Picture100

communications

 

SINGLE CARRIER MULTI-TONE modulation scheme

 

    Orthogonal Frequency Division Multiplexing (OFDM) is the most popular transmission technology in digital terrestrial broadcasting (DTTB), adopted by many DTTB standards. The great advantage of OFDMA is its robustness in the presence of multipath signal propagation. The immunity to multipath arises from the fact that the OFDMA system transmits information to the M orthogonal frequency carrier each of which operates at 1/M times the bit rate of the information signal. On the other hand, the OFDMA waveform exhibits noticeable envelope fluctuations resulting in a high peak-to-average power ratio (PAPR). Signals with a high PAPR require highly linear power amplifiers to avoid excessive intermodulation distortion. To achieve this linearity, the amplifiers have to operate with a large back off from their peak power. The result is low power efficiency (measured by-the ratio of transmitting power to the power dissipated), which places significant difficulties on portable wireless terminals. Another problem with the OFDMA scheme in wireless transmissions derives from the inevitable offset in frequency references among the transmitted and receiver terminals. Frequency offset destroys the orthogonality of the transmissions, thus introducing subcarriers interference. To overcome these drawbacks, 3GPP is studying a modified form of OFDMA for uplink transmissions in the “long-term evolution (LTE)” of cellular systems.  A modified version of OFDMA, called Single Carrier FDMA (SC-FDMA), has been described in several standard documents and researches. As in OFDMA, the transmitter in the SC-FDMA uses number orthogonal frequencies (subcarriers) to transmit information symbols. Compared to OFDMA, the mechanism significantly reduces variations in the envelope of the transmitted signal. Therefore, SC-FDMA signals have inherent lower PAPR then OFDMA signals. The logical solution would be to use a new SC FDMA modulation for future Broadband communication systems, especially for mobile satellite systems, TV Broadcasting, aeronautical communication, but it is possible only after a significant improvement in the SC FDMA technology. As it is well known the SC-FDMA uses Digital Fourier Transform (DFT) instead of Fast Fourier Transform (FFT), which is used in OFDM. Because of DFT complexity, the number point in DFT and IDFT blocks of SC-FDMA transmitter is limited correspondingly to 16 and 256 points. For broadcasting systems are needed DFT and IDFT with significantly more points (minimum 1024 and 4096). Therefore SC-FDMA up to date is used only for upstream of cellular devices. 

    While we were working on wavelet-based filtered multi-tone modulation (WFMT) for aeronautical application we noticed that for wireless communication the single channel WFMT is the optimal method. This modulation scheme, that we named SC-WFMT, is ideally suited for transmission in fading channel and may be used in a variety wireless communication systems.

    Our next development is useful for US terrestrial broadcast TV translation. It is fully backward-compatible with existing ATSC A53/A153 systems and at the same time has better performance than OFDM systems have. We call this scheme "Single Carrier Multi-Tone" (SCMT) to associate it with the physical principles which it built on.

    Guarneri Communication has been established for developing, producing and marketing the full programmable communication VLSI which realizes our proprietary methods of the standard technologies (DVB, ATSC). Our target is to develop universal broadband integrated circuit which can be used in terrestrial broadcasting, satellite broadcasting and cable TV systems.