Radio frame configuration

1. A method comprising:

storing, in a terminal device of a cellular communication system, definitions for a first radio frame configuration defining a frame structure of a radio frame comprising at least one sub-frame dedicated only for downlink transmission;

storing, in the terminal device, definitions for a second radio frame configuration defining a frame structure of a radio frame comprising at least one flexible special sub-frame configurable as either a downlink sub-frame or as an uplink sub-frame, wherein the flexible downlink sub-frame and the flexible uplink sub-frame both comprise an uplink part and a downlink part, and wherein both the uplink part and the downlink part carry at least one of control information and a reference signal;

receiving, from a network element of a cellular communication system, a radio signal carrying an information element indicating a radio frame configuration applied in a cell of the cellular communication system; and

selecting, on the basis of the received information element, a radio frame configuration amongst a set of radio frame configurations comprising at least the first radio frame configuration and the second radio frame configuration and using the selected frame configuration in communication with the network element in the cell.

2. The method of claim 1 , wherein the first radio frame configuration further comprises at least one sub-frame dedicated only for uplink transmission, and at least one special sub-frame comprising an uplink part and a downlink part.

3. The method of claim 1 , wherein the flexible special sub-frame comprises a further uplink part or a further downlink part depending on whether the flexible sub-frame is configured as the flexible uplink sub-frame or the flexible downlink sub-frame.

4. The method of claim 1 , wherein the terminal device stores definitions for a plurality of different radio frame configurations each defining said at least one flexible special sub-frame, wherein at least two of the plurality of different radio frame configurations have a different sub-frame length.

5. The method of claim 4 , wherein a radio frame configuration having a greater sub-frame length has a smaller sub-carrier spacing in sub-frames.

6. The method of claim 1 , further comprising in the terminal device:

receiving flexible special sub-frame and, together with the flexible special sub-frame, an indication of whether the flexible special sub-frame is a flexible special downlink frame or a flexible special uplink frame; and

applying to the received flexible special sub-frame a sub-frame structure mapped to the received indication.

7. An apparatus comprising:

at least one processor, and

at least one memory comprising a computer program code, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to:

provide a first radio frame configuration defining a frame structure of a radio frame comprising at least one sub-frame dedicated only for downlink transmission;

provide a second radio frame configuration defining a frame structure of a radio frame comprising at least one flexible special sub-frame configurable as either a flexible downlink sub-frame or as a flexible uplink sub-frame, wherein the flexible downlink sub-frame and the flexible uplink sub-frame both comprise an uplink part and a downlink part, and wherein both the uplink part and the downlink part carry at least one of control information and a reference signal; and

select, amongst a set of radio frame configurations comprising at least the first and second radio frame configuration, a radio frame configuration and causing transmission of a radio signal carrying an information element indicating the selected radio frame configuration.

8. The apparatus of claim 7 , wherein the first radio frame configuration further comprises at least one sub-frame dedicated only for uplink transmission and at least one special sub-frame comprising an uplink part and a downlink part.

9. The apparatus of claim 7 , wherein the processor, the memory, and the computer program code are configured to cause the apparatus to provide a plurality of different radio frame configurations each defining said at least one flexible special sub-frame, wherein at least two of the plurality of different radio frame configurations have a different sub-frame length.

10. The apparatus of claim 9 , wherein a radio frame configuration having a greater sub-frame length has a smaller sub-carrier spacing in sub-frames.

11. The apparatus of claim 7 , wherein the flexible special sub-frame of the second radio frame configuration has a greater maximum length of an uplink part than a maximum length of an uplink part of the special sub-frame of the first radio frame configuration.

12. The apparatus of claim 7 , wherein the flexible special sub-frame comprises a further uplink part or a further downlink part depending on whether the flexible sub-frame is configured as the flexible uplink sub-frame or the flexible downlink sub-frame.

13. The apparatus of claim 12 , wherein the further uplink part carries uplink data and the further downlink part carries downlink data.

14. An apparatus comprising:

at least one processor, and

at least one memory comprising a computer program code, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to:

store, in the at least one memory, definitions for a first radio frame configuration defining a frame structure of a radio frame comprising at least one sub-frame dedicated only for downlink transmission;

store, in the at least one memory, definitions for a second radio frame configuration defining a frame structure of a radio frame comprising at least one flexible special sub-frame configurable as either a downlink sub-frame or as an uplink sub-frame, wherein the flexible downlink sub-frame and the flexible uplink sub-frame both comprise an uplink part and a downlink part, and wherein both the uplink part and the downlink part carry at least one of control information and a reference signal;

receive, from a network element of a cellular communication system, a message carrying an information element indicating a radio frame configuration applied in a cell of the cellular communication system; and

select, on the basis of the received information element, a radio frame configuration amongst a set of radio frame configurations comprising at least the first radio frame configuration and the second radio frame configuration and using the selected frame configuration in communication with the network element in the cell.

15. The apparatus of claim 14 , wherein the first radio frame configuration further comprises at least one sub-frame dedicated only for uplink transmission, and at least one special sub-frame comprising an uplink part and a downlink part.

16. The apparatus of claim 14 , wherein the flexible special sub-frame comprises a further uplink part or a further downlink part depending on whether the flexible sub-frame is configured as the flexible uplink sub-frame or the flexible downlink sub-frame.

17. The apparatus of claim 16 , wherein the further uplink part carries uplink data and the further downlink part carries downlink data.

18. The apparatus of claim 14 , wherein the processor, the memory, and the computer program code are configured to cause the apparatus to store definitions for a plurality of different radio frame configurations each defining said at least one flexible special sub-frame, wherein at least two of the plurality of different radio frame configurations have a different sub-frame length.

19. The apparatus of claim 18 , wherein a radio frame configuration having a greater sub-frame length has a smaller sub-carrier spacing in sub-frames.

20. The apparatus of claim 14 , wherein the flexible special sub-frame of the second radio frame configuration has a greater maximum length of an uplink part than a maximum length of an uplink part of the special sub-frame of the first radio frame configuration.

21. The apparatus of claim 14 , wherein the processor, the memory, and the computer program code are configured to cause the apparatus to:

receive flexible special sub-frame and, together with the flexible special sub-frame, an indication of whether the flexible special sub-frame is a flexible special downlink frame or a flexible special uplink frame; and

apply to the received flexible special sub-frame a sub-frame structure mapped to the received indication.