## 2 Related Work  
The IoT paradigm is very well suited to the architectural requirements of smart city services and has therefore been previously considered in work in this field.  
[PERSON] _et al._[2] designed an urban IoT network based on the web service paradigm, deployed in Padova (Italy). This proof of concept monitors street lighting and air quality and allows remote identification of issues that demand the attention of maintenance staff through either a central control station or mobile devices.  
A different perspective is found in previous publications [7], where sensors and actuators of an IoT-enabled smart city scenario are considered service providers, just as in a cloud provisioning model. This proposal eliminates the barriers between physical (sensors and actuators) and logical (cloud service providers) worlds, creating a higher level of abstraction that allows the deployment of innovative ubiquitous smart city services.  
Taking a framework perspective, [PERSON] _et al._[8] identified the basic IoT elements needed in smart cities and described a case study for noise monitoring.  
Although the above works deal with the interaction through mobile devices between city staff and citizens and smart city services, they mainly focus on the communications architecture and do not describe methods to improve user interaction or proximity communications with AR technologies, as proposed in our work.  
We must remark, however, that AR interfaces have already been applied in smart city and smart building scenarios to deliver context-dependent information for both maintenance tasks and other purposes, such as tourist information services [9, 10, 11, 12]. For example, in Smart Santander, tourist information is delivered via AR interfaces on GPS-enabled smartphones, associating points of interest with cloud data. This is complemented with QRcode labels and RFID tags when a more precise location is needed [13, 14].  
Current smartphones allow the creation of powerful AR interfaces using video see-through display techniques [15]. To create an overlay with augmented information on top of the image shown to the user, the system needs to accurately identify and track relevant objects in the video stream field of view. This can be achieved with techniques such as video processing, RFID object labeling and user positioning.  
Static AR markers have been proposed to help to visualize sensor information. Two approaches have been considered: the placement of AR markers over the sensor nodes themselves [16, 17] or elsewhere, using the marker as a gate identifier to access sensor information [18, 19].  
In the field of dynamic markers, we are only aware of the work by [PERSON] _et al._[20], who proposed an AR marker with limited information printed using temperature-sensitive ink. This thermochromic marker enabled the devising of a four-range temperature sensor. In a smart city IoT environment, this would pose scalability and connectivity problems.  
Finally, while various authors have combined AR interfaces and IoT architectures in the same framework, they only proposed static AR markers as physical object IDs [21, 22]. In general, although the individual technologies that form part of our proposal have all been dealt with in considerable depth in the literature, none of the studies to date have considered the combination of AR and VLC to support proximity information services relying on IoT architectures (where the AR marker acts as an additional communications channel between a smart object and the user interface). In this paper, we show how this combination of technologies improves usability and efficiency in a smart city maintenance scenario.