Ravinder David Koilpillai

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.

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.

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.

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.

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.

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.

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.

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.

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.