This paper proposes a stabilized multichannel random access protocol based on slotted ALOHA for relay deployed cellular networks. To ensure the stability of random access, the proposed protocol dynamically controls the number of random access channels in a BS and a RS and the retransmission probability of the random access packets under heavy load conditions. A mathematical formula is also developed that derives an optimal partition ratio of the shared random access channels between a base station and a relay station without and with capture effect. Numerical results show that the proposed protocol can guarantee the required utilization and delay even in high offered load, which otherwise can cause bistable problem of slotted ALOHA.

Switch and stay combining (SSC) is an attractive diversity technique due to its low complexity and compatibility to resource-constrained wireless networks. This letter proposes a distributed SSC for partial relay selection networks in order to achieve spatial diversity as well as to improve spectral efficiency. Simulation results show that the performance loss (in terms of bit error probability) of the proposed networks relative to partial relay selection networks with selection combining is not substantial.

This letter proposes an intra-cell partial spectrum reuse (PSR) scheme for cellular OFDM-relay networks. The proposed method aims to increase the system throughput, while the SINR of the cell edge users can be also promoted by utilizing the PSR scheme. The novel pre-allocation factor γ not only indicates the flexibility of PSR, but also decreases the complexity of the reuse mechanism. Through simulations, the proposed scheme is shown to offer superior performances in terms of system throughput and SINR of last 5% users.

Usually the contact voltage drop or contact resistance of electromagnetic relays is observed only to identify if the contacts are failure or not on the manufactures' life tests. However, it is difficult to reveal the contact performance degradation because the variation of contact resistance may not be obvious. In this paper, a new life test technology was investigated to analyze the contact failure mechanisms and degenerative processes of electromagnetic relays by measuring their time parameters including closing time, opening time, over-travel time, rebound duration and gap time during each operation. Moreover, for the purpose of verifying the time parameters, the contact motion and contact morphology during life test were record by using a high speed camera. Both the variations of time parameters and information obtained from photos taken by high speed camera show that it involves three different degenerative phases during the whole life of a relay. The results also indicate this method is an effective technology to discriminate and diagnose the failure mechanisms for electromagnetic relays.

Cooperative relaying (CR) is a promising technique to provide spatial diversity by combining multiple signals from source and relay stations. In the present paper, the impact and use of the asymmetric property in bi-directional CR under asymmetric traffic conditions are discussed assuming that CR involves one communication pair and one relay station in a time division duplex (TDD) system. The asymmetric property means that the average communication quality differs for each transmission direction because of the difference in signal power between the combined signals for each direction. First, numerical results show the asymmetric property of bi-directional CR. Next, in order to evaluate the impact of the asymmetric property, the optimal relay position and resource allocation are compared to those in simple multi-hop relaying, which does not have the asymmetric property. Numerical results show that, in order to maximize the overall quality of bi-directional communication, the optimal relay position in CR depends on the offered traffic ratio, which is defined as the traffic ratio of each transmission direction, while the offered traffic ratio does not affect the optimal relay position in multi-hop relaying. Finally, the asymmetric property is used to enhance the overall quality. Specifically, a high overall quality can be achieved by, for example, opportunistically switching to the transmission direction with higher quality. Under asymmetric traffic conditions, weighted proportionally fair scheduling (WPFS), which is proposed in the context of downlink scheduling in a cellular network, is applied to transmission direction switching. Numerical results reveal that WPFS provides a high overall quality and that the quality ratio is similar to the offered traffic ratio.

This paper proposes an interference avoidance technique that allows wireless device with similar frequency bands to be operated adjacent to each other for compact mobile wireless routers (MWRs). This MWR implements two devices of Wireless LAN (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX). The MWR connects WLAN terminals to the backbone network by using WiMAX-WLAN relay. Generally, different frequency channels are assigned for the wireless systems assign in order not to interfere among multiple systems. However, mutual system interference is generated if the space between each device is very close and if the frequency using each system is adjacent. To suppress this interference, this paper proposes a novel interference avoidance technique that leverages IEEE802.11n Power Save Multi-Poll (PSMP). First, we clarify the conditions that raise the issues of mutual interference by experiment. Simulations are conducted to show that the proposed scheme outperforms the conventional schemes. Finally, the effectiveness of the proposed scheme is shown by the computer simulation.

In this paper, we study an opportunistic scheduling scheme for the TDMA wireless network with relay stations. We model the time-varying channel condition of a wireless link as a stochastic process. Based on this model, we formulate an optimization problem for the opportunistic scheduling scheme that maximizes the expected system throughput while satisfying the QoS constraint of each user. In the opportunistic scheduling scheme for the system without relay stations, each user has only one communication path between the base station and itself, and thus only user selection is considered. However, in our opportunistic scheduling scheme for the system with relay stations, since there may exist multiple paths between the base station and a user, not only user selection but also path selection for the scheduled user is considered. In addition, we also propose an opportunistic time-sharing method for time-slot sharing between base station and relay stations. With the opportunistic time-sharing method, our opportunistic scheduling provides opportunistic resource sharing in three places in the system: user selection in a time-slot, path selection for the selected user, and time-slot sharing between base station and relay stations. Simulation results show that as the number of places that opportunistic resource sharing is applied to increases, the performance improvement also increases.

In this letter, two receive diversity combining techniques are proposed for cooperative relay systems based on single-carrier frequency division multiple access (SC-FDMA) when relay station (RS) transmits the received signals from multiple mobile stations (MSs) together using one large size discrete Fourier transform (DFT). A simplified-MRC (S-MRC) technique performs diversity combining in the time-domain by using the estimated channel weights and initial estimates obtained by the SC-FDMA signal detector. An interference rejection-MRC (IR-MRC) technique performs diversity combining in the frequency-domain by adjusting the DFT spreading size at the receiver. It is shown by computer simulation that the proposed receive combining techniques achieve a significant diversity gain over the conventional techniques.

Multihop network is an approach utilizing distributed wireless stations for relaying. In this system, area size, coverage and total transmit power efficiency can be improved. It is shown by computer simulations that the cooperative relaying scheme provides transmit diversity effect, and can offer much better performance compared with that of non-cooperation case. To confirm this superior performance in actual environments, field trials using real time communication equipments are now being planned. This paper reports the design and the performance of wireless equipments for field trials.

This paper proposes two novel frame resource allocation schemes: Mixed bidirectional allocation scheme and Offset allocation scheme. They improve system capacity and latency performance unlike the conventional time-division duplex relay scheme which divides the frame structure into time segments for the access zone and time segment for the relay zones as in IEEE802.16j (WiMAX) systems. Computer simulations confirm that the two proposed schemes outperform the conventional schemes in terms of throughput and latency. An evaluation of the offset allocation scheme confirms that it improves the total throughput by about 85%, and reduces latency by about 72%, compared to the conventional schemes.

Generalized selection combining (GSC) was recently proposed as a low-complexity diversity combining technique for diversity-rich environments. This letter proposes a multi-hop Decode-and-Forward Relaying (MDFR) scheme in conjunction with GSC and describes its performance in terms of average bit error probability. We have shown that the proposed protocol offers a remarkable diversity advantage over direct transmission as well as the conventional decode-and-forward relaying (CDFR) scheme. Simulation results are also given to verify the analytical results.

This paper aims at analyzing the performance of an anonymous communication system 3-Mode Net with respect to the number of relay nodes required for communication and sender anonymity. As for the number of relay nodes, we give explicit formulas of the probability distribution, the expectation, and the variance. Considering sender anonymity, we quantify the degree of sender anonymity under a situation where some relay nodes collude with each other. The above analyses use random walk theory, a probability generating function, and their properties. From obtained formulas, we show several conditions for avoiding a situation where the number of relay nodes becomes large, and for providing high sender anonymity. Furthermore, we investigate the relationship between the number of relay nodes and sender anonymity, and give a condition for providing a better performance of 3 MN.

We present an adaptive power saving (APS) scheme to reduce downlink energy consumption of the transmit power in the cellular relay network (CRN). In the APS scheme, some cell topologies operating in 2-hop mode using relay stations (RSs) are converted to that of 3-hop mode and others are simultaneously converted to that of single-hop mode when the offered traffic load becomes very low, especially during night periods. By this means, we show the APS scheme outperforms the conventional CRN (CCRN) scheme in terms of energy consumption.

We propose an incremental relaying protocol in conjunction with partial relay selection with the aim of making efficient use of the degrees of freedom of the channels as well as improving the performance of dual hop relaying with partial relay selection (DRPRS). Specifically, whenever the direct link from the source to the destination is not favorable to decoding, the destination will request the help of the opportunistic relay providing highest SNR of the links from the source. Theoretical analyses, as well as simulation results, verify that our scheme outperforms the DRPRS scheme in terms of bit error probability.

We perform error probability analysis of the uncoded OFDM fixed gain Amplify-and-Forward (AF) relaying system with subcarrier permutation (SCP). Two SCP schemes, named: the best-to-best SCP (BTB SCP) and the best-to-worst SCP (BTW SCP) are considered. Closed-form expressions for the bit error rate (BER) of the above SCP methods are derived. Numerical results manifest that these SCP schemes may outperform one another, depending on the average channel conditions of the links involved. That is, BTB SCP is better at low signal-to-noise ratio (SNR) values, while BTW SCP prevails in the medium and high SNR regime. Thus, it could be concluded that OFDM AF relaying systems may switch from the BTB SCP to BTW SCP in order to achieve optimum BER performance. Moreover, using the derived end-to-end SNR probability density functions (PDF), tight upper bounds for the ergodic capacities of both SCP schemes are obtained.

In this letter, the outage performance of multi-hop multiple-input multiple-output (MIMO) relaying systems is analyzed for spatially correlated Rayleigh fading channels. We focus on nonregenerative MIMO decouple-and-forward (DCF) relaying in orthogonal space-time block code (OSTBC) transmission and provide its outage probability given the assumption of ideal relay gain. The outage obtained here is shown a lower bound for using practical gains, which gets tight at high SNR. We conduct numerical studies to assess the impact of the spatial correlation between antennas on the outage probability.

This paper proposes an efficient multiuser relay scheme in OFDMA systems. In the proposed scheme, multiple terminals transmit their data packets simultaneously in the same subband and multiple relay stations retransmit their received signals in different subbands after subband conversion. A base station (BS) extracts individual packets from received signals in the different subbands. In advance of data transmission, the BS selects appropriate terminals so that the BS can extract individual data packets successfully. Numerical results show that the proposed relay scheme achieves higher system throughput than the conventional relay scheme when scheduling is applied to a larger number of terminals than the number of relay stations.

Introducing adaptive online data compression at network-internal nodes is considered for alleviating traffic congestion on the network. In this paper, we assume that advanced relay nodes, which possess both a relay function (network resource) and a processing function (computational and storage resources), are placed inside the network, and we propose an adaptive online lossless packet compression scheme utilized at these nodes. This scheme selectively compresses a packet according to its waiting time in the queue during congestion. Through preliminary investigation using actual traffic datasets, we investigate the compression ratio and processing time of packet-by-packet compression in actual network environments. Then, by means of computer simulations, we show that the proposed scheme reduces the packet delay time and discard rate and investigate factors necessary in achieving efficient packet relay.

A cooperative relaying system with transmission scheduling is investigated. Cooperative relaying is composed of multiple links because the source sends the data to more than one receiver, and the destination receives multiple data transmitted by more than one transmitter. Therefore, if the source can transmit the data when the channel gains of the links are high, it is not clear which channel gains should be high in order to achieve high spectral efficiency. In the present letter, the spectral efficiency of a cooperative relaying system is theoretically derived under the assumption that the source transmits the data only when the channel gains of links are above certain threshold values. Numerical results reveal that a high spectral efficiency can be achieved by assuring a high channel gain for the link with the highest average received power among links to the destination.

A stochastic quasi-gradient algorithm is applied to design linear dispersion (LD) codes for two-way wireless relay networks under Rayleigh fading channels. The codes are designed to minimize an upper bound of the average pairwise error probability. Simulation results show that the codes obtained by the optimization technique achieve a coding gain over codes that are randomly generated based on the isotropic distribution.