1. EE359 – Lecture 18 Outline Announcements Review of Last Lecture
Last HW posted today, due Friday 12/9 at noon.
Final Exam is Wed., 12/14, 9:30-11:30, in MCCULL115, Final Review most likely 12/12, time/room TBD. More details Monday.
OHs today are 4-5, extra Ohs next week.
Review of Last Lecture
FFT Implementation of OFDM (DMT)
Challenges in OFDM
Overview of Spread Spectrum
Direct Sequence Spread Spectrum
ISI and Interference Rejection
PN Sequences and Maximal Linear Codes

2. Review of Last Lecture S ISI Countermeasures: Equalizers, MCM, SS
Multicarrier Mod.: B/N=BN=(1+b)/TN <<Bc
Can overlap subcarriers (separation of 1/TN)
Correction: This is twice as efficient for b=1 (not b=0)
For fading across subcarriers, equalization doesn’t help, can precode or do adaptive loading
Adaptive loading same as in flat-fading and MIMO
R/N bps
QAM
Modulator x
cos(2pf0t)
cos(2pfNt) S
R bps
Serial To
Parallel
Converter
R/N bps
QAM
Modulator f0
fN-1
2BN

3. FFT Implementation (DMT)
Efficient IFFT structure at transmitter
Reverse structure (with FFT) at receiver
IFFT performs frequency modulation
Cyclic prefix turns makes linear convolution circular: removes ISI between blocks
Received symbols scaled versions of transmitted symbols 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

4. 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

5. Pseudorandom Sequences
Autocorrelation determines ISI rejection
Ideally equals delta function
Maximal Linear Codes
No DC component
Large period (2n-1)Tc
Linear autocorrelation
Recorrelates every period
Short code for acquisition, longer for transmission
In SS receiver, autocorrelation taken over Tb
Poor cross correlation (bad for MAC) 1 -1
2n-1 Tc
-Tc

6. Direct Sequence Spread Spectrum
Bit sequence modulated by chip sequence
Spreads bandwidth by large factor (K)
Despread by multiplying by sc(t) again (sc(t)=1)
Mitigates ISI and narrowband interference
S(f)
s(t)
sc(t)
Sc(f)
S(f)*Sc(f)
1/Tb
1/Tc Tc
Tb=KTc 2

7. ISI and Interference Rejection
Narrowband Interference Rejection
Multipath Rejection (Two Path Model)
S(f)
I(f)
S(f)*Sc(f)
Info. Signal
Receiver Input
Despread Signal
I(f)*Sc(f)
aS(f)
S(f)
S(f)*Sc(f)[ad(t)+b(t-t)]
bS’(f)
Info. Signal
Receiver Input
Despread Signal

8. Main Points OFDM efficiently implemented using FFTs
OFDM challenges are PAR and timing/frequency offset.
Spread spectrum spreads signal over wide bandwidth for ISI/interference rejection
DSSS rejects interference by spreading gain
DSSS rejects ISI by code autocorrelation
Maximal linear codes have good autocorrelation properties but poor cross correlation