Deux methodes sont proposees pour reduire la puissance de crete ďun signal ofdm. Elles sont efficaces, souples et sans distorsion et elles apportent un accroissement faible de la complexite des calculs. De plus elles fonctionnent avec des nombres quelconques. La premiere methode produit plusieurs signaux multiporteurs et choisit celui qui a la plus faible puissance de crete pour la transmission. La seconde combine des sequences transmises partielles pour minimiser le facteur de crete. Apres ľanalyse theorique, les performances sont evaluees par simulation.

OFDM with reduced peak-to-average power ratio by multiple signal representation

The orthogonal frequency division multiplexing (OFDM) transmission technique applied in a multipath environment has a bit error rate (BER) comparable with a narrowband radio channel because the fading of each subcarrier is frequency-nonselective. To overcome this behavior and to reduce the BER a combination of OFDM and CDMA has been previously proposed. In this case the information bits are spread over many subcarriers. The present authors discuss the performance of OFDM-CDMA with BPSK or QPSK signalling over a frequency-selective Rayleigh fading channel. An important question is the detection strategy in an OFDM-CDMA system. The authors make a quantitative comparison of different detection algorithms: conventional correlation (equal gain, EGC), orthogonality restoring correlation (ORC), ORC with a threshold in order to suppress subcarriers with low signal strength (TORC) and an iterative improvement based on a maximum likelihood approach. With TORC and an optimal threshold a gain of more than 8 dB over conventional OFDM can be obtained at p/sub b/=10/sup -3/, if the fading of subcarriers is uncorrelated.

Comparison of different detection algorithms for OFDM-CDMA in broadband Rayleigh fading

A system and a method for transmitting coded information bits using multitone techniques on a frame-by-frame basis. For transmission, coded information bits are mapped into differential phase signals and absolute phase signals are generated based on differential phase signals. N complex symbols are then generated and inverse discrete Fourier transformed into N complex, time-domain samples, which are augmented with a cyclic prefix and suffix. The augmented N complex, time-domain samples are then separated into two real samples and converted into first and second baseband signals. The first and second baseband signals are impressed into quadrature carrier components to form in-phase and quadrature signal components which are combined into an RF signal prior to transmission. For reception, a received RF signal is non-coherently quadrature demodulated into first and second quadrature baseband signals. First and second quadrature sample sequences are generated by sampling the quadrature baseband signals in parallel. The added cyclic prefix and suffix are discarded, and N complex numbers are generated from the first and second quadrature (real) sample sequences. N channel-weighted tones are generated by performing a discrete Fourier transform on the N complex numbers. N phase difference signals are generated which are demapped into coded information bits according to a predetermined phase constellation.

An FFT-based multitone DPSK modem

An improved DAB transmission system with which a higher net data rate can be achieved while maintaining the same system quality, or the same system quality can be achieved with a much lower signal-to-noise ratio on the receiver side at the same net data rate. A symbol mapper is connected directly downstream from a convolution encoder instead of to the block generator; as in the case of prior DAB systems. A multi-stage channel encoder, to which the data substreams of a source data stream are distributed, is used. The source data stream is processed on the complex symbol level and not on the bit level, as in the case of prior DAB systems.

Method and system for the OFDM multicarrier transmission of digital broadcasting signals

In the OFDM methods, a plurality of modulated carriers are transmitted by frequency multiplexing, which results in an almost rectangular spectrum owing to the high number of carriers. In order to separate the carriers from one another again in the receiver, a fast Fourier transform is carried out such that each carrier can be cleanly separated from the others when the carriers are exactly orthogonal relative to one another. However, the orthogonality of the carriers can be disturbed by various causes. Furthermore, in the receiver the useful signal has to be separated from the undesired neighbouring channel signals by analog or digital filtration. In order to improve carrier and channel separation, the selectivity of FFT filtration can be increased by increasing the number of FFT components. However, this normally leads to an undesirably high increase in computing steps. The claimed configuration of the time window used within the context of the FTT and oversampling before the FFT render the calculation of some of the coefficients superfluous.

S. Breide

Papers

OFDM-Übertragungsverfahren für den digitalen Fernsehrundfunk