Pros and Cons of Multi-User Orthogonal Frequency Division Multiplexing Mashhur Sattorov, Sang-Soo Yeo, Heau Jo-Kang Division of IT Engineering, Graduate School, Mokwon University, Korea mashhurs@yahoo.com , ssyeo@mokwon.ac.kr , hjkang@mokwon.ac.kr Abstract: In the past decades, most researches focused on adaptive modulation on orthogonal frequency division modulation (OFDM) technique. However, nowadays, we can observe majority of scientific areas are engaging in to achieve advanced multi-path spectral efficiency, user/resource allocation in reliable as well as high data rate transmission on this modulation technique in order to achieve high quality of service (QoS) in 4G period. Therefore, obtaining this modulation scheme’s plus and minus concerns bring us to reach to develop better. Thus, in this thesis, we will focus on an overview of OFDM spread spectrum scheme’s pros and cons issues and to highlight hot solutions for disadvantages. I. Introduction OFDM is as discrete multi-tone modulation because, instead of a single carrier being modulated, a large number of subcarriers are modulated by digital modulation techniques like QAM (M-QAM), BPSK, QPSK, DQPSK, DQAM etc. This is a spread-spectrum technique that increases the efficiency of data communications by increasing data throughput because there are more carriers to modulate. In addition, problems with multi-path signal cancellation and spectral interference are greatly reduced by selectively modulating the clear carriers or ignoring carriers with high bit-rate errors. In OFDM system design, a number of parameters are up for consideration, such as the number of subcarriers, without a guard time the data receiving in purpose is impossible except on Blind channel [1], symbol duration, subcarrier spacing, modulation type per subcarriers, and the type of FEC coding. The choice of parameters is influenced by system requirements for instance, available bandwidth, required bit rate, tolerable delay spread and Doppler values. Thus, counted issues below are considered and divided into the advantages and disadvantages issues for OFDM. IEEE 802.16d uses Orthogonal Frequency Division Multiplexing (OFDM). IEEE 802.16e (mobile) uses Orthogonal Frequency Division Multiple Access (OFDMA). The WiMAX forum established that, initially, OFDM-256 will be used for fixed-service 802.16d (in 2004). It is referred to as the OFDM 256 FFT Mode, which means there are 256 subcarriers (figure 1 illustrates 5 subcarriers of OFDM signal in frequency domain) available for use in a single channel. Multiple Access on a channel is accomplished using TDMA. Alternatively, FDMA may be used. [2] Figure 1. Five OFDM subcarriers in frequency domain. II. Orthogonality on OFDM In CDM (Code Division Multiplexing) system, as the Walls code plays an important role, orthogonality of OFDM also main point for spreading signals as an unique, meaning receiver finds out distinguish integer cyclic number from each signal. Figure 2 shows the signals that even though they are a sum of sinusoids with each corresponding to subcarriers in time domain, they are orthogonal each other in frequency domain.