Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multi-carrier transmission technique in which a single high rate data stream is divided into multiple low rate data streams. These data streams are then modulated using subcarriers which are orthogonal to each other. In this way the symbol rate on each subchannel is greatly reduced, and hence the effect of intersymbol interference (ISI) due to channel dispersion in time caused by multipath delay spread is reduced.
Guard interval can also be inserted between OFDM symbols to reduce ISI further. The orthogonality between subcarriers can be maintained, even though the signal passes through a time-dispersive channel by cyclically extending the OFDM symbols into guard interval. The conservative approach in threshold based adaptation is by using the lowest quality subcarrier in each subband for controlling the adaptation algorithm [11]. It means that the lowest value of SNR will be used in mode selection. By using this method, the overall BER in one subband is normally lower than the BER target.
The main advantages of OFDM are its multipath delay spread tolerance and efficient spectral usage by allowing overlapping in the frequency domain. Another significant advantage is that the modulation and demodulation can be done using inverse Fast Fourier Transformation (IFFT) and Fast Fourier Transformation (FFT) operations, which are computationally efficient.
The performance of turbo-coded adaptive modulation are investigated in [4]. Three different modulation mode allocation algorithms were discussed and compared. Further studies on the application of turbo code in adaptive modulation and coding is conducted in [5]. This paper proposed an optimal approach based on prediction of the average BER over all subcarriers.
Therefore a better adaptation algorithm is used in this paper to provide a better trade off between throughput and overall BER by choosing a more suitable scheme for each subband. Instead of using the lowest SNR in each subband, the average value of the SNR of the subcarriers in the subband is going to be used.
Thus the advantage of using an adaptive scheme with transmission blocking is that the performance can be designed to meet a certain required BER. However the disadvantage is that the utilization of the transmission blocking results in transmission latency. The BER performance of adaptive modulation
The comparison is made only for the adaptive scheme utilizing transmission blocking. This is because the performance of adaptive scheme utilizing transmission blocking is more superior compared to adaptive scheme without transmission blocking.
The BER performance of adaptive modulation system was better than the most robust modulation mode 4QAM systems in terms of BER performance. This phenomenon was also observed in [14]. As can be seen, the BER remains constant at 10-3 after 25 dB since the scheme is designed to meet the BER target of 10-3. Another aspect of performance improvement is in terms of the decrease of SNR required to meet the BER target. Adaptive modulation scheme is able to meet the BER target at 25 dB compared to more than 28 dB for 4QAM fixed modulation scheme.
A better adaptation algorithm is used to improve the throughput performance. This algorithm utilizes the average value of the instantaneous SNR of the subcarriers in the subband as the switching parameter. The results show an improved throughput performance with considerable BER performance.
In summary, the AM scheme has its advantages when compared to individual fixed modulation modes in terms of its BER performance and throughput performance. Note however for the adaptive scheme without transmission blocking that the target BER can only be achieved if the channel SNR is higher than 24 dB respectively.