Chintha Tellambura - Profile on Academia.edu (original) (raw)

Papers by Chintha Tellambura

2010 Conference Record of the Forty Fourth Asilomar Conference on Signals, Systems and Computers, 2010

Joint bandwidth and power allocation in wireless multiuser decode and forward relay networks is p... more Joint bandwidth and power allocation in wireless multiuser decode and forward relay networks is proposed in this paper for maximizing the number of admissible users. A suboptimal greedy search algorithm is developed to solve the admission control problem efficiently at low complexity. The condition under which the greedy search is optimal is found. The way we derive such optimality condition for the greedy search can serve as a benchmark for other applications of greedy search. The advantages of the suboptimal greedy search algorithm compared to exhaustive search algorithm are shown.

2011 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2011

A problem of joint optimal bandwidth and power allocation in cognitive networks under fading chan... more A problem of joint optimal bandwidth and power allocation in cognitive networks under fading channels is considered. It is assumed that multiple secondary users (SUs) share the spectrum of a primary user (PU) using frequency division multiple access. The bandwidth and power are allocated so as to maximize the sum ergodic capacity of all SUs under the total bandwidth constraint of the licensed spectrum as well as different combinations of the peak/average transmit power constraints at the SUs and the peak/average interference power constraint imposed by the PU. Although the optimization problem is convex, its dimension and, thus, complexity may be high. Therefore, computationally efficient ways of solving the problem are of importance and are investigated here by finding structures of the optimal solutions to the problem under different combinations of the constraints.

2007 IEEE Wireless Communications and Networking Conference, 2007

Despite their high spectral efficiencies, multipleinput multiple-output (MIMO) systems suffer fro... more Despite their high spectral efficiencies, multipleinput multiple-output (MIMO) systems suffer from high cost and complexity due to multiple radio frequency chains at both link ends. A possible solution is to select a subset of the available antennas at transmitter and/or receiver based on maximal capacity or minimal error rates. In this paper, we propose a receive antenna selection algorithm to minimize the union bound on the vector error rate. By relaxing the antenna selection variables from discrete to continuous, we formulate the problem as a convex optimization problem. An efficient iterative method can be used to obtain the solution.

We propose a novel transmission protocol for multiway relay networks (MWRNs) in which the number ... more We propose a novel transmission protocol for multiway relay networks (MWRNs) in which the number of timeslots required for full mutual multi-way data exchange among K user nodes can be reduced to just two from (K − 1)/2 + 1 in the current state-of-the-art. The proposed MWRN adopts superposition-coded transmission, successive interference cancellation (SIC) reception, power-domain non-orthogonal multipleaccess (NOMA) and linear detection/precoding facilitated by massive multiple-input multiple-output (MIMO). First, the user nodes transmit their signals to a massive MIMO-enabled relay, where a linear detector based on maximal ratio combining criterion is used for signal reception. Next, the relay composes a superposition-coded signal for each user node and transmits towards the user nodes by using a linear precoder based on maximal ratio transmission criterion. User nodes perform SICbased decoding for retrieving symbols sent by the remaining K user nodes. Thus, our proposed MWRN protocol completes the full mutual multi-way data exchange among all users within two time-slots. We derive the achievable sum rate of it via the so-called worst-case Gaussian approximation and show that a significant spectral efficiency gain can be achieved over the existing MWRN counterparts.

IEEE Transactions on Communications, Mar 1, 2019

Spectrum harvesting is needed for large-scale wireless networks to access underutilized spectrum ... more Spectrum harvesting is needed for large-scale wireless networks to access underutilized spectrum and support multiple heterogeneous users. Cooperative spectrum harvesting allows for improved co-channel existence and intra/inter-cell interference mitigation, which dramatically improves spectral efficiency. However, good performance metrics to quantify cooperative spectrum harvesting (CSH) schemes are not available. For example, existing metrics such as data rate, error/outage probability, and multiplexing/diversity gains may not clearly distinguish large signal-to-interference-plus-noise ratio (SINR) scenarios and sum rate performance for multiple links. To overcome these limitations, we propose two new metrics called spectrum access level (SAL) and user participation level (UPL). The advantages of these metrics are: 1) achieving distinct upper bounds for multiple links; 2) upper bounds being evaluated directly by basic CSH system model; and 3) determining the performance at any power level of CSH schemes even if they are not interference-exempt. Moreover, a novel CSH system model is conceived to achieve satisfying spectrum access ability based on SAL and UPL, and an interference-exempt scheme is designed to achieve relevant upper bounds. Numerical results verify the efficiency of SAL and UPL, and the spectrum access ability of proposed system model with interference-exempt scheme.

IEEE Transactions on Wireless Communications, 2019

In this paper, we propose a novel successive twoway relaying (STWR) system that uses a pair of co... more In this paper, we propose a novel successive twoway relaying (STWR) system that uses a pair of conventional half-duplex (HD) relays to mimic a full-duplex two-way relay (FD-TWR). Although classical FD-TWR is spectral efficient and expands cell coverage, the proposed STWR utilizes existing HD infrastructure to boost the FD implementation, and offers bi-directional data exchange and low-complexity residual selfinterference (RSI) mitigation. To formulate STWR, we develop a unified signal model to facilitate the mitigation of generalized selfinterference (GSI). GSI consists of back-propagating interference due to two-way relaying, RSI of FD sources and inter-relay interference caused by the pairs of HD relays. Because the GSI channel matrix has a distinct row linearity, we propose an efficient digital approach to remove the GSI and design two low-complexity algorithms. These algorithms avoid RSI channel estimation, full-rank matrix, and complex matrix computation. Our analysis and simulations show that: 1) the proposed STWR achieves the multiplexing gain of the true FD-TWR; 2) the distance between the two HD relays should be optimized to achieve the highest spectral efficiency; 3) the STWR system with two algorithms can achieve a diversity order of one or two, respectively. Therefore, the STWR concept achieves a flexible tradeoff between performance and complexity, potentially enabling large-scale relay deployments.

IEEE Transactions on Wireless Communications, Feb 1, 2018

Soft iterative detection/decoding algorithms are fundamentally necessary for multiple-input multi... more Soft iterative detection/decoding algorithms are fundamentally necessary for multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) adopted in the 3rd Generation Long Term Evolution-Advanced (LTE-A) in order to increase the capacity and achieve high data rates. However, their high performance critically requires log likelihood ratio (LLR) computations with prohibitive complexity. This challenge will be addressed in this paper. We first use the the assumption of Gaussian transmit symbols to show the equivalence among several existing algorithms. We next develop a non-Gaussian approximation for high-order constellations, which paves the way for interference cancellation based detectors. Based on both Gaussian and non-Gaussian approximations, we thus develop several capacity-achieving iterative MIMO-OFDM demodulation and decoding algorithms. To this end, we adopt Kbest algorithms to take advantage of both types of approximations and the list decoder. Unlike existing algorithms, our proposed K-best algorithms make use of the a priori probabilities to generate the list. Simulations of standard-compliant LTE systems demonstrate that the proposed algorithms outperform existing ones.

The performance of a cooperative network with Cooperative Maximal Ratio Combining (C-MRC) is anal... more The performance of a cooperative network with Cooperative Maximal Ratio Combining (C-MRC) is analyzed. To this end, we propose a heuristic approximation to the total received SNR (signal-to-noise ratio) at the destination. The approximation is not only an accurate representation of the received SNR, but also amenable to performance analysis. We then derive the probability density function (PDF) of the approximate SNR. The PDF is used to derive expressions of the parameters required for the performance analysis of rate adaptive transmission, based on the use of a discrete set of modes of square M-ary quadrature amplitude modulation (QAM), with a cooperative network employing C-MRC. Analytical expressions for mode selection probability, the outage probability, the average spectral efficiency, and the average bit error rate are derived. Results show that performance of the system under C-MRC is comparable with other cooperative networks.

This paper investigates the impact of co-channel interference (CCI), imperfect channel state info... more This paper investigates the impact of co-channel interference (CCI), imperfect channel state information (CSI) and pilot contamination for multi-pair massive multiple-input multipleoutput (MIMO) two-way relay networks (TWRNs). We consider a multi-cell TWRN system consisting of single-antenna user nodes and amplify-and-forward (AF) relay nodes having very large antenna arrays. Under the aforementioned channel imperfections, the asymptotic signal-to-interference-noise ratio and asymptotic sum rate expressions are derived in closed-form whenever the number of relay antennas grows unbounded with respect to the number of user nodes. For perfect CSI case, the transmit power at the user nodes and the relay can be scaled down inversely proportional to the number of antennas at the relay. Moreover, for the imperfect CSI case, these transmit powers can only be scaled down inversely proportional to the square-root of the relay antenna count. Thus, even with imperfect CSI, the benefits of employing a massive MIMOenabled relay on transmit power savings are significant. Moreover, our analysis shows that although the detrimental effect of CCI can be asymptotically negated completely, the residual interference due to pilot contamination cannot be mitigated even in the limit of infinitely many relay antennas.

In this paper, an extension of raptor codes is introduced which keeps all the desirable propertie... more In this paper, an extension of raptor codes is introduced which keeps all the desirable properties of raptor codes, including the linear complexity of encoding and decoding per information bit, unchanged. The new design, however, improves the performance in terms of the reception rate. Our simulations show a 10% reduction in the needed overhead at the benchmark block length of 64,520 bits and with the same complexity per information bit.

2017 IEEE Wireless Communications and Networking Conference (WCNC), 2017

We analyze relay selection for an underlay cognitive radio (CR) two-way relay network (TWRN) with... more We analyze relay selection for an underlay cognitive radio (CR) two-way relay network (TWRN) with zero-forcing (ZF) transmission and receiving. The source and the destination nodes are massive multiple-input multiple-output (MIMO) enabled. Relays will perform amplify and forwarding (AF) while the destination and source nodes perform self interference cancellation. We first obtain asymptotic signal-to-interference-plus-noise ratio (SINR) values under the power scaling at the relay and end nodes. Then, we derive optimal power allocation schemes for the end nodes to satisfy the interference constraints at the primary user (PU). Based on these optimal values, we analyze the effect of relay selection on the sum rate. With the use of massive MIMO, the SINR and the sum rate will only depend on the pathloss coefficients of the channels and average noise levels. Thus, the relay selection can be done at the deployment stages of the system and most of the time it simplifies to selection of the relay with the highest number of antennas. Our simulation results validate the analytical asymptotic results and qualify CR massive MIMO TWRNs as a possible candidate for future wireless systems.

Journal of Science and Technology: Issue on Information and Communications Technology, 2017

We characterize the performance of energy detector (ED) over square-law, square-law selection, an... more We characterize the performance of energy detector (ED) over square-law, square-law selection, and switch-and-stay diversity combining schemes. The exact average probabilities of a miss (Pm), and a false alarm (Pf) are derived in closed-form. To derive Pm for versatile Nakagami-m and Rician fading channels, a twofold approach, using the probability density function (PDF) and the moment generating function (MGF), is applied. Using the PDF method, the achievable diversity order over the Nakagami-m channel is derived. However, this method becomes intractable when analyzing Pm of the aforementioned combiners in Rician channels, but the MGF method can handle this case. Our analysis helps to quantify the performance gains of ED due to diversity reception. Theoretical derivations are verified through numerical Monte-Carlo simulation results.

IEEE Transactions on Vehicular Technology, 2019

This article investigates a full-duplex base station using a generalized frequency division multi... more This article investigates a full-duplex base station using a generalized frequency division multiplexing (GFDM) transceiver with the radio frequency (RF) impairments including phase noise, carrier frequency offset (CFO) and in-phase (I) and quadrature (Q) imbalance. To fully focus on the RF impairment issue, we study the simple configuration of single uplink user and single downlink user. They both are half-duplex wireless. In the uplink, we study analog and digital self-interference (SI) cancellation and propose a complementary SI suppression method. Desired signal and residual SI powers and signal-to-interference ratio (SIR) are derived in closed form. Similarly, in the downlink, we derive desired signal power, co-channel interference signal power caused by the uplink user and SIR. The RF impairments degrade the efficiency of SI cancellation and affect GFDM more negatively than full-duplex orthogonal frequency division multiplexing (OFDM). Hence, we propose full-duplex GFDM receiver filters for maximizing the SIR for both uplink and downlink transmissions. Finally, the uplink and downlink rates and the uplink-downlink rate region are derived. Significantly, the optimal-filter based GFDM outperforms full-duplex OFDM by 25 dB higher SIR and an uplink rate increase of 500%. Index Terms-Full-duplex radios, generalized frequency division multiplexing (GFDM), radio frequency (RF) impairments, signal-to-interference ratio (SIR), filter design, rate region.

IEEE GLOBECOM 2008 - 2008 IEEE Global Telecommunications Conference, 2008

Recently, a maximum likelihood (ML) detection rule for differential unitary space time modulation... more Recently, a maximum likelihood (ML) detection rule for differential unitary space time modulation (DUSTM) under the existence of unknown carrier frequency offset (CFO) has been derived. However, the ML detection is based on the exhaustive search over all the unitary group codes. In this paper, we design an efficient detection algorithm for newly derived ML rule, by modifying the bound intersection detector (BID). Our proposed algorithm is seen as a generalization of the existing BID that is known to be an optimal detector for the conventional DUSTM. The simulation results show that the proposed algorithm can save a large portion of the computational complexity compared to the naive searching method.

IEEE Transactions on Communications, 2015

Relay selection strategies help to improve spectral and energy efficiencies, to enhance transmiss... more Relay selection strategies help to improve spectral and energy efficiencies, to enhance transmission robustness, or to reduce latency in multi-relay cooperative networks. Two novel relay selection strategies are proposed and analyzed here for multipleinput multiple-output (MIMO) amplify-and-forward (AF) two-way relay networks (TWRNs) with spatial multiplexing. Specifically, they are designed to maximize the effective end-to-end signal-to-noise ratio (SNR), and thereby minimize the overall outage probability or maximize the achievable sum rate. Interestingly, the first strategy amounts to maximizing the minimum of the eigenvalues of the Wishart matrices from the selected relay to the two user nodes. Counter-intuitively, the latter strategy amounts to maximizing the minimum of the determinant of the same Wishart matrices. The performance of these two strategies is investigated by deriving lower/upper bounds of the overall outage probability and the average sum rate approximations in closed-form. Further, the asymptotic high SNR approximations of the outage probability are derived, and thereby, the achievable diversity-multiplexing trade-off is quantified. This trade-off reveals that whenever the sum of relay antennas is fixed, the achievable diversity order is always a constant, and hence, the multiplexing gain can indeed be improved by equally distributing antennas among the available set of relays. Our results reveal that relay selection indeed significantly alleviates the inherent diversitygain loss associated with the use of available degrees of freedom for spatial multiplexing.

IEEE GLOBECOM 2007-2007 IEEE Global Telecommunications Conference, 2007

In this paper, we answer the question that "Can conventional differential unitary space time modu... more In this paper, we answer the question that "Can conventional differential unitary space time modulation (DUSTM) be applied when there is an unknown carrier frequency offset (CFO)?" and present a maximum likelihood (ML) detection rule for this scenario. We analyze the asymptotical performance of our ML detection and provide the code design criterion by using the modified diversity product. The analysis also brings the insight that our proposed decision rule is a new differential modulation scheme in both temporal and spatial domains. Various simulations are conducted, and the proposed algorithm is shown to be more robust to the CFO drifting than the existing double temporal differential modulation.

2007 IEEE International Conference on Communications, 2007

Frequency synchronization is one of the most important components in orthogonal frequency-divisio... more Frequency synchronization is one of the most important components in orthogonal frequency-division multiplexing (OFDM) systems. Recently, the discrete cosine transform (DCT) based OFDM system has received wide attentions due to several advantages. Hence, the study of frequency synchronization issue for this newly raised system is well on its time. To provide a thorough study, we consider the non-circular transmissions, and the results can be easily generated to circular transmissions if the elliptic variance is set to zero. We present three joint maximum likelihood (ML) carrier frequency offset (CFO) and phase offset (PO) estimators. From both the theoretical analysis and the numerical comparisons, we found new advantages of the DCT-OFDM over the traditional discrete Fourier transform (DFT) based OFDM. These advantages, as well as those already studied in the early works on DCT-OFDM, support the belief that the DCT-OFDM is a new promising multi-carrier modulation (MCM) scheme.

IEEE Wireless Communications and Networking Conference, 2006. WCNC 2006., 2006

In this paper, we consider efficient blind decoder design for orthogonal space-time block codes (... more In this paper, we consider efficient blind decoder design for orthogonal space-time block codes (OSTBCs). A general decision rule for blind OSTBC decoding is derived assuming a quasi-static flat multiple-input multiple-output (MIMO) Rayleigh fading channel. We use the linear dispersion representation of OSTBCs to derive a blind decoder that results in a quadratic minimization problem, which can be solved efficiently by semidefinite relaxation, sphere decoding or successive interference cancellation. To resolve phase ambiguity problems inherent in blind detectors, rather than using pilot symbols that results in a bandwidth loss, we propose novel totally blind decoders using dual constellations or a superimposed training scheme. To alleviate the computational burden, a minimum mean-squareerror (MMSE) channel estimator is also proposed to track the time-varying channel without using the blind decoder.

2011 IEEE Global Telecommunications Conference - GLOBECOM 2011, 2011

A novel transmit/receive (Tx/Rx) antenna selection strategy is proposed and analyzed for two-way ... more A novel transmit/receive (Tx/Rx) antenna selection strategy is proposed and analyzed for two-way multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay networks. This strategy involves choosing the best transmit and receive antennas at the two sources and the relay based on the minimization of the overall outage probability. The performance of the proposed selection strategy is quantified by deriving the overall outage probability and its high SNR approximation. Specifically, the diversity order is derived to obtain valuable insights into practical system designing. In particular, our results are extended to cater the multiple relay scenario, and thereby, a joint relay and Tx/Rx antenna selection strategy is proposed and analyzed. To this end, the overall outage probability, its high SNR approximation and diversity order are derived. Our numerical results show that the proposed selection strategies achieve the full diversity order. All the analyses are validated through Monte-Carlo simulations. 3 The system model in [9] is restricted to dual-antenna relay terminal.

IEEE GLOBECOM 2008 - 2008 IEEE Global Telecommunications Conference, 2008

Optimal pilots design and placement for the frequency offset and channel estimation in orthogonal... more Optimal pilots design and placement for the frequency offset and channel estimation in orthogonal frequencydivision multiplexing access (OFDMA) uplink systems are proposed. The received pilots of multiple users can always be demodulated, even if they are totally overlapped due to the large frequency offsets. With the knowledge of channel state information (CSI) at the receiver, the performance of the proposed frequency offset estimation is robust to the channel estimation errors. The frequency offset and CSI can be jointly estimated by employing the proposed pilots.

2010 Conference Record of the Forty Fourth Asilomar Conference on Signals, Systems and Computers, 2010

Joint bandwidth and power allocation in wireless multiuser decode and forward relay networks is p... more Joint bandwidth and power allocation in wireless multiuser decode and forward relay networks is proposed in this paper for maximizing the number of admissible users. A suboptimal greedy search algorithm is developed to solve the admission control problem efficiently at low complexity. The condition under which the greedy search is optimal is found. The way we derive such optimality condition for the greedy search can serve as a benchmark for other applications of greedy search. The advantages of the suboptimal greedy search algorithm compared to exhaustive search algorithm are shown.

2011 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2011

A problem of joint optimal bandwidth and power allocation in cognitive networks under fading chan... more A problem of joint optimal bandwidth and power allocation in cognitive networks under fading channels is considered. It is assumed that multiple secondary users (SUs) share the spectrum of a primary user (PU) using frequency division multiple access. The bandwidth and power are allocated so as to maximize the sum ergodic capacity of all SUs under the total bandwidth constraint of the licensed spectrum as well as different combinations of the peak/average transmit power constraints at the SUs and the peak/average interference power constraint imposed by the PU. Although the optimization problem is convex, its dimension and, thus, complexity may be high. Therefore, computationally efficient ways of solving the problem are of importance and are investigated here by finding structures of the optimal solutions to the problem under different combinations of the constraints.

2007 IEEE Wireless Communications and Networking Conference, 2007

Despite their high spectral efficiencies, multipleinput multiple-output (MIMO) systems suffer fro... more Despite their high spectral efficiencies, multipleinput multiple-output (MIMO) systems suffer from high cost and complexity due to multiple radio frequency chains at both link ends. A possible solution is to select a subset of the available antennas at transmitter and/or receiver based on maximal capacity or minimal error rates. In this paper, we propose a receive antenna selection algorithm to minimize the union bound on the vector error rate. By relaxing the antenna selection variables from discrete to continuous, we formulate the problem as a convex optimization problem. An efficient iterative method can be used to obtain the solution.

We propose a novel transmission protocol for multiway relay networks (MWRNs) in which the number ... more We propose a novel transmission protocol for multiway relay networks (MWRNs) in which the number of timeslots required for full mutual multi-way data exchange among K user nodes can be reduced to just two from (K − 1)/2 + 1 in the current state-of-the-art. The proposed MWRN adopts superposition-coded transmission, successive interference cancellation (SIC) reception, power-domain non-orthogonal multipleaccess (NOMA) and linear detection/precoding facilitated by massive multiple-input multiple-output (MIMO). First, the user nodes transmit their signals to a massive MIMO-enabled relay, where a linear detector based on maximal ratio combining criterion is used for signal reception. Next, the relay composes a superposition-coded signal for each user node and transmits towards the user nodes by using a linear precoder based on maximal ratio transmission criterion. User nodes perform SICbased decoding for retrieving symbols sent by the remaining K user nodes. Thus, our proposed MWRN protocol completes the full mutual multi-way data exchange among all users within two time-slots. We derive the achievable sum rate of it via the so-called worst-case Gaussian approximation and show that a significant spectral efficiency gain can be achieved over the existing MWRN counterparts.

IEEE Transactions on Communications, Mar 1, 2019

Spectrum harvesting is needed for large-scale wireless networks to access underutilized spectrum ... more Spectrum harvesting is needed for large-scale wireless networks to access underutilized spectrum and support multiple heterogeneous users. Cooperative spectrum harvesting allows for improved co-channel existence and intra/inter-cell interference mitigation, which dramatically improves spectral efficiency. However, good performance metrics to quantify cooperative spectrum harvesting (CSH) schemes are not available. For example, existing metrics such as data rate, error/outage probability, and multiplexing/diversity gains may not clearly distinguish large signal-to-interference-plus-noise ratio (SINR) scenarios and sum rate performance for multiple links. To overcome these limitations, we propose two new metrics called spectrum access level (SAL) and user participation level (UPL). The advantages of these metrics are: 1) achieving distinct upper bounds for multiple links; 2) upper bounds being evaluated directly by basic CSH system model; and 3) determining the performance at any power level of CSH schemes even if they are not interference-exempt. Moreover, a novel CSH system model is conceived to achieve satisfying spectrum access ability based on SAL and UPL, and an interference-exempt scheme is designed to achieve relevant upper bounds. Numerical results verify the efficiency of SAL and UPL, and the spectrum access ability of proposed system model with interference-exempt scheme.

IEEE Transactions on Wireless Communications, 2019

In this paper, we propose a novel successive twoway relaying (STWR) system that uses a pair of co... more In this paper, we propose a novel successive twoway relaying (STWR) system that uses a pair of conventional half-duplex (HD) relays to mimic a full-duplex two-way relay (FD-TWR). Although classical FD-TWR is spectral efficient and expands cell coverage, the proposed STWR utilizes existing HD infrastructure to boost the FD implementation, and offers bi-directional data exchange and low-complexity residual selfinterference (RSI) mitigation. To formulate STWR, we develop a unified signal model to facilitate the mitigation of generalized selfinterference (GSI). GSI consists of back-propagating interference due to two-way relaying, RSI of FD sources and inter-relay interference caused by the pairs of HD relays. Because the GSI channel matrix has a distinct row linearity, we propose an efficient digital approach to remove the GSI and design two low-complexity algorithms. These algorithms avoid RSI channel estimation, full-rank matrix, and complex matrix computation. Our analysis and simulations show that: 1) the proposed STWR achieves the multiplexing gain of the true FD-TWR; 2) the distance between the two HD relays should be optimized to achieve the highest spectral efficiency; 3) the STWR system with two algorithms can achieve a diversity order of one or two, respectively. Therefore, the STWR concept achieves a flexible tradeoff between performance and complexity, potentially enabling large-scale relay deployments.

IEEE Transactions on Wireless Communications, Feb 1, 2018

Soft iterative detection/decoding algorithms are fundamentally necessary for multiple-input multi... more Soft iterative detection/decoding algorithms are fundamentally necessary for multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) adopted in the 3rd Generation Long Term Evolution-Advanced (LTE-A) in order to increase the capacity and achieve high data rates. However, their high performance critically requires log likelihood ratio (LLR) computations with prohibitive complexity. This challenge will be addressed in this paper. We first use the the assumption of Gaussian transmit symbols to show the equivalence among several existing algorithms. We next develop a non-Gaussian approximation for high-order constellations, which paves the way for interference cancellation based detectors. Based on both Gaussian and non-Gaussian approximations, we thus develop several capacity-achieving iterative MIMO-OFDM demodulation and decoding algorithms. To this end, we adopt Kbest algorithms to take advantage of both types of approximations and the list decoder. Unlike existing algorithms, our proposed K-best algorithms make use of the a priori probabilities to generate the list. Simulations of standard-compliant LTE systems demonstrate that the proposed algorithms outperform existing ones.

The performance of a cooperative network with Cooperative Maximal Ratio Combining (C-MRC) is anal... more The performance of a cooperative network with Cooperative Maximal Ratio Combining (C-MRC) is analyzed. To this end, we propose a heuristic approximation to the total received SNR (signal-to-noise ratio) at the destination. The approximation is not only an accurate representation of the received SNR, but also amenable to performance analysis. We then derive the probability density function (PDF) of the approximate SNR. The PDF is used to derive expressions of the parameters required for the performance analysis of rate adaptive transmission, based on the use of a discrete set of modes of square M-ary quadrature amplitude modulation (QAM), with a cooperative network employing C-MRC. Analytical expressions for mode selection probability, the outage probability, the average spectral efficiency, and the average bit error rate are derived. Results show that performance of the system under C-MRC is comparable with other cooperative networks.

This paper investigates the impact of co-channel interference (CCI), imperfect channel state info... more This paper investigates the impact of co-channel interference (CCI), imperfect channel state information (CSI) and pilot contamination for multi-pair massive multiple-input multipleoutput (MIMO) two-way relay networks (TWRNs). We consider a multi-cell TWRN system consisting of single-antenna user nodes and amplify-and-forward (AF) relay nodes having very large antenna arrays. Under the aforementioned channel imperfections, the asymptotic signal-to-interference-noise ratio and asymptotic sum rate expressions are derived in closed-form whenever the number of relay antennas grows unbounded with respect to the number of user nodes. For perfect CSI case, the transmit power at the user nodes and the relay can be scaled down inversely proportional to the number of antennas at the relay. Moreover, for the imperfect CSI case, these transmit powers can only be scaled down inversely proportional to the square-root of the relay antenna count. Thus, even with imperfect CSI, the benefits of employing a massive MIMOenabled relay on transmit power savings are significant. Moreover, our analysis shows that although the detrimental effect of CCI can be asymptotically negated completely, the residual interference due to pilot contamination cannot be mitigated even in the limit of infinitely many relay antennas.

In this paper, an extension of raptor codes is introduced which keeps all the desirable propertie... more In this paper, an extension of raptor codes is introduced which keeps all the desirable properties of raptor codes, including the linear complexity of encoding and decoding per information bit, unchanged. The new design, however, improves the performance in terms of the reception rate. Our simulations show a 10% reduction in the needed overhead at the benchmark block length of 64,520 bits and with the same complexity per information bit.

2017 IEEE Wireless Communications and Networking Conference (WCNC), 2017

We analyze relay selection for an underlay cognitive radio (CR) two-way relay network (TWRN) with... more We analyze relay selection for an underlay cognitive radio (CR) two-way relay network (TWRN) with zero-forcing (ZF) transmission and receiving. The source and the destination nodes are massive multiple-input multiple-output (MIMO) enabled. Relays will perform amplify and forwarding (AF) while the destination and source nodes perform self interference cancellation. We first obtain asymptotic signal-to-interference-plus-noise ratio (SINR) values under the power scaling at the relay and end nodes. Then, we derive optimal power allocation schemes for the end nodes to satisfy the interference constraints at the primary user (PU). Based on these optimal values, we analyze the effect of relay selection on the sum rate. With the use of massive MIMO, the SINR and the sum rate will only depend on the pathloss coefficients of the channels and average noise levels. Thus, the relay selection can be done at the deployment stages of the system and most of the time it simplifies to selection of the relay with the highest number of antennas. Our simulation results validate the analytical asymptotic results and qualify CR massive MIMO TWRNs as a possible candidate for future wireless systems.

Journal of Science and Technology: Issue on Information and Communications Technology, 2017

We characterize the performance of energy detector (ED) over square-law, square-law selection, an... more We characterize the performance of energy detector (ED) over square-law, square-law selection, and switch-and-stay diversity combining schemes. The exact average probabilities of a miss (Pm), and a false alarm (Pf) are derived in closed-form. To derive Pm for versatile Nakagami-m and Rician fading channels, a twofold approach, using the probability density function (PDF) and the moment generating function (MGF), is applied. Using the PDF method, the achievable diversity order over the Nakagami-m channel is derived. However, this method becomes intractable when analyzing Pm of the aforementioned combiners in Rician channels, but the MGF method can handle this case. Our analysis helps to quantify the performance gains of ED due to diversity reception. Theoretical derivations are verified through numerical Monte-Carlo simulation results.

IEEE Transactions on Vehicular Technology, 2019

This article investigates a full-duplex base station using a generalized frequency division multi... more This article investigates a full-duplex base station using a generalized frequency division multiplexing (GFDM) transceiver with the radio frequency (RF) impairments including phase noise, carrier frequency offset (CFO) and in-phase (I) and quadrature (Q) imbalance. To fully focus on the RF impairment issue, we study the simple configuration of single uplink user and single downlink user. They both are half-duplex wireless. In the uplink, we study analog and digital self-interference (SI) cancellation and propose a complementary SI suppression method. Desired signal and residual SI powers and signal-to-interference ratio (SIR) are derived in closed form. Similarly, in the downlink, we derive desired signal power, co-channel interference signal power caused by the uplink user and SIR. The RF impairments degrade the efficiency of SI cancellation and affect GFDM more negatively than full-duplex orthogonal frequency division multiplexing (OFDM). Hence, we propose full-duplex GFDM receiver filters for maximizing the SIR for both uplink and downlink transmissions. Finally, the uplink and downlink rates and the uplink-downlink rate region are derived. Significantly, the optimal-filter based GFDM outperforms full-duplex OFDM by 25 dB higher SIR and an uplink rate increase of 500%. Index Terms-Full-duplex radios, generalized frequency division multiplexing (GFDM), radio frequency (RF) impairments, signal-to-interference ratio (SIR), filter design, rate region.

IEEE GLOBECOM 2008 - 2008 IEEE Global Telecommunications Conference, 2008

Recently, a maximum likelihood (ML) detection rule for differential unitary space time modulation... more Recently, a maximum likelihood (ML) detection rule for differential unitary space time modulation (DUSTM) under the existence of unknown carrier frequency offset (CFO) has been derived. However, the ML detection is based on the exhaustive search over all the unitary group codes. In this paper, we design an efficient detection algorithm for newly derived ML rule, by modifying the bound intersection detector (BID). Our proposed algorithm is seen as a generalization of the existing BID that is known to be an optimal detector for the conventional DUSTM. The simulation results show that the proposed algorithm can save a large portion of the computational complexity compared to the naive searching method.

IEEE Transactions on Communications, 2015

Relay selection strategies help to improve spectral and energy efficiencies, to enhance transmiss... more Relay selection strategies help to improve spectral and energy efficiencies, to enhance transmission robustness, or to reduce latency in multi-relay cooperative networks. Two novel relay selection strategies are proposed and analyzed here for multipleinput multiple-output (MIMO) amplify-and-forward (AF) two-way relay networks (TWRNs) with spatial multiplexing. Specifically, they are designed to maximize the effective end-to-end signal-to-noise ratio (SNR), and thereby minimize the overall outage probability or maximize the achievable sum rate. Interestingly, the first strategy amounts to maximizing the minimum of the eigenvalues of the Wishart matrices from the selected relay to the two user nodes. Counter-intuitively, the latter strategy amounts to maximizing the minimum of the determinant of the same Wishart matrices. The performance of these two strategies is investigated by deriving lower/upper bounds of the overall outage probability and the average sum rate approximations in closed-form. Further, the asymptotic high SNR approximations of the outage probability are derived, and thereby, the achievable diversity-multiplexing trade-off is quantified. This trade-off reveals that whenever the sum of relay antennas is fixed, the achievable diversity order is always a constant, and hence, the multiplexing gain can indeed be improved by equally distributing antennas among the available set of relays. Our results reveal that relay selection indeed significantly alleviates the inherent diversitygain loss associated with the use of available degrees of freedom for spatial multiplexing.

IEEE GLOBECOM 2007-2007 IEEE Global Telecommunications Conference, 2007

In this paper, we answer the question that "Can conventional differential unitary space time modu... more In this paper, we answer the question that "Can conventional differential unitary space time modulation (DUSTM) be applied when there is an unknown carrier frequency offset (CFO)?" and present a maximum likelihood (ML) detection rule for this scenario. We analyze the asymptotical performance of our ML detection and provide the code design criterion by using the modified diversity product. The analysis also brings the insight that our proposed decision rule is a new differential modulation scheme in both temporal and spatial domains. Various simulations are conducted, and the proposed algorithm is shown to be more robust to the CFO drifting than the existing double temporal differential modulation.

2007 IEEE International Conference on Communications, 2007

Frequency synchronization is one of the most important components in orthogonal frequency-divisio... more Frequency synchronization is one of the most important components in orthogonal frequency-division multiplexing (OFDM) systems. Recently, the discrete cosine transform (DCT) based OFDM system has received wide attentions due to several advantages. Hence, the study of frequency synchronization issue for this newly raised system is well on its time. To provide a thorough study, we consider the non-circular transmissions, and the results can be easily generated to circular transmissions if the elliptic variance is set to zero. We present three joint maximum likelihood (ML) carrier frequency offset (CFO) and phase offset (PO) estimators. From both the theoretical analysis and the numerical comparisons, we found new advantages of the DCT-OFDM over the traditional discrete Fourier transform (DFT) based OFDM. These advantages, as well as those already studied in the early works on DCT-OFDM, support the belief that the DCT-OFDM is a new promising multi-carrier modulation (MCM) scheme.

IEEE Wireless Communications and Networking Conference, 2006. WCNC 2006., 2006

In this paper, we consider efficient blind decoder design for orthogonal space-time block codes (... more In this paper, we consider efficient blind decoder design for orthogonal space-time block codes (OSTBCs). A general decision rule for blind OSTBC decoding is derived assuming a quasi-static flat multiple-input multiple-output (MIMO) Rayleigh fading channel. We use the linear dispersion representation of OSTBCs to derive a blind decoder that results in a quadratic minimization problem, which can be solved efficiently by semidefinite relaxation, sphere decoding or successive interference cancellation. To resolve phase ambiguity problems inherent in blind detectors, rather than using pilot symbols that results in a bandwidth loss, we propose novel totally blind decoders using dual constellations or a superimposed training scheme. To alleviate the computational burden, a minimum mean-squareerror (MMSE) channel estimator is also proposed to track the time-varying channel without using the blind decoder.

2011 IEEE Global Telecommunications Conference - GLOBECOM 2011, 2011

A novel transmit/receive (Tx/Rx) antenna selection strategy is proposed and analyzed for two-way ... more A novel transmit/receive (Tx/Rx) antenna selection strategy is proposed and analyzed for two-way multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay networks. This strategy involves choosing the best transmit and receive antennas at the two sources and the relay based on the minimization of the overall outage probability. The performance of the proposed selection strategy is quantified by deriving the overall outage probability and its high SNR approximation. Specifically, the diversity order is derived to obtain valuable insights into practical system designing. In particular, our results are extended to cater the multiple relay scenario, and thereby, a joint relay and Tx/Rx antenna selection strategy is proposed and analyzed. To this end, the overall outage probability, its high SNR approximation and diversity order are derived. Our numerical results show that the proposed selection strategies achieve the full diversity order. All the analyses are validated through Monte-Carlo simulations. 3 The system model in [9] is restricted to dual-antenna relay terminal.

IEEE GLOBECOM 2008 - 2008 IEEE Global Telecommunications Conference, 2008

Optimal pilots design and placement for the frequency offset and channel estimation in orthogonal... more Optimal pilots design and placement for the frequency offset and channel estimation in orthogonal frequencydivision multiplexing access (OFDMA) uplink systems are proposed. The received pilots of multiple users can always be demodulated, even if they are totally overlapped due to the large frequency offsets. With the knowledge of channel state information (CSI) at the receiver, the performance of the proposed frequency offset estimation is robust to the channel estimation errors. The frequency offset and CSI can be jointly estimated by employing the proposed pilots.