Mirette Sadek

Personal satellite communication: technologies and challenges

Communication regulatory bodies in many parts of the world have recently granted satellite operators the right to extend their networks by adding ground segments. This has opened the door, for the first time, to truly ubiquitous satellite coverage, thus revolutionizing the use of satellites in personal communications. In this article, we review personal satellite communication systems. Background on the evolution of satellite communications is given along with the basics of satellite systems. This is followed by a comprehensive discussion of the current challenges facing the future of personal satellite communication systems and some of the proposed solutions.

Novel hybrid soft-hard demodulation schemes for RQD constellation

Rotated constellation with Q-delay (RQD) is a new modulation scheme adopted by some wireless standards including the second generation of the digital video broadcasting for terrestrial (DVB-T2) system. This scheme offers significant improvement over unrotated modulation in terms of bit error rate (BER) performance due to its signal space diversity. In this paper, we present four novel computationally efficient demodulation schemes with hybrid soft-hard outputs used in hard demapping of data. The main idea is to take advantage of the unique projections the rotated cells have in the RQD system. The proposed schemes reduce the number of log likelihood ratio (LLR) computations. One proposed scheme gives the same BER performance as traditional LLR while reducing the computational complexity by 34% at SNR = 20 dB. Another scheme reduces the computations by 25% independent of the SNR level. The other two schemes offer a tradeoff between computational complexity and power efficiency.

Modified MIMO-OFDM Channel Estimation Technique for DVB-T2 Systems

This paper proposes an enhanced pilot-aided channel estimation algorithm for the second generation of the digital video broadcast for terrestrial (DVB-T2) standard. Two channel estimation techniques are discussed, the first is based on the 3-points averaging technique, while the second is based on studying the frequency domain pattern of the channel response. The second technique leads to a 0.5 dB reduction in the SNR needed to guarantee a specific bit error rate (BER) requirement when compared to the conventional estimation technique recommended by the DVB-T2 implementation guide lines.

Adaptive handoff for multi-antenna mobile satellite systems with ancillary terrestrial component

In integrated mobile satellite systems (MSSs) with ancillary terrestrial component (ATC), seamless handoff techniques will be fundamental for allowing users to switch between the space segment and the terrestrial component. On the other hand, introducing multiple-input multiple-output (MIMO) antenna technology in next-generation MSS-ATC systems promises the well-known advantages related to the MIMO implementation. For performing handoff analysis, the single-input single-output (SISO) channel model equivalent to the MIMO model needs to be developed, which is challenging in MSS-ATC networks because of the high complexity of the satellite channel, and the vast difference in the nature of the MSS and the ATC links. This challenge is tackled in this paper, where an optimal user-driven handoff algorithm for MSS-ATC systems is presented, with both types of links implementing MIMO. Furthermore, for a better signal prediction, Kalman filtering is proposed, which significantly increases the performance. Notably, the application of multi-antenna technology in MSS-ATC systems reveals significant gains in terms of handoff performance.

Comparison and performance evaluation of orthogonal frequency and multi-carrier code division multiple access systems

Future mobile communication systems aim to provide extremely high speed data transmission, especially in the downlink. Broadband orthogonal frequency code division multiple access (OFCDMA) with two-dimensional (2D) time and frequency domain spreading is becoming a very promising technique for high speed wireless communications due to its advantages over multi-carrier code division multiple access (MC-CDMA), direct sequence CDMA (DS-CDMA) and orthogonal frequency division multiplexing (OFDM). This paper presents a comparison, through simulation, between the performance of OFCDMA and MC-CDMA systems operating under the same condition. The paper also explains the basic structure of multi-carrier direct sequence CDMA (MC-DS-CDMA) technique, its advantages and OFCDMA system structure. It is shown that OFCDMA is superior to the above mentioned systems.

Controlled limited-exchange diffusion network model for cellular-based applications

In adaptive diffusion networks, one of the main challenges is the large volume of data exchange among nodes needed to arrive at a collective decision. In this study, a new model for adaptive diffusion networks is proposed which offers a tradeoff between the mean-square error performance of the system and the volume of data exchanged among network nodes while preserving the network convergence rate. Study of the mean-square stability of the network under the proposed algorithms is provided. Also, a study of the mean-error dynamic behaviour of the network is carried out. A closed-form expression for the overall network steady-state means-square error is derived and verified against simulated data. The proposed algorithm is applied to a cellular network location estimation problem, and delivers good performance even under 75% reduction in data exchange volume.

Hybrid analog and digital beamforming for space-constrained and energy-efficient massive MIMO wireless systems

Massive multiple-input multiple-output (MIMO) systems can achieve high data rates due to the large number of antennas at the base station when it serves a small number of users. However, deploying massive MIMO in current mobile networks faces a lot of practical issues such as limited physical space and power consumption. Packing large number of antennas in a limited physical space makes spatial correlation between the base station antennas inevitable. In this paper, we study the effect of correlation and we investigate the effectiveness of increasing the number of antennas given limited physical space. Hybrid analog and digital MIMO decoding is adopted to exploit the large array gain at a lower cost and power consumption. Also, different antenna-array structures are investigated and compared in terms of power consumption versus performance.

Performance evaluation of CoMP methods under RF calibration errors

Coordinated multipoint (CoMP) transmission-reception systems promise an enhanced cell-edge user downlink throughput, specially in time division duplexing (TDD) systems. Radio frequency (RF) calibration is necessary for proper CoMP operation. In this paper, we study the performance of different CoMP schemes and different precoders under RF mismatch calibration errors. Numerical results are provided which show the best CoMP mode candidates given different SNR and calibration error levels.

A modified hybrid RQD demapper based on novel hard to soft assigning algorithm

Rotated constellation with Q-Delay (RQD) is an innovative scheme that appeared to give better performance compared to the traditional unrotated modulation. The bit error rate (BER) decreases due to the RQD scheme signal space diversity. The authors presented four novel computationally efficient demodulation schemes with hybrid soft-hard outputs in [1] that took advantage of the unique projections the rotated cells have in the RQD system. The Coded Modulation (CM) system can utilize the hybrid computationally efficient demodulation schemes for obtaining better results in terms of BER and computation complexity. One of the hybrid schemes was tested with Low Density Parity Check (LDPC) coder/decoder through two proposed Hard to Soft Assigning algorithms (HSA). These algorithms assign estimated Log-Likelihood Ratio (LLR) values to the hard mapped bits, so that the LDPC decoder receives an all-soft stream. Numerical results show that HSA algorithms give better performance than the original all-soft stream at a moderate SNR range.

Channel Estimation for Alamouti Coded OFDM over Time Variant Channel with Different Receivers

In this paper, we investigate a semi-blind maximum a posteriori probability-based channel estimator for Alamouti coded OFDM system over time varying Rayleigh channels. This channel estimation is used to handle rapid variations of channel within a transmission block. With the estimate of the channel matrix for the Alamouti symbol interval, a zero-forcing (ZF) receiver can be applied to detect this symbol. Linear and Minimum Mean Square Error (MMSE) decoding algorithms are also introduced in this paper. These receivers take into consideration the variation of channel over two consecutive OFDM symbols, i.e., over Alamouti symbol. The performance of this estimator is compared with Least Square (LS) channel estimator via simulation. Moreover it is shown that using MMSE as a decoding algorithm provides better performance than conventional, linear or ZF decoding methods.