The use of multi-tone signals for the communication of information has been proposed for quite some time. In such systems, a plurality of tones are used to communicate symbols in parallel, with the total bandwidth between a transmitter and receiver device being a function of the number of tones being used to communicate the information.
Generally, a multi-tone signal includes N (N>0) complex symbols modulated on N distinct tones simultaneously in a symbol duration T:
            s      ⁡              (        t        )              =                  ∑                  k          =          1                N            ⁢                        A          k                ⁢                  cos          ⁡                      (                                          2                ⁢                π                ⁢                                                                  ⁢                                  f                  k                                ⁢                t                            +                              θ                k                                      )                                ,            f      ⁢                          ⁢      o      ⁢                          ⁢      r      ⁢                          ⁢      t        =                  [                  0          ,          T                ]            .      In the above equation, θk and Ak are respectively the phase (in radians) and amplitude of the complex symbol to be transmitted on tone k, and fk is the frequency of tone k. t is the time variable. A multi-tone signal comprises a plurality of single-tone signals, where each single-tone signal is a periodic signal. In a practical system, the periodic signals are transmitted for a symbol duration T, which is a finite time interval.
An OFDM signal is an example of the multi-tone signal with each distinct tone k representing a different subcarrier. In an OFDM signal the distinct tones which are used to form the OFDM signal are orthogonal over the symbol duration.
FIG. 1 illustrates a known OFDM transmission system 1. In the known system, for each period in which a symbol is to be transmitted, a digital complex symbol generator 2 generates a vector of digital complex symbols. The vector includes, e.g., one symbol per OFDM tone to be used. The vector of complex symbols are then transformed into a vector of complex time domain samples corresponding to a symbol period by an Inverse Fourier transform operator 3, e.g., an Inverse discrete or Inverse Fast Fourier Transform (IFFT) circuit. The time domain samples represent the discrete samples of the baseband signal to be transmitted during a symbol transmission period. This signal is essentially the sum of one or more sinusoid component signals, e.g., the OFDM tones. A single cyclic prefix is generated for the signal to be transmitted during a symbol transmission period. The cyclic prefix is added by the cyclic prefix generator 4 to the vector of time domain samples supplied by the Inverse Fourier transform operator 3. Generally, the cyclic prefix is usually a copy of the last few samples in the vector of the time domain samples and will therefore include all the OFDM signal's sinusoid components, e.g., tones. After the cyclic prefix is appended to the beginning of the samples supplied by the Inverse fourier transform operator 3, the signal samples pass through a filter 5. The filter 5 is used to limit out of band spectral emissions. The filtered samples are then converted to an analog signal by a digital to analog converter 6. The analog signal is then mixed with the carrier frequency by mixer 7 to generate a passband signal. The passband signal is then power amplified by amplifier 8 and transmitted to a communication channel through a single antenna 9.
Accordingly, in the known system shown in FIG. 1, a periodic sinusoidal signal to be transmitted is generated in the baseband, a cyclic prefix is added in the baseband, and then the signal is mixed to the passband prior to OFDM signal transmission.
While the concept of using a multi-tone or OFDM signal to communicate information is relatively well understood, the existing techniques for transmitting such a signal tend to be inefficient in terms of power utilization. The power inefficiency results from the multi-tone signal being a sum of a plurality of single tone signals, which normally leads to a high peak-to-average ratio in the resultant multi-tone signal. As a result, there is a need for improving the transmission techniques of multi-tone signals, e.g., OFDM signals. It is desirable that at least some new transmission techniques increase the power efficiency of multi-tone signals thereby making them better suited for use in systems such as wireless communication networks where long battery life and inexpensive power amplifiers of wireless devices are highly desirable.