Since the signal processing in modern radio receivers is carried out digitally in the intermediate-frequency band or in baseband, the received analog signal must be sampled and digitized. This process is referred to as analog/digital (AD) conversion. The sampling frequency must, as is known, be chosen to be sufficiently high to satisfy the Nyquist condition. The optimum sampling time is governed by the time of maximum energy in a received information unit (bit or chip). This optimum sampling time is, however, not known during the sampling process. This means that the sampled discrete-value received signal generally has a sampling time error (discrepancy between the sampling time used and the optimum sampling time).
The sampling time error of the discrete-value received signal is normally corrected by means of an early/late correlator and an interpolator. The early/late correlator receives the discrete-value received signal, which has been double oversampled, correlates it with a sequence that is known in the receiver, and compares the signal energies of the early and late samples over a specific time period. The early/late correlator uses this comparison to determine a sampling time error signal which indicates the discrepancy between the sampling time and the optimum sampling time (if, for example, the signal energy in the early samples is of the same magnitude as the signal energy in the late samples, the optimum sampling time is precisely in the center between the sampling times for the early and the late samples). This sampling time error signal is supplied to an interpolator. The interpolator recalculates the sample values of the discrete-value received signal at support points which are shifted with respect to the sampling times by the sampling time error that has been determined. The sampling time is thus readjusted computationally. The received signal values converted to the optimum sampling time are then produced at the output of the interpolator.
Since the early/late correlator determines the sampling time error signal from the received data, the sampling time error signal emitted from the early/late correlator is noisy. This leads to fluctuations in the drive for the interpolator, which are undesirable. The sampling time error signal therefore has to be filtered.
The filtering of the sampling time error signal is subject to contradictory requirements. On the one hand, the filter must have a narrowband filter characteristic (with a bandwidth of about 2 kHz for UMTS) in order to achieve a high degree of noise suppression. Relatively high-order filters are therefore required. On the other hand, the filter should have as short a delay (latency) as possible, and should involve as little implementation complexity as possible for cost and space reasons.
Furthermore, the receiver is used not only for data reception but also for measurement tasks, such as the measurement of signal power levels. While a high degree of noise suppression is required for data reception, the filter for filtering the sampling time error signal must stabilize as quickly as possible and must have short latency for the measurement tasks, which frequently are carried out only briefly, in order that the measurement takes as little time as possible.
In previous solutions, all the sampling time error values in a time slot are combined by summation to form a sampling time error value. These combined sampling time error values, which are calculated for each time slot, are averaged over two or more time slots. This results in a filter with a sufficiently good filter characteristic. The result of the filtering is in each case applied to the next time slot since this means that there is no need to temporarily store the received data.
The document WO 01 33793 A discloses the sampling time error of a sampled received signal being determined by means of an appropriate apparatus. The sampling time error signal determined in this way is then passed to a filter. The filtered sampling time error signal is used to control an interpolator, which produces a received signal which has been corrected by the sampling time error.