Implementing WiMax OFDM Timing and Frequency Offset Estimation

A Lattice Semiconductor White Paper

Introduction

Orthogonal Frequency Division Multiplexing (OFDM) is the basis of some WLAN and

WiMax air interfaces. It also has been proposed for use in next generation cellular

systems such as Super-3G and HSOPA (High Speed OFDM Packet Access) in the

3GPP standards group. OFDM has a number of advantageous features, such as good

tolerance to multi-path fading and inter-symbol interference (ISI). By using a number of

sub-carriers, the symbol length can be kept long and a guard period (the cyclic prefix)

used to mitigate ISI. Data is allocated to a number of sub-carriers so that any nulls in

the frequency domain do not knock out a whole allocation. This gives forward error

correction (FEC) a greater chance to recover the data in the receiver.

These advantages do come at a cost. First, the orthogonal modulated carriers need to

be generated. Fortunately, the Inverse Fast Fourier Transform (IFFT) algorithm can be

used to convert suitably constructed frequency domain signals into the required time

domain waveform. Equally, the receiver can use the FFT algorithm to convert back to

the frequency domain before de-modulation. These algorithms are efficient and can be

implemented in either software in a DSP or, for higher bandwidth and increased

processing capacity, in a DSP-enabled FPGA, such as the LatticeECP device. Another

cost associated with OFDM is the use of FFTs and the fact that the receiver has no

prior knowledge of either the symbol timing or exact frequency of the local oscillator at

the transmitter end.

Any time offset between the start of the orthogonal waveforms and the first sample

point used in the FFT will affect the result of a Fourier Transform. Also, any frequency

offset between transmitter and receiver, if not corrected, will result in a blurring of the