The effect of spatial diversity on the throughput and reliability of wireless networks is examined. Spatial diversity is realized through multiple independently fading transmit/receive antenna paths in single-user communication and through independently fading links in multiuser communication. Adopting spatial diversity as a central theme, we start by studying its information-theoretic foundations, then we illustrate its benefits across the physical (signal transmission/coding and receiver signal processing) and networking (resource allocation, routing, and applications) layers. Throughout the paper, we discuss engineering intuition and tradeoffs, emphasizing the strong interactions between the various network functionalities.

Fair queueing (FQ) algorithms, which have been proposed for quality of service (QoS) wireline/wireless networking, rely on the fundamental idea that the service rate allocated to each user is proportional to a positive weight. Targeting wireless data networks with a multicode CDMA-based physical layer, we develop FQ with time-varying weight assignment in order to minimize the queueing delays of mobile users. Applying dynamic programming, we design a computationally efficient algorithm which produces the optimal service rates while obeying 1) constraints imposed by the underlying physical layer and 2) QoS requirements. Furthermore, we study how information about the underlying channel quality can be incorporated into the scheduler to improve network performance. Simulations illustrate the merits of our designs.

The performance of an IEEE 802.11a wireless receiver, in the presence of multiple users transmitting ultra-wideband (UWB) interfering signals, is evaluated. The frequency band of interest for the 802.11a system is 5.21-5.23 GHz. The coexistence of the two systems is characterized in terms of 802.11a receiver's bit error rates and throughputs. Our simulations indicate that UWB interferers can cause severe degradation in the attainable throughput of the 802.11a system. However, as the UWB interferers move away from the 802.11a receivers, so does their negative effect on the throughput performance of the 802.11a system.

By employing more than one antenna at the transmitter and by properly coding data across the transmit antennas, physical layers (PHYs) with space-time block codes (STBCs) promise increased data rates with minimal decoding complexity at the receiver. This paper presents a comprehensive study of how the STBC gains at the PHY translate to significant network performance improvement in 802.11a wireless local area networks. We base our study on a detailed, across-all-layers, simulation of an 802.11a system. We have extended the network simulator with an implementation of the 802.11a PHY, which allows us to assess the impact of STBC not only at the PHY layer, but at the higher layers as well. An extensive set of simulations illustrates the merits of transmit diversity (in the form of STBC) and sheds light on how performance can be improved for transmission control protocol (TCP) traffic. Essentially, STBC presents to TCP a "smoother" wireless channel; this is corroborated by a brief theoretical analysis as well.

We examine multicarrier transmission over time-varying channels. We first develop the model for such a transmission scheme and focus particularly on OFDM-based schemes. We analyze the impact of time-variation within a transmission block which could arise both from Doppler spread of the channel and from synchronization errors. We propose a time-domain approach to mitigate the effects of such time-variations. This approach reduces to the familiar single-tap frequency-domain equalizer when the channel is block time-invariant. We also develop this in the context of multiple transmit and receive antennas and specialize the receiver to space-time block-coded systems. Finally we provide numerical results.

In this paper, we investigate OFDM transmission over fast fading channels. In such scenarios, the Doppler spread of the channel and synchronization errors cause intercarrier interference (ICI) and complicate channel estimation because the channel cannot be assumed constant during the transmission of an OFDM block. Channel estimation in rapidly time-varying scenarios becomes critical, an we propose a scheme for estimating channel parameters varying within a transmission block. Along with the channel estimation scheme, we also examine the issue of pilot tone placement and show that in time-varying channels it may be better to group pilot tones together into clumps equispaced onto the FFT grid; this placement technique is in contrast to the common wisdom for time-invariant channels. Finally we provide numerical results.

In block transmission systems, transmitter-induced redundancy using finite-impulse response (FIR) filterbanks can be used to suppress intersymbol interference and equalize FIR channels irrespective of channel zeros. At the receiver end, linear or decision-feedback (DF) FIR filterbanks can be applied to recover the transmitted data. Closed-form expressions are derived for the FIR linear or DF filterbank receivers corresponding to varying amounts of transmission redundancy. Our framework encompasses existing block transmission schemes and offers low implementation-cost equalization techniques both when interblock interference is eliminated, and when IBI is present as, e.g., in orthogonal frequency-division multiplexing with insufficient cyclic prefix. By applying blind channel estimation methods, our filterbank transmitters-receivers (transceivers) dispense with bandwidth consuming training sequences. Extensive simulations illustrate the merits of our designs.

Packet fair queueing (PFQ) algorithms have been extensively studied for provision of quality of service (QoS) guarantees in integrated services networks. Because of the fixed weight assignment, the inherent in PFQ delay-bandwidth coupling imposes limitations on the range of QoS that can be supported. We develop PFQ with deterministic time-varying weight assignments, and we propose a low-overhead algorithm capable of supporting arbitrary piecewise linear service curves which achieve delay-bandwidth decoupling. Unlike existing service-curve based algorithms, our time-varying PFQ scheme mitigates the punishment phenomenon, and allows sessions to exploit the extra bandwidth in under-loaded networks.

The traditional layered approach to wireless networks (network, data link, and physical layer) suboptimally utilizes the limited available capacity. We propose a quality of service wireless framework based on the joint design of a packet fair queuing scheduler, demand assignment medium access control (MAC) protocol, and multirate multipath-transparent CDMA-based physical layer. The unique code assignment procedure allows a direct mapping of the packet fair schedule to the physical layer transmission/reception, and supports transmission (service) rates of arbitrarily fine resolution. Judiciously designed user codes combat the harsh time-varying fading environment, and parsimonious code description minimizes overhead in the MAC. Simulations illustrate the merits of our designs.

Multiuser decision feedback (DF) receivers can be employed to improve the bit error performance at the physical layer of wireless networks. Given a wireless network framework capable of utilizing "capacity" gains at the physical layer, DF receivers elevate the provisioned QoS guarantees. This paper develops closed-form FIR DF receivers suitable for multirate multipath transparent CDMA transmissions with block spreading codes. Capitalizing on the multiuser interference elimination capabilities of our CDMA framework, our DF receivers outperform both the linear and the successive interference canceller (SIC) receivers. Simulations illustrate the merits of our designs.