some suggestion on antenna design inside a metal housing

Could u give some suggestion or guideline on inside antenna design inside a metal housing of a mobile phone? the antenna i design is monopole or PIFA . Do you think which is better for performance ? thans in advance.

PIFA is also a Monopole antenna.
Is derivated from Inverted-F, and Inverted F is derivated from Inverted-L, and Inverted-L is a bent Monopole antenna over a ground plane.
Due to its near-field current distribution PIFA is more suitable for metal housing.

by metal housing you mean the antenna is completed
or parcially sorrounded by metal? This is one of
main recommendation about antennas: does not
put them close or inside metals because will not
work. In most of the cases, however, is inevitable
that the antenna being modified by the near metals.
In that case the whole package is the antenna. Any
dielectric(plastic) or metal will modify the impedance
and resonance frequency of the antenna.