Toshiharu Kojima

A study of SLM PAPR reduction of OFDM signals without side information

Selected mapping (SLM) is a well-known technique to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM). SLM generally requires the side information for each OFDM symbol, which leads to decrease the transmitting efficiency. In addition, false side information detection degrades the bit error rate performance significantly. In this paper, we propose a novel SLM scheme without any side information by devising a phase rotation and simple scrambler detection in the receiver. Computer simulation results show that the proposed scheme achieves excellent performances in terms of both PAPR reduction and BER.

A fast acquisition AFC for helicopter satellite communications

In helicopter satellite communications, removal of time-varying Doppler shift due to helicopter maneuver is one of the most important issues. The conventional automatic frequency control (AFC) scheme for helicopter satellite communications removes Doppler shift exactly. However, its acquisition time is extremely long. This is caused by a delay element in the feed-back loop. In this paper, we propose a novel AFC scheme for helicopter satellite communications to reduce the acquisition time. The proposed AFC scheme consists of a feed-back loop and a feed-forward loop. This configuration excludes the delay element from the feed-back loop. Results of computer simulation show that the proposed AFC scheme significantly reduces its acquisition time.

A novel SLM PAPR reduction of OFDM signals without side information

Selected mapping (SLM) is a well-known technique to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM). SLM generally requires the side information for each OFDM symbol, which leads to decrease the transmitting efficiency. In addition, false side information detection degrades the bit error rate (BER) performance significantly. In this paper, we propose a novel SLM scheme without any side information by devising a phase rotation and simple scrambler detection in the receiver. Computer simulation results show that the proposed scheme achieves excellent performances in terms of both PAPR reduction and BER.

A novel AFC scheme with time diversity combining for helicopter satellite communications

In helicopter satellite communications, the periodic blockage of received signal and the carrier frequency fluctuation are the most important problems. In this paper, we propose a novel automatic frequency controller (AFC) scheme with time diversity combining. The proposed AFC generates the frequency error signal based on the time diversity combined signal. The signal-to-noise power ratio (SNR) of the frequency error signal is therefore prevented from falling to zero even in the condition of signal blockage. The proposed AFC thus tracks and removes the carrier frequency fluctuation due to Doppler shift more exactly than the conventional one. Results of computer simulation show that the proposed AFC improves the SNR of the frequency error signal significantly. It is meant that the proposed AFC also improves both of the acquisition characteristics and jitter performance even in the severe environments of the helicopter satellite communications.

An initial acquisition scheme for Walsh-Hadamard code division multiplexing

This paper addresses the problem of initial acquisition in code division multiplexing employing Walsh-Hadamard code (WHCDM). Unlike pseudo-noise codes used in direct-sequence spread spectrum communications, Walsh-Hadamard code has many side lobes in its autocorrelation function. Well-known initial acquisition schemes for direct-sequence spread spectrum communications, therefore, cannot be applied to WHCDM. In order to solve this problem, we propose an initial acquisition scheme for WHCDM with a novel approach. It detects the symbol timing on the basis of not the autocorrelation function of Walsh-Hadamard code but the energy variance of the signal after demultiplexing. The results of computer simulation confirm that the proposed scheme provides rapid acquisition.

An improved time diversity combining for helicopter satellite communications

Periodic blockage of the received signal due to rotor blades is one of the most important problems in helicopter satellite communications. Time diversity is a promising method in order to overcome the periodic blockage. In this paper, a novel time diversity combining scheme is proposed. It performs maximal ratio combining on the basis of the channel blockage information estimated with a pseudo bit error rate. The performance of the proposed scheme is evaluated by the computer simulation. The results show that the proposed scheme remarkably improves the bit error rate performance of the conventional one employing equal gain combining.

Performance of Walsh-Hadamard code division multiplexing in helicopter satellite communications

One of the most important problems in helicopter satellite communications is the periodic blockage of the received signal caused by rotor blades. Code division multiplexing (CDM) has an implicit time diversity effect overcoming the periodic blockage. Performance of CDM employing Walsh-Hadamard code (WHCDM) under the periodic blockage environment is investigated in this paper. First, we show that a simple WHCDM scheme has the implicit time diversity effect by computer simulation. Next, a WHCDM scheme combined with explicit time diversity is proposed. The proposed scheme improves the bit error rate performance of the conventional time diversity scheme in high speed data transmission.

A hybrid Walsh-Hadamard code division multiplexing for helicopter satellite communications

In helicopter satellite communications, periodic channel blockage due to rotor blades and Doppler shift caused by helicopter maneuvers are major issues. Code division multiplexing employing Walsh-Hadamard code (WHCDM) is a promising solution for the periodic blockage. However, automatic frequency control (AFC) removing Doppler shift limits the multiplexing factor of WHCDM to less than its maximum value because of its frequency discrimination performance. This limitation reduces the spectral efficiency in the conventional WHCDM. In this paper, we propose a novel WHCDM achieving the maximum spectral efficiency with the multiplexing factor of its maximum value minus one. In the proposed WHCDM, one Walsh-Hadamard code selected by information data is excluded from the transmitted signal. Results of computer simulation show that the bit error rate performance of the proposed WHCDM is slightly better than that of the conventional one in the higher Eb/N0 region.

Adaptive candidate selection scheme in QRM-MLD algorithm for MIMO detection

This paper proposes an adaptive candidate selection scheme in the QRM-MLD algorithm for MIMO detection. The QRM-MLD is a near-ML detection algorithm which can achieve a tradeoff between the BER performance and the computational complexity in the MIMO systems. In this paper, we adopt an adaptive candidate selection scheme into the QRM-MLD. First, similarly to the conventional QRM-MLD, the proposed detection applies a fixed number of the survived branches to achieve a near-ML performance. Next, in order to evaluate the reliability of the survived branches in each detection layer, we introduce a ratio function of the path metric to the smallest path metric among the survived branches. The survived branch with lower reliability has less children nodes as the candidates in the next detection layer, which can avoid a large amount of the path metric evaluations and sorting. Hence, the complexity of the proposed detection should be low. Numerical results exhibit that the proposed scheme achieves the near-ML performance with lower complexity compared to the conventional QRM-MLD.

A study on code synchronization for time diversity in helicopter satellite communications

In helicopter satellite communications, the periodic blockage of the channel due to the rotor blades degrades the bit error rate (BER) performance. Although time diversity is attractive in order to overcome the periodic blockage, the combining process in the receiver requires the code synchronization. The pre-detection combining is suitable for time diversity because it is superior to the post-detection combining in terms of the BER performance. However, a sync word cannot be applied to the code synchronization in the pre-detection combining. To solve this problem, we propose a novel code synchronization scheme. The proposed scheme estimates the timing offset between the transmitter and the receiver on the basis of the cross-correlation of all possible pairs of the encoded sequences to combine. Results of computer simulation show that the proposed scheme achieves excellent synchronization performance without sync word even in the severe environment of helicopter satellite communications.