EE359 – Lecture 13 Outline Annoucements Midterm announcements No HW this week (study for MT; HW...
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Transcript of EE359 – Lecture 13 Outline Annoucements Midterm announcements No HW this week (study for MT; HW...
EE359 – Lecture 13 Outline
AnnoucementsMidterm announcementsNo HW this week (study for MT; HW due
next week)
Midterm reviewIntroduction to adaptive
modulationVariable-rate variable-power
MQAMOptimal power and rate adaptationFinite constellation sets
Midterm Announcements
Midterm Thur Nov. 7, 6-8pm, Thornton 110Open book/notes (bring
textbook/calculators)Covers Chapters 1-7 No computers
Short review todayOHs this week:
Andrea: Tuesday 5-6pm and Wednesday 6-7pm
Mainak: Mainak’s: Tues 6-7 pm, Packard 364, Wed 7-8pm Packard 106, Thurs 1:30-2:30 pm, Packard 106
No HW this week (new HW posted Thurs)Midterms from past 3 MTs posted, 10
pts for taking one and solns to all exams
Review of Last Lecture
Maximal Ratio Combining
MGF Approach for Performance of MRC
Transmit diversityWith channel knowledge, similar to
receiver diversity, same array/diversity gain
Without channel knowledge, can obtain diversity gain through Alamouti scheme over 2 consecutive symbols
MMbbb dddpppPdpPP ...)(...)()()(...)()( 21**2*1
dg
PM
iiib
5.
0 12;
sin
1M
Adaptive ModulationChange modulation relative to
fading
Parameters to adapt:Constellation sizeTransmit powerInstantaneous BERSymbol timeCoding rate/scheme
Optimization criterion:Maximize throughputMinimize average powerMinimize average BER
Only 1-2 degrees of freedom needed for good performance
Variable-Rate Variable-Power MQAM
UncodedData Bits Delay
PointSelector
M(g)-QAM ModulatorPower: P(g)
To Channel
g(t) g(t)
log2 M(g) Bits One of theM(g) Points
BSPK 4-QAM 16-QAM
Goal: Optimize P(g) and M(g) to maximize R=Elog[M(g)]
Optimization Formulation
Adaptive MQAM: Rate for fixed BER
Rate and Power Optimization
Same maximization as for capacity, except for K=-1.5/ln(5BER).
P
PK
P
P
BERM
)(1
)(
)5ln(
5.11)(
P
PKEME
PP
)(1logmax)]([logmax 2
)(2
)(
Optimal Adaptive Scheme
Power Adaptation
Spectral Efficiency
else0
)( 0
0
11KKK
P
P
g
1
0
1
Kgk g
R
Bp d
K K
log ( ) .2
Equals capacity with effective power loss K=-1.5/ln(5BER).
Spectral Efficiency
K1
K2
K=-1.5/ln(5BER)
Can reduce gap by superimposing a trellis code
Constellation Restriction
Restrict MD(g) to {M0=0,…,MN}. Let M(g)=g/gK
*, where gK* is later
optimized.Set MD(g) to maxj Mj: Mj M(g).Region boundaries are gj=MjgK*, j=0,
…,NPower control maintains target BER
M(g)=g/gK*
gg0 g1=M1gK* g2 g3
0
M1
M2
OutageM1
M3
M2
M3
MD(g)
Power Adaptation and Average Rate
Power adaptation: Fixed BER within each region
Es/N0=(Mj-1)/K Channel inversion within a region
Requires power increase when increasing M(g)
Average Rate
1
1
0
0,)/()1()(
jKM
P
P jjjj
)(log 11
2
jj
N
jj pM
B
R
Efficiency in Rayleigh Fading
Sp
ectr
al
Eff
icie
ncy
(bp
s/H
z)
Average SNR (dB)
Main Points
Adaptive modulation leverages fast fading to improve performance (throughput, BER, etc.)
Adaptive MQAM uses capacity-achieving power and rate adaptation, with power penalty K.Comes within 5-6 dB of capacity
Discretizing the constellation size results in negligible performance loss.