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Quantumassisted and Quantumbased Communications
Figure 1. Comparison between classic serial, parallel and quantum computing. Assuming that only one of the eight keys unlocks the box, by employing serial computing we have to try each of the keys sequentially until one succeeds to unlock it. Classic parallel computing creates as many boxes as the available keys and tries all of them at once, requiring a large amount of resources. With quantum computing we are able to try all the keys in parallel on a single box. The box corresponds to a function, while the keys represent the legitimate inputs of the function. The key that unlocks the box is the input of the function which will lead to a desired output. By employing quantum computing, the function may be evaluated for the inputs in parallel, as in parallel computing, with the computational cost of a single evaluation, as in serial computing.
Quantumassisted MultiUser Wireless Systems 
The high complexity of numerous optimal classic communication schemes, such as the maximum likelihood (ML) multiuser detector (MUD), often prevents their practical implementation. In this work, we present an extensive review and tutorial on quantum search algorithms (QSA) and their potential applications, and we employ a QSA that ﬁnds the minimum of a function in order to perform optimal hard MUD with a quadratic reduction in the computational complexity when compared to that of the ML MUD. Furthermore, we follow a quantum approach to achieve the same performance as the optimal softinput softoutput classic detectors by replacing them with a quantum algorithm, which estimates the weighted sum of a function’s evaluations. We propose a softinput softoutput quantumassisted MUD (QMUD) scheme, which is the quantumdomain equivalent of the ML MUD. We then demonstrate its application using the design example of a directsequence code division multiple access system employing bitinterleaved coded modulation relying on iterative decoding, and compare it with the optimal ML MUD in terms of its performance and complexity. Both our extrinsic information transfer charts and bit error ratio curves show that the performance of the proposed QMUD and that of the optimal classic MUD are equivalent, but the QMUD’s computational complexity is signiﬁcantly lower. 
[4] FixedComplexity QuantumAssisted MultiUser Detection for CDMA and SDMA [More Publications] 
In a system supporting numerous users the complexity of the optimal Maximum Likelihood MultiUser Detector (ML MUD) becomes excessive. Based on the superimposed constellations of K users, the ML MUD outputs the specific multilevel Kuser symbol that minimizes the Euclidean distance with respect to the faded and noisecontaminated received multilevel symbol. Explicitly, the Euclidean distance is considered as the Cost Function (CF). In a system supporting K users employing Mary modulation, the ML MUD uses M^K CF evaluations (CFE) per time slot. In this contribution we propose an Early Stoppingaided DurrHoyer algorithmbased Quantumassisted MUD (ESDHA QMUD) based on two techniques for achieving optimal ML detection at a low complexity. Our solution is also capable of flexibly adjusting the QMUD's performance and complexity tradeoff, depending on the computing power available at the base station. We conclude by proposing a general design methodology for the ESDHA QMUD in the context of both CDMA and SDMA systems. 
[5] LowComplexity SoftOutput QuantumAssisted Multiuser Detection for DirectSequence Spreading and Slow SubcarrierHopping Aided SDMAOFDM Systems [More Publications] 
Lowcomplexity suboptimal multiuser detectors (MUDs) are widely used in multiple access communication systems for separating users, since the computational complexity of the maximum likelihood (ML) detector is potentially excessive for practical implementation. Quantum computing may be invoked in the detection procedure, by exploiting its inherent parallelism for approaching the ML MUDs performance at a substantially reduced number of cost function evaluations. In this contribution, we propose a softoutput (SO) quantumassisted MUD achieving a nearML performance and compare it to the corresponding SO ant colony optimization MUD. We investigate rank deficient directsequence spreading (DSS) and slow subcarrierhopping aided (SSCH) spatial division multiple access orthogonal frequency division multiplexing systems, where the number of users to be detected is higher than the number of receive antenna elements used. We show that for a given complexity budget, the proposed SODürrHøyer algorithm (DHA) QMUD achieves a better performance. We also propose an adaptive hybrid SOML/SODHA MUD, which adapts itself to the number of users equipped with the same spreading sequence and transmitting on the same subcarrier. Finally, we propose a DSSbased uniform SSCH scheme, which improves the system's performance by 0.5 dB at a BER of 10^(−5), despite reducing the complexity required by the MUDs employed. 
[6] NonCoherent Quantum Multiple Symbol Differential Detection for Wireless Systems [More Publications] 
In largedimensional wireless systems, such as Cooperative Multicell Processing (CoMP), mmWave and massive MIMO systems, or cells having a high user density, such as airports, train stations and metropolitan areas, sufficiently accurate estimation of all the channel gains is required for performing coherent detection. Therefore they may impose an excessive complexity. As an attractive design alternative, differential modulation relying on noncoherent detection may be invoked for eliminating the requirement for channel estimation at the Base Station, although at the cost of some performance degradation. In this treatise we propose lowcomplexity HardInput HardOutput (HIHO), HardInput SoftOutput (HISO), as well as SoftInput SoftOutput (SISO) Quantumassisted Multiple Symbol Differential Detectors (QMSDD) which perform equivalently to the optimal, but highly complex Maximum A posteriori Probability (MAP) MSDDs in multiuser systems, where the users are separated both in the frequency domain and in the time domain. When using an MSDD, the detection of a user’s symbols is performed over windows of differentially modulated symbols, hence they exhibit an increased complexity with respect to the Conventional Differential Detector (CDD), while simultaneously improving the performance of the system, especially at high Doppler frequencies. 
[7] Iterative QuantumAssisted MultiUser Detection for MultiCarrier Interleave Division Multiple Access Systems [More Publications] 
With the proliferation of smartphones and tablet PCs, the data rates of wireless communications have been soaring. Hence, the need for powerefficient communications relying on lowcomplexity multiplestream detectors has become more pressing than ever. As a remedy, in this paper we design lowcomplexity softinput softoutput quantumassisted multiuser detectors (QMUD), which may be conveniently incorporated into stateoftheart iterative receivers. Our design relies on extrinsic information transfer charts. Our QMUDs are then employed in multicarrier interleavedivision multipleaccess (MCIDMA) systems, which are investigated in the context of different channel code rate and spreading factor pairs, whilst fixing the total bandwidth requirement. One of our QMUDs is found to operate within 0.5 dB of the classical maximum a posteriori probability MUD after three iterations between the MUD and the decoders, while requiring only half its complexity, at a BER of 105 in the uplink of a rankdeficient MCIDMA system relying on realistic imperfect channel estimation at the receiver, while supporting 14 users transmitting QPSK symbols. 
[8] Joint QuantumAssisted Channel Estimation and MultiUser Detection for NonOrthogonal Multiple Access Systems [More Publications] 
Joint Channel Estimation (CE) and MultiUser Detection (MUD) has become a crucial part of iterative receivers. In this paper we propose a Quantumassisted Repeated Weighted Boosting Search (QRWBS) algorithm for CE and we employ it in the uplink of MIMOOFDM systems, in conjunction with the Maximum A posteriori Probability (MAP) MUD and a nearoptimal Quantumassisted MUD (QMUD). The performance of the QRWBSaided CE is evaluated in rankdeficient systems, where the number of receive Antenna Elements (AE) at the Base Station (BS) is lower than the number of supported users. The effect of the Channel Impulse Response (CIR) prediction filters, of the Power Delay Profile (PDP) of the channels and of the Doppler frequency have on the attainable system performance is also quantified. The proposed QRWBSaided CE is shown to outperform the RWBSaided CE, despite requiring a lower complexity, in systems where iterations are invoked between the MUD, the CE and the channel decoders at the receiver. In a system, where U = 7 users are supported with the aid of P = 4 receive AEs, the joint QRWBSaided CE and QMUD achieves a 2 dB gain, when compared to the joint RWBSaided CE and MAP MUD, despite imposing 43% lower complexity. 
[9] Quantumaided MultiUser Transmission in NonOrthogonal Multiple Access Systems [More Publications] 
With the research on implementing a universal quantum computer being under the technological spotlight, new possibilities appear for their employment in wireless communications systems for reducing their complexity and improving their performance. In this treatise, we consider the downlink of a rankdeficient, multiuser system and we propose the discretevalued and continuousvalued Quantumassisted Particle Swarm Optimization (QPSO) algorithms for performing Vector Perturbation (VP) precoding, as well as for lowering the required transmission power at the Base Station (BS), while minimizing the expected average Bit Error Ratio (BER) at the mobile terminals. We use the Minimum BER (MBER) criterion. We show that the novel quantumassisted precoding methodology results in an enhanced BER performance, when compared to that of a classical methodology employing the PSO algorithm, while requiring the same computational complexity in the challenging rankdeficient scenarios, where the number of transmit antenna elements at the BS is lower than the number of users. Moreover, when there is limited Channel State Information (CSI) feedback from the users to the BS, due to the necessary quantization of the channel states, the proposed quantumassisted precoder outperforms the classical precoder. 
Quantum Error Correction Codes 
The design of Quantum Turbo Codes (QTCs) typically relies on the analysis of their distance spectra, followed by MonteCarlo simulations. In this work, we extend the application of EXtrinsic Information Transfer (EXIT) charts to quantum turbo codes, which sets a lower bound on the achievable decoding performance, with the predicted convergence threshold lying only 0.3 dB below the Word Error Rate (WER) simulation results. Furthermore, we have conceived EXITchart aided code search algorithms for optimizing QTCs. Using the proposed algorithms, we have designed an optimal entanglementassisted QTC, whose performance is only 0.4 dB away from the corresponding hashing bound. 
[4] NearCapacity Code Design for EntanglementAssisted Classical Communication over Quantum Depolarizing Channels [More Publications] 
We have conceived a nearcapacity code design for entanglementassisted classical communication over the quantum depolarizing channel. The proposed system relies on efficient nearcapacity classical code designs for approaching the entanglementassisted classical capacity of a quantum depolarizing channel. It incorporates an Irregular Convolutional Code (IRCC), a Unity Rate Code (URC) and a softdecision aided Superdense Code (SD), which is hence referred to as an IRCCURCSD arrangement. Furthermore, the entanglementassisted classical capacity of an Nqubit superdense code transmitted over a depolarizing channel is invoked for benchmarking. It is demonstrated that the proposed system operates within 0.4 dB of the achievable noise limit for both 2qubit as well as 3qubit SD schemes. More specifically, our design exhibits a deviation of only 0.062 and 0.031 classical bits per channel use from the corresponding 2qubit and 3qubit capacity limits, respectively. The proposed system is also benchmarked against the classical convolutional and turbo codes. 
[5] The Road From Classical to Quantum Codes: A Hashing Bound Approaching Design Procedure [More Publications] 
Powerful quantum error correction codes (QECCs) are required for stabilizing and protecting fragile qubits against the undesirable effects of quantum decoherence. Similar to classical codes, hashing bound approaching QECCs may be designed by exploiting a concatenated code structure, which invokes iterative decoding. Therefore, in this paper, we provide an extensive stepbystep tutorial for designing extrinsic information transfer (EXIT) chartaided concatenated quantum codes based on the underlying quantumtoclassical isomorphism. These design lessons are then exemplified in the context of our proposed quantum irregular convolutional code (QIRCC), which constitutes the outer component of a concatenated quantum code. The proposed QIRCC can be dynamically adapted to match any given inner code using EXIT charts, hence achieving a performance close to the hashing bound. It is demonstrated that our QIRCCbased optimized design is capable of operating within 0.4 dB of the noise limit. 
[6] Fifteen Years of Quantum LDPC Coding and Improved Decoding Strategies [More Publications] 
The nearcapacity performance of classical lowdensity parity check (LDPC) codes and their efficient iterative decoding makes quantum LDPC (QLPDC) codes a promising candidate for quantum error correction. In this paper, we present a comprehensive survey of QLDPC codes from the perspective of code design as well as in terms of their decoding algorithms. We also conceive a modified nonbinary decoding algorithm for homogeneous CalderbankShorSteanetype QLDPC codes, which is capable of alleviating the problems imposed by the unavoidable lengthfour cycles. Our modified decoder outperforms the stateoftheart decoders in terms of their word error rate performance, despite imposing a reduced decoding complexity. Finally, we intricately amalgamate our modified decoder with the classic uniformly reweighted belief propagation for the sake of achieving an improved performance. 
[7] A FullyParallel Quantum Turbo Decoder [More Publications] 
Quantum Turbo Codes (QTCs) are known to operate close to the achievable Hashing bound. However, the sequential nature of the conventional quantum turbo decoding algorithm imposes a high decoding latency, which increases linearly with the frame length. This posses a potential threat to quantum systems having short coherence times. In this context, we conceive a Fully Parallel Quantum Turbo Decoder (FPQTD), which eliminates the inherent time dependencies of the conventional decoder by executing all the associated processes concurrently. Due to its parallel nature, the proposed FPQTD reduces the decoding times by several orders of magnitude, while maintaining the same performance. We have also demonstrated the significance of employing an oddeven interleaver design in conjunction with the proposed FPQTD. More specifically, it is shown that an oddeven interleaver reduces the computational complexity by 50%, without compromising the achievable performance. 
[8] Construction of Quantum LDPC Codes From Classical RowCirculant QCLDPCs [More Publications] 
Classical rowcirculant quasicyclic (QC) lowdensity parity check (LDPC) matrices are known to generate efficient highrate short and moderatelength QCLDPC codes, while the comparable random structures exhibit numerous short cycles of length4. Therefore, we conceive a general formalism for constructing nondualcontaining CalderbankShorSteane (CSS)type quantum lowdensity parity check (QLDPC) codes from arbitrary classical rowcirculant QCLDPC matrices. We apply our proposed formalism to the classical balanced incomplete block design (BIBD)based rowcirculant QCLDPC codes for demonstrating that our designed codes outperform their dualcontaining CSStype counterparts as well as the entanglementassisted (EA)QLDPC codes. 
[9] EXITchart Aided Quantum Code Design Improves the Normalised Throughput of Realistic Quantum Devices [More Publications] 
In this contribution, the Hashing bound of Entanglement Assisted Quantum Channels (EAQC) is investigated in the context of quantum devices built from a range of popular materials, such as trapped ion and relying on solid state Nuclear Magnetic Resonance (NMR), which can be modelled as a socalled asymmetric channel. Then, Quantum Error Correction Codes (QECC) are designed based on Extrinsic Information Transfer (EXIT) charts for improving performance when employing these quantum devices. The results are also verified by simulations. Our QECC schemes are capable of operating close to the corresponding Hashing bound. 
Quantumassisted Routing 
[1] QuantumAssisted Routing Optimization for SelfOrganizing Networks SelfOrganizing Networks (SONs) act autonomously for the sake of achieving the best possible performance. The attainable routing depends on a delicate balance of diverse and often conflicting QualityofService (QoS) requirements. Finding the optimal solution typically becomes an NPhard problem, as the network size increases in terms of the number of nodes. Moreover, the employment of userdefined utility functions for the aggregation of the different objective functions often leads to suboptimal solutions. On the other hand, Pareto Optimality is capable of amalgamating the different design objectives by providing an element of elitism. Although there is a plethora of bioinspired algorithms that attempt to address this optimization problem, they often fail to generate all the points constituting the Optimal Pareto Front (OPF). As a remedy, we propose an optimal multiobjective quantumassisted algorithm, namely the Nondominated Quantum Optimization algorithm (NDQO), which evaluates the legitimate routes using the concept of Pareto Optimality at a reduced complexity. We then compare the performance of the NDQO algorithm to the stateoftheart evolutionary algorithms, demonstrating that the NDQO algorithm achieves a nearoptimal performance. Furthermore, we analytically derive the upper and lower bounds of the NDQO algorithmic complexity, which is of the order of O(N) and O(N^{3/2}) in the best and worstcase scenario, respectively. This corresponds to a substantial complexity reduction of the NDQO from the order of O(N^{2}) imposed by the bruteforce (BF) method. Video 1. NDQO simulation for a 6node SON for 300 frames: The SON is assumed to be covering a (100x100) m square block area, where the Source Node (SN) and the Destination Node (DN) are located at the opposite corners of this square block and they are stationary. The Mobile Relay Nodes are denoted in the video by the red triangle marker and are referred to as R [top] [2] NonDominated Quantum Iterative Routing Optimization for Wireless Multihop Networks Routing in Wireless Multihop Networks (WMHNs) relies on a delicate balance of diverse and often conflicting parameters, when aiming for maximizing the WMHN performance. Classified as a Nondeterministic Polynomialtime hard problem (NPhard), routing in WMHNs requires sophisticated methods. As a benefit of observing numerous variables in parallel, quantum computing offers a promising range of algorithms for complexity reduction by exploiting the principle of Quantum Parallelism (QP), while achieving the optimum fullsearchbased performance. In fact, the socalled NonDominated Quantum Optimization (NDQO) algorithm has been proposed for addressing the multiobjective routing problem with the goal of achieving a nearoptimal performance, while imposing a complexity of the order of O(N) and O(N^{3/2}) in the best and worstcase scenarios, respectively. However, as the number of nodes in the WMHN increases, the total number of routes increases exponentially, making its employment infeasible despite the complexity reduction offered. Therefore, we propose a novel optimal quantumassisted algorithm, namely the NonDominated Quantum Iterative Optimization (NDQIO) algorithm, which exploits the synergy between the hardware and the quantum parallelism for the sake of achieving a further complexity reduction, which is on the order of O(N^{1/2}) and O(N^{3/2}) in the best and worstcase scenarios, respectively. Additionally, we provide simulation results for demonstrating that our NDQIO algorithm achieves an average complexity reduction of almost an order of magnitude compared to the nearoptimal NDQO algorithm, while having the same order of power consumption. Video 2. A brief introduction into quantumassisted multiobjective routing.
