EE359 – Lecture 19 Outline Announcements Final Exam Announcements HW 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 Synchronization RAKE Receivers Multiple Access Users separated by time, frequency, codes, or space
EE359 – Lecture 19 Outline. Announcements Final Exam Announcements HW 8 (last HW) due Friday 5pm (no late HWs) 10 bonus points for course evaluations online Projects due Dec. 6 Direct Sequence Spread Spectrum ISI and Inteference Rejection Spreading Codes and Maximal Linear Codes - PowerPoint PPT Presentation
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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
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: