EE359 – Lecture 18 Outline

Announcements last HW posted, due Thurs 12/4 at 5pm (no late HWs) Last regular class lecture, Monday 12/1, 9:30-10:45 (as

usual) Final info (coverage, format, extra OHs, etc) given in 12/1

lecture End-of-Quarter bonus lecture + course summary: 12/1, 4-

6pm here Final exam 12/8, 8:30am-11:30am in Thornton102 Final projects must be posted 12/6

Review of Last Lecture

Design issues in OFDM

Introduction to Spread Spectrum

ISI and Interference Rejection

Review of Last Lecture

Multicarrier Modulation: breaks data into N substreams (B/N<Bc); Substream modulated onto separate carriers

Overlapping substreamsMinimum substream separation is BN=1/TN

DFT ReviewUse cyclic prefix to make linear convolution circular

x

cos(2f0t)

x

cos(2fNt)

R bpsR/N bps

R/N bps

QAMModulator

QAMModulator

Serial To

ParallelConverter

Review Cont’dFFT Implementation of

OFDMUse IFFT at TX to modulate symbols on

each subcarrierCyclic prefix makes linear convolution of

channel circular, so no interference between FFT blocks in RX processing

Reverse structure (with FFT) at receiver

x

cos(2fct)

R bpsQAM

Modulator

Serial To

ParallelConverter

IFFT

X0

XN-1

x0

xN-1

Add cyclicprefix and

ParallelTo SerialConvert

D/A

TX

x

cos(2fct)

R bpsQAMModulatorFFT

Y0

YN-1

y0

yN-1

Remove cyclic

prefix andSerial toParallelConvert

A/DLPFParallel

To SerialConvert

RX

Yi=HiXi+ni

h(t) +

n(t)

OFDM Design Issues Timing/frequency offset:

Impacts subcarrier orthogonality; self-interference

Peak-to-Average Power Ratio (PAPR)Adding subcarrier signals creates large signal peaksSolve with clipping or PAPR-optimized coding

Different fading across subcarriersMitigate by precoding (fading inversion), adaptive

modulation over frequency, and coding across subcarriers

MIMO/OFDMApply OFDM across each spatial dimensionCan adapt across space, time, and frequencyMIMO-OFDM represented by a matrix, extends matrix

representation of OFDM alone (considered in HW)

Intro. to Spread Spectrum

Modulation that increases signal BWMitigates or coherently combines ISI Mitigates narrowband

interference/jammingHides signal below noise (DSSS) or makes

it hard to track (FH)Also used as a multiple access technique

Two typesFrequency Hopping:

Narrowband signal hopped over wide bandwidth

Direction Sequence: Modulated signal multiplied by faster chip

sequence

Direct Sequence Spread Spectrum

Bit sequence modulated by chip sequence

Spreads bandwidth by large factor (G) Despread by multiplying by sc(t) again

(sc(t)=1)

Mitigates ISI and narrowband interference

s(t) sc(t)

Tb=KTc Tc

S(f)Sc(f)

1/Tb 1/Tc

S(f)*Sc(f)

2

ISI and Interference Rejection

Narrowband Interference Rejection (1/K)

Multipath Rejection (Autocorrelation

S(f) S(f)I(f)S(f)*Sc(f)

Info. Signal Receiver Input Despread Signal

I(f)*Sc(f)

S(f) S(f)S(f)*Sc(f)[(t)+(t-)]

Info. Signal Receiver Input Despread Signal

S’(f)

Main Points OFDM challenges: timing/frequency offset, PAPR Subcarrier fading degrades OFDM performance

Compensate through precoding (channel inversion), coding across subcarriers, or adaptation

OFDM naturally combined with MIMOOrthogonal in space/freq; extended matrix

representation 4G Cellular and 802.11n/ac all use OFDM+MIMO

Spread spectrum increases signal bandwidth above that required for information transmission

Benefits of spread spectrum: ISI and narrowband interference rejectionAlso used as a multiuser/multiple access technique