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R1-084169 DL RS design for higher order MIMO


3GPP TSG RAN WG1 #55 Prague, Czech Republic, November 10 – 14, 2008 Agenda item: Source: Title: Document for: 11.3 Samsung Issues on DL RS Design for Higher Order MIMO Discussion and decision

R1-084169

1 Introduction
LTE advanced (LTE-A) is expected to achieve higher DL peak and average sector throughput than LTE and the LTE-A feasibility study includes the support of higher order MIMO in the DL. Higher order MIMO considers up to 8X8 multiantenna configuration to achieve the peak spectrum efficiency target [1]. This issue necessitates the design of additional reference signal (RS) for LTE-A. Key aspects for the RS design would be: Backward compatibility: LTE UEs shall be supported in LTE-A networks. This means any operation of LTE UEs should not be affected by the introduction of new RS; in particular, implementations associated with RSbased estimation functions (channel estimation, CQI/PMI feedback determination, RSSI measurement, etc) should not be affected. Low overhead: Higher order MIMO shall provide substantial gains over the existing MIMO schemes, even taking into account higher RS overhead. Otherwise, introducing higher order MIMO is counter-productive. In this sense, we have to investigate trade-off between the gain from better channel estimation and the loss from the additional RS overhead. Simplicity: The introduction of the additional RS should not be unnecessary complex. Several documents have considered the above aspects [2-4]. In this contribution, we identify the issues on the RS design to support higher order MIMO.

2 Classification of RS
RS can be categorized into several types according to its purpose. CQI-CRS: is the common RS for measurements. UEs determine CQI, PMI, and RI for feedback to the eNB by measuring CQI-CRS. CQI-CRS needs to be distributed over the entire band. DM-CRS: is the common RS for demodulation. Since multiple UEs use the DM-CRS for channel estimation, the precoding scheme chosen for a given UE cannot be applied to DM-CRS. Therefore, explicit signalling of the PMI should be included in the PDCCH when the PDSCH is precoded. If the PDSCH is precoded, the UE recognizes the precoding scheme from the PMI and obtains the amplitude and phase references of PDSCH modulation symbols by applying precoding to the channel estimates. NDM-DRS: is a non-precoded, dedicated RS for demodulation. A UE estimates the channel response by receiving its own NDM-DRS. If the PDSCH is precoded, the UE recognizes the precoding scheme from the PMI in the PDCCH and obtains the amplitude and phase references of PDSCH modulation symbols by applying precoding to the channel estimates. PDM-DRS: is a dedicated RS for demodulation. The same precoding scheme is applied to the PDM-DRS and the PDSCH. A UE obtains the amplitude and phase references of PDSCH modulation symbols by directly receiving its own PDM-DRS. The PMI information does not need to be included in PDCCH. Note that in Rel.8 the RS for antenna ports 0~3 are used as both CQI-CRS and DM-CRS (this type of RS is referred to as CQI/DM-CRS in this contribution) and the RS for antenna port 5 is a type of PDM-DRS.

3 Backward compatibility
To support up to 8 eNB transmitter antennas, respective RS from antennas 5-8 need to be transmitted in a sub-frame. Since legacy LTE UEs are not aware of the existence of additional RS, they will use the respective REs in the PDSCH demodulation/decoding process which will somewhat degrade the PDSCH BLER of LTE UEs.

3.1

Additional RS in the PDSCH region

If additional RS is located in the control region, this should not change the mapping rule of the control channels (PCFICH, PHICH, and PDCCH) to not affect legacy LTE UEs. Considering backward compatibility, mapping the additional RS onto predefined CCEs was suggested in [4]. With this approach, the location of additional RS cannot be optimized since the Rel.8 CCE interleaver is not optimized for channel estimation. Another concern is that channel estimation becomes complicated because the location of additional RS may change dynamically depending on the PCFICH value and the number of CCEs used for scheduling assignments. Furthermore, using PDCCH REs for additional RS transmission may often lead to scheduling limitations and throughput loss as the maximum PDCCH size of 3 OFDM symbols will be more often reached. Accordingly, this approach does not fulfil the requirements of low overhead and simplicity and is unlikely to provide robust performance. Among the multi-antenna techniques, all channels except PDSCH (PCFICH, PHICH, PDCCH, PBCH) use only transmit diversity schemes. However, the additional transmit diversity gain from using 8 instead of 4 antennas is marginal, especially if zero antenna correlation cannot be assumed and realistic channel estimation is considered. Because of negligible performance difference and in order to avoid introducing unnecessary complexity in the LTE-A design and implementation, the transmission of control channels should be kept from a maximum of 4 antennas as in Rel.8. This is anyway default for the PCFICH, the PHICH, and the PBCH as they need to be received by LTE UEs. The above observations lead to the following guideline: Additional RS is located only in the PDSCH region.

3.2

LTE-A subframes

Even though the additional RS are located in the PDSCH region, at least an RS used for measurement, CQI-CRS, needs to be transmitted periodically on preconfigured resources. Considering this requirement, we need to examine whether legacy LTE UEs can be scheduled on the PDSCH region where the additional RS is transmitted. Two possibilities exist: Resources containing the additional RS LTE-A UEs are aware of the RBs containing the additional RS in a subframe. Legacy LTE UEs can be assigned such RBs, without any prior knowledge of the additional RS presence, and the only effect will be some performance degradation. However, since the eNB scheduler already knows this performance degradation, it can simply adjust the PDSCH MCS to achieve a desired BLER. Introduction of LTE-A subframes Only LTE-A UEs can be scheduled PDSCH transmissions in LTE-A subframes. UEs can be informed the LTE-A subframes through an SIB. By using the signalling of MBSFN subframe configurations, the eNB can prohibit legacy UEs from monitoring LTE-A subframes for channel estimation. Since both LTE and LTE-A UEs should be able to receive SIB, the subframes delivering SIB will not be LTE-A subframes. This approach enables the optimization of the new RS structure since the requirement of backward compatibility is avoided. On the other hand, it introduces scheduling constraints since legacy UEs cannot be scheduled in LTE-A subframes. LTE-A subframes may also be used for other enhancements such as relays, CoMP, etc. Both approaches should be evaluated and compared in the SI phase in terms of throughput, scheduling constraints, and overall system impact for LTE and LTE-A UEs.

4 Issues on RS design for higher order MIMO
Following issues need to be discussed during the SI phase for the RS design: Issue 1: How to configure the resources on which CQI-CRS (or CQI/DM-CRS) are transmitted? Two options exist. The first is to introduce CQI-CRS (or CQI/DM-CRS) over the entire band in some sub-frames. The second is to specify CQI-CRS hopping patterns, similar to the UL sounding RS hopping patterns in Rel. 8. Issue 2: Whether to define LTE-A subframes?

As discussed earlier, introduction of LTE-A subframes has pros and cons. Due to hard partitioning between LTE-A and normal subframes, introduction of LTE-A subframes unavoidably leads to scheduling constraint that legacy UEs cannot be assigned PDSCH reception in LTE-A subframes. On the other hand, the RS structure can be optimized since LTE-A subframes do not need to be backward compatible. Issue 3: Whether to introduce sparse CQI-CRS? As mentioned above, a set of preconfigured resources should contain at least CQI-CRS corresponding to the additional antennas to enable LTE-A UEs to determine CQI, PMI, and RI. In designing CQI-CRS, if we follow the philosophy behind CRS design in Rel.8, the CQI-CRS can also be used as DM-CRS, i.e. CQI/DM-CRS. Overhead should be carefully considered in that case. Otherwise, to reduce CQI-CRS overhead, the CQI-CRS may be transmitted only in a few sub-frames every frame and the DM RS can be a DRS. Issue 4: Whether to apply precoding to the dedicated RS? In order to enable channel estimation for LTE-A UEs, DRS should be included in the allocated RBs if DM-CRS is not defined. The DRS can be either NDM-DRS or PDM-DRS depending on whether it is precoded or not. If the DRS is not precoded (NDM-DRS), the legacy LTE CRS can be reused as DM-CRS and only the RS corresponding to the additional antennas (e.g. antennas 5-8) needs to be added. The PMI should be signalled in the PDCCH. The bit-width of the new PMI will be larger than that of existing PMI, expecting larger codebook for higher order MIMO. NDM-DRS overhead does not depend on the transmit RI. If the DRS is precoded (PDM-DRS), the legacy LTE CRS cannot be reused as DM-CRS when higher order MIMO is applied to PDSCH transmissions. Since PDM-DRS captures the precoding applied to PDSCH, signalling the PMI in the PDCCH is not needed. The PDM-DRS pattern can be optimized in the sense of RS overhead reduction by rankdependent RS design. Since PDM-DRS is defined per layer, RS overhead can be reduced for low rank transmissions. The transmit RI is signalled in the PDCCH to enable rank-dependent RS design.

5 Conclusions
In this contribution, we addressed the key aspects to be considered for the RS design in LTE-A, such as backward compatibility, low overhead, and simplicity. Considering compatibility and simplicity, we suggest additional RS be located only in the PDSCH region. We also identified the following issues for consideration in the LTE-A SI phase regarding the RS design to support higher order MIMO: Issue 1: How to configure the resources on which CQI-CRS (or CQI/DM-CRS) are transmitted Issue 2: Whether to define LTE-A subframes Issue 3: Whether to introduce sparse CQI-CRS and rely on DRS for channel estimation Issue 4: Whether to apply precoding to the DRS We propose to examine the above issues when determining the RS pattern in LTE-A.

6 References
[1] TR 36.814 v0.0.0, “Technical Specification Group Radio Access Network; Further Advancements for E-UTRA Physical Layer Aspects” [2] R1-083685, “Support of DL Higher-Oder MIMO Transmission in LTE-Advanced,” NTT DoCoMo [3] R1-083869, “Design Consideration for Higher-Order MIMO in LTE-Advanced,” Nortel [4] R1-083827, “Common Reference Symbol Mapping/Signaling for 8 Transmit antennas,” Motorola


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