Kari Aho

Impact of Control Channel Limitations on the LTE VoIP Capacity

In this paper, we present a fully dynamic simulative analysis of the Downlink (DL) Voice-over-IP (VoIP) performance in 3G Long Term Evolution (LTE) with both Uplink (UL) and DL control channel constraints. In UL the Physical Uplink Control Channel (PUCCH) capacity affects the Channel Quality Indicator (CQI) resolution and in DL the Physical Downlink Control Channel (PDCCH) capacity has an impact to the amount of multiplexed users per Transmission Time Interval (TTI). The results indicate that with realistic control channel assumptions, semi-persistent packet scheduling outperforms dynamic packet scheduling.

Analysis of VoIP over HSDPA Performance with Discontinuous Reception Cycles

The aim of this paper is to evaluate how Discontinuous Reception (DRX) cycles and related timers take effect to Voice over IP (VoIP) performance when High Speed Downlink Packet Access (HSDPA) networks are in question. DRX cycles limit the scheduling freedom of users and increase battery saving opportunities in the User Equipment (UE) by allowing it to turn its receiver circuitry off for some periods of time. Prior work has concentrated mainly on optimizing the usage of radio resources when small bit rate delay critical services, like VoIP, are considered. However, the battery life of small handheld devices might become a limiting factor in providing satisfactory user experience. Thus, this paper evaluates the performance also from the battery life perspective when DRX cycles together with VoIP are considered. The performance is evaluated with a fully dynamic system level tool in which the mobility of the users, radio resource management functionalities and the interactions between them are explicitly taken into account. The study indicated that the longer the DRX cycle is the higher are the battery saving opportunities but at the same time VoIP over HSDPA capacity can be compromised. Capacity degeneration in pure VoIP traffic simulations was, however, possible to be mitigated by using an adequately long inactivity timer. In mixed traffic scenarios including both VoIP and Best Effort (BE) traffic, higher cell throughput was achieved by allowing more scheduling time for BE users with adequately long DRX cycle and a short inactivity timer.

Enhancing HSUPA system level performance with dual carrier capability

The purpose of this paper is to analyze how dual carrier capability can enhance High Speed Uplink Packet Access performance in comparison to using only single carrier. Dual carrier operation gives the User Equipment the possibility to transmit simultaneously using two 5 MHz bands, theoretically doubling the peak data rates and user throughput. The analysis is conducted with a system level simulation tool. This paper first indicates with single carrier simulations that, especially in small cells, terminals have spare power available for dual carrier operation. These observations are verified with dual carrier simulations by showing that the burst throughput can be practically doubled. In the larger cells only the users in good position can fully benefit from using dual carrier.

Impact of Practical Codebook Limitations on HSUPA Closed Loop Transmit Diversity

Different alternatives for uplink transmit diversity for HSUPA are being investigated in 3GPP. This paper compares the performance of two-antenna beamforming using closed loop feedback information against the baseline single antenna transmission. With beamforming, multiple transmit antennas are utilized and the UE transmitter applies a weight vector to the transmit antennas in order to amplify the received signal. In this paper, the impact of different codebook sizes, delays and update intervals related to antenna weights are studied utilizing a comprehensive system level simulator. The studies show that increasing codebook size provides relatively higher performance gain over the baseline. However, increasing the number of available beamforming antenna weights will also increase signaling overhead. In terms of weight update interval the best performance is achieved with one slot interval and if the interval is prolonged to the length of Transmission Time Interval (TTI), namely 3 slots, then the performance with higher mobile velocity can be compromised. Similarly, the results with different feedback delays show that additional delays in fast fading channel should be avoided.

Extended HSUPA coverage and enhanced battery saving opportunities with multiple TTI lengths

3GPP has specified that terminals can be configured to use either 2 or 10 ms transmission time interval in high speed uplink packet access systems. The purpose of this paper is to evaluate the benefit of exploiting a mixture of both of the transmissions time intervals within a cell instead of only one. The study is quantified by means of studying the achievable coverage of voice over IP and possible battery saving benefits. The analysis is conducted with a system level simulator modeling network and terminal behavior in detail. The paper indicates that utilizing a mixture of both transmission time intervals can extend coverage whilst providing enhanced battery saving opportunities.

Trade-Off between Increased Talk-Time and LTE Performance

The purpose of this paper is to analyze the trade-off conditions between battery saving opportunities and long term evolution network performance. To achieve this goal voice over IP with discontinuous reception is studied. Analysis is conducted with vast amount of different settings, including on duration, inactivity and discontinuous reception cycle timers. The quality of service and battery saving opportunities with discontinuous reception are evaluated with a dynamic system simulator which enables detailed simulation of multiple users and cells with realistic assumptions. This paper indicates high battery saving, i.e., increased talk-time opportunities without compromising the performance when discontinuous reception is properly adapted.

On HSUPA Open Loop Switched Antenna Transmit Diversity Performance in Varying Load Conditions

The Switched Antenna Transmit Diversity (SATD) scheme is a promising transmit diversity technique to improve the performance in High Speed Uplink Packet Access (HSUPA) systems. Antenna switching rests on a technique where only one antenna is active at a time even if the UE is equipped with multiple transmit antennas. Switching and antenna selection can be based on different methods and criteria and this paper provides simulative analysis of three different open loop antenna switching algorithms for the HSUPA systems. Furthermore, the focus will be on evaluating corresponding system level performance compared to the single Tx antenna performance when bursty traffic is assumed. The studies show that there are achievable gains and thus system performance may be enhanced by applying the antenna switching scheme, especially when the mobile velocity is low.

On Serving Cell Change Reliability in HSDPA Network

In Wideband Code Division Multiple Access (WCDMA) networks the Signaling Radio Bearer (SRB) is transmitted downlink to inform the user i.a. when it needs to make a handover. When transmitting e.g. Voice over IP (VoIP) on High Speed Downlink Packet Access (HSDPA) SRB is mapped on HSDPA user data channel, High-Speed Downlink Shared Channel (HS-DSCH) in order to maximize voice capacity. The lack of support for soft handover (SHO) of HS-DSCH may compromise the reception of SRB and thus also a possible incoming serving HSDPA cell change command especially at the cell borders. This can lead to serious VoIP service degradation due to prolonged camping on a poor cell and in the worst case even to call dropping, if the cell change command is not received before VoIP call drop criterion is met. This paper considers the performance of SRB transmission on source cell HS-DSCH during a handover process. Also a strategy of transmitting the serving cell change command from the target cell is analyzed. The study is done by the means of fully dynamic system simulations and they consider both macro-cell and manhattan scenarios. The results show that a relatively low SRB/handover error rate is experienced in a macro cell scenario but in a more challenging manhattan scenario the error rate is higher. Transmitting the serving cell change command from the target cell HS-SCCH results in similar handover failure rates in manhattan scenario than using SRB on HS-DSCH in macro-cell scenario when static power allocation for HS-SCCH is in use.

Switched antenna transmit diversity imperfections and their implications to HSUPA performance

3GPP is investigating uplink transmit diversity alternatives for HSUPA. This paper focuses on a transmit diversity scheme using switching between two transmit antennas without additional feedback information. The special focus of this paper is on various non-ideal conditions that occur in real implementations, such as antenna imbalance, correlation and the existence of multiple different SATD algorithms in the network simultaneously. The studies show that transmit antenna correlation somewhat decreases performance but still keeps gains over the baseline, antenna imbalance reduces situations where the switching is feasible and that using a mixture of algorithms can bring down the gains.

Non-regular network performance comparison between HSDPA and LTE

In this paper we study and benchmark the performance of 3rd Generation Partnership Project (3GPP) High Speed Downlink Packet Access (HSDPA) and 3GPP Long Term Evolution (LTE) Downlink with constant bit rate type traffic. In addition a non-regular simulation scenario is considered using realistic propagation data and base station positions related to Tokyo city downtown area. HSDPA was introduced in 3GPP Release 5 and in further releases the technology has been improved with e.g. receive and transmit diversity technologies and support for higher order modulation schemes. LTE was introduced in 3GPP Release 8 with completely new physical layer and improved Radio Resource Management (RRM) functionalities. The benchmarking is performed with fully dynamic system level simulators based on detailed and commonly accepted models for e.g. mobility, propagation, call generation and fading. The results show that LTE Release 8 outperforms HSDPA Release 8 in terms of spectral efficiency and user throughput. With 8 UEs per cell and LTE MIMO about 90% of UEs are fully satisfied, while about 45% of the UEs are fully satisfied with HSDPA MIMO. This results also about 30% gain in spectral efficiency even through with LTE the network is running half empty due to uneven UE distribution in non-regular network with variable cell sizes.