Peak to average power ratio reduction in wireless communication systems
According to an example aspect of the present disclosure, there is provided a method for transmitting a Discrete Fourier Transform-spread-Orthogonal Frequency Domain Multiplexed. DFT-s-OFDM, signal, the method comprising, converting, by a wireless transmitter, complex-valued symbols to polar domain symbols, wherein the complex-valued symbols are modulated using at least phase shift keying, processing, by the wireless transmitter, angle values of the polar domain symbols to avoid angle differences larger than π rad, or lower than −π rad, between consecutive angle values, interpolating, by the wireless transmitter, at least the processed angle values by an interpolation factor N/L, wherein L is a length of Fast Fourier Transform, FFT, and Nis length of Inverse FFT, IFFT, converting, by the wireless transmitter, the interpolated angle values into complex domain symbols and generating, by the wireless transmitter, a DFT-s-OFDM waveform comprising said complex domain symbols.
1 . A method comprising:
converting, by a wireless transmitter, complex-valued symbols to polar domain symbols, wherein the complex-valued symbols are modulated using at least phase shift keying;
processing, by the wireless transmitter, angle values of the polar domain symbols to avoid angle differences larger than IT rad, or lower than −π rad, between consecutive angle values;
interpolating, by the wireless transmitter, the processed angle values by an interpolation factor N/L, wherein L is a length of Fast Fourier Transform, FFT, and N is length of Inverse FFT, IFFT;
converting, by the wireless transmitter, the interpolated angle values into complex domain symbols; and
generating, by the wireless transmitter, a Discrete Fourier Transform-spread-Orthogonal Frequency Domain Multiplexed, DFT-s-OFDM, waveform comprising said complex domain symbols.
2 . A method according to claim 1 , wherein said converting the complex-valued symbols to polar domain symbols comprises determining an angle and a radius of the complex-valued symbols.
3 . A method according to claim 1 , wherein the complex-valued symbols are symbols modulated using pulse amplitude modulation, quadrature phase shift keying or M-quadrature amplitude modulation, wherein M indicates how many bits are transmitted per time interval or symbol for each unique amplitude/phase combination.
4 . A method according to claim 1 , wherein said interpolating comprises performing an L-point FFT and N-point IFFT.
5 . A method according to claim 1 , further comprising:
determining, by the wireless transmitter, that the complex-valued symbols are symbols modulated using phase shift keying or M-quadrature amplitude modulation with M less than four; and
setting, by the wireless transmitter, a radius of each polar domain symbol as one.
6 . A method according to claim 5 , further comprising:
determining, by the wireless transmitter, that the complex-valued symbols are symbols modulated using M-quadrature amplitude modulation with M greater than four; and
interpolating, by the wireless transmitter, a radius of the polar domain symbols by the interpolation factor N/L.
7 . A method according to claim 1 , further comprising:
transmitting, by the wireless transmitter, the generated DFT-s-OFDM waveform comprising said complex domain symbols to a wireless receiver.
8 . A method according to claim 1 , further comprising:
dividing, by the wireless transmitter, the DFT-s-OFDM waveform into B blocks of length N B ;
converting, by the wireless transmitter, symbols of each of said blocks into frequency-domain symbols using FFT of length N B ;
windowing and folding, by the wireless transmitter, said blocks comprising the frequency-domain symbols;
converting, by the wireless transmitter, said folded frequency-domain symbols of each block into time-domain blocks using IFFT of length N B ;
concatenating, by the wireless transmitter, all said time-domain blocks to generate another DFT-s-OFDM waveform; and
transmitting, by the wireless transmitter, said another DFT-s-OFDM waveform.
9 . An apparatus comprising at least one processing core and at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to:
convert complex-valued symbols to polar domain symbols, wherein the complex-valued symbols are modulated using at least phase shift keying;
process angle values of the polar domain symbols to avoid angle differences larger than π rad, or lower than −π rad, between consecutive angle values;
interpolate the processed angle values by an interpolation factor N/L, wherein L is a length of Fast Fourier Transform, FFT, and N is length of Inverse FFT, IFFT;
convert the interpolated angle values into complex domain symbols; and generate a Discrete Fourier Transform-spread-Orthogonal Frequency Domain Multiplexed, DFT-s-OFDM, waveform comprising said complex domain symbols.
10 . An apparatus according to claim 9 , wherein said converting the complex-valued symbols to polar domain symbols comprises determining an angle and a radius of the complex-valued symbols.
11 . An apparatus according to claim 9 , wherein the complex-valued symbols are symbols modulated using quadrature phase shift keying or M-quadrature amplitude modulation.
12 . An apparatus according to claim 9 , wherein said interpolating comprises performing an L-point FFT and N-point IFFT.
13 . An apparatus according to claim 9 , wherein the at least one memory and the computer program code are further configured to, with the at least one processing core, cause the apparatus at least to:
determine that the complex-valued symbols are symbols modulated using phase shift keying or M-quadrature amplitude modulation with M less than four; and
set a radius of each polar domain symbol as one.
14 . An apparatus according to claim 9 , wherein the at least one memory and the computer program code are further configured to, with the at least one processing core, cause the apparatus at least to:
determine that the complex-valued symbols are symbols modulated using M-quadrature amplitude modulation with M greater than four; and
interpolate a radius of the polar domain symbols by the interpolation factor N/L.
15 . An apparatus according to claim 9 , wherein the at least one memory and the computer program code are further configured to, with the at least one processing core, cause the apparatus at least to:
transmit the generated DFT-s-OFDM waveform comprising said complex domain symbols to a wireless receiver.
16 . An apparatus according to claim 9 , wherein the at least one memory and the computer program code are further configured to, with the at least one processing core, cause the apparatus at least to:
divide the DFT-s-OFDM waveform into B blocks of length N B ;
convert symbols of each of said blocks into frequency-domain symbols using FFT of length N B ;
window and fold said blocks comprising the frequency-domain symbols; converting said folded frequency-domain symbols of each block into time-domain blocks using IFFT of length N B ;
concatenate all said time-domain blocks to generate another DFT-s-OFDM waveform; and
transmit said another DFT-s-OFDM waveform.
17 . An apparatus comprising at least one processing core and at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to:
receive from a wireless transmitter a Discrete Fourier Transform-spread-Orthogonal Frequency Domain Multiplexed, DFT-s-OFDM, waveform comprising complex domain symbols; and
determine complex-valued symbols, wherein the determining comprises decimating at least angle values of said complex domain symbols by an interpolation factor N/L, wherein N is a length of Fast Fourier Transform, FFT, and L is length of Inverse FFT, IFFT.
18 . An apparatus according to claim 17 , wherein the complex-valued symbols are symbols modulated using quadrature phase shift keying or M-quadrature amplitude modulation, wherein M indicates how many bits are transmitted per time interval or symbol for each unique amplitude/phase combination.