A key parameter for any wireless communication system is the communication range. Range application requirements are the dedicating factor when we are talking about highly scalable smart metering or other related applications with a small portion of data to be transmitted from multiple sensors.
There are several technical options to increase range and power efficiency by reducing the data rate. Two solutions came out in long-range RF communication:
- Narrowband approach lies in scaling receiver bandwidth to the signal to reduce noise seen by the receiver.
- Adding coding gain on a higher rate signal to combat the high receiver noise in a wideband receiver.
WAVIoT NB-Fi (Narrowband Fidelity) protocol uses DBPSK on physical layer for signal transmission. End-nodes transmit a radio signal in 50 Hz bandwidth with a minimum bit rate of 50 bod. Narrow band approach and high energy for each bit of transmitted data provide excellent link budget and high noise immunity.
WAVIoT end-user devices are developed from scratch with top quality and energy-efficient components for improved performance. As a result, we have achieved a perfect radio communication capabilities (176 dBm link budget) and up to 20 years of end-node power autonomy.
WAVIoT 3-rd generation SDR gateways with optimized proprietary block coding approach and additional mathematics algorithms provide -154 dBm of receiver sensitivity.
This picture displays how NB-Fi signals utilize the spectrum. More than 200 simultaneous signals use only 5% of the spectrum with no collisions. The company believes such approach provides enough capacity for building truly wide-range telematic network for M2M and IoT purposes.
Some challenges of narrowband approach
The drawback of a narrowband system has traditionally been the higher requirements on the RF crystal. A frequency error on the RF crystal leads to an offset on the programmed RF frequency. If the offset gets too big, the signal will fall outside the channel, and be filtered out by the strong receive filters. Legacy narrowband systems typically use temperature-controlled oscillators (TCXOs). These have been more expensive than standard crystals, but the difference has been drastically reduced.
Today, however, the accuracy of standard crystals improved significantly, thus we can apply narrowband RF approach with a reasonable budget and effective spectrum utilization.
LoRa uses a unique modulation format with a spread spectrum technology an unmodulated carrier in an FM chirp, which has similarities to M-ary FSK. It means energy is spread across a wider band, though not in the same way DSSS is. This approach allows to use cheap oscillators (or crystals with a lot of drift) and get more stability on the receiver.
Some challenges of coding gain solution
Once coding gain is used, it is possible to demodulate the signal at -20 dB below the noise floor but it negatively affects the overall system performance.
The main drawback of using the coding gain solution is very low spectrum efficiency. The waste of spectrum is quite obvious as you transmit a lot of redundant data to compensate for the higher noise floor.
It is easy to see that in the same 125 kHz bandwidth used for LoRa coding, there is a room for 2 500 WAVIoT narrowband channels. Network capacity utilization is hence a major drawback of coding gain solutions.
The picture below displays how LoRa signals use the available frequency spectrum. 10 signals use more than 30% of the spectrum with several collisions.
Trading higher receives sensitivity for less spectrum efficiency (higher bandwidth) by spread spectrum goes against regulatory requirements and worldwide industry practice for better spectrum utilization. The growth in demand for wireless connectivity has increased the demands on radio spectrum all around the world.
Coding gain makes it possible to have several orthogonal codes in the same channel, but then the protection between these is given by the coding gain only. Having 10 dB coding gain will give less than 10-20 dB protection against another meter in the same channel.
WAVIoT narrowband system provides up to 65 dB protection from the adjacent/neighbor channel – a 45 dB difference compared to using LoRa coding gain. Practically 45 dB ensures a dramatic difference in robustness and coexistence in a real-life deployment, translating to a 45 dB improvement in sensitivity in the presence of interference.
Provided that the net data rate/throughput is the same for the two scenarios, the payload part of the packet will be of a similar length. However, the signal seen on the receiver is very different.
WAVIoT’s NB-Fi signal is clearly visible by receiver above the noise floor and can be reliably received with as little as 3 bytes of the preamble. At the same time, LoRa signal is hardly visible, as it is buried below the noise floor. Should you be willing to extract any meaningful information from this signal, you first need to accurately synchronize it to the coding scheme in order to get the required coding gain. Needless to say, this will require a very long preamble or leader sequence before the actual data can be received. When using schemes with high coding gain, the leader sequence will be by far the most dominating part of the message, further reducing the spectral efficiency.
The long leader sequence has a strong negative effect on the battery lifetime as a lot of redundant information must be transmitted to enable the receiver to find the wanted signal from below the noise floor.
WAVIoT narrowband communication is a proven way of achieving long-range RF communication and offering a superior availability and scalability versus LPWAN systems based on Lora coding gain principles. Higher requirements for oscillators are compensated by capabilities of the modern improved crystals.
LoRa is, so to say, very forgiving to simple oscillators (or crystals with a lot of drift), it allows to get more sensitivity on the receiver. But on the other hand, it reduces spectrum efficiency, i.e. the number of devices that can communicate in a given period of time and area, which means lower coexistence properties (protection against interference) and further reduces communication reliability. Another aspect to think of is, as was already mentioned, decreasing battery lifetime as coding gain signals require longer leader sequences to recover the information signal from the very strong noise component.
Making a comparison of the efficiency on receivers we may well note that WAVIoT GW220 gateway sensitivity of -154 dBm is far higher vs LoRa’s ( e.g. LL-BST-24) -132 dBm. The radio wave propagation theory represents that 20 dBm in the air will give us 10 times higher range or two additional concrete walls in an urban environment.
Our conclusions have focused on the use of narrowband versus coded wideband approach for long-range communications and corroborated with Texas Instrument white paper «Long-range RF communication: Why narrowband is the de facto standard».
Download white paper
Long-range RF communication: Why narrowband is the de facto standard
(2015.12.10, 0,7 Mb)