Associated Products:

DT402D, DT402D, DT402DCE, DT402DCE, DT500D, DT500D, DT500DCE, DT500DCE, DT602D, DT602D, DT602DE, DT602DE, UR92, UR92, UR92CE, UR92CE, UR93, UR93, UR93E, UR93E, UT4D, UT4D, UT4DCE, UT4DCE, UT6D, UT6D, UT6DE, UT6DE

KB1065: Clarifications of UR93 and DT602D/UT6D Duplex RF and network performance

This article was last updated on Nov. 30, 2020, 12:42 p.m. | Print Article | Leave Feedback

Clarifications of UR93 and DT602D/UT6D Duplex RF and network performance:

The new UR93 and DT602D and UT6D throttles employ the new Digitrax designed and manufactured RF28 Modular Certified 2.4GHz transceiver module. The RF28 is configured to interoperate with existing Digitrax Duplex UR92 LocoNet access points and e.g. DT500D and UT4D throttles worldwide. All Duplex units employ IEEE 802.15.4 PHY/MAC compatible devices using 2.4GHz Direct Sequence Spread- Spectrum (DSSS) modulation.

The RF28 was developed for:

a)  Smaller size. The venerable RF24 could not fit in the small UT6D, and this case was designed in 2017, so a new smaller Duplex support transceiver module was needed.

b)  The link power budget between the DT602D/UT6D and UR93 LocoNet access point (for a given RX level and Bit Error Rate) was increased by about +30dB in each direction. The actual gain number is affected by; production tolerances, temperature and operating frequency. In non-technical terms that is an effective power increase over the link of about 1,000 fold.

This is significantly more than the theoretical direct line-of-sight free space range-doubling requirement, which would only require a +6dB (x4) power increase to double RF range. The extra link power budget is particularly useful for mitigating RF signal absorption/reflection by variable intervening salt-water columns (human bodies) and antenna directivity/orientation and multipath reflections. Theoretical RF link ranges are an abstraction and largely unattainable in e.g. buildings and cluttered spaces on earth.

The close by environment can easily induce -20dB or more of RF directional sensitivity, if you simply change throttle orientation, with respect to; body/hand azimuth, height above a ground plane and reflection surfaces. Additionally, multipath effects from nearby metallic conductors and surfaces like; fading and Fresnel zones can cause acute path-loss changes in very small changes of 3D location and orientation angles. The 2.4GHz free-space wavelength is slightly less than 5”, and so e.g. moving just 2.5” can reverse an adding signal to a canceling signal, or deep fade. Antennas aggregate all incoming signal paths at varying phase (delay) and strength (voltage) to provide a vector sum to be detected.

Inadequate link power margin means loss of messages and/or short ranges.

c)  To provide superior fade and multi-path resistance the RF28 also employs a pair of high efficiency orthogonal space-diversity chip antennae. These constantly work in real time to find the optimal path to the other 802.15.4 Duplex transceiver that is exchanging data. The DT602D and UT6D both indicate the active antenna direction by the color of the Duplex Channel number displayed in the LCD status line, either Green or Blue. This will change as the RF28 is moved around or the ambient RF environment changes. This is not a quality metric, but an indication that the RF28 is working to always get the most reliable RF wireless link possible in the face of uncontrollable RF path propagation effects. Note that the chip antennae were selected for 0dB gain or non-directivity to further even-out reception reliability.

d)  The RF28 is FCC Modular Certified under Part15.247 at close to maximum allowed transmit power for the approximate 2.2MHz Occupied Bandwidth the 802.15.4 modulation generates with a 250KBps data rate. The CE required Transceiver Certification under ETSI EN300-328 V2.2 limits the Power Spectral Density below that which the FCC allows, so the RF28 transmits at about -8dB less transmit power when operating in CE version Duplex products. This simply reflects National requirements.

e)  The standard 802.15.4 throughput of 250KBps is optimal and efficient for low data rate requirements, such as a hand controller device like a throttle, because it is more bandwidth and power efficient than e.g. Wifi/ 802.11 high data rate systems.

Wifi systems have up to about a typical 20MHz occupied bandwidth or channel spacing. For a given Bit Error Rate, increasing the receive bandwidth for faster bit rates [at a given modulation complexity] means you lose sensitivity and so have to increase transmit power to restore a given margin on the link power budget. Otherwise you trade range vs link reliability. This fundamental of RF physics is dictated by noise power accumulated across the bandwidth degrading error performance. Wifi setup may also be more challenging for non-technical users who simply want to run trains and not configure a WAP, router or TCP/IP network.

f)  IEEE 802.15.4 employs a 32-chip DSSS bi-phase modulation scheme which gives significant processing gain against co-channel RF interference. The retired Digitrax Simplex links at 916.5MHz employed Narrow Band FM modulation at about 12KBps which provided an excellent range at a miniscule 0dBm transmit power and a measure of co-channel signal interference suppression due to FM capture effects.

g)  ETSI EN300-328 V2.2 requires both Receiver Blocking and Transmit Adaptivity/ Holdoff capability. Conforming well-engineered products can reliably operate in a mixed RF environment, assuming other devices do not “hog” the RF channel availability time, and block other users. The RF28 employs enhanced Duplex beacon capture and tracking logic.

h)  Using a DT602D/UT6D with an existing UR92 will still provide the benefit of about a +15dB link gain increase, so operations are compatible with older Duplex equipment, but the range and fade resistance are significantly improved. A DT602D/UT6D is able to automatically track and use more UR92 beacons around a larger layout.

I)  Adding a UR93 to an existing UR92 based Duplex layout will also provide the benefit of about a +15dB link gain increase to improve operations for all Duplex throttles. Note that all Duplex throttles will automatically start to track the higher signal UR93 beacon(s), and these UR93’s can handle any level of traffic, because they are not connection limited like e.g. a router or other connection device.

Because of the noted RF and system improvements, the RF28 based Duplex products approach prior Simplex throttle performance, but with many enhanced LocoNet capabilities.


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