IWR1443BOOST: using a lens/concentrator to improve performance of vital sign monitoring

I'm using IWR1443 to monitor vital signs like breath rates and heart rates. I notice that in the developers guide of vital sign monitoring there is an optional choice of using a lens to get better performance. "A lens/concentrator to direct the radar waves towards the chest (Optional, for improved performance)". 

we'd like to maintain the accuracy of breath rates and heart rates while increasing the distance to about 1m. So, the thought of lens can be really helpful.

But how can make it focus on the chest? like, what kind of lens do we need? what's its focal length? where should the lens placed? A guide book will be better.

Thanks in advance,

yuna