A study item for minimum performance requirements for interference Mitigation of Cell-specific Reference signals (CRS-IM) for LTE/LTE-Advanced (FDD/ TDD) homogeneous network was approved at the 3GPP RAN #59 meeting [3]. This technical report summarizes the work that RAN4 has accomplished in this study item to assess the feasibility of CRS-IM UE receivers for Homogenous deployment. These receivers attempt to mitigate the CRS interference that arises from users operating outside the serving cell. CRS-IM has been studied in the Rel-11 Work Item on FeICIC. Due to its benefits under the condition where interference from CRS dominates but is negligible from data assuming data RE muting, the CRS-IM has been standardized under the above operation conditions for heterogeneous deployments and the corresponding requirements were specified in Release 11. At the same time, the enhanced performance requirements in homogeneous network deployment for MMSE interference rejection combining (MMSE-IRC) receiver were extensively studied and specified in Release 11. The MMSE-IRC receiver can suppress both data and CRS interference without the need to differentiate them. However, the UE performance in homogenous deployment may be further improved by using the same CRS-IM capability as defined for FeICIC, especially when similar interference conditions arise under low traffic loading of neighbouring cells and under similar scope as in FeICIC i.e. synchronous network operation and UE reference receiver assuming single FFT processing. In past link level evaluations, this type of interference has been modelled as almost blank subframe, as defined in FeICIC WI, or full traffic, as defined in MMSE-IRC receiver. The study item has developed models for this interference in terms of their powers relative to the total other cell interference power, and their resource allocation according to traffic loading levels. Some of these previously identified scenarios studied in Release 11 for specifying MMSE-IRC receiver performance requirements and the corresponding link/system parameters are reused. The baseline detector structure is that of an MMSE-IRC detector as defined in [2] combined with a CRS-IM receiver. The same CRS-IM receiver defined for FeICIC is reused here. LTE throughput estimates are developed using link level simulations. In addition, system level performance is assessed to determine the gains that interference mitigation receiver might provide in throughput and coverage. Complexity issues associated with implementing these types of receivers are also discussed. 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 feasibility study. Clause 4 describes the receiver methods that can be applied to CRS-IM receivers. Clause 5 describes the network scenarios that were defined and used to generate the interference statistics, which were then used to develop the interference models described in clause 6. Clause 6 defines the interference models/profiles that were developed in order to assess the link level performance of enhanced receivers.

The objectives of this Study Items are:

• Identify the partial traffic loading levels and other realistic system level parameters (e.g. traffic and interference models including interference level, time offset between cells and frequency offset between cells) and performance metrics for studying the feasibility of CRS IM in a synchronized homogenous network assuming:
  • 3GPP Case 1 as a starting point
  • non-colliding CRS between serving and dominant aggressor/interfering cells and
  • non-colliding CRS between dominant aggressor/interfering cells
  • The homogeneous deployment and relevant system parameters should reuse as much as possible those defined in the Rel-11 performance study of MMSE-IRC
• Identify the baseline receiver which can be used for evaluating the gain of CRS IM in a synchronized homogenous network considering:
  • Reuse of CRS IM receiver assumed for Release 11 FeICIC.
  • Reuse of MMSE-IRC receiver as the baseline receiver. MMSE-IRC does not differentiate CRS or data interference when suppressing them and was assumed for Release 11 work item on interference rejection combining.
• Agree on interference levels, interference models and simulation parameters for link level evaluations.
• Evaluate the system level and link level gains of CRS IM with respect to the baseline MMSE-IRC receiver in a synchronized homogenous network deployment under the various loading levels identified.
  • Gains of CRS IM from 1 and 2 aggressor cells CRS shall be evaluated and compared.
  • Modeling of realistic CRS IM receiver for system level performance evaluation should be clarified and agreed if possible
• Investigate UE complexity tradeoffs e.g. tradeoff between number of interferers to cancel and the number of transmit antenna ports.
• The introduction of CRS IM based receiver requirements for homogenous deployments will only use the existing Release 11 signaling of CRS assistance information.