Techniques for supporting co-existence between 2-step random access and 4-step random access
Aspects of the present disclosure provide techniques and apparatus for wireless communication. In one aspect, a method is provided which may be performed by a wireless device such as a user equipment (UE) in a network supporting co-existence of a first type of random access and a second type of random access, which may be associated with different latency, overhead, quality of service (QoS), energy efficiency, and/or spectral efficiency.
1 . A method for wireless communications by a user equipment (UE) in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, comprising:
receiving configuration information from one or more network entities in the network, the configuration information indicating multiple random access channel (RACH) occasions, including at least one of the multiple RACH occasions that is indicated as supported for the UE initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the UE based on a first preamble root sequence index, to the one or more network entities and supported for the UE initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the UE based on a second preamble root sequence index that is different from the first preamble root sequence index, to the one or more network entities; and
performing the first type of random access with at least one network entity of the one or more network entities based, at least in part, on the configuration information, wherein the performing the first type of random access comprises:
transmitting a first message to the at least one network entity to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
receiving a second message associated with completion of the first type of random access, the second message transmitted by the at least one network entity in response to the first message.
2 . The method of claim 1 , wherein the second type of random access comprises: transmission of the second preamble to the at least one network entity to initiate the second type of random access;
reception of a random access response, the random access response transmitted by the at least one network entity in response to the second preamble;
transmission of a third message, in response to the random access response, to the at least one network entity, the third message comprising information transmitted on an uplink shared channel; and
reception of a fourth message associated with completion of the second type of random access, the fourth message transmitted by the at least one network entity in response to the third message.
3 . The method of claim 1 , further comprising receiving the configuration information in: a system information block (SIB), a master information block (MIB), a radio resource control (RRC) message, or a combination thereof.
4 . The method of claim 3 , wherein the SIB is scrambled by a group radio network temporary identifier (RNTI) for the first type of random access.
5 . The method of claim 2 , wherein the multiple RACH occasions comprise at least one of time, frequency, or space resource for initiating the first type or the second type of random access.
6 . The method of claim 2 , wherein the first preamble of the first type of random access comprises: a preamble sequence from a subset of one or more preamble sequences for the second type of random access, a preamble sequence other than the one or more preamble sequences for the second type of random access, or a combination thereof.
7 . The method of claim 6 , wherein the multiple RACH occasions comprise one or more RACH occasions for the first type of random access and occupy a subset of RACH occasions normally for the second type of random access, and wherein the subset of RACH occasions is for the one or more RACH occasions for the first type of random access only.
8 . The method of claim 7 , wherein the subset of RACH occasions for the first type of random access comprises a reduced periodicity, a reduced duty cycle, or a reduced density relative to the RACH occasions normally for the second type of random access.
9 . The method of claim 8 , wherein the multiple RACH occasions comprise: one or more RACH occasions for the first type of random access in a first field of a system information block (SIB), one or more other RACH occasions for the second type of random access in a second field of the SIB, or a combination thereof.
10 . The method of claim 9 , wherein the one or more RACH occasions for the second type of random access are used by a UE initiating the second type of random access, and wherein the initiating the second type of random access comprises a reduced periodicity, a reduced duty cycle, or a reduced density relative to the RACH occasions normally for the second type of random access.
11 . The method of claim 9 , further comprising:
determining the one or more RACH occasions for the first type of random access and the one or more RACH occasions for the second type of random access based on the configuration information; and
transmitting the first message in the determined one or more RACH occasions for the first type of random access and not in the determined one or more RACH occasions for the second type of random access.
12 . The method of claim 7 , wherein the configuration information further comprises at least one parameter indicating at least one of: a number of synchronization signal/physical broadcast channel (PBCH) blocks (SSBs), a number of contention-based preambles for each SSB, or a combination thereof.
13 . The method of claim 7 , wherein a first preamble format for the first type of random access is different from a second preamble format for the second type of random access.
14 . The method of claim 7 , wherein the configuration information further comprises at least one parameter specifically for the first type of random access.
15 . The method of claim 6 , wherein the multiple RACH occasions comprise one or more RACH occasions for the first type of random access and occupy a subset of RACH occasions for the second type of random access, and wherein the subset of RACH occasions is for both the one or more RACH occasions for the first type of random access and the one or more RACH occasions for the second type of random access.
16 . The method of claim 15 , wherein the one or more RACH occasions for the first type of random access are sent in: system information, radio resource control (RRC) messaging, or a combination thereof, for a UE performing the first type of random access only.
17 . The method of claim 15 , wherein the first preamble of the first type of random access comprises a preamble sequence other than the one or more preamble sequences for the second type of random access, and wherein the first preamble of the first type of random access is selected from non-contention based random access (non-CBRA) preambles associated with each of one or more synchronization signal/physical broadcast channel (PBCH) blocks (SSBs).
18 . The method of claim 17 , wherein a number of the non-CBRA preambles associated with each of the one or more SSBs is configured by at least one parameter.
19 . The method of claim 15 , wherein the subset of RACH occasions for both the one or more RACH occasions for the first type of random access and the one or more RACH occasions for the second type of random access is configured by at least one parameter, and wherein the subset of RACH occasions is associated with a same synchronization signal/physical broadcast channel (PBCH) block (SSB) index.
20 . The method of claim 6 , wherein the one or more RACH occasions comprise one or more RACH occasions that are only for the first type of random access and that are different from RACH occasions configured by the at least one network entity for the second type of random access.
21 . The method of claim 20 , wherein the one or more RACH occasions only for the first type of random access are sent in: system information, radio resource control (RRC) messaging, or a combination thereof, for a UE performing the first type of random access only.
22 . The method of claim 20 , wherein the configuration information further comprises at least one parameter indicating at least one of: a number of synchronization signal/physical broadcast channel (PBCH) blocks (SSBs), a number of contention-based preambles for each SSB, or a combination thereof.
23 . The method of claim 20 , wherein a first preamble format for the first type of random access is different from a second preamble format for the second type of random access.
24 . The method of claim 20 , wherein the configuration information further comprises at least one parameter specifically for the first type of random access.
25 . The method of claim 2 , wherein transmission of the payload of the first message is based, at least in part, on a modulation and coding scheme (MCS) for the payload, the MCS being: different from a MCS for the third message of the second type of random access, a subset of the MCS for the third message, or a combination thereof.
26 . The method of claim 25 , wherein the MCS for the payload is based, at least in part, on a size of the payload, a resource allocation of the payload, a waveform of the payload, a coverage condition of the UE, or a combination thereof.
27 . The method of claim 26 , further comprising receiving the configuration information in: a system information block (SIB), a radio resource control (RRC) message, or a combination thereof, wherein the configuration information further comprises configuration for the MCS for the payload.
28 . The method of claim 2 , wherein the configuration information further comprises slot format information only for the first type of random access.
29 . The method of claim 28 , further comprising receiving the configuration information in; a system information block (SIB), a radio resource control (RRC) message, or a combination thereof.
30 . The method of claim 28 , wherein the slot format information indicates a slot-based structure for the first message, a symbol-based structure for the first message, or a combination thereof.
31 . The method of claim 28 , wherein the configuration information further comprises one or more indications of: a number of slots containing one or more physical uplink shared channel (PUSCH) occasions and a number of time domain PUSCH occasions in each slot of the number of slots, and wherein each slot of the number of slots has a same time domain resource allocation.
32 . The method of claim 28 , wherein physical uplink shared channel (PUSCH) occasions that include a guard period are contiguous in time domain within a slot.
33 . The method of claim 1 , wherein the configuration information further comprises at least one of guard band configuration or guard time configuration for the first preamble and for the payload of the first message of the first type of random access.
34 . The method of claim 33 , wherein the at least one of the guard band configuration or the guard time configuration for the first preamble of the first message is same as guard band configuration or guard time configuration for the second preamble of a different first message of the second type of random access, respectively.
35 . The method of claim 33 , wherein the at least one of the guard band configuration or the guard time configuration for the payload is same as the at least one of the guard band configuration or guard time configuration for the first preamble of the first message, respectively.
36 . The method of claim 33 , wherein the at least one of the guard band configuration or the guard time configuration for the payload is different from the at least one guard band configuration or guard time configuration for the first preamble of the first message, respectively.
37 . The method of claim 33 , wherein the guard band configuration for the payload comprises a physical resource block (PRB).
38 . The method of claim 33 , wherein the at least one of guard band configuration or guard time configuration for the first preamble and for the payload of the first message is: received in a system information block (SIB), received in a master information block (MIB), received in a radio resource control (RRC) message, specified in a standard, or a combination thereof.
39 . The method of claim 33 , wherein intra-slot frequency hopping for each physical uplink shared channel (PUSCH) occasion of the first message is configured, and wherein a guard time between hops of the intra-slot frequency hopping is configurable.
40 . The method of claim 1 , wherein the UE comprises an Internet-of-Things (IoT) UE.
41 . A method for wireless communications by a network entity in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, comprising:
determining configuration information indicating multiple random access channel (RACH) occasions, including at least one of the multiple RACH occasions that is indicated as supported for a user equipment (UE) initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the UE based on a first preamble root sequence index, to at least one network entity and supported for the UE initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the UE based on a second preamble root sequence index that is different from the first preamble root sequence index, to the at least one network entity;
outputting the configuration information for one or more UEs in the network; and
performing the first type of random access with at least one UE of the one or more UEs based, at least in part, on the configuration information, wherein performing the first type of random access comprises:
receiving a first message from the at least one UE to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
outputting for the at least one UE, in response to the first message, a second message associated with completion of the first type of random access.
42 . The method of claim 41 , wherein the second type of random access comprises:
reception of the second preamble from the at least one UE to initiate the second type of random access;
transmission of a random access response to the at least one UE, the random access response transmitted by the network entity in response to the second preamble;
reception of a third message from the at least one UE in response to the random access response, the third message comprising information transmitted on an uplink shared channel; and
transmission of a fourth message associated with completion of the second type of random access to the at least one UE, in response to the third message.
43 . The method of claim 41 , further comprising transmitting the configuration information in: a system information block (SIB), a master information block (MIB), a radio resource control (RRC) message, or a combination thereof.
44 . The method of claim 43 , further comprising scrambling the SIB by a group radio network temporary identifier (RNTI) for the first type of random access.
45 . The method of claim 42 , wherein the multiple RACH occasions comprise at least one of time, frequency, or space resource for initiating the first type or the second type of random access.
46 . The method of claim 42 , wherein the first preamble of the first type of random access comprises: a preamble sequence from a subset of one or more preamble sequences for the second type of random access, a preamble sequence other than the one or more preamble sequences for the second type of random access, or a combination thereof.
47 . The method of claim 46 , wherein the multiple RACH occasions comprise one or more RACH occasions for the first type of random access and occupy a subset of RACH occasions normally for the second type of random access, and wherein the subset of RACH occasions is for the one or more RACH occasions for the first type of random access only.
48 . The method of claim 47 , wherein the subset of RACH occasions for the first type of random access comprises a reduced periodicity, a reduced duty cycle, or a reduced density relative to the RACH occasions normally for the second type of random access.
49 . The method of claim 48 , wherein the multiple RACH occasions comprise: one or more RACH occasions for the first type of random access in a first field of a system information block (SIB), one or more RACH occasions for the second type of random access in a second field of the SIB, or a combination thereof.
50 . The method of claim 49 , wherein the one or more RACH occasions for the second type of random access are used by a UE initiating the second type of random access, and wherein the initiating the second type of random access comprises a reduced periodicity, a reduced duty cycle, or a reduced density relative to the RACH occasions normally for the second type of random access.
51 . The method of claim 49 , wherein the at least one UE determines the one or more RACH occasions for the first type of random access and the one or more RACH occasions for the second type of random access based on the configuration information, and further comprising:
receiving the first message from the at least one UE in the determined one or more RACH occasions for the first type of random access and not in the determined one or more RACH occasions for the second type of random access.
52 . The method of claim 47 , wherein the configuration information further comprises at least one parameter indicating at least one of: a number of synchronization signal/physical broadcast channel (PBCH) blocks (SSBs), a number of contention-based preambles for each SSB, or a combination thereof.
53 . The method of claim 47 , wherein a first preamble format for the first type of random access is different from a second preamble format for the second type of random access.
54 . The method of claim 47 , wherein the configuration information further comprises at least one parameter specifically for the first type of random access.
55 . The method of claim 49 , wherein the multiple RACH occasions comprise one or more RACH occasions for the first type of random access and occupy a subset of RACH occasions for the second type of random access, and wherein the subset of RACH occasions is for both the one or more RACH occasions for the first type of random access and the one or more RACH occasions for the second type of random access.
56 . The method of claim 55 , wherein the one or more RACH occasions for the first type of random access are sent in: system information, radio resource control (RRC) messaging, or a combination thereof, for a UE performing the first type of random access only.
57 . The method of claim 55 , wherein the first preamble of the first type of random access comprises a preamble sequence other than the one or more preamble sequences for the second type of random access, and wherein the first preamble of the first type of random access is selected from non-contention based random access (non-CBRA) preambles associated with each of one or more synchronization signal/physical broadcast channel (PBCH) blocks (SSBs).
58 . The method of claim 57 , wherein a number of the non-CBRA preambles associated with each of the one or more SSBs is configured by at least one parameter.
59 . The method of claim 55 , wherein the subset of RACH occasions for both the one or more RACH occasions for the first type of random access and the one or more RACH occasions for the second type of random access is configured by at least one parameter, and wherein the subset of RACH occasions is associated with a same synchronization signal/physical broadcast channel (PBCH) block (SSB) index.
60 . The method of claim 52 , wherein the one or more RACH occasions comprise one or more RACH occasions that are only for the first type of random access and that are different from RACH occasions configured by the at least one network entity for the second type of random access.
61 . The method of claim 60 , wherein the one or more RACH occasions only for the first type of random access are sent in: system information, radio resource control (RRC) messaging, or a combination thereof, for a UE performing the first type of random access only.
62 . The method of claim 60 , wherein the configuration information further comprises at least one parameter indicating at least one of: a number of synchronization signal/physical broadcast channel (PBCH) blocks (SSBs), a number of contention-based preambles for each SSB, or a combination thereof.
63 . The method of claim 60 , wherein a first preamble format for the first type of random access is different from a second preamble format for the second type of random access.
64 . The method of claim 60 , wherein the configuration information further comprises at least one parameter specifically for the first type of random access.
65 . The method of claim 42 , wherein transmission of the payload of the first message is based, at least in part, on a modulation and coding scheme (MCS) for the payload, the MCS being: different from a MCS for the third message of the second type of random access, a subset of the MCS for the third message, or a combination thereof.
66 . The method of claim 65 , wherein the MCS for the payload is based, at least in part, on a size of the payload, a resource allocation of the payload, a waveform of the payload, a coverage condition of the UE, or a combination thereof.
67 . The method of claim 66 , further comprising transmitting the configuration information in: a system information block (SIB), a radio resource control (RRC) message, or a combination thereof, wherein the configuration information further comprises configuration for the MCS for the payload.
68 . The method of claim 42 , wherein the configuration information further comprises slot format information only for the first type of random access.
69 . The method of claim 68 , further comprising transmitting the configuration information in: a system information block (SIB), a radio resource control (RRC) message, or a combination thereof.
70 . The method of claim 68 , wherein the slot format information indicates a slot-based structure for the first message, a symbol-based structure for the first message, or a combination thereof.
71 . The method of claim 68 , wherein the configuration information further comprises one or more indications of: a number of slots containing one or more physical uplink shared channel (PUSCH) occasions and a number of time domain PUSCH occasions in each slot of the number of slots, and wherein each slot of the number of slots has a same time domain resource allocation.
72 . The method of claim 68 , wherein physical uplink shared channel (PUSCH) occasions that include a guard period are contiguous in time domain within a slot.
73 . The method of claim 41 , wherein the configuration information further comprises at least one of guard band configuration or guard time configuration for the first preamble and for the payload of the first message of the first type of random access.
74 . The method of claim 73 , wherein the at least one of the guard band configuration or the guard time configuration for the first preamble of the first message is same as guard band configuration or guard time configuration for the second preamble of a different first message of the second type of random access, respectively.
75 . The method of claim 73 , wherein the at least one of the guard band configuration or the guard time configuration for the payload is same as the at least one of the guard band configuration or guard time configuration for the first preamble of the first message, respectively.
76 . The method of claim 73 , wherein the at least one of the guard band configuration or the guard time configuration for the payload is different from the at least one guard band configuration or guard time configuration for the first preamble of the first message, respectively.
77 . The method of claim 73 , wherein the guard band configuration for the payload comprises a physical resource block (PRB).
78 . The method of claim 73 , wherein the at least one of guard band configuration or guard time configuration for the first preamble and for the payload of the first message is: transmitted in a system information block (SIB), transmitted in a master information block (MIB), transmitted in a radio resource control (RRC) message, specified in a standard, or a combination thereof.
79 . The method of claim 73 , wherein intra-slot frequency hopping for each physical uplink shared channel (PUSCH) occasion of the first message is configured, and wherein a guard time between hops of the intra-slot frequency hopping is configurable.
80 . The method of claim 41 , wherein the at least one UE comprises an Internet-of-Things (IoT) UE.
81 . An apparatus for wireless communication in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, the apparatus comprising:
at least one processor; and
memory coupled to the at least one processor, the memory comprising instructions executable by the at least one processor to cause the apparatus to:
receive configuration information from one or more network entities in the network, the configuration information indicating multiple random access channel (RACH) occasions, including at least one of the multiple RACH occasions that is indicated as supported for the apparatus initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the apparatus based on a first preamble root sequence index, to the one or more network entities and supported for the apparatus initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the apparatus based on a second preamble root sequence index that is different from the first preamble root sequence index, to the one or more network entities; and
perform the first type of random access with at least one network entity of the one or more network entities based, at least in part, on the configuration information, wherein the performing the first type of random access comprises:
transmitting a first message to the at least one network entity to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
receiving a second message associated with completion of the first type of random access, the second message transmitted by the at least one network entity in response to the first message.
82 . An apparatus for wireless communication in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, the apparatus comprising:
at least one processor; and
memory coupled to the at least one processor, the memory comprising instructions executable by the at least one processor to cause the apparatus to:
determine configuration information indicating multiple random access channel (RACH) occasions, including at least one of the multiple RACH occasions that is indicated as supported for a user equipment (UE) initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the UE based on a first preamble root sequence index, to at least one network entity and supported for the UE initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the UE based on a second preamble root sequence index that is different from the first preamble root sequence index, to the at least one network entity;
output the configuration information for one or more UEs in the network; and
perform the first type of random access with at least one UE of the one or more UEs based, at least in part, on the configuration information, wherein performing the first type of random access comprises:
receiving a first message from the at least one UE to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
outputting for the at least one UE, in response to the first message, a second message associated with completion of the first type of random access.
83 . An apparatus for wireless communication in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, the apparatus comprising:
means for receiving configuration information from one or more network entities in the network, the configuration information indicating multiple random access channel (RACH) occasions, including at least one of the multiple RACH occasions that is indicated as supported for the apparatus initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the apparatus based on a first preamble root sequence index, to the one or more network entities and supported for the apparatus initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the apparatus based on a second preamble root sequence index that is different from the first preamble root sequence index, to the one or more network entities; and
means for performing the first type of random access with at least one network entity of the one or more network entities based, at least in part, on the configuration information, wherein the means for performing the first type of random access comprises:
means for transmitting a first message to the at least one network entity to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
means for receiving a second message associated with completion of the first type of random access, the second message transmitted by the at least one network entity in response to the first message.
84 . An apparatus for wireless communication in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, the apparatus comprising:
means for determining configuration information indicating multiple random access channel (RACH) occasions, including one or more RACH occasions indicated at least one of the multiple RACH occasions that is indicated as supported for a user equipment (UE) initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the UE based on a first preamble root sequence index, to at least one network entity and supported for the UE initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the UE based on a second preamble root sequence index that is different from the first preamble root sequence index, to the at least one network entity;
means for outputting the configuration information for one or more UEs in the network; and
means for performing the first type of random access with at least one UE of the one or more UEs based, at least in part, on the configuration information, wherein the means for performing the first type of random access comprises:
means for receiving a first message from the at least one UE to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
means for outputting for the at least one UE, in response to the first message, a second message associated with completion of the first type of random access.
85 . A non-transitory computer-readable medium for wireless communication by a user equipment (UE) in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, the computer-readable medium comprising code, which when executed by at least one processor, causes the UE to:
receive configuration information from one or more network entities in the network, the configuration information indicating multiple random access channel (RACH) occasions, including at least one of the multiple RACH occasions that is indicated as supported for the UE initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the UE based on a first preamble root sequence index, to the one more network entities and supported for the UE initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the UE based on a second preamble root sequence index that is different from the first preamble root sequence index, to the one or more network entities; and
perform the first type of random access with at least one network entity of the one or more network entities based, at least in part, on the configuration information, wherein the performing the first type of random access comprises:
transmitting a first message to the at least one network entity to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
receiving a second message associated with completion of the first type of random access, the second message transmitted by the at least one network entity in response to the first message.
86 . A non-transitory computer-readable medium for wireless communication by a network entity in a network supporting co-existence of a first type of random access and a second type of random access, the first type of random access being a two-step random access and the second type of random access being a four-step random access, the computer-readable medium comprising code, which when executed by at least one processor, causes the network entity to:
determine configuration information indicating multiple random access channel (RACH) occasions, including at least one of the multiple RACH occasions that is indicated as supported for a user equipment (UE) initiating the first type of random access by transmitting at least a first preamble, from a first preamble set that is generated by the UE based on a first preamble root sequence index, to at least one network entity and supported for the UE initiating the second type of random access by transmitting at least a second preamble, from a second preamble set that is generated by the UE based on a second preamble root sequence index that is different from the first preamble root sequence index, to the at least one network entity;
output the configuration information for one or more UEs in the network; and
perform the first type of random access with at least one UE of the one or more UEs based, at least in part, on the configuration information, wherein performing the first type of random access comprises:
receiving a first message from the at least one UE to initiate the first type of random access based, at least in part, on at least one of the multiple RACH occasions, the first message comprising the first preamble and a payload; and
outputting for the at least one UE, in response to the first message, a second message associated with completion of the first type of random access.