EE359 – Lecture 19 Outline

AnnouncementsFinal Exam AnnouncementsHW 8 (last HW) due Friday 5pm (no late

HWs)25 bonus points for course evaluations

(online)Projects due Dec. 5 at midnight

Spreading Codes and Maximal Linear Codes

SynchronizationRAKE ReceiversMultiple Access

Users separated by time, frequency, codes, or space

Multiuser detection reduces interference in DSSS

Final Exam Announcements

Final 12/7/15 3:30-6:30pm in Thornton 102 (here)

Covers Chapters 9, 10, 12, 13.1-13.2 (+ earlier chps)

Similar format to MT, but longer: open book, notes.If you need a book or calculator, let us know

by 12/3/14 Practice finals posted (10 bonus points)

Turn in for solns, by exam for bonus pts Course summary and bonus lecture on

advanced topics tomorrow, 4-6pm, here (pizza/cake/beer) Will also have time for course evaluatoins

Final review and discussion section: Thur 12/3 1:30-3pm, Packard 364.

OHs leading up to final exam

MineToday after classWed: 2:30-4pmSat: 12/5 2:30-4pmMon: 12/7 10-11:30am

TAs: Thu 3-4 pm : Packard 364 (follows

discussion session and review session)

Fri 2-4 pm: 2-3 pm in Packard 109. 3-4 pm outside Packard 340 + email OH

Sat 4-5 pm, Sun 1-2pm: Packard 340

Review of Last Lecture:OFDM-MIMO, Spread

SpectrumMIMO-OFDM combines both

techniques; represented by a matrix in frequency/space.

SS: Modulation that increases signal BW to combat narrowband interference and ISIAlso used as a multiple access

techniqueTwo types: Frequency Hopping

and DSSSFH: Hops info. signals over a wide

bandwidthDSSS: modulates info signal by chip

sequence

s(t) sc(t)

Tb=KTc Tc

S(f)Sc(f)

1/Tb 1/Tc

S(f)*Sc(f)

Review Cont’dISI and Interference

RejectionNarrowband Interference

Rejection (1/K)

Multipath Rejection (Autocorrelation r(t))

S(f) S(f)I(f)S(f)*Sc(f)

Info. Signal Receiver Input Despread Signal

I(f)*Sc(f)

S(f) aS(f)S(f)*Sc(f)[ad(t)+b(t-t)]

Info. Signal Receiver Input Despread Signal

brS’(f)

Time-domain analysis for ISI and NB interference rejection show impact of spreading gain and code autocorrelation

Maximal Linear CodesAutocorrelation determines ISI

rejectionIdeally equals delta function

Maximal Linear CodesNo DC componentLarge period (2n-1)TcLinear autocorrelationRecorrelates every periodShort code for acquisition, longer for

transmissionIn SS receiver, autocorrelation taken

over TbPoor cross correlation (bad for MAC)

1

-1 N Tc -Tc

SynchronizationAdjusts delay of sc(t-t) to hit

peak value of autocorrelation.Typically synchronize to LOS

component

Complicated by noise, interference, and MP

Synchronization offset of Dt leads to signal attenuation by r(Dt)

1

-12n-1 Tc -Tc

Dtr(Dt)

RAKE Receiver Multibranch receiver

Branches synchronized to different MP components

These components can be coherently combinedUse SC, MRC, or EGC

x

x

sc(t)

sc(t-iTc)

xsc(t-NTc)

Demod

Demod

Demod

y(t)

DiversityCombiner

dk̂

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.

R1

R2

R3

x h1(t)x h21(t)

x

h3(t)

x h22(t)

Uplink and Downlink typically duplexed in time or frequency

7C29822.033-Cimini-9/97

Bandwidth Sharing Frequency Division

Time Division

Code DivisionCode cross-

correlation dictates interference

Multiuser Detection

Space (MIMO Systems)

Hybrid Schemes

Code Space

Time

Frequency Code Space

Time

Frequency Code Space

Time

Frequency

Code Division via DSSS

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 (near-far problem)

)()2cos(5.5.)()(5.5.

))(2cos()2cos()()()()2(cos)()()(ˆ

12210

212211

1220

222

111

trtaa

tttaa

c

T

cc

cccc

T

cc

fdddttstsdd

dttftftstststftststx

b

b

a

a

Multiuser Detection In CD semi-orthogonal systems and

in TD-FD-CD cellular systems, users interfereInterference generally treated as noise.Systems become interference-limitedOften 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 usersMaximum likelihood: exponentially

complex in N Successive interference cancellation:

Sequentially subtracts strongest interferer: Error propagation

Successive Interference Cancellation

Signal 1 Demod

IterativeMultiuserDetection

Signal 2Demod

- =Signal 1

- =

Signal 2

Why Not Ubiquitous Today? Power and A/D Precision

A/D

A/D

A/D

A/D

OFDMA and SDMA OFDMA

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

Main Points

Spreading code designs optimized for ISI and multiuser interference rejection

Synchronization depends on autocorrelation properties of spreading code.

RAKE receivers combine energy of all MPUse same diversity combining techniques as

before

Multiple users can share the same spectrum via time/frequency/code/space division

Multiuser detection mitigates interference through joint or successive detection