文章目录
Introduction
提到AS层的选网,不得不讲NRRC的状态,相比于EUTRAN,NR多了INACTIVE态。而选网就是发生在INACTIVE或IDLE态,如果AS处于CONNECTED态,则需要先释放连接才可以进行网络选择。
NAS和AS行为对比
选网在NAS和AS层在行为上的对比
从上述表格对比看:
PLMN选择和小区选择时:
AS层更多是在UE支持的RAT和BAND上进行搜索,属于选网的执行者;
NAS层则是根据一定策略选择合适的网络,属于选网的决策者。
AS行为之PLMN选择
AS层应该根据射频的BAND能力,在自己支持的所有band中搜索,对于每一个载频,UE只搜索最强小区并读取系统消息获取PLMNlist。(当然一般厂商不会这么做,有多重策略保证搜索的速度和精度,比如深搜和浅搜相结合的方法)。
在搜索时,在最强小区读到的一个或者多个plmn都要上报,满足高质量标准(RSRP值>=-110dbm)的plmn给NAS不携带RSRP值;如果不满足110dbm的plmn,要携带对应的rsrp值给NAS;
一旦选择了一个PLMN,AS应该在该PLMN内尝试camp on cell。
AS行为之小区选择
网络选择对于AS来说,其实是小区搜索。因为AS不知道当前有哪些网络,只有通过扫频或历史信息协助,才能扫描到当前可用的频率及小区,继而读取小区的系统信息,获取小区的PLMN list。
AS为了完成小区选择的,需要根据一定的标准选择一个质量最好的小区,这就是小区的测量。测量过程中evaluating Srxlev and Squal of non-serving cells。另外如果UE驻留在一个小区后,也需要周期性的搜索更好小区,即小区重选。
另外,5G中多了多波束的概念,因此在小区质量评估中,需要考虑多波束操作:
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if nrofSS-BlocksToAverage is not configured in SIB2/SIB4; or
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if absThreshSS-BlocksConsolidation is not configured in SIB2/SIB4; or
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if the highest beam measurement quantity value is below or equal to absThreshSS-BlocksConsolidation:
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derive a cell measurement quantity as the highest beam measurement quantity value, where each beam measurement quantity is described in TS 38.215 [11].
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else:
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derive a cell measurement quantity as the linear average of the power values of up to nrofSS-BlocksToAverage of highest beam measurement quantity values above absThreshSS-BlocksConsolidation.
以上加粗的均为系统消息中的配置。
小区类型
acceptable cell:
An “acceptable cell” is a cell on which the UE may camp to obtain limited service (originate emergency calls and receive ETWS and CMAS notifications). Such a cell shall fulfil the following requirements, which is the minimum set of requirements to initiate an emergency call and to receive ETWS and CMAS notification in an NR network:
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The cell is not barred, see clause 5.3.1;
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The cell selection criteria are fulfilled
suitable cell:
A cell is considered as suitable if the following conditions are fulfilled:
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The cell is part of either the selected PLMN or the registered PLMN or PLMN of the Equivalent PLMN list;
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The cell selection criteria are fulfilled
According to the latest information provided by NAS:
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The cell is not barred
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The cell is part of at least one TA that is not part of the list of “Forbidden Tracking Areas”, which belongs to a PLMN that fulfils the first bullet above.
barred cell:
cell barred的条件(满足任意一个):
- MIB中携带的cellBarred被置为barred
- SIB1中携带的Band等信息不支持
- RRC连接重建过程中小区选择时没有收到MIB或SIB。
当cellBarred,并且MIB指示intraFreqReselection为nonAllowed,对于小区重选,则要将该频点Barred。
reserved cell:
A cell is reserved if it is so indicated in system information, 运营商保留或other use,也是会在SIB消息中指示。
- SIB1中携带的cellReservedForOtherUse被置为TRUE(全PLMN配置)
- SIB1中携带的cellReservedForOperatorUse被置为reserved(per PLMN配置)
下期预告
下期讲小区选择中质量评估。