A work item for Enhanced CRS and 4Rx SU-MIMO Interference Mitigation Performance Requirements for LTE was approved at the 3GPP RAN #73 meeting [2]. This technical report summarizes the work that RAN WG4 has accomplished in this work item to investigate the feasibility of CRS-IM and SU-MIMO IM receivers. In the LTE Release 11 to 13, multiple UE interference mitigation (IM) receiver enhancements for the interference-limited environments were introduced, including LMMSE-IRC, CRS-IM, SU-MIMO IM, NAICS, and other advanced receivers. The substantial part of the respective 3GPP work was dedicated to the introduction of the CRS Interference Mitigation (CRS-IM) functionality, which was done in the scope of the Release 11 FeICIC WI and Release 12/13 CRS-IM SI and WI. The purpose of the introduced CRS-IM functionality is to specify the receiver mechanisms to mitigate the dominant CRS interferences from the neighbouring cells, which may become a limiting factor for the DL performance for the synchronous homogeneous deployments. As the result of the 3GPP work in the LTE Release 11-13, multiple CRS-IM UE demodulation performance requirements were defined covering some typical LTE operation scenarios. However, still only a subset of the important operation scenarios was addressed in the UE demodulation performance for other scenarios is not guaranteed:

• CRS-IM requirements are introduced for the case of the network deployments using 2 CRS APs only, meanwhile, the UE behaviour for the case of 4 CRS APs deployments is undefined.
• CRS-IM requirements is defined for the case using the same number of CRS APs in the serving and interference cells. Practically, different cells may use different CRS configurations.
• CRS-IM requirements are specified for the UEs equipped with 2 receive antennas only, while the UEs with 4 receive antennas, which are emerging in the market, may also benefit from using the CRS-IM and should be considered.

Therefore, the CRS-IM functionality is actually not defined for many important use cases and without proper UE demodulation requirements it is not possible to guarantee correct UE implementations. The work item [2] aims to investigate and introduce CRS-IM for the generic LTE deployment scenarios with various combinations of the number of CRS APs and number of UE receive chains. In the LTE Release 12 the performance requirements for UEs using enhanced interference cancellation and suppression SU-MIMO receivers were introduced. The advanced receivers such as reduced complexity ML (R-ML) were considered for the intra-cell SU-MIMO inter-stream interference handling. The performance requirements were introduced for the UEs equipped with 2 receive antennas. Recently in the LTE Release 13 the basic performance requirements for the 4 receive antennas UEs were introduced, however, the requirements for the SU-MIMO scenarios are assumed to be based on the simple LMMSE receiver. By using SU-MIMO advanced receiver, which can suppress or cancel the inter-stream interference, the significant gain can be achieved in the typical scenarios. This technical report captures the outcome of the work item technical studies. The content of each specific clause of the report is briefly described as follows. Clause 1 of this document defines the scope and objectives of this study. Clause 4 describes studies on the enhanced CRS-IM performance requirements. Clause 5 describes studies on the enhanced SU-MIMO IM performance requirements. Clause 6 captures the conclusions.

The objective of this report is to investigate feasibility and specify requirements for 1) CRS-IM receivers for the generic scenarios with different number of CRS APs (2, 4) and different number of UE receive antennas and 2) SU-MIMO IM receivers for UEs equipped with 4 receive antennas. The work item [2] has the following detailed objectives:

• Investigate feasibility and specify requirements for the CRS-IM receivers for the generic scenarios with different number of CRS APs (2, 4) and different number of UE receive antennas for synchronous networks:
  • Stage 1: Investigate performance benefits and feasibility of using CRS-IM receivers:
    • Identify target scenarios including deployment scenarios, interference models, and others
      • Reuse Rel-12 NAICS and Rel-13 CRS-IM WIs assumptions as the starting point
    • Evaluate the CRS-IM performance benefits
    • Identify reference CRS-IM receiver structure assumptions including at least number of cancelled cell(s), and IM algorithm.
      • The tradeoff between complexity and performance should be analyzed in order to define reference receivers.
  • Stage 2: Specify UE demodulation and CSI reporting performance requirements to verify practical CRS-IM operation for the identified scenarios based on the outcome of Stage 1.
  • The work should be done under the following assumptions:
    • The following scenarios are prioritized:
      • 2 RX antennas UEs with 4 CRS APs in the serving and interference cell(s)
      • 2 RX antennas UEs with a mix of 2 and 4 CRS APs in the serving and interference cell(s)
      • 4 RX antennas UEs with 2 and 4 CRS APs in the serving and interference cell(s)
    • CRS-IM can be used for different downlink physical channels:
      • First priority: PDSCH (both CRS- and DMRS-based transmission modes)
      • Second priority: DL control channels including PDCCH, PCFICH, PHICH and EPDCCH
    • Non-colliding CRS scenarios are primarily considered.
      • Colliding CRS scenarios are not precluded.
    • Homogeneous deployment scenarios are primarily considered
      • Heterogeneous deployment scenarios are not precluded.
    • Single carrier case.
    • Reference PDSCH receiver structure: LMMSE-IRC with CRS-IM.
• Investigate feasibility and specify requirements for the enhanced SU-MIMO inter-stream interference mitigation (SU-MIMO IM) receivers for the scenarios with 4 receive antennas UEs
  • Stage 1: Investigate performance benefits and feasibility of using SU-MIMO IM receivers for the scenarios with 4 receive antennas UEs.
    • Investigate and agree on target scenarios including layer number for spatial multiplexing, MCS, propagation conditions and others for performance evaluation
      • Reuse the assumptions for Rel-12 SU-MIMO IM (Type-C receiver) and Rel-13 4Rx MIMO performance requirements as starting point.
      • SU-MIMO scenarios
        • rank 2/3/4 SU-MIMO is considered
        • The maximum number of layers (e.g. rank3/4) should be determined based on feasibility study (e.g. operating SNR), realistic Tx EVM assumption and the study on performance gains, reference receiver complexity and testability, etc.
        • Strive to reuse agreed Tx EVM for 4Rx in Rel-13 if the related agreement can be reached
      • Consider 2 and 4 CRS APs scenarios
      • Channel correlations
        • Focus on the Medium, Medium A and High antenna correlation models
      • Modulation order: up to 256QAM
        • The considered modulation orders should be decided based on feasibility study (e.g. operating SNR), realistic Tx EVM assumption, performance gains, reference receiver complexity and testability, etc.
        • Strive to reuse agreed Tx EVM for 4Rx in Rel-13 if the related agreement can be reached.
    • Identify the reference receiver structure for SU-MIMO IM evaluation
      • R-ML is considered in high priority
    • Evaluate the performance of enhanced SU-MIMO IM receivers.
  • Stage 2: Specify UE demodulation performance requirements to verify enhanced SU-MIMO receivers for the UEs equipped with 4 RX antennas
    • Reference receiver structure and other test parameters are based on the outcome of Stage 1.
    • Single carrier case.