Optimization of User Association for MIMO Wireless Local Area Networks

PhD Thesis Proposal Defence

Title: "Optimization of User Association for MIMO Wireless Local Area Networks"


Mr. Wang Kit WONG


The ever-growing wireless bandwidth requirement has spurred recent development 
of Multiple-Input-Multiple-Output (MIMO) based physical-layer technologies, 
such as transmit beamforming, interference cancellation, and interference 
alignment. Such technologies substantially improve the performance of MIMO 
WLANs over single-antenna WLANs.

Users are often covered by multiple access points (APs) in today’s WLAN 
deployment. User association control is to assign or associate users in the 
overlapping regions to one of the APs for service. The default implementation 
always associates a user to the best signal AP. This leads to unsatisfactory 
throughput performance at congested APs and channel under-utilization at 
others. The performance of MIMO WLANs are highly related to user association. 
In this thesis, we study three challenging optimization problems in WLANs: 1) 
user association; 2) joint user association and random access control and 3) 
joint user association and AP grouping.

We first study the user association problem for MIMO WLANs. We formulate user 
association as an integer programming problem, which considers user migration 
cost. Then, we propose a factor-4 approximation algorithm to tackle the 
problem, which achieves similar performance as previous schemes but incurs much 
less user migration cost.

Secondly, we study the joint user association and random access control 
optimization problem. Random access control determines APs’ transmit 
probabilities, which is another factor affecting the performances of WLANs. APs 
associated with more users require higher channel access opportunities. On the 
other hand, APs with higher transmit probabilities should serve more users. 
Therefore, they are coupled. We propose a distributed algorithm termed CARA 
(Joint Client Association and Random Access Control) to tackle the joint 
optimization problem, which achieves higher throughput than sequential 

Finally, we study the joint user association and AP grouping optimization 
problem. Coordinated beamforming (CB) has emerged as a promising interference 
coordination approach for cooperative MIMO systems. CB allows interfering APs 
to transmit as a group. Due to heavy channel state information (CSI) feedback 
overhead, APs need to be partitioned into cooperation groups no larger than a 
certain size where only APs in the same group are able to cooperate with CB. We 
study the novel optimization problem of minimizing AP load by joint AP grouping 
and user association as they are coupled. Based on alternating direction 
optimization, we propose DAGA (Distributed Joint AP Grouping and User 
Association) to tackle the optimization problem. DAGA produces an approximated 
user association solution which is at most elogm (m is the number of APs) times 
of the optimum.

Date:			Friday, 21 December 2018

Time:                  	4:00pm - 6:00pm

Venue:                  Room 3494
                         (lifts 25/26)

Committee Members:	Prof. Gary Chan (Supervisor)
 			Prof. Cunsheng Ding (Chairperson)
 			Dr. Pan Hui
 			Prof. James Kwok

**** ALL are Welcome ****