In the fifth generation ($\mathbf{5 G}$) mobile networks, the growing energy consumption of base stations (BS) has made network energy saving (NES) an increasingly important issue. This paper proposed optimized schemes, including the BSoriented NES (B-NES), the user equipment (UE)-oriented NES (U-NES), and the hybird NES (H-NES), for the NES function (NESF) to make the BS energy-saving decision. The first two schemes employ heuristic algorithms, while the last scheme adopts the Harris Hawks Optimization (HHO) or Whale Optimization Algorithm (WOA), based on an evolutionary approach. With the proposed schemes, each UE periodically measures the received signal reference power (RSRP) of both the serving and neighboring BSs and reports candidate BSs with an RSRP at least as high as that of the serving BS to the NESF. Additionally, each BS periodically reports its current and predicted traffic load to the NESF. The information is utilized by the NESF to optimize network energy efficiency by determining the transition of each BS between energy saving and active states, while maintaining the quality of service of UEs. Simulation results show that the H-NES scheme achieves the best energy efficiency, however its computational complexity is the highest. In contrast, the UNES scheme, which has the lowest computational complexity, outperforms the B-NES scheme and performance lower than that of the H-NES scheme. In conclusion, the proposed schemes not only enable energy-efficient networks but also exhibit adaptability for deployment in the sixth-generation (6G) mobile networks.

Traditional handover mechanisms rely heavily on continuous user equipment (UE) measurements and frequent reporting to centralized gNBs. This approach introduces significant signaling overhead and latency, rendering it inadequate for latency-sensitive applications, such as extended reality (XR). This paper proposes a novel handover process that leverages geofencing and UE-based decision-making to improve handover performance. By introducing a geofencing approach, the UE remains in a measurement-free state within predefined regions, activating signal measurements only when exiting the geofence region. The UE independently evaluates the serving and neighbor gNBs and initiates a reverse handover request to the serving gNB, eliminating the need for periodic measurement reporting. Simulation results demonstrate that the proposed mechanism significantly reduces the number of measurements and signaling overhead, thereby improving scalability and resource efficiency.

In 5G cellular networks, base stations (BS) are the main contributors to energy costs, making their efficiency crucial, especially as we move toward 6G. This paper proposed a scheme that extends the radio resource management measurement for user equipment (UE) handover to support BS energy saving. UEs periodically measure the received signal reference power of the serving and neighboring BSs, reporting both the serving BS and the best BS with the maximum RSRP to the access and mobility management function (AMF). Additionally, BSs report their current and predictive cell traffic loads to the AMF. This information helps the AMF identify BSs that can enter energy-saving (ES) state or return to ACTIVE state, ensuring a balance between energy savings and quality of service. Such a distributed traffic prediction process reduces the AMF’s workload. The scheme also addresses potential handover issues to maximize BSs in ES state. Simulation results demonstrate that the proposed scheme achieves significant network energy savings without sacrificing the quality of services of UEs.

With the latest development of the low earth orbit (LEO) constellation, satellite communication is considered in 3GPP as a promising NR-based non-terrestrial network (NTN) solution for extending 5G service to underserved areas. Due to the inherent characteristics of NTN, such as long propagation delay and high speed movement, some NR functions need to be redesigned or amended. This paper focuses on the random access procedure where the UEs may or may not have GNSS capability. To reuse legacy RACH Occasion (RO) scheme, a UE is assumed to have its location information for tracking the ephemeris of LEO satellite and compensating the propagation delay of slang range; otherwise, the new design of RO scheme is inevitable. This paper considers the mixed environment where a portion of UEs have GNSS-capability. Three different RO schemes, which are backward compatible with legacy RO scheme, are proposed to resolve the potential issues. Simulation results show that the proposed RO schemes outperform the legacy RO scheme in terms of the average delay and probability of successful preamble transmission regardless of the UE with or without GNSS capability.

The Industrial Internet of Things (IIoT) is gradually shifting from traditional wired transmission to wireless access network aiming to reduce the complexity of factory wiring. Because of ongoing advancements in IIoT applications, the demanding for low latency and ultra reliability become mandatory. Thus, new industry network services such as Time-Sensitive Networking (TSN) and Ultra-reliable and Low Latency Communications (URLLC) are designed. This study proposes a cost-effective solution for industrial networks by utilizing the mature Bluetooth Low Energy (BLE) physical layer for IIoT data transmission. The concept is to deploy multiple pairs of BLE modules on IIoT devices and adjust the relevant operational parameters, including data transmission scheme, timer synchronization and frequency hopping mechanism, to improve the reliability and reduce the transmission delay. In order to validate the performance, we developed the whole system to ensure the packet latency is under 1ms and the error rate is less than 10−5.

Low earth orbit (LEO) satellite system becomes an emerging non-terrestrial network solution for future radio communications. To maximize the system capacity, an LEO satellite could utilize circularly polarized planar antenna array with the frequency reuse technique to form multiple earthmoving beams. With the increasing number of LEO satellites, inline interference from the geosynchronous equatorial orbit (GEO) satellite is an important issue to be resolved when the spectrum resource is shared. With the frequency hopping phenomena in earth-moving LEO networks, the inline interference could be minimized if the GEO users also perform the similar frequency hopping via coordination between LEO and GEO satellite networks. This paper proposes eight spectrum sharing schemes for GEO satellite networks to produce the frequency hopping sequence in earth-moving LEO network. We evaluate the system capacity, fairness between LEO and GEO users, and time costs of all schemes. Simulation results validate the advantage obtained from coordination, and show that the simple greedy scheme performs as good as the optimal scheme due to fixed frequency hopping patterns.

To tackle the problem of transmission uncertainty for ultra-reliable and low-latency communication (URLLC) in unlicensed band, existing studies have proposed many channel reservation based schemes categorized into single-channel reservation and multi-channel reservation schemes. The schemes integrate the full duplex radio and orthogonal busy tone technologies to reserve URLLC channel. URLLC channel reservation is accomplished by all URLLC devices continuously sending busy tones on the URLLC channel. Compared to the multi-channel reservation scheme, the single-channel reservation scheme is more friendly to the heterogeneous system, such as Wi-Fi, but it generates an unsatisfactory probability of successful channel reservations as it is difficult to find the common single channel for all URLLC devices especially when the traffic load of unlicensed band is above a certain level. This paper proposes a progressive single channel reservation (P-SCR) scheme to promote the success probability of channel reservation by relaxing simultaneousness restriction of URLLC channel reservation. The P-SCR scheme allows user equipment to reserve the common URLLC channel in a progressive manner and introduces an advanced preparation time to improve channel utilization and probability of successful URLLC channel reservations. The performance evaluation results showed that the proposed P-SCR scheme obviously outperforms the existing single- and multi-channel reservation schemes in terms of the probability of successful channel reservation.

Ultra-reliable and low latency communications (URLLC) is one of the major scenarios in IMT-2020. The increase of network traffic significantly downgrades the transmission quality of the licensed spectrum and makes the strict requirements of URLLC hard to be satisfied. Thus, offloading traffic from the licensed spectrum to the unlicensed spectrum is a promising solution to cope with this challenge. This paper proposes a beam-based channel reservation mechanism to occupy the channel in the unlicensed spectrum. We also investigate the influence of hidden node problem on system performance. Numerical results validated that the proposed scheme can achieve the requirement of URLLC.

IMT-2020 focuses on providing new services over diverse application scenarios, such as ultra-reliable low latency communications (URLLC). Regarding the increase of mobile traffic demand, the consensus of deploying 5G in unlicensed spectrum has been reached under the premise that the coexistence issue between 5G and Wi-Fi has been resolved. This paper proposes a hopping-based multi-channel coordination scheme, aiming at determining the minimal number of channels occupied for URLLC service without violating regulations. The proposed scheme uses the busy tone and full-duplex radio technologies to reduce the interference and collision. Moreover, by using the proposed scheme, the URLLC device can efficiently hop to the channel with less or no collision for data transmission. Numerical results validated that the proposed scheme actually achieves the reliability requirement of URLLC.

This paper introduces the concept of parallel transmission and proposes two novel retransmission schemes based on parallel transmission for uplink ultra-reliable and low latency communications. One scheme performs two transmission attempts over both dedicated and shared resources in the first transmission opportunity and the retransmission over the dedicated resource in the next one if necessary. The other scheme performs parallel retransmission over both types of resources if the initial transmission is unsuccessful. We formulate the system model to analyze the reliability and resource utilization of different retransmission schemes. Numerical results show that the proposed retransmission scheme has a higher reliability than existing ones. Moreover, one of the proposed schemes significantly uses less resource than other schemes.

LoRaWAN is a promising solution of Low-power wide area network (LPWAN) operating in unlicensed spectrum to support long range wireless services for Internet of Things (IoTs). However, with the growth of IoT devices deployed in a fixed geographic area, the immunity to interference on the communication increases significantly. Attackers may utilize such situation to jam packet transmission by emitting RF interference signal at the same time when a LoRa end node is sending data to the LoRa gateway. As a consequence, the transmission of the LoRa end node would fail due to collision which would in turn reduce the network performance. In this paper, we implement a LoRa jammer on commercial LoRa devices by modifying the open source and figure out the proper setting of the jammer through three scenarios aiming to evaluate the influence of LoRa transmission configuration on jamming performance. Specifically, the impact of non-orthogonality of LoRa transmission on jamming effect is investigated. Possible countermeasures for LoRaWAN are then presented to alleviate the jamming attacks.

The Listen-Before-Talk (LBT) is the main procedure for Licensed Assisted Access (LAA) to accomplish fair and friendly coexistence with other operators or technologies operating over unlicensed spectrum. However, in LBT, the lack of coordination with the other existing systems brings challenges in sustaining performance in the coexistence of LAA and WiFi networks. Specifically, the hidden node situation (HNS) and exposed node situation (ENS) cannot be effectively handled when both LAA and WiFi attempt to access the unlicensed spectrum at the same time. Thus, transmission failure might occur and the network performance would be degraded. In order to mitigate the influences caused by HNS and ENS, based on LBT, a Give-And-Take (GAT) mechanism with the Listen-Before-Receive (LBR) procedure is proposed in this paper. With GAT, the LAA sender conditionally gives up or takes back transmission opportunities, and thus the unlicensed spectrum could be efficiently shared between LAA and WiFi in terms of energy usage and throughput. Evaluation results show that the proposed GAT could conditionally obtain better network performance comparing to legacy LBT.

In indoor location environment, shadowing, multipath and reflection phenomena make the position estimation inaccurate. Large number of Access Points (APs) equipped with single antenna could obtain enough Receive Signal Strength (RSS) information to describe signal characteristics of the target space. However, it results in a large investment costs. This paper provides a novel indoor positioning solution using the multi-antennas architecture to increase the accuracy of location estimation. The system obtains enough RSS to compute the user location and provides a reasonable cost of deployments in indoor location environment. Experimental and numerical results show that the proposed indoor location estimation demonstrates a high accuracy of positioning and outstanding performance when RSS is collected by smart antenna technique. More specifically, the accuracy is increased from 1.34% to 92.4% and the error variance is reduced by 96% as compared to the traditional location fingerprint method.

In LTE TDD, some configurations which assign more subframes for Downlink (DL) are designed for heavy DL traffic. In such case, multiple HARQ ACK/NACKs for different DL subframes shall be transmitted in a given Uplink (UL) subframe. This situation will become worse while the Machine Type Communication (MTC) User Equipments (UEs) come into the LTE network because the number of the MTC UEs is very large and with several particular features. Indeed, to efficiently reduce the UL overhead caused by control signalling, the time bundling is adopted to combine multiple HARQ ACK/NACKs into an ACK/NACK by AND operation. However, the time bundling increases the number of transmissions and also degrades the overall system throughput. In this paper, we propose the Go-back-N (GBN) time bundling to decrease the number of transmissions and to increase the system throughput by strategically mixing the transmitted Transport Blocks (TBs) and the new coming TBs controlled by the transmit window. The simulation results show that the GBN time bundling can significantly improve the efficiency of HARQ feedback operation in LTE TDD. In other words, the overall throughput performance and the number of transmissions are improved especially for a large number of MTC UEs in LTE TDD network.

In wireless networks, the cyclic redundancy check (CRC) technique often cooperates with retransmission schemes to achieve reliable transmission. However, too many retransmissions in error-prone wireless environments not only create potential latency but also limit the overall bandwidth capacity. In general, an erroneously received packet is likely to possess some useful information, by which receivers can derive the original packet from a number of erroneously received copies. This study proposes a CRC-based packet recovery (CPR) mechanism to determine the original packet by minimizing the number of retries per packet. The CPR mechanism only buffers the first erroneously received copy and maintains a bitmap vector to relax the buffer requirement to derive the positions of inconsistent bits from erroneous copies. Based on these bit positions, the CPR mechanism generates all candidate packets, one of which is likely to be the correct packet. This study also designs two architectures with different numbers of CRC encoders to implement the proposed CPR mechanism for evaluations and comparisons. Performance evaluation results confirm that the proposed CPR mechanism can significantly reduce unnecessary retransmissions and buffer space consumption, compared with the existing mechanisms. Results also show that different architectures reveal a tradeoff between hardware cost and processing delay.

In IEEE 802.16 specifications, a CRC field can be attached in the rear of a WiMAX MPDU. Thus, each intermediate node (relay node or base station) in an 802.16-based multihop network can perform error detection on a received WiMAX MPDU. On the sender, the ARQ mechanism will be enabled to retransmit the MPDU if a transmission error occurs and an ACK message cannot be received. However, the ARQ mechanism is unsuitable for real-time traffic due to the delay consideration and the CRC field becomes an option to reduce the overhead of a WiMAX MPDU. Once the CRC field is removed from a WiMAX MPDU, the error occurrence cannot be detected by the intermediate nodes in time, and the erroneous data is still forwarded in the network but discarded by the destination. The resources of nodes in a network are wasted. Hence, the error detection mechanism should be retained for real-time traffic, but the overhead of CRC should be reduced. In this paper, we study an efficient discrete-error-checking scheme (DECS) to reduce the CRC overhead for real-time traffic. The DECS only needs several CRC checking bits, which are attached behind some WiMAX MPDUs. The DECS can efficiently provide the error detection for the intermediate nodes to discard these erroneous and useless data early. Thus, the utilization of node resources and wireless bandwidth can be improved significantly.

The mobile wireless metropolitan area networks (WMAN) architecture adopts power saving mechanism to prolong the battery lifetime of mobile devices. Due to the burst characteristic of non-real-time service, the exponentially growing sleep window, which is employed in the IEEE 802.16e and IEEE 802.16m, seems not the best approach in term of packet response time. To let the sleep window match the traffic pattern, base station (BS) should collect traffic information and then notify mobile station (MS) the appropriate sleep window size. As a solution, we propose an advanced power saving mechanism, namely A-PSM, which simply adjusts the sleep window according to the average packet inter-arrival time in order to improve the power saving capability. To reduce the negotiation overhead and accomplish the synchronization of sleep pattern between BS and MS, the information of measured inter-packet arrival time is piggybacked on every downlink packet sent from BS. Moreover, a parameter of tolerable delay is proposed to control the packet delay when the inter-packet arrival time suddenly becomes longer. As confirmed by the performance evaluation, the proposed A-PSM can easily achieve better power saving capability, as compared to the IEEE 802.16e and IEEE 802.16m under all kinds of traffic arrival rates.

Media streaming using P2P (peer to peer) technique has been implemented and widely deployed in the present time. In the traditional P2P media streaming, each peer, while downloading media contents, is at the same time simultaneously uploading the media contents to other peers, which will contribute to the overall available bandwidth. Traditionally before playing back the information, P2P media streaming requires to download and reassemble media content. However, time delay occurs when media contents were required to receive and reassemble from highly dynamic and autonomous mobile peers. Until now, delay free playback is still a challenge for peers to playback video instantly. Content Distributed Network (CDN) server provides user with original media contents and instant playback. Although, CDN seems to be satisfied with media streaming; the unique characteristics of media steaming, such as large file size, high bandwidth requirement, and sustained streaming duration can lead to limitation of services capacity. In this paper, we proposed a mixture of P2P and CDN system for media streaming in mobile environment, which can reduce both initial and average duration of playback and accommodating more users, therefore scalability has been improved.

The IEEE 802.11p architecture imposes a demand- ing reliable transmission requirement on both road-side-unit (RSU) and on-board-unit (OBU) in order to provide emergency messages, traffic information, and internet services. The IEEE 802.16e network, which has been designed as a centralized control system to provide the mobile services in a metropolitan area network (MAN), can assist the WAVE network to fulfill the requirement. As all OBUs equip with both IEEE 802.11p and IEEE 802.16 transceivers, the deficiency of the IEEE 802.11p contention-based medium access control protocol can be curbed by using the IEEE 802.16e centralized scheduling scheme. As a solution, we propose a novel scheme, namely coordinated scheduling scheme (CSS), which allows the IEEE 802.11p OBUs to access service channels in a contention-free manner by means of IEEE 802.16 centralized scheduling. As confirmed by the performance evaluation, the proposed CSS can achieve significant throughput improvement, as compared with the IEEE 802.11p standard.

Wireless mesh networks (WMNs) is a promising technology that provides broadband wireless Internet access. It is one of the most important challenges to establish multiple connections among stations (STAs) in WMNs. In this paper, we propose a novel automatic mesh scheme (AMS) of layer 2 to form WMN in IEEE 802.11 wireless networks. AMS establishes multiple connections from a STA to neighbor STAs and determines proper routing path effectively, where three objectives are optimized, i.e., minimizing the average hop count, balancing traffic load and without modifying the communicating procedure in IEEE 802.11 standard. AMS owns self-organization capacity, establishes multiple connections and determines routing path automatically. Our AMS only uses simple software functions without the cost of extra hardware overhead. Simulation results show that the proposed scheme significantly improves the system performance. Importantly, AMS that works very well has been implemented and exhaustively tested in a realistic test-bed.

To extend the battery life of an mobile station (MS) roaming in wireless metropolitan area networks (WMAN), the support of power saving mechanism is needed. However, the features of fast growing of sleeping window and fixed listening window in power saving class of type I (PSC-I), which is designed for handling non-real-time and bursty traffic, may consume much more power when the traffic intensity is time-varying. As a solution, we propose a universal power saving mechanism, namely UPSM, which adopts a constant sleeping window to reduce the negotiation overhead and alleviate the possible degradations of sleep ratio, energy saving, and mean packet delay caused from irregular packet inter- arrival gaps. Moreover, we utilize a score based metric for evaluations and also provide the mathematical analyses for figuring out appropriate settings. As confirmed by the performance evaluation, the proposed UPSM with specific constant sleeping window can easily achieve better power saving capability than PSC-I under any kind of traffic intensity.

The wireless access in vehicular environment (WAVE) architecture of intelligent transportation system (ITS) has been standardized in the IEEE 802.11p specification and it is going to be widely deployed in many roadway environments in order to provide prompt emergency information and internet services. A typical WAVE network consists of a number of WAVE devices, in which one is the road-side-unit (RSU) and the others are on-board-units (OBUs), and supports one control channel (CCH) and one or more service channels (SCH) for OBUs to access. The CCH is used to transport the emergency messages and service information of SCHs and any SCH is used to carry internet traffic of OBUs. However, the IEEE 802.11p contention-based medium access control protocol would downgrade transmission efficiency when the number of OBUs served by a RSU is large and all WAVE devices equipped with a single transceiver access to the same SCH. Synchronizing all WAVE devices to periodically and equally access the CCH and an SCH will cost as much as 50% channel resource wastage on SCH. As a solution, we propose an efficiency-improvement scheme, namely agent-based scheduling (ABS) scheme, which promotes the SCH throughput by means of choosing one OBU to be the agent to schedule the other OBUs contending the access opportunities on one SCH to access the other SCH served by RSU in a contention-free manner. Numerical results and simulation results demonstrate that the proposed scheme indeed improves the standard transmission efficiency.

Wireless access in vehicular environment (WAVE) architecture of intelligent transportation system has been standardized in the IEEE 802.11p specification. The WAVE network supports the features of multi-rate and multi-channel and it is going to be widely deployed in realistic roadway environments in order to provide traffic information and convenient services. However, the adopted contention-based medium access control protocol, which confronts the performance anomaly problem, would severely downgrade transmission efficiency between road-side-unit (RSU) and on-board-unit (OBU) because they may use diverse data transmission rates to access channel. As a solution, this paper proposes the vehicular channel access scheme (VCAS) to group a number of OBUs with similar transmission rates into one channel to optimize the channel throughput, while the group sizes of channels are controlled in order to fulfill the fairness requirement. To flexibly compromise the tradeoff between throughput and fairness, a marginal utility model is proposed in this paper. Simulation results demonstrate that the proposed VCAS with marginal utility provides a flexible way to handle versatile vehicular scenarios.

The new mobile wireless metropolitan area network (WMAN) architecture imposes a demanding performance requirement on the radio resource to provide broadband internet access. The radio resource is partitioned as bursts in time and frequency domains and used by mobile stations (MS) in an exclusive manner. The base station (BS) functionally serves as a resource controller for traffic to and from the MSs associated with it, and thus naturally generates the proper downlink (DL) and uplink (UL) MAPs for active MSs based on service and traffic requirements. However, the DL-MAP construction scheme in IEEE 802.16e OFDMA standard, which was designed for handling irregular traffic pattern of MS, often produces a large DL-MAP, as a certain small amount of data distribution to MSs will render a potential overhead of information elements (IE) in DL-MAP and limit the overall capacity. Moreover, the robustness requirement on MAP broadcasting would further cause severe system overhead. As a solution, we propose two exclusive condensed DL-MAP structures, which only carry partial information of each rectangular burst in order to reduce the size of IE in DL-MAP. For each condensed DL-MAP structure, the algorithms for BS to produce the condensed DL-MAP structure and the scheme for MS to precisely reconstruct the original DL-MAP structure are provided. As confirmed by the analytical results, the proposed condensed DL-MAP can achieve significant DL-MAP size reduction compared with standard DL-MAP structure.

Based on IEEE 802.16 specifications, the convergence sublayer (CS) of 802.16-based networks provides the function for heterogeneous upper layer protocols. The CS can encapsulate such as IP packets with variable lengths or ATM cells with fixed length in its MAC PDU and usually adds a CRC-32 field to the rear of the MAC PDU. Based on the CRC-32, each 802.16 relay node or BS can detect error occurrence on a MAC PDU and use ARQ mechanism to retransmit the corrupt MAC PDU. However, for real-time streaming (e.g., UDP streaming), the ARQ mechanism is not required and the CRC-32 is also removed. Once CRC-32 is removed from the MAC PDUs, PDU error will not be detected by the intermediate nodes in time and these corrupt IP packets or ATM cells will be still forwarded in the networks to waste radio bandwidth. Hence, the error detection mechanism could be maintained, but the large overhead of CRC-32 should be reduced for real-time streaming. In this paper, we propose an efficient discrete-error-checking scheme (DECS) to reduce the overhead of error detection for real-time streaming. The proposed DECS only needs a few checking bits inserted in the MAC PDU, and it can efficiently provide the error detection function. Moreover, a chase mechanism concurrently works on each relay node and BS to early discard these corrupt and useless IP packets or ATM cells. The concept of the DECS including the chase mechanism can be extensively implemented in many wire and wireless multi-hop networks to further improve bandwidth utilization.

The mobile wireless metropolitan area network (WMAN) architecture imposes a demanding reliable transmission requirement on both base station and mobile station to provide continuously bi-directional services. The transmitter will functionally allocate a buffer for outbound traffic, and thus naturally provide reliable transmission by using the acknowledgement scheme and the automatic repeat request (ARQ) mechanism. The IEEE 802.16 standard supports four report types for a receiver to adaptively report the binary reception status of the received ARQ block to the transmitter in the more efficient way. However, the standard does not recommend the strategy for selecting the most appropriate report type for certain report content, as using an improper report type will render more message overhead and cause long packet reception delay. As a solution, we propose a new strategy, namely smart ARQ report strategy (SARS), which evaluates the efficiency of each report type on encoding the binary report content to select the best report type with the minimal signal overhead and maximal report capability. Simulation results reveal that SARS can achieve significant overhead reduction as well as better response time than the strategy using single report type.

The mobile wireless metropolitan area network (WMAN) architecture imposes a demanding power management requirement on mobile stations (MSs) in order to prolong the lifetime of MS. The base station (BS) will functionally serve as a buffer for non-real-time traffic distribution to the multiple MSs associated with it, and thus naturally provide MSs with the power saving capability. However, the power management scheme in IEEE 802.16e standard, which was designed for handling sleep pattern of non-real-time traffic, often use timeout approach to trigger sleep mode, as a certain idle time prior entering sleep mode will render a potential latency and limit the overall power saving capacity. Moreover, the fast growing sleeping window would cause long packet reception delay. As a solution, we propose a new scheme, namely listening interval spreading approach (LISA), which redistributes the idle periods and a number of interim listening periods in order to reduce the response time of interactive traffic, such as HTTP applications. As confirmed by the performance evaluation, the proposed LISA can achieve significant response time reduction as well as better power saving efficiency than standard.

The standard IEEE 802.15.4 medium access control (MAC) protocol is designed for low-power transmissions in low-rate and short-distance wireless personal area networks (WPANs). The modified CSMA/CA contention protocol suffers from inefficient channel utilization that is mainly caused by no carrier sensing during backoff. Nodes usually perceive channel busy after backoff but channel may be idle during backoff period. Moreover, standard protocol can not get rid of collisions. In this article, we propose a rotational listening strategy (RLS) to improve the opportunity of success transmissions in IEEE standard protocol. In RLS, to behave like a contention-free network, channel access period is partitioned into a number of mini-slots, in which one mini-slot is allotted for one node, and channel sensing of a node is restricted on its own mini-slots. Simulation results demonstrate that RLS indeed improves the transmission efficiency and conserves energy.

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.

Communication technology of broadband wireless access (BWA) system designed for wireless metropolitan area networks (WMANs) has become matured. The IEEE 802.16 standard has the potential to be the global standard for WMANs. In this paper, we first point out the inefficient data transmission in IEEE 802.16 point-to-multipoint network and then propose the virtual direct link access (VDLA) scheme which is based on smart scheduling strategy and location reporting protocol. The VDLA allows pairs of subscriber stations to communicate directly without passing through the base station. Simulation results show that the IEEE 802.16 WMANs with VDLA achieves higher average network throughout when handling Intranet traffic.

IEEE 802.15.4 wireless personal area network (WPAN) requires devices to "frequently sleep to conserve power. Without hidden-device protection mechanism, collisions caused by hidden devices may last to frames from other later devices which may only hear some devices involving in previous collisions. Such collision chain problem (CCP) significantly degrades network throughput. This paper proposes a novel collision resolving scheme with P-frozen contention strategy (PFCS) to solve the CCP in WPAN. The coordinator could refer to two partially recognizable fields, frame length and source address, of corrupted frame to allocate dedicated bandwidth for correspondent device and to freeze its contention. Simulation results show that the proposed scheme can not only improve transmission efficiency and access delay but also conserve energy

In the IEEE 802.15.4 MAC protocol, peer-to-peer transmission between a pair of neighboring nodes is not easy due to nodes frequently staying at sleeping mode. However, in star topology, data exchanging between two non-hidden nodes are relayed by a network coordinator which spends double power consumptions for one valid frame transmission from sender to destination. This paper proposes a day and night access (DNA) scheme to enhance transmission efficiency and to minimize power consumption simultaneously. Simulation results demonstrate that DNA scheme outperforms IEEE 802.15.4 MAC standard in terms of data update success rate, data update latency and energy dissipation per update.

The meta-heuristic methods, genetic algorithms (GAs), are frequently used to obtain optimal solutions for some complicated problems. However, due to the characteristic of natural evolution, the methods slowly converge the derived solutions to an optimal solution and are usually used to solve complicated and offline problems. While, in a real-world scenario, there are some complicated but real-time problems that require being solved within a short response time and have to obtain an optimal or near optimal solution due to performance considerations. Thus, the convergence speed of GAs becomes an important issue when it is applied to solve time-critical optimization problems. To address this, this paper presents a novel method, named hyper-generation GA (HG-GA), to improve the convergence speed of GAs. The proposed HG-GA breaks the general rule of generation-based evolution and uses a pipeline operation to accelerate the convergence speed of obtaining an optimal solution. Based on an example of a time-critical scheduling process in an optical network, both analysis and simulation results show that the HG-GA can generate more and better chromosomes than general GAs within the same evolutionary period. The rapid convergence property of the HG-GA increases its potential to solve many complicated problems in real-time systems

The IEEE 802.11 standard and enhanced amendments have defined fourteen transmission rates (1/2/5.5/6/9/11/12/18/22/24/33/36/48/54 Mbit/s) for mobile stations to transmit and receive data frames. With the characteristic of modulation schemes, a higher level modulation scheme requires a higher signal-to-noise ratio (SNR) and, consequently, the data rate is inversely proportional with the transmission distance. Using a higher level modulation scheme, a higher network throughput can be expected; however, the frame error probability will also become higher. Doubtlessly, it is an open issue of selecting a proper modulation scheme for a pair of mobile stations in a time-varying indoor environment. This paper proposes a safe multiple access-rates transmission (SMART) scheme for enhancing the reliability of data transmission in the IEEE 802.11 multi-rate infrastructure wireless networks. The SMART scheme provides reliable transmission by reserving a retransmission period, which immediately follows the transmitted frame and is estimated from a lower transmission rate, for each transmitted frame. If any error occurs on the transmitted frame, the sender will retransmit it right away by using a lower transmission rate to make sure of successful retransmission. Otherwise, the reserved period will be taken by the access point (AP), which often has the longest waiting queue and is the bottleneck in infrastructure wireless networks. The efficiency of the proposed SMART scheme is evaluated by simulation. Simulation results show that the derived performance of the SMART scheme is significantly better than standard under the real environment with asymmetric traffic load.

In IEEE 802.11 infrastructure wireless local area networks (IWLANs), most of data frames are transmitted between access point (AP) and wireless stations (STAs). The data transmission in IWLAN is inefficiency since the excessive overhead generated from frequent handshakes between AP and STAs and contentions among STAs. For minimizing the bandwidth wastage, in this paper, we propose a new ACK-based polling strategy to efficiently coordinate and arbitrate the sequence of transmissions among STAs. The proposed strategy alters the consecutive handshake sequence between sender and receiver defined in IEEE 802.11 medium access control (MAC) protocol to minimize the handshaking overhead and to reduce or even eliminate the potential collisions in transmissions. Different from standard handshakes, the recipient of data frame postpones sending acknowledgement (ACK) control frame to schedule the timing of the next transmission from sender. The ACK frame is sent from receiver via standard contention protocol and the sender waits the ACK frame for the authorization to send the next data frame. The simulations and analytical results demonstrate that the proposed ACK-based contention strategy is able to improve the transmission efficiency by simply reducing the overheads of handshakes and collisions in the wireless networks.

In IEEE 802.11 infrastructure wireless local area networks (WLAN), all mobile stations (STAs) are coordinated by an access point (AP), which is a static device and plays the role of the bridge between wired and wireless networks. Such coordination is achieved by restricting all STAs to access the channel listened by AP. Within the 2.4 GHz unlicensed industry, science, and medicine (ISM) band defined in the IEEE 802.11 2.4 GHz physical layer (PHY) specifications, three of fourteen channels, which are independent and exclusive, can be used to transfer data packets at a same area concurrently. However, in most small/medium enterprises or home environments, one AP is sufficient for covering whole service area. This implies that the other two channels' capacity has being wasted by the single channel operation defined in standard. In order to overcome the drawback, we propose a new and simple CSMA based media access control (MAC) protocol, named wireless switch protocol (WSP), for promoting the IEEE 802.11 aggregate network throughput. This is simply achieved by allowing any pair of STAs in WLAN to exchange data packets in another idle channel after their handshaking with each other in the common channel, which is specified by AP. Simulation results show that the total network throughput of WSP is obviously depending on the time taken by the changing frequency channel and the intranet and Internet traffic distribution, where the intranet and Internet mean the data exchanged between mobile STAs, and between STA and wired host, respectively. If all data packets are intranet traffic and the traffic load is heavy, the improving ratio of derived goodput of proposed WSP and that of the IEEE 802.11 standard approximates 400%. In the worse case that all traffic is Internet traffic, the proposed WSP still obtains the similar throughput as that of IEEE 802.11 standard.

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.

We present a data flushing data transfer (DFDT) protocol for IEEE 802.11 wireless ad hoc network. The basic mechanism of DFDT is quite the same as the distributed coordination function (DCF) of the medium access control (MAC) of IEEE 802.11, which uses a random access delay backoff time after a busy medium condition and RTS/CTS dialogue before sending actual payload data (direct data/ACK could also be used). The enhancement introduced by DFDT is mainly produced by the compilation process (CP), which fits as many MAC layer packets as possible into one physical layer packet within the limit of a predetermined length. By using the CP, we lower the protocol overhead, the packet arrival rate of the physical layer, and network contention all with one action. DFDT takes the advantages of the RTS/CTS mechanism but has less the overhead. Simulation results backed by numerical analysis show growing improvement in performance, limited by the saturation of the network, as the network load gets higher.

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.

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.

This paper discusses the issue of routing packets over an IEEE 802.11 wireless ad hoc network with multiple data rates (1, 2, 5.5, and 11 Mb/s). With the characteristics of modulation schemes, the data rate of a wireless network is inversely proportional with the transmission distance. The conventional shortest path of the minimum-hops approach will be no longer suitable for the contemporary multi-rate/multi-range wireless networks (MR/sup 2/WN). We propose an efficient delay-oriented multi-rate/multi-range routing protocol (MR/sup 2/RP) for MR/sup 2/WN to maximize the channel resource utilization as well as to minimize the network end-to-end transfer delay. By analyzing the medium access delay of the IEEE 802.11 medium access control (MAC) protocol, the proposed MR/sup 2/RP is capable of predicting the end-to-end transfer delay of a routing path and find the best one. The proposed MR/sup 2/RP may choose a longer path but with less contention competitors and buffer queuing delay. Simulation results show that MR/sup 2/RP 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.

In this paper, we will discuss how to provide multiple-data-rate transmissions in a contemporary IEEE 802.11 infrastructure wireless network. According to the characteristics of modulation schemes, the highest data rate between a pair of mobile stations will be inversely proportional with the transmission distance. Therefore, a moving mobile station is requested to dynamically adjust its modulation scheme to achieve maximal network throughput as well as to keep the connection alive. In this paper, we propose a hybrid handshake protocol with a simple broadcasting protocol to help mobile stations to obtain the necessary location information. The critical fairness problem of multi-rate wireless networks is also discussed and solved in this paper.

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.

IEEE 802.11 is a standard, which specifies both the medium access control (MAC) and physical layers (PHY) for wireless local area networks (LANs). However, it cannot support real-time services for ad hoc networks. We propose a distributed bandwidth reservation protocol (DBRP), which is compliant with the IEEE 802.11 standard, to support both non-real-time (data) and real-time (voice) traffic over an ad hoc WLAN. The simulation results show that DBRP can provide a high channel utilization as well as a high quality of service (QoS). We also realize our proposed DBRP over a WLAN adapter and the measured results show that the DBRP performs much better then the conventional IEEE 802.11 standard.

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.

We discuss the issues of providing multiple data rates (1/2/5.5/11 Mbps) over an IEEE 802-11 infrastructure wireless network. With the characteristic of modulation schemes, the data rate of the wireless network is inversely proportional with the transmission distance. A moving mobile station needs to dynamically adjust the modulation scheme to keep the connection alive and to achieve the maximum network throughput. To do this, the mobile stations are requested to detect the distance between access point and itself from time to time. A simple broadcasting approach is proposed for mobile stations to detect their locations in the wireless network. Based on this information, the mobile node can easily determine the proper transmission rate.

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.

We design a novel fuzzy controller to dynamically adjust the packet length to protect against the interference from microwave oven over the IEEE 802.11 FHSS (frequency hopping spread spectrum) wireless LAN card. The idea of adjusting the packet length under the noise environment from the measurement results about the effects of microwave ovens over the wireless LAN card is referred by Lee, Sheu, Chen, Yu and Huaung (see APCC/OECC'99, p.817-820, 1999). Simulation results show that the designed fuzzy controller can effectively improve the transmission performance in terms of network throughput.

In the hybrid fiber/coax (HFC) architecture, over several hundreds subscribers in CATV (community antenna TV) network may cause serious collisions. In this paper, we propose a new network architecture which using an intelligent node (IN) to stand for a group of subscribers to request the demand resources. The IN has the ability to reduce the collision probability as well as the collision resolving period. The simulation results show that the proposed architecture in terms of throughput, buffer delay, and fairness outperforms the standard architecture.

A novel handoff algorithm based on grey theory is being proposed for reducing the handoff overhead in cellular communication environment. The design of the methods is intended to be used under a log-normal fading environment. The grey prediction model is used to predict the received signal strength indicator (RSSI) values from two base stations (BS). The decision whether performing handoff is based on the predictive RSSI value, the received RSSI value and n-1 past RSSI values. By changing the handoff probability of two regions (divided by the middle point of two separated BSs), the position of first handoff to be executed will close to the middle point of two separated BSs. The simulation results show that the proposed algorithm with grey theory obtains excellent performance because it not only minimizes the number of performing handoff, but also has a very short calculating time.

The integrated technology of a novel prediction system with a high prediction accuracy for indoor wireless LAN is proposed. Different from conventional methods, the genetic algorithm and neural network are employed to compute the environmental parameters and enhance the prediction accuracy, respectively.

In this paper, we propose a fuzzy bandwidth allocation controller (FBAC) to support two types of services: (i) restricted time-bounded service (RTBS), such as voice and video services, (ii) loose time-bounded service (LTBS), such as data service in wireless networks. Based on the FBAC, five different request assignment strategies are introduced. Simulation results demonstrate that the fuzzy controller with some proposed assignment strategies obtain high network performance, high fairness as well as an acceptable blocking probability.

In a wavelength division multiplexing (WDM) star coupler network, both data and signals issued by the transmitters at the nodes are combined by the passive star coupler (PSC) and then broadcasted back to all nodes. To serve a multicast request on WDM star coupler networks, a transmitter of the source node, a receiver of each destination node and a data channel have to be available at the same time. The multicast scheduling algorithm proposed by Borelia and Mukherjee (see Proceedings of IEEE ICC'95, p.1277-81, 1995) while incorporating arbitrary transceiver tuning times and propagation delays, can not fully utilize the channel bandwidth. In this paper, an efficient backtrack multicast scheduling algorithm (BMSA) for multicast packet transmissions on WDM-based local lightwave networks with a passive star topology is proposed. The simulation results show that the algorithm substantially improves the channel utilization.

A slot interleaved multiple access (SIMA) scheme for distributed-queue dual-bus (DQDB) metropolitan area networks is proposed. In this scheme, all the active nodes will access the slots in an interleaved fashion. This scheme is very efficient for solving the bandwidth domination problem and the priority domination problem, which cannot be solved readily by the DQDB medium access control (MAC) protocol with a bandwidth balancing mechanism. An enhanced SIMA (ESIMA) scheme is proposed to solve the bandwidth allocation problem, which cannot be solved by the DQDB MAC protocol. Simulation results show that SIMA and ESIMA perform much better than the DQDB MAC on throughput as well as the access delay.>