Eva Englund

Evolving 3G mobile systems: broadband and broadcast services in WCDMA

The third-generation WCDMA standard has been enhanced to offer significantly increased performance for packet data and broadcast services through the introduction of high-speed downlink packet access (HSDPA), enhanced uplink, and multimedia broadcast multicast services (MBMS). This article provides an overview of the key technologies used, the reasons behind their selection, and their integration into WCDMA. Performance results are also included to exemplify the performance possible in an evolved WCDMA network.

LTE evolution towards IMT-advanced and commercial network performance

This paper provides a high-level overview of Long Term Evolution (LTE) Rel-10, sometimes referred to as LTE-Advanced. First, a brief overview of LTE and some of its technologies are given and then the International Mobile Telecommunications (IMT)-Advanced requirements are discussed. LTE Rel-10 enhances the LTE with carrier aggregation, improved multi-antenna support, relaying and improved support for heterogeneous deployments. The paper is concluded with measurements from the first commercial LTE network, showing that live LTE network performance is well aligned with expectations.

The benefits of advanced receivers for high speed data communications in WCDMA

The benefits of using an advanced receiver, the generalized RAKE (G-RAKE) (see Bottomley, G.E. et al., IEEE J. Select. Areas Commun., vol.18, p.1536-45, 2000; Wang, Y.-P. E. and Bottomley, G.E., Proc. IEEE Veh. Technol. Conf., 2000), for high speed mobile Internet applications over wideband CDMA (WCDMA) are studied. Like the conventional RAKE receiver, G-RAKE is a linear receiver that combines the outputs of correlators (fingers) at different time lags. However, unlike RAKE, the finger placements and combining weights for G-RAKE can be optimized to suppress interference, which is achieved by accounting for the interference coloration introduced by dispersive channels. We show that, even for mildly dispersive channels, G-RAKE can increase the achievable peak data rates significantly. It is also shown that the enhanced link performance translates into system capacity gains of around 30-35%. Moreover, for file transfer applications, G-RAKE reduces the download time of a 200 KB file by approximately a half under moderate to high system loads (effects of TCP and Internet delays are accounted for). Therefore, advanced receivers provide significant perceivable benefits to both service operators (higher system capacity) and end users (higher effective data rates, shorter download time).

TCP performance over HS-DSCH

Both the performance of transmission control protocol (TCP) based applications and the effect of TCP on system capacity in a WCDMA system using high speed downlink shared channel (HS-DSCH) are evaluated. It is found that the HS-DSCH improves both the application performance and the system capacity. The effect of TCP on application performance is much stronger than the effect on the system capacity.

System performance of WCDMA enhanced uplink

We present the potential performance improvements attainable by new enhancements to the WCDMA uplink. Improvements can be expected in both system capacity and end-user perceived performance. In this paper we focus on the capacity gains of short TTI, hybrid ARQ and scheduling in node-B. The performance is studied from both the link-level and system-level perspectives. The link performance results focus on the gains obtained by the introduction of short TTI and hybrid ARQ with soft combining in node-B. In the system performance results, we also evaluate node-B based rate scheduling for both 2 ms TTI and 10 ms TTL. Our studies indicate that with the introduction of enhanced uplink the gain in uplink capacity can be in the order of 70-100%.

Impacts of higher order modulation on HS-DSCH system performance

To support high peak rates the possibility to use higher-order modulation is included in the WCDMA high-speed downlink shared channel (HS-DSCH). In an initial roll-out, higher-order modulation may not be available in all terminals and it is therefore interesting to understand to what extent enhancements offered by HS-OSCH depends on higher-order modulation. We study the impacts of higher-order modulation on HS-DSCH system performance. Our results indicate that performance gains when introducing HS-DSCH are significant, even if higher-order modulation is not available. When time dispersion is small, higher-order modulation may improve system throughput. To take advantage of higher-order modulation in environments with significant time dispersion advanced receivers structures should be considered. Furthermore, higher order modulation is found to be more sensitive to errors in the channel quality estimates.

WCDMA uplink enhancements for high-speed data access

To meet future demands for improved uplink packet data performance, enhancements to the WCDMA uplink are part of Re16 of the specifications, with completion date at the end of 2004. The main targets are improved performance for packet data services in terms of reduced delays, improved availability of high data rates, and improved capacity. The paper gives an overview of the design targets, the basic principles, and how they are integrated into WCDMA. Simulation results are used to exemplify the performance gains brought by introducing these enhancements into WCDMA.

High-data-rate availability in WCDMA enhanced uplink systems

The Wideband Code-Division Multiple-Access (WCDMA) standard has been evolved to further improve uplink high-data-rate coverage and to reduce delays. These enhancements are achieved by the use of fast scheduling, shorter transmission time interval and fast hybrid ARQ with soft combining. It has been shown that the use of high-data-rate radio access bearers is key to end-to-end uplink packet access performance [1]. In this paper, we study data rate availability in WCDMA Enhanced Uplink systems. Data rate availability as limited by user terminal power or own-signal interference in dispersive channels is investigated. We show that in dispersive channels the use of advanced receivers can further increase the availability of high data rates. Our results indicate that advanced receivers allow good availability of high-speed data access in dispersive channels.

Improved packet data performance for WCDMA using multi-antenna techniques

Multi-antenna systems have been proposed to improve data rates for third-generation networks, such as the highspeed downlink shared channel (HS-DSCH) packet-data mode in the WCDMA system. We consider the impact of incorporating two multiple transmit antenna systems into HS-DSCH. The first uses transmit diversity with channel knowledge at the transmitter together with a single receive antenna, while the second is an open-loop scheme with multiple transmit and receive antennas. In dispersive propagation environments, multiple access and intersymbol interference, resulting from multicode and multi-antenna transmissions, must be handled appropriately to avoid limiting overall performance. We show, through system simulations, that improved system and user throughputs can be obtained in realistic channel conditions due to the combined effects of channel and multiuser diversity.

Uplink transmission timing in WCDMA

In wireless network uplink communications, there is a tradeoff between transmission coordination to avoid overload situations, and distributed transmission decisions to adapt to fast channel variations. Here, uplink transmission timing (UTT) is proposed as a scheme to allow some load control support, while transmitting mainly when the channel is favorable. It utilizes channel state feedback in the form of power control commands, which already are available in the system. Simulations illustrate the transmission timing behavior, and also indicate that UTT is a power and inter-cell interference efficient scheme to transport data compared to traditional dedicated channels with continuous transmissions and to schemes where transmission decisions are random.