Multiplex communications – Channel assignment techniques
Reexamination Certificate
1998-02-20
2001-05-29
Chin, Wellington (Department: 2664)
Multiplex communications
Channel assignment techniques
C370S329000, C375S346000, C455S562100, C342S378000
Reexamination Certificate
active
06240098
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a device for space division multiplexing/demultiplexing of radio signals which are organized in frames and are transmitted in duplex on the same frequency and time channel, requiring no setting. It applies, in particular, to cellular radio communications between at least one base station and a plurality of mobile units.
2. Discussion of the Background
With the constant increase in demand for mobile communications and the limitation on the number of channels allocated to the operators of cellular radio communications networks, saturation problems will become critical in future years.
Conventional techniques currently make it possible to multiplex communications in frequency division (FDMA), in time division (TDMA) or in code division (CDMA).
FDMA (Frequency Division Multiple Access) consists in allocating a pair of frequencies to each communication (one for the down path and one for the up path). On its own, this very simple multiplexing technique affords substantially no opportunity for improving spectral efficiency.
TDMA (Time Division Multiple Access) consists in allocating a precise time interval to each communication. On its own, this technique which has already been optimized to improve the spectral efficiency within the scope of wireless transmission by utilizing the gaps present in the speech signal, gives little likelihood of further improvement in this regard.
CDMA (Code Division Multiple Access) consists in allocating each communication a code defining a frequency hop law over short time intervals. Although it does allow some improvement in the spectral efficiency (progressive saturation by signal degradation rather than abrupt saturation by service interruption, as in the case of the previous two techniques), this method remains involved and expensive to implement.
A number of ways of improving the capacities of cellular communications systems are currently being investigated, namely: diversification of the cells and the waveforms, with the use of smaller and smaller cells (“microcell” and “nanocell” of the ATDMA calibration project, referring to “Advanced Time Division Multiple Access”, for urban areas), and in particular a new multiplexing method which can be combined with existing techniques: Space Division Multiple Access (SDMA).
This technique is based on the idea that the radio signals corresponding to a plurality of links between a base station and mobile units may, in many cases, be spatially decorrelated.
Utilizing this decorrelation makes it possible to separate the spatial channels (even if they all occupy the same frequency and time channel) using a multipath transmission/reception system associated with an antenna base tailored to the frequency range which is used.
A number of methods using this concept have already been proposed. A first method is based on techniques for estimating direction of arrival, such as the system described in the patent by Richard Roy (US patent PCT/US92/10074, December 1991, entitled “Spatial Division Multiple Access Wireless Communication System”). A second method uses cyclostationarity characteristics of digital transmission signals to separate them by blind processing operations (U.S. Pat. No. 5,260,968, of Sep. 11, 1993, entitled “Method and Apparatus for Multiplexing Communications Signals through Blind Adaptive Spatial Filtering” by William A. Gardner and Stephan V. Schell).
For the first method, estimation of the directions of arrival of various radio sources on the basis of the signals received on an antenna base requires perfect control over the wavefront, which leads to the constraint of calibrating the antenna base which is used. Further to the extra cost introduced by setting, the standard deviation of the indirect estimate of the direction vector on the basis of the estimated direction of arrival and the setting table of the antenna base is much greater than that obtained by direct estimation methods. The direction vector corresponds to the value taken by the transfer function of the antenna base for the azimuth, elevation and carrier frequency of the incident electromagnetic wave transmitted by a source. This information is then used by the space division multiplex processing.
For the second method, 2nd order separation techniques using cyclostationarity require either differences in symbol rates or shifts in carrier frequencies of the signals to be separated in order to operate. These limitations make these techniques ill-suited to the channel configurations formed in radio communications between a base station and mobile units having decorrelated multiple routes which cannot be separated by the above methods and are therefore inoperable in most urban areas.
Lastly, to separate the various signals received, the antenna processing techniques mentioned above employ a so-called purely spatial filtering structure constituted by one complex gain per reception path. The anti-scrambling capacities of this type of structure, in terms of the number of independent interference sources which can be rejected (two decorrelated routes originating from the same source constitute two independent interference sources) are suboptimal. This is because the spatial filter adapts to one useful route followed by a source and rejects all the other routes, both the decorrelated useful routes followed by this source and the routes followed by the interference signals.
In the rest of the description, the terms “sensors” and “antennas” will be used interchangeably, as will the terms “multisensor”, “multipath” and “multisensor array” and “antenna base”, it being moreover known that an antenna base, or multisensor array, forms a number of transmission paths equal to the number of antennas or sensors.
SUMMARY OF THE INVENTION
The object of the invention is to overcome the deficiencies and limitations of the aforementioned methods.
To this end, the invention relates to a method for space division multiplexing/demultiplexing of radio signals which are organized in frames and are transmitted in duplex between at least one base station and a plurality of mobile units communicating on the same frequency and time channel by means of a multipath receiver and transmitter which are integrated in the base station and are coupled to an antenna base.
A first variant of the invention applies in particular to non-gaussian radio signals. The method relating to this first variant is characterized in that it consists:
in estimating the spatial information relating to each mobile unit on the basis of the signal received by the multipath receiver, for the reception and transmission frequencies, by means of blind source separation methods, and on the basis of this information,
in isolating by spatial filtering, possibly in the presence of multiple routes in the channel, the respective routes to each mobile unit whose power is greater than a determined threshold to allow space division demultiplexing, and
in simultaneously transmitting in the direction of the main route of each mobile unit, the signal which is intended for it, while protecting each mobile unit from the signals transmitted to the others by spatial filtering with cancelling constraints to allow the space division multiplexing.
A second variant of the invention applies to digital radio signals organized in frames, including sequences which are known a priori. The method relating to this second variant is characterized in that it consists:
in estimating the spatial information relating to each mobile unit, on the basis of the signal received by the multipath receiver, for the reception and transmission frequencies, by means of filtering methods, while utilizing the a priori known sequences, and on the basis of this information,
in isolating, possibly in the presence of multiple routes in the channel, the respective routes to each mobile unit whose power is greater than a determined threshold, and in demodulating the signal received by each of the mobile units linked with the base station, to permit s
Chevalier Pascal
Monot Jean-Jacques
Multedo Gilbert
Pipon Francois
Thibault Joël
"Thomson-CSF"
Chin Wellington
Nguyen Steven
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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