The present invention relates to signal transmission.
It relates more particularly to transmission of a signal over a plurality of frequency subcarriers simultaneously.
This is the case, for instance, when using OFDM (“Orthogonal Frequency Division Multiplexing”). OFDM in its application to 3G radiocommunication systems is detailed in the technical report 3GPP TR 25.814 V7.0.0, Release 7, “Technical Specification Group Radio Access Network; Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)”, published in June 2006 by the 3rd Generation Partnership Project (3GPP).
FIG. 1 shows an example of a signal transmission using OFDM. A signal carrying information elements must be transmitted from a transmitter 1 to a receiver 2.
Xn designates information to be transmitted. First, a module 3 of the transmitter 1 transforms this serial flow into a plurality of N parallel flows Xn,0, Xn,1 . . . Xn,N−1. As an example, each one of these flows may consist in a succession of bits having duration of Tu.
An Inverse Fast Fourier Transform 4 is then performed in the transmitter 1 from the N parallel flows Xn,0, Xn,1 . . . Xn,N−1. Once this operation is achieved, an OFDM symbol sm with duration of Tu is obtained.
This symbol is coded over a plurality of frequency subcarriers separated by a guard band so as to reduce interference there between. Besides, the frequency subcarriers are orthogonal since the minimal duration of an information element carried by each one of them is the inverse of the guard band value.
The OFDM symbol sm is then transmitted over a communication channel 5 which may be of different nature, such as a radio channel. The receiver 2, listening to the channel 5, receives a symbol ŝm. Information elements {circumflex over (X)}n,0, {circumflex over (X)}n,1 . . . {circumflex over (X)}n,N−1 are estimated from this symbol, each element corresponding to the information transmitted over one given frequency subcarrier of the plurality.
Finally, a conversion parallel to serial 7 allows obtaining an estimation {circumflex over (X)}n of the transmitted information Xn.
While OFDM is quite efficient against the Inter Symbol Interference phenomenon, a trouble is that it creates a signal envelope with a very large range resulting in strong variations in the instantaneous power. The Peak-to-Average Power Ratio (PAPR) may thus be quite high with OFDM.
As a consequence, devices having a transfer characteristic with saturation and using OFDM may suffer from such situation. This applies in particular to devices involved in transmission using OFDM, such as power amplifiers. Indeed, power amplifiers should avoid a saturation of the OFDM signal to be amplified and thus operate in a linear zone.
To avoid this, there is a need for reducing the PAPR in systems using OFDM. This need has been identified in the above mentioned technical report TR 25.814 (see paragraph 9.2.1.4). Circulated clipping and filtering are more particularly evoked as an efficient way of mitigating the PAPR.
The traditional way of using clipping is to suppress a group of adjacent frequency subcarriers—generally at an end of the frequency band—from the plurality of subcarriers over which the signal is to be transmitted. However, such clipping may induce an undesired loss of information.
An object of the present invention is to transmit a signal over a plurality of frequency subcarriers while reducing the PAPR and without loosing too much information.