Determination of PRACH occasions and PUSCH occasions for 2-step random access
Operation in unlicensed spectrum requires Clear Channel Assessment (CCA), where a transmitter must monitor the channel before transmitting, to ensure it is free. An exception is a brief gap that allows for Rx/Tx turn-around, so that an ACK can be transmitted following reception, without performing a new CCA. NR introduces a 2-step Random Access (RA) procedure, where MsgA includes both a PRACH and a PUSCH transmission. To minimize CCA delays in unlicensed spectrum, only those RACH opportunities (ROs) and PUSCH opportunities (POs) that result in a very small delay between the PRACH and PUSCH transmissions of MsgA should be employed. RO configurations are defined for LTE 4-step RA. Embodiments provides methods for selecting which of these be may be shared for 2-step RA, for selecting which should be used in case of collisions, for selecting ROs and POs when the configurations are not shared, and for communicating RO configurations to UEs.
1 . A method, performed by a wireless device operative in a wireless communication network, of performing a Random Access (RA) procedure, the method comprising:
receiving signaling indicating a plurality of pre-configured Random Access Channel (RACH) occasions (RO) which are candidates for specific Physical Random Access Channel (PRACH) transmissions of a 2-step procedure, wherein the signaling comprises a PRACH mask indicating index values with which to index a predefined table to ascertain which ROs are shared between the 2-step RA procedure and a 4-step RA procedure; and
transmitting, to the network, an RA preamble in one of the plurality of pre-configured ROs.
2 . The method of claim 1 , wherein the method comprises:
receiving signaling indicating a plurality of Physical Uplink Shared Channel (PUSCH) occasions (PO), each comprising a plurality of periodically recurring symbols in which a PUSCH transmission may occur;
receiving signaling indicating a subset of RA resources, each comprising one of the pre-configured RO immediately followed by a first symbol of one of the PO; and
transmitting to the network a PUSCH transmission in a PO associated with the transmitted RA preamble.
3 . The method of claim 1 , wherein the received indication of a plurality of pre-configured ROs comprises:
a flag indicating whether the plurality of candidate ROs includes only the last RO in a RACH slot or all ROs of the plurality of candidate Ros;
an integer N≥1 indicating that the plurality of candidate ROs consists of the last N ROs in a RACH slot; or
an indication of a bitmask indicating which pre-configured ROs are candidates for specific PRACH transmissions.
4 . The method of claim 1 , wherein the signaling indicating a plurality of pre-configured ROs includes or references a new or modified PRACH configuration table that includes short PRACH formats spanning only one PRACH occasion at the end of or within a PRACH slot.
5 . The method of claim 4 , wherein the PRACH configuration or RO is allocated for a 2-step RA, and the associated msgA PUSCH resources can be configured to follow the PRACH occasion within the PRACH slot within a predetermined duration.
6 . The method of claim 1 , wherein the plurality of pre-configured ROs comprises separately configured 2-step ROs for use in 2-step RA and 4-step ROs for use in 4-step RA.
7 . The method of claim 6 , further comprising, for each collision between 2-step ROs and 4-step ROs:
if the colliding 2-step RO is the last 2-step RO in a PRACH slot, invalidating colliding 4-step ROs and using the 2-step RO for 2-step RA; and
if the colliding 2-step RO is other than the last 2-step RO in a PRACH slot, invalidating the 2-step ROs and using the 4-step RO for 4-step RA.
8 . The method of claim 7 , further comprising, for each collision between 2-step ROs and 4-step ROs:
if the colliding 2-step RO is configured with a gap until the first symbol of an associated PUSCH transmission of less than a predetermined duration, invalidating colliding 4-step ROs and using the 2-step RO for 2-step RA; and
otherwise, invalidating the 2-step ROs and using the 4-step RO for 4-step RA.
9 . The method of claim 7 , further comprising:
if a 4-step RO collides with a 2-step msgA PUSCH resource allocation that is allocated with a gap from its associated preceding 2-step RO of less than a predetermined duration, invalidating colliding 4-step ROs and using the 2-step RO for 2-step RA; and
otherwise, invalidating the colliding msgA PUSCH resource allocation and using the 4-step RO for 4-step RA.
10 . The method of claim 1 :
wherein the plurality of pre-configured ROs comprises shared or separately configured 2-step ROs for use in 2-step RA and 4-step ROs for use in 4-step RA; and
wherein the signaling indicates only the last 2-step RO in a PRACH slot for 2-step RA.
11 . The method of claim 10 , wherein selecting only the last 2-step RO in a PRACH slot for 2-step RA comprises selecting only the last 2-step RO in a PRACH slot only if the wireless device is operating in unlicensed spectrum.
12 . The method of claim 1 :
wherein the signaling indicating a plurality of pre-configured ROs comprises a PRACH mask indicating which RO of a PRACH configuration can be used;
wherein the PRACH mask index indicates:
that only the last RO in a PRACH slot can be used;
that only the last k ROs in a PRACH slot can be used, where K is an integer ≥1;
that all ROs other than the RO in a PRACH slot can be used;
that only the ROs having an associated PUSCH resource allocation that immediately follows the RO can be used for 2-step RA; or
that only the ROs having an associated PUSCH resource allocation that immediately follows the RO with a gap of 25 usec or less can be used for 2-step RA.
13 . The method of claim 2 :
wherein the plurality of pre-configured ROs comprises separately configured 2-step ROs for use in 2-step RA and 4-step ROs for use in 4-step RA;
wherein one or more 4-step ROs are invalidated by one or more 2-step ROs;
wherein the plurality of pre-configured ROs is identified by a first configuration signaled to the wireless device; and
wherein the subset of RA resources comprises one or more ROs wherein the wireless device may only transmit a RA preamble associated with the PUSCH.
14 . A wireless device operative in a wireless communication network, comprising:
communication circuitry configured to wirelessly communicate with one or more nodes of the wireless communication network; and
processing circuitry operatively connected to the communication circuitry, the processing circuitry configured to cause the wireless device to:
receive signaling indicating a plurality of pre-configured Random Access (RA) Channel (RACH) occasions (RO) which are candidates for specific Physical Random Access Channel (PRACH) transmissions of a 2-step procedure, wherein the signaling comprises a PRACH mask indicating index values with which to index a predefined table to ascertain which ROs are shared between the 2-step RA procedure and a 4-step RA procedure; and
transmit to the network an RA preamble in one of the plurality of pre-configured ROs.
15 . A method, performed by a base station operative in a wireless communication network, of participating in a Random Access (RA) procedure, the method comprising:
sending, to one or more wireless devices, signaling indicating a plurality of pre-configured Random Access Channel (RACH) occasions (RO) which are candidates for specific Physical Random Access Channel (PRACH) transmissions of a 2-step procedure of a 2-step or prioritized RA procedure, wherein the signaling comprises a PRACH mask indicating index values with which to index a predefined table to ascertain which ROs are shared between the 2-step RA procedure and a 4-step RA procedure; and
receiving, from a wireless device, an RA preamble in one of the plurality of pre-configured ROs.
16 . The method of claim 15 , wherein the candidate for specific PRACH transmission is only the last RO in a slot.
17 . The method of claim 15 , wherein the method comprises:
sending, to the wireless device, signaling indicating a plurality of Physical Uplink Shared Channel (PUSCH) occasions (PO), each comprising a plurality of periodically recurring symbols in which a PUSCH transmission may occur;
sending, to the wireless device, signaling indicating a subset of RA resources, each comprising one of the pre-configured RO immediately followed by a first symbol of one of the PO; and
receiving, from the wireless device, a PUSCH transmission in a PO associated with the received RA preamble.
18 . The method of claim 15 , wherein the indication of a plurality of pre-configured ROs comprises:
a flag indicating whether the plurality of candidate ROs includes only the last RO in a RACH slot or all ROs of the plurality of candidate ROs;
an integer N≥1 indicating that the plurality of candidate ROs consists of the last N ROs in a RACH slot; or
an indication of a bitmask indicating which pre-configured ROs are candidates for specific PRACH transmissions.