[Imported] Do I need two frequencies for a licensed repeater?

[sbeadle]

>>Message imported from previous forum - Category:Trio Data Radios<<
User: joelw, originally posted: 2018-10-16 21:21:05 Id:25
This is a re-posting from the obsoleted (October 2018) "Schneider Electric Telemetry & SCADA" forum.

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**_jweder:
Do I need two frequencies for a licensed repeater?_**

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This can be a complex topic! Several issues here.

You can do this with one frequency or with two, depending on your needs. One frequency is often easier and cheaper to acquire from the licensing agency, but this can introduce issues which must be overcome._**

In terms of hardware, one option is the use of an ER450 or MR450 as a repeater. This will be a Store & Forward repeater. (receive the whole packet and only then re-transmit it) The other option is to use an EB450 full duplex repeater, or an ER450 with the Y option which does the same thing.

**NOTE**: Full duplex means the radio retransmits at the same time as it's receiving. No waiting like with a store & forward repeater. A duplexer is also required to share the antenna between the Tx and Rx without the Rx hearing the Tx. A full duplex repeater is fast but also more expensive.

When you're doing Store & Forward repeating using a remote radio (ER450 or MR450) you can repeat on the same frequency you received the signal on. A copule of issues arise from this however:

1) You need to Translate the Tx and Rx Stream ID's for the serial ports. Translating (same as repeating but each SID is changed to a new one before being re-sent) ensures that even if a remote radio accidentally hears both the repeated signal AND the direct signal from the host radio it won't get confused. Most RTU's won't like it at all if they hear two copies of the same message either! You set up translation in the same place as you set up repeating. Go to Stream Setup then Repeat/Translate Configuration. In the E Series you have to disable one serial port to allow this.

For example the Host radio transmits on SID 1. The repeater translates SID 1 to 3. The Remote radio receives on SID 3. It will ignore any messages it may hear (weakly) from the Host radio as the SID on that message is 1. When the Remote radio transmits it uses SID 4. The repeater translates that down to SID 2, and the Host radio is configured to receive SID 2. This sort of process is documented in the Trio training course, in the exercise on M Series repeating.

2) The other problem that can arise from using just one frequency is that the system can generate a lot of collisions. A direct (but very weak) signal can arrive at a radio that's trying to listen to a repeated signal at the same time. To minimize this problem you MUST turn on Carrier Detect collision avoidance in all radios!

Alternately, the use of two frequencies in a Store & Forward system ensures that there's no way a remote radio will ever hear the Host radio, as both are on the same Tx and Rx frequencies. They need the repeater in the middle, as it has the opposite frequencies for Tx and Rx. This type of system is preferred (if possible to get two frequencies) as there is no concern about collisions. The only concern with this type is that there can only be one repeater in the system. (unless it is very carefully designed and the environment allows)

The last repeater type to consider is the full duplex type. Here you use either an EB450 or an ER450 with the Y option, and a duplexer. This type works ina similar manner as the two frequency S&F option above. When you're doing full duplex you MUST use two frequencies, as there's no way a radio can transmit on the same frequency it's receiving on. In fact typically the requirement is a minimum 5 MHz separation, so that the duplexer can adequately keep the two signals apart.

A full duplex repeater is the fastest type, introducing far less latency (delay) than a store & forward repeater. It is however going to cost significantly more money. Also it must be understood that only one full duplex repeater should exist in any system. Having two would cause collisions if any remote could for any reason hear both repeaters.