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See detailOne-Bit Quantized Constructive Interference Based Precoding for Massive Multiuser MIMO Downlink
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Shahbazpanahi, Shahram UL et al

in One-Bit Quantized Constructive Interference Based Precoding for Massive Multiuser MIMO Downlink (2020, July 27)

We propose a one-bit symbol-level precoding method for massive multiuser multiple-input multiple-output (MU-MIMO) downlink systems using the idea of constructive interference (CI). In particular, we adopt ... [more ▼]

We propose a one-bit symbol-level precoding method for massive multiuser multiple-input multiple-output (MU-MIMO) downlink systems using the idea of constructive interference (CI). In particular, we adopt a max-min fair design criterion which aims to maximize the minimum instantaneous received signal-to-noise ratio (SNR) among the user equipments (UEs), while ensuring a CI constraint for each UE and under the restriction that the output of the precoder is a vector of binary elements. This design problem is an NP-hard binary quadratic programming due to the one-bit constraints on the elements of the precoder’s output vector, and hence, is difficult to solve. In this paper, we tackle this difficulty by reformulating the problem, in several steps, into an equivalent continuous-domain biconvex form. Our final biconvex reformulation is obtained via an exact penalty approach and can efficiently be solved using a standard block coordinate ascent algorithm. We show through simulation results that the proposed design outperforms the existing schemes in terms of (uncoded) bit error rate. It is further shown via numerical analysis that our solution algorithm is computationally-efficient as it needs only a few tens of iterations to converge in most practical scenarios. [less ▲]

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See detailRobust SINR-Constrained Symbol-Level Multiuser Precoding With Imperfect Channel Knowledge
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Ottersten, Björn UL

in IEEE Transactions on Signal Processing (2020), 68(1), 1837-1852

In this paper, we address robust design of symbol-level precoding (SLP) for the downlink of multiuser multiple-input single-output wireless channels, when imperfect channel state information (CSI) is ... [more ▼]

In this paper, we address robust design of symbol-level precoding (SLP) for the downlink of multiuser multiple-input single-output wireless channels, when imperfect channel state information (CSI) is available at the transmitter. In particular, we consider a well known model for the CSI imperfection, namely, stochastic Gaussian-distributed uncertainty. Our design objective is to minimize the total (per-symbol) transmission power subject to constructive interference (CI) constraints as well as the users’ quality-of-service requirements in terms of signal-to-interference-plus-noise ratio. Assuming stochastic channel uncertainties, we first define probabilistic CI constraints in order to achieve robustness to statistically known CSI errors. Since these probabilistic constraints are difficult to handle, we resort to their convex approximations in the form of tractable (deterministic) robust constraints. Three convex approximations are obtained based on different conservatism levels, among which one is introduced as a benchmark for comparison. We show that each of our proposed approximations is tighter than the other under specific robustness settings, while both of them always outperform the benchmark. Using the proposed CI constraints, we formulate the robust SLP optimization problem as a second-order cone program. Extensive simulation results are provided to validate our analytic discussions and to make comparisons with conventional block-level robust precoding schemes. We show that the robust design of symbol-level precoder leads to an improved performance in terms of energy efficiency at the cost of increasing the computational complexity by an order of the number of users in the large system limit, compared to its non-robust counterpart. [less ▲]

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See detailAn Approximate Solution for Symbol-Level Multiuser Precoding Using Support Recovery
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Ottersten, Björn UL

in IEEE International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Cannes 2-5 July 2019 (2019, August 29)

In this paper, we propose a low-complexity method to approximately solve the SINR-constrained optimization problem of symbol-level precoding (SLP). First, assuming a generic modulation scheme, the ... [more ▼]

In this paper, we propose a low-complexity method to approximately solve the SINR-constrained optimization problem of symbol-level precoding (SLP). First, assuming a generic modulation scheme, the precoding optimization problem is recast as a standard non-negative least squares (NNLS). Then, we improve an existing closed-form SLP (CF-SLP) scheme using the conditions for nearly perfect recovery of the optimal solution support, followed by solving a reduced system of linear equations. We show through simulation results that in comparison with the CF-SLP method, the improved approximate solution of this paper, referred to as ICF-SLP, significantly enhances the performance with a negligible increase in complexity. We also provide comparisons with a fast-converging iterative NNLS algorithm, where it is shown that the ICF-SLP method is comparable in performance to the iterative algorithm with a limited maximum number of iterations. Analytic discussions on the complexities of different methods are provided, verifying the computational efficiency of the proposed method. Our results further indicate that the ICF-SLP scheme performs quite close to the optimal SLP, particularly in the large system regime. [less ▲]

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See detailRobust Design of Power Minimizing Symbol-Level Precoder under Channel Uncertainty
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Ottersten, Björn UL

in IEEE Global Communications Conference (GLOBECOM), Abu Dhabi 9-13 December 2018 (2019, February 21)

In this paper, we investigate the downlink transmission of a multiuser multiple-input single-output (MISO) channel under a symbol-level precoding (SLP) scheme, having imperfect channel knowledge at the ... [more ▼]

In this paper, we investigate the downlink transmission of a multiuser multiple-input single-output (MISO) channel under a symbol-level precoding (SLP) scheme, having imperfect channel knowledge at the transmitter. In defining the SLP design problem, a general category of constructive interference regions (CIR) called distance preserving CIR (DPCIR) is adopted. In particular, we are interested in a robust SLP design minimizing the total transmit power subject to individual quality-of-service (QoS) requirements. We consider two common models for the channel uncertainty region, namely, spherical (norm-bounded) and stochastic. For the spherical uncertainty model, a worst-case robust precoder is proposed, while for the stochastically known uncertainties, we derive a convex optimization problem with probabilistic constraints. We simulate the performance of the proposed robust approaches, and compare them with the existing methods. Through the simulation results, we also show that there is an essential trade-off between the two robust approaches. [less ▲]

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See detailSymbol-Level Precoding Design Based on Distance Preserving Constructive Interference Regions
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Ottersten, Björn UL

in IEEE Transactions on Signal Processing (2018), 66(22), 5817-5832

In this paper, we investigate the symbol-level precoding (SLP) design problem in the downlink of a multiuser multiple-input single-output (MISO) channel. We consider generic two-dimensional constellations ... [more ▼]

In this paper, we investigate the symbol-level precoding (SLP) design problem in the downlink of a multiuser multiple-input single-output (MISO) channel. We consider generic two-dimensional constellations with any shape and size, and confine ourselves to one of the main categories of constructive interference regions (CIR), namely, distance preserving CIR (DPCIR). We provide a comprehensive study of DPCIRs and derive several properties for these regions. Using these properties, we first show that any signal in a given DPCIR has a norm greater than or equal to the norm of the corresponding constellation point if and only if the convex hull of the constellation contains the origin. It is followed by proving that the power of the noise-free received signal in a DPCIR is a monotonic strictly increasing function of two parameters relating to the infinite Voronoi edges. Using the convex description of DPCIRs and their characteristics, we formulate two design problems, namely, the SLP power minimization with signal-to-interference-plus-noise ratio (SINR) constraints, and the SLP SINR balancing problem under max-min fairness criterion. The SLP power minimization based on DPCIRs can straightforwardly be written as a quadratic programming (QP). We derive a simplified reformulation of this problem which is less computationally complex. The SLP max-min SINR, however, is non-convex in its original form, and hence difficult to tackle. We propose alternative optimization approaches, including semidefinite programming (SDP) formulation and block coordinate descent (BCD) optimization. We discuss and evaluate the loss due to the proposed alternative methods through extensive simulation results. [less ▲]

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See detailPower Minimizer Symbol-Level Precoding: A Closed-Form Sub-Optimal Solution
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Ottersten, Björn UL

in IEEE Signal Processing Letters (2018), 25(11), 1730-1734

In this letter, we study the optimal solution of multiuser symbol-level precoding (SLP) for minimization of the total transmit power under given signal-to-interference-plus noise ratio (SINR) constraints ... [more ▼]

In this letter, we study the optimal solution of multiuser symbol-level precoding (SLP) for minimization of the total transmit power under given signal-to-interference-plus noise ratio (SINR) constraints. Adopting the distance preserving constructive interference regions (DPCIR), we first derive a simplified reformulation of the problem. Then, we analyze the structure of the optimal solution using the Karush-Kuhn-Tucker (KKT) optimality conditions. This leads us to obtain a closed-form sub-optimal SLP solution (CF-SLP) for the original problem. Meanwhile, we obtain the necessary and sufficient condition under which the power minimizer SLP is equivalent to the conventional zero-forcing beamforming (ZFBF). Simulation results show that CF-SLP provides significant gains over ZFBF, while performing quite close to the optimal SLP in scenarios with rather small number of users. The results further indicate that the CF-SLP method has a reduction of order 1000 in computational time compared to the optimal solution. [less ▲]

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See detailSymbol-Level Precoding Design for Max-Min SINR in Multiuser MISO Broadcast Channels
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Ottersten, Björn UL

in International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Kalamata 25-28 June 2018 (2018, August 27)

In this paper, we address the symbol level precoding (SLP) design problem under max-min SINR criterion in the downlink of multiuser multiple-input single-output (MISO) channels. First, we show that the ... [more ▼]

In this paper, we address the symbol level precoding (SLP) design problem under max-min SINR criterion in the downlink of multiuser multiple-input single-output (MISO) channels. First, we show that the distance preserving constructive interference regions (DPCIR) are always polyhedral angles (shifted pointed cones) for any given constellation point with unbounded decision region. Then we prove that any signal in a given unbounded DPCIR has a norm larger than the norm of the corresponding vertex if and only if the convex hull of the constellation contains the origin. Using these properties, we show that the power of the noiseless received signal lying on an unbounded DPCIR is an strictly increasing function of two parameters. This allows us to reformulate the originally non-convex SLP max-min SINR as a convex optimization problem. We discuss the loss due to our proposed convex reformulation and provide some simulation results. [less ▲]

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See detailConstructive Interference for Generic Constellations
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL; Ottersten, Björn UL

in IEEE Signal Processing Letters (2018), 25(4), 586-590

In this letter, we investigate optimal and relaxed constructive interference regions (CIR) for the symbol-level precoding (SLP) problem in the downlink of a multiuser multiple-input single-output (MISO ... [more ▼]

In this letter, we investigate optimal and relaxed constructive interference regions (CIR) for the symbol-level precoding (SLP) problem in the downlink of a multiuser multiple-input single-output (MISO) channel.We define two types of CIRs, namely, distance preserving CIR (DPCIR) and union bound CIR (UBCIR) for any given constellation shape and size. We then provide a systematic way to describe these regions as convex sets. Using the definitions of DPCIR and UBCIR, we show that the SLP power minimization problem, minimizing either sum or peak (per-antenna) transmit power, can always be formulated as a convex optimization problem. Our results indicate that these regions allow further reduction of the transmit power compared to the current state of the art without increasing the computational complexity at the transmitter or receiver. [less ▲]

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See detailSymbol vs Block Level Precoding in Multi-beam Satellite Systems
Kayhan, Farbod UL; Haqiqatnejad, Alireza UL; Grotz, Joel et al

in Symbol vs Block Level Precoding in Multi-beam Satellite Systems (2018)

Precoding techniques for multi-beam satellite systems have received a considerable attention in recent years as a tool to mitigate the interference among the beams, and hence increasing the throughput ... [more ▼]

Precoding techniques for multi-beam satellite systems have received a considerable attention in recent years as a tool to mitigate the interference among the beams, and hence increasing the throughput. Our goal is to compare two main categories of precoding schemes, namely, the conventional linear block level precoding and the symbol level precoding. Focusing on power minimization problem with signal to interference plus noise ratio (SINR) constraints, symbol level precoding (SLP) has significant gains with respect to the zero forcing (ZF). However, the lower transmit power is achieved with a price: A higher computational complexity. Therefore, several sub-optimal SLP techniques have been proposed in the literature to overcome the complexity. While ZF has the lowest complexity among the techniques chosen in this paper, it is not an optimal linear block level precoder as far as power minimization is concerned. Therefore, in order to have a more complete picture, one needs also to consider optimal block level precoders. Our results indicate that in order to have a fair comparison, one needs to consider two different scenarios, namely, low and high SINR threshold regimes. While for low SINRs the optimal linear block level precoding scheme may provide a good solution with reasonable complexity, for high SINR threshold, the SLP techniques become more attractive. Our results also indicate that the performance of SLP highly depends on the chosen constellation space, and therefore a final conclusion can be achieved only after appropriately optimizing the constellation set. [less ▲]

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See detailOn low complexity detection for QAM isomorphic constellations
Kayhan, Farbod UL

in Kayhan, Farbod (Ed.) On low compelxity detection for QAM isomorphic constellations (2017, December)

Despite of the known gap from the Shannon's capacity, several standards are still employing QAM or star shape constellations, mainly due to the existing low complexity detectors. In this paper, we ... [more ▼]

Despite of the known gap from the Shannon's capacity, several standards are still employing QAM or star shape constellations, mainly due to the existing low complexity detectors. In this paper, we investigate the low complexity detection for a family of QAM isomorphic constellations. These constellations are known to perform very close to the peak-power limited capacity, outperforming the DVB-S2X standard constellations. The proposed strategy is to first remap the received signals to the QAM constellation using the existing isomorphism and then break the log likelihood ratio computations to two one dimensional PAM constellations. Gains larger than 0.6 dB with respect to QAM can be obtained over the peak power limited channels without any increase in detection complexity. Our scheme also provides a systematic way to design constellations with low complexity one dimensional detectors. Several open problems are discussed at the end of the paper. [less ▲]

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See detailUnified Satellite and Terrestrial ACM Design
Haqiqatnejad, Alireza UL; Kayhan, Farbod UL

in International Communications Satellite Systems Conference (ICSSC), Trieste 16-19 October 2017 (2017, October 17)

First step towards integrating satellite and terrestrial standards at the physical layer is to design a unified packet frame structure, and in particular, a unified adaptive coding and modulation (ACM ... [more ▼]

First step towards integrating satellite and terrestrial standards at the physical layer is to design a unified packet frame structure, and in particular, a unified adaptive coding and modulation (ACM) scheme without substantial loss in either of the systems. In this paper we introduce a methodology to design such a set of modulation and coding (MODCOD) combinations. In the first step, we design a set of base MODCODs for the AWGN channel. Then, for any other given channel model, we build a new set of MODCODs through a suitable transformation of the base MODCODs. We mainly focus on two types of channels: (a) non-linear satellite channels, and (b) AWGN channels with fading. We compare our results with latest digital video broadcasting standards, namely, DVB-T2 and DVB-S2X. [less ▲]

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See detailOptimized non-orthogonal multiplexing in Peak Power Limited Channels
Alagha, Nader; Kayhan, Farbod UL; Montorsi, Guido

Scientific Conference (2017, October)

In this paper, we investigate the performance of optimized overlay modulation schemes and compare the results with both time division multiplexing and superposition modulation assuming a peak power ... [more ▼]

In this paper, we investigate the performance of optimized overlay modulation schemes and compare the results with both time division multiplexing and superposition modulation assuming a peak power limited AWGN channel. We show that significant gains can be achieved by properly designing the constellation set, specially in the presence of large SNIR imbalance between the user's link quality. We introduce two realistic case studies and present the results for each scenario. [less ▲]

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See detailQAM to Circular Isomorphic Constellations
Kayhan, Farbod UL

in 8th Advanced Satellite Multimedia Systems Conference (ASMS/SPSC), Palma de Mallorca, Spain, 2016 (2016, September)

Employing high order constellations is inevitable in order to achieve higher spectral efficiency in satellite communication systems. In DVB-S2X standard, constellations with up to 256 points have been ... [more ▼]

Employing high order constellations is inevitable in order to achieve higher spectral efficiency in satellite communication systems. In DVB-S2X standard, constellations with up to 256 points have been included. However, optimizing such high order constellations is a difficult task. In this paper we propose a very simple constellation design based on the radial map from QAM constellations to non-uniform APSK constellations. Our method provides a systematic way to generate a family of optimized high order constellations. We show that gains larger than 0.5 dB can be obtained with respect to the DVB-S2X constellations with 256 points. Even though the proposed constellations are essentially sub-optimal (in terms of maximizing the mutual information), they provide a very competitive test benchmark for the performance of high order constellations. Moreover, the proposed construction potentially allows for an efficient one-dimensional soft detection by an inverse mapping to the QAM. Finally, further optimization techniques are discussed. [less ▲]

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See detailLow Complexity Transmit Processing for Multibeam Satellite Systems with Non-Linear Channels
Mengali, Alberto UL; Kayhan, Farbod UL; Shankar, Bhavani UL et al

in Proceedings of ASMS Conference (2016, September)

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See detailFour-Dimensional Constellations for Dual-Polarized Satellite Communications
Mazzali, Nicolò UL; Kayhan, Farbod UL; Shankar, Bhavani UL

in Proceedings on IEEE International Conference on Communications (2016, May)

In this paper, we investigate the performance of constellations optimized for transmissions in dual-polar mobile satellite applications. These four-dimensional constellations (inphase and quadrature per ... [more ▼]

In this paper, we investigate the performance of constellations optimized for transmissions in dual-polar mobile satellite applications. These four-dimensional constellations (inphase and quadrature per polarization) are designed for joint transmission over the two polarizations. Such constellations enhance the reliability of the system by providing certain redundancy into their design. Their performance is compared with transmission of independent 2D constellations over each polarization. As performance metrics, the pragmatic achievable mutual information and the bit error rate have been considered. The gains serve to indicate the need to further investigate 4D constellation design and its application in dual-polar MIMO systems. [less ▲]

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See detailHigh Order APSK Constellation Design for Next Generation Satellite Communication
Kayhan, Farbod UL

Scientific Conference (2016)

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See detailComparative Assessment of Orthogonal and Nonorthogonal Multiplexing Techniques for Differentiated Satellite Broadcasting Services
Kayhan, Farbod UL; Montorsi, Guido; Taricco, Giorgio et al

Scientific Conference (2015)

This paper presents a comparative assessment of two multiplexing techniques for providing differentiated classes of services over a realistic broadcasting satellite channel under variable link conditions ... [more ▼]

This paper presents a comparative assessment of two multiplexing techniques for providing differentiated classes of services over a realistic broadcasting satellite channel under variable link conditions. A distinctive characteristic of satellite broadcasting channels is the non-linear characteristics of high power amplifier on-board of the satellite. In this scenario, the conventional additive white Gaussian noise (AWGN) channel with average power limitation is no longer an accurate model and nonlinear characteristics have to be taken into account. Orthogonal multiplexing (time sharing) techniques combined with variable coding and modulation are compared with hierarchical modulations with the goal of maximizing the number of broadcast channels over a given transponder while maintaining a target service availability. Hierarchical modulations are shown to provide better performance than orthogonal time division multiplexing (time-sharing) schemes for the AWGN channel. Here we design a practical transmitter scheme based on the hierarchical modulation and maximize the throughput by optimizing the achievable mutual information for finite size constellations. This will provide a lower bound on throughput which can be achieved by non orthogonal multiplexing scheme. We compare the performance of hierarchical modulation (non-orthogonal multiplexing) and time sharing techniques (orthogonal multiplexing) supporting two different service quality and service availability requirements for broadcasting the same content. [less ▲]

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See detailConstellation Design in Four Dimensions under Average Power Constraint
Kayhan, Farbod UL; Mazzali, Nicolò UL; Shankar, Bhavani UL

in Proceedings on IEEE Symposium on Communications and Vehicular Technology in the Benelux (2015)

In this paper we address the problem of constellation design in four dimensional space (4D) under average power constraint. We compare the performance of best lattice constellations with those optimized ... [more ▼]

In this paper we address the problem of constellation design in four dimensional space (4D) under average power constraint. We compare the performance of best lattice constellations with those optimized without any constraint on their structure. Even though the lattice based constellations provide the best minimum distance, usually they do not allow for a good binary labelling. Therefore, in more realistic system scenarios, up to a 2 dB gain can be obtained by properly optimizing the constellation and the corresponding binary labelling. We also investigate the performance of 4D constellations obtained by the Cartesian product of two 2D constellations. [less ▲]

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