EE359 Lecture 18 Outline Announcements HW due Fri; last HW posted, due Friday 12/9 at 4 pm (no late HWs) MIMO decoder supplemental handout posted Lectures net week are Monday 12/5 12-1:20 (Thornton 102 with lunch) and Friday 12/9 9:30-11:30 (here, rm 18, Huang, with donuts) Final info (coverage, format, etra OHs, etc) given in 12/5 lecture End-of-Quarter bonus lecture+course summary will be 12/9 lecture2 from 4-6pm (12-2pm is backup) with lunch or dinner. Final eam 12/15, 12:15pm-3:15pm Final projects must be posted 12/5 at midnight. Spread Spectrum Direct sequence (DSSS) ISI and Interference Rejection of DSSS Time and Frequency Domain Analysis Introduction to Multiuser Systems
Review of Last Lecture OFDM: Overlapping Subcarriers and FFT Implementation Overlapping subcarriers reduces BW by factor of 2 Use IFFT at TX to modulate symbols on each subcarrier Cyclic prefi makes linear convolution of channel circular, so no interference between FFT blocks in RX processing Reverse structure (with FFT) at receiver R bps QAM Modulator Serial To Parallel Converter X 0 X N-1 IFFT 0 Add cyclic prefi and Parallel To Serial N-1 Convert D/A TX cos(2pf c t) n(t) h(t) + cos(2pf c t) LPF A/D Remove cyclic prefi and Serial to Parallel Convert y 0 y N-1 FFT Y 0 Y N-1 Parallel To Serial Convert QAM Modulator Y i =H i X i +n i RX R bps
Review Continued OFDM Design Issues Timing/frequency offset: Impacts subcarrier orthogonality; self-interference Peak-to-Average Power Ratio (PAPR) Adding subcarrier signals creates large signal peaks Solve with clipping or PAPR-optimized coding Mitigation for fading across subcarriers Precoding (fading inversion): Used in DSL as there is minimal deep fades, not used in wireless systems Adaptive modulation: data rate (and power) adapted to subcarrier SNR. Used in LTE and 802.11a-g-n-ac Coding across subcarriers: bits are encoded into a block code of length N for N subcarriers. Each coded symbol is sent on a different subcarrier.
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: 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 s(t) s c (t) S c (f) S(f) S(f)*S c (f) 1/T b 1/T c T c T b =KT c Spreads bandwidth by large factor (G) 2 Despread by multiplying by s c (t) again (s c (t)=1) Mitigates ISI and narrowband interference
ISI and Interference Rejection Narrowband Interference Rejection (1/K) S(f) S(f) I(f) S(f)*S c (f) I(f)*S c (f) Info. Signal Receiver Input Despread Signal Multipath Rejection (Autocorrelation r(t)) as(f) S(f) S(f)*S c (f)[ad(t)+b(t-t)] brs (f) Info. Signal Receiver Input Despread Signal Can coherently combine all multipath components via a RAKE receiver
Multiuser Channels: Uplink and Downlink Uplink (Multiple Access Channel or MAC): Many Transmitters to One Receiver. Downlink (Broadcast Channel or BC): One Transmitter to Many Receivers. h 1 (t) R 1 R 2 h 3 (t) h 22 (t) h 21 (t) R 3 Uplink and Downlink typically dupleed in time or frequency Full-duple radios are being considered for 5G systems
Bandwidth Sharing Frequency Division Code Space Time Time Division Frequency Code Space Time Code Division Code cross-correlation dictates interference Multiuser Detection Space (MIMO Systems) Hybrid Schemes Frequency Frequency Code Space Time 7C29822.033-Cimini-9/97
Main Points Spread spectrum increases signal bandwidth above that required for information transmission Benefits of spread spectrum: ISI/narrowband interference rejection by spreading gain Also used as a multiuser/multiple access technique Multiple users can share the same spectrum via time/frequency/code/space division