INTERFEROMETRIC PATTERN

INTRODUCTION AND BACKGROUND

This invention relates to a communication system and method and more particularly to a multiple-input and multiple-output (MIMO) system without relying on multi-path propagation.

MIMO systems rely on a plurality of de-correlated paths between transmit and receive antennas to transmit several data streams on the same frequency, thereby increasing channel capacity. Typically, in free space point to point radio communication systems only two fully de-correlated channels are possible. These are either two orthogonal or opposite circularly polarized transmissions and can be generated and received with no or low correlation between such polarization schemes. This allows for 2x2 MIMO systems in free space, but higher order MIMO like 4x4 or 8x8 are generally believed impossible without reflections within the channel. Hence, higher than 2x2 MIMO is achieved in practice utilizing chance reflections between transmit and receive locations.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide a communication system and method between cooperating transmit and receive antennas which achieves de-correlated channels, without relying or utilizing reflections causing multipathing and with which the applicant believes the aforementioned disadvantages may at least be alleviated or which may provide a useful alternative for the known antennas and methods.

SUMMARY OF THE INVENTION

According to the invention there is provided a communications system comprising:

- a source arrangement of periodic waves, the source arrangement being configured to generate an interferometric wave pattern, the interferometric wave pattern comprising a plurality of lobes each having a main axis diverging from an origin at the source arrangement and a null between any two adjacent lobes;

- at least one sensor arrangement for the waves, the sensor arrangement comprising at least a first sensor and a second sensor which are spaced from one another;

- a first signal source connected to the source arrangement via a phase shift arrangement to generate a first interferometric wave pattern which illuminates the first sensor more than the second sensor; and

- a second signal source connected to the source arrangement via the phase shift arrangement to generate a second interferometric pattern which is off-set from the first interferometric pattern and which illuminates the second sensor more than the first sensor.

The periodic waves may have a centre frequency of f _c which is associated with a wavelength A _c , the source arrangement may comprise at least a first wave source and a second wave source which are spaced a distance di from one another, there may be an angle Θ between any main axis and an adjacent null, the at least one sensor arrangement may be spaced a distance s from the wave source arrangement, the first sensor and the second sensor may be spaced a distance d2 from one another, so that

and so that a line between the spaced sensors subtend the angle Θ at the origin, the first signal source may be connected via the phase shift arrangement to the first and second wave sources, with a first phase difference between them, collectively to generate the first interferometric pattern; and the second signal source may be connected via the phase shift arrangement to the first wave source and to the second wave source, with a second phase difference between them, a difference between the first phase difference and the second phase difference being between 90 degrees and 270 degrees, collectively to generate the second interferometric pattern. Each wave source may comprise a transmitting antenna and each sensor may comprise a receiving antenna.

It is a fundamental property of antennas that the receiving pattern (sensitivity as a function of direction) of an antenna when used for receiving is identical to the far-field radiation pattern of the antenna when used for transmitting. This is a consequence of the reciprocity theorem of electromagnetics. Therefore, when radiation patterns are referred to in this specification, the antenna can be viewed as either transmitting or receiving or both, whichever is more convenient in the context.

Said difference between the first phase difference and the second phase difference may be 180 degrees.

The waves referred to in this specification may be acoustic waves or electromagnetic waves, including radio frequency waves and optical waves. In a preferred embodiment of the invention the frequency f _c of the radio frequency waves is larger than 5 GHz.

In some embodiments, typically point-to-multipoint systems more than at least one sensor arrangement may be provided and typically, k (k=2 n) sensor arrangements may be provided, each a respective distance Sk from the source arrangement and in compliance with the requirement

wherein Sk is the distance between the source arrangement and the k ^th sensor arrangement and d2k is the spacing between the at least two sensors of the k ^th sensor arrangement.

In other embodiments, the source arrangement may comprise at least first and second directional wave sources or antennae mounted in back to back configuration a distance smaller than di apart and which in use launch waves in opposite directions; and deflectors for the waves which deflectors are spaced the distance di apart, to reflect the waves towards the at least one sensor arrangement.

Similarly, at least one of said sensor arrangements may comprise sensors mounted in back to back configuration a distance smaller than d2 apart and associated deflectors for impinging waves which are spaced the distance d2 apart, to reflect the impinging waves towards the sensors.

The first and second wave sources of the source arrangement may be located on a first base line or axis and the source arrangement may further comprise at least third and fourth wave sources spaced on a second axis which is orthogonal to the first axis, to provide 4x4 MIMO decorrelation by having interferometric wave patterns in a first or horizontal orientation as well as interferometric wave patterns in a second or vertical orientation, which provides four (4) decorrelated wave paths. Similarly, the first and second sensors of the at least one sensor arrangement may be located on an axis parallel to the first axis and the sensor arrangement may further comprise at least third and fourth sensors which are spaced on an axis orthogonal to said parallel axis. The system may comprise more wave sources and sensors such that sufficient decorrelation is achieved to support higher order MIMO communication by virtue of the spacing between sensors, spacing between wave sources and link distance s between transmit source or transmit arrangements and sensor or receiver arrangements.

Hence, the invention further extends to a system as defined above comprising a plurality of source arrangements on the first axis each having a different spacing between its respective source elements. According to another aspect of the invention there is provided a method of communication comprising:

- utilizing a communications system comprising: a source arrangement configured to generate an interferometric wave pattern, the interferometric wave pattern comprising a plurality of lobes each having a main axis diverging from an origin at the source arrangement, a null between any two adjacent; and at least one sensor arrangement for the waves, the sensor arrangement comprising at least a first sensor and a second sensor;

- driving the source arrangement with a first signal source to generate a first interferometric wave pattern which illuminates the first sensor more than the second sensor; and

- driving the source arrangement with a second signal source to generate a second interferometric pattern which is off-set from the first interferometric pattern and which illuminates the second sensor more than the first sensor.

The periodic waves may have a centre frequency f _c which is associated with a wavelength A _c , the source arrangement may comprise at least a first wave source and a second wave source which are spaced a distance di from one another, there is an angle Θ between any main axis and an adjacent null, the sensors may be spaced a distance d2 from one another, so that

and so that a line between the spaced sensors subtend the angle Θ at the origin; the first signal source may be connected via a phase shift arrangement to the first and second wave sources, with a first phase difference between them, collectively to generate the first interferometric wave pattern; and the second signal source may be connected via the phase shit arrangement to the first wave source and to the second wave source, with a second phase difference between them, a difference between the first phase difference and the second phase difference being between 90 degrees and 270 degrees, collectively to generate the second interferometric pattern.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein:

figure 1 is a diagrammatic representation of an example embodiment of a communication system comprising a source arrangement and a sensor arrangement;

figure 2 is a diagrammatic representation of another example embodiment comprising a plurality of differently spaced sensor arrangements;

figure 3 is a diagrammatic representation of the configuration of figure 1 when used in reciprocal fashion, that is with the source arrangement acting as sensor arrangement and vice versa; and figure 4 is a diagrammatic representation of another example embodiment of a communication system utilizing closely spaced antennae and associated deflectors to achieve a required distance between spaced wave sources and a required distance between sensors.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An example embodiment of a communication system is generally designated by the reference numeral 10 in figure 1 .

The system comprises a source arrangement 12 of periodic waves having a centre frequency f _c , which is associated with a wavelength A _c . The source arrangement comprises at least a first wave source 14 and a second wave source 16 which are spaced a distance di from one another, to generate an interferometric wave pattern 18. The interferometric wave pattern 18 comprises a plurality of lobes 18.1 to 18.13. Each lobe has a respective main axis 20 and the main axes diverge from an origin 22 at the source arrangement. There is a null 24 between any two adjacent lobes and an angle Θ between any main axis and an adjacent null. The system further comprises a sensor arrangement 26 for the waves which sensor arrangement 26 is spaced a distance s from the wave source arrangement 12. The sensor arrangement comprises at least a first sensor 28 and a second sensor 30 which are spaced a distance d2 from one another so that

and so that an imaginary line between the spaced sensors 28,30 subtends the angle Θ at the origin 22. A first signal source 40 is connected via a phase shift arrangement 41 to the first and second wave sources 14, 16, with a first phase difference between them, collectively to generate a first interferometric wave pattern 18 (shown in solid lines in figure 1 ) which illuminates the first sensor 28 and not the second sensor 30. A second signal source 42 is connected via phase shift arrangement 41 to the first wave source 14 and to the second wave source 42 with a second phase difference between them, collectively and concurrently with the first interferometric pattern, to generate a second interferometric pattern 46 (shown in broken lines in figure 1 ) which is angularly off-set from the first interferometric pattern 18 and which illuminates the second sensor 30 and not the first sensor 28. A difference between the first phase difference and the second phase difference is between 90 degrees and 270 degrees.

The signal from the first signal source 40 is split into first and second paths. At least the first path may comprise a phase shift element (not shown) of the phase shift arrangement 41 . However, in the example embodiment, the first path is connected to the first wave source 14 and the second path to the second wave source 16, so that the first phase difference is zero degrees. The first and second wave sources collectively transmit the signal to generate the first interferometric wave pattern 18, which ideally directs a lobe peak to the first sensor 28 and a null to the second sensor 30, but otherwise such that the first interferometric pattern 18 maximises the signal difference between the first sensor 28 and the second sensor 30, such that at the first sensor 28, the signal is at least 6dB, preferably at least 10dB, larger than at second sensor 30.

The signal from the second signal source 42 is also split into first and second paths. The first path is combined with the first path of the first transmit signal and connected to the first wave source 16. The second path comprises a 180 degrees phase shift element 44 of phase shift arrangement 41 , so that the second phase difference is 180 degrees. Hence, in the example embodiment, the difference between the first phase difference and the second phase difference is 180 degrees. The first and second wave sources collectively transmit the signal from the second source 42 to generate the second interferometric wave pattern 46 which ideally directs a lobe peak to the second sensor 30 and a null to the first sensor 28, but otherwise such that the interferometric pattern 46 maximises the signal difference between the second sensor 30 and the first sensor 28, such that at the second sensor 30, the signal is at least 6dB, preferably at least 10dB, larger than at the first sensor 28. Hence, MIMO signal 1 at output 40 and MIMO signal 2 at output 42 are transmitted simultaneously via the system 10 and MIMO signal 1 is received at antenna 28 and not at antenna 30 whereas MIMO signal 2 is received at antenna 30 and not at antenna 28. Hence, at the sensor arrangement 26 the MIMO signals are de-correlated and recoverable separately by electronic circuitry (not shown) which is connected to the antennas 28,30 at the sensor arrangement 26.

As shown in figure 1 , in one preferred embodiment, the first signal source 40 and the second signal source 42 are the two outputs of a two by two

(2x2) multiple input multiple output (MIMO) transmitter and the two sensors 28, 30 may be connected to the inputs of a 2x2 MIMO receiver (not shown). The MIMO transmit and receive system is already equipped with integral signal splitting as well as phase shifting and path combination mechanisms with an associated MIMO algorithm, to adjust phasing between the transmit signal sources 40,42 and the wave sources 14, 16, to maximise the difference between the signal at the second sensor 30 versus the signal at the first sensor 28 for one signal source and vice versa for the second signal source. By utilizing the above wave source and sensor configuration dimensions the MIMO algorithm is able to configure phase shift arrangement 41 and the phase relationship as stated above, to achieve suitable decorrelation between the two signal sources, which is required for effective MIMO operation. In figure 2 is shown a similar system, but with k sensor arrangements each spaced a distance Sk (k=1 , 2 and 3) from the source arrangement 12 and in compliance with the requirement

wherein Sk is the distance between the source arrangement and the k ^th sensor arrangement and d2k is the spacing between the at least two sensors of the k ^th sensor arrangement. Hence, d22 is the distance between the two sensors of the 2 ^nd sensor arrangement which is spaced S2 from the source arrangement 12 and d23 is the distance between the two sensors at the 3 ^rd sensor arrangement which is spaced S3 from the source arrangement 12. Also at the 2 ^nd and 3 ^rd sensor arrangements the MIMO signals are de-correlated and recoverable separately by electronic circuitry (not shown) which is connected to the antennas at these sensor arrangements.

In figure 3, a reciprocal configuration is illustrated. In this configuration, the spaced antennas 28 and 30 which are spaced a closer distance di than the antennas 14 and 16 which are spaced a distance d2, are used as transmitting antennas and the antennas 14 and 16 are used as receiving antennas. It is well known that with closer spaced antennas, the lobes 50.1 to 50.3 of the interferometric pattern are broader and the diverging main axes are spaced at a bigger angle. Hence, the angle Θ2 between a main axis and an adjacent null is also larger than in the configuration of figure 1 . As shown in figure 3, MIMO signals transmitted from arrangement 26 are also de-correlated at arrangement 12, so that they may be recovered separately by electronic circuitry (not shown) which is connected to the antennas at arrangement 12.

In figure 4, yet another example embodiment of the system is shown and designated 100. The system 100 comprises a source arrangement 1 12 comprising at least a first directional wave source such as a directional antenna 102, for example, and a second directional wave source or antenna 104 mounted in back to back configuration a distance smaller than di apart and which in use launch waves in opposite directions as shown at 106 and 108. Deflectors 1 14 and 1 16 for the waves are provided and spaced the distance di apart, to reflect the waves towards the at least one sensor arrangement 126. Similarly, the at least one sensor arrangement 126 may comprise sensors 101 and 103 mounted in back to back configuration a distance apart smaller than d2. Respective associated deflectors 128 and 130, which are spaced the distance d2 apart, are provided to reflect impinging waves from the source arrangement 1 12 towards the antennae 101 and 103 respectively. It will be appreciated that there are many variations in detail on the system and method as herein defined and/or described without departing from the scope and spirit of the invention.

For example, phasing of a third MIMO signal at the source arrangement may be applied such that interferometer beams are in addition generated between the two sensor elements and autocorrelation between the two sensor elements may be used to increase the decorrelation between such a central beam (which will be approximately equally received by both sensor elements) from the above streams purely pointing at one or the other sensor element, thus creating an additional MIMO stream. Such decorrelation may be achieved by using a third and central sensing element and removing the residual signal received by this sensor element intended for the other two sensor elements, hence negating this "interference" and increasing the decorrelation of this central beam.

Further, there may be provided a central node comprising a plurality of source arrangements on the first axis each having a different spacing between its respective source elements and an adaptive controller which energizes the central node allows such as to achieve multiple streams to remote outer nodes with sensor elements spaced at unknown distance d2 and/or unknown link distances s to the outer nodes.