Stefan Wänstedt

LTE-Advanced - Evolving LTE towards IMT-Advanced

This paper provides a high-level overview of some technology components currently considered for the evolution of LTE including complete fulfillment of the IMT-advanced requirements. These technology components include extended spectrum flexibility, multi-antenna solutions, coordinated multipoint transmission/reception, and the use of advanced repeaters/relaying. A simple performance assessment is also included, indicating potential for significantly increased performance.

Streaming applications over HSDPA in mixed service scenarios

High-speed downlink packet access (HSDPA) is included in release 5 of the WCDMA specifications to increase downlink capacity and bitrates. This paper considers performance aspects of streaming applications over HSDPA in a mixed streaming and best-effort service scenario. Different scheduling algorithms are evaluated with the aim of providing sufficient quality-of-service for streaming. The simulation results show that reasonable streaming performance can be achieved without service differentiation if a somewhat fair scheduler is used, and that a streaming-aware scheduler further can protect streaming quality-of-service in high load conditions.

The impact of QoS support on the end user satisfaction in LTE networks with mixed traffic

How good is a best effort network for different services in multi-service LTE networks? In this paper, we try to answer the question in the context of capacity of an LTE network with users concurrently running multiple services. We present a system simulation study for two-service scenarios with VoIP and a second service represented by real-time video, mobile TV, or web surfing. We present results for a best effort network and compare to those for a network with QoS provisioning. The study demonstrates that traffic differentiation and service prioritization are particularly crucial when a delay-critical service, e.g., VoIP, is in combination with a delay-insensitive intensive traffic. By prioritizing VoIP, we achieve VoIP capacity comparable to that in pure VoIP simulations at a cost of a few-percent capacity loss of the second service. A simple model for network capacity assessment for the two network types is presented to support our observations theoretically.

Effects of QoS Scheduling Strategies on Performance of Mixed Services over LTE

3GPP is currently in the process of defining the long-term evolution (LTE) of 3G, which will be an all-IP network. Streamlining the system for packet services will lead to improvements in the form of higher bit-rates, lower latencies and a wider array of service offerings, but will also pose new challenges that need to be overcome. This paper investigates how call setup signaling will be affected as the load on the system increases and how the delivery of important signaling messages can be ensured. A traffic scenario where all users engage in both a voice over IP (VoIP) conversation and a video session is applied in an extensive network simulator. SIP signaling is used to set up the different media connections. We find that if all traffic is scheduled with equal priority, setup signaling might become heavily impaired and the delivery of signaling messages can not be ensured. However, if these messages are absolutely prioritized and always scheduled before other traffic, the length of call setups and terminations can be kept at almost constant values even if the system load is high. The results also indicate that other service qualities are not significantly affected by such a prioritization scheme.

Mixed Traffic HSDPA scheduling - Impact on VoIP Capacity

IMS multimedia telephony (MMTel) for cellular systems is currently being standardized in 3GPP. MMTel comprises a number of media components such as voice over IP (VoIP), real-time video, text messaging and file sharing. In this paper down-link scheduling performance of MMTel services for various scheduling algorithms is investigated in an HSDPA system. The studied service mixes include VoIP and file download (FTP), as well as VoIP and real-time video. The investigation of the schedulers is done over a wide range of traffic mix ratios, in order to give a full understanding of the performance. This paper shows that it is beneficial to have separate schedulers for each media flow. For an MMTel scenario with VoIP and file downloading, the best performance is achieved when using a delay scheduler for VoIP and a proportional fair (PF) scheduler for FTP. An extra priority weight for VoIP over FTP (or v.v.) makes it possible to control the wanted total service mix. With simultaneous VoIP and real-time video the best performance is achieved when both services utilize a delay scheduler, this since the VoIP and video flows shall be synchronized. However, the scheduler for the video flow requires different parameter settings, compared to the VoIP flow, in order to give the best performance.

Scheduling for improving system capacity in multiservice 3GPP LTE

We study the impact of scheduling algorithms on the provision of multiple services in the long term evolution (LTE) system. In order to measure how well the services are provided by the system, we use the definition of joint system capacity. In this context, we claim that scheduling strategies should consider the current satisfaction level of each service and the offered load to the system by each service. We propose a downlink-scheduling strategy according to these ideas named capacity-driven resource allocation (CRA). The CRA scheduler dynamically controls the resource sharing among flows of different services such as delay-sensitive and rate demanding ones. Moreover, CRA scheduler exploits the channel-quality knowledge to utilize the system resources efficiently. Simulation results in a multicell scenario show that the CRA scheduler is robust regarding channel quality knowledge and that it provides significant gains in joint system capacity in single and mixed service scenarios.

Realization and Performance Evaluation of IMS Multimedia Telephony for HSPA

The proposed voice over IP (VoIP) over HSPA solution targets high capacity as well as coverage and speech quality comparable to circuit switched (CS) speech. The VoIP over HSPA radio bearer realization includes one radio link control (RLC) un-acknowledged mode for the speech media. Parallel signaling radio bearers are also transmitted using HSPA access. Due to the flexibility of IP and HSPA, both narrow-band and wideband speech codecs work well. A delay sensitive scheduler is selected for the downlink transmission. For the uplink, the non-scheduled mode is selected. Coverage and quality can, with simulations, be shown to be at least as good as CS speech according to 3GPP ReI. 99 specifications. Similarly, capacity evaluations show that VoIP over HSPA has the potential of matching or exceeding CS speech capacity, depending on scenario. Finally, end to end simulations show that the maximum allowed delay of single links seldom occurs for uplink and downlink simultaneously, resulting in a lower perceived end-to-end delay than expected most of the time

Scheduling Support for Mixed VoIP and Web Traffic over HSDPA

HSDPA (high-speed downlink packet access), introduced in WCDMA release 5, provides a high-bandwidth shared channel with short transmission time interval (TTI). The short TTI together with appropriate scheduling enable HSDPA to support efficient multiplexing of traffic. We explain the performance of four scheduling algorithms when transmitting a traffic mix consisting of both conversational (VoIP) traffic and background (Web) traffic over the high-speed downlink shared channel (HS-DSCH) of HSDPA. We consider both cell throughput and user satisfaction. The proportional fair (PF), the maximum rate (MR) scheduler and two extended versions of MR, are tested for different VoIP scheduling delay budgets and varying load. To understand the behaviour of the schedulers, we use the ns-2 simulator extended with a model of HS-DSCH to simulate a mixed traffic scenario. Our results show that a scheduler that gradually increases the VoIP priority and considers the users current possible rate, performs well. A more drastic increase in VoIP priority is however needed when the delay budget is short. Furthermore, attempting to uphold quality for both VoIP and Web traffic makes the system sensitive to overload situations.

Effects of simultaneous circuit and packet switched voice traffic on total capacity.

Introducing packet switched (PS) voice in cellular systems is often discussed in terms of circuit switched (CS) voice replacement. In this study, we investigate the differences in downlink resource consumption or capacity in a WCDMA system with both CS and PS voice traffic on the same 5 MHz carrier. The PS traffic is gradually increased, representing a migration from CS to a PS only scenario. The study comprises both theoretical calculations and system simulations of downlink voice traffic. The results show that gradually introducing PS voice traffic on the same carrier as CS speech gives equally good or better voice capacity without compromising the QoS requirements. Further on, introducing PS voice on the same carrier can be done in a very flexible way. The CS admission control thresholds can control the power usage for CS, and PS voice can thereafter use the remaining base station power. This means that the CS admission control can decide the ratio of CS and PS voice users, and increase the PS ratio gradually as the PS voice traffic becomes more common

Efficient QoS over LTE — A scheduler centric approach

With increasing use of smart phones and mobile broadband, efficient differentiation of traffic is essential for system stability and user satisfaction. In this paper we propose a scheduler centric approach to traffic differentiation (called QoS in this paper) over LTE, in which the scheduler acts as a core entity assisted with congestion control and admission control. The scheduler is responsible for enforcing the wanted differentiation; it guarantees that high priority bearers, which are in danger of not fulfilling their QoS contract, receive necessary resources. The bearer based congestion control reacts on scheduling decisions and removes bearers with insufficient resources. The admission control algorithm takes decisions using load measurements from the scheduler. Simulation results with a mixed traffic scenario show that applying the proposed approach guarantees GBR bearers sufficient resources efficiently in the order of set priorities while background bearers utilize the remaining resources.