This paper proposes a new MAC protocol and physical channel control schemes for TDMA-TDD multi-slot packet channel. The goal of this study is to support both circuit-switched and packet-switched communications on the same resources and to enable high-speed packet transmission using a multi-slot packet channel. In the proposed channel control schemes, three points are taken into account; 1) effective sharing of time slots and frequencies with minimum impact on circuit communications, 2) compatibility with the existing access protocol and equipment, and 3) dynamic allocation of uplink and downlink slots. As for the MAC protocol, we adopt BRS (Block Reservation Scheme) and adaptive access control scheme to the proposed MAC protocol. In addition, to overcome the inherent disadvantage of TDD channels, packet scheduling and access randomizing control are newly proposed in this paper. The results of throughput and delay evaluations confirm that downlink capacity can be drastically enhanced by the dynamic allocation of uplink and downlink slots while corruption under heavy traffic loads is prevented by applying the adaptive traffic load control scheme.

VoIP (Voice over IP) is one of the real time applications that demand wireless LAN systems meet severe quality requirements which commonly involve 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 based on autonomous distributed control that allows wireless LANs 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%.

This paper introduces a new analytic method that uses modified state equations to evaluate the performance of PCSD (Packet Channel Sharing protocol for DCA systems) with the goal of increasing the spectrum efficiency of DCA systems by realizing channel sharing between circuit-switched calls and packets. The results of this analysis show that PCSD is more suitable for microcellular systems than cellular systems, and that PCSD system performance improves as the average holding time of circuit-switched calls increases. Moreover, this paper proposes a novel scheme to determine the optimum release delay time of packet channels in order to achieve high throughput for packets as well as high channel capacity for circuit-switched calls. The proposed scheme shows that the optimum release delay time for PHS (Personal Handy-phone System) is greater than 60 frames and less than 100 frames.

The implementation and experimental evaluations of distributed zero-forcing beamforming (DZFBF) for downlink multi-user multiple-input multiple-output (DL MU-MIMO) systems are presented. In DZFBF, multiple access points (APs) transmit to own desired stations (STAs) at the same time and using the same frequency channel while mitigating inter-cell interference. To clarify the performance and feasibility of DZFBF, we develop a real-time transmission testbed that includes two APs and four STAs; all are implemented using field programmable gate array. For real-time transmission, we also implement a simple weight generation process based on ZF weight using channel state information which is fed back from STAs; it is an extension of the weight generation approach used in DL MU-MIMO systems. By using our testbed, we demonstrate the real-time transmission performance in actual indoor multi-cell environments. These results indicate that DL DZFBF is more effective than DL MU-MIMO with time division multiple access.

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.

As wireless LAN systems become more widespread, the number of access points (APs) is increasing. A large number of APs cause overlapping cells where nearby cells utilize the same frequency channel. In the overlapping cells, inter-cell interference (ICI) degrades the throughput. This paper proposes an interference-aware multi-cell beamforming (IMB) technique to reduce the throughput degradation in the overlapping cells. The IMB technique improves transmission performance better than conventional multi-cell beamforming based on a decentralized control scheme. The conventional technique mitigates ICI by nullifying all the interference signal space (ISS) by beamforming, but the signal spaces to the user terminal (UT) is also limited because the degree of freedom (DoF) at the AP is limited. On the other hand, the IMB technique increases the signal space to the UT because the DoF at the AP is increased by selecting the ISS by allowing a small amount of ICI. In addition, we introduce a method of selecting the ISS in a decentralized control scheme. In our work, we analyze the interference channel state information (CSI) and evaluate the transmission performance of the IMB technique by using a measured CSI in an actual indoor environment. As a result, we find that the IMB technique becomes more effective as the number of UT antennas in nearby cells increases.

Combining heterogeneous wireless networks that cross licensed and unlicensed spectra is a promising way of supporting the surge in mobile traffic. The unlicensed band is mostly used by wireless LAN (WLAN) nodes which employ carrier sense multiple access/collision avoidance (CSMA/CA). Since the number of WLAN devices and their traffic are increasing, the wireless resource of the unlicensed band is expected be more depleted in 2020s. In such a wireless environment, the throughput could be extremely low and unstable due to the hidden terminal problem and exposed terminal problem despite of the large resources of the allocated frequency band and high peak PHY rate. In this paper, we propose user equipment (UE) centric access in the unlicensed band, with support by licensed band access in the mobile network. The proposed access enables robust downlink transmission from the access point (AP) to the UEs by mitigating the hidden terminal problem. The licensed spectrum access passes information on the user data waiting at the AP to the UEs and triggers UE reception opportunity (RXOP) acquisition. Furthermore, the adaptive use of UE centric downlink access is presented by using the channel utilization measured at the AP. Computer simulations confirm that licensed access assistance enhances the robustness of the unlicensed band access against the hidden terminal problem.

Efficient use of heterogeneous wireless access networks is necessary to maximize the capacity of the 5G mobile communications system. The wireless local area networks (WLANs) are considered to be one of the key wireless access networks because of the proliferation of WLAN-capable mobile devices. However, throughput starvation can occur due to the well-known exposed/hidden terminal problem in carrier sense multiple access with collision avoidance (CSMA/CA) based channel access mechanism, and this problem is a critical issue with wireless LAN systems. This paper proposes two novel schemes to identify starved access points (APs) and user equipments (UEs) which throughputs are relatively low. One scheme identifies starved APs by observing the transmission delay of beacon signals periodically transmitted by APs. The other identifies starved UEs by using the miscaptured beacon signals ratio at UEs. Numerous computer simulations verify that that the schemes can identify starved APs and UEs having quite low throughput and are superior to the conventional graph-based identification scheme. In addition, AP and UE management with the proposed schemes has the potential to improve system throughput and reduce the number of low throughput UEs.