ADVANCES IN SMART ANTENNAS
Adding smart antennas to an ad hoc network can, in some instances, actually decrease the network capacity. However, when added properly, that is, using cross layer optimization techniques, smart antennas can provide gains that are in excess of M-fold.
Wireless local area networks (WLANs) are becoming ubiquitous with rapid growth in both the home and enterprise markets. However, users are often not satisfied with the coverage and performance of these networks for several reasons. First, the quality of service (QoS) for each user may not be consistent. For example, the user may be too far away from an access point (AP), behind a wall, in a “dead” spot, or suffering from low data rate due to range and/or interference problems. Furthermore, the user may be working with a laptop or handset with a battery where the power drain of the WLAN may be unacceptably high, or may simply find that one AP cannot cover their house. Two key techniques that can be used to overcome these problems are smart antennas and ad hoc networking . Although smart antennas are a physical-layer technique and ad hoc networking is a media access control (MAC) layer technique, one should not assume that the two techniques can be implemented independently. That is, just adding a smart antenna technique that increases the capacity of a link M-fold does not necessarily mean that the capacity of the ad hoc network will also be increased M-fold. Indeed, as we show in this article, adding smart antennas to an ad hoc network can, in some instances, actually decrease the network capacity. However, when added properly, that is, using cross layer optimization techniques, smart antennas can provide gains that are in excess of Mfold. In this article, we first briefly describe smart antennas and discuss their properties that are useful in ad hoc networks. This includes the two basic types of smart antennas: directional (or multibeam) and adaptive arrays. We then briefly describe ad hoc networks, their implementation issues, and how smart antennas can be used to overcome these issues. Finally, we discuss how smart antennas can be easily added to ad hoc networks to gain most of their benefits.
SMART ANTENNAS A smart antenna is a multi-element antenna where the signals received at each antenna element are intelligently combined to improve the performance of the wireless system. The reverse is performed on transmit. These antennas can increase signal range, suppress interfering signals, combat signal fading, and increase the capacity of wireless systems.