Antenna port detection

1. A method, implemented by a user equipment, for detecting presence of a second transmit antenna port in a distributed antenna port system, the method comprising:

receiving cell-specific reference signals (CRS) on a first receive antenna port of the user equipment and a second receive antenna port of the user equipment, the CRS having been transmitted by a first transmit antenna port and a second transmit antenna port in the distributed antenna port system;

estimating reference signal received power (RSRP) measurements from the CRS for a first plurality of distinct transmit-receive antenna port combinations, wherein the estimating of the RSRP measurements comprises:

estimating a first RSRP measurement from the CRS for a first transmit-receive antenna port combination comprising the first receive antenna port and the first transmit antenna port;

estimating a second RSRP measurement from the CRS for a second transmit-receive antenna port combination comprising the first receive antenna port and the second transmit antenna port;

estimating a third RSRP measurement from the CRS for a third transmit-receive antenna port combination comprising the second receive antenna port and the first transmit antenna port;

estimating a fourth RSRP measurement from the CRS for a fourth transmit-receive antenna port combination comprising the second receive antenna port and the second transmit antenna port;

filtering the RSRP measurements so as to obtain filtered RSRP measurements;

estimating reference signal resource element received power (RSRERP) measurements from the CRS for a second plurality of distinct transmit-receive antenna port combinations, wherein the estimating of the RSRERP measurements comprises:

estimating a first RSRERP measurement from the CRS for the second transmit-receive antenna port combination;

estimating a second RSRERP measurement from the CRS for the fourth transmit-receive antenna port combination;

filtering the RSRERP measurements so as to obtain filtered RSRERP measurements;

determining a signal to interference and noise ratio (SINR) based on the filtered RSRP measurements and the filtered RSRERP measurements;

determining, based on comparing the SINR to a first threshold (SINRthr), whether the second transmit antenna port is present or not.

2. The method of claim 1 , further comprising, in response to determining that the second transmit antenna port is present, continuing measuring from the second transmit antenna port.

3. The method of claim 1 , further comprising, in response to determining that the second transmit antenna port is present, skipping reception on the first receive antenna port during a next measurement occasion.

4. The method of claim 1 , further comprising, in response to determining that the second transmit antenna port is not present, disabling measuring from the second transmit antenna port.

5. The method of claim 1 , where there are N r receive antenna ports and wherein SINR for measurement occasion m is determined as:

SINR

⁡

(

m

)

=

1

N

r

⁢

∑

i

=

0

N

r

-

1

⁢

RSRP

R

×

i

T

×

1

⁡

(

m

)

RSRERP

R

×

i

T

×

1

⁡

(

m

)

,

where RSRP Rxi Tx1 (m) is the filtered RSRP for the second transmit antenna port at the i:th receive antenna port, and where RSRERP Rxi Tx1 (m) is the filtered RSRERP for the second transmit antenna port at the i:th receive antenna port.

6. The method of claim 1 , further comprising:

determining an antenna power ratio (G) based on the filtered RSRP of the second transmit antenna port and the filtered RSRP of the first transmit antenna port;

determining whether the second transmit antenna port is present or not also based on comparing the G to a second threshold (Gthr).

7. The method of claim 6 , where there are N r receive antenna ports and wherein G is determined as:

G

⁡

(

m

)

=

1

N

r

⁢

∑

i

=

0

N

r

-

1

⁢

RSRP

R

×

i

T

×

1

⁡

(

m

)

RSRP

R

×

i

T

×

0

⁡

(

m

)

,

where RSRP Rxi Tx1 (m) is the filtered RSRP for the second transmit antenna port at the i:th receive antenna port and where RSRP Rxi Tx0 (m) is the filtered RSRP for the first transmit antenna port at the i:th receive antenna port.

8. A user equipment for detecting presence of a second transmit antenna port in a distributed antenna port system, the user equipment comprising:

a first receive antenna port and a second receive antenna port arranged to receive cell-specific reference signals (CRS), the CRS having been transmitted by a first transmit antenna port and a second transmit antenna port in the distributed antenna port system;

a processing circuit configured to:

estimate reference signal received power (RSRP) measurements from the CRS for a first plurality of distinct transmit-receive antenna port combinations, wherein to estimate the RSRP measurements the processing circuit is configured to at least:

estimate a first RSRP measurement from the CRS for a first transmit-receive antenna port combination comprising the first receive antenna port and the first transmit antenna port;

estimate a second RSRP measurement from the CRS for a second transmit-receive antenna port combination comprising the first receive antenna port and the second transmit antenna port;

estimate a third RSRP measurement from the CRS for a third transmit-receive antenna port combination comprising the second receive antenna port and the first transmit antenna port;

estimate a fourth RSRP measurement from the CRS for a fourth transmit-receive antenna port combination comprising the second receive antenna port and the second transmit antenna port;

filter the RSRP measurements so as to obtain filtered RSRP measurements;

estimate reference signal resource element received power (RSRERP) measurements from the CRS for a second plurality of distinct transmit-receive antenna port combinations, wherein to estimate the RSRERP measurements, the processing circuit is configured to:

estimate a first RSRERP measurement from the CRS for the second transmit-receive antenna port combination;

estimate a second RSRERP measurement from the CRS for the fourth transmit-receive antenna port combination;

filter the RSRERP measurements so as to obtain filtered RSRERP measurements;

determine a signal to interference and noise ratio (SINR) based on the filtered RSRP measurements and the filtered RSRERP measurements;

determine, based on comparing the SINR to a first threshold (SINRthr), whether the second transmit antenna port is present or not.

9. The user equipment of claim 8 , wherein the processing circuit configured to, in response to determining that the second transmit antenna port is present, continue measuring from the second transmit antenna port.

10. The user equipment of claim 8 , wherein the processing circuit configured to, in response to determining that the second transmit antenna port is present, skip reception on the first receive antenna port during a next measurement occasion.

11. The user equipment of claim 8 , wherein the processing circuit configured to, in response to determining that the second transmit antenna port is not present, disable measuring from the second transmit antenna port.

12. The user equipment of claim 8 , wherein the processing circuit configured to:

determine an antenna power ratio (G) based on the filtered RSRP of the second transmit antenna port and the filtered RSRP of the first transmit antenna port;

determine whether the second transmit antenna port is present or not also based on comparing the G to a second threshold (Gthr).

13. A computer program product stored in a non-transitory computer readable medium for detecting presence of a second transmit antenna port in a distributed antenna port system, the computer program product comprising software instructions which, when run on a processing circuit of a user equipment, causes the user equipment to:

receive cell-specific reference signals (CRS) on a first receive antenna port of the user equipment and a second receive antenna port of the user equipment, the CRS having been transmitted by a first transmit antenna port and a second transmit antenna port in the distributed antenna port system;

estimate reference signal received power (RSRP) measurements from the CRS for a first plurality of distinct transmit-receive antenna port combinations, wherein the estimating of the RSRP measurements comprises:

estimating a first RSRP measurement from the CRS for a first transmit-receive antenna port combination comprising the first receive antenna port and the first transmit antenna port;

estimating a second RSRP measurement from the CRS for a second transmit-receive antenna port combination comprising the first receive antenna port and the second transmit antenna port;

estimating a third RSRP measurement from the CRS for a third transmit-receive antenna port combination comprising the second receive antenna port and the first transmit antenna port;

estimating a fourth RSRP measurement from the CRS for a fourth transmit-receive antenna port combination comprising the second receive antenna port and the second transmit antenna port;

filter the RSRP measurements so as to obtain filtered RSRP measurements;

estimate reference signal resource element received power (RSRERP) measurements from the CRS for a second plurality of distinct transmit-receive antenna port combinations, wherein the estimating of the RSRERP measurements comprises:

estimating a first RSRERP measurement from the CRS for the second transmit-receive antenna port combination;

estimating a second RSRERP measurement from the CRS for the fourth transmit-receive antenna port combination;

filter the RSRERP measurements so as to obtain filtered RSRERP measurements;

determine a signal to interference and noise ratio (SINR) based on the filtered RSRP measurements and the filtered RSRERP measurements;

determine, based on comparing the SINR to a first threshold (SINRthr), whether the second transmit antenna port is present or not.