Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
2000-07-28
2004-11-02
Yao, Kwang Bin (Department: 2667)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
Reexamination Certificate
active
06813263
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of the so-called “intelligent” array antennas, and more in particular to a discrimination procedure of a wanted signal from a plurality of cochannel interferents received by array antennas of base transceiver stations for cellular telecommunication and relative receiver.
1. Background Art
The use in mobile radio environment of antenna consisting of one or more arrays of electromagnetic field sensors (array) is known, for instance, from the European patent application published under No. 0593822A1. This document in fact, describes, mentioning the first claim, “An arrangement of antennas for a base transceiver station including a plurality of antenna arrays, each array able to form a multiplicity of narrow radiation lobes, separated and partially overlapped in the azimuth plane, the arrays being positioned in such a way that all the lobes generated by the same give a substantial omnidirectional coverage on said plane; lobes formation means (beamforming) in azimuth and angular elevation for each said array; a plurality of radiofrequency transceivers each one to transmit and receive the signals relative to one or more channels; a switching matrix to connect each transceiver with one or the other array through said beamforming means; etc.”.
More in particular, a beam former, for example a Butler matrix, is associated to an antenna array. A beam former consists of phase shifting and adder circuits, used in reciprocal way both in reception and in transmission. Considering for instance reception, the beam former has a plurality of input ports for the signals coming from the sensors of the array , and a multiplicity of output ports, each one relative to a pre-set azimuth direction and corresponding to a particular combination of module and phase of input signals. A dual behaviour applies to transmission, where the same port selected during reception is used to transmit towards the mobile . The antenna array, and relative beam former, are therefore an essential part of a system capable to identify the direction of the signal transmitted uplink by a mobile, that the system follows condensing a narrow radiation lobe in which the power of the signal transmitted down-link towards the mobile itself is concentrated. This equals to an intelligent behaviour of the antenna, which deviates from the traditional utilization of antennas in the same sector of the technique. Thanks to the intelligent behaviour, the interference from cochannel channel is reduced and the re-utilization of the same frequencies in adjacent cells is made possible. The deriving advantage is considerable and consists in the possibility to increase the dimension of cells, at equal transmitted power, in low traffic areas or reduce the frequency re-utilization distance, increasing the number of carriers per cell in high traffic areas.
The estimate of wanted signal arrival direction avails of means for the measurement of signals present at output ports of the beam formers and of a processor evaluating the above mentioned measures and selecting the best direction, for instance that one for which the signal level is higher.
The antenna arrangement described in the mentioned application, overcomes the limits introduced by sectorial antennas in corner-excited cells, still widely employed in the mobile radio field, due to need of infrasectorial handovers as the position of the mobile part around common antennas varies, though remaining close to the same. The intelligent antenna system enables in fact a transceiver to be associated to any possible narrow radiation lobes, sunburst arranged on the azimuth plane, therefore the cell appears to the network as if it were equipped of omnidirectional antenna, but without the relative known drawbacks.
2. Background Art
Notwithstanding all the features mentioned above, the intelligent antenna arrangement described in the mentioned application, fails to exploit the capability of an antenna array to shape the overall radiation diagram so that nulls are steered in the interferents' directions. For example, if interfering and wanted signal arrival directions are different but contained in the same beam, no discrimination can be made between them. This stands also if beam is selected, after demodulation, on the basis of wanted signal quality.
In order to exploit the interference nulling capabilities of the antenna array a numerical (and not analog) beamforming has to be performed on digitalized signals from the sensors of the array.
Numerical beamforming is nothing more then a linear combination of base band signals from the array elements using a set of complex coefficients w.
The procedures for numerical beamforming mainly differ in the choice of the coefficient's set w and usually 2 different methods can be used (see [1] for a more detailed description and for a comparison based on system capacity):
1) All cochannel user arrival directions are estimated and coefficients w are to chosen with the constraint that the overall array gain is “nulled” in the interferent arrival directions. See also |2| for an overview of known art about beamforming and “null steering”.
2) Coefficients w are calculated so that the difference between the combined signal and a reference part of the signal is minimized.
Both these methods for coefficients w calculation suffer from the fact that they optimize uplink receiver performance but the coefficients w are not always reliable for downlink transmission.
For example, when uplink noise level is negligible and the interferent and wanted signal arrival directions are close each other, both methods try to reduce interference as much as possible. The result is often a very low level of the array gain in the direction of the wanted signal and thus only a small fraction of transmitted energy will be directed towards the user.
WO 95/22873 discloses a cellular system receiver including a spatial filter which has up to one input for each antenna element and one output for each spatial channel. An adaptor adjust the spatial filtering in order to enhance one of the channels while suppressing the other. A channel estimator calculates estimated impulse response for each selected channel based upon training sequences and the output from the spatial filter.
In particular adaptation means are disclosed suitable to calculate the weights w that implement the spatial filtering of signal coming from a sensor array. The above description however does not contain any detailed explanation about the proposed algorithm but only mention is made to known or general procedures like “direction of arrival estimation”, “minimisation of error signal” and “gradient optimisation method”. Neither the cost function to be minimised nor the method to find that minimum is provided in a detailed text.
EP 809323 discloses a signal processing apparatus e.g. for wireless communication system generating beam pattern which has its maximum gain along direction of wanted signal and maintaining gain toward direction of interfering signals in as low level as possible. In particular a method to calculate said weights w is disclosed and according to this method the weights are chosen so to maximizse received wanted signal power. Interference is reduced as a consequence of directivity in the resulting array diagram, but is not minimised placing nulls in the interfering signal arriving directions.
SUMMARY OF THE INVENTION
The scope of the present invention is to overcome the above mentioned drawbacks and to indicate a discrimination procedure of a wanted signal from a plurality of cochannel interferents received by array antennas of base transceiver stations for cellular telecommunication.
To attain these objects, scope of the present invention is a discrimination procedure of a wanted signal from a plurality of cochannel interferents received by array antennas of frequency division (FDMA), or time division (TDMA), or mixed FDMA/TDMA multiple access telecommunication systems, re-employing a same frequency group in adjacent
Birch & Stewart Kolasch & Birch, LLP
Siemens Mobile Communications S.p.A.
Yao Kwang Bin
LandOfFree
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