Norihiko Morinaga

A Carrier Interferometry based Channel Estimation Technique for One-Cell Reuse MIMO-OFDM/TDMA Cellular Systems

This paper proposes a channel estimation technique for multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM)/time division multiple access (TDMA) (MIMO-OFDM/TDMA) cellular systems based on the dynamic parameter controlled-OF/TDMA (DPC-OF/TDMA) systems. In the proposed scheme, impulse responses of all the transmit antennas are multiplexed in the time domain using a carrier interferometry (CI) signal, which is equivalent to an impulse signal, and each impulse response is discriminated using a time window with the length of the maximum delay time of delayed paths, in order to estimate a channel state information (CSI) using only 64 subcarriers. Computer simulation confirms that the proposed channel estimation scheme is effective even in the case of 64 subcarriers under MIMO and multi-cell conditions

Two simple modifications for improving IEEE802.11 DCF throughput performance

IEEE802.11 employs distributed coordination function (DCF) as a media access control method. In order to address the hidden terminal problem, the DCF equips the virtual carrier sensing function in addition to the traditional carrier sensing function, CSMA. The virtual carrier sensing is realized by the RTS/CTS short packets exchange and the NAV. The RTS/CTS exchange can keep terminals other than the sender and the receiver silent for ensuring a data packet transmitted correctly. The NAV is a timer that indicates the amount of time that the medium will be reserved. The first modification is to replace the NAV for RTS of DCF to the one used in the protocol MACA (the idea of RTS/CTS exchange is originally proposed in the MACA). Simulations show that this modification improves the throughput performance. On the other hand, the DCF is designed to use ACK packet for ensuring correct data transmission in the media access control layer, while the ACK did not used in the MACA. We secondly show that the use of the ACK reduces the throughput performance of DCF when the RTS/CTS exchange is in use, concluding that by omitting the ACK from the DCF, the performance can be improved. This paper not only shows that these two simple modifications can improve the performance of the DCF but also points out that the functions being implemented in the DCF do not always work well for realizing higher performance.

An OFDM Based Adaptive Modulation Scheme Employing Variable Coding Rate

This paper proposes an OFDM based adaptive modulation scheme employing variable coding rate (VCR OFDM AMS), which selects optimum modulation and coding scheme (MCS) realized by combination of several modulation schemes and coding rates. The OFDM AMS with multilevel transmit power control (OFDM AMS/MTPC) can realize high data rate transmission in the dynamic parameter controlled-orthogonal frequency and time division multiple access (DPC-OF/TDMA). The employment of OFDM AMS/MTPC, however, makes transceiver design rather complex. To solve this problem, we propose to improve throughput performances of the OFDM AMS without employment of the MTPC. The simple OFDM AMS, however, does not fully utilize transmit power for throughput improvement because there is surplus transmit power which corresponds to power margin over required signal to interference plus noise power ratio (SINR). Thus, in order to improve transmit power efficiency for throughput increase, we reduce the required SINR gaps between adjacent MCSs by introducing many coding rates. Furthermore, this paper presents an effective bit loading algorithm when multiple coding rates as well as modulation schemes are used. Computer simulation confirms that the proposed VCR OFDM AMS gives sufficient throughput performances as an alternative to the OFDM AMS/MTPC.

A New MAC Protocol for Avoiding Needless Transmission Deferment Induced by Missed RTS/CTS Handshake

IEEE802.11DCF employs RTS/CTS (request to send/clear to send) mechanism for mitigating effect of hidden terminals. The RTS/CTS mechanism sets transmission deferral timer to neighbor terminals by exchanging RTS and CTS between transmitter and receiver. Then, in the case that RTS/CTS exchange succeeds, effect of hidden terminals could be suppressed, otherwise, any neighbor which received RTS and/or CTS defers its new transmission needlessly although DATA packet corresponding to the previous RTS/CTS exchange will not be transmitted. In this paper, we propose Cancel CTS procedure in order to cope with the unnecessarily transmission deferment. Results of computer simulation confirms that our method well improves the throughput performance of IEEE802.11DCF.

The Minimum Number of Adaptive Array Antenna Elements for Interference Suppression in Ubiquitous Communication Environments

This paper investigates the minimum number of antenna elements of an adaptive array antenna (AAA) required to suppress unnecessary radiation to a single location in an indoor ubiquitous communication environment. In the investigation the relationship between the number of antenna elements of an AAA and its digitally-created antenna null width is demonstrated using antenna directivity control technique that has been proposed by the authors. Also, the angular spread around a terminal to a single location in an indoor communication environment is demonstrated with propagation measurements. Based on the computer simulation results and the angular spread obtained by propagation measurements, the authors specify that at least 6 antenna elements are required to suppress radiation to a single location in an indoor communication environment up to -60 dB.

A Broadcasting Method for Suppressing Hidden Terminals Effect on IEEE802.11DCF

IEEE802.11DCF is widely used wireless LAN standard. But, packet delivery ratio of broadcast transmission is much lower than unicast transmission because the IEEE802.11DCF executes broadcast transmission without RTS/CTS exchange and ACK process. Hence, many protocols have been proposed to improve reliability of the broadcast transmission. Most of these protocols are proposed based on collision recovery policy using ACK signal or NACK signal instead of ACK frame. Then, transmission overhead is increased by retransmission under the condition that network contains some hidden terminals since ACK or NACK signal could not prevent transmission of hidden terminals. This paper proposes a new broadcast protocol based on collision prevention policy using modified RTS/CTS exchange, and clarifies that our proposed protocol achieved highly reliability with computer simulations

A carrier interferometry based channel estimation technique for MIMO-OFDM/TDMA systems

SUMMARY This paper proposes a channel estimation technique for the dynamic parameter controlled — orthogonal frequency and time division multiple access (DPC-OF/TDMA) systems studied as one of the candidates of the beyond third generation (B3G) systems. In the proposed scheme, the impulse response, which represents the channel state information (CSI) is estimated using carrier interferometry (CI) which is equivalent to impulse signal transmission. Moreover, because the minimum number of subcarriers allocated to terminals is 64, in order to estimate a CSI with its spreading factor of 64, we employ a code-multiplexed CI signal for a cell search process and a time-domain-multiplexed CI signal for transmit antenna identification. Furthermore, we also propose a flexible CSI estimation scheme that supports two cases: multiple subchannel block assignment and MIMO transmission cases. Computer simulation confirms that the pro

A Method for Syncronous Transmissions with Exposing Terminal on the Basis of Overheating RTS Header Information

IEEE802.11DCF, which is a most widely used wireless LAN employs RTS/CTS handshake as an option for mitigating effects of hidden terminals. This handshake can prevent packet collisions by exchange RTS and CTS among transmitter and receiver in order to notify length of its transmission period to neighboring terminals. However, by the handshaking, terminals overhearing RTS and/or CTS prohibit its transmission needlessly even though the terminals can transmit packet without any damaging effect for on going transmission. This paper proposes a new method for synchronous parallel transmissions with exposing terminal on the basis of overhearing RTS header information.

A New Method for Improving MAC Level Broadcast Receive Ratio Using 2-Phase RTS/CTS Handshakes Sequence

IEEE802.11DCF which is widely used wireless LAN standard employs RTS/CTS exchange for improving reliability of unicast transmission. But, broadcast receive ratio transmission is much lower than unicast transmission because the IEEE802.11DCF proceeds broadcast transmission without RTS/CTS exchange and ACK process. In this paper, we propose a new method for improving broadcast receive ratio transmission using 2-Phase RTS/CTS handshakes sequence (DRTS).

A new MAC protocol for reducing effect of needless transmission deferment induced by missed RTS/CTS handshake

IEEE802.11DCF employs RTS/CTS (Request To Send/Clear To Send) mechanism for mitigating effect of hidden terminals. The RTS/CTS mechanism sets transmission deferral timer to neighbor terminals by exchanging RTS and CTS between transmitter and receiver. Then, in the case that RTS/CTS exchange succeeds, effect of hidden terminals could be suppressed, otherwise, any neighbor which received RTS and/or CTS defers its new transmission needlessly although DATA packet corresponding to the previous RTS/CTS exchange will not be transmitted. In this paper, we propose Cancel CTS, procedure in order to cope with the unnecessarily transmission deferment. Results of computer simulation confirms that our method well improves the throughput performance of IEEE802.11DCF.