This paper presents a novel slotted ALOHA that has an enhanced capture effect even if heavy traffic is offered. Capture event success probability is improved by controlling the probability of high power transmission permission. This makes it possible to capture the other packets if only one station increases its transmission power. This control method is the key issue of this paper since the capture effect is one of the most promising ways to achieve high throughput without interference (collision) detection/carrier sense. We modified the conventional slotted ALOHA to yield a practical and useful packet transmission scheme. This modified slotted ALOHA has a finite buffer that stores backlog packets. Heavy traffic may cause buffer overflow which implies packet loss. When a station of a wireless LAN or another data terminal intends to send a packet, its transmission power is controlled by the number of stations in an area that covers the same cell shaped by an access point. As a result, only one of the packets can be successfully sent by capturing the other packets. This capture effect is improved significantly by the proposed scheme. This modified and enhanced slotted ALOHA achieves the maximum throughput of 0.74, which is higher than the 0.47 achieved by transmission power control in a previous paper. This is double the throughput of the original slotted ALOHA.

This paper proposes a maximal ratio combining (MRC) frequency diversity automatic-repeat-request (ARQ) scheme which is suitable for high data rate orthogonal frequency division multiplexing (OFDM) signals. The proposed scheme regularly changes the subcarrier assignment pattern at every retransmission. This scheme sharply reduces the number of ARQ retransmissions in slow fading environments by virtue of the frequency diversity effect. Computer simulation results show that the proposed scheme reduces the number of retransmissions to about 3 at the accumulative correct packet reception rate (ACPRR) of 0.99 while the conventional schemes lead to more than 10.

The Personal Handy Phone System (PHS) employs autonomous distributed control and a dynamic channel allocation scheme. A base station does not have many traffic channels because its cell radius is only a few hundred meters. However, a base station can experience heavy traffic. In this case, the blocking probability will increase. Congestion control can be employed, but it can never solve this problem fundamentally. We propose a dynamic cell assignment scheme and evaluate the improvement in system performance by computer simulations and theoretically. The scheme overlaps the base stations in and adjacent to heavy traffic areas to equalize base station loading. Each base station broadcasts a congestion factor and each mobile station dynamically selects a base station to get a traffic channel. This dynamic cell assignment technique makes it possible to improve the blocking probability characteristics.

The paper proposes an autonomous distributed TDMA frame synchronization technique for street microcellular systems. With conventional autonomous distributed control, it is difficult to realize high accuracy synchronization because phase drift happens in a steady state. It is bothersome to preset a delay equalizer to eliminate phase drift. The proposed technique realizes high speed and high accuracy inter-base-station frame synchronization and causes no phase drift in the steady state. Computer simulation results confirm the new technique achieves stable TDMA frame synchronization.

This paper describes the performance of an outage free SDH (synchronous digital hierarchy) interface 256 QAM modem. An outage free DMR (digital microwave radio) is achieved by a high coding gain trellis coded SPORT QAM and super multicarrier modem. A new frame format and its associated circuits connect the outage free modem to the SDH interface. The newly designed VLSI's are key devices for developing the modem. As an overall modem performance, BER (bit error rate) characteristics and equipment signatures are presented. A coding gain of 4.7dB (at a BER of 10/sup /spl minus/4/) is obtained using SPORT 256 QAM and Viterbi decoding. This coding gain is realized by trellis coding as well as by increasing of transmission rate. Roll-off factor is decreased to maintain the same frequency occupation and modulation level as ordinary SDH 256 QAM modem.>