Methods and devices for determining a transmission rank
The invention relates to a method 20 in a base station 2 for determining a transmission rank. The base station 2 controls two or more transmit antenna ports 3 a, 3 b for supporting a multi-antenna transmission mode and for transmission of data on a channel for communication with a user equipment 4 . The method 20 comprises receiving 21 a rank indicator from a user equipment 4 , the rank indicator indicating the number of spatial multiplexing layers recommended by the user equipment 4 , and determining 22 the transmission rank based on a channel imbalance factor CIF, wherein the channel imbalance factor CIF quantifies a difference in receive power of the two or more transmit antenna ports 3 a, 3 b . The invention also relates to a base station, methods in user equipment, user equipment, computer programs, and computer program products.
1. A method in a base station for determining a transmission rank, the base station controlling two or more transmit antenna ports for supporting a multi-antenna transmission mode and for transmission of data on a channel for communication with a user equipment, the method comprising:
receiving a rank indicator from a user equipment, the rank indicator indicating the number of spatial multiplexing layers recommended by the user equipment, and
determining the transmission rank based on a channel imbalance factor CIF, wherein the channel imbalance factor CIF quantifies a difference in receive power of the two or more transmit antenna ports.
2. The method as claimed in claim 1 , wherein the determining of the transmission rank based on a channel imbalance factor CIF is performed for a received rank indicator larger than 1.
3. The method as claimed in claim 1 , wherein the determining of the transmission rank based on a channel imbalance factor CIF is performed for a received rank indicator equal to 2.
4. The method as claimed in claim 1 , wherein the determining of the transmission rank based on a channel imbalance factor CIF comprises:
comparing the channel imbalance factor CIF with a threshold value Γ th , and
determining the transmission rank to be equal to the received rank indicator if the channel imbalance factor CIF is equal to or smaller than the threshold value Γ th and determining the transmission rank to be equal to one if the channel imbalance factor CIF is larger than the threshold value Γ th .
5. The method as claimed in claim 4 , wherein the threshold value Γ th is set based on a probability that the transmission rank would give high data throughput.
6. The method as claimed in claim 5 , wherein the probability is determined by simulation in a communication system comprising the base station.
7. The method as claimed in claim 1 , wherein the channel is denoted by an uplink channel matrix
H
=
(
h
11
h
12
…
h
1
Nt
h
21
⋮
⋱
⋮
h
Nr
1
h
Nr
2
…
h
NrNt
)
where the i-th column represents the channel between the i-th transmit antenna at the user equipment and all the receive antennas at the base station, N t is the number of transmit antennas of the user equipment and N r is the number of receive antennas of the base station, and the channel imbalance factor CIF is determined by:
CIF
=
10
lg
max
i
h
i
min
i
h
i
,
where h i is the i-th column of the uplink channel matrix H, and ∥h∥ is the Frobenius norm of a vector h.
8. The method as claimed in claim 1 , wherein the channel is denoted by an uplink channel matrix
H
=
(
h
11
h
12
…
h
1
Nt
h
21
⋮
⋱
⋮
h
Nr
1
h
Nr
2
…
h
NrNt
)
and the channel imbalance factor (CIF) is determined by:
CIF
=
tr
(
H
H
H
)
det
(
H
H
H
)
,
where tr(H H H) denotes the trace of the matrix H H H and det(H H H) denotes the determinant of the matrix H H H, and H H is the Hermitian transposition of H, N t is the number of transmit antennas of the user equipment and N r is the number of receive antennas of the base station.
9. The method as claimed in claim 1 , wherein the channel is denoted by an uplink channel matrix
H
=
(
h
11
h
12
…
h
1
Nt
h
21
⋮
⋱
⋮
h
Nr
1
h
Nr
2
…
h
NrNt
)
where the i-th column represents the channel between the i-th transmit antenna at the user equipment and all the receive antennas at the base station, N t is the number of transmit antennas of the user equipment and N r is the number of receive antennas of the base station, and the channel imbalance factor, CIF, is determined by:
CIF
=
λ
m
ax
(
H
H
H
)
λ
m
i
n
(
H
H
H
)
,
where λ max (H H H) denotes the maximum eigenvalue of the matrix H H H, and λ min (H H H) denotes the minimum eigenvalue of the matrix H H H, N t is the number of transmit antennas of the user equipment and N r is the number of receive antennas of the base station.
10. The method as claimed in claim 1 , further comprising, prior to the determining of transmission rank, performing channel measurements for determining the uplink channel matrix H.
11. The method as claimed in claim 1 , further comprising utilizing the determined transmission rank in communication with the user equipment.
12. A base station for determining a transmission rank, the base station comprising a processor for controlling two or more transmit antenna ports for supporting a multi-antenna transmission mode and for transmission of data on a channel for communication with a user equipment, the base station comprising:
a receiver configured to receive a rank indicator from a user equipment, the rank indicator indicating the number of spatial multiplexing layers recommended by the user equipment, and
a processor configured to determine, for a received rank indicator larger than 1, the transmission rank based on a channel imbalance factor CIF, wherein the channel imbalance factor CIF quantifies a difference in receive power of the two or more transmit antenna ports.
13. A computer program for a base station for determining a transmission rank, the base station controlling two or more transmit antenna ports for supporting a multi-antenna transmission mode and for transmission of data on a channel for communication with a user equipment, the computer program comprising computer program code stored on a non-trans memory, which, when executed by a processor of the base station, causes the base station to:
receive a rank indicator from a user equipment, the rank indicator indicating the number of spatial multiplexing layers supported by the user equipment, and
determine, for a received rank indicator larger than 1, the transmission rank based on a channel imbalance factor CIF, wherein the channel imbalance factor CIF quantifies a difference in receive power of the two or more transmit antenna ports.
14. A method in a user equipment for determining a rank indicator, the user equipment comprising two or more receive antennas for supporting a multi-antenna reception mode and for receiving data on a channel used in communication with a base station, the method comprising:
performing channel measurements for determining a downlink channel matrix H, and
determining the rank indicator based on a channel imbalance factor CIF, wherein the channel imbalance factor CIF quantifies a difference in receive power of the two or more receive antennas.
15. The method as claimed in claim 14 , comprising selecting a rank indicator based on the determined downlink channel matrix H, wherein the rank indicator indicates a number of spatial multiplexing layers of recommended by the user equipment.
16. The method as claimed in claim 15 , wherein the determining the rank indicator is performed for a selected rank indicator larger than 1.
17. The method as claimed in claim 15 , wherein the determining the rank indicator is performed for a selected rank indicator equal to 2.
18. The method as claimed in claim 15 , wherein the determining of the rank indicator based on a channel imbalance factor CIF comprises:
comparing the channel imbalance factor CIF with a threshold value Γ th , and
determining the rank indicator to be equal to the selected rank indicator if the channel imbalance factor CIF is equal to or smaller than the threshold value Γ th and determining the rank indicator to be equal to one if the channel imbalance factor CIF is larger than the threshold value Γ th .
19. The method as claimed in claim 18 , wherein the threshold value Γ th is set based on signaling from a base station, based on one or more parameters predefined in the user equipment or based on a combination thereof.
20. The method as claimed in claim 14 , wherein the channel is denoted by a downlink channel matrix
H
=
(
h
11
h
12
…
h
1
Nt
h
21
⋮
⋱
⋮
h
Nr
1
h
Nr
2
…
h
NrNt
)
where the i-th column of the matrix H represents the channel between the i-th antenna port at the base station and all the receive antennas at the user equipment N t is the number of transmit antenna ports of the base station and N r is the number of receive antennas of the user equipment, and the channel imbalance factor CIF is determined by:
CIF
=
10
lg
max
i
h
i
min
i
h
i
,
where h i is the i-th column of the channel matrix H, and ∥h∥ is the Frobenius norm of a vector h.
21. The method as claimed in claim 14 , wherein the channel is denoted by a downlink channel matrix
H
=
(
h
11
h
12
…
h
1
Nt
h
21
⋮
⋱
⋮
h
Nr
1
h
Nr
2
…
h
NrNt
)
where the i-th column of the matrix H represents the channel between the i-th antenna port at the base station and all the receive antennas at the user equipment N t is the number of transmit antenna ports of the base station and N r is the number of receive antennas of the user equipment, and the channel imbalance factor (CIF) is determined by:
CIF
=
tr
(
H
H
H
)
det
(
H
H
H
)
,
where tr(H H H) denotes the trace of the matrix H H H and det(H H H) denotes the determinant of the matrix H H H, and H H is the Hermitian transposition of H.
22. The method as claimed in claim 14 , wherein the channel is denoted by a downlink channel matrix
H
=
(
h
11
h
12
…
h
1
Nt
h
21
⋮
⋱
⋮
h
Nr
1
h
Nr
2
…
h
NrNt
)
where the i-th column of the matrix H represents the channel between the i-th antenna port at the base station and all the receive antennas at the user equipment N t is the number of transmit antenna ports of the base station and N r is the number of receive antennas of the user equipment, and the channel imbalance factor CIF is determined by:
CIF
=
λ
m
ax
(
H
H
H
)
λ
m
i
n
(
H
H
H
)
,
where λ max (H H H) denotes the maximum eigenvalue of the matrix H H H, and λ min (H H H) denotes the minimum eigenvalue of the matrix H H H.
23. The method as claimed in claim 14 , comprising transmitting the determined rank indicator to the base station.
24. A user equipment for determining a rank indicator, the user equipment comprising two or more receive antennas for supporting a multi-antenna reception mode and for receiving data on a channel used in communication with a base station, the user equipment comprising a processor arranged to:
perform channel measurements for determining a channel matrix H, and
determine the rank indicator based on a channel imbalance factor (CIF), wherein the channel imbalance factor (CIF) quantifies a difference in receive power of the two or more receive antennas.
25. A computer program for a user equipment for determining a rank indicator, the user equipment comprising two or more receive antennas for supporting a multi-antenna reception mode and for receiving data on a channel used for communication with a base station, the computer program comprising computer program code stored on a non-transitory memory, which, when executed by a processor of the user equipment, causes the user equipment to:
perform channel measurements for determining a channel matrix H, and
determine the rank indicator based on a channel imbalance factor (CIF), wherein the channel imbalance factor (CIF) quantifies a difference in receive power of the two or more receive antennas.