I. Introduction
The use of mobile devices has increased dramatically for the past few decades. Although the fundamental function of mobile phones is to provide voice communication and SMS services, recent mobile phones offer a rich set of additional features like instant e-mailing, document editing, surfing on the internet and GPS support. However, mobile devices have two main drawbacks: limited screen sizes and limited keyboards. Entering text through keyboard is required to write an SMS or an e-mail, to fill a web form, to edit a document or even to enter an address in the map. Fig. 1 shows a standard headset keyboard which has only a 12-key keypad (also some devices may have additional function keys) to write all letters and this limited keypad is the main obstacle to a better human-computer interaction for mobile devices. Although many researchers work on alternative keyboard designs for mobile devices [1]–[2] and many devices on the market have mini or on-screen QWERTY keyboards, the majority of the market still has the traditional 12-key keypad and suffers from low text entry speed. Considering the growing market on mobile emailing, it is essential for the mobile phone developers to devise faster typing methods [3].
A typical mobile phone 12-key keypad design.