Abstract:
A Code Division Multiple Assess (CDMA) receiving station includes first finger ( 310 ), a second finger ( 312 ), and a searcher ( 314 ). Assignment and deassignment of second finger ( 312 ) are made in part by determining the gap between the first finger and the second finger, or between the first finger and an energy peak detected by the searcher. Assignment and deassignment of second finger ( 312 ) are also made in part by selectively slowing the Time Tracking Loop (TTL) of both fingers when appropriate thresholds are crossed.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to wireless communications, and has particular relation to how a second finger in a Code Division Multiple Access (CDMA) wireless receiver is to be assigned and deassigned. 
     2. Background Art 
     Code Division Multiple Access (CDMA) systems are so named because they use Code, as distinct from frequency or time, to Divide a frequency band into Multiple channels, each of which provides Access to a separate transmitter. The codes cycle through a known sequence. A different offset in the cycle is assigned to each transmitter. When a receiver sets its offset to the same value, it can demodulate the signal from the transmitter. When the values differ to any great extent, the received signal is perceived as only noise. Offset is measured in “chips”. Typical CDMA systems operate at 1.2288 megachips per second. There are 32,767(2 15 −1) offsets per cycle, so each cycle takes only 26.6 milliseconds. 
     A typical receiving station includes several receivers, each of which is known as a “finger”. Typically, a receiving station will have four fingers available. This allows the receiving station to receive signals from up to four different transmitting stations. More commonly, however, a single signal takes several paths (“multi-path”) from the transmitting station to the receiving station. One path will perhaps be line-of-sight, while a second reflects off a multi-story garage building, while a third reflects off both the building and a vehicle. Because these paths have different lengths, the signals arriving on these paths will have different offsets, perhaps by as much as 20 chips. The receiving station therefore assigns a different finger to each offset, demodulates the signal at each finger, and combines the demodulated signals into a combined signal, ready for further processing. 
     The typical receiving station also includes a fifth finger, called a “searcher”. It is not a true finger, since it is incapable of demodulating a signal. What it is capable of, however, is very quickly measuring whatever signal strength is present at a large number of different offsets. It thus scans for signals at new offsets. It also takes each of the four offsets which already have fingers assigned to them, and measures the signal strength a few chips ahead of, and a few chips behind, the assigned offset. 
     Suppose that the offset of the received signal has changed. Perhaps the vehicle has moved or the receiving station has moved, or some other change has taken place. This fact can be detected by the signal strength at the leading offset having increased and the signal strength at the lagging offset having decreased (or vice versa). A Time Tracking Loop (TTL) adjusts the finger&#39;s assigned offset accordingly. 
     There will be some times when signals from different multi-paths converge (for example, the vehicle drives into the garage building), and now only one finger needs to be assigned, with the other being deassigned. There will be other times when a single signal will diverge into different multi-paths (for example, the vehicle drives out of the garage building), and now two fingers need to be assigned. 
     Conventional methods have not been good in determining when to make an assignment and when to make a deassignment. When making assignment decisions, the searcher determines that there is an energy peak at an offset which is near an offset to which a finger is currently assigned. The searcher measures the gap between the offset of the energy peak and the assigned offset of the finger. If the gap exceeds a threshold, an additional finger is assigned. 
     Similarly, the gap between the offsets of two different fingers is measured. When the gap falls below a threshold (generally different from the threshold described in the previous paragraph, so as to create hysteresis), the weaker of the two fingers is deassigned. 
     SUMMARY OF THE INVENTION 
     An apparatus and method are disclosed in which assignment and deassignment of a second finger are made in part by determining the gap, and in part by selectively slowing the Time Tracking Loop (TTL) when appropriate thresholds are crossed. 
     In its broadest assignment aspect, a method for assigning a second finger in a Code Division Multiple Access (CDMA) wireless receiver includes the steps of: 
     determining a gap between the assigned offset of a first finger and the offset of an energy peak detected by a searcher; 
     assigning a second finger if the gap exceeds a first threshold; 
     slowing down the speed of a Time Tracking Loop for both the first finger and the second finger if the gap exceeds the first threshold but does not exceed a second threshold, which is greater than the first threshold; and 
     maintaining the speed of the Time Tracking Loop for both the first finger and the second finger if the gap exceeds the second threshold. 
     In its broadest deassignment aspect, a method for deassigning a second finger in a Code Division Multiple Access (CDMA) wireless receiver includes the steps of: 
     determining a gap between the assigned offset of a first finger and the assigned offset of the second finger; 
     deassigning a second finger and maintaining the speed of the Time Tracking Loop for the first finger if the gap is less than a third threshold; 
     slowing down the speed of a Time Tracking Loop for both the first finger and the second finger if the gap is not less than the third threshold, but is less than a fourth threshold, which is greater than the third threshold; and 
     maintaining, the speed of the Time Tracking Loop for both the first finger and the second finger if the gap exceeds the fourth threshold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a flowchart of a method  100  for assigning an additional finger. 
     FIG. 2 shows a flowchart of a method  200  for deassigning one of two fingers. 
     FIG. 3 shows apparatus  300  suitable for carrying out the foregoing methods. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a flowchart of a method  100  for assigning a second finger. In first step  102 , a determination is made as to whether the gap (a “first gap”) between the assigned offset of the current finger, and the offset of the nearby energy peak as determined by the searcher, exceeds a first threshold. The first threshold is preferably ¾ of a chip. If the gap does not exceed this threshold, then the first decision  104  is not to assign a second finger. 
     At second step  106 , a determination is made as to whether the gap between the assigned offset of the current finger, and the offset of the nearby energy peak as determined by the searcher, exceeds a second threshold, larger than the first threshold. The second threshold is preferably 1.5 chips. If the gap does not exceed this threshold (that is, it lies between the first and second thresholds), then the second decision  108  is to assign a second finger, and also to slow down the Time Tracking Loop (TTL) on both fingers. The amount of the slow-down is determined by trading off, for the application at hand, the importance of quickly tracking signals coming through both multi-paths, and not excessively assigning and deassigning the second finger. 
     When the gap is in this range of ¾ to 1.5 chips, the signals being received on separate multi-paths are sufficiently distinct that overall signal quality is improved by demodulating them separately. However, the assigned offsets of these fingers should not be quickly changed, even though the offsets of the two received signals appear to be quickly changing. If they are quickly changed, then the gap may suddenly get out of this range, only to return to this range just as suddenly. This would cause transient assignment and deassignment of an additional finger, or transient slow-down and speed-up of the Time Tracking Loop. Such transient activities are undesirable. 
     When the gap exceeds the second threshold (preferable 1.5 chips), then the third decision  110  is to assign the extra finger, and to keep the Time Tracking Loops of both fingers at their normal high speeds. It is of no concern that the gap might transiently fall below 1.5 chips, for the reasons discussed below in the explanation of FIG.  2 . 
     FIG. 2 shows a flowchart of a method  200  for deassigning one of two fingers. In third step  202 , a determination is made as to whether the gap (a “second gap”) between the assigned offset of the first finger, and the assigned offset of the second finger, is less than a third threshold. The third threshold is preferably the same as the first threshold, and thus preferably is ¾ of a chip. If the gap is less than this third threshold, then the fourth decision  204  is to deassign one of the fingers and to enable normal (high-speed) operation of the Time Tracking Loop of the remaining finger. 
     At fourth step  206 , a determination is made as to whether the gap between the assigned offsets of the fingers is less than a fourth threshold, larger than the third threshold. The fourth threshold is preferably the same as the second threshold, and thus preferably is 1.5 chips. If the gap is less than this fourth threshold (that is, it lies between the third and fourth thresholds), then the fifth decision  208  is to continue both fingers, and also to slow down the Time Tracking Loop (TTL) on both fingers. 
     When the gap is not less than the fourth threshold, then the sixth decision  210  is to continue everything as it is. This decision has two aspects. 
     First, if both TTLs have been operating at high speed (because the gap has exceeded 1.5 chips for considerable time), then they continue to operate at high speed. This is typically the situation where the two signals are not likely to merge, and therefore should be quickly and separately tracked. 
     Second if both TTLs have been operating at low speed (because the gap has only just now exceeded 1.5 chips), then they continue to operate at low speed. Thus, if the gap transiently falls back below 1.5 chips, nothing changes. It is only when the gap falls below ¾ of a chip that the signals are considered to have merged, and one of the fingers is deassigned. This is typically the situation where the two signals are likely to merge, eventually, and therefore should be slowly tracked so that they are not repeatedly merged and unmerged. 
     The method of FIG. 1 may readily be combined with the method of FIG.  2 . 
     FIG. 3 shows apparatus  300  suitable for carrying out the foregoing methods. A transmitting station  302  transmits a signal along a first multi-path  304  and a second multi-path  306  to a receiving station  308 . The receiving station  308  includes a first finger  310 , a second finger  312 , a searcher  314 , and a decider  316 . The decider is used to decide whether or not to assign or deassign a finger. First finger  310  receives along first multi-path  304 . 
     When second finger  312  is not yet receiving the signal along second multi-path  306 , the searcher  314  does receive the signal along second multi-path  306 . The searcher  314  determines the offset location of the energy peak created by the signal along the second multi-path  306 . The decider  316  may thus determine the gap between the first finger&#39;s assigned offset and the offset of the energy peak. The decider  316  then decides, as set out in FIG. 1, whether to assign the second finger  312  to this energy peak. 
     As is known in the art (and therefore not shown in FIG.  3 ), the Time Tracking Loop (TTL) of each finger may be included within that finger, or a common TTL may be used. 
     When second finger  312  is receiving the signal along second multi-path  306 , the decider  316  determines the assigned offsets of the two fingers, and thus determines the gap between their assigned offsets. The decider  316  then decides, as set out in FIG. 2, whether to deassign the second finger  312  from the second multi-path  306 . 
     INDUSTRIAL APPLICATION 
     This invention is capable of exploitation in industry, and can be made and used, whenever is it desired to assign or deassign a second finger. The individual components of the apparatus and method shown herein, taken separate and apart from one another, may be entirely conventional, it being their combination that is claimed as the invention. 
     While various modes of apparatus and method have been described, the true spirit and scope of the invention are not limited thereto, but are limited only by the following claims and their equivalents, and such are claimed as the invention.