Radio Transceiver

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Radio's

Wireless sensor network solutions for at-home health care and body worn sensing generally work in the ISM band. This of course generates an immediate challenge as the technology must be reliable, robust and inexpensive and deployed into environments where there is potentially heavy interference. One key issue researchers find is that performance figures generated in controlled environments bear little relationship to those experienced in the real world.

There are a number of potential wireless technologies which could be used the deployment of WBSN. These include 802.15.4/Zigbee, Bluetooth, UWB and 802.11. Each technology has it pro's and cons. Defining the requirements for any BSN deployment greatly impacts the choice of wireless technology solution. Initial solutions will likely focus on the deployment of a small number of sensors typically between 1 and 6 sensors connected to a aggregation device which communicates via WLAN protocol, WiMAX or mobile phone nextwork such as GSM, 3G etc. For the body worn sensors 802.15.4 and Bluetooth are de facto choices.

802.15.4 was designed specifically with sensors networks in mind. It has many desirable features which make is suitable for WBSN's e.g. low power etc. However it has many disadvantages including short range due to its low power profile (not a issue for the BSN but problematic for communications with an off body aggregation device), limited data rate (less than 250 kps) and its susceptibility to interference. Many of the medical applications being developed using 802.15.4 are based around motes. Motes are low-power devices with a limited amount of computing power to relay measured data to a data aggregator or gateway. These wireless mote based sensors offer the capability to capture continuous real-time data such as patient vital signs and relay this back to nurses and physician workstations. Many of these projects are still in the design and deployment phase with initial designs yet to be tested. One of the challenges which must be addressed is the issue of low power communications. The central hubs are likely to be 802.11 enable for broadband connectivity, therefore interfacing low-power, ad-hoc sensor networking devices may be a problem. Another issue which must be addressed is that wireless sensor networks ensure "best effort" delivery of data but do not provide transmission methods for critical data i.e. medical e.g. detection of a patient fall. While Zigbee and Bluetooth focus many similar application areas they have very different network architectures and effective ranges. Over the long term Bluetooth and 802.15.4 Zigbee may evolve to be complementary technologies.

Bluetooth

Bluetooth® is a low-cost, low-power, robust, short-range wireless communication protocol which was initially founded by Ericsson in 1994 to replace traditional mobile phone and computer cables with wireless links. It operates in the license free 2.4 GHz ISM band. Bluetooth® uses 79 1MHz channels to transmit data. Interference between other ISM band devices (802.11 and 802.15.4 devices) and other Bluetooth® piconets is minimized using frequency hopping spread spectrum (FHSS), where the carrier is rapidly switched (hops) among the 79 available channels. The frequency hopping sequence is controlled by the master device within the piconet. Other Bluetooth® interference reduction techniques include adaptive power control, [Channel Quality Driven Data Rate] (CQDDR) and Adaptive Frequency Hopping(AFH).

It was first developed as cable replacement between mobile phones, headsets, PDAs, laptops etc, but since then it has evolved to solve more general applications in the Personal Area Network (PAN) domain. The Bluetooth stack is quite complicated, giving it a rather large footprint, which means that it cannot be used in the most processing-power and memory constrained devices.

The Bluetooth core system consists of an RF transceiver, baseband, and protocol stack. The system is usually implemented partly in hardware and partly in software running on a microprocessor. The partitioning can be configured in different ways depending on the application. From solutions where the radio, protocol stack and application runs on a single chip to solutions where there is a separate radio chip, a processor running the lower layers of the stack and yet another processor running the upper layers of the stack and the application. Extensive documentation and analysis of Bluetooth® and its applications can be accessed from the Bluetooth® SIG's website

802.15.4/Zigbee

IEEE 802.15.4 is a specification of a low-power air interface, and the accompanying MAC protocol. 802.15.4 is a CSMA/CA MAC -based system, with a total of 27 channels specified in the frequency bands of 2.4 GHz, 902-928 MHz, and 868.3 MHz. Three different over-the-air data rates can be allocated: 16 data channels with a data rate of 250 kb/s, 10 channels with a data rate of 40 kb/s and 1 channel with a data rate of 20 kb/s. Such a network can choose one of the 27 channels depending on availability, congestion state, and data rate of each channel. It is optimized for short range communications (typically 30-50 meters), low data throughput with a 30ms network join time and supports a flexible topologies, i.e. star or peer-to-peer topologies. It also supports very large numbers of nodes, a single 802.15.4 network can accommodate up to 216 devices, which are assigned during the association procedure. It is designed to maximize energy efficiency at the physical and MAC layers. The duty cycle of communications in an 802.15.4 network is around 1 percent, resulting in very low average power consumption for static and dynamic environments. However, it is also up to higher protocol layers to observe the low duty cycle. Most power saving mechanisms in 802.15.4 are based on beacon-enabled mode.

The 802.15.4 defines only one third of the total number of primitives used within Bluetooth and is therefore suitable for simple devices with limited memory and computational capacity. Two different types of devices are defined: a full function device (FFD) and a reduced function device (RFD). An FFD can talk to RFDs and FFDs while an RFD can only talk to an FFD

IEEE 802.15.4 determines which radio hardware to use and Zigbee determines the content of messages transmitted by each network node. [ZigBee] (on top of 802.15.4) ensures interoperability by defining higher network layers and application interfaces. The simple complexity, low-cost, low-power features of 802.15.4 are intended to enable broad deployment of wireless networks able to run for years on standard batteries, for a typical monitoring application.

802.15.4 is part of the IEEE's 802.15 wireless personal-area network specification activities. It uses a simple (28K byte) packet-based radio protocol aimed at very low-cost, battery-operated sensors that can intercommunicate and send low-bandwidth data to a central receiving station.

802.15.4/ZigBee is built on the IEEE 802.15.4 standard and specifies the MAC and PHY (physical) layers. The "ZigBee" comes from higher-layer enhancements in development by a multi-vendor consortium called the Zigbee Alliance. For example, 802.15.4 specifies 128-bit AES encryption, while ZigBee specifies but how to handle encryption key exchange. 802.15.4/ZigBee networks are designed to run in the unlicensed frequencies, including the 2.4-GHz band in the U.S.

IEEE 802.15.4/ZigBee is intended for uses such as control of lights, security alarms, motion sensors, thermostats and smoke detectors, environmental monitoring etc. There are plans for Zigbee integration with residential gateways that merge traffic onto a broadband Internet connection. Zigbee has specific advantages over other short range protocols such as 802.11 and 802.15.4 for WSN applications as devices based on these protocols use too much power and the protocols are too complex (and thus more expensive) to be embedded in devices on very large scales [1, 2]. Unfortunately, the ZigBee Alliance has its protocol closed at the moment; additionally, it adds another protocol between the device and the global IP-based network.

Ultra Wide Band

Impulse-radio-based UWB technology has a number of inherent properties that are well suited to sensor network applications. In particular, impulse radio-based UWB systems have potentially low complexity and low cost; noise-like signals that are resistant to severe multi-path and jamming; very good time domain resolution, allowing for location and tracking applications

The low complexity and low cost of impulse radio UWB systems arise from the essentially baseband nature of the signal transmission. Unlike conventional radio systems, the UWB transmitter produces a very short time domain pulse that is able to propagate without the need for an additional radio frequency(RF)mixing stage. The RF mixing stage takes a baseband signal and "injects" a carrier frequency or translates the signal to a frequency that has desirable propagation characteristics. The very wideband nature of the UWB signal means it spans frequencies commonly used as carrier frequencies. Since high burst data rates are achievable with UWB systems, a sensor employing UWB can transfer its data payload quickly and spend much of the rest of the time "asleep" or in a low-power state [3].

Radio Chipsets

Bluetooth

802.15.4

References

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