1. Field of the Invention
This invention relates generally to a radio communication apparatus comprising part of a radio communication system such as a code division multiple access (CDMA) communication system. More specifically, it relates to a radio communication apparatus employing a rake receiver which combines the useful multipath components of a spread spectrum signal.
2. Description of the Background Art
The CDMA communication system has been standardized by the Telecommunications Industry Association (TIA) for use in North America and has adapted spread spectrum communication technology. The mobile station used in this system is described in the TIA/EIA Interim Standard xe2x80x98Mobile Station-Base station Compatibility Standard For Dual-Mode Wide Band Spread Spectrum Cellular Systemxe2x80x99 (Interim Standard IS-95 Section 6.2.2.1).
In accordance with this standard, each mobile station is provided with a rake receiver, having at least one searcher for independent pilot channel acquisition and at least three fingers for data acquisition. The searcher searches for a pilot channel transmitted by a base station sending a pseudo-random number (PN) binary code reference sequence with no modulation of information bits thereon. Reception of the pilot channel makes it possible for each mobile station to synchronize with any other channel and to select the paths/delays for which the energy of a received signal on the traffic channel is greatest. Based on the pilot channel received by the searcher, each finger receives a useful multipath component of a received signal at a different timing. A receiver of this type is described by Wang et al. in a paper titled xe2x80x98Portable Telephone for CDMA Cellular Systemxe2x80x99, Oki Technical Review, 150 Vol. 60 (August 1994).
In the CDMA communication system, for example, a conventional mobile station pursuant to the Interim Standard IS-95 includes generally one searcher and three fingers. The number of searchers and the number of fingers is fixed in conventional CDMA systems and does not change in response to transmission characteristics.
However, in the field of mobile radio communications employing a CDMA communication system, the transmission characteristics existing between the base stations and the mobile stations change continuously because, in general, the location of each mobile station relative to a base station changes. Thus, the strength of the signal received by a mobile station may be reduced and therefore searchers in excess of the predetermined number are required if all pilot channels of neighboring base stations are to be searched quickly.
As one solution to this problem, mobile stations have been provided with one or more additional searchers. However, this is inefficient because the additional searchers make it necessary to increase the size of the receiver, add components which are used during only a portion of the time the receiver is in operation, and increase the power consumed by the receiver.
Therefore, it is an object of the present invention to provide a radio communication apparatus including a rake receiver which adaptively assigns a searcher operation and a finger operation in response to the characteristics of the transmission path existing at a given time between the base and mobile stations.
To accomplish these objectives, a radio communication apparatus is provided which comprises:
at least one combination searcher/finger unit operable to perform (1) a searcher operation for detecting signals on the reception paths and (2) estimating the strength of each of the signals, and a finger operation for outputting a demodulated symbol in response to a command signal. The radio communication apparatus is further provided with an assignment unit for generating the command signal to assign either the searcher operation or the finger operation of a combination searcher/finger unit based on at least one of the following conditions:
(1) the number of useful information containing reception paths which can be demodulated; or
(2) the number of neighboring base stations; or
(3) the total signal strength of the useful reception paths; or
(4) the reception error rate of the combined signal; or
(5) the reception modes having an initial synchronizing mode for synchronizing with one of the base stations, a call waiting-mode for waiting for a call from the synchronized base station and a communication mode for communicating to the synchronized base station.