Mitsuru Uesugi

A study on time domain spreading for OFCDM

The paper presents computer simulation results on spreading in both the time and frequency domain for OFDM-CDMA. The results show that the most suitable layout on which spreading chips are mapped is different for some conditions (e.g. number of multiplexing codes, propagation condition, etc). For example, in the case of employing a number of multiplexing codes, spreading only in the time domain outperforms other layouts. On the other hand, in case of half multiplexing codes, spreading in both the time and frequency domain outperforms the others.

Constellation rearrangement and spreading code rearrangement for hybrid ARQ in MC-CDMA

This paper presents two techniques that rearrange the data between retransmissions and can enhance the diversity gain from hybrid ARQ (hybrid automatic repeat request). One is constellation rearrangement that rearranges the signal constellation of QAM signals between retransmissions. The other is spreading code rearrangement that changes the spreading code with which the same symbol is transmitted in each retransmission. Simulation results show that those proposed schemes outperform type-H hybrid ARQ with packet combining, or chase combining.

Interference cancellation method using DFE

Improvement of the frequency utilization factor is the prime consideration in mobile communication. The interference canceller would serve as the most effective means for efficiency improvement, but there are many problems to realize it. We, therefore, have proposed two types of interference canceller which require less amount of operation and confirmed their performances through simulation. To implement improvement in the performances of the proposed methods, we have analysed the performance deterioration factors and have already initiated improvement effort for one of the factors.

Wireless Access Method to Ensure Each User's QoS in Unpredictable and Various QoS Requirements

With the fast growth of Internet services, the demand for wireless high speed and advanced multimedia communications is rapidly increasing. Initial multimedia services have already been offered in the second generation systems (2-G). Currently, the third generation systems (3-G) are being deployed and it will support up to 2 Mbps packet oriented data services. But it is expected that the demand for higher data rate and large variety of high quality services will increase further more. Therefore, it is mandatory for the fourth generation systems (4-G) to take each users quality of service (QoS) control into account.In this paper, we firstly describe an overview on requirements and technical issues for the 4-G systems, and wireless QoS control issues considering End-to-End QoS control in the IP network. Next, we propose broadband wireless access method for the fourth generation mobile communication systems. The proposed method for downlink is based on MC-CDMA (Multi-Carrier Code Division Multiple Access) with Multi-Timeslot, in which total downlink resources are split into three aggregates according to the QoS classes in time domain. It is considered that one of the typical advantages for the MC-CDMA is the effect of frequency diversity. We evaluated the effect by computer simulation using ray-traced propagation model. Simulation results show that short-term fading can be relaxed. Furthermore, we propose Multi-Level (ML) ARQ as one of the link adaptation methods. Simulation results show that the proposed scheme has a good performance with throughput and average retransmission power.

A method to reduce the peak power with signal space expansion (ESPAR) for OFDM system

OFDM (orthogonal frequency division multiplexing) is an effective technique for high data rate wireless transmission. However, the OFDM system has a problem with the peak to average power ratio (PAPR) being very high. A high PAPR can cause severe distortion when transmitted through a nonlinear device. Therefore, because extreme power amplifier specification is required, high power consumption and large hardware is required. We propose a new method called ESPAR (Expansion of the signal Space for Peak to Average ratio Reduction) in order to reduce the peak power. ESPAR expands the signal space by modulating some subcarriers and not modulating the other subcarriers. The proposed system selects the symbol patterns with low signal power in the expanded signal space and transforms data sequences to the pattern. We evaluate the characteristics of the proposed system by computer simulation and show the advantage of ESPAR.

Block-Orthogonal Frequency Division Multiplexing in Multi-Path Fading Channel

Orthogonal frequency division multiplexing (OFDM) is robust against multi-path interference due to using a lot of low symbol rate sub-carriers. In order to mitigate the degradation due to multi-path interference, guard time interval (GI) is inserted into each OFDM symbol. However, redundant allocation to GI leads to loss of effective transmission power. We propose the Block-OFDM which can reduce the GI overhead ratio per frame compared with conventional OFDM. In Block-OFDM system frequency domain equalization (FDE) is carried out in order to remove multi-path interference. We evaluated Block-OFDM performance in a frequency selective fading channel and compared with that of conventional OFDM.

Artificial delay added space time turbo coding on OFDM in correlated fading channel

This paper proposes an enhanced data transmission technique for space time turbo coding (STTuC) on OFDM. The performance of STTuC on OFDM degrades drastically when fading channel correlation between systematic stream and parity stream is high. Artificial delay added space time turbo coding (ADA-STTuC) lower the fading channel correlation between systematic and parity in high fading channel correlation case, which leads to performance improvement. Simulation results with two transmit antennas and one receiving antenna shows the performance improvement even in perfectly correlated fading channel.

Symbol ranking type interference canceller (SRIC) for CDMA system

In order to further increase the capacity of the CDMA system, several kinds of interference canceller are proposed. We propose a novel interference canceller named SRIC (symbol ranking type interference canceller). SRIC is very feasible for its small hardware complexity compared with the other multi-user type interference cancellers and is easy to be introduced to the base station afterwards. We have nine variants of SRIC and confirmed their performance.

Enhanced Multi-Antenna Technologies on OFDM for Future Mobile Communication Systems

Various types of multi-antenna techniques have been discussed to realize high speed data transmission in mobile communication systems. Those multiple antenna systems obtain space diversity gain by utilizing multi-paths as independent channels from transmitter and receiver. One important issue to utilize the powerful diversity gains of those multi-antenna technologies is how to create independent channels between transmitter and receiver. Almost all of conventional multi-antenna technologies have been discussed with assuming that there is not any correlation among channels between transmitter and receiver. However in realistic world this assumption is not always true and the correlation among channels becomes very high. This high correlation drastically degrades the performance of those multi-antenna technologies. In this paper we present an enhanced transmission diversity method to solve the problem of high correlation among channels. The following contents of this paper mainly focus on the Space Time Turbo Coding in OFDM systems, however the concept of the presented method can be adopted in various types of multi-antenna systems.

A method to improve the transmission performance under slow to fast fading environment

In high speed mobile radio communications, multi-path fading is one of the more serious transmission difficulties, and an adaptive equalizer is known to be an effective solution, which can help to alleviate it. However, burst error often remains after equalization under a slow fading environment and most equalizers cannot track the fading channel fluctuations accurately under a fast fading environment. Therefore we propose a scheme that assures a stable transmission against both slow and first fading environment.