Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-05-08
2003-06-17
Bayard, Emmanuel (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S320000
Reexamination Certificate
active
06580749
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a code division multiple access (CDMA) receiver for use in a CDMA system and a method of controlling the CDMA receiver.
As a cellular mobile radio communication system, a wide variety of multiple access systems have been heretofore proposed and be adopted in the world. Among others, a recent tendency has been directed to a cellular mobile radio code division multiple access (CDMA) system which has a specific spreading code assigned to each channel and which will be simply called a CDMA system. In such a CDMA system, a modulated wave of an identical carrier frequency which is spread by each specific spreading code is transmitted as a radio signal from a transmitter side to a receiver side. Responsive to the radio signal, a CDMA receiver in the receiver side carries out synchronization operation by the use of each specific spreading code to identify a desired channel. In order to distinguish the channels from one another, different spreading codes are used to identify radio channels between the base station and the mobile stations.
In addition, it is to be noted that the radio signal is received through a plurality of paths, namely, a multipath in the CDMA system and, therefore, multipath fading should be removed from the radio signal in the CDMA receiver by accurately detecting a predetermined signal, such as a synchronization signal and/or a pilot signal.
Furthermore, it is to be considered in the cellular mobile radio communication system that each mobile station is moved through the radio service areas or cells from one to another with communication kept between each mobile station and a base station. In this event, the base stations must be switched from one to another without interrupting communication with each mobile station.
Taking the above into account, a CDMA receiver which is used in a mobile station has a rake receiver supplied with radio signals through the multipath and a searcher for searching such multipath signals to establish chip synchronization. In other words, the searcher is used to detect optimum reception timing from the radio signals and to inform the rake receiver of the optimum reception timing. This applies to the CDMA receiver which is used in each base station. Therefore, the following description will be mainly made about the CDMA receiver of the base station.
Practically, both the rake receiver and the searcher are given received data signals which are subjected to high frequency amplification and frequency conversion by a high frequency amplifier and an intermediate amplifier, respectively. In this event, the rake receiver is operable in response to the received data signals received through the multipath and comprises a plurality of finger receivers for demodulating the received data signal by the use of a spreading code into a demodulated signal. To this end, calculations are made about correlations between a spreading code of a desired channel and each received multipath signal to capture codes at each reception timing through each path. Thereafter, maximal-ratio combining is carried out to augment received signal strength. Such maximal-ratio combining is effective to alleviate an influence of the multipath fading and to improve a signal-to-noise (S/N) ratio.
On the other hand, the searcher has a plurality of correlators which are operable in response to the received data signals and delayed spreading codes to calculate correlation values between the received data signals and the delayed spreading codes and a plurality of adders for adding the respective correlation values to produce summed up correlation values. Moreover, the summed up correlation values are delivered to an effective path judgment circuit to determine an effective path and to supply the finger circuits of the rake receiver with reception timing signals representative of the effective path or optimum reception timing.
Herein, it is to be noted that the correlators of the searcher are in one-to-one correspondence to the adders.
Recently, recent attempts have been made to widen each of the cells in such a CDMA system to reduce a frequency of handoff operations for switching the base stations from one to another. This results in enlargement of a cell radius of each cell. Such enlargement of the cell radius brings about expanding or widening a searching range in the searcher of the CDMA receiver in each base station. In other words, the searcher must be widened in the searching range.
In order to widen or expand the searching range in the searcher, it is necessary to increase the correlators and the corresponding adders in number. Practically, the searching range is determined by a chip number of the spreading code and must have a predetermined resolution. In consequence, the correlators and the corresponding adders should be increased in number as the searching range is widened because the number of the correlators is equal to a product of the chip number and the resolution as well as the number of the adders.
Under the circumstances, the searcher is inevitably complicated in structure as the searching range is widened, because of a large number of the correlators and the adders. In addition, it is very difficult to make each base station small in size when the correlators and the adders are increased in number.
In other words, a conventional searcher has a fixed search window having a constant searching range where the searcher can search. The constant searching range is determined by the number of the correlators or the adders. The fixed search window is defined by a starting time instant which is equal to a reference reception timing of a base transceiver station (BTS) that has zero propagation delay. In other words, the fixed search window has no window offset. In order to expand or widen the constant searching range in the fixed search window, the searcher must increase the correlators and the adders in number.
It will be assumed that the CDMA receiver carries out reception operation under environment with a large propagation delay such as a case of having a large cell radius or the like. In this event, the conventional searcher must comprise a correlation calculator and an adder portion which have the large number of correlators and the large number of adders, respectively. This is because the searcher has the fixed search window and it is therefore necessary to make the correlators and the adders large in number in order to expand or widen the constant searching range. Accordingly, the conventional searcher is large in structure.
In addition, various prior arts related to the present invention are already known. By way of example, a method and an apparatus for performing search acquisition in a CDMA communications system is disclosed in U.S. Pat. No. 5,644,591 issued to Todd R. Sutton. According to Sutton, a large window of PN chip offset hypotheses are searched and if an energy signal is found that might indicate the presence of the pilot signal having one of the chip offsets of the large search window, then a search of a subset of offset hypotheses, or small window, is searched.
In a searcher according to Sutton, if the search window is swept and no hypothesis's energy exceeds the detection threshold, then searcher controller would begin sweeping the next search window. In other words, the searcher according to Sutton mainly aims to search reception timimg of an effective path or the pilot signal. It will be assumed that the reception timing lies in one of the both ends of the search window by changing the reception timing of the effective path caused by moving a mobile station. In this event, it is feared that it cannot decide whether this path lies in a current search window or the next search window (or a previous search window). Under the circumstances, it is difficult to stably carry out reception processing.
Japanese Unexamined Patent Publication of Tokkai No. Hei 10-173,629 or JP-A 10-173629 discloses “RECEIVING DEVICE” to precisely operate RAKE reception valid to code division multiplex system communic
Bayard Emmanuel
NEC Corporation
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