peak to average power ratio reduction for ofdm
DESCRIPTION
With the upcoming deployment of wideband wireless network with throughput greater than 100Mbps over high frequency bands such as 5-GHz band and the adopting of multicarrier modulations, more and more challenges are brought to system and hardware design. OFDM, frequently referred as multi-carrier modulation, is becoming the de facto standard for next-generation wideband wireless networks. However, one of the critical issues of OFDM as well as other multicarrier modulation scheme is its high peak-to-average power ratio (PAPR), which usually requires large backoff and highly efficient high power amplifier (HPA), large dynamic range analog-to-digital converter (ADC), high linearity up-converter, etc. These requirements lead to expensive hardware systems that are difficult to design. Hence it becomes more and more important to alleviate the burden of hardware design with employing advanced PAPR reduction technologies. In this tutorial contribution, OFDM RF design challenges and PAPR reduction technologies are presented. They are discussed in terms of both theory and implementation with many examples. Especially a patent search is given too. There are five major parts in this tutorial. In the first part, an introduction to OFDM, the RF design challenges and the PAPR issue is presented. We outline the challenges, which include high power peak, linearity limitation, image rejection, phase noise and distortion, etc., brought to each component of OFDM RF design. Here we focus on high peak power and try to solve the PAPR issue, which is defined from both signal processing and coding perspectives. The problem of PAPR issue is outlined from implementation perspective with the discussion of the effectiveness of signal clipping, which is known as one of the simplest PAPR reduction technique. In the second part, an overview of most popular PAPR reduction approaches is given. It includes coding, signal processing and filtering, selection mapping, signal constellation optimization, etc. The pros and cons of these approaches are compared in terms of performance and implementation complexity. In the third part, PAPR reduction techniques adopted in existing standards are presented and discussed. We cover some of the most important standards including GSM, WCDMA, LTE and UMB. In the fourth part, many PAPR reduction patent examples are presented, followed by a presentation of our recent contributions to PAPR reduction. Our approaches are simple and efficient, with low implementation complexity on the receiver. The conclusions and further works are outlined in the last part.In summary, this tutorial is intended to provide a comprehensive overview of PAPR reduction form OFDM for a wide array of audiences. It includes not only the background theory, implementation considerations and related standards but also our recent contributions.TRANSCRIPT
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Peak-to-Average-Power Ratio Reduction for OFDM
Shu WangLG Electronics Mobile Research, USA
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Introduction• Understanding the PAPR (Peak-to-Average-Power Ratio) of OFDM from
• a signal processing perspective, • a coding perspective, and• an implementation perspective.
• PAPR reduction in research and standards.• Known techniques: clipping, coding, PTS, SLM, dynamic PA backoff,
single-carrier modulation, etc.• Relevant standards: GSM, WCDMA, UMB, LTE, etc.
• Two PAPR reduction techniques are investigated for regular OFDM.1) PAPR reduction with group-based cyclic delays2) PAPR reduction with subcarrier remapping
• Three PAPR reduction schemes are investigated for layer-modulated OFDM1) Rotated Layer Modulation2) PAPR Reduction with Layer-Based Cyclic Delay3) PAPR Reduction with Group-Based Cyclic Delay
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The PAPR of OFDM
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Real Part of OFDM Signal
Imag
inar
y P
art o
f OFD
M S
igna
l
6.53dB
3.01dB
2 4 6 8 10 12 14 160
0.05
0.1
0.15
0.2
0.25
Peak-to-Average-Power Ratio (dB)
Pro
babi
lity
Den
sity
Fun
ctio
n
8PSK, L=128
LOts
tsPAPR
E
max
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PAPR: A Signal Processing Perspective
high PAPR sounds like rare event, can we just ignore it?
•The statistic properties of PAPR can be described by CCDF (complementary cumulative distribution function).
•And let’s do something a little bit heuristic.
• Assume the frequency-domain symbol is complex Gaussian distributed.
• When the number of subcarriers, L, become large, the instantaneous power of each OFDM signal chip can be modeled by a chi-distributed signal with two degree of freedom.
L
LCG
e
p
PAPR
PAPRCCDF
11
Pr1
Pr1
Pr
N=64, oversampling factor 4, Thompson, et al 2005
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PAPR: A Coding Perspective
What is the achievable region of triplets (R, d, PMEPR)?
2
22
2
2
2
max1
maxmax
jezzC
ts
tSE
tSPAPR
cc
Peak-to-Mean Envelope Power Ratio (PMEPR)
zzCzczcc LL wc
1110
c
jez
(L-1)-sphere
(L-1)-sphere cap with max Euclid distance r
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Rapp’s SSPA Model
•The knee factor P controls the smoothness of the SSPA characteristic. When P = 2, it is known to be a good representation of the HPA’s in the sub-10 GHz frequency range.
PP
Vv
vv
2
1
2
sat
in
inout
)||
(1
PA output voltage PA input voltage
Knee factor. Typically P=2~3
PA output saturation level
•AM/AM characteristics of the Rapp SSPA model, P=2
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PAPR: An Implementation Perspective
Source: Intersil
0 0.5 1 1.5 2 2.5 3 3.5-80
-70
-60
-50
-40
-30
-20
-10
0
10
frequency normalized to symbol rate
Upp
er h
alf o
f pow
er s
pect
rum
(dB
)
OFDM, P0=2, N1=256, 8*ovs, alpha=.125
BO=5 dB BO=7 dB BO=10 dB
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Challenges Brought By High PAPR
• Peak transmit power is limited by• Regulations• Interference. both in-band and out-of-band interference are
concerned.• Hardware Limitations, especially when the bill of materials and
power consumption are among the major concerns.
• High PAPR of OFDM signals, especially at the high carrier frequency, e.g. 2-5GHz, and with high-order modulations, brings new challenges for the implementations.• It demands the HPA with large backoff.• It demands the high power amplifier with better efficiency.• It requires the up-converter with high linearity.• It requires the ADC with large dynamic range. • It requires the LO with low phase noise level.
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PAPR Reduction in Standards
• GSM: The modulation is GMSK, which has a constant envelope and is optimized for amplifier PAPR requirement.
• WCDMA: DPDCH and DPCCH are I-Q/code multiplexed and complex scrambled. The complex scrambling codes are formed in such a way that the rotations between consecutive chips within one symbol period are limited to ±90o. The full 180 rotation can happen only between consecutive symbols.
• UMB: The scheduler adapts assignments of different ATs based on their PA characteristics and power limitation and schedule power-limited users away from the edge of spectrum allocation and other users on the remaining spectrum with taking into account user’s power limitation as well as channel selectivity across subbands.
• LTE: SC-FDMA, which has low PAPR in time domain but high PAPR in frequency domain, is adopted. SC-FDMA has about 1.5 for 16QAM and 2.5dB for QPSK in PAPR gain but less frequency diversity gain.
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Some Popular PAPR Reduction Technologies
• Clipping: In-band distortion mostly is negligible. But out-of-band distortion is more serious.
• Filtering and Signal Processing : • time-invariant linear filter results in less peak regrowth and lower PAPR than DFT
filter in general, if there is no spectral masking.• Partial Transmit Signaling (PTS): divide/Group into clusters and each of them is done
with a smaller IFFT. [Muller and Huber, 97 ] • Tone Reservation (TR): inserting anti-peak signals in unused or reserved subcarriers.
The objective is to find the time-domain signal to be added into the original time-domain signal such that PAPR is reduced. [Tellado, 00]
• Coding: The idea is to select a codeword with less PAPR. it still is an open problem to construct codes with both low PAPR and short Hamming distance.
• Selected Mapping (SLM): it is based on selecting one of the transformed blocks for each data block, which has the lowest PAPR. [Bauml, Fisher and Huber, 96]
• Constellation Optimization• Tone Injection (TI): the basic idea is to increase the constellation size so that each of
the points in the original basic constellation can be mapped into several equivalent points in the expanded constellation.
• Active constellation extension (ACE): similar to TI. [Krongold and Jones, 03]
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Examples: Partial Transmit Signaling and Selective Mapping
•Partial Transmit Signaling (PTS)• Input is divided into M clusters, • Each cluster is converted into time-domain with shorter IFFT• Combine the M output sequence to minimize the PAPR
•Selective Mapping (SLM)• This method is based on generating M statistically independent transformed
blocks for each data block and transmitting the one with the lowest PAPR.• Multiple data streams by M different sequences• Converted them independently into time domain with IFFT• Select the best sequence for transmission.
•In general, both of them require transmitting some side information about the identity of the selected block.
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Rotation and Delay (1/2)
nTtffFe fnTj 2
tfenffF tnfj 2
Time-domain shifting is equal to frequency-domain rotating
Frequency-domain shifting is equal to time-domain rotating
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Rotation and Delay (2/2)
• Frequency-domain subcarrier remapping brings time-domain rotation on OFDM subcarriers.• From a time-domain perspective, it is similar to PTS.• With PTS, however, the direct rotation of subcarriers in time
domain may change frequency-domain pilot pattern.
• Time-domain cyclic delays of subcarriers brings the rotation of subcarriers in frequency domain. • From a frequency-domain perspective, it is similar to SLM.• With SLM, however, the rotation of subcarrier with SLM is done
in frequency domain, which is separated from the PAPR detection by IFFT. This results in high calculation complexity and/or processing delay.
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The Proposed PAPR Reduction Schemes
• PAPR Reduction with Subcarrier Remapping• Multiple OFDM signals are generated based on different
symbol-to-subcarriers mapping rule• The OFDM signal of the lowest PAPR is selected for
transmission.
• PAPR Reduction with Group-Based Cyclic Delay• The input subcarriers are partitioned into multiple groups.• The PAPR of the sum output can be reduced with properly
adjusting the delay of each group’s IFFT output.
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PAPR Reduction with Subcarrier Remapping (1/2)
IFFT
Parallel to Serial and
Cyclic Prefix
Tx and/or Symbol-to-Subcarrier Mapping Format 1
IFFT
Parallel to Serial, and
Cyclic Prefix
Tx and/or Symbol-to-Subcarrier Mapping Format 2
IFFT
Parallel to Serial and
Cyclic Prefix
Tx and/or Symbol-to-Subcarrier Mapping Format N
Select the one with lowest PAPR
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PAPR Reduction with Subcarrier Remapping (2/2)
0 2 4 6 8 10 1210
-4
10-3
10-2
10-1
100
Peak-to-Average Power Ratio (dB)
CC
DF Gaussian Approx
with No Cyclic Delay
with Tx Format Selection, G=2
QPSK/OFDM, L=128
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PAPR Reduction with Group-Based Cyclic Delay (1/2)
Cyclic delay and Cyclic/Zero Prefix
Cyclic delay and Cyclic/Zero Prefix
IFFT
PAPR control
Combining and PAPR calculation
Dividing, Decomposition,
Grouping or Clustering, or
amplitude adjustment if
necessary
IFFT
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0 2 4 6 8 10 1210
-4
10-3
10-2
10-1
100
Peak-to-Average Power Ratio (dB)
CC
DF
Guassian approximation with no PAPR reduction with group-based cyclic delay, G=2 with partial transmit signaling, G=2
PAPR Reduction with Group-Based Cyclic Delay (2/2)
QPSK/OFDM, L=128
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Superposition Precoding and Layered Modulation
•Optimal broadcast channel capacity is achievable by superposing two users’ signal together.
•Superposition precoding with interference cancellation outperforms TDM and FDM schemes in most time.
•Layer modulation is one of the popular implementations of superposition precoding.
Achievable rates, ( Bergmans and Cover, 1974 ).
Base Layer: QPSK
Enhancement Layer: rotated QPSKQPSK/QPSK Hierarchical Modulation
θ
θ
01 00
11 10
01
00
11
10
0111
0011
0010
0110
1110
00010000
0101
0100
11111101
1100
10101011
1001
1000
b1b0
e1e0
α
β
2α
2β
b1e1b0e0
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Layered Modulation in Standards
•UMB/MediaFLO supports layered transmission of base/enhancement layers
• Extends coverage with layered source coding• Provides a more graceful degradation of reception.
•Besides using a dedicated DVB-H network, DVB-H service can also be embedded into DVB-T network using layered modulation.
• DVB-H service use the HP input while DVB-T services use LP.• The HP input can offer increased robustness in mobile environment
over the LP input• The LP input can serve higher bit-rate for fixed reception service
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PAPR Reduction for Layer-Modulated OFDM
• The enhanced layer modulation with rotating enhancement layer• The signal constellation of the enhancement layer is rotated for
• lowering PAPR,• lowering symbol error rate, and• Increasing achievable throughput.
• PAPR reduction with layer-based cyclic delay• The input two layers are individually processing with IFFT.• The enhancement layer is cyclic delayed and added into the based layer
in time domain.• With proper adjusting the delay, the PAPR can be reduced.
• PAPR reduction with group-based cyclic delay• The input subcarriers are partitioned into at least two groups.• The output PAPR can be reduced with properly adjusting the delay of
each group’s IFFT output.
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Hierarchical Modulation with Rotation (1/2)
Power and/or Phase
Adjustment
Power and/or Phase
Adjustment
Superpostion
and IFFT
PAPR detection and control
Parallel to Serial and
PAPR calculation
Layer 1 Symbols
Layer 1 Symbols
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Hierarchical Modulation with Rotation (2/2)
0 2 4 6 8 10 1210
-4
10-3
10-2
10-1
100
Peak-to-Average Power Ratio (dB)
CC
DF
Gaussian Approximation, L=128Regular QPSK/QPSK, P
2/P
1=0.01, L=128
Regular QPSK/QPSK, P2/P
1=0.09, L=128
Regular QPSK/QPSK, P2/P
1=0.25, L=128
Enhanced QPSK/QPSK, P2/P
1=0.01, L=128
Enhanced QPSK/QPSK, P2/P
1=0.04, L=128
Enhanced QPSK/QPSK, P2/P
1=0.09, L=128
Enhanced QPSK/QPSK, P2/P
1=0.16, L=128
Enhanced QPSK/QPSK, P2/P
1=0.25, L=128
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PAPR Reduction with Layer-Based Cyclic Delay (1/2)
Cyclic Delay
Cyclic Delay
IFFT
PAPR control
Combining and PAPR calculation
Layer-1 Symbols
IFFTLayer-2 Symbols
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0 2 4 6 8 10 1210
-4
10-3
10-2
10-1
100
Peak-to-Average Power Ratio (dB)
CC
DF
Gaussian Approximation, L=128Regular QPSK/QPSK, P
2/P
1=0.01, L=128
Regular QPSK/QPSK, P2/P
1=0.09, L=128
Regular QPSK/QPSK, P2/P
1=0.25, L=128
Cyc. Del. QPSK/QPSK, P2/P
1=0.01, L=128
Cyc. Del. QPSK/QPSK, P2/P
1=0.04, L=128
Cyc. Del. QPSK/QPSK, P2/P
1=0.09, L=128
Cyc. Del. QPSK/QPSK, P2/P
1=0.16, L=128
Cyc. Del. QPSK/QPSK, P2/P
1=0.25, L=128
Cyclically Delayed Hierarch. Modulations
Regular Hierarch. Modulations
PAPR Reduction with Layer-Based Cyclic Delay (2/2)
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PAPR Reduction with Group-Based Cyclic Delay (1/2)
Cyclic Delay
Cyclic Delay
IFFT
PAPR control
Combining and PAPR calculation
The input layered-
modulated symbols are divided into
multiple smaller groups of layered-
modulated symbols IFFT
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PAPR Reduction with Group-Based Cyclic Delay (2/2)
0 2 4 6 8 10 1210
-4
10-3
10-2
10-1
100
Peak-to-Average Power Ratio (dB)
CC
DF
Gaussian Approximation, L=128Regular QPSK/QPSK, P2/P1=0.01, L=128
Regular QPSK/QPSK, P2/P1=0.09, L=128
Regular QPSK/QPSK, P2/P1=0.25, L=128
with PAPR Reduction, P2/P1=0.01, L=128, G=2
with PAPR Reduction, P2/P1=0.04, L=128,G=2
with PAPR Reduction, P2/P1=0.09, L=128, G=2
with PAPR Reduction, P2/P1=0.16, L=128, G=2
with PAPR Reduction, P2/P1=0.25, L=128, G=2
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Conclusions (1/2)• PAPR reduction is a historic issue existing with the development of
wireless communication systems. • The high PAPR of OFDM bring higher requirements and more
challenges on the system implementation, which limit the actual performance of OFDM systems.• It brings higher requirements on HPA, ADC, heat dissipation,
signal processing, etc.• Two new PAPR reduction technologies are proposed.
• PAPR reduction with transmission format selection• Low processing delay• High complexity
• PAPR reduction with cyclic delay diversity• Simple and seamless PAPR reduction. No additional
demodulation overhead• Better demodulation. More diversity• Compatible with any modulation and coding scheme.
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Conclusions (2/2)
• Rotational layer modulation has lower PAPR than regular layer modulation, providing properly rotating the enhancement layer.• It has PAPR reduction gain, though it is not significant.• The PAPR reduction gain depends on the power splitting between layers.
• With cyclic delays, additional PAPR reduction is achievable• The performance of layer-based approach depends on the power splitting
between layers, while the group-based approach doesn’t.• In general, the group-based approach has a constant better performance
than the layer-based approach.• The group-based approach has the best performance of the three proposed
approaches.• Simple and seamless PAPR reduction. No additional
demodulation overhead• Better demodulation. More diversity• Compatible with regular modulations as well as the enhanced layer
modulation for high throughput.