5G Multi-RTT定位LPP/NRPPa信令全链路拆解

graph TB
    subgraph "5GC"
        AMF[AMF<br/>接入与移动管理]
        LMF[LMF<br/>位置管理功能]
        GMLC[GMLC<br/>网关移动位置中心]
        UDM[UDM<br/>统一数据管理]
    end

    subgraph "NG-RAN"
        gNB1[gNB/TRP 1]
        gNB2[gNB/TRP 2]
        gNB3[gNB/TRP 3]
    end

    UE[UE] -->|N1 NAS/LPP| AMF
    AMF -->|NLs Nlmf_Location| LMF
    AMF -->|N2 NGAP/NRPPa| gNB1
    AMF -->|N2 NGAP/NRPPa| gNB2
    AMF -->|N2 NGAP/NRPPa| gNB3
    LMF -->|NLg| GMLC
    GMLC -->|Le/MLp| LCS[LCS Client/AF]
    GMLC -->|Nh| UDM

    UE -.->|DL-PRS/SRS-Pos| gNB1
    UE -.->|DL-PRS/SRS-Pos| gNB2
    UE -.->|DL-PRS/SRS-Pos| gNB3

sequenceDiagram
    participant LCS as LCS Client/AF
    participant GMLC as GMLC
    participant UDM as UDM
    participant AMF as AMF
    participant LMF as LMF

    LCS->>GMLC: 1. LCS Service Request<br/>(目标UE标识, QoS)
    GMLC->>UDM: 2. Nudm_SDM_Get<br/>(SUPI → AMF Address)
    UDM-->>GMLC: 3. AMF Address + Privacy信息
    GMLC->>AMF: 4. Namf_Location_ProvidePositioningInfo<br/>(SUPI, QoS, LCS Correlation ID)
    AMF->>AMF: 5. LMF选择<br/>(基于NF Instance/区域)
    AMF->>LMF: 6. Nlmf_Location_DetermineLocation<br/>(SUPI, QoS, AMF Correlation ID)
    Note over LMF: LMF开始定位过程<br/>见第4节详细信令
    LMF-->>AMF: 7. Nlmf_Location_DetermineLocation Response<br/>(Location Estimate, Accuracy)
    AMF-->>GMLC: 8. Namf_Location_ProvidePositioningInfo Response
    GMLC-->>LCS: 9. LCS Service Response

sequenceDiagram
    participant UE
    participant gNB1 as gNB/TRP-1
    participant gNB2 as gNB/TRP-2
    participant gNB3 as gNB/TRP-3
    participant AMF
    participant LMF

    rect rgb(240, 248, 255)
        Note over UE,LMF: Phase 1: LPP Capability Exchange
        LMF->>AMF: 1a. Namf_Communication_N1N2MessageTransfer<br/>(LPP RequestCapabilities)
        AMF->>UE: 1b. DL NAS Transport<br/>(LPP RequestCapabilities)
        UE-->>AMF: 1c. UL NAS Transport<br/>(LPP ProvideCapabilities)
        AMF-->>LMF: 1d. Nlmf_Location_DetermineLocation<br/>(LPP ProvideCapabilities)
    end

    rect rgb(255, 248, 240)
        Note over UE,LMF: Phase 2: NRPPa TRP信息获取 + PRS配置
        LMF->>AMF: 2a. Namf_Communication_NonUeN2MessageTransfer<br/>(NRPPa TRP Information Request → gNB1/2/3)
        AMF->>gNB1: 2b. NGAP Downlink NAS Transport<br/>(NRPPa TRP Info Request)
        AMF->>gNB2: 2b. NGAP Downlink NAS Transport<br/>(NRPPa TRP Info Request)
        AMF->>gNB3: 2b. NGAP Downlink NAS Transport<br/>(NRPPa TRP Info Request)
        gNB1-->>AMF: 2c. NRPPa TRP Information Response<br/>(TRP坐标, DL-PRS配置, 定时)
        gNB2-->>AMF: 2c. NRPPa TRP Information Response
        gNB3-->>AMF: 2c. NRPPa TRP Information Response
        AMF-->>LMF: 2d. 转发所有TRP Information Response
    end

    rect rgb(240, 255, 240)
        Note over UE,LMF: Phase 3: LPP Assistance Data + SRS配置
        LMF->>AMF: 3a. Namf_Communication_N1N2MessageTransfer<br/>(LPP ProvideAssistanceData)
        AMF->>UE: 3b. DL NAS Transport<br/>(LPP ProvideAssistanceData)
        Note over UE: UE收到辅助数据：<br/>各TRP的PRS时频配置、<br/>TRP坐标、定时偏移
        LMF->>AMF: 3c. Namf_Communication_N1N2MessageTransfer<br/>(含NRPPa: SRS Configuration → gNB)
        AMF->>gNB1: 3d. NGAP: SRS Configuration<br/>(配置SRS-Pos资源)
    end

    rect rgb(255, 255, 240)
        Note over UE,LMF: Phase 4: PRS测量 + SRS发送
        Note over UE,gNB1: UE接收多个TRP的DL-PRS<br/>测量UE-RxTxTimeDifference per TRP
        UE->>gNB1: 4a. SRS-Pos上行信号
        UE->>gNB2: 4a. SRS-Pos上行信号
        UE->>gNB3: 4a. SRS-Pos上行信号
        Note over gNB1,gNB3: 各gNB测量：<br/>gNB-RxTxTimeDifference<br/>UL-SRS-RSRPP
    end

    rect rgb(248, 240, 255)
        Note over UE,LMF: Phase 5: 测量结果上报
        UE-->>AMF: 5a. UL NAS Transport<br/>(LPP ProvideLocationInformation)
        AMF-->>LMF: 5b. 转发LPP测量结果
        gNB1-->>AMF: 5c. NRPPa Measurement Response<br/>(gNB-RxTxTimeDiff, UL-SRS-RSRPP)
        gNB2-->>AMF: 5c. NRPPa Measurement Response
        gNB3-->>AMF: 5c. NRPPa Measurement Response
        AMF-->>LMF: 5d. 转发所有NRPPa测量结果
    end

    rect rgb(255, 240, 245)
        Note over LMF: Phase 6: Multi-RTT位置计算
        Note over LMF: RTT_i = UE-RxTx_i + gNB-RxTx_i<br/>distance_i = RTT_i × c / 2<br/>多边定位求UE坐标
        LMF-->>AMF: 6. Location Estimate + Accuracy
    end

comb-2 (N=2):  |X| |X| |X| |X| |X|  → 频域密度高，精度好
comb-4 (N=4):  |X| | | |X| | |X|  → 平衡
comb-6 (N=6):  |X| | | | |X| | |  → 抗FDM好，精度略降

UE Rx-Tx Time Difference = tUE,Rx - tUE,Tx

其中：
- tUE,Rx = UE接收到TRP-i的DL-PRS的时间
- tUE,Tx = UE发送SRS-Pos信号的时间

这个时间差反映了UE侧的收发定时关系

LMF → gNB: NRPPa TRP INFORMATION REQUEST
  └─ TRP-Information-Request-List
      └─ trp-ID (或请求全部TRP)
      └─ requested-TRP-Information-List
          ├─ trp-Location (TRP坐标)
          ├─ dl-PRS-ResourceSet (PRS配置)
          ├─ UL-SRS-Configuration (SRS配置)
          └─ gNB-RxTxTimeDifference (gNB定时)

gNB → LMF: NRPPa TRP INFORMATION RESPONSE
  └─ TRP-Information-Response-List
      └─ trp-Location (纬度/经度/海拔)
      └─ dl-PRS-ResourceSet
          └─ dl-PRS-Resource (每个PRS资源的配置)
          └─ dl-PRS-ID

LMF → gNB: NRPPa POSITIONING MEASUREMENT REQUEST
  └─ Measurement-Request-List
      └─ srs-Pos-Resource (SRS-Pos资源配置)
      └─ measurement-Quantity (请求的测量量)
          ├─ gNB-RxTxTimeDifference
          ├─ UL-SRS-RSRPP
          └─ UL-AoA (可选)
      └─ measurement-Timing (测量时间窗口)

gNB → LMF: NRPPa POSITIONING MEASUREMENT RESPONSE
  └─ Measurement-Response-List
      └─ gNB-RxTxTimeDifference-r16
          └─ value (纳秒级精度)
      └─ UL-SRS-RSRPP-r16
          └─ value (dBm)
      └─ trp-ID (关联测量到TRP)

gNB Rx-Tx Time Difference = tgNB,Rx - tgNB,Tx

其中：
- tgNB,Rx = gNB接收到UE的SRS-Pos信号的时间
- tgNB,Tx = gNB发送DL-PRS信号的时间

RTT_i = UE-RxTxTimeDiff_i + gNB-RxTxTimeDiff_i

距离估计：
d_i = RTT_i × c / 2    (c = 光速 ≈ 3×10^8 m/s)

多边定位方程组（3个以上TRP）：
d_i = √[(x - x_i)² + (y - y_i)² + (z - z_i)²]

通过最小二乘法求解UE坐标 (x, y, z)

flowchart TD
    A[LMF收到DetermineLocation请求] --> B{QoS精度要求?}
    B -->|>50m| C[选择E-CID]
    B -->|10-50m| D{UE在室内or室外?}
    B -->|<10m| F{多TRP可用?}
    D -->|室外| E[A-GNSS + E-CID]
    D -->|室内| G[DL-TDOA / Multi-RTT]
    F -->|是| H[Multi-RTT<br/>精度最优]
    F -->|否| I[DL-AoD / UL-AoA<br/>单站角度定位]
    C --> J{满足QoS?}
    E --> J
    G --> J
    H --> J
    I --> J
    J -->|是| K[返回Location Estimate]
    J -->|否| L[升级定位方法<br/>或组合多种方法]

# 过滤LPP消息（通过NAS Transport容器）
nas-5gs.nas_msg_elem.type == 0x67 && nas-5gs.payload_container_type == "LPP"

# 过滤NRPPa消息
ngap.procedureCode == 46  && nrppa

# 过滤LMF定位请求
http.request.uri contains "/nlmf-loc/v1/determine"