To connect more than 16 GTS (802.15.3) enabled devices with single PAN-Coordinator

Hi,

I need to use GTS slots. I am bit confused with the 802.15.4 standards. Can I use GTS with more than 16 devices.
1) In super frame it is mentioned as total number of slots as 16. Is it possible to increase.
2) As per ?GTS starting slot? part of ?GTS descriptor? of beacon, it is 4 bit in length. So whether only 16 slots and there by 16 devices are supported in GTS.
3) Beacon says 7 devices. So whether only 7 GTS devices are allowed.

Please let me know whether it is possible if I want to have more than 16 devices enabled with GTS.

Thanks & Regards
Vishnu Beema