Method and apparatus for open loop transmission in a multiple antenna wireless communication system

1. A method for precoding in an open loop transmission in a wireless multiple antenna radio communications system, wherein the system:

provides for rank adaptation for data transmission with selected transmission rank r being 1≦r≦R, where R is a maximum available transmission rank of the communication system,

communicates resource units (RU) containing resource elements (RE),

provides N available Demodulation Reference Signals (DM-RS) for communication channel estimation in each RU,

provides a number of M REs in each RU for data transmission, and

provides a number of E REs in each RU allocated for DM-RS transmission,

wherein the method comprises:

applying a number, N Used , of DM-RS:s, N Used ≦N, to the E allocated REs;

precoding the data at each of the M REs within each RU with a precoding matrix and the same precoding matrix is used for all of the M REs within each RU;

precoding each of N Used DM-RSs over the E allocated REs within each RU with one column of the precoding matrix which is same as that used for precoding M data REs; and

when precoding, using at least L≧2 different precoding matrices from a set of precoding matrices W to precode the M REs and the E allocated REs in at least one RU for at least one certain rank r transmission such that REs precoded with a first precoding matrix are interleaved with REs precoded with at least one additional, a second different precoding matrix, and wherein the number of used precoding matrices L is adapted to the transmission rank r and number N of available DM-RS.

2. The method according to claim 1 , wherein the number N of available DM-RS depends on the number E of REs allocated for DM-RS:S in an RU and a length of code used for Code Division Multiplexing of DM-RS:S over the E REs.

3. The method according to claim 1 , further comprising:

using N Used ≦N available DM-RS for rank r transmission; and

wherein number L of used precoding matrices for precoding data is a largest integer smaller than N Used /r.

4. The method according to claim 3 , wherein:

E=12 REs are used in one resource unit for DM-RS,

rank is chosen as either of 1 or 2,

a length of CDM code is 4 and there are N=4 CDM DM-RS available, and

number L of used precoding matrices is a largest integer smaller than N/r.

5. The method according to claim 3 , wherein:

E=24 REs are used in one resource unit for DM-RS,

rank is chosen as either of 3 or 4,

a length of CDM code is 4 and there are N=8 CDM DM-RS available, and

number L of used precoding matrices is a largest integer smaller than N/r.

6. The method according to claim 1 , wherein the number of E REs allocated for DM-RS in each RU depends on the selected transmission rank r.

7. The method according to claim 1 , wherein:

the same number N of available DM-RS:s are used for different rank r transmissions, and

a number of used precoding matrices is a largest integer smaller than N/r.

8. The method according to claim 1 , wherein:

the number of available DM-RS for different rank r transmissions is N r ,

same number of L precoding matrices is used for each transmission rank r, and

Number N Used of used DM-RS≦N r available DM-RS for each rank r transmission is N Used =L*r.

9. The method according to claim 8 , wherein:

the number of available DM-RS N r for different rank r transmissions is the same.

10. The method according to claim 1 , wherein:

a number of N Used DM-RS for rank r transmission is equal to r.

11. The method according to claim 10 , wherein:

the L precoding matrices used in one RU are obtained by permuting the columns or rows of one precoding matrix.

12. The method according to claim 1 , wherein:

multiple precoding matrices for only some available different transmission ranks is used in a resource unit.

13. The method according to claim 1 , wherein:

the resource unit is one Resource Block Pair in a Long Term Evolution mobile communications system, and the number of used precoding matrices in a number of consecutive Resource Block Pairs are at least the number of consecutive resource block pairs+1.

14. The method according to claim 1 , further comprising:

using cycling of L≧1 precoding matrices in a resource unit or in consecutive resource units.

15. The method according to claim 14 , further comprising:

performing layer permutation by permuting columns of the L precoding matrices and then use the permuted precoding matrices for precoding.

16. The method according to claim 1 , wherein:

each of N Used DM-RS in a resource unit defines one antenna port, and there are N Used defined antenna ports.

17. The method according to claim 16 , wherein:

each data transmission RE is associated with r of N Used defined antenna ports when the transmission rank is r.

18. The method according to claim 17 , wherein:

each data transmission RE is associated with same antenna ports with different order.

19. The method according to claim 1 , further comprising:

signalling a scheme of used L precoding matrices to a receiver of precoded resource elements in the system.

20. The method according to claim 1 , further comprising:

computing received channel coefficients using non precoded reference signals; and

extracting precoding matrices used for received precoded reference signals using the computed received channel coefficients.

21. A radio transmitter configured to precode in an open loop transmission in a wireless multiple antenna radio communications system, wherein the system:

provides for rank adaptation for data transmission with selected transmission rank r being 1≦r≦R, where R is a maximum available transmission rank of the communication system,

communicates resource units (RU) containing resource elements (RE),

provides N available Demodulation Reference Signals (DM-RS) for communication channel estimation in each RU,

provides a number of M REs in each RU for data transmission, and

provides a number of E REs in each RU allocated for DM-RS transmission, and

wherein the radio transmitter is configured to:

apply a number, N Used , of DM-RS:s, N Used ≦N, to the E allocated REs;

precode the data at each of the M REs within each RU with a precoding matrix and the same precoding matrix is used for all of the M REs within each RU;

precode each of N Used DM-RSs over the E allocated REs within each RU with one column of the precoding matrix which is same as that used for precoding M data REs; and

when precoding, use at least L≧2 different precoding matrices from a set of precoding matrices W to precode the M REs and the E allocated REs in at least one RU for at least one certain rank r transmission such that REs precoded with a first precoding matrix are interleaved with REs precoded with at least one additional, a second different precoding matrix, and wherein the number of used precoding matrices L is adapted to the transmission rank r and number N of available DM-RS.

22. A user equipment adapted to receive a transmission from a radio transmitter in a wireless multiple antenna radio communications system, wherein the system:

provides for rank adaptation for data transmission with selected transmission rank r being 1≦r≦R, where R is a maximum available transmission rank of the communication system,

communicates resource units (RU) containing resource elements (RE),

provides N available Demodulation Reference Signals (DM-RS) for communication channel estimation in each RU,

provides a number of M REs in each RU for data transmission, and

provides a number of E REs in each RU allocated for DM-RS transmission, and

wherein the user equipment is adapted to receive a precoded open loop transmission,

wherein a number, N Used , of DM-RS:s, N Used ≦N, is applied to the E allocated REs;

wherein the data at each of the M REs within each RU is precoded with a precoding matrix and the same precoding matrix is used for all of the M REs within each RU;

wherein each of N Used DM-RSs over the E allocated REs within each RU is precoded with one column of the precoding matrix which is same as that used for precoding M data REs; and

at least L≧2 different precoding matrices from a set of precoding matrices W are used to precode the M REs and the E allocated REs in at least one RU for at least one certain rank r transmission such that REs precoded with a first precoding matrix are interleaved with REs precoded with at least one additional, a second different precoding matrix, and wherein the number of used precoding matrices L is adapted to the transmission rank r and number N of available DM-RS.