Make Random Access Contentions Transparent by Orthogonal Complementary Codes in Wireless Communications

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Make Random Access Contentions Transparent by
Orthogonal Complementary Codes in Wireless
Communications

• Xiaohua (Edward) Li
• Department of Electrical and Computer Engineering
• State University of New York at Binghamton

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Outline

• Introduction
• Access request and detection
• Performance analysis
• Simulations
• Conclusions

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Introduction Random Access

• Random access efficient for heterogeneous
  traffic
• Problems of random access contention
• Throughput loss ? severe in high traffic load
• Delay increased ? difficult to maintain QoS
• Resolve contention to improve efficiency

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Introduction Contention Resolution

• Some traditional methods
• Slotted ALOHA, CSMA/CA
• Reservation-ALOHA, RTS/CTS
• TDMA-CDMA
• Common characteristics
• Treat problem in MAC layer only
• Collided packets simply discarded
• Do not utilize information of physical layer
  signals

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Introduction Contention Resolution

• Alternative methods separate collided signal
• Physical layer signal processing
• By, e.g., repeated transmission, multi-user
  detection, constant modulus, etc
• Difficulties
• Signal separation is difficult, suffers many
  practical problems, e.g., ill-channel conditions,
  complexity
• Solution
• joint physical/MAC layer design

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Proposed Method Basic Idea

• Use access request packets (ARP)
• Physical layer
• Separate collided (ARP) only
• Make collision transparent to MAC
• MAC layer
• Schedule transmission of ARP and data packets
• Make physical layer signal separation easy

• Orthogonal complementary codes
• Efficient and robust collision separation

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System Design

• Slotted channel access request slot, data slot
• All active users transmit ARP in the same access
  request slot
• Contentions exist in access request slot only,
  not in data slot

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Orthogonal Complementary Code

• OC code set properties
• with I flocks, J family/flock, L-bit code/family
• Processing gain JL
• Orthogonal among flocks, irrespectively shifting
• Orthogonal within each flock with non-zero
  shifting

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Access Request Packets

• Designed with OC codes
• Packet (slot) length
• Efficient for large number of users

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Access Request Detection
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Joint Physical/MAC Layer Design

• Protocol
• At the beginning of a frame, central controller
  asks for access request
• Active users transmit ARP
• Central controller detects access requests
• Assign data packet slots to active users
• Properties
• Efficient ARP structure, with user ID inherently
  embedded
• ARP collision separation efficient and robust to
  asynchronous, near-far, multipath

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Performance Analysis
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Consider Detection Error
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Consider Detection Error
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Simulations Throughput
Compare throughput theory and simulated
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Simulations Delay
Compare delay theory and simulated
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Simulations ARP Detection

• 60 users, guard length 10,
• Processing Gain 64, random channel with max
  length 5
• Random asynchronous delay (max 5)
• Random near-far (NF)

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Simulations ARP Detection

• Decision error rate and traffic load

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Conclusions

• Joint physical/MAC layer design to
• Resolve contentions, to improve efficiency
• Make contentions transparent, to support QoS
• Access request collision resolution with OC codes
• Efficient in computational complexity
• Robust to (ill) multipath channels, near-far
  asynchronous transmission