11-10-2016, 10:51 AM
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INTRODUCTION
OFDM is a multi-carrier system where data bitsB are encoded to multiple sub-carriers.
OFDM offers several advantages over single carrier system like better multi-path effect immunity, simpler channel equalization and relaxed timing acquisition constraints.
The frequencies used in OFDM system are orthogonal.
The OFDM is therefore able to provide higher data rate for the same BW.
OFDM TRANSCEIVER
• Each sub-carrier in an OFDM system is modulated in amplitude and phase by the data bits.
• Modulation techniques typically used are BPSK, QPSK, 16QAM, 64QAM etC
• The process of combining different sub-carriers to form a composite time-domain signal is achieved using Fast Fourier transform.
VLSI implementation
• Following are the advantages of this approach:
Lower gate count compared to DSP+RAM+ROM, hence lower cost.
• Low power consumption.
VLSI Implementation
Design Methodology
• Since most of these algorithms are coded in "C" or tools like Matlab, it is important to have a verification mechanism which ensures that the hardware implementation (RTL) is same as the "C" implementation of the algorithm.
Algorithm survey & simulation
• The algorithms are tweaked and fine tuned to get the required performance.
• The following algorithms/parameters are verified:
• Channel estimation and compensation for different channel models (Rayleigh, Rician, JTC, Two ray) for different delay spreads.
• Correlator performance for different delay spreads and different SNR.
• Frequency estimation algorithm for different SNR and frequency offsets.
• System tolerance for I/Q phase and amplitude imbalance.
• FFT simulation to determine the optimum fixed-point widths.
Hardware design
• Baseband interfaces with two external modules: MAC and Radio.
. Interface to MAC.
• Interface to Radio.
CONCLUSION
• Different algorithms implemented in OFDM modem are identified.
• It was found during the algorithm design that many blocks need complex multipliers and adders and therefore special attention needs to be given to optimize these circuits and maximize reusability.