Jessica Östergaard

Internet Access Performance in LTE TDD

The Time Division Duplex (TDD) uplink-downlink configuration of the 3GPP Long Term Evolution (LTE) determines how the ten subframes in a radio frame are divided between the downlink and the uplink. The specified configurations cover a wide range of allocations from a downlink-heavy resource distribution ratio (9:1) to an uplink-heavy ratio (2:3). In this paper, we compare the performance of Internet access using the TCP protocol in different downlink-heavy asymmetries. We find that the performance depends on many factors such as the transferred file size, the control channel errors and the downlink/uplink traffic mix. When the file size is small, TDD can not fully utilize its potentially higher downlink capacity because of longer uplink access delays as well as shortage of uplink resources in the chosen configurations. With an increased file size, this effect fades away and TDD provides higher download bit rates than FDD. The realized increase in bit rate is however not as high as the calculated increase in available downlink resources.

LTE Downlink 2X2 MIMO with Realistic CSI: Overview and Performance Evaluation

For downlink multiple input multiple output (MIMO) schemes, there is a tradeoff between downlink MIMO performance and uplink reporting cost. The mobile speed has an impact on this tradeoff since the reporting delays the measurements. This paper presents the major downlink MIMO modes in 3GPP Long Term Evolution (LTE) and their related channel state information (CSI) modes. The impact of realistic CSI is investigated through advanced system simulations, showing that the total performance degradation of realistic CSI reporting is at a surprisingly low 20% compared to full, recent channel knowledge. With realistic CSI reporting, a selected set of promising 2x2 MIMO applications are evaluated at different mobile speeds. The open-loop spatial multiplexing mode offers unexpectedly high cell throughput despite its limited feedback requirement. It performs well for a wide range of speeds, as it provides good robustness to delayed channel knowledge. Bitrates can only be further improved by closed-loop spatial multiplexing in stationary scenarios, where CSI delay does not hamper the precoding gain. These results confirm that there are MIMO modes suitable for both high and low speeds.

Real-Life Indoor MIMO Performance with Ultra-Compact LTE Nodes

With the ever increasing demand for mobile broadband service with very high bitrates to indoor users, there is large industry traction around different types of In Building Solutions (IBS) for Long Term Evolution (LTE). Multiple Input Multiple Output (MIMO) is a key feature in LTE enabling significantly improved user bitrates, and the indoor channel characteristics are also expected to be in favor for MIMO performance. Despite this, due to deployment constraints and design constraints on the IBS antenna design and size, many IBS deployments of today do not support MIMO. As the market evolves, this is finally changing and in this paper, the 2x2 MIMO performance is measured in real-life with small-size radio equipment, a commercial smartphone and a low-power radio head, both with a size of approximately a wavelength. Despite the small antennas, the performance is found to be very good supporting rank-2 transmission on almost the whole 2500 m2 office floor with only 100 mW transmission power. In more than half of the area, the user bitrate exceeds the maximum rank-1 bitrate, demonstrating the value of MIMO. Also in Line-Of-Sight (LOS) conditions, traditionally a weakness for MIMO performance, the indoor channel richness facilitates rank-2 transmission. The 2x2 MIMO performance is found to be limited rather by propagation loss than MIMO channel characteristics and antenna size, which demonstrates the potential for ultra-compact 2x2 MIMO indoor antenna heads.