Monisha Ghosh

Analysis of the effect of impulse noise on multicarrier and single carrier QAM systems

The paper first shows the equivalence of the Bernoulli-Gaussian impulse noise model in the discrete time domain to the continuous-time model of Poisson arriving delta functions with random area distributed according to the power Rayleigh probability density function. This equivalence is then used to develop a closed form expression for the probability of error for single carrier QAM that is easily evaluated. Furthermore, the performance of multicarrier modulation (MCM) is also analyzed using the same impulse noise model and it is shown that in most cases MCM performs better than single carrier systems, specifically when the probability of an impulse is not too high and the impulse power is moderate.

Emerging cognitive radio applications: A survey

Recent developments in spectrum policy and regulatory domains, notably the release of the National Broadband Plan, the publication of final rules for TV white spaces, and the ongoing proceeding for secondary use of the 2360-2400 MHz band for medical body area networks, will allow more flexible and efficient use of spectrum in the future. These important changes open up exciting opportunities for cognitive radio to enable and support a variety of emerging applications, ranging from smart grid, public safety and broadband cellular, to medical applications. This article presents a high-level view on how cognitive radio (primarily from a dynamic spectrum access perspective) would support such applications, the benefits that cognitive radio would bring, and also some challenges that are yet to be resolved. We also illustrate related standardization that uses cognitive radio technologies to support such emerging applications.

Spectrum Sensing for Dynamic Spectrum Access of TV Bands

In this paper we address the issue of spectrum sensing in cognitive radio based wireless networks. Spectrum sensing is the key enabler for dynamic spectrum access as it can allow secondary networks to reuse spectrum without causing harmful interference to primary users. Here we propose a set of integrated medium access control (MAC) and physical layer (PHY) spectrum sensing techniques that provide reliable access to television (TV) bands. At the MAC level, we propose a two-stage spectrum sensing that guarantees timely detection of incumbents while meeting the quality of service (QoS) requirements of secondary users. At the PHY level, we introduce FFT-based pilot energy and location detection schemes that can detect a TV signal on a TV channel at levels as low as -116 dBm. We have evaluated these schemes through simulation and prototyping and show their effectiveness, reliability, and efficiency. These mechanisms are also part of the current IEEE 802.22 draft standard which is based on cognitive radio technology.

Blind decision feedback equalization for terrestrial television receivers

In December 1996 the Federal Communications Commission (FCC) adopted the Grand Alliance (GA) system as the digital television broadcasting standard for the United States ending a seven-year-long search for a fully digital television standard. MPEG-2 was chosen as the video compression standard, and trellis-coded 8-vestigial sideband (VSB) with a training sequence was chosen as the transmission standard. The laboratory tests that were performed on the final two competing systems, 8-VSB with training sequence and 32-quadrature amplitude modulation (QAM) with blind equalization, showed a need for blind equalization in dynamic channels that could not be adequately handled by an equalizer training on the training sequence alone. Hence, the final GA system recommended the use of blind equalization in the receiver. In this paper, we describe the U.S. digital television transmission standard as it pertains to the equalization problem, typical transmission channel characteristics and the need for blind equalization in terrestrial television receivers.

First Cognitive Radio Networking Standard for Personal/Portable Devices in TV White Spaces

Recent FCC rules allowing unlicensed use on a secondary basis of the Television White Spaces (TVWS) promise a whole new set of possible applications. The first step towards realizing these applications is the creation and adoption of industry standards. In this paper we present the first such standard for personal/portable devices in the TVWS that complies fully with the existing FCC rules while retaining flexibility for use with other regulatory domains. We describe the physical (PHY) and medium access control (MAC) layers specified in the standard and present performance results to demonstrate the robustness and spectral efficiency of the proposed protocols.

Maximum-likelihood blind equalization

A new approach to blind equalization is investigated in which the receiver performs joint data and channel estimation in an iterative manner. Instead of estimating the channel inverse, the receiver computes the maximum-likelihood estimate of the channel itself. The iterative algorithm that is developed involves maximum-likelihood sequence estimation (Viterbi decoding) for the data estimation part, and least-squares estimation for the channel estimation part. A suboptimal algorithm is also proposed that uses a reduced-state trellis instead of the Viterbi algorithm. Simulation results show that the performance obtained by these algorithms is comparable to that of a receiver operating with complete knowledge of the channel.

Adaptive chip-equalizers for synchronous DS-CDMA systems with pilot sequences

It is well known that in a multiple-user synchronous DS-CDMA system with multipath fading, the widely used RAKE receiver is suboptimal. Recently there has been interest in the use of chip-equalizers before despreading in order to restore the orthogonality of the transmitted sequences at the receiver. However, the issue of adapting these equalizers fast enough to track a wideband fading channel is still an open problem. In this paper we first show that if the optimal MMSE equalizer taps for a pilot sequence are used for equalizing other users, the MSE performance is very close to that obtained by using the optimal taps for that user, i.e. the equalizer taps that are optimal for the pilot sequence are, when multiplied by a gain factor, close to optimal for any other user in the system. We then go on to show that a DS-CDMA system that uses multiple pilot sequences can very effectively track fast-fading channel variations using least-squares adaptation on the known pilot sequences.

Spectrum Sensing Prototype for Sensing ATSC and Wireless Microphone Signals

Spectrum sensing is the key enabler for dynamic spectrum access in the television (TV) bands as it can allow secondary networks to reuse spectrum without causing harmful interference to primary users. In this paper we describe a sensing prototype that has been developed to demonstrate robust sensing of TV signals as well as wireless microphone signals in the laboratory and field. We will present the algorithms as well as simulation, lab and field test results that validate the prototypes capability to identify these signals down to a level of -116 dBm.

Adaptive modulation for coded cooperative systems

Throughput plays a key role in data communications over a wireless link. In this paper, we use adaptive modulation for a cooperative relaying system with the goal of maximizing the data throughput. We compare the throughput performance of three different schemes with adaptive modulation: direct transmission, multihop and coded cooperation. We find that cooperation with adaptive modulation provides large throughput improvement over multihop and direct transmissions. We also observe that to maximize the data throughput in direct transmission and multihop, the modulation mode for each hop only depends on the channel quality from the transmitter to the receiver. However, in coded cooperation a system wide optimization is necessary.

Bluetooth interference cancellation for 802.11g WLAN receivers

It is well known that one of the main sources of interference in the 2.4 GHz unlicensed band is due to the presence of Bluetooth systems in this bands. Recently, the IEEE 802.11 committee adopted a draft standard called 802.11g that will provide data rates of up to 54 Mbps in the 2.4 GHz band using OFDM as the physical layer (PHY) modulation format. One of the main barriers to actually achieving the high data rates will be the presence of interferers like Bluetooth. In this paper we develop a PHY layer algorithm for simultaneously estimating the multipath channel and interference characteristics and using these estimates in the convolutional decoder in order to improve the system performance in the presence of Bluetooth interference. Simulation results indicate that substantial improvements in 802.11g performances can be obtained by this approach.