Frequency division duplex (FDD) massive MIMO backhaul for repeater small cells

1. A method for channel estimation in a massive multiple-input-multiple-output (MIMO) backhaul system, the method comprising:

transmitting, by a central node of the MIMO backhaul system to a small cell repeater terminal on the downlink, a downlink pilot signal carrying a set of pilot sequences that are mutually orthogonal in the frequency domain, each pilot sequence in the set of pilot sequences corresponding to an antenna of an antenna array at the central node;

receiving, at the central node, an uplink pilot signal carrying the set of pilot sequences transmitted by the small cell repeater terminal on the uplink, the received uplink pilot signal being a frequency converted retransmission of the downlink pilot signal; and

estimating at least one of an uplink channel and a downlink channel between the central node and the small cell repeater terminal based on the set of pilot sequences and the received uplink pilot signal.

2. The method of claim 1 , wherein the estimating step comprises:

correlating the received uplink pilot signal with at least one pilot sequence in the set of pilot sequences; and

estimating at least one of the uplink channel and the downlink channel based on the correlating step.

3. The method of claim 2 , wherein the antenna array includes M antennas, where M is a real number greater than or equal to 2, and the correlating step includes,

generating a M×M up/down matrix of correlation values, each correlation value being indicative of a correlation between one of the M received uplink pilot signals and a pilot sequence in the set of pilot sequences, and wherein

the estimating step estimates at least one of the uplink channel and the downlink channel based on the M×M up/down matrix.

4. The method of claim 3 , wherein the estimating step further comprises:

identifying a dominant singular value from among the correlation values in the M×M up/down matrix; and

estimating at least one of the uplink channel and the downlink channel based on the identified dominant singular value.

5. The method of claim 4 , wherein singular vectors associated with the dominant singular value are indicative of a relative value of a component of an uplink propagation vector for the uplink channel between the small cell repeater terminal and the central node.

6. The method of claim 4 , wherein singular vectors associated with the dominant singular value are indicative of a relative value of a component of a downlink propagation vector for the downlink channel between the central node and the small cell repeater terminal.

7. The method of claim 1 , further comprising:

transmitting data to the small cell repeater terminal based on the estimated downlink channel.

8. The method of claim 1 , further comprising:

receiving data from the small cell repeater terminal based on the estimated uplink channel.

9. The method of claim 1 , further comprising:

transmitting a first trigger signal to set the small cell repeater terminal into a training mode in which the small cell repeater terminal transmits only uplink pilot signals to the central node on the uplink.

10. The method of claim 9 , further comprising:

transmitting a second trigger signal to the small cell repeater terminal to set the small cell repeater terminal into normal mode and activate access activities by the small cell repeater terminal.

11. A Massive multiple-input-multiple-output (MIMO) central node, comprising: a Massive MIMO antenna array configured to,

transmit, to a small cell repeater terminal, a downlink pilot signal carrying a set of pilot sequences that are mutually orthogonal in the frequency domain, each pilot sequence in the set of pilot sequences corresponding to an antenna of the Massive MIMO antenna array, and

receive an uplink pilot signal carrying the set of pilot sequences transmitted by the small cell repeater terminal, the received uplink pilot signal being a frequency converted retransmission of the downlink pilot signal; and

a measurement extraction circuit configured to estimate at least one of an uplink channel and a downlink channel between the Massive MIMO antenna array and the small cell repeater terminal based on the set of pilot sequences and the received uplink pilot signal.

12. The Massive MIMO central node of claim 11 , wherein the measurement extraction circuit is further configured to,

correlate the received uplink pilot signal with at least one pilot sequence in the set of pilot sequences, and

estimate the at least one of the uplink channel and the downlink channel based on the correlation.

13. The Massive MIMO central node of claim 12 , wherein the Massive MIMO antenna array includes M antennas, where M is a real number greater than or equal to 2, and the measurement extraction circuit is further configured to,

generate a M×M up/down matrix of correlation values, each correlation value being indicative of a correlation between one of the received uplink pilot signals and a pilot sequence in the set of pilot sequences, and

estimate the at least one of the uplink channel and the downlink channel based on the M×M up/down matrix.

14. The Massive MIMO central node of claim 13 , wherein the measurement extraction circuit is further configured to,

identify a dominant singular value from among the correlation values in the M×M up/down matrix, and

estimate at least one of the uplink channel and the downlink channel based on the identified dominant singular value.

15. The Massive MIMO central node of claim 14 , wherein singular vectors associated with the dominant singular value are indicative of a relative value of a component of an uplink propagation vector for the uplink channel between the small cell repeater terminal and the Massive MIMO antenna array.

16. The Massive MIMO central node of claim 14 , wherein singular vectors associated with the dominant singular value are indicative of a relative value of a component of a downlink propagation vector for the downlink channel between the Massive MIMO antenna array and the small cell repeater terminal.

17. The Massive MIMO central node of claim 11 , further comprising: a pre-coding circuit configured to pre-code data for transmission to the small cell repeater terminal based on the estimated downlink channel; and wherein

the Massive MIMO antenna array is further configured to transmit the pre-coded data to the small cell repeater terminal on the downlink channel.

18. The Massive MIMO central node of claim 11 , further comprising:

a decoding circuit configured to decode data received from the small cell repeater terminal based on the estimated uplink channel.

19. The Massive MIMO central node of claim 11 , wherein the Massive MIMO antenna array is further configured to transmit a first trigger signal to set the small cell repeater terminal into a training mode in which the small cell repeater terminal transmits only uplink pilot signals to the Massive MIMO central node on the uplink.

20. The Massive MIMO central node of claim 19 , wherein the Massive MIMO antenna array is further configured to transmit a second trigger signal to the small cell repeater terminal to set the small cell repeater terminal into normal mode and activate access activities by the small cell repeater terminal.

21. A small cell repeater terminal for transmitting data through a massive multiple-input-multiple-output (MIMO) backhaul system, the terminal comprising:

a transceiver antenna configured to,

receive a trigger signal from a central node of the massive MIMO backhaul system on a downlink channel,

receive, from the central node, a downlink pilot signal carrying a set of pilot sequences that are mutually orthogonal in the frequency domain, each pilot sequence in the set of pilot sequences corresponding to an antenna of a Massive MIMO antenna array at the Massive MIMO central node, and

transmit, to the central node, an uplink pilot signal carrying the set of pilot sequences on the uplink; and

a shifter circuit configured to,

cause the small cell repeater terminal to enter a training mode in response to the received trigger signal, and

in the training mode, frequency convert the received downlink pilot signal to generate the uplink pilot signal carrying the set of pilot sequences.