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

Outline Introduction Access request and detection Performance analysis Simulations Conclusions

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

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

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

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

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

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

Access Request Packets Designed with OC codes Packet (slot) length: Efficient for large number of users

Access Request Detection

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

Performance Analysis

Consider Detection Error

Consider Detection Error

Simulations: Throughput Compare throughput: theory and simulated

Simulations: Delay Compare delay: theory and simulated

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)

Simulations: ARP Detection Decision error rate and traffic load

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