EE359 – Lecture 19 Outline Multiple Access Announcements Final Exam Announcements HW 8 (last HW) due Sat. 12/10 at 4pm (no late HWs) 25 bonus points for course evaluations (online) Projects due Dec. 10 at midnight Multiple Access Users separated by time, frequency, codes, or space Multiuser detection reduces interference in DSSS Cellular System and Frequency Reuse

Final Exam Announcements Final 12/15/16 12:15-3:15pm in Huang 18 (here) Covers Chapters 9, 10, 12, 13.1-13.2, 13.4, 14.1-14.3, 15.1-15.3 (+ earlier chps) Similar format to MT, but longer: open book, notes. If you need a book or calculator, let us know by 12/8/16 Practice finals posted (10 bonus points) Turn in for solns, by exam for bonus pts Course summary and bonus lecture on advanced topics Friday 9:30-11:30, here Final review and discussion section: Fri 12/9 from 4-6pm, Packard 364.

OHs leading up to final exam Mine This week: today and Friday after class Next week: Mon 12/12 1-2:30, Wed 12/14 12-2pm and by appt. TAs: Monday 12/5 4 pm - Email OH, Mainak Wednesday 12/7 3.30-4.30 pm Discussion Session, Milind followed by OH from 4.30-5.30 pm Thursday 12/8 10-11 am (Milind OH regularly scheduled on Friday shifted because of class), 4-5 pm OH, Mainak Friday 12/9 4-6 pm Final Review and OH, Milind Saturday 12/10 3-5 pm OH, Milind Monday 12/12 3-4 pm - Mainak, OH Tuesday 12/13 3-4 pm - Email OH, Mainak Wednesday 12/14 3-4 pm - OH, Mainak Thursday 12/15 9.30-10.30 am - OH, Milind Thur 3-4 pm in Packard 364

Review of Last Lecture DSSS ISI and Interference Rejection Narrowband Interference Rejection (1/K) Multipath Rejection (Autocorrelation r(t)) 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)] brS’(f) Info. Signal Receiver Input Despread Signal Time-domain analysis for ISI and NB interference rejection show impact of spreading gain and code autocorrelation

Review Cont’d RAKE Receiver Multibranch receiver Branches synchronized to different MP components These components can be coherently combined Use SC, MRC, or EGC x Demod y(t) sc(t) ^ dk Diversity Combiner x Demod sc(t-iTc) x Demod sc(t-NTc)

Multiuser Channels: Uplink and Downlink Downlink (Broadcast Channel or BC): One Transmitter to Many Receivers. Uplink (Multiple Access Channel or MAC): Many Transmitters to One Receiver. x h3(t) R3 x h22(t) x h21(t) x h1(t) R2 R1 Uplink and Downlink typically duplexed in time or frequency

Bandwidth Sharing Frequency Division Time Division Code Division Code Space Time Frequency Frequency Division Time Division Code Division Code cross-correlation dictates interference Multiuser Detection Space (MIMO Systems) Hybrid Schemes Code Space Time Frequency Code Space Time Frequency 7C29822.033-Cimini-9/97

Multiuser Detection In CD semi-orthogonal systems and in TD-FD-CD cellular systems, users interfere Interference generally treated as noise. Systems become interference-limited Often uses complex mechanisms to minimize impact of interference (power control, smart antennas, etc.) Multiuser detection (MUD) exploits the fact that the structure of the interference is known MUD structures for N users Maximum likelihood: exponentially complex in N Successive interference cancellation: Sequentially subtracts strongest interferer: Error propagation

OFDMA and SDMA OFDMA SDMA (space-division multiple access) Implements FD via OFDM Different subcarriers assigned to different users SDMA (space-division multiple access) Different spatial dimensions assigned to different users Implemented via multiuser beamforming (e.g. zero-force beamforming) Benefits from multiuser diversity

Random Access: ALOHA and Slotted ALOHA • Data is packetized. • Packets occupy a given time interval • Pure ALOHA – send packet whenever data is available – a collision occurs for any partial overlap of packets (nonorthogonal slots) – Packets received in error are retransmitted after random delay interval (avoids subsequent collisions). • Slotted ALOHA – same as ALOHA but with packet slotting – packets sent during predefined timeslots – A collision occurs when packets overlap, but there is no partial overlap of packets – Packets received in error are retransmitted after random delay interval.

Cellular System Design • Frequencies (or time slots or codes) are reused at spatially-separated locations  exploits power falloff with distance. Ideally, interference results in SINR above the desired target. The SINR depends on base station locations, user locations, propagation conditions, and interference reduction techniques. • Best efficiency obtained with minimum reuse distance • System capacity is interference-limited. Base stations perform centralized control functions (call setup, handoff, routing, etc.) 8C32810.43-Cimini-7/98

Main Points Multiple users can share the same spectrum via time/frequency/code/space division Multiuser detection mitigates interference through joint or successive detection When user transmissions are bursty, random access is more efficient Does not work well when channel overloaded Cellular systems designed based on the premise of frequency reuse Time/freq/codes reused at spatially separated locations Interference reduced by power falloff with distance