Design Principles for Wideband Wireless Communications

Cornell University, Ithaca NY

Investigators

Abstract

Abstract Broadband wireless communications has been identified as a national imperative; it is deemed essential for maintaining the country's premier position in information technology and accelerating the advance of the information age. This research is aimed at enhancing the performance of wideband wireless multiaccess systems by optimizing tradeoffs between coding and spreading, capitalizing on advantages afforded by spatial diversity, and developing techniques for accommodating multirate users. A goal of this research is to establish, through a careful modeling of time dispersion and angular dispersion effects in wideband channels, that signals spread continuously over the available time interval and frequency band can send data with high reliability. When spatial macrodiversity also is available, the investigators contend that signals should be coded over spatial and spectral dimensions and their energy spread "continuously" over time, frequency and space. Linear signal space separation schemes have received considerable attention as approaches for providing good tradeoffs between throughput and system complexity in single-cell multiuser environments. The efficacy of linear signal space separation schemes diminishes substantially, however, when one generalizes to multiple-cell environments. The goal in such realistic multicell interference channels is to maximize a certain extension of classical spectral efficiency herein called area-spectral efficiency. This research investigates the validity of the contention that signals widely dispersed in time, frequency and space (TFS-wide signals) are nearly optimum in such environments and do not require the use of linear signal separation at the receiver. The investigators study various techniques for creating TFS-wide signals in the wideband wireless environment, with an emphasis on multicarrier signaling approaches. Provided the forward channel does not fade quickly relative to the capacity of the channel available for feeding back information from the receiver(s) to the transmitter(s), DMT (discrete multitone) with dynamic bit loading should prove to be a viable data transmission technique. Combined with spreading that enables multiple access, DMT can be made to provide both user separation and Shannon-optimal power distribution relative to the short-term spectrum of the wideband channel with fading and interference seen by each user. For cases where dynamic bit loading proves infeasible, OFDM (i.e., DMT with an equal number of bits in each subchannel) is being explored as a candidate modulation scheme for broadband wireless signaling.