This paper analyzes the performance of linearly precoded time division duplex based multi-user massive MIMO downlink system under joint impacts of channel non-reciprocity (NRC) and imperfect channel state information. We consider a generic and realistic NRC model that accounts for transceiver frequency-response as well as mutual coupling mismatches at both user equipment (UE) and base station (BS) sides. The analysis covers two most prominent forms of linear precoding schemes, namely, zero-forcing (ZF) and maximum-ratio transmission (MRT), and assumes that only the statistical properties of the beamformed channel are used at the UE side to decode the received signal. Under the approximation of i.i.d. Gaussian channels, closed-form analytical expressions are derived for the effective signal to interference and noise ratios (SINRs) and the corresponding capacity lower bounds. The expressions show that, in moderate to high SNR, the additional interference caused by imperfect NRC calibration can degrade the performance of both precoders significantly. Moreover, ZF is shown to be more sensitive to NRC than MRT. Numerical evaluations with practical NRC levels indicate that this performance loss in the spectral efficiency can be as high as 42% for ZF, whereas it is typically less than 13% for MRT. It is also shown that due to the NRC, the asymptotic large-antenna performance of both precoders saturate to an identical finite level. The derived analytical expressions provide useful tools and valuable technical insight, e.g., into calculating the NRC calibration requirements in BSs and UEs for any given specific performance targets in terms of effective SINR or the system capacity bound.

In this paper, we study the effects of effective channel non-reciprocity in coordinated TDD multi-cell MIMO network based on weighted sum rate (WSR) maximization. More specifically, we focus on UE transceiver non- reciprocity while the base stations (BS) are assumed to be perfectly calibrated, and both centralized and decentralized beamforming schemes are considered. In the centralized scheme, the cost function is constructed in a central controller using antenna specific UL pilots for channel estimation from all the connected BSs in the network. Then, even though the transceiver frequency response (FR) mismatches at the UE side corrupt the effective channel reciprocity, it is shown to have only a trivial impact on the WSR objective in such centralized case. However, when decentralized beamforming is deployed, the optimization is carried out in each BS and the corresponding cost function and optimization process depend on information acquired by over-the-air signaling between the BSs and all the users using precoded UL pilots. In this case, it is then shown that the transceiver FR mismatches at the UE side can cause severe performance degradation and even influence the convergence properties of the sum-rate optimization process. Further insight is provided for improving the performance by modifying the weight calculations in the optimization process and connecting users with good cell separations. Then a convergence-aware processing algorithm is also proposed to improve the performance of the decentralized scheme under UE transceiver non-reciprocity. Numerical experiments demonstrate that efficient processing algorithms for calibrating UE transceiver mismatches to be less than -30dB to -35dB are, in general, required in the decentralized system in order to achieve performance close to the ideal case without any RF imperfections.

Internet of Things (IoT) and Machine-to-Machine (M2M) applications are typically characterized by moderate investment costs to the M2M devices and infrastructure, in addition to the high reliability and energy efficiency requirements. The new Sub-1 GHz WiFi standard, namely the IEEE802.11ah, is being introduced to address these requirements deploying its recently specified MAC and PHY features and mechanisms. In this paper, we present an extensive analysis office 802.11ah network performance by means of realistic system level simulations. In particular, we focus on realistic performance evaluation and enhancement study of the IEEE 802.11ah network when multi-access points (multi-APs) with relatively high number of associated stations (STAs) are considered. The performance evaluation of the multi-AP IEEE 802.11ah network considers one of the main proposed MAC enhancement schemes for collision reduction, namely, the Restricted Access Window (RAW) mechanism. The analysis results confirm the importance of this novel mechanism to improve substantially the overall system performance from both network throughput and energy efficiency perspectives. Overall, the technical findings reported in this article strengthen the prospects of IEEE 802.11ah as one of the key enabling technologies for wide-scale low-cost and energy-efficient M2M deployments and IoT applications in the future.

In this paper, we propose an efficient transceiver non-reciprocity estimation-compensation framework for precoded TDD multi-user MIMO-OFDM downlink transmission systems. General signal models are first developed to analyze the effects of transceiver non-reciprocity at both the base-station and user equipment sides. The analysis shows that transceiver non-reciprocity at the base-station causes inter-user interference and thus substantial performance degradation, while the impact of transceiver non-reciprocity at the user equipment side can be fairly easily handled with downlink detector processing. Next, an over-the-air (OTA) type pilot-based estimation algorithm is devised for efficient identification of base-station non-reciprocity parameters, which are then used in pre-compensating or precoding the multiuser data properly in the base-station. Compared to the existing work in the literature, the proposed approach does not require the use of feedback signaling or complicated channel matrix decomposition techniques for extracting the non-reciprocity parameters. The resulting link and system performance of the proposed estimation-compensation framework is then evaluated using extensive computer simulations in linear precoded TDD multiuser MIMO-OFDM system context. Based on the obtained results, the proposed estimation-compensation approach can provide a simple, practical and flexible solution to efficiently restore the channel reciprocity with reasonable calibration overhead.

This paper studies the impact of two implementation imperfections, namely, transceiver frequency-response non-reciprocity and antenna mutual coupling mismatches at the base-station and user equipment on the channel reciprocity assumption in TDD systems. Comprehensive signal models are first developed to analyze the joint effects of these two imperfections in the TDD multi-user MIMO-OFDM downlink transmission system context, covering both zero-forcing (ZF) and eigenbeamforming based transmitter processing. The corresponding performance degradation is then evaluated in terms of SINRs and maximum achievable sum-rate. The analysis shows that during downlink transmission, the transceiver non-reciprocity and antenna mutual coupling mismatches at the base-station introduce inter-user interference (IUI) and are the major causes for the resulting performance degradation. Implementation imperfections at user equipment side, in turn, only introduce inter-stream interference (ISI) that can be fairly easily suppressed in detector processing as part of effective precoded channels. In order to achieve throughputs close to the ideal case, transceiver frequency-response mismatches and antenna mutual coupling at the base-station side need to be extremely well calibrated, generally below 35-40dB in terms of relative mismatch levels.

In this paper, we present a performance comparison between IEEE 802.15.4 which specifies the physical and media access control layers for low-rate wireless personal area networks (LR-WPANs) like ZigBee, and IEEE 802.11ah, a new global WLAN standard using sub-1 GHz frequency band, in terms of throughput and energy consumption. Both standards are targeting low power applications with relatively high number of nodes as in IoT and M2M applications. The simulations results demonstrate the better performance of IEEE 802.11ah throughput mainly in congested networks. However in particular cases, the IEEE 802.15.4 outperforms the IEEE 802.11ah from energy consumption point of view.