1. 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

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 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.

4. 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)

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 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
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

8. 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

9. 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