Normally we recommend to have a complete ground plane in the layer below the module. Given the module is placed on layer 1, layer 2 should be a complete ground plane. All the ground pins of the module should have one via to ground layer as close to the pad as possible on customer PCB.
Avoid routing the RF signal through vias if possible. Routing on the same layer as the module give a nice microstrip to the antenna. The width of the routing should give 50 ohm impedance. The width can be calculated with microstrip impedance calculators online. (example: https://www.eeweb.com/toolbox/microstrip-impedance)

For balancing the PCB it might be desirable to also fill top PCB layer with ground. This can be done, but keep two things in mind.
– Do not fill ground under the module. This is not allowed by IPC rules.
– Keep the ground clear of RF routing. Having ground close to RF trace changes impedance and use a coplanar microstrip impedance calculator to evaluate the impedance that should be 50 ohm.

Last Update: August 8, 2017