# Digital Pre-Distortion of Carrier Frequency Offset for Reliable Wi-Fi Enabled IoTs
## 5 Digital Pre-Distortion of CFO  
Using preambles, a receiver can estimate the link quality such as the signal-to-interference noise ratio (SINR). The long training field is 8 us in length, and is composed of two identical 3.2 us symbols. Due to the symbol repetition, this long training field can be used to estimate SINR [19]. In order to improve the data rate, the CFO values of data packets transmitted through wireless channels can be estimated by the units of packets in the physical layer (PHY); then, the estimated CFO and an average value of the CFOs estimated from the packets in good SINR conditions are stored, an estimated CFO of a current data packet is compensated for, or the average CFO is compensated according to the wireless channel environment. The transmitter distorts and sends a data packet as much as the CFO, which is estimated at the receiver. Accordingly, the receiver can further reliably detect and decode the packets. As a result, the data rate is improved by minimizing the residual CFO.  
Figure 1 provides a flowchart and an algorithm illustrating the method of the proposed digital CFO predistortion. If a measured long-term SINR is larger than a predetermined threshold value (Threshold 1), it is quickly adapted to channel variation because the CFO is estimated and predistorted by the unit of the packet. If the long-term SINR variation is larger than a predetermined threshold value (Threshold 2), a short-term SINR is monitored to determine if the stored CFO is used or not. When the short-term SINR is smaller than Threshold 1, it is difficult to accurately estimate the CFOs of the data packets transmitted through the wireless channel. That is, the estimated error may be very significant. In this case, the data packets are restored by compensating for an average CFO of the CFOs  
estimated in good SINR conditions. If the short-term SINR is larger than Threshold 1, the transmitter predistors the CFO using the currently estimated CFO.  
As described above, the compensated CFO, which is compensated restoring a previously received packet, is distorted in advance for the next transmission packet. That is, in a communication environment, a transmitter transmits packets by distorting the compensated CFO in advance. Accordingly, a receiver receiving packets can accurately detect and decode packets. As a result, data can be stably and reliably transmitted and received. The channel state of a wireless channel is estimated when the data packet is transmitted through the wireless channel. That is, the SINR of the wireless channel is estimated, the estimated SINR is compared with a predetermined first threshold, and the channel state of the wireless channel is estimated based on the comparison result. When it is determined that the current link quality is worse than the link quality required to estimate the CFO, the current data packet is restored by compensating the estimated CFO of the current data packet to the estimated CFO of the previously transmitted data packet. When it is determined that the current link quality is better than the required link quality, the current data packet is restored by compensating the estimated CFO of the current data packet to the estimated CFO of the current data packet.  
Accordingly, packet restoration error and packet loss can be prevented by accurately estimating the CFO without failing to estimate the CFO of the data packet, which may occur when a wireless channel has a poor state such as that of a wireless channel having a small SINR, multipath fading, or large delay. Therefore, the data packet transmit rate, the data processing rate, and the coverage thereof are improved, and overall system performance is improved.