In this paper, we study the advantages of improper Gaussian signaling (IGS) with the existence of hardware impairments (HWI) and imperfect successive interference cancellation (SIC) in the downlink two-user power-domain non-orthogonal multiple access (PD-NOMA) systems. We first provide the input-output relationships for the MIMO system with HWIs and then derive the rate expressions for each user. The improper signals are assumed to be generated by widely linear precoding (WLP) and transmitted under the PD-NOMA scheme in the context of imperfect SIC. To exhibit the supremacy of IGS, we formulate two kinds of optimization problems concerned with the max-min rate and maximizing achievable sum rate (ASR) between users under the quality of service (QoS) and the total power constraints (TPC). To deal with the non-convex log-determinant objective functions, we adopt successive concave quadratic programming (SCQP) to obtain suboptimal results by iterations. Simulation results demonstrate that IGS cases outperform ZF precoders and conventional proper Gaussian signaling (PGS) counterparts in both max-min rate and ASR, particularly in high I/Q imbalance (IQI) and less efficient SIC scenarios. Besides, the difference in the performance degradation between imperfect SIC and HWIs is compared. Finally, we briefly classify the conditions where IGS could be preferable for signal transmission.

The aim of this paper is to further investigate the communication with Orbital Angular Momentum (OAM) waves between two OAM antenna arrays. The antenna arrays consist of individual dipoles, which together form a Uniform Circular Array (UCA). In this paper, two configurations that have already been discovered are assumed, in which there is no crosstalk between the individual modes. On the one hand, the vertical alignment already examined by many papers and on the other hand a horizontal alignment in which the dipoles have a certain rotation around the axis. The first investigation deals with the frequency dependency in OAM communication in the given setup. In a further investigation, the influence of the radius of the antenna arrays is conducted. The simulations will focus on the transmission of the individual OAM modes and the crosstalk between different OAM modes.

In this study, we compare the system performance by considering faster-than-Nyquist (FTN) signaling employing single carrier frequency domain equalization (SC-FDE) and orthogonal frequency division multiplexing (OFDM) under the constraint of a fixed total utilized bandwidth. In this work, FDE also includes the appearance of colored noise, and the corresponding FDEs are formulated based on its effect to improve detection reliability. The advantages of FTN signaling and the preferable choices of compression and roll-off factors based on the bit error rate (BER) and system throughput are demonstrated under multipath fading channels via the simulation results.

This paper proposes a deep-learning-based uplink hybrid multiple access scheme consisting of both sparse code multiple access (SCMA) and orthogonal frequency-division multiple access (OFDMA). SCMA improves the system throughput when the carrier-To-noise ratio (CNR) is high. However, SCMA performance is significantly degraded, compared to OFDMA, when the CNR is low. To overcome this problem, the proposed scheme introduces a combination of SCMA and OFDMA as a novel multiple access pattern. The scheme determines the appropriate pattern among SCMA-only, OFDMA-only, or their combination, by utilizing user position information through deep learning. The effectiveness of the proposed scheme is demonstrated in terms of system throughput under different user distributions via computer simulations.

This paper proposes a mode group selection method for orbital angular momentum (OAM) multiplexing to suppress inter-mode interference (IMI) owing to the misalignment of the beam axis. We have proposed an IMI suppression method employing even-numbered OAM modes, and its effectiveness has been successfully confirmed. Odd-numbered OAM modes are robust and used in addition to even-numbered modes to improve the robustness against the IMI, and the appropriate mode group is selected based on the system capacity. The effectiveness of the proposed method is demonstrated in terms of system capacity via computer simulations.

This paper proposes a transmit power control method to reduce power consumption and alleviate inter-cell interference for centrally controlled wireless LANs with multi-user multiple-input multiple-output (MU-MIMO). The main feature of the proposed method is to control the transmit power of the access points (APs) according to the relationship between the number of allocated users and AP antennas. The effectiveness of the proposed method compared to the conventional fixed transmit power method is demonstrated in terms of power consumption and system throughput via computer simulations.

This paper investigates the system capacity of polarized orbital angular momentum (OAM) multiplexing by comparing it with OAM multiplexing without polarization for the same number of antenna elements. Since the same number of antenna elements is assumed for both approaches, their performances can be compared under the same number of simultaneous OAM streams, thereby allowing the effectiveness of polarization on UCA-based OAM multiplexing to be examined. Numerical results, obtained through computer simulations, provide the mode selectivity and system capacity with respect to link distance and carrier frequency.

This paper investigates adjacent-channel interference (ACI) influence on asynchronous systems consisting of filter bank multicarrier (FBMC) and orthogonal frequency-division multiplexing (OFDM), under multipath fading channels. FBMC inhibits out-of-band (OOB) radiation more than OFDM, by subcarrier-wise filtering. This results in ACI reduction, while causing inter-symbol interference (ISI) and inter-carrier interference (ICI) under multipath fading channels. In this paper, we evaluate the bit error rate (BER) performance of asynchronous FBMC/OFDM systems, considering the ACI and simulations.

This paper proposes a deep-learning-based resource allocation method to adaptively control system capacity and fairness for multi-user multiple-input and multiple-output (MU-MIMO). In the proposed method, Tomlinson-Harashima precoding (THP) is used to enhance the transmission rate. Additionally, channel resources are appropriately allocated based on user scheduling techniques, i.e., semiorthogonal user selection (SUS) for throughput maximization and proportional fairness (PF) for fairness among users. The primary feature of the proposed method is that it appropriately allocates channel resources by utilizing the user position information and target fairness index (FI) through deep learning. This makes it possible to meet various service requirements. Numerical simulations are used to demonstrate the effectiveness of the proposed method in terms of system capacity and fairness under different MIMO configurations and user distributions.

This paper presents the theoretical system capacity of orbital angular momentum (OAM) multiplexing that employs single-carrier with frequency domain equalization (SC-FDE) in the presence of ground reflection. Considering that inter-symbol interference (ISI) remains even after inserting a guard interval (GI), in addition to inter-mode interference (IMI), we derive the effect of such residual IMI and ISI, and incorporate their impacts into the analysis of the system capacity. Moreover, in FDE, zeroforcing (ZF) and minimum mean square error (MMSE) are considered as the factors affecting the system capacity because the effect of noise enhancement differs among these two factors. In this study, we demonstrate theoretical results based on the tworay ground reflection model by using parameters of link distance and carrier frequency. The numerical results show that OAM multiplexing employing SC-FDE improves the system capacity compared with the case without FDE regardless of the link distance, and its effectiveness is remarkable in the cases of relatively lower carrier frequencies.

This paper investigates the impact of the antenna arrangement, such as the centralized and distributed schemes, on the theoretical system capacity of multiuser multiple-input and multiple-output (MU-MIMO) systems in the presence of terminal mobility. In our theoretical analysis, we assume that the propagation model of the centralized or distributed scheme is characterized by scattered waves that arrive at the mobile station (MS) from the same or different angles because of the antenna arrangement of the base station (BS), and theoretically analyze the multiuser interference (MUI) caused by terminal mobility. Furthermore, we compare the performances of the centralized and distributed schemes using the proposed analysis.

This paper investigates the system capacity of polarized orbital angular momentum (OAM) multiplexing considering the impact of polarization interference. Dipole antenna is considered as the practical radiation pattern and the composition of the received mode signal is clarified by analyzing the effect of cross-polarization interference from other modes, as well as both cross-polarization interference in a certain mode and co-polarization interference from different modes. Based on the interference analysis, system capacity is precisely evaluated by comparison with the isotropic antenna case, employing a link distance and carrier frequency parameter. The numerical results show that cross-polarization interference from other modes rises at the regular mode interval, presenting a significant impact on system capacity, particularly for relatively short link distances or low carrier frequencies.

This paper proposes an adaptive repetition control using terminal mobility to alleviate the packet collision for uplink grant-free (GF) ultra-reliable and low-latency communications (URLLC). In GF, the packet collision among mobile stations (MSs) becomes a serious problem because the grant-issuing procedure conducted in grant-based (GB) transmission is eliminated. Especially in the K-repetitions method, intra-cell interference is increased due to excessive repetitions, resulting in decreased reliability. In the proposed approach, the number of necessary repetitions is determined using fading correlation easily obtained from the terminal speed. This is because the time diversity benefit within repetitions is dependent on the fading correlation. Using the proposed repetition control, the number of repetitions can be reduced especially in the low fading correlation, alleviating the packet collision. The effectiveness of our proposed method is demonstrated in an actual multi-user URLLC scenario by computer simulations.

This paper proposes a coordinated multi-point (CoMP) transmission method based on deep learning for taxi radio systems to prevent inter-cell interference (ICI). In CoMP, it is essential to select whether to use simultaneous transmission or time division multiplexing (TDM) considering the effect of the ICI. The feature of the proposed method is to determine such a transmission mode by using vehicle position information as taxi radio systems have such position information. Moreover, the proposed method makes it possible to avoid the online system capacity calculation also required for CoMP thanks to the use of deep learning. The effectiveness of the proposed method is demonstrated in comparison with a traditional online calculation method under the practical scenario based on the taxi radio system in Japan.

This paper proposes a hybrid multiple access scheme using both non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) to overcome the inherent problems of NOMA. In NOMA, as multiple access is performed within the power domain, a small difference in the channel gain between users or an increase in the number of simultaneous users reduces the effectiveness. To cope with these problems, the proposed scheme applies a novel combination of NOMA and OMA as well as NOMA for multiple access patterns, and then determines the best pattern based on the system capacity. The effectiveness of the proposed scheme is demonstrated in comparison with conventional hybrid multiple access using NOMA and OMA through computer simulations.

This paper proposes a repetition control method using terminal mobility to alleviate the packet collision for uplink grant-free (GF) ultra-reliable and low-latency communications (URLLC). Because the time diversity benefit within repetitions is basically dependent on a fading correlation, the proposed approach determines the number of necessary repetitions using a fading correlation easily obtained from the terminal speed. Using the proposed repetition control, the number of repetitions can be reduced especially in low fading correlations, alleviating packet collision. The effectiveness of our proposed method is demonstrated in terms of latency by computer simulations.

This paper proposes an inter-mode interference suppression method that employs only even-numbered modes for uniform circular array (UCA)-based orbital angular momentum (OAM) multiplexing. Given that interference from adjacent OAM modes becomes severe when the beam axis is misaligned, the proposed method uses only the even-numbered modes while leaving the odd-numbered modes unused, which effectively prevents the inter-mode interference. Moreover, we verify the effectiveness of the proposed method by applying it to multiple-input and multiple-output (MIMO) with zero-forcing (ZF) alone or ZF with successive interference cancellation because severe inter-mode interference is caused by transmitting multiple streams from multiple UCAs in each mode. The effectiveness of the proposed method compared with the traditional method, which uses all OAM modes, is demonstrated in terms of the system capacity through computer simulations.

This paper proposes a centralized resource allocation method to alleviate the problem of intercell interference for multiuser multiple-input multiple-output (MU-MIMO) capable wireless LANs. In this method, the control engine connected to all access points (APs) predicts the system throughput considering intercell interference by using the number of detectable APs and users, which allocates users for APs to achieve the highest throughput. After conducting AP selection, each AP adopts semi-orthogonal user selection (SUS) as user scheduling to improve MU-MIMO performance. The effectiveness of the proposed method is demonstrated using computer simulations in comparison with the traditional maximum received-signalstrength-indicator-based (RSSI-based) AP selection using random user scheduling.

MU-MIMO-OFDM can be seen as a promising technique for achieving high speed and large capacity thanks to both spatial multiplexing and robustness against multipath fading. However, in MU-MIMO-OFDM, the guard interval (GI) length should be determined in accordance with the longest delay time among all MSs, which results in the significant degradation in transmission efficiency. In this paper, we propose the GI control scheme using theoretical system capacity for MU-MIMO-OFDM systems. In the proposed scheme, the GI length is determined to maximize the theoretical system capacity while allowing some amount of inter-symbol interference (ISI), which reduces the transmission overhead. Moreover, the proposed scheme theoretically analyzes the system capacity from the channel state information (CSI) using correlation calculation. The effectiveness of the proposed scheme is demonstrated in comparison with the traditional GI length scheme adjusted to the largest multipath delay by means of computer simulations.

This paper proposes a method that employs successive interference cancellation (SIC) to further enhance the capacity of uniform circular array (UCA)-based orbital angular momentum (OAM)-MIMO systems. The main feature of the proposed method is to stabilize the performance of UCA-based OAM-MIMO systems by conducting SIC based on the results of parallel interference cancellation (PIC), which exploits the additional full space diversity benefit. The development of the proposed approach is motivated by the feature that large differences in the received power of different UCAs are advantageous to SIC. Moreover, the impact of the misalignment of the beam axis on the effectiveness of the proposed method is also investigated by considering a more realistic scenario. Our numerical results show that the effectiveness of the proposed method increases with increasing number of UCAs, and its advantage is retained within a certain amount of the misalignment of the beam axis.

This paper proposes an efficient proportional fairness (PF) scheduling method using user distribution in MU-MIMO Tomlinson-Harashima precoding (THP). Although PF realizes fairness among users, heavy computational loads are experienced due to user selection, which must calculate the system capacity for all possible user combinations. To overcome this problem, the proposed approach partially allocates channel resources for semiorthogonal user selection (SUS), which causes unfairness while maintaining throughput maximization with low computational loads. This is inspired by the fact that user scheduling for throughput maximization such as SUS increases the degree of fairness under non- uniform user distributions. Hence, in the proposed method, the user distribution is measured by the spread of channel state information (CSI) power obtained from multiple antennas, and the resource allocation to PF and SUS is adaptively controlled based on this spread. Computer simulations are used to demonstrate the effectiveness of the proposed method in comparison with the traditional MU-MIMO THP with PF in terms of fairness among users and calculation costs.

This paper proposes a method that employs successive interference cancellation (SIC) to further enhance the capacity of uniform circular array (UCA)-based orbital angular momentum (OAM)- MIMO systems. The central feature of the proposed approach is to exploit the additional space diversity benefit in the remaining streams by means of SIC owing to the large difference in the received power among different UCAs in each OAM mode. Therefore, it is expected that the proposed method can further stabilize the performance of UCA-based OAM-MIMO along with the communication distance. Our numerical results show that the proposed method outperforms the traditional method without SIC in terms of the system capacity owing to the large received power difference among different UCAs.

This paper proposes a low complexity fair user scheduling method employing semi-orthogonal user selection (SUS) for multi-user multiple-input multi-output (MU-MIMO) systems. The intent of the proposed approach is to consider the spatial orthogonality of users into fair user scheduling of the MU-MIMO system, which enhances the system capacity when compared to the traditional round robin (RR) based user scheduling. In the proposed method, the spatial orthogonality is calculated based on SUS which only requires the calculation of inner products among the channel state information (CSI). Therefore, both the improvement of the system capacity and fairness can be achieved with a reduced computational load. Moreover, in contrast to proportional fairness (PF), the proposed user scheduling does not require the heavy calculation of all possible user combinations as is required in MU-MIMO precoding, which is an effective and practical approach. The effectiveness of the proposed method is demonstrated and compared with RR and PF in terms of fairness and computational load.

This paper investigates the system-level performance of multi-user MIMO (MU-MIMO) with different power normalization in consideration of user scheduling. In this paper, matrix normalization (MN) and vector normalization (VN) are taken as the power normalization methods. As for user scheduling, semiorthogonal user selection (SUS) or proportional fairness (PΓ) is adopted, because they are opposed to each other in terms of the quality indexes, such as system capacity and fairness. Specifically, we evaluate the impact of these power normalization methods on the quality indexes and clarify the favorable matching between the power normalization and user scheduling to achieve certain quality indexes.

This paper presents the theoretical system-level performance of multi-user multiple-input and multiple-output (MU-MIMO) Tomlinson-Harashima precoding (THP) with user scheduling. In our performance evaluation, proportional fairness (PF), which makes a reasonable compromise between fairness among users and the benefit of multi-user diversity, is implemented as a user scheduling technique, and the effect of modulo loss resulting from TRIP modulo operation at the receiver is taken into account using mod-Lambda channel-based analysis, which provides accurate theoretical performance. Moreover, considering the application to the PF metric, the performance of the mod-Lambda channel-based PF metric is compared with that of the traditional Shannon-Hartley theorem-based metric.

MU-MIMO-OFDM can be seen as an attractive approach to realize high system capacity under the limitation on hardware cost and frequency selective fading. However, MU-MIMO-OFDM has an inherent problem of nonlinear distortion which is more severe than SISO-OFDM case. In this paper, we propose the OFDM clipping and filtering scheme employing deterministic transmit power control to overcome the BER degradation due to the severe nonlinear distortion. In the proposed approach, the transmit power level can be determined by the closed form formula thanks to the fact that the transmit signals at each antenna follows Rayleigh distribution. The effectiveness of the proposed approach is verified in terms of the BER and throughput by means of computer simulations.

In recent years, wireless communications systems, such as 5th generation (5G) mobile systems, have been developed as a complex combination of various elementary technologies. Therefore, it is important to consider how to effectively and efficiently combine these technologies from the viewpoint of network operation and system configuration. In this context, theoretical analysis is essential as a means of bridging the gap between different technologies. Moreover, finding common objectives among different technologies is expected to reduce computational cost. In this paper, the combination of multiuser MIMO (MU-MIMO) and user scheduling, such as resource allocation or user selection, are taken up as examples based on an awareness of the above issues. As for network operation, MU-MIMO resource allocation based on theoretical channel capacity considering the effect of time selective fading is introduced. Moreover, the efficient combination of MU-MIMO employing Tomlinson Harashima precoding (THP) and user selection using space orthogonality as a common measure is presented.

This paper proposes a novel receiving antenna allocation scheme for downlink multiuser multiple input multiple output orthogonal frequency division multiplexing (MU-MIMO-OFDM) transmission; an access point (AP) simultaneously transmits data frames to a combination of allocated receiving antennas on a subcarrier basis at each station (STA). The proposed scheme combines a limited channel state information (CSI) feedback sequence with a receiving antenna decision method. In the proposed scheme, each STA estimates the channel responses of all receiving antennas by using the training preamble transmitted from an AP, and then feeds the channel response of the antenna with maximum norm back to the AP when the spatial correlation value between receiving antennas is higher than a threshold. Otherwise, each STA feeds full channel responses back to the AP. This scheme decreases the amount of CSI fed back while exploiting the spatial diversity gain, and the AP’s computational complexity is also decreased regarding the antenna allocation. Moreover, the receiving antenna decision method eliminates the overhead to notify the allocated antenna information from the AP to each STA by simply comparing its own receiving antenna powers. We clarify the effectiveness of the proposed scheme in our computer simulations using channel responses measured in an indoor environment. The results show that the proposed scheme maintains the channel capacity of the downlink MU-MIMO-OFDM transmission while greatly reducing the overhead and computational complexity.

This paper proposes a method for control overhead reduction by employing frequency correlation for MU-MIMO-OFDM THP with semi-orthogonal user selection (SUS). The feature of the proposed approach is to reduce the control overhead generated by subcarrier-based user selection results by limiting the sub-carriers according to the frequency correlation. The motivation behind this approach comes from the fact that the same user selection results could be applicable to neighboring subcarriers by increasing the frequency correlation. A simple theoretical formula containing a parameter for a power delay profile is adopted to facilitate the derivation of the frequency correlation. Frequency correlation is achieved by generating the power delay profile by averaging the channel state information (CSI) over multiple transmit antennas with the aid of MU-MIMO. Moreover, our efficient method, which ensures the benefit of ordering by reusing the SUS results for the ordering calculation, is adopted to coordinate SUS and THP ordering. The effectiveness of the proposed method is demonstrated in terms of both system capacity and control overhead reduction by comparing it with the subcarrier-based approach.

This paper investigates the influence of the number of uniform circular arrays (UCAs) on the system capacity in orbital angular momentum (OAM) multiplexing. In UCA-based OAM, although multiple UCAs can alleviate the difference in the received signal power among OAM modes, the maximum number of OAM modes is limited. The primary contribution of our work is to evaluate the waterfilling-based system capacity of UCA-based OAM with the number of UCAs at constant transmit power and number of antenna elements, to demonstrate the benefit of multiple UCAs according to the communication distance. Our numerical results show that the multiple UCAs stabilize the system capacity irrespective of the communication distance, whereas the single UCA is effective particularly in short-range communications.

This paper investigates the theoretical system capacity of multi-user multiple-input and multiple-output (MU-MIMO) Tomlinson-Harashima precoding (THP) in the presence of terminal mobility. Considering that MU-MIMO generally suffers from transmission performance degradation over time selective fading channels owing to terminal mobility, we incorporate the impact of mobility as well as the modulo loss peculiar to THP into the system capacity analysis. In particular, the multiuser interference (MUI) due to terminal mobility is theoretically derived and included as the undesired component in the mod-Lambda channel required for analyzing the effect of the modulo loss. The proposed analysis enables a fair comparison in terms of the system capacity between THP and linear precoding (LP), and demonstrates the superiority of THP over LP in the presence of terminal mobility.

This paper investigates an adaptive movable access point (AMAP) system and explores its feasibility in a static indoor classroom environment with an applied wireless local area network (WLAN) system. In the AMAP system, the positions of multiple access points (APs) are adaptively moved in accordance with clustered user groups, which ensures effective coverage for non-uniform user distributions over the target area. This enhances the signal to interference and noise power ratio (SINR) performance. In order to derive the appropriate AP positions, we utilize the k-means method in the AMAP system. To accurately estimate the position of each user within the target area for user clustering, we use the general methods of received signal strength indicator (RSSI) or time of arrival (ToA), measured by the WLAN systems. To clarify the basic effectiveness of the AMAP system, we first evaluate the SINR performance of the AMAP system and a conventional fixed-position AP system with equal intervals using computer simulations. Moreover, we demonstrate the quantitative improvement of the SINR performance by analyzing the ToA and RSSI data measured in an indoor classroom environment in order to clarify the feasibility of the AMAP system.

This paper investigates the theoretical system capacity of MU-MIMO Tomlinson-Harashima precoding (THP) with the ordering process. In this paper, the impact of the modulo loss peculiar to THP on the system capacity is taken into account, which makes us recognize the significance of its impact comparing with the Shannon-Hartley-based results. Moreover, the ordering process generally performed before THP to enhance the system capacity is also considered in our performance analysis. In detail, the impact of the modulo loss is analyzed by using the mod-A channel, where the result of the V-BLAST-like suboptimum ordering is reflected as the power normalization factor. The proposed performance analysis considers these two actual effects, which provides a fair comparison between THP with ordering and linear precoding (LP) as well as the impact of the modulo loss, which has great potential to improve user scheduling techniques that require accurate measurements of the instantaneous system capacity.

This letter proposes an efficient method for combining MU-MIMO Tomlinson Harashima precoding (THP) with user scheduling techniques to realize fairness among users as well as high system capacity. The concept of the proposed approach is to first adopt semiorthogonal user selection (SUS) for throughput maximization and then apply proportional fairness (PF) to the remaining users, which regains fairness lowered by using SUS for throughput maximization. Moreover, considering that THP adopted as a precoder for PF requires the calculation of ordering and precoding for each user combination, the proposed method adopts linear precoding (LP) instead of THP to relax the computational effort during operation of PF. This approach is inspired by the fact that LP achieves almost the same system capacity as THP with ordering when conducting PF. The effectiveness of the proposed method is demonstrated by comparing it to MU-MIMO THP with SUS or PF in terms of system capacity and fairness among users, by means of computer simulations.

The scintillation and bit error rate performance of optical wireless communication systems employing Laguerre-Gaussian (LG) beams were evaluated considering the size of the transmitted beam and receiving aperture. As a result, it was found that the system employing LG beams with a high radial number can reduce the effect of scintillation and improve communication quality.

This paper presents the performance evaluation of multi-user MIMO (MU-MIMO) employing Tomlinson Harashima precoding (THP) in consideration of user scheduling techniques from the viewpoint of system capacity and fairness. In this paper, the ordering process is adopted for THP, and both semiorthogonal user selection (SUS) and proportional fairness (PF) are considered as user scheduling techniques. Their opposite characteristics make it possible to establish the degree of matching between precoding and user scheduling techniques. The system-level performance with unequal CNR users is evaluated through computer simulations, and it is compared with the performance of linear precoding (LP).

This paper proposes an efficient ordering calculation method employing frequency correlation in multiuser multiple-input multiple-output orthogonal frequency division multiplexing (MU-MIMO-OFDM) Tomlinson Harashima Precoding (THP). Although the ordering process is effective to improve the transmission performance of THP, it requires a large amount of computational effort due to sub-carrier-based operation. To cope with this problem, the proposed method thins out the sub-carriers conducting the ordering process based on the frequency correlation. The basic idea behind this proposal is that the ordering results are considered to be the same among the neighboring sub-carriers in a relatively small delay spread. Moreover, the frequency correlation is derived from a delay profile which can be determined by multiple transmit antennas. The effectiveness of the proposed method is demonstrated in terms of calculation cost and the bit error rate (BER) by comparing with the traditional MU-MIMO-OFDM THP without thinning.

This paper proposes an adaptive movable access point (AMAP) system to enhance the system capacity in dense wireless-device environments. In the proposed approach, the positions of access points (APs) are adaptively moved in accordance with clustered user distribution, which is sure to be effective in ununiform user distribution due to dense device environments. In order to derive the appropriate positions of APs, we utilize k-means method and estimate the user's position which is essential for k-means method by means of receive signal strength indicator (RSSI) or time of arrival (ToA). The effectiveness of the proposed systems is evaluated in the presence of the positioning error obtained from the experimental results concerning RSSI and ToA and is demonstrated in comparison with the fixed position of AP with equal intervals.

This paper proposes an inter-symbol interference (ISI) suppression scheme employing periodic signals for network MIMO-OFDM systems. Network MIMO-OFDM suffers from the ISI due to long time difference of arrival ( TDOA) especially in large cells. In the proposed scheme, the periodic signals generated by only the even-numbered sub-carriers are adopted for ISI-affected users while the traditional non-periodic signals are used for ISI-free users. Moreover, to realize the transmission mode selection between the periodic and non-periodic signals, the theoretical derivation method of the BER is newly derived considering the impact of channel coding. The effectiveness of the proposed method is demonstrated in comparison with the traditional non-periodic transmission by means of computer simulations.

This paper proposes a simple coordinated multipoint (CoMP) transmission method for taxi radio systems in order to prevent inter-cell interference (ICI) without a significant renewal of facilities. The proposed method takes advantage of the interesting property that the ICI which is essential for CoMP can be represented by the communication distance easily obtained from vehicle position information in a 2-cells scenario. Moreover, the proposed approach can be adopted for more than 3 cells by combining the ICI between 2 cells. The proposed method exploits diversity reception in the case of time division multiplexing (TDM), which makes it possible to improve the channel capacity in overlapped area. The effectiveness of the proposed method is demonstrated in comparison with the case of no coordination (w/o CoMP) under the practical scenario based on the taxi radio system in Japan.

This paper proposes an enhanced decision feedback equalizer (DFE) for filter bank multicarrier (FBMC) systems over offset quadrature amplitude modulation (OQAM) based on a minimal mean square error (MMSE) criterion. For each subcarrier, besides one feedforward (FF) filter and one feedback (FB) filter, we add two intercarrier interference (ICI)-suppressing filters, which feed back detected precursors extracted from neighboring subchannels to enhance ICI suppression in the presence of multipath channels. Our simulation results indicate our proposal makes a considerable improvement comparing with MMSE-DFE under a severe ICI and ISI scenario despite few extra costs in computational complexity.

This paper proposes a downlink MU-MIMOOFDM transmission using simple receive antenna selection. In the proposal, base station transmits to users with selected receive antenna per subcarrier to obtain diversity gain. The proposal also decreases computational complexity by narrowing down the number of antenna combination in accordance with correlation value between receive antennas at each user. From experimental evaluations, it is indicated that the proposal offers high channel capacity with greatly reduced antenna combination.

This paper proposes an inter-symbol interference (ISI) suppression scheme employing periodic signals for network MIMO-OFDM systems. Network MIMO-OFDM suffers from the ISI due to long time difference of arrival (TDOA) among the users especially in large cells. In the proposed scheme, the periodic signals generated by only the even-numbered sub-carriers is adopted for suppressing the ISI and the appropriate transmission mode is chosen between the traditional non-periodic and periodic signals based on the theoretical BER obtained by means of correlation operation. The proposed approach makes the most of the useful properties that a certain ISI-affected user does not interfere at all with the other users, which enables us to make the countermeasure individually. The effectiveness of the proposed method is demonstrated in comparison with the traditional non-periodic transmission by means of computer simulations.

This letter derives the theoretical BER of coded SC-FDE transmission taking advantage of the traditional theoretical results of the instantaneous SINR. The proposed approach is inspired by the fact that the theoretical channel coding gain under AWGN is directly applicable to the traditional FDE performance due to the flat SINR characteristic within an FDE block. The proposed derivation method can provides the theoretical coded BER without any effort of actual signal processing such as modulation, demodulation, and channel coding. The validity of the proposed approach is confirmed by the agreement with the computer simulation results under frequency selective fading channels.

This paper proposes a simple coordinated multipoint (CoMP) transmission method for taxi radio systems. The feature of the proposed method is to realize CoMP only by using the vehicle position information, which enables us to free from a significant renewal of facilities. The proposed approach is inspired by the idea that the channel capacity based on the inter-cell interference (ICI) can be represented by the communication distance which is easily obtained from the vehicle position information. In addition, the proposed method is applicable to more than 3 cells by using the same approach applied to 2 cells. The effectiveness of the proposed method is demonstrated in comparison with the case of no coordination (w/o CoMP) under the practical scenario based on the taxi radio system.

This paper proposes an efficient method for combing multi-user MIMO (MU-MIMO) employing Tomlinson Harashima precoding (THP) with semiorthogonal user selection (SUS). Although the ordering of MU-MIMO THP and SUS create space diversity benefit and multi-user diversity benefit, respectively, a significant computational effort is required, which is expected to be relaxed while retaining the system level performance. The proposed method is inspired by the fact that the ordering calculations of MU-MIMO THP and SUS are commonly based on the spatial orthogonality. If that is the case, the result of the ordering of SUS is fully reused for the ordering of MU-MIMO THP, which can eliminate the calculation process of the ordering of MU-MIMO THP. The effectiveness of the proposed method is demonstrated in comparison with the traditional method with the ordering of MU-MIMO THP by means of computer simulations.

This paper proposes the theoretical derivation method of the Shannon capacity for nonlinearly amplified uplink Orthogonal Frequency Division Multiple Access (OFDMA) in the presence of terminal mobility. In our theoretical derivation, the impact of the intercarrier interference (ICI) arising from terminal mobility as well as the multiuser interference (MUI) caused by nonlinear amplification of the mobile stations (MSs) on OFDMA signals is taken into account. Considering the fact that these interferences differ with the sub-carrier allocation methods such as localized FDMA (LFDMA) and interleaved FDMA (IFDMA), we derive the probability density function (PDF) of the instantaneous signal-to-noise-plus-distortion-plus-interference ratio (SNDIR) for each sub-carrier allocation method. Since the SNDIR is successfully expressed in the closed form, the proposed formula enables us to evaluate the Shannon capacity without conducting time consuming computer simulations.

This paper proposes a novel modulo loss suppression scheme employing a theoretical BER formula for multi-user multiple-input multiple-output (MU-MIMO) Tomlinson-Harashima Precoding (THP) systems. The main feature of the proposed approach is to control the constellation distance by using the simple theoretical bit error rate (BER) formula in order to overcome the modulo loss. Behind this proposal, there is an interesting fact that the BER performance is dependent on the influence of both the thermal noise and the modulo loss and that its influence is determined by the constellation distance. In the proposed approach, we newly derive a theoretical simple BER formula for MU-MIMO THP and then find out the optimal constellation distance to suppress the modulo loss. The effectiveness of the proposed scheme is demonstrated in comparison to the traditional MU-MIMO THP with fixed distance by means of computer simulations.

This paper proposes the inter-symbol interference (ISI) suppression scheme employing periodic signals for coded MIMO-OFDM systems. The feature of the proposed approach is to choose the appropriate transmission mode composed of traditional non-periodic signals or periodic signals with only even-numbered sub-carriers according to the delay spread. To provide the selection criterion for choosing between these transmission modes, we develop the theoretical bit error rate (BER) estimation method using the correlation calculation for coded MIMO-OFDM. The effectiveness of the proposed scheme is demonstrated in comparison with the traditional non-periodic symbol transmission.

This paper proposes an antenna decision method for downlink multiuser MIMO (MU-MIMO) with receive antenna allocation per subcarrier. When receive antenna allocation is used, each receiver requires the information of the antenna allocation before receiving the data; for this an overhead signal is used. The proposed method decreases the overhead by deciding an antenna with comparison of received power at each antenna of the receiver. Computer simulations show that the proposed method offers highly accurate antenna decision and high channel capacity with one OFDM symbol.

This paper proposes an adaptive decision feedback channel estimation (DFCE) scheme using the knowledge of terminal speed in single-carrier frequency-domain equalization (SC-FDE). Since SC signals generally have the large envelope variation in frequency spectrum, the subcarrier signals with the small magnitude cause the noise enhancement. The proposed scheme adopts DFCE only for the subcarriers with the relatively large magnitude. Moreover, the proposed scheme adaptively controls the forgetting factor according to the channel condition such as noise dominant channel and time selectivity dominant channel. The feature of the proposed approach is to theoretically derive the effect of time selective fading by using the terminal speed and to detect the actual channel condition. The effectiveness of the proposed scheme is demonstrated in comparison with the traditional DFCE with the fixed forgetting factor by means of computer simulations.

This paper proposes an adaptive resource allocation scheme to cope with heterogeneous traffic for multiuser multiple-input multiple-output orthogonal frequency division multiple access (MU-MIMO-OFDMA). The feature of the proposed approach is to maximize the frame efficiency by simultaneously allocating frequency and space resources under different user's packet size and channel condition. However, since frequency and space resources generate numerous number of allocation patterns, we reduce the number of patterns thanks to the fact that broadband frequency channels in OFDMA provide almost the same performance. Computer simulation shows the effectiveness of the proposed scheme by comparing with the conventional scheme without resource allocation.

Nonlinear precoding multi-user multiple-input multiple-output using vector perturbation (VP MU-MIMO) can provide good frequency efficiency at the cost of computational complexity, which exponentially increases as the number of spatial multiplexed users increases. This is because the original VP precoding requires to exhaustively search the optimum transmit signal vector among all the combinations of each user's transmit signal candidates. To reduce the computational complexity, in this paper, we propose signal space conversion assisted vector ranking algorithm, which exploits the equivalency between the search of optimum transmit signal vector in the VP precoding and the signal detection from received signal in the general signal transmission. And, its transmission performance and computational complexity are evaluated by computer simulation.

This paper proposes the theoretical derivation method of the bit error rate (BER) performance for uplink Orthogonal Frequency Division Multiple Access (OFDMA) employing space diversity reception under nonlinear fading channels. The proposed method provides the simple series form formula which takes into account the space diversity benefit as well as two different subcarrier allocation methods such as localized FDMA (LFDMA) and interleaved FDMA (IFDMA). Hence, the proposed formula makes it possible to easily confirm the performance difference between LFDMA and IFDMA while computer simulations require high computational complexity. The validity of the proposed formula is confirmed by the agreement with the computer simulation results under various kinds of system parameters such as the subcarrier allocation methods, the number of MSs, the number of antennas, and the input back-off(IBO).

We propose a cross-layer switching method of orthogonal frequency division multiple access (OFDMA) and multiuser multiple input multiple output (MU-MIMO) for the future wireless local area network systems. The proposed method, employed on the medium access control layer, switches between OFDMA and MU-MIMO as the transmission method after processing by using physical layer information such as an overhead of channel state information feedback, station number, and data length in order to improve the transmission efficiency. Simulation results show that the proposed method achieves higher total throughput than conventional OFDMA or MU-MIMO where switching is not performed.

This paper proposes a deterministic approach employing transmit power control to overcome the BER degradation of OFDM clipping and filtering. The feature of the proposed method is to perform the transmit power control before clipping and filtering to prevent the nonlinear distortion due to clipping process and furthermore, the optimal transmit power is automatically controlled based in the signal-to-noise-plus-distortion ratio (SNDR) derived by the simple mathematical formula. Therefore, the proposed method is expected to straightforwardly improve the BER without any iterative process which is required in the conventional clipping and filtering method. The effectiveness of the proposed method is demonstrated in comparison with the traditional clipping and filtering method by means of computer simulations. Copyright © 2013 by the Institute of Electrical and Electronic Engineers, Inc.

This paper proposes the theoretical derivation method of both the frequency spectrum and the bit error rate (BER) performance for uplink Orthogonal Frequency Division Multiple Access (OFDMA) in the presence of the nonlinear distortion. Since OFDMA has two different subcarrier allocation methods such as localized FDMA (LFDMA) and interleaved FDMA (IFDMA), there is the difference, in terms of the influence of the nonlinear distortion at each mobile station (MS) on the out-band radiation and the BER degradation, between LFDMA and IFDMA. Therefore, it is meaningful to clarify this difference by means of the theoretical approach. The feature of the proposed approach is to represent both the frequency spectrum and the BER performance by the simple series form formula, which enables us to overview these performances without any time-consuming computer simulations. The validity of the proposed formula is confirmed by the agreement with the computer simulation results with the parameters of the number of MSs and the input back-off (IBO). © 2013 IEEE.

Multi-user MIMO (MV-MIMO) using Tomlinson-Harashima precoding (THP) has great potential to achieve high system capacity in wireless systems. On the other hand, hybrid ARQ (HARQ) is essential to realize the reliable wireless packet transmission and therefore, the combination of THP with HARQ can be seen as a promising approach to achieve reliable high-capacity wireless transmissions. However, this combination has a problem of severe performance degradation due to the accumulation of modulo loss which is the inherent drawback of THP. In this paper, we propose the HARQ scheme to cancel the accumulation of modulo loss in Multi-user MIMO THP systems. The main feature of the proposed approach is to add the same perturbation vector as the initial transmit packet to the retransmit packets in order to prevent the accumulation of modulo loss. The effectiveness of the proposed scheme is demonstrated in comparison with the conventional packet combining scheme by means of computer simulations.

Recent years, the increase in complicated wire harness increases the weight and consequently harms the environment friendliness especially in a luxury car. Therefore, it is expected that in-car network is effectively built by using wireless connection as well as wired connection. In this paper, we investigate the coverage area performance of MB-OFDM UWB in-car communication in the presence of plural mobile terminals. In our approach, simultaneous multi-user wireless transmission is realized by multi-user MIMO (MU-MIMO) employing linear precoding with restriction of the total transmit power which satisfies the UWB power density limitation. Numerical results show the influence of the number of terminals on the coverage area under in-car propagation and verify the applicability of MU-MIMO to MB-OFDM UWB in-car wireless communication. © 2012 IEEE.

This paper proposes a novel clipping and filtering method employing transmit power control to improve the bit error rate (BER) performance in MIMO-OFDM transmission. The feature of the proposed approach is to perform the transmit power control before clipping and filtering to alleviate the BER degradation caused by clipping and filtering. Since the transmit power control to satisfy maximum signal-to-noise-plusdistortion ratio (SNDR) tends to lower the transmit power level especially in a relatively large CNR, the combination of its transmit power control with clipping and filtering surely improves the BER performance while suppressing the out-of-band radiation. Considering that MIMO systems suffer from the nonlinear distortion much more than SISO systems, the performance evaluation is to be conducted on MIMO systems, and the effectiveness of the proposed approach is demonstrated in comparison with the traditional clipping and filtering method with the constant transmit power level.

Multi-user MIMO (MU-MIMO) employing Tomlinson Harashima Precoding (THP) is a powerful technique to enhance the system capacity in multi-user mobile communications systems. However, MU-MIMO has an inherent drawback in the degradation of the transmission performance due to the time variant fading channels which are caused by the mobility of the mobile station (MS). Considering that MU-MIMO is mainly applied to mobile communications systems, the performance degradation due to this terminal mobility has to be solved. This paper proposes a novel MU-MIMO THP transmission scheme for the sake of alleviating the performance degradation due to the terminal mobility. The feature of the proposed approach is to incorporate the concept of time division multiplexing (TDM) into MU-MIMO THP system, which enables the transmission of high mobility user to take precedence over that of the lower mobility user. Thanks to the use of TDM, the proposed scheme exploits the space diversity benefit because TDM decreases the number of MSs within a time slot. The effectiveness of the proposed scheme is demonstrated in comparison with the traditional MU-MIMO THP scheme by means of computer simulations. © 2011 IEEE.

MU-MIMO-OFDM is an effective technique to achieve high capacity and reliable wireless communications systems because both the robustness against multi path fading and the high spectrum efficiency can be effectively achieved by the combination of OFDM with MU-MIMO. However, MU-MIMO-OFDM significantly suffers from the nonlinear distortion which is a main drawback of OFDM. In this paper, we propose nonlinear distortion suppression scheme employing transmit power control for MU-MIMO-OFDM. The main feature of the proposed scheme is to apply the deterministic transmit power control to MU-MIMO-OFDM without any destruction of the space orthogonality. The effectiveness of the proposed scheme is demonstrated in comparison with the traditional MU-MIMO-OFDM over the nonlinear fading channels.

This paper proposes the theoretical derivation method of the bit error rate (BER) for M-ary QAM/OFDM signals using maximal ratio combining (MRC) reception under nonlinear fading channels. Since the BER performance especially in the presence of nonlinear fading channels is mostly obtained by time-consuming computer simulations, we provide the simple analytical tool in order to prevent its computational cost. The feature of the proposed formula is to be represented by the simple series form formula without any integral calculation and to be applicable to arbitrary modulation schemes. The validity of the proposed approach is confirmed by the agreement with the computer simulation results under various kinds of system parameters such as the input back-off (IBO) and the number of branches. © 2011 IEEE.

Multi-user MIMO (MU-MIMO) employing Tomlinson Harashima Precoding (THP) is a powerful technique to enhance the system capacity in multi-user mobile communications systems. However, MU-MIMO has an inherent drawback in the degradation of the transmission performance due to the time variant fading channels which are caused by the mobility of the mobile station (MS). Considering that MU-MIMO is mainly applied to mobile communications systems, the performance degradation due to this terminal mobility has to be solved. This paper proposes a novel MU-MIMO THP transmission scheme for the sake of alleviating the performance degradation due to the terminal mobility. The feature of the proposed approach is to incorporate the concept of time division multiplexing (TDM) into MU-MIMO THP system, which enables the transmission of high mobility user to take precedence over that of the lower mobility user. Thanks to the use of TDM, the proposed scheme exploits the space diversity benefit because TDM decreases the number of MSs within a time slot. The effectiveness of the proposed scheme is demonstrated in comparison with the traditional MU-MIMO THP scheme by means of computer simulations.

Permutation network plays an important role in the reconfigurable QC LDPC decoder for most modem wireless communication systems with multiple code rates and various code lengths This paper presents the generic permutation network (GPN) for the reconfigurable QC LDPC decoder Compared with conventional permutation networks this proposal could break through the input number restriction such as power of 2 and other limited number and optimize the network for any application in demand Moreover the proposed scheme could greatly reduce the latency because of less stages and efficient control signal generating algorithm In addition the proposed network processes the nature of high parallelism which could enable several groups of data to be cyclically shifted simultaneously The synthesis results using the 90 nm technology demonstrate that this architecture can be implemented with the gate count of 18 3k for WiMAX standard at the frequency of 600 MHz and 10 9k for WiFi standard at the frequency of 800 MHz

Recent years, increasing weight and deployment cost of complicated wire harness are revealing especially in a luxury car. Therefore, it is expected that in-car network is built by wireless communication having an excellent environment-compatibility, as well as wired network. In this paper, we investigate the applicability of multiband orthogonal frequency division multiplexing ultra-wideband (MB-OFDM UWB) to in-car wireless communication from the viewpoint of the coverage area and then describe the practical problems to be solved for in-car UWB wireless communication. © 2010 IEEE.

This paper proposes a coverage area prediction method for extremely reliable in-car MB-OFDM UWB communication. Since sensor and control networks inside a car require extremely high quality of service (QoS), it is almost impossible for computer simulations to provide very low bit error rate (BER). In the proposed approach, very low BER performance is obtained by approximating the simulated BER with simple exponential functions thanks to the fact that coded MB-OFDM UWB system does not theoretically introduce any error floor. In other words, once the simulated BER can be transformed into a simple mathematical formula such as exponential function, the coverage area can be predicted irrespective of the target QoS. By using this approach, we demonstrate the coverage area to satisfy the BER of 1 × 10-12.

Since the structured quasi-cyclic low-density parity-check (QC-LDPC) codes tor most modern wireless communication systems include multiple code rates, various block lengths. and the corresponding different sizes of submatrices in parity check matrix (PCM), the reconfigurable LDPC decoder is desirable and the permutation network is needed to accommodate any Input number (IN) and shift number (SN) for cyclic shift In this paper we propose a novel permutation network architecture for the reconfigurable QC-LDPC decoders based on Banyan network We prove that Banyan network ha, the nonblocking property or cyclic shift when the IN is power of 2, and give the control signal generating algorithm Through introducing the bypass network, we put forward the nonblocking scheme for any IN and SN In addition, we present the hardware design of the control signal generator. which can greatly reduce the hardware complexity and latency The synthesis results using the TSMC 0 mu m pin library demonstrate that the proposed permutation network can be implemented with the area of 0 546 mm(2) and the frequency of 292 MHz

Permutation network plays an important role in the reconfigurable QC-LDPC decoder for most modern wireless communication systems with multiple code rates and various code lengths. In this paper, we propose the variation Banyan network (VBN) based permutation network architecture for the reconfigurable QC-LDPC decoders and give the control signal generating algorithm for cyclic shift. Through introducing the bypass network, we put forward the nonblocking scheme for any input number and shift number. In addition, the optimized VBN is proposed for WiMAX and WiFi standard, which can shift at most 4 groups of input data, and greatly reduce the hardware complexity. The synthesis results using the 90nm technology demonstrate that the proposed permutation network can be implemented with the gate count of 18.3k and the frequency of 600 MHz. © 2010 IEEE.

This paper proposes an adaptive single-carrier modulation with frequency domain equalization (SC-FDE) scheme further enhancing frequency diversity benefit. We have so far proposed the periodic spectrum transmission realized by the even-numbered time domain samples, which further improves the bit error rate (BER) of SC-FDE in severe frequency selective fading channels. However, in frequency non-selective fading channels, the periodic spectrum transmission provides worse BER than the traditional non-periodic transmission. Therefore, the proposed adaptive transmission chooses the appropriate transmission mode by comparing the error vector magnitudes (EVM) of the periodic and non-periodic transmissions. Moreover, since the EVM is measured by the CAZAC-based pilot signals used for channel estimation, additional redundancy is not needed any more. Numerical results show that the proposed adaptive scheme always provides the best performance irrespective of the frequency selectivity in channels while its frequency selectivity gives some BER degradation in the periodic and non-periodic transmissions. © 2010 IEEE.

This paper derives the theoretical bit error rate (BER) of the multiband orthogonal frequency division multiplexing (MB-OFDM) system for ultra wideband (UWB) communication. The proposed derivation takes into account the fading correlation caused by time and frequency spreading, which is one of the most significant factors in MB-OFDM UWB system. Moreover, the proposed formula gives consideration to the frequency characteristics of propagation loss that are not negligible in UWB systems. The theoretical results by the proposed approach are compared with the computer simulation results in order to confirm the validity of the proposed derivation.

This paper proposes a simple series form formula of the bit error rate (BER) for M-ary QAM/OFDM signals under nonlinear fading channels. In the proposed derivation, the effect of the nonlinear distortion due to the soft envelope limiter is approximated by the additive Gaussian noise with the variance equal to the power of the clipped portion of the composite waveform. Since the BER is represented by the simple series form formula, the proposed approach frees us from the time-consuming computer simulations. Moreover, the proposed formula is applicable to arbitrary M-ary quadrature amplitude modulations (QAM). The validity of the proposed approach is confirmed by the agreement with the computer simulation results with parameters of the input back-off (IBO) and average CNR. © 2009 Springer-Verlag.

This paper proposes a simple series form formula of the bit error rate (BER) for DBPSK/OFDM signals under time selective nonlinear fading channels. The proposed approach derives the theoretical BER in the presence of nonlinear distortion and time selective fading effects. Since the BER is represented by the simple series form formula, the proposed approach frees us from the time-consuming computer simulations. The validity of the proposed approach is confirmed by the agreement with the computer simulation results under various kinds of system parameters such as the input back-off (IBO) and the normalized maximum Doppler frequency.

This paper proposes a bit metric derivation method for OFDM adaptive array in combination with channel coding. In the proposed approach, post-FFT processing is adopted as an OFDM adaptive array architechture and the bit metric required for channel coding is derived considering the effect of the residual CCI signals in the array output of each sub-channel. The transmission performance is evaluated in the presence of severe CCI channels by means of the computer simulations and the effectiveness of the proposed approach is demonstrated compared with the traditional bit metric method. ©2009 IEEE.

This paper investigates the coverage area of MB-OFDM UWB system under in-car propagation environments. The coverage area of each transmission speed to satisfy a certain quality of service (QoS) is predicted by combining the transmission performance of MB-OFDM UWB system with actual in-car propagation characteristics. Since the transmission performance of MB-OFDM strongly depends on the delay spread, the coverage area prediction takes into account the delay spread based on the distance between the transmitter and the receiver, as well as the propagation loss. The coverage area in the occupied scenario with four passengers is compared with that in the empty scenario. Computer simulation results show that the coverage area in the occupied scenario turns out to be much smaller than that in the empty scenario due to the large propagation loss and the decrease in the delay spread.

This paper proposes a simple series form formula of the bit error rate (BER) for DQPSK/OFDM with differential detection under comprehensive nonlinear fading channels. The proposed approach aims to derive the theoretical BER in the presence of both the nonlinear distortion and the inter-symbol interference (ISI) which are the essential factors in evaluating the comprehensive performance of OFDM signals. Since the proposed formula is composed of the simple series form expression, the time-consuming simulations could be avoided. The validity of the proposed approach is confirmed by the agreement with the computer simulation results under various kinds of system parameters such as the input-backoff (IBO) and the multipath delay.

This paper proposes a frequency diversity scheme using only even-numbered samples for single-carrier transmission with frequency-domain equalization (SC-FDE). In the proposed scheme, a periodical frequency spectrum generated by using only even-numbered samples in the time domain provides the frequency redundancy, which is utilized for frequency diversity. Moreover, in order to avoid the data rate reduction due to the decrease in the samples within one block, the high-level modulation is applied to each sample and the transmitting power of each sample can be doubled for the equivalent power transmission instead. Computer simulation results show that the proposed scheme provides a steeper BER curve than the typical SC-FDE over frequency selective fading channels, while the typical SC-FDE is more favorable than the proposed scheme over flat fading channels. Moreover, the proposed scheme still retains its characteristic even when channel estimation and channel coding are additionally taken into account.

This paper proposes a new channel estimation method and a new interference cancellation scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems in the presence of intersymbol interference (ISI). The proposed channel estimation method uses special training sequences (TSs) to have a desirable crest-factor of the transmitted training signal, and to prevent the influence of ISI on the channel estimation performance. By using the recommended training sequences, the ill-conditioned problem of the least square (LS) filter integrated in the proposed channel estimator can be avoided. The proposed interference cancellation scheme uses the estimated channel coefficients and the channel state information (CSI) to reproduce the interference components, which are then iteratively cancelled from the received signals. To reduced the error-floor of the demodulated symbols using for the calculations of the interference components, the so-called remodulation technique is also included in the proposed interference cancellation scheme. Simulation results show that the proposed channel estimation method outperforms conventional channel estimation methods, especially in the presence of ISI and if the signal-to-noise ratio (SNR) is larger than 15 dB. The combination of the proposed method with a space-time block code (STBC) to combat the interference influences results in an excellent system performance in terms of symbol error ratio (SER), In comparison with a STBC MIMO-OFDM system with sufficient guard interval (GI), this combination gains 1.52 dB of SNR at the same SER of 1.1 center dot 10(-6) even after performing only one iteration of interference cancellation.

This letter presents an iterative decision feedback channel estimation scheme for burst mode COFDM transmission. The feature of the proposed scheme is that the channel estimation using metrics comparison is applied to the initial stage of the iterative mechanism, which makes it possible to provide a reliable data stream at the initial stage. Computer simulation results show that the proposed approach provides better BER than the traditional iterative decision feedback channel estimation scheme irrespective of the number of iterations.

This paper presents the periodic spectrum transmission for single-carrier frequency-domain equalization (SC-FDE) employing space-time transmit diversity (STTD). The proposed scheme utilizes only even-numbered samples within a time-domain data block to create a periodical spectrum which provides frequency redundancy. Frequency redundancy is used for subchannel combining before FDE, which alleviates the loss of orthogonality caused by FDE under frequency selective fading channels. Therefore, the proposed scheme has an advantage of enhancing the overall frequency diversity benefit while keeping the space diversity benefit achieved by STTD. Computer simulation results show that the proposed scheme outperforms the typical SC-FDE using full samples over relatively severe frequency selective fading channels when applying 1-bit/sample transmission.

This paper proposes an inter-symbol interference (ISI) suppression scheme using the periodic signal waveform for fixed-rate coded OFDM systems. In this scheme, the periodic signal waveform composed of the even-numbered sub-carrier group is used for extending the guard interval, and the odd-numbered sub-carrier group is also enable to be available by means of frequency conversion in order to realize the sub-carrier group selection. This paper mainly focuses on the combining effect of the sub-carrier group selection and the channel coding scheme in the inter-symbol interference (ISI) condition. Computer simulation result show that the proposed scheme has the great potential to create the combined frequency diversity effect without the error floor even in the severe ISI condition.

This paper presents the performance of periodic spectrum transmission for single-carrier with frequency-domain equalization (SC-FDE) using channel coding. So far, we have proposed periodic spectrum transmission and showed the effectiveness of enhancing a frequency diversity benefit compared with the typical case when adopting a low rate transmission such as 1-bit/sample. In this paper, the impact of channel coding on the proposed scheme is intensively discussed compared with coded OFDM (COFDM) as well as the typical SC-FDE. This is because OFDM is sure to be the promising approach when taking channel coding into account. Computer simulation results show that the proposed scheme 1-bit/sample transmission outperforms COFDM irrespective of coding rate.

This paper proposes a fading compensation scheme for burst mode COFDM transmission based on the pilot symbols inserted at both ends. The feature of the proposed scheme is that the decision variables are determined by the metrics based on the two-way decision feedback channel estimation and compensation. Numerical results given by computer simulation showed the effectiveness of the proposed scheme.

This paper presents a closed form expression of an exact average bit error rate (BER) for a time-division duplex (TDD) transmit diversity scheme employing maximal ratio combining (MRC) over time selective flat Rayleigh fading channels. In the proposed analysis, the feed back delay which degrades the BER performance is taken into account. The results are generally applicable to an arbitrary modulation scheme, as well as an arbitrary number of transmitting branches. To confirm the validity of the proposed analysis, the theoretical results are compared with the simulated ones.

This paper presents an analytical expression of the bit error rate (BER) performance for DQPSK/OFDM signals using selection combining diversity reception under nonlinear fading channels. In the proposed approach, the soft envelope limiter is assumed to be a peak limited devices and the effects of nonlinear distortion are treated as the additive Gaussian noise with zero mean and suitable variance. This approach has an advantage of reducing the computational cost dramatically compared with the computer simulation because the proposed formula for predicting the BER performance consists of a simple series form. The theoretical results show perfect agreement with those obtained by the computer simulation. © 2005 IEEE.

This paper proposes an inter-symbol interference (ISI) suppression scheme using the sub-carrier group selection for orthogonal frequency division multiplexing (ORM) time division duplex (TDD) transmit diversity. Although transmit diversity is quite effective in improving the transmission performance, the ISI due to the long multipath delays causes the error floor and leads to a loss of the space diversity benefit. To cope with this degradation, the periodic time domain waveform obtained by the even-numbered sub-carrier group is used for the extension of the guard interval. Moreover, the odd-numbered sub-carrier group is also applied to exploit the frequency diversity benefit, which is realized by carrying out the simple frequency conversion at the receiver. To keep the constant transmission rate even in the sub-carrier group transmission, the high-level modulation scheme with power enhancement is applied. The computer simulation results show that the proposed scheme not only prevents the error floor due to the ISI but also achieves the space and frequency diversity benefit without the extension of the guard interval.

This paper presents a performance comparison between carrier interferometry coded OFDM (CI/COFDM) and typical coded OFDM (COMM) using multiple receive antennas. The feature of CI/COFDM with diversity reception is to combine the time diversity benefit introduced by channel coding with the frequency and space diversity benefits created by frequency-domain equalization (FDE). Simulation results for QPSK showed that, at the perfect channel estimation, CI/COFDM outperforms COMM under frequency and time selective fading channels because of the powerful time diversity benefit introduced by channel coding with time interleaving. However, at the imperfect channel estimation, this advantage of CI/COFDM over COFDM becomes very limited.

This paper proposes an inter-symbol interference (ISI) suppression scheme using only the even-numbered sub-carriers for the fixed-rate OFDM systems with the 2-dimensional modulation. The proposed scheme is based on the principle that the first half of the waveform in the time domain is the same as the second half when an OFDM symbol is composed of only the even-numbered sub-carriers. The feature of the proposed scheme is that, in the case of the maximum multipath delay beyond the duration of the guard interval, the OFDM symbol with only the even-numbered sub-carriers is transmitted in order to generate the extended virtual guard interval and that the high-level modulation with the sub-carrier power enhancement is applied to achieve the constant data rate. In addition, at the receiver, only the second half of the OFDM symbol is used for the FFT processing to avoid the ISI. Moreover, the condition of the maximum multipath delay is notified to the transmitter by using the feedback channel. Numerical results given by computer simulation showed that the proposed scheme provides far better bit error rate (BER) performance than the traditional OFDM transmission using all sub-carriers under the multipath delay beyond the duration of the guard interval.

Carrier interferometry orthogonal frequency division multiplexing (CI/OFDM) creates a full frequency diversity benefit without bandwidth expansion. In this paper, we present the bit error rate (BER) performance of carrier interferometry coded OFDM (CI/COFDM) employing multiple receive antennas over the frequency and time selective fading channels. The main feature of this architecture is to combine the time diversity benefit achieved by channel coding with the space and frequency diversity benefits exploited by CI/OFDM using space diversity reception. We also compare the BER performance of this architecture with that of typical coded OFDM (COFDM) employing subchannel space combining diversity reception. Numerical results obtained by the computer simulation showed that CI/OFDM outperforms OFDM when applying both space diversity reception and channel coding.

The transmission quality in mobile wireless communication systems is affected by not only AWGN but also multi-path fading. Particularly, the Doppler frequency, the delay spread and the Rician factor have a great influence upon the quality over fading channels. This letter proposes the approximate equation for easily calculating the BER in DS-CDMA systems over multi-path fading channels. The validity of the approximate equation is confirmed from the fact that the BER calculated by the equation coincides with that by the computer simulation.

The transmission quality in mobile wireless communications is affected not only by additive white Gaussian noise, but also by multipath fading, which drastically changes the amplitudes and phases of wireless signals. It is one of the key themes in direct-sequence code-division multiple-access systems to evaluate average bit error rate (BER) performances as parameters of the Doppler frequency, the delay profile, the number of simultaneous access users, and so on. This paper proposes the approximate equation for easily calculating the average BER over fading channels (multiray fading models). The performance evaluations are carried out in the two-ray model, the IMT2000 model, and the COST207 model. It is confirmed from the coincidence of approximate results with computer simulation results that the proposed approach is applicable to a variety of parameters.

This paper proposes a 2-dimensional linear propagation prediction (LPP) in maximal ratio combining (MRC) transmitter diversity for orthogonal frequency division multiplexing (OFDM) time division multiple access-time division duplex (TDMA/TDD) systems in order to overcome the degradation of the transmission performance due to the fast fading or the TDD duration. In the proposed scheme, the downlink channel condition of each sub-channel is predicted by interpolating the uplink fading fluctuation with both the amplitude and phase, and the predicted downlink channel condition is used for the weighting factor to employ MRC transmitter diversity. Numerical results obtained by the computer simulation show that the proposed 2-dimensional LPP with the second-order Lagrangeis interpolation predicts the downlink channel condition accurately under the fast fading or the long TDD duration. Moreover, in such a condition, the proposed LPP provides far better performance than the conventional I-dimensional LPP.

Orthogonal frequency division multiplexing (OFDM) systems can avoid the effect of multipath fading by absorbing delayed waves into guard interval. But as the effect of delayed waves over guard interval becomes stronger, it causes the bit error rate (BER) performance in OFDM systems to be more degraded. This paper proposes the approximate derivation method of the BER in DQPSK/OFDM systems over frequency selective fading channels. In this study, we will derive the approximate equation of the BER with differential coding/differential detection in the time domain. We confirm that the BER derived from the proposed approximate equation coincides with that by computer simulation even when such parameters as the Doppler frequency shift, the delay spread, the Rician factor and so on are varied.

Transmitter diversity is a powerful technique to improve the transmission quality of downlink in microcellular mobile communications systems. Under cochannel interference (CCI) at the base station (BS), the transmitter diversity is not necessarily effective, because the desired-plus-interference signal power used as a criterion of downlink branch selection is not always relative to the downlink propagation condition. This paper proposes the theoretical derivation of bit error rate (BER) performance in the transmitter diversity under CCI occurring at BS, as parameters of average SIR at BS, normalized Doppler frequency, and so on. It is confirmed from the correspondence of theoretical results with simulation results that the proposed theoretical approach is applicable to the CCI environments at BS.

This paper proposes a theoretical derivation method of the BER performance in maximal ratio combining (MRC) time division duplex (TDD) transmitter diversity, as parameters of Doppler frequency and so on. The proposed theoretical approach can be applied not only to single-carrier transmission under flat fading but also to orthogonal frequency division multiplexing (OFDM) under frequency selective fading. In this paper, we choose OFDM transmission, in addition to single-carrier transmission, as an example of the system model to obtain the numerical results, and the comparison between the theoretical results and the simulated ones is demonstrated to confirm the validity of the proposed theoretical approach.

New method for deriving the bit error rate (BER) of the BPSK signal in the cochannel interference is proposed, which utilizes the eye pattern of the interference signal, and is different from the conventional method based on the conversion of the interference components to thermal noise. The validity of the proposed derivation method is quantitatively evaluated in terms of the BER performance and is confirmed by comparing with the results obtained by the computer simulation.

The paper proposes a transmitter diversity scheme with a desired signal selection for the mobile communication systems in which the severe cochannel interference (CCI) is assumed to occur at the base station. The feature of the proposed scheme is that the criterion of the downlink branch selection is based on the desired signal power estimated by the correlation between the received signal and the unique word at the matched filter. Moreover, the unique word length control method according to the instantaneous SIR is applied to the proposed scheme, taking account of the uplink transmission efficiency. Computer simulation results show that the proposed scheme provides the better performance than the conventional transmitter diversity in the seven CCI environments, and that the unique word length control method applied to the proposed scheme decreases the unique word length without the degradation of the transmission quality, comparing with the fixed unique word length method.

We propose a novel SLR weighting postdetection combining diversity scheme with a new accurate SIR estimation method. The SIR is estimated and used as the weighting factor to compensate severe cochannel interference, one of the most important issues for PCS in terms of frequency utilization. The improvement offered by the proposal depends on SIR estimation accuracy. The SIR is, in this paper, estimated by a matched filter where the auto-correlation between received signal and unique word is calculated. Computer simulations confirm that the SIR of each diversity branch can be estimated easily and accurately by the proposed SIR estimation method. The proposed diversity scheme achieves a performance very close to that of ideal SINR weighting diversity under Rayleigh fading with severe cochannel interference. When average SIR = 10 dB and the number of branches (L) = 4, the proposed diversity scheme lowers the required E-b/N-0 by 5 dB at BER = 1 x 10(-3) compared to conventional maximal ratio combining diversity. This paper also presents the unique word length required to realize adequate performance, i.e., robustness against high-pitch Rayleigh fading.

New methods for deriving the bit error rate (BER) in the cochannel interference and the adjacent channel interference am proposed, which utilize the eye pattern of the interference signal and the probability density function of components in interference signals, respectively, and are different from the conventional method based on the conversion of the interference components to thermal noise. These methods are quantitatively evaluated and confirmed as the effective approach in terms of BER performance by the comparison between the results calculated by these methods and the results obtained by the experiment and computer simulation.