Teruya Fujii

All-SIP Mobility: Session Continuity on Handover in Heterogeneous Access Environment

Given the high performance of recent access networks and mobile terminals, it has become feasible for mobile nodes to support multiple ongoing sessions while they are moving. However, in the heterogeneous access network environment, the IP addresses of mobile nodes are forced to change when they switch networks. In this paper, we introduce a simple network platform on which inter-network handover can be efficiently realized. We then propose the SIP-MIP controlled mobility architecture, which prevents QoS degradation of real-time sessions while keeping TCP connections alive. We also propose the all-SIP mobility architecture; it literally uses only SIP components to maintain both TCP and UDP sessions. The proposed architecture eliminates the cost issue and residual drawbacks of using MIP. In addition, both architectures require little or no modification on the server side, which makes them more cost effective and easier to deploy on existing infrastructure.

SCS-MC-CDMA system with best effort cell structure

This paper proposes SCS-MC-CDMA system with best effort cell structure. 4th generation mobile communication (4G) system is expected to offer 100 Mbps or higher data rate communication in the high-frequency microwave band. The system is characterized by relatively long delay paths, high maximum Doppler frequency, high power consumption at the mobile terminals and small cell coverage caused by the high data transmission rates and the high-frequency band. All of these are serious problems that must be solved. The proposed SCS-MC-CDMA system can realize high communication quality even under delay spreads as long as a few microseconds and maximum Doppler frequency as high as 1000 Hz. Channel estimation is accurate since code-multiplexed pilot symbols are used, and MMSEC is adopted to reduce inter-code interference. Concerning power consumption, the system can assign a number of sub-carriers, which may be narrower than the whole bandwidth of the system, to each user according to the users data rate requirements. This lowers the power consumption, especially for low data rate users such as voice users. Moreover, the best effort cell structure overcomes the small cell coverage issue by changing the maximum data rate for each user adoptively according to his distance from the base station or channel quality.

Performance Evaluation of End-to-End Communication Quality of LTE

LTE (Long Term Evolution) is standardized in 3GPP (3rd Generation Partnership Project) to provide higher data throughput as well as lower latency for various IP based services including web browsing, VoIP (Voice over IP), video streaming and so on. In this paper, we conduct the performance evaluation of LTE by indoor and outdoor experiments. We evaluated both TCP throughput and UDP transmission delay. For the TCP throughput evaluation, we observed the TCP- uplink-limit problem in which the uplink performance degradation in low SNR (Signal to Noise Ratio) region affects on the downlink TCP throughput. We also analyzed the transmission delay of UDP packets. The delay distribution is affected by the traffic pattern, i.e. packet size and inter packet gap.

Delay profile modeling for wideband mobile propagation

In this paper, we study delay profile modeling for broadband mobile communication systems. Based on delay profile measurement data collected in urban and suburban areas, we create two delay profile models. One is a relative delay profile model in which each path is normalized to the first paths power. The other is an absolute delay profile model; it allows the propagation loss of each path to be calculated. We also propose a novel path delay profile prediction formula that takes account of the antenna height of BS, city structure, and distance from BS.

A study on channel estimation methods for MC-CDMA systems

In this paper, we propose a novel channel estimation method for time-domain spreading multi-carrier code division multi-carrier code division multiple access (MC-CDMA) systems. In time-domain spreading MC-CDMA systems, code-multiplexed pilot symbols are transmitted consecutively in both time and frequency domains. In our proposed method, weighted pilot symbols in both domains are coherently added to improve signal-to-noise ratio (SNR) of pilot symbols in channel estimation. However, the channel estimation quality degrades when the weights of pilot symbols are not selected appropriately according to the changes of propagation environments. We propose a method which the optimum weighting factors for pilot symbols are adaptively selected according to the changes of propagation environments, by measuring maximum Doppler frequency for the time domain and delay spread for the frequency domain at MS (mobile station). We evaluate our proposed method by computer simulation, and clarify improvement of channel estimation accuracy.

Field Experiment of CoMP Joint Transmission over X2 Interface for LTE-Advanced

Multiple base station cooperation techniques have been attracting much attention for the improvement in cell-edge throughput recently. In 3GPP, such techniques are referred to as CoMP and studied actively. Joint transmission is a promising technique in CoMP. In CoMP JT, previous studies have mainly focused on intra-eNB CoMP because it is relatively easy to implement. The intra-eNB CoMP JT in combination with optical fiber systems such as RRH or RoF can realize throughput improvement at cell edge. However, the number of RRHs being able to be connected to the same eNB is usually limited to a few because of the signal-processing capability of eNB. Therefore, CoMP JT can be used only within the cells connected to the same eNB, which makes it impossible to use CoMP JT between at any cell border. To enable all cell-edge UEs enjoy the merit of CoMP JT, CoMP JT based on a distributed cooperation approach using inter-eNB interface such as X2 interface has been proposed. In the distributed cooperation, CoMP JT can be realized in a distributed manner, so that CoMP JT can be used at any cell border. However, the previous studies focused on only concepts or evaluation by computer simulations. To verify the feasibility and its effect with real system, we developed a prototype system of CoMP JT realized on a distributed cooperation approach using inter-eNB interface. The technical details to realize it is shown in this paper. We also conducted laboratory and field experiments and demonstrated its feasibility. Also, we confirmed that drastic throughput improvement at cell edge can be realized with the real system.

Performance evaluation of TCP and UDP during LTE handover

LTE (Long Term Evolution) has been standardized in 3GPP (3rd Generation Partnership Project) to provide higher data throughput as well as lower latency for various IP based services including web browsing, VoIP (Voice over IP), video streaming and so on. In this paper, we evaluate the performance of TCP and UDP when handover is executed in an LTE system. The performance is degraded due to the interference between source and target eNBs (evolved NodeB). Also the handover interruption time negatively affects overall performance. We measure the TCP throughput and the transmission delay of UDP during the handover process by using our indoor and outdoor LTE testbeds. The results show that better TCP throughput performance is obtained by decreasing the A3-offset and the interruption time caused by a handover event is around 80ms.

Iterative MIMO Signal Detection with Inter-Cell Interference Cancellation for Downlink Transmission in Coded OFDM Cellular Systems

MIMO/OFDM is receiving considerable attention for next generation mobile radio communication systems. These systems require improved throughput performance to users at cell-edges. In our previous study, we introduced an enhancement of MLD (Maximum Likelihood signal Detection) algorithm, in which desired and interference signals are jointly detected, to improve the cell-edge throughput in the downlink of single- cell-frequency-reuse MIMO/OFDM cellular systems. In order to achieve higher cell-edge throughput in coded OFDM cellular systems, this paper proposes an iterative signal detection with ICI cancellation based on an enhancement of MAP (Maximum A posteriori Probability) algorithm in which signal path search space includes not only desired signal space but also interference signal space. Its transmission characteristics are evaluated by computer simulation and it is confirmed that the proposed iterative signal detection can achieve higher ICI cancellation and improve BLER (BLock Error Rate) and throughput performance more than the conventional MLD-based signal detection.

A Downlink Transmission Method for OFDM Cellular Systems with Inter-Cell Interference Cancellation Using Simplified MLD based on MMSE QRD-M Algorithm

For next generation mobile radio communication, one-cell-frequency-reuse OFDM systems are attracting considerable attention. In such systems, it is necessary to improve the throughput performance of users at the cell-edge. This paper introduces an effective signal detection method with ICI canceller for OFDM cellular systems. The proposed method is an enhancement of a simplified MLD (Maximum Likelihood Detection) in which the signal path search space includes not only the desired signal space but also the interference signal space. This algorithm uses the MMSE QR decomposition of the virtual MIMO channel matrix and the M-algorithm in order to reduce the computational complexity of the original MLD. Its transmission performance is evaluated by computer simulation and it is confirmed that the signal detection that the proposed method can improve the throughput performance of the MS at the cell-edge in fully-loaded OFDM cellular systems. This paper also clarifies the tolerance of the frequency offset difference between local oscillators in the adjacent base stations for the proposed signal detection method.

Improving Throughput by Multi-Cell Coordinated Vertical Plane Beam Control with Pre-Coding

Cellular mobile communication systems reuse the same frequency channel in geographically distant areas to improve the spectrum efficiency. However, co-channel interference among cells remains a critical issue. Mitigating co-channel interference leads to a further improvement in spectrum efficiency. One technique for mitigating co-channel interference is antenna beam tilting, which controls the vertical plane beam of base station antennas. In cellular mobile communication systems, antenna beam tilting is generally used by setting fixed beams at each base station. If the vertical plane beam pattern is changed dynamically for each mobile station, it is possible to significantly improve the SINR (Signal- to-Interference plus Noise Ratio), which drastically improves the spectrum efficiency. We propose here a vertical plane beam control method that uses pre-coding; the vertical plane beam pattern for each mobile station is easily set by controlling the phase of the baseband signal. The proposed method can be easily implemented with little or no modification of the antenna configuration at the base station. This system is effective for improving not only cell edge capacity but also overall cell capacity at the same time. A computer simulation shows that the spectrum efficiency of the proposed system is approximately twice that of conventional antenna beam tilting.