**1. Introduction**

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream. OFDM is a combination of modulation and multiplexing OFDM is a special case of Frequency Division Multiplexing (FDM), multiple user access is achieved by subdividing the available bandwidth into multiple channels that are then allocated to users.

However, OFDM uses the spectrum much more efficiently by spacing the channels much closer together. This is achieved by making all the carriers orthogonal to one another, preventing interference between the closely spaced carriers. Each carrier in an OFDM signal has a very narrow bandwidth (i.e. 1 kHz), thus the resulting symbol rate is low. This results in the signal having a high tolerance to multipath delay spread. One of the main reasons to use OFDM is to increase the robustness against frequency selective fading or narrowband interference. In a single carrier system, a single fade or interferer can cause the entire link to fail, but in a multicarrier system, only a small percentage of the subcarriers will be affected. Coded Orthogonal Frequency Division Multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission. This is to overcome errors in the transmission due to lost carriers from frequency selective fading, channel noise and other propagation effects.

OFDM overcomes most of the problems with both FDMA and TOMA. In FDMA many carriers are spaced apart in such a way that the signals can be received using conventional filters and demodulators . In such receivers, guard bands are introduced between carriers which results in lowering of spectrum efficiency. In OFDM sub carriers are mathematically orthogonal so that it is possible to receive signal without intercarrier interference. OFDM makes efficient use of spectrum by allowing overlap. OFDM is more resistant to frequency selective fading than single carrier systems due to dividing the channel into narrowband flat fading subchannels. OFDM eliminates ISi and IFI through use of a cyclic prefix . OFDM provides good protection against cochannel interference and impulsive parasitic noise. OFDM is less sensitive to sample timing offsets than single carrier systems (Seshadri Sastry et al., 2010 a) and (Seshadri Sastry et al., 2010 b) proposed a OFDM system with adaptive

modulation using fuzzy logic interface to improve system capacity with maintaining good error performance. Adaptive modulation systems using ordinary hardware decision making circuits are inefficient to decide or change modulation scheme according to given conditions. Using fuzzy logic in decision making interface makes the system more efficient. The results of computer simulation show the improvement of system capacity in Rayleigh fading channel.

(Kwang et al. 2009) proposed a multi-user multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system with adaptive modulation and coding to improve system capacity with maintaining good error performance. The results of computer simulation show the improvement of system capacity in Rayleigh fading channel.

(Li Yanxin et al.2007) presented a novel method for demodulating the QAM signals basing on adaptive filtering. The commonly used least mean square (LMS) error adaptive filtering algorithm is employed for studying the demodulating procedure and the performance of the novel adaptive QAM demodulation. The novel adaptive QAM demodulation does not need the adaptive filter completing convergence. Therefore, the sampling rate and processing speed are decelerated. The performance of the method in theory is compared with computer simulating results. It shows that the error rates in simulation agree well with that in theory. Also, it is indicated that the demodulation method has many advantages over conventional ones, such as the powerful anti-noise ability, the small transfer delay, and the convenient implementation with DSP technology.

(Kiyoshi Hamaguchi et al.) proposed an adaptive modulation system for land mobile communications that can select one of quadrature amplitude modulation levels as a suitable modulation for propagation conditions is described. The main characteristics of the system are a mode in which information cannot be transmitted under adverse propagation conditions and a buffer memory for maintaining the data transmission rate. In the paper they confirmed that the basic performances of the adaptive modulation system using the equipment they developed and they found the measured performance was consistent with computer simulation results. Further in paper it was also confirmed that the adaptive modulation system provided a noticeable improvement in spectral efficiency and transmission quality.

Sorour Falahati, Arne Svensson, Torbjom Ekman and Mikael Stemad proposed that when adaptive modulation is used to counter short - term fading in mobile radio channels, signaling delays create problems with outdated channel state information. The use of channel power prediction will improve the performance of the link adaptation. It is then of interest to take the quality of these predictions into account explicitly when designing an adaptive modulation scheme. They studied the optimum design of an adaptive modulation scheme based on uncoded M-QAM modulation assisted by channel prediction for the flat Rayleigh fading channel. The data rate, and in some variants the transmit power, are adapted to maximize spectral efficiency subject to average power and bit error rate constraints. The key issues studied here are how a known prediction error variance will affect the optimized transmission properties such as the SNR boundaries that determine when to apply different modulation rates, and to what extent it affects the spectral efficiency. The investigation is performed by analytical optimization of the link adaptation, using the statistical properties of a particular but efficient channel power predictor.

Optimum solutions for the rate and transmit power are derived based on the predicted SNR and the prediction error variance.

M.K.Wasantha and W.A.C.Fernando discussed an OFDM-CDMA system with adaptive modulation schemes for future generation wireless networks. Results presented in this paper show that adaptive systems can perform better than fixed modulation based systems both in terms of BER and spectral efficiency.
