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The Next Generation Challenge for Software Defined Radio
Mark Woh1, Sangwon Seo1, Hyunseok Lee1, Yuan Lin1, Scott Mahlke1, Trevor Mudge1, Chaitali Chakrabarti2, Krisztian Flautner3
1Advanced Computer Architecture Lab, University of Michigan2Department of Electrical Engineering, Arizona State University
3ARM, Ltd.
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3G Wireless
Large Coverage
Outdoor - High Mobility
Up to 14Mbps
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Expected Wireless Growth
The growth of wireless will require more bandwidth
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4G Wireless
What we need Adaptive high performance transmission system
Great candidate for SDR
Large Coverage – 100Mbps Coverage
Outdoor - High Mobility
Macro CellsPico Cells
Isolated HotSpots – 1Gbps Coverage
Indoor – Very Low Mobility
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Next Generation Wireless – 4G
MIMOdecoder
DEMOD(OFDM)
ChannelDecoder
MIMOencoder
MOD(OFDM)
ChannelEncoder
...
...
· IFFT
· FFT · STBC· VBLAST
...
· Turbo code· LDPC code
TX
RX
Antenna
...
3 Major Components to 4G Modulation/Demodulation
Multiple-Input Multiple-Out (MIMO)
Channel Decoder/Encoders
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Modulation - OFDM
0 fsc Nfsc
….….
-Nfsc -fsc
Properties of OFDM-High Spectral Efficiency-Low Intersymbol Intereference-Flat Fading Subcarriers
Can sustain high data rates withmultiple users
Can be implemented with IFFT/FFT
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Major Component of Modulation – FFT/IFFT
Very wide data level parallelism
Requires complex operations
complex mult
x[1]
eiw
complexadd
complexsub
x[0]
X[1]
X[0]
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MIMO (Multiple Input – Multiple Out)
Previously we used single antenna systems
Now we use multiple antennas to increase the channel capacity
Diversity - High Reliability
Space Time Block Codes (STBC)
Multiplexing – High Throughput
Vertical-BLAST (V-BLAST)
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Space Time Block Codes (STBC)
CombinerChannel
EstimationChannel
Estimation
Tx1 Tx2Transmit Antennas
x[1]
-x[2]*
x[2]
x[1]*
h11 h22
h21 h12
Channel
Rx1 Rx2
Time
Receive Antennas
n11
n12
n21
n22
~x[1]~x[2]
h11
h12 h22
h21
Noise
y11 = h11x[1] + h12x[2] + n11
y12 = -h11x[2]* + h12x[1]* + n12
y21 = h21x[1] + h22x[2] + n21
y22 = -h21x[2]* + h22x[1]* + n22
Received Signal
~x[1] = h11*y11 + h12y12* + h21*y21 + h22y22*
~x[2] = h12*y11 - h11y12* + h22*y21 - h21y22*
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STBC
Requires complex operations
Low Data Movement
Highly parallelizable
ComplexMultiply
Accumulate
Channel Estimation
h22 h21 h12 h11
y21 y11
y22* y12*
Receiver Antenna 1 and 2
~x[1] ~x[2]
Conjugate+Negation
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Vertical-BLAST (V-BLAST)
S/P
Mod
Mod
V-Blast Detector
Demod
Demod
M Transmitters R Receivers
1 2 3 4 1 2 3 4 1 2 3 4
5 6 7 8 5 6 7 8 5 6 7 8
9 10 11 12 9 10 11 12 9 10 11 12
13 14 15 16 13 14 15 16 13 14 15 16
Data Stream of 4 TxLinear Combination
of Data
Channel Estimation
Nulling Vector
1 2 3 4
Subtract Strong Signal
Repeat
Strongest Signal
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V-BLAST
Implementation Based on Square Root Method for V-BLAST Original requires repeated pseudo-inverse calculation for finding the
strongest signal
This algorithm has reduces complexity
Complexity Requires matrix operations on complex numbers
Many Matrix Transformations
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Channel Decoding
3G Technologies in 4G Viterbi
Turbo Decoder
New to 4G LDPC
Better performance characteristics compared to Turbo and Viterbi
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LDPC
0 1 0 1 1 0 0 1
1 1 1 0 0 1 0 0
0 0 1 0 0 1 1 1
1 0 0 1 1 0 1 0
H =
E0
L0 L1 L2 L3
E1 E2 E3
L4 L5 L6 L7
L Node Original Value Message from Check Nodes Decision
L0
L1
L2
L3
L4
L5
L6
L7
1
1
0
1
0
1
0
1
E1 → 0 E3 → 1 1
0
0
1
0
1
0
1
E0 → 0 E1 → 0E1 → 1 E2 → 0E0 → 0 E3 → 1E0 → 1 E3 → 0E1 → 0 E2 → 1E2 → 0 E3 → 0E0 → 1 E2 → 1
E0
L0 L1 L2 L3
E1 E2 E3
L4 L5 L6 L7
1 1 0 01 1 0 1
E0
L0 L1 L2 L3
E1 E2 E3
0
L4 L5 L6 L7
1 1 0 01 1 0 1
Message Sent = [ 1 0 0 1 0 1 0 1 ]
Message Recieved = [ 1 1 0 1 0 1 0 1 ]
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LDPC
Min-Sum Decoding Used
Regular LDPC code
Can get benefit from Wide SIMD Can do the Bit Node and Check Node
Alignment of Check and Bit nodes is a problem
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SODA PE Architecture
SIMDReg.File
EX
SIMDALU+Mult
SIMDShuffle
Net-work(SSN)
WB
ScalarALU
WB
EX
ScalarRF
LocalSIMD
Memory
LocalScalar
Memory
STV
AGURF
EX
WB
AGUALU
1. SIMD pipeline
2. Scalar pipeline
4. AGU pipeline
VTS
Pred.Regs
WB
SIMDto
Scalar(VtoS)ALU
RF
DMA
GlobalMemory
SODADSP
5. DMA
3. Localmemory
System
Interco
nn
ect
SIMD – 32 Wide, 16-bit datapath, Predicate Execution
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Key 4G algorithms
100 Mbps 1 Gbps
MCycle/s MCycle/s
FFT 2x360 4x360
IFFT 2x360 4x360
STBC 240 -
V-BLAST - 1900
LDPC 7700 4x18500
4G Workload on SODA
100 Mbps 4G requires 8Ghz SODA PE
1 Gbps 4G requires 20Ghz SODA PE
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SODA With Technology Scaling
0
500
1000
1500
2000
2500
3000
3500
4000
180nm 130nm 90nm 65nm 45nm 32nm 22nm
Fre
qu
en
cy
(M
hz)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Po
we
r (W
)
ITRS Scaled Frequency Fixed Scaled Frequency Scaled Power
180nm 130nm 90nm 65nm 45nm 32nm 22nm
Vdd (V) 1.8 1.3 1.1 1.1 1 0.9 0.8
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We can’t do any of 4G with technology scaling on one core Would 8GHz cores even be an energy efficient solution?
What about 1Gbps? Are we ever going to get a 20GHz core?
Cannot rely on technology scaling to give us 4G for free 4G SDR will require algorithmic and architectural innovations
SDR Challenges In 4G
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4G Algorithm-Architectural Co-design
Architectural improvements (SODA II) Specialized functional units
CISC-like complex arithmetic operations
Specialized data movement hardware Less strain on the memory system
Wider SIMD
How wide can we go?
More PEs
What does the interconnect look like?
Algorithmic optimization through parallelization Reduce intra-kernel communication
Reduce memory accesses
Arithmetic is much cheaper than data movement
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Thanks
Questions?
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Successive Cancelling for V-BLAST
V-BLAST successive interference cancelling (SIC)
The ith ZF-nulling vector wi is defined as the unique minimum-norm vector satisfying
Orthogonal to the subspace spanned by the contributions to yi due to the symbols not yet estimated and cancelled and is given by the ith row of H
~
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Alamouti Scheme
Assumption: the channel remains unchanged over two consecutive symbols Rate = 1 Diversity order = 2
Simple decoding
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Advantages of Software Defined Radio
Multi-mode operations
Lower costs Faster time to market
Prototyping and bug fixes
Chip volumes
Longevity of platforms
Enables future wireless communication innovations
Cognitive radio
UWB EDGE 802.16a
802.16a Bluetooth
802.11b WCDMA 802.11n
SDR