1. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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

2. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Outline Introduction Joint PHY/MAC Layer design Performance Analysis Simulations Conclusions

3. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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

4. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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?

5. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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.

6. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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 User n User 1User 2 Controller (User 0)

7. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Packet Flow in the Wireless Networks … Access request packets (Controller & Users) Data packets (Users) Access request packets (Controller & Users) Data packets (Users) … Problem: if there are too many unknown users, contentions in access request slots are high

8. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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

9. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Random Access Scheduling Protocol 1.Central controller ask all active users for access requests 2. Active users generate random numbers Form subgroups distributedly by the numbers Transmit access request packets according to the random numbers 3. 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 4.Go to 2

10. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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

11. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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

12. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Access Request Collision Resolution There are at most K different collided access codes, although there are unknown number of users Received collided signal

13. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Access Request Collision Resolution

14. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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)

15. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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

16. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Performance Analysis (cont.) System Efficiency: ratio of successful to total transmission in one frame  instantaneous throughput Without detection error: With detection error

17. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Performance Analysis (cont.) System Throughput: Packet delay:

18. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Simulations DER versus SNR, for offered load 0.5 and 1: satisfactorily low detection errors for even low SNR

19. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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.

20. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design Simulations (cont.) Packet delay vs. throughput: Has better (much smaller) packet delay than the ALOHA. With DER=0, 0.01, 0.05.

21. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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

22. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design -Questions? -Issues? -Comments?

23. X. Li, W. LiuICC 2003 - May 11, 2003A Joint Layer Design 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