Multiple Aggregation Levels

Why do we need multiple aggregation levels?

First, to support multiple DCI (Downlink Control Information) formats to improve resource utilization. We know that the DCI size varies a lot depending on the format and the channel bandwidth.  We can view PDCCHs with different aggregation levels as different sizes of container, which can increase the granularity of the resource utilization, instead of a “one size fits all” solution.

Second, to accommodate different RF conditions. The ratio between the DCI size and the PDCCH size indicates the effective coding rate. With the DCI format fixed, higher aggregation levels provide more robust coding and reliability for the UEs under poor RF conditions. For a UE in good RF conditions, lower aggregation levels can save resources.

Third, to differentiate DCIs for control messages and DCIs for UE traffic. Higher aggregation levels can be used for control message resource allocations to provide more protection. The aggregation level for control messages can be 4 or 8, while the aggregation level for UE-specific allocation can be 1 or 2 or 4 or 8.

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Aggregation Level

What is Aggregation Level to use ?

It is defined as number of CCE’s used for sending a control information. Its values can be 1,2,4 and 8.

Search space Number of PDCCH candidates
Type Aggregation level Size [in CCEs]
UE-specific 1 6 6
2 12 6
4 8 2
8 16 2
Common 4 16 4
8 16 2

Resource allocation to PDCCH

Explain Resource allocation to PDCCH? What is CCE ?

Elements allocated to the PDCCH are grouped into quadruplets (groups of 4 Resource Elements). The number of quadruplets(REG) available to the PDCCH is equal to the number of quadruplets within the set of OFDMA symbols signaled by the PCFICH, which have not already been allocated to the PCFICH.

Re source element quadruplets are grouped into Control Channel Elements (CCE). There are 9 quadruplets within a single CCE. i.e. 36 Resource Elements per CCE. The PDCCH uses QPSK modulation so a single CCE can transfer 72 bits.

1 CCE = 9 continuous REG’s ( Resource element Group )
1 REG = 4 RE ( Resource Element )
pdcch

RRC Connection Reestablishment

Why does RRC Connection Reestablishment trigger?

Below are the scenario when RRC Connection Reestablishment triggers:

  • RRC Connection Reconfiguration Failure
  • Upon detecting Radio Link Failure
  • Handover Failure
  • Mobility From E-UTRA Failure
  • Integrity Failure Indication Received From Lower Layers

Difference between X2 and S1 Hand Over

X2 Hand Over:

HO occurs when source and target eNBs are served within the same MME pool. The procedures relies on the presence of X2 interface between Source and Target eNB, which is summarized as follows,

  • Source eNB makes HO decision and setup a direct tunnel i.e X2 transport bearer between Source and target eNB.
  • Detach UE from Source eNB and Forward traffic from source eNB to Target.
  • Path switch procedure between Target eNB and MME
  • Releases S1 bearer of source eNB
  • Release X2 transport bearer for direct packet forwarding.

S1 Hand Over:

S1 handover is when If two eNodeBs are not connected with same MME or the X2 interfaces are not defined between eNB or when X2 procedure fails(due to unreachability/Error response etc). Summary of S1-HO is as follows,

  • Source eNB makes HO decision and setup a indirect tunnel i.e S1 bearer between Source eNB and SGW, and target eNB and SGW.
  • S1 bearer for UL setup between target and source eNB
  • Detach UE from Source eNB and indirect packet forwarding
  • No need for the Path switch procedure between Target eNB and MME, as MME is aware of HO
  • Releases S1 bearer of source eNB
  • Release S1 transport bearer for indirect packet forwarding.

If the two eNodeBs are connected with same MME, it is preferred to perform X2 based handover but there is no restriction in using S1 based handover even in this case. If two eNodeBs are not connected with same MME, you have to perform S1 based handover even in this case.

X2 handover is faster as compared to S1. X2 handover is defined only for intra LTE handover while S1 handover can be used for intra LTE, inter RAT.

Handover types in LTE

The Handover is the process of transferring an ongoing data session/Call from
one (source) cell connected to the core network to another (target) cell. Handovers are needed when UE moved out of its serving cell’s coverage or for load balancing purposes.

In mobile communication, Handover can either be Network controlled (i.e. HO decision is with network) or Mobile Evaluated (i.e. Mobile terminal makes HO decision and inform Network to arrange resources on target cells)

LTE uses both the approaches in a way that, LTE capable UE sends measurement report to network and based on this report; network directs UE to move to a target cell.

Handover Types in LTE:

  • Intra-LTE Handover:  Source and target cells are part of the same LTE network.
    • Handover using X2 Interface
    • Handover using S1 Interface

What is difference between X2 and S1 HO?

If more than one frequencies/carrier are being in use for LTE then HO can further be distinguished with (Inter or Intra eNodeB) InterFreq and (Inter or Intra eNodeB) IntraFreq Hand overs.

  • Inter-LTE Handover:  Handover happens towards other LTE nodes. (Inter-MME and Inter-SGW
    • Inter-MME Handover
    • Inter-MME/SGW Handover
  • Inter-RAT Handover: Handover between different radio technologies. For example handover from LTE to WCDMA.

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