Takeo Ichikawa

Novel multiple access protocol for voice over IP in wireless LAN

VoIP (voice over IP) is one of the real time applications which demand that wireless LAN systems achieve severe quality requirements in terms of delay time, jitter and packet loss. However, it is difficult for CSMA/CA (carrier sense multiple access with collision avoidance) to achieve the service quality demanded by VoIP if voice and data traffic coexist, so some form of priority control is needed. This paper proposes a novel multiple access protocol that allows wireless LANs based on autonomous distributed control to satisfy the VoIP requirements. This new protocol suits both VoIP and data traffic and executes priority control dynamically according to whether the VoIP packet collides with a data packet or another VoIP packet. The results of a theoretical analysis and computer simulations indicate its excellent performance. This proposed protocol reduces the delay time of VoIP packets by 54 to 70% compared with conventional CSMA/CA, even if the traffic load increases, provided that the packet loss probability is less than 3%.

Spatial Domain Resource Sharing for Overlapping Cells in Indoor Environment

As microcell wireless systems become more widespread, intercell interference among the access points will increase due to the limited frequency resource. In the overlapping cell scenario, radio resources should be shared by multiple cells. Although time and frequency resource sharing has been described in many papers, there is no detailed report on dynamic spatial resource sharing among multiple cells for microcell wireless systems. Thus, we present the effectiveness of spatial resource sharing among two access points. We introduce two scenarios based on the zero forcing method; one is the primary-secondary AP scenario and the other is the cooperative AP scenario. To evaluate the transmission performance of spatial resource sharing, channel matrices are measured in an indoor environment. The simulation results using the measured channel matrices show the potential of spatial resource sharing.

A Packet Combining Demodulation Scheme for Multi-Hop Wireless Systems using Network Coding

Network coding has been referred to as the way to improve the network throughput of multi-hop communication network systems. Therefore a lot of researches have focused on IP and MAC layers. However, this report focuses on the demodulation scheme for the physical layer network coding and propose a packet combining demodulation scheme for network coded multi-hop wireless systems. With this scheme, a system can receive the benefits of both cooperative transmission and network coding, and the communication quality is drastically improved. Numerical results show improvements in the packet error probability of the network coded system based on the IEEE 802.11a standard.

Development and experimental validation of downlink multiuser MIMO-OFDM in gigabit wireless LAN systems

Multiuser multiple-input multiple-output (MU-MIMO) has been proposed as a means to improve spectrum efficiency for various future wireless communication systems. This paper reports indoor experimental results obtained for a newly developed and implemented downlink (DL) MU-MIMO orthogonal frequency division multiplexing (OFDM) transceiver for gigabit wireless local area network systems in the microwave band. In the transceiver, the channel state information (CSI) is estimated at each user and fed back to an access point (AP) on a real-time basis. At the AP, the estimated CSI is used to calculate the transmit beamforming weight for DL MU-MIMO transmission. This paper also proposes a recursive inverse matrix computation scheme for computing the transmit weight in real time. Experiments with the developed transceiver demonstrate its feasibility in a number of indoor scenarios. The experimental results clarify that DL MU-MIMO-OFDM transmission can achieve a 972-Mbit/s transmission data rate with simple digital signal processing of single-antenna users in an indoor environment.

Proposal of wireless traffic control schemes for wireless LANs

This paper proposes two traffic control schemes to support the communication quality of multimedia streaming services such as VoIP and audio/video over IEEE 802.11 wireless LAN systems. The main features of the proposed scheme are bandwidth control for each flow of the multimedia streaming service and load balancing between access points (APs) of the wireless LAN by using information of data link, network and transport layers. The proposed schemes are implemented on a Linux machine which is called the wireless traffic controller (WTC). The WTC connects a high capacity backbone network and an access network to which the APs are attached. We evaluated the performance of the proposed WTC and confirmed that the communication quality of the multimedia streaming would be greatly improved by using this technique.

PHS with packet data communications (PHS-PD) protocol architecture

The Personal Handy-phone System (PHS) started operating in Japan in July 1995 and there were over 4.6 million users as of November 1996; it is now spreading to other countries in Asia and Oceania. We have developed the PHS with packet data communications (PHS-PD). It provides packet-switched data communications services in addition to existing voice services. This paper overviews PHS-PD and proposes the optimum protocol stack. The proposed stack provides high performance when connected to various public/private networks, and easy Internet access from the mobile environment. A PHS-PD experimental system is introduced.

A Master-Slave Channel Selection Scheme to Improve Fairness among Cells in IEEE 802.11 Wireless LANs

This paper proposes a master-slave channel selection scheme to improve fairness among cells in IEEE 802.11 wireless LANs. Each access point (AP) in our proposed scheme selects its operating channel in such a way that interference is minimized and the number of APs on each channel is balanced. Through numerous simulations and testbed experiments with UDP and TCP traffic flows, we show that the proposed scheme improves the minimum cell throughput and fairness among cells.

Indoor experiments on real-time multiuser MIMO transmission in wireless LAN systems

This paper presents indoor experimental results on an implemented real-time downlink multiuser multiple-input multiple-output (DL-MU-MIMO) transceiver for next generation wireless LAN systems. In the transceiver, channel state information (CSI) is estimated at each station (STA) and fed back to an access point (AP) on a real-time basis. At the AP, transmit beamforming weight for DL-MU-MIMO transmission is calculated by using the estimated CSI. This paper also proposes a fast computation scheme for computing transmit weight in real time. Indoor experiments with the DL-MU-MIMO transceiver demonstrate its feasibility. The experimental results clarify that DL-MU-MIMO transmission can achieve system throughput of greater than 500 Mbit/s with simple digital signal processing of single-antenna STAs in an indoor environment.

Frame transfer protocol with shortcut between wireless bridges

In this paper, we propose a frame transfer protocol with shortcut (FRASH): a novel frame transfer protocol suitable for wireless bridge networks based on the wireless LAN standard approved by IEEE 802.11 committee. FRASH has compatibility with conventional transparent bridge protocols for Ethernet/IEEE 802.3 bridges. Furthermore, it achieves higher throughput and less forwarding delay than a conventional protocol. This paper presents a detailed FRASH scheme and clarifies that FRASH improves throughput characteristics by more than 40% compared with the conventional protocol by computer simulations.

Channel Access Acquisition Mechanism Coupled with Cellular Network for Unlicensed Spectrum

Interworking among heterogeneous wireless networks across licensed and unlicensed spectra has gained much attention to support the surge in mobile traffic. Wireless LAN (WLAN) is well known as the dominant wireless network application in the unlicensed spectrum. Unlicensed spectrum access should follow carrier sense multiple access/collision avoidance (CSMA/CA) to share the wireless resource with existing WLAN nodes. However, the transmission throughput with CSMA/CA can be significantly degraded by the hidden terminal problem in environments where huge mobile traffic depletes the wireless resources. In this paper, we propose a channel access acquisition mechanism that uses licensed spectrum access. The proposed mechanism significantly reduces the impact of the hidden terminal problem by using both transmission and reception opportunities. So that the receiver can efficiently get reception opportunities, the information on the user data waiting at the transmitter is notified by using a licensed spectrum channel. Computer simulations show that the proposed access mechanism enables robust transmission even if the wireless resource at the receiver is depleted by transmission from nodes hidden from the transmitter.