Effects of Fixed Point FFT Implementation of Wireless LAN

Abstract

With the rapid growth of digital wireless communication in recent years, the need for high
speed mobile data transmission has increased. New modulation techniques are being
implemented to keep with the desire more communication capacity. Processing power has
increased to a point where orthogonal frequency division multiplexing (OFDM) has become
feasible and economical. Since many wireless communication systems being developed use
OFDM, it is a worthwhile research topic. Some examples of applications using OFDM
include Digital subscriber line (DSL), Digital Audio Broadcasting (DAB), High definition
television (HDTV) broadcasting, IEEE 802.11 (wireless networking standard).OFDM is a
strong candidate and has been suggested or standardized in high speed communication
systems.
This thesis analyzes the factor that affects the OFDM performance. The performance
of OFDM was assessed by using computer simulations performed using Matlab.it was
simulated under Additive white Gaussian noise (AWGN) channel conditions for different
modulation schemes like binary phase shift keying (BPSK), Quadrature phase shift keying
(QPSK), 16-Quadrature amplitude modulation (16-QAM), 64-Quadrature amplitude
modulation (64-QAM) which are used in wireless LAN for achieving high data rates.
One key component in OFDM based systems is inverse fast Fourier transform/fast
Fourier transform (IFFT/FFT) computation, which performs the efficient
modulation/demodulation. This block consumes large resources in terms of computational
power.this thesis analyzes, different IFFT/FFT implementation on performance of OFDM
communication system. Here 64-point IFFT/ FFT is used. FFT is a complex function whose
computational accuracy, hardware size and processing speed depend on the type of arithmetic
format used to implement it. Due to non-linearity of FFT its computational accuracy is not
easy to calculate theoretically. The simulation carried out here, measure the effects of fixed
point FFT on the performance of OFDM. Comparison has been made between bit error rate
of OFDM using fixed point IFFT/FFT and a floating point IFFT/FFT. Simulation tests were
made for different integer part lengths, fractional part lengths by limiting the input word
lengths to 16 bits and found the suitable combination of integer part lengths and fractional
part lengths which can achieve the best bit error rate (BER) performance with respect to
floating point performance. Extensive computer simulations show that fixed point
computation provides very near result as floating point if the delay parameter is suitably
selected.