We develop a novel differential spatial modulation (DSM) scheme for amplitude phase shift keying (APSK) modulation, which can either improve throughput or performance over DSM for PSK. Then we ...investigate the impact of time-varying fading on DSM. We find performance degrades if the fading is too fast due to differential detection. The impact of a long outer error control code (ECC) is also considered. Its performance is limited by the slowly varying channel required for differential detection. We consider using reconfigurable antennas to periodically change the channel conditions and hence significantly improve coded performance for DSM systems.
Millimeter-wave (mmWave) channel sparsity is widely exploited to reduce the complexity of compressed sensing and beamtracking algorithms for 5G, yet there are still many misconceptions about it: that ...it arises from pencilbeam antennas admitting just a few paths, that the channel in fact has just a few paths, and that the channel is always sparse. What is missing to confirm these conjectures is a measurement-based analysis, to our knowledge conducted by only two papers thus far, however, both at microwave (5 GHz), in a single environment, and based on just a few (<inline-formula><tex-math notation="LaTeX">\leq</tex-math></inline-formula>49) measurements. For conclusive analysis on mmWave sparsity, we have collected measurements with our high-resolution 3-D double-directional 60 GHz channel sounder in three indoor and two outdoor environments, comprising a total of 750 measurements while investigating both line-of-sight (LoS) and obstructed LoS (OLoS) conditions. We learned that in LoS, the channel is always sparse that sparsity drops notably from LoS to OLoS and that mmWave sparsity is inherent to the channel itself-not to pencilbeam antennas-since our sounder has omnidirectional field-of-view. We also propose a sparsity metric that accounts for channels with just a few paths, which we found to occur in some environments.
In the communication systems that use a linear modulation scheme for transmission, the fractionally-spaced (FS) samples of the received signal constitute a wide-sense cyclostationary time series. ...Hence, the standard Fourier transform techniques cannot be used to study the spectral characteristics of the received FS samples or to derive the transfer function (TF) of the corresponding digital minimum mean-square error (MMSE) receiver. In this paper, an analytical expression for the TF of the FS MMSE equalizer is derived, which includes the effects of the continuous-time to discrete-time (C/D) converter used at the receiver front end. Using this TF, the sources of instability of the FS least-mean-square (LMS) algorithm and the effects of the equalizer length and sampling phase on convergence of the LMS algorithm are explained. For stabilization of the FS LMS algorithm, conditions on the front-end C/D converter are provided, such that, when satisfied the LMS algorithm becomes more stable and the learning characteristics of the modified receiver are better than the leaky-FS LMS algorithm. Theoretical results are corroborated by simulations.
Fifth generation cellular networks must support dense user environments where close proximity users and highly correlated channels are prevalent. This study presents a novel decorrelating ...zero-forcing (DZF) scheme. It improves downlink zero-forcing (ZF) performance over multi-user multiple-input multiple-output channels when highly correlated users are present. DZF groups correlated users into pairs where the stronger user is labelled ‘near’ and the weaker user, ‘far’. For the far and unpaired users, the proposed design is similar to conventional ZF (CZF) in that the base station performs traditional ZF and the users employ the leading left singular vectors of the channel for processing. In contrast, the near users employ the second strongest left singular vector which decorrelates their effective channel with the paired far user. This reduces the noise inflation inherent in ZF for correlated channels. From simulations and analysis the authors demonstrate that DZF has both rate and fairness advantages over CZF when highly correlated users are present. Furthermore, they design a hybrid ZF (HZF) scheme, which harnesses the advantages of both CZF and DZF, hence providing robustness against the joint scheduling of semi-orthogonal and highly correlated users with very little additional complexity. Simulations clearly demonstrate that HZF has higher rates than CZF and DZF.
In this paper, we propose an iterative structure for channel estimation and data detection for multiple-input-multiple-output orthogonal frequency-division multiplexing systems (MIMO-OFDM) with ...insufficient cyclic prefix (CP) and a limited number of pilot subcarriers. Insufficient CP leads to increased intersymbol interference (ISI) and intercarrier interference (ICI). The interference corrupts pilot subcarriers used for channel estimation and affects the detection process. We propose a multistep channel estimation process, which is assisted by a small number of pilot subcarriers. First, the channel covariance matrix and the number of channel paths are acquired from the channel least square estimates at the pilot subcarriers. We then formulate a maximum likelihood process to approximate the time-domain channel. For data detection, a high-performance bidirectional M-algorithm (BDMA) is proposed for trellis-based equalization. Simulation results show that the channel estimate mean square error converges to the Cramer-Rao bound (CRB) after a few iterations. In addition, the achieved bit error rate (BER) can reach that of the sufficient CP case, even when the delay spread is much longer than the CP. 1 .
Pilot contamination occurs when cells simultaneously transmit the same pilot sequences, creating interference. Unsynchronizing the pilots can reduce pilot contamination, but it can produce data to ...pilot interference. In this paper, we investigate the impact of pilot contamination and other interference, namely data to pilot interference, on the performance of finite massive MIMO systems with synchronized and unsynchronized pilots. Two unsynchronized pilot schemes are considered. The first is based on an existing time-shifted pilot scheme, where pilots overlap with downlink data from nearby cells. The second time-shifted method overlaps pilots with uplink data from nearby cells. Results show that if there are small numbers of users, the first time-shifted method provides the best sum rate performance. However, for higher numbers of users, the second time-shifted method has advantage compared to other methods. We also show that time-synchronized pilot is not necessarily the worst case scenario in term of sum rate performance when shadowing effect are considered.
Two-hop relay networks for critical industries require high reliability within a limited spectrum. The lower hop between densely populated user equipment (UE) and the relay nodes (RNs) is assumed to ...have low spectral efficiency due to the inexpensive low-powered UEs equipped with omnidirectional antennas in non-line-of-sight (NLOS) conditions. Thus, spectrally efficient frequency reuse (FR) in the upper hop between RNs and base stations (BSs) is pivotal in enhancing the two-hop relay network's uplink coverage and capacity. To achieve this objective, we propose the deployment of higher order sectorisation (HOS) at BS and directional antennas (DAs) at RNs in the upper hop and propose rotational (RFR), double (DFR), and triple (TFR) frequency reuse schemes. These schemes allow FR by exploiting the orientation of the DAs at RNs and are compared with conventional distance-based FR. Furthermore, an optimisation method is formulated to partition the limited bandwidth between the two hops to increase the average end-to-end uplink capacity while using the proposed FR schemes on the upper hop. When evaluating system performance with HOS at BSs (upper hop) and RNs (lower hop) in irregular cellular layout, the statistical evaluation of uplink interference becomes computationally expensive. To tackle this issue, we developed a semi-analytical model to efficiently compute the statistical distribution of uplink interference with reduced time and complexity. It is shown that this model is an accurate tool for performance assessments, which also validate the end-to-end two-hop capacity improvements up to 73% and 110% using the proposed DFR and TFR schemes, respectively.
Ultrawideband (UWB) communication channels cause pulse distortion and result in many multipath components arriving at the receiver in clusters. In this paper, the effects of lossy dielectric walls on ...UWB pulses are analyzed. The frequency-dependent parameters of commonly used building materials are used to analyze the effects of multiple reflections and transmissions, material distortion, and interpulse interference on the UWB pulse waveform. The possibility of polarization-dependent distortion is discussed. Various thicknesses of walls and angles of incidence are considered. The distortion due to each effect is quantified in terms of maximum correlation coefficients (MCCs). The overall effect of the wall is modeled as a tapped-delay-line filter based on the MCC. The results are used to specify the multicluster structure of UWB channel models for indoor communication.
Non-orthogonal multiple access (NOMA) in the power domain serves multiple users at the same time and frequency slot. The significance of NOMA is its ability to increase the sum rate of the system and ...serve more users relative to orthogonal techniques. In this paper, we look at the high rate NOMA implementations where a single beamforming vector is shared by multiple users in the same cluster in a NOMA zero-forcing beamforming (NOMA-ZFBF) system. In order to maintain user fairness, it is important to guarantee the quality of service (QoS) of all clustered users. First, considering a target signal to interference plus noise ratio (SINR) for the weak user, i.e., guaranteeing the weak user's QoS (W-QoS), we propose a simple W-QoS-based NOMA (W-QoS NOMA) algorithm for NOMA cluster formation and power allocation that always exceeds the time division multiple access rates. Second, the correlation between the users plays a vital role during NOMA cluster formation in NOMA-ZFBF system. Therefore, we integrate the proposed W-QoS with partially correlated user clustering algorithms in order to analyze the effect of using the W-QoS technique. The numerical results confirm that the integration of W-QoS with partially correlated user clustering algorithms improves the sum capacity of the system. Third, considering an arbitrary correlation between users, we derive the signal to noise ratio (SNR)/SINR expressions of strong and weak users. These expressions show the importance of correlation factor which has a higher impact on the power penalty of the strong user, which in turn affects the clustering rate. Finally, we devise a mathematical technique using a perfectly correlated channel for the strong and weak users in each NOMA cluster to derive the exact SNR/SINR distributions of singletons, strong, and weak users. This gives a closed form expression for the expected sum rate, which gives an upper bound on performance. Furthermore, we investigate how close that realistic partially correlated user clustering can get to this bound.
Reconfigurable antenna arrays provide a means for efficient use of the spatial domain in wireless communication systems. Despite its potential, the topic is only briefly explored in the literature. ...In this paper, we present a comprehensive theoretical analysis of the performance of reconfigurable systems. We consider a receiver equipped with multiple reconfigurable antennas that pick the best state based on the channel between the transmitter and the receiver. For such a system, we derive a new expression for the moment generating function (MGF) of the received signal-to-noise ratio by employing maximal ratio combining. Based on the MGF, we analyze three important performance measures, specifically, achievable rate, error probability, and outage probability. Furthermore, we conduct an asymptotic analysis incorporating the correlation between reconfigurable states and show that a reconfigurable system can achieve a diversity order of the number of antennas times the number of reconfigurable states. Finally, we discuss the applicability of reconfigurable antennas in novel wireless networks with large antenna arrays and distributed antenna systems, highlighting the performance gains and requirement for fewer RF chains.
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