Telecommunications – Transmitter and receiver at separate stations – Having measuring – testing – or monitoring of system or part
Reexamination Certificate
2001-02-16
2004-11-23
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Having measuring, testing, or monitoring of system or part
C455S067150, C455S067160, C455S424000, C455S437000, C455S442000, C370S328000, C370S329000, C370S330000, C370S331000, C370S332000, C370S333000, C375S343000
Reexamination Certificate
active
06823175
ABSTRACT:
TECHNICAL FIELD
The object of the present invention is a process for simultaneously measuring the propagation characteristics of a plurality of radio-frequency channels. It finds an application in radio-communications with mobiles. The radio-frequency channels, whose characteristics are being measured, are here wideband.
STATE OF THE PRIOR ART
In a radio-communications system with mobiles, as for example in the GSM system or in the IS-95 system, a microwave signal is propagated between fixed base stations, connected to the telephone network, and mobile users, to be called hereinafter mobile stations. Each connection between a base station and a mobile station is made by means of a radio-frequency or mobile radio channel.
The microwave signal reaches the mobile station (in a so-called down direction) or the base station (in a so-called up direction) with more or less attenuated and phase offset echoes, a consequence of the obstacles encountered during propagation. The signal undergoes overall an attenuation which fluctuates according to the displacements of the mobile. Two fluctuation scales may be distinguished: rapid fluctuations, caused by interference, and slow fluctuations due to modifications in the environment or in mobile station-base station separation when the mobile is displaced over great distances. These attenuations affect in the first place the signal-to-noise ratio. But multiple paths also generate interference between symbols, which is all the more marked the higher the information rate.
To improve transmission performance, it is conceivable to transmit the same signal from several different sources (transmitter diversity) or to receive it in different places (receiver diversity). When one of the connections becomes too attenuated or is subject to too much interference between symbols, an automatic transfer (or “handover”) procedure is used. Processing algorithms are also conceivable which combine the signals coming from several channels.
A growing number of high bit rate transmission systems provide for the use of space diversity or polarisation diversity. Space micro-diversity (using several close transmit and receive antennae) or polarisation micro-diversity (dual polarisation transmission or reception) make it possible to counter multiple paths and rapid fading. Macro-diversity (connection between one mobile and several base stations) may also be used to this end. It also makes it possible to counter masking effects and to smooth transitions between cells.
It consequently becomes indispensable to have full control over measuring the propagation characteristics of the different channels which may be used in diversity so that new networks can be designed and deployed in the best way.
Measurement cycles in micro-diversity at mobile station level or in polarisation diversity at mobile station level do not raise any particular problems. The channel sensing signal is transmitted from the base station. The antennae are placed on a vehicle containing the reception means, these antennae being a few centimetres apart in space micro-diversity or superimposed in polarisation diversity. They are connected to a same sensing device, which will for example read alternately on one and on the other the signal delivered by the antennae. Rotating the vehicle wheel triggers the signal acquisitions. It is said that the configuration is “master distance” It is easy to recognise the exact position of each measuring point when going through the results.
Micro-diversity at base station level and macro-diversity are much trickier to achieve. These difficulties may be stressed by distinguishing “master time” methods and “master distance” methods:
a) “Master Time” Methods
In space micro-diversity at the base station, several antennae are a few centimetres apart. In polarisation diversity, they are superimposed.
The channel between the mobile station and the base station being reciprocal, the signal may very well be transmitted from the mobile station and be received in diversity at the base station. A receive end technician must always be present to trigger the beginning then the end of the measurements. Acquisitions occur at a regular rate; they are controlled by a receive-end clock. The method is called “master time”.
In this configuration, the vehicle must travel at a constant speed so that the exact location of the measuring points may be reconstituted. This is only possible over small sections and some routes may have to be ruled out because of road traffic.
In macro-diversity, the antennae are spaced apart by several hundred metres. Channel measurements in “master time” therefore require several sensing devices, in the event one at each station. A technician must be present on each site. The problems previously raised in respect of micro-diversity remain. To these may be added difficulties in synchronising measurements: the moments of triggering and then stopping the recordings must be common to the different sites.
b) “Master Distance” Methods
Different methods have been proposed for carrying out measurements in diversity at the base station or in macro-diversity, on two channels, and in a configuration of the “master distance” type.
A first solution, the most straightforward conceptually, consists in transmitting the same sensing signal on the two connections. Superimposition of the impulse responses of the two channels is obtained. These responses are disassociated if they are sufficiently offset in time. For this the transmitter sequences must be synchronised. This technology is described in the article by M. G. KADEL entitled “Measurement of wideband micro-and macro-diversity characteristics of the mobile radio channel” published in IEEE, Proc. of VTC, Stockholm, Sweden, 1994, pages 165 to 169.
Synchronisation of sequences is a priori possible in micro-diversity or in polarisation diversity. It is complex to achieve in macro-diversity since two independent transmitters are being worked with. The sequences can always be randomly offset by reinitialising a sequence of one or other of the transmitters. At all events, good transmitter synchronisation is only verified at the receive end by displaying the main impulse response peaks and by noting their effective separation. It is only on this condition that measurements of a section may begin.
The length of a sequence must be at least equal to twice the spread of delays of the impulse responses measured. In macro-diversity, the peaks of the two impulse responses shift independently of each other during displacement of the mobile. It is therefore necessary for there to be ample room for manoeuvre, especially when synchronisation is obtained randomly by reinitialising a transmitter.
The sequence length used in practice with this method is of the order of 100 &mgr;s. The method is difficult to apply to more than two transmitters, since recognition then synchronisation of sequences may prove difficult to achieve.
Another problem appears in macro-diversity since the oscillators of the two transmitters drift apart independently. They are not subject to the same clock and the impulse responses calculated may in the end overlap. The results become unusable when these drifts are substantial, which may unfortunately occur while it is difficult to verify stability at the time of measurement.
A second solution is described in the article by G. KADEL entitled “Simulation of the DECT System Using Wideband Channel Data Measured in Two Diversity Branches, Proceedings of the 2nd International Conference on Universal Personal Communications (ICUPC), Ottawa, pp. 546 to 550. This solution consists in offsetting the carrier frequency of the second transmitter relative to that of the first. In practice, an offset &Dgr;f of about 20 Hertz may be introduced. The composite signal received is demodulated to the frequency of the first transmitter. After processing, the two superimposed impulse responses may be observed. The slight frequency offset of the second transmitter introduces artificially a Doppler effect. The measurement remains of good quality si
Duponteil Daniel
Zayana Karim
Chin Vivian
France Telecom
Pearne & Gordon LLP
Yun Eugene
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