Introduction to WISA - Millennial Net

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Page 12/16 to White_Paper Introduction to WISA V2.doc 6 - GSGeneral WISA Principles and Features: A simple transmission control protocol is appliedwhere telegrams received by the base station are acknowledged. In case of a missingacknowledgement, the device will re-transmit the telegram (automatic repeat requestARQ). The short frames allow for several re-transmissions within the permissible delaywindow, and provide a sufficiently high reliability also with heavy disturbance.With frame-by-frame frequency hopping and antenna switching at the input module (basestation), the radio channel used for re-transmission will largely be independent of theprevious transmission, thus noticeably increasing the probability of successfultransmission. As any re-transmission occurs on the uplink slot and frequency allocated tothe particular SA, it will not affect the transmissions of any other SA.A special requirement for an energy-autonomous system, e.g. a sensor, is the extremelow-power requirement for communication. This is a challenge when combined with thereal-time requirement. The use of the sensors and actuators radio needs to be minimizedby exploiting the possibility of a more complex base station design. A minimized radio usealso minimizes interference to other users. The system has a continuous downlink, offeringsynchronization information to sensors. When a device (e.g. sensor) wakes up, it canimmediately find synchronization, which means less use of the receiver. Figure 9summarizes some important requirements of wireless field device automation andcompares WISA and a Bluetooth implementation by typical data.SA-A1SA-A2InputModule ASA-AiSA-B1InputModule BSA-B2SA-BjFigure 8: Topology of WISARequirementOperating FrequencyCommunication BandwidthRaw bit rateCommunication LatencyNominaltypicalWorst-case (heavy disturbance -> retransmitts; max. No. of act. slaves)Node DensityNumber of active slaves per masterNumber of parked slaves per masterPower ConsumptionActiveSleep mode* Infineon Flinkstone; ** 1-3s wake up time!WISA2.4 GHz1 Mbps (DL)4x1 Mbps (UL)2 ms< 5 ms20 ms≤ 120-200 mW0.1 mWBluetooth*2.4 GHz1 Mbps(DL/UL)5 ms18 ms> 100ms7(256 **)300 mW4 mWFigure 9: WISA vs. Bluetooth
Page 13/16 to White_Paper Introduction to WISA V2.doc 6 - GSWISA Profiles: Different profiles are used for the integration of different devices. Thegeneric WISA technology can be easily adapted to new applications. The definition of thepay-load in each packet can be redefined to cater for different applications having theirown profile. Currently there are different profiles defined for the system, one for thewireless proximity switches and one with 32 bit for digital I/O or analogue values. Thedifferent profiles can coexist and can be handled by the same input/output module.5. WISA-COM: Reliability/EMC, Coexistence and System PerformanceThe WISA system has not been optimised for pure data throughput but rather for highestreliability, minimum latency with many nodes and low power consumption in an industrialenvironment. Reliability is the most important requirement for the users, therefore WISAuses a number of measures to increase reliability.There are generally three possibilities to interfere with wireless communication in industry:1.) Interference/disturbance by industrial processes2.) Other users of the same frequency (Coexistence)3.) Environmental influence (e.g. multipath effects, moving devices)1.) After extensive measurements it can be stated that typical industrial processes ordevices have no harmonics above approximately 1,5 GHz. As an example arc welding andfoil welding has been identified to emit broadband noise up to this frequency, but nothigher. Then the main possible interference source are processes using the 2,4GHz ISMband (Microwave ovens/processes like drying) themselves. Therefore frequency planninghas to be done - use should be coordinated and a certain distance has to be keptdepending on the power levels.WISA-powerWISA-comArc weldingProcessesSpot weldingRFID, anti theft dev..Induction heatingSwitch Mode Power SuppliesMotor contr. invertersHigh HP MotorsISM TransmitterContactors relaysGrundfrequenzHarmonische101001.00010.000100.000Frequency in Hz1.000.00010.000.000100.000.0001.000.000.00010.000.000.000100.000.000.000Figure 10: Frequency areas of different processes or devices in industry
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