1. Field of the Invention
The present invention generally relates to communication systems. More particularly, the invention generally relates to controlling the signal-to-interference ratio (“SIR”) in a multi-data-rate communication system.
2. Background Information
Wireless communication systems (e.g., cellular telephone systems) may suffer from various channel impairments, such as fading, dropped calls, etc. To ameliorate the effects of such impairments, in the face of such impairments some communication systems strive to guarantee a prescribed quality of service (“QoS”). At least one metric directly related to QoS is the received signal-to-interference ratio (“SIR”). A high SIR value means that the power of the data signal is relatively high as compared to the average power of interference. Interference may stem from nearby cell phone transmissions and other sources. A low SIR value means that the average power of interference is relatively high compared to the power of the data signal.
A high SIR is generally desirable to transmit data error-free across a wireless medium. A high SIR, however, requires a higher transmit power level associated with the data to be transmitted than would be the case for a low SIR. That is, increasing transmission power leads to an increase in SIR. Increasing the transmission power of a cell phone may disadvantageously cause that phone's transmission to interfere with the transmissions of nearby cell phones. In general, therefore, it is desirable to have a cell phone transmit at a power level that is minimally sufficient to support the desired SIR. In some conventional cell phone systems, this power control may be implemented by each phone determining the number of transmission errors it is experiencing per unit of time. If the phone is experiencing too many errors, the phone may submit a request to the base station (i.e., cell tower) to which it is currently associated to step up transmission power. The cell tower may contain the centralized logic to balance the power load of its associated cell phones. If acceptable, the cell tower may grant a phone's request for more power. In response, the tower may step up the transmitted power level. On the other hand, if the phone is not experiencing too many errors, the phone may request permission from its tower to step down its transmission power. In this manner, a control loop is formed in an attempt to control the power level of each phone.
The aforementioned system generally works well when the communication data rate is constant. A problem may occur in communication systems in which the data rate may vary. Some systems employ variable block sizes and thus variable data rates. That is, the number of bits comprising each data block may vary, thereby resulting in variable data rates. The transmission power required to maintain a minimally sufficient SIR may be different between the different data rates. If the cell phone is currently transmitting at a particular data rate, the power level will automatically be stepped down to a minimum level as described above. However, the phone may then suddenly desire to jump to a higher data rate (e.g., the user may start speaking after a prolonged period of silence). At that point, the transmission power level of the phone may be too low to support the higher needed data rate for a minimally acceptable SIR. As a result, data may not be received correctly and/or the call may be dropped altogether. A solution to this problem is desirable.