## 2 IEEE 802.15.4-2015 Overview
### Ieee 802.15.4 Sln-Ofdm  
The SUN-OFDM (Orthogonal Frequency Division Multiplexing) physical layer was rolled into the IEEE 802.15.4-2015 version of the standard along with SUN-FSK (Frequency Shift Keying) and SUN-OQPSK (Offset Quadrature Phase Shift Keying). While FSK and OQPSK are two well-known modulation techniques that have been widely used in low-power wireless communications thanks to their simplicity, performance and low-cost, OFDM has been commonly used in other communication domains such as fixed and cellular access (e.g., xDSL, WiMAX and LTE), as well as wired and wireless LANs (e.g., PLC and Wi-Fi) due to the stringent processing, memory and energy consumption requirements.  
The SUN-OFDM PHY can operate in both the sub-GHz bands (400, 700, 800 and 900 MHz) and the 2.4 GHz (2400-2483.5 MHz) with data rates from 50 kbps to 800 kbps depending on the OFDM option and MCS (Modulation and Coding Scheme) value, as summarized in Table 1.  
As it can be observed in Table 1, SUN-OFDM defines four options, numbered from 1 to 4, which determine how many sub-carriers are grouped together in order to form an OFDM channel. Option 1, the largest, groups 104 sub-carriers and occupies a bandwidth of 1094 kHz with 1200 kHz of channel spacing. Option 4, the narrowest, groups 14 sub-carriers and occupies a bandwidth of 156 kHz with 200 kHz of channel spacing.  
On top of that, the MCS parameter, numbered from 0 to 6, determines how each sub-carrier is modulated (available modulation for each carrier are BPSK, QPSK and 16-QAM), whether _frequency repetition_ is applied (i.e., \(4\times\), \(2\times\) or no frequency repetition) and the FEC (Forward Error Correction) rate applied to the input data stream for each sub-carrier (i.e., \(R=1/2\) or \(R=3/4\)).  
Regardless of the OFDM option and MCS value, the sub-carrier spacing and symbol rate is constant: 10416-2/3 Hz and 8-1/3 ksymbols/s. Each symbol in the OFDM physical layer is 120 \(\upmu\)s long, although the actual symbol transmission time is divided into a BS (Base Symbol) time of 96 \(\upmu\)s and a CP (Cyclic Prefix) time of 24 \(\upmu\)s. For each symbol, the CP is a replica of the last 1/4 of the BS (24 \(\upmu\)s) and is copied at the front, making the signal periodic. The long duration of an OFDM symbol and the fact that it is cyclic makes it robust against long spread time environments resulting in multi-path propagation that may cause ISI (Inter-Symbol Interference) [9].  
As per the IEEE 802.15.4-2015 standard, SUN-OFDM frames are composed of a SHR (Synchronization Header), PHR (PHY Header) and PSDU (PHY Service Data Unit), as shown in Figure 2. The SHR is composed of a short and long preamble (STF and LTF, respectively), whereas the PHR contains a header with the receiver configuration for the PHY payload (modulation and data rate, frame length, scrambler configuration and a redundancy check). The OFDM header (SHR and PHR) is transmitted using the lowest supported MCS level for the OFDM option being used. That is, for OFDM1 and OFDM2, the header is transmitted using MCS0 (BPSK, R = 1/2, \(4\times\) repetition),  
Figure 1: IEEE 802.15.4-2015 OQPSK-DSSS frame format.  
for OFDM3 the header is transmitted using MCS1 (BPSK, R = 1/2, 2\(\times\) repetition), and, for OFDM4, the header is transmitted using MCS2 (QPSK, R = 1/2, 2\(\times\) repetition). Therefore, for any device implementing any of the OFDM options, implementation of the BPSK and QPSK is mandatory and support of 16-QAM is optional. Finally, the maximum length of the PSDU is 2047 bytes, large enough to transport a complete IPv6 packet without fragmentation.