Tuesday, 3 November 2015

The Random Access Procedure

1       The introduction of random access procedure
The LTE random access procedure comes in two forms, allowing access to be either contention-based or contention-free.
A UE initiates a contention-based random access procedure. In this procedure, a random access preamble signature is randomly chosen by the UE, with the result that it is possible for more than one UE simultaneously to transmit the same signature, leading to a need for a subsequent contention resolution process.
For the use-cases of a UE in RRC_CONNECTED state, but not uplink-synchronized, needing to receive and a UE in RRC_CONNECTED state, handing over from its current serving cell to a target cell, the eNodeB has the option of preventing contention occurring by allocating a dedicated signature to a UE, resulting in contention-free access. This is faster than contention-based access – a factor which is particularly important for the case of handover, which is time-critical.

1.1         Contention-Based Random Access Procedure
The contention-based procedure consists of four-steps as shown in the figure.


                                 
• Step 1: Preamble transmission;
• Step 2: Random access response;
• Step 3: Layer 2 / Layer 3 (L2/L3) message;
• Step 4: Contention resolution message.
Ø Step 1: Preamble transmission
The UE selects one of the 64 − Ncf available PRACH contention-based signatures, where Ncf is the number of signatures reserved by the eNodeB for contention-free RACH. The set of contention-based signatures is further subdivided into two subgroups, so that the choice of signature can carry one bit of information to indicate information relating to the amount of transmission resource needed to transmit the message at Step 3. The broadcast system information indicates which signatures are in each of the two subgroups (each subgroup corresponding to one value of the one bit of information), as well as the meaning of each subgroup. The UE selects a signature from the subgroup corresponding to the size of transmission resource needed for the appropriate RACH use case (some use cases require only a few bits to be transmitted at Step 3, so choosing the small message size avoids allocating unnecessary uplink resources), which may also take into account the observed downlink radio channel conditions. The eNodeB can control the number of signatures in each subgroup according to the observed loads in each group.
The initial preamble transmission power setting is based on an open-loop estimation with full compensation for the path-loss. This is designed to ensure that the received power of the preambles is independent of the path-loss; this is designed to help the eNodeB to detect several simultaneous preamble transmissions in the same time-frequency PRACH resource. The UE estimates the path-loss by averaging measurements of the downlink Reference Signal Received Power (RSRP). The eNodeB may also configure an additional power offset, depending for example on the desired received Signal to Interference plus Noise Ratio (SINR), the measured uplink interference and noise level in the time-frequency slots allocated to RACH preambles, and possibly also on the preamble format.
Ø Step 2: Random Access Response
The Random Access Response (RAR) is sent by the eNodeB on the Physical Downlink Shared CHannel (PDSCH), and addressed with an ID, the Random Access Radio Network Temporary Identifier (RA-RNTI), identifying the time-frequency slot in which the preamble was detected. If multiple UE had collided by selecting the same signature in the same preamble time-frequency resource, they would each receive the RAR.
The RAR conveys the identity of the detected preamble, a timing alignment instruction to synchronize subsequent uplink transmissions from the UE, an initial uplink resource grant for transmission of the Step 3 message, and an assignment of a Temporary Cell Radio Network Temporary Identifier (C-RNTI) (which may or may not be made permanent as a result of the next step – contention resolution). The RAR message can also include a back off indicator which the eNodeB can set to instruct the UE to back off for a period of time before retrying a random access attempt.
The UE expects to receive the RAR within a time window, of which the start and end are configured by the eNodeB and broadcast as part of the cell-specific system information. The earliest subframe allowed by the specifications occurs 2 ms after the end of the preamble subframe, as illustrated in the figure. However, a typical delay (measured from the end of the preamble subframe to the beginning of the first subframe of RAR window) is more likely to be 4 ms. The next figure shows the RAR consisting of the step 2 message (on PDSCH) together with its downlink transmission resource allocation message ‘G’ (on the Physical Downlink Control CHannel (PDCCH)).

If the UE does not receive a RAR within the configured time window, it retransmits the preamble. The minimum delay for preamble retransmission after the end of the RAR window is 3 ms. (If the UE receives the PDCCH signalling the downlink resource used for the RAR but cannot satisfactorily decode the RAR message itself, the minimum delay before preamble re-transmission is increased to 4 ms, to allow for the time taken by the UE in attempting to decode the RAR.)
Ø Step 3: Layer 2/Layer 3 (L2/L3) Message
This message is the first scheduled uplink transmission on the PUSCH and makes use of Hybrid Automatic Repeat reQuest (HARQ). It conveys the actual random access procedure message, such as an RRC connection request, tracking area update, or scheduling request. It includes the Temporary C-RNTI allocated in the RAR at Step 2 and either the C-RNTI if the UE already has one (RRC_CONNECTED UEs) or the (unique) 48-bit UE identity. In case of a preamble collision having occurred at Step 1, the colliding UEs will receive the same Temporary C-RNTI through the RAR and will also collide in the same uplink time-frequency resources when transmitting their L2/L3 message. This may result in such interference that no colliding UE can be decoded, and the UEs restart the random access procedure after reaching the maximum number of HARQ retransmissions. However, if one UE is successfully decoded, the contention remains unresolved for the other UEs. The following downlink message (in Step 4) allows a quick resolution of this contention. If the UE successfully receives the RAR, the UE minimum processing delay before message 3 transmissions is 5 ms minus the round-trip propagation time. This is shown in figure for the case of the largest supported cell size of 100 km.

Ø Step 4: Contention Resolution Message
The contention resolution message is addressed to the C-RNTI or Temporary C-RNTI, and, in the latter case, echoes the UE identity contained in the L2/L3 message. It supports HARQ. In case of a collision followed by successful decoding of the L2/L3 message, the HARQ feedback is transmitted only by the UE which detects its own UE identity (or C-RNTI); other UEs understand there was a collision, transmit no HARQ feedback, and can quickly exit the current random access procedure and start another one. The Use’s behavior upon reception of the contention resolution message therefore has three possibilities:
1)        The UE correctly decodes the message and detects its own identity: it sends back a positive Acknowledgement, ‘ACK’.
2)        The UE correctly decodes the message and discovers that it contains another Use’s identity (contention resolution): it sends nothing back (Discontinuous Transmission, ‘DTX’).
3)        The UE fails to decode the message or misses the DL grant: it sends nothing back (‘DTX’).
1.2         Contention-Free Random Access Procedure
The slightly unpredictable latency of the random access procedure can be circumvented for some use cases where low latency is required, such as handover and resumption of downlink traffic for a UE, by allocating a dedicated signature to the UE on a per-need basis. In this case the procedure is implied as shown in the figure. The procedure terminates with the RAR.



1       The introduction of random access procedure
The LTE random access procedure comes in two forms, allowing access to be either contention-based or contention-free.
A UE initiates a contention-based random access procedure. In this procedure, a random access preamble signature is randomly chosen by the UE, with the result that it is possible for more than one UE simultaneously to transmit the same signature, leading to a need for a subsequent contention resolution process.
For the use-cases of a UE in RRC_CONNECTED state, but not uplink-synchronized, needing to receive and a UE in RRC_CONNECTED state, handing over from its current serving cell to a target cell, the eNodeB has the option of preventing contention occurring by allocating a dedicated signature to a UE, resulting in contention-free access. This is faster than contention-based access – a factor which is particularly important for the case of handover, which is time-critical.

1.1         Contention-Based Random Access Procedure
The contention-based procedure consists of four-steps as shown in the figure.


                                 
• Step 1: Preamble transmission;
• Step 2: Random access response;
• Step 3: Layer 2 / Layer 3 (L2/L3) message;
• Step 4: Contention resolution message.
Ø Step 1: Preamble transmission
The UE selects one of the 64 − Ncf available PRACH contention-based signatures, where Ncf is the number of signatures reserved by the eNodeB for contention-free RACH. The set of contention-based signatures is further subdivided into two subgroups, so that the choice of signature can carry one bit of information to indicate information relating to the amount of transmission resource needed to transmit the message at Step 3. The broadcast system information indicates which signatures are in each of the two subgroups (each subgroup corresponding to one value of the one bit of information), as well as the meaning of each subgroup. The UE selects a signature from the subgroup corresponding to the size of transmission resource needed for the appropriate RACH use case (some use cases require only a few bits to be transmitted at Step 3, so choosing the small message size avoids allocating unnecessary uplink resources), which may also take into account the observed downlink radio channel conditions. The eNodeB can control the number of signatures in each subgroup according to the observed loads in each group.
The initial preamble transmission power setting is based on an open-loop estimation with full compensation for the path-loss. This is designed to ensure that the received power of the preambles is independent of the path-loss; this is designed to help the eNodeB to detect several simultaneous preamble transmissions in the same time-frequency PRACH resource. The UE estimates the path-loss by averaging measurements of the downlink Reference Signal Received Power (RSRP). The eNodeB may also configure an additional power offset, depending for example on the desired received Signal to Interference plus Noise Ratio (SINR), the measured uplink interference and noise level in the time-frequency slots allocated to RACH preambles, and possibly also on the preamble format.
Ø Step 2: Random Access Response
The Random Access Response (RAR) is sent by the eNodeB on the Physical Downlink Shared CHannel (PDSCH), and addressed with an ID, the Random Access Radio Network Temporary Identifier (RA-RNTI), identifying the time-frequency slot in which the preamble was detected. If multiple UE had collided by selecting the same signature in the same preamble time-frequency resource, they would each receive the RAR.
The RAR conveys the identity of the detected preamble, a timing alignment instruction to synchronize subsequent uplink transmissions from the UE, an initial uplink resource grant for transmission of the Step 3 message, and an assignment of a Temporary Cell Radio Network Temporary Identifier (C-RNTI) (which may or may not be made permanent as a result of the next step – contention resolution). The RAR message can also include a back off indicator which the eNodeB can set to instruct the UE to back off for a period of time before retrying a random access attempt.
The UE expects to receive the RAR within a time window, of which the start and end are configured by the eNodeB and broadcast as part of the cell-specific system information. The earliest subframe allowed by the specifications occurs 2 ms after the end of the preamble subframe, as illustrated in the figure. However, a typical delay (measured from the end of the preamble subframe to the beginning of the first subframe of RAR window) is more likely to be 4 ms. The next figure shows the RAR consisting of the step 2 message (on PDSCH) together with its downlink transmission resource allocation message ‘G’ (on the Physical Downlink Control CHannel (PDCCH)).

If the UE does not receive a RAR within the configured time window, it retransmits the preamble. The minimum delay for preamble retransmission after the end of the RAR window is 3 ms. (If the UE receives the PDCCH signalling the downlink resource used for the RAR but cannot satisfactorily decode the RAR message itself, the minimum delay before preamble re-transmission is increased to 4 ms, to allow for the time taken by the UE in attempting to decode the RAR.)
Ø Step 3: Layer 2/Layer 3 (L2/L3) Message
This message is the first scheduled uplink transmission on the PUSCH and makes use of Hybrid Automatic Repeat reQuest (HARQ). It conveys the actual random access procedure message, such as an RRC connection request, tracking area update, or scheduling request. It includes the Temporary C-RNTI allocated in the RAR at Step 2 and either the C-RNTI if the UE already has one (RRC_CONNECTED UEs) or the (unique) 48-bit UE identity. In case of a preamble collision having occurred at Step 1, the colliding UEs will receive the same Temporary C-RNTI through the RAR and will also collide in the same uplink time-frequency resources when transmitting their L2/L3 message. This may result in such interference that no colliding UE can be decoded, and the UEs restart the random access procedure after reaching the maximum number of HARQ retransmissions. However, if one UE is successfully decoded, the contention remains unresolved for the other UEs. The following downlink message (in Step 4) allows a quick resolution of this contention. If the UE successfully receives the RAR, the UE minimum processing delay before message 3 transmissions is 5 ms minus the round-trip propagation time. This is shown in figure for the case of the largest supported cell size of 100 km.

Ø Step 4: Contention Resolution Message
The contention resolution message is addressed to the C-RNTI or Temporary C-RNTI, and, in the latter case, echoes the UE identity contained in the L2/L3 message. It supports HARQ. In case of a collision followed by successful decoding of the L2/L3 message, the HARQ feedback is transmitted only by the UE which detects its own UE identity (or C-RNTI); other UEs understand there was a collision, transmit no HARQ feedback, and can quickly exit the current random access procedure and start another one. The Use’s behavior upon reception of the contention resolution message therefore has three possibilities:
1)        The UE correctly decodes the message and detects its own identity: it sends back a positive Acknowledgement, ‘ACK’.
2)        The UE correctly decodes the message and discovers that it contains another Use’s identity (contention resolution): it sends nothing back (Discontinuous Transmission, ‘DTX’).
3)        The UE fails to decode the message or misses the DL grant: it sends nothing back (‘DTX’).
1.2         Contention-Free Random Access Procedure
The slightly unpredictable latency of the random access procedure can be circumvented for some use cases where low latency is required, such as handover and resumption of downlink traffic for a UE, by allocating a dedicated signature to the UE on a per-need basis. In this case the procedure is implied as shown in the figure. The procedure terminates with the RAR.
1       The introduction of random access procedure
The LTE random access procedure comes in two forms, allowing access to be either contention-based or contention-free.
A UE initiates a contention-based random access procedure. In this procedure, a random access preamble signature is randomly chosen by the UE, with the result that it is possible for more than one UE simultaneously to transmit the same signature, leading to a need for a subsequent contention resolution process.
For the use-cases of a UE in RRC_CONNECTED state, but not uplink-synchronized, needing to receive and a UE in RRC_CONNECTED state, handing over from its current serving cell to a target cell, the eNodeB has the option of preventing contention occurring by allocating a dedicated signature to a UE, resulting in contention-free access. This is faster than contention-based access – a factor which is particularly important for the case of handover, which is time-critical.

1.1         Contention-Based Random Access Procedure
The contention-based procedure consists of four-steps as shown in the figure.


                                 
• Step 1: Preamble transmission;
• Step 2: Random access response;
• Step 3: Layer 2 / Layer 3 (L2/L3) message;
• Step 4: Contention resolution message.
Ø Step 1: Preamble transmission
The UE selects one of the 64 − Ncf available PRACH contention-based signatures, where Ncf is the number of signatures reserved by the eNodeB for contention-free RACH. The set of contention-based signatures is further subdivided into two subgroups, so that the choice of signature can carry one bit of information to indicate information relating to the amount of transmission resource needed to transmit the message at Step 3. The broadcast system information indicates which signatures are in each of the two subgroups (each subgroup corresponding to one value of the one bit of information), as well as the meaning of each subgroup. The UE selects a signature from the subgroup corresponding to the size of transmission resource needed for the appropriate RACH use case (some use cases require only a few bits to be transmitted at Step 3, so choosing the small message size avoids allocating unnecessary uplink resources), which may also take into account the observed downlink radio channel conditions. The eNodeB can control the number of signatures in each subgroup according to the observed loads in each group.
The initial preamble transmission power setting is based on an open-loop estimation with full compensation for the path-loss. This is designed to ensure that the received power of the preambles is independent of the path-loss; this is designed to help the eNodeB to detect several simultaneous preamble transmissions in the same time-frequency PRACH resource. The UE estimates the path-loss by averaging measurements of the downlink Reference Signal Received Power (RSRP). The eNodeB may also configure an additional power offset, depending for example on the desired received Signal to Interference plus Noise Ratio (SINR), the measured uplink interference and noise level in the time-frequency slots allocated to RACH preambles, and possibly also on the preamble format.
Ø Step 2: Random Access Response
The Random Access Response (RAR) is sent by the eNodeB on the Physical Downlink Shared CHannel (PDSCH), and addressed with an ID, the Random Access Radio Network Temporary Identifier (RA-RNTI), identifying the time-frequency slot in which the preamble was detected. If multiple UE had collided by selecting the same signature in the same preamble time-frequency resource, they would each receive the RAR.
The RAR conveys the identity of the detected preamble, a timing alignment instruction to synchronize subsequent uplink transmissions from the UE, an initial uplink resource grant for transmission of the Step 3 message, and an assignment of a Temporary Cell Radio Network Temporary Identifier (C-RNTI) (which may or may not be made permanent as a result of the next step – contention resolution). The RAR message can also include a back off indicator which the eNodeB can set to instruct the UE to back off for a period of time before retrying a random access attempt.
The UE expects to receive the RAR within a time window, of which the start and end are configured by the eNodeB and broadcast as part of the cell-specific system information. The earliest subframe allowed by the specifications occurs 2 ms after the end of the preamble subframe, as illustrated in the figure. However, a typical delay (measured from the end of the preamble subframe to the beginning of the first subframe of RAR window) is more likely to be 4 ms. The next figure shows the RAR consisting of the step 2 message (on PDSCH) together with its downlink transmission resource allocation message ‘G’ (on the Physical Downlink Control CHannel (PDCCH)).

If the UE does not receive a RAR within the configured time window, it retransmits the preamble. The minimum delay for preamble retransmission after the end of the RAR window is 3 ms. (If the UE receives the PDCCH signalling the downlink resource used for the RAR but cannot satisfactorily decode the RAR message itself, the minimum delay before preamble re-transmission is increased to 4 ms, to allow for the time taken by the UE in attempting to decode the RAR.)
Ø Step 3: Layer 2/Layer 3 (L2/L3) Message
This message is the first scheduled uplink transmission on the PUSCH and makes use of Hybrid Automatic Repeat reQuest (HARQ). It conveys the actual random access procedure message, such as an RRC connection request, tracking area update, or scheduling request. It includes the Temporary C-RNTI allocated in the RAR at Step 2 and either the C-RNTI if the UE already has one (RRC_CONNECTED UEs) or the (unique) 48-bit UE identity. In case of a preamble collision having occurred at Step 1, the colliding UEs will receive the same Temporary C-RNTI through the RAR and will also collide in the same uplink time-frequency resources when transmitting their L2/L3 message. This may result in such interference that no colliding UE can be decoded, and the UEs restart the random access procedure after reaching the maximum number of HARQ retransmissions. However, if one UE is successfully decoded, the contention remains unresolved for the other UEs. The following downlink message (in Step 4) allows a quick resolution of this contention. If the UE successfully receives the RAR, the UE minimum processing delay before message 3 transmissions is 5 ms minus the round-trip propagation time. This is shown in figure for the case of the largest supported cell size of 100 km.

Ø Step 4: Contention Resolution Message
The contention resolution message is addressed to the C-RNTI or Temporary C-RNTI, and, in the latter case, echoes the UE identity contained in the L2/L3 message. It supports HARQ. In case of a collision followed by successful decoding of the L2/L3 message, the HARQ feedback is transmitted only by the UE which detects its own UE identity (or C-RNTI); other UEs understand there was a collision, transmit no HARQ feedback, and can quickly exit the current random access procedure and start another one. The Use’s behavior upon reception of the contention resolution message therefore has three possibilities:
1)        The UE correctly decodes the message and detects its own identity: it sends back a positive Acknowledgement, ‘ACK’.
2)        The UE correctly decodes the message and discovers that it contains another Use’s identity (contention resolution): it sends nothing back (Discontinuous Transmission, ‘DTX’).
3)        The UE fails to decode the message or misses the DL grant: it sends nothing back (‘DTX’).
1.2         Contention-Free Random Access Procedure
The slightly unpredictable latency of the random access procedure can be circumvented for some use cases where low latency is required, such as handover and resumption of downlink traffic for a UE, by allocating a dedicated signature to the UE on a per-need basis. In this case the procedure is implied as shown in the figure. The procedure terminates with the RAR.

Budi Prasetyo

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