Generally, the OFDM scheme is widely used for wire/wireless communication fields. The OFDM is the scheme most frequently discussed in communication fields together with CDMA (code division multiple access). Yet, an OFDM or CDMA system has worse PAPR than other communication systems. In a communication system, performance of an output amplifier of a transmitter largely depends on PAPR. If the PAPR increases, a linear interval of the output amplifier should be correspondingly widened. This results in inefficient communications.
Importance of PAPR in an OFDM system is much greater than that in a CDMA system. The reason is explained as follows. In case of the CDMA system, since user signals are summed together in time domain, it is able to deliver the user signals by manipulating the user signals in time domain. On the other hand, in case of the OFDM system, user data exists in frequency domain. So, conversion between frequency domain and time domain should be carried out to find PARR in time domain. In particular, domain conversion work should be conducted in proportion to an application count in the process of selecting a minimum PAPR using various candidate codes or schemes in the OFDM system. So, system complexity may increase.
In the related art, PARR improving schemes can be classified into an improving scheme in frequency domain and an improving scheme in time domain. A phase randomization scheme, a selective mapping scheme, and the like belong to the improving schemes in frequency domain. And, a PTS (partial transmit sequence) scheme is a representative one of the improving schemes in time domain.
FIG. 1 is a block diagram to explain a method of diminishing PAPR in frequency domain according to a related art.
Referring to FIG. 1, assuming that a data vector to be transmitted in an OFDM system is {right arrow over (d)}=[d0, d1, . . . , dN-1]T, a signal transmitted in time domain can be obtained through inverse fast Fourier transform (IFFT) shown in Equation 1.{right arrow over (s)}=[s0, s1, . . . , sN-1]T=F−1{right arrow over (d)}  [Equation 1]
In Equation 1, F is a Fourier transform matrix. A vector {right arrow over (s)} is a signal to be transmitted via an antenna by being modulated into a carrier frequency. A variation of an absolute value of the transmission signal vector {right arrow over (s)} is represented as PAPR. And, the PAPR can be defined as Equation 2.
                              P          ⁢                                          ⁢          A          ⁢                                          ⁢          P          ⁢                                          ⁢          R                =                                            max                                                k                  =                  0                                ,                …                ⁢                                                                  ,                                  N                  -                  1                                                      ⁢                                                                            s                  k                                                            2                                                          1              N                        ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                                                      s                    k                                                                    2                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
As can be seen from Equation 2, if any one of vector components has an abnormally large value, the PAPR increases to degrade signal characteristics. To solve this problem, a method used in frequency domain can be represented as Equation 3.{right arrow over (d)}x=MSMP{right arrow over (d)}  [Equation 3]
In Equation 3, MS is a matrix (phase shift matrix) that changes a phase component of each data component of {right arrow over (d)} and MP is a matrix (position permutation matrix) that plays a role in changing a sequence of data component (Phase shift and position permutation block in FIG. 1).
In the related art PAPR improving scheme in frequency domain, in order to make PAPR attenuate according to Equation 3, signals in time domain are found using various combinations of MS and MP and the signal having the best performance is then selected. So, in order to execute PAPR improvement in frequency domain, N-sized IFFT should be used to find the PAPR for the various combinations of MS and M. And, complexity of Nlog2N is added each transform.
Unlike the above-explained performance improving method through time domain conversion after completion of data conversion in frequency domain, a PTS scheme is able to directly improve PAPR in time domain. In the PTS scheme, data symbols are grouped into predetermined groups without converting the data symbols in frequency domain and each of the groups is converted to time domain. Before summing the converted symbols in time domain into one, the symbols are multiplied by different phase patterns, respectively and are then summed together. However, since the PTS scheme needs a step of multiplying the symbols by the different phase patterns, respectively, it may raises complexity in system implementation.