In this thesis, we design a robust concatenated code for the Multiple-Input Multiple-Output (MIMO) system in the presence of slow Rayleigh fading with no channel side information at the transmitter (no CSIT) and perfect channel side information at the receiver (perfect CSIR). Since we are interested in the slow fading channel, outage capacity is used as the measure of performance. Good space-time codes can be designed so as to maximize the so-called rank and the determinant criteria. However, a practical system will concatenate a space-time code with an outer code at the transmitter and perform iterative decoding at the receiver. It is necessary to design the space-time code together with the outer code in practice. We will call this kind of code a concatenated space-time code. At the transmitter, we will consider the bit-to-symbol mapping and space-time code together as a space-time modulator and thus, Bit Interleaved Coded Modulation (BICM) and Multilevel coding (ML) can be applied to design outer codes for the nonbinary constellation. However, the concatenated space-time codes designed by these two methods can only be decoded with arbitrarily small error probability for a fixed channel realization and such designs are not robust over the ensemble of fading channels. Our approach of designing concatenated space-time code is to design an outer code for a space-time modulator such that the concatenated space-time code can be decoded with arbitrarily small error probability in a set of fixed channels which have the same capacity. Through this approach, we discovered a new design criterion for spacetime codes: a good space-time code should stabilize its Extrinsic Information Transfer (EXIT) charts. In other words, the robustness of a space-time code in the slow fading channel and its performance in iterative decoding can be visualized by the EXIT charts. The rank and the determinant criterion do not evaluate the performance of a space-time code in iterative decoding, but the new criterion does. Therefore, the new criterion is applicable to design concatenated space-time codes. Applying our approach and new criterion, a rate 7.2 bits/s/Hz concatenated space-time code is designed. The performance is close to the outage capacity, and the rate lost is 0.2 bits/s/Hz.

The objective of this thesis is to develop tools for the analysis and optimization of an iterative receiver. These tools can be applied to most soft-in soft-out (SISO) receiver components. For illustration purposes we consider a multi-user DS-CDMA system with forward error correction that employs iterative multi-user detection based on soft interference cancellation and single user decoding. Optimized power levels combined with adaptive scheduling allows for efficient utilization of receiver resources for heavily loaded systems.¶ Metric transfer analysis has been shown to be an accurate method of predicting the convergence behavior of iterative receivers. EXtrinsic Information (EXIT), fidelity (FT) and variance (VT) transfer analysis are well-known methods, however the relationship between the different approaches has not been explored in detail. We compare the metrics numerically and analytically and derive functions to closely approximate the relationship between them. The result allows for easy translation between EXIT, FT and VT methods. Furthermore, we extend the $J$ function, which describes mutual information as a function of variance, to fidelity and symbol error variance, the Rayleigh fading channel model and a channel estimate. These $J$ functions allow the \textit{a priori} inputs to the channel estimator, interference canceller and decoder to be accurately modeled. We also derive the effective EXIT charts which can be used for the convergence analysis and performance predictions of unequal power CDMA systems.¶ The optimization of the coded DS-CDMA system is done in two parts; firstly the received power levels are optimized to minimize the power used in the terminal transmitters, then the decoder activation schedule is optimized such that the multi-user receiver complexity is minimized. The uplink received power levels are optimized for the system load using a constrained nonlinear optimization approach. EXIT charts are used to optimize the power allocation in a multi-user turbo-coded DS-CDMA system. We show through simulation that the optimized power levels allow for successful decoding of heavily loaded systems with a large reduction in the convergence SNR.¶ We utilize EXIT chart analysis and a Viterbi search algorithm to derive the optimal decoding schedule for a multi component receiver/decoder. We show through simulations that decoding delay and complexity can be significantly reduced while maintaining BER performance through optimization of the decoding schedule.

Iterative processing has been shown to be very effective in multiuser space-time block coding (STBC) systems. The complexity and efficiency of an iterative receiver depend heavily on how the log-likelihood ratios (LLRs) of the coded bits are computed and exchanged at the receiver among its three major components, namely the multiuser detector, the maximum a posterior probability (MAP) demodulators and the MAP channel decoders. This thesis first presents a method to quantitatively measure the system complexities with floating-point operations (FLOPS) and a technique to evaluate the iterative receiver's convergence property based on mutual information and extrinsic information transfer (EXIT) charts.<p>Then, an integrated iterative receiver is developed by applying the sigma mappings for M-ary quadrature amplitude modulation (M-QAM) constellations. Due to the linear relationship between the coded bits and the transmitted channel symbol, the multiuser detector can work on the bit-level and hence improves the convergence property of the iterative receiver. It is shown that the integrated iterative receiver is an attractive candidate to replace the conventional receiver when a few receive antennas and a high-order M-QAM constellation are employed.<p> Finally, a more general two-loop iterative receiver is proposed by introducing an inner iteration loop between the MAP demodulators and the MAP convolutional decoders besides the outer iteration loop that involves the multiuser detection (MUD) as in the conventional iterative receiver. The proposed two-loop iterative receiver greatly improves the iteration efficiency. It is demonstrated that the proposed two-loop iterative receiver can achieve the same asymptotic performance as that of the conventional iterative receiver, but with much less outer-loop iterations.

Iterative processing has been shown to be very effective in multiuser space-time block coding (STBC) systems. The complexity and efficiency of an iterative receiver depend heavily on how the log-likelihood ratios (LLRs) of the coded bits are computed and exchanged at the receiver among its three major components, namely the multiuser detector, the maximum a posterior probability (MAP) demodulators and the MAP channel decoders. This thesis first presents a method to quantitatively measure the system complexities with floating-point operations (FLOPS) and a technique to evaluate the iterative receiver's convergence property based on mutual information and extrinsic information transfer (EXIT) charts.<p>Then, an integrated iterative receiver is developed by applying the sigma mappings for M-ary quadrature amplitude modulation (M-QAM) constellations. Due to the linear relationship between the coded bits and the transmitted channel symbol, the multiuser detector can work on the bit-level and hence improves the convergence property of the iterative receiver. It is shown that the integrated iterative receiver is an attractive candidate to replace the conventional receiver when a few receive antennas and a high-order M-QAM constellation are employed.<p> Finally, a more general two-loop iterative receiver is proposed by introducing an inner iteration loop between the MAP demodulators and the MAP convolutional decoders besides the outer iteration loop that involves the multiuser detection (MUD) as in the conventional iterative receiver. The proposed two-loop iterative receiver greatly improves the iteration efficiency. It is demonstrated that the proposed two-loop iterative receiver can achieve the same asymptotic performance as that of the conventional iterative receiver, but with much less outer-loop iterations.

L'utilisation de la modulation à phase continue (CPM) est particulièrement intéressante lorsque le canal de communication comporte une forte non-linéarité et un support spectral limité, en particulier pour la voie aller, lorsque l'on dispose d'un amplificateur par porteuse à bord du satellite, et pour la voie retour où le terminal d'émission travaille à saturation. De nombreuses études ont été effectuées sur le sujet mais les solutions proposées reposent sur la démodulation/décodage itératif des CPM couplées à un code correcteur d'erreur de type convolutif ou bloc. L'utilisation de codes LDPC n'a pas été à ce jour abordée de façon précise. En particulier, il n'existe pas à notre connaissance de travaux sur l'optimisation des codes basés sur des graphes creux adaptés à ce type de schémas. Dans cette étude, nous proposons d'effectuer l'analyse asymptotique et le design d'un schéma Turbo-CPM basé sur des graphes creux. Une étude du récepteur associé comportant les fonctions de démodulation sera également effectuée. / The use of the continuous phase modulation (CPM) is interesting when the channel represents a strong non-linearity and in the case of limited spectral support; particularly for the uplink, where the satellite holds an amplifier per carrier, and for downlinks where the terminal equipment works very close to the saturation region. Numerous studies have been conducted on this issue but the proposed solutions use iterative CPM demodulation/decoding concatenated with convolutional or block error correcting codes. The use of LDPC codes has not yet been introduced. Particularly, no works, to our knowledge, have been done on the optimization of sparse graph-based codes adapted for the context described here. In this study, we propose to perform the asymptotic analysis and the design of turbo-CPM systems based on the optimization of sparse graph-based codes. Moreover, an analysis on the corresponding receiver will be done.

Abstract Iterative receiver techniques, multiple-input – multiple-output (MIMO) processing, and orthogonal frequency division multiplexing (OFDM) are amongst the key physical layer technologies when aiming at higher spectral efficiency for a wireless communication system. Special focus is put on iterative detection, decoding, and channel estimation for a MIMO-OFDM system. After designing separately efficient algorithms for the detection, channel decoding, and channel estimation, the objective is to optimize them to work together through optimizing the activation schedules for soft-in soft-out (SfISfO) components. A list parallel interference cancellation (PIC) detector is derived to approximate an a posteriori probability (APP) algorithm with reduced complexity and minimal loss of performance. It is shown that the list PIC detector with good initialization outperforms the K-best list sphere detector (LSD) in the case of small list sizes, whereas the complexities of the algorithms are of the same order. The convergence of the iterative detection and decoding is improved by using a priori information to also recalculate the candidate list, aside from the log-likelihood ratios (LLRs) of the coded bits. Unlike in pilot based channel estimation, the least-squares (LS) channel estimator based on symbol decisions requires a matrix inversion in MIMO-OFDM. The frequency domain (FD) space-alternating generalized expectation-maximization (SAGE) channel estimator calculates the LS estimate iteratively, avoiding the matrix inversion with constant envelope modulation. The performance and computational complexity of the FD-SAGE channel estimator are compared to those of pilot based LS channel estimation with minimum mean square error (MMSE) post-processing exploiting the time correlation of the channel. A time domain (TD) SAGE channel estimator is derived to avoid the matrix inversion in channel estimation based on symbol decisions for MIMO-OFDM systems also with non-constant envelope modulation. An obvious problem, with more than two blocks in an iterative receiver, is to find the optimal activation schedule of the different blocks. It is proposed to use extrinsic information transfer (EXIT) charts to characterize the behavior of the receiver blocks and to find out the optimal activation schedule for them. A semi-analytical expression of the EXIT function is derived for the LS channel estimator. An algorithm is proposed to generate the EXIT function of the APP algorithm as a function of the channel estimate’s mutual information (MI). Surface fitting is used to get closed form expressions for the EXIT functions of the APP algorithm and the channel decoder. Trellis search algorithms are shown to find the convergence with the lowest possible complexity using the EXIT functions. With the proposed concept, the activation scheduling can be adapted to prevailing channel circumstances and unnecessary iterations will be avoided.

Abstract This thesis focuses on advanced multi-antenna receiver and transmission techniques to improve the utilization efficiencies of radio resources in broadband single carrier communications. Special focus is devoted to the development of computationally efficient frequency domain (FD) turbo equalization techniques for single and multiuser MIMO frequency selective channels. Another special emphasis is given to transmission power optimization for single user MIMO communications, which takes into account the convergence properties of the iterative equalizer. A new iterative FD soft cancellation (SC) and minimum mean square error (MMSE) filtering based joint-over-antenna (JA) multiuser MIMO signal detection technique for multiuser MIMO uplink transmission in frequency-selective channels is proposed. The proposed FD multiuser MIMO detection technique requires significantly lower computational complexity than its time-domain counterpart. Furthermore, significant performance gains can be achieved with the proposed JA turbo receiver compared to an antenna-by-antenna (AA) turbo receiver when the total number of transmitter antennas and users is larger than the number of receiver antennas, as well as in the presence of spatial correlation. The impact of existing linear precoding techniques, e.g, maximum information rate (MaxRate) and minimum sum mean square error (MinSumMSE), on the performance of frequency domain turbo equalization is investigated by utilizing extrinsic information transfer (EXIT) chart analysis. A novel transmission power minimization framework based on an EXIT analysis of single carrier MIMO transmission with iterative FD SC-MMSE equalization is then proposed. The proposed optimization framework explicitly takes into account the convergence properties of the iterative equalizer. The proposed convergence constrained power allocation (CCPA) technique decouples the spatial interference between streams using singular value decomposition (SVD), and minimizes the transmission power while achieving the mutual information target for each stream after iterations at the receiver side. The transmission power allocation can be formulated as a convex optimization problem. A special case having only two mutual information constraints is considered, for which the Lagrange dual function is derived and its dual problem is solved. Inspired by the Lagrange duality, two CCPA based heuristic schemes are developed. The numerical results demonstrate that the proposed CCPA schemes outperform the existing power allocation schemes. / Tiivistelmä Tässä työssä tutkitaan edistyksellisten moniantennivastaanotto- ja lähetysmenetelmien käyttöä radioresurssien tehokkuuden parantamiseen laajakaistaisessa yhden kantoaallon kommunikaatiossa. Työssä keskitytään erityisesti laskennallisesti tehokkaiden taajuustasossa suoritettavien iteratiivisten kanavakorjaintekniikoiden kehittämiseen yhden ja usean käyttäjän multiple-input multiple-output (MIMO) -kommunikaatiossa taajuusselektiivisen radiokanavan yli. Toinen tutkimuksen painopiste on lähetystehon optimointi yhden käyttäjän MIMO-kommunikaatiossa, jossa iteratiivisen kanavakorjaimen konvergenssiominaisuudet otetaan huomioon. Työssä ehdotetaan uudenlaista iteratiivista taajuustasossa suoritettavaa soft-cancellation (SC) ja minimum mean square error (MMSE) -suodatukseen pohjautuvaa joint-over-antenna (JA) monen käyttäjän ilmaisumenetelmää nousevan siirtokanavan tiedonsiirtoon taajuusselektiivisessa radiokanavassa. Ehdotettu tajuustasossa suoritettava usean käyttäjän MIMO-lähetyksen ilmaisumenetelmä vaatii selvästi vähemmän laskentatehoa verrattuna aikatason menetelmään. Tämän lisäksi ehdotetulla menetelmällä voidaan saavuttaa merkittävää suorituskykyhyötyä verrattuna antenna-by-antenna (AA) -pohjaiseen iteratiiviseen vastaanottimeen kun lähetysantennien ja käyttäjien kokonaislukumäärä on suurempi kuin vastaanotinantennien. Suorituskykyhyöty pätee myös tilakorrelaation tapauksessa. Työssä tutkitaan lisäksi olemassa olevien lineaaristen esikoodaustekniikoiden, esim. maximum information rate (MaxRate) and minimum sum mean square error (MinSumMSE), vaikutusta taajuustasossa suoritettavaan iteratiivisen kanavakorjaimen konvergenssiominaisuuksiin xtrinsic information transfer (EXIT) -analyysin avulla. Työssä ehdotetaan uudenlaista EXIT-analyysi-pohjaista lähetystehon minimointimenetelmää yhden kantoaallon MIMO-lähetykseen käyttäen iteratiivista taajuustason SC-MMSE-kanavakorjainta. Menetelmä ottaa huomioon iteratiivisen kanavakorjaimen konvergenssiominaisuudet. Ehdotettu convergence constrained power allocation (CCPA) -menetelmä erottaa tilatason häiriön lähetteiden välillä hyödyntäen singular value decomposition (SVD) -tekniikkaa ja minimoi lähetystehon ja saavuttaa samalla keskinäisinformaatiotavoitteet jokaiselle lähetteelle iteraatioiden jälkeen vastaanottimessa. Lähetystehon minimointiongelma voidaan muotoilla konveksiksi optimointiongelmaksi. Kahden keskinäisinformaatiorajoitteen erityistapaukselle johdetaan Lagrangen duaalifunktio ja ratkaistaan sen duaalifunktio. Työssä kehitetään lisäksi kaksi CCPA-pohjaista heuristista menetelmää. Numeeriset tulokset osoittavat ehdotettujen CCPA-pohjaisten menetelmien suoriutuvan paremmin verrattuna olemassa oleviin menetelmiin.

Massive MU-MIMO (Multiuser-Multiple Input and Multple Output) is a promising technology for 5G wireless communications because of its spectrum and energy efficiency. To combat the distortion from multipath fading channel, the acquisition of channel state information is essential, which generally requires the training signal that lowers the data rate. In addition, coarse quantization can reduce the high computational energy and cost, yet results in the loss of information. In this thesis, an iterative decision feedback receiver, including iterative Channel Estimation (CE) and equalization, is constructed for a Massive MU-MIMO uplink system. The impact of multipath distortion and coarse quantization can be gradually reduced due to the iterative structure that exploits extrinsic feedback to improve the CE and data detection, so that the data rate is improved by reducing training signals for CE and by using low precision quantization. To observe and evaluate the convergence behaviour, an Extrinsic Information Transfer (EXIT) chart method is utilized to visualize the performance of the iterative receiver. / Graduate

The increasing demand for high data rate transmission over broadband radio channels has imposed significant challenges in wireless communications. Accurate channel estimation has a major impact on the whole system performance. Specifically, reliable estimate of the channel state information (CSI) is more challenging for orthogonal frequency division multiplexing (OFDM) systems in doubly selective fading channels than for the slower fading channels over which OFDM has been deployed traditionally. With the help of a basis expansion model (BEM), a novel multivariate autoregressive (AR) process is developed to model the time evolution of the fast fading channel. Relying on pilot symbol aided modulation (PSAM), a novel Kalman smoothing algorithm based on a second-order dynamic model is exploited, where the mean square error (MSE) of the channel estimator is near to that of the optimal Wiener filter. To further improve the performance of channel estimation, a novel low-complexity iterative joint channel estimation and symbol detection procedure is developed for fast fading channels with a small number of pilots and low pilot power to achieve the bit error rate (BER) performance close to when the CSI is known perfectly. The new channel estimation symbol detection technique is robust to variations of the radio channel from the design values and applicable to multiple modulation and coding types. By use of the extrinsic information transfer (EXIT) chart, we investigate the convergence behavior of the new algorithm and analyze the modulation, pilot density, and error correction code selection for good system performance for a given power level. The algorithms developed in this thesis improve the performance of the whole system requiring only low ratios of pilot to data for excellent performance in fast fading channels. / Graduate

Abstract This thesis concentrates on joint optimization of transmit power allocation and receive filtering in multiuser, multi-antenna communications. Due to the increasing number of wireless devices, the design of energy-efficient communication links is becoming increasingly important. In cellular mobile communications, reducing the average power consumption in uplink transmission is beneficial for users in order to extend battery life and, hence, energy efficiency in general. However, the power consumption of the high power amplifier (HPA) at the transmitter depends on the peak power of the transmission. This thesis focuses on power allocation problems for single-carrier (SC) frequency division multiple access (FDMA) and orthogonal FDMA (OFDMA) transmission assuming iterative reception. The goal in the first scheme presented in this thesis is to reduce the average power consumption by designing a power allocation method that takes into account the convergence properties of an iterative receiver in multiuser uplink communications. The proposed scheme can guarantee that the desired quality of service (QoS) is achieved after a sufficient number of iterations. Reducing the peak-to-average power ratio (PAPR) in any transmission system is beneficial because it allows the use of inexpensive, energy-efficient power amplifiers. The goal in the second scheme presented in this thesis is to control the PAPR of the transmitted signal. Hence, in addition to the QoS constraint, the instantaneous PAPR constraint is derived for SC-FDMA and OFDMA transmission. Moreover, a statistical approach is considered in which the power variance of the transmitted waveform is controlled. The QoS and PAPR constraints are considered jointly and, therefore, the proposed power allocation strategy jointly takes into account the channel quality and the PAPR characteristics of the power amplifier. However, the PAPR constraint can be adopted to any SC-FDMA or OFDMA framework and it is not restricted to the scheme presented in this thesis. The objective of the optimization problems considered throughout the thesis is to minimize the sum power. The majority of the derived constraints are non-convex and therefore, two alternative successive convex approximations (SCAs) are derived for all the non-convex constraints considered. The numerical results show that the proposed power allocation strategies can significantly reduce the average transmission power of users while allowing flexible PAPR control. Hence, the proposed methods can be used to extend battery life for users and especially improve the QoS at the cell edges. / Tiivistelmä Väitöskirjassa tutkitaan lähettimessä tapahtuvan tehoallokoinnin sekä vastaanottimessa tapahtuvan signaalin suodatuksen yhteisoptimointia monikäyttöön suunnatussa langattomassa moniantennikommunikaatiossa. Langattomien laitteiden lukumäärän kasvaessa energiatehokkuuden merkitys tiedonsiirtolinkkien suunnittelussa korostuu. Soluihin perustuvassa langattomassa tietoliikenteessä keskimääräisen tehonkulutuksen pienentäminen ylälinkkilähetyksessä (käyttäjältä tukiasemaan) on tärkeää käyttäjän kannalta, sillä se pidentää laitteen akun kestoa. Lähettimen tehovahvistimen (high power amplifier (HPA)) tehonkulutus on kuitenkin verrannollinen lähetyksen huipputehoon. Väitöskirjassa luodaaan uusia menetelmiä sekä vertaillaan tehoallokointia yhden kantoaallon taajuustason monikäyttöön (single carrier frequency division multiple access (SC-FDMA)) ja ortogonaalisen taajuustason monikäyttöön (orthogonal FDMA (OFDMA)) perustuvissa lähetysteknologioissa. Työn ensimmäisessä osiossa tavoitteena on keskimääräisen tehonkulutuksen pienentäminen monen käyttäjän ylälinkkikommunikaatiossa suunnittelemalla tehoallokointimenetelmä, joka ottaa huomioon iteratiivisen vastaanottimen konvergenssiominaisuudet. Työssä ehdotettu menetelmä takaa vastaanotetun informaation halutun laadun (quality of service (QoS)) riittävän monen vastaanottimessa tehdyn iteraation jälkeen. Huipputehon ja keskitehon suhteen (peak to average power ratio (PAPR)) pienentäminen missä tahansa lähetyksessä on hyödyllistä, sillä sen ansiosta voidaan käyttää energiatehokkaampia ja halvempia tehovahvistimia. Työn jälkimmäisessä osiossa tavoitteena on kontrolloida lähetetyn signaalin huipputehon ja keskitehon suhdetta. Työn ensimmäisessä osiossa esitetyn QoS-rajoitteen lisäksi tehoallokointia rajoitetaan symbolisekvenssikohtaisella PAPR-rajoitteella SCFDMA- ja OFDMA-lähetyksessä. Lisäksi esitetään tilastollinen menetelmä, jossa rajoitetaan lähetetyn signaalin tehon varianssia. Kun käytetään yhtäaikaisesti QoS- ja PAPR-rajoitteita, voidaan tiedonsiirtokanavaan suunnitella optimaalinen tehoallokointi ottaen huomioon tehovahvistimen epälineaarisuudet. Työssä esitetty PAPR-rajoite on kuitenkin geneerinen, ja se voidaan sovittaa mihin tahansa SCFDMA- tai OFDMA- optimointikehykseen. Työssä esitettävien optimointiongelmien tavoitteena on käyttäjien summatehon minimointi. Suurin osa työssä esiintyvistä ongelmista on ei-konvekseja, joten siinä esitetään kaksi vaihtoehtoista peräkkäinen konveksi approksimaatio (successive convex approximation (SCA)) -menetelmää kaikille ei-konvekseille rajoitteille. Numeeriset tulokset osoittavat, että esitetyt tehoallokointimenetelmät pienentävät merkittävästi keskimääräistä tehonkulutusta mahdollistaen lisäksi adaptiivisen PAPR-kontrolloinnin. Väitöskirjassa esitettyjen menetelmien avulla voidaan pidentää mobiilikäyttäjien akun kestoa sekä erityisesti parantaa solun reunakäyttäjien palvelun laatua.