# Evaluation of IEEE802.15.4g for Environmental Observations
## 2 Materials and Methods
### IEEE802.15.4g PHYs  
The \"g\" amendment of the IEEE802.15.4 standard has been specifically designed to target SUN applications. Its maximum packet size is 2047 B, enough to carry a complete IP packet in one frame without the need to implement link-layer fragmentation. A longer communication range is needed by SUN applications, therefore the standard makes a more extensive use of the sub-GHz frequencies than the previous version of 2006, using O-QPSK. In sub-GHz bands, radio signals can cover longer distances with the same transmission power, compared to 2.4 GHz. In Europe, two of the most popular unlicensed ISM sub-GHz frequency bands for Non-Specific Short Range Devices are 433.05-434.79 MHz and 863-870 MHz [19; 20]. For this study, we use the latter because the frequency band is wider (7 MHz vs. 1.74 MHz), allowing more communication channels. This band also provides a good trade-off between range and antenna dimension (the recommended size of an antenna depends of the wavelength--\(\lambda\)--of the signal. For dipole antennas, using length \(\lambda/2\) is common. At 433 MHz, this translates to a 35 cm antenna, compared to a 17 cm at 863-870 MHz). The IEEE802.15.4g includes three modulations:  
* **Frequency Shift Keying (FSK)**. In transmission, it delivers good power efficiency thanks to the constant envelope of the signal. It may implement a Gaussian filter or be unfiltered. It uses convolutional coding as Forward Error Correction (FEC). This technique uses redundancy of information and gives the possibility to the receiver to correct errors by itself, without having to request a retransmission of the information. The price to pay is the reduced effective data rate, since more bits are needed to encode the same information. A FEC rate of 1/2 means that for 1 bit of information, 2 bits are transmitted. Data Whitening and interleaving are also optional. Data rates vary from 5 kbps to 400 kbps. SUN FSK devices are simple and do not require complex circuitry consuming high processing power, resulting in a less power-hungry technology. This PHY targets low data rates and high energy efficiency applications, e.g., smart metering. Most electric meters in the US use this PHY. * **Orthogonal Frequency Division Multiplexing (OFDM)**: This PHY is designed to provide high data rate communication in challenging environments presenting multi-path fading conditions, such as urbanized areas or indoor scenarios. Here, the transmitted signals bounce off obstacles, taking different paths and some of them arriving to the receiver at slightly different times (with a different phase). As a result, the overall received signal is the complex sum of those reflections, which can constructively or destructively (self-)interfere. Multi-path fading is also dependent on the frequency used, i.e., it affects different frequencies in different ways. In order to cope with this, OFDM spreads the information to be transmitted over multiple sub-carriers occupying different frequencies. Each sub-carrier is modulated according to a Modulation and Coding Scheme (MCS) value, and it can be BPSK, QPSK and 16-QAM. If part of the data is lost, it can be recovered using FEC techniques and frequency repetition (2 or more sub-carriers transporting the same information). OFDM devices require a more complex circuitry with higher power consumption in order to carry out sophisticated signal processing functions such as creating or demodulating OFDM symbols. This technology has been widely applied in high-end systems such as cellular networks; its use in low-power devices is new. Data rates vary from 50 kbps up to 800 kbps. Typical applications are security and surveillance systems, where high data rates are required. * **Offset Quadrature Phase Shift Keying (O-QPSK). ** It uses Direct Sequence Spread Spectrum (DSSS) or Multiplexed Direct Sequence Spread Spectrum (MDSSS), with data rates from 6 kbps up to 500 kbps. It shares some characteristics with IEEE802.15.4-2006, making multi-mode systems easier to design and more cost-effective [21].