Multistage beamforming of multiple-antenna communication system

1. A multistage beamforming circuit in a wireless communication network, the circuit comprising:

a data unit configured to:

implement a frequency domain beamforming stage by converting K received data streams into M precoding output streams in a frequency-domain, and

transform the M output streams to M orthogonal frequency-division multiplexing (OFDM) time domain signals;

the data unit comprising:

a frequency domain precoding module configured to receive and precode the K received data streams into the M precoding output streams by applying frequency domain precoding matrices:

M Inverse Fast Fourier Transform (IFFT) processing blocks for processing the M precoding output streams to yield the M OFDM time-domain signals, each IFFT processing block configured to:

receive a frequency-domain signal,

map the frequency-domain signal to resource elements in a frequency domain,

transform the frequency-domain signal to a stream of time domain samples, and

add a cyclical prefix to the stream of time domain samples yielding an OFDM time-domain signal; and

a remote radio head configured to implement a time-domain broadband beamforming stage by converting the M OFDM time-domain signals into N transmit streams of time-domain samples based on beamforming control signals received from the data unit, the beamforming control signals indicating a wideband precoding matrix or a set of indicators to construct the wideband precoding matrix, the remote radio head comprising a transmit antenna array configured to transmit the N transmit streams that together form broadcast beams and user-specific beams, the antenna array including a plurality of physical antennas,

wherein the number N of transmit streams is greater than the number M of precoding output streams, and

wherein N. M. and K are positive integers.

2. The multistage beamforming circuit of claim 1 , wherein the data unit further comprises:

N CRS CELL - SPECIFIC REFERENCE SIGNAL (CRS) PORTS CONFIGURED TO RECEIVE COMMON CONTROL SIGNALS AS FREQUENCY DOMAIN SIGNALS; AND

N CRS IFFT processing blocks for processing the common control signals to yield N CRS OFDM time-domain signals.

3. The multistage beamforming circuit of claim 1 , wherein the data unit further comprises:

an antenna virtualization module configured to receive N CRS common control signals as frequency domain signals, apply a common control signals specific antenna virtualization precoding to the N CRS frequency domain common signals to generate M virtualized common control signals; and

M adders configured to combine the M virtualized common control signals with the M precoding output streams.

4. The multistage beamforming circuit of claim 3 , wherein the antenna virtualization module is further configured to:

receive N CSI-RS frequency-domain CSI-RS signals, and apply a CSI-RS specific antenna virtualization precoding to the N CSI-RS frequency-domain CSI-RS signals to generate M virtualized CSI-RS signals; and

wherein the M adders are further configured to combine the M virtualized CSI-RS signals with the M precoding output streams.

5. The multistage beamforming circuit of claim 1 , further comprising a beamforming control module configured to receive uplink feedback including at least one of precoding matrix indicator (PMI) and channel-state-information (CSI), and based on the feedback, generate beamforming control signals including:

a first beamforming control signal configured to control the frequency domain precoding module to select the frequency domain precoding matrices, and

a second beamforming control signals configured to control a time-domain broadband beamforming module to select a wide-band precoding matrix.

6. The multistage beamforming circuit of claim 1 , wherein the remote radio head further comprises a time-domain broadband beamforming module that includes a preceding matrix having at least one of:

rows that are Discrete Fourier Transform (DFT) vectors, and

columns that are DFT vectors; and wherein the broadcast beams comprise:

a wide beam width cell-specific reference signal (CRS),

wide beam width channel-state-information reference signals (CSI-RS), and

wide beam width common control channels associated with the CRS and including at least one of: Physical Downlink Control Channel and Physical Broadcast Channel, and

wherein the user-specific beams comprise:

narrow beam width CSI-RS,

narrow beam width user equipment specific reference signal (UE-RS), and

narrow beam width UE data channels associated with UE-RS.

7. The multistage beamforming circuit of claim 1 , wherein the remote radio head comprises:

a time-domain broadband beamforming module configured to receive and precode the M output streams into the N precoded output signals using a wideband precoding matrix; and

N transmission paths respectively coupled to at least one of the physical antennas, each transmission path Including a series of a digital-to-analog converter, a mixer, and a power amplifier together configured to form a respective one of the N transmit streams using a respective one of the N precoded output signals.

8. The multistage beamforming circuit of claim 7 , wherein the remote radio head further comprises:

an antenna virtualization module configured to receive N CSI-RS time-domain CSI-RS signals, and apply a CSI-RS specific antenna virtualization precoding to the N CSI-RS time-domain CSI-RS signals to generate N virtualized CSI-RS signals; and

N adders configured to combine the N virtualized CSI-RS signals with the N precoded output signals.

9. The multistage beamforming circuit of claim 8 , wherein the antenna virtualization module is further configured to:

receive N CRS common control time-domain signals from the data unit, and

apply a common control signals specific antenna virtualization precoding to the N CRS time domain common signals to generate N virtualized common control signals, and

wherein the N adders are further configured to combine the N virtualized common control signals with the N precoded output signals.

10. The multistage beamforming circuit of claim 1 , further comprising a common public radio interface (CPRI) interface configured to transmit the M precoding output streams from the data unit to the remote radio head.

11. A base station for multistage beamforming in a wireless communication network, the base station comprising:

a data unit configured to implement a frequency domain beamforming stage, the data unit comprising:

a frequency domain precoding module configured to receive and precode K data streams into M precoding output streams in a frequency domain by applying frequency domain precoding matrices,

M pairs of an Inverse Fast Fourier Transform (IFFT) processing block coupled to M cyclic prefix processing block, each pair configured to transform the M precoding output streams into M orthogonal frequency-division multiplexing (OFDM) time-domain signals, wherein each IFFT processing block is configured to receive a frequency-domain signal, map the frequency-domain signal to resource elements in a frequency domain, transform the received frequency-domain signal to a stream of time domain samples, and each cyclic prefix processing block is configured to add a cyclical prefix to the stream of time domain samples to generate the M precoding output streams; and

a remote radio head (RRH) configured to implement a time-domain broadband beamforming stage by converting the M OFDM time-domain signals into N transmit streams of time-domain samples based on beamforming control signals received from the data unit, the beamforming control signals indicating a wide-band precoding matrix or a set of indicators to construct the wideband precoding matrix, the RRH comprising:

a time-domain broadband beamforming module configured to receive and precode the M output streams into N precoded output signals using a wide-band precoding matrix,

a transmit antenna array configured to transmit the N transmit streams that together form broadcast beams and user-specific beams, the antenna array including a plurality of physical antennas,

wherein the number N of transmit streams is greater than the number M of precoding output streams in the time domain, and

wherein N. M. and K are positive integers.

12. The base station of claim 11 , wherein the data unit further comprises:

N CRS cell-specific reference signal (CRS) ports configured to receive common control signals as frequency domain signals; and

N CRS IFFT processing blocks for processing the common control signals to yield N CRS OFDM time-domain signals.

13. The base station of claim 11 , wherein the data unit further comprises:

N CRS CELL - SPECIFIC REFERENCE SIGNAL (CRS) PORTS, EACH PORT CONFIGURED TO RECEIVE COMMON CONTROL SIGNALS IN THE FREQUENCY - DOMAIN;

M resource element mappers configured to map the M precoding output streams to resource elements in a frequency domain to generate M resource-element-mapped precoding output streams; and

N CRS RESOURCE ELEMENT MAPPERS CONFIGURED TO MAP THE COMMON CONTROL SIGNALS TO RESOURCE ELEMENTS IN THE FREQUENCY DOMAIN TO GENERATE N CRS RESOURCE - ELEMENT - MAPPED COMMON CONTROL SIGNALS;

an antenna virtualization module configured to apply a common control signals specific antenna virtualization precoding to the N CRS resource-element-mapped common control signals to generate M virtualized common control signals in the frequency domain; and

M adders, each of the M adders configured to combine the M virtualized common control signals with the M resource-element-mapped precoding output streams, and

wherein each of the M IFFT processing blocks is coupled to a respective one of the M adders and further configured to receive a combined frequency-domain signal from the one of the M adders as the received frequency-domain signal.

14. The base station of claim 11 , wherein the data unit further comprises:

N CRS CRS ports, each CRS port configured to receive one of N CRS common control signals in the frequency-domain;

N CSI-RS CSI-RS ports, each CSI-RS port configured to receive one of N CSI-RS CSI-RS signals in the frequency-domain;

an antenna virtualization module configured to apply a common control signals specific antenna virtualization precoding to the N CRS common control signals to generate M virtualized common control signals in the frequency domain, and to apply a CSI-RS specific antenna virtualization preceding to the N CSI-RS frequency-domain CSI-RS sign is to generate virtualized CSI-RS signals; and

M adders configured to combine the M virtualized CSI-RS signals with the M preceding output streams, each adder configured to combine a respective one of the M virtualized common control signals with a respective one of the M preceding output streams,

wherein each of the M IFFT processing blocks is coupled to a respective one of the M adders and further configured to receive a combined frequency-domain signal from the adder as the received frequency-domain signal.

15. The base station of claim 11 , wherein the time-domain broadband beamforming module comprises a preceding matrix having at least one of:

rows that are Discrete Fourier Transform (DFT) vectors, and

columns that are DFT vectors; and

wherein the broadcast beams comprise:

a wide beam width cell-specific reference signal (CRS),

wide beam width channel-state-information reference signals (CSI-RS), and

wide beamwidth common control channels associated with the CRS and including at least one of: Physical Downlink Control Channel and Physical Broadcast Channel, and

wherein the user-specific beams comprise:

narrow beam width CSI-RS,

narrow beam width user equipment specific reference signal (UE-RS), and

narrow beam width UE data channels associated with UE-RS.

16. The base station of claim 11 , wherein the remote radio head further comprises:

an antenna virtualization module configured to receive N CSI-RS time-domain CSI-RS signals, and apply a CSI-RS specific antenna virtualization precoding to the N CSI-RS time-domain CSI-RS signals to generate N virtualized CSI-RS signals; and

N adders configured to combine the N virtualized CSI-RS signals with the N precoded output signals.

17. The base station of claim 16 , wherein the antenna virtualization module is further configured to:

receive N CRS common control time-domain signals from the data unit, and

apply a common control signals specific antenna virtualization precoding to the N CRS common control time domain signals to generate N virtualized common control signals, and

wherein the N adders are further configured to combine the N virtualized common control signals with the N precoded output signals.

18. The base station of claim 11 , further comprising a beamforming control module configured to receive uplink feedback including at least one of precoding matrix indicator (PM) and channel-state-information (CSI), and based on the feedback, generate beamforming control signals including:

a first beamforming control signal configured to control the frequency domain precoding module to select the frequency domain precoding matrices, and

a second beamforming control signals configured to control a time-domain broadband beamforming module to select a wide-band precoding matrix.

19. A multistage beamforming method comprising: implementing a frequency domain beamforming stage by converting K data streams in a frequency-domain into M precoding output streams in a frequency domain, wherein converting the K data streams into M precoding output streams comprises receiving and precoding the K data streams into M precoding output streams by applying frequency domain precoding matrices;

transforming the M output streams to M orthogonal frequency-division multiplexing (OFDM) time-domain signals by:

mapping the M output streams to resource elements in a frequency domain to generate M mapped frequency-domain signals,

transforming the M mapped frequency-domain signals to M streams of time domain samples using Inverse Fast Fourier Transform (IFFT), and

adding a cyclical prefix to each of the M streams of time domain samples yielding the M OFDM time-domain signals:

implementing a time-domain broadband beamforming stage by converting the M OFDM time-domain signals into N transmit streams of time-domain samples based on beamforming control signals received, the beamforming control signals indicating a wide-band precoding matrix or a set of indicators to construct the wideband precoding matrix; and

transmitting, by a transmit antenna array including a plurality of physical antennas, the N transmit streams that together form broadcast beams and user-specific beams,

wherein the number N of transmit streams is greater than the number M of precoding output streams, and

wherein N. M. and K are positive integers.

20. The method of claim 19 , wherein converting the M OFDM time-domain signals into the N transmit streams comprises:

receiving and precoding the M OFDM time-domain signals into the N transmit streams of time-domain samples using a wide-band precoding matrix.

21. The method of claim 19 , further comprising one of:

applying a common control signals specific antenna virtualization precoding to common control signals to generate N virtualized common control signals in the time domain, and combining the N virtualized common control signals with the N transmit streams;

applying a common control signals specific antenna virtualization precoding to common control signals to generate M virtualized common control signals in the frequency domain, and combining the M virtualized common control signals with the M precoding output streams; and

further comprising one of:

applying a CSI-RS specific antenna virtualization precoding to time-domain CSI-RS signals to generate N virtualized CSI-RS signals, and combining the N virtualized CSI-RS signals with the N transmit streams; and

applying a CSI-RS specific antenna virtualization precoding to frequency domain CSI-RS signals to generate M virtualized CSI-RS signals, and combining the M virtualized CSI-RS signals with the with the M precoding output streams.

22. The method of claim 19 , wherein converting the M OFDM time-domain signals into the N transmit streams of time-domain samples comprises using a precoding matrix having at least one of:

rows that are Discrete Fourier Transform (DFT) vectors, and

columns that are DFT vectors; and wherein the broadcast beams comprise:

a wide beamwidth cell-specific reference signal (CRS),

wide beam width channel-state-information reference signals (CSI-RS), and

wide beamwidth common control channels associated with the CRS and including at least one of: Physical Downlink Control Channel and Physical Broadcast Channel, and

wherein the user-specific beams comprise:

narrow beamwidth CSRS,

narrow beamwidth user equipment specific reference signal (UE-RS), and

narrow beamwidth UE data channels associated with UE-RS.