The next problem in OFDM systems is Peak to Average Power Ratio (PAPR) problem. The input symbol stream of the IFFT should possess a uniform power spectrum, but the output of the IFFT may result in a non uniform or spiky power spectrum as most of transmission energy would be allocated for a few instead of the majority subcarriers [3].
This implies that the power amplifier must be oversized in terms of its average power requirement which means that the efficiency of the power amplifier (PA) will suffer dramatically, since its DC power consumption is determined by the peak power level. Since the DC power consumption of the PA represents a significant portion of the total DC power for a radio, traditionally designed OFDM modems are not power efficient.
This segment describes about the characteristics of the channel used in this project. Here, the Rayleigh fading channel is used to simulate the real channel model in the transmission of the OFDM system using adaptive modulation. The channel is modelled with the characteristics of the Rayleigh fading which follows the Rayleigh distribution that affects the transmitted signal passing through this channel.
The channel estimation and mode selection are done at the receiver side and the information is sent to the transmitter using a feedback channel [31]. In this model, the adaptation is done frame by frame. The channel estimator is used to estimate the instantaneous SNR of the channel. Based on the instantaneous SNR calculated, the best mode will be chosen for the next transmission frame. This task is done by the mode selector block.
When the signal is received at the receiver, the analog to digital converter converts the analog signal into digital form. While the signal is distorted in its amplitude and phase by the noise and channel characteristics, some amplitude and phase correction mechanism must be used in the signal to retrieve the actual signal. Then, this signal is transform to parallel form using the converter so that Fast Fourier Transform (FFT) is applied to the signal which converts the time domain signal to its frequency domain.
This graph reveals that spectral efficient higher order modulation schemes works well at higher SNR when the strength of the signal is very high and the effect of noise is very low. But, however, the efficiency of the higher order modulation is very low in the lower SNR where the strength of the signal is quite low.
The contrast to higher order modulation schemes, lower order modulation schemes are efficient at lower SNR or weak strength of the signal where, the effect of noise is strong while the throughput of lower order modulation schemes remains short compared to the spectral efficient higher order modulation schemes at higher SNR.
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.