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A METHOD OF PEAK TO AVERAGE POWER RATIO (PAPR)
FIELD OF INVENTION
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The present invention relates to a method of peak to average power ratio (PAPR)
reduction in OFDM systems.
BACKGROUND OF INVENTION
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Orthogonal frequency division multiplexing (OFDM) is considered as one of the most
significant technologies ideal for different communication and broadcasting systems.
This is because OFDM offers high spectral efficiency, robustness to frequency
selective fading, tolerant to radio-channel delay spread and power efficient.
Consequently, OFDM has been adopted as a standard for high data rate
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communications like worldwide interoperability for microwave access (WiMAX), Digital
Video Broadcasting (DVB) and Long Term Evolution (LTE).
However, one major drawback of an OFDM signal is its high peak-to-average power
ratio (PAPR). This stems essentially from the IFFT operation. In time domain, data
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symbols across subcarriers can add up to produce high peaks at some instant. For
example, in an OFDM system with 512 subcarriers and BPSK modulation, the PAPR at
the output can be as high as 10dB. PAPR of 10 dB means that for transmitting an
average power of 0.2W, the power amplifier should be able to handle power peaks of
2.0W which is 10 times higher. In other words the power amplifier cannot operate
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efficiently and hence consumes much power for transmission.
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US 2013/0094614A1 describes suboptimal strategies for combining partial transmit
sequences by reducing the complexity of searching the optimum set of phase vector.
However, this algorithm suffers from performance degradation. Also the number of
Inverse Fast Fourier Transform (IFFT) is not reduced and also there is still a need to
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send side information back to the sender.
US 8,416,869 B2 describes method of an OFDM system based on path finding. The
method is applied to the OFDM system using an ant colony optimization (ACO)
algorithm and a Partial Transmission Sequence (PTS) to reduce a PAPR, and an area
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of a computing circuit. The method reduces PAPR with less complexity. However the
number of IFFT is not reduced and also there is still a need to send side information.
KR 20030084291 (A) describes a method to exactly reconstruct the information at the
receiving end by transmitting the additional information related to the PTS. Here, good
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spectrum efficiency and BER performance are achieved, but complexity still remains a
drawback.
Hence, there is a need for a solution that will reduce complexity of searching for the
optimum set of phase vector due to the number of IFFT.
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SUMMARY OF INVENTION
Accordingly there is provided a method of peak to average power ratio (PAPR)
reduction, the method includes the steps of interleaving the partitioned output of
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Inverse Fast Fourier Transform (IFFT) operation and performing phase optimization,
such that multiplication operations are eliminated without having to send side
information and hence improving transmission efficiency.
The present invention consists of several novel features and a combination of parts
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hereinafter fully described and illustrated in the accompanying description and
drawings, it being understood that various changes in the details may be made without
departing from the scope of the invention or sacrificing any of the advantages of the
present invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be fully understood from the detailed description given herein
below and the accompanying drawings which are given by way of illustration only, and
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thus are not limitative of the present invention, wherein:
Figure 1 shows a block diagram of a method of peak to average power ratio (PAPR)
reduction;
Figure 2 shows a detailed block diagram of new optimization technique of peak to
average power ratio (PAPR) reduction;
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Figure 3 illustrates flowchart of interleaving process technique with N=8, M=4; and
Figure 4 shows a graphical representation of CCDF of PAPR of the method for different
N subcarriers compared to original OFDM for M=4.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention relates to a method of peak to average power ratio (PAPR)
reduction. A detailed description of a preferred embodiment of the invention is
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disclosed herein. It should be understood, however, that the disclosed preferred
embodiment is merely exemplary of the invention, which may be embodied in various
forms. Therefore, the details disclosed herein are not to be interpreted as limiting, but
merely as the basis for the claims and for teaching one skilled in the art of the
invention.
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The following detailed description of the preferred embodiment will now be described in
accordance with the attached drawings, either individually or in combination.
Figure 1 shows a block diagram of a method of peak to average power ratio (PAPR)
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reduction. The method includes the steps of interleaving output of Inverse Fast Fourier
Transform (IFFT) modulation and performing phase optimization, such that
multiplication operations are eliminated without sending side information to reduce
bandwidth.
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The method is low in complexity, in which the output of IFFT (modulator) is partitioned
and then interleaved and then phase optimization which eliminates multiplication
operations permanently is applied. This method offers significant improvement for
wireless transmitter in terms of increased power efficiency and reduced complexity and
hence reduced costs.
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As a bonus, this method obviates the need to send side
information which otherwise would have incurred more bandwidth as is used in
previous known solutions. The novelty of the method is in the combination of time
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interleaving and the new phase optimization technique, which reduce the systems cost
and increases power efficiency.
The method is to be applied in an OFDM system by using a single IFFT modulator. The
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most complex block in OFDM systems is the IFFT and by reducing the number of
IFFTs in the design of PAPR, the complexity will be reduced. An interleaver is also
included for increased reduction of PAPR and also enhances BER performance. An
encoder of size 2M-1×M-1 with number of (M -1) comparators and inverters, wherein M
denotes number of sub blocks is also included. This block is required to find the
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optimum phase sequence.
At the transmitter of the method, shown in Fig. 1, an incoming serial random data
vector:
  [X
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X ,..., X
]T
0, 1
N 1
(1)
is passed through a single N-point IFFT, which is represented by vector:
x  [ x , x ,..., x
]T
0 1
N 1
(2)
Then, the N-point IFFT x element is partitioned into M subblocks which are
represented by vectors {xm, m=1, 2, …, M} such that:
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x
M
x
m1
m
(3)
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Then, the output of M subblocks is passed to an interleaver, which reorders them in
specific ways as shown in Fig. 3. The M subblocks will be interleaved with each other:
x
M
 D( x )
m
(4)
m1
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where D (.) is Deinterleaving of x’m,{x’m, m=1, 2, …, M} are the interleaved elements of
xm. Then, a new phase optimization scheme is applied. This eliminates multiplication
operations permanently. Only two phase sequences {0, 1} are required. First, all phase
sequence possibilities are generated by using an Encoder of size 2M-1×M-1. b1 can be
set to zero, (b1=0) without loss of performance. This Encoder will generate all phase
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sequences which equals 2M-1 as shown in Fig. 2. For example, if M=4, the size of
Encoder is 8×3 as shown in Fig. 2 (i.e. b1=0 and {b2 b3 b4} = (000, 001, 010, 011,
100, 101, 110, 111). Then, the phase of each subblock is converted according to the
proposed weight of phase rotation as:
~
x
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M
 (1)
m1
bm
xm'
(5)
where bm= {0, 1}.
That is, the comparator will detect whether the phase factor is 0 or 1. If the weight of
phase factor is 0, the phase of elements of sub-block does not change and passes
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directly to summation unit; if weight of phase factor is 1, the phase is rotated by
passing through the inverter and then passes to summation unit. As the first step, b1
can be set to zero (b1=0) without loss of performance. Then, the PAPR of the
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combined signal can be calculated. If the PAPR at b2=0 is lower than PAPR at b2=1,
then, all the phase sequence possibilities beginning with 1 will be neglected and vice
versa. For instance, when M=4, if the PAPR at b2=1 is the lowest then all phase
sequences with b2=0 will be neglected (i.e. {0, 0, 0}, {0, 0, 1}, {0, 1, 0}, {0, 1, 1}). So,
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half of these sequences will be neglected, and each time half of the remaining of the
possibilities will be neglected until the phase factors sequence possibilities runs out.
At receiver side, there is an Encoder such as in transmitter. All candidate sequences
have an index each, respectively. We can implicitly insert it in specified positions in the
transmitted signal (i.e. by changing the behavior of the specified sample) and will be
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used by Encoder at receiver side to recover the phase sequence. So, the method does
not require sending the side information.
Fig. 4 shows the superior performance of this method against the original OFDM,
represented in terms of the Complementary Cumulated Distributed Function (CCDF) of
PAPR of the method, for different N subcarriers and M=4.
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The main advantage of this invention is the significant reduction in complexity by up to
98% compared to conventional PTS, and hence this reduces the hardware resource
needed while at the same time, it gives superior PAPR performance as shown in Fig.4.
The other salient feature of this scheme is that no side information need to be
transmitted which leads to increased transmission efficiency (TE). The main reason
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that the cost of a device will drop substantially is because it will not require expensive
high power amplifier at the transmitter due to the excellent PAPR performance.
The method is performed in feed forward optimization scheme in which only two phase
sequences are required to optimize the IFFT partitions. The interleaving is applied to
enhance the bit error rate (BER) performance and to also to enhance the optimization
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technique. By this way minimum PAPR is obtained without the need of feedback which
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results in reduced processing time, and less computation that leads to reduced
complexity. Moreover, one main aspect of this algorithm is that it only needs one IFFT
operation, which reduces its complexity significantly.
It is to be understood that the embodiments of the invention described are
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exchangeable for other variations of the same in order to be used in various
applications. The present embodiment of the invention is intended for, but not restricted
to, reducing complexity in OFDM.
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CLAIMS
1. A method of peak to average power ratio (PAPR) reduction, the method includes
the steps of:
i.
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interleaving the partitioned output of Inverse Fast Fourier Transform
(IFFT) modulation; and
ii.
performing phase optimization,
such that multiplication operations are eliminated without having to send
side information and hence reducing bandwidth.
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2. The method as claimed in claim 1, wherein the method includes determining
optimum phase sequence.
3. The method as claimed in claim 1, wherein the method increases transmission
efficiency as there is no side information needed.
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4. The method as claimed in claim 1, wherein the method reduces complexity of
conventional partial transmission sequence (PTS) by up to 98%.
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ABSTRACT
A METHOD OF PEAK TO AVERAGE POWER RATIO (PAPR)
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A method of peak to average power ratio (PAPR) reduction is provided, the method
includes the steps of interleaving output of Inverse Fast Fourier Transform (IFFT)
modulation and performing phase optimization, such that multiplication operations are
eliminated without having to send side information to reduce bandwidth.
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The most illustrative drawing: Figure 1