Chaitali Sengupta

An Integrated CAD Framework Linking VLSI Layout Editors and Process Simulators

As feature sizes in VLSI circuits extend into the far sub-micron range, new process techniques, such as using phase shifted masks for photolithography, will be needed. Under these conditions, the only means for the circuit designer to design compact and efficient circuits with good yield capabilities is to be able to see the effect of different design approaches on manufactured silicon, instead of solely relying on conservative general design rules. The integrated CAD framework accomplishes this by providing a link between a layout editor (Magic), advanced photolithographic techniques such as phase shifted masks, and a process simulator (Depict). This paper discusses some applications of this tool. A non- conventional process technique involving interferometric phase shifting and off-axis illumination has been evaluated using the tool. Also, a feature of the CAD framework which allows representation of a phase shifted mask, together with its layout analysis capability has been used to compact a piece of layout by inserting phase shifted elements into it.

On multipath channel estimation for CDMA systems using multiple sensors

This paper focuses on the design of a multiuser receiver structure for the reverse link of a code-division multiple-access communication system, in the presence of multipath effects and using an antenna array at the base station receiver. The algorithm presented solves the complex multidimensional problem of channel estimation in this complex scenario using a maximum-likelihood approach. This channel estimation technique requires the transmission of a training sequence or feedback of detected data. Once a composite channel-impulse response of each user is estimated, it is directly used in the detection process instead of first extracting the individual channel parameters, such as path delays and attenuation factors. The paper presents a framework that facilitates a computationally efficient solution to the combined problem of channel estimation and detection in a scenario involving multiple users, multiple paths, and multiple sensors at the receiver.

Maximum likelihood multipath channel parameter estimation in CDMA systems using antenna arrays

The problem addressed in this paper is the estimation of the channel parameters in a code division multiple access (CDMA) communication system, in the presence of multipath effects and multiple sensors at the base station receiver. The algorithm presented solves the problem by estimating a composite channel impulse response of each user, which can be directly used in the detection process to appropriately modify the spreading code of the user. In addition, the algorithm combines the benefit of spatial processing in the form of an antenna array at the receiver to gain an increase in performance of the system.

A comparison of the open loop transmit diversity schemes for third generation wireless systems

Multiple transmit antennas giving rise to diversity (transmit diversity) have been shown to increase the downlink (base station to the mobile) capacity in cellular systems. The third generation partnership project (3Gpp) for WCDMA FDD has chosen space time transmit diversity (STTD) as the open loop transmit diversity technique for two transmit antennas. On the other hand, the CDMA2000 has chosen space time spreading (STS) and orthogonal transmit diversity (OTD) as the open loop transmit diversity techniques. In this paper, we compare the open loop transmit diversity techniques of 3Gpp and CDMA2000. We show that STS involves a QPSK to non-QPSK constellation transformation followed by an STTD encoding of the resulting symbols. Thus, the performance of STS is the same as STTD, while the peak to average ratio (PAR) and complexity of STS is higher than STTD. We also show theoretically that the performance of STTD is 0.3-0.5 dB better than OTD. Further, STTD has another distinct advantage over OTD and STS in its application to the broadcast channels. During the power on and possibly during soft handoff, the mobile does not know the presence/absence of the diversity antenna on the primary/soft handoff base station. In this case, a blind detection of the diversity antenna is possible for STTD with a very small degradation in performance. This is not possible for OTD and STS. We thus show that STTD is a better open loop transmit diversity technique than OTD and STS.

Performance Analysis Framework and Vertical Handover Triggering Algorithms for Voice over WLAN/Cellular Network

The development of handheld user equipments (UE) capable of operating over both cellular networks and wireless local area networks (WLAN) is an important step towards the evolution of next-generation integrated networks. For real-time applications, maintaining a seamless connectivity and an acceptable level of quality is quite important. Efficient vertical handoff algorithms are required to ensure the best exploitation of the available resources under certain quality constraints. This paper proposes a realistic performance analysis framework upon addressing relevant factors that affect the performance of a voice application within a heterogeneous network environment. While the existing works in this field relies on received signal strength (RSS) samples to estimate the WLAN usability, this paper also proposes an algorithm that takes into account the real-time packet error pattern along with RSS.

Subspace‐based tracking of multipath channel parameters for CDMA systems

In this paper, we evaluate several issues in the application of subspace-based methods to tracking channel parameters in Code Division Multiple Access (CDMA) communication systems, in the presence of multipath fading. We focus on two aspects of the problem - the performance of Singular Value Decomposition (SVD) based schemes while tracking the time variations in the signal subspace, due to fading, and, the performance tradeoff involved in using several complexity reducing approximate schemes. The performance benefits to be obtained from application of subspace-based methods to channel estimation has been well studied. The aim of this work is to lay the groundwork for real time implementation of this computationally complex problem.

Efficient multiuser receivers for CDMA systems

We focus on the design of multiuser receiver structures for code division multiple access (CDMA) communication systems, in the presence of multipath effects and multiple sensors at the base station receiver. We present a flexible and extensible framework that allows the use of an estimated effective spreading code from the channel estimation phase, in the multiuser detection process. The effective spreading code captures all the channel parameters such as path delays, attenuation factors, and directions of arrival. Hence estimation of this one composite vector removes the necessity of estimating each individual parameter, thus reducing computational complexity. The results also show that this approach leads to better performance for multiuser detection, especially when the channel consists of a number of low energy paths in addition to a few discrete strong paths.

Fixed point error analysis of multiuser detection and synchronization algorithms for CDMA communication systems

Conventional correlation based single-user techniques for direct sequence code division multiple access (DS-CDMA) wireless communication systems are susceptible to performance degradation due to interference from other users. Previous research has focused on development of several multiuser techniques where information about multiple users is used to improve performance for each individual user. Due to performance benefits of these methods, they are attractive candidates for implementation in future cellular systems. In this paper we present an error analysis of fixed point implementation of some of these techniques.

Enhanced Microlithography Using Combined Phase Shifting and Off-axis Illumination

Off-axis illumination is a promising optical microlithography technique which can be used to improve the image quality of line-space patterns. With this method the image is produced by the zero and first order diffracted beams. Due to the intensity difference between these two order diffracted beams the contrast of the image cannot be unity. This paper demonstrates the optical enhancement that can be achieved by a combination of interferometric phase shifting and off-axis illumination. In such an arrangement the mask is illuminated symmetrically from both the front and back sides, and not two but in fact four–(two zero and two first)–order beams produce the image. We show experimentally that the contrast of the image can be improved if the phase difference between the reflected and transmitted beams is π, and the intensity of the transmitted beam is about 13% of the reflected beam. This improved quality image with feature sizes of 0.4 µm was recorded in a photoresist using an Ar+ ion laser operating at 457.9 nm.

Arithmetic acceleration techniques for wireless communication receivers

We develop techniques to accelerate the implementation of the next generation wireless communication algorithms in hardware. We discuss an implementation of a key computationally intensive baseband algorithm for joint multiuser channel estimation and detection for this purpose and study its real-time requirements. An analysis of the bottlenecks present in the algorithm is made. We present an acceleration technique using task decomposition to take advantage of the existing pipelining and parallelism flow in the algorithm. We show that an application specific system design with multiple processing elements is more effective than the conventional single processor approach as it can satisfy the high data rate requirements of the next generation wireless communication systems. Our analysis is done independent of the final mapping of the processing elements in hardware.