Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-02-24
2001-03-27
Vo, Don N. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S354000
Reexamination Certificate
active
06208695
ABSTRACT:
The present invention relates to the reception of multicarrier RF signals and to a corresponding apparatus.
BACKGROUND
Various methods for terrestrial transmission of digital broadcast signals are known, having modulation types like OFDM, QPSK and QAM. A main concern in connection with such systems is to find a system-conform signal in case a receiver is switched on or tuned to another channel. Examples for such broadcast signals are DVB (digital video broadcast), HDTV-T (hierarchical digital television transmission) and DAB (digital audio broadcast) signals.
Considerations on DAB receiver tuning concepts are so far based on the assumption that an acceptable number of fixed DAB center frequencies can be used, if necessary with small offsets of quite below the 16 kHz step size. A reasonable approach is given by the CEPT (conference Européenne des administrations des postes et des télécommunications) T-DAB allotment plan of July 1995. This approach would result in a reasonable time to scan the entire set of DAB frequencies and would require an acceptable size of extra memory for storing the corresponding numbers or to provide an adequate algorithm for the calculation during the tuning process.
In addition to the CEPT plan, broadcasters and national organisations are considering a lot of exceptions or, in general, offsets of up to several 100 kHz. The main reason behind these proposals is to reduce the influence from and to other services in the same or neighbouring frequency sections. Such scenario would increase the amount of fixed channel numbers to be tuned and subsequently the size of the receiver memory and the scanning time for the entire frequency set.
An accompanying proposal made by the industry is—in order to minimise the constraint for the receiver design—that all transmitters are obliged to transmit all other DAB transmitter frequencies of a wide area around that one which provides the information. However, it is questionable whether such method is practicable.
INVENTION
The applicant has carefully considered above ideas. These considerations are not limited to a specific transmission signal like DAB as specified in the ETSI (European telecommunications standards institute) standard ETS 300 401, November 1994, but can be applied to any signal which is conform to the selected main transmission system.
There are some possible ways for the introduction phase of such a transmission system:
A) A fixed number of additional (known) frequencies is added to the list given by CEPT, which increases the total amount of frequencies up to e.g. 50%. However, this has to be agreed on a world-wide or at least European-wide basis to allow simpler receivers to stick only to these numbers.
B1) A reasonable number of additional unknown frequencies is used.
B2) A reasonable number of additional, not generally defined frequencies is used and notified (possibly together with the known frequencies) via the transmitters in a wide area around the one referring to. This condition must be agreed to by all broadcasters on a world-wide or at least European-wide basis.
C) A general (possible) offset of n-times the defined frequency step size of 16 kHz is introduced.
The so-far-known proposals from the broadcasters result for Band III and L-band in 6(or 26)+23+9=38 (or 58, respectively) additional DAB frequencies which leads already in maximum to a 95% increase of frequencies with respect to the number based on the CEPT plan. A special European solution would require an increase of max. 57%.
The main question is whether this is an agreeable basis and whether further exceptions can be avoided. New services may require additional frequency numbers and may not accept the rule of transmitting the frequency information for surrounding transmitters. On the other hand, a solution with several offsets for all frequencies increases the number of tuning positions by e.g. 100% per additional offset position. This would have a great impact on the receiver design and also on the tuning performance.
There are some general requirements from a user's point of view. DAB receivers will be compared with and measured on the performance of well-known FM receivers. Good FM car radios normally run through the whole band (without an input signal) in a few seconds and will not miss any station received with reasonable power. Therefore, a user would not understand and finally not accept a DAB receiver that needs more than 5 or 10 seconds to decode the first station or ensemble.
Also, the situation that a receiver is not able to tune all transmitters or ensembles in the rated band (given by the receiver specification or data sheet) is not acceptable. This would require that all frequencies or channel numbers (within the 16 kHz raster) which can not definitely be excluded must be tuned.
The transmitted signal includes a multiplicity of modulated carriers (known as OFDM modulation which is described e.g. in “Data transmission by frequency division multiplexing using the discrete Fourier transform”, Weinstein, S. B. et al., IEEE Transactions on Communication Technology, Vol. COM-19, No. 15, October 1971 and “An orthogonally multiplexed QAM system using the discrete Fourier transform”, Hirosaki, B., IEEE Transactions on Communication Technology, Vol. COM-29, No. 7, July 1981). E.g. QPSK and/or QAM modulation can be used for these carriers. A certain amount of the total channel capacity may be assigned to synchronisation and channel estimation/correction data. One or more time frequency phase reference symbols may be included into the transmitted signal. These reference symbols can be modulated by a certain number of additional sequences suited for correlation, for instance CAZAC sequences (constant amplitude zero auto-correlation). The features and the decoding of such CAZAC sequences are described in EP-A-0529421. The DAB transmission signal is described e.g. in clauses 14.1 to 14.4 of ETS 300 401.
In a decoder, such reference symbols are evaluated subsequently to demodulation (including FFT), differential decoding, and re-conversion (i.e. arranging in the original signal sequence prior to shifting etc. for modulation) of the sequences by performing a correlation.
The resulting information is taken to correct
the frequency of at least one of the oscillators being used in the receiver for frequency conversion
or in a similar arrangement, e.g. a multiplier
or in case of PLL controlled oscillators for correcting the reference oscillator.
In the DVB system the reference symbols include additional M-sequences which allow to determine and to correct significant deviations of the normal receiver oscillator frequency or to determine and to correct the oscillator frequency if the transmitter frequency deviates from a given raster (offset).
Now, specific receiver problems and possible tuning strategies are considered. An important factor for all cases is the time needed by the receiver to reach a given tuning point with adequate accuracy and to exclude the possibility that there is a DAB-compliant signal. After evaluating this, the same procedure can be applied for the next tuning point, and so on.
If, however, a DAB-compliant signal (or a signal which is conform to any other pre-selected transmission system) has been found—even with an offset—the goal is reached and another strategy for the further processing is to be applied which is described below.
The normal tuning or searching procedure may comprise the following steps:
a) Settling the oscillator control and the PLL to the predefined values.
b) Null symbol detection and testing: delivers, with rather low probability, an information whether the signal is a DAB ensemble or not.
c) Mode detection and setting.
d) Coarse time synchronisation and AFC: requires FFT, includes reference symbol evaluation and allows to derive an information whether or not a DAB signal has been received.
e) Fine time synchronisation.
f) Fast Information Channel evaluation, further settings and compliance tests, based, for instance, on coding/decoding control, and de-interleaving.
Steps a
Klank Otto
Klausberger Wolfgang
Laabs Juergen
Akiyama Kuniyuki
Deutsche Thomson-Brandt GmbH
Tripoli Joseph S.
Vo Don N.
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