Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2614—Peak power aspects
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03828—Arrangements for spectral shaping; Arrangements for providing signals with specified spectral properties
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
  • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
  • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
  • H04B2201/70706—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation with means for reducing the peak-to-average power ratio

Definitions

• the invention relates to a method to reduce the peak-to-average power ratio of a radio communication signal, according to the preamble of claim 1. Further, it concerns a transmitter, a receiver and a communication system according to claims 9, 10, and 11 respectively.
• a low peak-to-average power ratio (PAPR) of the transmitted signal in uplink of a mobile communications system is important, because lower PAPR implies e.g. longer battery life for the same average transmitted power.
• SC-FDMA Single-Carrier Frequency Division Multiple Access
• DFT-spread OFDM Discrete Fourier Transform-spread Orthogonal Frequency Domain Multiplexing
• Figure 1 shows the transmitter structure for DFT-spread OFDM.
• Each block of M complex modulated symbols x n , n 0,l,...,M-l, is transformed by a DFT and results in M coefficients X&
• IDFT Inverse Discrete Fourier Transform
• a cyclic prefix is inserted.
• a time window may be applied after the cyclic prefix to reduce out-of-band emissions.
• the cyclic prefix and the time window do not change the PAPR of the signal.
• the transmitter in Figure 1 has a very similar effect on the power distribution of the transmitted signal as a time-domain processed single-carrier transmission with a sine pulse-shaping filter.
• the sine waveform decays very slowly and hence the signal at a certain time in general depends on a large number of modulated symbols, which introduces power variations in the transmitted signal and hence increases PAPR.
• Root-raised cosine (RRC) filters are widely used for pulse- shaping. These filters have the following properties, which are useful in numerous applications:
• the filters o can be implemented as FIR filters, and o give rise to Nyquist pulses after a matched filter. However, these properties are not important for DFT-spread OFDM, because o time-domain filtering can be replaced by spectrum-shaping in the frequency domain, and o frequency-domain equalization is performed regardless the impulse response of the channel including spectrum-shaping.
• the RRC filter gives shorter pulse shapes with increasing roll-off factor.
• the shorter pulse shape implies that the signal at a certain time instant depends on a small number of symbols, which gives a small variation of the signal amplitude, and hence lower PAPR than for the sine filter.
• the sine filter is a RRC filter with roll-off factor equal to zero.
• One way to reduce the PAPR of the signal in DFT-spread OFDM is to apply spectrum shaping by multiplying the output of the DFT component-wise with a spectrum-shaping sequence, see Document 2, as shown in Figure 2. hi case the signal is mapped on a larger set of sub-carriers than the size of the DFT, the input to the spectrum-shaping block is periodically extended before multiplication with the spectrum-shaping sequence.
• the spectrum shaping is described by the following equation,
• S k are the components of the spectrum shaping sequence and U is the number of occupied sub-carriers.
• the output of the IDFT is given by
• the only spectrum-shaping sequence S that has been proposed in prior art is sampled from the root-raised cosine (RRC) function, i.e. the transfer function of the RRC filter.
• RRC root-raised cosine
• a main object is consequently to propose a method that enables a lower PAPR for SC-FDMA, compared to SC-FDMA with RRC shaping.
• the present invention suggests a solution that under certain circumstances yields a PAPR that analytically can be proven to be minimised.
• this is accomplished by a method having the characteristics that are defined in claim 1, by a transmitter having the characteristics of claim 9, by a receiver having the characteristics of claim 10 and by a radio communication system having the characteristics of claim 11.
• a radio communication signal is filtered in the discrete frequency domain with a spectrum-shaping sequence.
• This sequence is chosen to possess a spectrum that is maximised in a finite interval, i.e. the energy outside the finite interval is minimised.
• the spectrum shaping sequence of the invention can be designed to have an adjustable parameter to trade off the peak-to-average power ratio to the required average signal-to-noise ratio.
• This parameter can constitute or correspond to the width of said finite interval.
• the spectrum shaping sequence of the invention can constitute substantially a discrete prolate spheroidal sequence or a product of a discrete prolate spheroidal sequence and a sampled function.
• the spectrum shaping sequence of the invention can constitute an approximation of a discrete prolate spheroidal sequence. Such an approximation is a Kaiser window.
• the spectrum-shaping sequence may also constitute a Kaiser window multiplied with a sampled function.
• the method of the invention could be implemented in a transmitter for a radio communication system. Preferably, such a transmitter would be communicating with a corresponding receiver for a radio communication system, including means for receiving and processing signals generated by the transmitter. Together they would form part of a radio communication system that would include at least one such transmitter and at least one such receiver.
• fig. 1 constitutes a block diagram of a method according to prior art
• fig. 2 constitutes a block diagram of the method of the invention
• fig. 3 illustrates a radio communication system employing the method of the invention.
• Fig. 2 constitutes a block diagram of the method of the invention.
• the invention consists of the selection of the index-limited spectrum-shaping sequence of length U, where U is greater than or equal to M, whose Fourier spectrum is such that the energy in a finite interval is maximised, i.e. the energy outside the finite interval is minimised.
• the function is not restricted to give a Nyquist pulse in the time domain after matched filtering, and hence it may induce inter-symbol interference. Inter-symbol interference does not increase the complexity of the receiver since it contains an equalizer anyway but it may increase the required average signal-to-noise ratio (SNR) for a given throughput. The required SNR increases with increasing main-lobe width of the Fourier spectrum of the spectrum-shaping function due to inter-symbol interference.
• SNR signal-to-noise ratio
• x n are modulated data symbols and s(t) is proportional to the Fourier series expansion (Fourier spectrum) of the spectrum shaping sequence S k , s(t) ⁇ . It is assumed that the amplitude of x n is constant (as for phase shift keying) and equals A. /is the centre frequency of the signal and I 7 is the time duration of the useful signal, i.e. the output signal from the IDFT, divided by L.
• the peak power is proportional to the energy of s plus cross-terms
• s(i) is the Fourier spectrum of S k , which means that s(t), ⁇ t ⁇ W , where W is of the order of 1/2M, is the main-lobe of the spectrum, while s(t), ⁇ t ⁇ > W contains the sidelobes of the spectrum.
• W is of the order of 1/2M
• s(t), ⁇ t ⁇ > W contains the sidelobes of the spectrum.
• the spectrum-shaping sequence that fulfils this criterion is the zero* 11 discrete prolate spheroid sequence, also labelled discrete prolate spheroidal window, see D. van de Ville et al., "On the N-Dimensional Extension of the Discrete Prolate Spheroidal Window," IEEE Signal Processing Letters, vol. 9, no. 3, pp 89-91, March 2002 (this paper is called Document 3 below).
• This index-limited sequence maximises the energy in a finite interval of its Fourier spectrum.
• the discrete prolate spheroidal sequences S®(U, W) are the normalised eigenvectors that satisfy
• the discrete prolate spheroidal window is the eigenvector S ⁇ that corresponds to the largest eigenvalue )P .
• W is an adjustment parameter, which allows adjustment of the main- lobe width in the Fourier spectrum of the spectrum-shaping function. While increasing t/and keeping the product UW constant, the discrete prolate spheroidal sequences approximate the (continuous) prolate spheroidal wave function.
• the Kaiser window has an adjustment parameter ⁇ , which allows adjustment of the main-lobe width in the Fourier spectrum of the spectrum-shaping function.
• the width of the main-lobe has opposite impacts on the required
• the Kaiser window spectrum shaping sequences have been evaluated by link-level simulations and their performance has been compared to RRC functions for QPSK.
• the Kaiser window spectrum shaping sequence introduces inter-symbol interference in the transmitted signal, which in general increases the required average SNR to achieve a certain throughput (spectral efficiency).
• the PAPR is reduced compared to a RRC spectrum shaping function.
• the selected performance measure is the throughput as a function of peak transmit power (99.9 percentile), which is proportional to the average received SNR plus PAPR (99.9 percentile) given that the average channel loss and the noise power is constant. The latter is ensured by keeping both noise bandwidth and noise spectral density constant in all simulations.
• the throughput is the product of the information bit rate and the ratio of transmitted blocks that are received correctly.
• the modulation and coding are fixed throughout the simulation, i.e. there is no link adaptation.
• For each modulation two information bit rates, Ri are simulated.
• the coded bit rate Rc is selected such that the modulated symbol rate equals the 3 dB bandwidth of the RRC function.
• the Kaiser window has been evaluated for the same coded bit rates as for the different RRC functions.
• the total bandwidths of the spectrum-shaping functions are selected to give similar out-of-band leakage for all functions, i.e. to be within the same spectral mask.
• the frame format is according to Document 1, Table 9.1.1-2. Other simulation parameters are given in Table 1 and spectrum-shaping function-dependent parameters are shown in Table 2.
• the invention also embraces a radio communication system, which for instance could consist of base station(s) 120 of a cellular system 100 and terminal(s) 130 communicating with said base station(s).
• the base station(s) and/or terminal(s) would include at least one transmitter with means for executing the method according to the invention.
• the base station(s) and/or terminal(s) would also include at least one receiver including means for receiving and processing signals generated by the transmitter.
• the spectrum .shaping of the invention can also be achieved with time-domain processing of a single-carrier signal with a pulse-shaping filter, in which the filter coefficients are given by the inverse Fourier transform of the spectrum-shaping sequence.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
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• 2005
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