IEEE Wireless Communications • April 2008 82 1536-1284/08/$25.00 © 2008 IEEE 270 300 240 10 90 0 -10 60 120 50 10 A CCEPTED FROM O PEN C ALL INTRODUCTION In recent years, the deployment of wireless local area networks (WLANs) 1 has increased dramati- cally. WLAN technology has found its way into the home, office, and public domain (hotels, restaurants, airports, etc.). Nowadays, almost every large terminal, for example, a laptop com- puter, and even a small terminal, such as a smart-phone, has built-in WLAN capability. Because the technology has improved in the area of security, and because various business models have shown their strengths, WLAN capability now is offered by most wireless network pro- viders. Although a WLAN now offers data rates of hundreds of megabits per second (Mb/s), the offered capacity often is not sufficient in multi- user scenarios and cannot be compensated by the installation of more access points. The only limiting factor for mass usage of WLANs at the moment is the pricing policy of the network pro- viders. In places where access is granted for free, such as in the office or in some conference cen- ters, the capacity per user sometimes is not even sufficient to run basic services, such as a virtual private network (VPN). If flat rates, as they are known in the wired world, are introduced for wireless, this observation probably also will hold for the aforementioned WLANs of network pro- viders. The interference problem is a major contrib- utor to the limited capacity of WLANs. First, the WLAN spectrum is located within the congested unlicensed industrial scientific and medical (ISM) band that is subject to interference from other communication systems such as Bluetooth and digital enhanced cordless telecommunica- tions (DECT). Second, due to the lack of avail- able spectrum, frequency reuse is so frequent that it causes severe co-channel interference problems in dense traffic areas. This is usually referred to as the hidden nodes problem [1] in WLAN that degrades network capacity. Third, the medium access (MAC) layer of a WLAN cannot solve the hidden nodes problem in the co-channel interference scenario, although it can be well solved by using the request to send/clear to send (RTS/CTS) [2] mechanism in a single hotspot scenario. This is referred to as the large interference range problem [3]. The interference problem can be mitigated greatly in the spatial domain by utilizing multiple antenna techniques. We classify the multiple antenna techniques into two classes, signal space techniques and beam space techniques. Signal space techniques defines a class of techniques, for example, multiple input multiple output (MIMO), time reversal, and diversity-combining techniques that require the instantaneous chan- nel state information (CSI) of each transmitting- receiving antenna pair at the receiver end and even at both ends. However, beam space tech- CHENGUANG LU, FRANK H. P. FITZEK, P ATRICK C. F. EGGERS, OLE KIEL JENSEN, GERT F. PEDERSEN, AND TORBEN LARSEN, AALBORG UNIVERSITY ABSTRACT In dense traffic areas, wireless local area net- works (WLANs) suffer from interference prob- lems due to the congestion of the open and unlicensed ISM band. To mitigate these prob- lems, a terminal-embedded beamforming frame- work is proposed. This beamforming is capable of focusing the transmission and the reception in the direction of the relevant access point. At the same time, the framework is backward compati- ble with existing WLAN networks. The beam- forming enabled terminal benefits in terms of capacity, security, and energy efficiency without requiring any changes on the network side or requiring new investment on the part of network providers. The beamforming solution is seen as an attractive value-added feature, as well as a low-cost solution for the future WLAN terminal design. This opens the door for mobile device manufacturers to include the proposed solution in their product line. In this work, the backward- compatibility challenges are addressed, and pos- sible solutions and limitations are discussed. A prototype design built on a laptop computer also is described. The experimental results show a significant capacity increase in both an interfer- ence-free scenario and an interference-limited scenario. T ERMINAL -E MBEDDED B EAMFORMING FOR W IRELESS L OCAL A REA N ETWORKS 1 In this article, WLAN refers to the IEEE 802.11 infra- structured wireless local area network. The beamforming solution is seen as an attractive value-added feature, as well as a low-cost solution for the future WLAN terminal design. This opens the door for mobile device manufacturers to include the proposed solution in their product line. Authorized licensed use limited to: National Chung Cheng University. Downloaded on January 12, 2009 at 04:17 from IEEE Xplore. Restrictions apply.