Multi-Goal (S11, OTA, SAR) Optimization of Mobile Phones

Model

In this project, we present a novel approach which allows to perform the effective optimization of entire CAD derived devices, embedded in complex environments. The new optimizer combines the advantages of enhanced Genetic Algorithms with the unique and superior speed of FDTD hardware acceleration. On the basis of a commercial mobile phone, the applied methods are outlined and demonstrated whereas the devices’ antenna is optimized with respect to return loss/matching, radiation performance and SAR under real usage conditions, i.e., including head and hand.

Simulation

A commercial mobile phone's CAD dataset (IGES) consisted of more than 200 distinguished parts. The integrated antenna was subsequently converted into a parameterized form leading to a total of 6 parameters, allowing to simulate and optimize the targeted multi-band performance in different operational situations. The optimization is performed in two steps: In the first step, a suitable and robust antenna design was developed for free-space by using the optimizer. In the second step, this antenna design was the initial solution to optimize S11, OTA performance and peak spatial SAR when the phone was operated next to the SAM head (touch position) including homogeneous hand phantoms

The resulting grid for free space phone optimization contained about 5 million FDTD cells, while the grid for the phone, head and hand simulations contained about 11 million cells. The optimization goal for the first optimization step  is to obtain a dual-band antenna which covers the bands from 890 MHz to 960 MHz and from 1710 MHz to 1880 MHz with a return loss better than  -8dB. SAR and far-field optimization were performed at 1747 MHz, looking for the minimization of the SAR and back radiation of the antenna towards the user’s head.

Results

The optimizations were run using SEMCAD X and hardware-accelerated workstations.  Using this configuration we were able to achieve simulation speeds of more than 450 million cells/s; enabling the resolution of 20 million cells problems in less than 10 minutes. The maximum number of iterations/evaluations was 280 (10 generations), achieving convergence of the problem in less than 3 days.

The optimization process ends if either the maximum number of iterations is reached or the optimization goal is achieved. The return loss of the patch antenna before and after optimization is shown in the last figure. The achieved antenna matching for this configuration is better than −8dB in the two specified bands.

The last step of the optimization procedure enabled us to reduce the initially obtained SAR and farfield back radiation values as well as to obtain better return loss in the multi-goal optimization. SAR optimization was performed at 1747 MHz which finally allowed – while maintaining the optimum return loss - to reduce the initially obtained SAR by more than 10%. The far-field back radiation reached a level better than -9dB respect to the maximum.

Ref. Multi-goal (S11, OTA, SAR) optimization of mobile phones using effective novel genetic algorithms, Microwave Engineering Europe, April 2007