Shiann-Tsong Sheu

IMT-advanced relay standards [WiMAX/LTE Update]

There are two candidates for IMT-Advanced (4 G) standards, LTE-Advanced by 3 GPP and 802.16 m by IEEE. This article focuses on relay architectures in 16 m and LTE-A, and discusses design principles and trade-offs leading to decisions in each standards group. Basically, 16 m relay and LTE-A Release 10 relay are very similar technologies where the relay is essentially an orthogonal frequency-division multiple access base station with a wireless backhaul link. However, some open issues, such as mobility, power saving, multihop architecture, transparent relaying, multi-arrier transmission, and cooperative transmission, are still left as challenges for engineers and researchers. This article provides insights to both relay standards that could be helpful for readers to fully comprehend practical ways of incorporating relays into 4 G wireless broadband networks.

A new multichannel access protocol for IEEE 802.11 ad hoc wireless LANs

The IEEE 802.11 wireless local area networks (WLANs) standard supports several equal-capacity communication channels which can be simultaneously shared and accessed by mobile stations. In such multichannel communication system, a mobile station basically can transmit on any of these channels based on a suitable access control protocol. However, with the feature of one transceiver per mobile station, the standard restricts mobile stations to operate in one selected channel and the other channel capacities are wasted inevitably. In this paper, we propose a new carrier sense multiple access (CSMA) based protocol, called multichannel access protocol (MAP), to support parallel transmissions in IEEE 802.11 ad hoc WLANs. To realize the proposed MAP protocol over contemporary ad hoc WLANs, the MAP protocol is not only compliant with the IEEE 802.11 standard but also taking one transceiver constrain into consideration. All mobile stations with MAP will contend for channel access right in a dedicated channel during a periodical contention reservation interval (CRI) and then transmit data frames over different channels by a channel scheduling algorithm (CSA). Given a number of requests, the problem of finding a proper schedule for these requests to be served on a multichannel system so that the longest channel busy period is minimal is known to be NP-hard (Hou, et al. 1994). The time complexity of proposed heuristic CSA is O(|X| log |X| + |X|M/sup 2/) where |X| and M denote the number of successful requests in the CRI and the number of available channels respectively. Simulation results show that the proposed MAP protocol with CSA achieves an obviously higher throughput than conventional IEEE 802.11 WLAN with single channel.

A bandwidth allocation/sharing/extension protocol for multimedia over IEEE 802.11 ad hoc wireless LANs

We propose a novel bandwidth allocation/sharing/extension (DBASE) protocol to support both asynchronous traffic and multimedia traffic with the characteristics of variable bit rate (VBR) and constant bit rate (CBR) over IEEE 802.11 ad hoc wireless local area networks. The overall quality of service (QoS) will be guaranteed by DBASE. The designed DBASE protocol will reserve bandwidth for real-time stations based on a fair and efficient allocation. Besides, the proposed DBASE is still compliant with the IEEE 802.11 standard. The performance of DBASE is evaluated by analysis and simulations. Simulations show that the DBASE is able to provide almost 90% channel utilization and low packet loss due to delay expiry for real-time multimedia services.

Grouping strategy for solving hidden node problem in IEEE 802.15.4 LR-WPAN

The medium access control (MAC) protocol defined in IEEE 802.15.4 standard is able to achieve low-power transmissions in low-rate and short-distance wireless personal area networks (WPANs). The modified CSMA/CA protocol used to minimize power consumption does not have the hidden-node protection mechanism, such as RST/CTS mechanism, for the sake of reducing the control overheads. Referring to previous research results, which proved that the probability of any two nodes in infrastructure network unheard each other is about 41%, the hidden-node problem (HNP) could result in inefficient data transmission in WPAN and quick power consumption. In this paper, we propose a simple and efficient grouping strategy to solve the IEEE 802.15.4 HNP without needing extra control overheads in data transmissions. The proposed strategy groups nodes according to their hidden-node relationships and then separates the periodic transmission period into several non-overlapping sub-periods, one for each group. The WPAN coordinator is responsible for detecting the hidden-node situation and performing the grouping procedure if necessary. In this paper, we also prove that the maximal number of groups in a WPAN is five. Simulation results demonstrate that the proposed strategy is able to improve the standard transmission efficiency and to conserve energy by eliminating the unnecessary collisions.

A highly reliable broadcast scheme for IEEE 802.11 multi-hop ad hoc networks

In wired networks, the broadcast data packets can be easily and safely delivered to destinations. Nevertheless, it is a big challenge to transfer the broadcast frames over the IEEE 802.11 based multi-hop ad hoc wireless networks due to the high bit error rate, the high collision probability, and the lake of acknowledgement (ACK). Unfortunately, most of routing protocols need the broadcast function to exchange important information between nodes. From our observations, the efficiency of the routing protocol, such as dynamic source routing (DSR) and ad-hoc on-demand distance vector (AODV), finding the path from source to destination is strongly depending on the supported broadcast scheme in the underlying media access control (MAC) protocol. In this paper, we first investigate the uncertain broadcast problem in the IEEE 802.11 MAC protocol while delivering the necessary broadcast frames. Since no acknowledgement will be sent by any recipient of the broadcast frame in IEEE 802.11 MAC protocol, we propose a highly reliable broadcast scheme to solve such uncertain problem. The proposed scheme, which is still compatible with standard, can efficiently minimize bandwidth consumption as well as propagation delay.

A novel delay-oriented shortest path routing protocol for mobile ad hoc networks

In wireless ad hoc mobile network, a host which desires to communicate with another host may need some intermediate nodes to relay data packets. To maximize the channel resource utilization and minimize the network transfer delay along the path, the shortest path with minimum hops approach is often adapted. However, by considering employing the medium access control (MAC) protocol, the minimum transfer delay from source to destination may be achieved by choosing a longer path but with less contention delay. We propose an efficient delay-oriented routing protocol for mobile ad hoc wireless networks. The expected access contention delay of the IEEE 802.11 protocol is analyzed to support the routing decision. Simulation results show that the derived path length in the proposed delay-oriented routing protocol is slightly higher than that of the conventional shortest path with minimum hops approach but it can significantly reduce both the average transfer delay and packet loss rate.

MR 2 RP: the multi-rate and multi-range routing protocol for IEEE 802.11 ad hoc wireless networks

This paper discusses the issue of routing packets over an IEEE 802.11 ad hoc wireless network with multiple data rates (1/2/5.5/11 Mb/s). With the characteristics of modulation schemes, the data rate of wireless network is inversely proportional with the transmission distance. The conventional shortest path of minimum-hops approach will be no longer suitable for the contemporary multi-rate/multi-range wireless networks (MR2WN). In this paper, we will propose an efficient delay-oriented multi-rate/multi-range routing protocol (MR2RP) for MR2WN to maximize the channel utilization as well as to minimize the network transfer delay from source to destination. By analyzing the medium access delay of the IEEE 802.11 medium access control (MAC) protocol, the proposed MR2RP is capable of predicting the transfer delay of a routing path and finding the best one, which has the minimum transfer delay from source to destination. The proposed MR2RP may choose a longer path but with less contention competitors and buffer queuing delay. Simulation results show that MR2RP performs the load balancing and fast routing very well, and its call blocking probability is obviously lower than that of conventional minimum-hops approach with fixed transmission rate.

DBASE: a distributed bandwidth allocation/sharing/extension protocol for multimedia over IEEE 802.11 ad hoc wireless LAN

In ad hoc networks, carrier sense multiple access (CSMA) is one of the most pervasive medium access control (MAC) schemes for asynchronous data traffic. However, CSMA could not guarantee the quality of real-time traffic. In this paper, we will propose a distributed bandwidth allocation/sharing/extension (DBASE) protocol to support multimedia traffics with the characteristics of variable bit rate (VBR) and constant bit rate (CBR) over ad hoc WLAN. Overall quality of service (QoS) will be guaranteed in DBASE. Such a bandwidth allocation procedure is based on a contention process that only occurs before the first successful access and a reservation process after the successful contention. If any real-time station leaves, the reserved bandwidth will be released by DBASE immediately. The designed DBASE protocol will not only allocate sufficient bandwidth for real-time stations but also permit them to extend bandwidth requirements on demand if there is any excess bandwidth left. Moreover, the proposed DBASE is still compliant with the IEEE 802.11 standard. In this paper, the system capacity of DBASE is analyzed and the performance of DBASE is evaluated by simulations. Simulations show that the DBASE is able to provide high channel utilization, low access delay and small delay variation for real-time services.

An Effective Path Selection Metric for IEEE 802.16-based Multi-hop Relay Networks

For usage models of coverage extension and throughput enhancement, a novel framework, Multi-hop Relay (MR) network, which consists of various kinds of stations such as the base station (MR-BS), the relay station (RS), and the mobile station (MS) is proposed by the study group under the IEEE 802.16 WG. With the help of RSs, data transmissions between MR-BS and MS become more reliable and effective. In the paper, we propose a new measurement, called effective radio resource index (ERRI), to indicate the effectiveness of radio resource of a link used to transmit data. The ERRI of a link is normalized and compared to the metric defined as the bandwidth unit required for transmitting a fixed amount of data using 64-QAM CC 5/6 with repetition 1. Additionally, a cost function of a relay path between MR-BS and MS is also proposed to efficiently facilitate the evaluation of path selection schemes for MR networks. Simulation results show that the relay path selection scheme with ERRI considerations is indeed effective in terms of network throughput.

The impact of RTS threshold on IEEE 802.11 MAC protocol

Wireless technologies and applications received great attention in recent years. The medium access control (MAC) protocol is the main element that determines the efficiency in sharing the limited communication bandwidth of the wireless channel in wireless local area networks (WLANs). The request-to-send/clear-to-send (RTSICTS) mechanism is an optional handshaking procedure used by the IEEE 802.11 wireless network to reduce the possibility of collision. The RTS-Threshold (RT) value which determines when the RTS/CTS handshaking mechanism should be used is an important parameter to investigate; since different RT values will produce different performance characteristics in data transmission. This paper presents an analysis of the influence of the RT parameter on the IEEE 802.11 wireless network, and gives a guideline to dynamically adjust the RT value. Simulation results of this paper show that the RTSICTS mechanism should be always turned on (RT = 0) to achieve an excellent performance while saving complex work designing a dynamic RT mechanism which will not have notable effect.