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Low-Cost Low-Power Radios: Bluetooth and Zigbee:

As radios decrease their cost and power consumption, it becomes feasible to embedded them in more types of electronic devices, which can be used to create smart homes, sensor networks, and other compelling applications. Two radios have emerged to support this trend: Bluetooth and Zigbee.

Bluetooth radios provide short range connections between wireless devices along with rudimentary networking capabilities. The Bluetooth standard is based on a tiny microchip incorporating a radio transceiver that is built into digital devices. The transceiver takes the place of a connecting cable for devices such as cell phones, laptop and palmtop computers, portable printers and projectors, and network access points. Bluetooth is mainly for short range communications, e.g.from a laptop to a nearby printer or from a cell phone to a wireless handset. Its normal range of operation is 10m (at 1mW transmit power), and this range can be increased to 100m by increasing the transmit power to 100mW.The system operates in the unlicensed 2.3GHz frequency band; hence it can be used worldwide without any licensing issues. The Bluetooth standard provides l asynchronous data channel at 723.2 Kbps.In this mode, also known as Asynchronous Connection-Less, or ACL, there is a reverse channel with a data rate of 57.6Kbps.The specification also allows up to three synchronous channels each at a rate of 64Kbps.This mode also known as Synchronous Connection Oriented or SCO,is mainly used for voice applications such as headsets, but can also be used for data. These different modes result in an aggregate bit rate of approximately 1Mbps.Routing of asynchronous data is done via a packet switching protocol based on frequency hopping at 1600hops per second. These are also a circuit switching protocol for the synchronous data.


Bluetooth uses frequency-hopping for multiple access with a carrier spacing of 1Mhz,Typically,up to 80 different frequencies are used, for a total bandwidth of 80Mhz.At any given time, the bandwidth available is 1Mhz,with maximum of eight devices sharing the bandwidth. Different logical channels can simultaneously share the same 80MHz bandwidth. Collision will occur when devices in different piconets, on different logical channels, happen to use the same hop frequency at the same time. As the number of piconets in an area increases, the number of collision increases, and performance degrades.

The ZigBee radio specification is designed for lower cost and and power consumption than Bluetooth. The specification is based on the IEEE 802.15.4 standard. The radio operates in the same ISM band as Bluetooth, and is capable of connecting 255 devices per network. The specification support data rates of up to 250Kbps at a range of up to 30m.These data rates are slower than Bluetooth, but in exchange the radio consumes significantly less power with a larger transmission range. The goal of ZigBee is to provide radio operation for months or years without recharging, thereby targeting applications such as sensor networks and inventory tags.

Bluetooth wireless technology is a low-cost, short-range, radio links between mobile PCs, mobile phones, and other portable devices. The Bluetooth specifications are released by the Bluetooth Special Interest Group (SIG), an industry group consisting of industrial leaders in the telecommunications, computing, and networking.

The Bluetooth system operates in the 2.4 GHz industrial, scientific, and medicine (ISM) band. It is based on a low-cost, short-range radio link integrated into a microchip, enabling protected ad hoc connections for wireless communication of voice and data in stationary and mobile environments. It enables use of mobile data in different ways for different applications. Due to its low-cost target, it can be envisaged that Bluetooth microchips will be embedded in all consumer electronic devices. The characteristics of the Bluetooth technology offer wide room for innovative solutions and applications that could bring radical changes to everyday life. Let us imagine a PDA (with a Bluetooth microchip) that automatically synchronizes with all the electronic devices in its 10 meter range when you arrive at your home. Your PDA can, for example, automatically unlock the door, turn on the house lights while you are getting in, and adjust the heat or air conditioning to your preset preferences. But not only the home can become a more comfortable environment when the access to information is fast and easy. Let us imagine arriving at the airport and finding a long queue at the check-in desk for seat assignment. You can avoid the queue using a hand-held device to present an electronic ticket and automatically select your seat.

A Bluetooth unit consists of a radio unit operating in the 2.4 GHz band. In this band, 79 different radio frequency (RF) channels that are spaced 1 MHz apart are defined. The radio layer utilizes the frequency hopping spread spectrum (FHSS) as its transmission technique. The hopping sequence is a pseudorandom sequence of 79 hop length, and it is unique for each piconet. The FHSS system has been chosen to reduce the interference of nearby systems operating in the same frequency range (for example, IEEE802.11 WLAN) and make the link robust. The nominal rate of hopping between two consecutive RF is 1600 hop/sec. A time division duplex (TDD) scheme of transmission is adopted. The channel is divided into time slots, each 625 _s in length, and each slot corresponds to a different RF hop frequency. The time slots are numbered according to the Bluetooth clock of the master. The master has to begin its transmissions in even-numbered time slots. Odd-numbered time slots are reserved for the beginning of the slaves’ transmissions. The transmission of a packet nominally covers a single slot, but it may last up to five consecutive time slots. For multislot packets, the RF hop frequency to be used for the entire packet is the RF hop frequency assigned to the time slot in which the transmission has begun. The RF change reduces the interference from signals coming from other radio modules. There are two types of physical links that can be established between Bluetooth devices: a synchronous connection-oriented (SCO) link, and an asynchronous connectionless (ACL) link. The first type of physical link is a point-to-point, symmetric connection between the master and a specific slave. It is used to deliver delay-sensitive traffic, mainly voice. In fact, the SCO link rate is 64 Kbit/s and it is settled by reserving a couple of consecutive slots for master-to-slave transmission and immediate slave-to-master response


Wireless communication
Andrea Goldsmith
Stanford University

image: Google images




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