Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1999-09-27
2003-02-11
Vo, Nguyen T. (Department: 2682)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C375S148000, C370S328000
Reexamination Certificate
active
06519477
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for channel estimation from received signals transmitted via a radio channel. The method can advantageously be used in mobile radio systems.
In a mobile radio system, information is transmitted from a transmitting radio station to a receiving radio station. This information arrives at the receiving radio station in the form of received signals. Due to various external influences, the received signals arrive at the receiving radio station via a number of propagation paths. The signal components which correspond to the various propagation paths arrive at the receiving radio station in the form of wave elements or partial waves at successive times. In the receiving radio station, there is now the problem of equalizing these signal components, which may further be influenced by other interference components, the problem of correcting the errors, and of decoding the transmitted information.
Parameters which take account of the channel conditions are determined within the receiving device, in order to evaluate the received signals. These parameters are, for example, channel impulse responses which are known, for example, from W. Koch, “Optimum and Sub-Optimum Detection of Coded Data Disturbed by Time-Varying Intersymbol Interference”, IEEE Proceedings 1990, pages 1679-84, and are expressed by channel coefficients. These channel coefficients, which are used in a channel model, are used to superimpose different, successively arriving signal components of a received signal on one another in a suitable manner.
It is furthermore known to transfer the received signals to the baseband and to A/D convert them for obtaining digitized received signals. The digitized received signals as well as the channel impulse responses are supplied to a detector, which equalizes the received signals and carries out an error correction. The signal symbols reconstructed in the output of the detector are then decoded in a decoder, for example in a Viterbi decoder.
It is also known to use so-called training sequences for adjusting or equalizing receiving radio stations in mobile radio systems, see Mouly and Pautet, “The GSM System for Mobile Communications”, 49. rue Louise Bruneau, F-91120 Palaiseau, France, 1992, pages 231-237. At predetermined times, the transmitting radio station transmits a sequence of digital signals which is known to the receiving radio station, that is to say whose data are available in undistorted form in the receiving radio station.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for channel estimation which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which allows channel impulse responses to be determined in an improved manner, such that the determination of the channel impulse responses is resistant to interference sources.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for estimating a channel from received signals transmitted via the channel, the method comprises the steps of receiving, with receiving sensors of a receiving device, a plurality of received signals composed of at least one subscriber signal marked with transmitter-specific fine structures, the at least one subscriber signal being transmitted by a plurality of wave elements having differing incidence directions at the receiving sensors; obtaining, in the receiving device, an incidence direction of at least one of the wave elements; and determining directionally selective channel impulse responses from the plurality of received signals and the incidence direction of the at least one of the wave elements.
In the case of the method according to the invention for channel estimation from received signals transmitted via a radio channel, a receiving device is assigned a number Ka of receiving sensors, via which Ka received signals, which are assigned to the receiving sensors, are received. The received signals are composed of at least one subscriber signal which has a transmitter-specific fine structure impressed on it, in which case a k-th subscriber signal, k=1 . . . K, is transmitted through the use of Kd wave elements or partial waves whose incidence direction at the receiving point differs. In a first method step, at least one direction of incidence of a wave element of a subscriber signal is evaluated so that, in a second method step, directionally selective channel impulse responses are determined from the Ka received signals and the at least one direction of incidence.
In many applications, for example in radar, sonar or seismic measurement systems having only one transmitter or reflector, or having only a small number of transmitters or reflectors—see e.g. U.S. Pat. No. 5,299,148 in this context—the number Ka of receiving sensors is greater than the number Kd of wave elements to be evaluated per subscriber, so that, using the method according to the invention, Ka-Kd fewer channel impulse responses need be determined. This also results in a reduction in the complexity of the channel estimation.
In addition, the influence of interference sources is considerably reduced by concentrating of the incidence directions of the wave elements. The channel estimation becomes more accurate.
In accordance with a preferred embodiment of the invention, the directionally selective channel impulse responses are determined from training sequences of the subscriber signals, wherein the training sequences form the transmitter-specific fine structures. If the subscriber signals contain training sequences, which are known at the receiver end, more accurate channel estimation is possible than by using data which still have to be detected in the receiving device. Furthermore, the method can thus easily be implemented in existing mobile radio systems.
A further advantageous embodiment of the invention provides that subscriber signals from a plurality of transmitters or reflectors are superimposed and arrive as the received signals in the receiving direction, wherein these signals are transmitted at the same time in one frequency channel. In this case, the subscriber signals can be separated using a CDMA method (Code Division Multiple Access). Direction-related and subscriber-signal-related channel estimation thus allows the accuracy of the determined channel impulse responses to be improved in CDMA mobile radio systems as well.
In the simplest case, it is possible to use direction information from an “a-priori-knowledge” which is obtained, for example, from geometric/geographic considerations of the locations of the mobile stations and/or base stations, and/or of interference sources as well. No further calculation effort is thus required for direction estimation.
If the receiving device does not have a sufficient a-priori-knowledge, then it is advantageous to determine the incidence directions of the wave elements and/or of interference signals from the received signals. Continuously updated values for the incidence directions are thus available. This is of interest particularly for applications in mobile radio systems.
High-resolution direction-estimation methods are used to determine the incidence directions of the wave elements. Such high-resolution direction-estimation methods, such as the MUSIC (Multiple Signal Classification) or ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) methods, use knowledge of the complex radiation characteristics of the receiving sensors and/or specific geometric preconditions for the configuration of the receiving sensors in order to carry out an accurate direction estimation, which involves little signal processing complexity.
The resistance to interference sources is further improved by considering additional information relating to at least one incidence direction and/or a correlation matrix of interference signals in order to determine the incidence directions of the wave elements.
According to an advantageous embodiment of
Baier Paul Walter
Blanz Josef
Haardt Martin
Papathanassiou Apostolos
Greenberg Laurence A.
Mayback Gregory L.
Ramos-Feliciano Eliseo
Siemens Aktiengesellschaft
Stemer Werner H.
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