{"id":168,"date":"2017-07-19T16:48:46","date_gmt":"2017-07-19T15:48:46","guid":{"rendered":"http:\/\/www.burningimage.net\/cattrack\/?p=168"},"modified":"2023-07-22T07:56:58","modified_gmt":"2023-07-22T06:56:58","slug":"gps-cat-tracker-transceiver","status":"publish","type":"post","link":"https:\/\/www.burningimage.net\/cattrack\/gps-cat-tracker-transceiver\/","title":{"rendered":"Which RF Transceiver?"},"content":{"rendered":"

I’ve chosen a Microcontroller and GPS module. \u00a0The last major element that I need to select is the narrow-band RF transceiver. \u00a0My priorities here are physical size, power consumption and the transmit\/receive range.<\/strong><\/p>\n

<\/p>\n

\"\"<\/p>\n

The way I’ll want to use the Transceiver is thus:<\/p>\n

    \n
  1. The transceiver sits in an ultra-low-power sleep mode for around 10 seconds.<\/li>\n
  2. After this, the transceiver wakes up and waits for a transmission from the base station for around 1 second.<\/li>\n
  3. If a transmission is received in the period above, e.g. a request for the collar’s position, then the collar wakes up the GPS module, gets a position fix and transmits the position back to the base station.<\/li>\n
  4. Once the position has been transmitted back to the basestation (perhaps multiple times for reliability) then it goes back to its ultra-low-power sleep mode.<\/li>\n<\/ol>\n

    I’ve looked at a few different protocols and ICs, summarised below:<\/p>\n

    Bluetooth<\/h4>\n

    Bluetooth is a fairly obvious protocol to investigate, given how ubiquitous it is these days. The generic term ‘Bluetooth’ could either be referring to Bluetooth 2\/3 (sometimes called Legacy Bluetooth) or Bluetooth 4 (sometimes called Bluetooth Low Energy or LE). \u00a0Although they share a name, the two protocols are completely different from each other.<\/p>\n

    Bluetooth 2\/3<\/h5>\n

    This older incarnation of Bluetooth is used for things like Bluetooth Speakers and over-the-air File Transfers. \u00a0In theory the maximum data rate is 3 Mbps – much faster than I am likely to need.<\/p>\n

    An easy way to get up and running with Bluetooth 2\/3 is to use a Bluetooth UART module. The cheapest of these is the HC-05 – popular among hobbyists. The beauty of these modules is that once you buy two and pair them up, you can send UART data into one module from your microcontroller and receive it from the other module, up to around 50 metres away. The module itself handles all of the faff and complexities around using the Bluetooth protocol.<\/p>\n

    \"\" Although incredibly easy to use and very effective, the HC-05 module isn’t suitable for use in CatTrack for a few reasons:<\/p>\n

      \n
    1. Range<\/strong> – the sensitivity of Bluetooth receivers is only around -90 dBm with a transmit power of < +10 dBm. \u00a0Ideally to get the range I want I’ll need the sensitivity to be better than -110 dBm, so Bluetooth is some way off.<\/li>\n
    2. Power consumption<\/strong> – Bluetooth 2\/3 is not designed for low power consumption.<\/li>\n<\/ol>\n

      In theory the size of the HC-05 module (27mm x 13mm) is too big for my requirements too, but I would not use the module in the form shown in the photo above. \u00a0I would be using the raw ICs. \u00a0For the HC-05 that means a BC417 Bluetooth IC and a small memory module.<\/p>\n

      Bluetooth 4 \/ LE<\/h5>\n

      Bluetooth 4 definitely addresses the power consumption issue of Bluetooth 2\/3. \u00a0It has been designed from the ground up to be able to run on a coin cell for around a year. \u00a0Exactly what I’m after! \u00a0It’s the protocol used in these ‘key finder’ type devices:<\/p>\n

      \"\"\"\"<\/p>\n

      However… the range is still quite a way off what I need. \u00a0Realistically you will only get around a 50 metre range with Bluetooth LE. \u00a0I need much more than that, hundreds of metres at least!<\/p>\n

      LORA<\/h4>\n

      LORA is a fairly new protocol that has been developed with range in mind. \u00a0It offers a sensitivity down to around -148 dBm and a transmit power of +20 dBm. \u00a0This gives a pretty amazing 168 dB link budget! \u00a0This translates, in theory, to miles and miles of range for CatTrack (depending on the antenna).<\/p>\n

      Semtech seem to pretty much hold the rights to LORA modulation. Their SX1276\/77\/78\/79<\/a> device looks to be the one to go for.<\/p>\n

      One thing you need to be aware of is that it isn’t possible to break the laws of physics. \u00a0In order to get such an incredibly good sensitivity, the bit rate must have to be very low indeed. \u00a0Sure enough, this can be seen in the datasheet:<\/p>\n

      \"\"<\/p>\n

      In order to achieve a sensitivity of -147 dBm, our data rate has to be only 24 bits per second. \u00a0This means it’d take a good few seconds to transmit our GPS position, which in turn means that because the transmitter is on for longer, our battery life will decrease.<\/p>\n

      Raw FSK<\/h4>\n

      The last transceiver I’ll look at will be a raw FSK transceiver. \u00a0You can get really cheap FSK transceivers for only a few pounds from eBay.<\/p>\n

      \"\"<\/p>\n

      I call these raw FSK transceivers because they are just that. \u00a0There’s no protocol or error correction like Bluetooth. \u00a0Literally all you can do is transmit either a 0 or a 1. \u00a0It’s then up to you to design your own protocol to interpret those 0s and 1s.<\/p>\n

      After a bit of research I found that TI’s range of devices <\/a>seemed to be the most full featured. \u00a0Of those devices, the CC1125 was the most suitable for my needs. \u00a0It offers the best sensitivity of all TIs devices, at -129 dBm when receiving at 300 bits per second.<\/p>\n

      LORA or FSK?<\/h4>\n

      Now I need to choose a device to go with! \u00a0I have my reservations around the LORA protocol. \u00a0I don’t think that in the real world it is realistic to expect the kind of range that the sensitivity implies.<\/p>\n

      The problem with LORA is that it is a wide-band system, compared to FSK’s narrow-band operation. \u00a0This can cause problems in the real world because in order for LORA to work, we need a completely quiet area of spectrum, up to 125 kHz wide. \u00a0If a signal pops up anywhere in this 125 kHz bandwidth whilst we are trying to transmit then the transmission will be corrupted. \u00a0Given that I plan to use an ISM band – there is a good chance that there will be a lot of other signals popping up from time to time in the same band.<\/p>\n

      Anecdotal reports indicate that when tested in the real world, LORA offers no range benefit over TI’s FSK range, due to LORAs wide-band operation.<\/p>\n

      Along with reading a white paper on why narrowband is the de facto standard for long-range communication<\/a> (which I admit may be a little biased based on who wrote it!), I think I’ll go with the CC1125 FSK device.<\/p>\n","protected":false},"excerpt":{"rendered":"

      I’ve chosen a Microcontroller and GPS module. \u00a0The last major element that I need to select is the narrow-band RF transceiver. \u00a0My priorities here are physical size, power consumption and the transmit\/receive range.<\/p>\n","protected":false},"author":1,"featured_media":172,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[9,4,5,10,13,6,16,8,14,15,7,11,12],"class_list":["post-168","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-posts","tag-bluetooth","tag-cc1125","tag-fsk","tag-hc-05","tag-lora","tag-narrowband","tag-protocol","tag-receiver","tag-semtech","tag-ti","tag-transmitter","tag-uart","tag-wireless"],"_links":{"self":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts\/168","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/comments?post=168"}],"version-history":[{"count":8,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts\/168\/revisions"}],"predecessor-version":[{"id":183,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/posts\/168\/revisions\/183"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/media\/172"}],"wp:attachment":[{"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/media?parent=168"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/categories?post=168"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.burningimage.net\/cattrack\/wp-json\/wp\/v2\/tags?post=168"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}