Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-03-02
2001-02-20
Hunter, Daniel S. (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S067150, C455S067700
Reexamination Certificate
active
06192238
ABSTRACT:
The present invention relates to a receiving method for a receiver of a digital cellular radio system, the receiver employing Viterbi detection and diversity reception, the method involving forming an estimated impulse response of a channel, and the estimated impulse response of the channel as well as the predetermined sequence comprised in the signal being symbol sequences.
The invention further relates to a receiver of a digital radio system, the receiver comprising a Viterbi detection means, diversity branches and a means for forming an estimated impulse response of the channel.
In a cellular radio system, the quality of the connection between a base station and a subscriber terminal equipment constantly varies. The variation results from disturbance present on the radio path and the attenuation of radio waves as a function of distance and time in the channel fading out. The connection quality can be measured for example by monitoring the strength level received. In order to partly compensate for the quality variations of the connection, power control can be used.
Digital cellular radio systems require a more accurate method for estimating the connection quality. Known quality parameters employed include, for example, bit error rate BER and signal to noise ratio SNR.
It is known to utilize the decisions of the Viterbi detection in estimating the SNR of the signal received. As a receiver, either a base station or a subscriber terminal equipment may be used. In prior art solutions, the Viterbi detection is completely performed for the burst received prior to determining the SNR. Due to the Viterbi algorithm often being too demanding a task for a digital signal processing program within processing time allowed by the receiver, separate Viterbi hardware has to be used. This is described in greater detail in J. Hagenauer, P. Hoeher: A Viterbi Algorithm with Soft-decision Outputs and its Applications, IEEE GLOBECOM 1989, Dallas, Tex., November 1989, which is incorporated herein as reference. Computing SNR on the basis of the differences between the signal received and the estimated signal calculated on the basis of the channel estimate are also dealt with in the European Patent 428199, which is also incorporated herein as reference.
It is well known that the SNR information, which is simple to compute by the method according to the invention, is required when using various kinds of diversity receivers. In diversity reception, the most typical diversity receivers combine the signals prior to or following the detection, and they comprise for example Selective combining, Maximal-ratio combining, and Equal-gain combining. The diversity signals are normally detected by using the Viterbi detection, whereby the signal combining takes place after the detection. However, the most advantageous way is to combine the signals prior to the detection because this results in a higher signal gain. A closer description on diversity receivers is given for example in William C. Y. Lee: Mobile Communications Engineering, Chapter 10, Combining Technology, pp. 291-336, McGraw-Hill, USA, 1982, which is incorporated herein as reference.
The present invention aims at realizing a method by means of which the SNR can be estimated directly from the signal received without employing the Viterbi detection, thus making it possible to combine the signals when using diversity receivers.
This object is achieved by a method set forth in the introduction, characterized by the steps of forming a reference signal from the estimated impulse response of the channel and the predetermined sequence within the signal, computing as a variance type computation the energy of noise associated with the signal from the reference signal and the predetermined sequence received from the channel, computing the SNR as a ratio between the reference signal energy and the noise energy, and combining the diversity branch symbols which temporally correspond with one another, and weighting the outputs of the adapted filters in each branch and the autocorrelation taps in the impulse response by the SNR of each branch.
The receiver according to the invention is characterized in that, in order to compute the SNR, the receiver comprises a means used for forming the reference signal from the estimated impulse response of the channel and the predetermined sequence within the signal, a means for computing the channel noise energy from the reference signal and the received predetermined sequence as a variance type computation, a means for computing the SNR as a ratio between the reference signal energy and the energy of the noise, and diversity branches combining means which combine the temporally corresponding symbols of the different branches, and a means for weighting the outputs of the adapted filters in each branch and the autocorrelation taps in the impulse response by the SNR of each branch.
The invention provides several advantages. By means of the method according to the invention the SNR can be estimated directly from the signal received without carrying out the Viterbi detection, or by implementing the method of the invention prior to the Viterbi detection by utilizing the predetermined sequence within the burst. The invention has two main stages: the first stage is the forming of the reference signal and the second stage is the computing of the variance, i.e. the noise energy, between the signal received and the reference signal. The reference signal will be obtained upon computing the convolution of the channel estimated impulse response, obtained by means of the predetermined sequence, and the predetermined sequence. The estimated noise energy is computed as a variance, or a function resembling variance, from the reference signal and the sequence received. The signal energy is obtained either from the energy of the taps in the channel estimated impulse response or by computing the energy of the samples, i.e. symbols, of the reference signal. By dividing a signal thus formed by noise, an estimated momentary SNR of the channel will be obtained.
By avoiding the use of the Viterbi algorithm, memory capacity and time consumed in the computing process will be saved. The value of the SNR thus obtained can be utilized in estimating the conditions on the channel, as an aid in bad frame estimation methods and for scaling in ML (maximum likelihood) metrics. In addition, the SNR can be utilized in diversity combining, and it is particularly practicable is cases the diversity signals are combined prior to the detection.
The preferred embodiments of the method according to the invention are also set forth in the attached dependent claims
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, and the preferred embodiments of the receiver according to the invention are set forth in the attached dependent claims
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REFERENCES:
patent: 5199047 (1993-03-01), Koch
patent: 5479446 (1995-12-01), Mourot
patent: 5533067 (1996-07-01), Jamal et al.
patent: 5563746 (1996-10-01), Bliss
patent: 5606580 (1997-02-01), Mourot et al.
patent: 5727032 (1998-03-01), Jamal et al.
patent: 5796535 (1998-08-01), Turtle et al.
patent: 701 334 (1996-03-01), None
IEEE Global Telecommunications Conference & Exhibition, Dallas, Texas, Nov. 27-30, 1989 pp. 1680-1686, J.Hagenauer et al.
William C.Y. Lee: Mobile Communications Engineering:, Chapter 10, Combining Technology, pp. 291-336, McGrae-Hill, USA, 1982.
Hunter Daniel S.
Nguyen Thuan T.
Nokia Telecommunications Oy
Pillsbury Madison & Sutro LLP
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