Pulse or digital communications – Receivers – Interference or noise reduction
Patent
1995-06-05
1997-09-16
Chin, Stephen
Pulse or digital communications
Receivers
Interference or noise reduction
375267, 455273, H04B 710, H04L 102
Patent
active
056688393
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to radio receivers operating in digital radio links and protected with diversity techniques and more precisely concerns a mixed-combination-strategy combiner for receivers operating a high capacity digital radio links and protected with space or angle diversity.
As known, in a radio link connection the bean transmitted can divide into several beams which reach the receiving antenna over different paths for reasons tied to propagation. This can bring fading of the reception signal caused by the interference of the different beams on the receiving antenna. The fadings are frequently a cause of interruption of radio link connections. It is also known that the fading can be flat or dispersive. In the former case all the frequencies of the spectrum in the band of the received signal are equally attenuated. In the latter case the attenuation strikes predominantly only some zones of the spectrum, producing a distortion of the amplitude-frequency response. In reality the two types of fading can occur simultaneously.
It is useful to note that the dispersive fading is evaluated by measurements of dispersion in the band of the reception signal, where by dispersion it is meant the ratio between the maximum and minimum amplitudes of the frequency spectrum in said band. The frequency for which occurs the greatest attenuation is called notch frequency (f.sub.notch).
In an individual receiver affected by fading, whether flat or dispersive, the error rate on the bits received, corresponding to the BER measured at the output of the demodulator, can exceed a certain threshold, so that the receiver is temporarily out of service. Therefore, in order to avoid this serious shortcoming, diversity reception techniques have been known for a long time. In cases of space or angle diversity, which more strictly concern the type of diversity to which the combiner which is the object of the present invention relates, reception equipment has been equipped with two or more receivers connected to a respective antenna or to their own feeders of said antenna. The outgoing signals from the receivers reach the inputs of a combiner which combines them appropriately to generate a single reception signal to be sent to the demodulator.
With the diversity criterion, the performance of the receiving equipment is considerably improved because the combined signal thus obtained has a much lower probability of going off-service than the individual signals received.
To implement the most appropriate combination strategy, it is essential to know theoretically and/or experimentally the transfer function of a transmitting channel affected by fading. For this purpose there have been proposed different mathematical models among which a well known model is the one proposed by Bell laboratories and known as the `three-beam model`. Of the three beams, one is the main one which reaches the receiving antenna directly, while the other two are echoes of the main beam, i.e. beams which reach the receiving antenna by completing multiple paths of different length. The transfer function proposed for the transmission channel comprises parameters whose values are inferable from knowledge of the fading statistics for the particular section considered. Knowing the type of modulation of the transmitted signal, the fading statistics and the total transfer function of the entire transmission system, it is possible to obtain useful information on the signal characteristics at the output of the demodulator, e.g. power, amplitude dispersion, S/N, BER, etc.
This being stated, it is the job of the combiners to act on some parameters of the diversity receivers in order to optimise a preselected characteristic of the combined signal.
Depending on the optimised characteristic, it is possible, generally speaking, to group the combiners in appropriate operating categories.
A first category includes the combiners which compensate for flat fading by carrying out a phasing of the signals present on the two
REFERENCES:
patent: 4261056 (1981-04-01), Barnett et al.
patent: 4354276 (1982-10-01), Karabinis
patent: 4710975 (1987-12-01), Okamoto et al.
Bernasconi Antonio
Dal Lago Antonella
Sirtori Maria Pia
Chin Stephen
Greenberg Laurence A.
Lerner Herbert L.
Phan Hai H.
Siemens Telecommunicazioni S.p.A.
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