«Sensors 2014, 14, 21151-21173; doi:10.3390/s141121151 OPEN ACCESS sensors ISSN 1424-8220 Article VibeComm: Radio-Free ...»
Sensors 2014, 14, 21151-21173; doi:10.3390/s141121151
VibeComm: Radio-Free Wireless Communication for Smart
Devices Using Vibration
Inhwan Hwang, Jungchan Cho and Songhwai Oh *
Department of Electrical and Computer Engineering and ASRI, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea; E-Mails: firstname.lastname@example.org (I.H.);
* Author to whom correspondence should be addressed; E-Mail: email@example.com;
External Editor: Vittorio M.N. Passaro Received: 3 September 2014; in revised form: 23 October 2014 / Accepted: 30 October 2014 / Published: 10 November 2014 Abstract: This paper proposes VibeComm, a novel communication method for smart devices using a built-in vibrator and accelerometer. The proposed approach is ideal for low-rate off-line communication, and its communication medium is an object on which smart devices are placed, such as tables and desks. When more than two smart devices are placed on an object and one device wants to transmit a message to the other devices, the transmitting device generates a sequence of vibrations. The vibrations are propagated through the object on which the devices are placed. The receiving devices analyze their accelerometer readings to decode incoming messages. The proposed method can be the alternative communication method when general types of radio communication methods are not available. VibeComm is implemented on Android smartphones, and a comprehensive set of experiments is conducted to show its feasibility.
Keywords: smartphones; vibration; wireless; communication
1. Introduction With the advent of the smartphone and the growth of smartphone users, short-range wireless communication between smartphones and between a smartphone and other devices has become an important capability of smartphones. A number of short-range wireless communication methods Sensors 2014, 14 21152 have been developed and embedded into smartphones, including Wi-Fi, Bluetooth and near-ﬁeld communication (NFC). However, the currently available short-range radio communication methods cannot be operated when just one of the communication devices is not equipped with radio communication components. In this paper, we propose a new short-range wireless communication method, named VibeComm, which utilizes the built-in vibrator and accelerometer in smartphones. The proposed method is motivated by the old-fashioned dot-and-dash communication method or Morse code, and it can show the possibility of a noveltype of wireless communication method without using radio.
We assume that the communicating smart devices are placed on a single object, such as a table or a desk. The transmitting device generates a sequence of vibrations, which is similar to Morse code.
The generated vibrations are propagated to the other devices through the object on which the devices are placed. The receiving devices decode the incoming message based on their accelerometer readings.
The proposed method can be used to communicate with devices with accelerometers, and no radio is required. Hence, the proposed method enables communication with a wide range of everyday objects that lack radio functionality, closing the gap between the cyber world and the physical world.
Compared to the existing short-range wireless communication methods, such as near-ﬁeld communication (NFC), radio-frequency identiﬁcation (RFID), Wi-Fi, ZigBee (IEEE 802.15.4) and Bluetooth, the proposed method has a number of desirable characteristics. NFC is used for short-range communication with a maximum communication range of 20 cm based on RFID technology . While a number of smartphone manufacturers are starting to package NFC chips in their smartphones, NFC is not yet widely available. In addition, NFC’s practical working distance is about 4 cm, while 20 cm is the theoretical maximum limit. The proposed method VibeComm can provide a longer communication range than NFC. Furthermore, two-way and one-to-many communication are available in VibeComm.
RFID is another wireless technology for non-contact communication using radio , but it suffers from similar shortcomings as NFC, as well as the cost of adding RFID tags is not negligible . While Wi-Fi , Bluetooth  and ZigBee  are widely available for longer range communication than NFC, they still need extra components that make it possible to communicate . Infrared data association (IrDA) was a widely-used short-range communication method for personal digital assistants (PDAs) and mobile phones before the introduction of smartphones. IrDA uses infrared to enable mobile phones to send and receive data up to 115 kb/s. However, its communication range is less than 1 m, and its angular coverage is between a 15- and 30-degree half-angle cone from the optical axis of the receiver device .
A number of non-radio-based communication methods have been proposed before. Komine et al.  proposed a new communication method based on LED lights. However, it requires a line of sight between communicating devices, and one-to-many communication is not possible without an extra apparatus. Arentz et al.  proposed the use of sound in the frequency range from 20 kHz to 23 kHz as an alternative to IrDA. However, the sound can be easily overheard, and the method is not robust against ambient noise. Chang et al.  introduced ComTouch, which allowed communication utilizing vibration. Their method was also motivated by Morse code, but they used the vibration signal to replace characters or sounds, especially for deaf/blind people. They presented a custom-made device that could generate vibrations. A sender sends a message by grabbing the device, and the received message is presented as a series of vibrations. Studer et al.  proposed Shot for secure communication. It requires direct contact between two devices, and they mutually communicate for identiﬁcation using Sensors 2014, 14 21153 vibration. However, the actual message is transmitted using radios, unlike VibeComm. VibeComm can be easily modiﬁed to transmit predeﬁned vibration patterns to help communication between a smartphone and a deaf/blind person.
With various sensors in a smartphone, such as an accelerometer, digital compass, gyroscope, GPS, microphone and camera, a number of researchers have applied various sensing data collected by smartphones to discover contexts about a user and her surroundings to deliver better services to the users. For example, a vibrator and an accelerometer in a smartphone, a combination that is utilized in VibeComm for communication, has been applied to the indoor localization problem [13,14]. Shafer  used a vibrator and accelerometer to classify seven pre-deﬁned locations using a support vector machine with time-series accelerometer readings. Kunze et al.  proposed a symbolic localization method through active sampling of acceleration and sound signatures. Their method used vibration and sound (short and high frequency beeps) to sample the response of the environment.
As an alternative to radio-based wireless communication, Yonezawa et al.  proposed a method to transfer data from a smartphone to a notebook by generating vibrations from a smartphone. It is assumed that a smartphone is directly in contact with a notebook for communication . In , the authors applied the method from  for pairing electronic devices. Again, two communicating devices are required to be directly in contact with each other. VibeComm is similar to , since it uses a vibrator and an accelerometer, but VibeComm does not require direct contact between devices, as well as two-way or one-to-many communication is possible in VibeComm. Since the vibration energy is dissipated in the object on which communicating devices are placed and there are echoes from object boundaries as vibrations bounce back, a more sophisticated communication system design is required for VibeComm.
The proposed method is ideal for low-rate off-line communication. As its communication medium is the object on which devices are placed, communication over a longer distance and broadcasting to multiple devices are possible using VibeComm. These features distinguish the proposed method from [15,16] and radio-based communication methods. One possible application of VibeComm is the task of conﬁguring a large number of devices simultaneously without using radios. For instance, consider a desk that is used by multiple users. Suppose that a number of electronics, such as a notebook, a digital alarm clock, a tablet, an MP3 player, a telephone and a TV, are placed on the desk, and each electronic device on the desk has predeﬁned user settings. If each user has her preferred settings for all electronics stored on her smartphone, then the proposed method can be used to conﬁgure all electronics on the desk for each user by simply placing the user’s smartphone on the desk.
The remainder of this paper is organized as follows. An overview of the proposed method is shown in Section 2. Section 3 discusses issues when designing the proposed method, and Section 4 describes the design of VibeComm. The implementation of the proposed method is described in Section 5, and the results from the experiments are discussed in Section 6. Two-way and one-to-many communication demonstrations are given in Section 7, and remaining issues are discussed in Section 8.
Sensors 2014, 14 21154
2. VibeComm Overview
An overview of VibeComm is shown in Figure 1. We consider the problem of packet-based communication between smart devices that are equipped with accelerometers and vibrators. The transmitting device takes an input from a user application and encodes the message into a series of packets. Each packet consists of frames, and each frame represents a bit of information. Each packet is transmitted from a transmitter by generating vibrations. A receiving device listens for incoming vibrations using its accelerometer and decodes incoming packets. The decoded message is then delivered to a user application. Due to the current hardware limitations discussed below, a sophisticated coding scheme cannot be applied. A simple coding scheme is applied to VibeComm, where the presence of vibration in a frame indicates a bit 1 and the absence of vibration represents a bit 0. In order to reduce power consumption, impulse-like vibration signals are used to encode a packet, instead of continuous vibrations. In this paper, we focus on sending and receiving text messages to demonstrate the feasibility of vibration-based wireless communication. In the next two sections, we discuss design issues, calibration of raw accelerometer readings and the design of the transmitter and receiver for VibeComm.
Figure 1. An overview of VibeComm.
A transmitter encodes a message into a series of vibrations. Vibrations propagate through the object on which the transmitter is placed. Receivers that are placed on the same object listen for incoming vibrations using accelerometers and decode them into messages.
3. Design Considerations
3.1. Time Synchronization and Inconsistent Sampling Frequency Time synchronization between a sending device and a receiving device is critical for reliable communication. When time synchronization fails, a frame can be read incorrectly, and a bit error will result. Unfortunately, the wall clock available in smart devices is not sufﬁcient, since there can be an unknown length of delay from the propagation of the vibration signal. Hence, it is required to synchronize time between a transmitter and receivers each time communication starts for successful communication.
In addition to time synchronization, we have observed that the sampling frequency of an accelerometer is inconsistent. Android-based smartphones support four different sampling rates for their accelerometers, and they are SENSOR DELAY NORMAL, SENSOR DELAY UI, SENSOR DELAY GAME and SENSOR DELAY FASTEST. We have empirically measured each sampling rate, and they approximately correspond to 6 ± 1 Hz, 17 ± 1 Hz, 50 ± 1 Hz and 100 ± 1 Hz, respectively.
Furthermore, the sampling rate varies between models. We have also observed that both the sampling rate and the sampling interval vary depending on the load and state of the Android OS. Hence, it is not possible to sample with an exact sampling frequency with current Android platforms.
An example is shown in Figure 2. We have transmitted a successive bit-stream of 1100011 from the sending device and examined the received data from the receiving device. Note that we have assumed that the sampling rate of the accelerometer is 17 Hz. The ﬁrst seven bits are decoded correctly. However, the next seven bits are decoded as 1110001, which is a one-bit shifted version of the transmitted signal.
This shows that a small misalignment can accumulate and cause a decoding error.
Figure 2. A successive transmission of 1100011.
A bit error occurs due to time synchronization and an inconsistent sampling frequency.
3.2. Effects of Echoes As the vibration signal propagates throughout the communication medium object, the signal can be reﬂected from boundaries and other objects placed on the communication medium. These reﬂected vibration signals can make decoding more challenging. We can reduce the effect of echoes by generating vibration for a short period of time. An example can be seen from Figure 2. In addition, this impulse-like vibration signal can reduce the power consumption of the transmitter. While the effect of echoes was not considered in [15,16], it becomes a signiﬁcant issue in VibeComm, as it is designed for communication over a longer distance through a rigid material.
3.3. Detection Sensitivity
Since the vibration signal strength varies depending on the communication medium type (the rigidness of the medium) and decays as the communication distance increases, the signal detection sensitivity has to be set appropriately. Unexpected circumstantial factors, such as interfering vibration introduced by other objects, can make this problem even more difﬁcult. The problem resulting from the rigidness of the communication medium can be solved to some degree by adjusting the detection threshold. As shown in Figure 3, the choice of the threshold value can make signiﬁcant differences in decoded messages.