System and method for beam-based physical random-access

1. A method in a wireless device for performing random access to a network node, the method comprising:

receiving a set of downlink beam-specific reference signals (BRSs) from the network node;

determining a preferred BRS based on the received signal power for each BRS in the set; and

selecting, based on the preferred BRS, a random-access preamble for transmitting a random-access attempt to the network node according to one or more pre-defined association rules defining an association between a random-access resource and a BRS, the selecting using a function that uses the preferred BRS as an input and provides the selected random-access preamble as an output, the function being specified in a standard; and

transmitting the selected random-access preamble to the network node, whereby the selection of the random-access preamble indicates to the network node which downlink beam is preferred by the wireless device to be used for downlink transmissions.

2. The method according to claim 1 , wherein the selecting comprises using a function for selecting the random-access preamble from the set of preambles based on the BRS, the function specified in a standard, or given by prior broadcast signaling.

3. The method according to claim 1 , wherein the selecting comprises selecting a time and/or frequency resource to be used for transmitting the random-access attempt.

4. The method according to claim 1 , wherein the selecting comprises selecting the random-access preamble based on one or more pre-defined association rules known to the wireless device.

5. A method in a network node for supporting random access from a wireless device, the method comprising:

transmitting a set of downlink beam-specific reference signals (BRSs);

detecting a random-access preamble in a signal received from the wireless device, said preamble detection indicating a preferred BRS of the set of downlink BRS, the preferred BRS being preferred by said wireless device according to one or more pre-defined association rules defining an association between a random access resource and a BRS, wherein the random access preamble is based on a function that uses the preferred BRS as an input and provides the random-access preamble as an output, wherein the function is specified in a standard; and

transmitting a random-access response in a same beam, and/or beam direction, and/or with same beamforming weights, as the preferred BRS indicated by the preamble detection.

6. The method according to claim 5 , further comprising

selecting an uplink beam for the detecting of the random-access preamble according to one or more pre-defined association rules between random-access preambles and uplink beams known at the network node.

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

selecting a time and/or frequency resource for random-access preamble detection according to one or more pre-defined association rules between random-access preambles and time/frequency resources known at the network node.

8. The method according to claim 5 , wherein the transmitting further comprises:

transmitting the random-access response according to the one or more pre-defined association rules known at the network node.

9. The method according to claim 5 , wherein the detecting further comprises searching a subset of random-access preambles in each uplink beam for which subset an associated BRS is transmitted in downlink, each BRS pointing out a subset of random-access preambles to be searched.

10. The method according to claim 5 , wherein random-access preamble sequences and resources are reused for BRSs associated with downlink beams having a pre-determined angular separation.

11. A wireless device configured to perform random access to a network node, the wireless device comprising a processor and a memory, the memory containing instructions executable by the processor to:

receive a set of downlink beam-specific reference signals (BRSs) from the network node;

determine a preferred BRS based on the received signal power for each BRS in the set; and

select, based on the preferred BRS, a random-access preamble for transmitting a random-access attempt to the network node according to one or more pre-defined association rules defining an association between a random-access resource and a BRS, the selecting using a function that uses the preferred BRS as an input and provides the selected random-access preamble as an output, the function being specified in a standard; and

transmit the selected random-access preamble to the network node, whereby the selection of the random-access preamble indicates to the network node which downlink beam is preferred by the wireless device to be used for downlink transmissions.

12. The wireless device according to claim 11 , wherein the processor is further configured to:

use a function for selecting a random-access preamble from the set of preambles based on the BRS, the function specified in a manual or standard, or given by prior broadcast signaling.

13. The wireless device according to claim 11 , wherein the processor is further configured to:

select a time and/or frequency resource to be used for transmitting the random-access attempt.

14. The wireless device according to claim 11 , wherein the processor is further configured to:

select the random-access preamble based on one or more pre-defined association rules known to the wireless device.

15. A network node configured to support random access from a wireless device, the network node comprising a processor and a memory, the memory containing instructions executable by the processor to:

transmit a set of downlink beam-specific reference signals (BRSs);

detect a random-access preamble in a signal received from the wireless device, said preamble detection indicating a preferred BRS of the set of downlink BRS, the preferred BRS being preferred by said wireless device according to one or more pre-defined association rules defining an association between a random access resource and a BRS, wherein the random access preamble is based on a function that uses the preferred BRS as an input and provides the random-access preamble as an output, wherein the function is specified in a standard; and

transmit a random-access response in a same beam, and/or beam direction, and/or with same beamforming weights, as the preferred BRS indicated by the preamble detection.

16. The network node according to claim 15 , wherein the processing means is further configured to:

select an uplink beam for detecting the random-access preamble according to one or more pre-defined association rules between random-access preambles and uplink beams known at the network node.

17. The network node according to claim 15 , wherein the processing means is further configured to:

select a time and/or frequency resource for random-access preamble detection according to one or more pre-defined association rules between random-access preambles and time/frequency resources known at the network node.

18. The network node according to claim 15 , wherein the processing means is further configured to:

transmit the random-access response according to the one or more pre-defined association rules known at the network node.

19. The network node according to claim 15 , wherein the processing means is further configured to:

search a subset of random-access preambles in each uplink beam for which subset an associated BRS is transmitted in downlink, each BRS pointing out a subset of random-access preambles to be searched.

20. The network node according to claim 15 , wherein random-access preamble sequences and resources are reused for BRSs associated with downlink beams having a pre-determined angular separation.

21. A wireless communication system comprising:

a wireless device and a network node, the network node operable to transmit a set of downlink beam-specific reference signals (BRSs) and the wireless device operable to:

receive the set of downlink BRSs from the network node;

determine a preferred BRS based on the received signal power for each BRS of the set of downlink BRSs;

select, based on the preferred BRS, a random-access preamble to be used to transmit a random-access attempt to the network node according to one or more pre-defined association rules defining an association between a random-access resource and a BRS, the selecting using a function that uses the preferred BRS as an input and provides the selected random-access preamble as an output, the function being specified in a standard; and

transmit the selected random-access preamble to the network node, whereby the selection of the random-access preamble indicates to the network node which downlink beam is preferred by the wireless device to be used for downlink transmissions;

the network node further operable to:

detect the preferred BRS of the wireless device based on the random-access preamble that the wireless device uses when transmitting the random-access attempt; and

transmit a random-access response in a same beam, and/or beam direction, and/or with same beamforming weights, as the preferred BRS.