Radio signals for mobile radio are subject to multipath propagation, that is to say reflections, scatter and diffraction of the transmitted radio signal on various obstructions in the propagation path results in generally two or more received signal versions in the receiver, which are shifted in time with respect to one another, and are attenuated to different extents. The functional principle of a RAKE receiver is based on first of all separately evaluating two or more of these received signal versions, and then superimposing them with the correct timing in order to achieve as high a detection gain as possible. The expression RAKE in this case provides an illustrative description of the structure of a receiver such as this, with the tines of the rake representing RAKE fingers, and the handle of the rake representing the higher-level received signal that is produced on the output side.
In UMTS systems (UMTS: universal mobile telecommunications system) for the third mobile radio generation, code division multiple access (CDMA) is used as the multiple access method. For CDMA, all of the subscribers use the same frequency range, but the radio signal is coded differently for or by each subscriber. The different CDMA coding allows subscriber separation.
During the CDMA coding process, each data symbol in the digital data signal to be transmitted has a subscriber-specific CDMA spreading code applied to it at the transmitter end. The elements of the CDMA spreading code sequence that is used for this purpose are referred to as chips. The time duration of a data symbol is an integer number Q of the chip time duration Tc, with 1/Tc corresponding to the chip rate. Q is the length (number of chips) in the CDMA spreading code sequence that is used.
CDMA despreading is carried out at the chip clock rate in the individual RAKE fingers. The chip time duration is known in the receiver, but it is necessary to determine and take account of the absolute timing of the chips of the received signal in each RAKE finger. This requires considerably more accuracy than the chip time duration Tc. In UMTS, the chip time duration is Tc=2.6 ms.
For this purpose, it is already known for each RAKE finger to have an associated circuit arrangement which samples the received signal with a high degree of oversampling (for example at 8 times the chip rate), with a different phase angle. An optimum sample value with an optimum phase angle is then selected separately in a sampling phase selection element for each RAKE finger on the basis of the maximum chip energy, and is then used for the rest of the signal processing.
Furthermore, German Laid-Open Specification DE 100 05 441 A 1 discloses a method in which a digital interpolator is used to select the optimum phase. Based on a data signal that is oversampled at twice the chip rate, this interpolator produces suitable intermediate values for intermediate sampling phases, which are then processed further in the RAKE finger at the chip clock rate.
The two implementation forms have the common feature that suitable phase angles of an oversampled data signal are selected in a sampling phase selection element for further processing.
The optimum sampling time of the received signal differs for each finger of the RAKE receiver, and is determined from the received symbols, in particular with the aid of the pilot symbols. The task of finding the optimum sampling time is carried out by a time error measurement device, which generally has a non-linear transmission characteristic between its sampling time error signal on the output side, from which the drive signal for the sampling phase selection element is generated, and the time error in the input side in the respective path of the RAKE finger. The actual time error in the respective path can be deduced from the output signal from the time error measurement device by reverse mapping.
The setting of the optimum sampling time by means of sampling phase selection is carried out with a restricted time resolution. For example, if interpolators according to the prior art are used as sampling phase selection elements, only up to three different intermediate values may be set. The interpolators can thus be implemented as digital filters with a small number of fixed filter coefficients. Owing to the fact that the time resolution of the sampling phase selection element is restricted, the sampling control signal that controls the sampling phase selection element is in discrete form. This necessitates an association between individual values of the discrete sampling control signal and individual quantization intervals of the sampling time error signal, whose values are continuous. This means that quantization intervals must be determined for the sampling time error signal in accordance with the requirements for the sampling phase selection element in the transmission characteristic of the time error measurement device, which indicates the relationship between the time error on the x axis and the sampling time error signal on the y axis. The transmission characteristic is referred to as an S curve, owing to its shape.
The shape of the S curve is governed on the one hand by the specific implementation of the time error measurement device, but on the other hand also by characteristics of the transmission path between the antenna and the input of the time error measurement device, in particular characteristics of the radio-frequency section and of the reception filter chain in the receiver. Furthermore, the characteristics of the transmission path between the antenna and the sampling phase selection element, in particular in the reception filter chain, but also between the time error measurement device and the sampling phase selection element, or characteristics of the implementation of the time error detector or of the sampling phase selection element, can result in shifts with respect to the origin of the S curve.
Until now, the quantization intervals for the S curve have typically been determined by means of a simulation model of the receiver, and are implemented permanently in the receiver. This has the disadvantage that quantization quality that results from this, and the resolution that is associated with this depend on the accuracy of the modelling of the reception path. Furthermore, component-dependent variations are possible inter alia with regard to the delay in the reception path, in the time error measurement device or in the sampling phase selection element, and these are not taken into account by the simulation model. In addition, temperature influences and ageing influences in the receiver cannot be covered by the simulation model, either. This means that the optimum sampling time is not always correctly set by the sampling phase selection element.