Effects of Cyclic prefix on OFDM system D C Shah B U Rindhe S K Narayankhedkar EXTC Department, EXTC Department, EXTC Department, Smt. Indira Gandhi college of Engg., Smt. Indira Gandhi college of Engg., Smt. Indira Gandhi college of Engg., Kopar Khairane, Navi Mumbai Kopar Khairane, Navi Mumbai Kopar Khairane, Navi Mumbai 022-27545242 022-27545242(D) 022-27545242(D) mrs.dhara_shah@rediffmail.com bu_rindhe1@rediffmail.com skniitb@yahoo.com ABSTRACT Orthogonal frequency division multiplexing (OFDM) is a block transmission technique with bandwidth efficient signalling scheme for wideband digital communications. In OFDM the spectrum of the individual carriers mutually overlaps. Nevertheless, the OFDM carriers exhibit orthogonality on a symbol interval if they are spaced in frequency exactly at the reciprocal of the symbol interval, which can be accomplished by utilizing the discrete time Fourier transform (DFT). In the baseband, complex-valued data symbols modulate a large number of tightly grouped carrier waveforms. The transmitted OFDM signal multiplexes several low-rate data streams each data stream is associated with a given subcarrier. The main advantage of this concept in a radio environment is that each of the data streams experiences an almost flat fading channel. In slowly fading channels, the inter-symbol interference (ISI) and inter-carrier interference (ICI) within an OFDM symbol can be avoided with a small loss of transmission energy using the concept of a cyclic prefix. With the development of modern signal processing technology, OFDM has become practical to implement and has been proposed as an efficient modulation scheme for applications ranging from modems, digital audio broadcast, to next generation high speed wireless data communications. The high speed wireless LAN standard IEEE 802.11a is based on OFDM. Also OFDM is been used for WIMAX and a candidate for 4G technology. This paper is an attempt to understand multiple effects of varying length of cyclic prefix for Additive White Gaussian Noise (AWGN) channel and with Rayleigh fading channel. Categories and Subject Descriptors C.2 Computer-Communication Networks. B.4.3 Interconnections (Subsystems) : Fiber optics General Terms Measurement, Performance, Experimentation, Theory, Verification. 1. INTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) is a multi- carrier transmission technique specially derived to combat the frequency selective fading severely affecting single carrier Figure 1. Block diagram of OFDM system with cyclic prefix systems. The OFDM system divides the available spectrum into many closely spaced orthogonal carriers, each being modulated by a low rate data stream. This will result in highly effective spectrum utilization compared to Frequency Division Multiple Access (FDMA) by removal of carrier spacing overhead. Orthogonality of carriers will prevent interference between the closely spaced overlapping carriers. Due to orthogonal nature, spectrum of each carrier has a null at the centre frequency of each of the other carriers in the system which results in no interference between the carriers. The narrow bandwidth of each carrier gives rise to low symbol rate and thus high tolerance to multi-path delay spread, as the delay spread must be very long to cause significant inter-symbol interference. Thus inter-symbol interference an important consideration while considering the performance of the signal during the various stages it passes while transmission. The detailed block diagram of an OFDM system including cyclic prefix is as shown in figure 1. The wideband data stream of binary digits modulated and mapped to a (possibly complex) symbol stream using some modulation constellation like BPSK, QAM, etc. By inverse multiplexing these symbols are de-multiplexed into N parallel streams. The constellation may be different. So some streams may carry a higher bit rate than others. An inverse FFT is computed on each set of symbol giving set of complex time domain samples these samples are then quadrature mixed to pass band in the standard way. The real and imaginary components are first converted to the analog domain using DAC’s ; the analog signals are then used to modulate cosine and sine waves at the carrier frequency ,fc respectively .These signals are then summed to give the transmission signal s(t). The receiver picks up the signal r(t) which Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. 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