Toshihisa Nakai

Error resilient video coding by dynamic replacing of reference pictures

We show the performance of the proposed error resilient video coding system, which dynamically replaces reference pictures in inter-frame coding according to the acknowledgement signals from the decoder. Scalability to the quality of the network is a feature of the system. A poor network condition in its way results in a certain picture quality and a good network condition enhances the picture quality. Two types of signaling, ACK and NACK, are evaluated. The evaluation includes the influences of the backward channel errors as well as those of the forward channel errors. ACK signaling outperforms NACK signaling especially under the existence of the backward channel errors which are highly correlated with the forward channel errors.

Adaptive equalizer for digital cellular radio

The authors study the performance of the decision feedback equalizer (DFE) for application to the US digital cellular telephone system. The performance of the DFE is very sensitive to the delay interval between two rays and considerably degrades around a delay of 0.2T, where T is symbol duration. To improve the performance, the authors propose to utilize the Coded Digital Verification Color Code (CDVCC) part at the middle of the TDMA (time division multiple access) frame as a training sequence of the DFE. A bit error rate below 3% at all delays up to 1T can be achieved when the average E/sub b//N/sub 0/ is 22 dB and the fading frequency is 80 Hz.<<ETX>>

Error resilient video coding controlled by backward channel signaling

We have proposed an error resilient video coding system, which dynamically replaces reference pictures in inter-frame coding according to the backward channel signaling. This system can prevent the temporal error propagation, because the encoder would not use the erred picture as the reference picture. This system has two modes, one is ACK mode and the other is NACK mode. ACK mode is effective in more erroneous condition, and NACK mode is effective in less erroneous condition. In this paper, we explain both modes and focus on the mode switching mechanism according to the error condition on the network so that the optimal performance can be achieved in any error conditions by the proposed system. We show the performance of our proposed system compared with the previous version of ITU-T H.263, which did not have a reference picture selection mode, by a computer simulation as well.

Reliable broadcast message authentication in wireless sensor networks

Due to the low-cost nature of sensor network nodes, we cannot generally assume the availability of a high-performance CPU and tamper-resistant hardware. Firstly, we propose a reliable broadcast message authentication working under the above-mentioned circumstances. The proposed scheme, although based on symmetric cryptographic primitives, is secure against anyone who knew the message authentication key as well as the malicious router nodes in multi-hop networks. The proposed scheme consists of three steps; (i) reliable broadcast of a message, (ii) legitimate acknowledgments from all the nodes in the network, and (iii) disclosure of the message authentication key. Secondly, we propose a way to reduce the amount of the stored information until the disclosure of the key, in which the server transmits the message integrity code of a message before transmitting the message. Finally, we consider the characteristic and the security issues of the proposed schemes.

A design consideration on TDMA cellular radio

A time-division multiple access (TDMA) cellular system using QPSK (quadrature phase shift keying) with coherent detection is studied. The required guard time between TDMA bursts and the effect of the unbalanced characteristics of an IF filter on a detected signal are analyzed. An attempt is made to minimize the guard time between TDMA burst signals using a window which suppresses the signal at both edges. As a result, it was found that the required guard time can be reduced to a 2.3 symbol duration in a 3-km radius cell. If a conventional IF filter (which is designed for an analog terminal) is adapted to a digital terminal, in cases of 1 kHz variation of the center frequency intersymbol interference caused by the IF filter would be about -20 dB. It is concluded that more sophisticated IF filters are needed for digital cellular terminals are needed than for analog cellular terminals.<<ETX>>

A decision feedback equalizer with a frequency offset compensating circuit for digital cellular radio

The performance of a decision feedback equalizer (DFE) with a frequency offset compensating circuit for the North American digital cellular system (NADC) is presented. The carrier frequency offset at the equalizers output deteriorates performance of the DFF. Using the unit upper triangular matrix diagonal matrix decomposition type recursive least squares algorithm as an adaptive algorithm, a frequency offset compensating circuit, which is composed of a phase locked loop (PLL), is used. Simultaneous control of the DFE and PLL is effectively utilized to improve the performance. A BER below 3% for all delays up to one symbol period can be achieved when the average E/sub b//N/sub 0/ is 20 dB, the fading frequency is 80 Hz and the carrier frequency offset is 200 Hz in the simulation results. The authors assume the two rays equal power multipath Rayleigh fading model, the same as the NADC specifications. The influence of quantization in the calculation is described. The mantissa-field occupies 8 bits and the exponent-field occupies 6 bits.<<ETX>>

Initial common secret key sharing using random plaintexts for short-range wireless communications

With the advent of radio frequency-based remote controls, we have to consider new security threats of eavesdropping and unauthorized controls in exchange for user-friendliness. Against these attacks, it is effective to encrypt and authenticate control commands. However, it will be a challenge to design how we establish a common secret key for encryption and authentication between a remote control and a controlled device. We propose new initial common key sharing protocol, which extends the over-the-air key transport in plaintext with suppressing the transmission power. Our proposed protocol increases the amount of transmission data by sharing the secret data to multiple random plaintexts, and makes it difficult for the eavesdroppers that are not located near the transmitter to obtain secret data by increasing the probability of the transmission error at the eavesdroppers. We evaluate the security level of our proposed protocol based on IEEE 802.15.4 specification, and show the relationships among the amount of transmission data, the bit error rate at eavesdroppers, and the number of trials to find the secret data through a brute force attack.