GB2353669A - Determination of inter cell interference - Google Patents

Abstract

A method of obtaining estimates of receiver noise plus inter cell interference by associating a channel estimate window with each simultaneous co-channel intra cell signal which interferes with the measurement of inter cell interference. Some or all of the positions within a correlation window in which no signal is present are identified, and averaging measurements taken at these positions to provide the desired estimates. These estimates can then be used to allow dynamic channel assignment (DCA) to be implemented more effectively.

Description

IMPROVEMENTS IN OR RELATING TO MOBILE TELECOMMUNICATIONS The present

invention relates to improvements in or relating to 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) and TDD.

(UMTS is an acronym for universal mobile telecommunication system as understood by persons skilled in the art.) As 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, either in the short or the long term to different cells. In the emerging 3 GPP standard, there are 15 TDD/TDMA time slots per ftame. Of these, some are permanently assigned for special purpose use. The remainder are available for assigning to cells. For example, if slots I to 12 are available, then a three cell re-use pattern could then be established in which, say, slots I to 4 were assigned to base station A, slots 5 to 8 to base station B and slots 9 to 12 to base station C. Within each allocation, some time slots could be assigned to uplink and others to downlink operation. (Uplink operation refers to data transfer from a mobile station to a base station, and downlink operation refers to data transfer from a base station to a mobile station within a telecommunications cell as is well known.) 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.

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.

In order to overcome these disadvantages, it is proposed within 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. In order to implement DCA, it is necessary for the various apparatus in the system, that is, the base stations and the mobile stations,, to perform various measurements. Specifically, each mobile station needs to measure the level of inter cell interference. Similarly, the base station may also need to measure the level of inter cell interference.

Within the TDMA/TDD structure of UTRA TDD,, 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.

It should be appreciated that UTRA TDD also contains a CDMA component wherein several spread spectrum modulated signals can be made contemporaneously in any given time slot. Thus, for example, 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.

Thus, 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.

In the downlink direction, all signals are added together before being transmitted from the base station site. In the uplink direction, 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 5 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 39505 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 midarnble codes for the different signal transmissions are obtained as cyclic shifts of an extension to the base code.

In a conventional receiver, 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 midarnble codes in other signals due to their non-ideal cross-correlation properties. In the approach described in the paper, a cyclic correlation is performed against the midamble code. The reference for the cyclic correlation is the 'inverse' of the midarnble 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. Thus, the output of the cyclic correlators will be a set of impulse responses for each of the signals, separated by the shifts.

However, 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. Thus, 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.

It is therefore an object of the present invention to provide a method of identifying some or all of the positions within a correlation window in which no signal is present.

It is a further object of the present invention to provide a method of averaging measurements taken at the identified positions to obtain estimates of receiver noise plus inter cell interference.

In accordance with one aspect of the present invention, there is provided 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.

Advantageously, by performing cyclic correlation against the midamble codes for neighbouring cells, it will be possible to obtain a richer characterisation of the interference environment, allowing more effective DCA to be implemented.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:

Figure 1 illustrates magnitudes for a set of codes for an uplink with eight codes; and Figure 2 illustrates impulse responses for a downlink set of channel estimates.

Figure I 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 I for clarity.

On the downlink, the path is the same for all midamble codes.

However,. it is still appropriate to transmit multiple midamble codes in order to provide information concerning the power transmitted on each signal in the time slot. This will apply if the same power is used for the midamble field as for the data fields for each signal. A downlink set of channel estimates is illustrated in Figure 2. Note, in this case that all impulse responses are scaled replicas.

The following discussion concerns the downlink in a telecommunications cell comprising a base station and at least one mobile station. 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.

Thus, 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.

In one method, if the measurement window is wider than the anticipated delay spread, then 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 ofii) 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.

Once the refined list of signal components has been obtained, it can be used to obtain a refined estimate of the inter cell interference from measurements in the positions when signal components were not found. As for the previous method, the process can be made more effective by averaging the measurements over multiple frames.

In the methods described so far, the signal components are identified at specific locations. In practice, it must be appreciated that 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.

If, however, 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:- No. Level (dB) 1 -10.8 2 -18.1 3 -26.4 4 -41.2 -40.9 6 -37.6 7 -39.9 8 -47.5 9 -59.8 -47.8 It is possible that 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, I OdB 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. In this case two options are available, either:- a) To identify the paths in the relevant time slot; or b) To use the BCCH time slot with a conservatively specified fixed window.

It should be appreciated that although the method of the present invention has been described with reference to the use of measurements in the BCCH time slot and midamble code for identifying signal path position, 10- the present invention is equally applicable to measurements on other codes and/or in other time slots.

Once the available positions for measurement of inter cell interference plus receiver noise have been identified, such measurements may be performed in any of the time slots in a cell of a base station which are either inactive and assigned to downlink operation, or currently unassigned to that base station. Not all of the measurement positions need to be used. Computational complexity can be reduced at the expense of increased measurement variance by taking fewer measurement samples. Moreover, it is not necessary to perform measurements in every time slot in every frame.

Furthermore, where 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. When measuring inter cell interference plus receiver noise in time slots which have not been assigned to a mobile station's affiliated base station, 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.

As mentioned earlier, further improvements to the DCA scheme can be obtained by having the mobile station correlate using the inverse midamble codes of its neighbouring base stations. This can be used in two different ways.

One way is to measure the signal energy in the neighbouring cell's BCCH. This would give a measure of the path loss to the neighbouring base station, given knowledge of that base station's transmitted power on its BCCH signal. When the mobile station signals this information back to the network, the latter can then predict the level of interference from that neighbouring base station at the mobile station given knowledge of the neighbouring base station's transmit power in all of its downlink slots on all assigned codes.

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. In this case, the mobile station is measuring inter cell interference as though it were affiliated to the neighbouring cell. Thus, 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. In this case, 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.

If the above procedure is performed for multiple midamble codes corresponding to the significant neighbouring base stations, then a complete characterisation of the radio paths involved in the system operation becomes possible.

Claims (8)

CLAIMS:

1. 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:- a) 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; b) identifying signal path positions for each channel estimate window; c) determining those positions within the channel estimate window which have no signal components; and d) using measurements in the positions having no signal components to provide the estimate of inter cell interference.

2. A method according to claim 1, wherein step b) comprises applying a threshold to measurements in each channel estimate window.

3. A method according to claim 2, wherein the threshold is set at a multiple of measured inter cell interference plus noise.

4. A method according to claim 1, wherein step b) comprises measuring energies within the channel estimate window, ranking the measured energies in descending order of magnitude, and selecting a predetermined number of the strongest measured energies as containing signal components, the remaining energies relating to positions in which there are no signal components.

5. A method according to any one of claims 1 to 4, wherein the channel estimate window is a fixed width window.

6. A method according to any one of claims 1 to 4, wherein the channel estimate window is amplitude dependent.

7. A method according to claim 6, further comprising the step of measuring the ratio of signal response to inter cell interference plus receiver noise to exclude signal positions from measurement.

8. A method according to claim 6 or 7, further comprising the step of determining the relative amplitudes of adjacent pairs of peak responses as indications of related responses below the threshold.