Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/005—Control of transmission; Equalising
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/20—Arrangements for detecting or preventing errors in the information received using signal quality detector

Definitions

• the present invention relates to improvements in or relating to 5 mobile telecommunications, and is more particularly concerned with co- channel interference estimation.
• the UMTS terrestrial radio access (UTRA) - time division duplex (TDD) system is based on a combination of code division multiple access (CDMA) and hybrid time division multiple access
• TDMA time division multiple access
• TDD Time Division Multiple Access 10
• UMTS is an acronym for universal mobile telecommunication system as understood by persons skilled in the art.
• UTRA TDD is a cellular system, a spatial re-use approach is employed in which the same frequency (ies) is (are) used in every cell but in which different time slots from the TDMA structure are assigned,
• Uplink operation refers to data transfer from a mobile station to a base station
• downlink operation refers to data
• the group of base stations A, B and C could be a representative 'cluster' of cells which repeats as is well-known to persons skilled in the art.
• time slot allocation Whilst allocating time slots in this way is possible, it has the disadvantage that it requires careful planning of time slot allocation. Moreover, the time slot allocation is inflexible as it is not easy to alter the allocation of time slots on an 'as needed' basis as traffic density alters.
• UTRA TDD to employ dynamic channel assignment (DCA) wherein the assignment of time slots is performed automatically in a fashion which tends towards optimal performance.
• DCA dynamic channel assignment
• each mobile station needs to measure the level of inter cell interference.
• the base station may also need to measure the level of inter cell interference.
• the transmission in each time slot consists of a signal 'burst' which comprises three components, namely, a first data field, a midamble field containing a midamble code which serves to provide a training sequence, and a second data field.
• UTRA TDD also contains a CDMA component wherein several spread spectrum modulated signals can be made contemporaneously in any given time slot.
• several spread spectrum modulated signals can be made contemporaneously in any given time slot.
• eight signals each using a different spreading code may be transmitted in a given time slot.
• Each signal will consist of its own unique data fields and midamble code.
• the first data fields for each signal are transmitted contemporaneously followed by the midamble codes for each signal (each midamble code being different), followed by the second data fields.
• all signals are added together before being transmitted from the base station site.
• one or more mobile stations may each transmit one or more signals in a given time slot.
• UTRA TDD uses a highly optimised structure for the midamble codes. This structure has been arranged to allow a channel estimate to be obtained from a given midamble code in such a way that interference from other midamble codes transmitted in the same time slot is substantially eliminated. Details of such a method are described in "Optimum and Suboptimum Channel Estimation for the Uplink of CDMA Mobile Radio Systems with Joint Detection", ETT, Vol. 5, No. 1, Jan-Feb 1994, pages 39-50, by Bernd Steiner and Peter Jung. It is to be noted that, although the title of the paper refers only to the uplink, the techniques described therein are also useful, and applicable, to the downlink of UTRA TDD. The paper describes a system in which, for a given base station, the same base code is used for all midamble codes. The different midamble codes for the different signal transmissions are obtained as cyclic shifts of an extension to the base code.
• a channel estimate is formed from the midamble code by correlating against the code thereof. However, if this is done, there will be substantial interference from the other midamble codes in other signals due to their non-ideal cross-correlation properties.
• a cyclic correlation is performed against the midamble code.
• the reference for the cyclic correlation is the 'inverse' of the midamble base code. This 'inverse' is obtained either by forming the inverse of a
• Toeplitz matrix formed from the base code and reversing the order of the first row, or by forming the discrete Fourier transform of the base code, taking the reciprocal of each value, and then performing the inverse discrete Fourier transform and reversing the order. Correlation against the reverse of a waveform is equivalent to convolution. Convolution of a code with its inverse essentially creates an impulse.
• the base code(s) is(are) selected by computer search to have a reasonably flat spectrum and, in particular to avoid deep troughs in the magnitude which would lead to peaks in the inverse spectrum.
• the shifts in the base code which provide the midamble codes are selected to exceed the maximum delay spread plus delay uncertainties for the anticipated radio channel.
• the output of the cyclic correlators will be a set of impulse responses for each of the signals, separated by the shifts.
• a considerable number of the locations across the cyclic correlator output may contain no signal energy. If there were no receiver noise or inter cell interference, and ignoring any effect due to the receiver and transmitter filtering which tend to extend the responses to multipath, the level in the "no signal" positions would be equal to zero. This arises directly from the cyclic convolution against the inverse code.
• measurement of the energy by forming the modulus square in the "no signal” positions provide measurement of the receiver noise and inter cell interference uncorrupted by the intra cell signals.
• a method of obtaining an estimate of inter cell interference in the presence of one or more simultaneous co-channel intra cell signals in a mobile telecommunications system comprising a plurality of cells, each cell having a base station and a plurality of mobile stations associated therewith, the method comprising the steps of, in one or more cells and one or more base stations or mobile stations :- associating a channel estimate window with each of the one or more simultaneous co-channel intra cell signals which interfere with measurement of the inter cell interference; identifying signal path positions for each channel estimate window; determining those positions within the channel estimate window which have no signal components; and using measurements in the positions having no signal components to provide the estimate of inter cell interference.
• Figure 1 illustrates magnitudes for a set of codes for an uplink with eight codes
• Figure 2 illustrates impulse responses for a downlink set of channel estimates.
• Figure 1 shows an example uplink with eight codes. However, it is to be noted that code 8 is not transmitted. It will readily be appreciated that the estimates formed are complex values, although only the magnitudes of these estimates are shown in Figure 1 for clarity.
• a Broadcast Control Channel (BCCH) is transmitted in one time slot in every frame from every base station. Because of its broadcast nature, this signal must be transmitted with enough power to reach every location in the cell. Thus, automatic transmit power control is not applied to this signal. Every mobile station affiliated to a given base station should be able to receive its BCCH and the associated midamble code should be strong.
• BCCH Broadcast Control Channel
• the channel estimate window associated with the BCCH should be an effective means of identifying the signal path positions. This can be done by applying a threshold to the measurements.
• the threshold can be determined in one of two methods.
• the margin can be used for measurements of inter cell interference plus receiver noise. If this margin is large enough to provide a large enough number of measurements, then no further action need be taken. However, on the other hand, if the margin is not sufficiently large, then the relatively poor estimate of inter cell interference plus receiver noise obtained from the measurements in the margin can be used to set the threshold level. The threshold level will be set at some suitable multiple of the measured inter cell interference plus noise. The other measurements are then compared against the threshold level and those which exceed the threshold level are designated as containing signal components. Note that the whole process can be made more effective in terms of fewer misdirected signal components and fewer falsely detected noise only components by averaging the measurements over several frames.
• An alternative method requires identifying the signal components which are applicable when there is no 'margin'.
• This method comprises the steps of:- i) Ranking the measured energies in descending order of magnitude and selecting the N strongest energies as containing the signal components.
• a suitable value for N would be in the range 3 to 12, for example. However, if there are fewer than N actual signal components, then noise only components will be falsely designated as containing signal components.
• This can be overcome by performing a two-stage process in which the first stage is to determine a threshold level against which the signal energies are compared to identify the signal components as described above, and the second stage is to rank the measured energies in descending order of magnitude.
• ii) Forming a measurement of the inter cell interference plus receiver noise using the positions in which signal components were not found. This measurement can then be used to set a threshold level in the same way as described earlier to identify the signal components.
• the refined list of signal components can be used to obtain a refined estimate of the inter cell interference from measurements in the positions when signal components were not found.
• the process can be made more effective by averaging the measurements over multiple frames.
• the signal components are identified at specific locations.
• the signals are filtered and then sampled once per chip. If the sampling takes place at the peaks of the filter response, then there will be nominally zero response in positions other than the peak response position to a particular multipath component. This is true because the combination of the transmitter and receiver filter responses is typically selected to provide an overall Nyquist response.
• the sampling is at any other position there will be responses at non zero levels in the surrounding positions. For example, if the sampling is exactly halfway between the peak response positions, there will be two equal amplitude responses accompanied by additional responses symmetrically disposed about the centre responses. If the transmit and receiver filter responses are both square root raised cosine in the frequency domain with excess bandwidth factor of 0.22 as specified for UTRA TDD, then the magnitude of these responses relative to the centre responses will be:-
• responses detected above the threshold level may have associated responses which are below the threshold.
• This problem can be overcome by setting a window around the detected responses in which inter cell plus receiver noise measurements will not be made. This can be done either in the form of a fixed width window, for the sake of simplicity, or an amplitude dependent window. In the case of the amplitude dependent window, the approximate ratio of the signal response to the inter cell interference plus receiver noise is measured. Then, any positions for which worst case sampling could lead to responses more than, say, lOdB below the level of the inter cell interference plus receiver noise, would be excluded from the measurements.
• a further refinement would be to look for adjacent pairs of peak responses. Depending on the relative amplitudes, the approximate levels of related responses could be predicted. It has been assumed that the positions of the signal multipath components are identified by measurements of the midamble code associated with the BCCH in the BCCH time slot. It will, however, be appreciated that the levels of received signal power and inter cell interference plus receiver noise will vary from time slot to time slot. Because the BCCH is a broadcast transmission, it should be transmitted at the maximum power. Thus, from the viewpoint of absolute signal power, the window set for the positions of signal components based on BCCH measurements should also be appropriate for other time slots. However, if the inter cell interference is lower in other time slots then the effect of signal presence in terms of corrupting the measurements will be more severe.
• the mobile station can determine that a particular code in a given time slot is not in use, all of the correlator positions across the measurement window for the code are available.
• measurements can be performed without correlating against the inverse of the base station's code since there is no intra cell interference to be eliminated in this case.
• An alternative way is to arrange for the mobile station to measure inter cell interference plus receiver noise using any of the techniques previously described except that the inverse midamble code is that of the neighbouring cell.
• the mobile station is measuring inter cell interference as though it were affiliated to the neighbouring cell.
• the measured inter cell interference is different from that measured in the normal case in that it does not contain the component due to the neighbouring base station but does contain what would normally be considered as intra cell interference.
• the measurements in the two cases can be subtracted to provide the difference between the intra cell signal power and the inter cell interference component from the selected neighbouring cell. Since the intra cell signal power can readily be measured, it then becomes possible to compute the component of inter cell interference from the selected neighbouring cells.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Quality & Reliability (AREA)
• Mobile Radio Communication Systems (AREA)

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• 2000
  • 2000-08-15 WO PCT/GB2000/003155 patent/WO2001013565A1/en not_active Application Discontinuation
  • 2000-08-15 CN CN 00811707 patent/CN1370360A/zh active Pending
  • 2000-08-15 EP EP00953331A patent/EP1205049A1/en not_active Withdrawn

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