IOSR Journal of Electronics and Communication Engineering (IOSR-JECE) e-ISSN: 2278-2834,p- ISSN: 2278-8735. Volume 5, Issue 3 (Mar. - Apr. 2013), PP 26-30 www.iosrjournals.org www.iosrjournals.org 26 | Page Comparative Study of OFDM and CDMA Technique Lalit Singh GariA 1 , Amit Shah 2 , Deepesh Rawat 3 1 (Asst. Prof.(Ece), Btkit,Dwarahat Uttarakhand, India) 2 ( Mtech (Dc 2 nd Year) Btkit,Dwarahat, Uttarakhand, India) 3 ( Mtech (Dc 2 nd Year) Btkit,Dwarahat, Uttarakhand, India) Abstract: Orthogonal frequency division multiplex (OFDM) modulation is being used more and more in telecommunication both in wired and wireless. This modulation technique has several advantages, reason for its increasing usage in communication. OFDM can provide high data rates, it can be implemented easily, it is spectrally efficient and with sufficient robustness to channel imperfections. On the other hand most third generation mobile phone systems are using Code Division Multiple Access (CDMA) as their modulation technique. For this reason, CDMA is also investigating so that the performance of both CDMA and OFDM can be compared. It is found that OFDM performs extremely well when compared with CDMA, and provide very high tolerance to multipath delay spread, channel noise, and peak power clipping. In addition to this it provides a high spectral efficiency The noise performance of OFDM is found to depend solely on the modulation technique used for modulating each carrier of the signal. The OFDM signal performance is found to be the same as for a single carrier system, using the same modulation technique and can provide large data rates with sufficient robustness to radio channel impairments The minimum signal to noise ratio (SNR) required for BPSK was ~7 dB, where as it was ~12 dB for QPSK and ~25 dB for 16PSK. CDMA was found to perform poorly in a single cellular system, with each cell only allowing 7-16 simultaneous users in a cell, compared with 128 for OFDM.1.25 MHz bandwidth and 19.5 kbps user data rate was used for it. This low cell capacity of CDMA was attributed to the use of non-orthogonal codes used in the reverse transmission link, leading to a high level of inter-user interference. Keywords: BER, CDMA, FFT, ISI, OFDM, OFDM, S/N I. INTRODUCTION The telecommunications’ industry is in the midst of a veritable explosion in Wireless technologies Wireless communication is one of the most active areas of technology development. Wireless Broadband Technologies allow the simultaneous delivery of voice, data and video over fixed or mobile platforms. Wi-Fi, WiMAX, LTE, UMB are some of the emerging technologies. With the increase demand there is a need to transmit information wirelessly, quickly, and accurately. To address this need, several technologies have been combined for high rate transmission with forward error correction techniques.Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a wired or a wireless system in which each signal is limited by a specific frequency band, and is modulated by data stream. Orthogonal Frequency Division Multiplexing[1] and is a special case of frequency division multiplexing, where the data is distributed over a large number of carriers, which are orthogonal to each other. OFDM[2] is spectrally efficient compared to the conventional FDM systems, since it doesn't need guard bands between adjacent channels. This orthogonality property of the carriers is main characteristic of OFDM, since the interference due to the other carriers is prevented when the receiver demodulates a particular carrier. Once the OFDM data are modulated to time signal, all carriers transmit in parallel to fully occupy the available frequency bandwidth [3].OFDM is becoming widely applied in wireless communications systems due to its high rate transmission capability with high bandwidth efficiency and its robustness with regard to multi-path fading and delay [4]. One of the principal advantages of OFDM is its utility for transmission at very nearly optimum performance in unequalized channels and in multipath channels. Inter-symbol interference (ISI)[5] and inter-carrier interference (ICI) can be entirely eliminated by the simple expedient of inserting between symbols a small time interval known as guard interval. One of the principal disadvantages of OFDM is sensitivity to frequency oıset in the channel. There are two deleterious effects caused by frequency oıset; one is the reduction of signal amplitude in the output of the filters matched to each of the carriers and the second is introduction of ICI from the other carriers which are now no longer orthogonal to the filters. Timing errors also affects OFDM system performance by reducing the delay spread robustness and by introducing phase shift in the received spectrum. Adaptive estimation of channel is necessary before the demodulation of OFDM signals since the wireless channel is frequency selective and time varying. Some of the main advantages of OFDM are efficient spectral usage and its multi-path delay spread tolerance by allowing overlapping in the frequency domain. Another significant advantage is that the modulation and demodulation can be done using IFFT and FFT operations, which are computationally efficient.