1. 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

2. Outline Introduction Access request and detection Performance analysis
Simulations
Conclusions

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. Access Request Detection

11. 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

12. Performance Analysis

13. Consider Detection Error

14. Consider Detection Error

15. Simulations: Throughput
Compare throughput: theory and simulated

16. Simulations: Delay
Compare delay: theory and simulated

17. 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)

18. Simulations: ARP Detection
Decision error rate and traffic load

19. 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