Method and network node for enabling measurements on reference signals

1. A method performed by a network node of a wireless network, for enabling mobility measurements performed by wireless devices, a set of resource blocks being distributed in a frequency domain across an available frequency bandwidth, and a pre-defined maximum average transmit power (Pavg) per resource block being available for transmission by the network node, the method comprising:

obtaining an allocation of a subset of resource blocks in the set of resource blocks, to be used for mobility reference signals intended for mobility measurements;

transmitting the subset of resource blocks using a first transmit power (P 1 ) per resource block which is higher than the pre-defined maximum average transmit power (Pavg) per resource block; and

transmitting other resource blocks in the set not included in the subset using a second transmit power (P 2 ) per resource block which is lower than the pre-defined maximum average transmit power (Pavg) per resource block so that the total transmit power used for transmitting the set of resource blocks does not exceed a total available maximum transmit power, the first transmit power (P 1 ) per resource block corresponding to a power increase (Pb) from a normalized power of 1 representing the pre-defined maximum average transmit power (Pavg) per resource block, the second transmit power (P 2 ) per resource block corresponding to a power decrease (Pd) from the normalized power, the power increase (Pb) being set to a wanted value, the power decrease (Pd) being determined based at least on the power increase (Pb), a total number (N) of resource blocks in the set, and a number (N_m) of resource blocks in the subset.

2. The method according to claim 1 , wherein the allocation of the subset is signalled to the wireless devices in one of dedicated messages and in a broadcast message.

3. The method according to claim 1 , wherein Pd is determined as:

Pd=1− N _ m*Pb /( N−N _ m ).

4. The method according to claim 3 , wherein the power increase Pb is set depending on the number of transmit antennas configured for the network node.

5. The A method according to claim 4 , wherein the power increase Pb is set for one of one and two antennas via which the mobility reference signals are transmitted.

6. The method according to claim 1 , wherein the first transmit power (P 1 ) per resource block is applied to extend a coverage area of the network node to increase mobility coverage in the wireless network.

7. The method according to claim 1 , wherein the network node is upgraded by increasing the number of transmit antennas, and the first transmit power (P 1 ) per resource block is applied to maintain a coverage area of the network node achieved before increasing the number of transmit antennas.

8. The method according to claim 1 , wherein the mobility reference signals are transmitted in resource elements of the resource blocks in the subset, each resource element corresponding to a spectrum allocated for a subcarrier and an Orthogonal Frequency-Division Multiplexing, OFDM, symbol.

9. The method according to claim 1 , wherein the allocation of the subset is one of configured by the network node and retrieved from a central node in the wireless network.

10. The method according to claim 1 , wherein the mobility reference signals are Cell-specific Reference Signals (CRS) for measurements of at least one of a Reference Signal Received Power (RSRP) and a Reference Signal Received Quality (RSRQ).

11. A network node arranged to enable mobility measurements performed by wireless devices, a set of resource blocks being distributed in frequency domain across an available frequency bandwidth, and a pre-defined maximum average transmit power (Pavg) per resource block being available for transmission by the network node, the network node being configured to:

obtain an allocation of a subset of resource blocks in the set of resource blocks, to be used for mobility reference signals intended for mobility measurements;

transmit the subset of resource blocks using a first transmit power (P 1 ) per resource block which is higher than the pre-defined maximum average transmit power (Pavg) per resource block; and

transmit other resource blocks in the set not included in the subset using a second transmit power (P 2 ) per resource block which is lower than the pre-defined maximum average transmit power (Pavg) per resource block so that the total transmit power used for transmitting the set of resource blocks does not exceed a total available maximum transmit power, the first transmit power (P 1 ) per resource block corresponding to a power increase (Pb) from a normalized power of 1 representing the pre-defined maximum average transmit power (Pavg) per resource block, the second transmit power (P 2 ) per resource block corresponding to a power decrease (Pd) from the normalized power;

set the power increase (Pb) to a wanted value; and

determine the power decrease (Pd) based at least on the power increase (Pb), a total number (N) of resource blocks in the set, and a number (N_m) of resource blocks in the subset.

12. The network node according to claim 11 , wherein the network node is configured to signal the allocation of the subset to the wireless devices in one of dedicated messages and in a broadcast message.

13. The network node according to claim 11 , wherein is determined as:

Pd=1− N _ m*Pb /( N−N _ m ).

14. The network node according to claim 13 , wherein the network node is configured to set the power increase Pb depending on the number of transmit antennas configured for the network node.

15. The network node according to claim 14 , wherein the network node is configured to set the power increase Pb for one of one and two antennas via which the mobility reference signals are transmitted.

16. The network node according to claim 11 , wherein the network node is configured to apply the first transmit power (P 1 ) per resource block to extend a coverage area of the network node to increase mobility coverage in the wireless network.

17. The network node according to claim 11 , wherein the network node is upgraded by increasing the number of transmit antennas, and the network node is configured to apply the first transmit power (P 1 ) per resource block to maintain a coverage area of the network node achieved before increasing the number of transmit antennas.

18. The network node according to claim 11 , wherein the network node is configured to transmit the mobility reference signals in resource elements of the resource blocks in the subset, each resource element corresponding to a spectrum allocated for a subcarrier and an Orthogonal Frequency-Division Multiplexing, OFDM, symbol.

19. The network node according to claim 11 , wherein the network node is configured to one of configure the allocation of the subset and to retrieve the allocation of the subset from a central node in the wireless network.

20. The method according to claim 11 , wherein the mobility reference signals are Cell-specific Reference Signals (CRS) for measurements of at least one of a Reference Signal Received Power (RSRP) and a Reference Signal Received Quality (RSRQ).