# Real-Time Mitigation of the Mobility Effect for IEEE 802.15.4g SUN MR-OFDM
## 1 Introduction  
Low-power wide-area networks are attracting the attention of researchers worldwide because of their ability to connect low-power devices over large geographical areas. In the vision for the internet of things (IoT), LPWA technologies are seen as complementary and alternative technologies that support the conventional cellular and short-range wireless technologies for smart cities and machine-to-machine applications [1]. LPWA technology provides unique feature sets, including long-range, low-cost scalability, and low-power devices. The communication technology solution based on IEEE 802.15.4, which supports the mesh networking topology, has limitations in its communication range and data rate. Considering these problems, the IEEE 802.15.4g smart utility networks (SUNs) standard was proposed with a long communication range of several hundred meters and a data rate of 800 kbps. SUNs can also support a reliable mesh-routing protocol, which is expected to be a promising solution for mesh sensor networks. The IEEE 802.15.4g SUN [2] is an amendment to the IEEE 802.15.4 standard [3], which has been processed to offer a global standard that facilitates large-scale process control applications, such as smart-grid networks. Three optional physical layers (hereinafter PHYs) have been proposed for SUNdevices: multi-rate multi-regional frequency shift key (MR-FSK), multi-rate multi-regional orthogonal frequency shift key (MR-OPSK), and multi-rate multi-regional offset quadrature phase shift key (MR-O-QPSK). MR-FSK offers a good modulation scheme with a constant envelop signal that increases the transmission power efficiency. The MR-O-QPSK (DSSS) PHY converts multi-mode systems to more cost-effective and simpler designs. The multi-regional orthogonal frequency-division multiplexing (OFDM) was designed to supply higher rates in the delay spread channels. With the three PHYs, the MR-OFDM provides parameters to deal with the main standard scopes defined in the IEEE project authorization request, which is as follows [4]:  
* Operation in the license-exempt frequency bands, such as 700 MHz to 1 GHz and in the 2.4-GHz band;
* Application mainly to outdoor communications;
* Data rate of IEEE 802.15.4 SUN to be at least 40 kbps and not greater than 1 Mbps;
* Mechanisms to coexist with other systems in the same bands, such as the IEEE 802.11 standard, IEEE 802.15 standard, and 802.16 standard. The OFDM system is a special form of frequency-division multiplexing that divides the high rate stream into lower date streams called sub-streams, and simultaneously transmits all sub-streams through orthogonal subcarriers. In OFDM systems, the cyclic-prefix (CP) is added as the guard interval to combat inter-symbol interference (ISI), which seriously affects high rate systems due to multipath fading. Furthermore, the cyclic-prefix interval does not carry a signal, and it can reduce the system performance. Therefore, the length of the CP must be carefully chosen to balance between ISI reduction and effective performance. The principle disadvantage of OFDM is its sensitivity to frequency offsets caused by the Doppler shift and the asynchronous frequency of the local oscillators (LOs), which are also known as carrier frequency offsets (CFO). A CFO is a non-ideality in the baseband system design, which occurs when the LO signal in the receiver does not match the carrier signal in the transmitter. This phenomenon can occur in two cases: (i) frequency mismatch in the transceiver oscillators, and (ii) the Doppler effect as the transmitter and the receiver move relative to each other. When the CFO becomes present in the OFDM system, there is a frequency shift in the receiver's signal, which leads to non-orthogonality among the subcarriers. This can result in inter-carrier interference (ICI). The CFO includes two parts: integer frequency offset (IFO) and fractional frequency offset (FFO). IFO causes a circular shift of the subcarriers on the receiver side, which causes a significant degradation in the bit error rate (BER) performance.