Tag Archives: LTE

LTE-A in Unlicensed Band (LTE-U)

Qualcomm has recently floated the idea of deploying LTE in unlicensed bands, particularly focusing on the 5GHz band, which is currently used mostly for WiFi. According to a document (RP-131635) submitted to the upcoming 3GPP plenary meeting, the proposal is to deploy LTE as Supplemental Downlink (SDL) in 5725-5850 MHz in USA, with the PCell (Primary Cell) always operating on a carrier in a licensed band. Verizon has also submitted a Work Item Proposal (RP-131680) to to introduce the new band for SDL usage. There’s also a Study Item proposal from Ericsson (RP-131788) is the rapporteur to study the modifications necessary to the LTE radio.

These documents can be downloaded from the 3GPP FTP site.

In addition, there’s a presentation from Qualcomm on the same topic.

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RLM, RLF and RRC re-establishment

Radio Link Monitoring (RLM) is one of the important procedures in LTE. It is used to keep track of the radio link condition so that appropriate steps can be taken if Radio Link Failure (RLF) is declared. The figure below (taken from T-doc R2-133859) captures the RLM process:

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From the same T-doc:

PCell radio link monitoring is to determine whether the PCell radio link should be considered as failed (i.e. radio link is worse than Qout for time period determined by N310). If so, UE performs two actions; 1) stopping autonomous uplink transmission by releasing SPS, CQI, SRS, SR and 2) starting cell selection procedure to find a cell providing acceptable radio link.

PCell Functionality in LTE-Advanced

In LTE Rel-1o, the concept of Primary Cell (PCell) and Secondary Cell (SCell) was introduced to support Carrier Aggregation. The PCell is more or less like the serving cell in the non-CA case. The functions provided by the PCell are summarized below (see Section 7.5 of 3GPP TS36.300 for more details).

–     Provides Security inputs

–     Provides NAS mobility functions

–     Have always Uplink and Downlink resources: Carrier frequency (FDD) or UL/DL subframes (TDD)

–     Used for PUCCH transmission

–     Used for RRC connection re-establishment

–     Used for Radio Link Monitoring

–     Can be changed only by Handover

–     Cannot be deactivated

–     Cannot be cross scheduled

–     Semi-persistent resources can only be configured for the PCell

–     UE acquires system information of PCell from the Physical Broadcast Channel (PBCH)

Performance of CS Fallback from LTE to UMTS

One of the key challenges for telcos deploying LTE networks is to provide voice services. While many solutions have been proposed, Circuit-Switched Fallback (CSFB) and Voice over LTE (VoLTE) are currently the two most talked about options. A new paper in the IEEE Communications magazine looks at CSFB performance in great detail. Here’s an excerpt from the abstra

Analysis results in well optimized deployments show that on an average, MO/MT call-setup time for CSFB from LTE to UMTS is around 1 sec greater than legacy UMTS. However, the results can vary depending on the network configuration and the conditions of the measurement. To illustrate the optimization of CSFB performance in real networks, we also highlight the principal call set up optimization and implementation factors impacting CSFB call setup delay and success rates. This article demonstrates that in well optimized networks, CSFB to UMTS call setup success rates can be very close to those achieved in the legacy UMTS systems.

Link to the paper

The reader is also referred to an Alcatel-Lucent whitepaper on the same topic.

Options for Providing Voice over LTE and Their Impact  on the GSM/UMTS Network

Mobility State Estimation in LTE

As part of the HetNet mobility enhancements for LTE, one of the topics being discussed in RAN2 is the accuracy of mobility state estimation (MSE) mechanism. I came across a nice description of the existing MSE scheme in a RAN2 doc (R2-115919). The 3GPP TS36.331 (RRC specification) describes MSE for UEs in RRC_CONNECTED mode as follows:

Step 1: The UE counts handovers during time period t-Evaluation and t-HystNormal.

Note: The UE shall not count consecutive handovers between same two cells.

Step 2: The UE detects its own mobility state based on the number of handovers:

  • If the number of handovers during the time period t-Evaluation exceeds n-CellChangeHigh

–            the UE enters the High-mobility state

  • If the number of handovers during the time period t-Evaluation exceeds n-CellChangeMedium but does not exceed n-CellChangeHigh

–            the UE enters the Medium-mobility state

  • else if criteria for either Medium- or High-mobility state is not detected during the time period t-HystNormal

–            the UE enters the Normal-mobility state

Step 3: The UE scales the timeToTrigger (TTT) based on its mobility state:

  • if the High-mobility state is detected:

–            use the TTT value multiplied by sf-High

  • else if the Medium-mobility state is detected:

–            use the TTT value multiplied by sf-Medium

  • else

–            no scaling is applied

LTE Release 12: Small Cell Enhancements – Higher Layer Aspects

One of the 3GPP Rel-12 Study Items focuses on Small Cell Enhancements.from the perspective of higher layers. There are 3 scenarios under consideration:

Scenario 1:  Macro and small cells on the same carrier frequency, connected via non-ideal backhaul.

Scenario 2: Macro and small cells on different carrier frequencies, connected via non-ideal backhaul.

Scenario 3: Only small cells on one or more carrier frequencies, connected via non-ideal backhaul.

The challenges posed by these scenarios are summarized in the table below.

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To address these challenges, 3GPP’s RAN Working Group 2, is currently working towards a set of solutions, with the following design goals in mind:

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The idea of ‘dual connectivity’ has more or less been accepted as the way forward. This term refers to operation where a given UE consumes radio resources provided by at least two different network points connected with non-ideal backhaul. Each eNB involved in dual connectivity for a UE may assume different roles and these roles do not necessarily depend on the eNB’s power class and can vary among UEs.

Within the dual-connectivity framework, there are two potential solutions being discussed:

Inter-node radio resource aggregation: This solution is aimed at improving per-user throughput by aggregating radio resources in more than one eNB for user plane data transmission.ImageRRC Diversity: This solution is targeted at improving mobility robustness by transmitting handover related RRC signalling could from/to a potential target cell. The RRC diversity scheme could also be applied for handovers from the macro to pico cells, between macro or between pico cells.

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Further details of these solutions can be found in TR36.842.