This paper proposes an energy consumption oriented grouping scheme in terminal collaborative multi-input multi-output reception. In previous studies, user equipment (UE) grouping criteria based on system throughput and user fairness have been proposed. However, these approaches do not take terminal energy consumption into account. This paper provides a novel UE grouping scheme that incorporates cumulative energy consumption. Numerical results obtained through computer simulation demonstrates that the proposed scheme reduces cumulative energy consumption while maintaining system throughput and user fairness.

In this paper, the appropriate strength of augmented responses is investigated in linear multi-input multioutput (MIMO) detection. The augmented responses is used to generate a square channel matrix for applying linear MIMO detection schemes in overloaded MIMO systems. It is originally proven that the strength of augmented responses should be as small as possible. If gradient descent direction random walk MIMO detection is applied, the augmented responses limits the step size of the gradient descent algorithm. Thus, the strength of the augmented responses effect on the convergence rate of the gradient descent algorithm and bit error performance. Numerical results obtained through computer simulation show that the optimum values of the augmented responses exists.

This paper proposes energy consumption oriented grouping in terminal collaborative beamforming (BF) transmission for the uplink of the fifth generation mobile communication. By utilizing both millimeter-wave and Sub-6 bands, BF transmission through terminal collaboration enhances received signal power. However, previous studies have not taken energy consumption of each user equipment (UE). Therefore, this paper introduces a grouping scheme with cumulative energy consumption. Each UE leaves and rejoins a group for terminal collaborative BF transmission according to its cumulative energy consumption. Numerical results obtained through computer simulation show that the proposed scheme minimizes the amount of reduction in system throughput while maintaining fairness among UEs. The proposed scheme reduces the cumulative energy consumption by a factor of about 0.6 with little costs of a user throughput and a fairness index.

This paper proposes a minimum mean square error (MMSE) equalization-based multi-input multi-output (MIMO) demodulation scheme with Metropolis-Hastings random walk along a gradient descent direction. The conventional scheme using intermediate search points works effectively for higherorder modulation symbols. However, its performance is deteriorated for lower-order modulation symbols. The proposed scheme suppresses the coefficients in the offdiagonal components of a channel matrix by applying MMSE equalization and improves performance in low signal-to-noise power ratio conditions even for lowerorder modulation symbols. Numerical results obtained thorough computer simulation show that the proposed scheme improves by about 2.0 dB at a bit error rate of $10^{-2}$ when the number of iteraitons is 10 for QPSK symbols with 64 transmit and receive antennas.

The erasure demodulation is possible to reduce a block error rate (BLER) and extend the communication range of a wireless system. In the erasure demodulation a demodulator regards a decoded bit as an erasure bit if its corresponding likelihood at the output of the decoder is smaller than a threshold. The demodulator outputs both “0“ and “1“ for the erasure bit and CRC error detection following the erasure demodulation checks which output is correct. As one of those outputs must be a correct bit, BLER performance improves. On the other hand, as the number of erasure bits increases, the number of CRC decoding operations grows exponentially. Thus, in this paper, a trade-off between computational complexity and performance improvement is investigated Packets are treated as an erasure overflow state if the number of erasure bits in each packet exceeds a threshold. They are counted as the packets with date recovery errors (DREs). Numerical results obtained through computer simulation show that the proposed scheme improves the DRE probability.

This paper proposes the affiliation macrocell switching of picocells in a distributed antenna system (DAS). In a conventional DAS, a macrocell is composed of a fixed number of picocells. On the other hand, in the proposed scheme, picocells can switch their affiliation to adjacent macrocells so that the throughputs of cell-edge users improve. The proposed scheme aims to improve a system throughput with the moderate increase of backhaul complexity. Numerical results obtained through computer simulation show that the proposed scheme improves the system throughput by approximately 0.3 bps/Hz/cell and maintains user fairness. Moreover, the system throughput improves regardless of the distance between distributed antennas.

This paper proposes user equipment (UE) grouping schemes that take user fairness into account in collaborative multiple-input multiple-output reception. In previous research, the criterion for UE grouping to improve system throughput is proposed. However, the user fairness is deteriorated if these schemes focus on the system throughput. In this paper, the range of the received signal power is restricted to reduce the difference among user throughput. Numerical results obtained through computer simulation show that there is a trade-off relationship between the user throughput and the user fairness via the range of the received signal power.

This paper evaluates the throughput performance of uplink beamforming transmission through terminal collaboration. In the proposed scheme multiple user equipments (UEs) form a group and transmit signals synchronously to enhance the power of a received signal at a base station. The signal transmitted through terminal collaboration is selected according to proportional fairness scheduling. Thus, fairness among UEs is maintained simultaneously with system throughput improvement. Numerical results obtained through computer simulation show that the system throughput grows as the number of UEs in the group increases. It is also shown that the proposed scheme increases the system throughput by about 0.5-1.5 bps/Hz.

In this paper, belief propagation (BP) multi-input multi-output (MIMO) detection with maximum ratio combining (MRC) and minimum mean square error (MMSE) pre-cancellation is proposed for overload MIMO. In the proposed scheme, MRC is applied before MMSE pre-cancellation. The BP MIMO detection with MMSE pre-cancellation leads to a reduction in diversity gain due to the decreased number of connections between variable nodes and observation nodes in a factor graph. MRC increases the diversity gain and contributes to the improvement of bit error rate (BER) performance. Numerical results obtained through computer simulation show that the BERs of the proposed BP MIMO detection with MRC and MMSE pre-cancellation are about 0.5 dB better than those of the conventional BP with MMSE pre-cancellation at a BER of 10−3.

In this paper, coordinated multi-point transmission (CoMP) with attribution macro cell switching in distributed antenna transmission is proposed. The proposed scheme is applied to a macro cell system in which each macro cell consists of multiple micro cells. In the proposed scheme, macro cells can be expanded and overlapped with each other, and each micro cell may switch its attribution macro cell. Inter-cell interference is mitigated in conjunction with CoMP transmission. Numerical results obtained through computer simulation show that the largest improvement in a system throughput is realized with three user equipments per macro cell when an inter-antenna distance is 50 meters. The switching frequency corresponds to the amount of the improvement in the throughput. The switching happens less frequently with a longer inter-antenna distance. Thus, the proposed scheme improves the system throughput than the conventional scheme in a dense distributed antenna system.

In this paper, a nonlinear quantized precoding and gain adjustment scheme for low-resolution digital-analog converters (DACs) in a massive multiple-input multiple-output (MIMO) system is proposed. The nonlinear quantized precoding determines transmit antenna outputs with a transmit symbol and channel state information. In a full-digital massive MIMO system, low-resolution DACs are used to suppress power consumption. Conventional precoding algorithms for low-resolution DACs do not optimize transmit antenna gains individually. Thus, in this paper, a gain adjustment scheme that optimizes individual transmit antenna gains and the DAC outputs of a virtual block antenna formed by multiple antennas is proposed. Numerical results obtained through computer simulation show that the proposed precoding scheme achieves bit error rate performance close to that of the conventional Gibbs sampling based algorithm with much smaller antenna gains on a i.i.d Rayleigh fading channel.

This paper proposes a new index modulation (IM) scheme for a multi-carrier system, in which spreading codes are selected by index bits. In the proposed scheme OFDM transmission is carried out without spreading codes on some of the subcarriers. At the same time, IM with spreading codes is applied. One of the spreading codes corresponding to index bits is selected in each subcarrier, and the selected code is modulated with assigned coding bits. By increasing the number of coding bits transmitted at a time, the coding rate of the system is reduced, and the overall performance is improved. The OFDM signal and the spreading signal by IM co-exists and interfere with each other. At a receiver, soft replica symbols are generated from the soft output of a demodulator or a decoder and subtracted from the received signal to cancel the interference. Numerical results obtained through computer simulation results show the performance improvement of about 3.0 dB at a bit error rate of 10−4compared with the conventional multicarrier transmission and 6-path Rayleigh fading channels.

In this paper, belief propagation (BP) multiple-input multiple-output (MIMO) detection with minimum mean square error (MMSE) pre-cancellation matrix switching in overloaded MIMO systems is proposed. Loops in a factor graph degrades the demodulation performance of the BP scheme. Therefore, several schemes that suppresses the negative impact of loopy BP have been proposed. Among them, the application of MMSE pre-cancellation prior to BP has been proposed. In the scheme, the effects of loops in the factor graph is suppressed by MMSE pre-cancellation. However, many loops with large coefficients remain in the factor graph even after MMSE pre-cancellation. Therefore, the proposed scheme applies MMSE pre-cancellation matrix switching every after a preset number of BP iterations in order to suppress the occurrence of large beliefs in the loops. Numerical results obtained through computer simulation show that the proposed scheme achieves better BER performance than BP with MMSE pre-cancellation without matrix switching. The proposed scheme outperforms the conventional without switching by 0.7 dB at BER= 10-3.

In the fifth-generation mobile communication system (5G) mixed numerology is standardized to meet the requirements of various services. In this paper, mixed numerology signal transmission in which multiple signals with different numerologies is assumed. As a conventional pilot signal, demodulation reference signal (DMRS) in a forth generation mobile communication (4G) is applied. However, under mixed numerology non-orthogonal multiple access (NOMA), DMRS can not realize orthogonality between pilot signals. As a solution, new pilot signal design based on a Zadoff-Chu sequence is proposed. The proposed pilot signal assignment scheme improves orthogonality between signals with different numerologies and, as a result, reduces bit error rate (BER) performance. Numerical results obtained through computer simulation show that the proposed pilot signal assignment scheme improves the performance by about 2.0 dB at a BER of 1$0^{-2}$.

Channel estimation is required to demodulate the multiple-input multiple-output (MIMO) signals and there are blind and non-blind channel estimation schemes. One of non-blind channel estimation schemes is using Gibbs sampling. In channel estimation using Gibbs sampling, when the number of received data symbols is small, estimation accuracy may be deteriorated owing to the unreliability of weights. In order to solve this problem, this paper introduces a coefficient for adjusting the weights used in channel estimation with Gibbs sampling. When the number of data symbols is small, larger coefficient values are found to be more suitable because they suppress the fluctuation of channel estimation. Numerical results obtained though computer simulation show that the proposed scheme improves by about 5dB as compared with a decision directed scheme in a 4 × 4 MIMO system when the number of data symbols is one.

In this paper, user set elimination in the allocation sequences of round-robin (RR) scheduling for distributed antenna transmission (DAT) is proposed. In prior research, the initial phase selection of the user equipment (UE) allocation sequences in RR scheduling has been investigated. However, all of UE sets are included without taking their throughputs into account. Thus, a UE set is assigned even through its total throughput is the lowest. The proposed scheme eliminates the specific UE sets from the RR scheduling sequence according to the expected total throughput of those UE sets. In addition, the initial phase selection is applied to the remaining UE sets. Numerical results obtained through computer simulation show that fairness index as well as system throughput improve when the ratio of eliminated UE sets is small.

In this paper, two-stage BP detection is proposed for overloaded MIMO. In the two-stage BP detection scheme, the BP with MMSE pre-cancellation algorithm followed by the normal BP detection is applied. In overloaded MIMO systems, the loops in a factor graph degrade the demodulation performance of BP detection. MMSE pre-cancellation reduces the number of connections or coefficient values in the factor graph and improves the convergence characteristics of posteriori probabilities. Numerical results obtained through computer simulation have shown that the BERs of the proposed two-stage BP detection are about 1.0-1.5dB better than those of the conventional BP with MMSE pre-cancellation when the MMSE block size is four and the number of MMSE blocks is one.

In this paper, a macro cell switching scheme for distributed antennas is proposed. In conventional distributed antenna transmission (DAT), a macro cell to which each antenna belongs is fixed. Though a cell-free system has been investigated because of its higher system throughput, the complexity of front-haul can be inevitably large. To increase the flexibility of resource allocation in the DAT with moderate front-haul complexity, the macro cell switching of distributed antennas (DAs) is proposed. In the proposed scheme, DAs switch their attribution macro cells depending on the amount of pre-assigned resources. Numerical results obtained through computer simulation show that the proposed scheme realizes a better system throughput than the conventional system especially when the number of user equipments (UEs) is smaller.

In this paper, a non-linear precoding algorithm with low out-of-band (OOB) radiation in a massive multiple-input multiple-output (MIMO) system is proposed. Massive MIMO is a system with more than a few hundred antennas implemented in a base station and is characterized by better spectral efficiency and higher throughput. Full digital massive MIMO may require lower resolution in digital-to-analog converters (DACs) since each high-resolution DAC consumes a large amount of power. In massive MIMO systems with low resolution DACs, an output signal designed by nonlinear processing has been investigated. The conventional scheme focuses only on a sum rate or errors in received signals and causes large OOB radiation. This paper proposes an optimization criterion that takes OOB radiation power into account. Gibbs sampling is used as an algorithm to search for a sub-optimal solution for this criterion. Numerical results obtained through computer simulation show that the proposed criterion reduces the mean OOB radiation power by a factor of 10 as compared with the conventional criteria. Furthermore, the proposed criterion achieves larger sum rate over all subcarriers as compared with an error based criterion. On the other hand, as compared with a sum rate based criterion, the throughput on each subcarrier shows less variation and that demands a smaller number of link adaptation options although the average sum rate of the proposed criterion is smaller.

In this paper, block parallel Gibbs sampling (BPGS) multiple-input multiple-output (MIMO) detection is proposed. In a conventional Gibbs sampling scheme, MIMO detection is carried out sequentially symbol-by-symbol. The proposed scheme divides a symbol vector to blocks and updates candidate transmit symbols in parallel in a block so that the total number of iterations in a unit period increases. Furthermore, maximum ratio combining (MRC) is adopted to BPGS to improve accuracy in this paper. Numerical results obtained through computer simulations show that bit error rate performance under high bit-energy-to-noise-spectrum-density conditions improves with the proposed scheme. It is also shown that the block size of three achieves the best performance when number of antennas is 16×16 and the number of iterations is 50.

In this paper, mixed Gibbs sampling multiple- input multiple-output (MIMO) detection with forcible search is proposed. In conventional Gibbs sampling MIMO detection, the problem of stalling occurs under high signal-to-noise ratios (SNRs) which degrades the detection performance. Mixed GS (MGS) is one solution to this problem. In the MGS, random sampling is carried out with a constant probability without judging if a current search is at a local minimum. In the proposed scheme, combined with MGS, multiple candidate symbols are forcibly changed when the search is captured by a local minimum. The search restarts away from the local minimum which effectively enlarges the search area in a solution space. Numerical results obtained through computer simulation show that the proposed scheme achieves better performance in a large scale MIMO system with QPSK signals.

In this paper, the application of minimum mean square error (MMSE) pre-cancellation prior to belief propagation (BP) is proposed as a detection scheme for overloaded multiple-input multiple-output (MIMO). In overloaded MIMO, the loops in a factor graph deteriorate the demodulation performance with BP. Therefore, the proposed scheme applies the MMSE pre-cancellation prior to BP and reduces the number of the loops. Furthermore, it is applied to the selected transmit and receive nodes so that the condition number of an inverse matrix in the MMSE weight matrix is minimized in order to suppress the residual interference and the noise after the MMSE precancellation. It is shown by numerical results obtained through computer simulation that the proposed scheme achieves better bit error rate (BER) performance than BP without the MMSE pre-cancellation. The proposed scheme improves the BER performance by 4.6-5.0 dB at a BER of 5.0 × 10-3as compared with conventional BP. Numerical results also shows that the MMSE pre-cancellation reduces the complexity in BP by a factor of 1/896 in terms of the number of multiplication operations.

In this paper, a precoder design algorithm using spatial signal synthesis with selected multiple antenna subsets for hybrid multiple-input multiple-out-put (MIMO) systems is proposed. The hybrid MIMO systems are expected to be implemented in high frequency bands. To reduce power consumption and increase the bandwidth of a transmit signal, the resolution of phase shifters and the number of quantization levels in digital-analog (D/A) converters have to be limited. A baseband precoder needs to take the effect of quantization into account. On the other hand, when the number of transmit antennas is larger than that of receive antennas, spatial signal synthesis can be realized. The selection of antenna sets also needs to take the effect of quantization into account. In this paper, a transmit antenna selection scheme that improves the spectral efficiency by spatial signal synthesis with multiple antenna subsets is proposed. Numerical results obtained through computer simulation show that the proposed scheme with two antenna subsets of four transmit antennas and two receive antennas improves the spectrum efficiency by about 1.0 bit/s/Hz when the number of transmit and receive are 4.

One of key technologies in the fifth generation mobile communications is a distributed antenna system (DAS). In the DAS inter-user interference effects on its spectrum efficiency as distributed antennas are located closely. Round-robin (RR) scheduling is known as a scheme that achieves a trade-off between computational complexity and spectrum efficiency. In this research user equipment (UE) allocation scheme for RR scheduling are proposed. The proposed scheme determines the phases of UE allocation sequence that is pre-determined for low complexity. Numerical results obtained through computer simulation have shown that maximum selection that sequentially searches the phase with the maximum tentative throughput realizes the largest spectrum efficiency next to full search. The system throughput increases as the number of UEs increases in 2-user RR scheduling.

In this paper, the latency of non-orthogonal multiple access (NOMA) grant-free (GF) transmission in uplink is evaluated by link and system level simulations. In order to cancel interference among NOMA signals, multiuser detection (MUD) consisting of minimum mean square error demodulation followed by successive interference cancellation is applied. Retransmission happens only when a packet error occurs after serial interference cancellation (SIC). Therefore, the latency is reduced as compared to GF transmission without MUD. Numerical results obtained through system level simulation show that the application of MUD in uplink GF transmission improves the latency and increases the successful transmission rate without retransmission by a factor of about 3.0.

The fifth generation mobile communication systems are expected to support the Internet-of-Things (IoT) applications. One of the problems in massive connections is the assignment of pilot sequences. For supporting a large number of the IoT devices, non- orthogonal sequences have to be assigned and a receiver in a base station needs to estimate channel impulse responses (CIRs) by solving a joint optimization problem. In this paper, a new channel estimation scheme for the IoT devices is proposed. The proposed scheme is based on a repeating weighted boosting search algorithm and modifies the generation process of candidate CIR vectors for faster convergence in search iterations. Instead of calculating the sum of weighted candidate CIR vectors, the proposed scheme generates a candidate CIR vector that is assumed to be closer to the global optimum. In this case, the proposed scheme improves the convergence rate as compared to the conventional scheme though the mean square errors (MSE) of the solutions is worse. If the number of subcarriers is limited, the MSE of the estimated CIR vectors with the proposed algorithm is equivalent to that with the conventional scheme. Thus, the proposed scheme is less complex and suitable for massive connections. Numerical results obtained through computer simulation have shown that the proposed scheme achieves faster convergence by about 17-35% as compared to the conventional scheme.

Throughput analysis of the usage of joint maximum-likelihood detection (MLD) in a distributed antenna system (DAS) with multiple mobile terminal scheduling is evaluated in this paper. In DAS, mobile terminals are closer to desired antennas which improves system throughput and increases frequency utilization efficiency. However, since there are more antennas in a cell, interference can occur between transmitted signals from the antennas when multiple mobile terminals are scheduled. Therefore, we propose a novel mobile terminal scheduling method and the use of joint MLD to reduce the effects of interference. A system level simulation shows that the usage of joint MLD in densely packed DAS provides better system throughput.

In this paper, a metric function for Gibbs sampling multiple-input multiple-output (MIMO) detection is proposed. In conventional Gibbs sampling MIMO detection, an exponential function is used in the calculation of a metric for the selection of candidate symbols. However, the exponential function can be implemented by a look-up table and may require a large amount of memory. This paper proposes a metric function based on a simple fraction. The proposed metric substitutes the exponential function though it increases the number of multiplication operations. It is shown by numerical results obtained through computer simulation that the proposed metric function improves the performance under a high signal-to-noise ratio condition in a large scale MIMO system since its curve is close to that of the exponential function when an input metric distance approaches to zero.

In this paper, complexity reduction schemes for Gibbs sampling multi-input multi-output (MIMO) detection with maximum ratio combining are proposed. In a conventional Gibbs sampling MIMO detection algorithm, the Gibbs sampling is directly applied to a received signal. Thus, a squared Euclid distance between the received signal vector and a candidate symbol vector is calculated as a metric and it requires (2 × No. of received antennas) multiplication operations. On the other hand, in a proposed algorithm, each candidate symbol is updated with a metric calculated by two multiplication operations. However, after each iteration, another metric is also need to be calculated to select the best candidate symbol vector. To reduce the number of multiplication operations, a summation and subtraction metric (SSM) is applied. Furthermore, as an initial transmit symbol vector, a zero vector is applied in the conventional and proposed Gibbs sampling MIMO detection algorithms since the receiver can avoid to calculate the pseudo inverse of a channel matrix. The bit error rate performance and the complexities of these schemes are compared with that of QR decomposition with M-algorithm (QRM)-maximum likelihood detection (MLD). Numerical results obtained through computer simulation show that the proposed Gibbs sampling MIMO detection algorithm is less complex when the numbers of transmit signals and received antennas are more than 32 × 32.

This paper proposes an adaptive frequency offset scheme in an amplify-and-forward (AF) relay station (RS). When the RSs are at a cell edge, frequency resources are partially assigned between them and a base station (BS) to avoid inter-cell interference. On the other hand, the RS is expected to use a higher frequency band for transmission in the next generation mobile communication system. The whole frequency band can be used in the transmission between the RS and a mobile station (MS) while the RS uses only the restricted bandwidth in the low frequency band. In the proposed AF relay scheme, the frequency of the signal from the RS is shifted adaptively in accordance with the frequency response of the channel between the RS and the MS. In this scheme, the relay transmission is assumed to use a wider bandwidth between the RS and the MS than that between the BS and the RS. By making use of the better frequency response between the RS and the BS owing to the adaptive frequency offset scheme the total throughput of the cell improves. It is shown with numerical results obtained through computer simulation that for a two-user case the proposed scheme improves the total throughput by about 1.2 to 1.3 times with proportional fairness scheduling especially at a low Eb/N0condition.

In this paper, a precoder design algorithm using Gibbs sampling for hybrid multiple-input multiple-output (MIMO) systems is proposed. The hybrid MIMO systems are expected to be implemented in high frequency bands. To reduce power consumption and increase the bandwidth of a transmit signal, the number of phase shifters and the number of quatntization levels in digital-analog (D/A) converters have to be limited. A baseband precoder needs to take the effect of quantization into account. Owing to the quantization, it is difficult for a conventional precoder design algorithm with orthogonal matching pursuit (OMP) to reach the optimal solution. On the other hand, the proposed precoder design algorithm using Gibbs sampling is robust to quantization errors. Numerical results obtained through computer simulation show that the proposed algorithm achieves spectral efficiency of 50 bit/s/Hz at a signal-to-noise ration of 20 dB for a quantization level of 24 on an independent Rayleigh fading channel.

In this research minimum mean square error (MMSE) interference rejection is applied to a receiver in a distributed antenna networks (DAN). In the DAN, not only the desired signals, but also the interference signals may reach from nearby antennas. For the overloaded signal situation, non-linear detection schemes such as joint maximum likelihood detection can be applied to the received signals. However, the amount of metric calculations in the joint maximum likelihood detection (MLD) increases exponentially with the number of signal streams. Therefore, MMSE interference rejection followed by a MLD detection is proposed. The effect of residual interference after the MMSE interference rejection is evaluated. Numerical results obtained through computer simulation show that the performance of the proposed scheme is about 2.5 dB worse at a bit error rate (BER) of 10~3as compared with that of the joint MLD while the complexity reduces by a factor of 4 for QPSK signal streams. The BER performance degradation can be suppressed to about 1.0 dB by adjusting the value of the coefficient in the MMSE matrix.

This paper applies non-orthogonal multiple access (NOMA) with joint detection to a heterogeneous network (HetNet). The HetNet is a system in which pico base stations are placed in a macrocell and shares frequency resources with a macro base station. In the HetNet, the offloading of traffic to the pico base stations is limited since the transmission power of the macro base station is large. To achieve better spectrum efficiency with offloading, NOMA HetNet is proposed. In the proposed scheme, the desired signal is extracted with joint detection to realize offloading to a pico base station. Numerical results obtained through system level simulation show that NOMA doubles the total throughput by enhancing traffic offloading.

This paper proposes a channel estimation scheme with scattered pilots in generalized frequency division multiplexing (GFDM) with multiple subcarrier bandwidths. In the proposed scheme, the pilot symbols are assigned to narrow bandwidth subcarriers. Thus, it is possible to avoid inter-subsymbol interference that is inevitable in GFDM with unified bandwidth subcarriers. On a GSM-TU channel, the mean square error (MSE) of the proposed GFDM is equivalent to that of the OFDM and bit error rate (BER) performance for QPSK symbols is about 1dB better than that of the conventional GFDM at a BER of 10-3.

This paper presents a multiuser (MU)-multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) scheme that realizes signal separation among mobile terminals by spreading codes. In conventional multi-user MIMO systems, the most significant problem is co-channel interference among mobile terminals. The calculation of weight coefficients requires the channel state information (CSI) of all the terminals. Owing to a Doppler shift, the accurate CSI is difficult to obtain for mobile terminals. Signals to the other stationary terminals cause severe interference and beam nulling can not be realized appropriately. Thus, this paper proposes an MU-MIMO-OFDM scheme with orthogonal spreading codes that separates signals for mobile terminals. Numerical results obtained through computer simulation show that the proposed scheme improves bit error rate (BER) performance by 2.5dB at a BER of 10-3in comparison with the conventional MU-MIMO on a Rayleigh fading channel.

In this paper, Gibbs sampling multi-input multi-output (MIMO) detection with maximum ratio combining (MRC) is proposed and it is applied to the uplink with a massive MIMO receiver. In conventional Gibbs sampling schemes, the Gibbs sampling algorithm is directly applied to received signals. When it is applied to signals with higher order modulation symbols, a stalling problem occurs. Though a random restart scheme is proposed to solve the stalling problem, it needs adaptive restart criteria that makes hard to estimate the total amount of computational complexity. In the proposed scheme, each candidate symbol is updated with a metric calculated by multiplying corresponding MRC coefficients to the received signal vector. In this case, the convergence speed of the metric for the received signals even with higher order modulation symbols improves and the complexity of metric calculation for the update of a transmit candidate symbol vector can be reduced. Numerical results obtained through computer simulation have shown that the proposed scheme reduces the number of multiplication operations by a factor of 1/6.7 when it is applied to signals modulated with QPSK. It is also shown that the proposed scheme improves the convergence speed of the metric for Eb/N0≥ 10dB when 16QAM symbols are transmitted. Furthermore, the proposed mixed Gibbs sampling achieves the same BER performance with smaller complexity as compared to that of a QR decomposition with M-algorithm (QRM)-maximum likelihood detection (MLD) when the number of transmit signals is more than 32.

In this paper, the measurement results of field experiments for correlation reduction precoding with a repetition code in an overloaded MIMO systems are presented. Measurements are conducted in three propagation conditions that are line-of-sight (LOS), non-line-of- sight (NLOS), and a mobile environment. Numerical results obtained through the measurements show that amplitude shift and phase rotation precoding improves demodulation performance. It is also presented that the precoding scheme reduces bit error rate (BER) in the mobile environment.

In wireless communications for industrial automation, industrial equipments frequently communicate each other through the transmission of a small amount of data. For a random access system, a frame format that multiplexes a data signal to IEEE 802.11 short preamble sequence has been proposed to improve a spectrum efficiency. This frame format enables a receiver to realize timing synchronization, channel estimation, frequency offset compensation, and data transmission simultaneously within a preamble period. In receivers for wireless LAN communication systems, a direct conversion architecture has been widely employed because of its low production cost and low power consumption. However, this architecture suffers from a DC offset mainly caused by the leakage of a local oscillator (LO) signal. In order to eliminate the DC offset, a high-pass filter (HPF) is often inserted before an A/D convertor. When the gain of a low noise amplifier (LNA) changes in the presence of the DC offset, the level of the DC offset varies in the middle of the short preamble signal. With the time-varying DC offset, the received signal suffers from the long transient response of the HPF. In the use of the proposed frame format, the transient response causes severe deterioration on a bit error rate (BER) performance. In the previous research, a differential filter has been applied in a digital domain to eliminate the residual DC offset and estimate a frequency offset. In order to obtain better BER performance with time-varying DC offset elimination, the differential filter outputs with different delay values are summed together. It is shown that the set of differential filters with specific delay values can reduce the BER. The BER performance for two differential filter branches with the delays, dl= {4, 5}, improves the error floor by a half than that with the single branch only with the delay, dl= 4, when the number of the receive antennas is 3 on an Indoor Residential A channel model.

This paper proposes a generalized frequency division multiplexing (GFDM) modulation scheme that transmits a signal with different subcarrier bandwidths. In a receiver, the GFDM signal is demodulated by using a zero forcing (ZF) algorithm or a minimum mean square error (MMSE) algorithm and the BER performance of these algorithms is related to the condition number of a modulation matrix. This matrix can be optimized by adjusting the roll-off factor of subcarrier filters. It is shown that the performance of the proposed GFDM is about 0.02 dB better than that with a roll-off factor of 0 at a BER of 10-3on an AWGN channel. On the other hand, on the multipath fading channels, the BER performance improves as the subcarrier bandwidth increases because of frequency diversity.

Magnetic resonance wireless power transfer is expected to be one of the prevalent schemes because of its high efficiency and long transmission range. However, in this scheme, there is a problem that the characteristics of a transmission channel changes in according with the distance between coil antennas. This paper investigates the performance of data transmission with an array antenna in a wireless power transfer system. In the assumed system, the same antennas for wireless power transfer are used for data transmission. The frequency response of the channel for data transmission changes in according with the distance between the antennas. The assumed system uses multiple transmit antennas and beamforming is realized with ideal phase shifts to transmit symbols in the transmitter. Numerical results obtained through computer simulation show that the proposed scheme can mitigate the dependency of the bit error rate (BER) to the distance between the antennas. It is within 1-3 dB at the BER of 10-5 with two transmit antennas. In addition, the system with two transmit antennas achieves 2 dB or more improvement in term of the BER than with a single antenna for specific antenna distances.

This paper presents a low complexity metric for joint maximum-likelihood detection (MLD) in overloaded multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) systems. In overloaded MIMO systems, a nonlinear detection scheme such as MLD combined with error correction coding achieves superior performance than that of a single signal stream with higher order modulation. However, MLD demands large computational complexity because of multiplications in the selection of candidate signal points. Thus, a Manhattan metric has been used to reduce the complexity. Nevertheless, it is not accurate and causes performance degradation in overloaded MIMO systems. Thus, this paper proposes a new metric that is calculated with summations and bit shifts. New numerical results obtained through computer simulation show that the proposed metric improves bit error rate (BER) performance more than 0.2dB at the BER of 10^(-4) in comparison with a Manhattan metric.

This paper presents a codeword metric calculation scheme in two step joint decoding of block coded signals in an overloaded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system. In the previous literature, the two step joint decoding scheme has been proposed for the complexity reduction of joint maximum likelihood decoding in the overloaded MIMO system. However, the two step decoding scheme has not been combined with an outer code. The selection of the candidate codewords in the inner block code may not always be able to provide the metric of a binary coded symbol for the outer code. The metric of the binary coded symbol calculated in the first stage is used in the proposed scheme. It is shown that the proposed two step decoding scheme shows better performance than a bit flipping based scheme and reduces the complexity by about 0.013 for 4 signal streams with the cost of bit error rate degradation within 0.5dB.

This paper proposes an open-loop correlation reduction precoding scheme for overloaded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. In overloaded MIMO-OFDM systems, frequency diversity through joint maximum likelihood (ML) decoding suppresses performance degradation owing to spatial signal multiplexing. However, on a line-of-sight (LOS) channel, a channel matrix may have a large correlation between coded symbols transmitted on separate subcarriers. The correlation reduces the frequency diversity gain and deteriorates the signal separation capability. Thus, in the proposed scheme, open-loop precoding in order to reduce the correlation between codewords transmitted on different signal streams is employed at the transmitter of an overloaded MIMO system. The proposed precoding scheme changes the amplitude as well as the phase of the coded symbols transmitted on different subcarriers. Numerical results obtained through computer simulation show that the proposed scheme improves the bit error rate performance on Rician channels.

In this paper, a peak-to-average power ratio (PAPR) reduction scheme without side information (SI) is propossed. Cyclic-selective mapping (cyclic-SLM) has been applied to reduce the PAPR of a transmit orthogonal frequency division multiplexing (OFDM) signal. The cyclic-SLM generates a signal candidate by combining an original signal and its cyclic shifted signal. The Cyclic SLM requires side information regarding the cyclic shift to recover the signal while it will reduce the throughput of a wireless OFDM system. The propossed scheme applies delayed correlation based estimation to omit the transmision of the SI. In the receiver side, the amount of the cyclic shift is estimated with exploiting the correlation between a guard interval sequence and a received signal. Numerical results show that the cyclic-SLM with the delayed correlation (DC) provides significant PAPR reduction while it realizes the accuracy of the shift estimation close to 100% at E_b/N_0 of 8 dB. It also achieves the PAPR that is equivalent to the conventional SLM and the bit error rate degradation is slightly different from that with perfect shift estimation.

Wireless machine-to-machine (M2M) has been applied to a very wide area of industry and has been used for today's communication systems. Automatic control of industrial machinery is one of the applications of wireless M2M and requires high reliability and efficiency of data transmission. In order to shorten frame length, a frame format that multiplexes a data signal to the IEEE 802.11a short preamble sequence has been proposed. This paper evaluates the performance of an iterative frequency offset compensation scheme in terms of a timing synchronization probability and a bit error rate (BER). Three iterations of frequency offset compensation achieve about 0.06 improvement on the timing synchronization probability and the BER degradation is suppressed within 3dB from the theoretical BER performance.

In order to support high data rate communications, the third generation partnership project (3GPP) long term evolution (LTE) implements multiple-input multiple-output (MIMO) systems. In addition, to achieve excellent performance, LTE also employs turbo codes as the forward error correcting (FEC) standard. Although conventional turbo codes give satisfactory performance in the ideal MIMO, significant performance degradation occurs in an overloaded MIMO system when the number of transmit antennas is larger than the number of receive antennas. This paper proposes a joint decoding scheme for turbo codes in overloaded MIMO-OFDM systems. In joint turbo decoding, the calculations of soft information are based on the bit combinations from all of streams. In addition, a super-trellis diagram is used to combine the trellis diagrams. Numerical results obtained through computer simulation show that the proposed scheme achieves better performance by 2.0dB for 4 transmit antennas and 2 receive antennas in the BER of 10-5. The proposed scheme also achieves throughput improvement with the ratio close to the theoretical overloaded MIMO capacity.

In recent years, wireless power transfer has been attracting a great deal of attention. In order to realize efficient power transfer, it is necessary to communicate data such as a frequency, required power, or error tolerance. In the proposed system, because of the use of the same antennas for power transmission and data transmission, the frequency response of a channel for the data transmission changes owing to load fluctuation and the distance between antennas. This paper investigates data transmission performance in the wireless power transfer system with frequency response estimation at the transmitter side. The numerical results obtained through computer simulation show that the proposed scheme can estimate the frequency response of the channel and the difference between the expected bit error rate (BER) and the BER with the estimation error is less than 0.5 dB at the BER of 10-3.

Non-orthogonal multiple access improves spectral efficiency through sharing the same frequency resource with multiple wireless terminals. In previous researches, it has been shown that a log likelihood ratio (LLR) calculated from a signal on the sharing spectrum should be masked to zero if the power of an interference signal is stronger than that of a desired signal. This implies that the spectrum is not shared by the terminal receiving the weaker signal. In this paper, a spectrum sharing scheme with the use of interference cancelation and LLR calculation on the basis of the probability density function of residual interference is proposed. The residual interference remains owing to a channel estimation error and the LLR calculation takes that into account. Numerical results obtained thorough computer simulation show that the proposed scheme improves the performance by 1 dB with ideal channel estimation and 2 dB with channel estimation error at the BER of 10-4as compared to that with a conventional LLR calculation scheme.

Non-orthogonal multiple access improves spectral efficiency through sharing the same frequency resource with multiple wireless terminals. In the previous researches, it has been shown that a log likelihood ratio (LLR) calculated from a signal on the sharing spectrum should be masked to zero if the power of an interference signal is stronger than that of a desired signal. This implies that the spectrum is not shared by the terminal receiving the weaker signal. In this paper, a spectrum sharing scheme on the basis of interference cancelation and a LLR calculation with the probability density function of residual interference is proposed. Numerical results obtained thorough computer simulation show that the proposed scheme improves the performance by 1 dB at the BER of 10-4as compared to that of the conventional scheme.

Overlapped FFT based energy detection has been proposed as a signal detection scheme in dynamic spectrum access. The overlapped FFT scheme increases the number of FFT frames to reduce the variance of squared noise and improves the detection performance. As the FFT frames are overlapped, correlation values between the frames affect to the detection performance. This paper proposes the window functions which decrease the correlation values between adjacent FFT bins. Numerical results obtained through computer simulation show that novel window functions generated by upsampling a Hamming window improves the detection performance by 0.09.

Non-orthogonal access improves the spectral efficiency through sharing the same frequency resource with multiple wireless terminals. In the previous researches, it has been shown that a log likelihood ratio (LLR) calculated from a signal on the sharing spectrum should be masked to zero if a power of an interference signal is stronger than that of a desired signal. This implies that the spectrum is not shared by the terminal receiving the weaker signal. In this paper, a spectrum sharing scheme on the basis of interference cancelation and LLR weighting to reduce the effect of interference in low density parity check (LDPC) decoding is proposed. Numerical results obtained thorough computer simulation show that the proposed scheme improves the performance by 2dB at the BER of 10-2as compared to that of the system without weighting and overlapping to the parity bits is less sensitive to the BER performance relative to the case for overlapping to the systematic bits.

This paper investigates the relationship between antenna position and data communication performance in a magnetic resonance wireless power transfer (MRWPT) system. In MRWPT information such as the types of equipments, the required amount of electrical power, or the timing of power transfer should be transmitted from the antenna. It is assumed here that power transfer coils in the MRWPT system are employed as antennas for data communication. The frequency characteristic of the antennas change due to coil displacements. The power transfer coils are modeled as a band pass filter (BPF) and the frequency responses of the filter are presented in this paper. The characteristics of the filter are derived through circuit simulation and resulting data communication performance is evaluated. Numerical results obtained through computer simulation show that the bit error late (BER) performance can be improved by controlling the center frequency of the communication link.

The fourth generation mobile communication system is expected to be implemented on frequency bands higher than the bands already assigned for IMT-2000. Therefore, to compensate propagation loss, relay schemes have been investigated. Amplify-and-Forward (AF) relay has advantages in terms of lower complexity and time slot usage. However, the coupling effect causes the loop signal between transmit and receive antennas in the relay station and it can degrade the performance. Thus, in this research, a combining scheme of loop signals and original signals in the frequency offset AF relay scheme is proposed. It is shown that diversity gain and bit error rate (BER) performance vary according to the set of frequency offsets in the relays and modulation schemes. The system with 64 quadrature amplitude modulation (QAM) suffers from interference owing to frequency offsets.

A diversity scheme with Fractional Sampling (FS) in OFDM receivers has been investigated recently. FS path diversity makes use of imaging components of a desired signal transmitted on an adjacent channel. In the previous literature, a non-orthogonal access scheme over multiple channels with iterative interference cancellation (IIC) and FS was proposed. The conventional scheme has shown that the imaging component is transmitted non-orthogonally and path diversity is realized with FS and IIC. However, the conventional scheme does not take into account a spatial multiplexing transmission scheme such as multiple-input multiple-output (MIMO). Therefore, this paper proposes a MIMO-FS scheme with IIC and evaluates its performance through computer simulation. It is shown that in the MIMO situation with channel coding the proposed system improves the performance by 1.5dB at the BER of 10-5compared to a system without imaging components.

Cognitive radio is expected to improve the efficiency of frequency usage, capacity, and dependability of a wireless system. In order to change the radio parameters flexibly, the CR terminal should measure the radio environments on adjacent channels as well as the channel for communication. This paper proposes an overlapped FFT energy detection of adjacent channels with frame summation. The proposed scheme constructs new frame through summation and reduces the energy of the noise at the outputs of FFT. Numerical results obtained through computer simulation have shown that the overlapped FFT with no frame summation and with frame summation improve the probability of detection by about 2% and 3%, respectively, as compared to that without overlapping on the adjacent channel.

Recent interest in wireless power transfer research has been attracting a great deal of attention. To transfer power efficiently and safely in wireless power transfer system, information, such as frequency, required power and element values, need to be transmitted reliably. However, the bandwidth, which is used for exchanging information, is affected by the change of load at the receiver when it is charging. This paper investigates the effect of load fluctuation in data communication using orthogonal frequency division multiplexing (OFDM) modulation in resonant-type wireless power transfer systems. The equivalent circuit used in the transmitting and receiving antennas is a band pass filter (BPF) and its bandwidth is evaluated through circuit simulations. Numerical results obtained through computer simulation show that the bit error rate (BER) performance in the presence of the load fluctuation is affected by the power efficiency at feeding end.

Software defined radio (SDR) is a technology that allows a single terminal to support various kinds of wireless systems by changing its software to reconfigure the wireless terminal. As a software defined radio (SDR) receiver platform, Sora has been developed recently, which is based on a general purpose processors (GPPs). In the SDR receiver, timing synchronization of OFDM signal consumes a significant amount of computational power in the GPP. In this paper, a new time synchronization scheme for the GPP based SDR platform is proposed. The proposed scheme reduces the amount of data transmission between the memory and the GPP. It is shown through the experiment that the proposed scheme reduces the number of cycles for timing synchronization by 1/5.

This paper presents performance of a multiuser MIMO-OFDM (MU- MIMO) system with Fractional Sampling (FS) on a street canyon model. The MU-MIMO system has been investigated to improve the channel capacity in cellular systems. However the capacity of the MU-MIMO is deteriorated in an urban environment such as a street canyon due to a high correlation between antennas. Through FS it is possible to separate multipath components and realize diversity. FS works effectively when the correlation between the users are high. Numerical results through computer simulation shows that the capacity improves with the FS by about 3.5 bps/Hz.

Direct conversion receivers in orthogonal frequency division multiplexing (OFDM) systems suffer from direct current (DC) offset, frequency offset, and IQ imbalance. We have proposed an IQ imbalance estimation scheme in the presence of DC offset and frequency offset, which uses pilot subcarriers in the frequency domain. In this scheme, the DC offset is eliminated by a differential filter. However, the accuracy of IQ imbalance estimation is deteriorated due to the intercarrier interference (ICI) components caused by frequency offset. To overcome this problem, a new IQ imbalance estimation scheme with intercarrier interference self-cancellation pilot symbols in the frequency domain has been proposed in this paper. Numerical results obtained through computer simulation show that estimation accuracy and bit error rate (BER) performance are improved.

A diversity scheme with Fractional Sampling (FS) in OFDM receivers has been investigated recently. FS path diversity makes use of the imaging components of the desired signal transmitted on the adjacent channel. In this paper nonorthogonal access over multiple channels with iterative interference cancellation (IIC) and FS is proposed. The proposed scheme transmits the imaging component non- orthogonally on the adjacent channel. In order to accommodate the imaging component, it is underlaid on the other desired signal. Through diversity with FS and IIC, non-orthogonal access on multiple channels is realized. Our propose scheme can accommodate nonorthogonal signals with limited diversity gains.

This paper presents a signal de-multiplexing scheme in branch metric calculation of soft decision decoding for a spatially multiplexed MIMO system. In a conventional MIMO system, de-multiplexing is carried out over the symbols received by multiple antenna elements and is separated from decoding. Therefore, the de-multiplexing requires multiple uncorrelated antenna elements in order to realize a full rank channel matrix. This leads to the limitation of the system capacity by the number of the receive antenna elements. Instead of splitting de-multiplexing and decoding, in this paper, a de-multiplexing scheme at branch metric calculation in soft decision decoding is proposed. Based on ideal interleaving, independence among the coded symbols is assumed. Thus, the full rank channel matrix with the size of minimum free distance can be realized for signal de-multiplexing. As examples of the proposed system, the performance of repetition codes, block codes, and convolutional codes with MMSE de-multiplexing and soft decision Viterbi decoding on a Rayleigh fading channel is investigated. It is shown through numerical analysis that a lower BER can be achieved with a larger minimum free distance for the same normalized transmission rate that is given as the product of the coding rate and the number of multiplexed signal streams.

Fractional sampling (FS) has been investigated to achieve path diversity with a single antenna in an OFDM system. However, in the FS-OFDM system, it is necessary to oversample a received signal and it leads to large power consumption in a small terminal. To shift the signal processing burdens for FS from a receiver to a transmitter, a precoded transmit path diversity scheme in OFDM has been proposed. It achieves path diversity without oversampling the received signal at the receiver side on time invariant channels. However, it has not taken time variant channels into consideration. On time variant channels, inter-carrier interference (ICI) deteriorates the bit-error-rate (BER) performance. In this study, the precoding for both ICI suppression and path diversity combining at the transmitter side has been proposed. Computer simulation shows that both the conventional scheme and the proposed scheme can achieve transmit path diversity. On a two path Rayleigh fading channel, the BER performance of the proposed scheme is better than that of the conventional scheme by about 1 dB. On ultra wideband (UWB) channels, the performance of the proposed scheme is 7–10 dB better at BER=10−2 than that of the conventional scheme. It can achieve ICI suppression and path diversity without any specific signal processing at the receiver. The effect of the ICI suppression is more significant as the number of the multipath is larger.

We address the Primary User (PU) detection (spectrum sensing) problem, relevant to cognitive radio, from a finite random matrix theoretical (RMT) perspective. Specifically, we employ recently-derived closed-form and exact expressions for the distribution of the standard condition number (SCN) of uncorrelated and semi-correlated random dual central Wishart matrices of finite sizes in the design Hypothesis-Testing algorithms to detect the presence of PU signals. In particular, two algorithms are designed, with basis on the SCN distribution in the absence (H_0) and in the presence (H_1) of PU signals, respectively. Due to an inherent property of the SCN's, the H_0 test requires no estimation of SNR or any other information on the PU signal, while the H_1 test requires SNR only. Further attractive advantages of the new techniques are: a) due to the accuracy of the finite SCN distributions, superior performance is achieved under a finite number of samples, compared to asymptotic RMT-based alternatives; b) since expressions to model the SCN statistics both in the absence and presence of PU signal are used, the statistics of the spectrum sensing problem in question is completely characterized; and c) as a consequence of a) and b), accurate and simple analytical expressions for the receiver operating characteristic (ROC) — both in terms of the probability of detection as a function of the probability of false alarm (P_D versus P_F) and in terms of the probability of acquisition as a function of the probability of miss detection (P_A versus P_M) — are yielded. It is also shown that the proposed finite RMT-based algorithms outperform all similar alternatives currently known in the literature, at a substantially lower complexity. In the process, several new results on the distributions of eigenvalues and SCNs of random Wishart Matrices are offered, including a closed-form of the Marchenko-Pastur's Cumulative Density Function (CDF) and extensions of the latter, as well as variations of asymptotic the distributions of extreme eigenvalues (Tracy-Widom) and their ratio (Tracy-Widom-Curtiss), which are simpler than those obtained with the "spiked population model".

We address the Primary User (PU) detection (spectrum sensing) problem, relevant to cognitive radio, from a finite random matrix theoretical (RMT) perspective. Specifically, we employ recently-derived closed-form and exact expressions for the distribution of the standard condition number (SCN) of uncorrelated and semi-correlated random dual Wishart matrices of finite sizes, to design Hypothesis-Testing algorithms to detect the presence of PU signals. An inherent characteristic of the SCN/RMT-based approach, is that no signal-to-noise ratio (SNR) estimation or any other information on the PU signal is required. On top of this property, other attractive advantages of the new techniques are: a) due to the accuracy of the finite SCN distributions, superior performance is achieved under a finite number of samples, compared to asymptotic RMT-based alternatives; b) since expressions to model the SCN statistics both in the absence (H0) and presence (H1) of PU signal are used, the statistics of the spectrum sensing problem in question is completely characterized; c) as a consequence of a) and b), accurate and simple analytical expressions for the receiver operating characteristic (ROC) - both in terms of PD as a function of PF and in terms of PA as a function of PM - are yielded. It is also shown that the proposed finite RMT-based algorithms outperforms all similar alternatives currently known in the literature, at a substantially lower complexity.

A diversity scheme with Fractional Sampling (FS) in OFDM receivers has been investigated recently. In the previous literature it has been shown that performance improvement with path diversity through FS depends on the bandwidth of the total channel response including transmit and receive baseband filters. However, if the signal on the adjacent channel exists, that signal causes interference to the desired signal with FS. In this paper, an adjacent channel interference cancellation scheme has been proposed for the FS OFDM receiver. The proposed scheme demodulates the signal on the adjacent channel and regenerates it to be subtracted from the received signal. Numerical results through computer simulation show that the proposed scheme achieves BER improvement under low signal-to-interference-ratio (SIR) conditions.

To achieve diversity with a single antenna, a fractional sampling (FS) scheme in OFDM has been proposed. FS achieves path diversity with a higher sampling rate and multiple demodulation branches. In the previous literatures for FS, the sampling interval is fixed even though the initial phase selection of the sampling has been investigated. The fixed sampling interval limits the diversity gain through FS. Thus, in this study, a non-uniform sampling point selection scheme according to the frequency response of a channel is proposed. Numerical results through computer simulation show that the proposed scheme improves the BER performance by about 2 dB when the number of the demodulation branches is 2. While the proposed scheme further improves bit error rate (BER) performance it requires a larger rate and more computational complexity for the selection of the sampling points. Therefore the complexity reduction for the sampling point selection is also investigated. The complexity reduction scheme eliminates the specific sets of the sampling points and reduces the computational complexity of the sampling point selection for the proposed scheme by a factor of 2. Computer simulation also shows that the low complexity version of the proposed scheme even achieves the equivalent BER compared to the original proposed scheme.

This paper presents a user/sampling point selection algorithm for a multiuser MIMO-OFDM system with Fractional Sampling (FS). Through FS, it is possible to separate multipath components and obtain diversity gain in an OFDM system. Therefore, the capacity of the multiuser MIMO-OFDM system increases by 1-2 bit/sec/Hz. This paper also proposes a suboptimum sampling point selection algorithm. It is shown that the suboptimal algorithm shows the capacity close to that of the optimal one. However, when the oversampling ratio increases, the difference on the capacity also grows. Thus, there is a tradeoff between the computational complexity and the capacity improvement.

In this paper, a DF relay scheme using different frequency band is proposed. The communication between the Relay Station (RS) and the Base Station (BS) uses lower frequency band than that between the Mobile Station and the Base Station. By communicating in the lower frequency band, high-order modulation should be employed in order to maintain the throughput. Numerical results through computer simulation show that by combining two unequally modulated signals, the BER performance improves more effectively.

In this paper, a new feedback equalization scheme to suppress inter-carrier interference (ICI) in an OFDM system using scattered pilot is investigated. On a fast fading channel severe ICI occurs due to a Doppler shift and it deteriorates a bit error rate (BER) seriously because of small subcarrier spacing. In an ISDB-T receiver the equalization is mainly processed in a frequency domain because the scattered pilot is transmitted over the subcarriers. However, the frequency domain equalization may not suppress severe ICI in the case of the fast fading channel with a large Doppler shift. The proposed equalization scheme uses the scattered pilot symbols transformed in a time domain as the reference signal for feedback taps. Numerical results through computer simulation show that the proposed scheme improves the BER performance especially with low carrier-to-noise ratio (CNR) conditions.

To achieve diversity with a single antenna, a fractional sampling (FS) scheme in OFDM has been investigated. In addition, to obtain more diversity gain through FS, a sampling point selection (SPS) scheme has been proposed. However, it requires a large number of multiplication operations for the SPS. This leads to large power consumption and costs for additional circuits. In this paper, a low-complexity SPS scheme is proposed. There is the trade-off between the diversity gain and the estimation accuracy. The serial process of the channel estimation requires only a DFT block with the size of N/4 as the additional circuit.

In this report, an adaptive modulated orthogonal frequency division multiplexing (OFDM) scheme that provides efficient data transmission for correlated receiver antenna arrays is proposed. Previously reported adaptive modulation schemes generally select the modulation scheme based on the estimated carrier-to-noise ratio (CNR) on each subcarrier. However, in antenna array communication in addition to the CNR, the spatial fading correlation should also be considered to evaluate the system performance. Especially, when the Time Shifted Sampling (TSS) technique is applied which overcomes the spatial correlation for some subchannels of an OFDM symbol, the spatial correlation varies dramatically through subcarriers. In this work, modulation scheme is selected based on the CNR and the spatial correlation coefficient for each subcarrier when the TSS technique is employed at the receiver. Numerical results through computer simulation show that the proposed scheme outperforms the conventional systems in achieving a target BER and establishing reliable communication while having almost the same transmission rate.

In the IEEE802.11n WLAN standard, orthogonal frequency division multiplexing (OFDM) modulation is employed. Diversity techniques are implemented to overcome multipath fading in the OFDM systems. A fractional sampling (FS) scheme is one of the diversity techniques with a single antenna. This scheme also can be applied in a MIMO-OFDM system to increase its capacity. In this paper, the effect of FS in a WLAN system following the IEEE802.11n standard is investigated through experiments. Numerical results through the experiments indicate that diversity gain through FS can be obtained in the NLOS conditions.

Fractional sampling (FS) and Doppler diversity equalization in OFDM receivers can achieve two types of diversity (path diversity and Doppler diversity) simultaneously on time-varying multipath channels. However FS with a higher sampling rate requires the large amount of complexity in demodulation. In this paper, a novel sampling point selection (SPS) scheme with MMSE equalization in FS-OFDM receivers is proposed. On fast time-varying multipath channels, the proposed scheme selects the appropriate samples from the fractionally sampled signals. Through the computer simulation, it is demonstrated that with the proposed scheme, both path diversity gain and Doppler diversity gain can increase as compared to a conventional non-SPS scheme.

In OFDM receivers, an analog-to-digital converter must convert a signal with large bandwidth and high resolution. A ΣΔ modulator is a type of A/D converters that has very high resolution. However, it has lower bandwidth than the other types of A/D converters because the quantization noise of the ΣΔ modulator increases in high frequency. Moreover, the oversampling rate of the ΣΔ modulator needs to be decreased for reduction of power consumption. In this paper, performance of an OFDM receiver with the ΣΔ modulator using a subcarrier weighting scheme is investigated. The symbol weighting scheme is used with a soft-decision Viterbi decoder in the receiver, and the symbol on each subcarrier is weighted by a coefficient derived from the output noise spectrum of the ΣΔ modulator. The numerical results obtained through computer simulation show that the BER performance is improved by about 2-3 dB on multipath fading channels.

Orthogonal frequency and code division multiplexing (OFCDM) has received large attention as a modulation scheme to realise high data rate transmission. The OFCDM system with fractional sampling (FS) is investigated. FS is a diversity scheme with a single antenna, which achieves path diversity through oversampling and parallel signal reception. However, the correlation among noise components may deteriorate the bit error rate performance at the receiver as the number of subcarriers and oversampling ratio increases. To overcome this problem, the alternative spreading code is used for the FS OFCDM system. Numerical results through computer simulation show that the proposed scheme can improve the performance of the receiver with the large oversampling ratio and the number of subcarriers.

In the IEEE 802.11a WLAN standard, orthogonal frequency division multiplexing (OFDM) modulation is employed. Diversity techniques are effective way to overcome multipath fading in the OFDM systems. A fractional sampling (FS) scheme is one of the diversity techniques with a single antenna. Through FS it is possible to separate multipath components and obtain diversity gain in OFDM. Though FS has been analysed numerically, the experiment of FS on actual indoor channels with real filter characteristics has not been carried out. In addition, though it has been suggested that in FS diversity a sharp filter limits the diversity gain, this limitation has not been confirmed especially with measurements. Therefore we investigate the effect of filter bandwidth in FS through simulation and an experiment. Numerical results through the simulation and the experiment indicate that the FS can achieve path diversity and improves the performance by about 2 dB if the bandwidth of the baseband filter is larger than the minimal bandwidth.

This paper presents the performance of a Fractional Sampling OFDM receiver with delay diversity. Delay diversity achieves transmit diversity by intentionally ingenerating paths which have different delays at a transmitter side. However, the correlations between subcarriers are not always low. The proposed scheme which combines the delay diversity and the FS offers diversity gain by reducing the correlation between the sub-carriers. The proposed scheme improves the BER performance by about 2dB and the improvement depends on the amount of the delay.

Direct conversion receivers in orthogonal frequency division multiplexing (OFDM) systems suffer from direct current (DC) offset, frequency offset, and IQ imbalance. We have proposed an IQ imbalance estimation scheme in the presence of DC offset and frequency offset, which uses preamble signals in the time domain. In this scheme, the DC offset is eliminated by a differential filter. However, the accuracy of IQ imbalance estimation is deteriorated when the frequency offset is small. To overcome this problem, a new IQ imbalance estimation scheme in the frequency domain has been proposed in this paper. The IQ imbalance is estimated with pilot subcarriers. Numerical results obtained through computer simulation show that estimation accuracy and bit error rate (BER) performance can be improved even if the frequency offset is small.

In this paper, an interference suppression scheme by MMSE combining in OFDM-antenna array with time shifted sampling (TSS) is proposed. An array antenna at a base station has been thoroughly investigated to increase the uplink capacity. The performance of the uplink is not only limited by the correlation between the antenna elements, which strongly depends on the spatial aspects of the channel, as well as the interference from the terminals. Numerical results through computer simulation show that the proposed scheme improves bit error rate performance because of interference suppression by MMSE combining and diversity reception by TSS at the same time.

In this paper, an optimum insertion/deletion point selection algorithm for fractional sample rate conversion (SRC) is proposed. The direct insertion/deletion technique achieves low complexity and low power consumption as compared to the other fractional SRC methods. Using a multiple set insertion/deletion technique is efficient for reduction of distortion caused by the insertion/deletion step. When the conversion factor is (N ±¿)/N, the number of possible patterns of insertion/deletion points and the number of combinations for multiple set inserters/deleters grow as ¿ increases. The proposed algorithm minimizes the distortion due to SRC by selecting the patterns and the combinations.

Through fractional sampling (FS) it is possible to separate multipath components and achieve path diversity. However, if no path component whose delay corresponds to the sampling point, FS cannot obtain diversity gain. In this paper, a novel scheme to improve the performance with FS over a sparse multipath channel is proposed. The proposed scheme uses multiple transmit antennas and transmit multiple signals with fractional delays. The performance improvement with the proposed scheme is confirmed through computer simulation. It is shown that the proposed scheme increases the capacity of a MIMO-OFDM system by a factor of 1.5 to 2.

In a direct-sequence / spread-spectrum (DS/SS) system, a RAKE receiver is used to improve a bit error rate (BER) performance. The RAKE receiver can collect more signal energy through independent paths and achieve path diversity. The RAKE receiver obtains further diversity gain through fractional sampling. However, the power consumption of the RAKE receiver increases in proportion to a sampling rate and does not always maximize the signal-to-noise ratio (SNR). Therefore, the sampling rate selection schemes have been proposed to reduce the average sampling rate without degrading the BER. These schemes select the tap positions and the sampling rate depending on channel conditions. In this paper, sampling rate selection schemes for the DS/SS system are investigated through an experiment. Numerical results show that the power consumption can be reduced even through the experiment without the degradation of the BER.

A direct conversion architecture reduces the cost and power consumption of a receiver. However, a direct conversion receiver may suffer from direct current (DC) offset, frequency offset, and IQ imbalance. This paper presents an IQ imbalance estimation scheme for orthogonal frequency division multiplexing (OFDM) direct conversion receivers. The proposed IQ imbalance estimation scheme operates in the presence of dynamic DC offset and frequency offset. The proposed scheme calculates IQ imbalance from a simple equation. It employs the knowledge of the preamble symbols of the IEEE 802.11 a/g standards, while it does not require the impulse response of the channel. Numerical results through computer simulation show that the bit error rate (BER) performance for the proposed IQ imbalance estimation scheme has a degradation of about 4 dB with a large DC offset, frequency offset, and IQ imbalance.

In this paper, a metric weighting scheme on a coded fractional sampling orthogonal frequency division multiplexing (FS OFDM) system is investigated. FS achieves path diversity with a single antenna through oversampling and subcarrier-based maximal ratio combining (MRC). Though the oversampling increases diversity order, correlation among noise components may deteriorate bit error rate (BER) performance. To clarify the relationship between the impulse response of the pulse shaping filter and the BER performance, five different pulse shaping filters are evaluated in the coded FS OFDM system. Numerical results obtained through computer simulation show that the metric weighting based on the Frobenius norm improves BER performance of the coded FS OFDM system.

Impulse radio ultra wideband (IR-UWB) can achieve high resolution in ranging because it uses a very short pulse with a duration of less than 1 ns. In order to reduce cost and power consumption, a ranging system with high-speed comparators has been proposed. In this system, it is necessary to reduce noise power through averaging the comparator outputs. The authors evaluate the effect of clock offset on the averaging process, and a two-stage averaging scheme for the IR-UWB ranging system is proposed. Through computer simulation, it is proved that the proposed scheme can reduce the effect of the clock offset with appropriate numbers of initial averaging.

A direct conversion architecture reduces the cost and power consumption of a receiver. However, a direct conversion receiver may suffer from direct current (DC) offset, frequency offset, and IQ imbalance. This paper presents an IQ imbalance estimation scheme for orthogonal frequency division multiplexing (OFDM) direct conversion receivers. The proposed IQ imbalance estimation scheme operates in the presence of dynamic DC offset and frequency offset. The proposed scheme calculates IQ imbalance from a simple equation. It employs the knowledge of the preamble symbols of the IEEE 802.11 a/g standards, while it does not require the impulse response of the channel. Numerical results obtained through computer simulation show that the bit error rate (BER) performance for the proposed IQ imbalance estimation scheme has a degradation of about 4 dB with a large DC offset, frequency offset, and IQ imbalance.

A diversity scheme with fractional sampling (FS) in OFDM receivers is investigated recently. When a sharp filter is employed, the correlation of noise samples among the adjacent subcarriers increases, and the performance of this scheme is deteriorated. Therefore, low-complexity subcarrier based MMSE combining scheme with the subblock of the noise covariance matrix is proposed in this paper. Numerical results through computer simulation show that the MMSE combining scheme proposed in this paper can outperforms a conventional MRC scheme and a subcarrier based MMSE combining scheme, when the sharp filter is employed.

An essential condition for diversity reception is that the fading distributions between the individual received signals of an antenna array are uncorrelated. In this paper, a new technique to improve the performance of transmission with the correlated Rayleigh-fading signals is proposed. In conventional array systems, individual receivers start sampling the received signals at the same time with the same sampling rate. On the other hand, in the proposed scheme, the received signals are again sampled with the same rate, however the sampling points are shifted in each receiver. Numerical results through computer simulation show that with the correlated received signals, by applying the proposed technique the correlation can be reduced to a sufficient level for diversity reception.

In this paper, sampling rate selection path diversity (SRSPD) scheme for direct-sequence/spread-spectrum (DS/SS) is proposed. In DS/SS communication systems, oversampling may be employed to increase the signal-to-noise ratio (SNR). However, oversampling enlarges the power consumption because signal processing of the receiver has to be carried out at a higher clock rate. In order to reduce the power consumption, the proposed SRSPD scheme selects the tap positions of the RAKE receiver and the sampling rate depending on the characteristics of the channel.

Software-defined radio (SDR) is a technology of widespread interest in wireless communications. With the SDR terminal, packets may be transmitted through multiple wireless communication links. The terminal may recognize the channel conditions and select one of the links. For this purpose, the channel estimation is required. In this paper, the channel estimation for IEEE802.11b WLAN system is investigated through an experiment. In order to estimate the impulse response of the channel, a pseudo-inverse matrix with a threshold is employed. The results indicate that the channel can be estimated precisely with an optimum threshold.

In WLAN systems, direct conversion architecture has been implemented. Though this architecture reduces the cost and power consumption, it may suffer from DC offset, frequency offset, and IQ imbalance. This paper presents a frequency offset estimation scheme for OFDM direct conversion receivers. In this paper, a frequency offset estimation scheme in the presence of time-varying DC offset and IQ imbalance is proposed. The novelty of this scheme is in its usage of a differential filter to reduce DC offset. Evaluation of the presented estimation scheme was carried out on an OFDM communication system designed to the specifications of the IEEE 802.11a/g standard for WLANs. Numerical results through computer simulation show that the proposed frequency offset estimation scheme works well even with large DC offset and IQ imbalance.

Through fractional sampling, it is possible to separate multi- path components and achieve diversity. However, power consumption grows as the sampling rate increases. In this paper, a novel sampling rate selection scheme in OFDM according to multipath environments is proposed. Numerical results through computer simulation show that the proposed sampling rate selection scheme reduces power consumption by decreasing the sampling rate when delay spread is small.

The influence of timing jitter on quadrature charge sampling for a direct conversion receiver has been evaluated. In contrast to voltage sampling, the charge sampling mixer integrates the signal current instead of tracking the signal voltage. The charge sampling mixer has been applied to RF direct sampling in wireless receivers. The influence of timing jitter on charge sampling has been analysed in some literatures. However, in wireless receivers, quadrature sampling is required in order to demodulate I-phase and Q-phase signals. Different from charge sampling, timing jitter causes crosstalk between these signals. The influence of timing jitter on quadrature sampling is investigated.

In this paper an adjacent channel interference (ACI) cancellation scheme for multi-channel reception with undersampling and investigated. Low-IF receiver architecture is used in the multi-channel reception scheme. However, in this system, the signal in the adjacent channel causes interference to the desired signal. ACI cancellation scheme with analog filter bank has been proposed to mitigate the influence from the adjacent channel. Undersampling technique is applied in this system in order to lower the required sampling frequency and power consumption. The effects of the adjacent channel to the undersampling technique in this scheme is investigated through the experiment

In software defined radio (SDR), implementation of RF front-end and analog-to-digital converter (ADC) is an important issue. One type of new schemes proposed for SDR is discrete time receiver (DTR), which processes analog signal directly. In the DTR architecture, the received signal is sampled at radio frequency (RF) and channel selection and demodulation are carried out in the digital domain. This architecture achieves reduction of on-chip components and enables one-chip receiver. However, in this architecture, the sampling jitter generated from phase noise of phase locked loop (PLL) may deteriorate the performance. In this paper, the phase noise of the PLL is modeled and the influence of the phase noise to the DTR is analyzed. Moreover, the performance of the DTR is evaluated in terms of modulation schemes and signal bandwidth

Recently, the direct conversion scheme has been actively investigated for the purpose of cost miniaturization and low power consumption of wireless receivers. IQ imbalance is one of the problems for the direct conversion receiver. In the direct conversion receiver, a /spl pi//2 phase shifter and a double balance mixer (DBM) for the radio frequency (RF) signal is required. However, it is difficult to obtain the accurate phase shifter and the linear DBM over broad bandwidth. As a result, the balance between in-phase (I) and quadrature-phase (Q) of the demodulated signals may be lost. In the case of OFCDM modulations, this IQ imbalance causes intercarrier interference (ICI) in the demodulated signals. In this paper, the decision directed scheme for IQ imbalance compensation is proposed.

In this letter, a parallel analog-digital (A/D) conversion scheme with a filter bank for low intermediate-frequency receivers is presented. The analysis filters of the filter bank divide the frequency components of the received signal, and achieve parallel A/D conversion. Therefore, the required conversion rates and the resolution of the A/D converters can be reduced and the receiver can demodulate wideband signals. As the analysis filters consist of analog components, their coefficients include errors. These errors cause mutual interference between signals in orthogonal frequencies. In order to remove this interference, a decorrelating compensation scheme is proposed.

A digital compensation scheme for coefficient errors of a complex filter bank in low-IF receivers is presented. The complex filter bank is employed to suppress DC offset and image signals in the low-IF receivers and relax the requirements on the conversion rate and resolution of A/D converters. The proposed compensation scheme regenerates interference due to the coefficient errors and subtracts it from the digital signal converted by an A/D converter. The proposed scheme also improves the effective resolution of A/D converters.

In this paper we investigate a new codeword assignment method for source-channel encoding of AC coefficients with multicode DS/SS modulation. With multicode DS/SS modulation, the total amount of energy to transmit an image is more significant than the total number of bits. The proposed method shows better performance than the conventional codeword assignment method for larger AC coefficients.

A new intracell orthogonalization technique for a code division multiple access (CDMA) wireless local communication system is posed. On the forward link of the system, it is necessary to achieve high bit rate transmission and to reduce the complexity of the receiving terminal. To meet these two requirements, cyclically shifted spreading sequences are utilized. However, it is necessary to orthogonalize the multiple signals since these sequences are cross-correlated. One way of achieving orthogonalization is pre-decorrelation. However, the orthogonality does not clearly appear in a multipath fading channel because of the interchip interference caused by multipath. Therefore in this paper, pre-decorrelation is combined with multicarrier modulation. The results obtained through computer simulation show that the proposed technique can achieve orthogonality in a multipath fading channel. Also the proposed technique makes efficient forward link power control possible, and reduces intercell interference.

In this paper, a new initial sequence acquisition technique for multicarrier time division duplex (TDD) CDMA systems are proposed and evaluated on a Rayleigh fading channel. The proposed technique transmits preambles utilized for initial sequence acquisition on multiple carriers at the same time in order to achieve frequency diversity. Soft decision detection on multiple carriers is employed. The numerical results show that the proposed technique improves required Eb/N/sub o/ by more than 10 dB for the probability of correct acquisition of 0.99. The proposed technique is suitable for TDD CDMA systems since a smaller number of preamble sequences for acquisition is desirable in TDD packet transmission.

This paper suggests that multicarrier modulation reduces the complexity and the delay caused by the multiuser interference cancellation process utilizing convolutional codes. For spread spectrum multiple access, multiuser interference (interference due to signals from other users) limits the performance of the communication link. To remove this interference, a multiuser interference cancellation technique which utilizes orthogonal convolutional codes has been proposed for the uplink (mobiles to a base station) of the cellular code-division multiple-access (CDMA) systems. However, this technique requires large interleavers and huge memory, or artificial multipath diversity and a RAKE system to achieve sufficient coding gain if it is applied to wireless indoor communications and fading is slow compared to the data rate. To reduce the complexity of the canceller, multicarrier modulation is employed as it provides frequency diversity gain and coding gain without the interleavers or a RAKE system. This paper shows that multicarrier modulation reduces the complexity of the canceller and still provides sufficient coding gain in order to cancel the multiuser interference. A canceller with decoding in the initial decision and multicarrier modulation improves the capacity by a factor of 1.4 as compared with a canceller without decoding.

This paper proposes a new parallel co-channel interference cancellation technique which utilizes orthogonal convolutional codes. Co-channel interference (CCI) limits the performance of a spread spectrum multiple access communication link. Several CCI cancellation techniques have been proposed to remove this interference. Of particular interest are techniques which do not require the receiver to have knowledge of the cross-correlation between user sequences. These techniques reconstruct the CCI based on the initial decisions regarding the signals from the other users. However, these techniques leave residual interference after cancellation caused by errors in these initial decisions. To improve the initial decisions and reduce the residual interference, our proposed scheme utilizes the error correcting capability of orthogonal convolutional codes. This paper evaluates the performance of this scheme. We show that, given a processing gain of 128 for up to about 40 users, the performance of the proposed CCI canceller approaches the performance of a system without multi-user interference. We also show that the proposed CCI canceller offers an improvement in capacity by a factor of 1.5/spl sim/3 over that of a conventional canceller.