# OFDM Network Optimization Using a QPSK Based on a Wind-Driven Genetic Algorithm
## 2 Materials and Methods  
### Related Work  
This section discusses some previous works that studied QPSK modulation, OFDM channel performance improvement, and bandwidth optimization in fading channels. An OFDM signal is obtained by performing an Inverse Fast Fourier Transform (IFFT) on a QPSK signal followed by the addition of a cyclic prefix (CP) [12]. OFDM modulation is either conducted using quadrature phase-shift keying (QPSK) or quadrature amplitude modulation (4-QAM) [13]. These two modulation techniques can be modified into higher-order modulation schemes, e.g., QPSK constellations can be doubled resulting in 8-PSK and 4-QAM can also be increased up to 64-QAM before major distortions start to emerge. In [14], a hybrid extended phase shift modulation strategy is introduced to bring stability to a system that is dominated by non-linear components. This methodology shows that the conventional modulation techniques can be enhanced resulting in better channel performance. The authors successfully had channel performance improvement by enhancing the modulation techniques, but their main challenge was with optimal parameter selection. The authors in [15] presented a methodology of having a constant envelope waveform output from modulation that would be fed into the channel. The methodology offered good spectral efficiency. However, since most data are received in digital form as binary digits, this methodology did not fully address the mitigation of bit errors in communication systems. The authors in [16] presented a simulation methodology for OFDM networks. The methodology was intended to break down an OFDM system and highlight the blocks from where distortions or errors might emanate. The methodology proved to be very useful for OFDM network analysis but did not offer a substantial number of solutions to the problems encountered on the various channel sub-blocks, e.g., noise and attenuation. The authors in [17] did a performance analysis for PSK and ASK modulation under uncertain and noisy conditions. The analysis showed that the customary PSK has better output than other modulation techniques. However, regardless of its displayed superiority, it still had massive bit errors and distortions resulting in loss of information. This raised more questions on whether the customary PSK should be considered for 5G and the subsequent technologies considering the need for high data throughput and bandwidth efficiency. The authors challenged fellow researchers to come up with further PSK enhancement techniques. In [18], the authors did a raw comparison of PSK and QAM in an OFDM system. They compared these modulation techniques in their customary form, i.e., without any enhancements. The QPSK technique gave stable results, but it was not study against ICI and ISI. Phase ambiguity was its biggest drawback. The amplitude modulation techniques produced comparably good results, but they were not adaptive to channel conditions and were also very susceptible to synchronization errors. The authors in [19; 20] analyzed the various encoding techniques and challenged fellow researchers to develop or enhance the customary techniques. They recommended that fellow researchers should start by enhancing the basic modulation techniques, i.e., QPSK or 4-QAM before attempting the higher-order modulation techniques so that any synchronization errors can be easily noted and rectified. In [21], a hybrid QPSK modulation technique was presented. This technique is made by rearranging the phases and amplitudes of the modulating signal. This was done in an attempt to mitigate detection errors in fading channels. This technique gave significantly better results in noisy optical channels than the conventional modulation techniques. Nonetheless, the technique suffered greatly from phase ambiguity resulting in some errors.