In the pan-European digital cellular radio system known as Groupe Speciale Mobile (GSM) each of the RF channels is divided into timeslots of approximately 0.577 ms duration. The modulating bit rate for a GSM carrier is 270.838 kbit/s which means that the timeslot 2 in FIG. 1 corresponds to 156.25 bit durations. During this time period the RF carrier is modulated by a data stream, the extent of which is termed a "burst". In other words, a burst represents the physical content of a timeslot. The timeslots 2 are grouped together in sets of eight consecutive timeslots as one TDMA frame 1, as illustrated in FIG. 1. (TDMA is an acronym for time division multiple access). A physical channel is defined by specifying both a RF channel (or, in the case of frequency hopping, a sequence of RF channels) and a TDMA frame timeslot number. Hence for a given RF channel the system has available to it eight physical channels.
There are two main types of logical channels within the GSM system known respectively as traffic channels (TCHs) and control channels (CCHs). The traffic channels are intended primarily to carry encoded speech or user data, whereas the control channels carry signalling and synchronization data between the base station and the mobile station e.g. mobile telephone. The control channels include data which instructs the mobile station in which timeslots they are to receive bursts i.e. "concerned reception timeslot" and data from which the mobile station derives its timing. Because of the TDMA, the transmission rate of the radio path is very high, the multipath propagation specific for the radio path appears during the reception as a rapid Rayleigh fading of the envelope of the RF signal, and also as interference between the detected bits, in other words, the field strength received by the mobile telephone varies very rapidly. In order to take account of this, it is conceivable that the received signal is the sum of independent Rayleigh-fading signals and signals having a different delay, and the reception can be planned accordingly. Since the multipath propagation causes linear distortions on the channel, a linear receiver is needed in a digital radio-telephone to allow correction of these distortions. In this case, automatic gain control (AGC) has to be used in the receiver amplifiers, aiming at keeping the level of the received signal constant before the Analog-to-Digital Converters (ADC's) 4. In digital radio-telephones, the AGC function is generally accomplished in the same way as in AM or SSB receivers, in which the AGC is accomplished by using an adjustable amplifier 3 controlled with analogue voltage Vc as an intermediate frequency amplifier, which attenuates the signal the more, the greater its amplitude. The characteristics of the actual AGC amplifiers and their positioning in the receiver will not be discussed here. FIG. 2 is a schematic circuit diagram showing AGC in a typical cellular radio receiver system.
In a TDMA system, for instance a GSM system, the dynamic range of the AGC amplifier 3 must be wide, as does the maximal gain, in order to prevent deterioration of the signal/noise ratio as the gain is being reduced. The signal/noise ratio deteriorates because the resolution is not sufficient for pure conversion of a small signal when ADC's are used. As noted above, the signal level is maintained constant before the ADC 4 with the help of an AGC amplifier 3.
Conventionally, the field strength of the received signal is measured and averaged over a specific time span, and thus the mean field strength is calculated. When the required signal level is known, the required gain of the AGC amplifier 3 can be calculated with the aid of the mean field strength.
In a TDMA system this involves a mean calculation of the field strengths of the concerned reception time slots. The mean calculation is typically carried out over a long time span, of the order, for example, of approximately one second. One 4.62 ms frame in GSM has one reception timeslot of a duration of 0.577 ms, as discussed above, which means that the signal power of the concerned reception timeslots are averaged over the course of several frames, and the average obtained is utilised in the AGC. This control method may be sufficient on a traffic channel.
There will be a problem where there is frequency hopping if a frequency identical to a signalling frequency is included in the frequency hopping sequence, because at this frequency the power may be different from the signal power of the other concerned timeslots.
There will also be a problem when a mobile telephone is in a monitoring state, i.e. in which it monitors the paging channel of the base station.
In this situation the gain of the AGC amplifier 3 may be spurious. For example, in a GSM system, a mobile telephone does not have to monitor the paging channel (PCH) continuously--the minimum requirement being to monitor the PCH once in about two seconds. Each time, the telephone monitors this channel for about 20 ms. In fact, the network can instruct the mobile telephone to monitor the PCH more frequently. The field strength of the received signal may however have changed markedly since the previous reception, especially when the mobile telephone is in a rapidly moving vehicle. In this case, the mean calculation of field strength measurements made during the monitoring of the PCH yields a very poor idea of the field strength prevailing since the previous measurement.