In this paper, an energy detection scheme based on the pseudo bit error rate (BER) estimation is investigated. In the proposed scheme, the basic strategy is to use the pseudo BER to estimate the signal-noise-ratio (SNR) of primary signal for energy detection at the cognitive users. Threshold and sensing length are updated with the estimated SNR afterward. We have shown that this modified energy detector outperforms traditional energy detector with the noise uncertainty and reliably alleviate the SNR wall effect.

We consider secret key agreement based on radio propagation characteristics in a two-way relaying system where two legitimate parties named Alice and Bob communicate with each other via a trusted relay. In this system, Alice and Bob share secret keys generated from measured radio propagation characteristics with the help of the relay in the presence of an eavesdropper. We present four secret key agreement schemes: an amplify-and-forward (AF) scheme, a signal-combining amplify-and-forward (SC-AF) scheme, a multiple-access amplify-and-forward (MA-AF) scheme, and an amplify-and-forward with artificial noise (AF with AN) scheme. In these schemes, the basic idea is to share the effective fading coefficients between Alice and Bob and use them as the source of the secret keys. The AF scheme is based on a conventional amplify-and-forward two-way relaying method, whereas in the SC-AF scheme and the MA-AF scheme, we apply the idea of physical-layer network coding to the secret key agreement. In the AF with AN scheme, the relay transmits artificially generated noise, as well as channel information signal, in order to conceal the latter. Simulation results show that the MA-AF scheme outperforms the other schemes in Rayleigh fading channels, whereas the AF with AN scheme is suitable for Rician fading channels.

We consider secret key agreement based on radio propagation characteristics in two-way relaying system where two legitimate parties named Alice and Bob communicate with each other via a relay. In this system, Alice and Bob share secret keys generated from their radio propagation characteristics by the help of the relay. In this paper, we present two secret key agreement schemes: an amplify-and-forward (AF) scheme and a multiple-access amplify-and-forward (MA-AF) scheme. In both of the schemes, the basic idea is to share the effective fading coefficient between Alice and Bob and use it as the source of secret keys. The AF scheme is based on a conventional amplify-and-forward two-way relaying method, while the MA-AF scheme utilizes the inherent combining of signals provided by simultaneous transmissions over a multiple-access channel in order to share the secret keys more securely efficiently. We analyze eavesdropping strategy in terms of eavesdropper's location and show that if the eavesdropper is located near the relay and can receive signals from the relay without multipath fading and noise, the AF scheme is not secure. Simulation results show that the MA-AF scheme is more secure and efficient than the AF scheme.

We consider information reconciliation in secret key agreement from a fading channel where two legitimate parties utilize channel estimates of their fading channel as correlated random variables to generate a secret key. The information reconciliation is a process of correcting the discrepancies between the legitimate parties' keys by public discussion in the presence of eavesdropper, where the amount of the information revealed in the public discussion is needed to be as little as possible. In this paper, we propose information reconciliation protocols using the reliability values of channel estimates in order to correct errors effectively. The proposed information reconciliation protocols are modified versions of a protocol used in quantum key distribution called sliced error correction using Cascade. Simulation results show that the proposed protocols can correct errors with less the number of disclosed bits and less the number of communications than the conventional sliced error correction using Cascade.

As a countermeasure for eavesdroppers in wireless communication, a secret key agreement scheme using a variable directional antenna named ESPAR antenna was developed. In this paper, we describe an improved key generation method using multi-level quantization and parity check in the secret key agreement scheme. It enhances key agreement probability in legitimate parties efficiently without reducing the security. There was a trade-off between the key agreement probability and the security. Simulation result shows that the proposed method realizes higher security than the conventional one in the key agreement system using ESPAR antenna.

The paper proposes a new multiuser interference cancellation system in MC-CDMA transmissions. In the proposed system, spreading codes having sinusoidal waveforms are used. It is shown that, by using the spreading scheme, the spreading and multiplexing processes are accomplished by IFFT in the transmitter and FFT in the receiver. Using the unique property of the spreading scheme, the effect of the multiuser interference due to multipath can be limited only between specific combinations of the spreading codes, therefore MLSE by the Viterbi algorithm can reasonably be applied. Simulation results indicate that significant improvement is obtained by the proposed system when compared with conventional MC-CDMA systems using the Walsh codes and MMSEC (minimum mean sequence error combining).

This paper proposes a new parallel transmission scheme using cyclic codes generated from a modified M-sequence, which is biased with constant direct current (dc) on a M-sequence and inserted guard chips before and after this biased M-sequence. In the proposed system, the orthogonality of codes is kept not only between parallel transmission channels but also between direct and delayed waves within the guard chips. In this paper we apply a code and chip timing joint recovery scheme based on the feature of the guard chips of a modified M-sequence and evaluate the bit error rate performances by computer simulations. As a result, the proposed system can transmit several Mbps with better quality in comparison with conventional multi-code code division multiplexing (CDM) transmission.

This paper proposes a new type of greatly reduced-complexity sequence estimation in which stronger power delay waves are selected and combined. The proposed method has a similar BER performance and a greatly reduced complexity than MLSE. A high-bit-rate transmission system with a rate of 10 Mbit/s, /spl pi//4-QPSK/TDM for multimedia data transmission in land mobile communications is implemented. In adopting a 4-channel TDM, a user bit rate of 2 Mbit/s can be achieved. The system can compensate for delayed waves of up to 1.8 /spl mu/sec and sustains a maximum Doppler frequency (f/sub d/) of 240 Hz.

Co-channel interference disturbs the spectral efficiency in cellular radio systems. A co-channel interference reduction method using a constant modulus algorithm (CMA) adaptive array antenna is proposed. When we apply an adaptive array antenna to reduce co-channel interference, a co-channel interference signal may be captured. In this method, the replica of the interference signal is generated and then eliminated from the received signal, and thus the desired signal can be obtained. Computer simulation results of GMSK modulation confirm that a CMA adaptive array antenna that uses this method can reduce co-channel interference more than a conventional CMA adaptive array antenna.

Field and laboratory experiments of a slow frequency hopping/16-ary quadrature amplitude modulation (SFH-16QAM) system for land mobile communications are presented. For interference diversity with high quality and high capacity on 16QAM, the system uses an FH scheme with a interference immunity decoding to further enhance immunity from co-channel interference, The laboratory experimental results show the effectiveness for reducing the bit error rate (BER) under strong interference. Field experiments have been conducted to demonstrate the BER performance in suburban Tokyo. The results show that the obtained BER is close to that of laboratory experiments and that the system can provide better performance than a system without FH.

This paper proposes a slow frequency hopping (FH) system using 16-ary quadrature amplitude modulation (QAM) for land mobile communications. The system uses pilot-symbol-assisted QAM for high spectral efficiency and uses orthogonal slow FH and forward error correction (FEC) with symbol interleaving for high-quality transmission over frequency-selective fading channels. The proposed system also uses an interference-detection-assisted decoding scheme to further enhance immunity from co-channel interference. Computer simulation shows that the proposed system can provide high-quality transmission over channels with frequency-selective fading and co-channel interference. It also shows that the interference-detection-assisted decoding scheme is especially effective for reducing the bit error rate (BER) when the interference level for each frequency of FH is dispersed.

This paper proposes a complexity-reduced decision feedback equalization (DFE) scheme for 16-ary quadrature amplitude modulation (16 QAM) over frequency-selective fading channels. The proposed DFE uses bidirectional equalizing (BDE), and space diversity combining (SDC) to achieve high-spectral-efficiency, high-quality data transmission. To reduce the amount of computation of both BDE and SDC for 16 QAM, this paper proposes a tap gain interpolation scheme, new selection schemes of both the equalizing direction of the BDE and the selection of diversity branch. Using these techniques, a 16 QAM/time division multiple access (TDMA) system with 448 ksymbol/s is designed. The system performance is evaluated by computer simulation, assuming a maximum delay of 13 msec. The results confirm that the proposed system is effective and also simple for high-speed data transmission in land mobile communications.

We developed an experimental equalizer for wide-band 16 QAM/TDMA in land mobile communications that works under frequency-selective fading environments, and performed an laboratory experiment. Frame synchronization is important for an equalizer to work properly. In conventional frame synchronization, false synchronization due to delayed-wave occurs under non-minimum-phase condition, leading to degraded performance. We propose a new frame synchronization method that examines the direct-wave and delayed-wave signal power ratio, and even under non-minimum-phase condition, can hold synchronization to the direct-wave, to maintain the equalizer performance. BER performance in the experiment is improved about 10 times at Eb/N0 equal to 20 dB with delay of 3 to 5 symbols in the maximum Doppler-frequency equal to 40 Hz.

Japan's CS spacecraft was launched successfully in December 1977 and placed at 135°E longitude in a geostationary orbit. On-orbit performance of the satellite was verified quite well with the exception of two sets ofK-band transponders. Substantial communication experiments have been conducted for more than half a year starting in May, 1978. In the satellite loop-back configuration at the Main Fixed Earth Station, 1872 and 972 channels of FDM-FM telephony signals can be transmitted through anyK- andC-band transponder, respectively, with signal quality better than 50 dB inS/(N + D). The quality of FM-TV signals is also good (more than 59 dB in unweightedS/NatK-band). Basic transmission characteristics of PCM-PSK signals with various modulation parameters were measured with good results. However, it became clear that spin modulation effects must be considered for estimating the BER performances, especially atK-band. DPCM-TV signal transmission tests were also conducted with satisfactory results. PCM-TDMA systems were verified to work well atK-band, including functions such as frame and burst synchronization, initial acquisition and so forth. Diversity switching functions will be tested in a later experimental phase. In propagation experiments, rainfall attenuation, excess noise temperature due to rain, and effects of the ionosphere have been studied. Although insufficient data have been accumulated, some estimation of the necessary link margin against the rainfall attenuation atK-band was obtained. In general, confidence has been attained that some CS-like system withK-band links will be suitable for practical use. The experiments are now being conducted and will continue through the nominal mission period of 3 years.