CAREER: Key Problems in Wireless Communications

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This research can be classified into two major themes, motivated by the following key bottlenecks for wireless systems: the scarcity of radio bandwidth, and the problem of interfacing inherently lossy wireless links with high-speed wireline networks where loss occurs mainlv due to congestion. The first research theme is concerned with enhancing the capacity and performance of direct-sequence CDMA, which holds great promise as a robust workhorse technology for future wireless systems. Since multiple-access interference imposes a major limitation on CDMA performance two broad methodologies are considered for alleviating its effects: decentralized adaptive reception and centralized multi-user reception. The goal is to further the theoretical development of both these methodologies with a view to developing practical receiver structures. The issues addressed include devising methods for making adaptive interference suppression work over the severe time variations of a wireless multiple-access channel, supporting multiple classes of traffic (and hence multiple rates) using direct-sequence CDMA, inventing near optimal adaptive nonlinear receivers, incorporating antenna diversity into interference suppression schemes with reasonable complexity, and deriving receivers based on multi-user detection and estimation principles for systems to which adaptive methods do not apply. The second research theme is the development of a theory for designing interfaces that ensure compatibility between wireless and wireline networks while minimizing the use of precious wireless bandwidth. The problem formulations, while of broader interest, are motivated by the fact that two wireline technologies with which a wireless network must be compatible are the emerging ATM networks and TCP/IP. The problems considered include the formulation of service guarantees over a lossy wireless link and the design principles for implementing them, optimization of wireless bandwidth for a broadcast or multiple-access wireless channel by using transmission control based on the channel state seen by each receiver, manipulation of the feedback mechanisms used by TCP/IP flow control and ATM rate control by the wireless interface to provide good end-to-end performance over a heterogeneous network with both wireless and wireline links, and the design of a flow control mechanism that does not suffer from the shortcomings of TCP/IP.