International Journal of Computer Applications (0975 – 8887) Volume 52– No.5, August 2012 1 A Novel Approach for Reduction in CFO at Channel Estimator in WiMax OFDM System Minal Saxena Research scholar MANIT Bhopal INDIA Kavita Khare Associate Professor MANIT Bhopal INDIA ABSTRACT During Multicarrier transmission in OFDM(Orthogonal Frequency Division Multiplexing), the signal suffers due to fading environment and results in timing and frequency errors at the receiver end.An efficient timing and frequency offset estimation algorithm has been designed in VHDL using ISE XILINX 10.1. The maximum value at which the timing offset is achieved and input signal with a delay are compared to get the frequency offset. This operation ensures that the frequency offset calculation is done at the best time, i.e., when the correlation over the actual received symbol cyclic prefix is complete. The values for frequency offset has been obtained after simulations as .00034232 and timing offset has been observed as1.864ns.The design is achieved at 75.5 Mhz and uses 2% of the total memory of the Virtex-6 Device. The design when implemented was found to reduce 95% of carrier frequency offset(CFO) error, than the performance of a simple frequency estimator. 1. INTRODUCTION During the past few years, there has been an explosion in wireless technology. This growth has opened a new dimension to future wireless communications whose ultimate goal is to provide universal multimedia communication without regard to mobility or location with high data rates. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique, many carriers, each one being modulated by a low rate data stream share the transmission bandwidth. Orthogonality is a property that allows multiple information signals to be transmitted perfectly over a common channel and detected without interference. The OFDM is a digital modulation scheme that can support high-speed video communication along with audio with elimination of Intersymbol Interference (ISI) and Interchannel Interference (ICI) [1]. At the same time, it can accommodate more number of users showing the spectral efficiency. Because of high capacity transmission of OFDM ,It has been applied to Digital transmission system,such as Digital audio broadcasting(DAB) system ,Digital video broadcasting TV(DVB-T)system, Asymmetric digital subscriber line(ADSL),Ultra wideband(UWB) system, IEEE 802.11a/g Wireless local area network(WLAN),IEEE 802.16 Worldwide interoperability for microwave access (WiMax)systems and HIPERLAN2(High performance local area network).Its application in mobile communication is more complex especially because of the mobility of the mobile user, thus more exact symbol and frequency offset control must be used to ensure that the subcarrier remain orthogonal. During the transmission the signal suffers due to the fading environment and hence results in two major problems. One problem is the unknown time instant to start sampling a new OFDM symbol, known as Timing Offset. A second problem is the mismatch of the oscillator frequencies between the transmitter and receiver resulting in frequency offset. The demodulation of a signal with an offset in the carrier frequency can cause large bit errors and may degrade the performance of a symbol synchronizer [2]. Also, Sensitivity to a timing offset is higher in multi-carrier systems than in single-carrier systems .It is therefore important to estimate this offset and minimize its impact. Frequency offset is estimated at the instant the start of the frame is detected, therefore its value is directly related to timing offset. Many algorithms have been designed in past to estimate the values of these offsets, but this approach looks forward to minimize the values of these offsets using VLSI technology. 2. ESTIMATION METHODS To send the transmitter signal to the receiver with the features to combat the channel problems, some characteristics of the channel must be estimated, maybe in terms of delay between the transmitter and receiver or in terms of the channel impulse response. Channel estimation is basically estimating the channel impulse response at the receiver. Broadly classifying the channel estimation methods, one is the Training sequence based method and the second is the blind method [3]. Channel estimation is essential for removing ISI and noise, and is used in diversity combining, Maximum Likelihood (ML) detection, angle of arrival estimation, etc. Cyclic Prefix/Pilot is basically a reference carrier/tone or reference signal/symbol that is known at the receiver end in terms of position of sequence/pattern and used for the channel estimation because it provides channel state information as it has undergone the most recent channel behavior. It acts as a guard interval in time domain to eliminate the effect of multipath delay spread. A different approach using superimposed pilots has been proposed in [4] where performance of a channel estimator is analyzed for 32 carriers.it has applied the PSK modulation technique. The use of cyclic prefix in the proposed algorithm analyses the channel estimator performance for 256 carriers. The design constraints and data flow are discussed in the next section. 3. DATA FLOW Work done in past, has been mostly the analysis of the SNR and BER on the channel estimation technique applied that too for pilot schemes.The logic applied for estimation using the cyclic prefix than pilot symbols is better in approach, because Cyclic prefix contains a part of the data in the sent frame, thus converting a linear convolutive channel to a circular convolutive channel ,hence improving the the correlator performance.