Takeo Ohseki

Burst Construction and Packet Mapping Scheme for OFDMA Downlinks in IEEE 802.16 Systems

In this paper, we propose a burst construction and packet mapping scheme in the orthogonal frequency-division multiple access (OFDMA) downlinks of IEEE 802.16 systems. In the standard of the systems, there are some restrictions on the usage of downlink radio resources and they are defined in the physical layer (PHY) specification. One of the main restrictions is that a rectangular region, which is called Burst, is defined on a two-dimensional domain of time and frequency, and packets must be allocated within the region. However, how to define the burst and allocate data packets within the burst is left implementation dependent. Therefore, we consider a burst construction and packet mapping scheme that is easy to realize on systems and attains efficient usage of radio resources. By computer simulation, it has been confirmed that the proposed scheme can decrease not only the control data ratio within the rectangles, but also control data that must be transmitted at the head of every frame, which can result in higher throughput.

Broadcast methods for inter-vehicle communications system

This paper presents novel broadcast methods for an inter-vehicle communications system for ITS. During inter-vehicle communications, it is essential that emergency information be broadcast to surrounding vehicles. Emergency information such as emergency-vehicle-approach information and traffic accident information are needed by vehicles in a particular area. By limiting the broadcast direction, the proposed methods can provide broadcasts to a particular area and avoid mistakenly notifying other areas where the information is not needed. We developed a simulation and experimental system for the proposed method and confirmed that the information is broadcast to the desired area.

Performance analysis of adaptive downlink modulation using OFDM and MC-CDMA for future mobile communications system

To meet the strong demand for broadband multimedia services by both nomadic and mobile users, it is important to increase the bit rate of future mobile communications systems. To enhance system capacity, novel technologies or new concepts for improving system capacity are indispensable. Performance analysis for adaptive downlink modulation using orthogonal frequency division multiplexing (OFDM) and multicarrier CDMA (MC-CDMA) is presented. The basic concept of the proposal method is that a time frame is divided into two sub-frames, one is for OFDM and the other is for MC-CDMA, and the base station (BS) allocates the preferred modulation scheme to each user in each time slot in accordance with their service requirements and link conditions such as the received signal strength indication (RSSI) level and interference signal strength. Simulation results show that the throughput performance of the proposed system is better than that of the system using OFDM or MC-CDMA only.

Multihop Mobile Communications System Adopting Fixed Relay Stations and its Time Slot Allocation Schemes

In this paper, we present a multihop mobile communications system utilizing fixed relay stations to enhance the coverage of the service area. A multihop mobile communications system was originally proposed as a system in which only mobile stations relay communication between other mobile stations and a base station to solve the problem of poor coverage expected in a future cellular system. In the system, however, a probability of a mobile station being able to connect with a base station via their relay functions is governed by a probability of mobile stations existing in its neighborhood. Therefore, we introduce fixed stations as relay stations in addition to relay functions of mobile stations to enhance the probability of link conductivity between a base station and a mobile station. By adopting this multihop architecture, a mobile station which needs relay functions can choose relay stations among fixed relay stations or other mobile stations. Moreover, we also examine its time slot allocation methods. We confirmed that the ratio of successful connection between mobile stations and base stations improved about 0.15 by introducing just 4 fixed relay stations in a certain environment

Simple Implementation of QoS Wireless Packet Scheduling in IEEE 802.16 Systems

In this paper, we propose a Quality of Service (QoS) packet scheduling algorithm for the IEEE 802.16 system and evaluate the algorithm on hardware. The proposed algorithm controls packets based on a deadline for each packet determined from QoS parameters. The feature of our algorithm is that it can simply control the various QoS classes defined in the standard using the deadline. As a result, QoS parameters, such as minimum traffic rate and maximum latency, would be guaranteed by scheduling packets based on the deadline. We then implemented and evaluated the algorithm with a testbed. There have been few reports of an evaluation on IEEE 802.16 hardware before. We confirmed that QoS with high priority is guaranteed with the proposed algorithm.

Throughput Gain of Fractional Frequency Reuse with Frequency Selective Scheduling in SC-FDMA Uplink Cellular System

SC-FDMA is a multiple access technique adopted in LTE uplink transmission. SC-FDMA, as well as OFDMA, can improve system throughput by assigning different frequency portions of system bandwidth to different user equipments (UEs) adaptively based on their channel quality information. In SC-FDMA uplink cellular system, interference arising from UEs in neighboring cells degrades system performance, especially the performance of the cell-edge UE. To overcome this drawback, many papers propose the Fractional Frequency Reuse (FFR) technique and analyze its influence. However, these studies have different conclusions on the effectiveness of FFR, and the throughput gain of FFR depends on the evaluating conditions. In this paper, we reveal the conditions where FFR is effective by demonstrating the throughput gain of FFR in SC-FDMA uplink cellular system. In order to analyze the throughput gain of FFR, we focus on the performance relationship between FFR and frequency selective scheduling (FSS). From the analysis, the throughput gain of FFR is small because of the degradation of the FSS gain, and FFR is effective when the following conditions are met; (i) the number of UEs is small, (ii) the multipath delay spread is large or equal to 0.

Fast outer-loop link adaptation scheme realizing low-latency transmission in LTE-Advanced and future wireless networks

In this paper, a fast outer-loop link adaptation (OLLA) scheme is proposed to realize low-latency transmission. OLLA generally compensates for the estimated signal-to-interference-plus-noise ratio (SINR) that is used to optimize the selection of a modulation and coding scheme (MCS) based on the reception of a positive acknowledgement (ACK) or a negative acknowledgement (NAK) for a Hybrid ARQ (HARQ) initial transmission. The proposed scheme controls the size of the compensation in the estimated SINR based on the time elapsed after a user equipment (UE) transits from an idle state to an active state. The computer simulation result shows that the proposed scheme can achieve a high throughput just after UE starts its data transmission that is otherwise impossible if using the conventional scheme. This performance helps shorten the latency of transmission, especially in small-packet applications.

Performance evaluation of LTE-Advanced downlink adopting higher order modulation in small cells

In 3GPP Release 12, 256QAM (Quadrature amplitude modulation) is expected to be adopted in downlinks of the LTE-Advanced standard as a new modulation order for the purpose of further enhancement in frequency efficiency. However, if a UE (user equipment) is configured to be able to select 256QAM in downlink transmission, the UE cannot select several MCSs (modulation and coding schemes), which was possible in the previous standard. This point is one of the performance deterioration factors associated with 256QAM. Evaluating and clarifying the impact which this deterioration factor has on system performance is of commercial significance because it leads to the development of an effective application method for 256QAM, which will be required in the deployment phase after standardization completion. Therefore, this paper presents some actual methods by which 256QAM is configured to UEs in LTE-Advanced heterogeneous networks, and evaluates their performance by computer simulation, so as to clarify the above-mentioned impact. The results of the simulation make it clear that it is desirable to configure UEs in small cells so that 256QAM can always be selected.

Multihop mobile communications system using MC-CDMA in forward links

This paper proposes a multihop mobile communications system using an MC-CDMA transmission technique in the forward links. In the system, it is assumed henceforth that each mobile station has a relaying function where a frequency band is used both in the 1st and 2nd hops. Then, we examined a radio resource allocation method in the forward links. In this method, an orthogonal codeword is assigned for each group that consists of one or more mobile stations and the resources of time and frequency are distributed to each link within a group. To clarify the characteristics of this method, we compared the proposed system and a conventional one without relaying, in terms of the base station connection rate, throughput, and so on. Simulation results indicate that the proposed system improves the connection rate without further orthogonal codeword consumption, and user throughput in an appropriate mobile station density.

On symbol timing for OFDM based mobile communications systems

This paper deals with symbol timing synchronization issue for broadband OFDM based mobile communication system. A symbol timing approach using adaptive threshold is proposed. Several OFDM training symbol formats and the corresponding timing metrics are evaluated. Simulations am carried out using Rayleigh fading model at 5 GHz band, with the velocity being 100 km/h and 150 km/h. The performances of the simulation results demonstrate that the proposed symbol timing method is feasible for mobile telecommunications.