19-04-2012, 11:38 AM
Effects of Fixed Point FFT on the Performance of OFDM in Wireless LAN
[attachment=20337]
INTRODUCTION
With the rapid growth of digital wireless communication in recent
years, the need for high speed mobile data transmission has
increased. Many wireless communication systems being
developed use OFDM to achieve high data rates. OFDM is a
strong candidate and has been suggested or standardized in high
speed communication Systems like WLAN [1]. OFDM technique
has taken a long time to be of prominence practically. The
hardware solution, which makes use of multiple modulators and
demodulators, was somewhat impractical for use in the civil
systems. The ability to define the signal in the frequency domain,
in software on VLSI (very large scale integration) processors, and
to generate the signal using the inverse Fourier transform is the
key to its current popularity.
OFDM systems are better than single-carrier systems in multi-path
fading channel environment [2]. OFDM is used in many high data
rate transmission systems, for example, digital video broadcasting
(DVB), IEEE 802.11, IEEE 802.16, HIPERLAN Type II, many
derivatives of digital subscriber line (DSL) and Home networking
etc [3, 4]. It is projected that OFDM systems are the strongest
candidate for 4G systems.
This paper investigates the effects of finite word length in FFT
implementation for WLAN application. More particularly the
investigation shows that performance of WLAN using floating
point computation can be achieved by using fixed point
implementation, if number of bits used for fraction part is selected
suitably.
The paper is organized as follows. Following the introduction, the
fundamental concepts of OFDM and principles behind OFDM is
outlined in Section 2. Section 3 discusses on WLAN standards.
Section 4 and 5 discusses the Floating-point and the Fixed-point
representation respectively. Section 6 is dedicated for discussion
on experimental results. Finally, section 7 provides the concluding
remarks.
OFDM
OFDM is the most popular scheme now for higher bit rate
applications. This has built in orthogonality and works on basis of
simple frequency analysis. Spectrum utilization in OFDM is
much higher [5]. Guard band protects interferences. OFDM is
implemented using fast computation of FFT. The incoming serial
data is first converted form serial to parallel and grouped into ‘x’
bits each to form a complex number. The number ‘x’ determines
the signal constellation of the corresponding sub carrier, such as
BPSK, QPSK, 16 QAM, 64QAM, and 256 QAM. The complex
numbers are modulated in the base band by the inverse FFT
(IFFT) and converted back to serial data for transmission. The
receiver performs the inverse process of the transmitter [6].
WIRELESS LAN STANDARDS
The Institute of Electrical and Electronics Engineers (IEEE) has
established a committee to setup standards for Local Area and
Metropolitan Area Networking named the “802 LMSC” (LAN
MAN Standards Committee). Within this committee the 802.11
workgroup has task of developing the standards for wireless
networking [7]. Within this 802.11 workgroup, there are task
Permission to make digital or hard copies of all or part of this work for
personal or classroom use is granted without fee provided that copies are
not made or distributed for profit or commercial advantage and that
copies bear this notice and the full citation on the first page.
CONCLUSION
From the simulation studies conducted, it is seen that word length
of 8 bits is not sufficient to achieve desired BER performance
with respect to floating point performance for any combination of
integer part lengths and fractional part lengths. In case of input
word length of 16 bits, it is seen that fixed-point FFT provides
nearly similar performance to floating point FFT if the fraction
size parameter is suitably selected. Again it also seen that the
fraction part is of half the size of word length, the system provides
best performance.
[attachment=20337]
INTRODUCTION
With the rapid growth of digital wireless communication in recent
years, the need for high speed mobile data transmission has
increased. Many wireless communication systems being
developed use OFDM to achieve high data rates. OFDM is a
strong candidate and has been suggested or standardized in high
speed communication Systems like WLAN [1]. OFDM technique
has taken a long time to be of prominence practically. The
hardware solution, which makes use of multiple modulators and
demodulators, was somewhat impractical for use in the civil
systems. The ability to define the signal in the frequency domain,
in software on VLSI (very large scale integration) processors, and
to generate the signal using the inverse Fourier transform is the
key to its current popularity.
OFDM systems are better than single-carrier systems in multi-path
fading channel environment [2]. OFDM is used in many high data
rate transmission systems, for example, digital video broadcasting
(DVB), IEEE 802.11, IEEE 802.16, HIPERLAN Type II, many
derivatives of digital subscriber line (DSL) and Home networking
etc [3, 4]. It is projected that OFDM systems are the strongest
candidate for 4G systems.
This paper investigates the effects of finite word length in FFT
implementation for WLAN application. More particularly the
investigation shows that performance of WLAN using floating
point computation can be achieved by using fixed point
implementation, if number of bits used for fraction part is selected
suitably.
The paper is organized as follows. Following the introduction, the
fundamental concepts of OFDM and principles behind OFDM is
outlined in Section 2. Section 3 discusses on WLAN standards.
Section 4 and 5 discusses the Floating-point and the Fixed-point
representation respectively. Section 6 is dedicated for discussion
on experimental results. Finally, section 7 provides the concluding
remarks.
OFDM
OFDM is the most popular scheme now for higher bit rate
applications. This has built in orthogonality and works on basis of
simple frequency analysis. Spectrum utilization in OFDM is
much higher [5]. Guard band protects interferences. OFDM is
implemented using fast computation of FFT. The incoming serial
data is first converted form serial to parallel and grouped into ‘x’
bits each to form a complex number. The number ‘x’ determines
the signal constellation of the corresponding sub carrier, such as
BPSK, QPSK, 16 QAM, 64QAM, and 256 QAM. The complex
numbers are modulated in the base band by the inverse FFT
(IFFT) and converted back to serial data for transmission. The
receiver performs the inverse process of the transmitter [6].
WIRELESS LAN STANDARDS
The Institute of Electrical and Electronics Engineers (IEEE) has
established a committee to setup standards for Local Area and
Metropolitan Area Networking named the “802 LMSC” (LAN
MAN Standards Committee). Within this committee the 802.11
workgroup has task of developing the standards for wireless
networking [7]. Within this 802.11 workgroup, there are task
Permission to make digital or hard copies of all or part of this work for
personal or classroom use is granted without fee provided that copies are
not made or distributed for profit or commercial advantage and that
copies bear this notice and the full citation on the first page.
CONCLUSION
From the simulation studies conducted, it is seen that word length
of 8 bits is not sufficient to achieve desired BER performance
with respect to floating point performance for any combination of
integer part lengths and fractional part lengths. In case of input
word length of 16 bits, it is seen that fixed-point FFT provides
nearly similar performance to floating point FFT if the fraction
size parameter is suitably selected. Again it also seen that the
fraction part is of half the size of word length, the system provides
best performance.