Multiplex communications – Generalized orthogonal or special mathematical techniques – Fourier transform
Patent
1995-07-28
1997-03-11
Olms, Douglas W.
Multiplex communications
Generalized orthogonal or special mathematical techniques
Fourier transform
370478, 370484, H04L 506
Patent
active
056109083
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for transmitting digital data by a frequency division multiplex using a large number of closely-spaced carriers.
Orthogonal frequency division multiplex (OFDM) signals have been proposed for various applications, of which the first was digital audio broadcasting (DAB), also known as digital sound broadcasting (DSB); see Leonard J. Cimini, Jr., "Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing", IEEE Transactions on Communications, Vol. COM-33, No. 7, July 1985, pages 665 to 675. An OFDM signal consists of a large number of carriers, closely-spaced in frequency, and each modulated with data. Typically there will be several hundred or even more carriers.
The proposed DAB system involves: processes, to overcome frequency selectivity, using many simultaneous narrowband carriers, gain in all fixed and mobile receiving conditions, and a Viterbi maximum-likelihood decoding algorithm to provide a large coding gain, and possibly an outer block code concatenated with the convolution inner code.
Such a signal maybe termed a Coded Orthogonal Frequency Division Multiplex (COFDM).
For further background information concerning the COFDM proposal, reference may for example be made to: Broadcasting to Mobile Portable and Fixed Radio Receivers", TBC 88, IEE Conference Publication No. 293, pages 349 to 353. IEEE Transactions on Consumer Electronics, Vol. 35, No. 3, August 1989, pages 493 to 503, which includes a block diagram of a proposed receiver. Review--Technical, No. 246, April 1991, pages 87 to 112, which outlines the construction of both transmitter and receiver. Channels--Part I . . . " IEEE Transactions on Communications, Vol. 39, No. 5, May 1991, pages 783 to 793. Digital Audio Broadcasting, Montreux 8-9 June 1992; eg. Le Floch, "Channel Coding and Modulation for DAB", Proceedings pages 99-109
In the proposed DAB system using COFDM, quadrature phase shift keying (QPSK) modulation, also termed 4-PSK modulation, is used to modulate the carriers prior to transformation by a fast Fourier transform circuit. In principle higher order PSK modulation systems could be used.
In an OFDM signal relatively high voltage peaks can occur for short periods of time when the various carriers are in phase. The ratio between the possible peak power and the mean power increases as the number of carriers increases. A problem then occurs when the signal is transmitted through a practical device (such as an amplifier), as the device must have a linear transfer characteristic with a large amount of "headroom" to prevent non-linear effects from occurring. In a transmitter network the amount of headroom required may significantly affect the cost of the transmitters. This headroom may be reduced if the instantaneous peak-to-mean power ratio of the signal can be reduced.
One method of reducing the peak-to-mean power ratio would be to use some of the data capacity of the system, so that the phase of a number of carriers can be chosen in each symbol to prevent large variations in the overall signal envelope from occurring. For DAB this is not possible because no spare data capacity exists for this purpose.
Another possibility would be to introduce a 45.degree. phase shift on half the carriers; for example, on alternate carriers. This would ensure that the carriers could not all add up in phase at once. In practice, however, we have found that little improvement is obtained this way.
It might be thought that the desired reduction in peak power could be achieved simply by limiting the signal. However, this process introduces intermodulation products. In an OFDM system, the intermodulation products fall exactly on existing carriers, and so effectively change the amplitudes and phases of these carriers.
European Patent Application EP-A-0 499 560 (and corresponding Canadian Patent Application CA-A-2 059 455) describes a receiver designed for coherent demodulation of an OFDM signal. At the receiver the received OFDM signal i
REFERENCES:
patent: 4495620 (1985-01-01), Steele et al.
patent: 4881241 (1989-11-01), Pommier et al.
E. Feig et al. "Digital Implementation of Frequency Division Multiplexing on Peak-Limited Channels", IEEE, International Conference on Acoustics, Speech, and Signal Processing ICASSP, 1989, vol. 2, pp. 1364-1367.
B.T. Tan et al. "Crest Factor Minimisation in FDM PSK Systems", Electronics Letters, vol. 26, No. 13, 1990, pp. 859-861.
D.G.W. Ingram et al. "Application of Transform Techniques to Digital Line Codes and Data Transmission", Electronics Letters, vol. 18, No. 22, 1982, pp. 949-950.
L. Cimini, Jr. "Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing", IEEE, Transactions on Communications, vol. COM-33, No. 7, 1985, pp. 665-675.
D. Pommier et al. "New Prospects for High-Quality Digital Sound Broadcasting to Mobile, Portable and Fixed Radio Receivers", IBC 88, IEE Conference Publication, No. 293, pp. 349-353.
B. Le Floch et al. "Digital Sound Broadcasting to Mobile Receivers", IEEE Transactions on Consumer Electronics, vol 35, No. 3, Aug. 1989, pp. 493-503.
G. Plenge "DAB--A new sound broadcasting system Status of the development--Routes to its introduction", EBU Review-Technical, No. 246, Apr. 1991, pp. 87-112.
E.F. Casas et al. "OFDM for Data Communication Over Mobile Radio FM Channels--Part I: Analysis and Experimental Results", IEEE Transactions on Communications, vol. 39, No. 5, May 1991, pp. 783-793.
Digital Audio Broadcasting, "Strategies for the introduction of DAB worldwide Experimental evidence and recent system developments. Concepts for DAB into the 21st century", Proceedings, First International Symposium on Digital Audio Broadcasting, Jun. 1992, pp. 5-17 by D. Pommier, p. 75 by G. Plenge, pp. 83 and 96 by G. Stoll and pp. 99-109 by B. Fluch.
"Advanced digital techniques for UHF satellite broadcasting. Collected papers on concepts for sound broadcasting into the 21st century". European Broadcasting Union. EBU Technical Center of the occasion of demonstrations of experimental UHF satellite sound broadcasting equipment during the ITU WARC-ORB (2) Conference, Geneva, Sep. 1988, pp. 1-99.
Maddocks Mark Charles D.
Shelswell Peter
British Broadcasting Corporation
Oliver Milton
Olms Douglas W.
Phillips Matthew C.
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