1. EE359 – Lecture 19 Outline Announcements Final Exam Announcements HW 8 (last HW) due Sunday 5pm (no late HWs) Bonus lecture today 6-8pm (pizza/cake); Hewlett 103 10 bonus points for course evaluations online Projects due end of this week Introduction to Spread Spectrum Direct Sequence Spread Spectrum ISI and Inteference Rejection Spreading Codes and Maximal Linear Codes Synchronization RAKE Receivers Multiuser Spread Spectrum

2. Final Exam Announcements Final Wed., 12/14, 8:30-11:30, Gates B12 (here) Covers Chapters 9, 10, 12, 13.1-13.2 (+ earlier chps) Similar format to MT, but longer: open book, notes. Practice finals posted by Wed (10 bonus points) Turn in to Pat or Nima for solns, by exam for bonus pts Bonus Lecture (Course review; advanced topics) today 6-8pm in Hewlett 103. Review Session: Thu, Fri, Sun, or Mon? Extra OHs in advance of the final Me: 12/12 and 12/13 11:30-12:30 and by appt. Nima: 12/12 and 12/13 5-6pm.

3. Review of Last Lecture FFT Implementation of OFDM Design Issues PAPR, frequency offset, fading, complexity MIMO-OFDM v x cos(2  f c t) R bps QAM Modulator Serial To Parallel Converter IFFT X0X0 X N-1 x0x0 x N-1 Add cyclic prefix and Parallel To Serial Convert D/A TX x cos(2  f c t) R bps QAM Modulator FFT Y0Y0 Y N-1 y0y0 y N-1 Remove cyclic prefix and Serial to Parallel Convert A/D LPF Parallel To Serial Convert RX

4. Intro. to Spread Spectrum Modulation that increases signal BW Mitigates or coherently combines ISI Mitigates narrowband interference/jamming Hides signal below noise (DSSS) or makes it hard to track (FH) Also used as a multiple access technique Two types Frequency Hopping: l Narrowband signal hopped over wide bandwidth Direction Sequence: l Modulated signal multiplied by faster chip sequence

5. Direct Sequence Spread Spectrum Bit sequence modulated by chip sequence Spreads bandwidth by large factor (G) Despread by multiplying by s c (t) again (s c (t)=1) Mitigates ISI and narrowband interference s(t) s c (t) T b =KT c Tc Tc S(f) S c (f) 1/ T b 1/ T c S(f) * S c (f) 2

6. ISI and Interference Rejection Narrowband Interference Rejection (1/K) Multipath Rejection (Autocorrelation  S(f) I(f) S(f) * S c (f) Info. Signal Receiver Input Despread Signal I(f) * S c (f) S(f)  S(f) S(f) * S c (f)[  (t)+  (t-  )] Info. Signal Receiver Input Despread Signal  S ’ (f)

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

8. Synchronization Adjusts delay of s c (t-  ) to hit peak value of autocorrelation. Typically synchronize to LOS component Complicated by noise, interference, and MP Synchronization offset of  t leads to signal attenuation by  (  t) 1 2 n -1 T c -T c tt  t)

9. RAKE Receiver Multibranch receiver Branches synchronized to different MP components These components can be coherently combined Use SC, MRC, or EGC x x s c (t) s c (t-iT c ) x s c (t-NT c ) Demod y(t) Diversity Combiner dkdk ^

10. Multiuser DSSS Each user assigned a unique spreading code; transmit simultaneously over same bandwidth Interference between users mitigated by code cross correlation In downlink, signal and interference have same received power In uplink, “close” users drown out “far” users (  1 >>  2 : near-far problem)    

11. Main Points Spread spectrum increases signal bandwidth above that required for information transmission Benefits include ISI and interference rejection, multiuser technique DSSS rejects ISI by code autocorrelation Maximal linear codes have good autocorrelation properties but poor cross correlation Synchronization depends on autocorrelation properties of spreading code. RAKE receivers combine energy of all MP Use same diversity combining techniques as before