1. Field of the Invention
The present invention relates to a CDMA receiver, and more particularly to a method of measuring a delay profile, detecting a plurality of paths with high signal power within the measured range, and allocating the plurality of paths to finger receivers.
2. Description of the Related Art
DS-CDMA (Direct Sequence-Code Division Multiple Access) is a method in which a plurality of communicators use the same frequency band for communication. In this method, communicators are identified by the use of spreading codes.
In mobile communications, when a plurality of radio waves simultaneously propagate to the same receiver, interference occurs between the plurality of radio waves with different propagation delay times because of varying propagation path lengths of the radio waves. In DS-CDMA communication, information data is spread with spreading codes at a higher rate with a shorter period than propagation time. For this reason, radio waves with different propagation delay times can be separated and extracted.
Movements of a mobile station change the positional relationship between the mobile station and a base station, and hence the delay profile, which is signal power distribution with respect to delay time, also changes over time. Signals propagating along paths other than paths in which a base station is directly viewed from a mobile station are varied in accordance with the Rayleigh distribution.
In DS-CDMA communication, a plurality of path signals separated in terms of time with different propagation delay times and changing in accordance with the Rayleigh distributions are in-phase combined (rake combined) to obtain a diversity effect, thereby improving reception characteristics. Specifically, the diversity effect obtained by performing the rake combination makes it possible to reduce the transmission power required to achieve certain reception quality (bit error rate). In other words, interference power with other users inside or outside the same cell is reduced, so that subscriber capacity can be increased in a certain frequency bandwidth.
However, the relative positional change of a mobile station with respect to a base station as described above causes variations in delay profile as well as variations in delay time of paths to be rake combined. Therefore, in a mobile communication environment, a receiver requires a multipath search, tracking function for following variations in delay profile to allow instantaneous rake combination of the plurality of paths that can provide maximum signal power.
For example, as shown in FIG. 1, a CDMA receiver as described above comprises multipath search unit (multipath detecting means) 24 in a communication environment, rake combination receiver 25 for in-phase combining (rake combining) a plurality of paths, antenna 21, high-frequency reception circuit 22, and A/D (analogue/digital) converter 23.
In such a conventional CDMA receive system, multipath search unit 24 measures a delay profile, selects several paths with high signal power within the delay profile, and notifies the timing of the paths to rake combination receiver 25. Rake combination receiver 25 performs despread for each path based on the timing information for rake combination to obtain a path diversity effect.
Rake combination receiver 25 may have means (path tracking means) for tracking the movement of a particularly specified path, in which case multipath search unit 24 need notify path information to rake combination receiver 25 at least initially or at regular intervals. The CDMA receiver and multipath search method are disclosed in Japanese Patent Application Laid Open No.181704/1997 or the like.
On the other hand, a conventional method of allocating multipath to fingers is illustrated in FIG. 2. In the method, at step S81, a delay profile for propagation paths is first measured with a matched filter or sliding correlator. Next, at steps S82 and S83, upper correlation peaks are detected from the measured delay profile, the number of the detected upper correlation peaks being equal to the number of fingers possessed by a rake receiver. At step S84, the detected path timing is allocated as despread timing for each finger.
In the aforementioned conventional method of allocating a plurality of paths to finger receivers, when pilot interpolation synchronous detection is performed in which pilot symbols included periodically in a received signal are used to estimate phases of respective information symbols using the phase of the pilot symbol as a reference phase, the phase of the pilot hops at the switching of paths allocated to a finger, thereby temporarily failing to provide symbol synchronization with the pilot interpolation. As a result, reception characteristics are deteriorated unless the switching of respective finger paths is minimized as much as possible.
Additionally, in the aforementioned method, when a delay profile has the third correlation peak and the fourth correlation peak with substantially equal powers in a rake receiver having three fingers, the third path timing and the fourth path timing are frequently interchanged depending on variations in propagation path. This causes frequent switching of allocated path timing (despread timing) in one of the fingers, resulting in significantly deteriorated reception characteristics.
Taking a receiver with one finger as another example for simplifying description, when a delay profile with two paths at equal levels is measured, path A or path B is allocated to the finger depending on slight variations in received level. Such frequent switching of paths leads to deteriorated reception characteristics.
It is an object of the present invention to provide a CDMA receiver for improving reception characteristics by preventing frequent changes of path allocation to finger receivers, and a method for allocating a plurality of paths to finger receivers.
The CDMA receiver according to present invention has calculating means, operational means, and search means. The calculating means calculates a state weighting coefficient based on the present path allocation to a plurality of finger receivers. The operational means performs a predetermined operation between the calculated state weighting coefficient and a delay profile. The search means searches for paths based on the weighted delay profile.
According to one embodiment of the present invention, the CDMA receiver further includes allocating means. The allocating means allocates the plurality of paths detected by the search means to the plurality of finger receivers respectively.
Specifically, in the CDMA receiver according to the present invention, state weighting calculated on the basis of the present state of allocation to rake fingers is performed for a measured delay profile in a multipath search unit of a rake receiver, which provides hysteresis for path switching level of fingers to make it possible to prevent frequent switching of paths which leads to deteriorated characteristics. This leads to improvement in reception characteristics.
Additionally, path timing for finger receivers can be protected even when a path is temporarily lost due to shadowing or the like, resulting in improved reception characteristics. Furthermore, since conditional branch decision in path switching or the like is simplified, configurations of hardware or software can be simplified.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.