Coded DS-CDMA Systems with Iterative Channel Estimation and no Pilot Symbols

In this paper, we describe direct-sequence code-division multiple-access (DS-CDMA) systems with quadriphase-shift keying in which channel estimation, coherent demodulation, and decoding are iteratively performed without the use of any training or pilot symbols. An expectation-maximization channel-estimation algorithm for the fading amplitude, phase, and the interference power spectral density (PSD) due to the combined interference and thermal noise is proposed for DS-CDMA systems with irregular repeat-accumulate codes. After initial estimates of the fading amplitude, phase, and interference PSD are obtained from the received symbols, subsequent values of these parameters are iteratively updated by using the soft feedback from the channel decoder. The updated estimates are combined with the received symbols and iteratively passed to the decoder. The elimination of pilot symbols simplifies the system design and allows either an enhanced information throughput, an improved bit error rate, or greater spectral efficiency. The interference-PSD estimation enables DS-CDMA systems to significantly suppress interference.Comment: To appear, IEEE Transactions on Wireless Communication

Joint Decision-Directed Channel and Noise-Variance Estimation for MIMO OFDM/SDMA Systems Based on Expectation-Conditional Maximization

A joint channel impulse response (CIR) and noise-variance estimation scheme is proposed for multiuser multiple-input–multiple-output (MIMO) orthogonal frequency-division multiplexing/space-division multiple access (OFDM/SDMA) systems, which is based on the expectation-conditional maximization (ECM) algorithm. Multiple users communicating over fading channels exhibiting a range of different characteristics are considered in this paper. Channel estimation becomes quite challenging in this scenario since an increased number of independent transmitter–receiver links having different statistical characteristics have to be simultaneously estimated for each subcarrier. To cope with this scenario, we design an ECM-based joint CIR and noise-variance estimator for multiuser MIMO OFDM/SDMA systems, which is capable of simultaneously estimating diverse CIRs and noise variance. Furthermore, we propose a forward error code (FEC)-aided decision-directed channel estimation scheme based on the ECM algorithm, which further improves the ECM algorithm by exploiting the error correction capability of an FEC decoder for iteratively exchanging information between the decoder and the ECM algorithm

Haberleşme sistemlerinde gözü kapalı turbo denkleştirme algoritmalarının geliştirilmesi

06.03.2018 tarihli ve 30352 sayılı Resmi Gazetede yayımlanan “Yükseköğretim Kanunu İle Bazı Kanun Ve Kanun Hükmünde Kararnamelerde Değişiklik Yapılması Hakkında Kanun” ile 18.06.2018 tarihli “Lisansüstü Tezlerin Elektronik Ortamda Toplanması, Düzenlenmesi ve Erişime Açılmasına İlişkin Yönerge” gereğince tam metin erişime açılmıştır.Bu çalışmada, tek-giriş tek-çıkış ve çok-giriş çok-çıkış haberleşme sistemlerinde gözü kapalı turbo denkleştirme problemi ele alınmıştır. DPSK ve MSK modülasyon kullanan SISO haberleşme sistemleri için üç farklı gözü kapalı turbo denkleştirici (alıcı) geliştirilmiştir. Geliştirilen gözü kapalı alıcılar, yumuşak bilgi kullanarak kanal parametrelerini kestirmektedirler. Kanal kod çözücüden gelen yumuşak bilginin kestirimde kullanılması, kanal parametrelerinin daha doğru kestirilmelerini sağlamaktadır. Ayrık-zamanlı FIR filtre olarak modellenen haberleşme kanallarının kanal katsayılarını kestirmek için bu tezde kullanılan algoritmalar: EM, LMS ve RLS algoritmalardır. SISO haberleşme sistemleri için geliştirmiş olduğumuz gözü kapalı turbo denkleştirme algoritmaları, MIMO haberleşme sistemlerine de uyarlanmıştır. Bu uyarlama sonucunda, MIMO sistemler için iki adet yarı gözü kapalı turbo denkleştirici (gözü kapalı alıcı) elde edilmiştir. İlkönce, matematiksel olarak gözü kapalı turbo denkleştirme algoritmaları çıkartılmış ve akabinde de bilgisayar programları bu çıkartılan algoritmalara göre yazılmışlardır. Geliştirilen gözü kapalı alıcılar birbirleri ile bit hata oranı cinsinden bilgisayar simülasyonları yoluyla karşılaştırılmışlardır. İlaveten, SISO ve MIMO haberleşme sistemleri için yapılan turbo denkleştirme (kanal parametrelerinin mükemmel bilindiği varsayımı altında) çalışmaları sonucunda elde edilen bit hata oranı eğrileri çizdirilmiştir. Geliştirilen gözü kapalı turbo denkleştiricilerin (alıcıların) bit hata oranı performansları çok etkileyicidir (Proakis B kanalı ve DPSK modülasyonu kullanan SISO haberleşme sistemleri için SNR > 8 dB olduğunda BER=0, her bir alt kanal hafıza uzunluğu L=1 olan frekans seçici Rayleigh sönümlemeli kanalı ve DPSK modülasyonu kullanan MIMO (2Tx, 2Rx) sistemler için ise SNR > 10 dB olduğunda BER=0 olmaktadır). SUMMARYIn this study, blind turbo equalization problem for single-input single-output and multiple-input multiple-output communication systems is investigated. Three different blind turbo equalizers (receiver) are developed for both DPSK and MSK modulation in the SISO communication systems. Developed blind receivers use soft channel estimation. Using of the soft information that comes from the channel decoder provides more correct channel estimation. Communication channels are modelled as discrete-time FIR filter and its coefficients are estimated with EM, LMS and RLS algorithms in this study. Developed blind turbo equalization algorithms for SISO systems are adapted to MIMO communication systems. This adaptation produces two semi blind turbo equalizers for MIMO systems. Firstly, blind turbo equalization algorithms are derived mathematically and then computer programming are done according to these algorithms. Developed blind receivers are compared with each other in terms of bit error rate performances via computer simulations. In addition, bit error rate curves of turbo equalization (channel parameters are perfectly known) for both SISO and MIMO systems are plotted. BER performances of developed blind turbo equalizers are very promising (simulation values of the developed receivers for SISO communication systems in which Proakis B channel and DPSK modulation are used: when SNR >8 dB, BER=0. Values of the developed receivers for MIMO (2Tx, 2Rx) communication systems where Rayleigh fading channel whose each subchannel has same memory length (L=1) and DPSK modulation are used: when SNR > 10 dB, BER=0)

Comparison of EM-based algorithms for MIMO channel estimation

Iterative channel estimation can improve the channel-state information (CSI) with respect to noniterative estimation. New iterative channel estimators based on the expectation-maximization (EM) algorithm are proposed in this paper. A first estimator, called the unbiased EM (UEM), is designed to unbias the EM estimates. A second estimator is then put forward, which is based on the expectation-conditional-maximization (ECM) algorithm, and its complexity is lower than that of the EM. An unbiased ECM (UECM) estimator is also proposed. Although the unbiasedness of the UEM and UECM estimators is not rigorously proved, the use of these names is explained in the paper. The new estimators are compared with well-known ones, such as the EM, the decision-directed (DD), and the data-aided (DA) estimators. Simulations are reported for a turbo receiver operating over frequency-selective multiple-input multiple-output channels. It is shown that the UEM channel estimator outperforms the EM, and that the ECM-based estimators are very close to the EM-based ones