Roopsha Samanta

Codebook Adaptation for Quantized MIMO Beamforming Systems

Quantized multiple-input multiple-output (MIMO) beamforming systems use predesigned codebooks for the quantization of transmit beamforming vectors. The quantized vector, which is conveyed to the transmitter using a low-rate feedback channel, is used for transmission to provide significant diversity and array gain. The codebook for quantization is a function of the channel distribution, and is typically designed for fixed channel distributions. In this paper, we propose a channel adaptive feedback strategy for arbitrary channel distributions, and present a simple codebook design methodology based on the channel statistics. The codebook for quantization is dynamically chosen rom a structured set of pre-designed codebooks, called the code set, wherein all codebooks are derived from one mother codebook. Simulations illustrate that the proposed method can improve error rate performance in correlated and/or channels with strongly line-of-sight components

Frame theoretic quantization for limited feedback MIMO beamforming systems

In an uncorrelated Rayleigh fading channel, multiple-input multiple-output (MIMO) beamforming-combining systems can attain full diversity and significant array gain using channel knowledge at the transmitter. In the absence of channel knowledge at the transmitter, a beamforming vector can be quantized at the receiver using a fixed pre-determined codebook of beamforming vectors and the codeword index can be sent to the transmitter. Designing optimal codebooks (that maximize expected SNR) is an open problem and vector quantization (VQ) and Grassmannian line packings (GLP) have been proposed as possible solutions. In this paper, a mutual information based codebook design criterion is introduced and equiangular frames (EF) are proposed as beamforming codebooks satisfying this criterion. Besides providing a systematic way to design codebooks, EFs result in higher receive signal-to-noise ratio compared to codebooks designed using VQ or GLP.

Automatic Generation of Local Repairs for Boolean Programs

Automatic techniques for software verification focus on obtaining witnesses of program failure. Such counterexamples often fail to localize the precise cause of an error and usually do not suggest a repair strategy. We present an efficient algorithm to automatically generate a repair for an incorrect sequential Boolean program where program correctness is specified using a pre-condition and a post-condition. Our approach draws on standard techniques from predicate calculus to obtain annotations for the program statements. These annotations are then used to generate a synthesis query for each program statement, which if successful, yields a repair. Furthermore, we show that if a repair exists for a given program under specified conditions, our technique is always able to find it.

Qlose: Program Repair with Quantitative Objectives

The goal of automatic program repair is to identify a set of syntactic changes that can turn a program that is incorrect with respect to a given specification into a correct one. Existing program repair techniques typically aim to find any program that meets the given specification. Such “best-effort” strategies can end up generating a program that is quite different from the original one. Novel techniques have been proposed to compute syntactically minimal program fixes, but the smallest syntactic fix to a program can still significantly alter the original program’s behaviour. We propose a new approach to program repair based on program distances, which can quantify changes not only to the program syntax but also to the program semantics. We call this the quantitative program repair problem where the “optimal” repair is derived using multiple distances. We implement a solution to the quantitative repair problem in a prototype tool called Qlose (Quantitatively close), using the program synthesizer Sketch. We evaluate the effectiveness of different distances in obtaining desirable repairs by evaluating Qlose on programs taken from educational tools such as CodeHunt and edX.

Cost-aware automatic program repair

We present a formal framework for repairing infinite-state, imperative, sequential programs, with (possibly recursive) procedures and multiple assertions; the framework can generate repaired programs by modifying the original erroneous program in multiple program locations, and can ensure the readability of the repaired program using user-defined expression templates; the framework also generates a set of inductive assertions that serve as a proof of correctness of the repaired program. As a step toward integrating programmer intent and intuition in automated program repair, we present a cost-aware formulation - given a cost function associated with permissible statement modifications, the goal is to ensure that the total program modification cost does not exceed a given repair budget. As part of our predicate abstractionbased solution framework, we present a sound and complete algorithm for repair of Boolean programs. We have developed a prototype tool based on SMT solving and used it successfully to repair diverse errors in benchmark C programs.

Joint space-time interference cancellation and channel shortening

In this paper, we present a framework for the design of space-time equalizers that perform joint interference cancellation and channel shortening in multi-user multiple-input multiple-output frequency-selective channels. The space-time filter coefficients and the shortened channel vectors are jointly optimized to minimize the interference-plus-noise using training data. The receiver design is completed by using an appropriate Viterbi equalizer in the second stage for inter-symbol interference equalization. The design is adapted to two different detection schemes- independent detection and ordered successive interference cancellation. Simulation results have shown acceptable symbol error performance with both detection schemes.

Succinct Representation of Concurrent Trace Sets

We present a method and a tool for generating succinct representations of sets of concurrent traces. We focus on trace sets that contain all correct or all incorrect permutations of events from a given trace. We represent trace sets as HB-Formulas that are Boolean combinations of happens-before constraints between events. To generate a representation of incorrect interleavings, our method iteratively explores interleavings that violate the specification and gathers generalizations of the discovered interleavings into an HB-Formula; its complement yields a representation of correct interleavings. We claim that our trace set representations can drive diverse verification, fault localization, repair, and synthesis techniques for concurrent programs. We demonstrate this by using our tool in three case studies involving synchronization synthesis, bug summarization, and abstraction refinement based verification. In each case study, our initial experimental results have been promising. In the first case study, we present an algorithm for inferring missing synchronization from an HB-Formula representing correct interleavings of a given trace. The algorithm applies rules to rewrite specific patterns in the HB-Formula into locks, barriers, and wait-notify constructs. In the second case study, we use an HB-Formula representing incorrect interleavings for bug summarization. While the HB-Formula itself is a concise counterexample summary, we present additional inference rules to help identify specific concurrency bugs such as data races, define-use order violations, and two-stage access bugs. In the final case study, we present a novel predicate learning procedure that uses HB-Formulas representing abstract counterexamples to accelerate counterexample-guided abstraction refinement (CEGAR). In each iteration of the CEGAR loop, the procedure refines the abstraction to eliminate multiple spurious abstract counterexamples drawn from the HB-Formula.

Joint Interference Cancellation and Channel Shortening in Multiuser-MIMO Systems

In this paper, a two-stage receiver structure for interference cancellation in multiuser spatially multiplexed multiple-antenna systems is presented. A space-time equalizer is used in the first stage for joint coantenna/cochannel-interference suppression and shortening of the effective channel for each transmit stream of the desired user. The channel shortening, combined with independent detection, helps reduce the complexity of the second stage Viterbi equalizer, which is used for separate intersymbol-interference equalization for each of the streams. Three objective functions are proposed for determining the coefficients of the space-time equalizer using a direct training data-based approach, which does not require estimation of the interferers channel. Simulation results show good symbol error performance as compared to existing algorithms with asynchronous multiple-input multiple-output interferers

Robustness Analysis of Networked Systems

Many software systems are naturally modeled as networks of interacting elements such as computing nodes, input devices, and output devices. In this paper, we present a notion of robustness for a networked system when the underlying network is prone to errors. We model such a system

From Non-preemptive to Preemptive Scheduling Using Synchronization Synthesis

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