BACKGROUND AND DESCRIPTION OF RELATED ART High-speed wireline or fiber optic connections of backbone networks interconnecting nodes of a terrestrial radio ac- cess network are previously known. It is also known to in- terconnect radio base stations with microwave links provid- ing interconnections of moderate data rates.

Increased antenna area of prior art microwave link antennas increases signal quality, but also increases irradiated mi- crowave power as does transmission power increases. An in- creased antenna area can be achieved by arranging a plural- ity of smaller area antenna elements in an array.

Efficient modulations and signal constellations offer re- lieved power requirement, or improved performance if micro- wave power is maintained, as number of signal points in the signal constellation increases.

American Patent Application US2003/0125040 discloses a sys- tem for multiple-input multiple-output (MIMO) communica- tion. A MIMO channel formed by NT transmit antennas and NR receive antennas is decomposed into Nc independent channels

also referred to as spatial sub-channels, where Nc<min {NT, NR}. Data is processed prior to transmission based on channel state information.

American Patent Application US2002/0039884 reveals a radio communication system with a transmitter having a plurality of transmitter antennas and a receiver having at least one antenna. Thereby a plurality of paths with various charac- teristics are formed between the transmitter antennas and the at least one receiver antenna. Data is assigned one or more categories. Depending on categories and path charac- teristics, the data is mapped to one or more of the trans- mitter's parts and antennas.

American Patent Application US2002/0039884 describes a ra- dio communication system with a transmitter having a plu- rality of transmitter antennas and a receiver having at least one antenna. Data tags indicate data importance or other requirements. Data is assigned one or more catego- ries. Depending on categories and path characteristics, the data is mapped to one or more of the transmitter's parts and antennas.

3rd Generation Partnership Project (3GPP): Technical Speci- fication Group Radio Access Network, Physical layer aspects of UTRA High Speed Downlink Packet Access (Release 4), 3G TS 25. 848 v4.0. 0, France, March 2001, describes MIMO open loop signal processing of MIMO transmitter and receiver in section 6.5.

Bell Labs Technical Journal, autumn 1996 : G. Foschini, "Layered Space-Time Architecture for Wireless Communica- tion in a Fading Environment When Using Multi-Element An- tennas"shows that under fading conditions with statisti- cally uncorrelated identically distributed propagation

channels, the bandwidth constrained channel capacity of a MIMO channel, CMIMO, scales on average as CMIMO=CSISO#min {M, N}, (1) where Corso is channel capacity of a SISO channel. For a band limited (bandwidth B) AWGN (additive white gaussian noise) channel the SISO channel capacity equals Cs, so=B-1092 (1+SNRsiso) [bits/sI, (2) where SNRsIso is the SISO channel signal to noise ratio.

Figure 1 schematically illustrates N transmitter antenna elements « T1, T2,..., TN » and M receiver antenna elements « R1, R2,..., RM# in MIMO communications. Between the various transmitter and receiver antenna elements there are propa- gation channels « hll, hl2,... hlM,..., hN ».

The individual propagation channels, that are SISO (Single Input Single Output) channels, form a MIMO channel.

C. Schlegel and Z. Bagley,"Efficient Processing for High- Capacity MIMO Channels"submitted to JSAC, MIMO Systems Special Issue: April 23,2002 reveals estimation of optimum channel capacity of a MIMO system for a known MIMO-channel described by channel matrix H by means of singular value decomposition, SVD.

U#S#VH=SVD{H}, (3) where U and V are unitary matrices, S is a resulting diago- nal matrix with singular values in the main diagonal, and Vu is a Hermitian transformed matrix V.

None of the cited documents above discloses particular an- tenna configurations related to communications distance with line of sight, LOS, MIMO communications.

SUMMARY OF THE INVENTION Next generation radio access networks are expected to be required to support peak user data rates in the order of magnitude of 30 Mbps-1 Gbps. With a vast amount of base stations, it would be advantageous to interconnect base stations over radio links for flexibly connecting/discon- necting links of a mobile station active set of radio links with the base station as the mobile station moves.

Present radio link solutions do not offer sufficient data rates of aggregate user data, as to/from a base station, including a plurality of high user data rate at reasonable power levels for reasonably sized element antenna aper- tures.

Consequently, there is a need of antennas of large aper- tures providing required data rates at reasonable transmis- sion power for reasonably sized element antenna apertures.

It is consequently an object of the present invention to achieve an antenna configuration for line of sight commun- cation useful for providing low error rates at moderate transmission power within limits as may be required by due authorities.

It is also an object to achieve a system flexible to dif- ferent transmission ranges and wavelength ranges.

An object is also to offer high data rates for low trans- mission power levels as regards antenna properties.

Another object is to achieve an antenna configuration adapted to particular communications distance and wave- length.

Finally, it is an object to relieve the dependency on tan- gible interconnections, such as wire lines or optical fi- bers, for interconnection of base stations or other nodes of a telecommunications system. Such interconnections are generally associated with great initial investment costs and maintenance costs.

These objects are met by a method and system of antennas configured for a particular communications distance over line of sight links providing multiple input multiple out- put communications links.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates N transmitter antenna elements and M receiver antenna elements in MIMO communia- tions.

Figure 2 schematically illustrates a spherical wave-front background to the invention.

Figure 3 illustrates example capacity versus SNRsIso for four-element LOS MIMO linear array, according to the inven- tion, and four-element linear array non-LOS MIMO.

Figure 4 illustrates a square grid LOS MIMO antenna array, according to the invention.

Figure 5 shows a linear LOS MIMO antenna array, according to the invention.

Figure 6 depicts a spatially oversampled antenna array, according to the invention.

Figure 7 demonstrates hexagonal antenna element packing, according to the invention.

Figure 8 has an antenna array with circular element pack- ing, according to the invention.

Figure 9 displays a clustered directional hybrid with eight groups of clustered antenna elements for eight chan- nels MIMO, according to the invention.

Figure 10 shows a clustered directional hybrid with four groups of clustered antenna elements for four channels MIMO, according to the invention.

Figure 11 illustrates a clustered directional hybrid with two groups of clustered antenna elements for two channels MIMO, according to the invention.

Figure 12 comprises plotted capacity per bandwidth vs. nor- malized SNR for MIMO communications with square grid LOS MIMO antennas for various levels of clustering at both re- ceiver and transmitter side, according to the invention.

Figure 13 shows an LOS MIMO antenna array with director elements, according to the invention.

Figure 14 depicts schematically an LOS MIMO antenna with a grid of interconnected rods or tensed wires to which the antenna elements are attached, according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In backbone networks based on wireless communications it is important to achieve capacity to handle data rates of ag- gregate traffic, where individual peak user data rates are in the order of 100 Mbps to 1 Gbps.

Fixed fiber optical networks are not always applicable.

They are often associated with great costs, provide little or no flexibility and occupy extensive ground space.

Prior art Multiple Input Multiple Output, MIMO, communica- tions systems most commonly are designed to utilize scat- tering and, therefore, requires a scattering environment.

The present invention is not dependent on such scatterers and suits line of sight communication very well. A theo- retical reason for this is its exploitation'of spherical wave fronts and associated phase differences.

Figure 2 schematically illustrates propagation paths and principles of the invention. Respective propagation paths « P11 », « P12 », « P13 » between a transmitter antenna « T1 » and receiver antennas « R1 », « R2 », « R3 » differ slightly in length due to a spherical wave front property of the transmitted signal. The small differences in path lengths « 611 », « 612 », « 613 » add to the communications distance D. With path pij as a reference for the communications distance, &, j equals zero.

I. e. when pll is selected as reference then 5n=0. The an- tenna configuration according to the invention essentially maximizes MIMO channel capacity for great signal to noise ratios, SNR, in respect of the spherical wave front prop- erty for LOS communications. This is in contrast to, e. g., maximizing antenna directivity as illustrated in and ex- plained in relation to figure 12 below.

With each MIMO sub-channel operating close to its maximum theoretical performance, according to the configuration of the invention, great performance gains are achieved.

Figure 3 illustrates example capacity versus SNRsaso for LOS MIMO and non-LOS MIMO (fading uncorrelated channels) for a four-element linear array. The gain of LOS MIMO as com-

pared to non-LOS MIMO in terms of capacity increase or SNR gain is the vertical or horizontal difference between the curves, respectively. The SNR gain implies, e. g. , in- creased noise immunity or reduced transmission power re- quirement.

Radio Access Networks, RANs, are frequently realized with base stations connected in series, with at least one of the serialized base stations serving as an anchor to the core network. Consequently, the individual links between base stations may carry data traffic of a plurality of base sta- tions. With individual peak user data-rates in the range of 100 Mbps-1 Gbps required peak rates of data links be- tween base stations could be expected to be in the range of 1-100 Gbps.

Prior art radio data links is not known to provide data rates of more than one Gbps for the spectrum efficiency achieved with the invention. The major two reasons for this are that there are practical limits on signal constel- lation sizes, practical and regulatory constraints on available radio spectrum, and power limits.

Prior art relies upon uncorrelated channels between the various antenna elements. This could e. g. be the case for channels fading due to scattering. However, the invention points out that exploitation of the spherical property of wave fronts results in ideal MIMO gain in absence of scat- terers. According to the invention square grid LOS MIMO antenna array and linear LOS MIMO antenna arrays are pre- ferred, see figures 4 and 5 respectively. This does not exclude circular or hexagonal packing as a means to in- crease antenna elements surface density as illustrated in figures 7 and 8 respectively. In the hexagonal packing of figure 7 the respective distances between (at most 6) near-

est neighboring antenna elements « Antenna element » are all essentially equal « d ». Spatially oversampled and clustered antenna arrays, see figure 6 and 9 respectively, are pre- ferred for some situations. Figures 10 and 11 show some other clustered directional hybrids for 16 antenna elements « Antenna element ».

With reference to figures 9-11, while the total number of antenna elements, N, are kept constant equal to 16 elements the respective number of groups of elements, k, of the fig- ures varies. In figure 9 there are eight groups with two antenna elements « Antenna element » each. Within each group the antenna elements « Antenna element » are positioned suf- ficiently close for signals to add coherently in phase, thereby generating a directivity gain. Figure 11 illus- trates an example realization with four groups, each of four antenna elements « Antenna element ». In figure 11, an example for N=16 and k=2 is illustrated. In the figures each group of antenna elements « Antenna element » generates a MIMO sub-channel. With Nlk antenna elements for each MIMO sub-channel on receiver and transmitter side, the total achievable gain is (Nlk) 2, since both sides contribute to the gain. If equivalent isotropic radiated power, EIRP, is at its maximum level allowed, the gain at transmitter side is achieved as a reduction of transmit power and not in in- creased received, power or energy per symbol. Assuming an SNR gain of (N/k) 2 for grouped directional antennas with k groups, equations (1) and (2) transform into <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> 2<BR> <BR> <BR> Ccluseered=B k log2 (l+ k2SNRsIso) [bits/s].<BR> <BR> <BR> <BR> zig There are SNR ranges where MIMO communications with clus- tered elements antennas outperform MIMO with the same num- ber of antenna elements, not being clustered. As noted in

figure 12 a channel capacity increase is achieved with clustering particularly for poor transmission conditions (small SNR). Figure 12, plots the channel capacity per bandwidth Cclustered/B for MIMO communications with clustered antenna elements versus SNR « SNRsIso » normalized to SISO com- munications conditions, and where k is the number of clus- ters of antenna elements at transmitter and receiver ends, kc= [1, N]. The figure illustrates performance for an example of 16 antenna elements according to equation (4), with SISO performance of N=l antenna element antennas included for reference.

Typically high SNR conditions prevail in short range commu- nications. Consequently, gain increase by unclustered MIMO communications with great number of antenna elements is preferred for short-range communications.

In figure 5, for an optimum MIMO system and for a communi- cations distance D much greater than element separation d the distance a=d (N-1) is specified by where the approximation in equation (6) holds for great number of antenna elements N. For N=16 antenna elements « Antenna element », the approximation error is about 7%.

Table 1 illustrates element separation, d, of a transmitter- receiver pair of linear MIMO antennas versus communications distance, D, at some example wavelengths, A, equal to 3 mm, 7.9 mm and 42.9 mm. Element separation d [m] Distance D [km] A=3 mm A=7. 9 mm =42. 9 mm 0.2 0. 55 0.9 2.1 2 1. 7 2.8 6.5 20 5.5 8.9 20.7 200 17. 3 28. 1 65. 4 Table 1: Linear MIMO antenna, N=2.

For the square grid LOS MIMO antenna array in figure 4, the distance a, corresponding to that of equation (5) for linear

arrays, is determined to where the approximation in equation (8) holds for great number of antenna elements N. For N=16 antenna elements « Antenna element », the approximation error is about 33%.

An important observation is that for the square grid LOS MIMO antenna array in figure 4 the distances a and d get relatively smaller in proportion to the fourth root of N, whereas for the linear array of figure 5 the distance de- pendency is proportional to the square root of N.

With the antenna area A=a2, and using the approximation in equation (8), the MIMO channel capacity, CMIMO=NCsiso, ex- pressed in terms of channel capacity for a SISO system, Cszso, with the example design of figure 4 according to the invention is

In figure 4 and equation (7) the antenna elements « Antenna element » are assumed to be electrically active elements, supplying a voltage or current to a receiver. However, as illustrated below basically the same distance relations hold for antenna elements being directors guiding received electromagnetic field to electrically active antenna ele- ments.

Figure 13 illustrates an alternative realization with di- rector elements « Director » mounted on supports « Supports ».

The directors « Director » direct electromagnetic fields re- ceived and electromagnetic fields to be transmitted, pref- erably with one director per electrically active antenna element « Active elements ». Preferably the directors « Di- rector » are pure reflectors but can also be made of dielec- tric material. The supports « Supports » are designed not to shadow, or only have a small shadowing impact on, the elec- trically active antenna elements « Active elements ». The positioning of the directors is preferably in accordance with equation (5) and (7) for a linear and square grid LOS MIMO antenna respectively. The relevant distance d is es- sentially equal to the separation distance. of the projec- tion of the directors onto a plane, the plane being perpen- dicular to the LOS transmission path to the other re- ceiver/transmitter end. Advantages achieved by the reali- zation of figure 13 in addition to those mentioned above are, e. g. , simplified wiring of the antenna elements and the antenna elements spanning a smaller distance range thereby being mechanically robust. Also, by adjusting the directors the electrically active antenna elements need not always be repositioned even if communications distance changes.

The dependency of a, A and d on D for an LOS MIMO antenna has practical implications, addressed by the invention. An obvious solution to the problem of getting a, to the commu- nications distance D, appropriately matched element dis- tance, d, is to manufacture custom-made antennas. From a cost perspective, however, a more attractive solution is manufacturing of a set of antenna models for MIMO communi- cations, each designed for a range of communications dis- tances D, and upon installation selecting an antenna model within the set that best matches the communications dis- tance. Another embodiment is realized by individually ad- justable antenna elements. Preferably this is realized by a grid « Grid » of interconnected rods or tensed wires to which the antenna elements « Antenna element » are attached as illustrated in figure 14. The wires or rods are pref- erably connected to. a frame « Frame ». Models that are elec- tromechanically adjustable comprise electromechanical mo- tors to which the rods are connected, such that the rods to which the antenna elements « Antenna element » are attached may move along the frame. A further embodiment of adapting an LOS MIMO antenna to communications distance D uses spa- tial oversampled antennas as schematically illustrated in figure 6 and activating the antenna elements by signal processing providing best performance at actual communica- tions distance. The particular element distribution may be varied, e. g. as illustrated in figures 7 and 8. An impor- tant issue of the invention is that active elements are distributed such that their mutual distances reflects com- munications distance (distance between transmit and receive antennas) and wavelength such that the spherical properties of the radio wave can be exploited.

It is observed that as transmitter and receiver antennas form an antenna pair for a communications link, the element

distance d of e. g. a transmitter antenna can be reduced if the element distance of the corresponding receiver antenna of the communications link is increased in proportion to the distance reduction of the transmitter antenna. If ele- ment distance of receiver antenna, dR, are reduced in rela- tion to d, transmitter-side antenna element distance, dT, should be increased (in relation to d) in proportion thereto. Consequently, the distance d of equations (5) and (7) is the geometrical average of receiver and trans- mitter antenna element distances dR and dT, respectively.

The invention does not only cover planar antenna configura- tions, but also three-dimensional configurations as illus- trated in figures 15-17. Figure 15 illustrates a two-layer square grid LOS MIMO antenna with two layers of antenna elements each on a square grid. Figures 16 and 17 illus- trate realizations with equal distance between all nearest neighboring antenna elements. In figure 16 the antenna elements are positioned to the vertices of a cube and in figure 17 the antenna elements are positioned to the verti- ces of a tetrahedron.

Various embodiments of the invention also cover different realizations of signal processing at transmitter and re- ceiver ends. The processing is necessary for adaptation to prevalent channel conditions. At receiver or transmit side, determining channel singular values as described in relation to equation (3) and singular value decomposition can be achieved by digital signal processing of base band signals. If determined at transmitter side, information on channel matrix, H, need to be transferred from receiver side, or the channel matrix otherwise estimated at trans- mitter side, see figure. For a 2X2 channel matrix, singu- lar value decomposition can also be achieved by a 3-dB hy-

brid to perform multiplication or weighting as need be, op- erating on high-frequency signals. Also, for channel ma- trices greater than 2X2 a generalization of a 3-dB hybrid, a Butler matrix directional coupler, may be used. A fur- ther embodiment realizes the processing by means of an ar- rangement of microstrip or waveguides, also operating on high-frequency signals. At receiver side, channel equali- zation requires processing. This processing can be per- formed by any of the processing realizations described for transmitter side, or received signal can be equalized by means of zero forcing, for which the received signal being multiplied by the inverse matrix of channel matrix H, or by means of minimum mean square error, MMSE, for which the mean square error is minimized, the various processing re- alizations giving rise to further embodiments.

If there is multipath propagation, this is preferably in- corporated into the singular value decomposition at trans- mitter side through feedback information. Corresponding information can also be derived through channel reciprocity if the reverse direction channel matrix is determined at transmitter side (the transmitter side also comprising ra- dio receiver). Another solution comprises a self-tuning antenna, optimizing performance at receiver side, transmit- ter side or both. The antenna element positioning is then adapted to channel propagation properties corresponding to a measured channel matrix, H. This can be achieved by, e. g. a stochastic gradient algorithm. Particularly for fixed positioned antenna elements, they may require the an- tenna elements to be re-distributed for optimum perform- ance. For an electromechanically adjustable element an- tenna the optimization can be achieved by automatic posi- tion adjustments of the antenna elements. The different solutions to multipath propagation can also be combined.

The concept of the present invention combines well with other known means to increase throughput, such as transmis- sion at both vertical and horizontal polarization or trans- mission at left-hand and right-hand circular polarization, or different coding of different sub-channels depending on their respective channel quality, which further demon- strates the usefulness of the invention. Such combinations are also within the scope of this invention.

The invention is not intended to be limited only to the em- bodiments described in detail above. Changes and modifica- tions may be made without departing from the invention. It covers all modifications within the scope of the following claims.