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K Giridhar
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K Giridhar
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K Giridhar
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Giridhar, Krishnamurthy
Krishnamurthy, Giridhar
Giridhar, K.
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50 results
Now showing 1 - 10 of 50
- PublicationAn efficient suboptimum detector based on linear prediction in Rayleigh flat-fading channels(01-04-2001)
; ; It is well known that the optimum detector based on linear prediction, for frequency-nonselective (flat) fading channels, requires MP states for M-ary signalling, where P is the order of the prediction filter required to decorrelate the received signal. In this paper, we first derive a suboptimal linear predictive-based (SLP) detector for M-ary PSK by making a high SNR assumption. We then demonstrate using the concept of isometry that for M-ary PSK signalling, only MP-1 states are required for this SLP detector. The complexity reduction is achieved with no loss in symbol-error-rate performance compared to the case where MP states are used. As a corollary, we also show that for a first-order prediction filter and M-ary PSK signalling, the SLP detector is identical to the conventional differential detector. © 2001 Elsevier Science B.V. - PublicationAn iterative DFE receiver for MIMO SC-FDMA uplink(01-12-2014)
;Dhivagar, B. ;Kuchi, KiranEver since the adoption of the single carrier frequency division multiple access (SC-FDMA) scheme for the uplink in the Long Term Evolution (LTE) standard, there has been much interest in improved receiver algorithms for the same. Successive interference cancelation (SIC) based turbo receivers (Turbo SIC) have been proposed to tackle the multi-stream interference (MSI) for transmission modes such as spatial multiplexing (SM) and multiuser (MU) multiple input multiple output (MIMO). In addition, decision feedback equalizers (DFE) have been discussed as an alternative to the simple linear minimum mean square error (LMMSE) based receivers to suppress intersymbol interference (ISI) caused by the frequency selective multi-path channel. This paper proposes a DFE receiver variant based on parallel interference cancelation (PIC) with maximum likelihood detection (MLD) and ordered SIC (OSIC) principles to suppress ISI and MSI. Simulations show that the proposed receiver performs better than the existing SIC and DFE based receivers and achieves the matched filter bound (MFB) for both coded and uncoded systems. - PublicationNonlinear techniques for the joint estimation of cochannel signals(01-12-1997)
; ;Shynk, John J.Mathur, AmitCochannel interference occurs when two or more signals overlap in frequency and are present concurrently. Unlike in spread-spectrum multiple-access systems where the different users necessarily share the same channel, cochannel interference is a severe hindrance to frequency- and time-division multiple-access communications, and is typically minimized by interference rejection/suppression techniques. In this paper, rather than using interference suppression, we are interested in the joint estimation of the information-bearing narrow-band cochannel signals. Novel joint estimators are proposed that employ a single-input demodulator with oversampling to compensate for timing uncertainties. Assuming finite impulse-response channel characteristics, maximum likelihood (ML) and maximum a posteriori (MAP) criteria are used to derive cochannel detectors of varying complexities and degrees of performance. In particular, a (suboptimal) two-stage joint MAP symbol detector (JMAPSD) is introduced that has a lower complexity than the single-stage estimators while accruing only a marginal loss in error-rate performance at high signal-to-interference ratios. Assuming only reliable estimates of the primary and secondary signal powers, a blind adaptive JMAPSD algorithm for a priori unknown channels is also derived. The performance of these nonlinear joint estimation algorithms is studied through example computer simulations for two cochannel sources. © 1997 IEEE. - PublicationIndex and constellation order lowering for OFDM with index modulation(01-05-2020)
;Abhijith Nambi, S.Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a promising variant of OFDM where indices of symbol carrying sub-carriers also convey information bits besides the complex symbols themselves. Recently, a lower order modulation variant has been proposed for OFDM-IM, where the baseband constellation is lowered with appropriate rotations and index reuse, so as to lower the number of sub-blocks whose pairwise error probability is inversely related to the signal to noise ratio (SNR). In this work, we demonstrate that the number of symbol carrying sub-carriers and hence the number of used indices can also be lowered along with modulation order lowering. This reduces the aforementioned number of worst case sub-block pairs even further. The resulting unused sub-carriers can now be used to enhance the spectral efficiency as well. Numerical results reveal SNR gains up to 5dB for uncoded systems and 2dB for coded systems. - PublicationStudying the effect of delay diversity on a DS-CDMA downlink(01-01-2005)
;Jagannathan, SumanthIn this paper, we study the error rate performance of a direct sequence CDMA downlink similar to the CDMA2000 standard. While analytical computation of the error rate is possible in DS-CDMA systems under certain simplifying assumptions, computer simulations are perhaps the only means to quantify the error rate for realistic channel models. Since the delay spread of the multipath channel varies with distance, we investigate the error performance as a function of the distance from the base station. The computer simulations reveal certain interesting results which to our knowledge have not explicitly appeared in open literature. We compare the RAKE error rate performance under situations of no handoff, soft handoff, independent fading, correlated fading, chip-spaced sampling, sub-chip sampling, and pilot channel cancellation at the mobile receiver. These results indicate that narrow-band DS-CDMA downlink has certain severe shortcomings, and that it may be vital to ensure line of sight propagation for the “near” users, and exploit base-station (macro) diversity for the “far” users (in soft handoff) to ensure adequate error rate performance. © 2005 Taylor & Francis Group, LLC. - PublicationResource Allocation for D2D Links in the FFR and SFR Aided Cellular Downlink(01-10-2016)
;Gupta, Shruti ;Kumar, Suman ;Zhang, Rong; ; Hanzo, LajosDevice-to-device (D2D) communication underlying cellular networks, allows direct transmission between two devices in each other's proximity that reuse the cellular resource blocks in an effort to increase the network capacity and spectrum efficiency. However, this imposes severe interference that degrades the system's performance. This problem may be circumvented by incorporating fractional frequency reuse (FFR) or soft frequency reuse (SFR) in OFDMA cellular networks. By carefully considering the downlink resource reuse of the D2D links, we propose beneficial frequency allocation schemes, when the macrocell has employed FFR or SFR as its frequency reuse technique. The performance of these schemes is quantified using both the analytical and simulation results for characterizing both the coverage probability and the capacity of D2D links under the proposed schemes that are benchmarked against the radical unity frequency reuse scheme. The impact of the D2D links on the coverage probability of macrocellular users (CUs) is also quantified, revealing that the CUs performance is only modestly affected under the proposed frequency allocation schemes. Finally, we provide insights concerning the power control design in order to strike a beneficial tradeoff between the energy consumption and the performance of D2D links. - PublicationEnhancement in spectral efficiency using transmit-side channel shortener for MISO-OFDM systems(01-01-2014)
;Singh, IstdeoTime-domain channel shortening filters (CSFs) are used in orthogonal frequency division multiplexing (OFDM) systems to increase the spectral efficiency. We consider transmit-side CSF to reduce the cyclic prefix (CP) length required for the OFDM symbol. We first construct a simple CSF scheme which, however, has a poorer spectral efficiency (SE) when compared to a system with adequate CP length (which we refer to as a full-CP system). We then propose a modified CSF (MCSF) structure that exploits the null-space of an under-determined system of equations, and provides independent equivalent channels to the receiver. The SE of the MCSF is shown to be generally higher than the full-CP system when the chosen CP length is significantly smaller than the channel delay spread. Simulated BLER performance for turbo-coded multi-antenna OFDM systems reveals that the MCSF can provide up to 2 dB gain in (Eb/N0} when compared with the full-CP system for the same transmit information rate. © 2014 IEEE. - PublicationTensor-based spatial smoothing (TB-SS) using multiple snapshots(01-05-2010)
;Thakre, Arpita ;Haardt, Martin ;Roemer, FlorianTensor-based spatial smoothing (TB-SS) is a preprocessing technique for subspace-based parameter estimation of damped and undamped harmonics. In TB-SS, multichannel data is packed into a measurement tensor. We propose a tensor-based signal subspace estimation scheme that exploits the multidimensional invariance property exhibited by the highly structured measurement tensor. In the presence of noise, a tensor-based subspace estimate obtained via TB-SS is a better estimate of the desired signal subspace than the subspace estimate obtained by, for example, the singular value decomposition of a spatially smoothed matrix or a multilinear algebra approach reported in the literature. Thus, TB-SS in conjunction with subspace-based parameter estimation schemes performs significantly better than subspace-based parameter estimation algorithms applied to the existing matrix-based subspace estimate. Another advantage of TB-SS over the conventional SS is that TB-SS is insensitive to changes in the number of samples per subarray provided that the number of subarrays is greater than the number of harmonics. In this paper, we present, as an example, TB-SS in conjunction with ESPRIT-type algorithms for the parameter estimation of one-dimensional (1-D) damped and undamped harmonics. A closed form expression of the stochastic Cramr-Rao bound (CRB) for the 1-D damped harmonic retrieval problem is also derived. © 2006 IEEE. - PublicationEfficient synchronization and frequency tracking for cellular Reuse-I OFDMA systems(01-01-2007)
;Bhargava, Y.Orthogonal Frequency Division Multiplexing (OFDM) systems are highly sensitive to frequency synchronization errors which cause inter-carrier interference, leading to degradation in the system performance. In this paper, a pragmatic timing and frequency synchronization scheme for reuse-I OFDMA systems that is compatible with IEEE 802.16d/e wireless MAN standard is proposed. The algorithm uses cyclic prefix correlation technique for estimating and tracking the OFDMA frame boundary and fractional frequency offset. Once the preamble is captured, the integral part of the frequency offset and cell-specific preamble code can be estimated. This novel scheme is computationally efficient and robust to co-channel interference and multipath fading. Simulation results show that the proposed algorithm gives excellent performance even at very low signal to interference noise ratios. © 2007 by the IETE. - PublicationPredicting the affordable rate in interference-limited cellular systems using higher-order Markov models(01-01-2016)
;Pulliyakode, Saishankar Katri; ;Hanzo, LajosIn cellular broadband access systems, such as 3GPP long-term evolution, the user equipment (UE) feeds back a quantized rate metric to the base station (eNodeB), in order to perform rate adaptation. The periodicity of this rate feedback is fixed so as to minimize the overhead without eroding its benefits. However, between two feedback instants n and n+δ, the actual rate that the UE can correctly decode might change due to the: 1) Doppler shift and 2) change in the active set of interferers. Hence, to fully exploit the benefits of adaptation, an accurate prediction of the attainable rate is required. In this context, we argue that a non-parametric approach to rate prediction is necessary. Since the selected rate is from a set of discrete values, the rate prediction problem is mapped by us onto a discrete sequence prediction problem, and we construct higher order Markov models for the discrete sequences using source encoding algorithms. We then propose two distinct rate-prediction algorithms. One of them is the adaptive maximum a posteriori estimator, while the other is the adaptive Bayesian risk-based estimator. Both of these algorithms simultaneously estimate the best Markov model for each UE and then perform prediction based on the estimated model.