Personal Area Networks (cont.)

Using the body as a communication medium! How is this possible?

In PAN, to communicate between two devices using body as a medium we need a PAN Transmitter and PAN receiver, both battery powered. The PAN transmitter capacitatively couples a small displacement current through the human body to a receiver. The Transmitter need not be in direct contact with the skin. A capacitor is a small device with two metal plates separated from each other that can store an electric charge. Whenever an alternating current flows in a circuit with a capacitor in the circuit, a displacement current flows between the two plates of the capacitor, which are electrically isolated from each other. This displacement current is real and is what is transferred from the PAN device to the body. In a PAN, the transmitter electrode facing the body and the skin act as a capacitor. By modulating the electric current, the displacement current between the transmitter and the body can be modulated. As a result, data can be transferred across to the receiver (by means of current flowing through the body, which also interacts with the body in the same way). As in any electric circuit we need a return path for the circuit to be completed; this return path is provided by the "earth ground," which includes all conductors and dielectrics in the environment that are in close proximity to the PAN devices. The earth ground needs to be electrically isolated from the body to prevent shorting of the communication circuit. You will have a clear understanding by looking at the figure below.

The Transmitter capacitatively couples the electric field to the body. Here the transmitter plate and the body act as a capacitor (A). A normal electric current flows through the body and a part of it reaches the other end where the body and receiver plate act as a capacitor B. Here again, the electric current flows between the body and the receiver as a displacement current. The other electrode of the receiver facing the earth and the ground act as a capacitor C and the ground and the other elctrode of the transmitter act as capacitor D. Since the ground is at zero potential (always), electric fileds are setup between the ground and the electrodes and thus the return path for the current is formed. Therefore current (and therefore data) can be sent from the transmitter to the receiver just like any other electric circuit. The transmitters and receivers basically consist of an encoder and/or a decoder followed by the transmitting and receiving circuitry. So, typically the encoder will encode the input data in a suitable form (discussed later) and feed to a transmitting circuitry which converts the data into electric current. At the receiver the electric current is received, amplified, and then converted into data. The decoder then decodes the data into the bits of information that can be processed by the device. The figure below shows a typical transmitter. A receiver is similar with TRX replaced by a RX and a decoder replacing an encoder.

The PAN is based on the seven-layer ISO 7498 network standard [4] of the International Organization for Standardization. Various devices can interact using some form of time multiplexing. Currently time multiplexing is being used. Code division multiplexing could be another exciting alternative.

Passing electric currents through the body: Scary thought, isn't it ?

Don't worry. Electric currents are flowing through your body all the time - your nerves, senses and thought patterns all hinge on delicate electric transmissions. The current used in PANs is one-billionth of an amp (i.e. a nanoamp) - lower than the electrical current that flows naturally through our body. Every time you comb your hair, you're creating 1,000 times more electrical charge than a PAN connection. The natural salinity of the human body makes it an excellent conductor of electrical current. PAN technology takes advantage of this by creating an external electric field that passes an incredibly tiny current through the body, over which data is carried.

Modulation strategies

Two modulation strategies have been evaluated for PANs. The on-off keying where data is represented by the presence or absence of electric current. This scheme is very simple but to improve the signal-to-noise ratio (SNR), higher voltage should be applied. Direct sequence spread spectrum modulates the carrier with a pseudonoise (PN) sequence, producing a broadband transmission much greater than the message bandwidth. Symbol-synchronous PN modulation is used where a message bit one is represented by transmitting the entire PN sequence, and a message bit zero is represented by transmitting the inverted PN sequence. The signal-to-noise performance increases with the length of the PN sequence. For spread spectrum the switches are driven by the PN sequence, and the integrated result, which is the correlation, is compared to two thresholds. If the correlation is greater than a positive threshold, the message bit is one. If the correlation is less than a negative threshold, the message bit is zero. If the correlation is between these thresholds (the dead zone), no message bit is received.

Data transmission speeds

Theoretically PAN devices can communicate at 417 Kbits per second, if a robust SNR of 10db is assumed. The PAN transceiver prototype implements a modest 2400 bits-per-second modem. Telephone modems have pushed modulation and digital signal processing techniques to their practical limits. The application of modem telephony techniques to PAN devices may deliver channel capacities of 100 Kbits per second. Data compression will also increase the effective capacity of a PAN communication channel.

Power and Security

The nearfield communication strategy adopted by PANs give them a clear edge over RF based solutions. The extremely small amounts of currents involved make sure that PAN devices would not drain the precious battery. Also using low frequency and low power would ensure that the signal would not propagate very far beyond the body; thus, only devices worn by the user, or by people or devices in direct contact with the user, could detect it. The near-field effect used to make PAN possible has many advantages over other methods of short-range wireless communication. Near-field electrostatic coupling is proportional to electrode surface area. Far-field transmission efficiency is maximized by matching the impedance of the transmitter to free space, typically by using a half-wavelength antenna. PAN devices 25 to 80 millimeters long would require a carrier of several gigahertz for efficient transmission. Since the energy consumed by electronic components increases with frequency, any increase in the frequency of the carrier beyond that required to contain the information increases energy consumption. Near-field communication can operate at very low frequencies (0.1 to 1 megahertz) that can be generated directly from inexpensive microcontrollers. For example, the prototype PAN transmitter operates at 330 kilohertz (KHz) at 30 volts with a 10-picofarad electrode capacitance, consuming 1.5 milliwatts discharging the electrode capacitance. A majority of this energy is conserved (recycled) by using a resonant inductance-capacitance (LC) tank circuit. Even at low powers, RF waves can be noticed at quite some distance from the source. This is because the RF waves attenuate with distance squared whereas the nearfield elecric fields attenuate with distance cubed, providing a distinct advantage as far as security is concerned.

Inspite of the short-range, security can still be a problem. Touching a body in PAN could be as good or as bad as tapping into a telephone line, therefore a security method using encryption should be adopted that makes the PAN data look like a stream of random bits to would-be eavesdroppers.

PAN Devices

Any device we carry around can potentially act as a PAN device. Your watch, pager, mobile phone, PDA, identification badge, credit card and even your shoe! Now we can realize how PAN can help removing the redundancy of IOs and storage capacities I was mentioning in the beginning. Your watch can be a natural choice of a display. Your PDA can act as a storage point. Your mobile phone/ pager can act as an interface medium to the outside world. With this, not only can the devices become less bulky, but also the cost of owning these devices can also fall dramatically. Consider the following scenario. You get a paging message on your pager hooked to your belt. Using PAN the pager uploads the message to a watch. You are alerted by a beep and you can read the message from your watch. At the same time your pager sends the information to your PDA in your waist pocket which accesses your address book to find the sender's phone number. The message can also be stored there for future reference. The PDA sends this information over to your mobile phone through your body and the number is already dialed for you. Just press Go and you can talk to the person. Your mobile doesn't need address books and storage capacitiy, you pager doesn't need displays. The PDA can act as a server of information and all other devices access it for any information. All these devices form an instant network and talk to each other transparently without your intervention, sharing resources and capabilities and as a result making life much simpler for you.

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