Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-06-02
2003-02-11
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S342000
Reexamination Certificate
active
06519276
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to cellular communication systems, and more particularly to a method for an improved sequential searcher operation in a mobile telecommunication system supporting a variable data rate transmission.
2. Description of the Related Art
A mobile telecommunication system satisfying the IS-95 standard and standards based on IS-95 uses a searcher and rake receiver which demodulates and uses the signal energy of all paths to find a best receive path among multiple receive paths for demodulation of a reverse channel. Multiple receive paths occur as a consequence of both movement of a mobile station and changes in the wireless telecommunication environment thereby causing continuous changes in the receive-delay time. In such an environment, it is necessary to establish a path having the strongest received signal among the multiple receive paths (i.e., signals), each path having a different delay time. The searcher and rake receiver are used for this purpose.
A prior system which discloses such a method is disclosed in U.S. Pat. No. 5,644,591 entitled “Method and Apparatus for Performing Search acquisition in a CDMA Communication System”. The '591 patent calculates the energy level of a received signal in accordance with a pseudorandom noise (PN) sequence, selects a necessary PN sequence and decodes the received signal synchronized to the selected sequence.
A reverse channel of a mobile telecommunication system employing code division multiple access (CDMA) technology transmits walsh symbols at a rate of 4.8 kHz. Six consecutively transmitted walsh symbols make up a single power control group (PCG) with a length of 1.25 millisecond (ms) and 16 consecutive PCGs make up a frame of 20 ms. That is, a gating process divides each 20 ms frame into 16 power control groups.
In a system supporting a 9.6 Kbps rate, one frame corresponds to 192 bits (i.e. full rate) and is composed of 576 code symbols and 96 walsh symbols. Each of the 16 PCG contain 12 bits, composed of 36 code symbols and 6 walsh symbols.
Starting with the first PCG, a searcher searches for a signal at a search position corresponding to a particular delay time within a walsh symbol. The searcher may determine at the first search position whether a signal is received or not by either detecting a signal greater than a predetermined high threshold value when a signal is received, or by detecting a signal lower than a predetermined low threshold value when a signal is not received (i.e. walsh symbol). The searcher records the result of searching including delay time information and moves to the next search position.
The search position represents a delay time and the delay time means a reflection path. That is, the searcher searches for the signal of walsh symbols at a specific delay time. A plurality of search positions, for example, one hundred search positions, may exist within a walsh symbol. The base transceiver station (BTS) stores the received signals in receiving order and divides the signals into walsh symbols in accordance with time. That is, the walsh symbol is a kind of time unit and it has signal.
When each of the six walsh symbols in the PCG have been searched, the searcher calculates an average of the six search results and determines whether a signal is detected or not and then moves to the next PCG. The recorded delay time information is used to control a rake receiver.
While the search operation is capable of determining whether a signal is received or not before searching each of the six walsh symbols in the time required to receive a single PCG group (i.e., 6×4.8 kHz cycles), the searcher in the prior art system does not stop the search operation at the point of determination but performs the search operation unconditionally until all six search iterations have completed, thereby wasting processing time.
A sequential searcher is constructed to avoid wasted processing time. Once the sequential searcher can determine whether a signal is received or not while searching signals at a walsh symbol/rate of 4.8 kHz, the sequential searcher moves to a first search position of the next PCG and continuously performs the search operation. Therefore, the sequential searcher can perform more search operations during the same time period.
In other words, if the sequential searcher detects the existence of a signal at search position
10
of walsh symbol
3
of PCG
0
, the sequential searcher starts searching for signals, not from search position
10
of walsh symbol
4
, but from search position
11
of walsh symbol
3
of new PCG
1
. That is, the search position of the next PCG from the PCG where the existence of a signal is detected.
If no signal is determined to be received, the sequential search repeats the search operation at the same search position a maximum of 6 times. Here, limiting the search to a maximum of six times at the same position is intended to perform the search for a whole PCG. That is, since 6 times 4.8 kHz is equivalent to a PCG, the searcher detects the existence of a signal once each power control group. After searching six times, the search result at the corresponding searcher position is used to control a rake receiver.
Since IS-95 and standards based on the IS-95, however, support variable data rate transmission, the 16 PCGs of each 20 ms frame may or may not carry a signal depending on the data rate.
Turning now to the drawings,
FIG. 1
illustrates an exemplary embodiment of a variable data rate transmission of a reverse CDMA channel as defined by IS-95B. As illustrated, since all PCGs are used in the case where a system transmits at 9600 bps, a frame transmits all 16 PCGs. On the other hand, when a system transmits at 4800 bps, the system uses half of PCGs, therefore only 8 PCGs are transmitted. Similarly, 4 PCGs are transmitted in the case of 2400 bps and 2 PCGs are transmitted in case of 1200 bps.
In the case of 4800 bps, the system selects 8 PCGs, while 4 PCGs are selected in the case of 2400 bps, and 2 PCGs are selected in the case of 1200 bps within a frame. The selection of PCGs is pseudo randomized in accordance with a data burst randomizing function. Selective transmission of PCGs appears as noise to those with no knowledge about the randomizing function of a system; by contrast, the transmission is a very orderly selection to those with knowledge of the randomizing function.
If a sequential searcher is directly applied to a system which uses a variable data rate in the aforementioned manner the sequential searcher performs the searching operation over all power control groups (PCGs) without considering whether a corresponding PCG carries a signal and may deteriorate demodulation performance. That is, it is already known by virtue of the variable data rate that certain PCGs will not carry a signal.
A non-sequential searcher, by contrast, detects a signal 6 times unconditionally at a particular position, but the sequential searcher stops searching at a current position and moves to next searcher position if the sequential searcher judges that a signal exists or not during any iteration of the six detection attempts.
That is, since the sequential searcher moves to the next searcher position of the next walsh symbol, if the sequential searcher judges the existence of a signal before the search operation is performed a maximum of 6 times, the starting position for the sequential searcher to start signal detection is not fixed. In such a case, the sequential searcher may perform the searcher operation over both PCGs carrying a signal and PCGs not carrying a signal.
For example, if the 6 iteration search operation starts from a walsh symbol located at the middle of a PCG, a sequential search performs signal searching from the rest of the walsh symbols of the PCG to a particular walsh symbol of the next PCG. Then, if the next PCG does not carry a signal in accordance with a variable data rate transmission, the sequential searcher performs searching over a noise signal and causes a problem in no
Ahn Jae-min
Kim Hee-sub
Dilworth & Barrese LLP
Samsung Electronics Co,. Ltd.
Tse Young T.
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