auxiliary beam pair enabled aod estimation for...
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Auxiliary Beam Pair Enabled AoD Estimation for Large-scale mmWave MIMO Systems
www.profheath.org
Dalin Zhu, Junil Choi and Robert W. Heath Jr.
Wireless Networking and Communications Group Department of Electrical and Computer EngineeringThe University of Texas at Austin
This work is funded by a research gift from Huawei Technologies, Inc.
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Hybrid precoding in mmWave MIMO
Most prior work assumes a channel estimate is available for configuration
A combination of digital and analog beamforming/combining
BasebandPrecoding
1-bitADC
DAC
1-bitADCDAC
RFChain
RF Precoding
1-bitADCADC
1-bitADCADC
BasebandCombining
RF Combining
FBB FRF WBBWRF
RFChain
RFChain
RFChain
channel
Effective channel at baseband
Prior work on channel estimation in mmWave MIMO
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* A. Alkhateeb, O. E. Ayach, G. Leus, and R. W. Heath Jr., ̀ `Channel estimation and hybrid precoding for millimeter wave cellular systems,'' in IEEE J. Sel. Top. Signal Process, vol. 8, pp. 831-846, Oct. 2014** H. Ghauch, T. Kim, M. Bengtsson, and M. Skoglund, ̀ ` Subspace estimation and decomposition for large millimeter-wave MIMO systems,'' submitted to IEEE J. Sel. Top. Signal Process., arXiv preprint arXiv:1507.00287, Jul. 2015*** C. Kim, T. Kim, and J.-Y. Seol, ̀ ` Muti-beam transmission diversity with hybrid beamforming for MIMO-OFDM systems,'' in IEEE Global Telecomm. Conf. (GLOBECOM’13), pp. 61-65, Dec. 2013.
u Compressed sensing based approaches *ª Assumes sparsity in the angular domain of the channelª Requires a prior knowledge of the number of the propagation paths
u Subspace based methods **ª Echo-based subspace estimation between the transmitter and receiverª Requires a lot of training between the transmitter and receiver
u Exhaustive search based approaches ***ª Searches for the best pairs of analog transmit and receive steering vectorsª Training overhead is high
High-resolution estimate of channel
information
Low training and feedback overhead
Feasible channel estimation method is required
Contributions: Path-based channel estimator based on auxilary beam pairs (ABPs)
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Exhuastive search via grid of beams
Each beam is probed
Receiver determines which beams have the strongest signals
The resolution for AoD angle is limited by grid sizeTransmitter
Receiver
Index of preferred beams are sent to the transmitter
Quantization error
True AoD
Quantized AoD
Angle estimation in monopulse radar systems
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Receive beam pair
Signal detectionAmplitude comparison
Angle estimation
Echo signal
Custom designed analog receive beams
Projections of on and
S. M. Sherman, ``Monopulse principles and techniques,'' Artech House, 1984
Beams distinguished by sending with different polarization,
code, or time
Basic design principle of auxiliary beam pairs (1/2)
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. . .
. . .
scatter
: boresight angle of n-th ABP
: steering angle of
: steering angle of
: dominant path’s AoD
: n-th ABP
Transmit array response vector:
System setup
Auxiliary beam pair that covers the AoD:
Pairs of analog transmit beams are formed to cover a given angular range
Exact AoD can be extracted from the ABP that covers the AoD
Basic design principle of auxiliary beam pairs (2/2)
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is invertible with respect to
If
. . .
. . .
Amplitude comparison
Angle estimation
Calculate received signal strengths
Quantized version of
Quantization options
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Distribution of the ratio metric Distribution of the estimated AoD
More codewords can be allocated in densely distributed portions
Quantizing the ratio metric provides more quantization resolution
Uniformly distributed codewords
Procedure for estimating the AoD using ABP
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Auxiliary beam pairs are probed
A set of ratio metrics are calculated, each corresponds to a ABP
The AoD is estimated using the quantized ratio metric
Transmitter
ReceiverThe ratio metric with the highest
received signal strength is quantized
Deployment scenarios using ABP-based method (1/3)
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Beam finding for control channels
Base station Base station Base station
UE
UE UE
UE UE
UE
Layer-1: system-specific information Layer-2: cell-specific information Layer-3: user-specific information
Beam finding process for control channels is facilitated via ABP design
Beamforming range for the next layer is optimized
using the estimated AoD
Deployment scenarios using ABP-based method (2/3)
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Beam finding for control channelsAn example of control channels beamforming using ABP
Base station
UEbeam-a
beam-b The UE decodes system-specific control information from beam-a
The UE calculates a ratio metric with respect to the ABP formed by
beam-a and beam-b
Base station
UE
The UE quantizes the ratio metric and sends it back to the base station
Base station
UE
Using the quantized ratio metric, the base station estimates the AoD
Base station
UE
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2 3
The probing range for layer-II beamforming is determined around
the estimated AoD
Base station
UE
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beam-a
beam-b
Layer-I
Layer-II
Deployment scenarios using ABP-based method (3/3)
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Hybrid precoding for data channels
. . .
. . .
. . . . . .
. . . . . .
Base
band
Pre
code
r
DAC
Mixers Phase shifters
PA
Antenna array
Phase shifters Mixers
ADC
Base
band
Com
bine
rDAC
ADC
Antenna array
Multi-path’s AoDs/AoAs can be estimated via ABP
Analog and digital precoder and combiner are optimized using the high resolution estimates of AoDs/AoAs via ABP design
Numerical results (1/2)
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MSE of AoD estimation
: total # of transmit antennas
Parameters
: total # of receive antennas: total # of transmit RF chains
: total # of receive RF chains
: total # of data streams
Channel assumption
: total # of channel paths (6)
ª AoDs/AoAs take continuous valuesª AoDs/AoAs are uniformly distributed
Performance metric: MSE of AoD estimation
In radians
With increase in the number of antennas, more and narrower auxiliary beams are probed, which improves
the estimation performance
Promising MSE performance of AoD estimation can be obtained
Numerical results (2/2)
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Spectral efficiency performance
: total # of transmit antennas
Parameters
: total # of receive antennas: total # of transmit RF chains
: total # of receive RF chains
: total # of data streams
Channel assumption
: total # of channel paths (6)
ª AoDs/AoAs take continuous valuesª AoDs/AoAs are uniformly distributed
Assume , the performance gap is marginal
Multi-path’s AoDs/AoAs estimation performance via ABP design is promising
Conclusionsu High-resolution AoD/AoA estimation method via ABP is possible
ª Suited for mmWave MIMO with directional beamformingª Quantizing the ratio metric results in better performanceª Applicable to beamformed control channels and data channels design in practice
u Future workª ABP-based AoD/AoA estimation assuming arbitrary antenna array geometryª Exploiting TDD channel’s reciprocity to enable mmWave MIMO precodingª Simultaneously employing multiple RF chains to facilitate the estimation
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Questions?
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Backup Slides
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System model for hybrid precoding
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Steering angle
transmit signal vector,
baseband precoder,
Complex Gaussian noise vector
baseband combiner,
analog precoder :analog combiner
similarly defined to
Analog precoder and combiners are simple spatial matched filters
Spatial channel model
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Total number of propagation paths
channel matrix Path-r’s gain
AoD (spatial frequency) to be estimated
AoA (spatial frequency) to be estimated
Assuming ULA
u Received signal with respect to each beam in the ABP (noiseless)
u Received signal strength with respect to each beam in the ABP
u A ratio metric via the difference and sum of and
Detailed derivations of ABP design principle
20is invertible with respect to
If
Performance evaluation of quantization options
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MSE of quantizing the estimated AoD:
MSE of quantizing the ratio metric:
The performance gap between two quantization options is marginal
The receiver requires knowledge of ABP parameters from the transmitter to estimate and quantize the AoD
In radians
Estimation of multiple angles using multiple RF chains (1/2)
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Beam finding for control channelsAn example of multiple AoDs estimation
u Multiple analog beams can be simultaneously probed by the TX and RX
u Consider noiseless reception and a given receive probing ,
u Assume that path-r’s AoD , and
Total number of transmit probings Total number of receive probings
Desired transmit ABP that covers path-r’s AoD Their positions in
Estimation of multiple angles using multiple RF chains (2/2)
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Beam finding for control channelsAn example of multiple AoDs estimation
u The ratio metric that characterizes path-r’s AoD is calculated as
u The quantized version of the ratio metric is fed back to the transmitteru Path-r’s AoD is estimated by the transmitter using the ratio metricu The above process iterates until all paths’ AoDs have been estimated