The technology disclosed provides the ability for a subframe to be configured as a “flexible” subframe. As a result, at least three different types of subframes in a TDD system may be configured: a downlink (“DL”) subframe, an uplink (“UL”) subframe, and a “flexible” subframe. The use of flexible subframes is determined based on a primary TDD configuration, and in a preferred example, on the existing primary TDD configuration in the network. If there is secondary TDD configuration, flexible subframes may be determined based on both the primary and secondary configurations, e.g., using specific rules. Also, the HARQ feedback timing for downlink (DL) transmissions may be determined based on the secondary TDD configuration. Preferred examples ensure that uplink (UL) feedback does not collide with a flexible subframe used for DL transmission. The technology preferably is compatible with legacy UEs.