Apparatus and method for controlling peak to average power ratio (PAPR)

An apparatus for controlling PAPR in an OFDM communication system and method thereof are disclosed, by which implementation is simplified and enhanced PAPR characteristics are provided. The present application includes outputting GNb data symbols by oversampling Nb parallel data symbols, spreading the oversampled data symbols using DFT, and mapping the spread signal to subcarriers.

This application claims the benefit of the Korean Patent Application No. 10-2006-0023113, filed on Mar. 13, 2006, which is hereby incorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to controlling PAPR. Although the present disclosure is suitable for a wide scope of applications, it is particularly suitable for decreasing PAPR characteristics in an orthogonal frequency division multiplexing (hereinafter abbreviated OFDM) system.

2. Discussion of the Related Art

Generally, in the OFDM system, signal processing is performed on a channel tending to have frequency-selective fading in a frequency domain to bring about flat fading. So, the OFDM system enables more efficient communications. Due to this advantage, the OFDM is widely adopted by wireless communication systems.

Meanwhile, the OFDM system has a problem of a peak to average power ratio (hereinafter abbreviated PAPR). If the PAPR is big, a power amplifier having a large linear interval to amplify a signal corresponding to a peak power is needed. Yet, a product cost for manufacturing the power amplifier having the large linear interval is too high. In case that a power amplifier has a small linear interval, a signal amplified in a non-linear interval gets distorted.

To decreasing the PAPR, various methods have been proposed. As an example of the methods, variants of OFDM like SC-FDMA, offset DFT-SOFDM and precoded DFT-S-OFDM are provided. In these methods of modifying OFDM signal generation, PAPR characteristics of a transmission signal is enhanced in a manner of spreading a transmission data vector by DFT before mapping data to subcarriers in a frequency domain and then mapping the data to the subcarriers. In the methods of modifying OFDM signal generation, it is in common that a signal is spread by DFT. In particular, since the data signal mapped by the subcarriers is performed by IDFT at a final transmission step, powers of signals having peak powers are cancelled out to reduce a power variation of the final transmission signal.

FIG. 1is a block diagram of an example of an OFDM signal generator supporting SC-FDMA,

Referring toFIG. 1, a serial-to-parallel converting unit11converts a data symbol inputted in series to a parallel signal. A signal spreading unit12performs a dispreading on the paralleled data symbol in a frequency domain using Discrete Fourier Transform (hereinafter abbreviated DFT) before generating an OFDM signal. Equation 1 indicates a method of dispreading a parallel signal using NbsNbDFT matrix.
sF=FNb×Nbsx[Equation 1]

In Equation 1, ‘N’ indicates the number of subcarriers provided to an OFDM signal, ‘sx’ indicates a data symbol vector, ‘sF’ indicates a vector of which data is spread in a frequency domain, and ‘sTx’ indicates an OFDM symbol vector transmitted in a time domain. Moreover, ‘FNb×Nb’ is a DFD matrix of ‘NbsNb’ used in dispreading a data symbol.

A subcarrier mapping unit13maps the spread vector sFto subcarrier using a subcarrier allocating pattern. An Inverse Discrete Fourier Transform (hereinafter abbreviated IDFT) unit14transforms the signal mapped to the subcarrier into a signal in a time domain. Equation 2 represents Inverse Discrete Fourier Transform.
sTx=FN×N−1sF[Equation 2]

In Equation 2, FN×Nis NbsNbDFT matrix used to transform a signal in a frequency domain to a signal in a time domain and sTxis a signal generated in a time domain by IDFT, A parallel-to-serial converting unit15converts the parallel signal converted in a time domain to a serial signal. A cyclic prefix inserting unit16inserts a cyclic prefix in a signal to avoid interference between OFDM symbols and then transmits the signal.

The offset DFT-S-OFDM system improves the PAPR performance of the SC-FDMA. Yet, the offset DFT-S-OFDM system requires a considerable load of calculations to make its implementation complicated. In particular, in the process of performing DFT dispreading, DFT matrix is applied in a manner of separating an input symbol into a real part and an imaginary part. So, the calculation load increases to make the corresponding implementation more complicated. Hence, the demand for a system, of which implementation is simpler than the offset DFT-S-OFDM system with PAPR performance better than that of the offset DFT-S-OFDM system, rises.

SUMMARY OF THE INVENTION

Accordingly, the present application is directed to control PAPR and method thereof that substantially obviates one or more problems due to limitations and disadvantages of the related art.

An object of the present application is to provide an apparatus for controlling PAPR in an OFDM system and method thereof, by which implementation is simplified and by which enhanced PAPR characteristics are provided.

Additional advantages, objects, and features of the application will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the application. The objectives and other advantages of the application may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with the purpose of the application, as embodied and broadly described herein, a method of controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, according to the present application includes the steps of outputting GNbdata symbols by oversampling Nbparallel data symbols, spreading the oversampled data symbols using DFT (discrete Fourier transform), and mapping the spread signal to subcarriers.

In another aspect of the present application, a method of controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, includes the steps of spreading Nbparallel data symbols using DFT (discrete Fourier transform), shifting a phase of each of the spread data symbols by a phase value corresponding to a case of oversampling the corresponding data symbol, summing the spread data symbol and the phase-shifted data symbol, and mapping the summed symbols to subcarriers.

In another aspect of the present application, an apparatus for controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, includes an oversampling unit outputting GNbdata symbols by oversampling Nbparallel data symbols, a discrete Fourier transform unit spreading the oversampled data symbols using DFT (discrete Fourier transform), and a subcarrier mapping unit mapping the spread signal to subcarriers.

In another aspect of the present application, an apparatus for controlling PAPR (peak to average power ratio), which is applied to a mobile communication system, includes a discrete Fourier transform unit spreading Nbparallel data symbols using DFT (discrete Fourier transform), a phase shift unit shifting a phase of each of the spread data symbols by a phase value corresponding to a case of oversampling the corresponding data symbol, a signal summing unit summing the spread data symbol and the phase-shifted data symbol, and a subcarrier mapping unit mapping the summed symbols to subcarriers.

It is to be understood that both the foregoing general description and the following detailed description of the present application are exemplary and explanatory and are intended to provide farther explanation of the application as claimed.

DETAILED DESCRIPTION OF THE INVENTION

In case that a signal is spread in a frequency domain based on DFT, if a length of a data vector is Nb, a maximum number Nbof data vectors appear in a transmission signal. Hence, it is able to decrease PAPR in a manner that input data appear in the transmission signal as many as possible. And, oversampling can be performed to increase a size of the input data with maintaining a size of the transmission data. In particular, after oversampling has been performed on the data vectors, the oversampled vectors are transformed to be transmitted.

FIG. 2is block diagram of an OFDM system according to a first embodiment.

Referring toFIG. 2, a serial-to-parallel converting unit21converts a data symbol inputted in series to a parallel signal. And, an oversampling unit22oversamples Nbdata symbols G-times (oversampling coefficient) to extend a number of input symbols by G times with maintaining a number of independent data symbols. In the following description, an example of ‘G=2’ is explained.

First of all, assuming that a signal sxwhich is inputted to the oversampling unit22after being converted in parallel by the serial-to-parallel converting unit21is [sx(0), sx(1) 0 sx(Nb−1)]T, Equation 3 indicates a method of oversampling data symbols.
sxo=sxo1+sxo2[Equation 3]

In Equation 5, sxo2is exemplarily generated by shifting sxo1by one by one. Yet, sxo2can be generated by shifting sxo1by an arbitrary k.

A discrete Fourier transform unit23performs DFT on the oversampled signal to bring about a dispreading effect in a frequency domain. And, the DFT can be expressed as Equation 6.
sFo=sFo1+sFo2[Equation 6]

In this case, F2NbsNbis a transform matrix consisting of upper Nbof F2Nbs2Nb. A subcarrier mapping & frequency window unit24maps the DFT-performed signal to subcarrier and then executes a frequency window.

The frequency-domain signal mapped to the subcarrier is inputted to an inverse Fourier transform unit25. The inverse Fourier transform unit25performs Inverse Fast Fourier Transform (hereinafter abbreviated IFFT) on the input signal to transform it into a time-domain signal. And, a parallel-to-serial converting unit26converts the time-domain transformed parallel signal to a serial signal.

A cyclic prefix (hereinafter abbreviated CP) inserting & time window unit24to eliminate interference between OFDM symbols adds a CP to the serial signal and then executes a time window.

FIG. 3is block diagram of an OFDM system according to a second embodiment of the present application.

Referring toFIG. 3, the respective components of sFo1and sFo2in Equation 7 and Equation 8 can be represented as Equation 9.

In Equation 9, if the oversampled vector is transformed, the result is related to an original signal SF. And, sFo1and sFo2differ from each other in a phase component. Hence, the OFDM system shown inFIG. 2can be implemented by the method shown inFIG. 3.

Similar to the former embodiment shown inFIG. 2, a serial-to-parallel converting unit31converts a data symbol inputted in series to a parallel signal. And, a discrete Fourier transform unit32performs spreading on the parallel-converted data symbol using DFT matrix of Nb×Nb.

As shown in Equation 9, if phases of the respective spread input symbols are shifted and summed, it brings about the same effect of oversampling.

So, a phase shifting unit33shifts a phase of the input symbol in a manner shown in Equation 9. And, a summing unit34generates a signal resulting from summing an input phase shifted symbol and an non-shifted symbol.FIG. 3shows a case that a value of G (oversampling coefficient) is 2. Yet, the number of input signals inputted to the summing unit34is normally ‘G’. For instance, if G is 3, a signal outputted from the discrete Fourier transform unit32, a signal resulting from performing a first phase shift on the signal outputted from the discrete Fourier transform unit32, and a signal resulting from performing a second phase shift on the signal outputted from the discrete Fourier transform unit32are summed up.

A subcarrier mapping & frequency window unit35maps the DFT-performed signal to subcarrier and then executes a frequency window. The frequency-domain signal mapped to the subcarrier is inputted to an inverse Fourier transform unit36. The inverse Fourier transform unit36transforms the inputted signal into a time-domain signal by performing inverse fast Fourier transform (hereinafter abbreviated IFFT) on the inputted signal. A parallel-to-serial converting unit37converts the parallel signal transformed into the time-domain signal to a serial signal. And, a cyclic prefix (hereinafter abbreviated CP) inserting & time window unit38to avoid interference between OFDM symbols adds CP to the serially converted signal and then executes a time window.

FIG. 4is a graph of PAPR performance of an OFDM system according to one embodiment.

First of all, inFIG. 4, if N=512, Nb=128 and if subcarriers are allocated to a continuous subcarrier group, PAPR performance of each system is represented as CCDF (complementary cumulative distribution function) over PAPR.

Referring toFIG. 4, a curve41indicates PAPR performance of an OFDM system in case of not using a system for PAPR performance enhancement, and a curve42indicates PAPR performance in case of applying SC-FDMA system.

A curve43indicates PAPR performance in case of applying Offset-DFT-S-OFDM SC-FDMA, and a curve44indicates PAPR performance of an OFDM system according to the present application.

FIG. 5is a graph of PAPR performance of an OFDM system according to another embodiment.

InFIG. 5, if N=512, Nb=64 and if subcarriers are allocated to a continuous subcarrier group, PAPR performance of each system is represented as CCDF (complementary cumulative distribution function) over PAPR.

Referring toFIG. 5, a curve51indicates PAPR performance of an OFDM system in case of not using a system for PAPR performance enhancement, and a curve52indicates PAPR performance in case of applying SC-FDMA system.

A curve53indicates PAPR performance in case of applying Offset-DFT-S-OFDM SC-FDMA, and a curve54indicates PAPR performance of an OFDM system according to the present application.

Accordingly, the present application provides the following effect or advantage.

Most of all, the present application enables PAPR to be efficiently controlled with a simple structure for implementation.