Electronic Products & Technology

Enhancing wireless performance on every body

By Dr Gareth Conway, CEO & co-founder, AntennaWare   

Electronics Wireless IoT Wearable Technology antenna Editor Pick wearable wireless

Human body among the most difficult platforms for deploying wireless systems

With advances in the front-end physical layer of wireless systems, extra gains of up to 20dB in non-line-of-sight link budgets are demonstrated in comparison to conventional practice. A combined antennas and propagation approach is explored to improve the physical layer performance of wireless body-centric networks. The techniques and understanding from the experimental measurements can be used to solve wireless connectivity problems on broader platforms, such as close to metal for other communication applications.

Introduction

In a world where the population just want ‘wireless that works’ with ubiquitous communication devices that seamlessly connect in any environment on any host platform that devices are positioned on, there are still significant performance gains to be exploited with careful physical layer choices. The human body is often seen as one of the most difficult platforms for the deployment of wireless systems, due to the variability of the properties across different positions on the body, and across the different morphologies of the human population.

BodyWave UWB antennas serve as a BoA solution to the challenges of wireless body blocking and detuning.

Conductive platforms make wireless communication difficult, particularly when the device is mounted close to the structure. Just like metal, the human body appears electrically conductive to electromagnetic waves, and appears more conductive as frequency is increased. A lot can be learned from devices designed to operate in close proximity to the human body, as it is a ‘lossy’ and dynamic platform, that obstructs and drastically limits wireless coverage in the microwave frequency range (300+ MHz). Improvements in the physical layer performance are advantageous for more robust and reliable connectivity, longer communication ranges, lower transmit powers or simply more confidence in design before product deployment.

Wireless systems operating close to the human body

The physical layer of the wireless system, namely the antenna, is the part of the system that interacts with the environment that it is placed in. Placing the antenna close to different materials, will impact the performance of the antenna. It can change the impedance of the antenna and its radiation characteristics. Antennas that are sensitive to being placed next to conductive materials, such as printed antennas, or chip antennas with bespoke printed circuit board (pcb) keep-out areas, often exhibit the greatest performance variation. Nonetheless, the wireless performance of the system then becomes very dependent on the host platform.

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|S21| Path gain measurements around the head at 6.5GHz. Source: Antennaware

For some applications, where the platform is known, and the materials are well defined, the antenna can be optimised specifically for that platform. However, for applications such as the distribution of wireless sensors, where they are expected to consistently perform, wherever they may be deployed, needs a different approach to achieve optimal and reliable communication links. The human body presents a unique challenge for wireless technology, with complexities, that mean the wireless performance for devices in close proximity to tissue are difficult to characterise and ensure robust and reliable operation on every person, in every application use case.

The right approach

The antenna is one of the most vital components of any wireless system. It is responsible for converting an in-circuit guided alternating current produced by a transmitter into a free-space, propagating EM wave. There are a number of metrics which are important when considering an antennas performance, but, for difficult platforms, the top three are: input impedance stability, total radiation efficiency and the correct radiation pattern to meet the propagation needs of the application. To evaluate end-to-end communication link performance, Received Signal Strength Indicator (RSSI) or forward path gain (|S21|), are important metrics to quantify the propagation mode effectiveness. It is advisable to test for worst case body-blocking scenarios, with non-line of sight (NLoS) propagation, and no significant nearby objects for the electromagnetic waves to reflect and diffract off (low multipath environments).

Numerical FDTD simulation showing E-field cross section of
torso with body shadowing.

In this type of environment the received signal strength will be weakest, and performance is marginal at distance. A communication system that performs effectively in NLoS anechoic environments, is not dependant on the proximity of nearby objects for multipath communication. Thus, has more reliable performance, as the platform moves through or is deployed in different environments.

Therefore, to get the best quality of performance from a wearable wireless device it must strive to exhibit the following characteristics on different people and at different antenna-body spacing distances on the same person:

  • Stable Return Loss |S11| or Resonant Frequency.
  • Minimal power absorption into the tissue
  • Predictable or repeatable Radiation Efficiency Reduction
  • Overcome Body Shadowing (Limited Wireless coverage)

And finally, tested and validated for worst-case scenarios, i.e when the wireless received signal weakest and the communication link becomes marginal or drops-out.

NLoS performance enhancement

When all of the above is understood, and the right antennas are used, gains of 10-20dB in worst case NLoS can be achieved compared to conventional printed antennas and chip antenna approaches. The gain can be validated in the forward path gain, measured between the transmitting and receiving wearable antennas. The path gain of the antennas |S21| was measured from ear-ear, for the BodyWave antenna in comparison to a chip and printed monopole UWB antennas. The path gain results in Figure 2 show that even for a short link around the head, that the path loss could be as great at 95dB, when using printed and chip antennas as the antenna under test (AUT).

Conclusion

An antenna that is agnostic to the platform holds the key to unlocking the wireless performance and differentiating product performance. To mitigate the effects of near-field coupling from the antenna to the tissue, and the absorption of energy into the tissue, the antenna must be decoupled from the platform. Printed antennas for pcbs and ceramic chip antennas are not the right approach. They were never designed to be placed close to a difficult platform, thus radiate inefficiently and are subject to varying impedance caused by near-field coupling. By launching the electromagnetic waves to support on-body propagation, in addition to isolating the antenna from the tissue by placing above the pcb ground-plane, significant performance gains of up to 20dB have been demonstrated.

To achieve optimal performance from the wireless link on difficult platforms, and maximise available link budget, it is recommended to start with the antenna radiation characteristics and selection, rather than treating antennas as an afterthought. The step-change improvement in link budget performance by overcoming body blocking could translate to more than doubling the communication distance, significantly reducing the power and battery size, or simply give a more robust and reliable quality connectivity, which is core to wearable audio, sport and healthcare applications.

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Dr Gareth Conway, CEO & co-founder, AntennaWare

AntennaWare is a UK-based developer of commercial antennas designed for wireless wearables – in sectors including IoT, audio, healthcare and sports.

antennaware.co.uk

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