ITERATIVE DETECTION, DECODING, AND CHANNEL ESTIMATION IN MIMO-OFDM, ACTA UNIVERSITATIS OULUENSIS C Tec h n i c a 3 5 8

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

ISBN-10:
978-951-42-6219-7
Kieli:
eng.
Sivumäärä:
139 s.
Tekijät:
Ylioinas Jari
Tuotekoodi 013985
20,00 €