A high-speed wireless LAN

ireless local area networks (WLANs) small enough for portable computing devices have transmission rates up to a few Mbps, the lower end of rates obtained in IEEE 802-compliant wired LANs. WLANs can provide a useful service when the application demands and number of users are low. Accommodating more users and multimedia traffic requires much higher performance, from several tens of Mbps to over 100 Mbps. Achieving wireless access at these high rates is difficult. WLAN systems face technical problems similar to those encountered in outdoor wide-area radio-based systems, including the limited available bandwidth and fading noise due to multipath interference and blockage. The goal in WLAN system design is to transmit at the maximum information rate with an acceptable probability of error and minimum equipment complexity, power, and cost. Competing approaches use either infrared radiation or radio waves in the microwave or millimeter-wave bands. The success of radio-based indoor WLAN systems depends on access to the radio spectrum. There is fierce competition for the small amount of unallocated spectrum in the microwave bands. Therefore, research on high-speed WLANs has focused on infrared and millimeter-wave carriers. 1 With a virtually unlimited and unregulated spectrum available, many researchers have proposed and developed infrared-carrier techniques for wireless indoor access to LANs. So far, the most practical infrared techniques use diffuse infrared links; they have demonstrated raw link transmission rates up to 50 Mbps in cells a few meters in radius. 2 Nevertheless, we believe that the prospect of economically achieving reliable infrared transmission rates much above 10 Mbps for truly portable terminals in the next five years is poor. The obstacles are the high power requirements and the delay spread of the diffusely reflected (multipath) received signals. Overcoming these obstacles requires remarkable breakthroughs: either equalizers to accommodate the delay spread or adap-tive directional links that limit the delay spread while maintaining an acceptable signal to noise ratio. We have developed a radio solution at millimeter-wavelength frequencies, where the spectrum is sufficient to accommodate link speeds of hundreds of Mbps. Using a test bed with burst-mode transmission capability and an experimental 40-GHz radio, we have demonstrated a picocellular approach with a range of approximately 10 meters and link rates up to 185 Mbps. In addition, we have built a prototype modem with a raw link rate of 54 Mbps for use in a high-speed indoor WLAN demonstrator. The spectral band from 54 to 65 …