Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-07-02
2001-12-11
Tse, Young T. (Department: 2634)
Pulse or digital communications
Receivers
Automatic frequency control
C375S343000, C375S366000, C375S368000, C370S509000
Reexamination Certificate
active
06330293
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and a receiver for receiving multicarrier digital signals.
BACKGROUND OF THE INVENTION
Modulation types such as OFDM, QPSK and QAM can be used for terrestrial transmission of digital television and broadcast radio signals (referred to in general as broadcast radio signals in the following text). Examples of such broadcast radio signals include DVB (digital video broadcast), HDTV-T (hierarchical digital television transmission) and DAB (digital audio broadcast). The fundamental principles of the DVB system are specified in ETS 300 744.
The data in digital broadcast radio signals are arranged in two-dimensional (time and frequency, called “temporal-spec-tral” in the following text) frames which have a time duration of TF and, in the case of ETS 300 744, comprise 68 OFDM symbols. Four frames form a superframe. Various transmission modes may be used in the transmission systems for digital audio or video signals mentioned above. In the case of ETS 300 744, symbols of duration Ts are in each case formed from 1705 carriers (2K mode) or from in each case 6817 carriers (8K mode) at different frequencies.
The 2K mode is particularly suitable for individual transmitters and small SFN networks (single frequency networks) with limited distances between transmitters.
The 8K mode can be used for individual transmitters and for small and large SFN networks.
The symbols have a time duration of Ts with a wanted part of duration Tu and a guard interval of duration &Dgr;. The guard interval is formed by cyclic continuation of the wanted part, and is arranged before the latter, in time. All symbols contain data and reference information. Each symbol may be regarded as a group of cells, one cell corresponding to each carrier.
Apart from the actual video, audio or other data, the frames contain scattered pilot cells (scattered pilots) continuous pilot signals and TPS carriers or pilots (transmission parameter signaling). These are described, for example, in Sections 4.4 to 4.6 of ETS 300 744, March 1997.
The pilot cells or carriers contain reference information whose transmitted value is known by the receiver. The continuous pilot signals may coincide with a scattered pilot cell, for example in every fourth symbol. The value or contents of the scattered and continuous pilot signals are derived, for example, from a pseudo-random binary sequence W
k
for each of the transmitted carriers k. The sequence W
k
may also define the start phase of the TPS carrier information. The pilot cells or carriers may be used at the receiver end for frame synchronization, frequency synchronization, time synchronization, channel estimation and transmission mode identification. The receiver manufacturer is free to chose whether and how these options are used at the receiver end.
EP-A-0 786 889 describes a corresponding system for use with DAB.
SUMMARY OF THE INVENTION
An important consideration in the context of such systems is to find a system-conformal signal in the situation where a receiver is switched on or is tuned into another channel. To do this, the receiver has to be able to distinguish between different services, for example to distinguish digital signals from analogue signals or digital DVB signals from digital DAB signals. Both digital signals and analogue signals (for example PAL television signals) may occur in certain frequency bands, in which case the center frequencies may differ from the specified channel mid-frequencies.
The invention is based on an improved method for tuning when receiving multicarrier digital signals, or for checking the system conformity of such received digital signals.
At the receiver end, coarse time synchronization linked to mode detection and, possibly and additionally, coarse AFC (automatic frequency correction) are carried out initially both for searching for and identifying received signals, as well as for continuously monitoring them.
In coarse time synchronization, the time signal is correlated with the time signal shifted by a wanted symbol length Tu. This correlation may be carried out more than once, for example five times per data frame. In this correlation, signal samples of different length Tu are used, depending on the respective mode, and the correlation result maxima obtained from this are then used to deduce the present mode (for example 2K or 8K modes). If no usable correlation result maximum is obtained, the correlation steps may be repeated.
The guard interval used is determined, and a sampling window is then positioned, based on the interval between the maxima and/or their amplitudes, taking account of the mode. This can be done by once-off setting of a counter which is synchronized to the symbol sequence (Tu+&Dgr;) and outputs a time window of duration Tu. In the following text, this time window is also called the sampling window or FFT window. A basic oscillator used in this case, and thus the position of the window as well, are corrected in subsequent steps via fine time synchronization.
Once the mode has been identified correctly and the sampling window has been positioned approximately correctly, an FFT can be carried out, with a length corresponding to the mode. Instead of an FFT, the invention, in an entirely general form, allows the use of a Fourier transformation or any other transformation which allows frequency-spectral representation of the time domain in the frequency domain. Once the signal has been converted in this way, pilot cells are taken from it in accordance with the intended arrangement layout, and are correlated with the values provided in accordance with the specification. According to the specification, 45 spectrum positions in the case of the 2K mode and 177 spectrum positions in the case of the 8K mode, for example, are occupied by continuous pilot signals. For example, ±16 such sets (over ±16 carrier intervals) are used for correlation in the 2K mode, and ±64 such sets (over ±64 carrier intervals) in the 8K mode. The correlation steps carried out provide a correlation maximum and, possibly, a number of secondary maxima of lower amplitude in the immediate vicinity. The frequency offset of the baseband signal can be determined from the position of the maximum. This result is used for coarse correction of the frequency, for example by means of a multiplier M arranged upstream of the FFT section, so that the frequency error for further steps is less than ±1/2 carrier interval.
However, it is a precondition that the position of the maximum was known in advance with sufficient reliability and an accuracy of better than ±1/2 carrier interval. The following calculation can be carried out in order to estimate the position 1
real,s
of the maximum more accurately:
1
real,s
=
1
max,s
+W
1max,s,1
/(
W
1max,s,
+W
1max,s,1
)*sgn(
1
max,s,1
−1
max,s
),
where “sgn” is the mathematical sign of the position difference, the greatest maximum has the value W
1max,s
and is located at the position 1
max,s
, and the next smaller maximum value (of the same polarity) is designated W
1max,s,1
and is located at the position 1
max,s
+1 or 1
max,s
−1, designated 1
max,s,1
.
These calculations can be simplified by using the two values—the maximum and the next smaller maximum—in the sequence of the 1 values. The possible positions are then designated 1
1,s
(the first position) and 1
2,s
, in which case the maximum may occur either at 1
1,s
or at 1
2,s
. The mathematical sign function then disappears:
1
real,s
=1
1,s
+W
12,s
/(
W
11,s
+W
12,s
).
A plurality of such results (obtained successively in time), preferably three, may be combined, filtered or processed together in order to improve the AFC. The next frequency evaluation may be carried out at a greater interval, for example a total of 3 to 6 evaluations may be carried out per frame for the purpose of synchronization monitoring, in order to keep the computation complexity within reasonable orders of magnitude.
The intermediate value or more accurate
Klank Otto
Klausberger Wolfgang
Laabs Jurgen
Deutschethomson-Brandt GmbH
Hermann E. P.
Tripoli J. S.
Tse Young T.
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