Codebook sub-sampling for CSI feedback on PUCCH for 4Tx MIMO

1. A method of channel state information (CSI) feedback, comprising:

generating, by a user equipment, a first report coding a Rank Indicator (RI);

generating, by the user equipment, a second report coding a first precoding matrix indicator (PMI 1 ) associated with the index of a first precoding matrix (W 1 ), a second precoding matrix indicator (PMI 2 ) associated with the index of a second precoding matrix (W 2 ), and a Channel Quality Indicator (CQI), and

wherein coding the PMI 2 employs codebook sub-sampling as follows, for RI=1 and 2:

Relationship between the second PMI

value and W2 index (i 2 )

Value of the

Chosen W2 index

second PMI

for sub-sampling

RI

(I PMI2 )

(i 2 )

1

0-3

2I PMI2 + 4 · └I PMI2 /2┘

2

0-3

I PMI2 + 2 · └I PMI2 /2┘

wherein a CSI feedback signal comprises the first and second report; and transmitting, by the user equipment, the CSI feedback signal.

2. The method of claim 1 , wherein indices of precoding matrices W 1 and W 2 are jointly encoded with CQL.

3. The method of claim 1 , wherein the index of W 1 is jointly encoded with the index of W 2 .

4. The method of claim 1 , wherein indices PMI 1 and PMI 2 are jointly encoded with CQI.

5. The method of claim 1 , wherein PMI 1 is jointly encoded with PMI 2 .

6. The method of claim 1 , wherein the CSI feedback signal is transmitted in a Physical Uplink Control Channel (PUCCH) transmission.

7. The method of claim 1 , wherein coding the PMI 1 employs codebook sub-sampling as follows, for RI=1 and 2:

Relationship between the first PMI

value and W1 index (i 1 )

Value of the first

Chosen W1 index

PMI

for sub-sampling

RI

(I IPMI1 )

(i 1 )

1

0-3

4I PMI1

2

0-3

4I PMI1 .

8. A user equipment (UE), for transmitting channel state information (CSI) feedback, comprising:

circuitry for generating a first report comprising a Rank Indicator (RI);

circuitry for generating a second report comprising a first precoding matrix indicator (PMI 1 ) associated with the index of a first precoding matrix (W 1 ), a second precoding matrix indicator (PMI 2 ) associated with the index of a second precoding matrix (W 2 ), and a Channel Quality Indicator (CQI),

wherein the PMI 2 employs codebook sub-sampling as follows, for RI=1 and 2:

Relationship between the second PMI

value and W2 index (i 2 )

Value of the

Chosen W2 index

second PMI

for sub-sampling

RI

(I PMI2 )

(i 2 )

1

0-3

2I PMI2 + 4 · └I PMI2 /2┘

2

0-3

I PMI2 + 2 · └I PMI2 /2┘;

and

circuitry for transmitting the first and second reports.

9. The UE of claim 8 , wherein indices of precoding matrices W 1 and W 2 are jointly encoded with CQI.

10. The UE of claim 8 , wherein the index of W 1 is jointly encoded with the index of W 2 .

11. The UE of claim 8 , wherein indices PMI 1 and PMI 2 are jointly encoded with CQI.

12. The UE of claim 8 , wherein PMI 1 is jointly encoded with PMI 2 .

13. The UE of claim 8 , wherein the transmitted first and second reports comprise a CSI feedback signal.

14. The UE of claim 13 , wherein the CSI feedback signal is transmitted in a Physical Uplink Control Channel (PUCCH) transmission.

15. The UE of claim 8 , wherein coding the PMI 1 employs codebook sub-sampling as follows, for RI=1 and 2:

Relationship between the first PMI

value and W1 index (i 1 )

Value of the first

Chosen W1 index

PMI

for sub-sampling

RI

(I PMI1 )

(i 1 )

1

0-3

4I PMI1

2

0-3

4I PMI1 .