NB-IoT, LTE-M - classic mistakes of IoT developers - Part 1

NB-IoT, LTE-M - classic mistakes of IoT developers - Part 1

The new LPWAN technologies of 3GPP save us a lot of energy and increase coverage. At the same time, NB-IoT and LTE-M modules are much cheaper than the previously cheapest cellular GSM / GPRS. To make all this happen, which is promised to us by the marketing of 3GPP, the manufacturers and the network operators, the developer of the wireless IoT devices has to take many actions. The promises are similar for LoRaWAN and Sigfox.

To ensure that the requested effects occur, the wireless IoT developer must leave his path and must break new ground. To do this, one should not rely on the statements from advertising and marketing but turn to experienced consulting. A manufacturer's FAE is a good place to start. Since there is more than one manufacturer, you can also ask several manufacturers. If one is not a good buying customer, the FAE will most likely not invest too much time to give advice. On top of that, he is committed to his employer with his employment contract.

It's like in a restaurant. If you as a guest want to order goulash soup and the waiter only has tomato soup, he will tell you how healthy tomato soup is. His job is to sell.

The field application engineer (FAE) of a manufacturer supports the start-up with an idea all the way to the car manufacturer with the demand of a few million units a year. He gets a basic salary and a commission on the sales in his region. From this, you can see that as an IoT developer you are not always important to the FAE. We have the same situation with the FAEs of the distributors.

If you as an IoT developer only ask and never buy anything or buy from the competition, sooner or later you will no longer get any advice. Marketing is certainly the wrong advisor because these people usually lack technical knowledge. The salesman is often the wrong person to ask because he may lack technical knowledge as well. Even if he has the knowledge, the FAE will only give comprehensive advice if it is important for his turnover or if he can give the advice without much effort.

It is therefore highly likely that honest advice can only be obtained from an independent consultant or from several consultants in parallel. For technical consulting, the consultant should have the appropriate training and experience. You have to pay this consultant. In the end, this is cheaper than developing a wireless IoT device with a malfunction or based on the wrong radio technology.

If the IoT developer lacks the budget, then we usually recommend a mixture of consulting and reading books and other sources. Based on the budget, a combination of online workshops and reading/learning is created. This takes a little longer than anything based on consulting and is a good compromise.

In extreme cases, such as antenna design, you often have no choice but to outsource. Buying a MegiQ VNA for only 4000 Euros is no substitute for years of study in RF engineering specialising in integrated antennas. Buying the simulation software CST for antennas for 50000 Euro also does not migrate the IoT developer to an antenna developer.

A demo application on the Arduino or Raspberry Pi is still far from a device in series production. There is also a long way to go before a microcontroller programmed in C really only needs 2 uA in standby mode.

The right radio technology has been chosen, the antenna design is perfect, the code in C works stably, the tools for the plastic housing have been produced, sales/marketing/support has been organised and nothing has been forgotten. Everything has been done right.

Then the IoT device just has to comply with the applicable regulations. The thousands of pages for, for example, RED / CE or FCC, ETSI EN 303 645 V2.1.1 (Cybersecurity for consumer IoT) and legal regulations on product safety have been imbibed by the IoT developer with their mother's milk and the IoT device goes through certification without any problems.

If you have all this know-how in you or in your team, you can stop reading here. Then you can save yourself the time of sending me a request for consulting to harald.naumann (at) lte-modem.com and relax and go into production.

Listen and ask for advice for your wireless IoT design

To make things better or to make them work, you have to look for and find specialists. Linkedin is a good place to start. The profiles of the contacts are open. The IoT developer can see where the potential consultant has worked before and what they have published. A good job history with a mix of professional articles, books and professional presentations sounds promising. References from the consultant's clients can also be helpful. You may also need more than one consultant or one with a good consultant with a network of specialists.

We don't need to look for the one who can do everything. The one who claims to be able to do everything can be sorted out right away.

So that the advice and listening in this article does not become a dry run, we will take up a problem with NB-IoT and LTE-M and explain it in detail.

If you are still reading, I would like to point out that all the text is created in my spare time. All these many words and sentences are not paid for by anyone. No manufacturer, no network operator, no employer or anyone else pays for the effort. In one of the next issues of the IoT M2M Times, I will give the possibility of sponsoring. Makers, small startups to large corporations are allowed to donate and fund. Everyone will receive a receipt or invoice and I will declare the payments in taxes or encourage other experts to work out measurement series for me.

Please send non-binding commitments for sponsoring to harald.naumann (at) lte-modem.com or via Linkedin Messager.

NB-IoT / LTE-M Reducing energy consumption with PSM

The solution to the reduce energy consumption in NB-IoT and LTE-M is quite simple. If the IoT developer does not leave his path and uses the new LPWAN technology of 3GPP like his previous GSM /GPRS module, he will find that his great NB-IoT / LTE-M module on the same PCB will end up consuming more energy than his old GSM/GPRS module. Various actions are required to ensure that low energy consumption will be achieved. Describing all the necessary measures would fill a book. In addition, different applications require different measures. We will only take a closer look at one possibility here.

Saving energy with GSM /GPRS

The only way to save energy with GSM / GPRS is to switch off the radio module. The Gillette service button is a very good example of saving energy with the GSM module. The microcontroller in the PCB only needs 500 nA in super-deep-sleep mode. At 500 nA, the timer is active and the button is scanned periodically. The code for establishing the connection works on the GSM module and the microcontroller serves as a watchdog. When transmitting with 33 dBm (2 Watt) transmission power, the GSM module requires approx. 250 mA on average with GPRS Class 8. In receive mode, it is approx. 5 mA. The timer that wakes up the service button every 2 weeks is not a Real Time Clock (RTC) but an RC timer. An RTC is too expensive and requires too much energy due to the oscillator with quartz. If you breathe on the PCB with the microcontroller, the current increases from 500 nA to 1000 nA. The current of another 500 nA flows through the humidity in the breath.

You can find more about the Gillette service button in my IoT M2M Cookbook and in my blog.

• Excerpt IoT M2M Cookbook
• Article on the Gillette service button

The Gillette service button was the first order button that could be bought by end customers in the supermarket. The standby time was about 10 years. 22000 were produced. NB-IoT was not known and specified at that time.

Saving energy with PSM in NB-IoT and LTEM

The option of switching off the NB-IoT /LTE-M module is not allowed to be used. The IoT developer is not allowed to use the well-proven procedure as with the Gillette service button for NB-IoT and LTE-M. Exceptions prove the rule. We will deal with the exception later.

If the radio module is put into power-save mode (PSM), then it only needs 3 uA. The special thing about PSM is that the base station remembers our registration (attach). When we register again (Reattach), much less energy consumption is required. The current during transmission is approx. 200 to 250 mA at 20 to 23 dBm transmission power. At the same distance from the base station and therefore the same link budget, the transmission speed is significantly higher than with GSM/GPRS and the power requirement is similar. The amount of energy is calculated from voltage x current x time. With the same current and voltage, only the transmission speed determines the amount of energy required. However, the real saving in energy comes from the simplified registration (reattach) at the base station. The most energy required for the transmission of 12 to 512 bytes will be approx. 400 mWs with the simplified registration at 144 dB link budget. At 154 dB it is already 5000 mWs significantly more. With a full registration (attach), 400 mWs now becomes 2000 mWs and 5000 mWs become 25000 mWs. 24 hrs PSM costs 3 uA x 3.3 volts x 24 hours x 3600 seconds/hours = 855 mWs.

We ignore the fact that we actually have to subtract a few seconds from the 24 hours for establishing the connection and sending. We round up the 855 mWs generously and calculate with 900 mWs.

900 mWs for PSM plus 400 mWs result in 1300 mWs and without PSM it becomes 2000 mWs with 144 dB link budget. At 154 dB link budget it will be 900 mWs plus 5000 mWs and thus 5900 mWs. Without PSM it becomes approx. 25000 mWs.

Without PSM, the NB-IoT and LTE-M module must search the LTE-M network again. A search run in a band takes approx. 1 to 1.5 minutes depending on the size of the band. If the field strength is good, it takes less time. 3.3 volts x 5 mA x 60 seconds give 990 mWs. To the 25000 mWs we add about 1000 mWs. We have therefore 26000 mWs without PSM versus 5000 mWs with PSM.

The assumed values for sending 12 to 512 bytes were taken from the study on the energy consumption of NB-IoT, LoRaWAN and Sigfox. The energy consumption for attach and reattach was determined using an Excel file for the NB-IoT chipset from Huawei.

NB-IoT and LTE-M without PSM end up consuming more energy than the consumption with PSM. Further energy can be saved with NB-IoT RAI.

NB-IoT and LTE-M PSM

The time the NB-IoT/LTE-M module spends in power save mode is influenced by two timers. The first timer (T3324) determines how long the radio module opens a receive window after sending the e.g. 12 to 512 bytes. The second timer (T3412) determines how long the base station remembers the registration (attach) and thus enables a simplified registration (reattach). The delta for the timer for the simplified registration subtracted with the time for the reception window results in the time in which the radio module remains in the so-called power-save mode. If no receive window is required, this time can be selected as short as possible to avoid unnecessary energy consumption. If neither of the two timers AT commands is set, then the default settings of the radio module are selected. The two timers are communicated by the radio module to the base station as a request. The base station responds and sets the timers in the response. The theoretical maximum value for the PSM mode is approx. 413 days. You can send this wish to the base station and assume that it will certainly be rejected. The so-called PSM mode has the disadvantage for the mobile network operator that it must store sleeping radio modules in its database for new simplified registration (reattach). The longer this time is selected, the more sleeping devices the network operator would have to manage in its database. Another reason for rejecting the timer for sleeping with a maximum value can also be that the base station finds that far too many modules would wake up at the same time, leading to an overload in the mobile network. On top of that, the maximum values for the timer are specified differently in different mobile networks. The wireless module therefore sends a request to the base station, receives an answer and has to be satisfied with the answer. This answer can be different for the reasons mentioned above.

Example: Recyclables container and water meter

A recyclable material container generally sends its fill level and position once a week. If the bin has a motion sensor, then it is only necessary to activate the GNSS module once and transmit the position. A bin does not fill up infinitely fast. The parameters for the IoT device in the bin are often set only once and then retained. However, in order to be able to change values in the device, it must either open a reception window after each transmission or, for example, once a month. A request to change the parameters could then be executed once a month. However, the saved reception window would reduce energy consumption. Everyone must decide for themselves which operating mode is the right one for each case.

With a water meter, the water consumption is monitored, and with new types of water meters even the operating pressure of the water pipe. In this case, consideration must also be given to whether and how often parameters should be changed. If a recyclable material container or water meter exceeds a limit value for any reason, it can automatically switch from power save mode to transmission mode at any time. In the case of the recyclables container, for example, this is the case when the fill level becomes too high. Another limit value is, for example, the movement of the container although a movement is not to be expected. An unexpected movement can indicate a stolen container. For water meters that monitor water pressure, a sudden drop in pressure may indicate that a pipe has broken somewhere. If many water meters report a pressure drop at the same time, it can be deduced which water pipe is affected and appropriate measures can be taken to eliminate the fault.

The timer for the reception window is also only treated as a request by the base station. An extremely long window is also annoying for the network operator. A long reception window means that he cannot use the radio channel for other subscribers. So if you requested an extremely long window of theoretically 185 minutes, you can assume that the base station will definitely reject it. The length of the reception window, in turn, is determined by the amount of data you expect the base station to receive. With a very good field strength, the transmission speed with NB-IoT and LTE-M is significantly faster than with poor field strength. Conversely, this means that the software developer should adapt the reception window to the field strength in order to reduce the energy consumption again. If no reception window is expected after transmission, then this timer can be set to the shortest possible value to reduce unnecessary energy consumption. It is best to use the so-called RAI mode. With RAI, the reception window is not opened after transmission. More about this later in another article.

Summary Power Save Mode

If you use your LTE-M or NB-IoT module in the same way as your GSM module before, you will be doomed. The energy consumption with the two new LPWAN technologies will be higher than with his good old GSM module. Even if the power-save mode is used, the software developer must set the two timers in his software himself and send them to the base station as a request. The response from the base station then determines how his device must behave in the field. The answer from the base station can be different in different mobile radio networks. The developer can dynamically adjust the timer for the reception window to the field strength at his location. If no reception is required at all, then the smallest possible timer can be used there. If the network supports RAI operation, then this should be used to avoid the unnecessary reception window. This decision must also be anchored in the algorithm of the software. If there is no RAI, then the software must change dynamically and use the smallest possible timer. If the IoT developer does not select his own timer, then the default timer of the radio module is used and energy is wasted unnecessarily. We will go into more detail about the RAI operation mentioned several times in another article.

Outlook for the IoT / M2M Times

This edition on with the focus on energy consumption is based on five enquiries about energy consumption at the same time. One project uses LTE-M like GPRS and switches off hard. It keeps re-establishing the connection although it would be better to keep the connection and use the timer T3324. In another project, the IoT developer declares that GPRs uses less energy than NB-IoT and LTE-M. He is right - he does not use PSM! In the next project, the software developer has no experience with LTE-M or NB-IoT and other radio technologies with timers. He wants constant accessibility on battery and long battery life. And two other tasks involve developing texts for network operators on PSM and energy saving. They all want the same thing. The IoT devices and the level of knowledge is unequal. Even with this text, I will only be able to guide 4 on the 5 in the right direction. The rest can only be done with individual consulting at the level of the respective contact person.

Non-binding donation commitment:

If you have read this far, I look forward to receiving an e-mail with a non-binding commitment for a donation in a later issue at harald.naumann (at) lte-modem.com or via messanger. Thanks in advance.

Imprint

• Harald Naumann, Ludwig-Kaufholz-Weg 4, 31535 Neustadt, Germany
• Contact: harald.naumann (at) lte-modem.com, Phone: +49-5032 801 9985, Mobile:+49-152-33877687