Certification of wireless positioning systems/real time location systems Realtime Precision Everywhere
Introduction Wireless positioning systems/real time location systems use active mobile units and active fixed infrastructure to determine the position of mobile units Several techniques are used, either based on received signal strengh (RSSI) or signal-path propagationtime methods Signal-path propagation-time methods are either round-trip time-offlight (RTOF) or time-difference of arrival methods (TDOA) RTOF and TDOA systems are far more precise (cm range) than RSSI systems (m-range) RTOF and TDOA system accuracy and robustness require as much bandwidth as possible
Communication and Ranging RTOF and TDOA systems are hybrid systems: They do communication and ranging The communication part is necessary to exchange messages with station identifiers, measurement slots, align the stations and pre-synchronize for distance measurements Bandwidth for communication is usually small or medium, small data rate Ranging signals try to use as much bandwidth as possible Optimum signal shapes for communication and ranging differ -> 2 different signal shapes are used Examples: 2.4 GHz communication and UWB ranging (Ubisense), 5.8 GHz FSK and FMCW (Symeo), communication 2.4 GHz 802.15.4 and range-burst 2.4 GHz (Essensium)
Normation and Regulation Regulations SRD regulation EN300440 for ISM bands 2.4, 5.8 Ghz UWB regulation EN302500 (6-8.5), EN302065 (3.3-4.8 GHz, 6-9 GHz), EN302435 (2-8GHz) Communication EN300328 at 2.4 GHz Different RTLS companies follow different paths SRD EN300440 used by Symeo UWB EN302500 used by Ubisense EN300328 is used by Essensium, Nanotron How do the hybrid devices fit into a pure communication regulation?
EN300328 EN300328 is designed for communication systems It allows out-of-band emissions from undesired modulation products unavoidable in communication systems and spurious emissions below a spurious emission limits It specifies a max. bandwidth of 83.5 MHz, typical 802.15.4 systems use ~2MHz channel bandwidth Unintended spurious domain limit is -30 dbm Spurious measurement setup is 100 khz resolution bandwidth and 30 khz video bandwidth
EN300328 2.4 GHz One system on the market uses a special modulation modus (sharp edge transition) to intentionally generate desired large bandwidth ranging signals outside the allocated band The emissions are below the out-of band limitations for unintentended emissions (out-of-band domain and spurious) EN 300328: Transmitter unwanted emissions in the out-of-band domain are emissions when the equipment is in transmit mode, on frequencies immediately outside the necessary bandwidth which results from the modulation process, but excluding spurious. Spurious emissions are unintended emissions from the normal operation of the devices The spurious emissions are suppressed by the averaging nature of the EN300328 spurious domain measurement method This reduces the visibility of the special ranging signals in the EN300328 measurements In our opinion, such system should be governed by UWB regulations Symeo - Absolute Positioning
Example One system on the market is currently being marketed for using an edgedetection approach with very short rise-time signal pulses to get the large bandwidth for ranging signals The sharp edges are generated on purpose with an additional switch It is advertised to use 200 MHz bandwidth for ranging at 2.4 GHz Simulated spectra With the published data a simulation of the resulting spectrum shows the out-of-band domain measurements The out-of-band emissions are suppressed by the averaging nature of the EN300328 spurious domain method Symeo - Absolute Positioning
Conclusion The EN300328 out-of-band/spurious emission limit is exploited for ranging signals by some companies Systems using this approach have an advantage over UWB systems in terms of more transmit power -> less infrastructure cost Systems using this approach have an advantage over ISM band systems in terms of bandwidth -> less complicated setup, less redundancy required Does the EN300328 cover both communication signals and positioning signals of an RTLS? Is it allowed to exploit the out-of-band/spurious domain for desired ranging signals when the communication signals comply with the EN300328 or does such a system fall into the UWB regulation?