## 2 Basic Concepts and Work Scope
### Mobile-Device-Based Positioning  
In a study centered on radio-network-based localization services [9], a categorization is presented differentiating which is the entity responsible for the position estimation. In one category, a radio mobile device is responsible for the estimation of its own position. In another category, the network is responsible for estimating the position of the radio mobile device. A more concise categorization is used in the standards for cellular radio networks, where the positioning methods are classified into mobile-based (also known as user-based or user-equipment-based), mobile-assisted, network-based, and network-assisted [49; 50; 51; 52]. Similarly, we differentiate between cases in which the position estimate is performed by a communications or positioning infrastructure and by a mobile positioning device. In this context, the positioning device is a mobile entity intended to estimate its own position. Here, these cases are referred to as _infrastructure-based positioning_, shown in Figure 6a,b, and _mobile-device-based positioning_, shown in Figure 6c,d,e. In the former, the position estimate is made by the communications or positioning infrastructure. In the latter, the position estimate is made by the mobile positioning device. In the case of fingerprinting-based positioning using radio signals, the infrastructure utilizes fingerprints from uplink signals, whereas mobile devices utilize fingerprints from downlink signals. In the infrastructure-based positioning case, the positioning process is carried out in two steps; see Figure 6a. First, the devices transmit radio signals in uplink. Then, the radio signal information is collected as fingerprints in a distributed manner on the infrastructure side from all of the base stations involved, and the position estimate is computed using a dedicated device. Second, the position estimate is communicated to the positioning consumer. In this case, the consumer of the position estimate may not exactly be called the positioning device. In the literature, it is simply called the transmitter, or the positioning tag in some cases, as it is the device that generates the signal toward the infrastructure. A variation of the infrastructure-based positioning case is shown in Figure 6b, in which arrays of antennas for MIMO, massive MIMO [53], cell-free massive MIMO [54; 55; 56], Large Intelligent Surfaces (LISs) [57], or reconfigurable intelligent surfaces (RISs) are used at the base stations for positioning purposes, with the mobile users transmitting in uplink. Some examples of fingerprinting-based positioning with uplink transmission are [58; 59; 60; 61; 62; 63; 64; 65] for massive MIMO, [66; 67] for cell-free massive MIMO, [68] for LISs, and [69; 70] for RISs. In the mobile-device-based positioning case, the device receives the radio signals transmitted in downlink by the infrastructure and collects these as fingerprints; see Figure 6c. Then, the device itself or an additional supporting device estimates the position without further involvement of the infrastructure. A variation of this case is shown in Figure 6d, in which the base stations utilize MIMO, or massive MIMO, antenna arrays for positioning in downlink. In addition, some works propose the use of RISs for the downlink case. Some examples of fingerprinting-based positioning with downlink transmission are [71] for MIMO, [72; 73] for massive MIMO, and [74; 75] for RISs. Finally, we present a variation of the mobile-device-based positioning case with the signals in downlink, for the case of fingerprinting-based positioning with spatial side information, in Figure 6e. In this case, the downlink radio signals are received by the 2D sensor array to exploit the spatial side information. The processing of the fingerprints is performed in the device itself or using an additional supporting device. We do not consider in our scope the infrastructure-based positioning case.