To achieve long-range communication, a single-hop long-range wide area network (LoRaWAN) must sacrifice throughput. In contrast, multi-hop LoRaWAN can extend the communication range while maintaining high throughput by introducing relay nodes between a transmitter and a receiver. However, several challenges arise in multi-hop LoRaWAN. First, the hidden-node problem can lead to packet collisions because relay nodes operate autonomously and may not detect other nodes’ transmissions. Second, the acknowledgment (ACK)-based retransmission control process (ACK-RCP) reduces the throughput due to the duty cycle constraint imposed on unlicensed bands. To address these challenges, this paper proposesa monitoring resource activity-based retransmission control process (MRA-RCP)that determines whether retransmission is necessary byoverhearing signal transmission in the next hop. Thus, it does not require explicit feedback from the receiving node. The simulation results show that the proposed MRA-RCP can increase the number of successfully delivered packets to 187 % of that of the system with the conventional ACK-RCP. Furthermore, experimental evaluations with commercially available LoRaWAN nodes show that the proposed MRA-RCP achieves 192 % of the packets successfully delivered, compared to the conventional ACK-RCP in a real LoRaWAN environment.

Wireless communication systems under high mobility and multipath conditions suffer from severe channel variations in time and frequency. To overcome this issue, this study extends a differential space-time line coding (DSTLC) scheme to an orthogonal frequency-division multiplexing framework, i.e., a differential space-time-frequency line-coded (DSTFLC) system. In the DSTFLC system, communication performance depends on how the DSTFLC symbols are arranged on the time-frequency resource grid. To address this issue, three resource grid mapping (RGM) schemes are proposed: time-first RGM, frequency-first RGM, and Gilbert (generalized Hilbert) RGM schemes. As verified by the numerical results, three RGM schemes exhibit distinct benefits under different channel conditions, namely, time-varying channels with short delay spreads, frequency-selective channels in low-mobility scenarios, and time-varying frequency-selective channels. The proposed RGM schemes would provide practical design guidelines for deploying various Internet of Things applications under dynamic channel conditions.

Low-power wide area networks (LPWANs) are wireless communication systems that enable long-distance communication with low power consumption, and have attracted attention as a wireless infrastructure for Internet of Things (IoT) systems. However, the long-distance nature of LPWANs can cause the hidden node problem, which results in a lower packet delivery ratio (PDR). To address the hidden node problem, this paper proposes a channel assignment method. First, we derive an approximate formula for PDR under the impact of hidden nodes, and then propose an optimal channel assignment method based on simulated annealing, using the derived formula. Simulation results confirm that the proposed method can mitigate the degradation of PDR and increase the capacity of LPWAN networks.

Low-power wide-area networks (LPWANs), which achieve low-power consumption, enabling long-term battery operation and long-range communication capabilities, have emerged as a new standard for realizing massive wireless sensor networks (WSNs). LPWANs are becoming increasingly popular due to low introduction costs, which stem from features, such as using unlicensed bands and low-cost nodes. LPWANs are particularly useful for Internet of Things (IoT) applications that periodically collect information about specific observation targets. However, LPWAN generally adopts a simple medium access control (MAC), which significantly degrades communication quality due to packet collisions when the traffic load increases. Thus, MAC design is critical for realizing large-scale LPWANs. Carrier sense multiple access (CSMA) can autonomously avoid packet collisions. However, its performance is drastically deteriorated due to the hidden node problem in large-scale LPWANs. This article proposes an autonomous distributed MAC strategy that can suppress the hidden node problem by utilizing traffic periodicity. The proposed method is designed carefully considering LPWAN-specific constraints, such as duty cycle limitations in unlicensed bands, low clock accuracy of nodes, and limited downlink communication opportunities. From numerical results, the proposed method improves the packet delivery rate (PDR) performance by up to approximately by 29%, 9%, and 8% compared to ALOHA, CSMA-x, and the state-of-the-art LoRa MAC, respectively.

Smart factories operating autonomously through a cyber-physical system (CPS) need to recognize the location-dependent wireless environment accurately for highly reliable communications and highly accurate information aggregation from automated guided vehicle and sensors. Recognizing the wireless environment is also essential for designing wireless power transfer to sensors simultaneously with data transmission. One of the methods for recognizing the wireless environment is a radio environment map (REM), which visualizes the statistical results of measured received power and communication quality by linking them to location information. This method requires node’s location information that is difficult to accurately locate indoors in a practical environment. In this paper, we propose a method to construct a REM with nodes whose locations are estimated by using a position estimation method. Measurement nodes recognize the wireless environment while autonomously estimating their position. We measured the received power of private 5G, a type of private network, in a smart factory and constructed a REM using a position estimation method based on bluetooth low energy beacons. Using the proposed method, a radio propagation environment could be inferred with an accuracy of about $5[\mathrm{~dB}]$ in root mean squared error (RMSE).

The evolution of sensor networks is indispensable in modern society, with environmental sensing technologies playing a critical role. Packet communication, commonly used in many systems, is not suited for efficient transmission of environmental information due to the excessive data volume of headers. This paper focuses on the physical wireless parameter conversion sensor networks (PhyC-SN), examining how this innovative approach addresses the issues of packet communication and significantly reduces energy consumption. PhyC-SN allows for simultaneous environmental recognition through the frequency distribution of received signals by switching the carrier frequencies according to sensor information. However, PhyC-SN cannot generate heatmaps that depict the spatial distribution of sensor information. Thus, this study proposes a new method that combines Mobile Data Gathering Stations (MDGS) with PhyC-SN to enable the generation of heatmaps. By leveraging dynamic position control of MDGS and aggregation of sensor data, this combination allows for precise environmental recognition linked with sensor location information. This integration facilitates the construction of an energy-efficient sensor network, offering a sustainable technology solution.

With the development of the Internet of Things (IoT), Long Range Wide Area Network (LoRaWAN) that enables communication over long distances with low power consumption has attracted attention in recent years. LoRaWAN adopts LoRa modulation as the physical layer technology, with high noise immunity but a low data rate. Until now, various studies have attempted to increase the data rate of LoRaWAN by increasing the number of bits per LoRa symbol. Several studies increase throughput by using an algorithm to separate LoRa signals by transmitting multiple LoRa signals using the same spreading factor. Signal separation requires complex algorithms on the demodulator, and they have yet to try to increase the overall system throughput by increasing the number of semi-orthogonal channels. This paper proposes Fractional Spreading Factor (FSF)-LoRa, which introduces fractional spreading factors. This is achieved by introducing a new parameter called the Fractional Coefficient (FC). FSF-LoRa increases the number of new semi-orthogonal channels. Computer simulations show that the proposed FSF-LoRa with FC of 0.5 can improve the system throughput by about more than 6% compared to the conventional LoRa.

Low Power Wide Area Networks (LPWANs), including Long-Range Wide Area Network (LoRaWAN), that enable long-distance communication with low-power consumption have been attracting attention along with the development of the Internet-of-Things (IoT). Carrier Sensing (CS) is mandated before packet transmission in some regions and countries. However, the hidden node problem becomes more serious as the communication area becomes larger. Packet-Level Index Modulation (PLIM) assigns indexes to the frequency channel and time slot of packet transmission to increase throughput. This paper proposes a PLIM-based method to enable the GateWay (GW) to obtain the sensing relationships among nodes without explicit signaling. The obtained information can be used for node clustering and resource allocation to alleviate the hidden terminal problem. Computer simulation results show that the proposed method improves Packet Delivery Rate (PDR) by up to 13% compared to the conventional method using PLIM. And can achieve a similar PDR performance as the situation where the inter-node distance is ideally known.

Low-power wide area networks (LPWANs), such as long-range wide area networks, are increasingly adopted as a communication standard for wireless sensor networks. LPWAN has been adopted for systems that periodically collect data from sensors, such as in environmental monitoring. However, continuous packet collisions may occur in periodic traffic systems owing to the adoption of a simple random-access scheme in LPWAN. In addition, the use of low-cost nodes results in clock drift, rendering the synchronization of nodes in the system challenging. Thus, this study proposes a wireless resource allocation scheme to avoid continuous packet collisions under the adverse effect of such clock drift. In the proposed scheme, the gateway determines the transmission offset and frequency channel for each node utilizing the periodic traffic feature and clock drift. Based on computer simulation results, the proposed scheme can improve the packet delivery rate by over 20% compared with benchmark methods.

In recent years, long-range wide-area network (LoRaWAN) has attracted considerable attention due to its ability to realize massive machine-type communication (MTC); however, its throughput is limited by the duty cycle (DC). Packet-level index modulation (PLIM) can increase throughput by utilizing a data packet’s frequency channel and transmission timing as the information-bearing index. In PLIM, a node selects a specific transmission resource (frequency channel and timing) within a frame and transmits a packet. If the node cannot transmit the packet at the specific transmission resource, it discards the packet. This packet discard results in throughput degradation of each node. Thus, this paper proposes an index mapping scheme that divides a frame into multiple subframes to provide each node with multiple transmission opportunities. A node performs channel activity detection (CAD) at the selected transmission resource within a subframe; if the node cannot transmit a packet, it moves to the next subframe. The proposed scheme adaptively maps the information bit sequence onto a transmission resource within each subframe based on the wireless environment and communication quality. Computer simulation results show that the proposed scheme improves throughput by increasing the transmission opportunities and reducing the packet discard rate under the constraint of DC.

With the fast growth of Internet-of-things (IoT) and machine-to-machine (M2M) communications technology, low- power wide area networks (LPWAN) such as long-range wide area network (LoRaWAN) are attracting attention. One of the main applications of LPWAN is information collected from a large number of sensor nodes, where network traffic is generally dominated by periodic uplink (UL) traffic. In LPWAN, each sensor node transmits packets at arbitrary timing, which causes packet collisions and degrades the system communication quality. Especially in the case of periodic UL, continuous packet collisions may occur. Therefore, this paper proposes a centralized radio resource allocation scheme that avoids packet collisions of periodic traffic in LPWANs, taking into account the characteristics of the periodical traffic. The computer simulation results show that the proposed scheme can improve the average packet delivery ratio (PDR) by 18% and the age-of-information (AoI) performance compared to the ALOHA protocol.

Low-power wide area networks (LPWANs), which realize long-distance communication with low-power consumption, are suitable for wireless sensor networks (WSNs). In particular, long-range wide area network (LoRaWAN) has been actively studied in recent years. Since wireless power transfer (WPT) charges the battery of terminals by sending high-frequency signals, it can avoid operational disruption for battery replacement and unnecessitate wiring. However, if it is not well designed, WPT may cause interference to other wireless communication systems using the same frequency band. This article proposes a simple yet effective WPT interference cancelation method, which is suitable for LoRaWAN, to take a step to move the coexistence of LPWAN and WPT forward. The proposed method estimates the initial phase of the interfering signal by utilizing the packet frame structure of LoRaWAN and removes the interfering signal from the received signal during the signal detection process. Computer simulation results show that the proposed method can achieve almost the same symbol error rate (SER) performance as the system without interference, even under WPT interference.

Low-power wide-area network (LPWAN), which includes the long-range wide-area network (LoRaWAN) protocol, is an enabling technology that satisfy low power consumption and long-range communication. In many applications of LPWAN, an end node (EN) transmits data at regular intervals. Based on this, we had previously proposed the concept of packet-level index modulation (PLIM) to increase the number of information bits transmitted by one data packet. In PLIM, the generation interval between two consecutive packets is split into multiple time slots. Each EN transmits its packet in a specific combination of time slot and frequency channel, which represents the index, to convey additional information bits. To retrieve additional information, the gateway (GW) detects the time slot in which the data packet is being transmitted. Therefore, accurate synchronization between EN and GW is essential. However, clock drift occurs due to the inexpensive real-time clock oscillator on each EN, which results in timing misalignment between each node and the GW. This article proposes a simple clock drift estimation and compensation method for the PLIM. An experimental measurement is performed to model the clock drift. The numerical results obtained using the clock drift model show that it can accurately detect the time slot index under the influence of clock drift. Furthermore, PLIM is implemented on a commercial LoRaWAN node and GW to demonstrate its practicability.

In low power wide area network (LPWAN), intra-system and inter-system interferences have become crucial issues due to a huge number of terminals and the use of the same frequency channels among LPWAN systems. Thus, we propose a channel allocation method for one of the LPWAN systems, long range wide area network (LoRaWAN), according to the radio environment to suppress these interferences. First, a LoRa gateway (GW) estimates an empirical cumulative distribution function (ECD F) of inter-system interference signal power in each frequency channel. Based on the obtained ECDF, the GW allocates LoRa nodes to the frequency channels in ascending order of the inter-system interference time ratio. Then, the GW reallocates the LoRa nodes to the frequency channels by a search algorithm based on a formula of intra-system interference probability so that this probability on each channel becomes lower than a threshold while maintaining the intersystem interference suppression. The simulation results show that our proposed method can suppress packet losses compared to the conventional random channel selection.

Long range wide area network (LoRaWAN) enables low-power and long-range communication suitable for wireless sensor networks (WSNs). Since LoRaWAN is a decentralized network, packet collision may occur if multiple nodes send data packets simultaneously. It is necessary to adjust the node's transmission interval properly to avoid such packet collisions. This paper proposes a transmission interval control strategy that takes advantage of an acknowledgment (ACK) signal from a gateway (GW) to each node. Instead of transmitting a controlling signal, the GW selects one of two receiving windows for ACK packet transmission to indicate 0 or 1. Thus, it does not incur any overhead. The proposed method adaptively changes the transmission interval of each LoRaWAN node by utilizing the ACK signal returned from the GW. The proposed method uses a simple prediction method that is computationally simple and can provide good properties even when there are constraints on the systems associated with observing gentle changes, such as environmental monitoring. Computer simulation results show that the proposed method can reduce the number of transmitted packets while achieving the same observation accuracy level as conventional methods.

Abstract In a wireless sensor network consisting of a large number of sensor nodes (SNs), an efficient communication scheme is crucial to boost data collection efficiency. Furthermore, SN is required to communicate by the low transmission power to prolong its battery life. However, SNs located at the edge of the communication area may fall into the outage as the signal‐to‐noise power ratio fails to surpass a certain threshold. Moreover, signal collision due to the interferers may deteriorate the packet delivery rate. An unmanned aerial vehicle‐base station (UAV‐BS) is an attractive solution to lower outage probability and enhance communication efficiency. Here, outage probability and capturing failure probability performance of a rotational angle division multiple access (RADMA), which is realized by UAV‐BS with an array antenna, are analytically evaluated. The numerical performance evaluation is conducted to elucidate the performance improvement of RADMA under low power wide‐area network setup. Numerical simulation results show that RADMA can suppress the packet loss ratio 77%, shorten the transmission time 25%, and reduce the required transmission energy 20%.

A low power wide area network (LPWAN) is one of the promising technologies intended for Internet of Things (IoT). The LPWAN features a wide communication area that is a few kilometers to a few tens of kilometers; low power consumption is achieved using a battery. This study measures the transmission characteristics of a LoRaWAN which is one of the LPWAN. Therefore this paper shows the measured results in field experiment of certain cases: a road along the Tamagawa river in Tokyo, inside and around campuses, and a medium city environment. These measured results indicate that it is difficult to accurately perform a carrier sensing for LoRaWAN. Subsequently, we propose a method to realize the accurate spectrum sensing for spectrum sharing considering sensing sensitivity.

A reliable vehicle‐to‐vehicle communication (V2VC) is essential to enable safe and highly automated driving system. Addition of a relay station (RS) can improve packet delivery rate (PDR) of CSMA/CA‐based V2VC. However, the improvement is limited by packet collision at RS due to a hidden terminal (HT). This HT problem can be alleviated by employing sectorised receive antenna at RS, i.e. sectorised relay‐assisted (SR‐) V2VC. However, SR‐V2VC still faces another problem, i.e. packet congestion at RS under high traffic conditions due to the improved packet reception rate. This leads to packet drop at RS and hence limits the improvement brought by SR‐V2VC. Consequently, it is crucial to introduce a kind of traffic congestion avoidance methods to RS. In this article, the authors propose SR‐V2VC with a payload combining forwarding (PCF) method (SR‐V2VC/PCF) and theoretically evaluate the system performance using a CSMA/CA collision model. Large‐scale computer simulations are also conducted to confirm the performance improvement brought by SR‐V2VC/PCF. It is shown that the lowest broadcast PDR of 62% for the non‐relay system is improved to higher than 81% by SR‐V2VC/PCF and the directivity of the sector antenna has impact on the benefit of the proposed scheme.

Previously, a scheme in [1] was proposed for the outdated channel state information (CSI) problem, for data transmission in time division duplex (TDD) systems. In user movement environment, the actual channel of data transmission at downlink time slot is different from the estimated channel due to channel variation. In this paper, the effect of different user mobility on TDD downlink multiuser distributed antenna system is investigated. An efficient autocorrelation based feedback interval technique is proposed and updates CSI at less the cost of the downlink time slots. In the proposed technique, the frequency of CSI feedback for different users is proportional to their speed. Cooperative clusters are formed to maximize sum rate where channel gain based antenna selection and user clustering based on signal-to- interference plus noise ratio (SINR) threshold is applied to reduce computational complexity. Numerical results show that sum rate superiority of the proposed scheme over the user mobility.

In this paper we propose a Hybrid Group Paging scheme to overcome radio access networks (RANs) overload problem due to simultaneous access from massive number of machine-type communications (MTC) devices. Compared to the existing methods, the proposed scheme supports dynamic priority assignment among different sub-groups by combining features from both pull-based and push-based approaches. The performance of the proposed scheme is analyzed using recursive contending user estimation (RCE) method, and shown to agree well with the simulation results.

In this paper, we present a user equipment (UE) based distributed traffic steering mechanism between long-term evolution (LTE) and Wi-Fi networks. An agent residing in each UE evaluates the traffic condition of the network it is currently connecting to and makes the traffic steering decision. The evaluation is either periodic or event-driven such as access denial in the admission control due to network congestion. The learning mechanism enables each UE to use the locally available information at the UE and select the proper network under dynamic network conditions. The computer simulation results show that the proposed mechanism achieves low outage probability and small number of network switching with even less information than or almost the same as the existing method. We have also implemented the proposed traffic steering mechanism as an APP on android platform and verified that the proposed mechanism works effectively in real-time testing.

In this paper, we will first motivate the heterogeneous network as an evolutionary path to the fifth generation (5G) communications. We then present an agile software defined heterogeneous network architecture which virtualizes the various radio access networks such as cellular basestations and Wi-Fi access points and the various carrier frequencies from both licensed and unlicensed bands. The software defined heterogeneous network architecture can therefore support much more efficient resource utilization and meet the quality of service requirement. We will also share our work in context-aware Wi-Fi-cellular network traffic steering and mobility management as two use cases in the software defined heterogeneous network.

The channel state information (CSI) is used to optimise data transmission in time division duplex (TDD) systems, which is obtained at the time of channel estimation. The actual channel of data transmission at downlink time slot is different from the estimated channel due to channel variation in user movement environment. In this paper the impact of different user mobility on TDD downlink multiuser distributed antenna system is investigated. Based on mobility state information (MSI), an autocorrelation based feedback interval technique is proposed and updates CSI and mitigate the performance degradation imposed by the user speed and transmission delay. Cooperative clusters are formed to maximize sum rate and a channel gain based antenna selection and user clustering based on SINR threshold is applied to reduce computational complexity. Numerical results show that the proposed scheme can provide improved sum rate over the non cooperative system and no MSI knowledge. The proposed technique has good performance for wide range of speed and suitable for future wireless communication systems.

We consider a time-division duplex (TDD) two-tier network, comprising macro- and femtocells coexisting in a co-channel operation mode, in which inter- and intra-tier interference are incurred. The proposed cell selection accounts for both uplink and downlink capacities. Comparison is made with path loss based and received signal strength based cell selections. Through simulation, we evaluate the performance of indoor and outdoor users, in terms of cdf, average capacity and 5%-outage capacity under different parameters. Our results indicate appealing performance of the proposed capacity based cell selection under TDD two-tier cellular networks.

We propose a time slot based transmission strategy, referred to as adaptive coordinated napping (CoNap), for energy saving in cellular networks under time-varying traffic demand. In adaptive CoNap network, multiple neighboring base stations (BSs) form a cluster and each BS operates in either a transmit mode (TM) or a nap mode (NM) in each time slot. The dynamic assignment of TM and NM to each BS is implicitly coordinated among multiple BSs. This implicit coordination is realized by a binary general flickering pattern matrix (FPM) through adaptively selected mapping matrix (MM). To track the time-varying traffic demand, we develop an adaptive algorithm to dynamically select the appropriate MM from a predefined MM set by taking into account the network quality of service (QoS) requirement. Our numerical results based on a realistic energy consumption model in a cellular network show that as high as 40% saving can be achieved without compromising the network QoS.

QR decomposition based near maximum likelihood (ML) block detection significantly improves the transmission performance of uplink single-carrier (SC) multi-user multiple-input multiple-output (MU-MIMO). Hybrid automatic repeat request (HARQ) is an indispensable error control technique for high quality packet data transmission. The achievable diversity gain of HARQ depends on the packet combining strategy. In uplink MU-MIMO HARQ, received signal may consist of new packets and retransmitted packets as retransmission for each user acts independently. In this paper, a recursive QR packet combining scheme suitable for uplink SC MU-MIMO HARQ is proposed, which takes into account the number of retransmissions for each user in the detection order. The proposed scheme helps reduce the computational complexity and storage requirement significantly. Moreover, it improves the packet error rate (PER) performance significantly over the conventional bit-level log likelihood ratio (LLR) packet combining, as shown by our computer simulation results.

In this paper, dynamic traffic sharing and base station (BS) muting are incorporated into a clustered cellular system to reduce the total power consumption. Neighboring sectorized BSs form a cluster. Several BSs within the cluster can be muted as long as quality-of-service (QoS) requirement is met. Since the muted BS does not join the transmission, all the users within the cluster need to be served by the active BSs. A power consumption model is also established in this paper which is modeled as a function of traffic load, and incorporates the fixed, variable power consumptions in the system, as well as the power amplifier efficiency. Based on this model, the power saving gain under the effect of the additional traffic load due to BS muting will be discussed. The computer simulation results show that about 30% total power saving is achieved by dynamic traffic sharing and BS muting with clustered BS system.

In this paper, we propose to minimize the downlink transmission power through inter-sector cooperative relaying. One common resource block (RB) is shared by two users near the boundary between two neighbouring sectors, and a half-duplex shared relay node (SRN) is introduced along the sector boundary to assist the transmission. The SRN and the two basestation (BS) sectors cooperate to transmit information to the two users. The time sharing factor and frequency sharing factor are jointly optimized to minimize the total network power while satisfying the transmission rate constraint. We also consider rate splitting technique to optimize the cooperation between SRN and BS in order to further reduce the power consumption. Joint optimization is then performed on the message splitting, time and frequency sharing factors. Numerical results show that our proposed scheme can save total network power as high as 10 dB over the system without SRN.

Power minimization of a cooperative decode-and-forward (DF) half-duplex relaying, where two users communicate with one destination node (D), is studied in this paper. Recognizing the difference in link qualities, one user not only transmits its own message to D but also acts as a relay node (R) for the other user. To achieve total network power minimization, we consider two strategies: (i) a rate splitting strategy (RSS) which splits the whole message into two sub-messages with one being transmitted via R and the other directly to D, (ii) a transmission power boosting strategy (TPBS) in the broadcast phase that allocates more power than needed to ensure correct decoding of the message at R. We first consider joint optimization of rate splitting, a time sharing factor (between the broadcast phase and cooperation phase), and transmission powers. We then consider the second scenario where the time sharing factor is fixed. We prove that TPBS is not required for the first scenario, but is necessary for the second scenario to minimize the total network power. We also obtain channel conditions for which the optimal solution is to employ a multiple access channel (MAC) strategy, i.e., without relaying. Finally we show through numerical results that significant power saving is achieved by our proposed strategies.

ABSTRACT QR decomposition‐based near maximum likelihood block detection significantly improves the transmission performance of uplink single‐carrier (SC) multi‐user multiple‐input multiple‐output (MU‐MIMO). Hybrid automatic repeat request (HARQ) is an indispensable error control technique for high‐quality packet data transmission. The achievable diversity gain of HARQ depends on the packet combining strategy. In uplink MU‐MIMO HARQ, the received signal may consist of new packets and retransmitted packets as retransmission for each user acts independently. In this paper, a recursive QR packet combining scheme suitable for uplink SC MU‐MIMO HARQ is proposed, which takes into account the number of retransmissions for each user in the detection order. The computational complexity and required storage size can be significantly reduced by the proposed scheme. Moreover, it improves the packet error rate performance and throughput performance significantly over the conventional bit‐level log likelihood ratio packet combining, as shown by our computer simulation results. Copyright © 2012 John Wiley & Sons, Ltd.

Vector Coding (VC) is a transmission scheme that can obtain path diversity gain while orthogonalizing multiple symbols. It has been shown that VC can provide good bit error rate (BER) performance compared to orthogonal frequency division multiplexing (OFDM). In addition VC has been proven to be the optimal partition of time domain channel. However, In VC, the eigenvectors of the channel matrix must be available at both transmitter and receiver. Thus, orthogonalization and path diversity gain can be achieved only if perfect channel state information (CSI) is available at both ends. However, there exists channel estimation error in practice. In addition feedback delay of estimated CSI causes mismatch of CSI at both ends, which degrades the performance of VC severely. To see the feasibility (implementability) of VC, it is very important to evaluate the effects incurred by channel estimation error and feedback delay on the performance of VC. In this paper, we analyze VC system in the presence of channel estimation error and feedback delay. We show how the transmit and receive weights selection affects the performance of VC. Numerical (Theoretical) results are confirmed by computer simulations.

Power minimization of a cooperative decode-and-forward (DF) half-duplex relaying is considered in this paper. In the system considered, two source nodes (S) try to communicate with one destination (D). Recognizing the difference in link qualities, one of source nodes not only transmits its own information to D but also acts as a relay for the other S. An optimization problem is formulated to minimize total network power by adjusting transmission powers and a time sharing variable. Since the objective function is not convex, an optimal solution cannot be obtained with low complexity. Instead, we approximate the objective function by a convex function to obtain a suboptimum time sharing factor among transmission phases. Then, transmission power is calculated based on the obtained suboptimum time sharing factor. Power saving of the system considered is compared to multiple access channel (MAC) with optimum power allocation.

In this paper, we consider a two-phase cooperative relay transmission setup with relay's private information (CRT-RPI). A half-duplex decode-and-forward (DF) relay not only helps the source-to-destination transmission, but also transmits its own private information mRto the destination. Distributed space-time coding is employed across the source and the relay to obtain cooperative diversity gain for the source message, and superposition coding between the source and relay messages is used at the relay to transmit mR. We derive the achievable rate region of CRT-RPI and design i) the optimal time sharing factor between two phases and ii) the power allocation factors for the two messages, based on the rate region. Numerical results are also provided to verify the effectiveness of our proposed optimization schemes.

To achieve a high speed transmission with limited frequency bandwidth, multiple-input multiple-output (MIMO) has been gaining much attention. QR-decomposition based successive interference canceller (SIC) can provide good detection performance with reasonable complexity. In MIMO system, there could be spatial antenna correlation among antennas and this degrades the transmission performance. The performance severely degrades owing to the correlation among transmit antennas in the case of uplink transmission. In this paper, we propose a frequency band allocation in MIMO system based on received power difference among users that each user utilizes several frequency bands that overlap with other users'. At the receiver, QR-decomposition based SIC is performed by utilizing the received power difference. It is shown that the proposed scheme yields superior error rate performance over the conventional scheme by computer simulations.

We propose an incremental relaying scheme using joint Tx/Rx frequency-domain equalization (FDE) for single-carrier (SC) transmission. If a packet sent by a source node (S) has been correctly decoded at a relay node (R), but not at the destination node (D), retransmission is cooperatively done by S and R. Assuming that the channel state information (CSI) is shared by S, R, and D, joint Tx/Rx FDE is performed. We derive a set of optimal/suboptimal Tx/Rx FDE weights among S, R, and D, based on the minimum mean square error (MMSE) criterion under total transmit power constraint of S and R. Computer simulation verifies the effectiveness of the proposed scheme.

Single-carrier frequency-division multiple-access (SC-FDMA) with frequency domain equalization (FDE) can provide good performance even in a frequency selective fading channel. In this paper, we focus on a special kind of SC-FDMA, i.e., interleaved frequency-division multiple-access (IFDMA) be cause of its peak-to-average power ratio (PAPR) property and frequency diversity gain. FDE requires accurate channel estimation. In the case of fast time varying fading channel, decision directed channel estimation (DDCE) is effective. However, in IFDMA, amplitude of each sub carrier may fluctuate depending on the combination of data symbols, then estimation accuracy may degrade. That can be avoided by multiplying information data sequence by random phase sequence. The performance depends on the selection of the sequence. In this paper, selection criterion suitable for IFDMA with DDCE is proposed. The effect of the proposed method is evaluated by computer simulation. The results will show the proposed method can lower the error floor of the system.

Single-carrier frequency division multiple access (SC-FDMA) has been decided to be adopted in the uplink of the future wireless communication system. Interleaved frequency division multiple access (IFDMA), whose transmitted signal is generated in the time domain, is a candidate of SC-FDMA. The number of comb like spectrum and the spectral interval between each spectrum depend on the number of transmission data symbols Q in one IFDMA block and repetition number L. In the case of large Q and small L, large frequency diversity gain can be obtained due to the number of spectrum is increased. However, inter-chip interference (ICI) owing to the multipaths environment increases. In the multi-user environment, it is difficult to get multi-user diversity due to the decrease of the degree of freedom for channel allocation to each user with large Q and small L. On the other hand, in the case of small Q and large L, frequency diversity gain becomes smaller and ICI is decreased. Furthermore, multi-user diversity gain can be obtained due to the increase of the degree of the freedom for channel allocation. Thus, it is quite interesting to see how the parameters in IFDMA, i.e., Q and L, affect the average bit error rate (BER) performance in the multi-user environment. In this paper, we derive average BER performance of IFDMA in the multi-user environment theoretically by taking into account of frequency diversity, multi-user diversity and ICI when Q and L are changed and then average BER performance of IFDMA is compared to that of OFDM. In addition, the validity of the theoretical analysis is confirmed by computer simulation.

The uplink transmission performance, e.g., bit error rate (BER) performance, of multi-carrier-code division multiple access (MC-CDMA) is limited by the multi-access interference (MAI). Recently, we proposed a FD-PR/MC-CDMA, which uses frequency domain phase rotation and symbol mapping onto equi-distantly spaced sub-carriers. In FD-PR/MC-CDMA, multiple users can be multiplexed without causing MAI as long as the number of multiplexed users is smaller than the spreading factor devided by the maximum channel length. However, the number of users to be accommodated is limited by the length of the channel impulse response and the spreading factor. To increase the number of users or the data rate per user, the code multiplexing is effective, but this results in the performance degradation. In this paper, an iterative interference cancellation is applied to FD-PR/MC-CDMA and the impacts of the spreading factor and the length of the channel impulse response are evaluated by computer simulation.

To increase the data rate without signal bandwidth expansion, a combination of MC system with multiple-input multiple-output (MIMO)-space division multiplexing (SDM) is attractive. MIMO-SDM transmission performance is governed by min {Nt, Nr}, where Ntand Nrare the numbers of transmit and receive antennas, respectively. For downlink transmission, many receive antennas cannot be equipped at a mobile station due to space limitation. Therefore, the achievable data rate is limited by the number of receive antennas. In this paper, we propose a virtual MIMO-SDM for multi-carrier-code division multiple access (MC-CDMA) that can use the signals received via different propagation paths as virtual receive antennas. The equivalent number of receive antennas can be increased by a factor of the number of distinct paths of the channel. We confirm, by computer simulation, the effectiveness of virtual MIMO-SDM in a frequency-selective fading channel.

The bit error rate (BER) performance of multicode MC-CDMA severely degrades due to the inter-code interference (ICI) in a strong frequency-selective channel. In, a spreading code group construction method for MC-CDMA was proposed. The Walsh-Hadmard (WH) codes are divided into a number of code groups such that the code orthogonality can be maintained within each subgroup even in a frequency-selective channel; any code pair taken from different groups is not orthogonal. The number of spreading codes in each group is determined by the maximum time delay difference of the channel. In this paper, we point out that the number of codes in each group is not determined by the maximum time delay difference but the distribution of time delay differences among the propagation paths in the channel and thus, more orthogonal spreading codes can exist in each code group. The conditional BER is derived taking into account the interference from other code groups and the achievable downlink BER performance is numerically evaluated in a frequency-selective Rayleigh fading channel.

Multicarrier transmission techniques (i.e., orthogonal frequency division multiplexing (OFDM) and multi-carrier code division multiple access (MC-CDMA)) are the promising candidates for wireless access technique in future cellular communication systems because of their capability to eliminate the multipath interference. Multiple-input multiple-output (MIMO)-space division multiplexing (SDM) can increase the transmission rate without bandwidth expansion. A combination of multi-carrier technique and MIMO is a promising technique. However, the transmission performance of MC-CDMA severely degrades because of the inter-code interference (ICI) arising from the orthogonality distortion owing to the frequency- selective fading. In this paper, we derive the channel capacity formula for MC-CDMA MIMO taking into account the ICI and co-channel interference (CCI) in a cellular environment. Many ICI cancellers have been proposed. The cellular channel capacity of MC-CDMA MIMO with perfect ICI cancellation is numerically evaluated and is compared to that of OFDM MIMO in a cellular environment. The impacts of the frequency-reuse factor and the number of propagation paths are also discussed. The numerical computation results show that the cellular channel capacity of MC-CDMA is larger than that of OFDM.

Recently, we proposed an iterative modified QR-decomposition based M-algorithm (QRD-M) using decoding results of cyclic redundancy check (CRC) code for OFDM MIMO multiplexing. The symbol-wise CRC decoding results are used in the transmit antenna ranking and in the modified M-algorithm. Hereafter we call this iterative QRD-M as symbol-wise iterative modified QRD-M. In this paper, to further improve the transmission performance of OFDM MIMO multiplexing, we replace the use of symbol-wise CRC decoding results by the use of bit-wise CRC decoding results. The symbol replica candidates for the transmitted symbol are re-ordered using the bit-wise CRC decoding results in the modified M-algorithm. Computer simulation results show that when 16QAM data-modulation is used, an Eb/N0reduction of about 0.6~0.7 dB can be achieved for average packet error rate (PER)=10-2compared to the symbol-wise scheme.

To increase the transmission rate without bandwidth expansion, multiple-input multiple-output (MIMO) technique has been attracting much attention. When MIMO technique is applied to a cellular system, the MIMO channel capacity is affected by the interference from neighboring co-channel cells. In this paper, we evaluate the outage probability of MIMO downlink channel capacity in a cellular system, taking into account the frequency-reuse factor, path loss exponent, standard deviation of shadowing loss, and base station (BS) transmit power. We discuss about the cellular channel capacity. Furthermore, we compare the cellular MIMO channel capacity with those of other multi-antenna transmission techniques such as space time block coded multiple-input single-output (STBC-MISO) and single-input multiple-output (SIMO). We show that the optimum frequency reuse factor is 3 irrespective of type of multi-antenna transmission scheme for the outage probability of 10 %. We also show that the cellular MIMO channel capacity is always higher than those of SIMO and MISO.

Maximum likelihood detection with QR decomposition and M-algorithm (QRD-M) is one of the detection methods in multiple-input multiple-output (MIMO) multiplexing. We recently proposed a Turbo coded iterative modified QRD-M with decision directed channel estimation and showed that the packet error rate (PER) performance close to the ideal channel estimation case can be obtained. In the iterative modified QRD-M, signal ranking is performed based on both received signal-to- interference plus noise power ratio (SINR) and results of cyclic redundancy check (CRC) decoding and then, signal detection is performed by modified M-algorithm based on the results of CRC decoding. In this paper, we apply irregular low density parity check (irregular LDPC) codes to the previously proposed iterative modified QRD-M. The a posteriori probability of each bit (including parity bit) is necessary for decision directed channel estimation and modified M-algorithm in the iterative modified QRD-M. Unlike Turbo decoding, the a posteriori probability of each bit can be obtained in LDPC decoding. Thus, by using LDPC codes, more accurate signal detection is possible than using Turbo codes. To achieve more powerful detection of decoding errors, CRC codes can be jointly used with parity check equation of LDPC codes. It is shown by the computer simulation that the performance of the LDPC coded iterative modified QRD-M is superior to that of the Turbo coded iterative modified QRD-M.

Maximum likelihood detection with QR decomposition and M-algorithm (QRM-MLD) is one of the detection methods in multiple-input multiple-output (MIMO) multiplexing. We recently proposed an iterative QRM-MLD with decision directed channel estimation and showed that better transmission performance can be obtained compared to that with conventional pilot assisted channel estimation. In this paper, to further improve the transmission performance while keeping computational complexity low, we modify previously proposed iterative QRM-MLD by applying cyclic redundancy check (CRC) coding. In the proposed method, transmitted signals are ranked according to their CRC check results and signal-to-interference plus noise power ratio (SINR), before performing modified QRM-MLD. The number of iterations and that of surviving symbol replica candidates in iterative channel estimation/signal detection are adaptively controlled by CRC check results. Thus, the computational complexity of the proposed method can be kept low. Computer simulation results show that the loss in the average received signal energy per bit-to-noise power spectrum density ratio Eb/N0required for the average packet error rate (PER) of 10-2is approximately 0.5 dB from full MLD with ideal channel estimation for QPSK modulation