Massive multiple-input multiple-output (MIMO) realizes simultaneous transmission to a large number of mobile stations (MSs) and improves frequency utilization efficiency. It is drawing attention as the key technology of the fifth-generation (5G) mobile communication systems. The 5G system is going to be implemented in a high frequency band and massive MIMO beamforming (BF) is applied to compensate propagation loss. In the conventional BF scheme, a transmit beam is selected based on the power of received signals over subcarriers. The signal on a different subcarrier is transmitted with a different directivity. To improve the accuracy of beam selection, this paper proposes a transmit beam selection scheme for massive MIMO. The proposed scheme calculates the expected responses of the signals over the subcarriers based on the relative directivity between a base station (BS) and a MS. The MS calculates the correlation between the received signals and each of the expected response sequences. It then selects the beam with the highest correlation value. It is shown in this paper that the proposed scheme can improve the average signal-to-noise ratio of a received signal by about 1.5dB as compared with that of the power based search scheme. It is also shown that the proposed scheme with limited response coefficients can reduce the computational complexity by a factor of 1/100 while it still increases the average SNR by about 1.0dB.

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 is employed at the transmitter of an overloaded MIMO system in order to reduce the correlation between codewords transmitted on different signal streams. 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. It is also shown that the proposed scheme greatly suppresses the performance degradation on an independent Rayleigh fading channel even though the amplitude of the coded symbols varies.

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. To increase the diversity gain with FS the bandwidth of the transmit signal has to be enlarged. This leads to the reduction of spectrum efficiency. In this paper non-orthogonal access over multiple channels in the frequency domain 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 proposed scheme can obtain diversity gains for non-orthogonal signals modulated with QPSK.

In this paper, the performance of narrow band interference (NBI) rejection scheme for direct sequence spread spectrum (DS/SS) is analyzed. A single-tapped complex FIR filter is used for filtering a chip code to suppress NBI. In this system, the spectrum of transmitted signal has a null at an arbitrary frequency. By choosing filter coefficients, we place this null at NBI center frequency to mitigate the effect of NBI. The performance of this scheme is theoretically analyzed and validated by simulation. We also compare the effectiveness against BPSK interference between the chip code filtering and received signal filtering. The results indicate the chip code filtering is effective against single-tone and BPSK interference, and gains better performance than the received signal filtering at certain level of SNR.

Currently, multiband orthogonal frequency division multiplexing (MB-OFDM) is considered to be one of the modulation schemes of UWB and is being actively investigated. It is necessary to provide low-cost receivers for consumers to receive wide support for the MB-OFDM system. Such receivers can be achieved by utilizing direct-conversion architecture. Direct-conversion architecture suffers from IQ imbalance. IQ imbalance causes intercarrier interference (ICI) in the demodulated signals. In this paper, a new scheme of IQ imbalance compensation using transmit diversity is proposed. This scheme enables the system to achieve frequency diversity and simultaneously compensates for the influence of IQ imbalance. It is shown that the performance of the proposed scheme is better than that of the conventional IQ imbalance compensation scheme.

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 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 the proposed scheme, the combination of received symbols which satisfies orthogonality conditions is used for compensation of IQ imbalance. Therefore, in addition to the pilot symbols, the received symbols can be used in order to improve the accuracy of the compensation matrix and BER can be reduced.

In this paper an adjacent channel interference (ACI) cancellation scheme with undersampling for multi-channel reception is proposed and investigated. Low-IF receiver architecture is used in the multi-channel reception scheme. In this system, signal in the adjacent channel causes interference to the desired signal. The ACI cancellation scheme with analog filter bank has been proposed to mitigate the influence from the adjacent channel [10]. 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 examined and discussed.

Capacity of Cellular DS/CDMA systems depends on an amount of co-channel interference (CCI). One of the effective schemes to eliminate the CCI and improve the capacity is CCI cancellers which remove the CCI by subtracting all the regenerated signals of the interfering users. These cancellers, however, suffer from the residual interference due to the symbol errors in the initial decision. Therefore, a canceller which employed error correction in the initial decision has been proposed. In this system, two Viterbi decoders per one user are needed. Therefore, the amount of calculation increases and this causes additional signal processing delay which is not preferable, especially for voice transmission. Here we propose three fast decoding methods by simplifying the second Viterbi decoder which is used for decoding after the cancellation. Method-1 uses information of the first Viterbi decoder. Method-2 utilizes information of the second correlator instead of that of the first Viterbi decoder. Method-3 is the combination of method-1 and method-2. It uses information from both the first Viterbi decoder and the second correlator. The results obtained from the computer simulation show that the ACS reduction ratio reaches up to 80% within 0.5 dB degradation in Es/No.

Pre-decorrelation is a method of achieving orthogonalization between multiple signals on the forward link. This technique can achieve orthogonalization in a flat fading channel, however, the orthogonality does not clearly appear in a multipath fading channel because of interchip interference. In order to eliminate the effect of multipath and prevent interchip interference, multicarrier modulation can be employed. In this paper we propose a multicarrier pre-decorrelation technique which combines multicarrier modulation with pre-decorrelation. Computer simulation results show that the proposed technique can achieve orthogonalization in a multipath fading channel.

In the fourth generation mobile communication system, the frequency band higher than the bands already used for IMT-2000 has been assigned. However, the higher radio frequency increases the propagation loss. To cope with the propagation loss, relay transmission has been investigated. In this paper, a transmission diversity scheme with frequency offset in amplify-and-forward (AF) relaying is proposed. In this scheme, the frequency of the OFDM signal is shifted at the relay station. A different amount of frequency offset is given in each relay station and the signals from the relay stations are separated and combined by MMSE detection at the base station. The numerical results obtained through computer simulation show that diversity is realized and the BER performance is improved by 1-2 dB at the BER of 10-2. When the number of the RSs increases, more BER improvement is achieved.

Wireless communications for the control of industrial equipments need to send a large amount of short packets frequently and to improve frame efficiency. The OFDM frame of wireless local area networks has short preambles that are used for timing synchronization and coarse frequency offset estimation. As the short preambles are repeated in a time domain, they occupies subcarriers intermittently. Therefore, in this paper, a new frame format with OFDM modulation in which data symbols are orthogonally multiplexed with the preamble symbols in the frequency domain is proposed. Two preamble sequences that are based on an IEEE802.11g short preamble sequence and a Zadoff-Chu sequence are examined. The ratio of transmission powers between the pilot subcarrier and the data subcarrier is also varied. The timing synchronization probability with those sequences has been evaluated on different channel models. It is shown through the experiment that the synchronization performance is almost the same as that without data multiplexing at Es/N0 of more than 8dB.

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. However, this window function suffers more from adjacent channel interference than a conventional window. Therefore, this paper also proposes a two step detection scheme to achieve higher detection performance and to avoid the influence of the adjacent channel signal. Numerical results also indicate that the proposed scheme improves the detection performance and reduces the effect from the adjacent channel signal.

Delayed correlation has been used to detect orthogonal frequency division multiplexing symbols with cyclic prefix in spectrum sensing. Because of the frequency offset, the outputs of the delayed correlation do not lie only on the real axis of a complex plane. Therefore, the absolute value of the outputs of the delayed correlation is employed. Furthermore, with the use of a filter bank, the number of the outputs of the delayed correlators increases and the averaging over the outputs decreases the noise variance. This paper proposes a new delayed correlation scheme that uses a filter bank and employs the absolute of the outputs of delayed correlation. The proposed scheme improves the probability of detection as the number of the branches of the delayed correlators increases. In the case of 6 branches, the proposed scheme reduces the required sample energy by 1dB the probability of detection of 0.9.

A novel cumulant based MUSIC like DOA estimation algorithm for multicarrier modulation has been proposed in this paper. While the conventional MUSIC algorithm is not applicable to a correlation matrix calculated from received signals transmitted over the different carriers, the proposed algorithm can estimate the DOA of the signals with multicarrier modulation. The proposed algorithm does not require the sensor array responses for the frequency range of the interest and the initial phases of the carriers. With the proposed algorithm the number of signals whose DOA are estimated can be increased and the accuracy of the DOA estimation can be improved by employing larger number of carriers.

This paper presents the analysis of in-band interference caused by pulse-based ultra-wideband (UWB) systems. The analysis contains both plain Impulse Radio UWB (IR-UWB) and Transmitted Reference UWB (TR-UWB) systems as a source of interference. The supposed victim is a narrowband BPSK system with a band-pass filter. The effect of pulse-based UWB systems is analyzed in terms of bit error rate. The analysis is given in terms of the specific combinations of pulse repetition frequency and center frequency of the narrowband bandpass filter. In those situations, the UWB interference cannot be modeled as a Gaussian noise. It also manifests situations in which the victim is under the severest or the slightest interference from TR-UWB. According to its result, the analysis is validated via simulation.

In this paper, a new fractional sample rate conversion (SRC) scheme based on a direct insertion/cancellation scheme is proposed. This scheme is suitable for signals that are sampled at a high sample rate and converted to a lower sample rate. The direct insertion/cancellation scheme may achieve low-complexity and lower power consumption as compared to the other SRC techniques. However, the direct insertion/cancellation technique suffers from large aliasing and distortion. The aliasing from an adjacent channel interferes the desired signal and degrades the performance. Therefore, a modified direct insertion/cancellation scheme is proposed in order to realize high performance resampling.

In this paper, a non-linear precoding algorithm with low out-of-band (OOB) radiation is proposed for massive multiple-input multiple-output (MIMO) systems. Massive MIMO sets more than one hundred antennas at each base station to achieve higher spectral efficiency and throughput. Full digital massive MIMO may constrain the resolution of digital-to-analog converters (DACs) since each DAC consumes a large amount of power. In massive MIMO systems with low resolution DACs, designing methods of DAC output signals by nonlinear processing are being investigated. The conventional scheme focuses only on a sum rate or errors in the received signals and so triggers large OOB radiation. This paper proposes an optimization criterion that takes OOB radiation power into account. Gibbs sampling is used as an algorithm to find sub-optimal solutions given this criterion. Numerical results obtained through computer simulation show that the proposed criterion reduces mean OOB radiation power by a factor of 10 as compared with the conventional criterion. The proposed criterion also reduces OOB radiation while increasing the average sum rate by optimizing the weight factor for the OOB radiation. As a result, the proposed criterion achieves approximately 1.3 times higher average sum rates than an error-based criterion. On the other hand, as compared with a sum rate based criterion, the throughput on each subcarrier shows less variation which reduces the number of link adaptation options needed although the average sum rate of the proposed criterion is smaller.

This paper presents a joint decoding scheme for the overloaded multiple input multiple output (MIMO)-orthogonal frequency division multiplexing (OFDM) system. In the overloaded MIMO system, the number of receive antenna elements is less than that of transmit antenna elements. It has been shown that under the overloaded condition the performance of joint detection deteriorates while diversity reduces the amount of performance degradation caused by signal multiplexing. Thus, this paper proposes a maximum likelihood joint decoding scheme of block coded signals in the overloaded MIMO-OFDM system. The performance of joint decoding over Rayleigh fading channels is evaluated through simulation and experiments. The simulation shows that the diversity through block coding prevents any performance degradation in the joint decoding of 2 Hamming coded signal streams. However, there are differences between numerical results obtained through computer simulation and experiments owing to channel estimation errors.

In a multiple-input multiple-output (MIMO) system, maximum likelihood detection (MLD) is the best demodulation scheme if no a priori information is available. However, the complexity of MLD increases exponentially with the number of signal streams. Therefore, various demodulation schemes with less complexity have been proposed and some of those schemes show performance close to that of MLD. One kind of those schemes uses a Gibbs sampling (GS) algorithm. GS MIMO detection that combines feedback from turbo decoding has been proposed. In this scheme, the accuracy of GS MIMO detection is improved by feeding back loglikelihood ratios (LLRs) from a turbo decoder. In this paper, GS MIMO detection using only feedback LLRs from a turbo decoder is proposed. Through extrinsic information transfer (EXIT) chart analysis, it is shown that the EXIT curves with and without metrics calculated from received signals overlap as the feedback LLR values increase. Therefore, the proposed scheme calculates the metrics from received signals only for the first GS MIMO detection and the selection probabilities of GS MIMO detection in the following iterations are calculated based only on the LLRs from turbo decoders. Numerical results obtained through computer simulation show that the performance of proposed GS turbo MIMO detection is worse than that of conventional GS turbo MIMO detection when the number of GS iterations is small. However the performance improves as the number of GS iterations increases. When the number of GS iterations is 30 or more, the bit error rate (BER) performance of the proposed scheme is equivalent to that of the conventional scheme. Therefore, the proposed scheme can reduce the computational complexity of selection probability calculation in GS turbo MIMO detection.

A channel equalization technique on a time division duplex CDMA/TDMA system for wireless multimedia networks is investigated, and the bit error rate performance of the system is theoretically analyzed. The assumed network connects mobile terminals to a node of ATM based high speed LAN through a radio central unit. Only human interface facilities are implemented into the terminal so that users access integrated services through the node of the network. The uplink (from a mobile terminal to a radio central unit) employs a CDMA scheme to transmit human interface signals and the downlink employs a TDMA scheme to transmit display interface signals. Both the CDMA and the TDMA signals occupy the same frequency band. To mitigate bit error rate degradation due to fading, the radio central unit estimates the impulse response of the channel from the received CDMA signals and subtracts the replica signal to cancel the major intersymbol interference (ISI) component. Numerical results using the Nakagami-m fading model and recent propagation measurements show that the proposed TPC technique compensates the fading attenuation and the proposed CEQ cancels the major ISI component. The bit error rate performance of the downlink with the proposed CEQ is superior to that with the DFE by 12dB of the symbol SNR at the BER=10-6 over a specular channel, and the system with the proposed CEQ achieves a BER=10-6 at the symbol SNR=12dB. Furthermore, the channel equalizer is implemented without increases in complexity of the terminal because all the processing on the equalization is carried out only in the radio central unit.