
reduce the convergence rate to the targeted PAPR. However,
techniques based on repeated clipping and filtering that have been
implemented for OFDM systems require several iterations to converge
to the desired PAPR level, which implies that it is not an
efficient algorithm for hardware implementation [J. Armstrong,
"Peak-to-average power reduction for OFDM by repeated clipping and
frequency domain filtering," IEE Electronics Letters, vol. 38, no.
5, pp. 246-247, February 2002], [S. H. Leung, S. M. Ju, and G. G.
Bi, "Algorithm for repeated clipping and filtering in
peak-to-average power reduction for OFDM," IEE Electronics Letters,
vol. 38, no. 25, pp. 1726-1727, December 2002].
[0008] Hence, a need remains in the art for an improved method for
reducing the PAPR in wideband communication systems that is able to
eliminate several iterations to converge to the desired PAPR level
and to simplify the hardware implementation for multi-carrier
systems, such as OFDM and WCDMA.
SUMMARY OF INVENTION
[0009] Accordingly, the present invention has been made in view of
the above problems, and it is an object of the present invention to
provide a novel efficient method of peak cancellation (PC) for
reducing the PAPR for wideband communication system applications.
To achieve the above objects, according to an embodiment of the
present invention, the technique is based on a method of repeated
clipping and filtering. While conventional repeated peak
cancellation (RPC) requires several iterations so as to converge
into the targeted PAPR, since filtering causes peak re-growth, the
present invention is able to eliminate several iterations, which
subsequently saves hardware resources by means of the proper
scaling factor.
BRIEF DESCRIPTION OF DRAWINGS
[0010] Both the foregoing and further objects and advantages of the
invention can be more fully understood from the following detailed
description taken in conjunction with the accompanying drawings in
which:
[0011] FIG. 1. is a schematic diagram showing a multi-stage scaled
repeated peak cancellation (SRPC) method.
[0012] FIG. 2. is a schematic diagram showing a preferred
embodiment of the present invention.
[0013] FIG. 3A. is a schematic diagram showing a noise shaper for
multi-carrier.
[0014] FIG. 3B. is a schematic diagram showing a noise shaper for
single-carrier.
[0015] FIG. 3C is a schematic diagram showing an embodiment of a
clipper
[0016] FIG. 4A. is a graph showing a peak cancellation pulse in
time domain before filtering, after filtering at each stage,
respectively (Prior Art).
[0017] FIG. 4B. is a graph showing peak cancellation pulse in time
domain before filtering, after filtering, and after filtering and
scaling at each stage, respectively.
[0018] FIG. 5. is a graph showing simulation results of the PAPR
versus EVM for four WCDMA carriers using just clipping method, the
PW method and the SRPC method of the present invention
respectively.
[0019] FIG. 6. is a graph showing simulation results of the ACLR
versus PAPR for four WCDMA carriers using the PW method, the RPC
method, and the SRPC method of the present invention
respectively.
[0020] FIG. 7. is a table showing performance comparisons of
simulation results of the RMS EVM for different number of WCDMA
carriers using the PW method, the RPC method, and the SRPC method
of the present invention respectively.
[0021] FIG. 8. is a graph showing simulation results of the PDF for
four WCDMA carriers using the SRPC method of the present invention
respectively.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0022] The conventional repeated peak cancellation (RPC) method can
effectively reduce the PAPR. However, the RPC method requires
several iterations to converge to the desired PAPR level, which
implies that it is not an efficient algorithm for hardware
implementation. Instead, the present invention applies a scaling
factor to the peak cancellation pulse after the noise shaper but
inside the peak cancellation loop. The objective is to achieve
fewer iterations during processing and thereby reduce the PAPR and
EVM. Compared to the conventional RPC method, an embodiment of the
present invention achieves lower PAPR for, for example, four WCDMA
carriers although approach is expandable into an unlimited number
of carriers. The method provided by the present invention is
therefore referred to hereinafter as Scaled Repeated Peak
Cancellation (SRPC).
[0023] Various embodiments of the SRPC method according to the
present invention are described in detail below with reference to
the accompanying drawings.
[0024] FIG. 1. is a schematic diagram showing an embodiment of the
multi-stage SRPC method. As illustrated, the baseband signal x(n)
101 goes through the first SPC 102 with a scaling factor
.alpha..sup.(0) 107, and z.sub.n.sup.(1) 105 is the output from the
first iteration of the peak cancellation. After the i-th iteration,
the resulting signal can be represented by Z 110.
[0025] In the SRPC method of the present invention, as illustrated
in FIG. 2, the baseband signal x(n) 201 first passes through the
clipper 202. The clipper 202 output, c.sub.n, can be written as
follows:
c n = { A x n , x n > A 1 , x n .ltoreq. A ##EQU00001##
[0026] where A is the clipping threshold level. The clipped pulse
or peak cancellation pulse, p.sub.n can be written as
p.sub.n=x.sub.n-x.sub.nc.sub.n
[0027] Finally the PAPR reduced signal, z.sub.n 212 is described
by
z n = x n - d - .alpha. pf n = x n - d - .alpha. p n h n
##EQU00002##
where pf.sub.n, h.sub.n, and .alpha. denote the output signal of
the noise shaper 206, the impulse response of the low pass filter
(LPF), and the scaler 208, respectively. * denotes the convolution
operation.
[0028] As shown in FIG. 3a for multi-carrier operation, the peak
cancellation pulse 301 is frequency translated by (On), filtered,
frequency translated back to baseband and combined. This is because
the out-of-band emissions reside between the different carriers and
cannot be filtered out by line pass filter 304, as opposed to the
single carrier applications in FIG. 3b where only one finite
impulse response (FIR) filter 304 can be used. The FIR filters 304
for the multi-carriers have the same coefficients as that of a
signal carrier FIR filter 304. There is peak re-growth beyond the
clipped signal. This occurs because the resultant peak cancellation
pulse (p.sub.n) 301 is filtered by the noise shaper and
subsequently subtracted from the delayed input signal. This has the
net effect of increasing the peaks beyond that of the clipped
signal. Let z.sub.n 212 be the output signal and z.sub.n.sup.(1)
105 be the output from the first iteration. After the i-th
iteration, the resulting signal 110 can be represented by
z n ( 2 ) = z n ( 0 ) - .alpha. ( 1 ) pf n ( 1 ) ##EQU00003## z n (
3 ) = z n ( 2 ) - .alpha. ( 2 ) pf n ( 2 ) = z n ( 0 ) - .alpha. (
1 ) pf n ( 1 ) - .alpha. ( 2 ) pf n ( 2 ) ##EQU00003.2##
##EQU00003.3## z n ( i ) = z n ( i - 1 ) - .alpha. ( i ) pf n ( i )
= z n ( 0 ) - j = 1 i .alpha. ( j ) pf n ( j ) ##EQU00003.4##
[0029] The scaler, .alpha..sup.(i), 109, at i-th iteration can be
calculated as
.alpha. ( i ) = max ( p m ( i ) ) max ( pf n ( i ) )
##EQU00004##
[0030] The envelope of the input signal has a Rayleigh distribution
according to the central limit theorem, so that the maximum
magnitude of the clipping pulse can be numerically found once the
threshold level is set. This implies that the maximum magnitude of
the filtered pulse can be accordingly determined.
[0031] Referring next to FIG. 3C, an embodiment of a clipper in
accordance with the invention is shown in schematic block diagram
form. In the embodiment shown, a clipper comprises an amplitude
calculator 325 which receives the input signal and provides it to a
comparator 327 and a lookup table (LUT) 329. A clipping threshold
signal 331, which can be preset or variable according to the
desired implementation, provides a second input to the second input
to the comparator 327, and also provides an input to a multiplier
333. The output of the LUT provides the second input to the
multiplier, the output of which is provided to a mux 335. The
output of the comparator 327 provides a "select" input to the mux
335, while a constant 337 provides the second signal input to the
mux. Thus, it can be appreciated that the mux selects either the
output of the multiplier or a constant, depending on the comparison
between the amplitude of the input signal and the clipping
threshold. It will be appreciated by those skilled in the art that
numerous alternatives and equivalents to the embodiment of FIG. 3C
can be constructed given the teachings herein, and the illustrated
embodiment is therefore not intended to be limiting and is just one
of many that perform the requisite clipping function.
[0032] FIGS. 4a and 4b represent peak cancellation pulses in the
time domain for the prior art and the present invention,
respectively. As shown in FIG. 4b, applying the scaling factor
results in less iteration when compared to FIG. 4a. Therefore, this
scaling factor significantly reduces the computational load, which
saves hardware resources in an implementation. According to
numerical simulations, it has been found that two or three
iterations of the SRPC is sufficient.
[0033] In examining the performance of an embodiment of the SRPC
method, 3.sup.rd Generation Partnership Project (3GPP) standard
specifications state that the EVM and ACLR at 5 MHz offset should
be less than 17.5% and -45 dBc, respectively. The scrambling codes
and the time offsets of the time slot duration for multi-carriers
test model 1 (TM1) of the WCDMA downlink system is based on 3GPP TS
25.141, Section 6.1.1 of Release 6 (2002-12). The numerical
simulations used a signal that is TM1 with 64 dedicated physical
channels (DPCH) and 614,400 input samples (one radio frame at 61.44
Msamples/sec) that are processed in MATLAB. A low pass FIR filter
with 129 taps was designed to meet out-of-band distortions
specifications of -77 dBc.
[0034] FIG. 5. is a graph showing simulation results of the PAPR
with respect to EVM for four WCDMA carriers using the peak
windowing method with an 85 tap Hamming window length, just
clipping, and an embodiment of the present invention's SRPC method
with three stages of the present invention respectively, through
which the performance of the PAPR reduction of the three methods
can be compared. In the figure, the solid line with diamond markers
represents the performance with just clipping; this sets the lower
bound on the PAPR and EVM. It obviously has a large out-of-band
spectral radiation. The three-stage PC compressed the PAPR by 0.8
dB more than the single stage at an EVM of 10%. Using the SRPC
technique, the PAPR can be suppressed to approximately 5.7 dB at a
fixed 10% of EVM after only three stages, while 6.7 dB is
achievable with the PW method based on four WCDMA carrier input
signal. It should be noted that even a single stage of the proposed
algorithm outperforms the PW technique and it requires only two
iterations to obtain the same performance that is achieved by seven
iterations of the conventional RPC method.
[0035] FIG. 6. is a graph showing simulation results of the ACLR
versus PAPR for four WCDMA carriers using the peak windowing
method, the conventional RPC method, and the SRPC method of the
present invention respectively. In the figure, the PW technique has
a critical disadvantage that degrades ACLR as opposed to
conventional RPC and SRPC method. The original input signal has an
ACLR of approximately -77 dBc. Another point to note is that the
conventional RPC and SRPC methods deteriorate the ACLR up to
approximately 2 dB as the clipping threshold is reduced. This is a
result of the decrease in the average power as clipping becomes
more significant.
[0036] FIG. 7. is a table showing performance comparisons of
simulation results of the RMS EVM for different numbers of WCDMA
carriers using the PW method, the RPC method, and the SRPC method
of the present invention respectively. Simulations were performed
for a different number of carriers. For a single carrier, all three
techniques represent a similar ability in terms of EVM and PAPR.
However, the PW method still allows the ACLR to be compromised,
unlike the other two methods. The conventional RPC method requires
more than five iterations which increase its complexity, while the
proposed SRPC method only requires two iterations. It is not
possible for the PW method to achieve a PAPR of 5.5 dB, for the
three carrier and four carrier cases, even without considering EVM
and ACLR. This is because the window significantly alters many
input samples due to the large clipping, which significantly
changes the average power.
[0037] FIG. 8. is a graph showing simulation results of the PDF for
four WCDMA carriers using the SRPC method of the present invention
respectively. In the figure, the solid line shows the PDF of the
original input signal and the PDF at each stage of three stage SRPC
method is illustrated. The PDF difference can be minimized in the
region of samples with magnitude less than 1 V, as illustrated in
FIG. 8.
[0038] In summary, the SRPC method of the present invention,
compared to the conventional RPC method, could reduce PAPR more
effectively since the SRPC method is able to eliminate several
iterations, which subsequently saves hardware resources. In four
WCDMA carriers, the present invention could achieve the state of
the art performance for WCDMA applications.
[0039] Although the present invention has been described with
reference to the preferred embodiments, it will be understood that
the invention is not limited to the details described thereof.
Various substitutions and modifications have been suggested in the
foregoing description, and others will occur to those of ordinary
skill in the art. Therefore, all such substitutions and
modifications are intended to be embraced within the scope of the
invention as defined in the appended claims.
* * * * *