## 2 Literature Review  
According to the literature, there are two main types of olfactory displays: Wearable and devices placed in environment. The wearable devices are attached on the user's body or head [10]. This type of displays ensures the delivery of scents to the user but it has the drawback that the user is aware of the device, which can cause discomfort and therefore, the experience may not be as immersed as it should be. On the other hand, utilizing olfactory displays that are placed in the environment is not disturbing the user, since the user does not have to wear any hardware. However, a key limitation in this type of olfactory devices is that the scent may not reach the user, especially if it is placed far from the user or due to the weakness of the scent resulting from the air movement. Each type is subdivided into different categories. The external devices also known as \"placed in environment\" are divided based on the scent generation technique that is used in the air canon, natural vaporization and air flow. While the wearable devices are divided according to their placement on the user to either \"body\" or \"head mounted\". The paper in [11] introduces an olfactory display called inScent. A wearable device worn as necklace, it emits different scents upon receiving a mobile notification known as sensitization in which scents are used for delivering information. The scents are emitted automatically based on different scenarios either through a predefined name or the contents of a message or may be based on the timing as in calendar events. Scents are chosen on behalf of the user's preference, thus the cartridge can be exchanged. To generate and control the amount of scents to be produced, they use heating followed by fans' air for delivering to the user. Heating of a wearable device can be risky and also the fan produces a loud sound that can cause discomfort for the user. Recently, several researchers focused on designing a fashionable wearable device known as Essence [12]. They are developing a lightweight necklace-shape olfactory comfortable enough to be worn on a daily basis and controlled wirelessly via an Android application. The device can either release scent manually in which the user pulls down the string that will send data for the release of the scent or may be receiving some data from the Smartphone such as location and time. They also release the scent based on the heart rate, brain activity or electro dermal activity. The device can be controlled by someone to release the scent. However, the device can only release one scent thus it is not practical to have one scent that will be released according to the user's circumstances. Smelling Screen device in [13] can generate scent, based on the image shown in the Liquid Crystal Display (LCD), or by placing four fans on the corners of the screen indicating the image position in the screen resulting from the airflow of these fans. These devices did not operate with games, applications, or a movie but rather on random images which were shown on different corners of the screen. An inexpensive olfactory display has been developed in [14]. The device uses the Arduino Uno microcontroller as it is economical and capable of controlling the olfactory display. They also used fans to generate the airflow for scent delivery. The device was tested in different experiments involving games, advertising and procedural memories. For gaming, they used the Unity-based Tuscany. They had to modify the environment by adding a bowl of oranges that released the scent once the player came near it. However, this approach is not generic as most games are not designed to support olfactory displays. The researchers also developed their own application, presented in [15]. Researchers in [16] present an olfactory display that is simple, economical and capable of releasing 8 scents based on timed events. The scent is generated by a heating process to vaporize the essential oils and water that is used to clean the air from the previous scents.