WO2013143617A1

WO2013143617A1 - A line-of-sight multiple-input multiple-output communication system

Info

Publication number: WO2013143617A1
Application number: PCT/EP2012/055874
Authority: WO
Inventors: Mikael Coldrey; Björn Johannisson
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2012-03-30
Filing date: 2012-03-30
Publication date: 2013-10-03

Abstract

The invention relates to a reconfigurable multiple-input multiple- output system for wireless communication over a wireless link. The reconfigurable MIMO system comprise at least three spatially separated antenna elements, at least two radio units, a digital baseband unit and a switch unit that is arranged to coupled the digital baseband unit to at least two of the at least three spatially separated antenna elements, via at least of the two radio units, in at least a first and a second configuration.

Description

A LINE-OF-SIGHT MULTIPLE-INPUT MULTIPLE-OUTPUT COMMUNICATION SYSTEM

TECHNICAL FIELD The present invention relates to the field of wireless communication systems and, in particularly, to line-of-sight (LOS) multiple- input multiple-output (MIMO) communication systems for wireless communications.

BACKGROUND

Radio spectrum is a scarce resource and therefore it is important to use spectral efficient technologies. One such technology is a multiple-input multiple-output (MIMO) system that uses multiple antennas at the transmitter and at the receiver to convey multiple parallel data streams over the wireless channel. Conventionally, MIMO techniques are used in multipath non-line-of- sight (NLOS) wireless channels where different data streams are transmitted over different channel paths before being reconstructed at the receiver. However, for this to work the wireless channel needs to support enough multipath propagation.

It has been shown that MIMO techniques also can be used in line-of-sight (LOS) channels that do not have any significant multipath propagation or only have a very weak NLOS paths compared to the direct LOS path. The LOS channel is characterized by a strong single path which usually only can convey a single data stream (or two if using orthogonal polarizations). However, with careful placement of the antennas with respect to the frequency and distance between the transmitter and receiver, a MIMO channel can be achieved that supports transmission/reception of multiple data streams also in the LOS case.

The longer the distance is between the transmitter and receiver, the larger antenna separations are needed to resolve the multiple data streams. The higher the frequency is, the smaller antenna separations can be used. For example, the optimal antenna separation for a symmetric 2x2 (i.e., 2 transmitter and 2 receiver antennas) LOS MIMO hop of 1 km is 1 .58 m at 60 GHz.

A correct antenna separation is very important in a LOS MIMO system. For example, if the antenna separation is optimized for a certain frequency and an intended hop length, then it will not work well for another hop length that is substantially different from the intended hop length. During the antenna design phase the exact hop length is usually unknown, and therefore there is a great need for a single antenna solution that can support a wide range of hop lengths since each hop length requires an antenna solution tailored to that particular hop length. Also, it is currently not economical to produce a system that supports several different antenna solutions.

Furthermore, when antenna separation is small, it is an advantage from deployment, installation and mounting perspective if the LOS MIMO antenna solution can be confined in a single unit.

Thus, finding a way to offer a LOS MIMO system that can support a wide range of hop lengths is therefore highly sought for.

SUMMARY OF THE INVENTION

With the above description in mind, then, an aspect of the present invention is to provide a LOS MIMO system capable of supporting a wide range of hop lengths which seeks to mitigate, alleviate, or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination.

A first aspect of the present invention relates to a reconfigurable MIMO system for wireless communication over a wireless link, comprising at least three spatially separated antenna elements, at least two radio units, a digital baseband unit, characterized in that a switch unit is arranged to coupled said digital baseband unit to at least two of said at least three spatially separated antenna elements, via at least two radio units, in at least a first and a second configuration. In this way a wide range of hop lengths can be supported and yet keep the antenna compact.

The MIMO system wherein said switch unit may be a radio frequency switch coupled to said at least three spatially separated antenna elements and to said at least two radio units.

The MIMO system wherein said switch unit may be a digital switch coupled to said digital baseband unit and coupled to said at least two radio units. The MIMO system wherein said switch unit may be integrated in said digital baseband unit. In this way the switch may be integrated into the baseband unit reducing the number of separate units in the overall system.

The MIMO system wherein said at least three spatially separated antenna elements may be arranged with at least a first (d1 ) and a second (d2) spacing between said at least three spatially separated antenna elements, wherein said at least a first (d1 ) and a second (d2) spacing's are not equal. In this way a number of antenna configurations with different spatial separation may be created.

The MIMO system wherein said at least three spatially separated antenna elements may be arranged along a line.

The MIMO system wherein said at least three spatially separated antenna elements may be co-polarized antenna elements adapted for co- polarized MIMO communications over said wireless link.

The MIMO system wherein said at least three spatially separated antenna elements are dual-polarized antenna elements adapted for dual- polarized MIMO communications over said wireless link.

The MIMO system wherein said wireless link is a wireless line-of- sight (LOS) MIMO link.

A second aspect of the present invention relates to a method of configuring a multiple-input multiple-output system for wireless communication over a wireless link, comprising the steps of configuring at least three spatially separated antenna elements, at least two radio units, and a digital baseband unit for wireless communication over said wireless link characterized in arranging a switch unit to coupled said digital baseband unit to at least two of said at least three spatially separated antenna elements, via at least two radio units, in at least a first and a second configuration. The method may further comprise coupling said switch unit to said at least three spatially separated antenna elements and to said at least two radio units.

The method may further comprise arranging said at least three spatially separated antenna elements, with at least a first (d1 ) and a second (d2) spacing between said at least three spatially separated antenna elements, such that said at least a first (d1 ) and a second (d2) spacing's are not equal. The method may further comprise arranging said at least three spatially separated antenna elements along a line.

The method may further comprise adapting said at least three spatially separated antenna elements for co-polarized MIMO communications over said wireless link. The method may further comprise adapting said at least three spatially separated antenna elements for dual-polarized MIMO communications over said wireless link. The method wherein said configuring of said switch unit may be based on a hop length and a frequency.

A third aspect of the present invention relates to a wireless multiple-input multiple-output communication system comprising of at least two nodes wherein at least one of said at least two nodes is a reconfigurable multiple-input multiple-output (MIMO) system according to claims 1 -9.

Any of the features in the first and the second aspect of the present invention disclosed above may be combined, in any way possible, as to form different embodiments of the present invention. All of the benefits described in conjunction with the first aspect of the present invention may in the same way be applied to the second aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features, and advantages of the present invention will appear from the following detailed description of some embodiments and variants of the invention, wherein some embodiments or variants of the invention will be described in more detail with reference to the accompanying drawings, in which:

Fig. 1 shows a block diagram of a wireless MIMO system according to an embodiment of the present invention; and

Fig. 2 shows another block diagram of a wireless MIMO system according to an embodiment of the present invention; and Fig. 3 shows flowchart describing a method according to the present invention; and Fig. 4 shows a capacity versus hop length plot according to an example of the present invention; and

Fig. 5 shows a wireless multiple-input multiple-output communication system according to an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference signs refer to like elements throughout the description. As described in the background section, finding a way to offer a

LOS MIMO system that can support a wide range of hop lengths is highly sought for.

Figure 1 shows a MIMO system 101 for wireless communication, according to an embodiment of the present invention, capable of reconfigure itself to a 2x2 MIMO system over a wide range of hop lengths.

The MIMO system 101 disclose a switch-based reconfigurable non-uniform antenna array solution comprising three spatially separated antenna elements 102, 103, 104, wherein the first 102 and the second 103 antenna element is separated by a spacing d1 , and the second 103 and the third 104 antenna element is separated by a spacing d2. The antenna is thus non-uniform meaning that the spacing's d1 and d2 are not equal (d1≠d2). By only using two antenna elements 102, 103, 104 out of the three available antenna elements 102, 103, 104 at a time, and due to the fact that the spacing's d1 and d2 between the antenna elements 102, 103, 104 are not equal, different antenna separation states tuned to different hop lengths may be achieved.

The switching between the different antenna elements 102, 103, 104 may in the embodiment in figure 1 be done at radio frequency (RF) using a switching unit 105. The switching unit 105 is typically comprised of microwave switches 106, 107, 108. In the block diagram in figure 1 the switching unit is implemented using three microwave switches 106, 107, 108 arranged to coupled a digital baseband unit 1 1 1 to at least two of the at least three spatially separated antenna elements 102, 103, 104, via at least two radio units 109,1 10, in different configurations. In this way, three different antenna separation states (i.e. the first antenna element 102 to the second antenna element 103, the first antenna 102 element to the third antenna element 104 and the second antenna element 103 to the third antenna element 104) can be achieved by a single reconfigurable antenna system, according to the embodiment of the present invention.

Thus, if both the transmitter- and receiver-side of a wireless MIMO communication system employ such reconfigurable MIMO systems as shown in figure 1 , the overall MIMO communication system will be able to choose between 6 unique states (unique meaning non-equivalent). In this way the best (i.e. most suitable for a given situation) of the 6 available states can be chosen, preferably by using an automated process by for example measuring the MIMO channel quality (e.g., channel rank or capacity) or measuring the range (hop length), for a certain frequency. The best available state may also be set manually. The antenna spacing's d1 and d2 of the nonuniform antenna array are preferably chosen as to avoid deep reductions in throughput for the hop length range of interest.

Figure 2 shows a block diagram of a MIMO system 201 according to an embodiment of the present invention. The MIMO system 201 also disclose a switch-based reconfigurable non-uniform antenna array solution comprising three spatially separated antenna elements 202,203,204, wherein the first 202 and the second 203 antenna element is separated by a spacing d1 , and the second 203 and the third 204 antenna element is separated by a spacing d2. Also in this embodiment the spacing's d1 and d2 are not equal (d1≠d2) so that different antenna separation states tuned to different hop lengths may be achieved.

The main difference between the embodiments of the present invention shown in figure 1 and figure 2 is in the way the switching of the antenna elements is implemented. The switching between the different antenna elements 202,203,204 is in the embodiment in figure 2 done digitally, in a digital switch unit 209, in the digital baseband unit 208. In this embodiment the switching unit 209 is typically comprised of a digital switch fabric which may (or not) be similar in its realization to the switches 106, 107, 108 in the switch unit 105 in figure 1 . The digital switch unit 209 connects the "rest" of the digital baseband unit 208 to the antenna elements 202,203,204 via radio units 205,206,207, one for each antenna element 202,203,204. In this way the digital switched solution 201 , in the implementation shown in figure 2, has a redundant radio unit 205,206,207 which is not used for certain configurations. The redundant radio unit may in some embodiments be turned off or set in a sleep mode to conserve energy. In the same manner as in the case with the MIMO system disclosed in figure 1 the MIMO system disclosed in figure 2 is capable to achieve three different antenna separation states by a single reconfigurable antenna system.

Both embodiments of the present invention in figure 1 and figure 2 are described with a three element non-uniform antenna array to be used in a 2x2 order MIMO configuration. However, the idea of using a non-uniform array (reconfigurable or not) can of cause be extended to any order MIMO configuration. The at least three spatially separated antenna elements 102, 103, 104,202,203,204 may be arranged along a single line or they may be arrange in another pattern which is favourable for a certain setup.

The at least three spatially separated antenna elements 102, 103, 104,202,203,204 in the MIMO systems in figure 1 and figure 2 may be co-polarized antenna elements adapted for co-polarized MIMO communications over a wireless link or they may be dual-polarized antenna elements adapted for dual-polarized MIMO communications over a wireless link. The wireless link (or channel) may either be a wireless LOS MIMO link or a non-LOS MIMO link.

Figure 3 shows a flowchart 300 describing a method of how to set up a MIMO system 101 ,201 according to the present invention described in conjunction with figure 1 and 2 above. In a first step 301 the at least three spatially separated antenna elements, at least two radio units, and a digital baseband unit is configured for wireless communication over a wireless link. In a second step 302 a switch unit is arranged to be coupled the digital baseband unit to at least two of the at least three spatially separated antenna elements, via at least two radio units, in at least a first and a second configuration. The configuring of the switch unit is based on the specific hop length and frequency. The MIMO system may then be used to communicate over for instance a LOS MIMO link having a specific hop length and frequency.

The method may also include that the switch unit, in an embodiment of the present invention, is coupled to the at least three spatially separated antenna elements and to the at least two radio units.

The method may also include that the at least three spatially separated antenna elements, with at least a first (d1 ) and a second (d2) spacing between the at least three spatially separated antenna elements, is arranged such that the at least a first (d1 ) and a second (d2) spacing's are not equal. The at least three spatially separated antenna elements may in an embodiment of the present invention be arranged along a line.

The method may also include that the at least three spatially separated antenna elements, in an embodiment of the present invention, is adapted for co-polarized MIMO communications over the wireless link, while they in another embodiment of the present invention is adapted for dual- polarized MIMO communications over said wireless link.

Figure 4 shows a plot 400 with the capacity (bps/Hz) 401 versus hop length (meters) 402 for six unique states 403 (described in the box in the plot where d_t is the antenna separation at the transmitter and d_r is the antenna separation at the receiver). In this example the hop length range is from 15m to 1000m and the frequency is 60GHz. The possible separation at each antenna array is d1 =0.4m, d2=0.8m, and d1 +d2=1 .2m, respectively. Path loss and antenna gain are excluded in the capacity calculation. The plot clearly shows that the maximum throughput is achieved by choosing different antenna separations (by switching between the different antenna configurations) in such way that one ride on the peaks of the capacity curves 404 in figure 4. The plot also shows the capacity penalty by using a fixed (non-reconfigurable) antenna separation that is not optimized for the correct hop length. For example, using a 0.4m separation at both transmitter and receiver for a 600m hop length gives approximately a 5 bps/Hz capacity loss compared to the capacity attained by the disclosed reconfigurable antenna solution which in this case would be reconfigured to a 1 .2m antenna separation at both transmitter and receiver.

Figure 5 illustrates a wireless multiple-input multiple-output communication system 500 comprising of at least two nodes 501 ,502 wherein at least one of said at least two nodes 501 , 502 is implemented using a reconfigurable MIMO system 101 , 102. The wireless multiple-input multiple-output communication system 500, comprising the at least two nodes 501 ,502, may be configured as a microwave radio link adopted for wireless backhaul. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" "comprising," "includes" and/or "including" when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should be regarded as illustrative rather than restrictive, and not as being limited to the particular embodiments discussed above. The different features of the various embodiments of the invention can be combined in other combinations than those explicitly described. It should therefore be appreciated that variations may be made in those embodiments by those skilled in the art without departing from the scope of the present invention as defined by the following claims.

Claims

1 . A reconfigurable multiple-input multiple-output (MIMO) system (101 ,201 ) for wireless communication over a wireless link, comprising: - at least three spatially separated antenna elements

(102, 103,104,202,203,204);

- at least two radio units (109, 1 10,205,206,207);

- a digital baseband unit (1 1 1 ,208);

characterized in that

a switch unit (105,209) is arranged to couple said digital baseband unit

(1 1 1 ,208) to at least two of said at least three spatially separated antenna elements (102, 103, 104,202,203,204), via at least two radio units (109, 1 10,205,206,207), in at least a first and a second configuration.

2. The MIMO system (101 ) according to claim 1 , wherein said switch unit (105) is a radio frequency switch coupled to said at least three spatially separated antenna elements (102,103, 104) and to said at least two radio units (109, 1 10).

3. The MIMO system (201 ) according to claim 1 , wherein said switch unit (209) is a digital switch coupled to said digital baseband unit (208) and coupled to said at least two radio units (205,206,207).

The MIMO system (201 ) according to claim 3, wherein said switch unit (209) is integrated in said digital baseband unit (208).

The MIMO system (101 ,201 ) according to any previous claims, wherein said at least three spatially separated antenna elements (102, 103, 104,202,203,204) are arranged with at least a first (d1 ) and a second (d2) spacing between said at least three spatially separated antenna elements (102, 103, 104, 202,203,204), wherein said at least a first (d1 ) and a second (d2) spacing's are not equal.

6. The MIMO system (101 ,201 ) according to any of previous claims, wherein said at least three spatially separated antenna elements (102, 103,104, 202,203,204) are arranged along a line.

7. The MIMO system (101 ,201 ) according to any of the previous claims, wherein said at least three spatially separated antenna elements (102, 103, 104,202,203,204) are co-polarized antenna elements adapted for co-polarized MIMO communications over said wireless link.

8. The MIMO system (101 ,201 ) according to any of claims 1 -6, wherein said at least three spatially separated antenna elements (102, 103, 104,202,203,204) are dual-polarized antenna elements adapted for dual-polarized MIMO communications over said wireless link.

9. The MIMO system (101 ,201 ) according to any of the previous claims, wherein said wireless link is a wireless line-of-sight (LOS) MIMO link.

10. A method of configuring a multiple-input multiple-output (MIMO) system (300) for wireless communication over a wireless link, comprising the steps of:

- configuring (301 ) at least three spatially separated antenna elements, at least two radio units, and a digital baseband unit for wireless communication over said wireless link;

characterized in

- arranging (302) a switch unit to couple said digital baseband unit to at least two of said at least three spatially separated antenna elements, via at least two radio units, in at least a first and a second configuration.

1 1 . The method (300) according to claim 10, further comprising:

- coupling said switch unit to said at least three spatially separated antenna elements and to said at least two radio units.

12. The method (300) according to any of claims 10-1 1 , further comprising:

- arranging said at least three spatially separated antenna elements, with at least a first (d1 ) and a second (d2) spacing between said at least three spatially separated antenna elements, such that said at least a first (d1 ) and a second (d2) spacing's are not equal.

13. The method (300) according to any of claims 10 -12, further comprising:

- arranging said at least three spatially separated antenna elements along a line.

14. The method according to any of claims 10 - 13, further comprising:

- adapting said at least three spatially separated antenna elements for co-polarized MIMO communications over said wireless link.

15. The method according to any of claims 10 - 13, further comprising:

- adapting said at least three spatially separated antenna elements for dual-polarized MIMO communications over said wireless link.

16. The method according to any of claims 10 - 15, wherein said configuring of said switch unit is based on a hop length and a frequency.

17. A wireless multiple-input multiple-output communication system (500) comprising of at least two nodes (501 ,502) wherein at least one of said at least two nodes is implemented using a reconfigurable multiple-input multiple-output (MIMO) system according to claims 1 -9.