浅析UE小区同步

在工作中有时发现LTE UE始终不能与LTE callbox仪器做信令连接，但RF测量显示接收RSSI值似乎正常, 进一步可发现UE在小区同步中失败, 且LTE modem timer计数的frame boundary值很大, 接收SNR值变差, 即UE接收解调性能不正常。

该问题一般和外部的晶振和AFC控制电路相关, 其提供给芯片内部RF PLL的参考频率, 如果该参考频率偏差太大，会导致UE在小区同步时失败.

UE通过小区搜索过程试图接入LTE系统, 其中包括一系列同步步骤，使得UE确认必要的时间和频率参数, 可以解调下行信号、按正确时序发射上行信号和获取一些关键的系统参数.

LTE中有三种同步要求，

1)      symbol timing acquisition by which the correct symbol start is determined;

2)      carrier frequency synchronization which mitigates the effect of frequency errors resulting from Doppler shift and errors from electronics;

3)      sampling clock synchronization;

有两种小区搜索过程, 其一是UE初始同步，另一中是检测相邻小区为切换做准备, UE会用到每个小区都广播的两个特殊信令, Primary Synchronization Sequence (PSS) and SecondarySynchronization Sequence (SSS).

The detection of these signals allows theUE to complete time and frequency synchronization and to acquire useful systemparameters such as cell identity, cyclic prefix length, and access mode(FDD/TDD).

At this stage, the UE can also decode the Physical Broadcast Control Channel (PBCH) and obtain important system information.

Synchronization signals are transmitted twice per 10 ms radio frame. The PSS is located in the last OFDM symbol of the first and 11^th slot of each radio frame which allows the UE to acquire the slot boundary timing independent of the type of cyclic prefix length.

The PSS signal is the same for any given cell in every sub-frame in which it is transmitted (the PSS uses a sequence known as Zadoff-Chu).  

The location of the SSS immediately precedes the PSS – in the before to last symbol of the first and 11^th slot of each radio frame. The UE would be able to determine the CP length by checking the absolute position of the SSS. 

The UE would also be able to determine the position of the 10 ms frame boundary as the SSS signal alternates in a specific manner between two transmissions (the SSS uses a sequence known as M-sequences). 

In the frequency domain, the PSS and SSS occupy the central six resource blocks, irrespective of the system channel bandwidth, which allows the UE to synchronize to the network without a priori knowledge of the allocated bandwidth. The synchronization sequences use 62 sub-carriers in total, with 31 sub-carriers mapped on each side of the DC sub-carrier which is not used. This leaves 5 sub-carriers at each extremity of the 6 central RBs unused. 

Reference: 

1, 3GPP 36.300 (10.1.5) : Overall description of RACH Process, 3GPP standard for RACH Process

2, The Physical Layer, LTE white paper

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