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Project overview

  • Project name: Enabling LTE CA handover to secondary cell
  • Abstract: The goal of the project is to implement handover to secondary cell, i.e. implement the ability to switch primary cell and reconfigure secondary carriers over RRC in runtime.

Technical Approach

In current LTE carrier aggregation (CA) model of ns-3 the handover among different component carriers is not supported. This is because the current LTE CA model does not treat different component carriers as different cells. Additionally, the primary component carrier is fixed and there is no procedure that allows change of a primary component carrier. In order to allow handover among component carriers, it is necessary to enhance SAP interfaces between RRC and component carrier manager (CCM), and also between CCM and lower layers (MAC, PHY) to allow change of primary component carrier, and also to extend the model so that each component carrier is treated as independent cell.

Currently cells have several identifiers. Firstly, according to LTE specifications, each cell has its own Cell ID. UE can learn this ID by communicating with eNB, so it always corresponds to Cell ID on eNB. Secondly, for the needs of scheduler, which is implemented according to LTE Femtocell Scheduler API, each cell is identified internally within eNB by its 8-bit identifier (componentCarrierId).

On the one hand, UE is unnecessarily aware of eNB configuration. componentCarrierIds are configured by LteHelper, so PHYs on eNB and UE correspond one-to-one. However, they don't have to, and it may be convenient to reserve componentCarrierId = 0 for primary CC on each UE. It is even possible that number of PHYs on eNB and UE is not equal. On the other hand, entities such as LteEnbNetDevice have "CellId" attributes, which correspond to "primary" component carrier. As "primary" component carrier is a property of UE, not eNB, it is necessary to remove such attributes.

As distinction between primary and secondary carrier is done on each UE, any flags that identify primary carrier should be removed from eNB and scenario configuration. For example, ComponentCarrier class has "PrimaryCarrier" boolean attribute which should be removed.

General plan is as follows:

  1. Improve architecture of LTE CA to make sure secondary cells are treated the same way as primary cells by eNB and frequency channels can be switched easily on UE.
  2. Implement minimally feasible version of handover procedure on eNB and UE and write unit tests for this procedure.
  3. Implement necessary signalling on RRC to make handover negotiation possible.
  4. Implement handover algorithm to make automatic handover possible.
  5. Write a scenario to validate implemented functionality and provide an example of usage for implemented functionality.

Milestones and Deliverables

Phase 1 (May 30, 2017 - June 30, 2017)

In Phase 1 I made UEs receive secondary cell configuration over the wireless channel during initial cell selection instead of requiring them to be preconfigured and made it possible to use any of the component carriers as primary, including scenarios where different UEs have different primary component carriers.

  1. Made LteHelper create one SpectrumChannel for all carrier frequencies to allow PHYs to switch frequency without need to change the channel object.
  2. Assigned different physical cell IDs to different component carriers of the same eNB.
  3. Extended UeManager to track which cell ID is the primary component carrier for attached UE instead of assuming that primary cell is always on some fixed frequency.
  4. Fixed serialization of RRC messages carrying secondary component carrier configuration and modified UE RRC to read it during initial connection configuration.
  5. Made eNB transmit SIB1 (SystemInformationBlockType1) and SIB2 messages on all frequencies.
  6. Implemented test suite to test that UE can attach to any component carrier (not just the 0-th one) during initial cell selection. Run it with ./ -s lte-secondary-cell-selection
  7. Implemented test suite to test that throughput scales linearly with number of CCs. It proves that initial configuration is correct in both downlink and uplink channels as UE transmits CQI (channel quality indicator) in uplink and eNB transmits data to UE in downlink using high MCS. Run it with ./ -s lte-aggregation-throughput-scale

Report and code review:

Code is already merged upstream:

Phase 2 (June 30, 2017 - July 24, 2017)

In phase 2 I made it possible to trigger UE handover with and without enabled CA (carrier aggregation) in inter-eNB, intra-eNB, inter-frequency and intra-frequency scenarios. Previously only inter-eNB intra-frequency handover was supported and only in non-CA scenarios.

  1. Modified EPC code to make it possible to identify eNB by any of its cell IDs instead of the first one.
  2. Made sure LteEnbRrcProtocolIdeal is able to deliver messages regardless of which CellId is used.
  3. Extended API to make it possible to specify target component carrier for handover.
  4. Extended existing test suite to make it possible to change the number of CCs. This test implements inter-eNB intra-frequency handover scenario and checks that delay is within limits. Run it with ./ -s lte-handover-delay
  5. Implemented a test suite to extensively test all mentioned above handover cases (inter/intra-eNB, inter/intra-frequency, various number of CCs). Run it with ./ -s lte-primary-cell-change

Report and code review:

Code passed review and is ready for merge.

Phase 3 (July 28, 2017 - August 21, 2017)

  1. Updated MeasResults serialization according to 3GPP TS 36.331
  2. Created one measurement object for each CC to allow setting up measurement reports for SCells
  3. Made it possible to setup A1 and A2 events for cells other than PCell
  4. Extended existing A3 and A2/A4 algorithms to setup multiple MeasurementIds for each component carrier
  5. Implemented a test suite to test that A3 algorithm can perform inter-frequency handover based on A3 event measurements. Run it with ./ -s lte-secondary-cell-handover

Project summary

In this section I briefly describe what I have done according to Work Product Submission Guidelines.

In the project I have implemented handover to secondary cell in NS-3 LTE model with enabled carrier aggregation. Implementation of this functionality allows developing and evaluating handover algorithms in multi-carrier scenarios.

The original implementation was mostly based on LTE Release 9 and supported only static carrier aggregation configuration, so all UEs had to use the same frequency for primary cell. I extended the model with LTE Release 10..12 improvements to support UE-specific configuration and dynamic reconfiguration. I improved the model to treat each component carrier as an independent cell able to support UEs configured to use it as their primary component carrier. To support dynamic reconfiguration I implemented previously unsupported intra-eNB inter-frequency handover and extended event-based measurement framework (A1, A2, A3, A4 events) to make it possible to use existing and implement new handover algorithms which make decisions based on secondary cell channel conditions besides already supported primary carrier measurements. All added functionality is tested by either implementing new test suites or extending existing ones.

The project was divided into three milestones:

  1. Make UE (User Equipment) receive configuration via RRC (Radio Resource Control) protocol and test the ability of UE to attach to any component carrier of eNB (Enhanced Node B) and receive configuration in runtime.
  2. Make it possible to trigger manual handover in inter/intra-eNB and inter/intra-frequency cases. Test that handover is successfully performed.
  3. Make it possible to collect measurements from secondary cells and perform automatic handover to secondary cell based on received measurements.

All patches are available in the form of Rietveld code reviews:

Most recent version of the code ready for merge is available at Bitbucket. The easiest way to obtain the code is to clone Mercurial repository from Bitbucket.

At this point all the planned milestones are reached. First milestone code is already merged upstream, second passed review and third is prepared for merge and submitted for review.


Phase 1 statistics

$ hg diff -c bb02103e2b8d | diffstat
28 files changed, 1136 insertions(+), 383 deletions(-)

Phase 2 and Phase 3 merge statistics:

$ hg diff -r 969140f099bb -r 413811934b31 | diffstat
53 files changed, 1537 insertions(+), 818 deletions(-)

Fixed bugs

Some of the bugs encountered and fixed during GSoC are:

Future work

Although all the planned work is done, here is a list of things that I plan to do after GSoC:

  • Resolve any comments that may arise during code review and merge phase 2 and phase 3 code upstream.
  • Fix a noncritical bug I discovered in RRC message serialization during project implementation.
  • Implement A6 event measurements. A6 event is triggered when some neighbor cell becomes better than secondary cell. A6 event configuration is transmitted in ASN.1 extension and serialization of extensions is not supported by NS-3 ASN.1 code yet, so it is impossible to implement right now and requires ASN.1 serialization API changes.