This invention relates to time division multiple-access (TDMA) radiotelephone systems.
In radiotelephone systems, such as TDMA mobile phone systems, automatic gain control (AGC) is needed in the base stations to adjust for variations in amplitude of the incoming radio signals. In most systems, this is accomplished by detecting the signal at a baseband or intermediate frequency (IF) level and feeding it back to an attenuator at the IF portion of the station. (See, for example, U.S. Pat. No. 4,829,593 issued to Hara which utilizes a feedback loop for AGC, and also employs a feed forward portion to increase the dynamic range of a digital-to-analog converter.) While such an approach is generally adequate, certain systems, such as the Group Speciale Mobile (GSM), require a fast access time (generally less than 30 .mu.sec) and, therefore, a fast automatic gain control which cannot be easily achieved with standard feedback circuits. The problem is exacerbated by the fact that the burst for a random access channel (RACH) has a time interval (typically less than 325 .mu.sec) which is less than a time slot interval (generally approximately 577 .mu.sec) and can appear at various positions in the time slot. Thus, it is difficult to detect the presence of a RACH burst and still provide a fast AGC.
Several proposals have been made to provide a fast AGC for time division multiplex access systems. For example, U.S. Pat. No. 5,184,349 issued to Riotdan suggests a combination of feedback and feed forward circuitry where the input signal is detected by producing a voltage proportional to the log of the amplitude. The resulting signal is digitized, and then applied to a control processor which adjusts a series of attenuators. The presence of a RACH burst is determined by the detected signal passing a certain threshold value. European Patent Application No. 0 372 759 A2 discloses a circuit where the input is detected at an IF stage, digitized, and used to control a gate for passing an AGC signal in a feedback loop to an IF amplifier. Again, the presence of a RACH burst is determined by passing a threshold value. The AGC value established for a RACH burst is also used as a preliminary value for the traffic bursts.