Evolved Universal Terrestrial Radio Access (E-UTRA); Study on measurement gap enhancement for LTE
The present document is the technical report for the Study Item on measurement gap enhancement in LTE.
The current measurement gap configuration and the corresponding RRC signalling were first introduced in Rel.8 back in 2008. Since then, some fundamental evolutions in LTE have been realized. These include numerous new technologies (e.g. carrier aggregation, FeICIC, CoMP, dual-connectivity, etc.), new network topologies (e.g. Heterogeneous Network with small cells) and increased number of deployed bands and frequencies.
The main challenges of the existing measurement gap and the motivations for the further enhancement can be summarized in the following four aspects:
- Network impact and UE scheduling opportunity
- Up to 15% of DL/UL resources are restricted from reception and transmission due to the existing measurement gap configuration
- Multiple Rx chains equipped in CA capable UE can potentially provide extra degrees of freedom to do the measurement more efficiently, but they are not fully utilized at the moment.
- Challenges with multiple Rx chains and single chip RF-IC implementation
- UE can only be configured with single measurement gap pattern even equipped with multiple Rx chains
- This significantly limits network and UE's flexibility to balance the measurement delay, power consumption and spectrum efficiency, which can benefit from multiple RF chains.
- Harmonic interference and/or synthesizer operations due to one Rx chain can result in the interruption on the other Rx chains
- The scenarios where involving synthesizer operating include, but not limited to, radio turned on/off, inter-frequency measurement, SCell activation/deactivation and/or SCell measurements.
- The existing measurement gap pattern is inefficient to handle the Inter-RF chain interference issue. This unnecessarily imposes significant implementation restriction.
- UE power consumption and mobility
- The potential benefit has been recognized to save UE power consumption by differentiating the measurement requirements for the different frequency layers, e.g. coverage, offloading, etc. However, it is not easy to be realized based on the existing measurement gap configuration.
- The mobility performance can be considerably improved with measurement gap enhancement by reducing the measurement delay of coverage layers.
- With the ever-increasing number of deployed bands and frequencies, the existing inter-frequency/inter-RAT measurement mechanism faces even more challenges to meet the performance requirements.
- Limitations of the existing measurement gap pattern are identified in various technical areas
- No enhancement has been made even though the importance of measurement gap enhancement has been well recognized.
- The measurement gap related discussions are diverse and spread over quite a few topics.
- It is desirable to address different measurement gap related issue in a single SI and the eventual enhancement and design should be versatile enough to serve different purposes.
All in all, the effectiveness of new technologies and new network topologies evolved since Rel.8 not only greatly relies on the measurement accuracy and reporting delay, but also on the measurement efficiency and the associated power consumption at UE. All these issues motivate some effort to further investigate the measurement gap enhancement. It is desirable to design an all-weathered and future-proofed inter-frequency/inter-RAT measurement solution by jointly considering all aspects, including but not limited to delay, power consumption, flexibility and network performance.