Mehdi Rahmati

uwMIMO-HARQ: Hybrid ARQ for Reliable Underwater Acoustic MIMO Communications

Achieving robust and reliable data communications in the harsh underwater environment is still a challenging issue due to the fast-changing and unpredictable nature of the acoustic channel. Notwithstanding the use of acknowledgement messages at the link layer, data transmission protocols still require more efficient error control strategies so to achieve a higher link reliability. A novel solution based on hybrid automatic repeat request (HARQ) is proposed that exploits the diversity gain offered by independent links in an underwater acoustic Multiple Input Multiple Output (MIMO) system. Irrespective of the channel regime, the proposed scheme aims at reducing the probability of retransmission and at increasing the link reliability via packet-level codeword selection and waterfilling coding. Encoding and decoding algorithms at the transmitter and receiver, respectively, are designed, multiple evaluation metrics are defined, and computer-based simulation results are presented to quantify the performance improvement of the proposed methods.

Interference Cancellation in Multiuser Acoustic Underwater Networks Using Probabilistic SDMA

Combating interference is an important yet challenging issue for multiuser communications especially in the harsh underwater acoustic environment. In this paper, a novel Angle-Of-Departure (AOD)-based technique is proposed, which accounts for the inherent position uncertainty of underwater propeller-driven Autonomous Underwater Vehicles (AUVs) or buoyancy-driven gliders. A new probabilistic Space Division Multiple Access (SDMA) technique is studied using confidence interval estimation, and an effective approach to manage interference statistically is discussed. Also, an optimization problem is proposed to mitigate multiuser interference while keeping the transmitter antennae beam width at a desirable value so to find a trade off among (i) spreading the beam towards the receiver to combat position uncertainty, (ii) focusing such beam to minimize dispersion, and (iii) minimizing interference to other vehicles in the surrounding. Solutions and algorithms are proposed to overcome the multiuser interference via a hybrid SDMA-Time Division Multiple Access (TDMA) method. Simulation results show that the solution mitigates statistical interference, lessens packet retransmission rate, and obtains Signal-to-Interference-plus-Noise Ratio (SINR) gain and rate efficiency over conventional TDMA and SDMA methods.

Performance improvement of STBC CDMA system in correlated MIMO channels

In this paper the effect of spatial correlation in Code Division Multiple Access (CDMA) Space-Time Block Coding (STBC) system is investigated. Having some information about the channel of each active user in the transmitter, a linear precoder can be exploited in downlink situation to mitigate the effect of correlation in multiuser system environment which has not been addressed before. Simulation results illustrate that despite the number of interfering users, the anti-correlation precoder could mitigate the performance degradation caused by correlation.

Optimizing energy consumption in DMF cooperative system with mobile relay using adaptive modulation

In this contribution, the energy consumption of a mobile relay DMF (Demodulate and Forward) cooperative system is optimized by using adaptive modulation technique. The required adaptation thresholds will be achieved through solving an optimization problem. In the mobile relay system, the modulation order of source and relay varies adaptively, regarding the current location of the relay. A new definition for the effective and total energy of the system is proposed. Energy efficiency and monitoring metrics, and BER performance will be used to evaluate the proposed method. The simulation results show that the energy consumption is optimized through this algorithm, for any given location of mobile relay.

A Transmitter Model for Performance Improvement and Interference Mitigation of Multi-user STBC-CDMA System in Correlated MISO Channels

In this contribution, multi-user Space-Time Block Coding (STBC) Code Division Multiple Access (CDMA) system is simulated to investigate the effect of multi-user interference along with the spatial correlation of Multiple-Input Single-Output (MISO) channels. The correlative model is considered for MISO channel. Simulation results declare that spatial correlation degrades the performance of STBC CDMA system. On the other hand, simulations show a huge degradation due to multi-user interference. In order to improve the performance of this system, a transmitter model that utilizes two distinct precoders is proposed in this paper. An anti-correlation precoder and an interference cancellation precoder are used in this model. Simulation results illustrate that this model could achieve a satisfactory performance improvement.