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Multicarrier Modulation

Multicarrier Modulation. Outline. Introduction to ISI Countermeasures Multicarrier Modulation Overlapping Substreams Fading across subcarriers OFDM FFT Implementation DFT analysis TX and RX design OFDM and OFDM-MIMO Design Issues. ISI Countermeasures. Equalization

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Multicarrier Modulation

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  1. MulticarrierModulation

  2. Outline • Introduction to ISI Countermeasures • Multicarrier Modulation • Overlapping Substreams • Fading across subcarriers • OFDM FFT Implementation • DFT analysis • TX and RX design • OFDM and OFDM-MIMO Design Issues

  3. ISI Countermeasures • Equalization • Signal processing at receiver to eliminate ISI, must balance ISI removal with noise enhancement • Can be very complex at high data rates, and performs poorly in fast-changing channels • Not that common in state-of-the-art wireless systems • Multicarrier Modulation • Break data stream into lower-rate substreams modulated onto narrowband flat-fading subchannels • Spread spectrum • Superimpose a fast (wideband) spreading sequence on top of data sequence, allows resolution for combining or attenuation of multipath components.

  4. S cos(2pf0t) cos(2pfNt) x x Multicarrier Modulation • Breaks data into N substreams • Substream modulated onto separate carriers • Substream bandwidth is B/N for B total bandwidth • B/N<Bc implies flat fading on each subcarrier (no ISI) R/N bps QAM Modulator R bps Serial To Parallel Converter R/N bps QAM Modulator

  5. Overlapping Substreams • Can have completely separate subchannels • Required passband bandwidth is B. • OFDM overlaps substreams • Substreams (symbol time TN) separated in RX • Minimum substream separation is BN/(1+b). • Total required bandwidth is B/2 (for TN=1/BN) B/N fN-1 f0

  6. Fading Across Subcarriers • Leads to different BERS • Compensation techniques • Frequency equalization (noise enhancement) • Precoding • Coding across subcarriers • Adaptive loading (power and rate)

  7. cos(2pfct) cos(2pfct) LPF A/D D/A Serial To Parallel Converter x x FFT Implementation of OFDM • Use IFFT at TX to modulate symbols on each subcarrier • Cyclic prefix makes linear convolution of channel circular, so no interference between FFT blocks in RX processing • Reverse structure (with FFT) at receiver X0 x0 TX Add cyclic prefix and Parallel To Serial Convert R bps QAM Modulator IFFT XN-1 xN-1 RX Y0 y0 Remove cyclic prefix and Serial to Parallel Convert R bps QAM Modulator Parallel To Serial Convert FFT yN-1 YN-1

  8. OFDM Design Issues • Timing/frequency offset: • Impacts subcarrier orthogonality; self-interference • Peak-to-Average Power Ratio (PAPR) • Adding subcarrier signals creates large signal peaks • Different fading across subcarriers • Same mitigation techniques as in MCM: Precoding to invert fading, coding across subcarriers, and adaptative loading over time most common • MIMO/OFDM • Apply OFDM across each spatial dimension • Can adapt across space, time, and frequency

  9. Main Points • ISI can be mitigated through equalization, multicarrier modulation (MCM) or spread spectrum • Today, equalizers often too complex or can’t track channel. • MCM splits channel into NB flat fading subchannels • Fading across subcarriers degrades performance. • Compensate through precoding (channel inversion), coding across subcarriers, or adaptation

  10. Main Points • OFDM efficiently implemented using IFFTs/FFTs • Block size depends on data rate relative to delay spread • OFDM challenges: • PAPR; timing/frequency offset; fading across subcarriers • 4G Cellular, Wimax, 802.11n all use OFDM+MIMO • OFDM removes ISI, MIMO gives diversity/multiplexing • Adapt across space, time, and frequency

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