第469页 19.3.1 Positioning Reference Signals (PRS)

PRS的目的：
In order to increase the probability that the UE can detect sufficient neighbour cells, and therefore achieve good OTDOA positioning reliability, special ‘positioning subframes’ are designated in LTE Release 9. 

PRS为何能够做到降低相邻小区PDSCH的干扰呢，如何能保证相邻小区的PRS信号质量足够好呢？
Positioning subframes are designed to aid the ‘hearability’ of neighbour cells by reducing the interference and increasing the RS energy: typically they do not carry any Physical Downlink Shared CHannel (PDSCH) data, but provide PRSs in addition to the Release 8 cell-specific RSs.

PRS的位置（以双天线为例）
双天线的PRS位置

根据该RB中PRS的位置，可以确认PRS与PDCCH和CRS不重叠，而且按照PCI模六设计，复用系数大于CRS的模三，PRS的发射功率可以更大。
而且可以通过关闭定位帧的PDSCH发射来减少PDSCH导致的干扰，PRS应尽在定位帧中发射。
The requirements for the time within which a UE must be capable of reporting an RSTD measurement after receiving OTDOA assistance information can be found in [7].

UE上报RSTD测量结果，系统为了减少UE搜索相邻小区的PRS过程，系统应基于地理位置给出一个简化的候选列表集（不同于传统的邻区集），另外PRS差异应该也要给出，从而加速PRS的RSTD测量。