Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-11-20
2001-12-04
Vo, Don N. (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C370S347000
Reexamination Certificate
active
06327315
ABSTRACT:
This application is the national phase of International application PCT/FI97/00295 filed May 20, 1997 which designated the U.S.
FIELD OF THE INVENTION
The present invention relates to a method for estimating an impulse response in a digital radio system, where the signals to be transmitted comprise bursts formed of symbols, which bursts comprise a known training sequence, in which method the received signal is sampled, and a first estimate is calculated for the impulse response by means of the known training sequence.
BACKGROUND OF THE INVENTION
In a typical cellular radio environment, the signals between a base station and a subscriber terminal equipment propagate on several routes between a transmitter and a receiver. This multi-path propagation is mainly caused by signal reflections from surrounding surfaces. Signals travelling on different routes arrive at the receiver at different times because of a different propagation delay. This holds true for both directions of transmission. The multi-path propagation of a signal can be monitored in a receiver by measuring an impulse response of the received signal, signals arriving at different times being visible as peaks proportional to their signal strength.
FIG. 1
illustrates a measured impulse response by way of example. Time is on a horizontal axis
100
and the strength of the received signals is on a vertical axis
102
. Peaks
104
,
106
,
108
of the curve indicate the strongest multi-path propagated components of the received signal.
In prior art solutions the impulse response is estimated by means of a known training sequence added to the burst.
FIG. 2
illustrates by way of example a normal burst of the GSM system comprising start and end bits
200
,
202
, actual data in two parts
204
,
206
and a known training
208
sequence placed in the middle of the burst. In a normal burst, the length of the training sequence is 26 bits. In known solutions, such as in the GSM system, the impulse response is estimated by cross-correlating the received samples with a known training sequence. 16 bits of a 26-bit-long training sequence are used for estimating each impulse response tap. When the quality of the received signal is poor, it is difficult to attain a reliable estimation result with known methods. This will impair the performance of the receiver.
CHARACTERISTICS OF THE INVENTION
The object of the present invention is to accomplish a more accurate method than the present methods for estimating an impulse response.
This will be attained with a method as shown in the preamble, which is characterized by making preliminary decisions by means of the first impulse response estimate, calculating an error value of the estimated samples and the received samples calculated by means of the preliminary decisions, calculating a second estimate of the impulse response by minimizing said error value, calculating a new estimate for the impulse response by combining the first and second estimates to one another, and by using the estimate obtained in this way when estimating the received sequence again. The present invention also relates to a receiver in a digital radio system where the signals to be sent comprise bursts formed of signals, which bursts comprise a known training sequence, which receiver comprises means for sampling the received signal, and means for calculating a first estimate for the impulse response by means of the known training sequence. The receiver of the invention is characterized in that the receiver comprises means for making preliminary decisions of the received samples by means of the first impulse response estimate, means for calculating an error value of the estimated samples and the received samples calculated by means of the preliminary decisions, means for calculating a second estimate of the impulse response by minimizing said error value, means for calculating a new estimate for the impulse response by combining the first and second estimates to one another, and means for estimating the received sequence again by using the estimate obtained in this way.
The method and the receiver of the invention have several advantages in comparison with prior art solutions. The accuracy of the impulse response estimate is greater in the solution of the invention than in prior art methods. Furthermore, if the speed of the terminal equipment is high, the training sequence in the middle of the burst cannot alone give a correct picture of the impulse response as the impulse response may change even during a burst. With the solution of the invention, it is possible to calculate an impulse response separately for the start portion of the burst and for the end portion of the burst.
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Patent Abstract of Japan 07095107, M. Namekata.
Nguyen Dung X.
Nokia Telecommunications Oy
Pillsbury & Winthrop LLP
Vo Don N.
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