This article studies the cell-free massive multi-input multi-output (MIMO) two-way relay systems with low-resolution analog-to-digital converters (ADCs). Primarily, we analyze the normalized mean square error (NMSE) and derive a closed-form expression for NMSE’s expectation${\mathrm {Exp}}_{\mathrm {nmse}}$. Asymptotic analysis showcases that, with an infinite number of access point (AP) antennas M or ideal ADCs,${\mathrm {Exp}}_{\mathrm {nmse}}$converges to a finite value. Particularly, as pilot power decreases with M in a power law, the channel estimation performance can be maintained at a desired level. Secondly, we derive two closed-form expressions of spectral efficiencies (SE) for multiple-access channel (MAC) phase and broadcasting (BC) phases, respectively. The corresponding analysis implies that, as AP number$L\rightarrow \infty $, the SE tends to infinity with MAC phase and approaches to constant with BC phase. Interestingly, all SE expressions can reduce to the conventional cases with high-resolution ADCs. Finally, based on geometric programming, an effective power successive approximation (PSA) scheme is provided to maximize the sum SE. Results prove that the proposed PSA scheme can significantly improve the SE compared to baseline benchmarks, particularly for median AP antenna regime. All the derived closed-form expressions are validated through Monte Carlo simulations.

Channel measurement plays an important role in the emerging 5G‐enabled Internet of Things (IoT) networks, which reflects the channel quality and link reliability. In this paper, we address the channel measurement for link reliability evaluation in filter‐bank multicarrier with offset quadrature amplitude modulation‐ (FBMC/OQAM‐) based IoT network, which is considered as a promising technique for future wireless communications. However, resulting from the imaginary interference and the noise correlation among subcarriers in FBMC/OQAM, the existing frequency correlation method cannot be directly applied in the FBMC/OQAM‐based IoT network. In this study, the concept of the block repetition is applied in FBMC/OQAM. It is demonstrated that the noises among subcarriers are independent by the block repetition and linear combination, instead of correlated. On this basis, the classical frequency correlation method can be applied to achieve the channel measurement. Then, we also propose an advanced frequency correlation method to improve the accuracy of the channel measurement, by assuming channel frequency responses to be quasi‐invariant for several successive subcarriers. Simulations are conducted to validate the proposed schemes.

In this paper, an improved Tabu-search preamble assignment algorithm is proposed in cell-free massive multiple input multiple output (MIMO) to find the optimal preamble assignment solution with a low complexity. Different from the existing Tabu-search algorithm, the proposed one exploits the large-scale fading knowledge to refine the solution space so that only a small number of solutions that are more likely to mitigate preamble collision are explored. As a result, the complexity of Tabu search for preamble assignment can be significantly reduced. Simulation results demonstrate that the proposed algorithm is able to provide a good tradeoff between the performance and complexity compared to the state-of-the-art algorithms. In particular, it can achieve an over 50% complexity reduction with only a negligible performance loss in terms of the achievable sum rate compared to the existing Tabu-search algorithm.

In this article, we propose a novel waveform with low cyclic prefix (CP) overhead for narrow-band Internet-of-things (NB-IoT) systems. Compared with the classical orthogonal frequency-division multiplexing (OFDM), the proposed waveform employs the concept of symbol repetition on multiple successive OFDM symbols (SR-OFDMs) and only one CP is occupied by multiple SR-OFDM symbols; hence, the CP overhead is reduced significantly, improving the spectral and energy efficiencies in NB-IoT systems. It is proved that the proposed SR-OFDM can effectively fight against the multipath fading channels by a simple single-tap equalization, avoiding the mutual interference among symbols. In addition, it is shown that the proposed SR-OFDM and classical OFDM have the same size of the inverse discrete Fourier transform (IDFT) at the transmitter, which indicates the same subcarrier spacing. Especially, it is demonstrated that the signals of multiple symbols are separated at the receiver, and the demodulation of each symbol can be performed independently, which is helpful to reduce the complexity of symbol demodulation. To evaluate the proposed SR-OFDM, simulations have been done by considering the Stanford university interim (SUI) channels, which have large channel delay spread and exhibit high frequency selectivity.

In this paper, by considering the average mean squared error (AMSE) of channel estimation, we primarily obtain the closed-from expressions of the probability density function (PDF) and cumulative distribution function of AMSE for the least squares (LS)/minimum mean squared error (MMSE) estimation method as the line-of-sight (LOS) component is known, where the asymptotic analysis is executed in Rayleigh fading and strong LOS conditions. Secondly, the closed-form expressions for the expectation of AMSE ( Expamse) and variance of AMSE ( Varamse) are acquired, where Varamseis inversely proportional to the number of antennas ( M). As M becomes infinite, the PDF of AMSE at Expamsehas an order of root M. When the pilot power decreases with M in a power law, the LS case keeps deteriorating while the MMSE case converges to a constant which basically depends on the Rician K-factor. Next, the spectral efficiency is investigated by considering AMSE. When Expamseaccelerates, the spectral efficiency of the LS method keeps dropping and that of the MMSE method firstly is degraded and then is improved to a constant except Rayleigh fading. Finally, all results are validated via simulations.

In filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) systems, a large pilot overhead is required due to the existence of the imaginary interference. In this paper, we present an approach to reduce the pilot overhead of channel estimation. A part of pilot overhead is used for transmitting data, and compensating symbols are required and designed to remove the imaginary interference. It is worthwhile to point out that the power of compensating symbols can be helpful for data recovery; hence, the proposed approach decreases the overhead of pilots significantly without the cost of additional pilot energy. In addition, the proposed scheme is extended into multiple input multiple output systems without the performance loss. Compared with the conventional preamble consisting of 3 columns symbols, the pilot overhead is equivalent to 2 column symbols in the proposed preamble. To verify the proposed preamble, numerical simulations are carried out with respects to bit error ratio.

The massive Machine Type Communications (mMTC) is one of the three generic services to be supported by 5G wireless systems. To fulfill the ever-increasing network access demand from a large number of Machine Type Devices (MTDs), this paper develops a cluster-based group paging scheme. Specifically, with the proposed scheme, MTDs are divided into clusters and the group paging period is decomposed into two parts: intracluster access period and inter-cluster access period. In the intracluster access period, preamble reuse is adopted for facilitating the access request transmissions from MTDs in each cluster to its cluster head. In the inter-cluster access period, only cluster heads send access requests to the base station.To evaluate and optimize the access efficiency of the proposed scheme, the probability of successful access of each MTD is characterized, based on which the maximum probability of successful access and the corresponding optimal number of clusters and optimal length of the intra-cluster access period are obtained as explicit functions of key system parameters including the number of preambles and the number of MTDs. A comparative study of the access efficiency for group paging with clustering and without clustering is conducted, which reveals the critical threshold in terms of the number of MTDs, above which clustering is beneficial. The analysis is verified via extensive simulations. It is shown that the access performance of the proposed scheme significantly outperforms that of the traditional group paging scheme, especially in massive access scenarios.

Unmanned aerial vehicle (UAV) working as an aerial station can gather the instantaneous information to guarantee the low-latency communication for the smart grid network. In this paper, we firstly construct a practical model of the end-to-end delay with considering the bit-error-ratio (BER) requirement of the communication link, and optimize the UAV’s trajectory to minimize the end-to-end delay between the UAV and the smart grid terminals, in which the critical-mission terminals (CMTs) or non-critical-mission terminals (NCMTs) send the individual information to the flying UAV. Although this non-convex problem is difficult to solve, we propose a trajectory design scheme based on Q-learning. To reduce the delay of CMTs, we design the different reward function for CMTs and NCMTs. The promising advantage of proposed scheme is that some NCMTs closed to CMTs may obtain the priority service to reduce the waiting delay. Simulation results show that our proposed scheme obtains almost 17% performance gain comparing to the benchmark schemes.

In this article, we aim to reduce the pilot overhead of channel estimation in multiple input multiple output (MIMO) systems employing filter bank multicarrier (FBMC). The key idea is the employment of the concept of symbol repetition in frame structure. Firstly, we propose an MIMO-FBMC system based on block repetition, in which complex-valued symbols are transmitted instead of only real-valued symbols in classic MIMO-FBMC systems without block repetition. Then, we present two low-overhead channel estimation approaches in the block-repetition based MIMO-FBMC system and only two columns of symbols are required as the pilot overhead. However, the existence of the block repetition results in the increased sensitively to the channel coherence time. Then, the two channel estimation approaches are extended into classic MIMO-FBMC systems without block repetition. It is proved that, the proposed methods exhibit significant advantage in terms of pilot overhead, compared with the conventional interference approximation method (IAM) and frequency-division multiplexing (FDM) methods. To evaluate the performances of the proposed approaches, we also derive the Cramér-Rao Bound (CRB) bound, and carry out numerical summations with respects to both mean square error and bit error ratio metrics.

In this paper, we address differential non-coherence detections in uplink massive multiple input multiple output (MIMO) systems. Utilizing the channel statistics information, a special 16 quadrature amplitude modulation (QAM) can be adopted to achieve differential detection in uplink massive MIMO systems, which avoids costs of pilots or channel estimation. In addition, a higher-order extension of this differential 16-QAM is also presented in this paper. Moreover, it is found that users in different cells can transmit symbols simultaneously without causing interference to each other by jointly processing signals from all base stations if the number of receive antennas goes to infinity. For comparison, we also provide the performances of the differential amplitude phase shift keying (DAPSK) and coherence detection with pilot contamination in uplink massive MIMO systems, respectively. Simulation results show that the proposed differential 16-QAM scheme exhibits significant advantage over 16-DAPSK, and exhibits a large performance gain compared with the coherence detection with pilot contamination.

In this paper, we analyze the noise distribution and correlation characteristic on each subcarrier in OQAM-OFDM systems, and present a method based on the maximum likelihood (ML) criterion to improve the channel estimation. Then, the performance results are obtained by considering an IEEE 802.22 channel model and the proposed method is compared to the conventional frequency domain averaging (FDA) method. Simulation results show that the proposed method is an efficient method to improve the channel estimation.