Multihop Wireless Communications Channels

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Multihop WirelessCommunications Channels

• John Boyer
• Supervisors David D. Falconer and Halim
  Yanikomeroglu
• Broadband Communications Wireless Systems
  Centre
• Department of Systems and Computer Engineering
• Carleton University

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Outline

• Introduction
• Motivation
• Relaying and Diversity
• Multihop versus Multiroute Diversity
• System Model
• Simulation Results
• Implementation Considerations
• Contributions
• Future Research

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Introduction

• Multihop Wireless Communications Channel (MWCC)
• Mobile terminals have the ability to relay
  information when they are neither the initial
  source nor the final destination
• 4 channel models using 2 relaying models
• Decoded relaying intermediate terminals
  digitally decode and re-encode the received
  signal from immediately preceding terminal
• Amplified relaying intermediate terminals
  amplify the received signal from immediately
  preceding terminal
• Decoded relaying with diversity intermediate
  terminals combine, digitally decode and re-encode
  the received signal from all preceding terminals
• Amplified relaying with diversity intermediate
  terminals combine and amplify the received signal
  from all preceding terminals

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Motivation

• Traditional cellular infrastructures are quickly
  approaching theoretical information capacity
  levels
• New systems paradigms are required to meet demand
• Potential benefits of MWCCs include
• Increased coverage area
• Decreased transmit power
• Increased information capacity
• Increased flexibility and robustness
• Decreased deployment overhead
• Mathematical characterizations are required in
  order to quantify potential performance benefits
  and highlight important implementation
  considerations

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Relaying and Diversity

• Decoded Relaying versus Amplified Relaying
• Considerations are noise propagation, error
  introduction, and delay
• Decoded relaying does not propagate noise along
  the channel, introduces the possibility of
  decoding error at each intermediate terminal, and
  experiences delay due to intermediate terminal
  decoding as well as signal propagation
• Amplified relaying does propagate noise along the
  channel, introduces the possibility of decoding
  error at the destination terminal, and
  experiences delay due solely to signal
  propagation
• Multihop versus Multihop Diversity
• Considerations are performance and complexity
• Multihop diversity has better performance but
  entails significantly more complex receiver
  structures and protocols

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Multihop versus Multiroute Diversity
a) Multihop diversity
b) Multiroute diversity
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Multihop versus Multiroute Diversity

• Multihop Diversity
• Reception of signals from previous terminals
  along a single route
• Natural generation of multiple secondary signals
• Always provides a gain
• Multiroute Diversity
• Reception of signals from different terminals
  along multiple routes
• Artificial generation of multiple secondary
  signals
• Only provides a gain when multiple routes are
  statistically similar and signal to noise ratios
  are fairly high

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System Model
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System Model

• Each terminal Ti transmits the signal
• Each terminal Ti receives the signal
• Each terminal Ti has a signal to noise ratio

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System Model

• Each terminal Ti has a probability of error for
  BPSK
• Each terminal Ti has a probability of outage

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Simulation Results

• Channel models are compared using a normalized
  total power constraint
• Graphs present probability of error versus
  normalized singlehop signal to noise ratio
• 1st simulation presents results for fixed
  intermediate terminal positions with n hops
• 2nd simulation presents results for variable
  intermediate terminal position with 2 hops

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Simulation Results

• 1st simulation validates the theory and indicates
  that significant performance improvements can be
  realize through the use of multihop channels,
  especially those employing multihop diversity
  combining
• 2nd simulation indicates the performance
  improvements are fairly sensitive the to the
  location of the intermediate terminals and
  highlights the care that must be taken when
  selecting intermediate terminals
• Multihop Diversity gt Multihop gt Singlehop
• Amplified Relaying gt Decoded Relaying

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Implementation Considerations

• Receiver tracking speed
• Results in severe performance degradation
• Improved decoded relaying receiver models
• Possible use of error metrics from previous
  terminals
• Relaying channel allocation
• Relaying in same channel possible in future
• Feedback and feedforward interference
• Signal to interference plus noise ratio is
  limited by processing gain
• Propagation and processing delay
• Decoded relaying experiences significantly more
  delay

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Implementation Considerations

• Interference distribution and power control
• Redistribution of interference
• Power control equated to propagating routing
  information
• Multiple access schemes
• Diversity channels only applicable for CDMA and
  TDMA
• Global combining versus local combining with TDMA
• Adaptive modulation
• Only applicable with decoded relaying multihop
  channel
• Terminal complexity
• Limits the maximum number of hops

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Contributions

• Novel multihop channel models
• Multihop versus multiroute diversity
• Analysis of informative simulations
• Comparison of relaying and diversity schemes
• DRMD improves when intermediate closer to source
• ARMD improves when intermediate closer to
  destination
• Amplified relaying useless without fast tracking
• SINR limited by system processing gain
• Implementation considerations

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Future Research

• Refining theoretical results (reducing
  approximations)
• Formalizing argument against multiroute diversity
• Characterizing performance versus tracking error
• Derivation of optimal decoded relaying receiver
  model
• System level characterizations including
  coverage, interference distribution, throughput
  distribution, and capacity
• Integration of mobility models
• Application as link metrics for ad-hoc routing
  protocols