ECSE 6592 Wireless Ad Hoc and Sensor Networks Spatial Diversity in Wireless Networks

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ECSE 6592 Wireless Ad Hoc and Sensor Networks
Spatial Diversity in Wireless Networks

• Hsin-Yi Shen
• Nov 3, 2005

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Introduction

• Main characteristic in wireless channels-
  randomness in users transmission channels and
  randomness in users geographical locations
• Diversity- Convey information through multiple
  independent instantiations of random attenuations
• Spatial diversity- through multiple antennas or
  multiple users

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Wireless Channel Characteristics

• Three kind of attenuations-path loss, shadowing
  loss, fading loss
• Path loss Signals attenuate due to distance
• Shadowing loss absorption of radio waves by
  scattering structures
• Fading loss constructive and destructive
  interference of multiple reflected radio wave
  paths

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Attenuation in Wireless Channels
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Wireless Channel Characteristics

• Key parameters of wireless channels-coherence
  time, coherence bandwidth
• If symbol periodgtcoherence time, the channel is
  time selective
• If symbol periodlt channel delay spread, the
  channel is frequency selective

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MIMO Channel Model
H(kl) is the lth tap of the Mr x Mt channel
response matrix, z is noise vector
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Theoretical Consideration

• Information-Theoretic results for
  multiple-antenna channels
• Information-Theoretic results for multi-user
  channels
• Diversity order
• Design consideration

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Information-Theoretic results for multiple
antenna channels (1)
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Information-Theoretic results for multiple
antenna channels (2)

• Assume the receiver had access to perfect channel
  state information through training or other
  methods

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Information-Theoretic results for multiple
antenna channels (3)

• At high SNR the outage probability is the same as
  frame error probability in terms of SNR exponent
• For given rate, we can compare performance
  through an outage analysis

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Information-Theoretic results for multi-user
channels

• Two types of topology- multiple access channel
  and broadcast channel

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Information-Theoretic results for multi-user
channels
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Diversity Order and multiplexing gain
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Relation between Diversity Order and Multiplexing
Gain
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Relation between rate and SNR
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Design Consideration

• Space time code with low decoding complexity and
  achieving maximum diversity order
• Trade-off between diversity order and rate
• If system is delay-constrained, design with high
  diversity order and lower data rate
• Fairness for resource sharing between users
• Cross layer design

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Signal transmission

• Transmitter Techniques- spatial multiplexing,
  space-time trellis code and block codes
• Receiver techniques- joint equalization with
  channel estimation, space-time code decoding

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Spatial Multiplexing (Bell Labs Layered
Space-Time Architecture, BLAST)

• Multiple transmitted data streams are separated
  and detected successfully using a combination of
  array processing (nulling) and multi-user
  detection (interference cancellation) techniques
• A broadband channel scenario using a MIMO
  generalization of classical decision feedback
  equalizer (DFE)
• The nulling operation is performed as
  feed-forward filter and the interference
  cancellation operation is performed by the
  feedback filter

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Spatial Multiplexing-continued

• May have error propagation
• The presence of antenna correlation and the lack
  of scattering richness in the propagation
  environment reduce the achievable rates of
  spatial multiplexing techniques
• Enhancement Use MMSE interference cancellation,
  perform ML detection for first few streams

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Space time coding

• Improve downlink performance without requiring
  multiple receive antennas
• Easily combined with channel coding
• Do not require channel state information at the
  transmitter
• Robust against non-ideal operating conditions

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Space-time Trellis codes

• Maps information bit stream into Mt streams of
  symbols
• Decoding complexity increases exponentially as a
  function of the diversity level and transmission
  rate
• Example

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Space time block codes
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Cons and pros of space time block codes

• Achieve full diversity at full transmission rate
  for any signal constellation
• Does not require CSI at the transmitter
• ML decoding involves only linear processing at
  the receiver
• Does not provide coding gain
• A rate-1 STBC cannot be constructed for any
  complex signal constellation with more than two
  transmit antennas
• Simple decoding rule valid only for flat-fading
  channel where channel gain is const over two
  consecutive symbols

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Tradeoff between diversity and throughput

• BLAST achieves max spatial multiplexing with
  small diversity gain
• Space time codes achieves max diversity gain with
  no multiplexing gain
• Linear dispersion codes (LDC) achieve higher
  rate with polynomial decoding complexity for a
  wide SNR range
• Build in the diversity into the modulation

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Build Diversity into modulation
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Receiver techniques

• Coherent and non-coherent techniques
• Coherent technique require channel state
  information by channel estimation or training
  sequences and feed this to joint
  equalization/decoding algorithm
• Non-coherent techniques does not require CSI and
  more suitable for rapidly time-varying channels

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Joint Equalization/Decoding techniques

• M-BCJR algorithm at each trellis step, only M
  active states associated with the highest metrics
  are retained
• Significant reduction in the number of
  equalizer/decoder states

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Sphere decoder

• Suitable for codes with lattice structures
• Perform ML search with low computation complexity

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Joint Equalization/Decoding of space time Block
codes

• Eliminate inter-antenna interference using a low
  complexity linear combiner
• Single-carrier frequency domain equalizer (SC-FDE)

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Performance of SC-FDE
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Non-coherent techniques

• Does not require channel estimation
• Include blind identification and detection
  schemes
• Exploit channel structure (finite impulse
  response), input constellation (finite alphabet),
  output (cyclostationarity) to eliminate training
  symbols
• Use ML receiver which assumes statistics about
  channel state but not knowledge of the state
  itself

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Summary in signal transmission

• Mitigate fading effect by using space diversity
• Use MIMO to realize spatial rate multiplexing
  gains
• Use equalization techniques (ex M-BCJR, SC-FDE)
  to mitigate channel frequency selectivity
• Use channel estimation and tracking, adaptive
  filtering, differential transmission/detection to
  mitigate time selectivity

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Networking issues

• Medium sharing resource allocation
• Mobility and routing
• Hybrid networks

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Resource allocation

• Allocation criteria rate-based criteria and
  job-based criteria
• Rate-based criteria provide average data rates to
  users which satisfy certain properties
• Job-based criteria schedule data delivery in
  order to optimize various QoS guarantees based on
  the job requests

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Resource allocation-Rate-Based QoS criteria

• Utilize the multi-user diversity inherently
  available in wireless channels
• Schedule users when their channel state is close
  to peak rate it can support
• gt inherent unfairness
• Keep track of the average throughput Tk(t) and
  rate Rk(t), transmit the user with the largest
  Rk(t)/ Tk(t) among the active users
• If channel is slow time-varying, introduce random
  phase rotations between the antennas to simulate
  fast fading

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Impact of spatial diversity

• Multi-antenna diversity provide greater
  reliability by smoothening channel variations
• Multi-user diversity utilize the channel
  variability across users to increase throughput
• Choose diversity techniques according to channel
  conditions, mobility and application constraints
• For example, low delay-applications with high
  reliability requirement may use multi-antenna
  diversity with space time codes

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Hybrid Networks

• Two approaches to increasing TCP efficiency in
  hybrid networks
• Reduce error rate in wireless channel by using
  more sophisticated coding schemes, such as
  space-time codes
• Use explicit loss notification (ELN) to inform
  the sender that the packet loss occurred due to
  wireless link failure rather than congestion in
  wired part

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Space time code and TCP throughput

• STBC-enhanced 802.11a achieves a particular
  throughput value at a much lower SNR value than
  the standard 802.11a
• STBC modify the SNR region under which a
  particular transmission rate should be chosen
• STBC increase the transmission range and improve
  robustness of WLANs

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STBC-enhanced 802.11a

• The difference between STBC 802.11a and 802.11a
  becomes smaller when channel quality is
  sufficiently good
• STBC-802.11a can switch to faster transmission
  mode at much lower SNR values

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Conclusion

• In wireless networks, power and spectral
  bandwidth are limited
• Limitation on signal processing at terminal and
  requirement of sophisticated resource allocation
  techniques due to variation in capacity
• Spatial diversity improves data rates and
  reliability of individual links
• Space time codes improves link capacity and
  system capacity through resource allocation

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Future works

• Space time code design
• Implementation issues-low-cost multiple RF chains
  and low-power parallelizable implementation of
  STC receiver signal processing algorithm
• Receiver signal processing-the development of
  practical adaptive algorithm that can track rapid
  variation of large number of taps in MIMO channel
  and/or equalizer
• Standardization activities

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Reference

• 1S. N. Diggavi, N. Al-Dhahir, A. Stamoulis, and
  A.R. Calderbank, Great Expectations The Value
  of Spatial Diversity in Wireless Networks,
  Proceeding of The IEEE, Vol. 92, No. 2,
  pp219-270, Feb 2004
• 2 Sergio Verdu, Multiuser Detection,
  Cambridge University Press, 1998