Mahmoud Ammar

Multi-stage SIC structure for uplink UMTS multiuser receiver over multipath Rayleigh channels

This paper deals with uplink direct-sequence code division multiple access (DS-CDMA) transmissions over mobile radio channels. A new interference cancellation scheme for multiuser detection is presented. It is based on a modified multistage Successive Interference Cancellation (SIC) structure that enables efficient detection in multipath propagation environments, thanks to a single user RAKE combiner incorporated in each unit of the SIC structure. The behaviour of the suggested receiver is checked in the context of the Universal Mobile Telecommunication System (UMTS). Simulation results are presented for multipath flat fading Rayleigh channels. It is shown that the proposed detector is resistant to the near-far effect, and that small performance loss is obtained compared to the single-user bound.

A Generalised Wideband Space-Time MIMO Channel Simulator Based on the Geometrical Multi-Radii One-Ring Model

This paper extends the geometrical one-ring multi- input multi output (MIMO) channel model for a frequency flat fading process in a land mobile radio system with respect to frequency-selectivity. Our approach enables the design of efficient simulation models for space-time MIMO channels under isotropic scattering conditions for any numbers of transmit and receive antennas. A simulation study is performed into the space- time cross correlation to gain understanding of the modification of some relevant parameters. A conventional four transmit four receive 4 x 4 MIMO radio channel is analyzed.

Analysis of Full Volterra and Sparse Volterra Nonlinear Equalization for Downlink LTE System

In this work, we propose Full Volterra and Sparse Volterra channels equalization algorithms for downlink LTE system. For estimation purpose, the LTE standard provides known data as pilot symbols and exploits them through a coherent detection to improve system performance. The references symbols location is similar to diamond pattern through time and frequency domain. Our goal is to show how to use the Full Volterra and Sparse Volterra equalizers over frequency domain in order to mitigate linear and nonlinear effects introduced by power amplifier and multipath channel. Downlink LTE Performances System in term of BLER, EVM (%) and Throughput versus SNR are presented and discussed. Then we compare our results with basic RLS equalizer curves. Moreover, it is shown that the Full Volterra and Sparse Volterra models can significantly reduce interferences and nonlinear effects and consequently improve the performances of LTE Downlink System.

Interpolation based channel estimation methods for DS-CDMA systems in Rayleigh multipath channels

This paper presents a comparative investigation of different channel estimation methods for pilot based coherent RAKE combining in DS-CDMA transmission systems. The channel estimation algorithms are based on interpolation of the channel statistics from pilot symbols. The methods for channel estimation are analyzed and compared in terms of BER performance in a multipath channel. Third order Lagrange interpolation is found to be a good choice for channel estimation since it exhibits the best performance with small computational complexity. Mean square error (MSE) for the discussed methods are also presented by means of computer simulations.

Toward Performance Improvements of RAKE Receiver with Pilot Power Allocation and Lagrange Filtering

In this paper a novel channel estimation scheme is proposed for a RAKE receiver operating in a time varying multi-path channel. The approach is an extension of the well known linear interpolation channel estimator, which is based on interpolating the channel estimates from pilot symbol sequence. In this paper parallel pilot-symbol aided channel estimation is considered. The proposed technique manages to combine the obtained samples over one chip duration using a Lagrange interpolation filter, and thereby enhances the signal-to-noise ratio and improves the quality of channel estimates. We also investigate optimal power assignment for the pilot and data channels. Simulation results allowed us to pinpoint optimum pilot-to-data channel power ratio for the best bit error performance.

Iterative Soft Multipath Interference Cancellation Assisted by Hybrid ARQ with Constellation Rearrangement for HSDPA System

High speed downlink packet access (HSDPA) is the major evolution of UMTS downlink. It provides peak data rates up to 10.8 Mbps. This is reached by radio link adaptivity, which includes fast scheduling, adaptive modulation and coding (AMC), and hybrid automatic repeat request (HARQ). The performance of AMC suffers from multipath interference introduced by the channel especially when using 16-QAM modulation. This is because of the variation in bit reliabilities caused by the bit-mapping onto the signal constellation. In this paper, we present a soft iterative multipath interference canceller (MPIC) for suppressing the multipath interference. We also propose to use HARQ Chase combining with the constellation rearrangement technique in joint application with MPIC. This can equalize the variation of bit reliabilities by using different mapping rules over HARQ transmissions. It is proven that the performance of adaptive modulation and coding is significantly improved after only two HARQ transmissions.

Analysis of full volterra nonlinear equalizer for downlink LTE system

In this work, we propose a Full Volterra channel equalization for downlink LTE system. For estimation purpose, the LTE standard provides known data as pilots symbols and exploits them through a coherent detection to improve system performance. These drivers are placed in a hybrid way to cover up both time and frequency domain. Our aim is to precise how to adapt the Full Volterra technique to the LTE standard in order to compensate linear and nonlinear effects introduced by power amplifier and multipath channel. We present the comportment of Full Volterra equalizer in term of BLER, EVM (%) and Throughput versus SNR. Then we compare our results with basic RLSequalizercurves. Moreover, it is shown that the Full Volterra model can significantly reduce interferences and nonlinear effects and consequently improves the performances of LTE Downlink System.

Multi-user detection in DS-CDMA systems: a conjugate-gradient implementation

This paper deals with direct-sequence code division multiple access (DS-CDMA) transmissions over mobile radio channels. An iterative conjugate gradient (CG) implementation of the decorrelator detector is studied in the case of asynchronous transmissions over multipath channels. This implementation is based on a sliding window technique. Simulation results are presented for flat fading Rayleigh multipath channels, showing that the proposed detector is resistant to the near-far effect and that low performance loss is obtained, compared to the single-user bound.

MPIC and space-time rake detectors on MIMO HSDPA system using generalized Alamouti codes

It is well known that HARQ technique generates delays du to the retransmissions. Thus, the high speed real time services quality, in High Speed Downlink Packet Access (HSDPA), depends on the number of the retransmissions. Motivated by the important diversity that multiple transmitting antennas and multiple receiving antennas (MIMO) can provide, we study the possibility to avoid HARQ retransmissions by using evolved detector as Multipath Parallel Interference Canceller (MPIC) associated to MIMO. Based on generalized Alamouti codes, then, we demonstrate that MIMO 4×4 + MPIC detector could give better performance than MIMO 2×2 Space-time Rake taking into account the HARQ context.

A new SIC-HARQ receiver for the WCDMA enhanced uplink system

In this paper, we propose a new interference canceller (SIC-HARQ) for the WCDMA enhanced uplink system which is the combination of the hybrid ARQ technique (HARQ) with an improved successive interference canceller (SIC). The proposed SIC detector is an iterative receiver which consists of a multiple stages. It is capable of eliminating multiple user interferences (MUI) by generating and subtracting the users contribution from the received signal. At the last stage, when there are users which have transmission errors, the erroneous packets are saved at the receiver buffer and negative acknowledgements (NACK) are sent to the transmitters which in response retransmit the packets. The retransmitted packets are maximum ratio combined (MRC) with the previously saved ones. In this manner, the obtained packet after combining is more reliable than the individually transmitted packet. This increases the probability of correct decoding and increases the performance of the improved SIC detector. We have shown that the proposed SIC-HARQ receiver improves the block error rate (BLER) results of the data physical channels. This enhancement achieves, for the modulation and coding scheme 1 (MCS1), almost 5dB at the BLER equal to 10-2.