Reference signaling scheme using compressed feedforward codebooks for multi-user, multiple-input multiple-output (MU-MIMO) systems
A multi-user multiple-input multiple-output (MIMO) downlink beamforming system with limited feed forward is provided to enable precoding matrix information to be efficiently provided to a subset of user equipment devices, where zero-forcing transmit beamformers are computed at the base station and assembled into a precoding matrix. The precoding matrix is encoded using a compact reference signal codebook for forward link signaling, either by sending bits indicating the index of the transmission matrix used, or by transmitting one or more precoded pilots or reference signals wherein the pilot signals are precoded using vectors uniquely representative of the transmission matrix used which includes candidate reference signal matrices which meet a predetermined condition number requirement, such as a condition number threshold. The precoding matrix information is extracted at the user equipment devices using the compact reference signal codebook and used by the MMSE receiver to generate receive beamformers.
1. A base station configured for use in a multi-user, multiple-input multiple-output (MIMO) system, comprising:
a wireless transceiver;
a processor;
a non-transitory computer readable apparatus having a storage medium comprising at least one computer program stored thereon, the at least one computer program, when executed on the processor, causes the processor to:
receive channel state estimate information from a plurality of mobile stations;
generate transmit beamforming vectors;
assemble the transmit beamforming vectors into a reference signal matrix;
select an index from a compressed reference signal codebook corresponding to the reference signal matrix, where the compressed reference signal codebook comprises a plurality of candidate reference signal matrices, each of which is associated with a predefined performance metric and meets a predetermined threshold for said metric; and
feed forward the index that is representative of the assembled reference signal matrix to at least one of the plurality of mobile stations for use in generating receive beamforming vectors at the at least one of the plurality of mobile stations.
2. The base station of claim 1 , wherein the at least one computer program causing the processor to generate transmit beamforming vectors comprises causing the processor to use a zero-forcing beamformer to output transmit beamforming vectors.
3. The base station of claim 1 , wherein the predefined performance metric for each of the plurality of candidate reference signal matrices is a condition number that is lower than a predetermined threshold value.
4. The base station of claim 1 , wherein the at least one computer program further causes the processor to use the transmit beamforming vectors at the base station as weighting vectors for signals transmitted by the base station to the at least one of the plurality of mobile stations.
5. The base station of claim 3 , wherein the predetermined threshold value is a fixed threshold value.
6. The base station of claim 3 , wherein the predetermined threshold value is a variable threshold value.
7. The base station of claim 3 , wherein the at least one computer program further causes the processor to dynamically adjust the predetermined threshold value.
8. A base station configured for use in a multi-user, multiple-input multiple-output (MIMO) system, comprising:
a wireless transceiver;
logic configured to receive channel state estimate information from a plurality of mobile stations;
logic configured to generate transmit beamforming vectors;
logic configured to assemble the transmit beamforming vectors into a reference signal matrix;
logic configured to select an index from a compressed reference signal codebook corresponding to the reference signal matrix, where the compressed reference signal codebook comprises a plurality of candidate reference signal matrices, each of which is associated with a predefined performance metric and meets a predetermined threshold for said metric; and
logic configured to feed forward the index that is representative of the assembled reference signal matrix to at least one of the plurality of mobile stations for use in generating receive beamforming vectors at the at least one of the plurality of mobile stations.
9. The base station of claim 8 , wherein the logic configured to generate transmit beamforming vectors comprises logic configured to use a zero-forcing beamformer to output transmit beamforming vectors.
10. The base station of claim 8 , wherein the predefined performance metric for each of the plurality of candidate reference signal matrices is a condition number that is lower than a predetermined threshold value.
11. The base station of claim 8 , further comprising logic configured to use the transmit beamforming vectors at the base station as weighting vectors for signals transmitted by the base station to the at least one of the plurality of mobile stations.
12. The base station of claim 10 , wherein the predetermined threshold value is a fixed threshold value.
13. The base station of claim 10 , wherein the predetermined threshold value is a variable threshold value.
14. The base station of claim 10 , further comprising logic configured to dynamically adjust the predetermined threshold value.