1. Field of the Invention
The present invention relates to a transmission rate determination technology. In particular, the invention relates to a transmission rate determination method for changing a transmission rate during communication, and a base station apparatus, a terminal apparatus, and a communication system using the same.
2. Description of the Related Art
In wireless communication systems, transmission channels vary with time. Among technologies for increasing the transmission rate in such channel environment is adaptive modulation scheme. The adaptive modulation scheme controls its modulation mode in accordance with the channel environment. For example, if the channel environment is determined to be unfavorable, a modulation mode of high reliability is used for data transmission. If the channel environment is determined to be favorable, on the other hand, a modulation mode of high information volume is used for data transmission. When a base station apparatus and a terminal apparatus included in a wireless communication system communicate by TDD (Time Division Duplex) scheme, slots are typically transmitted for an up link and a down link alternately in regular order.
Each of the slots contains a preamble to be used for estimating the channel environment, modulation mode information indicating the modulation mode used for transmission, and information symbols. When the transmission channels of TDD scheme are reversible, the base station apparatus receives a slot, detects the C/N (Carrier to Noise Ratio) of the transmission channels, delay spread of the transmission channels and so on from received level information including a received baseband signal and RSSI (Received Signal Strength Indicator), and estimates the channel environment at the next timing of transmission. Besides, based on this estimation result, the base station apparatus selects a modulation mode to be used for transmission (See reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Japanese Patent Application Laid-Open No. 2002-290246.
Under the circumstances, the inventor has become aware of the following problems. When the adaptive modulation processing is performed, the transmission rate ideally becomes suitable for the channel quality. Nevertheless, the actual channel environment is fluctuating due to such reasons as the movement of the terminal apparatus, and suitable modulation modes may differ between when the channel quality is measured and when signals are transmitted actually. In particular, if the channel quality upon actual signal transmission degrades from when measured, the transmission of signals by using the modulation mode determined from the measured transmission quality can cause transmission errors easily.
The information to be communicated between the base station apparatus and the terminal apparatus includes not only data signals but also important information to control communication, such as a disconnection request from the terminal apparatus and an emergency message. Such important information is typically retransmitted when it causes a transmission error because of the communication using a modulation mode unsuitable for the actual channel quality. When the information is of extreme urgency, a larger transmission delay caused by the retransmitting can sometimes affect the control of the entire communication system. The retransmitting of the information also lowers the transmission efficiency.