PRS Muting Pattern Assignment to Optimize RSTD Measurement Acquisition for OTDOA Positioning in 3GPP LTE (2015)

The periodic Positioning Reference Signal (PRS) has been specially introduced in the 3GPP LTE Release 9 [1,2,3,4] Standard to enable high accuracy ranging measurements. PRS can be transmitted with higher power for improved level of hearability. PRS has a frequency re-use factor of six, as defined by the Physical Cell Identifier (PCI). PRS tones of cells in the same frequency group collide, which gives rise to cross-correlation interference. This may result in a failure to detect cells due to the near-far problem, where a closer-by cell will drown the signal of a far away cell of the same frequency group. The resulting loss in hearability can be particularly limiting in dense urban deployments or deployments with poor PCI network planning. To address this issue, the 3GPP LTE Standard provides the option of PRS muting so as to introduce orthogonality in the time domain between cells of the same frequency group. PRS is transmitted in pre-defined groups of positioning subframes, termed as “positioning occasions”. A muting pattern determines a sequence of PRS occasions for which the signal is either on or off (muted). When neighboring cells of the same frequency group are assigned orthogonal muting patterns, cross correlation interference is eliminated. In this work we use the terminology “muting scheme” and “muting pattern,” the distinction being that a muting scheme is a method to assign muting patterns to individual cells of a network. Boosting hearability through effective interference management requires an optimal muting pattern assignment. The objective of this work is to prove the value of muting and to evaluate various muting schemes to optimize PRS measurement acquisition. 

In this work, we created a simulation framework to evaluate the performance of various muting pattern assignments. The simulator was qualified against live data. Sub-urban, urban and dense urban deployments, which are modeled on existing Base Station Almanacs (BSA), were simulated. Once assigned, a muting pattern repeats periodically. The standard constrains the pattern length to be 2, 4, 8 or 16 bits. We studied muting pattern assignments with all four lengths and characterized them further with the notion of scheduling frequency, which is the number of occasions in one repetition period where the PRS of each cell is not muted. We generate randomized muting patterns for every possible pattern length-scheduling frequency combination. The choice of length and the scheduling frequency may impact the time to first fix (TTFF). 

In evaluating performance of muting schemes, we derived an upper performance bound by simulating a scenario where a virtual pattern length is equal to the size of the BSA and the scheduling frequency is equal to one. This allows each cell in turn to be the only one transmitting PRS with full power, effectively removing all cross-correlation interference. The lower bound is the performance when no muting is applied. The goal is to design a method that quantifies how much improvement is realized with respect to no muting and also how degraded the performance is with respect to the upper bound. 

We defined a performance metric called the hearability gain characteristic which captures all the properties of a muting scheme. It is normalized with respect to the upper and lower performance bounds and is evaluated over 16 PRS occasions (maximum allowed by the Standard). It captures the cell acquisition rate (which is the rate at which unique cells are detected over time and is tied to the TTFF) as well as the overall hearability at the end of the 16 occasions. The characteristic exposes the performance tradeoffs of the muting scheme and can be used to develop a score with which to rank various schemes. 

We found that for various network densities, the length 8 and 16 muting schemes with duty cycle values of 50%, offered good tradeoffs between cell acquisition rate and overall hearability. The muting scheme proposed is easy to generate and deploy by the carrier, as these require no apriori knowledge of the BSA geometry. These schemes approach the upper bound to overall hearability in the line of sight (LOS) scenarios. Moreover, the hearability loss due to attenuation, blockage and fading and the recovery time are lowest for these schemes. The average improvement in hearability as compared with no muting is about 60% in LOS scenarios and about 55% when there is blockage or fading. 

The main message of this work is that applying muting offers a large advantage in PRS hearability compared to no muting. Carriers are in the initial stages of deploying PRS muting, therefore a study of how best to utilize this feature in achieving an optimal tradeoff between TTFF and sensitivity, especially in challenging areas (indoor locations, downtown areas underground parking etc.) is important and timely. To this end, our work serves as a guideline for generating and deploying optimal PRS muting pattern assignments.