A method is disclosed for PHICH resource allocation in a wireless communication system that supports flexible TDD UL-DL configuration and in which different TDD UL-DL configurations are available and the TDD UL-DL configuration used by first UEs which operate according to the flexible TDD configuration can be different to the TDD UL-DL configuration used by second UEs which operate according to a long term TDD UL-DL configuration. The method comprising allocating a PHICH resource for both first UEs and second UEs according to LTE timing rules applicable to the TDD UL-DL configuration in use by second UEs.