A variety of mobile data communications services based on cellular phones and the PHS have recently been developed and used widely. Currently, the maximum transmission rate is limited to 64kbps in PHS public mobile data communications services. It is desired that higher transmission rate be supported to meet the requirements of mobile multimedia applications.
In order to achieve high-speed mobile data communications, the next generation mobile communications system called IMT-2000 has been extensively studied and standardized, but the widespread implementation of the IMT-2000 will require a huge cost and a bit long. On the other hand, wireless LAN systems supporting transmission rate higher than 1Mbit/s have already been developed. However, the mobility of a wireless LAN user is normally limited to indoor use.
Communications Research Laboratory (CRL) of Japanese MPT has therefore developed a 384kbit/s - PHS experimental system based on existing standards, infrastructures, and devices in order to provide a transmission rate suitable for IMT-2000 services in vehicular environments. This system can provide a transmission rate of 384kbit/s by using the 64kbit/s PHS data communications protocol, which is known as the PIAFS (PHS Internet Access Forum Standard). The PPP (Point-to-Point Protocol) Multilink protocol is used to bundle six 64kbit/s PHS to achieve 384kbit/s transmission rate. The PIAFS and the PPP Multilink protocol in the experimental system are used for TCP/IP transmission, which is not only suitable for multimedia services transmission but is also in wide use.
CRL conducted transmission experiments not only in laboratory environments but also in field environments to evaluate performances of such a high bit rate transmission. CRL experimental system can provide excellent performance at low cost, because it uses existing standards and commercially available hardware and software. Throughput performance of the system was evaluated using FTP under various radio propagation environments.
Evaluation results of file transfer indicated that the system can achieve high average of throughput if the radio propagation environment is stable in both outdoor and indoor environments. However, if the radio propagation environment is unstable such as the case where fluctuation of multipaths fading exist, and if many frame errors occur in burst, the roundtrip time in TCP/IP level becomes longer and timeout retransmissions will be triggered. It seems that one of the problems is the fact that both TCP/IP and PPP Multilink protocol are not designed to address the problem of frequent packet loss on wireless links. CRL is thinking that further study is necessary for optimizing TCP/IP and PPP Multilink protocol for wireless environment.

Fig. 1 384-kbps high-speed PHS experimental system

Fig. 2 Example of PHS time slots