CDMA data transmission systems, for example for use in a 3GPP-WCDMA-FDD device, usually have a “rake” receiver comprising various “fingers”, with each finger being assigned to a delayed path (data transmission path) [lacuna] received signal.
The number of active fingers and the delay in each “finger” in a rake receiver are assigned on the basis of a data transmission path searching unit. The operating parameters for the data transmission path searching unit are assigned by a “finger” management unit.
In wireless data stream transmission systems, signals pass via different data transmission paths to which different “fading” can be applied.
Since every data transmission path has a different length and the data stream signals propagate on the different paths at approximately the same data transmission speed, the data stream signal arrival times at the data stream receiver differ, in some cases substantially, for the different data transmission paths.
The data transmission path searching unit is now used to determine an arrival time for the data stream signals from the different data transmission paths. In line with the 3GPP standard (UMTS), the data stream is made up of frames and slots (data frames and data slots). A data frame has, by way of example, a duration of 10 milliseconds (ms) and contains 15 data slots. Each data slot has 2560 chips, which means that the chip frequency in this example is 3.84 MHz.
Since the bandwidth of a CDMA system is usually high, a chip period is very small, which means that delays on different data transmission paths are usually greater than one chip period.
These delayed arrival times for the various propagation paths result in data symbols transmitted at various times being superimposed at the receiver, an effect which is called intersymbol interference (ISI) and, without suitable countermeasures, has disadvantageous consequences for data reception. To compensate for ISI and to make advantageous use of the diversity of the various propagation paths, CDMA systems usually involve the use of a technique which is used to receive the data stream signals from all the relevant data transmission paths separately, and they are then combined.
On the basis of the prior art, this is done in a rake receiver, which is a data stream receiver which receives as many multipath data stream signals as possible. The rake receiver combines the signals from all these paths to produce a data stream signal which is as “interference free” as possible and which is stronger than the individual components. Individual paths are found by cross-correlating a reference pattern with the received signal.
The estimation of “power delay profiles” (PDPs) is fundamental to the operability of a rake receiver. The power delay profiles for different data transmission paths are estimated, by way of example, by a mobile UMTS receiver in order to determine the amplitude or a power and the delayed timing of the data transmission paths for data streams with regard to a receiver timing reference.
The power delay profile is determined by means of a correlation using a (primary or secondary) “pilot channel” (CPICH—Common Pilot Channel) which transmits a predetermined symbol sequence. In conventional manner, a complex correlation is provided between the incoming signal (r(i)), which is sampled at double the chip rate, and a known, complex conjugate pilot sequence signal p*(i), likewise sampled at double the chip frequency, in line with the general relationship, where Ncorr is the correlation length.
                              corr          _                compl            ⁡              (        n        )              =                            1                      N            corr                          ⁢                              ∑                          i              =              0                                                      N                corr                            -              1                                ⁢                                                                      r                  _                                ⁡                                  (                                                            2                      ⁢                      i                                        +                    n                                    )                                            ·                              p                _                                      *                          (                              2                ⁢                i                            )                        ⁢                                                  ⁢            n                              =      0        ,  1  ,  2  ,          ⁢  …  ⁢          ,            L      -      1        20  
In the case of the transmission diversity which is possible in the UMTS standard, it is necessary to perform this correlation for the data transmission paths of an “antenna 1” and of an “antenna 2”. In this context, the expressions “antenna 1” and “antenna 2” denote two different transmission devices in a data stream transmitter, for example antennas, so that at least two different data transmission paths are produced. In this context, the correlation length Ncorr is an even-numbered multiple of a pilot sequence symbol length, i.e. an even-numbered multiple of a CPICH symbol length of 256 chips. Although increasing the correlation length Ncorr increases the signal-to-noise ratio for a power delay profile estimation in a desirable manner, on the other hand this power delay profile estimation then becomes more sensitive to any sampling clock error. For this reason, a power delay profile estimation needs to be averaged over time.
The operability of the rake receiver is based quite fundamentally on correct positioning of a particular number of rake fingers on multiple data transmission paths in order to be able to combine the power thereof and to obtain a diversity boost.
Conventional methods use a “PDP (Power Delay Profile) determination device” in order to determine those data transmission paths which have the highest power. The output of each PDP determination device is compared with a threshold value, and all peak values in the received signal above this threshold are processed further by a finger positioning block.
Disadvantageously, the setting of a threshold value is extremely critical with regard to the detection of incorrect data transmission paths. Particularly in environments with a low signal-to-noise ratio or a high level of noise, which can be brought about, in particular, by a moving, mobile data stream receiver on account of fading effects, the amplitude distributions of noise and amplitude distributions of data transmission paths can overlap such that exact setting of a threshold value is no longer possible.
In conventional manner, the PDP determination device is in the form of a correlation filter, for example, which has the further disadvantage that secondary maxima are produced which simulate invalid data transmission paths, i.e. shadow data transmission paths.
If the threshold value is set to be too low, then a power delay profile determination is severely disturbed by noise, and not all peak values which are linked to a correlation of received signal and pilot sequence signal correspond to actual, i.e. valid, data transmission paths.
Disadvantageously, the peak values of the useful signal and of the noise signal are subject to statistical processes, which means that a relation between data transmission paths and noise peak values in the case of power delay profile determination is a statistical process which is dependent on an existing noise scenario.
In particular, conventional methods have disadvantages to the effect that data transmission paths are not correctly identified if, as is unavoidable in the case of mobile systems, new data transmission paths appear and existing data transmission paths disappear, which means that uniform monitoring of the data transmission paths is necessary.
It is therefore an object of the present invention to provide an apparatus for receiving a data stream which can be transmitted via at least one data transmission path in which a level of accuracy for data transmission path detection is improved as compared with conventional methods, with noise peak values differing from data transmission path peak values, and shadow data transmission paths being able to be avoided.