New Structures for Space-Time Coding

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New Structures for Space-Time Coding
Beamforming and TheirEffects on the Connectivity
of Wireless Ad-hoc Networks

• Hamid Jafarkhani
• Deputy Director
• Center for Pervasive Communications and Computing
• University of California, Irvine

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Important Issues in Space-Time Coding

• Diversity
• Performance
• Bandwidth efficiency
• Complexity
• Fitness in the overall system
• Flexibility

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Recent Advances

• Code design for non-coherent detection
• Improving the rate of block codes
• Improving the performance of trellis codes
• Combining array processing and space-time coding
• Combining with OFDM, beamforming,

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Advantages of Orthogonal Space-Time Block Codes

• Simple decoding Each symbol is decoded
  separately using only linear processing.
• Maximum diversity They provide the maximum
  possible diversity shown by theory.
• Interesting mathematical structure

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Disadvantage of Orthogonal Space-Time Block Codes

• Full rate codes do not exist for more than two
  transmit antennas

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Quasi-Orthogonal Space-Time Block Codes

• Full rate codes are possible if we allow pairwise
  (instead of symbol) decoding
• Adding rotation provides full diversity

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Simulation Results

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A Parameterized Class of Space-Time Block Codes

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Coding Gain Distance

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Set Partitioning for QPSK

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Example (Super-Orthogonal Space-Time Trellis Code)

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Coding Gain Distance (CGD)
number of States Rate Bits/sec/Hz Min CGD SOSTTC Min CGD STTC
4 1 64 32
4 2 16 4
4 2.5 4 -
4 3 2.54 -
2 1 48 16
2 2 12 -
8 3 2.54 2

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Simulation Results

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Decoding Complexity
Decoding Complexity (per symbol)
STTC (4-state, QPSK) 156
SOSTTC (4-state, QPSK) 58
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Advantage of SOSTTC

• Systematic method for code construction
• Combined coding gain/diversity gain
• Simplified ML decoding
• Closed form performance evaluation
• Extension to SQOSTTC for four transmit antennas

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Block Diagram of a Transmit Beamforming System
Bit stream for Ant-1
Input Bits
Encoder
Bit Stream for Ant-2
Receiver
Base Station
Mobile Station
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Shortcomings of Channel Feedback from Receiver

• Channel estimation error at the receiver.
• Quantization loss.
• The delay between estimation time and the time
  that feedback is used.

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Channel Feedback Quality

• If the feedback quality drops too low, the
  beamforming scheme should gradually fall back to
  the non-beamformed scheme.
• Perfect Channel Feedback Beamforming
• No Channel Feedback Space-Time Coding
• What shall we do in between?

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Linear beamforming scheme for STBCs
Feedback CSI
STBC Encoder (OSTBC/QSTBC)
Multiply with Beamforming Matrix P
Linear Processing at receiver
Input Bits
CPC
Decode Bits
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Beamforming Matrix Design Criterion
Beamforming term
Diversity term
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Simulation result (1)
Performance comparison between beamformed OSTBC
and beamformed QSTBC, PSK constellations, four Tx
antennas, 1 RX antenna, 3bits/channel use,
channel feedback quality 0.9 .
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A Simplified SOSTTC Beamforming Scheme
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Our Goal

• Finding the optimal configuration for the given
  channel feedback quality.
• Finding an analytical solution that is easily
  tractable.
• Adapting the system to channel conditions.

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Why is it promising?

• Low complexity
• Good performance
• Identical to optimal beamforming for perfect
  channel feedback and identical to STBC for no
  channel feedback.
• Adaptive structure for different configurations

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Special challenges for Adhoc Networks

• Nodes may have different resources
• Power
• Size
• Level of mobility
• Number of antennas
• As a result, nodes may use different modulation,
  coding, and beamforming methods

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Connectivity

• Conventional connectivity mesures do not work and
  may not be meaningful.
• There is a need for new connectivity metrics
  specially for hybrid networks that include nodes
  with different number of antennas.

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Geometric Disk Model

• Two nodes are connected if their distance is
  smaller than the transmission radius.
• Drawback Disk models do not reflect the wireless
  networking reality.

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SINR Model

• Two nodes are connected if the signal to noise
  and interference ratio is bigger than a
  threshold.
• Drawbacks
• SINR does not reflect coding/diversity impacts.
• A given SINR translates to different capacities
  and symbol error rates (SERs).

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Sample QPSK SER-SINR Plots
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Connectivity
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Our Goal

• Defining new connectivity metrics based on
  capacity and SER.
• Studying the effects of STC on the connectivity
  of wireless adhoc networks.
• Studying the effects of using multiple antennas
  in the connectivity of hybrid networks.