Some mobile radio standards use the TDMA method (Time Division Multiple Access) in order to transmit data. In this case, the two stations each transmit and receive alternately. In order to improve the immunity to interference sources, the mobile radio standards additionally use a so-called FHSS method (Frequency Hopping Spread Spectrum), in which they change the frequency after each transmitted or received packet. One example of a mobile radio standard such as this is Bluetooth, which has a channel bandwidth of 1 MHz and has a total of 79 channels available in the free frequency range that is used from 2.4 to 2.485 GHz. The time in which transmission or reception takes place, a so-called time slot, has a length of at least 625 μs, although data is transmitted or received for only 405 μs. The remaining time can be used to change to the new frequency.
Since the Bluetooth standard is operated in the unlicensed 2.4 GHz ISM frequency band, it is necessary to use the FHSS method that has been mentioned in order to make it possible to compensate for interference caused on a channel by the random frequency changes. As the free ISM frequency range is used to an increasing extent, interference in individual channels is becoming more frequent, however, so that the probability of finding a channel that has already been used after a frequency hop is rapidly rising. This is primarily a result of the new mobile radio standards 802.11b and 802.11g. These use a frequency bandwidth of about 20 MHz, which can lead to interference in a number of the channels that are used by Bluetooth. For this reason, so-called adaptive frequency hopping (AFH) has been developed as a further development, which allows Bluetooth appliances to reduce the number of channels used and to use the number of usable channels selectively.
However, to do this, a statement is required on the quality of each individual one of the 79 channels in order then to make it possible to make a selection of the usable channels. The statement on the quality is produced with the aid of an RSSI measurement (Receive Signal Strength Indicator), which indicates the signal strength in the measured frequency channel at the time of the measurement. There are a number of possible ways to carry out a measurement such as this.
One concept provides for RSSI measurements to be carried out rather than normal data transmission in the unused channels during the time slots. During a measurement such as this, it is necessary for no useful data to be transmitted in the measured channels during the measurement. However, a method such as this considerably reduces the available data rate while at the same time increasing the power consumption, since the oscillator and the phase-locked loop always have to pass through an initial transient process to a new channel.
One alternative concept is to use the 220 μs that has been mentioned at the end of each time slot. No useful data is transmitted during this time, so that an RSSI measurement can be carried out in another channel without having to take account of any useful data stream that may be present. However, the time which is available for the initial transient process for the phase locked loop when switching to a new channel, for the RSSI measurement and the renewed initial transient process for switching to a further channel for transmitting or receiving useful data is in this case very short. This leads to considerably increased power consumption and to a more complex, and thus more expensive, phase locked loop.
An object of the invention is therefore to provide a simple circuit arrangement as well as a method for quality measurement in a frequency channel.