LTE-U: Update from 3GPP

LTE-unlicensed or LTE-U was once again a major topic of discussion at the 3GPP RAN plenary meeting last week. Although no Study/Work Items were approved regarding the usage of LTE in unlicensed spectrum, a half-day workshop is planned for sharing ideas on LTE-U. It will be held after RAN #64 meeting on 13.06.2014.

An earlier workshop on the same topic was organised in Jan 2014, attended by 40-odd companies. A summary of that workshop is available in RP-140060. The key points are given below.

Possible use cases / scenarios Main discussion focused on Operator-deployed small cells

  • Indoor and outdoor hotspot
  • Primary cell on licensed spectrum aggregated with secondary cell on unlicensed spectrum
  • Dual connectivity and stand-alone operation were discussed as well

Other scenarios such as user-deployed small cells, Wireless backhaul were also discussed. Potential Technical Requirements

  • Multi-technology coexistence and fairness – Especially LTE – WiFi
  • Multi-operator coexistence and fairness – Especially LTE – LTE
  • In-device coexistence
  • Regulatory requirements, e.g., radar protection

Possible bands Main discussion focused on 5GHz band, considering:

  • Middle (5.4GHz) and upper part (5.8GHz)
  • DL-only and/or UL and DL
  • The need for a (almost) global band was discussed

It was expressed that LTE in unlicensed should be designed to work in (almost) any un-licensed band, e.g. not tailored to a specific band. Other bands (e.g., 3.5GHz in US, TVWS) were also discussed. Potential Technical Features The following new and existing features were discussed

  • FDD-TDD Carrier Aggregation, cross-carrier scheduling
  • Listen-before-talk, carrier sensing, RTS-CTS, Setting 802.11 NAV by sending CTS
  • Small cell on/off and discovery, subframe muting
  • Dynamic frequency/carrier selection
  • Transmit power control
  • Flexible/dynamic duplex
  • CA of more than 100MHz, larger channel BW

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.

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:

Image

 

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