The radio stripe (RS) concept has emerged as a viable and practical implementation of the cell-free (CF) massive multiple-input-multiple-output (mMIMO) paradigm, where a sequence of distributed access points (APs) sharing a common stripe cooperatively serve user-equipments (UEs) over both uplink (UL) and downlink channels. In such networks, active eavesdroppers can launch pilot-spoofing attacks, fully informed of the legitimate pilot sequences and AP-selection scheme, to contaminate UL channel estimation (CE), thereby degrading overall throughput and leaking confidential data to the adversary. To counteract spoofing, we analyze a bi-objective AP-selection framework that jointly maximizes (1) the network sum spectral efficiency (SE) and (2) the secrecy spectral efficiency (SSE) of the attacked UEs. Our results traces the Pareto frontier of sum SE versus attacked UE SE, revealing the inherent trade-offs between throughput and security.

The ability to provide reliable data rates across the several intended coverage areas has made massive multiple-input multiple-output (m-MIMO) cell-free (CF) a crucial technology for future sixth-generation (6G) systems. However, CF networks also introduce new vulnerabilities, specifically in terms of security and network integrity. For that, to complement the conventional cryptographic algorithms, physical layer security (PLS) can be an effective strategy for acquiring essential wireless network information in order to create important authentication mechanisms against spoofing attacks. To this end, we propose leveraging the wireless channel and the access point selection (APS) allocation schemes as authentication methods to prevent network attacks that attempt to impersonate legitimate user communications. Our results show that monitoring these parameters provides valuable insights about the network performance and the impact under active eavesdropping influence. Consequently, this approach enables a statistical detection of illegitimate spoofing attempts through alarms, which can be further reinforced by upper-layer authentication protocols to enhance the security performance of wireless networks.

The radio stripe (RS) concept has been emerging as a practical implementation of the cell-free (CF) massive multiple-input-multiple-output (MMIMO) network where all access points (APs) that perform the signal processing required to serve the users’ equipment (UEs) share a common stripe. In such a system, an uplink (UL) power optimization must be performed to increase the system’s spectral and power efficiencies (SE and PE). In this work, we develop a set of multi-objective UL power optimization models and algorithms based on the differential evolution (DE) meta-heuristic (MH) considering the optimal sequential linear processing (OSLP) and maximum ratio combining (MRC) detection schemes. The bi-objective approaches aim at deriving nondominated solutions in models to: 1) maximize the max-min fairness (MMF) and max-sum rate (MSR) metrics; 2) maximize the MMF and minimize transmission power (MTP); 3) maximize the MSR and minimize the MTP. A tri-objective approach is also developed to maximize the MMF and MSR and minimize the MTP. Solutions displaying interesting trade-offs are obtained with the proposed models and algorithmic approaches, compared with the case where the UEs employ full power in the UL. Results show that improved SE and PE can be attained with the proposed bi-objective and tri-objective optimization approaches.

The current applications’ requirement for higher data rates typically results in an increase in the signal’s Peak-to-Average Power Ratio (PAPR), which harms the transmitter’s Power Amplifier (PA) efficiency. The Magnitude Modulation (MM) technique was developed to address this issue in bandlimited single-carrier (SC) communications, leaving a trade-off between the attainable PAPR reduction and an increase in the bit error rate (BER) upon MM signal demodulation. Because Deep Learning (DL) algorithms are proving to be applicable in the majority of fields, two models based on current state-of-the-art Artificial Neural Networks (ANNs) are proposed in this paper to replace conventional demodulators. It is also shown that DL demodulators can leverage on the symbols’ memory that is embedded within the transmitted MM signal, caused by the MM procedure. Both systems’ BER performance is analyzed and compared, demonstrating that it is possible to mitigate the MM effect on demodulation.

With the recent advancements in wireless networks, Joint Communication and Sensing (JCAS) has become a growing field that is expected to be included in next-generation standards. However, not only is the current performance of the sensing ability still lacking to be used in real-world scenarios, proper security of such privacy-invasive technology has not been fully explored. To this end, we propose the creation of a more robust framework, capable of cross-domain detection and long-term analysis for improved detection, which will also serve as the basis for a security and privacy analysis of the threat landscape and solutions in this field.

One of the most promising practical implementations of the cell-free massive MIMO network is based on the radio stripe system, which considers that the access points are sequentially connected, sharing the same fronthaul link while serving all user equipment using the same time-frequency resources. This paper considers an uplink power allocation technique that aims to enhance the network spectral efficiency (SE), modelled as an optimization constrained problem explicitly considering the max-min fairness case. Instead of relying on exact globally optimal approaches, with high computational complexity, three different alternative meta-heuristic (MH) based on simulated annealing, differential evolution and particle swarm optimization approaches are explored. The SE performances along with the total run times of the MH approaches are analyzed and compared with the bisection method.

Large intelligent surfaces (LISs) is an evolution of massive MIMO systems allowing for huge capacity gains. To reduce implementation complexity, it is convenient to implement the LIS using panels that are either activated or deactivated, and associated to terminals according to their propagation characteristics to the panels. The associated spatial resource allocation to maximise the terminals’, as well as the overall, bit rates can lead to complex optimisation problems.In this paper we consider resource allocation for panel-based LIS surfaces. We present an iterative sequential algorithm for determining the set of active panels and the respective panel-terminal association for the maximisation of the minimum terminal rate. Our algorithm is decentralised and has low complexity. Moreover, it approaches the performance of much more complex, quasi-optimum algorithms.

One of the most promising technologies to be employed in beyond 5G networks is based on the concept of cell-free (CF) massive MIMO, in which a predetermined set of access points (APs) jointly cooperate in the data transmission and reception to/from the user equipment (UE). In order to efficiently manage radio resources, the CF central processing unit implement uplink power control policies. These policies aim to optimize a given network utility function. In this paper, we investigate the max-min fairness optimization problem, in which the spectral efficiency performance of the UE with the worst channel conditions is prioritized, taking into account a per-UE power constraint and assuming linear maximum ratio combining at APs. Existing solutions have typically relied on second-order cone programming with convex approximations, which exhibit high computational complexity and scalability issues. Therefore, meta-heuristics (MHs) are explored as alternative optimization schemes, capable of providing (near)-optimal solutions with reasonable computational effort. Three MH approaches with different operation principles are compared. Numerical results show that the differential evolution algorithm exhibits the best trade-off between solution quality and run time, being able to reach (near)-optimal solutions faster than the bisection approach while coping with the scalability issues of the geometric programming-based algorithm.

Current methods for optimization of low-density parity-check (LDPC) codes analyze the degree distribution pair asymptotically as block length approaches infinity. This effectively ignores the discrete nature of the space of valid degree distribution pairs for LDPC codes of finite block length. While large codes are likely to conform reasonably well to the infinite block length analysis, shorter codes have no such guarantee. We present and analyze an algorithm for completely enumerating the space of all valid degree distribution pairs for a given block length, code rate, maximum variable node degree, and maximum check node degree. We then demonstrate this algorithm on an example LDPC code of finite block length. Finally, we discuss how the result of this algorithm can be utilized by discrete optimization routines to form novel methods for the optimization of small block length LDPC codes.

Polar codes have been shown to provide an effective mechanism for achieving physical-layer security over various wiretap channels. A majority of these schemes require channel state information (CSI) at the encoder for both intended receivers and eavesdroppers. In this paper, we consider a polar coding scheme for secrecy over a Gaussian wiretap channel when no CSI is available. We show that the availability of a shared keystream between friendly parties allows polar codes to be used for both secure and reliable communications, even when the eavesdropper knows a large fraction of the keystream. The scheme relies on a predetermined strategy for partitioning the bits to be encoded into a set of frozen bits and a set of information bits. The frozen bits are filled with bits from the keystream, and we evaluate the security gap when the cyclic redundancy check-aided successive cancellation list decoder is used at both receivers in the wiretap channel model.

A Large Intelligent Surface (LIS) is a recently proposed concept, especially suitable for high speed indoor communications and industrial internet of things (IoT) applications. Basing the LIS on smaller panels has clear advantages in terms of flexibility and mass production of its elements. In this paper we consider a panel-based LIS and we study the interplay of the panel size, the number of baseband outputs per square meter of deployed surface, the total activated surface area, the number of baseband outputs per panel, the terminal density and the ensuing minimum terminal rate. Our performance results show that it is desirable to employ smaller panels when the terminal density increases, but this means more outputs per m2, and higher overall LIS implementation complexity. It was observed that we can surpass such increase by working with higher fractions of the LIS area. Furthermore, we present an empirical equation stating the number of outputs per panel needed to ensure that all terminals are reasonably served. These results are useful for the LIS design in practical scenarios.

The Time-Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing (TIBWB-OFDM) waveform enables to achieve greater confinement in the signal spectrum. This improves with the increase of the window roll-off since the out of band (OOB) radiation drops. However, the TIBWB-OFDM block duration also increases, which leads to a decrease in the transmission rate for a given band, limiting, therefore, the spectral efficiency gains of the system. In this paper, we propose an alternative method for TIBWB-OFDM symbol construction by allowing a partial overlap between adjacent windowed OFDM symbols that compose it. This improves the spectral efficiency at the expense of introduced interference between the signal’s blocks, that deteriorates the Bit Error Rate (BER) performance. To avoid these BER performance losses, we propose a modified receiver design that includes the cancellation of block overlapping effects in the time domain. The proposed receiver is shown to be able to achieve excellent performance, even with the strong overlapping effects of a TIBWB-OFDM system with high spectral efficiency.

We propose adding an irregular quadrature amplitude modulation (QAM) constellation to a wireless transmission scheme in order to obtain greater control over the signal-to-noise ratio (SNR) required to successfully decode the signal. By altering the separation between adjacent symbols, the minimum required SNR is raised without degradation in the performance of the scheme. This allows the system to adapt to preferable channel conditions for the authorized user, making it harder for eavesdroppers to intercept and decode the transmission, thus making the communication safer. In addition, we show that by overlaying a coset code onto the QAM constellation, a new, stronger security gap metric can be further improved. Results show the effectiveness of this strategy with an interleaved coding for secrecy with a hidden key (ICSHK) scheme.

We propose a coding methodology for physical layer security with adaptive characteristics, whereby adaptive we mean that the system must be tunable to different operational points/signal-to-noise ratio levels of both the legitimate receiver and the eavesdropper. Based on interleaving and scrambling as techniques that shuffle the original message before transmission, we consider puncturing over an interleaving/scrambling key and/or over the message as a mechanism to provide the required adaptability to channel conditions. The proposed techniques have shown suitable adaptability to different channel quality levels of the legitimate receiver and eavesdropper, while still guaranteeing the desired reliability for the legitimate receiver and secrecy against the eavesdropper.

This paper presents a practical physical-layer security scheme based on coding methodologies combined with self-jamming to combat advantaged eavesdroppers, i.e., eavesdroppers that may possess an equal or even better channel than the legitimate receiver. We introduce a strengthened security gap notion, where reliability is assured by typical bit-error rate (BER) measurements, but secrecy is guaranteed by considering the entire distribution of messages upon reception, instead of average measures. Relying on this new security gap notion, we then propose a scheme that combines concatenated coding with self-jamming by the legitimate receiver for effective security and reliability even when eavesdroppers possess a channel with equal or better conditions than the legitimate receiver.

We propose an adaptive secrecy scheme using polar codes with random frozen bits for a general wiretap channel, in which to protect the data from a potential eavesdropper, part or all of the frozen bits are randomly generated per message. To assess the secrecy level of the proposed scheme, three types of decoding strategies are evaluated: a matching decoder which knows the positions of all inserted bits inside the blocklength and tries to estimate them using the same decoding techniques, a blind decoder which treats all the frozen bits as the same value, and a random decoder which considers those dynamic bits as random at the receiver. Results are presented in terms of the system security gap, assuming an adaptive decoding strategy. It is shown that the system achieves combined secrecy and reliability. The proposed scheme does not assume knowledge of the eavesdropper's channel when defining the indices of information and frozen bits.

In this work, the authors consider phase shift keying (PSK)‐type signals with very compacted spectrum and low envelope fluctuations obtained through the use of magnitude modulation (MM) techniques and stringent roll‐off pulse shaping filtering. The analytical bit error rate (BER) of PSK‐type signals combined with MM is assessed, for both flat fading and time‐dispersive channel scenarios. The authors use a Gaussian model to approximate the statistical distribution of the distortion error introduced by MM to enable a closed‐form evaluation of the BER. The proposed expression depends only on the constellation order and the Kullback–Leibler divergence of the MM factors' probability density function from the Gaussian one, and it was found to be very accurate.

This study considers massive multiple‐input multiple‐output (m‐MIMO) schemes employing orthogonal frequency division multiplexing (OFDM) or single‐carrier with frequency‐domain equalisation (SC‐FDE) modulation, concerning an uplink transmission. The authors study the performance and complexity for both conventional zero forcing (ZF) and minimum mean squared error (MMSE) (which require the inversion of channel matrix) and iterative frequency domain equalisation receivers based on equal gain combining (EGC) and maximum ratio combining (MRC) concepts, that do not require matrix inversions. It is shown that, although matrix inversions are generally the more complex operations for relatively balanced systems (i.e. with similar number of transmit and receive antennas), this is not generally true for systems where the channel matrix is ‘very tall’ (i.e. much more receive than transmit antennas), at least in terms of the number of multiplications and sums. This means that the advantage on a reduced complexity of MRC/EGC‐based equalisers with respect to ZF/MMSE is, in fact, limited. However, the performance advantages combined with the possibility of parallel receiver implementations, make those techniques particularly interesting for m‐MIMO schemes, either employing OFDM or SC‐FDE modulations.

In this paper, we fill a void between information theoretic security and practical coding over the Gaussian wiretap channel using a three-stage encoder/decoder technique. Security is measured using Kullback-Leibler divergence and resolvability techniques along with a limited number of practical assumptions regarding the eavesdropper's decoder. The results specify a general coding recipe for obtaining both secure and reliable communications over the Gaussian wiretap channel, and one specific set of concatenated codes is presented as a test case for the sake of providing simulation-based evaluation of security and reliability over the network. It is shown that there exists a threshold in signal-to-noise ratio (SNR) over a Gaussian channel, such that receivers experiencing SNR below the threshold have no practical hope of receiving information about the message when the three-stage coding technique is applied. Results further indicate that the two innermost encoding stages successfully approximate a binary symmetric channel, allowing the outermost encoding stage (e.g., a wiretap code) to focus solely on secrecy coding over this approximated channel.

This paper proposes a method to provide secrecy for digital communications with arbitrarily large quadrature amplitude modulation (QAM) constellations for transmission over a Gaussian fading wiretap channel. This is accomplished by breaking the constellation down into nested quadrature phase-shift keying (QPSK) symbols and randomizing the assignment between message bits and modulated symbols using channel state information (CSI). If enough random bits can be generated from CSI it becomes possible to uniquely map an arbitrary message to any symbol in the large QAM constellation. The proposed method can thereby provide perfect secrecy while maintaining high reliability by exclusively assigning minimum-distance-mapped constellations through the randomization for use by the legitimate decoder.

This paper presents a testbed implementation and evaluation of coding for secrecy schemes in a real environment through software defined radio platforms. These coding schemes rely on interleaving and scrambling with randomly generated keys to shuffle information before transmission. These keys are then encoded jointly with data and then hidden (erased) before transmission, thus only being retrievable through parity information resulting from encoded data. An advantage of the legitimate receiver (e.g. a better signal-to-noise ratio) on the reception of those keys provides the means to achieve secrecy against an adversary eavesdropper. Through this testbed implementation, we show the practical feasibility of coding for secrecy schemes in real-world environments, unveiling the usefulness of interleaving and scrambling with a hidden key to reduce the required advantage over an eavesdropper. We further describe and present solutions to a set of issues that appear when doing practical implementations of security schemes in software defined radio platforms.

Time-Interleaved Block Windowed Burst-OFDM (TIBWB-OFDM) was recently proposed as a new waveform candidate for future 5G systems and beyond. In this paper, it is shown that this technique is, in fact, easily combined with MIMO systems, offering promising gains in terms of power and spectral efficiency. In addition, since TIBWB-OFDM is built on the mature OFDM technique, it enables fast forward implementation of a frequency domain equalization with inter-stream MIMO suppression, maintaining the simplicity of the equalizer at an affordable cost. Bearing the complexity requirement in mind, three equalization techniques are studied, namely low-complexity iterative maximum rate combining (MRC) and equal gain combining (EGC) receivers, that do not require costly matrix operations, and an iterative block decision feedback equalizer (IB-DFE) receiver that, although requiring matrix inversions, can provide excellent results by taking advantage of single-carrier type nature of the TIBWB-OFDM block. We show that, on the one hand, IB-DFE receiver exhibits superior gains and can be successfully employed in scenarios with moderate number of antenna-elements, where its complexity can be actually manageable. On the other hand, it is shown that iterative MRC and EGC offers excellent performances for high values of receiving antenna-elements.

This paper proposes a way of turning the equalisation of multiple- input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) signals simpler through the employment of low complexity iterative frequency domain equalisation (FDE) receivers based on equal gain combining (EGC) and maximum ratio combining (MRC) concepts, that do not require computing the inverses of high dimensional channel matrices. Performance results show that MIMO- OFDM schemes employing this type of techniques can achieve excellent bit error rate (BER) performances over severe time- dispersive channels and, therefore, approach or even outperform receivers that require inverting channel matrices, such as zero forcing (ZF) and minimum mean squared error (MMSE), after a few iterations.

This paper presents a new technique for providing the analysis and comparison of wiretap codes in the small blocklength regime over the binary erasure wiretap channel. A major result is the development of Monte Carlo strategies for quantifying a code's equivocation, which mirrors techniques used to analyze forward error correcting codes. For this paper, we limit our analysis to coset-based wiretap codes, and give preferred strategies for calculating and/or estimating the equivocation in order of preference. We also make several comparisons of different code families. Our results indicate that there are security advantages to using algebraic codes for applications that require small to medium blocklengths.

Massive MIMO (m-MIMO) schemes operating at mmWave bands are being proposed to be used in next generation wireless systems, bringing new challenges both in terms of detection techniques and the definition of channel models. In this paper, frequency-domain iterative detection schemes which do not require matrix inversions and are suitable for m-MIMO systems employing Single- Carrier with Frequency-Domain Equalization (SC- FDE) modulations are considered. These techniques are evaluated using a general clustered channel model for multilayer m-MIMO systems. Performance results show that these techniques can approach the Matched Filter Bound (MFB) with just a few iterations, even with significant correlation between different antenna elements.

This paper considers the use of the linear amplification with nonlinear components (LINC) technique for the power amplification of spectrally compact offset quadrature phase shift keying (OQPSK) signals allowing the use of highly efficient, low cost, and strongly nonlinear high power amplifiers (HPAs). However, the performance of the LINC signal separation and power combining procedures decreases with the rise of the signal's peak-to-average power ratio (PAPR). A new ring-type magnitude modulation (RMM) method is proposed for OQPSK signals that limits both its maximum and minimum complex envelope excursions avoiding zero crossings, without spreading the transmitted signal's spectrum. The performance results show that band-limited OQPSK signals whose envelope has low fluctuations produce LINC components with a narrower spectrum, with a considerable impact on the LINC transmitter regardless of the type of combiner chosen: when using a passive/matched combiner, the transmitter's power efficiency is significantly increased without spreading the combined signal's spectrum; for the highly efficient non-linear Chireix combiner, there is a reduction of the amount of spectral leakage produced by nonlinearly combining the LINC signal components. Finally, an iterative decoding scheme is also proposed, which employs estimates of the received symbols' RMM coefficients to compensate the RMM distortion.

The exploration of the physical layer characteristics of the wireless channel is currently the object of intensive research in order to develop advanced secrecy schemes that can protect information against eavesdropping attacks. Following this line of work, in this manuscript we consider a massive MIMO system and jointly design the channel precoder and security scheme. By doing that we ensure that the precoding operation does not reduce the degree of secrecy provided by the security scheme. The fundamental working principle of the proposed technique is to apply selective random rotations in the transmitted signal at the antenna level in order to achieve a compromise between legitimate and eavesdropper channel capacities. These rotations use the phase of the reciprocal wireless channel as a common random source between the transmitter and the intended receiver. To assess the security performance, the proposed joint scheme is compared with a recently proposed approach for massive MIMO systems. The results show that, with the proposed joint design, the number of antenna elements does not influence the eavesdropper channel capacity, which is proved to be equal to zero, in contrast to previous approaches.

The expectations for fifth generation (5G) systems involve handling huge throughputs together with power consumption. In order to address this issue, massive multiple-input and multiple-output (MIMO) schemes were developed exploring the fact that any M-ary constellation can be decomposed as several offset quadrature phase shift keying (OQPSK) signals, to be amplified by efficient saturated amplifiers and transmitted separately. Since nonlinear amplifiers should be employed with constant envelope signals to avoid nonlinear distortion, a ring-type magnitude modulation (RMM) technique was proposed for bandlimited OQPSK signals to mitigate its envelope fluctuations while maintaining its spectral characteristics. This technique was shown to improve the power amplification efficiency of linear amplification with nonlinear components (LINC) transmitters, although with some degradation of the of the detection performance. This paper proposes an iterative decoding scheme relying on the accuracy of the first iteration's log likelihood ratio (LLR) computation to estimate the RMM coefficients of the received symbols. Several LLR computation methods are compared with the ideal case where the receiver knows each symbol's RMM coefficients, and it is shown that a few iterations are enough to obtain a bit error rate (BER) performance close to the best case scenario.

In the next generation wireless systems, where m-MIMO and mmWave should be combined with beamforming techniques, array optimization at receiver ensuring a good SINR level can become a very large problem. Meta-heuristics can be an adequate approach to solve this problem with a mild computational effort. In this work, different meta-heuristics are implemented and applied to this problem, with Differential Evolution presenting promising connection setup time results that expectedly would enable its use in m-MIMO systems.

The growing progress in wireless communication services lead to a demand in high data rates, spectral efficiency and flexibility requirements. The recently proposed Block-Windowed Burst Orthogonal Frequency Division Multiplexing (BWB-OFDM) transceiver proved to be a reliable alternative scheme to face these current demands. BWB-OFDM employs smoother, non- rectangular windows, allowing a power spectral density similar to the filtered OFDM approach; also, it packs together several OFDM symbols, with the addition of a sole zero- padding to accommodate the multipath channel's propagation delay. This means better overall power and spectral efficiencies. Nevertheless, the system has the same drawback of OFDM when transmitting over hostile channel conditions, such as deep fading in time-dispersive channels. To overcome this problem, a new Time-Interleaved BWB- OFDM (TIBWB- OFDM) transceiver is proposed. This scheme employs interleaving on the time-samples of each BWB- OFDM block, thus creating a sort of diversity at the frequency domain, aiming to preserve the data symbols severely corrupted by the channel's deep fades. The new TIBWB-OFDM transceiver presents considerable power gains relatively to the BWB-OFDM,while maintaining their spectral efficiency.

Strongly nonlinear amplifiers have lower implementation complexity, together with much higher amplification efficiency than linear amplifiers. However, they are only recommendable when the signals at their input have very low envelope fluctuations, something that in general is incompatible with signals with high spectral efficiency (i.e. employing high-order constellations). It is proved, although, that spectral efficient M-ary constellations can be decomposed as a sum of several OQPSK signals that can be amplified separately, thus bringing a new life for this modulation scheme. Constant- envelope OQPSK-type signals can be obtained using its equivalent minimum shift keying (MSK) representation, but don't possess the required compact spectrum close to the minimum Nyquist frequency. This can only be achieved by employing narrowband Nyquist filters, which on the other hand causes the signal's envelope to fluctuate. Considering that, this paper proposes a ring-type magnitude modula- tion (RMM) method for narrowband offset quadrature phase shift keying (OQPSK) signals that considerably reduces its envelope fluctuations with- out spreading the transmitted signal's spectrum. Simulation results show that the reduction of the signal's dynamic range and peak-to-average power ratio (PAPR) allows the use of highly efficient, nonlinear saturated high power amplifiers (HPAs) without spreading the signal's spectrum and with minimum bit error rate (BER) performance degradation.

We propose a systematic concatenated coding scheme based on the combination of interleaving with powerful channel codes and jamming for wireless secrecy under the practical assumption of codes in the finite blocklength regime. The basic idea lies in generating a short random key that is used to shuffle/interleave information at the source, Alice. This key is then sent to the legitimate receiver, Bob, during a brief period of advantageous communication over the eavesdropper Eve (e.g., due to more interference from a jammer). Finally, the key is decoded at Bob to properly deinterleave the original information. Bob receives a better quality version of the interleaving key, therefore having the needed advantage over Eve. Information reliability is provided by a strong inner code, while security against Eve results from the proper selection of the outer code and interference levels over the key. We propose a methodology for selection of the outer code with reliability and security constraints. For that, we introduce bit error complementary cumulative distribution function metrics, suitable for security and reliability analysis of error correcting codes.

Base station (BS) cooperation architectures provide meaningful capacity gains, as they allow mobile terminals (MTs) in neighbor cells to share the same physical channel. Combining uplink signals received by the different BSs by employing iterative frequency-domain algorithms allows for the correct separation of the signals transmitted by each MT. To transmit those signals, MTs rely on single-carrier (SC) modulation with frequency-domain equalization (FDE). Those SC-FDE transmissions combined with iterative block decision feedback equalization (IB-DFE) techniques have been shown to be a effective mean to transmit cyclic-prefix (CP) assisted blocks within broadband wireless systems. In this paper, we aim to combine the aforementioned concepts of uplink transmission based on IB-DFE with BS cooperation in a clever manner. Our proposed scheme improves both the frequency reuse factor and the MTs' performance at cell edges, with further improved performance when low-density parity-check (LDPC) codes and turbo IB-DFE equalization schemes are considered.

Iterative block decision feedback equalization (IB-DFE) algorithms are proven to be powerful tools when detecting several uplink signals sharing the same physical channel. However, those algorithms may face some constraints -- the IB-DFE has some difficulty picking certain signals up if they are considerably weaker than the rest and the computational effort grows exponentially with the number of signals to be detected. In this paper we present a clustered multiuser detection scheme, based on an IB-DFE algorithm. It is demonstrated that if the mobile terminals (MTs) have enough power and are distributed in clusters, with considerable power difference between them, it is possible to successfully detect them all while containing the computational complexity. Our scheme also requires fewer receiving antennas than the original IB-DFE implementations and, with the aid of low-density parity- check (LDPC) and turbo- equalization, it is able to reach high performance levels with low power requirements.

The development of new mobile communications systems faces several important challenges, namely demanding for high bit rates, high spectral efficiency, low energy consumption and the usage of hostile channels. In order to fulfill these requirements for block-based single carrier transmissions, we propose to use high order QAM constellations combined with envelope control Magnitude Modulation (MM) techniques and bit-interleaved coded modulation with LDPC codes. Also, the receiver uses powerful MM specialized IB-DFE solutions to combat the channel distortion and noise. The obtained results have shown significant improvements in terms of energy efficiency and bit error rate.

Magnitude modulation (MM) techniques applied to single carrier (SC) transmissions provide the potential for gains in power efficiency by reducing the signal's peak-to-average power ratio, allowing efficient power amplification at the transmitter's side. This paper presents an accurate analytical model to measure the intrinsic symbol error rate (SER) due to MM distortion for systems employing high-order constellations and proposes a new feedback based soft-iterative magnitude demodulation (SiMD) scheme for the receiver with proper estimation of the MM factors used in transmission in order to compensate for the MM distortion introduced at the transmitter. It is shown that this technique allows the design of MM highly power efficient SC transceivers with high spectral efficiency.

The PAPR (Peak-to-Average Power Ratio) is widely employed to measure the magnitude of the envelope fluctuations of a given signal. It is particularly used to define the required amplifier back- off for multi-carrier modulations such as OFDM (Orthogonal Frequency Division Multiplexing) schemes. However, the PAPR increases with the block size (i.e., the number of subcarriers, in the OFDM case), but the signal's distribution converges to a Gaussian distribution for a large number of subcarriers. Therefore, one might ask if the PAPR is indeed the best parameter to define the amplifier's back-off. In this paper we address this issue. It is shown that the required amplifier's back-off for a given non-linear distortion level, i.e. referred to the input amplifier saturation power, is almost independent of the number of subcarriers and, therefore, the PAPR is not the most suitable to define it. Therefore, we recommend the use of a parameter related to the instantaneous envelope power distribution to define the amplifier back-off.

Polyphase magnitude modulation (MM) has been shown to be a robust and effortless mean to improve the efficiency of a transmitter's high power amplifier (HPA), due to the real-time reduction of the peak to average power ratio (PAPR). MM's technique flexibility allows us to include the MM system on any existing single-carrier (SC) based transmission system with clear benefits on the achieved bit error rate \emph{vs} overall signal power to noise ratio. This paper analyzes the efficiency of MM when added to a multi- input multi-output (MIMO) system, using a block-based SC transmission combined with iterative block decision feedback equalization (IB-DFE). To improve the IB-DFE performance for low power signals, we consider an additional scheme where low-density parity-check (LDPC) coding and turbo equalization are added. Simulation results show a net power efficiency enhancement, particularly for systems with channel coding, confirming MM as a major asset for high performance communication systems.

The linear amplification with nonlinear components (LINC) amplification concept is very important on the developing of the modern mobile and satellite communications systems due to the use of inexpensive high power amplifiers (HPA). This paper analyses the effectiveness of combining an efficient PAPR reducing technique based on magnitude modulation (MM) when applied to a LINC amplification system. The simulation results show that the inclusion of the MM on the LINC processing chain is very favourable in terms of spectral regrowth introduced by signal's clipping distortion. The system is also more robust to gain and phase imbalances between amplifiers with only a marginal bit error rate (BER) loss for an uncoded transmission scenario.

The control of the envelope's peak power by reducing overall peak-to-average power (PAPR) of transmitted RF signals can improve significantly the power efficiency of wireless communication systems both for single-carrier (SC) and multi-carrier (MC) transmissions. Multi-stage polyphase magnitude modulation (MPMM) has been recently proposed as an efficient PAPR reduction technique for SC signals. In this paper, we present a proof-of-concept prototype implementation in FPGA of the MPMM method. A prior study of the MPMM performance under fixed-point arithmetic is carried on, justifying decision criteria taken on MPMM's design targeted to FPGA.

In this paper we consider broadband wireless systems with high power efficiency. For this purpose we employ MM (Magnitude Modulation) techniques to reduce the PAPR (Peak-to-Average Power Ratio) of the transmitted signals, allowing efficient power amplification. Designed transceivers combine MM with SC-FDE (Single-Carrier with Frequency-Domain Equalization) methods upon iterative-block decision feedback equalization (IB-DFE). Since IB-DFE schemes designed for conventional SC signals are not suitable when we employ PAPR-reduction techniques, the effect of MM is taken into account on the estimation reliability factor that drives IB-DFE convergence. Results for the new proposed MM- IBDFE transceiver present gains above 1.5dB in the overall power efficiency, even when considering severely time-dispersive channels.

An efficient multistage polyphase magnitude modulation (MPMM) scheme was recently proposed to solve the problem of controlling the envelope's power peak of single carrier modulated signals, band limited by root-raised cosine (RRC) pulse shaping filters, in order to maximize the efficiency of the transmitter's high power amplifier (HPA). This paper addresses the statistical analysis of MPMM coefficients and performs an analytical study of the average power and the peak-to-average power ratio (PAPR) of the magnitude modulated transmitted signals. It is shown that the average power losses due to MPMM are largely compensated by the outstanding back-off reduction gains, leading to an effective reduction on the transmitted signal's PAPR. The analytical study also shows that these losses are comparable to those observed with the clipping method (less than 1dB), without generating the undesirable spectral regrowth effect.

Magnitude Modulation is a technique proposed with success for QPSK and OQPSK in order to maximize power and bandwidth of small satellite earth stations, by reducing PAPR of the modulated signal at the HPA input. To meet the ever growing demand for higher data rates, higher order modulations have been considered. This paper shows that it is possible to use the magnitude modulation concept for the M-QAM case, even considering the huge number of symbol combinations. The constellation and RRC symmetries are explored, allowing considerable reduction on look-up table computation complexity and storage requirements. Experimental results show considerable gains (>2dB) in PAPR and back-off reduction for 16-QAM with negligible BER losses.

Low-Density Parity-Check (LDPC) codes are among the best error correcting codes known and have been adopted by data transmission standards, such as DVB-S2 or WiMax. They are based on binary sparse parity check matrices and usually represented by Tanner graphs. LDPC decoders require very intensive message-passing algorithms, also known as belief propagation. This paper proposes a very compact stream-based data structure to represent such a bipartite Tanner graph, which supports both regular and irregular codes. This compact data structure not only reduces the memory required to represent the graph but also puts it in an appropriate format to gather data into streams. This representation also allows to map the irregular processing behavior of the Sum Product Algorithm (SPA) used in LDPC decoding into the stream-based computing model. Stream programs were developed for LDPC decoding and the results show significant speedups obtained either using general purpose processors, or graphics processing units. The simultaneous decoding of several codewords was performed using the SIMD capabilities of modern stream-based architectures available on recent processing units.

State-of-the-art decoders for Low-Density Parity-Check (LDPQ codes adopted by the DVB-S2 standard, explore the periodicity M = 360 features of the selected special LDPC- IRA codes. This paper addresses the generalization of a well known M-kernel parallel hardware structure and proposes an efficient partitioning by any factor of M, without memory addressing overhead and keeping unchanged the efficient message mapping scheme. The method provides a simple and efficient way to reduce the decoder complexity. Synthesizing the proposed decoder architecture for N = {45,90,180} parallel processing units using an FPGA family from Xilinx shows a minimum throughput above the minimal 90 Mbps.