Multiplex communications – Generalized orthogonal or special mathematical techniques
Patent
1996-08-22
1998-03-10
Kizou, Hassan
Multiplex communications
Generalized orthogonal or special mathematical techniques
370503, 375344, 455202, H04J 1100
Patent
active
057269734
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an arrangement for detecting and synchronizing radio signals modulated according to OFDM (orthogonal frequency division multiplexing). The invention complements and simplifies a known method for frequency control of clocks for mixing down from intermediate frequency (IF) to baseband frequency and of clocks for the baseband signal. The known method does not include control of phase errors in the down conversion from IF and timing errors in sampling.
2. Discussion of the Background
OFDM is a digital modulation method for primarily radio signals when nearly all signal processing at the baseband is done with digital components such as signal processors and specific circuit solutions. The transmitted signal, however, is analog which is why a number of analog components are needed for processing the radio signal before conversion occurs to or from digital baseband representation. These analog components occur primarily in the upwards and downwards mixing of the signal to RF and IF, that is to say channel filtering and, respectively, sampling. A large proportion of the synchronization problems in the system originates from deviations or other departures in these components.
Previously known methods for controlling the synchronization are based on the utilization of two known symbols, subcarriers, in the OFDM frame. From the phase variation of these subcarriers the error signals for a control system for controlling the oscillators for further down conversion from IF and for sampling of the baseband signal are calculated. On demodulation of the OFDM frame in the receiver equipment, the frequency deviations of the sampling and IF clock have given rise to what is assumed to be a moderate phase shift of all subcarriers. In the method, the phase difference between two frames is measured and the phase difference used for estimating the total deviation of the signal. Since the method is based on differential phase measurement between data frames, this implies that when the frequency error has been controlled, the error lags in phase and time since these deviations have not been determined in absolute terms. The technical problem of the invention is therefore to produce an arrangement which involves detection of the absolute phase shift, that is to say the factual phase deviation in relation to the known phase.
Within this field, a number of documents is also known.
WO 92/05646 shows a receiver for receiving OFDM signals. The document discusses control of local oscillators and sampling clock.
U.S. Pat. No. 5,228,025 describes a method for sending digital data to mobile users with the aid of COFDM. According to the document, "service words" are used for controlling the local oscillator.
U.S. Pat. No. 5,206,886 relates to a method and a device for correcting, with the aid of a pilot tone, phase and frequency errors and for correcting clock errors in a multi-carrier wave modem.
U.S. Pat. No. 5,228,062 describes a method and a device for correcting phase and frequency errors and for correcting clock errors in a multi-carrier wave modem.
SUMMARY OF THE INVENTION
In brief, the solution comprises a method which is based on detecting absolute phase errors for both synchronization symbols and where the phase is controlled with the IF clock and the timing error is controlled with the sampling clock. This involves the IF signal being mixed down to the baseband. This is controlled by a numeric oscillator. On the basis of the difference between frequencies in a transmitter and receiver and instability in crystals, the down conversion frequencies are continuously controlled by digitally varying the frequency between the oscillators. After down conversion, two quadrature components are obtained which are then sampled into a digital representation via A/D converters. The sampling unit is controlled by an oscillator for being able to sample the signal at the correct time and, respectively, the correct frequency. By means of this control
REFERENCES:
patent: 5602835 (1997-02-01), Seki et al.
patent: 5608764 (1997-03-01), Sugita et al.
Brimoussa A.
Kizou Hassan
Telia AB
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