EE359

1
EE359 Lecture 17 Outline

• Announcements
• OHs today moved to 130-230
• Lecture Wednesday will be 1130-1245 (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

2
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

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
Multicarrier Modulation
R/N bps
QAM Modulator
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/NltBc implies flat fading on each subcarrier (no
  ISI)

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/(1b).
• Total required bandwidth is B/2 (for TN1/BN)

B/N
f0
fN-1
6
Fading Across Subcarriers

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

7
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
Add cyclic prefix and Parallel To Serial Convert
R bps
QAM Modulator
IFFT
XN-1
xN-1
8
OFDM Challenges

• Peak-to-average power ration
• 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

9
Main Points

• ISI can be mitigated through equalization,
  multicarrier modulation, or spread spectrum
• Today, equalizers often too complex or cant
  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