When the requirements of internet of things (IoT) and machine-type communication (MTC) are considered, the 5G technology needs to support high spectral efficiency and massive connectivity of users/devices, and the demand for low latency, short packet duration, low power, high mobility and diverse service types. In this paper, first a new multiple access technique belonging to the class of non-orthogonal multiple access (NOMA) based on multi-code signaling (MCS) is proposed for the uplink data transmission mode in severe multi-path fading channels in order to handle the key challenges of 5G technology. This new technique combines the index modulation in code domain with channel division multiple access (ChDMA) on which a novel receiver architecture is structured at significantly reduced complexity. Second, an efficient message passing algorithm (MPA), which realizes iterative maximum a posteriori probability (MAP) detection with linear complexity in the number of interfering users, is proposed. The novel feature of the proposed MPA stems from the use of Ungerboeck factorization, where the whitening filter is avoided before MAP detection. The joint task of equalization and multi-user detection (MUD) is fulfilled via the introduced Ungerboeck type sum-product algorithm (SPA) framework. This architecture for general NOMA transmission, operating on the resultant Ungerboeck type factor-graph (FG) with bidirectional feedback, achieves dramatic complexity reduction (compared to conventional FG based algorithms).

In this paper, we propose a generic receiver based on optimum maximum likelihood sequence estimation (MLSE) employing a generalized Ungerboeck metric with significantly reduced complexity for the general class of M-ary quasi orthogonal signaling (MOS) in severe multipath fading channels. First, by using the factor graph (FG) of the obtained generalized Ungerboeck metric for MOS and sum-product algorithm (SPA) framework, a highly efficient reduced state sequence estimation (RSSE) algorithm is proposed that operates on forward and backward directions and is directly applied to unwhitened channel matched filter (CMF) and code matched filter outputs at symbol rate. The proposed structure, generating the a posteriori probabilities (APP) by bidirectional RSSE recursions, are substantiated with symbol rate bidirectional decision feedback (BDF) based on surviving paths in order to eliminate the post- and pre-cursor inter-symbol interference (ISI) as well as multi code interference (MCI) due to the non-ideal properties of the signaling waveforms and multipath channel. It appears as the unification of Ungerboeck type reduced trellis equalization framework for MOS and achieves near-optimum performance especially for channels with large dispersion. Second, we identify a bias term through an error probability analysis which helps us improve the proposed receiver's performance. Furthermore, a tight approximation for the bit error rate (BER) of the proposed receiver is derived. These analyses lead to significant insight on the selection of system parameters and clearly demonstrate the high performance and efficiency of the proposed scheme by a proper choice of the signaling parameters.

This paper presents a simplified maximum a posteriori (MAP) estimator, which yields channel taps in OFDM systems under fading conditions using a parametric correlation model, assuming that the channel is frequency selective, slowly time varying and Gaussian. Expressions for the variance of estimation error are derived to evaluate the performance of the MAP estimator. The relation between the correlation of subchannels taps and error variance and the effect of signal to noise ratio (SNR) are investigated. The performance of the simplified MAP estimator is analyzed using measured channel data. Approximations are developed in order to make the MAP estimator practical. The simulations show that the proposed estimator operates satisfactorily.

A study on finding new pulse shapes for binary CPM (Continuous Phase Modulation) signals to achieve better spectral and detection performance is carried out. Under the constraint that the power spectral density stays below the GSM spectral envelope, pulse shape is optimized to minimize the error probability. Polynomials are utilized in representing pulse shapes. The performance depends on the degree of the polynomials and the pulse length (L). With 8thdegree polynomials having 4 free parameters, optimization is performed for the case L = 3. The performance of the CPM signal, having the pulse shape found by optimization, is compared with that of GMSK, currently being used as the GSM modulation scheme, and a gain of 1.1 dB is obtained with the optimum pulse shape (L = 3). Increasing pulse length, much better results are attained. With L = 7, the gain goes up to 2.3 dB with a considerable decrease in the sidelobes.

In this paper, subcarrier synchronization techniques for OFDM systems are studied. These techniques are evaluated for frequency dispersive, frequency nondispersive, time varying, and time invariant channels. DVB-T parameters are taken as reference. It has been observed that one needs an effective subcarrier synchronization measure in order to combat degrading effects of dispersive, time varying channels. Because of this reason, we propose new receiver structures to solve synchronization problems for such channels. In the proposed receiver structure, PLL trackers for subcarriers synchronization are used. The PLL's are operated by using an expectation maximization (EM) based phase offset estimation algorithm. According to the performance evaluations made by computer simulations, the proposed receivers improve the system performance on a large scale.

In this study, we investigate the performance of adaptively loaded OFDM (orthogonal frequency division multiplexing) under Rayleigh fading with ideal maximal ratio diversity combining (MRC). We consider two approaches for adaptive loading: maximization of throughput subject to total transmit power and subcarrier error rate constraints and minimization of subcarrier error rate subject to total transmit power and target bit rate constraints. We demonstrate the performance improvement of adaptive loading over fixed loading in both cases. A tight lower bound on the average spectral efficiency of the throughput maximizing adaptive loading method under Rayleigh fading with diversity combining is obtained. For the error rate minimization approach, an optimum integer bit loading algorithm is presented which is computationally simpler than the optimum Hughes-Hartogs algorithm.

An acquisition method for direct-sequence spread spectrum signals is proposed. This method is a combination of an N-tap matched filter and a long M*N-tap matched filter, where N and M are the length of the spreading pseudo-noise sequence, and the length of the header sequence, respectively. Differential phase-shift keying modulation is used. The analytical performance analysis of the method is derived in terms of detection and false alarm probabilities. The performance is evaluated at various fading conditions with additive white Gaussian noise. An equal tap finite impulse response filter is used to increase the performance by combining multipath components of the channel.

A new algorithm for multiple emitter direction finding by using a single antenna moving along a circular trajectory is proposed. The problem is formulated by taking the Doppler frequency shift, caused by the movement of the antenna, into account, and by assuming that the information, hidden in the incoming signals, does not change in the observation duration. The proposed direction finding algorithm is, therefore, based on single snapshot processing and also on the linear prediction method developed by Tuncay Koc (see Ph.D. Dissertation, Middle East Technical University, Ankara, Turkey, 1996) and by Tuncay Koc and Tanik (see Signal Processing, vol.56, no.1, p.17-31, 1997). It has no restriction on the correlation of the incoming signals since it performs a single snapshot processing. The algorithm performance is investigated through computer simulations and it is observed that the root-mean-square errors of the direction of arrival estimates are less than one degree in most cases. Therefore, it seems reasonable to use the proposed direction finding algorithm in some particular applications, for instance, in identifying the environment for calibration purposes.

Code-hopping, which is an alternative spreading strategy to the static spreading code assignment of CDMA systems, is evaluated as a method to combat multiuser interference in S-CDMA systems. The uplink capacities of conventional and code-hopping S-CDMA are determined and compared by simulations carried out on a typical S-CDMA system for three nondispersive channels: no-fading, Rayleigh fading and Rician fading. The cellular structure is not composed of regular hexagons, but irregularly shaped natural cells where affiliation of the mobile user depends on the paths to the neighboring base stations. Results are presented for two different values of blocking ratio which is defined as the ratio of the number of rejected call attempts to the number of total call attempts, revealing significant uplink capacity increases provided by code-hopping in both fading and no-fading environments. Furthermore, it is observed that the increase in capacity is greater in fading channels than in no-fading ones.

The performance of the MUSIC (Multiple Signal Classification) algorithm in determining the bearing of a single emitter from the signals of an array of sensors is investigated. The causes and cures of ambiguities in bearing estimation, due to array response duplications, are addressed first. The statistical properties of unambiguous bearing estimates are then considered. In particular, expressions for the first two moments of the error in these estimates are presented. These expressions, being more general than those that have appeared in the literature, are not limited to a line array of isotropic elements; hence they include the effects of antenna pattern and array geometry on estimation accuracy. The numerical examples verify the analytical moment expressions.>

The theory for a computationally efficient, frequency domain adaptive line enhancer (ALE) structure, which uses the N-point DFT (discrete Fourier transform) for N-point input data blocks, is extended. It is shown that unless the input block covers an integer number of cycles, the SNR (signal/noise ratio) gain of this structure is lower than that of a time-domain ALE with the same number of taps. Analytical results for an ALE with a sinusoidal input indicate that independent weight updating in the discrete frequency domain is far from being optimum, unless the frequencies coincide, which is extremely improbable. The optimum solution is shown to be a matrix transformation on the input. The authors also propose a suboptimum structure that improves the performance over the independent-weight-updating scheme.>

Several non-minimum-bandwidth partial-response-signaling (PRS) techniques were compared in the above paper (ibid., vol.COM-35, p.475-80, April 1987). One of the criteria used for the comparison was the robustness to the timing jitter. The commenter shows that according to this criterion the duobinary signaling technique is superior to the dicode scheme at low values of the roll-off factor, in contradiction to the conclusion in the original study. In reply the author agrees that the comment is correct and that unnormalized and normalized figure-of-merit plots show different aspects of timing jitter sensitivity as a function of excess bandwidth.>

The noniterative method of adaptive channel equalization proposed by Butler and Cantoni [1] is investigated. Its start-up time is improved remarkably by choosing the period of the training sequence judiciously and performing the estimation of channel parameters exactly over one period. The resulting version of the algorithm is as fast as the fastest known iterative algorithms, such as Godard's Kaiman algorithm [2].