Mehran Behjati

Energy Efficient and High Capacity Tradeoff in Distributed Antenna System for a Green Cellular Network

Two main concerns for designing a wireless system are more network capacity and less energy consumption. Recently, distributed antenna system (DAS) has received considerable attention due to its potential to provide higher spectral efficiency (SE) and uniform coverage for cellular networks. In this regard, this paper compares the performance of DAS with centralized antenna system (CAS) in LTE-A system in terms of energy efficiency (EE), where practical restrictions such as out-of-cell interference, path loss, and small scale fading are taken into account. Furthermore, the EE and system power consumption are investigated under three different cell-load scenarios (high, moderate, and low load) where different numbers of antennas are activated and remaining of antennas are under sleep mode. Finally, based on the tradeoff between power-saving and EE, two optimal DAS antenna deployments are proposed for low and moderate cell-load scenarios. The results reveal that DAS considerably outperforms CAS in terms of EE and by optimal deploying antennas of DAS significant power-saving and EE are achievable. The proposed methodology achieved savings of up to 27.63% in terms of energy savings in a macrocell with guarantee of a high capacity of data.

Limited Feedback MU-MIMO in LTE-A System: Concepts, Performance, and Future Works

AbstractIn wireless communications, interference is known as a major restricting factor in achieving system capacity. Therefore, in the Long-Term evolution (LTE) system, a transmitter and a receiver collaborate together to tackle the interference. To do so, in the frequency division duplex system, information of downlink channel must be shared among the transceivers via a limited feedback technique. In this regard, this paper discusses the downlink transmission of multi-user multiple-input multiple-output with limited feedback in the LTE-Advanced system. Extensive studies are conducted on all steps of downlink transmissions and receptions which include techniques such as receive antenna combining, channel state information (CSI) acquisition, CSI quantization, precoding, and user scheduling. By utilizing a LTE-A standard compliant simulator, the link-level performance of the system is evaluated under different receive antenna combiners and quantization codebooks. Moreover, the main system constraints such as channel delay and imperfect CSI are applied to the system which results show that the system performance severely degrades under imperfections, especially when the number of transmit antennas increases. The results mainly reveal that to achieve the capacity of LTE-A, with the presence of interference, delay, and limited feedback, more sophisticated methods are required. Hence, this study precisely addresses the future works on CSI quantization, precoding, and user scheduling.

Performance of limited feedback MU-MIMO in distributed antenna systems with different schedulers

Distributed antenna system (DAS) acts as an effective solution to mitigating interference and path-loss by decreasing the access distance for users, and increases the cell coverage and system capacity as well. This study evaluates performance of scheduling methods on the DAS where practical system constraints are considered, such as path-loss, out-of-cell interference and limited feedback. Zero-forcing multiuser MIMO precoding is utilized as downlink transmission strategy, when imperfect channel state information is available at the transmitter. System performance is evaluated by empirical cumulative density functions of the cell throughput, where a long-term evolution advanced (LTE-A) standard compliant simulator is utilized for simulation. It is demonstrated that by spreading the transmit antennas throughout the cell more cell throughput is achievable. Moreover, by utilizing an appropriate scheduling algorithm, more potential of DAS can be extracted and leads to substantial cell throughput.

Compensation of energy-efficiency degradation due to antenna-muting in distributed antenna systems for a green cellular network

The main criteria for designing the upcoming cellular systems are: More network capacity and less energy consumption. Recently, distributed antenna system (DAS) has received considerable attention due to its potential to provide higher spectral efficiency and uniform coverage for cellular networks. In this regard, this article compares the performance of DAS with centralized antenna system (CAS) in LTE-Advanced system in term of cell throughput and energy efficiency (EE), where practical restrictions such as out-of-cell interference, path-loss, and small-scale fading are taken into account. Furthermore, the EE degradation is investigated under two different sleep-mode scenarios (moderate and low loaded networks) where different number of antennas are activated. Finally, we investigate the impact of channel bandwidth enhancement on the compensation of EE degradation due to the antenna muting during the sleep modes. The results reveal that DAS considerably outperforms CAS in term of EE (up to 56.6%). Furthermore, the simulation results show that during the sleep modes, by optimally assigning channel bandwidth, the EE-loss, not only can be compensated, but also EE can be significantly improved, where the proposed methodology guarantees a considerable power-saving for the cellular network (up to 27.63%).

Investigation of pilot training effect in massive-MIMO TDD system

Massive-MIMO system is widely addressed by the literature as one of the main contributors for the next generation of the cellular network (5G). In the Massive-MIMO system the number of transmit antennas is much larger than the number of served users. Hence, it promises higher spectral efficiency by improving the rank of the overall channel matrix and localizing the interference. To mitigate interference (especially between adjacent sub-sectors), the accuracy of estimated channel is highly important. While, channel estimation in Massive-MIMO systems will cause the technical challenge of pilot contamination, due to the large number of antennas. In this regard, this paper addresses the effect of pilot training during channel estimation in downlink and uplink transmission of TDD Massive-MIMO system. The results present the performance of channel estimation in term of mean square error under different channel estimation techniques, various number of transmit antennas, and different length of pilot training. Moreover, results are compared under two different scenarios; without and with interference (pilot contamination). The results highlight the importance of pilot contamination avoidance in the Massive-MIMO system.

Wiener-based smoother and predictor for massive-MIMO downlink system under pilot contamination

One of the challenges in massive-MIMO system is pilot contamination during the channel estimation process. Pilot contamination can cause error or inaccurate channel estimation process for future fifth generation (5G) downlink transmissions. This paper considers using a Wiener-based filter to smooth and predict the channel estimation to reduce the pilot contamination for more accurate CSI during channel estimation. The simulation results show that the Wiener-based smoothing and predicting technique reduces the effect of pilot contamination and increases the accuracy of CSI during channel estimation process. Wiener smoother (WS) is implemented based on Wiener-based filtering technique. The previous estimated CSI and weight coefficient vector are used to smooth the current estimated CSI by using block data formulation to reduce the effect of pilot contamination. However, WS technique suffers from pilot contamination due to pilot training. This motivates the development of two Wiener predictors (WP), known as WP1 and WP2. The WP1 and WP2 run a prediction technique for CSI and number of pilot training during the prediction period, which is missing from the original WS. Comparison results show that the proposed WS and WP outperforms the conventional minimum mean square error and least square, in terms of channel estimation error and per-cell rate. WP2 perform better than WS and WP1 because of the algorithm complexity that required more information to be updated, stored and processed for prediction. Thus, WP2 requires large computation and matrix operation compared to WS and WP1. The results indicate that the channel estimation error due to pilot contamination can be reduced by using the Wiener-based approaches.

The world-first deployment of narrowband IoT for rural hydrological monitoring in UNESCO biosphere environment

The success of a rural wireless monitoring system depends on establishing a reliable wireless link over the TCP/IP communication protocol in a challenging terrain and elevation profile. Several studies have shown that link reliability in a rural area can neither be predicted with high accuracy nor precisely modeled using existing mathematical channel modeling tools. Hence, the use of the empirical approach to infer wireless link reliability. This work focuses on the revival of a rural hydrological/water monitoring system with emphasis on the wireless link located in Tasik Chini, a lake with UNESCO biosphere status. The contributions of this study include: understudy the link reliability of a centralized wireless sensor network infrastructure system using the 2G and Long Range (LoRa) wireless network, the performance limitation of the low data wireless sensor network in a rural environment, approaches to revive rural water station monitoring center and finally highlight potential opportunities in rural wireless communications.

Investigation of rate-loss due to limited feedback in LTE-Advanced distributed antenna systems

In wireless communications, interference is known as a major restriction in achieving system capacity. Recently, distributed antenna system (DAS) has received considerable attention due to its potential to mitigating interference and path-loss. In LTE-A system, a transmitter and a receiver are collaborated together to tackle the interference. To do so in the frequency division duplex system, channel state information (CSI) of the downlink channel must be shared amongst transceivers via a limited feedback technique. In this regard, this article evaluates and compares the performance of DAS and centralized antenna system (CAS) under imperfect downlink CSI, where practical system restrictions such as out-of-cell interference, path-loss, and small-scale fading are taken into account. The system performance is evaluated in term of cell throughput, where a LTE-A standard compliant simulator is utilized for simulation. The results reveal high dependency of system performance to the accuracy of CSI, where system throughput severely degrades under imperfect CSI. In contrast to CAS, DAS is more robust again imperfect CSI, where under imperfect CSI, DAS significantly outperforms CAS in term of cell-throughput.