Patent Publication Number: US-2004042411-A1

Title: Method and apparatus for controlling the rate of path searching in a communications system

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field of the Invention  
       [0002] The present invention relates in general to the detection of new path delays and corresponding path energies for a channel in a cellular radio communications system, and in particular to controlling the power efficiency of a path searcher unit by varying the search rate during channel estimation.  
       [0003] 2. Description of Related Art  
       [0004] Access to and use of wireless networks is becoming increasingly important and popular for business, social, and recreational purposes. Users of wireless networks now rely on them for both voice and data communications. Furthermore, an ever increasing number of users demand both an increasing array of services and capabilities as well as greater bandwidth for activities such as Internet surfing. To address and meet the demands for new services and greater bandwidth, the wireless communications industry constantly strives to improve the number of services and the throughput of their wireless networks.  
       [0005] To that end, the wireless communications industry intends to continue to improve the capabilities of the technology upon which it relies and that it makes available to its customers by developing next-generation system(s). Protocols for a next-generation standard that is designed to meet the developing needs of wireless customers are being standardized by the 3 rd  Generation Partnership Project (3GPP). This set of protocols is known collectively as Universal Mobile Telecommunications System (UMTS).  
       [0006] The UMTS network consists of three independent domains namely a core network, a UMTS Terrestrial Radio Access Network (UTRAN) and User Equipment (UEs). The UEs may include for example, mobile stations, mobile terminals, etc. The main function of the network is to provide switching, routing, and transit for user traffic.  
       [0007] An important consideration of mobile terminals in a communications system is that of stand-by time. Stand-by time of a mobile terminal represents the maximum battery life of a mobile terminal when not in use for a conversation. To increase the stand-by time of a mobile terminal, the power consumption when the mobile terminal is turned on but not in use should be minimized. Power consumption can be minimized by powering off different components in the mobile terminal, such as signal processing circuitry, when the mobile terminal is not in use. For example, power consumption can be minimized by limiting the time a path searching unit searches for the presence of path delays.  
       [0008] The main object of the path searcher unit is to determine path delays. In conventional procedures, the path searcher unit periodically searches for path delays. In other words, the path searcher unit is activated during fixed intervals of time in order to determine path delays having higher power or higher Signal-to-Interference ratio (SIR) than the previously known path delays. The rate at which path delays appear is a complex function of terminal speed and the environment. Because the path searcher unit is a high power consuming function, there is a need to improve the power efficiency of the path searcher unit while searching for path delays for a channel in a communications system.  
       SUMMARY OF THE INVENTION  
       [0009] Embodiments of the present invention include a method and apparatus for controlling a rate of path searching for a channel in a communications system. In accordance with an embodiment of the invention, there is provided a method in which a search is performed for new path delays in a multipath communications environment. An estimated number of new path delays are identified during a specific time period. The identified number of new path delays are compared to at least one threshold value. The comparison results are utilized for altering the rate of path searching.  
       [0010] In accordance with another embodiment of the invention, controlling a rate of path searching for a channel is implemented in an apparatus that includes a searcher unit for searching new path delays in a multipath communications environment. The apparatus further includes a control unit for identifying a number of new path delays and comparing the number of new path delays to at least one threshold value. Furthermore, the apparatus includes a logic unit for selectively altering the rate of path searching based on the results of the comparison. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings, wherein:  
     [0012]FIG. 1 is a block diagram of a conventional wireless UMTS communication system, in which the present invention can be implemented;  
     [0013]FIG. 2 is a simplified block diagram of a conventional system for detecting path delays for a channel in a communications system;  
     [0014]FIG. 3 is a flow diagram of a conventional procedure for detecting path delays for a channel in a communications system;  
     [0015]FIG. 4 is a block diagram that shows pertinent details of an exemplary searcher unit that can be used to implement the functions of the searcher unit(s) shown in FIGS. 3 and 5;  
     [0016]FIG. 5 is a block diagram of an apparatus for controlling the rate of path searching in a communications system in accordance with an embodiment of the present invention; and  
     [0017]FIG. 6 is a flow diagram of a method for controlling the rate of path searching in accordance with principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0018] Reference is now made to the Drawings, wherein like reference characters denote like or similar parts throughout the various FIGURES. In a preferred embodiment, the present invention is implemented in a UMTS system. Accordingly, the following description and examples focus primarily on the use of embodiments of the invention in connection with a UMTS system. As will be appreciated by persons of ordinary skill in the art, however, the invention can be implemented in other wireless communication systems, such as, for example, CDMA and TDMA.  
     [0019] Referring now to FIG. 1, there is illustrated a block diagram of a UMTS wireless communications system  100 . The network  100  includes a core network  120  and a UMTS Terrestrial Radio Access Network (UTRAN)  130 . The UTRAN  130  is composed of, at least partially, a number of Radio Network Controllers (RNCs)  140 , each of which may be coupled to one or more neighboring base stations (BSs)  150 . Each BS  150  is responsible for a given geographical cell and the controlling RNC  140  is responsible for routing user and signaling data between the BS  150  and the core network  120 . All of the RNCs  140  may be directly or indirectly coupled to one another. The UMTS network  100  may further include multiple user equipment (UEs)  110 . The UE  110  may include, for example, mobile stations or mobile terminals that communicate with the base stations  150  via radio interfaces.  
     [0020] As known in the art, radio channels are severely impacted by the presence of multipath propagation. In multipath propagation, the propagating signal is reflected from a number of objects in the physical environment, and the various multipath components from the different radio paths arrive at a receiving device at slightly different times due to the time delays associated with the multipath propagation.  
     [0021] Referring now to FIG. 2, a simplified block diagram of a conventional system  200  for detecting path delays for a channel by a path searching unit is illustrated. The system  200  shown includes at least one path searcher unit  202 . A function of the path searcher unit  202  is to detect/find path delays corresponding to different paths in a multipath environment. The path searcher unit  202  in the conventional system periodically searches for path delays. This means that the path searcher unit  202  is activated at fixed intervals of time and performs a path searching operation at the same rate even when no path delays exist. This detection and searching process provides, for example, the path delay information and also provides channel estimations, if so desired. In short, the primary function of the one or more path searching units  202  is to detect path delays for different paths in a multipath environment, where each different path has a different propagation time.  
     [0022] The output of the one or more path searcher units  202  are coupled to the control unit  206 . A control signal from the control unit  206  includes path delay estimates that are used to set correct delays in a RAKE receiver unit  208 . Each Rake receiver component (or RAKE finger)  208   a - 208   n  demodulates the information received on a propagation path that corresponds to one of the received path delay estimates.  
     [0023] Referring now to FIG. 3, there is shown a flow diagram of a conventional procedure  300  for detecting path delays for a channel by a path searching unit. In step  305 , the path searcher unit  202  is activated or turned on. The purpose of the path searcher unit  202  is to acquire accurate estimates of the time of arrival and amplitude of various multipath components (step  310 ). At step  315 , path delay information acquired by the path searcher unit  202  is conveyed to the RAKE receiver.  
     [0024] Once the path delay information is received by the RAKE receiver, the path searcher unit  202  is deactivated or turned off (step  320 ). At step  325 , the process waits for the expiration of a constant time delay, after which the path searcher unit  202  is again activated to detect path delays by repeating steps  305 - 320 . The conventional procedure for detecting path delays requires the path searcher unit  202  to perform the path searching operation at the same rate even when no path delays exist. Each such path search consumes power and thereby reduces the stand-by time of the mobile terminal  110 .  
     [0025] Referring now to FIG. 4, there is shown a block diagram that shows pertinent details of an exemplary path searcher unit  400 . The exemplary searcher unit  400  includes a code matched filter  406 , which is matched to a pilot sequence of a dedicated data channel being used. The absolute value squared  408  of the complex signal output from the matched filter  406  is non-coherently accumulated in an integrate and dump unit  410  because of the frequency offset of the input signal complex signal. A path selection unit  412  searches for the highest peaks in the output from the integrate and dump unit  410  by comparing each peak with a predetermined threshold value. The path delays associated with the highest peak signal values are output to the control unit  306  (FIG. 3), to be used for selecting RAKE finger  308   a - 308   n  (FIG. 3).  
     [0026] In accordance with principles of the present invention, the stand-by time of a mobile terminal  110  can be improved by controlling the search rate of a path searcher unit  202  during channel estimation. To increase the stand-by time of a mobile terminal  110 , the search rate of a path searcher unit within the mobile terminal  110  while searching for new path delays is selectively altered based upon the number of new path delays detected during a specific time period.  
     [0027] If it is determined by the path searcher unit  202  or by a control unit in response to data provided by the path searcher that the number of new path delays detected is considerably small or if no new path delays exist, the search rate of the path searcher unit  202  can be decreased. However, if it is determined by the path searcher unit  202  that the number of new path delays detected is considerably large or if new path delays exist, the search rate of the path searcher unit  202  can be increased. Thus, by varying the search rate of the path searcher unit  202  in accordance with the path profile estimates, the stand-by time of the mobile terminal  110  can be prolonged.  
     [0028] Referring now to FIG. 5, there is illustrated an apparatus  500  for controlling the rate of path searching in accordance with an embodiment of the present invention. The path searcher unit  520  is periodically activated to detect new path delays. More specifically, the rate of path searching or the periodic interval during which the path searcher unit  520  is active is varied based upon the number of new path delays detected in comparison to at least one threshold value. The number of new path delays detected is utilized to calculate a timer value T that indicates the rate of path searching.  
     [0029] The timer value T is generally based on the number of new path delays detected by the path searcher unit  520 . If the number of new path delays detected is greater than a threshold value τ, the timer value T is decreased thereby activating the path searcher unit more frequently and after shorter periodic delays. This increases the rate of path searching.  
     [0030] However, if the number of new path delays detected is less than the threshold value τ, the timer value T is increased thereby activating the path searcher unit less frequently and after longer periodic delays. Thus, the rate of path searching is decreased. In one embodiment, the timer value T is increased if any new path delays are detected and is only decreased if no new path delays are detected.  
     [0031] In an exemplary embodiment of the present invention, a front end receiver (FeRx)  505  is enabled to begin receiving data y(k). Due to multipath propagation, the various multipath components from the different radio paths arrive at a receiving device at slightly different times due to the delays associated with the multipath propagation. The received signal y(k) is provided to a RAKE receiver  510 . As known in the art, the RAKE receiver remedies the effect of multipath propagation by using several baseband correlators to individually process several signal multipath components. The correlator outputs are combined to achieve improved communications reliability and performance.  
     [0032] The received signal y(k) is simultaneously provided to the path searcher unit  520  which periodically searches for new path delays. The path searcher unit  520  may contain one or more correlators which scan the time delay for a short period of time looking for strong signals. The path searcher unit  520  provides the path delay estimates to the control unit  515 . The control unit  515  utilizes the path delay estimates to determine whether new path delays exist.  
     [0033] If it is determined by the control unit  515  that no new path delays exist, the control unit  515  increases the timer value T. This timer value T is loaded into a timer, which counts down from the loaded timer value T and activates the path searcher unit  520  after a time interval equal to T. In other words, the control unit  515  increases the delay between the periodic intervals at which the path searcher unit  520  searches for new path delays. Thus, the rate of path searching is reduced.  
     [0034] However, if it is determined by the control unit  515  that new path delays exist, the control unit performs a comparison of the number of new path delays detected with a threshold value τ. If it is determined that the number of new path delays detected is greater than the threshold value τ, the control unit  515  decreases the timer value T. This timer value T is loaded into the timer. As stated above, the timer counts down from the loaded timer value T and activates the path searcher unit  520  after a time interval equal to T. The timer value T, in this case, is set such that it decreases the delay between the periodic intervals at which the path searcher unit  520  will search for new paths, thus, increasing the rate of path searching.  
     [0035] In an illustrative embodiment, the threshold value τ is effectively set such that 0&lt;τ&lt;1. More specifically, if no new path delays exist or if the number of new path delays detected (n) is less than the threshold value T (i.e., n=0), the timer value T loaded into the timer equals T=T*2, where T can be any maximum T value.  
     [0036] However, if it is determined that the number of new path delays detected is greater than the threshold value τ (i.e., n≧1) , the maximum timer value T loaded into the timer equals T=T/2, where T can be any minimum T value. Thus, the rate of path searching is altered by varying the delay between the periodic intervals at which the path searcher unit  520  searches for the presence of new path delays.  
     [0037] In another embodiment of the present invention, the number of new path delays detected (n) are compared to multiple threshold values τ1and τ2. As stated above, the path searcher unit  520  provides the path delay estimates to the control unit  515 . If it is determined by the control unit  515  that new path delays exist, the control unit performs a comparison of the number of new path delays detected with a first threshold value τ1. For example, the first threshold value τ1 can be effectively set to identify situations in which a relatively large number of new path delays exist.  
     [0038] If it is determined that the number of new path delays detected (n) is greater than the first threshold value τ1 (e.g., n≧3), the timer value T loaded into the timer equals T=T/4, down to some minimum T value. In other words, the value of T will be substantially decreased, but it will not go below a predetermined minimum T value (e.g., T=max (T/4, min value T)) . This rapidly decreases the delay between which the path searcher unit  520  searches for new path delays, thus, increasing the rate of path searching.  
     [0039] However, if it is determined that the number of new path delays detected (n) is less than the first threshold value τ1, then a second comparison is performed between the number of new path delays detected with a second threshold value τ2. For example, the second threshold value τ2 can be effectively set such that 0&lt;τ2&lt;1. If it is determined that the number of new path delays detected is less than the first threshold value τ1 but is greater than the second threshold value τ2 (i.e., τ2&lt;n&lt;τ1), the timer value T loaded into the timer equals T=T/2, down to some minimum T value (e.g., T=max(T/2, min value T)).  
     [0040] However, if the number of new path delays detected is less than the first threshold value τ1 and is also less than the second threshold value (i.e., n&lt;τ2), the timer value T loaded into the timer equals T=T*2. Thus, the rate of path searching is decreased. This example results in an increase in the path search rate if new paths exist and a decrease in the path search rate if no new paths exist.  
     [0041] Although the foregoing example uses only two thresholds, it will be understood by persons of ordinary skill in the art that any number of thresholds can be used in connection with the invention. In addition, the mechanism for increasing or decreasing the path search rate is not limited to the disclosed examples but can use any type of algorithm or process for increasing or decreasing the path search rate. It will be understood that, for some number of new path delays (e.g., n=1), it is possible for the timer value T to remain unchanged. In such a case, for example, the path search rate would decrease at times when no new paths are detected, would remain unchanged if one new path is detected, and would increase if more than one new path is detected.  
     [0042] In yet another embodiment of the present invention, the control unit  515  uses the output x(k) from the RAKE receiver  510  to determine if one or more of the strongest signal paths disappear during a short interval of time. An estimated Signal-to-Interference ratio (SIR), or a measure of the signal strength relative to background noise can be utilized to vary the rate of path searching. More specifically, during the time the path searcher unit  520  is periodically inactive, the control unit checks to see if at least one of the strongest signal paths is decreasing in SIR at or above a certain rate.  
     [0043] In a preferred embodiment of the present invention, if one of the strongest SIR for a rake finger is decreasing (y) dB during (z) slots, then the timer value T loaded into the timer is immediately set to zero and the path searcher unit is activated instantaneously. This will help to ensure that the receiver continues to receive a combined signal of sufficient strength to adequately detect the transmitted information. However, if none of the strongest signal paths are decreasing in SIR at or above a certain rate, then the timer value T remains unchanged at least until the next time it is determined whether there are any new paths.  
     [0044] Referring now to FIG. 6, there is illustrated a flow diagram  600  of a method for controlling the rate of path searching in accordance with principles of the present invention. Although the steps of the method are depicted in a particular sequence, it will be appreciated by persons of ordinary skill in the art that certain steps of the method do not necessarily follow a strict sequence but can be rearranged and/or performed simultaneously. At step  602 , the path searcher unit  520  is activated for a fixed period of time to search for new path delays. The path searcher unit  520  forwards path delay estimates, which are used to determine whether new path delays exist.  
     [0045] At step  604 , the received path delay estimates are monitored to determine if any new path delays exist. If it is determined that no new path delays exist, a timer value equal to T=T*2 (step  606 ) is loaded into the timer at step  616 . In other words, since no new path delays were detected at step  606 , the rate of path searching is reduced by increasing the periodic time delay during which the path searcher unit  520  searches for the presence of new path delays.  
     [0046] However, if it is determined at step  604  that new path delays exist, then at step  608 , a comparison is performed between the number of new path delays detected with a first threshold value. If the comparison indicates that the number of new path delays is greater than the first threshold value (step  608 ) a timer value equal to T=T/4 (step  610 ) is loaded into a timer at step  616 . More specifically, since the number of new path detected was greater than a first threshold value at step  608 , the rate of path searching is increased by loading a timer with a value that significantly decreases the periodic time delay during which the path searcher unit  520  searches for the presence of new path delays, thereby quickly increasing the rate of path searching.  
     [0047] On the other hand, if the comparison at step  608  indicates that the number of new path delays is less than the first threshold value, a comparison is performed at step  612  between the number of new path delays detected with a second threshold value. If it is determined that the number of new path delays detected is greater than the second threshold value (step  612 ) a timer value equal to T=T/2 (step  614 ) is loaded into a timer at step  616 . More specifically, since the number of new path detected was less than the first threshold value at step  608 , but greater than the second threshold value (step  612 ), the rate of path searching is increased but not as quickly as at step  610 .  
     [0048] However, if it is determined at step  612  that the number of new path delays is both less than the first threshold value (step  608 ) and less than the second threshold value (step  612 ), the rate of path searching remains unchanged and the timer is loaded at step  616  with a value that does not change the periodic time delay during which the path searcher unit  520  searches for the presence of new path delays.  
     [0049] It will be understood that the embodiment of the invention depicted in FIG. 6 represents a process with three threshold comparisons one at each of steps  604 ,  608 , and  612 . In other embodiments, it is possible to use more or fewer thresholds. For example, step  612  could be omitted, and a negative result of the comparison at step  608  would lead to step  614 . This omission would results in a change to the timer value T after each and every path search.  
     [0050] Additionally, to take into account any significant changes in path delays while the path searcher unit  520  is inactive, the Signal-to-Interference ratio (SIR) , or the measure of the signal strength relative to background noise, is constantly monitored at step  618 . Based on this monitoring, it is determined at step  620  if one or more of a selected number of the strongest signal paths disappear or otherwise decrease at or above some predetermined amount during a short interval of time. If it is determined at step  620  that none of the signal paths have disappeared or are decreasing at or above a certain rate, the SIR monitoring continues at step  618 .  
     [0051] If it is determined that at least one of the signal paths disappears or decreases at or above a certain rate, then at step  622  a timer value equal to T=0 is loaded into the timer at step  616 , thus, activating the path searcher unit  520  instantaneously. Generally, the process of steps  618 - 622  is performed in parallel with steps  602 - 616 .  
     [0052] Although preferred embodiment(s) of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it is understood that the present invention is not limited to the embodiment(s) disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.