Xiaohua Li and Wenyu Liu

1. Smart Contention Resolution Random Access Wireless Networks With Unknown Multiple Users: A Joint Layer Design Approach Xiaohua Li and Wenyu Liu Department of Electrical and Computer Engineering, State University of New York at Binghamton, Binghamton, NY 31902 Motorola Inc., Libertyville, IL 60048 A Joint Layer Design

2. Outline Introduction Joint PHY/MAC Layer design Performance Analysis Simulations Conclusions A Joint Layer Design

3. Introduction Issues: • Wireless spectrum is becoming scarce • Advanced access schemes required to improve efficiency to improve throughput to support packet priority/QoS • Random channel access is more preferred • Need resolve contentions A Joint Layer Design

4. Introduction(cont.) Issues:(cont.) • Contention resolution is challenging in wireless networks • Traditional ways: operate in MAC layer, collided signals discarded instead of utilized • Signal processing ways: separate collided signal in PHY layer Questions: Benefit from both PHY/MAC layer? A Joint Layer Design

5. Introduction(cont.) Proposed Solution: • Joint PHY/MAC layer design • PHY: signal separation or multiuser detection • MAC: smart packet scheduling • Compared with traditional ways: • collided signal is used instead of discarded • Compared with other signal processing ways • Preserves packet priority, support QoS • Works with unknown multiple users • Computationally efficient and robust, i.e., does not suffer ill-channel or near-far conditions. A Joint Layer Design

6. User 1 User 2 User n Controller (User 0) System Model of Wireless Networks • The wireless network has a central controller and multiple unknown mobile users: • Central controller: PHY collision separation, MAC packet scheduling A Joint Layer Design

7. Packet Flow in the Wireless Networks • Problem: • if there are too many unknown users, contentions in access request slots are high A Joint Layer Design

8. Packet Flow in the Wireless Networks (cont.) • Solutions: divide and conquer • Procedure: • Each active user generate I random number • Each random number is corresponding to a unique access request packet • Actives users are grouped according to I or access request packets A Joint Layer Design

9. Random Access Scheduling Protocol • Central controller ask all active users for access requests • Active users generate random numbers • Form subgroups distributedly by the numbers • Transmit access request packets according to the random numbers • Central controller detects collided packets • If there are active users from different subgroups, there access request packets collide • Collided packets can be detected successfully • From collided signal, central controller know active subgroups • Central controller asks each subgroup one-by-one to request again • Go to 2 A Joint Layer Design

10. Random Access Scheduling Protocol (cont.) • Packet priority: • Users generate random numbers according to priority • Central controller gets priority information after separating collided access requests • Central controller arrange active users to transmit data packets according to their priorities. • Efficiency: • Contention exists in access request slots only • If active users have different random integer, no collision happens for data packet transmission • Size of access request slots is reduced by divide-and-conquer • Collision detection: • Orthogonal codes can be used because of the reduced access request slot size A Joint Layer Design

11. Access Request Collision Resolution • Key point: need K different access request packets (codes) only • One codes for each group instead of user • All active users in a group transmit the same code, and will be divided in the next turn • Each user generate random number • find from • Select the corresponding code to transmit as access request packet A Joint Layer Design

12. Access Request Collision Resolution • There are at most K different collided access codes, although there are unknown number of users • Received collided signal A Joint Layer Design

13. Access Request Collision Resolution A Joint Layer Design

14. Access Request Collision Resolution • Active group/user detection (collision separation) • Construct received sample vector • Apply detector to calculate decision metrics • Compare with threshold to determine whether each group k is active (feedback to MAC) A Joint Layer Design

15. Performance Analysis Probability of Data Packet Collision: where A is the maximum range of the random number, u is the number of active users contending for channel access at the same time A Joint Layer Design

16. Performance Analysis (cont.) System Efficiency: ratio of successful to total transmission in one frame  instantaneous throughput Without detection error: With detection error A Joint Layer Design

17. Performance Analysis (cont.) System Throughput: Packet delay: A Joint Layer Design

18. Simulations DER versus SNR, for offered load 0.5 and 1: satisfactorily low detection errors for even low SNR A Joint Layer Design

19. Simulations (cont.) Throughput vs. offered traffic load : Achieve much higher throughput than ALOHA and CSMA. Especially, if the DER is not too low, it achieves almost the offered traffic load up to unity. A Joint Layer Design

20. Simulations (cont.) Packet delay vs. throughput: Has better (much smaller) packet delay than the ALOHA. With DER=0, 0.01, 0.05. A Joint Layer Design

21. Conclusions • Use signal processing, specifically, multiuser detection principles, to resolve random access contentions. • Central controller schedules random access request packet detection • Collisions are resolved through signal separation in systems with unknown multiple users • Enhance throughput, support QoS A Joint Layer Design

22. ? ? ? • Questions? • Issues? • Comments? ? ? A Joint Layer Design

23. Thank You !!! Xiaohua Li Department of Electrical and Computer Engineering State University of New York at Binghamton Binghamton, NY 12309 Email: xli@binghamton.edu Wenyu Liu Personal Communication Sector Motorola Inc. Libertyville, IL 60048 Email: wenyu_liu@yahoo.com A Joint Layer Design