Orod Raeesi

Performance evaluation of IEEE 802.11ah and its restricted access window mechanism

In this paper we provide an analytical model to compute the throughput and energy consumption of IEEE 802.11ah, the new Sub-1 GHz WiFi standard. The analytical model assumes known collision and error probabilities and applies to both basic and RTS/CTS access mechanisms. Comparison with simulation results shows that the model is extremely accurate in predicting the system throughput and energy consumption. We also investigate the IEEE 802.11ah system including the new restricted access window (RAW) mechanism, and compare it to the basic scheme. The obtained results show that the RAW mechanism can provide substantial improvements in the system performance, in terms of throughput, packet delay and energy consumption, in particular in highly-loaded dense network scenarios. These findings affirm and substantiate 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.

Performance comparison between slotted IEEE 802.15.4 and IEEE 802.1 lah in IoT based applications

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.

Performance Enhancement and Evaluation of IEEE 802.11ah Multi-Access Point Network Using Restricted Access Window Mechanism

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.

Analysis of Channel Non-Reciprocity Due to Transceiver and Antenna Coupling Mismatches in TDD Precoded Multi-User MIMO-OFDM Downlink

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.

Closed-form analysis of channel non-reciprocity due to transceiver and antenna coupling mismatches in multi-user massive MIMO network

In this paper, we analyze the impact of channel non-reciprocity due to two implementation imperfections, namely, transceiver frequency-response and antenna mutual coupling mismatches at the base-station side in multi-user massive MIMO system context Signal models are first developed to characterize the joint effects of these two imperfections in precoded multi-user downlink transmission, covering both zero-forcing (ZF) and maximum ratio transmission (MRT) based transmitter processing. Then closed-form expressions for evaluating the resulting performance degradation are derived in terms of effective SENR at terminal receiver input as well as lower bound of maximum achievable sum-rate in the system. Based on the derived results, it is also possible to directly evaluate non-reciprocity calibration requirements at the BS with given performance targets, e.g., desired effective SENR for each user or the overall system throughput, at given SNR level. The analysis also shows that the ZF precoding can be sensitive to the channel non-reciprocity problem even in the case that large amounts of antennas are deployed while the impact of implementation imperfections on MRT precoded system is less severe.

Impact of Power Amplifier Nonlinearities in Multi-User Massive MIMO Downlink

In this paper, we investigate the impact of power amplifier (PA) nonlinear distortion in pre-coded multi-user large antenna or massive MIMO downlink systems. First, detailed signal and system models are derived for the received signal at single-antenna user equipment (UE) under channel-aware linear precoding in the base-station combined with behavioral models for the individual PA units, covering both single-carrier and multi-carrier modulation schemes. Based on the derived models, it is shown that the PA induced nonlinear distortion can also combine coherently in the channel, depending on the relative differences between the phase characteristics of the different PA units and the corresponding distortion terms. Furthermore, it is also shown that the impact of nonlinear PAs and the resulting linear and nonlinear multi-user interference, quantified in terms of the received signal-to- interference-plus-noise ratio (SINR), is largely dependent on the effective or observable linear gain in the UE receiver demodulation stage. By observing only the instantaneous direct linear gain, the PA induced nonlinear distortion has a substantial impact on the effective SINR, even if very large number of TX antennas is adopted relative to the number of spatially multiplexed UEs. On the other hand, if the statistically averaged linear gain can be observed, the impact of nonlinear PAs is far less severe. These findings give thus new insight, not only to the core impact of nonlinear PAs in massive MIMO systems but also to the downlink reference signal design, radio frame design and radio resource management in time, in order to facilitate the estimation of the statistically averaged linear gains in the receivers within the scheduled transmission and processing blocks.

Efficient Estimation and Compensation of Transceiver Non-Reciprocity in Precoded TDD Multi-User MIMO-OFDM Systems

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.

On the Effects of UE Transceiver Non-Reciprocity in Coordinated TDD Multi-Cell MIMO Network

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.