IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

A near maximum a posteriori (MAP)-optimal soft detector that outputs a posteriori probabilities (APP's) for multiple-input multiple-output (MIMO) systems in flat fading channels is proposed. This is referred to as 'reduced state maximum a posteriori (RSMAP)' algorithm. This detection algorithm is based on BCJR algorithm and also uses ideas from reduced state sequence estimation (RSSE) and set partitioning. This algorithm is shown to be near optimal and the computational complexity to implement it is estimated. Finally we also show that applying the well known max-log approximation on this algorithm results in nearly same performance at much lower complexity. © 2012 IEEE.

Department of Electrical Engineering

2012 International Conference on Signal Processing and Communications, SPCOM 2012

Set-partitioning based forward/backward soft decision algorithms for MIMO detection

Voice services over Adaptive Multiuser channel in One Slot (VAMOS) performance in the presence of GMSK interferer is presented. Widely-Linear (WL) MMSE filtering is used to cancel the co-channel GMSK interferer while performing-QPSK detection. In this paper, two ways of estimating the Sub Channel Power Imbalance Ratio (SCPIR) are discussed. Due to the colouration of the resultant impairment after filtering, WL metric is proposed which incorporates error covariance between I/Q components in the metric computation. WL RSSE is used as the equalizer. Simulation results using the proposed receiver architecture show significant performance important over conventional VAMOS receivers. © 2011 IEEE.

Proceedings of the International Symposium on Wireless Communication Systems

SAIC receiver algorithms for VAMOS downlink transmission

MIMO systems promise significant capacity gains. When perfect CSI is available at the transmitter, precoding can further improve the performance. However, the performance is limited by the eigen layer having the least SNR. Coordinate Interleaved Spatial Multiplexing has been proposed recently, in which the performance of weaker eigen layers is improved at the expense of a degraded performance in the stronger layers. It has been showed that interleaving improves the overall performance of the system when the eigen layers have different diversities. Previous work on CISM assumes equal power to all the eigen layers. In this paper, we propose a power allocation approach to improve the performance of the scheme. We derive expressions for power allocation to the interleaved streams based on an optimality criterion, which tries to minimize the error probability. Simulations confirm our analytical results and show the improvement in performance. © 2010 IEEE.

2010 International Conference on Signal Processing and Communications, SPCOM 2010

A novel power allocation approach for coordinate interleaved spatial multiplexing

Multiple antenna wireless technology has the potential to enable high data rate wireless applications. To realize the spatial diversity offered by a MIMO system, either the receiver has to perform complex signal processing (such as ML decoding) or the transmitter has to preprocess the signals which requires channel knowledge at the transmitter, often through a feedback link. In this paper, we propose a new space-time signaling scheme for an Nr x Nt MIMO system that improves the diversity gain of the weak layers by encoding information along multiple dimensions and interleaving the co-ordinates of the symbols over all the layers. We prove analytically that the proposed scheme achieves full spatial diversity of NtNr. We also present simulation results that confirm our analytical results and show the superior performance of our method in comparison with existing methods. When compared with other diagonally layered schemes, the proposed scheme requires a very low feedback of log2Nt! bits and attains full diversity at the cost of a slight increase in coding and decoding complexity. ©2010 IEEE.

World University root node

Proceedings of 16th National Conference on Communications, NCC 2010

A new diagonally layered spatial multiplexing scheme with limited feedback

Performance of spatial multiplexing multiple-input multiple-output (MIMO) wireless systems can be improved with channel state information (CSI) at both ends of the link. This paper proposes a new linear diagonal MIMO transceiver, referred to as co-ordinate interleaved spatial multiplexing (CISM). With CSI at transmitter and receiver, CISM diagonalizes the MIMO channel and interleaves the co-ordinates of the input symbols (from rotated QAM constellations) transmitted over different eigenmodes. The analytical and simulation results show that with co-ordinate interleaving across two eigenmodes, the diversity gain of the data stream transmitted over the weaker eigenmode becomes equal to that of the data transmitted on the stronger eigenmode, resulting in a significant improvement in the overall diversity. The diversity-multiplexing tradeoff (DMT) is analyzed for CISM and is shown that it achieves higher diversity gain at all positive multiplexing gains compared to existing diagonal transceivers. Over rank n MIMO channels, with input symbols from rotated n-dimensional constellations, the DMT of CISM is a straight line connecting the endpoints (0,NtNr) and (min{Nt,Nr}, 0), where Nt, and Nr} are the number of transmit and receive antennas, respectively. © 2006 IEEE.

Fulltext

IEEE Transactions on Wireless Communications

Co-ordinate interleaved spatial multiplexing with channel state information

Cooperative relay communications can significantly improve the reliability of data transmission over wireless fading channels by implementing spatial diversity in a distributed fashion. Orthogonal decode and forward (ODF) relaying achieves maximum diversity provided by the source-destination and relaydestination channels, but suffers from low spectral efficiency. We propose a novel ODF protocol having higher spectral efficiency without sacrificing the diversity gain. We analyze the symbol error probability of the proposed protocol and also present simulation results. © 2009 IEEE.

IEEE Communications Letters

Orthogonal decode and forward relaying with improved spectral efficiency

Multiple antenna wireless technology and Orthogonal frequency division multiplexing (OFDM) have the potential to enable high data rate wireless applications. While the multiple-input multiple-output (MIMO) wireless techniques exploit spatial domain for improving the quantity and reliability of the data transmitted, OFDM converts the frequency-selective broadband channel into a bank of narrow band frequency-flat channels, allowing low complexity equalization at the receiver. To realize the spatial diversity offered by the multiple antennas in a MIMO-OFDM system, either the receiver has to perform complex signal processing (such as ML decoding) or the transmitter has to pre-process the signals which requires channel knowledge at the transmitter, often through a feedback link. In this paper, we propose a new space-time signaling scheme that requires limited amount of feedback and exploits the spatial diversity. We focus on 2×2 MIMO-OFDM systems and, through error analysis, show that the proposed method achieves full spatial diversity of 4. We also present simulation results that confirm our analytical results and show the superior performance of our method in comparison with existing methods.

GENERAL

IEEE Region 10 Annual International Conference, Proceedings/TENCON

A new space-time signaling scheme for MIMO-OFDM systems with limited feedback

When channel state information is available at the transmitter, we can diagonalize a MIMO channel with SVD transceivers. However, outage of the weaker eigenchannels limits the performance of such a transceiver. In the current work, we propose and analytically characterize co-ordinate interleaving of multi-dimensional symbols over sets of eigenchannels. We show that in our method, the diversity multiplexing trade-off (DMT) is determined by the strongest eigenchannel in each set. We also calculate the optimal (DMT) for different possible sets and constellation dimensions with rate allocation among the sets.© 2008 IEEE.

IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC

Signal space diversity for spatial multiplexing

Vertical layering with successive interference cancellation (SIC) at the receiver (e.g. V-BLAST) is a popular spatial multiplexing scheme that achieves high data rates over multi-antenna wireless channels. However, SIC suffers from low diversity gains and recently, with limited feedback, a diversity optimal SIC receiver has been proposed based on greedy QR decomposition (GQR-SIC). But the low diversity gain of the first decoded layer degrades the overall diversity gain. In this paper, we present a new diagonally layered spatial multiplexing scheme that employs GQR-SIC receiver but achieves significantly higher diversity gains. In the proposed scheme, the diversity gain of a weak layer is improved through interleaving the co-ordinates of the symbols transmitted over the weak layer and a strong layer. In this work, we focus on spatial multiplexing with only two layers and analyze the diversity multiplexing tradeoff to show that the proposed scheme achieves maximum diversity gain. When compared with other diagonally layered schemes, the proposed scheme has low decoding complexity. © 2008 IEEE.

GLOBECOM - IEEE Global Telecommunications Conference

A new diagonally layered spatial multiplexing scheme with partial channel knowledge

Spatial multiplexing over multiple-input multiple-output (MIMO) channels significantly improves the data rates over wireless channels. With channel knowledge at both ends of the link, many spatial multiplexing schemes have been proposed in literature, showing improved performance with respect to some performance measure. Co-ordinate interleaved spatial multiplexing (CISM) [1] is one such scheme that has been shown to achieve improved error rate performance. In this paper, we analyze CISM from diversity-multiplexing gain tradeoff (DMT) perspective. We focus on CISM over MIMO channels having only two degrees of freedom and show that the DMT of CISM is a straight line connecting the two extreme points (0, 2m) and (2,0) where m = max{Nt, Nr}. This result shows that CISM outperforms many other schemes that require same amount of feedback. © 2008 IEEE.

IEEE Vehicular Technology Conference

Diversity multiplexing gain tradeoff of co-ordinate interleaved spatial multiplexing

Co-ordinate Interleaving (CI) along with rotated constellations improves diversity gain over fading channels [1]. We propose employing CI over parallel Gaussian channels for improving the sum mutual information. © 2007 IEEE.

Forty-first Annual Conference on Information Sciences and Systems, CISS 2007 - Proceedings

Co-ordinate interleaving over parallel Gaussian channels

In this paper, a new low-complexity frequency-domain based technique is proposed for channel equalization and symbol detection of an Enhanced Data Rate for Global Evolution (EDGE) cellular system, which uses 8-PSK modulation. The proposed method exploits the present of tail symbols in the EDGE slot structure and uses them as partial Cyclic Prefix in the frequency domain equalizer. For channels with longer memory than the tail length, the receiver computes a channel shortening filter such that, when the filter is cascaded with receive chain, results in a shorter effective impulse response. The parameters of the channel shortening are set such that the effective impulse response memory length is equal to the tail length. The performance of the proposed system is demonstrated by computer simulations. © 2007 IEEE.

Proceedings of ICSCN 2007: International Conference on Signal Processing Communications and Networking

A low complexity EDGE demodulator based on FDE and impulse response shortening

IETE Technical Review (Institution of Electronics and Telecommunication Engineers, India)

Broadband to empower rural india

Spatial multiplexing (SM) over multiple-input multiple-output wireless channels provides significant capacity gains. In a SM scheme, the eigenmode having the least signal-to-noise ratio (SNR), degrades the overall error rate performance. In this paper, we propose co-ordinate interleaved spatial multiplexing that maximizes the minimum SNR over all eigenmodes. This linearly decodable SM scheme needs the knowledge of the right singular vectors of the channel at the transmitter, and the singular values and left singular vectors at the receiver. We derive the SNR expressions for the proposed scheme and compare its performance with other closed-loop schemes using computer simulaztions. © 2006 IEEE.

ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings

Co-ordinate interleaved spatial multiplexing with channel knowledge at transmitter and receiver

In a multipath fading channel, when there are many interfering CDMA signals, the multipath causes loss of orthogonality between different signals leading to intracell interference. The Generalized RAKE receiver, proposed recently in the literature, is based on the concept that this interference is not white but colored. Therefore, unlike the conventional RAKE receiver, GRAKE tries to match to the channel as well as whiten the interference. The GRAKE gives a typical improvement of 1–3 dB in SNR for a moderate increase in complexity. In this paper, we quantify the benefits of using the GRAKE in place of the RAKE in a WCDMA downlink under realistic conditions as specified in the 3GPP standards. The comparison is made both in the presence and absence of channel coding. After performing channel coding, the block error rate and hence the system throughput is evaluated for both receivers in a 144 kbps test case. It is demonstrated that the GRAKE provides significant improvement in BER performance, which translates to higher data throughput, as well as increased capacity. © 2005 Taylor & Francis Group, LLC.

IETE Journal of Research

Throughput enhancement in WCDMA using the generalized rake receiver

Linear detection of the downlink signals of a direct-sequence code division multiple-access (DS-CDMA) system has seen significant research in recent years. Notable amongst such efforts is the Generalized RAKE (based on ML criterion) and the RAKE receiver based on symbol-level MMSE. Both these approaches aim to suppress interference even while exploiting diversity. We show analytically that these two approaches are exactly equivalent.

Equivalence of the linear symbol-level MMSE and the generalized RAKE

In this letter, Doppler spread estimation in digital mobile radio systems is described. A maximum-likelihood approach is derived, and used to develop suboptimal approaches with lower complexity. The proposed algorithms rely on periodic channel estimation and can be used in both TDMA and CDMA systems.

Doppler spread estimation in mobile radio systems

This paper describes the physical layer for a High Penetration Paging (HPP) mode, that is a highly integrated adjoint to a cellular system. Through the use of very low rate coding and periodically inserted synchronization bursts, the HPP mode enables a cellular user to locate and maintain a minimal communication link with a cellular network at 15 to 20 dB below the normal operating Signal to Noise Ratio (SNR). In this paper, the ANSI-136 [1] air interface standard is used as the underlying cellular system. The same concept can be extended to other systems as well.

IEEE International Conference on Communications

Physical layer study of high penetration paging for cellular systems

In this paper, efficient and practical approaches for Doppler spread estimation in wireless mobile radio systems are described. A Hypothesis-testing approach, given the channel autocorrelation estimate, is utilized for Doppler spread estimation. The channel autocorrelation is estimated slot-by-slot using the knowledge of the channel estimates over the known fields of a TDMA burst, and averaged over several slots to reduce the effect of noise. In practical systems, the Doppler spread must be estimated in the presence of frequency offsets between the transmitter and the receiver. Hence, an algorithm that decouples Doppler spread estimation from automatic frequency compensation (AFC) is also presented. In addition to the mean estimation error performance, the convergence and the tracking ability of the algorithms are evaluated via simulation using ANSI 136 Rev-B modulation and signal transmission format.

2000 IEEE Wireless Communications and Networking Conference

Doppler spread estimation for wireless mobile radio systems

In this paper, Doppler spread estimation in digital mobile radio systems is described. An optimal approach (in the sense of maximum likelihood estimation) for Doppler spread estimation is derived. Suboptimal approaches, with trading off in performance and complexity, are developed from the optimal algorithm. In practical systems, the Doppler spread must be estimated in the presence of frequency offsets between the transmitter and the receiver. Hence, an algorithm that decouples Doppler estimation from automatic frequency compensation (AFC) is also presented. The proposed algorithms rely on channel estimation over the known fields of a TDMA burst. Such algorithms can be applied easily to systems with pilot symbols or a pilot channel.