Ofdm System, Adaptive Modulation And Coding

Orthogonal Frequency Division Multiplexing (OFDM), which employs orthogonal overlapping subcarriers to modulate the signals, has been attracted much attention in recent time as a favorable option for future generation wireless networks due to its ability to overcome many impairments in wireless channel. Since OFDM is a multi-carrier system, Adaptive Modulation and Coding (AMC) techniques can be effectively and efficiently used to enhance the system performance in terms of both BER/FER and overall system capacity. In a word, OFDM system in conjunction with AMC technique performs better than fixed modulation based systems both in terms of FER and spectral efficiency.The main idea of AMC is that high modulation order and weak channel coding can be used to boost up the average throughput when the channel states is good, and low modulation order and strong channel coding can be used to increase error protection when the channel states is bad. By matching the variable modulation level with variable rate channel coding, the physical bandwidth is maintained constant.Firstly, we consider adaptive algorithms in Adaptive Modulation (AM), including five fixed threshold adaptation algorithms and subband BER estimator adaptation algorithm. Based on the AM algorithms which are mentioned above, AMC can be implemented in OFDM system. In this thesis, we also consider a method that dividing all the OFDM subcarriers into several groups which are called subbands.