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Computationally Simple OFDM Todor Cooklev IPFW Wireless Technology Center cooklevt@ipfw.edu. Introduction Simple OFDM Preliminary Results Conclusions and directions for future research. Presentation Overview. Conventional OFDM.
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Computationally Simple OFDM Todor Cooklev IPFW Wireless Technology Center cooklevt@ipfw.edu
Introduction Simple OFDM Preliminary Results Conclusions and directions for future research Presentation Overview
Conventional OFDM Conventional OFDM is based on the discrete Fourier transform; IFFT performs multicarrier modulation
Simple OFDM idea • Can we simplify the DFT/FFT operations? • What would be the system performance • Can we simply the algorithms further? • Make a detailed comparison
The DFT, implemented by the Fast Fourier Transform algorithm (FFT), can be simplified via the rounded sine and cosine functions Any number of quantization levels can be used 5-level: -1, -0.5, 0, 0.5, 1 9 level: add -0.75, -0.25, 0.25, 0.75 to the above Rounded DFT idea
Definition of standard DFT: Using the relationship: the rounded DFT is derived with substitution as follows: Simple OFDM k = 0,1,…,N – 1 k = 0,1,…,N – 1
Two possible inverses • Approximate - the conjugate transpose • Exact – possible, but in general is not multiplierless
Simulation • Models of conventional OFDM were created to compare with standard OFDM performance characteristics • Many simulations have be run using the following key parameters: • 64 parallel data channels (64 point DFT/RDFT) • QPSK and 16 QAM modulation schemes • Quantization levels of 5, 9 and 17 • Pairing of IFFT with RDFT
Preliminary Results Comparison of conventional OFDM bit error rate with 5 level rounded system
Preliminary Results Comparison of conventional OFDM bit error rate with 9 level rounded system
Preliminary Results Comparison of conventional OFDM bit error rate with 17 level rounded system
Preliminary Results Comparison of conventional OFDM bit error rate with 5 level RDFT paired with standard IFFT
Benefits • Currently ASICs are used to implement OFDM • A dedicated OFDM chip (ASIC) contains multiple FFTs • Reduction in chip area • Make possible reconfigurable hardware implementation (FPGA) or even an entirely software implementation (on a DSP or GPP)
What we have achieved • Built a software simulation of the new idea • The rounded transform leads to viable OFDM communication systems at specific quantization levels • With performance that approaches conventional OFDM systems • With reduced computational complexity
Future Research • Investigate system performance when faced with carrier offset error • Investigate system performance when faced with inter-symbol interference (short cyclic prefix) • Apply the results to systems with multiple antennas • Develop new and simplified algorithms • Investigate the implementation in greater detail