Background
With the proliferation of health, commercial, and industrial applications that require an accurate location of the consumer or enterprise, the critical importance of positional accuracy continues to be one of the most interesting topics since World War II [1]. The critical role of positional accuracy, indoor and outdoor, is at the heart of implementing public safety regulations [2] . The 3G iPhone was the first consumer device to provide seamless integration of three positioning technologies: Assisted GPS (A-GPS), Wi-Fi positioning, and cellular network positioning [3]. 5G networks have many capabilities that are still not ready to be exploited. However, one critical capability is the positioning and location services that we see as a major use case that has the potential. As explained below, 3GPP standards took a significant step in developing LBS into the native architecture of 5G NR and 5GCS.
In this study, we are examining the accuracy of the positioning techniques with a focus on exploring the performance of the newly developed methods/algorithms associated with the new generations of these techniques. For cellular networks, 5G networks will be considered.
Literature Review
In this section, we discuss the development of positional accuracy over the last four decades. Three sections are dedicated to this discussion, with each covering one technique. A fourth section follows to cover the development of the comparative studies done so far.
To better understand the status-que of positioning across the three technologies, it is necessary to understand the historical evolution in cellular networks from 1G to 5G, as discussed in [10]. There have been very accurate algorithms that enable an ultra-accurate positioning such as quantum fingerprinting [11]. Another technique for GNSS high accuracy is discussed in [12]. The ultimate use case for positioning is to be used indoors and outdoors. To this end, [13] discusses the hybrid indoor positioning algorithm for cellular and Wi-Fi Networks. In [14], a GIS-based wave propagation model to solve the GSM-based localization problem when methods such as triangulation are not applicable due to the lack of measurements from more than one base station.
Cellular Positioning
Drivers
Emergency Services
The main driver for cellular positioning was public safety manifested by emergency services. Under the E911 services, the US FCC has been pushing the operator to implement the positioning functionality for UEs making emergency calls since 1990. Fines were imposed for any non-compliant operator, which was paused until the introduction of 3G networks [10]. With the 4G and 5G introduction, FCC adopted a biding decision for operators to provide horizontal and vertical positioning [2]. The Europeans followed a similar path with a less legal-binding mandate under the Coordination Group on Access to Location Information by Emergency Services (CGALIES) starting in 2000 [10].
Location-Based Services (LBS)
Commercial applications of positioning were coming soon after the public safety [1]. These applications were diverse and delivered different services, such as navigation, mapping, and geo-marketing, to different industries and consumer segments. These applications were even extended to the operators themselves. They started using positioning to enhance their own network using self-organizing networks (SON) features, for example, using the positioning data [10].
Positioning Cellular Techniques
There are two main categories of positioning methods: mobile-based and network-based. Mobile-based is the technique when the UE itself calculates its location by using signal measurements from terrestrial or/and satellite transmitters. The assistance data from the network can be exploited to perform the signal measurements and the position calculation. Network-based: The network location server computes the position of the mobile device by means of signal measurements performed by the network with respect to the mobile device or signal measurements performed and sent by the mobile device to the network.
Despite the method, there are five broad techniques that encompass the used algorithms [10], [17], and [19]. The five methods are listed below with a brief description.
Trilateration
The intersection between optimized geometric forms, hyperbolas or circles, for example, is created by distance measurements between the UE and reference transmitters or receivers. This intersection gives the location. Measurements that can be used are time of arrival (ToA), time difference of arrival (TDoA), Uplink TDoA (UTDoA), Observed TDoA (OTDoA), or received signal strength (RSS). [10], [17], and [19].
Triangulation
This is the earliest technique that started with the first generation of cellular networks. It has been developed over time and is still in use. The direction or angle of arrival (DoAn or AoA) of the received signals is used to estimate the position by using the intersection of at least two known directions of the incoming signal. [10] and [17].
Proximity
This is the least used technique used now due to its limited accuracy. It is the most widely adopted method in 2G, GSM, networks. The known transmitter position is assigned to be the position of the terminal. An example is the cell-ID method, where the position provided is the one of the serving base station. [10] and [19].
Scene analysis
It is also known as fingerprinting and its derivatives, such as quantum fingerprinting. It is a pattern-matching algorithm that finds the best match for a certain signal measurement, such as RSS, time delay, or channel delay spread, from a database of fingerprints. [10], [17], and [11].
Hybrid
Hybrid is the most complex technique where more than one technique from the previous four techniques are combined.
These four techniques are depicted in Fig. 1 from [10].
Figure 1: Fundamental positioning techniques using radio signals.[10]
Evolution
The journey of positioning functionality started with 1G analog networks that worked on Frequency Division Multiple Access (FDMA) such as Advanced Mobile Phone System (AMPS). Then moved to the digital circuit-switched 2G networks that uses Time Division Multiple Access (TDMA) such as GSM. The 3G network introduction made a step-up where the channels that are used became wide, 5 MHz, and used Code Division Multiple Access (CDMA). It also paved the way to include the Positioning Reference Signal in the 3GPP standards. This has been continued to the 4G and 5G networks with more efficient implementation. It was included in 3GPP standard 36.305 and TR 36.355.
5G
5G networks, both NR and 5GC, have made a significant enhancement in treating the positioning functionality. First, it is now built into the architecture a new module, Location Management Functionality (LMF), starting from Release 16, which is frozen now. is central in the 5G positioning architecture. The LMF receives measurements and assistance information from the next generation radio access network (NG-RAN) and the mobile device, otherwise known as the user equipment (UE), via the access and mobility management function (AMF) over the NLs interface to compute the position of the UE. Due to the new next generation interface between the NG-RAN and the core network, a new NR positioning protocol A (NRPPa) protocol was introduced to carry the positioning information between NG-RAN and LMF over the next generation control plane interface (NG-C). This module allows Non-Stand-Alone (NSA) deployment, i.e. 4G, to coexist with the 5G SA deployment, as shown in the architecture in Fig. 2.
Figure 2: UE Positioning Overall Architecture applicable to NG-RAN. [9]
Here are the elements that are added to the 4G networks in 5G starting from Release. 16 and 17:
- The delay error variance decreases in the order of the square of the bandwidth as the bandwidth increases. However, the angle variance is completely independent of the bandwidth. NR provides significant bandwidth improvement over LTE; while LTE provides a maximum of 20 MHz, NR provides up to 100 MHz in frequency range 1 and 400 MHz in frequency range 2.
- Received power is inversely proportional to all estimate variances. In NR, received power can be increased by beamforming. This is especially more important for numerologies with higher subcarrier spacings.
- NR provides five different choices for subcarrier spacing: 15 kHz, 30 kHz, 60 kHz, 120 kHz and 240 kHz. The subcarrier-spacing is a bit peculiar since it gives a linear increase to the angle variances while at the same time giving only a linear decrease to the delay variance. This effect is derived from the noise variance increasing linearly with the subcarrier spacing. A natural way to counter this is to increase the RX power.
- Different antenna patterns, in terms of spacings and number of polarizations in relation to rows and columns in antenna array, etc. do not affect the delay variance, but rather only the total number of antenna elements in the array matter. For the angle estimates, the variance is proportional to the inverse square of the antenna spacing. Furthermore, the number of rows and columns, respectively, of the antenna array gives a cubic decrease in the angle estimate variances. Typically, NR equipment carries a larger number of antennas.
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