Jorge Cabrejas

MAC Layer Performance of Different Channel Estimation Techniques in UTRAN LTE Downlink

Long Term Evolution (LTE) is the new standard proposed by the 3GPP to evolve towards 4G. Evolved UTRAN (E-UTRAN) specifications are currently completed and research groups are studying the performance of the last Release 8. Nevertheless, these studies lack a full modeling of the MAC layer because they either leave out retransmissions and turbo coding or assume ideal channel estimation. This paper uses an accurate LTE MAC layer simulator to perform a complete downlink LTE performance study. Results compare different channel estimation techniques showing significant difference among them, most of all regarding the robustness of the estimator against errors. Finally, LTE system performance assessment is presented employing a realistic channel estimator.

Multi-User Non-Coherent Detection for Downlink MIMO Communication

Current cellular technologies are based on the concept of coherent communication, in which the channel matrix used for demodulation is estimated via reference or pilot signals. Coherent systems, however, involve a significant increase of the signalling overhead, especially when the number of transmission points is increased or when the mobile channel changes rapidly, which motivates the use of non-coherent techniques. This letter extends the use of non-coherent communications to a multi-user (MU) multiple-input multiple-output (MIMO) framework by combining superposition coding with a reduced-complexity detection method. Numerical results confirm that our scheme achieves higher user rates than non-coherent MU transmission based on time multiplexing. In addition to the well-known sum-rate gain of MU systems, an extra performance gain given by downlink non-coherent MU communication is shown and qualitatively justified.

Time-Frequency Grassmannian Signalling for MIMO Multi-Channel-Frequency-Flat Systems

In this paper, we consider the application of non-coherent Grassmannian signalling in practical multi-channel-frequency-flat multiple-input multiple-output (MIMO) wireless communication systems. In these systems, Grassmannian signalling, originally developed for single-channel block-fading systems, is not readily applicable. In particular, in such systems, the channel coefficients are constant across time and frequency, which implies that spectrally-efficient signalling ought to be jointly structured over these domains. To approach this goal, we develop a concatenation technique that yields a spectrally-efficient time-frequency Grassmannian signalling scheme, which enables the channel coherence bandwidth to be regarded as an additional coherence time. This scheme is shown to achieve the high signal-to-noise ratio non-coherent capacity of MIMO channels when the fading coefficients are constant over a time-frequency block. This scheme is also applicable in fast fading systems with coherence bandwidth exceeding that of one subchannel. The proposed scheme is independent of the symbol duration, i.e., the channel use duration, and is thus compatible with the transmit filter designs in current systems.

Application of MIH for the Lightweight Deployment of LTE-Advanced Systems through Mobile Relaying

In a conventional cellular network end users connect directly to a Base Station (BS). Mobile relaying allows establishing an indirect two-hop link between the end user, called Mobile Node (MN), and the BS through a Mobile Relay (MR). This spreads out the cell coverage and increases the cell-edge throughput hence improving fairness among nodes. This article is focused on a Long Term Evolution Advanced (LTE-A) cellular network where MNs and MRs are connected through a Wireless Fidelity (WiFi) ad-hoc connection. It is proposed the use of Media Independent Handover (MIH) signaling to define an efficient dynamic routing mechanism for MR in this framework. The proposed mechanism, called MIH-Driven Relay Selection Mechanism (MIDRES), detects which is the best direct or indirect link with the BS based on information collected using MIH messages. The MNs or MRs send MIH messages when experiencing bad channel conditions, that is detected thanks to predefined thresholds. Then, the BS starts a polling process, again supported by MIH signaling, and performs optimal route selection either through the LTE-A radio interface or through a WiFi ad-hoc interface. This article examines the implementation of this mechanism and obtains the optimal thresholds that maximize operational performance. Moreover, the potential benefit of this LTE-compliant mobile relaying solution is evaluated using a calibrated simulation tool. The results show significant savings in cost of network deployment.

Use of mobile network analytics for application performance design

With the 5G technology, data traffic is going to grow by a factor of 1000, while the number of connected devices is likely going to be two orders of magnitude higher. With smartphones being cornerstone in our daily lives, understanding mobile network performance is critical for providing a superior user experience and, consequently, determining the success of an application. This paper presents a solution that uses the radio parameters measured by a mobile terminal to determine the best Application Protocol (APPP) for a service, so as it could adapt to the varying network conditions. From the training of an inference system with actual Mean Opinion Score (MOS) data, it will be possible to discern which radio Key Performance Indicators (KPIs) are best suited to characterize the state of the network and make the best possible decision. Results show how the decision system based on only three radio KPI is able to determine the user application experience with a success of up to 83%. Thanks to the use of this approach, application developers may fill the gap of knowledge between network KPIs and user experience.

Non-Coherent Open-Loop MIMO Communications Over Temporally-Correlated Channels

This paper investigates the use of non-coherent communication techniques for open-loop transmission over temporally-correlated Rayleigh-fading MIMO channels. These techniques perform data detection without knowing the instantaneous channel coefficients. Three non-coherent Multiple Input Multiple Output (MIMO) schemes, namely, differential unitary space-time modulation, differential space-time block code, and Grassmannian signaling, are compared with several state-of-the-art training-based coherent schemes. This paper shows that the non-coherent schemes are meaningful alternatives to training-based communication, specially as the number of transmit antennas increases. In particular, for more than two transmit antennas, non-coherent communication provides a clear advantage in medium to high mobility scenarios.

On the integration of Grassmannian Constellations into LTE networks: A link-level performance study

This paper presents Grassmannian signaling as a transmission scheme that can be integrated in Long Term Evolution (LTE) to support higher user speeds and to increase the throughput achievable in the high Signal to Noise Ratio (SNR) regime. This signaling is compared, under realistic channel assumptions, with the diversity transmission modes standardized in LTE, in particular, Space-Frequency Block Coding and Frequency-Switched Transmit Diversity for two and four transmit antennas, respectively. In high-speed scenarios, and even with high antenna correlation, Grassmannian signaling outperforms the LTE diversity transmission modes starting from four transmit antennas. Furthermore, in the high SNR regime, Grassmannian signaling can increase the link data rate up to 10% and 15% for two and four antennas, respectively.