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## OFDM Based WLAN System

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**OFDM Based WLAN System**Song Ziqi Zhang Zhuo**Wireless Channel**• Wireless channel is always unpredictable • Multipath reception is the unique characteristic of wireless channels • impairments present at the channel**What is Multipath**• More than one transmission path between transmitter and receiver • Received signal is the sum of many versions of the transmitted signal with varying delay and attenuation**Effect of Multipath on Received Part**• Delay Spread leads to ISI (Inter Symbol Interference) • Received signal at any time depends on a number of transmitted bits • Needs equalizer to recover data • Data Rates increase: Even WORSE!!**Multi-Carrier System**• Single carrier system • signal representing each bit uses all of the available spectrum • Multicarrier system • available spectrum divided into many narrow bands • data is divided into parallel data streams each transmitted on a separate band**What is OFDM?**• OFDM is a multicarrier system • uses discrete Fourier • Transform/Fast Fourier • Transform (DFT/FFT) • sin(x)/x spectra for subcarriers • Available bandwidth is divided into very many narrow bands • Data is transmitted in parallel on these bands**Why is OFDM?**• Most broadband systems are subject to multipath transmission • Conventional solution to multipath is an equalizer in the receiver • high data rates - equalizers too complicated • With OFDM there is a simple way of dealing with multipath • relatively simple DSP algorithms**How does OFDM solve Multipath Problem**• Data is transmitted in parallel • longer symbol period • e.g. for N parallel streams, symbol period is N times as long • Cyclic prefix • trick to avoid residual ISI**Transmitted in Parallel Without Interference**• Each subcarrier has a different frequency • Frequencies chosen so that an integral number of cycles in a symbol period • Signals are mathematically orthogonal**Disadvantages of OFDM**• Strict Synchronization Requirement • Peak-to-Average Power Ratio(PAPR)**What shall we achieve?**• Simple OFDM model including transmitter and receiver • Input random data with several modulation schemes for testing • Channel model • Demodulate the output data then compare it with the input random data**The OFDM Model**• At the transmitter • At the receiver**Channel Model**• Assumed to have a finite-length impulse response • Including Additive White Gaussian Noise**OFDM Model Design Parameters**• Number of subcarriers: 64 • CP length:8 • Subcarrier frequency spacing: sample rate/64 • Modulation type per subcarrier: QPSK • Number of multipath: 5 • SNR of the channel: 30DB**More**• BPSK and QAM modulation • Channel Estimation • Gain and phase compensation