WO2004112269A1 - Positionnement d'une fenetre de recherche de trajets multiples - Google Patents

Positionnement d'une fenetre de recherche de trajets multiples Download PDF

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Publication number
WO2004112269A1
WO2004112269A1 PCT/EP2004/004946 EP2004004946W WO2004112269A1 WO 2004112269 A1 WO2004112269 A1 WO 2004112269A1 EP 2004004946 W EP2004004946 W EP 2004004946W WO 2004112269 A1 WO2004112269 A1 WO 2004112269A1
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Prior art keywords
gravity
center
search window
values
delay
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PCT/EP2004/004946
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English (en)
Inventor
Elias Jonsson
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Telefonaktiebolaget Lm Ericsson (Publ)
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Priority claimed from EP03388046A external-priority patent/EP1487127B1/fr
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Publication of WO2004112269A1 publication Critical patent/WO2004112269A1/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7073—Synchronisation aspects
    • H04B1/7075—Synchronisation aspects with code phase acquisition
    • H04B1/70754—Setting of search window, i.e. range of code offsets to be searched
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7113—Determination of path profile
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
    • H04B1/7117—Selection, re-selection, allocation or re-allocation of paths to fingers, e.g. timing offset control of allocated fingers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
    • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
    • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
    • H04B2201/70707—Efficiency-related aspects

Definitions

  • the invention relates to a method of detecting multipath components in a time-varying fading radio channel in a digital wireless communications system in which individual multipath components of a signal transmitted through said channel are received with individual delays within a range of possible delay values.
  • the method comprises the steps of calculating repetitively a delay profile indicating a magnitude for each of a number of individual delay values; searching repetitively for new multipath components in a search window constituting a subset of said range of possible delay values; and positioning said search window based on at least one previously calculated delay profile.
  • the invention further relates to a receiver having means for detecting multipath components in a time-varying fading radio channel, and a corresponding computer program and a computer readable medium.
  • the physical channel between a trans- mitter and a receiver is typically formed by a radio link.
  • the transmitter could be a base station, and the receiver could be a mobile station, or vice versa.
  • the transmit antenna is not narrowly focused towards the receiver. This means that the transmitted signals may propagate over multiple paths.
  • the receiver may receive multiple instances of the same signal at different times, i.e. with different delays, because different portions of the signal are reflected from various objects, such as buildings, moving vehicles or landscape details.
  • portions with similar propagation distances combine at the receiver and form a distinct multipath component.
  • the effect of the combining depends on the instantaneous relationship of the carrier wavelength and distance differences, and it may thus for a given multipath component be either enhancing or destructive.
  • the combining leads to significant decrease of the magnitude, or fading, of the path gain for that path.
  • CDMA Code Division Multiple Access
  • WCDMA Wideband Code Division Multiple Access
  • PN pseudo-random noise
  • chips The number of chips used to spread one data bit, i.e. chips/bit, may vary, and it depends, at least in part, on the data rate of the channel and the chip rate of the system.
  • the received signal In the receiver the received signal must be despread and demodulated with the same spreading code using the same chip rate to recover the transmitted data. Furthermore, the timing of the demodulation must be synchronized, i.e. the despreading code must be applied to the received signal at the correct instant in time, which can be difficult due to the multipath effects mentioned above.
  • the performance of a CDMA receiver is improved if the signal energy carried by many multipath components is utilized. This is achieved by using a RAKE receiver, where each multipath component is assigned a despreader whose reference copy of the spreading code is delayed equally to the path delay of the corresponding multipath component.
  • the outputs of the de- spreaders, i.e. the fingers of the RAKE receiver are then coherently com- bined to produce a symbol estimate.
  • the RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for all paths.
  • SNR signal-to-noise ratio
  • the signal energy from as many physical paths as possible should be collected.
  • tracking as many different physical paths as possible significantly increases the robustness of reception, since this reduces the probability of a simultaneous deep fade of all paths, a phenomenon leading to serious and sometimes catastrophic degradation of the block error rate (BLER).
  • BLER block error rate
  • the structure of the propagation channel i.e. the absolute and relative delays of the individual multipath components, does not remain constant over time. Due to relative movement of the transmitter, the receiver and the objects in their vicinity, the delays of existing paths change, old paths disappear and new paths appear. In addition, the frequency offset between the transmitter and receiver circuits gives rise to a slow clock drift, which manifests itself as a gradual movement of the whole delay profile along the time axis. To ensure proper operation of the RAKE receiver, the changing delays of all known mul- tipath components should be tracked, and new paths should be discovered quickly after they appear.
  • the impulse response of a channel may be evaluated by transmitting a known pilot signal which is then despread and demodulated in the receiver.
  • a known pilot signal which is then despread and demodulated in the receiver.
  • the Common Pilot Channel CPICH
  • the received signal is correlated with the scrambling code for the cell and the CPICH channelization code, i.e. the CPICH symbols are de- spread and then accumulated. This is done for a given number of time de- lays.
  • the strength of the accumulated signal for a given delay reflects the power contained at this time-delay. If it is judged to be higher than a specified noise floor, we say that it represents a multi-path.
  • An approach to delay estimation in a RAKE receiver implementation could involve evaluation of the impulse response of the channel over the whole range of possible delays of the channel.
  • the resulting delay profile which may be a complex delay profile (CDP) or a power delay profile (PDP)
  • CDP complex delay profile
  • PDP power delay profile
  • the processing and power consumption expense of frequently executing this full path searching routine is usually prohibitive.
  • measurements of the signal quality from neighbouring cells should also be periodically reported, with the purpose of choosing the cell or set of cells that best can serve the user equipment.
  • the receiver should be able to monitor up to eight cells per frequency, and up to three frequencies should be monitored. Thus, a total of up to twenty-four cells need to be monitored.
  • typical implementations may use path searchers with observation windows shorter than the full search area.
  • the path searcher resolution may be reduced and additional densely-sampled windows of de- spreaders may be used that produce higher-resolution estimates of certain areas of the delay profile.
  • the path searcher stage is used periodically to re-scan the delay range with the purpose of detecting new paths.
  • a current power delay profile may comprise a set of power values for a num- ber of delay values. This set may represent the set of currently tracked paths, in which case the delay values will usually not be contiguous. It may also represent a contiguous region over which the path search is conducted. Typically, a path searcher correlates over a consecutive number of time delays, spaced at quarter, half, or chip distance. Spacing the delays equidistantly lends to a less complex hardware design. Other ways of representing the power delay profile are also possible.
  • the task is to determine a suitable position for the path searcher window.
  • a typically used method for determining a suitable path searcher window posi- tion for the next path search activation is based on computing the center of gravity (COG), also known as the mean excess delay, of the presently known power delay profile estimate.
  • COG center of gravity
  • the path searcher window is placed so that most of the channel power would be covered by the window.
  • the center of gravity estimate is used as the center position of the path searcher window, but the window may also be positioned asymmetrically around the center of gravity estimate.
  • a typical shape of the power delay profile exhibits exponential decay, i.e. the energy is concentrated towards the beginning of the region of interest. For a reasonable coverage the window could thus be placed e.g. 1/3 ahead of and 2/3 behind the center of gravity value.
  • the same idea can be applied to some other statistical measure related to the distribution of the power delay profile.
  • the center of gravity gives a consistent and reliable estimate of the true energy concentration in the channel.
  • the signal-to-noise ratio of the power delay profile is poor, and/or when the power delay profile used to compute the center of gravity is significantly time-varying due to fading, the center of gravity result tends to exhibit different problems.
  • the noise-induced component of the power delay profile values will cause a bias term which shifts the center of gravity result towards the average (non-power weighted) delay of all the entries of the power delay profile.
  • the size of the bias term depends on how far from each other the true center of gravity and the "average delay” are separated, and on the signal-to-noise ratio. In many practical cases, the bias term may be large enough to shift the path searcher window away from significant portions of the true power delay profile.
  • the power delay profile values may be thresholded, which will remove a certain fraction of the noise- only samples and reduce the bias.
  • noise removal assumes the use of a rather high threshold, which in turn may remove the channel components from the power delay profile as well, and thus distort the power delay profile.
  • the noise effect may also be reduced by noise subtraction, where the average noise power in the power delay profile is estimated, and instead of the original power delay profile values, values reduced by the average noise power are used in the center of gravity computation. In this way the bias is reduced, but since the noise floor cannot be estimated exactly, it cannot be removed completely, and a residual bias effect remains.
  • the window must be posi- tioned precisely in order to include all clusters of energy in the power delay profile. Any temporary fluctuation in the power delay profile seen by the center of gravity algorithm, e.g. due to fading, may offset the computed center of gravity value, and the window will be placed so that some of the true paths will be located outside the window. Thus the method might fail to catch any multipath delays that might appear or exist outside the path searcher window. As an example, two paths may be located within 4 chips of each other, and a third path is located 77 chips from the second one.
  • the path searcher window may be on chip resolution, and the length of the path-searcher window may be 80 chips.
  • the cell-searcher finds the third path, and we then update the window position by centring the new window on the center of gravity of the power delay profile, we would fail to find the first two paths and thus loose two thirds of the energy transmitted by the base-station. In subsequent center of gravity computations only the remaining paths are used, and the window shifts away from the lost (but physically still present) paths. This leads to increased interference levels, reduced signal-to-interference ratio and reduced diversity, and in some cases it can cause loss of synchronization with the transmitter side.
  • EP 1 257 067 shows a searcher in which the search range is successively extended when no active path is detected.
  • the search range may be extended until it covers the whole maximum delay range. If and when paths of sufficient quality are found, the normal size search window may be used again.
  • this solution does not solve the problem mentioned above, because paths of sufficient quality can actually be detected, and therefore the search range would not be extended.
  • the normal search window is actually wide enough, but the known methods of positioning the search window tend to position the window so that some important paths are not covered.
  • the invention shall provide a low complexity hardware implementation with a low memory solution for bookkeeping the signal quality from each cell.
  • the object is achieved in that the positioning of said search window is at least intermittently based on a delay profile calculated for a range of delay values that is wider than said search window.
  • the method further comprises the step of alternating between searching a number of times in a search window positioned based on a delay profile calculated for a previous search window; and searching a number of times in a search window positioned based on a delay profile calculated for a range of delay values wider than said search window, further computational resources are saved because the wider window is only used intermittently.
  • the method further comprises the steps of calculating a delay profile for the wider range of delay values a number of times; calculating a filtered delay profile from said number of calculated delay profiles; and using said filtered delay profile as the delay profile calculated for the wider range of de- lay values, it is ensured that very short noise spikes are not erroneously detected as paths, because they are filtered out.
  • the method further comprises the step of generating the wider range of delay values by combining a previous search window and at least one additional window comprising delay values adjacent to the previous search window.
  • the wider range of delay values may be generated by combining the previous search window with two additional windows having the same size as the previous search window and being arranged contiguous to each other symmetrically around the center of the previous search window.
  • the method further comprises the steps of calculating a center of gravity for said previously calculated delay profile; and positioning the search window around said calculated center of gravity, and in one embodiment the search window is positioned with the calculated center of gravity as its center.
  • the method may further comprise the steps of calculating a center of gravity for each of said previous search window and the two additional windows; cal- culating a combined center of gravity by combining the center of gravity for said previous search window with the center of gravity for at least one of the two additional windows; and positioning the search window around said combined center of gravity.
  • the step of calculating a combined center of gravity may expediently comprise the step of calculating a relative center of gravity value for each of the two additional windows by subtracting the center of gravity for said previous search window from the center of gravity calculated for each of the two addi- tional windows. Further, the step of calculating a combined center of gravity may comprise the steps of calculating said relative center of gravity values for the two additional windows a number of times; and calculating a filtered relative center of gravity value for each of the two additional windows from said number of calculated relative center of gravity values.
  • the method may further comprise performing repetitively the steps of resetting said filtered relative center of gravity values; calculating said relative center of gravity values for the two additional windows a number of times; and updating for each pair of new relative center of gravity values said filtered relative center of gravity values by including the pair of new relative center of gravity values; and calculating said combined center of gravity by combining the center of gravity for said previous search window with at least one of said updated filtered relative center of gravity values.
  • the features of the method described above and in the following may be implemented in software and carried out in a data processing system or other processing means caused by the execution of program code means, such as computer-executable instructions.
  • the program code means may be loaded in a memory, such as a RAM, from a storage medium or from another computer via a computer network.
  • the computer program means may be received embodied as a data signal.
  • the described features may be implemented by hardwired circuitry instead of software or in combination with software.
  • the invention further relates to a receiver having means for detecting multipath components in a time-varying fading radio channel in a digital wireless communications system in which individual multipath components of a signal transmitted through said channel are received with individ- ual delays within a range of possible delay values.
  • the receiver is adapted to calculate repetitively a delay profile indicating a magnitude for each of a number of individual delay values; search repetitively for new multipath components in a search window constituting a subset of said range of possible delay values; and position said search window based on at least one previously calculated delay profile.
  • a receiver When the receiver is further adapted to base the positioning of said search window at least intermittently on a delay profile calculated for a range of delay values that is wider than said search window, a receiver is provided that is able to detect multipath components located or appearing outside the path search window that is used for reporting paths to other parts of the receiver.
  • the receiver has a low complexity hardware implementation with a low memory solution for bookkeeping the signal quality from each cell.
  • the receiver When the receiver is further adapted to alternate between searching a number of times in a search window positioned based on a delay profile calculated for a previous search window; and searching a number of times in a search window positioned based on a delay profile calculated for a range of delay values wider than the search window, further computational resources are saved because the wider window is only used intermittently.
  • the receiver is further adapted to calculate a delay profile for the wider range of delay values a number of times; calculate a filtered delay profile from said number of calculated delay profiles; and use said filtered delay profile as the delay profile calculated for the wider range of delay values, it is ensured that very short noise spikes are not erroneously detected as paths, because they are filtered out.
  • the receiver is further adapted to generate said wider range of delay values by combining a previous search window and at least one additional window comprising delay values adjacent to the previous search window.
  • the receiver may further be adapted to generate the wider range of delay values by combining the previous search window with two additional windows having the same size as the previous search window and being arranged contiguous to each other symmetrically around the center of the previous search window.
  • the receiver may further be adapted to calculate a center of gravity for said previously calculated delay profile; and position the search window around said calculated center of gravity, and in one embodiment the receiver is further adapted to position the search window with said calculated center of gravity as its center.
  • the receiver may further be adapted to calculate a center of gravity for each of said previous search window and said two additional windows; calculate a combined center of gravity by combining the center of gravity for said previous search window with the center of gravity for at least one of said two additional windows; and position the search window around said combined center of gravity.
  • the receiver may further be adapted to calculate a relative center of gravity value for each of said two additional windows by subtracting the center of gravity for said previous search window from the center of gravity calculated for each of said two additional windows. Further, the receiver may be adapted to calculate said relative center of gravity values for the two addi- tional windows a number of times; and calculate a filtered relative center of gravity value for each of the two additional windows from said number of calculated relative center of gravity values.
  • the receiver may further be adapted to reset said filtered relative center of gravity values; calculate said relative center of gravity values for the two additional windows a number of times; and update for each pair of new relative center of gravity values said filtered relative center of gravity values by including the pair of new relative center of gravity values; and calculate said combined center of gravity by combining the center of gravity for said previous search window with at least one of said updated filtered relative center of gravity values.
  • the receiver may expediently be adapted for use in a Wideband Code Division Multiple Access system.
  • the invention also relates to a computer program and a computer readable medium with program code means for performing the method described above.
  • figure 1 shows an example of multiple paths between a base station and a mobile station
  • figure 2 shows a power delay profile for the paths illustrated in figure 1 ,
  • figure 3 illustrates the use of a path searcher in combination with a RAKE receiver
  • figure 4 shows a sampled power delay profile corresponding to the profile shown in figure 2
  • figure 5 shows a flow chart illustrating the steps of a part of a path searcher
  • figure 6 illustrates a situation where the known center of gravity method fails to find all relevant paths
  • figure 7 shows how an extended window is intermittently used in a path searcher
  • figure 8 shows a flow chart illustrating the use of extended window shown in figure 7
  • figure 9 shows different locations three windows that together form an extended window
  • figure 10 shows a flow chart illustrating an embodiment using the combina- tion of three windows as an extended window
  • figure 11 shows a flow chart illustrating a part of the flow chart of figure 10 in more detail.
  • Figure 1 shows a situation in which a base station 1 and a mobile station 2 of a wireless communications system communicate with each other.
  • a signal transmitted from the base station 1 is received by the mobile station 2.
  • the transmitted signal travels along multiple paths from the base station to the mobile station.
  • there is a direct and unobstructed propagation path 3 but in addition to this direct path, reflections from objects in the surroundings cause a number of indirect paths to exist. Two such paths are shown in the figure.
  • One indirect path 4 is reflected from a house 5, while another path 6 is caused by reflection from another building 7.
  • the power P received at the mobile station 2 as a function of the time t may look as illustrated in figure 2, which shows an example of a power delay profile.
  • the power delay profile shows all signals received at the mobile station, including noise and interference signals. However, only the peaks in the power delay profile correspond to the multipath components of the transmitted signal. Together these peaks form the impulse response of the channel.
  • the peak P 3 received at the time t a corresponds to the direct path 3 in figure 1
  • the peaks P b and P 0 received at the times t b and t G respectively, correspond to the indirect paths 4 and 6 in figure 1.
  • the delay of the path 6 (corresponding to the peak P 0 ) is larger than the delay of the path 3 (corresponding to the peak P 3 ).
  • the mobile station 2 and the base station 1 may be adapted for use in e.g. a Code Division Multiple Access (CDMA) system or a Wideband Code Division Multiple Access (WCDMA) system, and in that case the mobile station 2 may use a RAKE receiver, which is capable of identifying and tracking the various multipath signals for a given channel. In this way the energy or power of several multipath components can be utilized in the receiver. As mentioned above, this may be achieved by using a RAKE receiver, where each multi- path component is assigned a despreader whose reference copy of the spreading code is delayed equally to the path delay of the corresponding multipath component. The outputs of the despreaders, i.e. the fingers of the RAKE receiver, are then coherently combined to produce a symbol estimate.
  • the RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for all paths. The signal energy from as many physical paths as possible should be collected.
  • the delay of existing paths change, old paths disappear and new paths appear.
  • the changing delays of all known multipath components should be tracked, and new paths should be discovered quickly after they appear.
  • most of the time the delays of newly appeared paths are relatively similar to those of the old, known paths, and thus they may be detected by searching in the same neighbourhood.
  • the impulse response of the channel could be evaluated over the whole range of possible delays in the channel, and the resulting delay profile could then be subjected to peak detection and the peak locations reported to the RAKE receiver as the delay estimates.
  • the processing and power consumption of executing such a full path searching routine frequently is usually so high that it cannot be realized in a practical system.
  • infrequently activated limited-range path searchers are typically used for detecting new paths and, in some implementations, for re-detecting temporarily faded existing paths. This is illustrated in figure 3, in which a path searcher 11 is used in combination with a RAKE receiver 13.
  • the path searcher 11 is a device that periodically computes instantaneous impulse response estimates (complex or power) over a range of delays that only constitutes a fraction of the maximum delay spread allowed by the system. This reduced range of delays is also called a path search window.
  • the complex or power values for a given delay value may be estimated e.g. by correlating the received data for pilot symbols with an appropriately delayed copy of the spreading sequence. Since the path searcher 11 is mainly used to detect only the existence of paths, its output resolution may be lower than that required by the RAKE receiver 13.
  • the RAKE receiver 13 itself uses a general delay estimation algorithm 12, which is able to extract the path positions and find their delays with sufficient accuracy, once they are discovered by the path searcher 11.
  • an important task in the path searcher is to position the path searcher window so that new paths can be expected to be included in the window.
  • new paths will with high probability appear in the vicinity of the currently known paths, and thus the path search window for a new search is normally placed so that it covers the currently known paths.
  • This can be done in different ways.
  • One method is the center of gravity method, but as mentioned this method suffers from a number of problems.
  • the delay profile of the received signal will not be available as a continuous curve as the one illustrated in figure 2. Instead, it will consist of a number of sample values. This is illustrated in figure 4, which shows a sampled power delay profile corresponding to the continuous delay profile shown in figure 2.
  • the available estimates of the power delay profile constitutes a contiguous sequence of equally spaced samples.
  • the power values Y a , Y b and Y c at the delay values ⁇ ⁇ % and ⁇ c correspond to the peaks P a , P b and P 0 shown in figure 2.
  • EJI 0 and RSCP inform the network of the quality of a given cell.
  • RSCP and RSSI are computed, which then give E c U 0 .
  • a path searcher computes RSCP and RSSI. It correlates the received signal with the scrambling code for the cell and the CPICH channelization code, i.e. the CPICH symbols are despread and then accumulated. This is done for a given number of time delays. The strength of the accumulated signal for a given delay ⁇ d reflects the power Y d contained at this time delay. If it is judged to be higher than a specified noise floor, it is said to represent a multipath and its power contributes to the RSCP.
  • Block 101 we despread the pilots, usually the CPICH, for a given set of time delays.
  • Block 102 we multiply the received despread pilots by the known complex conjugate of the pilot symbols. This essentially gives an approximation of the radio channel for this time delay.
  • Block 103 we add a number N c of these de-rotated pilot symbols. This is called coher- ent summation and reduces the noise inherent with the de-rotated pilots/radio channel estimates.
  • Block 104 we square the output of Block 103 for each time delay resulting in the value X d .
  • a number N nc of the X ⁇ values are summed.
  • the result is the output power value Y d for the delay with index d .
  • This is called non-coherent summation and the purpose is again to combat noise.
  • Coherent summation is more effective than noncoherent summation in suppressing noise, but also requires that the radio channel does not change during a coherent summation.
  • Y d is the output for delay with index d .
  • D delays d l,...,D , and that the corresponding delays are spread equidistantly.
  • D IOO as a recommended value.
  • a subset ⁇ d of the power values Y d can be defined as the subset of the index set ⁇ 1...J3 ⁇ selecting the N d smallest Y d values.
  • N d D-20.
  • ⁇ d be a subset of the remainder set ⁇ .
  • d r ⁇ d; d e ⁇ l... D) and d £ ⁇ d ⁇ .
  • ⁇ we can choose ⁇ to consist of the indexes / fulfilling
  • Z is interpreted as the noise power that is subtracted from the peak power S d . This is because S d contains both the noise and signal energy (if it exists) from the multipath delay.
  • the center of gravity (COG) method is now described.
  • the center of gravity for the time delays is computed as
  • ⁇ d is the time delay corresponding to the index d .
  • the computed center of gravity values may further be filtered, i.e.
  • the index n enumerates the activations of the path-searcher over time.
  • the filtered center of gravity value is used as the center position of the path searcher window, but it should be noted that the window may also be positioned asymmetrically around the center of gravity value. Because of the space loss, the shape of the power delay profile may exhibit exponential decay, i.e. the energy is concentrated towards the beginning of the region of interest. For a reasonable coverage the window could thus be placed e.g. 1/3 ahead of and 2/3 behind the center of gravity value.
  • the situation may have changed as shown in the lower part of figure 6, where the multipath component Y b has disappeared, while two new components Y 0 and Y d have appeared 50 and 54 chips from Y 0 , respectively.
  • the new components Y 0 and Y d are located outside the search window centered between Y a and Y b , and thus they will not be found by the path searcher. This means that two thirds of the energy transmitted from the base station is lost. Further, they will not be used in the calculation of the next center of gravity, and therefore, as shown, the search window will continue to be centered around Y a , even for many subsequent searches.
  • the search window would have been positioned as the alternative search window shown at the bottom of figure 6, and the two new paths would have been detected in the next search.
  • the search window is actually wide enough, it is just not positioned correctly.
  • this problem could be solved by simply using a wider search window, but performing all the necessary calculations for a sufficiently wider window would increase considerably the memory consump- tion and the needed computational resources, and remembering that 24 cells need to be monitored continuously, such a solution will not be possible in a mobile terminal.
  • a method is described below, in which the size of the search window itself is maintained, while the center of gravity is calcu- lated based on a range of delay values that is larger than the search window.
  • One way of implementing the larger range of delay values for the calculation of the center of gravity is to perform the calculation of the center of gravity (and the path search, although the path search itself may also be performed at a higher rate) with the normal search window as described above, and then intermittently, e.g. for every 5 th or 10 th search, search for paths in a wider or extended window and use these for the calculation of the center of gravity.
  • N ⁇ 5
  • n enumerates the number of activations of the center of gravity calculation.
  • the normal search window WQ e.g.
  • a structure of a corresponding method is illustrated in the flow chart 200 in figure 8, which is activated each time a new center of gravity is to be calcu- lated. It is noted that when the receiver is switched on for the first time, the system has to be initiated, which is not shown in the figure. Typically, this can be done with a cell searcher, which is an initial search that delivers only one path and that is only done once.
  • step 202 If the normal window W 0 is to be used, the relevant paths in that window are found in step 202. Typically, this will of course be an integrated part of the normal path search process. Having found the path components, the center of gravity can now be calculated in step 203 as described above. In step 204 the position of the next search window is then adjusted to correspond to the new center of gravity.
  • step 201 If it was decided in step 201 that it was time to use the extended search window W ext for the next center of gravity calculation, the paths that are present in that wider window are found in step 205. This can be done by simply performing the normal path search process for the complete extended search window, or the path search process is performed for the normal search win- dow and then combined with a special search performed only for those parts of the extended window that are not included in the normal window.
  • the center of gravity can be calculated in step 206 in the same way as in step 203, but of course including all paths found in the extended search window. Then as before, the position of the next search win- dow is then adjusted in step 204 to correspond to the new center of gravity.
  • the calculation of the center of gravity based on a wider window than the one used for the normal path search could be done intermittently, e.g. for every 5 th or 10 th search.
  • the wider window may also be used for each center of gravity calculation, although the reduction in memory consumption and computational resources is then rather small, but if the path search is performed at a higher rate than the center of gravity calculations, there will still be a reduction when the normal search window is used for those path searches that are not combined with a center of gravity calculation.
  • the wider window needs only be used for the calculation of the center of gravity, while only paths in the normal are stored and reported to the delay estimator.
  • the path searcher can be run a total of three times, once for the reference window W 0 , i.e. the window we base our measurement reporting on, and then for two other window locations W_ x and W 1 .
  • the path searcher computes values Y d for a set of delays indexed by d ⁇ ,...,D and the delays are equi- distantly spaced.
  • These two windows can be of the same size as W 0 , but they may also have different sizes. Further, they may be positioned in different locations in relation to W 0 .
  • Figure 9 shows different locations of the win- dows W 0 , W_ x and W x , and in this case W_ x and W x are shown with the same size as W 0 .
  • the windows W_ x and W x are shown contiguous to W 0 , so that the three windows together form a window three times the size of W 0 .
  • the windows W_ x and W x may also be contiguous to each other so that an end point of each coincides with the midpoint of window W 0 , as shown in the upper example, or they may even overlap each other as shown in the middle.
  • window configuration where one of the end points of window W_ x and W x coincides with the midpoint of window W 0 , i.e. the upper configuration in figure 9.
  • An extended window is used intermittently, i.e. once for every N ext center of gravity calculations, and in this example the extended window is further used a number N ext , U p d at e of times, and the results filtered, before the center of gravity is actually updated.
  • the example is illustrated in the flow chart 300 in figure 10.
  • the spread pilots are supplied to the path searcher hardware block in step 301. From these pilot signals the new multipath components Y d , the new RSCP value and the new center of gravity are computed for the normal search window W 0 in step 302. All these calculations have been described earlier. Filtered versions of the RSCP value and the center of gravity are then computed for window W 0 in step 303, also using the calculations presented earlier.
  • step 304 the method checks if paths outside of the normal search window W 0 need to be investigated, i.e. if the extended window should be used. If the answer is ' No', the location of the path searcher window is updated in step 305 with the new center of the window given by the filtered center of gravity. If the answer is ⁇ es ' , we proceed to step 306, where an ex- tended search over a wider window will be conducted.
  • index n enumerates the number of activations that adjust the window W 0 (step 305) using the (normal or extended) center of gravity method, i.e. activations of the flow chart 300.
  • m enumerates the activations of the extended window method described below.
  • we can select l ⁇ N ⁇ ⁇ lO with N 6xI S as a recommended value.
  • step 305 we adjust the location of the new window W 0 .
  • step 401 a number of filter parameters and other parameters are chosen, such as filter parameters X 03 X 1 for the RSCP or E 0 /I 0 values and the filter parameter ⁇ for the difference in center of gravity delays between the different windows, i.e. W 0 , W_ x and W x .
  • filter parameters X 03 X 1 for the RSCP or E 0 /I 0 values and the filter parameter ⁇ for the difference in center of gravity delays between the different windows, i.e. W 0 , W_ x and W x .
  • N m is the number of center of gravity calculations between each calculation with the extended window
  • N extupdaie is the number of extended window calculations before the center of gravity is actually updated.
  • step 402 the new RSCP values for the two windows JFL 1 and W x are computed as it was done for window W 0 in step 302. Observe that the RSCP values for W_ x and W x need only use the delays that do not belong to the W 0 window.
  • step 403 the center of gravity values ⁇ _ x and ⁇ x for windows W_ x and W x are computed. It is observed, that in this embodiment the center of gravity values ⁇ _ x and ⁇ x for W_ x , and W x only use the delays that do not belong to the W 0 window.
  • step 404 the filtered center of gravity ⁇ f lt computed in step 303 is then subtracted from the computed center of gravity values ⁇ 1 and ⁇ x for windows W_ x and W x in order to obtain the relative center of gravity values ⁇ _ j and A 1 . It is noted that ⁇ $ is the center of window W 0 .
  • the relative center of gravity values ⁇ _, and A 1 are calculated as
  • RSCP%$ A 1 RSCP ⁇ 1 + (L-A 1 )RSCP_ 1 , and
  • RSCPJff A 1 RSCPJgI + 0- -K)RSCP, (1 )
  • step 303 a filtered RSCP value was also compute for win- dow W 0 as
  • RSCP ⁇ A Q RSCP$ fi +(l -A 0 )RSCP 0 .
  • the filters are reset in step 409, i.e. ⁇ M _ j - ⁇ _ J ana A fi ⁇ t ⁇ - A 1 .
  • step 411 If mmodN ⁇ ⁇ O , the values RSCP ⁇ , BSCPJff , A ⁇ 1 and ⁇ j£> are just stored in step 411 until the next time we revisit the flow chart 400. Only the values RSCP$$ and ⁇ ⁇ ) are updated, but this has already been done in step 303.
  • the center of gravity is updated in step 412 using the extended method.
  • the center ⁇ +1) of the new position of the window W 0 may be computed as a weighted average of the window W 0 and one of the two windows W_ ⁇ and W 1 , depending on which one has the largest RSCP value.
  • RSCPJffi > , ⁇ J +1) is calculated as
  • center ⁇ +1) of the new position of the window W 0 can also be computed in other ways.
  • One other way is to weight all three windows instead of only two.
  • r ⁇ +1) may also be calculated as
  • a further possibility is to store the absolute values of the center of gravity values for the windows W_ x and W x , instead of their relative values A 1 ⁇ l 1 and A ( %$ • If the path delay profile is fixed, but the clock drift is high, this method is, however, less advantageous.
  • step 412 After the center of gravity has been updated in step 412 or the filtered values stored in step 411 , the process returns to step 305 where the position of the new window W 0 is adjusted in according to the new center of gravity that was calculated in either step 303 or step 412.

Abstract

Des composantes multitrajets d'un signal envoyé sur un canal radio numérique à variation temporelle sont reçues avec des retards individuels compris dans une plage de valeurs de retard possibles. Le procédé consiste à calculer de manière répétitive un profil de retard qui indique une longueur pour une pluralité de valeurs de retard individuelles; à rechercher de manière répétitive de nouvelles composantes multitrajets dans chaque fenêtre de recherche (W0) ; et à positionner la fenêtre de recherche (W0) sur la base d'au moins un profil de retard calculé auparavant. Le positionnement de la fenêtre (W0) est au moins basé de manière intermittente sur un profil de retard calculé pour une plage (Wext ; W0 , W-1, W1) de valeurs de retard qui est plus étendue que la fenêtre de recherche. De cette manière, on dispose d'un procédé qui permet de détecter les composantes multitrajets en dehors de la fenêtre de recherche de trajet qui est utilisée pour rapporter les trajets à d'autres parties du récepteur. Un récepteur correspondant comporte une réalisation matérielle à faible complexité dotée d'une solution mémoire de base.
PCT/EP2004/004946 2003-06-13 2004-05-06 Positionnement d'une fenetre de recherche de trajets multiples WO2004112269A1 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
EP03388046.9 2003-06-13
EP03388046A EP1487127B1 (fr) 2003-06-13 2003-06-13 Méthode et système pour le placement d'une fenêtre de recherche multitrajet
US47905403P 2003-06-16 2003-06-16
US60/479,054 2003-06-16

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CN1787392A (zh) * 2005-12-16 2006-06-14 凯明信息科技股份有限公司 定时跟踪和捕捉装置
CN100420329C (zh) * 2006-08-09 2008-09-17 华为技术有限公司 一种上行专用物理信道搜索方法及装置
US7693521B1 (en) 2004-08-04 2010-04-06 Sprint Spectrum L.P. Method and system for mobile station handoff
US7702355B2 (en) 2005-04-14 2010-04-20 Telefonaktiebolaget L M Ericsson (Publ) Cell selection in broadcast and multicast communication environments
US7894816B1 (en) 2005-03-16 2011-02-22 Sprint Spectrum L.P. Method of selecting carrier frequency for call origination

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EP0942539A2 (fr) * 1998-03-10 1999-09-15 Sony Corporation Méthode à réception et récepteur d'un signal à spectre étalé
US20010046221A1 (en) * 2000-05-18 2001-11-29 Thomas Ostman Radio receiver and channel estimator
US6370397B1 (en) * 1998-05-01 2002-04-09 Telefonaktiebolaget Lm Ericsson (Publ) Search window delay tracking in code division multiple access communication systems
GB2368238A (en) * 2000-10-17 2002-04-24 Ubinetics Ltd A method of searching a code space
EP1276248A1 (fr) * 2001-07-11 2003-01-15 Telefonaktiebolaget L M Ericsson (Publ) Surveillance du retard d'une fenêtre de recherche dans des systèmes de communication à accès multiple par répartition de codes

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EP0942539A2 (fr) * 1998-03-10 1999-09-15 Sony Corporation Méthode à réception et récepteur d'un signal à spectre étalé
US6370397B1 (en) * 1998-05-01 2002-04-09 Telefonaktiebolaget Lm Ericsson (Publ) Search window delay tracking in code division multiple access communication systems
US20010046221A1 (en) * 2000-05-18 2001-11-29 Thomas Ostman Radio receiver and channel estimator
GB2368238A (en) * 2000-10-17 2002-04-24 Ubinetics Ltd A method of searching a code space
EP1276248A1 (fr) * 2001-07-11 2003-01-15 Telefonaktiebolaget L M Ericsson (Publ) Surveillance du retard d'une fenêtre de recherche dans des systèmes de communication à accès multiple par répartition de codes

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7693521B1 (en) 2004-08-04 2010-04-06 Sprint Spectrum L.P. Method and system for mobile station handoff
US7894816B1 (en) 2005-03-16 2011-02-22 Sprint Spectrum L.P. Method of selecting carrier frequency for call origination
US7702355B2 (en) 2005-04-14 2010-04-20 Telefonaktiebolaget L M Ericsson (Publ) Cell selection in broadcast and multicast communication environments
CN1787392A (zh) * 2005-12-16 2006-06-14 凯明信息科技股份有限公司 定时跟踪和捕捉装置
CN100420329C (zh) * 2006-08-09 2008-09-17 华为技术有限公司 一种上行专用物理信道搜索方法及装置

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