# A Survey on Adaptive Data Rate Optimization in LoRaWAN: Recent Solutions and Major Challenges
## 2 Technology Overview
### LoRa  
LoRa is the physical (PHY) layer of the LoRa technology which utilizes Chirp Spread Spectrum modulation and Forward Error Correction (FEC) that trades-off data rate for sensitivity, within a fixed BW. This combination enables long transmission ranges with low power characteristics as the chirp signals use the available bandwidth immediately. This characteristic makes the chirp signals robust from interference and noise. Each LoRa packet consists of a preamble of 10 chirps and six synchronization chirps followed by the data. Multiple data bits (chips) can be modulated by one chirp. The SF parameter determines the number of bits that are modulated. For instance, SF7 means a chirp can be encoded with seven bits. Transmitting a data packet with a higher SF implies additional bits are encoded into the chirp, resulting in additional Time on Air (ToA) and a reduced data rate, although it enhances robustness to noise. Three other parameters are involved in LoRa modulation that is BW, TP, and CR. Three bandwidth settings are available namely 125 KHz, 250 KHz and 500 KHz. The available code rate settings are 4/5, 4/6, 4/7 or 4/8 [15]. These parameters together are used to calculate the data rate, also known as the LoRa modulation bit rate \(R_{b}\). Equation (1) expresses the relationship between data transmission rate and CR, BW, and SF [16].  
\[R_{b}\ = SF*\frac{BW}{2^{SF}}*CR, \tag{1}\]  
where:  
SF = Spreading Factor  
BW = Modulation Bandwidth  
CR = Code Rate.  
Tuning the parameters above results in a number of the end-to-end transmission characteristics such as data rate, error correction capacity and communication range to be variable [17]. Although a combination of SF, BW and CR are possible theoretically, practically, SF and BW are combined to form the data rate, depending on the LoRaWAN regional parameters specification [9]. The specification document specifies the different regulatory requirements of LoRaWAN depending on the location of the deployment. In North America, for instance, there are 64, 125 kHz LoRa UL channels that have been specified, centered on a 200 kHz raster. Eight 500 kHz UL channels as well as eight, 500 kHz DL channels have also been specified. The gateways have the capacity of up to 64, 125 kHz UL channels and eight 500 kHz UL and DL channels. In Europe, LoRaWAN defined ten channels, eight of which have multiple data rates from 250 bps to 5.5 kbps. The specification also has one high data rate LoRa channel at 11 kbps, and one frequency shift keying (FSK) channel at 50 kbps. The European Telecommunications Standards Institute (ETSI) allows maximum output power in Europe of +14 dBM, except the G3 band which allows +27 dBm. Duty cycle restrictions exist under ETSI but there are no limitations on maximum transmission or ToA. As an example, the regional settings EU863-870 used in Europe and US902-928 used in North America have the data rates ranging from SF7 BW125 to SF12 BW125 and SF7 BW125 to SF10 BW125, respectively. The ADR adjusts the data rate depending on the available link budget. The larger the SF applied, the further the signal travels and the less interference present at the receiver. With the bandwidth fixed at BW125 as given in the example, lowering the SF increases the data rate, thereby minimizing the ToA for data packets making the gateway more sensitive to noise. The end nodes that are nearest to a gateway transmit using the lowest SF thereby prolonging the battery life because of the effect of ToA. More distant sensors transmit at a higher SF but the data rate will be low. A compromise is made between battery power and range considering that a higher SF allows for gateways to connect to end nodes further away due to higher reception sensitivity. Table 1 provides the characteristics of the LoRaWAN technology.