# Multi-User Activity Recognition Using Plot Images Based on Ambiental Sensors
## 1 Introduction  
Human activity recognition has found applications in many different fields such as homes and offices [1]. Due to the emergence of intelligent environments like homes and offices, human activity recognition has become increasingly necessary. This is motivated by the desire to transform the environment in which a person performs day-to-day activities to improve the quality of life. In [2], it was mentioned that these intelligent environments can help monitor disabled or elderly people who need permanent supervision. The system can be enabled to detect unusual activities that do not align with the monitored patient's routine. Human activity recognition, or HAR, can be divided into two large categories based on the types of sensors involved:  
* Ambiental sensors;
* Wearable sensors. Human activity can be described as a sequence of intended actions since we define human activities as actions that are intentional and not just a reaction to environmental impulse [3; 4; 5]. Also, when we consider multi-user activities, we look at activities where we have more than one participant. Activities that are executed at the same time are defined as an activity group [6]. Activities that are performed independently at the same time, and there are no intersections between them, are considered multi-individual activities. If the activities intersect, they are considered hybrid activities, a mix between multi-individual activities and group activities. These hybrid activities can be further classified into three subcategories based on the interaction between activities:  
* Simultaneous activities;
* Collaborative activities;
* Concurrent activities. The simultaneous activities category requires each user to perform an activity that is not related to other activities and takes place at different locations in an environment such as a house. Collaborative activities specifically involve identical actions in the same location. The category of concurrent activities requires users to perform different actions in the same location. In the same activity, each individual has a potentially different behavior. Ambient sensors generate data that do not provide direct information about user identification. Wearable sensors present a disadvantage, especially for certain activities, even if they would be the best solution for identifying users. The compromise solution would be to extract each user's behavior based on the ambient sensors' data and then identify the users according to the behaviors [7]. The activity recognition and person localization/identification come with their own set of problems. Locating people requires a high-precision interpretation of the sensor data. Also, between locations without sensor coverage, an approximation of the user's position must be used with as much accuracy as possible [8]. In the past, only RFID (radiofrequency identification) was used as a stable method for identifying users; today, several types of devices can be used. For example, devices can communicate via Bluetooth, Ultra-Wideband (UWB), and ZigBee, but the major disadvantage of these devices is that they have to be worn at all times [9]. Fixed ambient motion detection devices have their disadvantages; for example, infrared sensors are sensitive to heat sources or strong light, and surveillance cameras have reduced performance in some poor visibility conditions [10]. Most people own a smartphone and carry it for extended periods; this can be a motion detection solution. Although the activities, which can be recognized with the data obtained from these devices, are limited, they can play an important role in the identification of the user. The adopted solution must be a compromise, neither too uncomfortable for the user nor too complex for the environment, and it must also respect privacy rules [11]. The human activity recognition process involves activities carried out at home or in indoor spaces, as well as activities outside of residential spaces.