Let's start with several important aspects of the downlink channel:
- Half-duplex mode. Usually in LPWAN systems the downlink channel is operated in half-duplex mode. In half-duplex mode a data packet is either sent or received by the system, but the system cannot do both at the same time. In half-duplex mode, the gateway receiver " gags" during the return message transmission and skips all possible uplink messages on all reception channels. If the gateway serves a thousand devices, it cannot hear messages from all the others while transmitting a message to only one device.
- Duplex mode. Organising a duplex mode is extremely difficult, both technically and legally. It is almost never used in LoRa solutions. For some UNB systems duplex mode is practically impossible using 868 MHz for one-way uplink channel channel and 433 MHz for downlink channel (power limitation 10 mW) or 446 MHz PMR (the last is forbidden for such use).
- Increased power of the downlink channel of UNB systems. UNB systems require higher power to operate the downlink channel, to provide symmetric communication to the gateway, it is necessary to raise power from 25W to 0.5-2W as SigFox does, but this is not allowed in many countries. Such power is possible in the 446 MHz PMR band, but this band is not for IOT data. Using allowed power immediately significantly limits the range of the UNB system. That is why there are no details about downlink channel and its parameters on website of UNB system manufacturers.
- Work in receipt operation mode. Using of acknowledgement messages during the standard LPWAN gateway operation in half duplex mode significantly limits the gateway capacity (see 1). Each acknowledgement jams the reception from other devices, and it is unclear what the device should do if the acknowledgement is partially available because of uncertain downlink channel reception.
- Operation in polling mode. Operation in polling mode (class C for LoRaWAN) implies constant "listening" to the radio air. In this case, the device operates continuously, which makes it practically impossible to run on battery power.
- Firmware update of the ED (endpoint device) over the air. LPWAN ED firmware can be updated over the air, but this will be as ineffective as possible. Updating the device’s firmware is a long process and this action takes all the airtime, during this time the gateway is not able to work with the devices in normal mode and loses information. Remote devices at the receiving edge will require constant repetition of information, which will unpredictably increase the time that the gateway will be out of the reception of information from other EDs. Device reflash mode via LPWAN looks nice in the lab, but in practice we strongly advise against using it, as a rule, LPWAN device is a simple enough device - it does not need FUOTA, if only due to developer's fault.
- Security. The downlink channel must be highly secured, as it allows you to control the devices (switch on, switch off, change data, modes and keys), not just receive data from it. Interception of ED control by a malicious intruder can have very serious consequences. The downlink channel carries much more risks in using LPWAN systems than does the one-way uplink channel channel alone.
For example, most modern long-range radio security systems operate on one-way uplink channel and still fulfil all the requirements for professional object security equipment. Such systems are using special regular "channel control" parcels for communication control, and this is the most energy-efficient way of control.
One-way GoodWAN uplink channel has the following advantages:
- Longer battery life;
- Higher system network capacity;
- Higher information security;
- Control & monitoring over one-way uplink channel;
- Reduced cost of the endpoint devices;
- Easier installation and maintenance of the endpoint devices.