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EE359 – Lecture 18 Outline

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- Review of Last Lecture
- Multicarrier Modulation
- Overlapping Substreams
- OFDM FFT Implementation
- OFDM Design Issues

- Decision-Feedback Decoders
- Linear ML receiver except for error propagation

- Sphere Decoder (near ML):
- Only searches within a sphere of received symbol.

- Other MIMO design issues
- Space-Time Codes, Adaptive techniques, Limited FB

- ISI Countermeasures
- Equalization, multicarrier (OFDM), spread spectrum

S

cos(2pf0t)

cos(2pfNt)

x

x

R/N bps

- 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)

QAM

Modulator

R bps

Serial

To

Parallel

Converter

R/N bps

QAM

Modulator

- 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

cos(2pfct)

cos(2pfct)

LPF

A/D

D/A

Serial

To

Parallel

Converter

x

x

- 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

- Timing/frequency offset:
- Impacts subcarrier orthogonality; self-interference

- Peak-to-Average Power Ratio (PAPR)
- Adding subcarrier signals creates large signal peaks

- MIMO/OFDM
- Apply OFDM across each spatial dimension
- Can adapt across space, time, and frequency

- Different fading across subcarriers
- Mitigate by precoding (fading inversion), coding across subcarriers, and adaptative loading over time

- MCM splits channel into NB flat fading subchannels
- Can overlap subcarriers to preserve bandwidth
- OFDM efficiently implemented using IFFTs/FFTs
- Block size depends on data rate relative to delay spread

- OFDM challenges: PAPR; timing/frequency offset, MIMO
- Subcarrier fading degrades OFDM performance
- Compensate through precoding (channel inversion), coding across subcarriers, or adaptation

- 4G Cellular, Wimax, 802.11n all use OFDM+MIMO
- Adapt across space, time, and frequency