Technical Field of the Invention

This invention relates to the field of multiple input multiple output (MIMO) antennas.

Background to the Invention

Antennas are used for transmitting and receiving signals in various wireless applications. For instance antennas are widely used for communications, search and rescue, security and other military applications. Antennas are not only standalone devices, but can also be integrated into many different types of products ranging from antennas integrated as body wearable devices, antennas integrated into handsets / mobile personal digital assistants (PDAs) and vehicle / platform mounted antennas with associated systems. These different applications will have their own performance requirements that include, but are not limited to, weight, compactness, ergonomics, ruggedness and power consumption.

There is an ongoing demand for antenna improvements, particularly in relation to antenna efficiency, compactness and operating bandwidth. A developing approach to increasing antenna bandwidth is to provide more than one 'channel' or 'port' for transmitting/receiving signals on a single antenna device. The availability of a plurality of channels in an antenna is often referred to as multiple input multiple output (MIMO). MIMO antennas, whilst providing increased operating bandwidth, also have utility in improving channel efficiency by improving signal to noise ratio, mitigating multipath effects. This is in part due to an ability to acquire channel state information prior to transmitting, but also by providing a means for receiving data across multiple channels and enacting antenna diversity techniques to minimise correlation between the channels and enhance signal reception.

Spatially diverse MIMO antennas comprise multiple radiating structures spatially separated. Correlation between these radiating structures in principle increases the closer the radiating structures are placed to each other. This makes the manufacture of spatially diverse MIMO antennas for compact devices particularly challenging.

Therefore it is an aim of the present invention to provide a MIMO antenna that mitigates this issue.

Summary of the Invention

According to a first aspect of the invention there is provided a MIMO antenna comprising first and second spatially separated planar inverted-F antenna (PIFA) elements arranged parallel each other in an antenna plane to radiate in substantially the same direction, the first and second PIFA elements comprising respective first and second antenna feeds, the antenna feeds being arranged in respective first and second feed planes, wherein the first feed plane is perpendicular to the second feed plane.

An antenna is suitable for transmitting or receiving signals using electromagnetic radiation which may be at radio frequencies. The MIMO antenna is intended to be a compact antenna package (for instance an antenna configuration in a mobile phone) comprising a plurality of antenna elements operating collaboratively. The term 'spatially separated' is intended to mean the first and second PIFA elements are physically separated from each other (they are distinct elements), albeit within an overall compact antenna package. The spatial separation may be of the order of the wavelength at which the antenna elements operate. The spatial separation of the antenna elements is typically no more than half of the wavelength at which the antenna elements operate. Spatial separation of the antenna elements provides the technical advantage of spatial diversity. Spatial diversity is an antenna diversity technique that uses multiple antennas physically separated from each other to provide the separate antenna channels of an antenna system.

There are an increasing number of physical obstacles to line of sight transmission in the modern world. These obstacles cause varying amounts of scattering/reflection of signals and diffraction effects. The net result can be multipath effects i.e. a single transmitted signal being received by an antenna over multiple paths and combined, constructively or destructively. Spatial diversity techniques are advantageous in mitigating these multipath effects because each antenna will receive a signal that has independent phase characteristics owing to the fact that each antenna element or port is sufficiently uncorrelated from its neighbour. Therefore where one 'channel' suffers from multipath interference, a second channel (or antenna) will still be usable.

The PIFA elements are arranged in an antenna plane (the antenna plane being a geometrical plane). The PIFA elements may be arranged inside an antenna housing (for instance on a surface under a protective radome). The PIFA elements are suitable for arranging proximal to each other (for instance within a mobile phone case) as an antenna package. The arrangement of the elements is such that they radiate in substantially the same direction.

Planar antennas can be made directional through use of a relatively large ground plate. However an effect of using such a ground plate is to make the planar antenna itself narrowband. The inventor has shown that a PIFA can be configured to provide a wideband frequency response. A PIFA generally comprises a radiating top plate and a ground plate connected by a feed and a shorting pin. The inventor has shown in GB2539327 that by precisely configuring a PIFA, a wideband directional antenna element can be manufactured. In particular, configuring the radiating top plate of PIFA to be triangular in shape allows a constant topology to be scaled for different frequencies of operation. A triangular top plate enables a compact design, but with an increase in electrical length of the top plate, which is particularly beneficial in the VHF band of operation where a conventional rectangular top plate would be prohibited from operating at certain frequencies in applications requiring compact antenna design (such as body wearable or mounted applications). PIFA antennas may be used to operate across continuous frequency ranges or particular frequency bands.

The inventor has shown that a beneficial reduction in coupling between the PIFA elements can be achieved by optimising the relative orientation of their respective feeds. A feed of a PIFA connects the radiating top plate of the PIFA with the ground plate. The feed of the PIFA is arranged in a feed plane (a geometrical plane) perpendicular to the radiating top plate. By ensuring that for two PIFA elements, their respective feed planes (and therefore respective feeds) are arranged to be angled to each other (for instance both orientated vertically, but also perpendicular to each other), coupling between the PIFA elements as a result of their spatial proximity, can be reduced. Therefore two PIFA elements in a single antenna package can be modified to form a compact MIMO antenna. This reduction in coupling has been shown to be optimal when the antenna feeds are arranged to be in feed planes that are perpendicular to each other.

The PIFA elements are arranged in the antenna plane to both point in the same direction (with respect to their boresight) but also to be parallel with respect to their overall physical rotation within the antenna plane. This provides an overall antenna structure with closely spaced but similarly orientated antennas that radiate in substantially the same direction.

In some embodiments of the invention the first PIFA and second PIFA elements further comprise respective first and second parasitic radiators, the first parasitic radiator being arranged in the first feed plane, the second parasitic radiator being arranged in the second feed plane. The impedance bandwidth of an antenna can be increased through use of one or more parasitic radiators to provide an antenna with wideband operation. The parasitic radiators on the first and second PIFAs are mounted on and connected to the respective ground plates, and are configured with pre-determined height and width. The inventor has provided -through use of carefully positioned parasitic radiators- a MIMO antenna that provides wideband frequency operation. The inventor has shown that further restricting the arrangement of the first and second parasitic radiators to be in the first and second feed planes respectively, the performance of the parasitic radiators is optimised. A parasitic radiator being 'in the feed plane' means the parasitic radiator mounting position and orientation is in/extends along the feed plane. Further parasitic radiators (beyond one per PIFA) may be used. The inventor has shown that in some embodiments, one PIFA element may comprise one parasitic radiator, and a second PIFA element may comprise two parasitic radiators, for instance.

In preferred embodiments of the invention the MIMO antenna operates at frequencies between 1800MHz and 6000MHz. In other embodiments the MIMO antenna operates at frequencies between 800MHz and 2500MHz. Precise configuration of the PIFA antenna elements and/or parasitic radiators enables directional elements to be manufactured that are also wideband. For instance a single parasitic radiator on each antenna element within a MIMO antenna can achieve a wideband performance across the 1800MHz to 6000MHz band. Alternatively providing a single parasitic radiator on a first antenna element, but two parasitic radiators on a second antenna element, within a MIMO antenna, can deliver the wideband performance over 800MHz to 2500MHz.

In preferred embodiments of the invention the first and second PIFA elements are housed within a protective radome. A ruggedized antenna may be required in certain applications. The protective radome may be formed from a hardened polymer such as plastic. The radome is intended to protect the antenna from abrasion or impact, but is also transparent to the electromagnetic radiation with which the antenna operates. Mounting tabs may be provided on for instance the ground plate of the first and second PIFA elements in some embodiments, to enable mounting inside the radome.

The first and second PIFA elements are intended primarily to comprise rigid metal sheet material. However, in some embodiments, the antenna elements may be formed from flexible materials such as metal impregnated textiles (for instance if being incorporated into a body wearable garment).

According to a second aspect of the invention, there is provided a vehicle comprising the MIMO antenna of the first aspect of the invention. The space available for mounting compact antennas to vehicles is limited, and as such a compact antenna such as the MIMO antenna of the invention - that offers space saving benefits owing to the ability to position antenna elements closer together with reduced coupling - are attractive. The MIMO antenna may be mounted on a side of a vehicle on an exterior surface, or may be concealed behind vehicle bodywork.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure la shows a three dimensional illustration of an embodiment of a MIMO antenna according to the first aspect of the invention; and

Figure lb shows a plan view of the MIMO antenna in Figure la.

Detailed Description

Figure la shows a three dimensional illustration of an embodiment of a MIMO antenna (10) according to the first aspect of the invention. The antenna (10) comprising a first PIFA element (11) and a second PIFA element (12) mounted onto a platform (13) such that they are parallel each other and in the same antenna plane and such that they are spatially separated. Antenna elements (11, 12) radiate in the same direction, away from the platform (13). Each antenna element (11, 12) comprises a ground plate (15a, 15b), radiating top plate (14a, 14b), feed (16a, 16b), shorting pin (17a, 17b), and parasitic radiator (18a, 18b). The configuration of the feeds (16a, 16b) for the antenna elements (11, 12) is such that they are perpendicular to each other in order to reduce coupling between antennas (11, 12).

Figure lb shows a plan view (19) of the MIMO antenna in Figure la. Shown in the figure is platform (13) and antenna elements (11, 12) with their respective radiating top plates (14a, 14b), ground plates (15a, 15b), feeds (16a, 16b) and parasitic radiators (18a, 18b). The figure is illustrative in that top plates (14a, 14b) appear transparent in order to highlight the inventive configuration of the feeds (16a, 16b) and radiators (18a, 18b). With reference to first PIFA element (11), feed plane Ά' is labelled. Both feed (16a) and parasitic radiator (18a) for the first antenna element (11) are arranged to be in the feed plane 'A'. The inventor has shown this provides optimum performance of parasitic radiators. With reference to second PIFA element (12), feed plane 'B' is labelled, and feed (16b) and radiator (18b) are shown aligned into plane 'B'. Feed planes Ά' and 'B' are perpendicular to each other, thereby providing the antenna elements (11, 12) with reduced mutual coupling.

Greater than one parasitic radiator may be present with each antenna, and other performance enhancing features such as non-rectangular PIFA top plates, slots cut in the PIFA top plates, and PIFA ground plane structures may be present.