Low Power Wide Area Networks (lpwan): Technology Review And Experimental Study on Mobility Effect

LPWAN MODULATION

The LPWAN technologies by their characteristics have a range of tens of kilometers to a few kilometers in both rural and urban areas respectively [11]. Most of the LPWAN technologies slow down their modulation rates in order to put more energy in each transmitted bit or symbol so that the receivers are able to decode even the weakest signals without any errors. In general the receiver sensitivity for LPWAN technologies is around -150dBm [11]. The two modulation techniques adopted by LPWAN technologies are the narrowband modulation and the spread spectrum technique.

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   Narrow Band Modulation: In this technique, the signals are encoded at a very low bandwidth of 25kHz or less thus providing a high link budget. The overall spectrum can be shared very efficiently by multiple links as each carrier signal needs a very
   

low band space for itself. Since multiple links are tightly packed in the spectrum, there is minimum effect of noise on each of these narrowbands. Since the effect of noise is so low it becomes easier for the receivers to decode the receiver signal thus reducing the cost. LPWAN technologies like WEIGHTLESS-P [66] and NB-IoT [2] use narrowband modulation techniques. Some technologies like SigFox [57] use ultra narrow band (UNB) which squeezes the signals further in bandwidths of 100Hz, which further reduces the effect of noise and increasing the number of end devices that can be handled at once. The ultra narrow band restrictions reduce the rate at which data can be send over the channel, thus making it suitable for applications where efficient data transmission is a priority over latency and over a long range [51]. Technologies like TELENSA [61] and WEIGHTLESS-N [66] uses ultra narrow band modulation.

2. 
   Spread Spectrum Modulation: In this technique, the narrow band signal is spread over a much wider frequency closer to the noise level while maintaining the same power level. Since the transmitted wave is very close to the noise level it becomes less susceptible to external interference and also becomes harder to read for intruders. Since the signal is buried below the noise floor, powerful receivers are required to decode it. This technique suffers from inefficient use of spectrum space as a narrow band signal is spread over a wider band. This drawback is overcome by LPWAN technologies having multiple end devices using orthogonal sequences or different channels at the receivers end. This helps to increase the over all network capacity as the end devices using orthogonal sequences or different channels are able to simultaneously decode the received signal. The spread spectrum modulation technique is used in networks which require a higher degree of robustness for interference [50]. Chirp Spread Spectrum (CSS) and Direct sequence Spread Spectrum (DSSS) are variations of spread spectrum techniques used by LoRa [59] and INGENU-RPMA [20] respectively.
   

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