Low-complexity iterative frequency domain decision feedback equalization

Single-carrier transmission with frequency domain equalization (SC-FDE) offers a viable design alternative to the classic orthogonal frequency division multiplexing technique. However, SC-FDE using a linear equalizer may suffer from serious performance deterioration for transmission over severely frequency-selective fading channels. An effective method of solving this problem is to introduce non-linear decision feedback equalization (DFE) to SC-FDE. In this contribution, a low complexity iterative decision feedback equalizer operating in the frequency domain of single-carrier systems is proposed. Based on the minimum mean square error criterion, a simplified parameter estimation method is introduced to calculate the coefficients of the feed-forward and feedback filters, which significantly reduces the implementation complexity of the equalizer. Simulation results show that the performance of the proposed simplified design is similar to the traditional iterative block DFE under various multipath fading channels but it imposes a much lower complexity than the latter

Error performance analysis of n-ary Alamouti scheme with signal space diversity.

Masters Degree. University of KwaZulu-Natal, Durban.In this dissertation, a high-rate Alamouti scheme with Signal Space Diversity is developed to improve both the spectral efficiency and overall error performance in wireless communication links. This scheme uses high modulation techniques (M-ary quadrature amplitude modulation (M-QAM) and N-ary phase shift keying modulation (N-PSK)). Hence, this dissertation presents the mathematical models, design methodology and theoretical analysis of this high-rate Alamouti scheme with Signal Space Diversity.To improve spectral efficiency in multiple-input multiple-output (MIMO) wireless communications an N-ary Alamouti M-ary quadrature amplitude modulation (M-QAM) scheme is proposed in this thesis. The proposed N-ary Alamouti M-QAM Scheme uses N-ary phase shift keying modulation (NPSK) and M-QAM. The proposed scheme is investigated in Rayleigh fading channels with additive white Gaussian noise (AWGN). Based on union bound a theoretical average bit error probability (ABEP) of the system is formulated. The simulation results validate the theoretical ABEP. Both theoretical results and simulation results show that the proposed scheme improves spectral efficiency by 0.5 bit/sec/Hz in 2 × 4 16-PSK Alamouti 16-QAM system compared to the conventional Alamouti scheme (16-QAM). To further improve the error performance of the proposed N-ary Alamouti M-QAM Scheme an × N-ary Alamouti coded M-QAM scheme with signal space diversity (SSD) is also proposed in this thesis. In this thesis, based on the nearest neighbour (NN) approach a theoretical closed-form expression of the ABEP is further derived in Rayleigh fading channels. Simulation results also validate the theoretical ABEP for N-ary Alamouti M-QAM scheme with SSD. Both theoretical and simulation results further show that the 2 × 4 4-PSK Alamouti 256-QAM scheme with SSD can achieve 0.8 dB gain compared to the 2 × 4 4-PSK Alamouti 256-QAM scheme without SSD

Exploiting diversity in wireless channels with bit-interleaved coded modulation and iterative decoding (BICM-ID)

This dissertation studies a state-of-the-art bandwidth-efficient coded modulation technique, known as bit interleaved coded modulation with iterative decoding (BICM-ID), together with various diversity techniques to dramatically improve the performance of digital communication systems over wireless channels. For BICM-ID over a single-antenna frequency non-selective fading channel, the problem of mapping over multiple symbols, i.e., multi-dimensional (multi-D) mapping, with 8-PSK constellation is investigated. An explicit algorithm to construct a good multi-D mapping of 8-PSK to improve the asymptotic performance of BICM-ID systems is introduced. By comparing the performance of the proposed mapping with an unachievable lower bound, it is conjectured that the proposed mapping is the global optimal mapping. The superiority of the proposed mapping over the best conventional (1-dimensional complex) mapping and the multi-D mapping found previously by computer search is thoroughly demonstrated. In addition to the mapping issue in single-antenna BICM-ID systems, the use of signal space diversity (SSD), also known as linear constellation precoding (LCP), is considered in BICM-ID over frequency non-selective fading channels. The performance analysis of BICM-ID and complex N-dimensional signal space diversity is carried out to study its performance limitation, the choice of the rotation matrix and the design of a low-complexity receiver. Based on the design criterion obtained from a tight error bound, the optimality of the rotation matrix is established. It is shown that using the class of optimal rotation matrices, the performance of BICM-ID systems over a frequency non-selective Rayleigh fading channel approaches that of the BICM-ID systems over an additive white Gaussian noise (AWGN) channel when the dimension of the signal constellation increases. Furthermore, by exploiting the sigma mapping for any M-ary quadrature amplitude modulation (QAM) constellation, a very simple sub-optimal, yet effective iterative receiver structure suitable for signal constellations with large dimensions is proposed. Simulation results in various cases and conditions indicate that the proposed receiver can achieve the analytical performance bounds with low complexity. The application of BICM-ID with SSD is then extended to the case of cascaded Rayleigh fading, which is more suitable to model mobile-to-mobile communication channels. By deriving the error bound on the asymptotic performance, it is first illustrated that for a small modulation constellation, a cascaded Rayleigh fading causes a much more severe performance degradation than a conventional Rayleigh fading. However, BICM-ID employing SSD with a sufficiently large constellation can close the performance gap between the Rayleigh and cascaded Rayleigh fading channels, and their performance can closely approach that over an AWGN channel. In the next step, the use of SSD in BICM-ID over frequency selective Rayleigh fading channels employing a multi-carrier modulation technique known as orthogonal frequency division multiplexing (OFDM) is studied. Under the assumption of correlated fading over subcarriers, a tight bound on the asymptotic error performance for the general case of applying SSD over all N subcarriers is derived and used to establish the best achievable asymptotic performance by SSD. It is then shown that precoding over subgroups of at least L subcarriers per group, where L is the number of channel taps, is sufficient to obtain this best asymptotic error performance, while significantly reducing the receiver complexity. The optimal joint subcarrier grouping and rotation matrix design is subsequently determined by solving the Vandermonde linear system. Illustrative examples show a good agreement between various analytical and simulation results. Further, by combining the ideas of multi-D mapping and subcarrier grouping, a novel power and bandwidth-efficient bit-interleaved coded modulation with OFDM and iterative decoding (BI-COFDM-ID) in which multi-D mapping is performed over a group of subcarriers for broadband transmission in a frequency selective fading environment is proposed. A tight bound on the asymptotic error performance is developed, which shows that subcarrier mapping and grouping have independent impacts on the overall error performance, and hence they can be independently optimized. Specifically, it is demonstrated that the optimal subcarrier mapping is similar to the optimal multi-D mapping for BICM-ID in frequency non-selective Rayleigh fading environment, whereas the optimal subcarrier grouping is the same with that of OFDM with SSD. Furthermore, analytical and simulation results show that the proposed system with the combined optimal subcarrier mapping and grouping can achieve the full channel diversity without using SSD and provide significant coding gains as compared to the previously studied BI-COFDM-ID with the same power, bandwidth and receiver complexity. Finally, the investigation is extended to the application of BICM-ID over a multiple-input multiple-output (MIMO) system equipped with multiple antennas at both the transmitter and the receiver to exploit both time and spatial diversities, where neither the transmitter nor the receiver knows the channel fading coefficients. The concentration is on the class of unitary constellation, due to its advantages in terms of both information-theoretic capacity and error probability. The tight error bound with respect to the asymptotic performance is also derived for any given unitary constellation and mapping rule. Design criteria regarding the choice of unitary constellation and mapping are then established. Furthermore, by using the unitary constellation obtained from orthogonal design with quadrature phase-shift keying (QPSK or 4-PSK) and 8-PSK, two different mapping rules are proposed. The first mapping rule gives the most suitable mapping for systems that do not implement iterative processing, which is similar to a Gray mapping in coherent channels. The second mapping rule yields the best mapping for systems with iterative decoding. Analytical and simulation results show that with the proposed mappings of the unitary constellations obtained from orthogonal designs, the asymptotic error performance of the iterative systems can closely approach a lower bound which is applicable to any unitary constellation and mapping

Interference-free Spectrum Sharing In Cognitive Radio Based On Space Time Coding

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2016Thesis (M.Sc.) -- İstanbul Technical University, Instıtute of Science and Technology, 2016Yeni nesil telsiz iletişim sistemleri ve gezgin sistemler daha yüksek veri hızlarının yanında, maksimum bant genişliği kullanımına da gereksinim duyarlar. Son yıllarda, literatürde bazı teknikler telsiz sistemlerin yüksek veri hızlarını destekleyeceği çeşitli şekillerde ortaya konulmuştur. Ancak, yüksek veri hızlarını destekleme gereksinimini karşılamada, spektrumun çoğunun lisanslı kullanıcılara ait olduğu spektrum tahsis tablosunda olduğu gibi spektrum yetersizliği problemiyle karşılaşılır. Bununla birlikte, spektrum ölçümleri frekansa, coğrafi konuma ve zamana bağlı olarak spektrumun büyük bir kısmının sürekli olarak kullanılmadığını göstermektedir. Sonuç olarak, yüksek veri hızlarını desteklemek için spektrum dağılımını kabul edilebilir servis kalitesine göre yöneterek kaynakların verimli bir şekilde kullanılması gerekmektedir. Bu nedenle, telsiz iletişimde mevcut spektrumu paylaşarak spektrum yetersizliği problemini ortadan kaldırmak için pek çok yaklaşım ortaya atılmıştır. İki farklı telsiz sistemin önceliklerine göre aynı frekans bandında çalıştığı bilişsel radio (CR) spektrum kullanımını arttırmak ve spektrum yetersizliğini çözmek için potansiyel bir yol olarak kabul edilmiştir. CR ilerideki yeni nesil kablosuz sistemler için yenilikçi bir teknik ve gelişen bir araştırma alanıdır. CR’da lisanslı bandlara erişmek için ikincil kullanıcılar (SU) tarafından araya yerleştirme, altına serme ve üstüne serme teknikleri kullanılabilir. Araya yerleştirme tekniğinde, SU’lar birincil kullanıcılara (PU) girişim yaratmadan spektrum boşluklarını kullanmak için PU’ların varlığını ya da yokluğunu sezerler. Altına serme tekniğinde, eğer PU’lardaki girişim önceden belirlenmiş bir girişim seviyesinin altındaysa, SU’lara lisanslı spektrumu kullanma hakkı verilir. Üstüne serme tekniğinde ise, lisanssız kullanıcılar genellikle lisanslı spektrumu paylaşmak için spektrum sahiplerinin yararına işbirliği yaparak bir bedel öderler. Literatürde karma ve toplamsal kodlama teknikleri gibi farklı spektrum paylaşım türleri önerilmiştir. Toplamsal kodlamada SU, PU’nun ve kendi işaretini birleştirir, sonra birincil alıcıya (PR) ve ikincil alıcıya (SR) yayın yapar. Ancak bu teknik her iki kullanıcının alıcılarında önemli girişime neden olur. Sistemin, spektrumdan yarar sağlamak ve PU ile işbirliği yapmak için farklı teknikleri bir araya getirdiği karma yöntemlerden de yaralanılabilir. İletilen işarete kanalların sönümleme etkisini azaltabilen, modülasyon çeşitlemesi olarak da bilinen bir çeşitleme tekniği işaret uzayı çeşitlemesidir (SSD). SSD fazladan band genişliği ya da güç kullanmadan sönümlemeli kanallarda başarımı artırır. SSD işleminde veri iletiminin öncesinde modülasyonlu işaret kümeleri belli bir açı ile döndürülür ve döndürülen işaretler arasındaki minimum çarpımsal uzaklık maksimize edilir. Döndürülmüş işaret kümesine koordinat serpiştirmeli dik tasarım (CIOD) uygulandıktan sonra düzensiz bir modülasyonlu işaret kümesi elde edilir. Normalde işbirlikli haberleşmede verinin alıcıya iletilmesi birden fazla zaman aralığı aldığı için tüm sistemin band verimliliği düşer. Ancak, SSD ve CIOD tekniklerini sistem yapısında kullanmak sistemin karmaşıklığını artırmadan işbirlikli sistemlerde band verimliliğini kolaylıkla artırabilir. Bu tezde, üstüne serme yöntemiyle çalışan bilişsel radyo için iki farklı spektrum paylaşım protokolü önerilmiştir. İlk protokolde, birincil sistem her biri bir antenle donatılmış bir birincil verici (PT) ve bir birincil alıcı (PR) ile ikincil sistem ise her biri bir antenle donatılmış bir ikincil verici (ST) ve bir ikincil alıcı (SR) dan oluşmaktadır. Birincil sistem ile işbirliği çerçevesinde, SU’nun PU ya ait spektrum bandını kullanmasına izin verilmektedir. Bu çalışmada hem spektral verimliliği ve çeşitlemeyi artırmak hem de iki kullanıcının karşılıklı girişimini ortadan kaldırmak amacıyla üç zaman aralığında iki kullanıcının iletimini sağlayan CIOD uzay zaman kodlaması kullanan bir iletim protokolu önerilmektedir. İlk zaman aralığında, PT kendi işaretini PR ve ST’ye iletir ve ST iletilen işareti çözmeye çalışır. ST, PU nun işaretini doğru çözerse, serpiştirilmiş birincil işaret koordinat çiftini PR’ye ikinci zaman aralığında iletilmektir ve PR, PT ve ST’den aldığı işaretler yardımıyla işareti çözmeye çalışır. Ancak, ST’nin PT’den aldığı işareti doğru çözemediği durumda, PR sadece ilk zaman aralığında PT den gelen işarete dayanarak iletilen işareti çözmeye çalışır. Son olarak, ST üçüncü zaman aralığında kendi (ikincil) işaretini SR’ye iletir. Önerilen protokolde ikincil kullanıcıya ayrı bir zaman aralığı ayrılması iki kullanıcı arasındaki girişimi kaldırırken iki birincil işaretin CIOD yardımıyla iki zaman aralığında iletilmesi üç kanal kullanımı başına iki birincil işaretin iletilmesini sağlamakta, böylece iki kanal kullanımı başına bir birincil işaretin iletildiği referans sistemlere göre kanalın daha verimli kullanılmasını sağlamaktadır. PT ile ST arasında bir kritik uzaklık tanımlanmakta, ST, PT ye bu uzaklıktan daha yakın olduğu sürece PU nun servis kesilme olasılığı spektrum paylaşımı olmadığı durumdakine eşit veya daha düşük olmaktadır. Rayleigh sönümlemeli kanalda, PU ve SU nun servis kesilme olasılıkları için kapalı biçimde kuramsal ifadeler türetilmekte, hedeflenen farklı hızlar için protokolun başarımı değerlendirilmektedir. Kuramsal sonuçların bilgisayar benzetimleriyle bütünüyle uyum içerisinde olduğu görülmektedir. Önerilen protokol için hem PU nun hem de SU nun başarımında ST de toplamsal kodlamanın kullanıldığı literatürde verilen referans protokole göre önemli kazançlar elde edilmektedir. Diğer yandan birincil kullanıcının bit hata olasılığı için kuramsal bir üst sınır elde edilmekte, sonuçlar bilgisayar benzetimleriyle desteklenmektedir. Beklendiği gibi bilgisayar benzetim sonuçları artan işaret-gürültü oranı ile kuramsal üst sınıra yaklaşmaktadır. Son olarak, ikincil kullanıcının BER başarımı klasik doğrudan iletime eşdeğerdir. 4QAM ve 16QAM modülasyonları ve ST ile SR arasındaki uzaklığın farklı değerleri için elde edilen BER başarım eğrileri önerilen protokolun toplamsal kodlamalı referans protokole üstünlüğünü ortaya koymaktadır. İkincil protokolde birincil kullanıcı, birer antenli PT ve PR den oluşurken ikincil kullanıcı için ST iki antenli olup SR bir antenlidir. ST bir önceki protokolden farklı olarak en büyük oran birleştirme (MRC) ve Alamouti uzay-zaman kodlama tekniklerinden yararlanmaktadır. Protokol yine üstüne serme tekniğine dayalı olup spektral verimliliği artırma, çeşitleme sağlama ve alıcılarda girişimi önleme amacıyla CIOD ve Alamouti uzay zaman kodlama teknikleri bir arada kullanılmaktadır. Protokol PU nun iki, SU nun bir işaretini üç zaman aralığında iletmektedir. Birinci zaman aralığında PT iki birincil işaretin yalnız gerçel ve sanal kısımlarını PR ve ST ye iletmekte, ST iki anteni yardımıyla MRC uygulayarak bu iki birincil işareti yalnız gerçel ve sanal bileşenlerinden çözmekte, ikinci zaman aralığında ilişkin sanal ve gerçel bileşenleri bir araya getirerek birinci anteninden PR ye iletmektedir. Kendi ikincil işaretini ise aynı anda ikinci anteninden SR ye iletmektedir. Üçüncü zaman aralığında ise ikinci zaman aralığında ilettiği işaretlere Alamouti uzay zaman blok kodlaması uygulayarak her iki anteninde iletmektedir. ST, PT den gelen işareti çözemezse ikinci ve üçüncü zaman aralıklarında Alamouti koduna uygun biçimde kendi işaretini iletmektedir. Protokol hem PR hem de SR de Alamouti kodlamanın getirdiği tek simge çözme özelliğiyle girişimi ortadan kaldırmakta ayrıca her iki kullanıcı için uzay çeşitlemesi sağlamaktadır. Tezde birincil kullanıcının bit hata olasılığı için kuramsal bir üst sınır türetilmekte, üst sınırın geçerliliği bilgisayar benzetim sonuçlarıyla desteklenmektedir. PT, ST ve PR arasındaki farklı uzaklık değerleri için sonuçlar irdelenmekte, protokolun sağladığı BER başarım kazancı referans protokolle karşılaştırılarak ortaya konmaktadır. Diğer yandan, ikincil kullanıcının BER başarımı klasik Alamouti kodununkine eşdeğer olmakta, ST ile SR arasındaki uzaklığın çeşitli değerleri ve 16QAM modülasyonu için irdelenmekte, doğrudan iletim durumu ve toplamsal kodlamanın kullanıldığı referans protokolle karşılaştırılarak üstünlüğü ortaya konmaktadır.Next generation wireless communications and mobile systems demand higher data rates with maximum bandwidth utilization. Over the past years, some techniques in the literature are provided in various ways where wireless systems should support high data rates. However, this requirement is faced with spectrum scarcity accommodation problem as in spectrum allocation chart where majority of the spectrum is under license. Nevertheless, spectrum measurements show that most of the spectrum is not continuously used based on the frequency, geographical location and time. As a result, this requirement necessitates the efficient use of the available resources by managing spectrum distribution to provide acceptable service quality. Therefore, several approaches in wireless systems are introduced in literature for sharing the available spectrum to overcome the spectrum scarcity problem. Cognitive Radio (CR) is considered as a potential solution to boost the spectrum utilization and solve the spectrum scarcity in which two different wireless systems could operate in the same frequency band with respect to their priority. CR is an innovative technique and developing area of research for the next wireless generation systems. In CR, to access the licensed bands interweave, underlay and overlay patterns can be used by the Secondary Users (SUs). In the interweave protocol, SUs sense the existence or nonexistence of Primary Users (PUs) to use spectrum holes without causing interference to PUs. In the underlay protocol, SUs are certified to use the licensed spectrum if the interference caused at PUs is below a predetermined interference level. In the overlay protocol, unlicensed users generally pay a price to share a licensed spectrum by cooperating for the benefit of spectrum owners. Different kinds of spectrum sharing model such as hybrid and superposition coding techniques are proposed in the literature. In superposition coding SU combines the PU's and its own signal and then broadcast to both Primary Receiver (PR) and Secondary Receiver (SR). However, this technique causes severe interference in both receivers. Two CR patterns can be combined to approach the hybrid mode where in this case the system has flexibility to operate in different protocols to obtain benefits from the spectrum and cooperate with PU. One diversity technique that could mitigate the fading channels effect on the transmitted signal is Signal Space Diversity (SSD) also known as modulation diversity. SSD provides performance improvement over fading channels without using extra bandwidth or power. In SSD, constellation points of the modulation are rotated by an angle before the transmission which maximize the minimum product distance of the rotated constellation, and after applying Coordinate Interleaved Orthogonal Design (CIOD) on the rotated constellation an irregular modulation points are acquired. Normally, in cooperative communications overall system spectral efficiency and rate are decreased due to the fact that data takes more than one time slot to be transmitted to the receiver. However, using SSD and CIOD technique in the system configuration, one can easily enhance the overall spectral efficiency and rate in cooperative systems without adding any complexity to the system. In this thesis, two different spectrum sharing protocols for cognitive radio operating in overlay mode are proposed. In the first protocol, the primary and secondary system is comprised of PT, PR, ST and SR equipped all with one antenna. SU is allowed to use the shared spectrum band in accordance to cooperate with primary system. We take advantages of the SSD and CIOD concepts in this three-phase overlay protocol to enhance the spectral efficiency, rate and diversity as well as to provide single symbol decoding. In the first transmission time slot, PT broadcasts its signal to PR and ST where ST tries to decode the transmitted signal from PT. If ST correctly decodes the PU's signal, ST forwards the coordinate interleaved signal pair to PR in the second time slot and PR tries to estimate the symbol by received signals from PT and ST. However, in the case that ST could not correctly decode the signal received from PT, PR decodes the symbol only from PT. Finally, SU's signal is transmitted to SR by ST at the third time slot. Due to the fact that a specific time slot is dedicated to SU data transmission, this protocol avoids interference. A critical distance between PT and ST is obtained such that as long as ST is located within that distance the outage probability of PU will be equal or lower than the case that of without spectrum sharing and ST will benefit from the band. The outage probabilities for PU and SU over Rayleigh fading channels are derived in a closed-form expressions and depicted for different target rates. Note that, the theoretic outcomes match perfectly with simulation results. It is shown that a significant performance improvement in the proposed scheme is notable both for PU and SU in comparison to the reference protocol where superposition coding is used at ST. Furthermore, an upper bound on the Bit Error Probability (BEP) of primary system is obtained and supported via simulation results. The simulation results agree well with the theoretical upper bound in the high Signal to Noise Ratio (SNR), respectively. Finally, for the secondary system, BEP performance is the same as classical direct transmission. The BEP performance for 16-QAM and 4-QPSK modulation for different values of the distance between ST and SR are depicted where in both cases simulation. The results confirm the efficiency of the proposed spectrum sharing compared to the reference protocol. In the second protocol, PU consists of a pair of PT and PR with one antenna and for SU, ST is equipped with two antennas to use the benefits of Maximum Ratio Combining (MRC) and Alamouti techniques and SR has only one antenna. A CR protocol which is configured on overlay mode to share the spectrum of the primary system is presented. SSD, CIOD and Alamouti concepts are used together to benefit from the single symbol decoding, diversity gain and to increase the overall system rate and spectral efficiency. The protocol comprises of three time slots to transmit PU's and SU's data. In the first time slot, PT transmits the primary signal to PR and to both antennas of ST. Then, ST uses the MRC technique to decode the transmitted signal. If ST correctly decodes the transmitted signal from PT, it applies Alamouti coding to transmit the primary and its own symbols to PR and SR in the second and third time slots. Nevertheless, if ST could not correctly decode the signal pair received from PT, PR will estimate the symbol from the direct link PT$\to$PR. Meanwhile, secondary signal is transmitted in the second and third time slots which provides diversity which enhances SU system performance. This protocol guarantees an interference-free communication for both users by considering the fact that PR and SR could extract their own signal based on the single symbol decoding technique provided by Alamouti coding. An upper bound for the BEP of the primary system is derived and it is shown that computer simulation results are in perfectly match with the theoretical upper bound in high SNR which validates the theoretical derivations. Results for different values of the distance between PT, ST and PR are obtained where in all cases, they validate the significant improvement in the Bit Error Rate (BER) performance for the proposed protocol. For the secondary system, BEP performance is the same as classical Alamouti code and is depicted for various values of the distance between ST and SR for classical 16-QAM and is compared with the straightforward transmission scheme and with the reference scheme where SU uses nearly all of its power to transmit the PU's signal and SU is in outage in most cases and operates a relay.Yüksek LisansM.Sc

Spatial diversity in MIMO communication systems with distributed or co-located antennas

The use of multiple antennas in wireless communication systems has gained much attention during the last decade. It was shown that such multiple-input multiple-output (MIMO) systems offer huge advantages over single-antenna systems. Typically, quite restrictive assumptions are made concerning the spacing of the individual antenna elements. On the one hand, it is typically assumed that the antenna elements at transmitter and receiver are co-located, i.e., they belong to some sort of antenna array. On the other hand, it is often assumed that the antenna spacings are sufficiently large, so as to justify the assumption of independent fading. In this thesis, the above assumptions are relaxed. In the first part, it is shown that MIMO systems with distributed antennas and MIMO systems with co-located antennas can be treated in a single, unifying framework. In the second part this fact is utilized, in order to develop appropriate transmit power allocation strategies for co-located and distributed MIMO systems. Finally, the third part focuses on specific synchronization problems that are of interest for distributed MIMO systems

UNDERWATER COMMUNICATIONS WITH ACOUSTIC STEGANOGRAPHY: RECOVERY ANALYSIS AND MODELING

In the modern warfare environment, communication is a cornerstone of combat competence. However, the increasing threat of communications-denied environments highlights the need for communications systems with low probability of intercept and detection. This is doubly true in the subsurface environment, where communications and sonar systems can reveal the tactical location of platforms and capabilities, subverting their covert mission set. A steganographic communication scheme that leverages existing technologies and unexpected data carriers is a feasible means of increasing assurance of communications, even in denied environments. This research works toward a covert communication system by determining and comparing novel symbol recovery schemes to extract data from a signal transmitted under a steganographic technique and interfered with by a simulated underwater acoustic channel. We apply techniques for reliably extracting imperceptible information from unremarkable acoustic events robust to the variability of the hostile operating environment. The system is evaluated based on performance metrics, such as transmission rate and bit error rate, and we show that our scheme is sufficient to conduct covert communications through acoustic transmissions, though we do not solve the problems of synchronization or equalization.Lieutenant, United States NavyApproved for public release. Distribution is unlimited

Joint signal detection and channel estimation in rank-deficient MIMO systems

L'évolution de la prospère famille des standards 802.11 a encouragé le développement des technologies appliquées aux réseaux locaux sans fil (WLANs). Pour faire face à la toujours croissante nécessité de rendre possible les communications à très haut débit, les systèmes à antennes multiples (MIMO) sont une solution viable. Ils ont l'avantage d'accroître le débit de transmission sans avoir recours à plus de puissance ou de largeur de bande. Cependant, l'industrie hésite encore à augmenter le nombre d'antennes des portables et des accésoires sans fil. De plus, à l'intérieur des bâtiments, la déficience de rang de la matrice de canal peut se produire dû à la nature de la dispersion des parcours de propagation, ce phénomène est aussi occasionné à l'extérieur par de longues distances de transmission. Ce projet est motivé par les raisons décrites antérieurement, il se veut un étude sur la viabilité des transcepteurs sans fil à large bande capables de régulariser la déficience de rang du canal sans fil. On vise le développement des techniques capables de séparer M signaux co-canal, même avec une seule antenne et à faire une estimation précise du canal. Les solutions décrites dans ce document cherchent à surmonter les difficultés posées par le medium aux transcepteurs sans fil à large bande. Le résultat de cette étude est un algorithme transcepteur approprié aux systèmes MIMO à rang déficient