Fluctuating two-ray (FTR) channel model was shown to effectively characterize millimeter wave (mmWave) communication channels. In this article, we adopt FTR to investigate amplify-and-Forward (AF) mmWave relaying system. Two communications scenarios are considered corresponding to the presence and absence of a direct link between the transmitter and receiver. Outage probability and symbol error rate (SER) are then analytically obtained as performance metrics. The results are further compared with the corresponding metrics obtained based on conventional channel models including Nakagami-m and two-wave with diffuse power (TWDP). Especially, for the high-SNR regime, our analyses indicate that performance evaluations based on the conventional models significantly deviate from that of based on the FTR model. Our results provide quantitative insights on the importance of model selection in design and performance evaluations of relay-based mmWave systems. Moreover, for the high-SNR regime, we carry out asymptotic analysis and obtain a low-complexity expression for the achieved AF relaying gain. Such an expression provides a quantitative measure on whether or not AF relaying outperforms no-relaying in a given setting. Extensive numerical and simulation results are provided to confirm the accuracy of the analysis and investigate system performance in different settings.

Massive multiple‐input multiple‐output (MIMO) technology employs hundreds of antennas at the base station (BS) of a cellular system. Activating hundreds of antennas requires hundreds of radio‐frequency (RF) chains that lead to high cost and high energy consumption. Antenna selection techniques reduce the number of RF chains while curtailing the resulting performance loss. In this study, the authors consider massive‐MIMO downlink with zero‐forcing (ZF) precoding in a single cell. In ZF precoding with antenna selection, the number of users must always be less than or equal to the number of selected antennas, which may require adopting user selection (scheduling) algorithms. They propose a joint user and antenna selection algorithm to increase BS's energy efficiency (EE) and show that its performance is very close to that of the optimum exhaustive‐search based scheme. Using an analytical approximation for EE of the proposed scheme in asymptotically large numbers of users region, they find the optimum number of antennas to be selected.

Incremental relaying is a spectral-efficient relaying approach which introduces itself as one of the possible solutions to the spectrum scarcity problem for future generations of cellular communications systems. In this paper, we study threshold amplify-and-forward incremental relaying for correlated fading channels, i.e., fading channels for which the channel gain amplitude varies during the time interval of a block transmission; however, the variation occurs in a correlating fashion. From this aspect, these channels are different from both block-fading and fast-fading channels. Due to the correlating nature of the channels, it is possible to compress channel state information (CSI) sequences and employ the compressed CSI sequences as side information to carefully select the bits transmitted by the relay. This idea is already employed in previous works to increase the spectral efficiency; however, those works consider block-fading channels and also assume maximal ratio combining (MRC) at receiver. In case of MRC, explicit expressions for the end-to-end signal-to-noise ratio (SNR) exist, which enable packet error rate analysis by aid of Moment Generating Functions of the SNR. However, to apply MRC, the receiver requires all channel gains (including source-relay channel gain). In case of correlated channels, communicating all real-valued channel gains, requires a large overhead. This fact renders applicability of MRC for correlated channels. We apply the more practical Equal-Gain Combining method. We derive series approximations for symbol error rate, given M-QAM and MPSK modulations, and assuming Nakagami-m fading channels. We also analyze the spectral efficiency by assuming Markov models for the channels. We compare the spectral efficiency in case of correlated and fast-fading channels and show that, unlike fast-fading channels, correlated fading channels allow for a spectrally efficient scheme.

Energy efficiency (EE) is an important criterion in devising emerging mobile networks. One of the techniques that has been proposed to reduce energy consumption of base stations (BSs) in cellular networks is BS sleep mode. In this work, the effect of sleep mode on throughput, coverage and EE of relay assisted heterogeneous cellular networks is analysed. A cooperative relay‐BS sleep mode‐based algorithm for heterogeneous networks is proposed and its performance analytically evaluated. It is shown that the proposed algorithm has a considerably higher EE compared with a homogeneous network without sleep mode or a heterogeneous network with relay‐only sleep mode.

In many modern wireless systems that operate based on availability of channel state information at transmitter (CSIT), one of the main bottlenecks is updating CSIT of the time varying channel. In multiple‐input multiple‐output (MIMO) broadcast channels, CSIT is required for user scheduling and precoding. Most existing CSI feedback reduction techniques reduce the number of feedback terms by allowing only a limited number of users to feedback CSI, or allowing only partial/limited information of user channels to be send back to the transmitter. In this work, CSI is reduced over several time‐slots of transmission, and not in a single snapshot at the beginning of each time‐slot. A time‐varying MIMO broadcast channel with limited‐capacity feedback links is considered. An efficient threshold‐based feedback technique that maximises net‐throughput is proposed. The proposed scheme assigns different CSI feedback rates over several time‐slots to different groups of users proportional to their channel strength. Analysis and simulation results for zero‐forcing beamforming precoding show net‐throughput superiority of the proposed scheme over the case of perfect CSI.

Updating channel state information at the transmitter (CSIT) in time varying channels, is one of the main bottlenecks in wireless communication systems. In MIMO broadcast channels CSIT is required for user scheduling and precoder design. A time varying MIMO broadcast channel with limited-capacity feedback links is considered. An efficient threshold-based feedback technique based on allowing CSIT errors only for certain users is proposed. In the proposed scheme, the frequency of CSI feedback for different users is set to be proportional to their channel strength. Hence, CSI feedback is substantially reduced. Analysis and simulation results show net throughput superiority of the proposed scheme over the case of perfect CSIT. For large numbers of users, it is shown that how increase in the number of users can compensate the sum rate loss due to errors caused by partially outdated channel.

Spatially multiplexed multicarrier code-division multiplexing (SM-MC-CDM) is a multiple-input multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) communication technique with multiple antennas used for spatial multiplexing and with frequency domain spreading on each antenna. Unified successive interference canceller (U-SIC) is an efficient detector recently introduced for SM-MC-CDM. This paper presents an analytical approach to the performance of zero-forcing (ZF) U-SIC for SM-MC-CDM communications. For a system with an equal number of transmit and receive antennas, an approximation for the probability density function of post-detection signal-to-noise ratio (SNR) is used to derive a closed-form analytical upper bound and approximations for the probability of error and ergodic capacity. It is shown that SM-MC-CDM with ZF U-SIC is able to achieve higher diversity order than that achieved by ZF and minimum mean squared error (MMSE) V-BLAST detectors used on each subcarrier of a MIMO-OFDM system with the same number of subcarriers. The diversity order obtained increases with the number of subcarriers. It is also shown that the ergodic capacity of the system decreases with increasing number of subcarriers.

In multiple-input multiple-output (MIMO) broadcast channels, multiuser diversity is exploited by scheduling data transmission to users with best channel conditions. To find the best set of users, the base station requires knowledge of user channels, which for the case of non-reciprocal uplink and downlink channels may lead to a very heavy feedback overhead. In particular, well known transmission schemes for asymmetric MIMO downlink channels with more transmit than per user receive antennas, such as beamforming, require availability of full channel state information (CSI) at the transmitter. In this paper, two multiuser MIMO downlink techniques are presented, which only require partial CSI at the base station. The proposed schemes represent a combination of zero-forcing (ZF) receiver processing with MIMO point-to-point eigenmode transmission and transmit antenna selection. The results show that the scheme which is able to suppress inter-stream interference while maintaining maximum spatial multiplexing and high multiuser diversity gain, achieves higher system throughput.

In multiple-input multiple-output (MIMO) broadcast channels, multiuser diversity is exploited by scheduling data transmission to users with best channel conditions. To find the best set of users, the base station requires knowledge of user channels, which for the case of non-reciprocal uplink and downlink channels will lead to a great increase in feedback overhead. On the other hand, scheduling based on partial channel state information (CSI) often results in a great loss in system throughput. In this paper, a multiuser MIMO technique is presented for MIMO broadcast channels which only requires partial CSI at the base station and achieves a relatively high system throughput. The proposed scheme is a combination of MIMO point-to-point eigenmode transmission with zero-forcing (ZF) zero-forcingat the receivers. Spatial multiplexing is considered and the optimum number of data streams assigned to each of these two schemes in order to maximize the sum-rate is derived. The results show that with a negligible increase in feedback overhead compared to the case where only ZF linear receiver processing is adopted, the proposed scheme leads to a significant increase in the sum-rate.

Spatially multiplexed multi-carrier code division multiplexing (SM-MC-CDM) communications with turbo coding is considered over correlated and uncorrelated multiple input multiple output (MIMO) fading channels. Two link transmission strategies for SM-MC-CDM are proposed and their bit error rate performances are compared. The proposed system models combine successive interference cancellation and turbo decoding at the receiver end to mitigate the effect of interference and noise. By comparing the performance of the proposed systems with that of turbo coded MIMO-OFDM with VBLAST detector, we evaluate the effect of frequency domain spreading on the system performance.

We propose a sub-optimum detection method for spatially multiplexed multicarrier code division multiplexing (SM-MC-CDM) communications. Spreading the data in frequency domain results in additional diversity gain while using multiple antennas at the transmitter and the receiver may result in capacity gain due to spatial multiplexing and/or further diversity gain. To take advantage of the diversity and the multiplexing offered by SM-MC-CDM, low complexity and efficient detectors have to be designed. In this paper, the performance of our proposed detector is compared with a number of sub-optimum detectors using simulations over uncorrelated and correlated frequency selective MIMO channels. The results demonstrate that the new algorithm outperforms existing methods of comparable complexity.

The performance of a joint Rake and minimum mean square error (MMSE) equalizer receiver for high data rate ultra-wideband communications is studied in this paper. The proposed receiver combats intersymbol interference by taking advantage of the Rake and equalizer structure. The receiver performance is investigated using a semi-analytical approach and Monte-Carlo simulations. The effects of the number of Rake fingers and equalizer taps on the error performance are examined. For an MMSE equalizer at low to medium SNR, the number of Rake fingers is the dominant factor to improve system performance. while at high SNR the number of equalizer taps plays a more significant role in reducing error rates. The effect of channel estimation error on the performance of the Rake-MMSE-equalizer is also studied through simulations in this paper.