The invention lies in the electronics field. More specifically, the invention relates to frequency-stabilized circuit configurations, in particular to a transmitting/receiving configuration with a high-frequency section with a reception mixing stage, which receives an RF received signal and converts the RF received signal to an analog IF received signal by down-mixing with an adjustable mixing frequency, and a signal processing circuit. The latter includes an A/D converter which converts the analog IF received signal to a digital received signal, a digital filter which is connected downstream of the A/D converter and emits a filtered digital received signal, and a frequency estimator which receives the filtered digital received signal. Circuit configurations of this type are used in communications terminals (base station and mobile station) in mobile communications systems.
In practice, only a tightly constrained bandwidth range is generally available for the transmission of messages. In order to allow as many messages as possible to be transmitted, it is necessary to utilize the available bandwidth range as efficiently as possible. Firstly, multiple access methods such as time-division multiplex (TDMA, time division multiple access), frequency division multiplex (FDMA, frequency division multiple access), code division multiplex (CDMA, code division multiple access), and space division multiplex (SDMA, space division multiple access), as well as combinations of the multiple access methods allow flexible, requirement-oriented utilization of the available bandwidth. Secondly, best-possible utilization of the available bandwidth range must also be ensured in the hardware.
In mobile radio technology, the available total bandwidth is subdivided into traffic channels with a predetermined channel bandwidth, with a subscriber being assigned a specific traffic channel when dialing into the mobile radio network. The radio-frequency section of the mobile communications terminal (referred to as the mobile station from here on) is tuned to the assigned channel (mid-)frequency by means of the reception mixing stage, and signal components which are outside the channel bandwidth range are removed from the received signal by means of suitable filters (bandpass filters or low-pass filters) in the intermediate frequency (IF) or baseband range.
There is a risk during filtering, that frequency regions of the received signal in which information is carried may be filtered out inadvertently. The reasons for this are as follows:
When the mobile station is moving relative to the stationary base station, the Doppler effect results in a frequency shift between the RF received signal received by the mobile station and the radio signal transmitted by the base station at the predetermined channel frequency. This Doppler frequency shift is transferred by the down-mixing process to the analog IF received signal and to the digital received signal, where it results in a mismatch between these signals and the spectral pass band of the downstream filter.
Furthermore, slow drifts and rapid time fluctuations in the mixing frequency used for down-mixing the RF received signal in the reception mixing stage contribute to undesirable signal losses. Such drifts and fluctuations in the mixing frequency are caused by temperature drifts and phase noise in the oscillator that is used.
It has already been known from the prior art for frequency correction bursts (FCB) to be transmitted at regular time intervals in the radio signal transmitted by the base station. The FCB is searched for in the radio-frequency section of the mobile station using a frequency pattern with a pattern width of, for example, 20 kHz. The FCB can be determined to the accuracy of the pattern width by tuning to that pattern frequency which has the maximum FCB received signal strength. The mixing frequency is then readjusted as a function of the determined pattern frequency. This makes it possible to compensate for relatively slow frequency shifts caused by the Doppler effect and drifts in the mixing frequency.
It has already been known for oscillators with low phase noise to be used in order to reduce rapid frequency fluctuations. However, these have the disadvantage that low-noise oscillators are relatively expensive.
U.S. Pat. No. 5,241,688 describes a circuit for frequency synchronization of a mobile radio receiver. On detection of an FCB, an I/Q decoder in the digital signal processing section of the mobile radio receiver produces a control signal, which is supplied to a local oscillator where it compensates for the frequency offset. This control signal is produced in the I/Q decoder by estimating the signal energy downstream from adaptive bandpass filtering.
The object of the present invention is to provide a circuit configuration which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which is suitable for use in a communications terminal, can be produced economically, and allows good spectral utilization of a traffic channel with a predetermined bandwidth.
With the above and other objects in view there is provided, in accordance with the invention, a circuit configuration, comprising:
a radio-frequency section having a reception mixing stage configured to receive an RF received signal and to convert the RF received signal to an analog IF received signal by down-mixing with an adjustable mixing frequency; and
a signal processing circuit connected to the radio-frequency section and having
an A/D converter for converting the analog IF received signal to a digital received signal;
a digital filter connected to receive the digital received signal from the A/D converter, the digital filter having a given pass band and outputting a filtered digital received signal;
a channel estimator for estimating a transfer function of a radio channel connected to the digital filter; and
a frequency estimator contained in the channel estimator and connected to receive the filtered digital received signal from the digital filter, the frequency estimator continuously determining a first frequency correction control signal representative of a frequency offset between a frequency of the analog IF received signal and a frequency characteristic of the pass band of the digital filter, and the frequency estimator outputting the first frequency correction control signal for readjusting the mixing frequency in the radio-frequency section.
In other words, the first frequency correction control signal, which is required for readjustment of the mixing frequency, is not generated, as is normally done in the prior art, in the radio-frequency section, but is determined by calculation in the frequency estimator.
The frequency estimator is implemented in the channel estimator. The invention is thus dependent only on appropriate programming of the channel estimator (which is required in any case for mobile radio applications), and can thus be implemented economically and in a hardware-efficient manner in the circuit configuration.
The reception mixing stage can down-mix to a frequency range at a reduced frequency, or else directly to baseband (direct down-conversion). The term IF frequency used herein is intended to refer to any frequency below the carrier frequency, including baseband.
The circuit configuration is preferably designed not only as a receiver but also as a transmitter. The signal processing circuit then, furthermore, has a digital modulator and a D/A converter, and the radio-frequency section is equipped with a transmission mixing stage. An analog transmission signal which is produced by the digital modulator and is emitted by the D/A converter is supplied to the radio-frequency section and is converted in the transmission mixing stage, by up-mixing, to an RF transmission signal, using a further mixing frequency which is set as a function of the first frequency correction control signal. The advantages stated with respect to the receiving section of the circuit configuration according to the invention apply equally to the transmission section.
In accordance with an added feature of the invention,
the signal processing circuit further includes a digital modulator and a D/A converter;
the radio-frequency section has a transmission mixing stage; and
an analog transmission signal produced by the digital modulator and output by the D/A converter, is supplied to the radio frequency section and is converted to an RF transmission signal in the transmission mixing stage by up-mixing with a further mixing frequency set in dependence on the first frequency correction control signal.
In accordance with an additional feature of the invention, there is provided an oscillator having a controllable oscillator frequency, the oscillator receiving the first frequency correction control signal for controlling the oscillator frequency, and wherein at least one of the mixing frequency and the further mixing frequency is derived from the controlled oscillator frequency. In other words, the mixing frequency and/or the further mixing frequency is derived from the oscillator frequency.
In accordance with another feature of the invention,
a radio received signal, transmitted by a base station, contains a cyclically recurring frequency correction burst signal component, in the form of a sinusoidal oscillation; and
the frequency estimator estimates the frequency offset by evaluating a signal component of the filtered digital received signal, on which the frequency correction burst signal component of the radio received signal is based. In order, therefore, to determine the frequency offset, a radio signal transmitted by a base station preferably contains a frequency correction burst signal component (FCB), which recurs cyclically, in the form of a sinusoidal oscillation, and the frequency estimator estimates the frequency offset from an evaluation of a signal component of the filtered digital received signal, on which the frequency correction burst signal component (FCB) of the radio signal is based.
The frequency correction burst signal component (FCB) is preferably transmitted by the base station every 10 to 100 ms. The first frequency correction control signal can then be determined at the same rate, that is to say likewise every 10 to 100 ms. This rate is sufficient to correct slow frequency shifts, such as those which result from drift of the oscillator crystal as a result of temperature changes.
With the above and other objects in view there is also provided, in accordance with the invention, a circuit configuration, comprising:
a radio-frequency section with a reception mixing stage connected to receive an RF received signal and converting the RF received signal by downmixing to an analog IF received signal; and
a signal processing circuit having
an A/D converter converting the analog IF received signal to a digital received signal;
a digital filter connected downstream of the A/D converter, the digital filter having a given pass band, and outputting a filtered digital received signal; and
a frequency estimator connected to receive the filtered digital received signal, the frequency estimator continuously determining a second frequency correction control signal representative of a frequency offset between a frequency of the analog IF received signal and a frequency characteristic of the pass band of the digital filter; and
wherein the second frequency correction control signal is used for at least one of a spectral reprocessing of the digital received signal upstream of the digital filter and a readjustment of the pass band of the digital filter.
Similarly to the configuration summarized above, a (second) frequency correction control signal is also determined here by calculation in the frequency estimator, and this signal is representative of the frequency offset between the analog IF received signal and a frequency that is characteristic of the pass band of the digital filter. In contrast to the situation in the first-outlined solution, the second frequency correction control signal is, however, not used for readjustment of the mixing frequency, but either for spectral reprocessing of the digital received signal upstream of the digital filter, or for readjustment of the pass band of the digital filter.
In this solution as well, the measures according to the invention are carried out by calculation, without any need for additional complexity in the hardware area.
The circuit configuration is preferably likewise equipped with further circuit devices for transmitting a radio signal. In this case, the signal processing circuit furthermore has a digital modulator and a D/A converter, with a modulated digital transmission signal, which is provided by the digital modulator, then converted by the D/A converter to an analog transmission signal. The second frequency correction control signal is supplied to the digital modulator, with the digital modulator changing the frequency of the modulated digital transmission signal as a function of the second frequency correction control signal.
According to one preferred embodiment of the invention, the frequency estimator uses the moment calculation method in order to estimate the frequency offset. It has been found that this method allows the frequency offset to be determined particularly accurately and with little complexity.
The invention can provide for the frequency estimator to in each case redetermine the second frequency correction control signal for each of the data symbols in the filtered digital received signal. This determination of the second frequency correction control signal symbol-by-symbol results in the best possible time resolution. High time resolution allows effective correction for rapid frequency fluctuations which are caused by phase noise in the oscillator.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a frequency stabilized transmitting/receiving configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.