## 1 Introduction
To improve the mechanical stability of the system and better protect the users' privacy, the intraoral version of the TDS (iTDS) is designed to locate inside the mouth on a dental retainer that claps onto the teeth [12, 27, 28]. The first prototype of the iTDS, referred to as the iTDS-1 in this paper, was implemented in the form of a palatal retainer [12]. Palatal dental retainers locate in the palatal vault space, between the tongue and the palate, a space naturally designated for tongue movements in speech and a temporary reservoir of food during ingestion [29]. Several smart intraoral devices have been implemented as palatal dental retainers, such as the TTK, the ITCS, and the oral electro-tactile display [12, 13, 17, 30]. Figure 1a,b shows the actual implementation of the TTK and the iTDS-1, occupying the palatal vault space. There are several drawbacks in locating the iTDS-1 in the palatal vault space. Firstly, the iTDS-1 limits the palatal space and impedes proper tongue posture and movement necessary for clear speech [29]. Secondly, limiting the palatal space potentially degrades the iTDS performance because that space is needed for spacing out the tongue command locations and facilitating their easy selection. Intraoral space is also needed for passage of the tongue from one command location to another rapidly and naturally. Therefore, any object on the way might hinder the natural tongue movement [12]. Finally, when the iTDS is worn on the palate, it blocks the palatal tactile feedback that helps users to acquire a better sense of the magnetic tracer position and reproduce accurate tongue commands. To preserve the palatal vault space, a new arch-shaped iTDS, which is referred to as the iTDS-2, has been designed to be located on the lower jaw, filling the buccal shelf area. The buccal shelf area, a space between molar teeth and inner cheeks, works as a primary stress-bearing area of the mandibular arch and has been used for appliances, such as a mouth-piece or a denture [31]. Figure 1c,d show examples of smart intraoral appliances with built-in electronics, designed to be worn on the lower jaw, in the buccal  
Figure 1: Intraoral appliances with built-in electronics: (**a**) Tongue-Touch Keypad; (**b**) iTDS-1; (**c**) X2 impact mouth-guard and (**d**) Intellidrug intraoral drug-delivery system. shelf area. We have selected the lower jaw over the upper jaw, to minimize protrusion of the lips and preserve the intraoral space. The intraoral anatomy provides less space between the upper teeth and upper lip than that the lower teeth and lip [29]. Moreover, when the mouth is open, it is easier for the tongue to maintain its relative position with respect to the lower jaw than the upper jaw. This makes it easier to issue the iTDS commands. Nonetheless, a detailed comparative assessment of various intraoral spaces in terms of efficacy and comfort level for a wireless, non-contact tongue-based control surface is needed. In addition to the shape change resulting in the aforementioned advantages, the iTDS-2 has a more robust wireless communication capability compared to the iTDS-1, which is essential for the ATs that users fully depend on for their activities of daily living. The dual-band transmitter (Tx) in the iTDS-2 has a higher output power to provide the required SNR at a designated operating distance. We have carefully designed an antenna for the 432 MHz band to maximize the efficiency of radiated power towards the receiver (Rx). To reduce the probability of malfunction caused by strong external interference, the iTDS-2 automatically switches between its two bands based on the wireless link quality. A built-in super regenerative receiver (SR-Rx) in the iTDS-2 not only detects the wireless link quality in the 27 MHz band but also communicates with the TDS universal interface (TDS-UI) to receive the wireless link quality of the 432 MHz band. Accordingly, the iTDS-2 can switch between the 27 MHz and 432 MHz bands without any physical intervention by users.