Avneesh Agrawal

Text segmentation in mixed-mode images

Block based algorithms have found widespread use in image and video compression. However, popular algorithms such as JPEG, which are very effective in compressing continuous tone images, do not perform well with mixed-mode images which have a substantial text component. With a growing number of applications where such images occur, e.g., color facsimile, digital libraries and educational videos, there are advantages in being able to classify each block as being text or continuous tone. With such a classification, different compression parameters or even algorithms may be employed for the two kinds of data to obtain high compression with minimal loss in visual quality. In this paper we analyze and compare four methods for block classification in mixed mode images, namely variance, absolute-deviation, edge, and DCT based methods. Our evaluation of each scheme is based on the accuracy of segmentation, robustness across different types of images and sensitivity to the threshold used for segmentation. Our results show that DCT based segmentation offers the best accuracy and robustness. Another advantage of DCT is that it is compatible with standards like JPEG, MPEG and H.261.<<ETX>>

Power control for successive interference cancellation with imperfect cancellation

This paper proposes and analyzes an iterative power control scheme for use with successive interference cancellation (SIC) in the presence of cancellation errors. SIC is shown by Andrews and Meng (see IEEE Transactions On Wireless Communications, Oct. 2001), to increase the capacity of cellular CDMA systems significantly, even if the signal cancellation is imperfect due to estimation errors. However, an important complication of SIC relative to conventional CDMA receivers is that a specific non-uniform distribution of powers must be assigned to the users in order for the system to function robustly. This paper proposes a simple up/down distributed iterative power control scheme for DS-CDMA systems employing SIC. We analyze its feasibility region and prove that it converges to close to the optimum solution even in the presence of estimation errors. The total received power is shown to be a reliable metric for admission control. This analysis considers both multi-rate CDMA where each user has a different target signal-to-interference-and-noise ratio (SINR), and asynchronous power control where user power updates occur asynchronously.

Preamble design, system acquisition, and determination in modern OFDMA cellular communications: an overview

The wide choices of deployment parameters in next generation wireless communication systems, such as flexible bandwidth allocation, synchronous/ asynchronous modes, FDD/TDD, full/half duplex, and configurable cyclic prefix duration, etc., present significant challenges in preamble and system acquisition design. This article addresses these challenges, as well as the solutions provided by the next generation wireless standards, such as the IEEE 802.20 Mobile Broadband Wireless Access (MBWA) standard and the 3GPP LTE standard. The design facilitates the maximal flexibility of the system configuration and yet has low overhead, low acquisition latency, and low complexity. System acquisition and determination techniques are also discussed in detail.

Field Results on MIMO Performance in UMB Systems

The paper presents MIMO field performance results observed using a ultra mobile broadband (UMB) testbed network. We evaluate metrics such as antenna correlations and channel condition number to characterize the MIMO channel. Results show that low condition numbers, which are beneficial to MIMO, are prevalent for a majority of the coverage area in our network. We demonstrate that the use of MIMO provides gains of the order of 20-40% over SIMO transmissions. These gains are made possible by the use of cross-polarized transmit antennas and advanced UMB features that allow dynamic MIMO vs. SIMO transmission selection based on channel conditions. These results are obtained in a truly mobile, wireless wide-area deployment, which makes them unique. Our results point to the viability and value of MIMO in future mobile wireless networks.

Preamble Design in Ultra Mobile Broadband Communication Systems

The preamble design for a highly flexible wireless communication system can be challenging. This paper describes a preamble design for ultra mobile broadband (UMB) communication systems that allows flexible deployment, such as flexible bandwidth allocation, synchronous/asynchronous modes, FDD/TDD, full/half duplex, and configurable cyclic prefix duration, etc. The UMB preamble design facilitates the flexibility and yet has low overhead, low acquisition latency, and low complexity.

Preamble Design and System Acquisition in Ultra Mobile Broadband Communication Systems

The wide choices of deployment parameters in next generation wireless communication systems, such as flexible bandwidth allocation, synchronous/asynchronous modes, FDD/TDD, full/half duplex, and configurable cyclic prefix duration, etc., present significant challenges in preamble and system acquisition design. This paper addresses these challenges, as well as the solutions provided by the 3GPP2 Ultra Mobile Broadband (UMB) standard. The proposed preamble design facilitates the maximal flexibility of the system configuration and yet has low overhead, low acquisition latency, and low complexity. Although the design is discussed under the UMB context, it also serves as a preamble design paradigm for an OFDMA communication system in general.

Multi-antenna Techniques in Ultra Mobile Broadband Communication Systems

Theoretically, multi-antenna techniques offers significant capacity benefits, but realizing these gains in deployed wireless systems remains a very challenging problem. MIMO has been used in wireless LAN systems with great success. However, due to the hugely varying spatial profiles and velocity patterns of the access terminals, integrating MIMO techniques in a wireless WAN system has been a difficult design problem. This paper introduces the design paradigms of Ultra-Mobile Broadband, which is poised to become the successor of EVDO as the next generation of high speed wireless system. UMB allows the use of MIMO in various forms to suit the deployment scenario, and elicit as much gain as possible.

MIMO in wireless WAN — the UMB system

Theoretically, MIMO offers significant capacity benefits, but realizing these gains in deployed wireless systems remains a very challenging problem. MIMO has been used in wireless LAN systems with great success. However, due to the hugely varying spatial profiles and velocity patterns of the access terminals, integrating MIMO techniques in a wireless WAN system has been a difficult design problem. This paper introduces the design paradigms of ultra-mobile broadband, which is poised to become the successor of EVDO as the next generation of high speed wireless system. UMB allows the use of MIMO in various forms to suit the deployment scenario, and elicit as much gain as possible.

Transmit optimization for frequency division duplex multi-antenna systems

This paper proposes a transmission scheme for frequency division duplex (FDD) multi-antenna systems. The proposed scheme achieves rates close to capacity without requiring explicit feedback of channel state information (CSI) from the receiver. For FDD systems, the conventional assumption is that the channels on the forward and reverse links are completely uncorrelated and hence the transmitter has to rely on feedback from the receiver in order to implement any transmit optimization. However, this assumption of completely uncorrelated channels on the two links is not always a valid assumption. It is argued that in several situations, specifically those in which the base station antenna array is situated high above the terrain, there is partial correlation between the two links. This paper presents a transmission scheme that exploits this correlation between the forward and reverse links in a multi-antenna FDD system. The scheme is designed to be robust in that the performance degrades gracefully in the absence of any correlation between the two links. Simulations show that the proposed scheme indeed achieves near-optimal performance under many different channel conditions.