EE359 – Lecture 17 Outline Announcements Review of Last Lecture OHs today moved to 1:30-2:30 Lecture Wednesday will be 11:30-12:45 (30 minute shift) Will have project comments this week Review of Last Lecture ISI Countermeasures Multicarrier Modulation Overlapping Substreams Fading Across Subcarriers FFT Implementation OFDM Challenges

Review of Last Lecture MIMO Systems With perfect channel estimates at TX and RX, decomposes into r independent channels r-fold capacity increase over SISO system Demodulation complexity reduction Can also use antennas for diversity (beamforming) Leads to capacity versus diversity tradeoff in MIMO

ISI Countermeasures Equalization Multicarrier Modulation 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.

Multicarrier Modulation R/N bps QAM Modulator x cos(2pf0t) cos(2pfNt) S R bps Serial To Parallel Converter R/N bps QAM Modulator 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)

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 f0 fN-1

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

FFT Implementation Efficient IFFT structure at transmitter Reverse structure (with FFT) at receiver Cyclic prefix makes linear convolution circular, so there is no interference between input blocks ISI can be removed, but SNR/subchannel is the same Subcarrier orthogonality must be preserved Compromised by timing/frequency offset and fading. X0 x0 Serial To Parallel Converter R bps Add cyclic prefix and Parallel To Serial Convert QAM Modulator x cos(2pfct) D/A IFFT XN-1 xN-1

OFDM Challenges Peak-to-average power ration Intercarrier Interference Adding multiple substreams can result in high peak signal values Impacts amplifier efficiency Solutions include clipping and coding Intercarrier Interference Frequency and timing offset causes interference between carriers Mitigated by minimizing the number of subcarriers and non-rectangular pulse shaping

Main Points ISI can be mitigated through equalization, multicarrier modulation, or spread spectrum Today, equalizers often too complex or can’t track channel. Multicarrier is alternative to equalization Splits data stream into NB flat fading substreams Can overlap subcarriers Fading across subcarriers degrades performance. Compensate through coding or adaptation OFDM efficiently implemented using FFTs OFDM challenges include PAR and timing/frequency offset