Nanda Kishore Chavali - Academia.edu (original) (raw)

Papers by Nanda Kishore Chavali

2015 IEEE International Conference on Signal Processing, Informatics, Communication and Energy Systems (SPICES), 2015

2004 International Conference on Signal Processing and Communications, 2004. SPCOM '04., 2004

in the start of the frame. In this paper, for the same preamble as in 161, we propose a new timin... more in the start of the frame. In this paper, for the same preamble as in 161, we propose a new timing metric, and considering an ideal scenario we show that it yields a sharp peak at the correct symbol boundary. We then analyze the metric and evaluate its mean at the symbol boundary and in its neighborhood for the case

2014 International Conference on Signal Processing and Communications (SPCOM), 2014

2013 International Conference on Advances in Computing, Communications and Informatics (ICACCI), 2013

ABSTRACT The IEEE 802.11n is a high throughput (HT) wireless LAN standard that supports multiple ... more ABSTRACT The IEEE 802.11n is a high throughput (HT) wireless LAN standard that supports multiple input, multiple output orthogonal frequency division multiplexing (MIMO-OFDM) technology with maximum of 4 transmitting and 4 receiving antennas. This technology has been extended to 8 transmitting and 8 receiving antennas in upcoming very high throughput (VHT) wireless LAN standard, IEEE 802.11ac. Spatial expansion (SE) is a transmit diversity scheme used to map fewer space time streams to more transmit antennas to obtain spatial diversity. One of the key tasks at the receiver in such MIMO-OFDM systems is the channel estimation. We discuss a technique to improve the accuracy of channel estimation and the impact of SE on it. We compare the performance of the improved channel estimation technique with different antenna configurations of SE using simulations.

2008 IEEE 19th International Symposium on Personal, Indoor and Mobile Radio Communications, 2008

In this paper, we adapt four existing timing metrics for frame synchronization in OFDMA mode of W... more In this paper, we adapt four existing timing metrics for frame synchronization in OFDMA mode of WMAN. Among the methods considered, first method uses cyclic prefix information present in the symbol, second method uses the symmetry in the time domain preamble data, third method exploits a priori knowledge of the preamble PN sequence, and the fourth method exploits the conjugate symmetry in the time domain preamble. It is shown that direct application of these timing metrics is not possible for frame synchronization of OFDMA mode of WMAN. Modifications to these timing metrics are presented to adapt them to OFDMA mode of WMAN. Simulations are conducted for AWGN and frequency selective channels to demonstrate the performance of the modified timing metrics.

Proceedings of the International Conference on Advances in Computing, Communications and Informatics - ICACCI '12, 2012

ABSTRACT IEEE 802.11ac is an upcoming standard for wireless LAN physical layer and MAC layer for ... more ABSTRACT IEEE 802.11ac is an upcoming standard for wireless LAN physical layer and MAC layer for very high throughput (VHT) communication with targeted throughput greater than 1 Gbps [1] below 6 GHz band. The IEEE 802.11ac (also termed as VHT wireless LAN) physical layer is based on orthogonal frequency division multiplexing (OFDM) modulation and signal processing with multiple antennas at transmitter and receiver (MIMO). A device compliant to IEEE 802.11ac standard should be backward compatible to wireless LAN standards IEEE 802.11a and IEEE 802.11n. The VHT Physical layer supports legacy frame format, high throughput (HT) mixed frame format, HT green-field frame format and VHT mixed format [2, 3]. The VHT wireless LAN receiver has to synchronize for any of these frame formats. The OFDM modulation is sensitive to timing synchronization in multipath fading channel. The cyclic shift diversity (CSD) used in VHT wireless LAN introduces pseudo multipath at the receiver and causes error in the symbol boundary estimate. Hence, there is a need for a robust and adaptive time synchronization that suits for all frame formats in VHT wireless LAN. Without the accurate time synchronization, there will be inter symbol interference (ISI) and inter carrier interference (ICI) in the demodulated data, and cause many bit errors in the decoded data. In this paper, we propose a robust and adaptive timing synchronization technique for VHT Wireless LAN device compliant to IEEE 802.11ac standard. We will show the need for the adaptive window length in time synchronization theoretically. The proposed method works well for all frame formats and BW options of the IEEE 802.11ac standard. We will give simulation results to show the usefulness of the method.

TENCON 2008 - 2008 IEEE Region 10 Conference, 2008

Orthogonal Frequency Division Multiple Access (OFDMA) gains increasing attention for broadband, h... more Orthogonal Frequency Division Multiple Access (OFDMA) gains increasing attention for broadband, high data rate fixed/mobile communications. As the basic modulation technique in OFDMA is orthogonal frequency division multiplexing (OFDM), it is severely affected by carrier frequency offset (CFO). The CFO causes loss of orthogonality among the subcarriers. In this paper, we propose a method for joint estimation of CFO and

2009 Annual IEEE India Conference, 2009

Orthogonal frequency-division multiplexing (OFDM) is a well known multi-carrier digital modulatio... more Orthogonal frequency-division multiplexing (OFDM) is a well known multi-carrier digital modulation scheme in which a transmitted data is split into multiple streams that are modulated at different frequencies. The OFDM system is very sensitive to frame and frequency synchronization errors. In this paper, we propose a new OFDM preamble for accurate frame synchronization of OFDM systems. The preamble is generated

TENCON 2008 - 2008 IEEE Region 10 Conference, 2008

In this paper, we propose a new method for frame synchronization in OFDMA mode of WMAN. The schem... more In this paper, we propose a new method for frame synchronization in OFDMA mode of WMAN. The scheme is based on the preamble suggested for OFDMA mode of WMAN where the preamble consists of real subsymbols and OFDMA data symbols contain complex subsymbols for each frame. The proposed method is independent of frequency offset and results in sharp peak at

2002 IEEE International Conference on Personal Wireless Communications, 2002

Channel coding adds redundant bits to the original information bits in order to detect and correc... more Channel coding adds redundant bits to the original information bits in order to detect and correct possible errors occurring during transmission. In GSM (global system for mobile communications), we use both block coding and convolutional coding for error control. One of the channel coding schemes is CS-1 (see ETSI EN 300 909 v8.5.1, 1999), which employs shortened the cyclic fire code for burst error correction. We propose an efficient way of decoding the shortened binary cyclic fire code. The proposed scheme takes 224 clock cycles for syndrome computation and another 224 clock cycles for error trapping and correction. This decoding method is much less complex than conventional schemes and slightly less complex compared to the recently patented algorithm (Ramesh et al, "Shortened Fire code. Error-Trapping Decoding Method and Apparatus", US Patent 5,936,978). The proposed decoding scheme is applicable also to any binary shortened cyclic code.

2014 International Conference on Contemporary Computing and Informatics (IC3I), 2014

Abstract—IEEE 802.16-2004 and IEEE 802.16e-2005 standards specify OFDMA as a physical layer techn... more Abstract—IEEE 802.16-2004 and IEEE 802.16e-2005 standards specify OFDMA as a physical layer technique for fixed and nomadic BWA systems, respectively. OFDMA is very sensitive to time and frequency offsets. In this paper, a preamble based downlink frame synchronization method for OFDMA mode of WMAN is analyzed in frequency selective fading channels. Expressions for mean and variance of the metric are

EURASIP Journal on Wireless Communications and Networking, 2006

ABSTRACT Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for ... more ABSTRACT Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitting data at very high rates over time dispersive radio channels. In an OFDM system, frame synchronization and frequency offset estimation are extremely important for maintaining orthogonality among the subcarriers. Recently, several techniques have been proposed for frame synchronization in OFDM system. In multipath environment, the transmitted signal arrives at the receiver through direct and multiple delayed paths. In some cases, it is possible that power of the signal arriving through delayed path may be larger than that of the direct path (earliest path if there is no direct path). In those cases, estimate of the frame boundary may be shifted by a quantity equal to the delay of the dominant path. In such cases, there will be intersymbol interference (ISI) in the demodulated symbols unless the frame boundary estimate is preadvanced such that it dwells in the ISI-free portion of cyclic prefix or at the symbol boundary. In this paper, we propose a method for estimating the shift in the frame boundary estimate using a preamble having two identical halves. We assume that frame boundary and frequency offset estimation have been performed prior to the estimation of the shift. We also examine the quality of the frequency offset estimate when the frame boundary estimate is shifted from the desired value. The proposed method is applied to downlink synchronization in OFDM mode of WMAN (IEEE 802.16-2004). We use simulations to illustrate the usefulness of the method and also to support our assertions.

EURASIP Journal on Wireless Communications and Networking, 2006

Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitt... more Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitting data at very high rates over time dispersive radio channels. In an OFDM system, frame synchronization and frequency offset estimation are extremely important for maintaining orthogonality among the subcarriers. In this paper, for a preamble having two identical halves in time, a timing metric is proposed for OFDM frame synchronization. The timing metric is analyzed and its mean values at the preamble boundary and in its neighborhood are evaluated, for AWGN and for frequency selective channels with specified mean power profile of the channel taps, and the variance expression is derived for AWGN case. Since the derivation of the variance expression for frequency selective channel case is tedious, we used simulations to estimate the same. Based on the theoretical value of the mean and estimate of the variance, we suggest a threshold for detection of the preamble boundary and evaluating the probability of false and correct detections. We also suggest a method for a threshold selection and the preamble boundary detection in practical applications. A simple and computationally efficient method for estimating fractional and integer frequency offset, using the same preamble, is also described. Simulations are used to corroborate the results of the analysis. The proposed method of frame synchronization and frequency offset estimation is applied to the downlink synchronization in OFDM mode of wireless metropolitan area network (WMAN) standard IEEE 802.16-2004, and its performance is studied through simulations.

2015 IEEE International Conference on Signal Processing, Informatics, Communication and Energy Systems (SPICES), 2015

2004 International Conference on Signal Processing and Communications, 2004. SPCOM '04., 2004

in the start of the frame. In this paper, for the same preamble as in 161, we propose a new timin... more in the start of the frame. In this paper, for the same preamble as in 161, we propose a new timing metric, and considering an ideal scenario we show that it yields a sharp peak at the correct symbol boundary. We then analyze the metric and evaluate its mean at the symbol boundary and in its neighborhood for the case

2014 International Conference on Signal Processing and Communications (SPCOM), 2014

2013 International Conference on Advances in Computing, Communications and Informatics (ICACCI), 2013

ABSTRACT The IEEE 802.11n is a high throughput (HT) wireless LAN standard that supports multiple ... more ABSTRACT The IEEE 802.11n is a high throughput (HT) wireless LAN standard that supports multiple input, multiple output orthogonal frequency division multiplexing (MIMO-OFDM) technology with maximum of 4 transmitting and 4 receiving antennas. This technology has been extended to 8 transmitting and 8 receiving antennas in upcoming very high throughput (VHT) wireless LAN standard, IEEE 802.11ac. Spatial expansion (SE) is a transmit diversity scheme used to map fewer space time streams to more transmit antennas to obtain spatial diversity. One of the key tasks at the receiver in such MIMO-OFDM systems is the channel estimation. We discuss a technique to improve the accuracy of channel estimation and the impact of SE on it. We compare the performance of the improved channel estimation technique with different antenna configurations of SE using simulations.

2008 IEEE 19th International Symposium on Personal, Indoor and Mobile Radio Communications, 2008

In this paper, we adapt four existing timing metrics for frame synchronization in OFDMA mode of W... more In this paper, we adapt four existing timing metrics for frame synchronization in OFDMA mode of WMAN. Among the methods considered, first method uses cyclic prefix information present in the symbol, second method uses the symmetry in the time domain preamble data, third method exploits a priori knowledge of the preamble PN sequence, and the fourth method exploits the conjugate symmetry in the time domain preamble. It is shown that direct application of these timing metrics is not possible for frame synchronization of OFDMA mode of WMAN. Modifications to these timing metrics are presented to adapt them to OFDMA mode of WMAN. Simulations are conducted for AWGN and frequency selective channels to demonstrate the performance of the modified timing metrics.

Proceedings of the International Conference on Advances in Computing, Communications and Informatics - ICACCI '12, 2012

ABSTRACT IEEE 802.11ac is an upcoming standard for wireless LAN physical layer and MAC layer for ... more ABSTRACT IEEE 802.11ac is an upcoming standard for wireless LAN physical layer and MAC layer for very high throughput (VHT) communication with targeted throughput greater than 1 Gbps [1] below 6 GHz band. The IEEE 802.11ac (also termed as VHT wireless LAN) physical layer is based on orthogonal frequency division multiplexing (OFDM) modulation and signal processing with multiple antennas at transmitter and receiver (MIMO). A device compliant to IEEE 802.11ac standard should be backward compatible to wireless LAN standards IEEE 802.11a and IEEE 802.11n. The VHT Physical layer supports legacy frame format, high throughput (HT) mixed frame format, HT green-field frame format and VHT mixed format [2, 3]. The VHT wireless LAN receiver has to synchronize for any of these frame formats. The OFDM modulation is sensitive to timing synchronization in multipath fading channel. The cyclic shift diversity (CSD) used in VHT wireless LAN introduces pseudo multipath at the receiver and causes error in the symbol boundary estimate. Hence, there is a need for a robust and adaptive time synchronization that suits for all frame formats in VHT wireless LAN. Without the accurate time synchronization, there will be inter symbol interference (ISI) and inter carrier interference (ICI) in the demodulated data, and cause many bit errors in the decoded data. In this paper, we propose a robust and adaptive timing synchronization technique for VHT Wireless LAN device compliant to IEEE 802.11ac standard. We will show the need for the adaptive window length in time synchronization theoretically. The proposed method works well for all frame formats and BW options of the IEEE 802.11ac standard. We will give simulation results to show the usefulness of the method.

TENCON 2008 - 2008 IEEE Region 10 Conference, 2008

Orthogonal Frequency Division Multiple Access (OFDMA) gains increasing attention for broadband, h... more Orthogonal Frequency Division Multiple Access (OFDMA) gains increasing attention for broadband, high data rate fixed/mobile communications. As the basic modulation technique in OFDMA is orthogonal frequency division multiplexing (OFDM), it is severely affected by carrier frequency offset (CFO). The CFO causes loss of orthogonality among the subcarriers. In this paper, we propose a method for joint estimation of CFO and

2009 Annual IEEE India Conference, 2009

Orthogonal frequency-division multiplexing (OFDM) is a well known multi-carrier digital modulatio... more Orthogonal frequency-division multiplexing (OFDM) is a well known multi-carrier digital modulation scheme in which a transmitted data is split into multiple streams that are modulated at different frequencies. The OFDM system is very sensitive to frame and frequency synchronization errors. In this paper, we propose a new OFDM preamble for accurate frame synchronization of OFDM systems. The preamble is generated

TENCON 2008 - 2008 IEEE Region 10 Conference, 2008

In this paper, we propose a new method for frame synchronization in OFDMA mode of WMAN. The schem... more In this paper, we propose a new method for frame synchronization in OFDMA mode of WMAN. The scheme is based on the preamble suggested for OFDMA mode of WMAN where the preamble consists of real subsymbols and OFDMA data symbols contain complex subsymbols for each frame. The proposed method is independent of frequency offset and results in sharp peak at

2002 IEEE International Conference on Personal Wireless Communications, 2002

Channel coding adds redundant bits to the original information bits in order to detect and correc... more Channel coding adds redundant bits to the original information bits in order to detect and correct possible errors occurring during transmission. In GSM (global system for mobile communications), we use both block coding and convolutional coding for error control. One of the channel coding schemes is CS-1 (see ETSI EN 300 909 v8.5.1, 1999), which employs shortened the cyclic fire code for burst error correction. We propose an efficient way of decoding the shortened binary cyclic fire code. The proposed scheme takes 224 clock cycles for syndrome computation and another 224 clock cycles for error trapping and correction. This decoding method is much less complex than conventional schemes and slightly less complex compared to the recently patented algorithm (Ramesh et al, "Shortened Fire code. Error-Trapping Decoding Method and Apparatus", US Patent 5,936,978). The proposed decoding scheme is applicable also to any binary shortened cyclic code.

2014 International Conference on Contemporary Computing and Informatics (IC3I), 2014

Abstract—IEEE 802.16-2004 and IEEE 802.16e-2005 standards specify OFDMA as a physical layer techn... more Abstract—IEEE 802.16-2004 and IEEE 802.16e-2005 standards specify OFDMA as a physical layer technique for fixed and nomadic BWA systems, respectively. OFDMA is very sensitive to time and frequency offsets. In this paper, a preamble based downlink frame synchronization method for OFDMA mode of WMAN is analyzed in frequency selective fading channels. Expressions for mean and variance of the metric are

EURASIP Journal on Wireless Communications and Networking, 2006

ABSTRACT Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for ... more ABSTRACT Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitting data at very high rates over time dispersive radio channels. In an OFDM system, frame synchronization and frequency offset estimation are extremely important for maintaining orthogonality among the subcarriers. Recently, several techniques have been proposed for frame synchronization in OFDM system. In multipath environment, the transmitted signal arrives at the receiver through direct and multiple delayed paths. In some cases, it is possible that power of the signal arriving through delayed path may be larger than that of the direct path (earliest path if there is no direct path). In those cases, estimate of the frame boundary may be shifted by a quantity equal to the delay of the dominant path. In such cases, there will be intersymbol interference (ISI) in the demodulated symbols unless the frame boundary estimate is preadvanced such that it dwells in the ISI-free portion of cyclic prefix or at the symbol boundary. In this paper, we propose a method for estimating the shift in the frame boundary estimate using a preamble having two identical halves. We assume that frame boundary and frequency offset estimation have been performed prior to the estimation of the shift. We also examine the quality of the frequency offset estimate when the frame boundary estimate is shifted from the desired value. The proposed method is applied to downlink synchronization in OFDM mode of WMAN (IEEE 802.16-2004). We use simulations to illustrate the usefulness of the method and also to support our assertions.

EURASIP Journal on Wireless Communications and Networking, 2006

Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitt... more Orthogonal frequency division multiplexing (OFDM) is a parallel transmission scheme for transmitting data at very high rates over time dispersive radio channels. In an OFDM system, frame synchronization and frequency offset estimation are extremely important for maintaining orthogonality among the subcarriers. In this paper, for a preamble having two identical halves in time, a timing metric is proposed for OFDM frame synchronization. The timing metric is analyzed and its mean values at the preamble boundary and in its neighborhood are evaluated, for AWGN and for frequency selective channels with specified mean power profile of the channel taps, and the variance expression is derived for AWGN case. Since the derivation of the variance expression for frequency selective channel case is tedious, we used simulations to estimate the same. Based on the theoretical value of the mean and estimate of the variance, we suggest a threshold for detection of the preamble boundary and evaluating the probability of false and correct detections. We also suggest a method for a threshold selection and the preamble boundary detection in practical applications. A simple and computationally efficient method for estimating fractional and integer frequency offset, using the same preamble, is also described. Simulations are used to corroborate the results of the analysis. The proposed method of frame synchronization and frequency offset estimation is applied to the downlink synchronization in OFDM mode of wireless metropolitan area network (WMAN) standard IEEE 802.16-2004, and its performance is studied through simulations.