LTE: TTI Bundling

Why Bundling?

TTI Bundling optimizes the uplink (cell edge) coverage for services like VoLTE. For services such as VoIP, the packet size is small and the inter-arrival time of VoIP packets is constant (i.e., AMR codec provides one packet every 20ms during active period and one silence indicator (SID) at 160ms). In a cell edge scenario, the probability of decoding PUSCH as error is more so instead of waiting for HARQ feedback from eNB, the UE performs initial transmission and re-transmissions in 4 consecutive Transmit Time Intervals (TTIs)

Once TTI Bundling is enabled/activated the UE send the same packet but with different error detection and correction bits in 4 consecutive TTIs

TTI Bundling reduces a lot of signalling overhead as the eNB doesn’t need to send Ack/Nack for every (re-) transmission instead it sends Ack/Nack for the entire bundle

Latency is also reduced as no waiting time (for HARQ feedback) is required between the re-transmissions

The simulation results indicated that TTI bundling provides a cell edge gain of more than 4 dB in terms of the sustainable path loss

TTI Bundling Configuration

• Configuration of TTI Bundling is dedicated i.e., per UE basis. TTI Bundling might be configured by eNB in situation like UE is operating at its maximum output Tx Power
• TTI Bundling IE is under RadioResourceConfigDedicated => mac-MainConfig => ul-SCH-Config
• TRUE indicates that TTI bundling is enabled while FALSE indicates that TTI bundling is disabled.
• TTI bundling can be enabled for FDD and for TDD only for UL/DL configurations 0, 1 and 6
• For TDD, E-UTRAN does not simultaneously enable TTI bundling and Semi-Persistent Scheduling (at least until rel-10)
• E-UTRAN does not simultaneously configure TTI bundling and SCells with configured uplink (uplink Carrier Aggregation)
• Default configuration of TTI Bundling is ‘release’
• The UE indicates the support of TTI Bundling with FGI 28

 TTI Bundling Operation

Initial Bundle Transmission

For DCI0 detected in subframe n-4, the UE shall transmit the first PUSCH in subframe n and non-adaptive retransmissions in n+1, n+2, and n+3 with corresponding Redundancy Versions set as 0, 2, 3, and 1

UE shall expect Ack/Nack in subframe n+7

Bundle re-Transmission

For DCI0 and/or Nack detected in subframe n-9, the UE shall adjust the first PUSCH transmission in n and non-adaptive retransmissions in n+1, n+2, and n+3 with corresponding Redundancy Versions set as 0, 2, 3, and 1

UE shall expect Ack/Nack in subframe n+7

• Number of TTIs in a bundle is always equal to 4
• Each transmission/re-transmission of a bundle uses same HARQ Process
• Within a bundle HARQ re-transmissions are non-adaptive and triggered without waiting for feedback for previous transmissions
• The HARQ feedback of a bundle is only received for the last TTI of the bundle regardless of whether a transmission in that TTI takes place or not (e.g. when a measurement gap occurs). Say for example, in above figure, measurement gap starts in subframe#1 (the measurement GAP is until subframe#6) ⇨ The UE should still expect HARQ feedback in subframe#7 even though (re-) transmissions in subframes #1, #2, and #3 didn’t actually occur
• A retransmission of a TTI bundle is also a TTI bundle. As shown in the figure above, after receiving NACK at subframe#7, the UE performs retransmission as a bundle starting from subframe #16
• For FDD, when the TTI bundling is enabled, there shall be 4 uplink HARQ Processes. For TDD, there are 3 HARQ processes for UL/DL Configuration 0 and 6 whereas 2 HARQ processes used in case of UL/DL Configuration 1
• For transmission of PUSCH that corresponds to Msg3 TTI bundling does not apply

LTE: Activate Dedicated EPS Bearer Context Reject

·        The UE may reject the ACTIVATE DEDICATED EPS BEARER CONTEXT REQUEST from the MME by sending an ACTIVATE DEDICATED EPS BEARER CONTEXT REJECT message.

·        The ACTIVATE DEDICATED EPS BEARER CONTEXT REJECT message shall include the EPS Bearer Identity and an ESM Cause value indicating the reason for rejecting the dedicated EPS bearer context activation request.

·        The ACTIVATE DEDICATED EPS BEARER CONTEXT REJECT message contains an ESM cause that typically indicates one of the following values:

#26: insufficient resources

#31: request rejected, unspecified

#41: semantic error in the TFT operation

#42: syntactical error in the TFT operation

#43: invalid EPS bearer identity

#44: semantic error(s) in packet filter(s)

#45: syntactical error(s) in packet filter(s) or

#95 – 111: protocol errors

·        After receiving the ACTIVATE DEDICATED EPS BEARER CONTEXT REJECT message, the MME shall stop the timer T3485 and abort the dedicated EPS bearer context activation procedure

Reference 3GPP TS 24.301

Example: ACTIVATE DEDICATED EPS BEARER CONTEXT REJECT message

LTE: Semi-Persistent Scheduling

In the case of Dynamic Scheduling, the UE can get scheduling assignments/grants in every subframe. This gives the network full flexibility in assigning the resources to the UE at the cost of transmission of resource allocation information on PDCCH in every subframe. This also gives the flexibility of varying the resource allocation based on the reported channel conditions

For services such as VoIP, the packet size is small and the inter-arrival time of VoIP packets is constant (i.e., AMR codec provides one packet every 20ms during active period and one silence indicator (SID) at 160ms). The control signaling overhead (PDCCH) is too much for the E-UTRAN in order to support a large number of VoIP users. So, the optimal solution is to allocate the resources at once and let the UE use these resources instead of allocating the resources periodically.

How does the eNB configure the UE with SPS?

The eNB can configure the UE with SPS at any time but, typically this is done at the time of dedicated bearer establishment for the VoIP service. SPS can be configured/re-configured by RRC at any time using SPS-Config. This SPS-Config includes the configuration for semiPersistSchedC-RNTI (sps-CRNTI), sps-ConfigDL and sps-ConfigUL.

SPS can be configured only in the uplink (sps-ConfigUL), or in the downlink (sps-ConfigDL) or in both directions. Configuration of SPS doesn’t mean that the UE can start using SPS grants/assignments. The eNB has to explicitly activate SPS as explained below, in order for the UE to use SPS grants/assignments.  So, SPS configuration and activation are two different things, eNB first configures the UE with SPS and then activates the same. The eNB can explicitly release SPS without release SPS RRC configuration

SPS Activation/Release

When configuring SPS in any direction either UL or DL, SPS C-RNTI is mandatorily provided by the eNB. Soon after the UE is configured with SPS C-RNTI, the UE is configured by higher layers to decode PDCCH with CRC scrambled by the SPS C-RNTI.

A UE shall monitor PDCCH with CRC scrambled by the SPS C-RNTI in every subframe as the eNB can activate/re-activate/release SPS at any time using Downlink control information (DCI)

DCI Format 0 is used to activate/release SPS in UL; DCI Format 1/1A/2/2A/2B/2C is used to activate SPS in DL. In order to release SPS in DL, only DCI Format 1A can be used. The received DCI format on SPS C-RNTI can be a grant/assignment for a retransmission or for activation/re-activation/release of SPS. 3GPP TS 36213 has tabulated the validation procedure for activation/re-activation/release of SPS. A UE shall validate a SPS assignment PDCCH only if all the following conditions are met:

·     the CRC parity bits obtained for the PDCCH payload are scrambled with the SPS C-RNTI

·      the new data indicator field is set to ‘0’. In case of DCI formats 2, 2A, 2B and 2C, the new data indicator field refers to the one for the enabled transport block

Validation is achieved if all the fields for the respective used DCI format are set according to Table 9.2-1 or Table 9.2-1A. If validation is achieved, the UE shall consider the received DCI information accordingly as a valid SPS activation or release.  If validation is not achieved, the received DCI format shall be considered by the UE as having been received with a non-matching CRC

SPS Configuration details

The following high level IEs are provided by RRC.

  

SPS-ConfigDL has the following IEs (for the FDD mode of operation)

SPS-ConfigUL has the following IEs (for the FDD mode of operation)

SPS Operation in Uplink direction

For UL SPS, SPS C-RNTI, semiPersistSchedIntervalUL, implicitReleaseAfter and optionally p0-Persistent values are configured by RRC. After configuring SPS in UL direction, the eNB can send DCI Format 0 on SPS C-RNTI to activate the SPS in UL direction. After receiving activation command, the UE should consider that the uplink grant has been configured and should consider sequentially that the N^th grant occurs in the subframe for which the below condition satisfies:

(10 * SFN + subframe) = [(10 * SFN_{start time} + subframe_{start time}) + N * semiPersistSchedIntervalUL] modulo 10240

Where SFN_{start time} and subframe_{start time} are the SFN and subframe, respectively, at the time the configured uplink grant were (re-) initialized.
So, the UE shall use the grant provided in the SPS activation DCI Format 0 once every semiPersistSchedIntervalUL subframes and shall transmit PUSCH using this grant. If there is no data to be transmitted, the Multiplexing and Assembly entity shall provide MAC PDU containing zero MAC SDU to be transmitted on Semi-Persistent Scheduling resource. The UE shall clear the configured uplink grant immediately after implicitReleaseAfter number of consecutive new MAC PDUs each containing zero MAC SDUs, on the Semi-Persistent Scheduling resource. This is called as implicit release of UL SPS configured grant. After this the UE shall not use this configured grant for new transmissions at the occasions satisfying the above equation. Note that, retransmissions for Semi-Persistent Scheduling can continue after clearing the configured uplink grant

The eNB can explicitly send DCI Format 0 to indicate SPS release. Upon receiving DCI Format 0 which indicates SPS release, the UE should clear the configured uplink grant

Optionally, the eNB can configure the UE with p0-Persistent which is used to calculate the transmit power of PUSCH using SPS C-RNTI if the eNB wants the UE to use different power as compared to PUSCH on C-RNTI. Basically, in the PUSCH Tx power calculation, the UE should use p0-NominalPUSCH-Persistent and p0-UE-PUSCH-Persistent in place of p0-NominalPUSCH and p0-UE-PUSCH respectively. If the IE p0-Persistent absent, the UE shall use p0-NominalPUSCH and p0-UE-PUSCH in PUSCH Tx power calculation

SPS Operation in Downlink direction

For DL SPS, SPS C-RNTI, semiPersistSchedIntervalDL, numberOfConfSPS-Processes and n1PUCCH-AN-PersistentList values are configured by RRC. After configuring SPS in DL direction, the eNB can send DCI Format 1/1A/2/2A/2B/2C on SPS C-RNTI to activate the SPS in DL direction. From this point on the UE should consider that the downlink assignment has been configured and should consider sequentially that the N^th assignment occurs in the subframe for which the below condition satisfies:

(10 * SFN + subframe) = [(10 * SFN_{start time} + subframe_{start time}) + N * semiPersistSchedIntervalDL] modulo 10240

Where SFN_{start time} and subframe_{start time} are the SFN and subframe, respectively, at the time the configured downlink assignments were (re-) initialised.

The UE shall use this configured assignment once in every semiPersistSchedIntervalDL subframes and decode PDSCH (no corresponding PDCCH).

In order to transmit HARQ response (Ack/Nack) on PUCCH, the UE would need to calculate frequency resources. In the dynamic scheduling case, a PDCCH (DCI) is present, so the UE calculates Ack/Nack resources based on n_CCE. For the SPS case, from the DCI format indicating DL SPS activation, the TPC command for PUCCH field shall be used as an index to one of the four PUCCH resource values (n1PUCCH-AN-PersistentList) configured by RRC.

If the eNB wants to release configured assignment without releasing RRC configuration, it would explicitly send DCI Format 1A to indicate SPS release. Upon receiving DCI Format 1A which indicates SPS release, the UE should clear the configured downlink assignment

Another important aspect is the HARQ process ID calculation. In case of dynamic scheduling the corresponding DCI format indicates the HARQ process ID, whereas in the case of SPS, there is no PDCCH corresponding to PDSCH. The HARQ process ID needs to be calculated based on the below equation:

HARQ Process ID = [floor (CURRENT_TTI/semiPersistSchedIntervalDL)] modulo numberOfConfSPS-Processes

where CURRENT_TTI = [(SFN * 10) + subframe number]. Current TTI is the TTI in which PDSCH has been received on the configured assignment

Re-transmission Handling

Re-transmission handling on SPS C-RNTI (UL or DL) is an important aspect. When using dynamic scheduling, the eNB would use same NDI (New Data Indicator) as was sent for initial (new) transmission to indicate the grant/assignment received is for a re-transmission. When using SPS, the grant/assignment for new transmission is not provided every time as it is configured already. It has been already explained for SPS activation/release, that NDI is set to 0. If the eNB wants to send a grant/assignment for a retransmission it would toggle NDI i.e., NDI is always set to 1 for a re-transmission

Notes:

o   When Semi-Persistent Scheduling for uplink or downlink is disabled by RRC, the corresponding configured grant or configured assignment shall be discarded

o    Semi-Persistent Scheduling is supported on the PCell only

o    The UE should clear any configured downlink assignments and uplink grants when timeAlignmentTimer expires or during MAC reset or when UE has transmitted the scheduling request for maximum number of times (dsr-TransMax)

o    Except for handover or releasing SPS, E-UTRAN does not reconfigure sps-Config (RRC) when there is a configured downlink assignment or a configured uplink grant


SPS Occasions can be calculated using the below tool!

sps-Interval           activation-SFN           activation-Subframe          UL-DL Config
10 20 32 40 64 80 128 160 320 640                                                         0 1 2 3 4 5 6

uplink SPS?            downlink SPS?             Is TDD?             twoIntervalConfig?

SPS Occasions will be displayed here

LTE: Downlink Resource Allocation Type 1

Resource Allocation Type 1 uses the same number of RA bits as in Resource Allocation Type 0 but allows Resource Allocation on an individual RB level even for larger bandwidths.

The Resource Allocation Information consists of N_RBG bits which will indicate the UE about a set of VRBs (Localized Type) from one of the P RBG subsets. The number of RBG subsets (P) is same as RBG size (P) in RA Type 0 which is as given below:

In an RBG subset number p, where 0 ≤ p < P, it is possible to allocate every Pth RB starting from pth RB and this allocation can be done using a bitmap.

A total of ⎾N_RB^DL­­/P⏋bits are required for resource allocation as in RA Type 0. Resource Block assignment consists of 3 fields.

The First field with ⎾log₂ (P)⏋ bits used to indicate the selected RBG subset number among P RBG subsets.

The second field with one bit is used to indicate a shift of the resource allocation span within a subset. A bit value of 1 indicates shift is triggered. Shift is not triggered otherwise

The third field indicates a bitmap, where each bit of the bitmap addresses a single VRB in the selected RBG subset. The VRB is allocated to the UE if the corresponding bit value in the bit field is 1, the VRB is not allocated to the UE otherwise. The portion of the bitmap used to address VRBs in a selected RBG subset has size N_RB^TYPE1 and is defined as:

N_RB^TYPE1 = ⎾N_RB^DL­­/P⏋— ⎾log₂ (P)⏋ — 1

The addressable VRB numbers of a selected RBG subset start from an offset, ∆_shift(p)   to the smallest VRB number within the selected RBG subset, which is mapped to the MSB of the bitmap. The offset is in terms of the number of VRBs and is done within the selected RBG subset. If the value of the bit in the second field for shift of the resource allocation span is set to 0, the offset for RBG subset p is given by ∆_shift(p) = 0. Otherwise, the offset for RBG subset p is given by ∆_shift(p) = N_RB^RBGsubset(p) — N_RB^Type1, where the LSB of the bitmap is justified with the highest VRB number within the selected RBG subset. N_RB^RBGsubset(p) is the number of VRBs in RBG subset p and can be calculated by the following equation,

Consequently, when RBG subset p is indicated, bit i for i = 0, 1, …, N_RB^Type1 — 1 in the bitmap field indicates VRB number

Example:

Let us consider N_RB^DL = 15. From the above table, number of RBG subsets (P) is 2. In this case, 1-bit (log₂ (2)) is required to indicate the RBG subset number. One more bit is used to indicate whether a shift is used or not.  N_RB^TYPE1 = 8-1-1 = 6 bits are used for actual resource allocation bitmap. The resource allocation is illustrated below: 

Let the resource allocation bits are 00110011. MSB indicates RBG subset number, second bit indicates if shift is enabled or not. Remaining 6 bits indicate the bitmap of resource allocation of PRBs. In this case, RBG subset 0 is chosen without a shift. Using the remaining 6 bits 110011, RB numbers 0, 1, 8, and 9 are allocated to the UE. Similarly, if resource allocation bits are 11110011 ⇨ RB numbers 3, 6, 11, and 14 are allocated to the UE.

Reference: 3GPP TS 36.213

LTE: Downlink Resource Allocation Type 2 and Uplink Resource Allocation Type 0

This type of resource allocation is mainly used for contiguous RB allocations for uplink (RA Type0) and for compact scheduling of downlink assignments (RA Type 2).

In this type, the resource block assignment information indicates to a scheduled UE a set of contiguously allocated localized VRBs or distributed VRBs.

In case of resource allocation signaled with PDCCH DCI format 1A, 1B or 1D, one bit flag indicates whether localized VRBs or distributed VRBs are assigned (value 0 indicates Localized and value 1 indicates Distributed VRB assignment) . In the case of resource allocation signaled with PDCCH DCI format 1C, only distributed VRBs are assigned.

Localized VRB allocations for a UE vary from a single VRB up to a maximum number of VRBs spanning the system bandwidth.

For indicating contiguous RB assignment, starting position of the RB (RB_start) and the number of RBs is required. Let us consider RB_start = 0^th RB, the number of combinations possible = N_RB. Similarly when RB_start = 1^st RB, then the number of possible combinations are N_RB – 1 and so on. There are N_RB.(N_RB + 1)/2 combinations possible in total.

For downlink, PDCCH DCI format 1A, 1B or 1D, a type 2 resource allocation field consists of a Resource Indication Value (RIV) corresponding to a starting resource block (RB_start) and a length in terms of virtually contiguously allocated resource blocks L_CRBs.

For uplink, a resource allocation (type 0) field in the scheduling grant consists of a resource indication value (RIV) corresponding to an RB_start and a length in terms of contiguously allocated physical resource blocks (L_CRBs ≥ 1). The RIV value for both uplink and downlink is defined by:

        RIV = N_RB (L_CRBs — 1) + RB_start                                            if (L_CRBs — 1) ≤ ⎿ N_RB /2⏌

                = N_RB (N_RB — L_CRBs + 1) + (N_RB — 1 — RB_start)         otherwise

Example:

Let us consider N_RB = 6 ⇨ N_RB.(N_RB + 1)/2 = 21 ⇨  5-bits are required for indicating any RIV value ranging from 0 to 20. The RIV values for N_RB = 6 are illustrated below. Let RB_start = 2 and L_CRBs = 3, from the above equations RIV = 14

Reference: 3GPP TS 36.213

LTE: Downlink Resource Allocation Type 0

In Resource Allocation Type 0, Resource Block assignment information includes a bitmap indicating a set of Resource Block Groups (RBGs) to the scheduled UE. An RBG is a set of consecutive PRBs. 

In RA Type 0, the signaling overhead is reduced since the bitmap is defined on a group of RBs. The size of RBG (P) is a function of system bandwidth as shown in the table below:

The total number of RBGs (N_RBG) for downlink system bandwidth N_RB^DL­­ is given by N_RBG = ⎾N_RB^DL­­/P⏋where ⎿N_RB^DL­­/P⏌of the RBGs are of size P and if N_RB^DL­­ mod P  > 0 then 

one of the RBGs is of  size N_RB^DL­­ –  P.⎿N_RB^DL­­/P⏌.  

The bitmap is of size N_RBG bits with one bitmap bit per RBG such that each RBG is addressable

An RBG is allocated to the UE if the corresponding bit value in the bitmap is 1, the RBG is not allocated to the UE otherwise

From the above table it could be noted that for smaller bandwidths (N_RB^DL ≤ 10) the value of P = 1 (RBG size = 1) which means that each RB can be addressed with a bit in the bitmap.

The RBG size (P) is increased with system bandwidth as smaller values of P would require more number of bits to address the entire bandwidth

Example:

Let us consider N_RB^DL = 15. From the above table, P = 2, so total number of RBGs N_RBG = 8 out of which 7 RBGs are of size 2 RBs and one RBG is of size 1 RB. The total number of bits required for resource allocation is equal to N_RBG which is 8 in this case. The bitmap is done as shown in the below:

In this example if the resource allocation information is 11000001, this means RBGs 1, 2 and 8 (5 RBs in total) are allocated and the remaining RBGs are not allocated to the UE.


Reference: 3PGP TS 36.213

LTE: Resource Allocation

The information on which Resource Blocks (RBs) are allocated (Resource Allocation) for both Uplink and Downlink needs to be signalled to the UE.

The Resource Allocation information is carried on Downlink Control Information (DCI) by Physical Downlink Control Channel (PDCCH). Resource Allocation field is one of the major field in the DCI.

In the case of Uplink, the allocated RBs have to be contiguous in order to guarantee single-carrier property (SC-FDMA is used in the Uplink). The contiguous nature of resource allocation requires less number of bits but the scheduler in the eNB will have less flexibility in allocating the resources.

  

In the case of Downlink, the allocated RBs doesn’t need to be contiguous which will require more bits to signal the Resource Allocation but the eNB’s scheduler will have more flexibility in allocating the resources.

In LTE, ResourceAllocation Type 0, Resource Allocation Type 1 and Resource Allocation Type 2 are defined.

Resource Allocation Type 0 and Resource Allocation Type 1 are used for non-contiguous resource allocation (only for Downlink) which uses bit-map based signaling. 

For allocating contiguous resource blocks in the Uplink, Resource Allocation Type 0 is used where as same for downlink it is called as Resource Allocation Type 2.

The UE shall interpret the Resource Allocation field depending on the PDCCH DCI Format detected. 

For Uplink, Resource Allocation information is conveyed on PDCCH DCI Format 0 using Resource Allocation Type 0.

In the Downlink, PDCCH DCI Formats 1, 2, 2A and 2B uses Resource Allocation Type 0 or Type 1  whereas PDCCH DCI Formats 1A, 1B, 1C and 1D uses Resource Allocation Type 2

Click here for Resource Allocation Type 0 in the Downlink. Check here for Downlink Resource Allocation Type 1. 

Uplink Type 0 and Downlink Type 2 are explained in post 'Downlink Resource Allocation Type2 and Uplink Resource Allocation Type0'.

Reference: 3GPP TS 36.213