教育部補助「行動寬頻尖端技術跨校教學聯盟第三期計畫-行動寬頻網路與應用-小細胞基站聯盟中心」 Cloud RAN: 雲端無線接取網路與應用 課程單元：多基地台協同運作 (CoMP) (Draft) 計畫主持人：許蒼嶺 授課教師：萬欽德

CoMP (多點協作) Coordinated Multi-point Transmission

CoMP CoMP is an inter-cell cooperation technology that enables more than one transmission cell to communicate with a UE to achieve better throughputs at cell edge areas by reducing inter-cell interference CoMP cooperating cells share channel information of a UE, and based on the information, transmission cell(s) are decided. Before transmit data, UEs measure their channels, and report the resulting Channel State Information (CSI) to their base stations. CSI information includes Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), and Rank Indicator (RI). PMI: Base stations deliver more than one data stream (layer) through Tx antenna. Precoding matrix shows how individual data streams (layers) are mapped to antennas.

CoMP Transmission Points The CoMP Set RRM Measurement Set CoMP Measurement Set CoMP Cooperating Set CoMP Transmission Points Serving Cell Source: :http://raghudevareddy-lte-a.blogspot.tw/2014/02/lte-coordinated-multi-point-comp-basics.html The CoMP Set: = {serving cell ∈ CoMP transmission points ∈ CoMP cooperating set ∈ CoMP measurement set ∈ RRM measurement set

Types of DL CoMP(CB) Coordinated beam-forming (CB) Tx or Rx for a UE from single, semi-statically selected point Pre-coding coordinated among points; beam towards UE of interest, nulls towards victims Channel knowledge required Exchange of coordination information

Types of DL CoMP (DPS) Dynamic Point Selection (DPS) Also called DCS(Dynamic Cell Selection) Tx or Rx for a UE from single, dynamically selected, point At preparation phase, DPS works the same way as JT in that multiple cells share the same data. At data transmission phase, channel quality of UEs is checked in each subframe, and data is sent by the one cell that has the minimum path loss. Other cells that are not selected are muted. Tight synchronization is not required, HARQ is performed at the cell that is actually sending data.

DPS Dynamic cell selection (JP/DCS) involves user-plane (PDSCH) transmission from one point at a time within a CoMP cooperating set. DCS causes the UE to be served by the most favorable transmission cell at any given point of time. The serving cell can be selected based on channel variation, resource availability, and so on. The switching between transmission point can be sub frame by sub frame basis. Thus allowing a dynamic change in the transmission point that is transparent to the UE DCS is different from the handover in Rel-8, which is shown in the figure. Source: :http://raghudevareddy-lte-a.blogspot.tw/2014/02/lte-coordinated-multi-point-comp-basics.html

Types of DL CoMP(JT/TR) Joint Transmission/Reception (JT/JR) TX or RX for a UE from multiple points Multiple cells transmit the same data concurrently by using the same radio resources (frequency and time) Processing centralized with tight synchronization between JT cells is required HARQ is performed by serving cell only 2 scenarios :Intra eNB and Inter eNB

JT/JR Scenarios Scenario 1 - Intra-site Three-sectors intra site coordination cluster Data is sent from BBU to RU1 and RU2 Scenario 2 - Inter-site Coordination among 9 sectors of three sites Fiber between the sites

3GPP CoMP Scenarios cont. Scenario 3 - multi-cell Each node/point has an individual cell-ID Scenario 4 - shared/combined cell Small and macro cell belonging to the same coordination area, use the same cell ID

JT Types In JT the same (RB) of the PDSCH is transmitted from multiple cells associated with a UE-specific demodulation reference signal (DM-RS) among coordinated cells (i.e., from non-serving cell(s) as well as the serving cell) Coherent JT:  The RRH are coordinated by the corresponding eNB and are transmitting the data time-synchronized. The UE sends CSI feedback to all the cooperating cell sites. The phase of the transmitted signal can be adjusted to the CSI in such a way that the receiver is able to combine them at symbol level coherently. Non-coherent JT:  Cell-edge users calculate channel quality indicators (CQIs) and report them to their serving cells without providing thus channel phase information. Network does not have information concerning the relationship of the channels among the cooperating cells. Under this situation, the received signals arriving at the UE cannot be coherently combined. Coherent = 協調的,連貫的,一致的,相干的，相參的

Intra-Site(eNB) JT Intra-eNB JT Data is sent from BBU to RU1 and RU2 Sending same data concurrently to UE by allocating the same frequency and time resources. Because the same data is sent, the speed would not double, but reception performance would be improved. Between JT cells, transmission latency (or X2 latency if in different base stations) should be sufficiently low.

Inter-eNB JT In a distributed architecture: IP data is sent from BBU1 to BBU2. Data signal is sent from BBU1 and BBU2 to RU1 and RU2, respectively In a centralized architecture: IP data is sent from BBU1 to Centralized CoMP Coordinator. Data signal is sent from CoMP Coordinator to BBU1 and BBU2 IP data = signalling

CoMP scenarios defined in 3GPP R11 and R12 In the CoMP scenarios defined in 3GPP Release 11 and Release 12, CoMP is used: within a single eNB (intra- eNB) with ideal backhaul only - in restricted areas in Release 11 among eNBs (inter-eNB) with non-ideal backhaul as well - in more expanded areas in Release 12 . According to NGMN3, ideal backhaul refers to one with latency of 2.5 µsecs or less and capacity of 10 Gbps or higher, whereas non-ideal backhaul refers to one with latency of 5~30 msecs (or even higher). Release 11 allows for intra-eNB CoMP only. Due to this restriction, CoMP standardization in Release 11 is defined only for physical layer operations. CoMP coordination between eNBs with different vendors is not possible. In Release 12, CoMP scenarios and technologies have evolved to coordinate with neighbor base stations in non-ideal backhaul networks more practical network environments - like Microwave, DSL, Cable, PON, etc Source: http://www.netmanias.com

CoMP scenarios defined in 3GPP R11 and R12 Scenario ❶: CoMP used among macro cells belonging to one site in one macro eNB (e.g. Cells 2 and 3 in Cell Site 1) - intra-site Scenario ❷: CoMP used among macro cells belonging to different sites in one macro eNB (e.g. Cell 2 in Cell Site 1 and Cell 4 in Cell Site 2) - inter-site Scenario ❸: CoMP used between a macro cell and one of its small cells (small RRHs) that has a different cell ID than the macro cell Scenario ❹: CoMP used between a macro cell and one of its small cells (small RRHs) that has the same cell ID as the macro cell Scenario ❺: CoMP used among macro cells belonging to different macro eNBs (e.g. Cell 3 in eNB 1 and Cell 7 in eNB 2) Scenario ❻: CoMP used between a macro cell and one of its small cells (small cell eNBs) Scenario ❼: CoMP used among small cells (small cell eNBs) in a macro cell (macro cell and small cells use different channel, but small cells share the same channel)

Comparison of Scenarios Reference: http://www.netmanias.com 3GPP TR 36.819, CoMP operation for LTE physical layer aspects (Release 11) 3GPP TR 36.874, CoMP operation for LTE with non-ideal backhaul (Release 12) NGMN, RAN Evolution Project, "Backhaul and Fronthaul Evolution"

DL CoMP Call Flow

DL CoMP Call Flow Step 1: From SRSs received on cooperating eNBs, the network will identify UEs that benefit from CoMP and related eNBs. Step 2: The up-to-date SRS measurements should be shared between cooperating eNBs. Joint scheduling will be done over the cooperating eNBs, and the scheduling information needs to be shared among cooperating eNBs Step 3: The received signal information (from I/Q samples to hard bit decisions) should be shared among cooperating eNBs. JP: For Joint Reception the PUSCH transmitted by the UE is received jointly at multiple points (part of or entire CoMP cooperating set) at a time to improve the received signal quality. CS/CB: The scheduling and pre-coding selection decisions are made with coordination among points corresponding to the CoMP cooperating set. But the PUSCH data is intended for one point only.

DL CoMP 啟用評估 DCS方式的DL CoMP可以改善細胞邊緣UE所受到的同頻干擾，以提升細胞邊緣用戶的通訊品質和傳輸速率。 與JT及CBF相比，DCS方式的DL CoMP的要求更低，例如細胞發射天線數為2、UE高速移動、或細胞中沒有Beamforming使用者等情況下，JT和CBF的DL CoMP便無法使用，DCS方式的DL CoMP可作為一種補充應用。 DL COMP開啟後可以提升邊緣用戶的MCS，亦即提升細胞邊緣涵蓋，從而提升邊緣用戶HO成功率，並減少邊緣用戶HO過程中的傳輸速率下降。 在eNodeB計算資源配置優先順序時，相較於關閉DL CoMP，邊緣用戶由於平均傳輸速率提升，資源配置優先順序會有所下降。

TM 10 TM10 is defined in 3gpp rel-11 It enables DL CoMP operation and is configurable per serving cell  TM10 offers serving cell to configure UE to evaluate and report multiple sets of CSI-RS; thereby, allowing evaluation of multiple transmission points in CoMP measurement set. TM-10 also supports use of UE specific DMRS (2 UE DMRS scrambling IDs) for DL transmission.  It supports use of DCI-1A & new DCI format 2D, used for CS/CB or to allow PDSCH resource element mapping when using JP

DL CoMP Summary CoMP (DL) CS CB DPS JT/JR Resource Domain Frequency Frequency , Spatial Frequency , Time, Spatial # of transmission points Single Multiple Share information between neighbor cell CSI, Scheduling CSI, Scheduling, Beamforming CSI, Scheduling, Data Interference avoidance O X Receiver performance improvement Duplicated data reception from neighbor cells Time synchronization Backhaul type (Based on X2 latency tolerance) Ideal backhual, Non-idea backhaul Ideal backhual CSI = Channel State Information

UL CoMP 利用不同細胞之天線，接收手機上傳之訊號 得到訊號合併增益 消除細胞邊緣的干擾

UL CoMP for type 1 CoMP UEs Type-1 CoMP UEs UL CoMP 功能 啟用 eNB 選擇 type-1 CoMP Ues 及協作 細胞 A type-1 CoMP UE 在細胞邊緣 UE 在細胞邊緣回報並啟動event A3訊號交遞 Event A3條件︰當服務細胞與鄰 近細胞之RSRP差值達一定值時(建 議值:-10dB)

Gain of type 1 CoMP UEs eNB利用不同細胞的天線進行聯合訊號接收 eNB利用cell 1及cell 2的天線，將UE1的訊號接收後進行 結合，並得到訊號結合增益

Performing UL CoMP for type 2 CoMP UEs The eNB 選擇 type-2 CoMP UEs 及協作細胞 如何選擇 type-2 CoMP UE ? UE當受到type 1CoMP UE干擾時 ，即稱為type 2 UE 如何選擇協作細胞 ? Type 1 CoMP UE的服務細包對 Type 2 CoMP UE而言即是協作 細胞

Gain of type 2 CoMP UEs eNB利用不同細胞的天線進行聯合訊號接收 eNB利用Cell1 及Cell 2的天線接收UE2的訊號，將其結合後達 到消除干擾的增益

UL CoMP 啟用評估 在次都會區及郊區等站台間距大的場景下，不建議開啟本功能。 Intra site 之UL CoMP功能，可有效提升細胞內上、行平均傳輸速率和細胞邊緣用戶上行平均傳輸速率 ，建議在密集都會區和一般都會區（RU/RRU間距1000公尺以內）開啟本功能︰ 在次都會區及郊區等站台間距大的場景下，不建議開啟本功能。 在高話務量場景下（下行PRB利用率90%以上、上行PRB利用率90%以上 、CCE利用率80%以上、CPU利用率80%以上等），滿足以上條件之一或 多個時，建議先進行網路訊號交遞(Handover)優化，以減少A3事件量 測回報的信令處理所帶來的影響。 開通本功能前，請先檢查環境是否存在PIM干擾，若有PIM干擾時，先 解決PIM干擾，再開啟本功能。 SFN和UL CoMP功能同時打開時，SFN細胞與其鄰近細胞(可為一般細胞 或SFN細胞)之間的SRS起始sub frame配置要錯開，因UE會在起始SRS sub frame上被分配SRS資源，而eNB會選擇其中一個鄰近SFN細胞作為 協作細胞(指intra site場景)，若二個細胞間的SRS配置於起始sub frame相同的話，將無法選擇SFN細胞做UL CoMP。

CoMP 測試案例 測試場景: UE速率偏低仍未啟動CoMP功能 CoMP UE數量過多 CoMP啟動後成效不佳 eNodeB 拒絕啟動CoMP功能 UE　CoMP啟用/釋放太過頻繁

CoMP 觀察指標

CoMP Summary UL joint reception/processing possible with Rel-8 UEs UL CoMP UL joint reception/processing possible with Rel-8 UEs Large potential, especially for HetNet scenarios DL CoMP Coordinated scheduling possible with Rel-8 UEs Standardization of CoMP feedback not finished, still some work inRel-12/13 Medium performance potential on DL

CoMP Mechanism Coordinated scheduling eICIC – X2 assisted Enables interference handling between macro and metro layer for Small cell by coordinated scheduling for cell edge users. In an interference limited scenario, this improves cell edge bitrates. eICIC – X2 assisted CA Based Non CA based, ABS or RPSF Combined cell – Tx and Rx point selection Improved capacity in the multi-sector cell scenario by utilizing UE specific demodulation reference symbols . This enable an area splitting gain for Small cell since user data only need to be transmitted from the best transmission point.