Adaptive modulation uses different modulation schemes according to the instantaneous state of the channel condition. So, during the lower SNR or strength of the message signal, where the effect of noise is quite higher, it uses lower order modulation schemes to cope against the noise effect on the signal so that the signal could be successfully transmitted to the receiver. Even the graph shows that the throughput of adaptive modulation is less than 10 bps at lower SNR.
Hence, the overall graphical representation of BER in OFDM system using adaptive modulation shows that the BER is higher at lower SNR due to low strength of the signal while at higher SNR; BER is almost zero for adaptive modulation. The rate of fall of BER is steady at low SNR which is due to the steady increase in the strength of signal.
Thus, this shows that the BPSK, QPSK and 8-PSK are superior to adaptive modulation in terms of BER over all SNR while 16-PSK, 32-PSK and 64-PSK remain inferior against adaptive modulation which proves that the low order modulation schemes are robust to the higher order modulation schemes in terms of noise.
As the number of iteration in this project is limited to certain times, so the BER of adaptive modulation at lower SNR remains high; this is not acceptable in this project. Thus, with the increase in number of iteration, the BER of adaptive modulation decreases significantly below the predicted target which shows the effectiveness of the adaptive modulation in terms of BER in OFDM system.
For lower order modulation schemes, like BPSK, QPSK and 8-PSK, the throughput is higher at lower value of SNR while the higher order modulation schemes such as 16-PSK, 32-PSK and 64-PSK has lower throughput output. Thus, due to the adaptive technique, the throughput of adaptive modulation remains fairly in between these two set of modulation schemes.
Adaptive modulation which uses the feature of all the lower and higher order modulation schemes shows the most efficient throughput compared to all other modulation schemes regardless of the channel quality and the noise which shows the dominant characteristics of the adaptive technique.
According to the design of the system, ICI is not considered with the use of singular value decomposition in the case of channel estimation thus resulting in non optimum channel-SNR to feedback to the transmitter. The main aim of this research is to show the efficient performance of adaptive modulation over fixed modulation scheme in OFDM system in terms of BER and throughput which is effectively and successfully studied and analysed.
Thus, the BER and throughput of the different modulation schemes over the slowly varying Rayleigh channel are investigated and analysed. From the observation, it shows that BPSK modulation scheme is better than any other modulation schemes including adaptive modulation in terms of BER while adaptive modulation is far better in throughput than any other fixed modulation schemes.
The BER of the system is significantly low using uncoded message signal. However, with the use of coding , the BER of the OFDM system may drastically reduced resulting in more higher order of modulation schemes such as 128-PSK, 256-PSK and 512 PSK to be used to transmit. Hence, ultimately increasing the spectral efficiency of the system.
In wireless communication, particularly in OFDM system, adaptive modulation is the most convincing tool to increase the spectral efficiency on time-varying and slowly fading channels while keeping a predictable BER. However, reliable channel estimation is required to obtain accurate channel state information (CSI) so that the favourable modulation scheme can be applied to the message signal.