Debarati Sen

Beamforming for Millimeter Wave Communications: An Inclusive Survey

The remarkable growth of wireless data traffic in recent times has driven the need to explore suitable regions in the radio spectrum to meet the projected requirements. In pursuance of this, millimeter wave communications have received considerable attention in the research fraternity. Due to the high path and penetration losses at millimeter wavelengths, antenna beamforming assumes a pivotal role in establishing and maintaining a robust communication link. Beamforming for millimeter wave communications poses a multitude of diverse challenges due to the large channel bandwidth, unique channel characteristics, and hardware constraints. In this paper, we track the evolution and advancements in antenna beamforming for millimeter wave communications in the context of the distinct requirements for indoor and outdoor communication scenarios. We expand the scope of discussion by including the developments in radio frequency system design and implementation for millimeter wave beamforming. We explore the suitability of millimeter wave beamforming methods, both, existing and proposed till midyear 2015, and identify the exciting new prospects unfolding in this domain.

An Efficient Frequency Offset Estimation Scheme for Multi-Band OFDM Ultra-Wideband Systems

A frequency offset estimation scheme is presented for coarse frequency offset estimation of Ultra-Wideband (UWB) Multi-Band Orthogonal Frequency Division Multiplexing (MB- OFDM) systems. Cramer Rao Lower Bound (CRLB) for variance of the estimated frequency offset is derived and compared with the simulated results of the proposed scheme. Performance of the MB-OFDM system in terms of the Bit Error Rate (BER) with suggested frequency offset estimator is also reported. The algorithm is tested by computer simulation over multiple UWB channel models (CM), CM1: 0-4m line-of-sight, CM2: 0-4m non-line-of-sight (NLOS), CM3: 4-10m NLOS, and CM4: 25 nsec. delay spread as suggested by IEEE 802.15 Channel Modeling sub-committee. With higher number of iteration the algorithm shows improvement of the system performance by 8.8 dB in both CM1 and CM2; 7.6 dB for CM3 and 8.3 dB for CM4 at BER=10-4.

Symbol timing synchronization for ultra-wideband (UWB) multi-band OFDM (MB-OFDM) systems

A low-complexity correlation based symbol timing synchronization (CBTS) algorithm for UWB MB-OFDM systems is presented. The correlation based scheme attempts to locate the start of the FFT window during frame synchronization sequence (FS) of the received preamble by firstly correlating the received samples with known training samples and then identifying the first significant multi-path component by comparing the correlated samples of two consecutive OFDM symbols against a predefined threshold. Performance of the proposed algorithm is measured through mean-squared error (MSE) of timing estimation and probability of synchronization. Each of the 100 channel realizations for UWB channel model1 (CM1) (0-4 m LOS) and CM2 (0-4 m NLOS) are simulated for 1000 noisy channel realizations for 17 dB SNR. Synchronization probability and MSE for CM1 and CM2 are reported for 320 Mbps data rate.

A Comprehensive Analysis of Spectrum Handoff Under Different Distribution Models for Cognitive Radio Networks

The static frequency allocation in wireless communication became a major concern for efficient spectrum utilization. Also due to spatio-temporal variation, some of the frequency channels are not utilized efficiently. The prologue of open spectrum and dynamic spectrum access (DSA) methodology provides the secondary (unlicensed) users, supported by cognitive radios (CRs) to opportunistically utilize the unused spectrum bands. When a primary user returns to the engaged spectrum band, secondary users should release it immediately through proper handing off to another spectrum band available to the network satisfying the quality of service (QoS) of both the network. Hence the focus of the new spectrum management policies is DSA technology based on CR. Spectrum mobility is considered to be the subsequent big challenge in CR technology. Spectrum mobility is associated with spectrum handoff which is directly associated with link maintenance and QoS. In this paper, we scrupulously investigate and analyze the probability of spectrum handoff under diverse realistic primary and secondary user traffic models. We have established a state of the art standard generalized form of probability of spectrum handoff without switching delay considering secondary user call duration and residual time of availability of spectrum holes as measurement metrics for diverse distribution functions designed for tele-traffic analysis. We thoroughly investigate different distribution models for the residual time under both zero switching delay and finite switching delay conditions. The switching delay (tr) between spectrum holes comprises of spectrum sensing time and transition delay for realizing all the related parameters. A comprehensive simulation results are presented to validate the generalized theory established in this paper.

Long-Term Clock Synchronization in wireless sensor networks with arbitrary delay distributions

Clock synchronization is a crucial issue in the operation of wireless sensor networks. Although the existing synchronization algorithms under linear clock model assumptions perform well for short periods, they will become problematic for applications with long-term requirements. In this paper, we consider a more realistic and flexible relationship model for two clocks and exploit a Taylor expansion to approximate the relationship. Based on this model and a two-way time message exchange procedure, an estimation algorithm is proposed to recover the relationship and then achieve the synchronization. Finally, simulation results demonstrate that the proposed algorithm improves the accuracy of synchronization as compared to existing algorithms in many scenarios, and is also robust to different distributions of random delays.

An Energy Based Symbol Timing Synchronization Scheme for MB-OFDM Based Ultra-Wideband Communication

An ultra-wideband channel is expected to exhibit different delay characteristics in different bands. In this paper, wideband channel delay characteristics are studied and delay parameters are found considerably different over frequency bands 3.1-4.6 GHz. Based on this observation, an adaptive timing synchronization scheme (ATS) which estimates and maintains the timing delays of each band separately is proposed. The performance of ultra-wideband (UWB) Multi-band OFDM (MB- OFDM) system with ATS algorithm is studied through computer simulations. It is shown that ATS gives signal-to-noise ratio (SNR) improvement of 1.7 dB at bit error rate (BER) of 2times10-2, 2.2 dB at BER of 7x10-3, and 1.1 dB at BER of 5x10-3 for CM4, CM3, and CM2 respectively for uncoded MB-OFDM system over a non-adaptive synchronization scheme [C.W. Yak et al., 2005].

A New Frequency Offset Estimation Scheme For Ultra-Wideband MB-OFDM Systems

In this paper a frequency offset estimation scheme is presented for coarse frequency offset estimation of multi-band OFDM (MB-OFDM) based ultra-wideband (UWB) systems considering different normalized carrier frequency offsets and different channel frequency responses in three different bands. The Cramer Rao Lower Bound (CRLB) for variance of the estimated frequency offset is derived and compared with the simulated results of the proposed scheme. Performance of the MB-OFDM system in terms of the Bit Error Rate (BER) with suggested frequency offset estimator is reported. The algorithm is tested by computer simulation over 100,000 noisy realizations in each of the UWB channel models (CM), CM1, CM2, CM3, and CM4 as suggested by IEEE 802.15 channel modeling sub-committee.

A Multi-Band Timing Estimation and Compensation Scheme for Ultra-Wideband Communications

An Ultra-Wideband (UWB) channel exhibits frequency dependent delay characteristics in different bands of transmission. In [6] we studied and quantified the differences of delay parameters over frequency bands 3.1-4.6 GHz in several UWB channel models (CM). Based on this observation, we proposed an adaptive timing synchronization scheme (ATS) which estimates and maintains the timing delays of each band separately. In this paper, we modify our ATS algorithm [6] by optimal selection of the threshold for timing estimation to reduce the mean-squared-error (MSE) and increase the synchronization probability of the estimator. The performance of UWB Multi- band OFDM (MB-OFDM) system with modified ATS algorithm is studied through computer simulations. We show that modified ATS gives signal-to-noise ratio (SNR) improvement of 1.7 dB at Bit Error Rate (BER) of 2times10-2, 2.2 dB at BER of 7times10-3, and 1.1 dB at BER of 5times10-3 for CM4, CM3, and CM2 respectively for uncoded MB-OFDM system over a non-adaptive synchronization scheme [5]. Also, algorithmic computational complexity analysis of modified ATS is provided for its usability evaluation.

A Frequency Offset Estimation Scheme for OFDM Based UWB Systems

Frequency offset estimation is one of the crucial stages for frequency synchronization in OFDM based systems such as MB-OFDM UWB systems. A frequency offset estimation scheme is proposed for coarse frequency offset estimate. Variance of the proposed frequency offset estimated scheme is compared with the Cramer Rao lower bound (CRLB). BER performance of the proposed scheme is also reported

A Semi-Blind Channel Estimation Algorithm for Massive MIMO Systems

In this letter, we propose a semi-blind iterative space-alternating generalized expectation maximization (SAGE)-based channel estimator for massive MIMO systems. The method updates the pilot-based minimum mean square error (MMSE) estimate iteratively with the help of the SAGE algorithm. Pilot-based estimators require additional pilot symbols for enhancing accuracy, whereas the proposed estimator uses data symbols. Several of the existing estimators assume complete information on large scale fading coefficients of the interfering cells which requires heavy overhead. However, the proposed estimator solves this problem by an estimate obtained from the received samples. Further, we show that the proposed estimator converges almost in one iteration, and achieves appreciable improvement over the existing pilot-based and data-aided estimators in a pilot contaminated scenario.