International Journal of Scientific and Research Publications, Volume 3, Issue 11, November 2013 1 ISSN 2250-3153 www.ijsrp.org Review on OFDM a Brief Survey Vishal Pasi * , Prateek Nigam ** , Dr. Vijayshri Chaurasia ** * Digital Communication Scholar RKDF Ist Bhopal ** Electrical Engineering Scholar AISECT University Bhopal Abstract- Orthogonal frequency-division multiplexing (OFDM) effectively mitigates intersymbol interference (ISI) caused by the delay spread of wireless channels. Therefore, it has been used in many wireless systems and adopted by various standards. In this paper, we present a comprehensive survey on OFDM for wireless communications. We address basic OFDM and related modulations, as well as techniques to improve the performance of OFDM for wireless communications, including channel estimation and signal detection, time- and frequency-offset estimation and correction, peak-to-average power ratio reduction PAPR, intercarrier interference (ICI) and multiple-input– multiple-output (MIMO) techniques. We also describe the applications of OFDM in current systems and standards. Index Terms- Channel estimation , frequency-offset estimation, intercarrier interference (ICI), multicarrier (MC), multiple input – multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM), peak-to-average power reduction, timeoffset estimation, wireless standards. I. INTRODUCTION rthogonal frequency division multiplexing (OFDM) is a multicarrier multiplexing technique, where data is transmitted through several parallel frequency sub channels at a lower rate. It has been popularly standardized in many wireless applications such as Digital Video Broadcasting (DVB), Digital Audio Broadcasting (DAB), High Performance Wireless Local Area Network (HIPERLAN), IEEE 802.11 (WiFi), and IEEE 802.16 (WiMAX). It has also been employed for wired applications as in the Asynchronous Digital Subscriber Line (ADSL) and power-line communications. The ever increasing demand for very high rate wireless data transmission calls for technologies which make use of the available electromagnetic resource in the most intelligent way. Key objectives are spectrum efficiency (bits per second per Hertz), robustness against multipath propagation, range, power consumption and implementation complexity. These objectives are often conflicting, so techniques and implementations are sought which offer the best possible trade off between them. The Internet revolution has created the need for wireless technologies that can deliver data at high speeds in a spectrally efficient manner. However, supporting such high data rates with sufficient robustness to radio channel impairments requires careful selection of modulation techniques. Currently, the most suitable choice appears to be OFDM (Orthogonal Frequency Division Multiplexing). 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. Error correction coding can then be used to correct for the few erroneous subcarriers. The concept of using parallel data transmission and frequency division multiplexing was published in the mid-1960s [1, 2]. OFDM is a special case of multi-carrier modulation. Multi- carrier modulation is the concept of splitting a signal into a number of signals, modulating each of these new signals to several frequency channels, and combining the data received on the multiple channels at the receiver [3]. In OFDM, the multiple frequency channels, known as sub-carriers, are orthogonal to each other [4]. II. BASIC OFDM Let {sn,k} σ−1 k=0 with E|s n,k | 2 = ı 2 s be the complex symbols to be transmitted at the nth OFDM block, then the OFDM modulated signal can be represented by where Ts, Δf, and N are the symbol duration, the sub- channel space, and the number of sub-channels of OFDM signals, respectively. For the receiver to demodulate the OFDM signal, the symbol duration should be long enough such that TsΔf = 1, which is also called the orthogonal condition since it makes e −j2πkδft orthogonal to each other for different k. With the orthogonal condition, the transmitted symbols s n,k can be detected at the receiver by if there is no channel distortion. The sampled version of the baseband OFDM signal s(t) in (1) can be expressed as which is actually the inverse discrete Fourier transform (IDFT) of the transmitted symbols {sn,k} σ−1 k=0 and can efficiently be calculated by fast Fourier transform (FFT). It can easily be seen that demodulation at the receiver can be performed using DFT instead of the integral in (5). O