Coded-OFDM based Massive MIMO communications

Abstract

This thesis aims to analyze the performance of the maximal-ratio combining (MRC) and zero-forcing (ZF) detections in the orthogonal frequency-division multiplexing (OFDM) based massive multiple-input, multiple-output (MMIMO) system over the frequencyselective Rayleigh fading channel. Binary and quadrature phase shift keying (BPSK, QPSK), and M-ary quadrature amplitude modulation (QAM) schemes are focused by this research work. The mathematical expression of the co-channel interference (CCI) and effective noise (EN) for MRC detection is derived, and is chosen to analyze the probability distribution function (PDF) of the CCI and EN, bit error probability (BEP), PDF of signal-to-interference-plus-noise ratio (SINR), and outage probability of the output SINR. Furthermore, by assuming that the diagonal components of the Gram matrix approaches a constant, this thesis proves that the PDF of the CCI and EN for BPSK and QPSK modulation tends to the Gaussian distribution, whereas the PDF for M-QAM approaches the combination of scaled Gaussian distribution, i.e., the Gaussian mixture model (GMM). Thus, the analysis for the performance metrics is then simplified by utilizing the approximate PDFs. Monte-Carlo simulation results confirm that the outcome from the derived equation and the approximation closely matched those obtained by simulation. Moreover, there are deviations between the Gaussian distribution and the exact PDF of the CCI and EN, especially in a higher Eb/N0 region. Thus, an enhanced soft-output MRC detection is proposed for the coded-OFDM-MMIMO system. The detector utilizes the derived PDF of CCI and EN instead of the conventional Gaussian distribution to produce soft-information in forms of the log-likelihood ratio (LLR) for iterative decoders. Newton’s method is additionally chosen to simplify the computation for the LLRs. According to the numerical results, the proposed detector provided a better bit error rate (BER) performance than that of the classical, low-complexity, soft-output detections, especially at a higher Eb/N0 region. Likewise, the performance metrics for the OFDM-MMIMO system, utilizing ZF detection, are derived in this thesis. The PDF of the EN and BEP for the detection were previously derived in a research work, thus, this thesis improves the accuracy for the derived equations by increasing the Neumann series expansion (NSE) to second order. The PDF of signal-to-noise ratio (SNR) and the outage probability of output SNR are then derived by employing the derived PDF of the EN. Furthermore, an asymptotic closedform expression for the EN PDF, in forms of the Gaussian distribution, and the noise variance are derived in this thesis for simplifying the performance analysis. In addition, the derived noise variance for ZF detection is chosen to estimate the LLR instead of the approximate noise variance for the low-complexity soft-output ZF detection. The requirement for real arithmetic operators of the proposed detection is thus significantly reduced, whereas its BER is slightly lower than that of the classical detection. Focusing on a 10 × 200 coded-OFDM-MMIMO system, 97.81% of multiplications, required for producing the LLR from the estimated symbol, were minimized by utilizing the proposed detection. Therefore, the derived equations can be efficiently utilized for reducing the computational complexity of the soft-output ZF detection.