Increasing TCP Throughput and Fairness in Cognitive WLAN over Fiber Alireza Attar, Haoming Li, Victor C. M. Leung University of British Columbia 2332 Main Mall, Vancouver, BC, Canada Email: {attar, hlih, vleung}@ece.ubc.ca Abstract���The performance of legacy WLAN systems will significantly degrade as the number of active stations (STA) in the network increases, rendering satisfactory coverage of busy hot spots challenging in practice. A novel centralized architecture that coordinates several WLAN access points is the exploitation of Radio over Fiber (RoF) technology. In this paper we focus on spatial diversity techniques to enhance the performance of the newly proposed Cognitive WLAN over Fiber (CWLANoF) compared to legacy WLAN Extended Service Set (ESS). More specifically we develop an optimal transmitter-side beamforming technique and investigate the TCP performance enhancement as a result of usage of this scheme. Furthermore, the issue of fairness will also be discussed, where we introduce a fairness measure based on the distribution of TCP throughput among existing STAs in the network. The numerical results verify the superior performance of CWLANoF compared to WLAN ESS. I. INTRODUCTION A challenge in providing a reliable wireless access using Wireless LAN (WLAN) technologies, is to cope with the number of active users. It is well known that the network throughput will significantly decrease as the number of active users increases due to the higher collision probability. One solution to this problem is to cover the target location (such as enterprise offices, hotels, airports, conference rooms etc.) with more than one WLAN Access Point (AP), i.e., a spatial load balancing strategy. The IEEE 802.11x family of standards envisions the Extended Service Sets (ESSs) mode of WLAN operation, comprised of multiple APs, each forming its own Basic Service Set (BSS). However, the CSMA/CA nature of accessing the shared medium results in increased collisions in such an architecture due to hidden node or exposed node problems. A recent innovative solution to help coordinate a dis- tributed set of wireless APs is the integration of (optical) Fiber and Wireless (FiWi) [1]. In FiWi scenarios, a Passive Optical Network (PON) provides a high bandwidth backhaul for the wireless APs, facilitating coordinated service deliv- ery [2], [3]. An Optical/Electrical signal conversion and vice versa between the APs and a centralized coordinator enables transmitting/receiving radio signals over such an optical fiber backbone, a technique known as Radio over Fiber (RoF) [4]. The PON backbone might utilize Time or Wavelength Divi- sion Multiplexing, referred to as TDM-PON or WDM-PON respectively. In the former architecture two separate optical Fig. 1. Comparison of legacy WLAN ESS with TDM-PON and (logical) WDM-PON architectures. fiber lines, one for the uplink and one for the downlink communications, are shared by all APs. Any packet to or from the APs is replicated in the shared optical medium through simple splitters, as shown in Fig. 1. On the other hand, in the WDM-PON architecture the transmissions to and from the AP are separated by allocating a dedicated optical wavelength to each AP, thereby creating the logical star topology in Fig. 1. In CWLANoF, the APs are referred to as Remote Antenna Units (RAU) and it is clear that no processing power is available at RAUs. We focus on the WDM-PON architecture in this paper, however, in Section V we will briefly elaborate on the practical considerations when a TDM-PON architecture is utilized. Previous studies in [5], [6], [7], [8] investigated the feasi- bility of carrying a wide-band signal through RoF systems, specifically, multiple WLAN channels over low-cost multi- mode optical fibers. It was demonstrated that medium access control of WLAN in RoF systems is not affected by the optical delay [9], [10], [11]. However, most of these studies are concentrated on the design of optical-electric converter compo- nents. Furthermore, only simulcasting mode in the experiments on multiple WLAN channels over RoF were examined. The full potential of this architecture facilitated by the central 2010 24th IEEE International Conference on Advanced Information Networking and Applications 1550-445X/10 $26.00 �� 2010 IEEE DOI 10.1109/AINA.2010.76 1069