Mimo and Smart Antenna Techniques for 802.11a/b/g Networks
Introduction
Convenience and affordability have made Wi-Fi the dominant home networking technology. Its popularity has also raised the awareness of current Wi-Fi network limitations, spurring a rush to productize alternative technologies such as next generation HomePlug and UWB. With the combined advantage of mobility, ubiquity and massive volume, the chances of dislodging Wi-Fi's reign in the home are remote, especially since 802.11n, the next generation Wi-Fi standard body, is already working on addressing many of the current shortcomings.
The .11n proposals have centered on a wireless technology called MIMO (Multiple-In, Multiple-Out), and proprietary MIMO chipsets are already available in the market for experimentation. Ruckus Wireless BeamFlex™ technology applies the principles of MIMO to enhance today's 802.11a/b/g networks. This paper will discuss the wireless LAN problems and how MIMO addresses them. It will also examine the important implementation choices in MIMO and the BeamFlex approach.
What's the Problem?
The most frequent complaints about Wi-Fi home networks are inadequate range and spotty coverage. Performance fluctuations, often masked by the burstiness of data applications such as web surfing, become immediately apparent when the network is asked to support latency and throughput sensitive real-time applications such as online gaming and video streaming where instantaneous and stable bandwidth is necessary. Eventually, as broadband access speeds are upgraded to multi-megabits, the need for higher data rates than the current physical layer maximum of 54 Mbps will also become important.
The Wi-Fi Woes
Signal strength and noise level are the key determinants of wireless performance and range, (see Sidebar 1). It is well known that radio signals weaken with distance and impediments in the signal path. For example, an 802.11g network in a typical home loses performance to the point of being unusable at about 70 to 100 feet (23-33 meters), especially if there are intervening materials such as walls and doors that absorb or scatter the Wi-Fi signals, (see Sidebar 2, next page). But even the strongest signals can become undecipherable in the presence of “loud” noise, (nearby noise is always “loud”), resulting in receive errors and retransmissions. Beside the typical thermal and electromagnetic noise that exists in all homes, other major sources of noise are radio frequency (RF) interference, co- and adjacent channel interference and multipath interference.
