Chun Yang, QuNav, Thomas Pany, Petra Weitkemper, Universität der Bundeswehr München, Germany

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Abstract:

The fourth generation (4G) long term evolution (LTE) standard signals are well specified in the 3GPP standards. However, some aspects are left to operators as implementation options that depend on the actual deployment environment and local traffic patterns. To better understand such practical issues of in-the-air LTE signals, several experiments were carried out near Munich, Germany, which collected data of frequency division duplex (FDD) downlink frames for the analysis presented in this paper. From experimental data, LTE signals are observed in multiple frequency bands at a site and signals from different cells may be present on the same frequency carrier. In the latter case, signaling patterns are found to be coordinated in the data in such a way to avoid collision among the neighboring cells. Various orthogonal frequency division multiplexing (OFDM) signal components, namely, cyclic prefix (CP), primary synchronization signal (PSS), secondary synchronization signal (SSS), and cell-specific reference signals (CRS) are described in the paper together with their properties, rationale of design, and possible ways to exploit them for navigation, that is, generation of the time of arrival (TOA) estimates and pseudoranges for positioning. As a focus of this paper, antenna port (AP) is a unique concept introduced in 4G LTE and its actual implementation depends on a particular operator. AP is a logical entity and cannot be simply equated to a physical antenna. It is important as it is one of the key parameters that define the CRS. In other words, each antenna port is associated with a specific reference signal; and a receiver needs to use this reference signal to estimate the channel corresponding to the specific antenna port regardless of its physical origin. As such, the use of antenna ports does not cause any problem in channel estimation/equalization and subsequent data demodulation/decoding for communications. However, when CRS is used to estimate TOA from the channel impulse response (CIR) for timing, ranging, and ultimately positioning, it may present a potential ambiguity as to where the physical transmit antenna’s phase center is for the purpose of precise ranging. Unfortunately, this critical aspect is not well addressed in the 4G LTE-based opportunistic positioning literature. In this paper, experimental data are used to illustrate the effect of antenna ports on TOA estimation. Understanding of such effects is prerequisite for precision timing and ranging especially with carrier phase. The paper provides essential and insightful information directly useable for developing advanced TOA estimation algorithms and ultimately for navigation with LTE signals.