1. 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

2. Final Exam Announcements
Final 12/15/16 12:15-3:15pm in Huang 18 (here)
Covers Chapters 9, 10, 12, , 13.4, , (+ 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.

3. OHs leading up to final exam
Mine
This week: today and Friday after class
Next week: Mon 12/12 1-2:30, Wed 12/ pm and by appt.
TAs:
Monday 12/5 4 pm - OH, Mainak
Wednesday 12/ pm Discussion Session, Milind followed by OH from pm
Thursday 12/ 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/ pm OH, Milind
Monday 12/ pm - Mainak, OH
Tuesday 12/ pm - OH, Mainak
Wednesday 12/ pm - OH, Mainak
Thursday 12/ am - OH, Milind Thur 3-4 pm in Packard 364

4. 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

5. 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)

6. 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

7. 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
7C Cimini-9/97

8. 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

9. 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

10. 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.

11. 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.)
8C Cimini-7/98

12. 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