Insertion of downlink demodulation reference signals into OFDM frames

1. A method of inserting downlink demodulation Reference Signals into Resource Blocks of slots within Orthogonal Frequency Division Multiplexing (OFDM) frames at an OFDM transmitter forming part of an OFDM system, the transmitter having at least one antenna and the system having at least one transmission layer, the method including:

for each antenna, inserting cell-specific Reference Signals at sub-carriers indices evenly spaced by a fixed sub-carrier spacing; and

selectively inserting UE-specific Reference Signals into nodes of a rectangular lattice formed from unused Resource Elements in a group of Resource Blocks of two consecutive slots, the nodes being located at the same sub-carrier indices as the cell-specific Reference Signals and further being equally spaced by a fixed number of symbol indices within the group of Resource Blocks,

wherein for systems using 5 transmission layers, the selectively inserting UE-specific Reference Signals comprises inserting 4 UE-specific Reference Signals for three of the layers and 2 UE-specific Reference Signals for the other two layers.

2. A method according to claim 1 , wherein the UE-specific Reference Signals are selectively inserted by:

spacing UE-specific Reference Signals for a same layer and a same symbol index by twice the fixed sub-carrier spacing; and

spacing UE-specific Reference Signals for a same layer and a same sub-carrier by twice the fixed symbol spacing.

3. A method according to claim 1 , wherein the fixed symbol spacing equals 3 symbols.

4. A method according to claim 1 , wherein the fixed sub-carrier spacing equals 3 sub-carriers.

5. A method according to claim 1 , wherein, for each layer, the UE-specific Reference Signals are inserted at 2 symbol indices.

6. A method according to claim 1 , wherein the UE-specific Reference Signals are selectively inserted so as to maximize the spacing from each other in the lattice.

7. A method according to claim 1 , wherein the slots are configured to include a normal Cyclic Prefix.

8. A method according to claim 1 , wherein the slots are configured to include an extended Cyclic Prefix.

9. A method of inserting downlink demodulation Reference Signals into Resource Blocks of slots within Orthogonal Frequency Division Multiplexing (OFDM) frames at an OFDM transmitter forming part of an OFDM system, the transmitter having at least one antenna and the system having at least one transmission layer, the method including:

for each antenna, inserting cell-specific Reference Signals at sub-carriers indices evenly spaced by a fixed sub-carrier spacing; and

selectively inserting UE-specific Reference Signals into nodes of a rectangular lattice formed from unused Resource Elements in a group of Resource Blocks of two consecutive slots, the nodes being located at the same sub-carrier indices as the cell-specific Reference Signals and further being equally spaced by a fixed number of symbol indices within the group of Resource Blocks,

wherein for systems using 6 transmission layers, the selectively inserting UE-specific Reference Signals comprises inserting 4 UE-specific Reference Signals for two of the layers and 2 UE-specific Reference Signals for the other four layers.

10. A method according to claim 9 , wherein the UE-specific Reference Signals are selectively inserted by:

spacing UE-specific Reference Signals for a same layer and a same symbol index by twice the fixed sub-carrier spacing; and

spacing UE-specific Reference Signals for a same layer and a same sub-carrier by twice the fixed symbol spacing.

11. A method according to claim 9 , wherein the fixed symbol spacing equals 3 symbols.

12. A method according to claim 9 , wherein the fixed sub-carrier spacing equals 3 sub-carriers.

13. A method according to claim 9 , wherein, for each layer, the UE-specific Reference Signals are inserted at 2 symbol indices.

14. A method according to claim 9 , wherein the UE-specific Reference Signals are selectively inserted so as to maximize the spacing from each other in the lattice.

15. A method according to claim 9 , wherein the slots are configured to include a normal Cyclic Prefix.

16. A method according to claim 9 , wherein the slots are configured to include an extended Cyclic Prefix.

17. A method of inserting downlink demodulation Reference Signals into Resource Blocks of slots within Orthogonal Frequency Division Multiplexing (OFDM) frames at an OFDM transmitter forming part of an OFDM system, the transmitter having at least one antenna and the system having at least one transmission layer, the method including:

for each antenna, inserting cell-specific Reference Signals at sub-carriers indices evenly spaced by a fixed sub-carrier spacing; and

selectively inserting UE-specific Reference Signals into nodes of a rectangular lattice formed from unused Resource Elements in a group of Resource Blocks of two consecutive slots, the nodes being located at the same sub-carrier indices as the cell-specific Reference Signals and further being equally spaced by a fixed number of symbol indices within the group of Resource Blocks,

wherein for systems using 7 transmission layers, the selectively inserting UE-specific Reference Signals comprises inserting 4 UE-specific Reference Signals for one of the layers and 2 UE-specific Reference Signals for the other six layers.

18. A method according to claim 17 , wherein the UE-specific Reference Signals are selectively inserted by:

spacing UE-specific Reference Signals for a same layer and a same symbol index by twice the fixed sub-carrier spacing; and

spacing UE-specific Reference Signals for a same layer and a same sub-carrier by twice the fixed symbol spacing.

19. A method according to claim 17 , wherein the fixed symbol spacing equals 3 symbols.

20. A method according to claim 17 , wherein the fixed sub-carrier spacing equals 3 sub-carriers.

21. A method according to claim 17 , wherein, for each layer, the UE-specific Reference Signals are inserted at 2 symbol indices.

22. A method according to claim 17 , wherein the UE-specific Reference Signals are selectively inserted so as to maximize the spacing from each other in the lattice.

23. A method according to claim 17 , wherein the slots are configured to include a normal Cyclic Prefix.

24. A method according to claim 17 , wherein the slots are configured to include an extended Cyclic Prefix.