It is difficult to receive GPS signals from GPS satellites directly in many skyscrapers environments. So, many reflected GPS signals make the calculated distance error performance worst because of their longer pseudo ranges than those of the direct signals. The elevation mask is one of the most effective technologies to improve calculated distance error performances by eliminating the reflected signals as possible. But, assuming that the reflected signals are eliminated for position calculation, the number of receivable GPS signals might be less than 4 by larger elevation mask in many skyscrapers environments. Therefore, it is likely to use the pseudo ranges of the reflected signals aggressively which are eliminating by elevation mask. In this paper, we propose a planar approximation method and an estimation model that can estimates the number of reflections and the incident angles by the received GPS signal power. By computer simulation, our proposal improves the calculated distance error performances, for example maximum distance error performance had been improved by 182.4m compared to the conventional GPS position calculation.

In order to improve calculated distance errors of GPS receivers in urban areas, a relay type GPS had previously been proposed. In relay type GPS, target GPS receivers receiving can calculate their positions by assists of the many mobile terminals around them which calculate their own positions by GPS calculation and transmit them via the same GPS signal format. Therefore, the relay type GPS's calculated distance error performances are significantly affected by those of the mobile terminals around them. To solve this problem, we have already evaluated the mobile terminals' calculated distance error improvements by using elevation mask. However, elevation mask cannot use the direct signals from the receivable lower elevation GPS satellite than the elevation mask angles. In this paper, a new relay type GPS applying received signal power mask to the mobile terminals is proposed. The simulation results indicate the proposed relay type GPS improves the calculated distance error performances compared to those with elevation mask. For example, by applying received signal power mask of -140 dBm to the mobile terminals, the proposed scheme obtains 6.64m improvement of the 24 hours average calculated distance error.

Currently, demands for broadband communication are increasing in satellite communications. However, there is a problem that it is difficult to use of frequency bands effectively due to scattered unused bands caused by DAMA (Demand Assigned Multiple Access). In order to solve this problem, DSDT (Direct spectrum division transmission) has been developed. In this scheme, the single carrier modulated signal is finely divided into sub-spectra in the frequency domain on the transmission side and they are allocated to the unused bands over a satellite transponder. Then they are combined on the reception side, and finally the original single carrier modulated signal is recovered. However, there is a problem that the PAPR (Peak-to-average power ratio) increases in direct spectrum division transmission as the number of spectrum division increases. In order to improve the PAPR degradations, we propose a new direct spectrum division transmission applied by clipping and filtering method, which can be easily realized without transmitting the PAPR control information to the receiving side. Moreover, this paper indicates the proposed scheme improves the degradation due to the adjacent channel interference (ACI) from the DSDT spectra applied by clipping. The computer simulation results show that the PAPR performances of the DSDT with clipping and filtering are improved without BER (Bit error rate) degradation by setting the adequate clipping thresholds and adjacent channel interference due to clipping can be reduced by filtering.

This paper studies a multi-satellite multi-beam system with single frequency reuse which applies the MIMO (Multi Input Multi Output) technique. To improve the spectral efficiency of satellite systems in support of broadband communications, MIMO is the one of the most important approaches. Since it was difficult to achieve high spectral efficiency by simply combining a multi-beam system with single frequency reuse and a multiple satellite system, this paper newly proposes a pre- coding technique that takes account of the frequency offsets caused by the multiple satellites. We then evaluate our proposal by a cumulative distribution function (CDF) related to channel capacity and show the improvement of the spectral efficiency compared to the conventional satellite systems.

This paper describes performance evaluation results of SSB (Single Side Band) transmission of DFTs-OFDM (Discrete Fourier Transform Spreading Orthogonal Frequency Division Multiplexing) using multi-level BPSK (Binary Phase Shift Keying) through nonlinear HPA (High Power Amplifier). SSB-DFTs-OFDM is generated by eliminating USB (Upper Side Band) or LSB (Lower Side Band) spectrum in frequency domain after DFT spreading of multi-level BPSK signals, thus Hilbert transformer is not necessary to generate SSB signals. The simulation results show that SSB-DFTs-OFDM achieves almost the same PAPR (Peak to Average Power Ratio) and ACLP (adjacent channel power leakage) as conventional DFTs-OFDM and lower PAPR and ACLP than OFDM and required bandwidth can be reduced by half when BPSK modulation is employed. Furthermore, SSB-DFTs-OFDM using multi-level BPSK shows less error floor and/or Eb/N0 degradation compared with conventional DFTs-OFDM using QAM modulation in some cases when HPA OBO (Output Back Off) is small. SSB-DFTs-OFDM is an attractive alternative to DFTs-OFDM for WLAN and cellular applications.

Delay-tolerant networks (DTNs) are a promising network architecture which can provide reliable multi-hop message transmission between participating mobile nodes in an unfavorable environment that is prone to link disruption and disconnection by replicating and relaying messages without any need of physical infrastructure. Additionally, DTNs can also operate as cooperative DTNs to extend the coverage of other type of networks by carrying the messages that originate from farther away base station in a multi-hop fashion to the base station. In this paper, we focus on these cooperative DTNs and propose a novel routing scheme, ring distribution routing (RDR), that controls the replication and forwarding based on the source node surroundings. This paper also analyzes the reliability and buffer efficiency in RDR. Furthermore, we show that RDR provides the reliable and immediate message delivery in any environment through extensive computer simulations.

Interference detection and suppression schemes are proposed for coded OFDM systems in the presence of narrowband interference. Previously proposed interference detection schemes perform with a recursive forward error correcting (FEC) decoding, thus its calculation is heavy. The proposed scheme is based on an expectation maximization (EM) algorithm to detect the interference before the FEC decoding process. The complexity is reduced because the FEC decoding is performed only once. Simulation results indicate that the proposed scheme achieves almost the same bit error rate performance as the conventional scheme, while decreasing the times of the FEC decoding.

Sub-spectrum suppressed transmission has been proposed to achieve highly efficient frequency usage on satellite transponder for satellite communication systems. In our proposal, Direct Spectrum Division (DSD) is employed to directly divide a single-carrier modulation signal into sub-spectra and some sub-spectra are suppressed at the transmitter in order to reduce the total occupied bandwidth. In this paper we developed a prototype transceiver that achieves real-time processing of DSD-based sub-spectrum suppressed transmission and realizes autologous spectrum regeneration (ASR) to reduce inter symbol interference (ISI) due to the sub-spectrum suppression. This paper presents indoor experimental results of the prototype and clarifies that frequency utilization efficiency is increased up to 147% by proposed scheme.

Superposed multicarrier transmission scheme is known to improve frequency utilization efficiency when several wireless systems share the same spectrum. To suppress the effect of interference, forward error correction (FEC) metric masking is proposed. In the technique, log-likelihood ratio (LLR) that corresponds to superposed band is set to zero, because received bits that correspond to superposed band is unreliable. However, to apply FEC metric masking, the information about superposed band must be known at the receiver beforehand. Furthermore, the received bits contain channel estimation error, which is a cause of degradation. In this paper, we propose an iterative estimation technique of power of undesired signal (noise, interference, and channel estimation error) for superposed multicarrier transmission. We use the estimated power of the undesired signal to calculate the LLR that takes the channel estimation error into account, since including this extra information about the channel improves the BER. The proposed scheme estimates the power of undesired signal on each subcarrier, thus the information about superposed band is not required. Simulation results show that the accuracy of estimating undesired signal becomes more reliable as the number of estimation increases, so that BER becomes better as a result of iterative estimation.

This paper proposes to apply fractional frequency reuse (FFR) to our inter-cell interference (ICI) cancellation method in order to further improve spectral efficiency on large-scale base station cooperation. The proposed method realizes quasi-decentralized cooperation and can significantly reduce preprocessing complexity. System level simulations are conducted under practical conditions of estimation error on channel state information (CSI). The results clarify that the proposed method can achieve higher spectral efficiency than cooperative MU-MIMO, 11% on average value and 185% higher at the CDF=5%, even in imperfect CSI conditions.

Superposed multicarrier transmission is known to improve frequency utilization efficiency when several wireless systems share spectrum. In the superposed multicarrier transmission scheme, interference can be suppressed by setting the corresponding initial log likelihood ratio (LLR) for forward error correction (FEC) decoder to zero or proper values determined by the desired to undesired power ratio (DUR). Since this technique needs information of superposed subcarriers and DUR, in this paper we propose a superposed band and interference power estimation technique based on the residual power. Our proposed scheme adopts robust LLR for initial LLR and averages the residual power over all packets even if these contain bit errors. The superposed band is detected by comparing the averaged residual power with a threshold. Simulation results indicate that the proposed method performs faster interference detection than the conventional scheme.

This paper describes a QoS control scheme for a wide area ubiquitous wireless network which is designed to accommodate many wireless terminals (WTs), such as sensors and actuators, in a large cell area. The purpose of this paper is to establish a QoS control scheme in a media access control (MAC) layer that can hold the transmission delay time of high priority (HP) class traffic within a predefined value regardless of how much low priority (LP) class traffic there is. Several QoS control schemes for wireless communication use have been proposed. However, in the wide area ubiquitous wireless network, an access point (AP) accommodates many WTs and the AP traffic volume often drastically changes. Therefore, conventional schemes sometimes cannot control the QoS of HP traffic. To solve this problem, we propose a QoS control scheme that calculates a suitable initial back-off window size (IWS) of random access (RA) for each QoS class by using equations derived from a Markov chain behavior model. The proposed scheme adjusts the window size so as to prevent increased transmission delay of HP traffic. The scheme's performance is clarified by computer simulation.

Sub-spectrum suppressed transmission based on DSD (Direct Spectrum Suppression) is a technology that can increase frequency utilization efficiency of single carrier satellite communication systems. In order to suppress ISI (Inter Symbol Interference) caused by the sub-spectrum suppression, proposed ASR (Autologous Spectrum Regeneration) can equalize distorted waveform iteratively. In this paper we optimize ASR processing in two points of view; computational complexity and combination of MCS. First, distribution of iteration for turbo decoding, that is necessary per ASR iteration, is optimized to reduce computational complexity. Second, we evaluate various combinations of adaptive suppression and MCS (Modulation and Coding Scheme) to maximize frequency utilization efficiency. Based on computer simulation, we show those performances and clarify their optimization settings.

The spread of the Internet has given rise to strong demands for real-time services such as voice and streaming video. Since these services require guarantees that may include transmission rate, delay, and delay variance, the network, including the wireless access networks, must offer quality of service (QoS) guarantees. However, no radio access system fully supports QoS controls. In this paper, we propose a new QoS control scheme that uses back pressure traffic control and alters the Inter Frame Space (IFS) length of stations (STAs) according to the traffic circumstances of the wireless local area network (WLAN). It measures traffic of each user and controls the amount of data transmitted in order to guarantee the throughput of each user. This paper shows that the problem of “priority inversion” occurs and that the QoS is degraded when conventional schemes are employed in the medium access control (MAC) layer. The effectiveness of the proposed scheme is validated by comparison against conventional schemes. The results reveal that the proposed scheme can guarantee QoS in terms of throughput even if there is lot of best effort traffic and improve transmission delay performance.

This paper proposes an iterative inter-cluster interference (ICI) cancellation method for cooperative base station systems on the downlink/uplink. System level simulations were conducted with the parameter sets of cluster size and iteration order. The result showed that even in the imperfect CSI case, the spectral efficiency performance of the proposed method with the optimum parameter set is comparable to that of cooperative transmission/reception via MU-MIMO which can completely remove inter-cell interference. In addition, since the proposed method can be practiced in a simple manner, it also has a remarkable advantage to reduce computational complexity. It is a practical way to realize the nationwide wireless systems.

Superposed multicarrier transmission scheme is known to improve frequency utilization efficiency where several systems share spectrum without any spectrum spreading techniques. In superposed multicarrier transmission scheme forward error correction (FEC) coding is applied over subcarriers so that the effects of interference caused by superposition is mitigated by interleaving effects. When the interference power is large, the effects of FEC become smaller particularly owing to the mis-setting of initial log likelihood ratio (LLR). To solve this problem, FEC metric masking is proposed where initial LLRs for superposed subcarriers are replaced by neutral value in the receiver followed by decoding. This technique is effective, however, it needs the information about superposed frequency band. In this paper we propose a scheme to detect superposed band by using FEC metric masking. In the proposed scheme, the subcarriers where FEC metric masking is applied are changed and packet error rate (PER) is measured. When the FEC metric masking is applied to the superposed subcarriers, PER is minimized. Based on the change of PER, we can detect the superposed band. We evaluate the proposed scheme by computer simulation and show that it can detect the superposed band with high probability.

This paper presents the feasibility of a wireless network coding prototype system based on time division multiple access using the global positioning system to facilitate the synchronization of wireless nodes. We evaluate the system throughput of a system where wireless network coding and data aggregation are jointly applied in a saturated traffic environment based on Ethernet including 1,518 and 218 bytes data lengths assuming web browsing and VoIP applications. Experimental results show that wireless network coding with aggregation improves the system throughput by approximately 1.85-fold compared to a system without wireless network coding or aggregation.

In this paper, we propose subcarrier suppressed transmission for OFDMA (Orthogonal Frequency Division Multiple Access) in order to improve frequency utilization efficiency of STICS (Satellite/Terrestrial Integrated Mobile Communication System), which shares the same frequency band between satellite and land mobile systems. In OFDMA introducing channel rotation scheme, the proposed scheme can achieve as similar multi-user performance as single user. By computer simulation, we show the interference suppression performance and clarify the improvement ratio of the total average spectral efficiency.

In this paper, we propose a sub-spectrum suppressed transmission scheme for a single carrier modulation signal to increase the frequency utilization efficiency of satellite communication systems. A single-carrier modulated signal is directly divided into sub-spectra using a filter bank, and one of the sub-spectra is suppressed at the transmitter in order to reduce the total occupied bandwidth. We also propose an iterative equalization process using a sub-spectrum replica to improve the degradation due to inter-symbol interference caused by sub-spectrum suppression. By computer simulations, we show the reception performance improvement and the increase of the frequency utilization efficiency.

This paper proposes a new Acknowledge packet transmission scheme for bidirectional two-hop wireless network coding systems based on IEEE802.11a standard in order to achieve higher TCP throughput. The proposed scheme utilizes the subcarriers that carry zero-padding bits added to MAC protocol data unit (MPDU) for a relay node to identify which node transmits ACK packet against the XOR-ed packet. Computer simulations verify that the proposed scheme can improve TCP throughput performance by 3.4 Mbps when adaptive modulation scheme is applied.

This paper proposes a method for applying interlink correlation to evaluate a multiuser-multiple input multiple output (MU-MIMO) system and describes measurement of a multi-link MIMO propagation channel. The proposed method is validated through comparison of measurement results to those of the conventional MIMO propagation channel model. The results show that the proposed method employing the Weichselberger model can handle the interlink correlation property, and accurately reconstructs a MIMO propagation channel.

This paper considers two-hop wireless systems employing network coding and a carrier sense multiple access (CSMA) protocol. Network coding is a recent and highly regarded technology for the capacity enhancement of multiple unicast and multisource multicast networks. The two-hop wireless CSMA systems are often involved with the hidden node problem, but the impact of the hidden nodes on network coding has not been analyzed in theory. This paper provides explicit expressions of the throughput for single-relay two-hop wireless CSMA systems both without and with network coding. The throughput can be obtained from these expressions for given system parameters even when end nodes via the relay node are hidden each other. Furthermore it is shown that the transmit probability of the relay node is a design parameter that is crucial for maximizing the achievable throughput for CSMA systems with network coding. It is clarified that the throughput for CSMA systems can be enhanced as compared with that for slotted ALOHA systems in case of non-hidden end nodes whereas it deteriorates considerably in case of hidden end nodes.

In this paper, we propose a subcarrier suppressed transmission scheme that improves the frequency utilization efficiency of STICS (Satellite/Terrestrial Integrated Mobile Communication System), which shares the same frequency band between satellite and land mobile systems. Applying our proposal to the OFDM-based land system overcomes the resource assignment limits of the frequency sharing scheme and yields new land spectrum resources. Computer simulations elucidate the interference suppression performance and the improvement in total average frequency utilization efficiency.

This paper deals with two representative bidirectional traffic cases in two-hop wireless relay access systems employing network coding and a slotted ALOHA protocol. Network coding is a recent and highly regarded technology for capacity enhancement of multiple unicast and multisource multicast networks. The relay nodes are generally involved with unbalanced multidirectional traffic, but the impact of the unbalanced traffic on network coding has not been analyzed. This paper provides closed-form expressions for the throughput and packet delay for two-hop bidirectional traffic cases both with and without network coding even if the buffers on nodes are unsaturated. The analytical results are mainly derived by solving queueing systems for the buffer behavior at the relay node. The results show that the transmission probability of the relay node is a design parameter that is crucial for maximizing the achievable throughput of slotted ALOHA systems with network coding in two-hop bidirectional traffic cases.

In this paper, we propose the superposed multicarrier transmission scheme; it allows multiple signals to share the same frequency band without recourse to spectrum spreading. We also introduce a novel interference suppression technique which requires no change of existing wireless standards. It decodes FEC (Forward Error Correction) blocks after the metrics of interfered subcarriers are replaced by neutral value in the receiver. It minimizes their reliability and thus their impact on error correction performance. Computer simulations indicate that the proposed scheme with FEC Metric Masking can achieve higher total frequency utilization efficiency than conventional spectrum allocation even if a high power interference signal is present and CINR estimation is severely degraded on interfered subcarriers.

This paper deals with two representative unbalanced traffic cases for two-hop wireless relay access systems employing network coding and a slotted ALOHA protocol. Network coding is a recent and highly regarded technology for capacity enhancement with multiple unicast and multisource multicast networks. We have analyzed the performance of network coding on a two-hop wireless relay access system employing the slotted ALOHA under a balanced bidirectional traffic. The relay nodes will generally undergo this unbalanced multidirectional traffic but the impact of this unbalanced traffic on network coding has not been analyzed. This paper provides closed-form expressions for the throughput and packet delay for two-hop unbalanced bidirectional traffic cases both with and without network coding even if the buffers on nodes are unsaturated. The analytical results are mainly derived by solving queueing systems for the buffer behavior at the relay node. The results show that the transmission probability of the relay node is a design parameter that is crucial to maximizing the achievable throughput of wireless network coding in slotted ALOHA on two-hop unbalanced traffic cases. Furthermore, we show that the throughput is enhanced even if the traffic at the relay node is unbalanced.

In this paper, to evaluate the basic performance of MIMO cooperative transmission systems, a simple prediction method which calculation complexity is decreased for evaluating indoor MIMO cooperative transmission is proposed. 5.2 GHz indoor MIMO propagation measurements are carried out in line of sight condition. And the frequency utilization efficiency and eigenvalues are evaluated by measurement data. Simple prediction method of MIMO channel matrix, which takes into account the phase difference of direct waves based on the antenna position, is used to simulate the MIMO channel. The simulation results are then compared to the measurement results. Based on this comparison the proposed prediction method can accurately estimate the characteristics of indoor MIMO cooperative transmission systems.

In this paper, we propose a hidden node problem aware routing metric for wireless LAN (WLAN) mesh networks. To counter the hidden node problem, we utilize the packet collision ratio. This packet collision ratio is estimated from neighbor information, including the transmission time ratio and the neighbor list, as gathered by state announcement packets periodically. Simulation results for a VoIP (Voice over Internet Protocol) communication show that with the acceptable increased control overhead in network due to neighbor information, our proposed routing metric improves the average latency and the packet loss rate performance, and thus allows more concurrent VoIP calls, compared to conventional routing metrics which do not take account of the hidden node problem.

This paper proposes a new signal detection scheme that combines zero-forcing (ZF) and K-best algorithms for orthogonal frequency division multiplexing with space division multiplexing (OFDM/SDM) systems. Among various signal detection algorithms for SDM signals, maximum likelihood detection (MLD) is one of applicable approaches, which achieves the optimal performance. However, it suffers from exponential computational complexity against the number of transmit antennas and modulation order. Thus, we consider the K-best algorithm for the reduction of the computational complexity. We proposed the modified K-best algorithm, which exploits the ZF algorithm for initial symbol estimation. The initial symbol estimation improves the decoding accuracy of the original K-best algorithm. Therefore, the proposed scheme achieves both the reduction of the MLD algorithm's computational complexity and the improvement of the original K-best algorithm's decoding accuracy. Computer simulation results show that the performance degradation from the MLD algorithm is suppressed to just 1 dB or so in the required E/sub b//N/sub o/ for packet error rate (PER) = 10/sup -2/, when either 16 quadrature amplitude modulation (16QAM) or 64QAM is applied with three transmit and three receive antennas. In these cases, about 87% and 99% fewer metric computations are required than the MLD algorithm, respectively. It is presented that the proposed algorithm offers significant reduction of the computational complexity compared to the MLD algorithm.

This paper describes an SDM-COFDM (space division multiplexed-coded orthogonal frequency division multiplexing) prototype employing a hierarchical ICI (inter-channel interference) canceller for MIMO (multi-input multi-output)-based broadband wireless access systems. The prototype has two types of ICI cancellers: multiplicative and subtractive. The multiplicative canceller multiplies the received signals by the estimated inverse propagation coefficient matrix. In addition, the subtractive one generates ICI replicas, subtracts the replicas from the received symbols, weights the subtracted signals by the estimated propagation coefficients, and combines them to obtain diversity gain. Experimental results show that the newly developed prototype with 2/spl times/2 antennas obtains a 1.4 dB improvement in required CNR at the PER of 0.1 compared to that achieved with only the multiplicative canceller. It achieves more than 100 Mbit/s per 18 MHz bandwidth by using 64QAM with the coding rate of 3/4 in frequency selective fading environments.

This paper introduces a novel SDM-COFDM (space division multiplexed-coded orthogonal frequency division multiplexing) prototype for MIMO (multi-input multi-output) channel-based broadband wireless access systems. The developed prototype reduces the inter-channel interference in the SDM transmission by suing a simple feed-forward compensator that simply multiplies the received data symbols by the estimated inverse propagation coefficient matrix at each OFDM subcarrier. Moreover, in order to improve the accuracy of estimating the inverse propagation coefficient matrix, the STC (space-time coded) preamble and the common AFC (automatic frequency control) circuit have been implemented. Furthermore, the likelihood weighting, based on SNR (signal to noise power ratio) of each OFDM subcarrier, is used to improve the error correction performance of soft decision Viterbi decoding. Experimental results show that the prototype with 2/spl times/2 antennas achieves double the transmission bit rate without expanding the occupied frequency bandwidth, while achieving SISO (single-input single-output) transmission quality in fading environments. The prototype offers 108 Mbit/s per 20 MHz by using 64 QAM with the coding rate of 3/4.

This paper proposes space division multiplexed coded orthogonal frequency division multiplexing (SDM-COFDM) scheme using inter-channel interference canceller for multi-input multi-output (MIMO) channels. It reduces the degradation due to inter-channel interference in SDM transmission with a feed-forward canceller multiplying the received symbols by the estimated propagation inverse matrix for each OFDM subcarrier. Moreover, the proposed likelihood-weighting scheme, which is based on each subcarrier's signal-to-noise power ratio (SNR), enhances the error correction performance or soft decision Viterbi decoding. Computer simulation shows that the SDM-COFDM scheme achieves more than 100 Mbit/s per 20 MHz in frequency selective fading environments with the same carrier-to-noise ratio (CNR) as the single channel COFDM.

The paper proposes a half chip offset QPSK (quadrature phase shift keying) modulation CDMA (code division multiple access) scheme coupled with time diversity and high-coding-gain FEC (forward error correction) for wireless LAN (local area network) systems. It is clarified that the proposed scheme realizes compact spectrum characteristics in hard limited channels and excellent Pe performance for high-speed data transmission with simple hardware configuration. The experimental results show that by employing time diversity with FEC, the half chip offset QPSK CDMA scheme realizes a required Eb/No of 14.2 dB at a Pe of 10/sup -5/ in Rician fading environments with fD of 10 Hz and delay spread of 40 nsec.