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Virtual solution could spell the end for antennas

The fast pace of evolution of the wireless industry puts familiar time-to-market pressure on the engineering of every new mobile device. Being in the heart of every mobile product, the design of the RF front-end and, in particular, the antenna, becomes especially cumbersome - every product currently requires a fully customised antenna.

This antenna needs to meet many challenges including: new technologies like LTE and MIMO emerging; more antennas required; more frequency bands; constraints of size, weight and power; and the use of more discrete RF front-end components such as matching networks.

Beyond this, antenna design has traditional been constrained by an accepted correlation between physical size and performance. A well-known fundamental principle in antenna design is that an antenna must keep a minimum size relative to the longest operating wavelength to radiate efficiently. Beyond a certain size limit, a further antenna reduction results in a rapidly decreasing bandwidth and efficiency. An antenna enters into the 'small antenna' regime when its overall size is smaller than ? (wavelength)/ p. In a mobile system and considering a longest operating wavelength at a frequency of 824 MHz (the start of the frequency band of the US mobile networks), such a limit is around 120mm, about twice the top edge of a mobile phone where the antenna is usually located.

This means that most modern mobile phone antenna, even those integrated in current large smartphones, operate within the small antenna regime, so to reduce size means overcoming a fundamental wall that has constrained antenna evolution for decades.

Meeting all these needs without slowing down the time to market is clearly a problem, but one that Fractus believes it has solved with its Virtual Antenna technology. The Fractus solution is based on the mXTEND antenna booster that is capable of replacing conventional handset antennas of large dimensions by miniature and off-the-shelf, standard mobile antenna components. The solution can be effectively standardised across multiple handsets sharing the same platform while featuring different form factors.

Typically, mobile antenna solutions are designed in such a way that a single antenna element is intended to provide multiband performance. It means that multiple operating wavelengths must be packed into this single element, thus leading to complex antenna geometries and large dimensions. The Fractus' antenna booster, featuring a size which is typically x10 times smaller than a customised antenna, can replace current technology. The Virtual Antenna technology is based on the excitation of the ground plane inherently present in any handset platform. When using this technology the radiation is mainly provided by this ground plane. By properly selecting the arrangement and form factor of the element with respect to this ground plane, the ground plane radiation can be optimised. This fact allows minimising considerably the size of the element in an order of magnitude with respect to other handset antenna solutions.

Another key enabling technology is the radiofrequency system, i.e. the matching network. Current handset antenna solutions are commonly connected to a Front End Module (FEM) through a single input/output port. This fact increases the matching network and FEM complexity. In particular, more sophisticated matching networks, filtering, and power amplifier stages are required to split and process each frequency band separately, which increases the complexity, losses, and costs of the overall system. Fractus Virtual Antenna technology combines one or more mXTEND antenna boosters with one or more specifically designed matching network to provide multi-port or single port antenna front-end that matches the RF circuitry of the mobile front-end. The matching network must still be customised according to each application, although the FEM can be the same as that used in commercially available handset solutions.

The electromagnetic performance of the solution is tested regarding an evaluation board having the typical dimensions to those associated to current smartphones (120 mm length and 60 mm width). Two mXTEND Antenna Boosters are respectively placed at the corners of a transversal edge of the evaluation board. Each booster is intended to provide operation in a particular frequency region. In this sense, each booster is connected to a matching network specifically designed to cover, on one hand, the low frequency region (824-960MHz) comprising the communication standards (GSM850 and GSM900), and on the other hand, the high frequency region (1710-2690MHz) including the communication standards (GSM1800/DCS, GSM1900/PCS, UMTS, LTE2100, LTE2300, and LTE2500). The solution is capable of providing hepta-band operation through miniature elements occupying a volume of 250 mm3.

The matching network for the low frequency region comprises a series inductor, a broadband mechanism comprising a parallel resonator, and a fine tuning stage formed by a series capacitor. The matching network for the high frequency region consists in a "T" matching network comprising a series inductor, a shunt inductor, and a series capacitor. In this case, the evaluation board further includes two UFL3cables to connect each mXTEND Antenna Booster to each SMA connector, giving a two port solution.

One of the advantages of this two port solution with respect to current single input/output port designs mainly relies on the simplification of the matching network and the FEM. In this solution, no additional matching network or filtering stages are required to merge a two input/output port solution into a single port solution. Accordingly, the number of reactive elements required is minimised, and with them, complexity and ohmic losses. Furthermore, since the two operating regions, low and high frequency region, are not merged into a single input/output port, there is no need to split them with multiplexers. In this way, the number of filtering stages in the FEM is also reduced and consequently, losses and complexity are again minimised.

One of the parameters that antenna engineers will be familiar with is Voltage Standing Wave Ratio (VSWR), which relates to the fraction of power that is actually delivered to the radiating system. Usually, in mobile communication a VSWR equal to or lower than 3 or 4 is required to ensure proper performance.

The performance in terms of VSWR of the Virtual Antenna solution is similar to that provided by other traditional solutions but with the advantage of the reduced size and the standardisation - the same mXTEND antenna booster can be used in different handset platforms, thus the solution avoids the need of integrating customised antenna solutions.

The Virtual Antenna solution completely replaces the conventional antenna. The presence of a ground plane to be boosted is required but as this is inherently present in any handset platform it does not mean any extra elements need to be introduced.

So, for a fully featured, globally used smartphone, Fractus says only two mXTEND antenna boosters are currently required, one for the low frequency region and the other one for the high frequency region. Other configurations can be further used to include the new LTE standards.

Samples and evaluation boards are already available and the mXTEND is being currently tested in some proje

Author
Tim Fryer

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