International Journal of Computer Applications (0975 – 8887) Volume 127 – No.6, October 2015 6 Implementation of OFDM Transceiver using GMSK and QPSK Encoding Technique Gunjan Negi Student, ECE GRD Institute of Management and Technology Dehradun, India Arun Kumar Assistant Professor, ECE GRD Institute of Management and Technology Dehradun, India Anuj Saxena Information Technology Karnataka State University Karnataka, India ABSTRACT OFDM (Orthogonal Frequency Division Multiplexing) transmissions are arising as important modulation techniques because of its robustness against interference. Various modulation schemes have been used to implement OFDM. In this paper, the OFDM transceiver system is implemented using MATLAB. Gaussian Minimum Shift Keying modulation technique has been implemented in the proposed OFDM. The bit error rate (BER) performance has been evaluated in AWGN (Additive White Gaussian Noise) channel and is compared to Quadrature Phase Shift Keying modulation scheme. General Terms Bit error rate, Matlab, Additive White Gaussian Noise. Keywords Orthogonal Frequency Division Multiplexing, Gaussian Minimum Shift Keying, Quadrature Phase Shift Keying 1. INTRODUCTION OFDM can be implemented by using various modulation techniques. Researchers have proposed various modulation techniques like QPSK, QAM, and MSK etc. This paper demonstrates the simulation of an OFDM transceiver using Gaussian Minimum Shift Keying modulation technique and Quadrature Phase Shift Keying technique. The paper is organized as follows. Section II gives the implementation of the OFDM transceiver with OFDM system requirements and specifications. Section III gives the experimental results of system evaluation in term of simulation environment. Section IV includes conclusions of OFDM implementation. 1.1 System Design The general structure of OFDM transceiver system using Matlab simulation is illustrated in figure 1. Figure 1: Block Diagram of OFDM Transceiver 1.1.1 Transmitter In an OFDM system, the data block of N symbols, denoted by X=[X 0, X 1, X 3 , XN -1 ] T , is modulated by a set of orthogonal sub- carrier, f k , {k=0, 1, 2, N-1}, where T denotes the transpose. In OFDM system to sustain the orthogonality of the signals, the spacing Δf between neighboring subcarriers is set to be a multiple of 1/T. i.e., Δf=m1/T, where T is the duration of an OFDM symbol, then the transmitted OFDM symbol is given by x(t)=1/N    j2πft 0<=t>=T (1) The Input image is converted to source data and is passed through the encoder. Convolution codes are used to encode the data. Then the signal is passed through interleaver.Interleaving increases the resistance to fading. Binary to decimal convertor converts the binary vector to decimal number. 1.1.1.1 Modulator (a) GMSK modulator The signal is then passed through modulator (GMSK). Gaussian Minimum Phase Shift Keying is a continuous phase modulation scheme which is generated by filtering NRZ data with a Gaussian shaping filter. The GMSK modulated waveform is characterized by the BT product. Where, B is the bandwidth of the Gaussian filter and T is the duration of the data bit. The resulting signal is represented by S(t) = a I (t) cos(πt/2T) cos(2πf c t) – a Q (t)sin(2πf c t) (2) Where, a I (t) and a Q (t) are the even and odd information respectively. a I (t) has pulse edges on t= {-T,T,3T..} and a Q (t) on t={0,2T,4T…..}. The carrier frequency is f c . This equation can be rewritten in a form of phase and frequency modulation, S(t) = cos[2πf c t + b k (t)πt/2T + ᴓ k ] (3) where b k (t) is +1 when a I (t)=a Q (t) and -1 if they are of opposite signs, and is 0 if a I (t) is 1, and otherwise. Therefore, the signal is modulated in frequency and phase, and the phase changes continuously and linearly. (b)QPSK modulator In QPSK modulation, the input binary data is first converted to bipolar NRZ type of signal. The demultiplexer divides the signal into two separate bit streams. Here, b e (t) represents even numbered sequence and b o (t) represents odd numbered sequence. The bit stream b e (t) modulates carrier √P s cos( 2πf c t)