Repetition-Combining in FullMAX™ Radio
Advanced Signal Recovery Technology to Mitigate Deep RF Fades in
Longer Trains
Ondas Networks FullMAX™ radios utilize a unique repetition-combining feature which enables the radios to dramatically improve received signal performance without any increases in transmit power. This repetition-combining combining technique is extremely useful in RF environments that experience severe fading such as Head-of-Train / End-of-Train, where multiple signal paths, varying train speed, and path loss from long distances between transmitter and receiver all contribute to the level of fading. The IEEE 802.16t standard requires both PtMP and DPP operating modes to support a repetition rate of up to 128, enabling this technique to significantly improve communications in low packet / message rate applications which are common in railroad communications systems.
Head-of-Train Radio
How Repetition-Combining Works
With 802.16t time diversity, an Ondas Networks radio utilizes repetition – combining to accumulate the power of multiple, unusable repetitions of signal to combine them into a usable signal. By recovering messages which would have previously been lost by legacy radios, a FullMAX radio maintains communications even when the link suffers from severe fading. Through this repetition-combining technique, an Ondas 802.16t radio dramatically improves the received signal performance without any increases in transmission power. This is especially useful in applications where an increase in transmit power is impractical due to limitations of the source of power itself such as an end-of-train radio.
To further quantify the advantages of repetition combining in a real-world application, let's further explore the Head-of-Train / End-of-Train (HOT-EOT) communication environment. As noted above, the HOT-EOT environment is subject to deep RF fades which have been exacerbated by the impact of longer train lengths. To attempt to mitigate the impact of fading, a common technique is the use of “repetition-selection,” where the same message is repeated multiple times to increase the likelihood that at least one of the messages can be properly decoded:
Example A: Repetition-selection works only when at least one of the repetitions is error-free. In this case, the message is succesfully received.
However, in the case of severe RF fading none of the messages are properly decoded due to an inadequate receiver sensitivity to offset the deep fade. The impact of this fade may continue to disrupt multiple attempts to communicate between the HOT and EOT.
Example B: Repetition-selection fails when none of the repetitions is error-free. In this case, the message is not successfully received.
Unlike simple message retransmission used in repetition-selection, in repetition-combining each codeword within the burst is repeated and all repetitions are combined prior to decoding. The repetitions of each codeword are combined at the receiver. In the case where a FullMAX™ radio is configured for 8x repetition rate, the improvement in receiver sensitivity is 9 dB, i.e., the receiver sensitivity of an Ondas Networks radio operating in an EOT device is – 126 dBm. This can be also considered equivalent to increasing the transmit power of the EOT and HOT by 9 dB.
To state it another way, with a 39 dBm (8 Watts) Power Amplifier (PA) at the EOT/HOT the performance of the Head-of-Train / End-of-Train link with 8 repetitions is the same as the performance of a single repetition NGHE with a 48 dBm (65 Watts) PA. This improved receiver sensitivity of a FullMAX™ radio offsets the need for any increases in transmission power.
Example C: Repetition-combining (time diversity) helps recover the error bits from the error-free sections of the other two copies of the message.
To learn more about the Next-Generation HOT - EOT communications, check out our whitepaper.