【IoT-NTN】系统消息SIB31信令分析

3GPP卫星通信发展迅速， TS36.331 R17中新增SIB31携带星历信息，本文对SIB31的信令内容进行了分析。

SystemInformationBlockType31 分析报告

一、概述

本文档详细描述了SystemInformationBlockType31（简称SIB31）的结构和内容，该信息块用于向非地面网络（NTN）中的服务小区提供卫星辅助信息。SIB31仅针对NTN小区进行信号传输，包含了卫星轨道参数、网络时间对齐（NTA）参数以及其他关键信息，以支持UE（用户设备）与卫星网络的同步和通信。

SystemInformationBlockType31 information element

-- ASN1STARTSystemInformationBlockType31-r17 ::= SEQUENCE {servingSatelliteInfo-r17		ServingSatelliteInfo-r17,lateNonCriticalExtension		OCTET STRING					OPTIONAL,...,[[	servingSatelliteInfo-v1820	ServingSatelliteInfo-v1820		OPTIONAL	-- Need OR]]
}ServingSatelliteInfo-r17 ::=	SEQUENCE {ephemerisInfo-r17			CHOICE {stateVectors				EphemerisStateVectors-r17,orbitalParameters			EphemerisOrbitalParameters-r17},nta-CommonParameters-r17			SEQUENCE {nta-Common-r17					INTEGER (0..8316827)		OPTIONAL,	-- Need OPnta-CommonDrift-r17				INTEGER (-261935..261935)	OPTIONAL,	-- Need OPnta-CommonDriftVariation-r17	INTEGER (0..29479)			OPTIONAL	-- Need OP},ul-SyncValidityDuration-r17		ENUMERATED {s5, s10, s15, s20, s25, s30, s35, s40,s45, s50, s55, s60, s120, s180, s240, s900},epochTime-r17					SEQUENCE {startSFN-r17					INTEGER (0..1023),startSubFrame-r17				INTEGER (0..9)}																OPTIONAL,	-- Need OPk-Offset-r17					INTEGER (0..1023),k-Mac-r17						INTEGER (1..512)				OPTIONAL,	-- Need OP...
}ServingSatelliteInfo-v1820 ::=	SEQUENCE {satelliteId-r18				SatelliteId-r18					OPTIONAL,	-- Need ORreferenceLocation-r18			CHOICE {fixedReferenceLocation-r18			ReferenceLocation-r18,movingReferenceLocation-r18			ReferenceLocation-r18}															OPTIONAL,	-- Need ORdistanceThresh-r18				INTEGER(0..65535)			OPTIONAL	-- Need OR
}-- ASN1STOP

二、SIB31结构

SIB31采用ASN.1进行定义，主要包括以下几个部分：

ServingSatelliteInfo-r17：此部分提供了关于服务卫星的详细信息，包括卫星的星历信息、网络时间对齐（NTA）参数等。
- ephemerisInfo-r17：星历信息，可以选择以状态向量或轨道参数的形式提供。
- nta-CommonParameters-r17：包含网络时间对齐的通用参数，如NTA值、NTA漂移率和NTA漂移率变化。
- ul-SyncValidityDuration-r17：上行同步有效时长，定义了卫星星历数据和NTA参数的有效期。
- epochTime-r17：星历数据和NTA参数的参考时间点。
- k-Offset-r17 和 k-Mac-r17：与帧定时相关的偏移量参数，用于调整下行和上行帧之间的同步。
ServingSatelliteInfo-v1820（可选）：此部分在R18/V20版本中引入，提供了额外的服务卫星信息，如卫星ID、参考位置和距离阈值。
- satelliteId-r18：卫星的唯一标识符。
- referenceLocation-r18：卫星的参考位置，可以是固定位置或移动位置。
- distanceThresh-r18：与参考位置的距离阈值，用于启动基于位置的测量。

三、关键字段描述

SystemInformationBlockType31 field descriptions

distanceThresh

Distance from the serving cell reference location and is used in location-based measurement initiation in RRC_IDLE (as specified in TS 36.304 [4]) and RRC_CONNECTED. Each step represents 50m.

epochTime

Epoch time of the satellite ephemeris data and common TA parameters, see TS 36.213 [23]. This field also indicates the epoch time for the reference location of earth moving cells if present. The reference point for epoch time of the serving satellite ephemeris and Common TA parameters is the uplink time synchronization reference point.

epochTime is the starting time of a DL subframe indicated by startSFN and startSubframe. For serving cell, the startSFN indicates the current SFN or the next upcoming SFN after the frame where the message indicating the epochTime is received.

If the field is absent, the epoch time is the starting time of the DL subframe corresponding to the end of the SI window during which the SI message carrying SIB31(-NB) is transmitted.

E-UTRAN always includes epochTime when SIB31(-NB) is provided through dedicated signalling.

In case of handover or conditional handover, this field is based on the timing of the target cell, i.e. the startSFN and startSubFrame number indicated in this field refers to the SFN and sub-frame of the target cell, and UE considers the target cell epoch time (indicated by the startSFN and startSubFrame in this field) to be the frame nearest to the frame where RRCConnectionReconfiguration message is received.

k-Mac

Scheduling offset used when downlink and uplink frame timing are not aligned at the eNB, see TS 36.213 [23]. Unit in ms.

If the field if absent, the UE uses the (default) value of 0.

k-Offset

Scheduling offset used in the timing relationships in NTN, see TS 36.213 [23]. Unit in ms.

nta-Common

Network-controlled common TA, see TS 36.213 [23]. Unit of μs.

Step of 32.55208 ×10-3 μs. Actual value = field value * 32.55208 ×10-3.

If the field is absent, the UE uses the (default) value of 0.

nta-CommonDrift

Drift rate of the common TA, see TS 36.213 [23]. Unit of μs/s.

Step of 0.2 ×10-3 μs/s. Actual value = field value * 0.2 ×10-3.

If the field is absent, the UE uses the (default) value of 0.

nta-CommonDriftVariation

Drift rate variation of the common TA, see TS 36.213 [23]. Unit of μs/s2.

Step of 0.2 ×10-4 μs/s2. Actual value = field value * 0.2 ×10-4.

If the field is absent, the UE uses the (default) value of 0.

orbitalParameters

Instantaneous values of the satellite orbital parameters. The signalled values are valid at least for the duration as defined by ul-SyncValidityDuration and epochTime.

referenceLocation

Reference location of the NTN (quasi-)earth fixed cell or earth moving cell, used in location-based measurement initiation in RRC_IDLE (as specified in TS 36.304 [4]) and RRC_CONNECTED if distanceThresh is also configured. If configured by an earth moving cell, the broadcast reference location corresponds to the epoch time and is also used in the evaluation of Event D2 and CondEvent D2, and the UE derives the real-time reference location based on the serving satellite ephemeris, see TS 36.304 [4].

stateVectors

Instantaneous values of the satellite state vectors. The signalled values are valid at least for the duration as defined by ul-SyncValidityDuration and epochTime.

ul-SyncValidityDuration

Validity duration of the satellite ephemeris data and common TA parameters, i.e. maximum time duration (from epochTime) during which the UE can apply the satellite ephemeris without acquiring new satellite ephemeris, see TS 36.213 [23]. Unit in second.

Value s5 corresponds to 5 seconds, value s10 corresponds to 10 seconds and so on.

The ul-SyncValidityDuration is only updated when at least one of epochTime, nta-CommonParameters, ephemerisInfo is updated.

distanceThresh：从服务小区参考位置到某点的距离阈值，用于在RRC_IDLE和RRC_CONNECTED状态下启动基于位置的测量。每步代表50米。
epochTime：卫星星历数据和通用TA参数的参考时间点，也是地球移动小区参考位置的参考点。如果字段不存在，则使用传输SIB31消息的SI窗口结束时的DL子帧开始时间作为参考点。
k-Mac 和 k-Offset：这两个字段与下行和上行帧定时未对齐时的调度偏移量有关，单位分别为ms。如果字段不存在，则UE使用默认值0。
nta-CommonParameters：包括网络时间对齐值（nta-Common-r17）、漂移率（nta-CommonDrift-r17）和漂移率变化（nta-CommonDriftVariation-r17），用于调整UE与网络之间的时间对齐。
orbitalParameters 和 stateVectors：分别表示卫星轨道参数的瞬时值和卫星状态向量的瞬时值，这些值在ul-SyncValidityDuration和epochTime定义的有效期内有效。
ul-SyncValidityDuration：定义了卫星星历数据和通用TA参数的有效期，单位为秒。这个值在epochTime、nta-CommonParameters或ephemerisInfo更新时才会更新。

三、关键字段详细解释

1. EphemerisOrbitalParameters

EphemerisOrbitalParameters信息元素提供了以地心惯性坐标系（ECI）格式表示的卫星星历。它是ephemerisInfo-r17字段中轨道参数选择的一个具体实现。

-- ASN1STARTEphemerisOrbitalParameters-r17 ::= SEQUENCE {semiMajorAxis-r17					INTEGER (0..8589934591),eccentricity-r17					INTEGER (0..1048575),periapsis-r17						INTEGER (0..268435455),longitude-r17						INTEGER (0..268435455),inclination-r17						INTEGER (-67108864..67108863),anomaly-r17							INTEGER (0..268435455)
}-- ASN1STOP

EphemerisOrbitalParameters field descriptions

anomaly

Mean anomaly M at epoch time, see NIMA TR 8350.2 [110]. Unit in radian.

Step of 2.341* 10-8 rad. Actual value = field value * (2.341* 10-8).

eccentricity

Eccentricity e, see NIMA TR 8350.2 [110].

Step 1.431 * 10-8. Actual value = field value * (1.431 * 10-8).

inclination

Inclination i, see NIMA TR 8350.2 [110]. Unit in radian.

Step of 2.341* 10-8 rad. Actual value = field value * (2.341* 10-8).

longitude

Longitude of ascending node W, see NIMA TR 8350.2 [110]. Unit in radian.

Step of 2.341* 10-8 rad. Actual value = field value * (2.341* 10-8).

periapsis

Argument of periapsis w, see NIMA TR 8350.2 [110]. Unit in radian.

Step of 2.341* 10-8 rad. Actual value = field value * (2.341* 10-8).

semiMajorAxis

Semi major axis a, see NIMA TR 8350.2 [110]. Unit in meter.

Step of 4.249 * 10-3 m. Actual value = 6500000 + field value * (4.249 * 10-3).

semiMajorAxis-r17：半长轴，表示卫星轨道椭圆的长半轴长度。单位为米，实际值通过给定的步长和基准值（如6500000米）计算得出。
eccentricity-r17：偏心率，表示卫星轨道椭圆的形状。实际值通过给定的步长和字段值计算得出。
periapsis-r17：近地点角距，定义了卫星轨道与参考平面的交点与近地点之间的角度。单位为弧度，实际值通过给定的步长和字段值计算得出。
longitude-r17：升交点赤经，定义了卫星轨道平面与参考平面的交线与春分点的角度。单位为弧度，计算方法同上。
inclination-r17：轨道倾角，表示卫星轨道平面与参考平面的夹角。单位为弧度，计算方法同上。
anomaly-r17：平近点角，在某一特定时刻，卫星与近地点之间的角距离。单位为弧度，计算方法同上。

这些轨道参数共同描述了卫星在某一时刻的位置和速度，对于UE准确计算与卫星之间的距离和相对运动至关重要。

2. EphemerisStateVectors

EphemerisStateVectors信息元素提供了以地心地固坐标系（ECEF）格式表示的卫星位置和速度状态向量。这些向量对于UE精确计算与卫星之间的距离和相对位置至关重要。

-- ASN1STARTEphemerisStateVectors-r17 ::=	SEQUENCE {positionX-r17					PositionStateVector-r17,positionY-r17					PositionStateVector-r17,positionZ-r17					PositionStateVector-r17,velocityVX-r17					VelocityStateVector-r17,velocityVY-r17					VelocityStateVector-r17,velocityVZ-r17					VelocityStateVector-r17
}PositionStateVector-r17 ::= INTEGER (-33554432..33554431)VelocityStateVector-r17 ::= INTEGER (-131072..131071)-- ASN1STOP

EphemerisStateVectors field descriptions

positionX, positionY, positionZ

X, Y, Z coordinate of satellite position state vector in ECEF. Unit in meter.

Step of 1.3 m. Actual value = field value * 1.3.

velocityVX, velocityVY, velocityVZ

X, Y, Z coordinate of satellite velocity state vector in ECEF. Unit in meter/second.

Step of 0.06 m/s. Actual value = field value * 0.06.

positionX-r17, positionY-r17, positionZ-r17：分别表示卫星在ECEF坐标系中的X、Y、Z坐标位置。单位为米，实际值通过给定的步长（1.3米）和字段值计算得出。
velocityVX-r17, velocityVY-r17, velocityVZ-r17：分别表示卫星在ECEF坐标系中沿X、Y、Z轴的速度分量。单位为米/秒，实际值通过给定的步长（0.06米/秒）和字段值计算得出。

这些位置和速度状态向量提供了卫星在特定时刻的精确位置和移动方向，使得UE能够准确预测卫星的未来位置，从而实现精确的同步和通信。

四、总结

SystemInformationBlockType31为非地面网络（NTN）中的服务小区提供了关键的卫星辅助信息，包括卫星轨道参数、网络时间对齐参数以及其他重要参数。这些信息对于UE与卫星网络的同步和通信至关重要，确保了网络的高效运行和通信质量。通过ASN.1定义的SIB31结构清晰、完整，便于实现和解析。