05-05-2012, 12:39 PM
Design and Implementation of Autocorrelation and CORDIC Algorithm for OFDM Based WLAN
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Introduction
As we all know that wireless LAN or WLAN is a wireless local area network where two or more
systems are linked to each other without using wires. To enable communication between the systems or
devices, WLAN takes help of spread spectrum technique or OFDM modulation technique based on
radio waves. The advantage one acquires with WLAN is the mobility to move around a certain
coverage area and still be connected to the network.
With the advent of WLAN in the late 70’s, advancements have been made progressively with
time by various research organizations keeping in mind by the protocol standards. The WLAN
hardware originally was expensive from the time of its advent that its primary use was that of an
alternative technology to wired LAN places where cabling was difficult or a mere impossibility.
Orthogonal Frequency Division Multiplexing
A combination of modulation and multiplexing constitutes to orthogonal frequency division
multiplexing in other words OFDM. Independent signals that are a sub-set of a main signal are
multiplexed in OFDM and also the signal itself is first split into independent channels, modulated by
data and then re-multiplexed to create the OFDM carrier. Orthogonality of the sub-carriers is the main
concept in OFDM. Keeping in mind the integral area under one period of a sine or a cosine wave is
zero and that the carriers are all sine/cosine wave, we can conclude that when we multiply a sinusoid of
frequency m by a sinusoid of frequency n, the area under the product constitutes to zero. Therefore, in
general for all integers’ n and m, sinmx, sinnx, cosmx and cosnx are all orthogonal to each other. These
frequencies are called harmonics. This property is the key to understanding OFDM [2]. The property of
orthogonality allows simultaneous transmission on a lot of sub-carriers in a tight frequency space
without interference from each other. This acts as an undue advantage in OFDM [3][4][5].
Coordinate Rotation Digital Computer (CORDIC)
CORDIC stands for COordinate Rotation DIgital Computer. CORDIC is a simple and efficient
algorithm to calculate hyperbolic and trigonometric functions. All of the trigonometric functions can be
computed or derived from functions using vector rotations. Vector rotation can also be used for polar
to rectangular and rectangular to polar conversions, for vector magnitude, and as a building block in
certain transforms such as the DFT and DCT [7][8]. The CORDIC algorithm provides an iterative
method of performing vector rotations by arbitrary angles using only shifts and adds. The algorithm,
credited to Volder [2] is derived from the general (Givens) rotation transform as shown in equations (i)
and (ii).
OFDM Top Level Design
the block diagram of the top level of OFDM [5]. We can see from the figure that the main
components of OFDM are transmitter and the receiver.
The main building blocks of the transmitter section are the quadrature phase shift keying
(QPSK) modulator, a serial to parallel converter block, an inverse fast Fourier transform (IFFT)
generator, a cyclic prefix block where 4 bits are added to the signal to avoid inter symbol interference
(ISI) and inter carrier interference (ICI) and a parallel to serial converter block. The receiver section
also has the same blocks but in reverse order and in place of a QPSK modulator and IFFT we have a
QPSK demodulator and an FFT generator.