1. A method implemented in a base station used in a wireless communications system, the method comprising: (a) receiving, from a user equipment, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2; (b) wherein the codebook index 12 is determined according to a table comprising wherein codebook indices 12 are for physical uplink control channel (PUCCH) mode 2-1 codebook subsampling with 4 antenna ports in LTE (Long Term Evolution).

2. A method implemented in a user equipment used in a wireless communications system, the method comprising: transmitting, to a base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2; wherein the codebook index 12 is determined accordinq to a table comprising: wherein codebook indices 12 are for physical uplink control channel (PUCCH) mode 2-1 codebook subsampling with 4 antenna ports in LTE (Long Term Evolution).

3. A base station used in a wireless communications system, the base station comprising: a receiver to receive, from a user equipment, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI, the first PMI and the second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2; and a selector to select a precoding matrix according to the first PMI and the second PMI, wherein the codebook index 12 is determined according to a table comprising:

4. A user equipment used in a wireless communications system, the user equipment comprising: a transmitter to transmit, to a base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2; wherein the codebook index 12 is determined according to a table comprising: transmitting, from a user equipment to a base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2;

6. A wireless communications system comprising: a base station; and a user equipment to transmit, to the base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2; wherein the codebook index 12 is determined according to a table comprising: wherein codebook indices 12 are for physical uplink control channel (PUCCH) mode 2-1 codebook subsampling with 4 antenna ports in LTE (Long Term Evolution).

We Claim:
1. A method implemented in a base station used in a wireless communications system,
the method comprising:
(a) receiving, from a user equipment, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2;
(b) wherein the codebook index 12 is determined according to a table comprising
wherein codebook indices 12 are for physical uplink control channel (PUCCH) mode 2-1 codebook subsampling with 4 antenna ports in LTE (Long Term Evolution).
2. A method implemented in a user equipment used in a wireless communications
system, the method comprising:
transmitting, to a base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2;
wherein the codebook index 12 is determined accordinq to a table comprising:

wherein codebook indices 12 are for physical uplink control channel (PUCCH) mode 2-1 codebook subsampling with 4 antenna ports in LTE (Long Term Evolution).
3. A base station used in a wireless communications system, the base station
comprising:
a receiver to receive, from a user equipment, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI, the first PMI and the second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2; and
a selector to select a precoding matrix according to the first PMI and the second PMI, wherein the codebook index 12 is determined according to a table comprising:

4. A user equipment used in a wireless communications system, the user equipment
comprising:
a transmitter to transmit, to a base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2;
wherein the codebook index 12 is determined according to a table comprising:

transmitting, from a user equipment to a base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2;
6. A wireless communications system comprising:
a base station; and
a user equipment to transmit, to the base station, for 4 antenna ports rank indication (RI), a first precoding matrix indicator (first PMI), and a second PMI associated with a codebook index 12 , the first PMI and the second PMI being used for reporting information for a preferred precoding matrix from the user equipment to the base station, wherein the codebook has a double codebook structure W1.W2, wherein the first PMI corresponds to Wl and the second PMI corresponds to W2, and wherein the codebook index 12 is used for selecting W2;
wherein the codebook index 12 is determined according to a table comprising:

wherein codebook indices 12 are for physical uplink control channel (PUCCH) mode 2-1 codebook subsampling with 4 antenna ports in LTE (Long Term Evolution).