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Does NB-IoT work underground?

Shallow and deep indoor penetration with LPWAN is yesterday’s news. The new record for signal penetration through walls and floors was achieved in Munich on August 10, 2018 with NB-IoT.

NB-IoT underground signal

NB-IoT signal strength in the Munich subway

Munich is a very interesting location for tests and comparisons of LPWAN performance because all the important LPWAN technologies are represented in the city.

Due to the great summer heat in the office, our Munich team continued the NB-IoT tests in the tunnels of the Munich underground.

With NB-IoT you are able to receive a signal in the tunnels. In the photo montage, you can see +CSQ: 12,99 as the output of the terminal program. The formula is -113 dBm +2 dB x 12 = -89 dBm. The field strength at Goetheplatz station varies between -89 dBm and -106 dBm. According to the datasheet, the receiving limit is -129 dBm. At 10 dB more (i.e. -119 dBm), there is approx. 6,000 bps according to the diagram. At -109 dBm and more we reach approximately 21 kbs because this is valid for a link budget better than 144 dB. At -89 dBm receiving level, the NB-IoT module will surely regulate back its 23 dBm transmission power. 129 dBm is necessary to reach the maximum link budget of 164 dB. With -89 dBm we are 40 dB above the minimum.

NB-IoT signal level versus transmission speed

Throughput v link budget

MB-IoT throughput verus link budget

The antenna selected for the NB-IoT test is a monopole and therefore dependent on the size of the ground plane of the PCB. Due to the antenna’s positioning close to the pavement, the near field of the antenna is already reduced. This is not good for radio waves and leads to higher losses. The NB-IoT module still receives a very high signal in the subway tunnel even at pavement level. Interesting practical tests are at eye level of a terrier, a taller police dog, a ladies handbag .  In addition the Hata Radio Propagation model can be used to calculate the loss of radio energy due to road pavement, soil and steel reinforced concrete in the subway tunnel.

SIGFOX in Munich

SIGFOX refers to its network coverage, but sadly no locations or density of its gateways. You can easily determine this by activating a SIGFOX-Eval-Kit and visiting some public buildings in Munich. Choose the floor so that the city limits of Munich are covered, or the coverage according to the network map. The buildings can be selected on Wikipedia. Then the locations are transferred to Google Maps, a screenshot is made, transferred to PowerPoint, circles are drawn, the radius is determined and at the end calculated backward with the Hata Radio propagation model. With an estimated 156 dB link budget for SIGFOX, the range of the radio wave is probably primarily dependent on the earth’s curvature and the chosen floor of the 90 to 150 metre high buildings. Once the points for emission have been determined, you are on your way. It would also be possible to drive around by car and send messages. Since SIGFOX is limited to 1 % duty cycle, you can only send a message every 10 minutes on average. The transmission via the high buildings is probably easier.

LoRaWAN in Munich

Determining LoRaWAN in Munich is easy. There is no public LoRaWAN in Munich right now and therefore no coverage for our Terrier, police dog or ladies handbag. However, the LoRaWAN of public utilities is mentioned for completeness, which has announced 10 LoRaWAN gateways. In addition there is the intention to launch a public LoRaWAN. (Gust, 2018).

LoRaWAN in Munich

LoRaWAN – average area per gateway

We divided the area of Munich by 10 to calculate the average area per LoRaWAN gateway. This area was converted into a circle and projected onto the map of Munich. The map section was taken from the online map of the Bundesnetzagentur. The Bundesnetzagentur in Germany shows all base stations of cellular network operators and the height of the antennas. Each orange triangle stands for a base station of the three German network operators.

NB-IoT in Munich

There are several ways to determine the number of NB-IoT locations. The NB-IoT modules display the cell ID via the “AT” command. Subway tests are perfect for limiting the range of base stations. The base station will probably no longer be reached at the next railway station. The NB-IoT-tests were done in the railway station Goethestraße because this was the closest to our office. We had a signal in the office and courtyard. The station on Goethestraße has a total of four stairways. This makes further interesting tests possible. Our office has an underground car park. In the driveway, you can make phone calls with your smartphone. It would now be interesting to know how far back we can still transmit by NB-IoT in the underground car park.

The silver bullet for location search and measurement is the NB-IoT test system with a customised version of the akorIoT Micro on Quectel BG96. Our customer has been developing radio network test systems for cellular network operators for over 25 years. They hold Qualcomm’s licenses for reading the lower software layers and can evaluate a great deal of data. I will borrow such a test system and measure the NB-IoT network in Munich.

The mid-range version of the NB-IoT tests uses the new akorIoT SensPRO. Besides the multimode module Quectel BG96, the SensPRO holds a GNSS module Quectel L76 and a RAM memory on I2C. The SensPRO can be freely programmed with the Arduino SDK or ATMEL Studio. The interfaces have been published. Everyone therefore receives a programmable NB-IoT data collector for €149.00.

Since the SensPRO is the size of a credit card, it is also a freely programmable tracker for the terrier, police dog, handbag, container, toolbox and much more. In addition to NB-IoT, the BG96 can also use GPRS and LTE-M. If no NB-IoT is available, a roaming SIM card will be able to register with all the cellular networks in Munich. Almost all orange triangles in the map above are available for connections to the cloud server. “Waldo” and “Lassie” can therefore be found anywhere in Munich or even in foreign countries.

Do you want to test and compare NB-IoT or SIGFOX in Munich? Do not hesitate to order an akorIoT SensPro and test it first while enjoying a free coffee in our office. Then it is time to go to the subway station at Goethestraße, and from there you can continue to other places in Munich. Interesting places to visit are those which according to the coverage map have no coverage, because I would expect you to find coverage at these places as well. We would be happy to arrange a tour for you. At the end of the tour, an NB-IoT live test can take place in one of the popular Munich beer gardens. We would be happy to invite you for a beer, exchange experiences and book a hotel near the subway station Goethestraße.

If you are interested in an antenna matching, a customised antenna or a development of a whole IoT device, do not hesitate to contact us. Furthermore, the akorIoT team is happy to hold workshops on the customer’s location for decision making. Feasibility studies, prototypes with function or consulting are also part of the services.

August 12, 2018

Author: Harald Naumann

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