Hung Tuan Nguyen

Carrier aggregation for LTE-advanced: functionality and performance aspects

Carrier aggregation is one of the key features for LTE-Advanced. By means of CA, users gain access to a total bandwidth of up to 100 MHz in order to meet the IMT-Advanced requirements. The system bandwidth may be contiguous, or composed of several non-contiguous bandwidth chunks that are aggregated. This article presents a summary of the supported CA scenarios as well as an overview of the CA functionality for LTE-Advanced with special emphasis on the basic concept, control mechanisms, and performance aspects. The discussion includes definitions of the new terms primary cell (PCell) and secondary cell (SCell), mechanisms for activation and deactivation of CCs, and the new cross-CC scheduling functionality for improved control channel optimizations. We also demonstrate how CA can be used as an enabler for simple yet effective frequency domain interference management schemes. In particular, interference management is anticipated to provide significant gains in heterogeneous networks, envisioning intrinsically uncoordinated deployments of home base stations.

A Time Reversal Transmission Approach for Multiuser UWB Communications

We propose and evaluate the performance of the time reversal technique in impulse radio ultrawideband (UWB) communications. The evaluation was based on measured channel impulse responses of a 4times1 multiple input single output (MISO) system in the UWB frequency bandwidth of 3 to 5 GHz with both vertical and horizontal polarization at the receiver. The results show that there is a great potential in combining time reversal and UWB technique with respect to both reducing the receiver complexity and improving the system performance. Simultaneous communication is illustrated with 5 users, each with a bit error rate (BER) of less than 10-3 at an average signal to noise ratio (SNR) of 15 dB

MU-MIMO in LTE Systems

A relatively recent idea of extending the benefits of MIMO systems to multiuser scenarios seems promising in the context of achieving high data rates envisioned for future cellular standards after 3G (3rd Generation). Although substantial research has been done on the theoretical front, recent focus is on making Multiuser Multiple-Input Multiple-Output (MUMIMO) practically realizable. This paper presents an overview of the different MU-MIMO schemes included/being studied in 3GPP standardization from LTE (long-term evolution) to LTE Advanced. MU-MIMO system concepts and implementation aspects have been studied here. Various low-complexity receiver architectures are investigated, and their performance assessed through link-level simulations. Appealing performance offered by low-complexity interference aware (IA) receivers is notably emphasized. Furthermore, system level simulations for LTE Release 8 are provided. Interestingly, it is shown that MU-MIMO only offers marginal performance gains with respect to single-user MIMO. This arises from the limited MU-MIMO features included in Release 8 and calls for improved schemes for the upcoming releases.

The potential use of time reversal techniques in multiple element antenna systems

Based on outdoor measurements we study the feasibility of applying the time reversal techniques (TR) in multiple element antenna (MEA) wireless communication systems. It is demonstrated that the use of TR in wireless communication has a promising potential in mitigating the effect of channel dispersion and especially in reducing the cochannel interference where a margin of 18 dB interference reduction has been obtained.

Time reversal in wireless communications: a measurement-based investigation

Interference caused by other users in a multi-user environment degrades the system performance significantly. Conventionally, user separation in a multi-user environment is achieved by signal separation in time, frequency or code. The use of multiple elements antenna (MEA) systems can apply separation of the users in space by targeting the transmitted power to the user of interest. Inevitably in that case, the separation is not ideal and there is interference on the other users in the systems. Moreover in a wideband transmission, frequency selective fading causing inter symbol interference (ISI) is a big challenge for point to point wireless communications. In this paper, we study the feasibility of applying the time reversal technique (TR) in multiple input single output (MISO) systems to alleviate the effect of ISI. The ability of TR to focus the signal on the receiver of interest in a multi-user environment is also considered. The studies are supported by measured complex channel impulse responses with 10 MHz bandwidth centered at 2.14 GHz. For an 8times1 MISO system, using TR the root mean square delay spread is reduced by a factor of 2. To evaluate the capability of the TR in reducing the ISI, simulation of the bit error rate (BER) was made. The irreducible BER of the TR-MISO system was shown to be lower than that of the SISO system by at least an order of magnitude. By using TR and assuming that two users are communicating simultaneously with the same BS the two signals have 17 dB of isolation

On the performance of one bit time reversal for multi-user wireless communications

Time reversal (TR) technique can be used to achieve temporal focusing (compression of the effective channel impulse response in time) and spatial focusing (concentration of the transmit energy in space). In this paper the performance advantage and its trade off in the traditional time reversal system and its simplified version the one bit time reversal systems are discussed. Analytical solutions for the temporal and spatial focusing metrics are derived and served as the base for the comparison on the performance of the two TR systems. It is showed that although there is some degradation in the temporal focusing performance of the one bit TR system, its spatial focusing performance surprisingly outperforms that of the traditional TR system. The analytical investigations are verified using measurement data from recent UWB measurement campaign.

Capacity and performance of MIMO systems under the impact of feedback delay

Multiple input multiple output (MIMO) systems promise better performance and significant increases in channel capacity by utilizing the rich scattering environment to obtain diversity gain and spatial multiplexing. Maximum system capacity and performance can be achieved if the instantaneous channel state information (CSI) is known at both sides of the transmission link. In order to have such improvements, the CSI at both link ends must be updated timely. However, the updating process is always subject to non-ideality. We investigate the effect of delay in updating CSI on the MIMO spectral efficiency and performance with system capacity and BER as benchmarks.

Partial one bit time reversal for UWB impulse radio multi-user communications

With the purpose of further reducing the complexity of the system, while keeping its temporal and spatial focusing performance, we investigate the possibility of using partial one bit time reversal (TR) system for impulse radio ultra wideband multi-user wireless communications. The results show that, by optimally selecting the number of used taps in the pre-filter the performance of the partial one bit TR system can outperform the full one bit TR system. In some cases, the temporal and spatial focusing performance of the partial one bit TR system appears to be compatible to that of the original TR system.

Characterization of the Indoor/Outdoor to Indoor MIMO Radio Channel at 2.140 GHz

In this paper the radio channel characteristics of the 8 × 4 MIMO system consisting of a base station and a small terminal equipped with multiple antennas for indoor-indoor and outdoor-indoor scenarios are presented. We study the large-scale variation and small-scale characteristics of the measured channel coefficients. Although the mean received power is very much dependent on the measured location, the channel capacity seems to be unchanged when the receivers location is altered. The data collected from different scenarios (e.g. measurement locations, antenna setting) were used to investigate the advantage of having the knowledge of the channel at both ends of the transmission link. It is shown that using the water filling algorithm there is indeed an increase in the channel capacity. At low SNR, the benefit of knowing the channel at both link ends observed in the measurement data is much higher than which can be obtained in the channel matrix with usual assumption on identical independently distributed components. Using the small-scale and large-scale information in the formulation of the channel capacity we show that in our measurement, the variation of the mean received power has a greater influence on the change of the overall system performance than the change in the environmental multipath scattering property.

Multi-user MIMO and Carrier Aggregation in 4G systems: The SAMURAI approach

The market success of broadband multimedia-enabled devices such as smart phones, tablets, and laptops is increasing the demand for wireless data capacity in mobile cellular systems. In order to meet such requirements, the introduction of advanced techniques for increasing the efficiency in spectrum usage was required. Multi User -Multiple Input Multiple Output (MU-MIMO) and Carrier Aggregation (CA) are two important techniques addressed by 3GPP for LTE and LTE-Advanced. The aim of the EU FP7 project on “Spectrum Aggregation and Multi-user-MIMO: real-World Impact” (SAMURAI) is to investigate innovative techniques in the area of MU-MIMO and CA with particular focus on the practical, real-life, implementation and system deployment aspects. In the present paper, we provided an overview of the up-to-date SAMURAI contributions together with a description of the SAMURAI demonstrators developed as core part of the project.