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Adaptive Modulation. Outline. Adaptive MQAM: optimal power and rate Finite Constellation Sets Update rate Estimation error Estimation delay Practical Considerations in Adaptive Modulation. Adaptive Modulation. Change modulation relative to fading Parameters to adapt: Constellation size - PowerPoint PPT Presentation
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1
Adaptive Modulation
2OutlineAdaptive MQAM: optimal power and rateFinite Constellation SetsUpdate rateEstimation errorEstimation delayPractical Considerations in Adaptive Modulation
3Adaptive ModulationChange modulation relative to fading
Parameters to adapt: Constellation size Transmit power Instantaneous BER Symbol time Coding rate/scheme
Optimization criterion: Maximize throughput Minimize average power Minimize average BER
Only 1-2 degrees of freedom needed for good performance
Adaptive Technologies Rate Control
Fixing symbol rate, using multiple modulation schemes or constellation sizes• Fairly easy, used in current systems, such as GSM, IS-136 EDGE or 802.11a,
et. al. Fixing the modulation, changing the symbol rate
• Varying signal bandwidth is impractical and complicates bandwidth sharing.
Power Control Adapting the transmit power alone is generally used to compensate
for SNR variation due to fading. To maintain a fixed bit error probability, or a constant received SNR. The power adaptation thus inverts the channel fading so that the
channel appears as an AWGN channel to the modulator and demodulator.
Adaptive Technologies (cont) Error Control
Adapt the instantaneous BER subject to an average BER constraint.• Not an adaptive technique, since the transmitter does not adapt to SNR.
Error probability is typically adapted along with some other form of adaption such as constellation size or modulation type.
Hybrid techniques Adapt multiple parameters of the transmission scheme, including rate,
power, coding, and instantaneous error probability. Joint optimization of the different techniques is used to meet a given
performance requirement. Rate adaption is often combined with power adaptation to maximize
spectral efficiency.
6Variable-Rate Variable-Power MQAM
UncodedData Bits Delay Point
SelectorM(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
The rate and power of MQAM are varied to maximize spectral efficiency while meeting a given instantaneous Pb target.
Goal: Optimize P(g) and M(g) to maximize R=Elog[M(g)]
7Optimization FormulationAdaptive MQAM: Rate for fixed BER
Rate and Power Optimization
Same maximization as for capacity, except for K=-1.5/ln(5BER).
PPK
PP
BERM )(1)(
)5ln(5.11)( ggggg
PPKEME
PP
)(1logmax)]([logmax 2)(2)(
ggggg
8Optimal Adaptive SchemePower Adaptation
Spectral Efficiency
0
0
1 1( )0 else
K KKPP
gg g g gg
1
0g
1gK
gk g
RB
p dK K
log ( ) .2
g
gg
g g
Equals capacity with effective power loss K=-1.5/ln(5BER).
9Spectral Efficiency
K1
K2
K=-1.5/ln(5BER)
Can reduce gap by superimposing a trellis code
10Constellation RestrictionRestrict 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
0M1
M2
OutageM1
M3
M2
M3
MD(g)
11Power 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
00,)/()1()(
ggggggg jKM
PP jjjj
)(log 11
2
jj
N
jj pM
BR ggg
12Efficiency in Rayleigh Fading
13Practical ConstraintsConstellation updates: fade region duration
Error floor from estimation errorEstimation error at RX can cause error in absence of noise
(e.g. for MQAM)Estimation error at TX causes mismatch of adaptive power
and rate to actual channelError floor from delay: let r(t,t)=g(t-t)/g(t).
Feedback delay causes mismatch of adaptive power and rate to actual channel
Mjj
jj TT
NN
1
t
regionin fademax at ratecrossinglevel
regionin fademin at ratecrossinglevel
spreaddelay
AFRD
1
j
j
M
j
N
N
T
t
14Detailed FormulasError floor from estimation error (gg)
Joint distribution p(g,g) depends on estimation: hard to obtain. For PSAM the envelope is bi-variate Rayleigh
Error floor from delay: let =g[i]/g[i-id].p(|g) known for Nakagami fading
ˆ/target
0
ˆ ˆ.2[5 ] ( , )K
bP BER p d dg g
g
g g g g
^
^
ggg ddppBERPb )()|(]5[2.0 0
target
15Main 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.
Constellations cannot be updated faster than 10s to 100s of symbol times: OK for most dopplers.
Estimation error and delay lead to irreducible error floors in adaptive MQAM
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