Frans Laakso

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.

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.

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.

Performance of absolute and recursive feedback methods with HSUPA closed loop transmit diversity

3GPP is investigating uplink transmit diversity alternatives for High Speed Uplink Packet Access. This paper presents a closed loop beamforming transmit diversity scheme where NodeB determines the transmit antenna weight vectors and feedback is used for signaling the optimal weights to the user equipment. Two promising feedback method candidates are presented and benchmarked in various conditions on system level against the baseline performance without transmit diversity. In each time slot the absolute feedback method signals the whole weight vector information, while the recursive feedback method signals only a single bit and the weight vector is recursively calculated from multiple bits sent over consecutive time slots. The results show that both feedback methods are capable of providing gain over the baseline in simulated conditions. However, while the recursive feedback method requires less signaling bits per slot, it is more vulnerable to the effects of weight signaling errors due to the signaling error propagation.

Beamforming Transmit Diversity Using Power Control Commands for High Speed Uplink Packet Access

3GPP is investigating uplink transmit diversity alternatives for High Speed Uplink Packet Access (HSUPA). This paper studies beamforming transmit diversity where the UE transmitter applies a weight vector to the transmit antennas. In contrast to the traditional beamforming where transmitter is aware of the channel state through channel state feedback, a practical scheme which relies on existing power control commands to calculate the weight vector is investigated. This approach allows autonomous determination of the weight vector by the UE. Additionally, an ideal algorithm which always selects the optimal weight vectors is presented in order to obtain upper boundary for the performance. Both algorithms are benchmarked in various conditions on system level against baseline performance without transmit diversity. The studies show that ideally open loop beamforming is capable of providing gain in simulated conditions. However, the power control command dependent realistic algorithm is able to provide gain only when the low velocity Pedestrian A 3 kmph channel is used. With more complex and higher velocity Vehicular A 30 kmph channel the realistic algorithm is not able to provide gain and reaches only baseline level performance.

Introducing switched antenna transmit diversity for high speed uplink packet access

3GPP is investigating uplink transmit diversity alternatives for HSUPA. This paper addresses uplink transmit diversity from the perspective of open loop where additional feedback information is not available. Furthermore, the focus will be on switched antenna transmit diversity where only one antenna is active at the time even though UE is equipped with multiple transmit antennas. Five different algorithms are presented and benchmarked in system level in various conditions against baseline performance without Tx diversity. The studies show achievable gains when the UE velocity is low but practical algorithms get only fraction of the possible gain and improper algorithms can lead to even performance loss.

Impact of Amplitude Component on HSUPA Closed Loop Transmit Diversity Performance

3GPP is investigating uplink transmit diversity alternatives for High Speed Uplink Packet Access (HSUPA). This paper addresses uplink transmit diversity from the perspective of Closed Loop Beamforming (CLBF) when amplitude component is included in the beamforming codebook. This allows transmitter to divide the total transmit power unequally to transmit antennas based on the feedback from the NodeB. This study includes the investigation of the potential benefits of antenna selection and amplitude adaptation as part of the feedback. Furthermore, the focus will be on evaluating corresponding system level performance when long term antenna imbalance is assumed. The studies show that there are achievable gains and thus system performance may be enhanced by applying the amplitude component, especially when the antenna imbalance is high. However, the trends indicate that gains are mainly seen by the cell edge users, thus introducing amplitude component in the codebook increases coverage and fairness.

Uplink weight signaling for HSUPA closed loop transmit diversity

3GPP is investigating uplink transmit diversity alternatives for High Speed Uplink Packet Access. This paper studies closed loop beamforming transmit diversity where NodeB determines transmit antenna weight vectors and additional feedback is used for signaling the optimal weights to the user equipment. The used transmit antenna weights are signaled back to the NodeB in order to ensure correct decoding. This approach is benchmarked against the results where the used weights are not signaled back to the NodeB. Performance is analyzed in various conditions on system level against the performance without uplink weight signaling. The results show that signaling the used weights increases the performance over the case without uplink signaling especially with higher bit error rates. However, uplink weight signaling requires additional signaling bits which may not be justified if weight feedback bit error rates are expected to be low. Additionally when uplink weight signaling is used, even if the weight signaled in downlink were correctly received, signaling errors can happen in uplink which also will result in incorrect decoding at the NodeB.

Streamlining HSUPA TTI Lengths without Compromising HSUPA Capacity

This paper proposes a s.c. 2 ms range extension scheme for High Speed Uplink Packet Access (HSUPA) networks to avoid potential coverage problems and usage of multiple TTI lengths. With range extension UEs in poor radio conditions are configured to send bundles of 2 ms TTI transmissions without HARQ feedback to improve their coverage. This paper analyzes the performance of range extension in comparison with normal 2 ms TTI performance with multiple inter-site distances. The analysis is conducted considering VoIP performance due to its high sensitivity to additional delays and packet loss. The results acquired by using system level simulation tool indicate that using 2 ms range extension can improve system capacity in larger cells where normal 2 ms TTI can result into coverage problems. Moreover, utilizing repeated 2 ms TTIs instead 10 ms TTIs enables increased battery saving opportunities when UL DTX is applied.

Introducing Dual Pilot Closed Loop Transmit Diversity for High Speed Uplink Packet Access

The alternatives for HSUPA uplink transmit diversity are being currently investigated in 3GPP. This paper addresses the uplink transmit diversity from the perspective of closed loop beamforming where NodeB determines transmit antenna weights, and additional feedback is used for signaling the optimal weights to the user equipment. More precisely, this paper introduces a novel transmit diversity technique which involves transmitting two non-beamformed DPCCH (pilot) signals from two antennas whereas the actual data part of the signal is beamformed. Additionally, the dual pilot transmit diversity technique is benchmarked on system level in various conditions against the baseline system performance without transmit diversity. The studies show cell level throughput gains up to 53 % in Pedestrian A channel and up to 26 % in Vehicular A channel comparing to the baseline system. The highest relative gains are achieved in highly loaded cells with long inter-site distances.