Multi-antenna system feed device and wireless link terminal equipped with such a device

A multi-antenna system feed device and a terminal including such a device is suggested.The device includes at least:    A branch feeding an antenna, for instance, will be common to two consecutive combiners of the system. The suggested multi-antenna system feed device applies in particular to the extension of multi-antenna or sector antenna systems, used especially in devices with Multiple Inputs/Outputs of the MiMo type and more specifically to mesh network architectures.

This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/EP2008/059616, filed Jul. 22, 2008, which was published in accordance with PCT Article 21(2) on Jan. 29, 2009 in English and which claims the benefit of French patent application No. 07 05376, filed Jul. 24, 2007.

This invention relates to a multi-antenna system feed device and a terminal including such a device. It applies more particularly to the extension of multi-antenna or sector antenna systems, used in particular with multiple input/output devices referred to as being of the MiMo type, an acronym for “Multiple Input—Multiple Output” to standards 802.11 or 802.16. These concepts improve in a noteworthy manner the efficiency of transmission systems by maximizing the capacity of the transmission channels. The invention also applies to mesh networks in which the use of multi-antenna systems permits data to be routed towards the various nodes of the network by the beam forming technique.

Ad hoc mobile networks are defined by a group of mobile nodes connected together through a wireless medium. These nodes can be organized freely in a dynamic manner on their own to create a random and temporary topography of networks referred to as ad hoc, thus allowing people and terminals to interconnect in areas where there is no predefined communications infrastructure.

A new type of network derived from this concept is coming into being. It concerns mesh networks based on a combination of fixed nodes and mobile nodes, interconnected by wireless links.

Many studies are being carried out to improve the capacity of these mesh networks by alternatives using in particular multiple RF (radio-frequency) systems, MiMo techniques or beam shaping antennas. The multiple RF system technique is more especially a way of increasing the network capacity using attenuation, also called fading, independent at various frequencies, with frequency orthogonality. Similarly, multiple antenna systems of the MiMo type, both for transmission and reception, improve the capacity and integrity of wireless links by using antenna diversity and space multiplexing . . . .

Diversity which offers the receiver several responses independent of the transmitted signal is a powerful technique for dealing with interference and fading. Nevertheless, when the interference is at a high level and derives from multiple access, as is the case with a mesh network, the diversity of the antennas alone is not enough to improve the signal.

To deal with interference, smart antennas or adaptative network antennas, to improve radiation efficiency and the possibility of filtering out the sources of interference. To do this, we use antenna beam forming thus generating an effective high gain radiation pattern in the direction of the signal received or transmitted and at low gain in the other directions. Directional transmission control may suffice to ensure high rate transmission with a high level of spatial reuse.

This technique for a mesh network however requires being able to direct the transmitted signal to one or several of the selected antennas while preserving performance in terms of insulation between antennas. This latter constraint is closely linked to the radiation pattern control in a given direction.

The problem that arises does not come from selecting one antenna out of the N antennas, encountered in wireless link systems and generally managed by a more or less comprehensive RF switching device but in particular in the supply and selection of a multi-antenna system, more generally of the multi-sector type, allowing simultaneous signal transmission towards one channel or even N antennas or sectors.

One purpose of this invention is to resolve the problem of isolation between antennas. Accordingly, the purpose of the invention is a multi-antenna system feed system:a set of Wilkinson combiners, one combiner branch feeding an antenna with the branches connected to inputs to a feed point;a set of switches connected between the antennas and the combiners with each switch switching a combiner branch to its corresponding antenna with the antenna being connected to the line when the switch is closed;

Each combiner consists of two cascade-connected basic Wilkinson combiners, a base combiner comprising a terminal resistance between the quarter-wave lines, an additional line whose length is a multiple of the wavelength being connected between each terminal resistor and each quarter-wave line.

An additional line has, for instance, the same impedance as the quarter wave line.

In an advantageous embodiment, a branch feeding an antenna, for instance, will be common to two consecutive combiners of the system.

For example, each combiner consists of two Wilkinson cascade-connected basic combiners while one branch includes in series the quarter wave lines of the two combiners.

In an advantageous embodiment, the switches are, for instance, non-reflective.

In one possible configuration, in the open state, a switch connects its corresponding branch to an impedance whose value is approximately equal to the characteristic impedance of the combiner.

A switch can be connected to the corresponding antenna by a transmission line having impedance of 50 ohms.

The antennas can be antenna sectors of the same antenna.

For instance, the antenna system consists of Vivaldi type antennas.

In one possible embodiment, the device is located on a two-sided circuit with the first side supporting and forming a first part with:The antennas;The switches;The quarter wave lines of the basic combiners;The terminal resistors of these combiners;

And on the other side, supporting the following and forming a second part:The quarter wave lines of the other basic combiner;The additional lines;The terminal resistors of these other combiners; links between these sides ensure connection between the two parts.

One purpose of the invention is also having a wireless interconnection terminal equipped with a multi-antenna system having a feed device for the antennas according to any of the previous claims.

FIG. 1illustrates an example of a mesh network using wireless link technologies referred to as WiFi and Wimax. A group of terminals1communicates with a transmitter-receiver mounted at the top of a tower2. These terminals1form a set of fixed and mobile nodes. These are terminals devices of the MiMo type operating at standards 802.11 or 802.16. The fixed nodes are connected to the transmitter by a wireless link3of the Wimax type to standard 802.16. The mobile nodes are connected together by a wireless link4of the WiFi type to standard 802.11. Terminals5, for instance computers or mobile phones of any type, can also be integrated into the wireless networks4of the mobile nodes. As indicated previously, these nodes can self-organize themselves freely in a dynamic manner to create a random and temporary topography of networks referred to as “ad hoc”, thus allowing people and terminals5to interconnect in areas where there is no predefined communications infrastructure.

FIG. 2shows a simplified example of the architecture of a multi-antenna wireless link terminal1used in particular in the network ofFIG. 1. The terminal has transmission and reception antennas20. It also includes a baseband circuit21to standards 802.11 or 802.16, an RF interface circuit conforming to the standards used and an antenna access management device, for transmission or reception, used for directing the signal transmitted on the signal received towards one or several antennas selected simultaneously. For instance, this device includes a series of radio frequency switches23and links25each connecting an antenna to an RF interface circuit22. This interface circuit22is itself connected to reception and transmission circuits that are also of known types. During transmission, this circuit appears as an RF feed to the antennas. The switches are controlled by a control circuit24.

It appears that an interface stage inserted between the antenna feed point and the antennas themselves would ensure the necessary criteria of isolation between the antennas and even preserve good matching from the feed standpoint. But whatever technology is used, the penalty for inserting this stage at this precise point of the RF system leads to first to degrading the reception sensitivity by the addition of insertion losses and also in increasing the transmission power to compensate for these losses. These drawbacks lead to thinking in terms of feeding the antenna systems directly from the feed point, that is in star mode. After the optimization of a concept like this, simulation however demonstrated that it is unlikely to hope for isolation of better than around 12 dB between the antennas, far from sufficient to make the most of directivity performance for a mixed type network application, that is with mobile nodes and fixed nodes.

FIG. 3illustrates a broadband Wilkinson type combiner. More specifically, this combiner is a cascade set-up of two conventional Wilkinson combiners31,32. Indeed, the bandwidth of a Wilkinson type combiner can be increased by the cascade connection of two conventional basic combiners. Each basic Wilkinson combiner has two quarter wave transmission lines311,312, therefore having length λ/4, each having a characteristic impedance of Z1for the first combiner and a characteristic impedance of Z2for the second combiner. The cascade is produced in such a way that the branches of the second combiner32connect like terminal resistors to the branches of first combiner31. Load resistors33,34are connected to the outputs of the branches of second combiner32. The input of first combiner31, forming the input of the overall combiner, is loaded by a third load resistor35and is connected to the first combiner through a line including characteristic impedance36. A terminal resistor37,38is connected between the two branches of each of the basic combiners.

The characteristic impedances of the transmission lines and the terminal resistance values can be optimized to obtain the required isolation in a given frequency band. There is then a trade-off between the isolation performance and the effective bandwidth.

The implementing of a device according to the invention is based in particular on:A specific extension of the architecture of a Wilkinson broadband combiner;The use of each branch311,312of the combiner on two consecutive antennas or in two consecutive sectors;A modulo λ unbalance of one of the branches of the combiner allowing the actual setup of the device;A switching system for the antennas or sectors of the non-reflective type.

As far as the cascade extension of the combiners is concerned, if we consider this solution for the feeding of two antennas or two consecutive antenna sectors, performance from the standpoint of matching and isolation can be respectively around 20 dB and 30 dB. However, this solution requires the simultaneous feeding of at least four antennas or antenna sectors.

FIG. 4is a block diagram representing the use of Wilkinson type combiners with respect to the invention. A device according to the invention extends this solution based on broadband Wilkinson combiners to N sectors or antennas by simultaneously using each branch of the combiner both for the sector order n−1 and for the sector order n+1 as illustrated inFIG. 4. The example ofFIG. 4concerns the case of an antenna with four sectors41,42,43,44of the Vivaldi type. Considering the first two antenna sectors41and42, they are fed respectively from the central point, not shown, by first branch401and second branch402of a first Wilkinson type combiner45. Similarly, second and third sectors42,43are fed simultaneously by first branch402and second branch403of second combiner46, with branch402feeding second sector42being common to first and second combiners45,46.

Similarly, third and fourth sectors43,44are fed respectively by first branch before03and a second branch404of a third combiner47and the fourth and first sectors44,41all fed respectively by first branch404and second branch401of a fourth combiner48. Branch403feeding third sector43is common to second and third combiners46,47, while branch404feeding fourth section44is common to third and fourth combiners47,48and branch401feeding first sector41is common to fourth and first combiners48,45. The input of each combiner is also connected to the central feed point. This architecture can be repeated in this way, depending on the numbers of antennas or antenna sectors being used.

FIG. 5illustrates the principle of antenna or antenna sector switching for architecture of the type shown inFIG. 4. The system ofFIG. 5allows the selection of a signal to be transmitted simultaneously towards one or several antenna sectors1,42,43,44thus permitting a modification to the overall radiation pattern according to the network protocol management being used, for instance a network mixing together the fixed terminals and mobile terminals. Therefore, it is important to avoid the least deactivation of one or several simultaneous sectors which could modify the isolation and matching performance of the overall antenna.

According to the invention, antenna switching is carried out behind the feed system based on Wilkinson combiners by a set of selector switches51,52,53,54for instance, of the non-reflective types. In particular, this allows:On the one hand, avoiding any unbalance of the combiner(s) because one or several antenna channels are not deactivated and therefore since one of the branches of this or these combiners no longer see the load presented by the antenna,In addition, remaining almost insensitive to the isolation performance of the set of switches.

The Wilkinson combiners used are of the same type as used inFIG. 3.FIG. 5shows the four branches401,402,403,404feeding respectively the first41, second42, third43and fourth sectors44, each branch being common to two consecutive combiners. As an example, two combiners46,47are shown inFIG. 5whereby the branch403feeding the third sector is common to these to combiners. Each combiner45,46,47,48is for instance made up of two basic combiners and each branch includes in series the quarter wave lines60,59of the cascade-connected combiners.

Branches401,402,403,404are connected at the input to central feed point50. At the output, each branch is connected to a selector switch51,52,53,54.

When a switch is open, its corresponding branch is connected to impedance55so that the branch is loaded on this impedance55. To render the selector switch non-reflective for instance, this load impedance55equals the characteristic impedance of the combiner, for instance 50 ohms. When a selector switch is closed, it connects its corresponding branch to its antenna or its associated antenna sector, or for instance via a line having characteristic impedance Z3, for instance 50 ohms.

A device as illustrated inFIG. 5thus preserves in all the active sectors the same isolating performance whatever the switching performed on selector switches51,52,53,54. Furthermore, this solution makes it possible to maintain a PIRE (Equivalent Radiated Isotropic Power) as a constant by per sector, equal for instance to the power at output50of the power amplifier minus 6 dB, to the exclusion of feed circuit losses and the gain of the overall antenna. In particular, this for simply ensuring maximum emitted power per sector41,42,43,44while allowing for the regulations and standards in force.

Nevertheless, this solution requires that part of the power transmitted by the power amplifier is absorbed by loads55of the non-reflective selector switches. If the amplifier output power is sufficient, this is not constraining and may even simplify the control of the emitted power for mesh network management purposes.

In the reception direction, no loss of sensitivity regarding selector switches51,52,53,54needs to be allowed for because the selected antenna sector is directed towards emission point50.

FIG. 6illustrates another embodiment that guarantees electrical performance in the frequency spectrums used while providing for practical implementation. Indeed, one difficulty in using a solution of the type shown inFIG. 5can come from its practical implementation, especially in the frequency field in which 2.4 GHz WiFi band to Wimax applications whose frequency bands are placed respectively at 2.7 GHz, 3.5 GHz or 5.8 GHz, or yet again in WiFi bands in the 5 GHz range.

To obtain optimum behavior from the combiners in these radiofrequency fields, terminal resistors37must be located as close as possible to each of the quarter wave transmission lines. Considering, for instance, a Vivaldi type antenna as illustrated inFIG. 4for an application at 5.8 GHz, the quarter wave line of a combiner measures 7.4 mm. Accordingly, a cross feed401,402,403,404as illustrated inFIG. 4, representing for instance the first four quarter wave sections having an impedance Z1means connecting terminal resistors37at the ends of the cross, that is at a distance of approximately 10 mm at 5.8 GHz. Such a distance is particularly prohibitive and considerably degrades the matching and especially the isolation performance, possibly rendering the solution ineffective.

The set up in figure in6guarantees electrical performance while ensuring compatibility with an embodiment that can be implemented practically. In this typical embodiment, to minimize the lengths of terminal resistors37, one of the branches of the combiner is unbalanced by an additional line61whose length is a multiple of wavelength λ, having, for instance the same characteristic impedance Z1as initial branch60. Accordingly, each terminal resistor37is connected between a branch60having a length λ/4 and a branch61having a length 5λ/4 as illustrated inFIG. 6. This set up is advantageously suited for multi layers circuit, having for instance two layers a front face and a rear face.

FIGS. 7aand7bshow an example of the implementation of the set up ofFIG. 6on a double sided printed circuit in whichFIG. 7ashows one face andFIG. 7bshows the other. On the first face, illustrated inFIG. 7a, the following are located:Antenna sectors41,42,43,44, for instance, in the form of patches;Selector switches51,52,53,54;The quarter wave lines59of the second basic combiners;Terminal resistors38of these combiners;

On the second side, illustrated inFIG. 7b:Quarter wave lines61of the first basic combiner;Modulo λ 60 length lines equal to wavelength λ in the example shown;Terminal resistors37.

Links71between these sides ensure connection between the two parts. Measurements have shown that a circuit as illustrated inFIGS. 7aand7bproduces matching in excess of 20 dB throughout the WiFi band, included in particular between 5.15 GHz and 5.875 GHz with isolation in excess of 30 dB, guaranteeing very good decorrelation between the antenna patterns.

In particular, the invention is ideally suited to multi-antenna systems or multi-sector antennas used in MiMo systems and especially for mesh network architectures. Through its performance in terms of isolation between antennas, the invention will considerably improve the radiation efficiency and the possibility of filtering out interference. Control of directional transmission will thus allow high rate transmission with a high level of spatial re-use.

The typical embodiment presented in the figures includes four antennas or antenna sectors. Naturally, it is possible to apply the invention to a greater number of antennas.

A device according to the invention may be used advantageously to equip a wireless link terminal, for instance of the type shown inFIG. 2. In this case, switches23and links25system is replaced by a device according to the invention as described previously, connected at the input to interface22and at the output to the antennas.