LOS-MIMO microwave radio link channel estimation for rank deficient channels

1 . A computer-implemented method, performed in a network node, for estimating a line-of-sight, LOS, multiple-input multiple-output, MIMO, radio propagation channel condition, wherein a LOS-MIMO radio link is arranged to communicate a nominal number of MIMO data streams over the radio link from a first site to a second site, the method comprising:

configuring the LOS-MIMO radio link to communicate a reduced number of MIMO data streams smaller than the nominal number of MIMO data streams from the first site to the second site over the radio link;

transmitting the reduced number of MIMO data streams from the first site to the second site;

determining a LOS-MIMO channel phase condition at the second site, based on the reduced number of MIMO data streams transmitted from the first site,

communicating the determined LOS-MIMO channel phase condition from the second site to the first site; and

estimating the LOS-MIMO radio propagation channel condition at the first site based on the LOS-MIMO channel phase condition communicated from the second site.

2 . The method according to claim 1 , further comprising:

transmitting the reduced number of MIMO data streams from the first site to the second site as a time-interleaved transmission pattern over transmit antennas at the first site.

3 . The method according to claim 1 , further comprising:

periodically transmitting time-overlapping pilot signals from at least two transmit antennas at the first site.

4 . The method according to claim 1 , further comprising:

transmitting a MIMO data stream, of the reduced number of MIMO data streams, from one transmit antenna at the first site and a pilot signal from another transmit antenna at the first site.

5 . The method according to claim 1 , further comprising:

determining the LOS-MIMO channel phase condition by correlating a LOS-MIMO radio signal received at one antenna to a corresponding LOS-MIMO radio signal received at another antenna of the second site.

6 . The method according to claim 1 , further comprising:

determining the LOS-MIMO channel phase condition by correlating a detected LOS-MIMO data stream to a LOS-MIMO radio signal received at one antenna and to a corresponding LOS-MIMO radio signal received at another antenna of the second site.

7 . The method according to claim 1 , further comprising:

communicating the determined LOS-MIMO channel phase condition from the second site to the first site periodically.

8 . The method according to claim 1 , further comprising:

filtering the LOS-MIMO channel phase condition prior to communicating the LOS- MIMO channel phase condition from the second site to the first site.

9 . The method according to claim 1 , further comprising:

comparing the determined LOS-MIMO channel phase condition to a pre-determined acceptance criterion, and communicating the result of the comparison from the second site to the first site.

10 . The method according to claim 1 , further comprising:

communicating any of: an equalizer setting of the second site, mean-squared error, MSE, of the second site, a bit error rate, BER, of the second site, a packet error rate, PER, of the second site to the first site, and a received signal strength, RSS, of the second site.

11 . The method according to claim 1 , further comprising:

transmitting a reduced number of MIMO data streams from the second site to the first site;

determining a LOS-MIMO channel phase condition at the first site, based on the reduced number of MIMO data streams transmitted from the second site;

communicating, from the first site to the second site, the LOS-MIMO channel phase condition determined at the first site; and

estimating the LOS-MIMO radio propagation channel condition at the second site based on the LOS-MIMO channel phase condition communicated from the first site.

12 . The method according to claim 1 , further comprising:

adjusting the number of active MIMO data streams sent over the LOS-MIMO radio link based on the estimated LOS-MIMO radio propagation channel condition.

13 . The method according to claim 1 , further comprising:

requesting a LOS-MIMO channel condition report from the second site by triggering transmission of a request message from the first site.

14 . A network node for estimating a line-of-sight, LOS, multiple-input multiple-output, MIMO, radio propagation channel condition, wherein a LOS-MIMO radio link is arranged to communicate a nominal number of MIMO data streams over the radio link from a first site to a second site, the network node comprising:

processing circuitry;

a network interface coupled to the processing circuitry; and

a memory coupled to the processing circuitry, wherein the memory comprises machine readable computer program instructions that, when executed by the processing circuitry, causes the network node to:

configure the LOS-MIMO radio link to communicate a reduced number of MIMO data streams smaller than the nominal number of MIMO data streams from the first site to the second site over the radio link;

transmit the reduced number of MIMO data streams from the first site to the second site;

determine a LOS-MIMO channel phase condition at the second site, based on the reduced number of MIMO data streams transmitted from the first site;

communicate the determined LOS-MIMO channel phase condition from the second site to the first site; and

estimate the LOS-MIMO radio propagation channel condition at the first site based on the LOS-MIMO channel phase condition communicated from the second site.