Bit Error Rate Performance Analysis of Channel Estimated Adaptive OFDM System Srinu Pyla Dr. K Padma Raju Dr. N. Bala Subrahmanyam Assistant Professor Professor & Director of DAP Professor & HOD G V P College of Engg. (A) J N T U University G V P College of Engg. (A) Visakhapatnam, India Kakinada, India Visakhapatnam, India Abstract: Modern communication systems are designed to support multiple applications such as data, voice, video and multimedia transmission, hence they require high data rate, spectral efficiency and inter symbol interference (ISI) free transmission. Orthogonal frequency division multiplexing (OFDM) meets the above requirements but fluctuations in Signal to Noise Ratio is quite common due to variations in envelope of OFDM signal which degrades system performance. System performance can be improved either by using Space time coding or Adaptive modulation (AM) schemes. In this work, adaptive modulation is considered due to its low complexity and optimum spectrum utilization over space time coding. In this scheme, channel state information (CSI) is fed to the transmitter to adopt the order of modulation in order to maintain constant bit error rate (BER) irrespective of the channel conditions. Here channel estimation has done using Least Square (LS), Minimum Mean Square Error (MMSE) and interpolation along with comb type pilot symbol assisted channel estimation algorithms. In this work, a new scheme has implemented to improve the BER performance by integrating channel estimation and adaptive modulation and this new scheme results superior performance over individual methods. Keywords: Bit Error Rate (BER), Channel Estimation, Comb type pilot, Least Square (LS), Minimum Mean Square Error (MMSE), adaptive modulation and Signal to Noise Ratio (SNR) . I. INTRODUCTION Modern communication systems are designed to support multiple applications such as data, voice, audio, video and multimedia transmission, and therefore they require high data rate, spectral efficient and inter symbol interference (ISI) free transmission [8]. Orthogonal frequency division multiplexing (OFDM) meets the above requirements and also provides multipath fading free transmission but fluctuations in Signal to Noise Ratio is quite common due to variations in envelope of OFDM signal and causes system performance degradation [6]. Because of its numerous advantages, OFDM widely used in many applications such as wireless local area networks, terrestrial digital video broadcasting, Wi-Max, LTE-Advanced etc… [12]. In wireless multipath propagation, the transmitted signal undergoes many reflections due to presence of several objects in the transmission path and multiple versions of the transmitted signal receive at different time intervals [8], and leads to fluctuations in Signal to Noise Ratio and increases bit error rate. BER can be controlled either by using Space time coding or Adaptive Modulation schemes. For implementation of adaptive modulation communication system, channel state information (CSI) is required. Moreover the transfer function of the channel in OFDM looks unequal both in time and frequency domains. Hence dynamic channel estimation is required [1]. The most popular channel estimation scheme is pilot channel estimation and can be performed mainly in two ways, one is either by inserting pilot tones into each subcarrier of OFDM symbols (block type pilot) or by inserting pilot tones into each OFDM symbol (comb type pilot) [5]. Block type pilot is efficient in case of slow fading channel and comb type pilot is beneficial in frequency selective fading channel [4, 7]. BER performance of comb type is better over block type [3]. In addition to these schemes there are several techniques for channel estimation such as Adaboost, Blue, LS, MMSE, Interpolation, recursive least square (RLS) etc. Many of the channel estimation schemes for OFDM are under the assumptions of a slow fading channel in which the transfer function of the channel is being stationary but in practical scenario it is time varying. So in such a case the channel parameters estimation is preferable in every OFDM data block [2]. In this paper, channel estimation at pilot frequencies has being carried out using comb type pilot arrangement through LS, MMSE and at data frequencies channel interpolation is using linear, spline and cubic. The performance of all schemes have been compared by measuring bit error rate with M- ary PSK and QAM digital modulation schemes by considering additive white Gaussian noise (AWGN) channel. For further improvement of BER performance a new scheme is implemented by integrating channel estimation and adaptive modulation. Adaptive modulation is famous for efficient spectrum utilization and moreover it maximizes linkup time, information rate and minimizes transmission errors. The results show the BER performance of this integrated scheme is superior over individual methods This paper is organized as follows. In section II OFDM system with channel estimation is described. Section III discusses various channel estimation algorithms. Section IV describes adaptive modulation. Section V describes proposed method i.e adaptive OFDM with channel estimation. Simulation results are shown in Section VI. Finally the International Journal of Computer Science and Information Security (IJCSIS), Vol. 14, No. 2, February 2016 88 https://sites.google.com/site/ijcsis/ ISSN 1947-5500