GB2513169A

GB2513169A - A method for controlling interference cancellation

Info

Publication number: GB2513169A
Application number: GB1307051.1A
Authority: GB
Inventors: Yuan Zhang; li-ke Huang; Chris Hayler
Original assignee: Aeroflex Ltd
Current assignee: Aeroflex Ltd
Priority date: 2013-04-18
Filing date: 2013-04-18
Publication date: 2014-10-22
Anticipated expiration: 2033-04-18
Also published as: GB2513169B; GB201307051D0

Abstract

This application is concerned with interference cancellation in an OFDM receiver affected by multi-cell interference. The claimed invention relates specifically to determining whether interference cancellation should be carried out. The receiver receives a signal containing a desired signal portion from a serving cell and an interfering signal portion from an adjacent cell. Pilot signals transmitted from the neighbouring cell may significantly interfere with data carrying resource elements of the desired signal. The receiver estimates the power in the desired signal portion and the power in the interfering signal portion, calculates the difference between the estimated and interfering power estimates and compares the difference with a threshold. Based on the result of the comparison a decision is made on whether to perform interference cancelation. In an embodiment, the power of the interference portion is estimated by generating a channel estimate from de-patterned pilot symbols, estimating pilot symbols on the basis of information relating to operating parameters of the neighboring cell and taking the product of the channel and pilot estimates. The invention may also involve estimating a bandwidth of the transmitter of the interference, or/and the number of antennas of the transmitter.

Description

A METHOD FOR CONTROLLING INTERFERENCE CANCELLATION

Technical Field

The present invention relates to a method of controlling interference cancellation.

Background to the Invention

In order to support higher data rates in mobile telecommunications networks, the third generation partnership project (3GPP) introduced a new air interface based on orthogonal frequency domain multiple access (OFDMA) techniques as the long term evolution (LTE) of the UMTS network.

One problem that arises in LTE cellular networks is inter-cell interference. This problem is particularly pronounced when a receiving User Equipment (UE) is located at or near a cell boundary. An example of such as situation is shown generally at 10 in the schematic illustration of Figure 1. In the illustrated example, a UE 12 receives signals from a Node B 14 (identified as eNBI in Figure 1) serving the cell in which the UE 12 is located, but may also receive signals intended for other UEs from Node Bs 16, 18 (identified as eNB2 and eNB3 in Figure 1) serving nearby cells, which may intcrfcrc with thc dcsircd signal from thc Node B 14 scrving thc cell in which thc liE 12 is located, leading to reduced reception quality at the UE 12.

Efforts have been made to address this issue. In one scheme, adjacent Node Bs communicate with each other to schedule data transmissions, such that when one is transmitting data to a target liE its adjacent Node Bs, which could interfere with the reception of the data by the target UE, cease transmitting data. Whilst this scheme can be effective, it does not address a problem of inter cell interference arising from transmission of a pilot signal by Node Bs adjacent to the Node B serving the cell in which the receiving UE is located, since transmission of the pilot signal cannot be intcrruptcd.

Summary of Invention

According to a first aspect of the present invention, there is provided a method of controlling the operation of an interference cancellation scheme in a receiver, the method comprising the steps of: receiving a signal containing a desired signal portion and an interfering signal portion; estimating the power in the desired signal portion; estimating the power in the interfering signal portion; calculating a difference between the estimated power in the interfering signal portion and the desired signal portion; and if the difference so calculated betters a threshold, commencing operation of the interference cancellation scheme.

Using the method of the present invention, the interference cancellation scheme of the receiver can be selectively activated, thereby reducing power consumption when the level of interference is insufficient to warrant operation of the scheme, but permitting activation of the interference cancellation scheme at more significant levels of interference.

The method may further comprise estimating a bandwidth of a transmitter of an interfering cell used to transmit the interfering signal portion of the received signal.

The method may further comprise estimating a number of antennas of a transmitter of an interfering cell used to transmit the interfering signal portion of the received signal.

The interference cancellation scheme may comprise a method for cancellation of adjacent cdl intcrfcrcncc in a signal received by a receiver of a cellular communication system comprising: generating a channel estimate for a propagation channel between a transmitter of an interfering cell and the receiver; receiving information relating to operating parameters of the transmitter of the interfering cell; generating, using the information so received, a pilot symbol estimate tbr a pilot symbol transmitted by the transmitter of the interfering cell; calculating an estimated interference symbol by multiplying the pilot symbol estimate by the channel estimate; and subtracting thc estimatcd interfcrencc symbol from a signal rcccivcd at thc receiver.

Generating the channel estimate for the propagation channel may comprise generating dc-patterned pilot symbols from thc rcccivcd signals and performing a moving average filtering operation on the dc-patterned pilot symbols.

Rccciving information rclation to opcrating parametcrs of the transmittcr may comprise performing a cell search to detect an identifier of the transmitter of the interfering cell.

Receiving information relating to operating parameters of the transmitter may further comprisc cstimating a bandwidth and a numbcr of antcnnas of the transmittcr.

Estimating the bandwidth of the transmitter may comprise calculating a power value of the channel estimate.

The method may further comprise generating log likelihood ratios (LLRs) for data symbols contained in the received signal and scaling the value of the LLRs so generated.

The LLR values may be reduced by a constant scaling factor.

According to a second aspect of the invention there is provided a receiver for a cellular communication system, the receiver being configured to: receive a signal containing a desired signal portion and an interfering signal portion; estimate the power in the desired signal portion; estimate the power in the interfering signal portion; calculate a difference between the estimated power in the interfering signal portion and the desired signal portion; and if the difference so calculated betters a threshold, commence operation by the receiver of an interference cancellation scheme.

The receiver may be further configured to estimate a bandwidth of a transmitter of an interfering ccli used to transmit the interfering signal portion of the received signal.

The receiver may be further configured to estimate a number of antennas of a transmitter of an interfering cell used to transmit the interfering signal portion of the received signai.

The receiver may be further configured to: generate a channel estimate for a propagation channel between a transmitter of an interfering cell and the receiver; receive information reiating to operating parameters of the transmitter of the interfering cell; generate, using the information so received, a pilot symbol estimate for a pilot symbol transmitted by the transmitter of the interfering cell; calculate an estimated interference symbol by multiplying the pilot symbol estimate by the channel estimate; and subtract the estimated interference symbol from a signal received at the receiver.

The receiver may comprise an interference cancellation unit which comprises: a de-patterning unit configured to generate dc-patterned pilot symbols from a signal received by the receiver; and a moving average fiher configured to perform a moving average filtering operation on the dc-patterned pilot symbols.

The receiver may be configured to perform a cell search to detect an identifier of the transmitter of the interfering cell.

The receiver may be further configured to estimate a bandwidth and a number of antennas of the transmitter.

The receiver may further comprise a power calculation unit, the power calculation unit being configured to estimate the bandwidth of the transmitter by calculating a power value of the channel estimate.

The receiver may further comprise a processor configured to generate log likelihood ratios (LLRs) for data symbols containcd in thc rcccivcd signal and to scaic thc valuc of the LLRs so generated.

The processor may be configured to reduce the LLR values by a constant scaling factor.

Brief Description of the Drawings

Embodiments of the invention will now be described, strictly by way of example only, with rcfcrence to thc accompanying drawings, of which: Figure 1 is a schematic representation of part of a cellular network in which a FE is located in a serving cell of the cellular network and is subject to inter-cell interference from adjacent cells of the cellular network; Figure 2 shows an exemplary OFDM resource grid for a receiver in a cellular communications network in which there is one serving cell and one interfering cell; Figurc 3 is a schcmatic rcprcscntation of an architccturc for a rcccivcr; Figure 4 is a schematic representation of an architecture for a symbol rate interference cancellation unit of thc rcceivcr architccturc illustratcd in Figurc 3; Figure 5 shows an exemplary OFDM resource grid for a signal transmitted using two antennae; and Figure 6 is a flow diagram illustrating a proccss for controlling intcrfcrcncc cancellation in the receiver architecture illustrated in Figure 3.

Description of the Embodiments

Referring first to Figure 2, an OFDM resource grid for a receiver in a cellular communications network in which there is one serving cell and one interfering cell is shown generally at 20. In this casc both the scrving ccli and the adjacent ccli arc served by transmitters of the single input single output SISO) type. In Figure 2 it is assumed that there is no frequency offset between the serving cell and the interfering ceH, and that any time offset between the serving eeH and the interfering cell is within the cyclic prefix (CP) length of a transmitted pilot signal. Thus, a pilot symbol Ro contained within a pilot signal transmitted by interfering cell appears in a resource element (RE) of the receiver resource grid intended for data transmitted by the serving cell, causing interference (shown with a dotted background in Figure 2).

A signal model of a resource element R0 at a pilot position in the resource grid of the serving cell can be written as xkO) = Hf (1) p(i) + k()' (I) where H' (I) and p') (/) denote, respectively, the channel coefficients and pilot symbol of the serving cell (referred to as Cell 1) at the kth OFDM subcarrier, and n4 denotes noise on the kth OFDM subcarricr. This equation applies for single input multiple output (SIMO) and MIMO cases, since the pilot REs are orthogonal among the antennae of a SIMO transmitter. As this equation reflects only the pilot position of the serving cell, there is no interference term, and thus channel estimates of the sewing cell based on the pilot signal may not reflect the actual level of interference at the data positions of a received signal.

A signal model of a resource element at a data position in the resource grid of the serving cell which is affected by inter-cell interference from a pilot signal of an adjacent cell can be written, for the SISO ease, as x4 (1) = h' (I) cI' (I) + ;(2) (I) (2) (1) + n1 (/), wherehf(i)denotes the channel coefficients of the serving cell (Cell 1) experienced by the kth subcarrier of a data signal transmitted by the transmitter of the serving cell, d1 (i) dcnotcs the data symbol on thc kth subcarricr of thc data signal, h2 (1) denotcs the chaimel coefficients of the interfering cell (Cell 2) experienced by the kth subearrier of an interfering pilot signal transmitted by the transmitter of the interfering cell, and 2) (1) denotes the pilot symbol on the kth subcarrier of the interfering pilot signal.

For a SIMO case, a signal model of a resource element at a data position in the rcsourcc grid of thc serving cell which is affected by inter-cell interference can be written as = h10. d(i) + phT(1) . p(2)(f) + (2) LX2 ()j h (OJ Lh(i) L2 @)J or x(i) = II'(i)ct'(i) + + whilst for a 2x2 MIMO case the signal of the resource element at a data position of the serving cell which is affected by inter-cell interference can be written as -h7 (i) h (iH 1? (i) wf ()l d[1 (H + H[2 H. (2) + (3 H2 (i)J -h (0 MY (i)j w (1) wY (i)J d (H Lh2) (i)j p i) ) or x(i) = Ht1(i)W(i) d1(i) + H(2)(p(2)(f) + n(i).

Comparing equation (2) with equation (3), it can be seen that for the SIMO ease and the MIMO case the interference term H2(/)p2(i) is the same. This is because the pilot signal is allocated orthogonally among antennae. For one RE of the data signal received by a receiver such as a UE in the serving cell, only the pilot symbol from a single antenna of a transmitter such as a Node B in an interfering cell will interfere.

However, the number of interfering REs will increase, since the number of pilot symbols transmitted in the 2x2 MIMO case is twice that of the SIMO case, resulting in more severe inter-cell interference.

Accordingly, there is a need to reduce the effect of inter-cell interference arising from pilot signals transmitted by transmitters in adjacent cells.

Figure 3 is a schematic representation of a receiver architecture which can be used in a receiver of a cellular communications system to reduce the effects of this inter-cell interference. It will be appreciated that the schematic representation of Figure 6 presents the architecture as a series of functional blocks, but that the functional blocks do not necessarily represent physical components of a "real world" implementation of the architecture, but are instead intended to represent processing operations undergone by a received signal.

The receiver architecture is shown generally at 60 in Figure 3, and includes parallel first and second fast Fourier transform (FFT) units 62, 64, which are operative to receive encoded data from upstream components of the receiver and to reverse an inverse fast Fourier transform (lEFT) process which takes place in a transmitter transmitting pilot or data symbols to the receiver. Outputs of the FFT units 62, 64 are input to a symbol rate processor 66, which performs symbol-rate processing on symbols received by the architecture 60. Outputs of the symbol rate processor 66 are connected to inputs of a bit rate processor 68, which is configured to perform bit-rate processing on data bits output by the symbol rate processor 66.

The receiver architecture 60 also includes a symbol rate interference cancellation unit and a bit rate interference cancellation unit 72. The operation of the symbol rate interference cancellation unit 70 and the bit rate interference cancellation unit is controlled by an interference cancellation control unit 74.

The symbol rate interference cancellation unit 70 is configured to estimate the interference experienced by the receiver in the sewing cell as a result of the transmission of the pilot signal by a transmitter of a neighbouring cell, and to subtract the resulting estimate of the interference from the signal received at the receiver in the serving cell, so as to at least partially cancel the interference.

From equations (2) and (3) above, it can be seen that the interference term in the signal model of a resource element affected by inter-cell interference is fi(f)p2(i), whcre (2) denotes the pilot symbol of the interfering pilot signal. Thus, it is this interference term that is estimated by the symbol rate interference cancellation unit 70 and subtracted form the received signal.

The symbol rate interference cancellation unit 70 estimates the propagation channel H2experienced by the interfering pilot symbols, and, using the estimated propagation channel, generates an estimate of the interference, which is subsequently subtracted from the received signal to at least partially cancel the interference in the received signal.

Applying zero-forcing dc-patterning to equation (2) gives = . =H(i)+H°(i). d(z) + n(/) (4) p (i) p (i) pi) where ñ2 (i) is a vector of dc-patterned pilot symbols. In equation (4) the data RE of the sewing cell represented by the term 1111) (/). C) becomes interference. p (/)

Usually the power of a data RE is relatively high, so the estimation in equation (4) cannot be very accurate. The expectation of equation (4) can be expressed as E(H(2) (i))= E1H(2) w +110) (i) 2)(o + 10) *) fi = E(E(2) (O)= E(H' (I)). EL (2 + E[ The tes E9 d°(i) and E[ 9 approximate to zero, as it can be assumed that L (/) p the expectation of a data symbol in the serving cell is zero. This is because di?,) is a complex symbol with QPSK, 16QAM or 64QAM modulation, and it can be assumed that the different modulation points have an equal probability, since the whole constellation is systematic. The expectation of a data symbol is therefore zero, and so E(fF2)(f4 E(H°(/)) (5) Equation (5) gives a good estimate of the mean value of the inference channel H2 experienced by the interfering pilot symbols.

Equation 5) is equivalent to using a moving average filter in the time domain on the dc-patterned pilot symbols to generate an estimate of the mean value of the inference channel (2) If the length of the moving average filter is within the coherence time of thc channel, fI(i)can be approximated by E(H(2)(i)). Thus, the proccdurc undertaken by the symbol rate interference cancellation unit 70 to estimate the interference channel H2 experienced by the interfering pilot symbols can be expressed as iflustrated in Figure 4.

As can be seen, following the FFT operation performed by FFT unit 62 or 64, the received signal is passed to a dc-patterning unit 82 of the symbol rate interference cancellation unit 70 to generate the dc-patterned pilot symbols H (i). The dc-patterned pilot symbols are passed to a moving average filter 84, which outputs E(H(2) (/)), which is used by the symbol rate interference cancellation unit 70 as the estimate of the interference channel H2 experienced by the interfering pilot symbols.

In order to estimate the interference caused by the transmitted pilot symbols of the interfering cell, those pilot symbols must also be estimated by the symbol rate interference cancellation unit 70.

The pilot symbols transmitted by the interfering transmitter are predefined, based upon operating parameters of the transmitter, including a cell identifier (ID), its bandwidth and number of antennas. The symbol rate interference cancellation unit 70 estimates the pilot symbols p(2J in a manner that will be described below.

The receiver performs a cell search for nearby cells, and from this the cell ID of the interfering transmitter is known. The cell search information including this cell ID is passed to the symbol rate interference cancellation unit 70. The symbol rate interference cancellation unit 70 estimates the bandwidth and number of antennas of the interfering transmitter, as will be described below, and uses the resulting estimates of the bandwidth and number of antennas to estimate the pilot symbols (2) of the interfering pilot signal.

In one embodiment, the symbol rate interference cancellation unit 70 initially sets the bandwidth and number of antennas of the interfering transmitter to a maximum possible value. For example, the bandwidth may be set to a value of 20 MHz, while the number of antennas may be set to 4. The channel estimates described above are performed using these parameters. If the actual bandwidth of the interfering transmitter is smaller than the maximum (i.e. 20 MHz in this example), the power of pilot symbols outside the signal band will be zero.

This can be expressed mathematically by re-writing equation (5), bearing in mind that H2(i) = 0 for pilot symbols outside the actual bandwidth of the interfering transmitter. Thus E(A (2) (0) = EH (1) (/) +11 (U. d°Q) + p (i) p (0) As before, the ten EL d(U(1) 9 and E[ 9 approximate to zero, as it can be pr(j) assumed that the expectation of a data symbol in the serving cell is zero, for the reason set out above. Thus EfrI()(j))rc 0 (6) This suggests that the power of the channel estimates, i.e. Ig (ir)(oF %r the pilot symbols in the interfering signal can be used to detect the signal bandwidth of the interfering signaL In practice, it is not necessary to calculate the power of the channel estimates at all possible pilot positions, since the system band is symmetrical around DC, and thus the channel estimates fbr only one half of the system band need be calculated.

In LTE, the smallest possible signal bandwidth is 1.4 MHz, and this bandwidth contains 6 resource blocks. Thus, any resource block within a band extending thm DC to 0.7 MHz (i.e. half the smallest possible signal bandwidth, since the system band is symmetrical about DC) will always include valid pilot symbols. This band extending from DC to 0.7 MHz is referred to as the in-band area, since it will always contain valid pilot symbols. A power calculation unit 86 of the symbol rate interference cancellation unit 70 calculates the average power of the channel estimates fbr the pilot symbols in this in-band area, as = fIEfr012 This avenge power is averaged over a predetermined number N of subframes. The average power so calculated is stored and used as a reference.

The maximum possible system bandwidth is 20 MHz, and initially the interference calculation unit 70 assumes that the bandwidth of the interfering cell is equal to this maximum. A candidate bandwidth, representing a possible actual bandwidth of the interfering cell, is selected by the interference calculation unit 70, and from this candidate bandwidth an out of band measurement area is identified, as the bandwidth between the maximum bandwidth and the candidate bandwidth. For example, if a candidate bandwidth of 15 MHz were selected by the interference cancellation unit, the out of band bandwidth would be thc 5 MHz band bctwccn 15 MHz and 20 MHz.

The moving average filter 84 is applied to dc-patterned pilot symbols in this out of band area to calculate channel cstimatcs E(H(2) (i))for / = 0, 1, 2, ... , L-I, where L is the number of pilot symbols in the out of band observation area.

The power calculation unit 86 then calculates the average power of the out of band channel estimates, as 1 [-I -2 ,utband = 7; EH)1 This average power is averaged over a predetermined number N of subframes. A ratio of the average power of the out of band pilot symbols to the average power of the inband pilot symbols is calculated and compared to a predefined threshold. If this ratio utband is greater than the threshold s the symbol rate interference nhanct cancellation unit 70 determines that the candidate bandwidth is the actual bandwidth of the interfering cell. If the ratio is smaller than the threshold a the symbol rate interference cancellation unit 70 determines that the candidate bandwidth is not the actual bandwidth of the interfering cell, and a new candidate bandwidth is selected, and the process described above is repeated until the actual bandwidth of the interfering cell has been determined.

A similar approach can be applied to antenna number estimation in an antenna number estimation unit 88 of the interference cancellation control unit 74.

It is known that every resource block in a. signal transmitted by the interfering cell has the same pattern of pilot symbols. Thus, only one resource block of the interfering signal need be observed to determine the number of antennas used by the interfering cell. The target resource block for observation should be selected from those in the 1.4 MHz range around DC, since these resource blocks will always be present in a transmitted signal, as 1.4 MHz is the minimum bandwidth of a transmitted signal.

Thus, the detection of the number of antenna used by the interfering cell can be pcrformcd independently ofthc detection of thc bandwidth of the intcrfcring signal.

Figure 5 illustrates a resource block for a two-antenna transmission. The resource elements marked RO represent resource elements containing pilot symbols transmitted by a first antcnna (rcfcrrcd to as antcnna port 0). These arc always present, since in order for fransmission of a signal to take place at least one antenna must be used. The resource elements marked Ri represent resource elements containing pilot symbols transmitted using a second antenna (rcfcrrcd to as antcnna port 1). Thcsc pilot symbols are only present when a signal has been transmitted using two or more antennae. Otherwise, the resource elements occupied by the RI pilot symbols will be used for data transmission. The process used by the symbol rate interference cancellation unit 70 makes use of this fact, as will be described below.

In a first step, the moving average fiher 84 is applied to pilot symbols from antenna port 0 of the selected resource block to calculate channel estimates EH(2) (O)for i = 0, 1, 2 L-I, wherc L is the number of port 0 pilot symbols in the selected resource block.

Next, the power calculation unit 86 calculates the average power of these channel estimates, as The symbol rate interference cancellation unit 70 then assumes that a frirther antenna port ii (where ii = 1, 2 or 3, since there can only be a maximum of 4 transmit antenna) was used to transmit the interfering signal. The moving average filter 84 is applied to pilot symbol positions from antenna port n of the selected resource block to calculate channel estimates EH(2)(i))for i = 0, 1, 2 L-I, where L is the number of port n pilot symbols in the selected resource block, and the average power of the port n pilot symbols in the selected resource block is calculated as A ratio ofp to p0 is calculated and compared to a predetermined threshold fi, and if the ratio exceeds the threshold J3, the symbol rate interference cancellation unit 70 Pu determines that n antennae were used to transmit the interfering signal. Otherwise, a new value of n is selected and the process described above is repeated until the correct n has been determined.

When the antenna number and signal bandwidth have been estimated, the interference cancellation control unit 74 uses the antenna number and bandwidth estimates to generate pilot symbol estimates. This may be achieved, for example, by inputting the antenna number and bandwidth estimates into a pilot symbol determination unit 90, which may be, for example, a look-up table containing predefined pilot sequences for particular bandwidth and antenna number values.

Once the interference channel H2 and the pilot symbols p2W have been estimated, the interference term Ht2(i)j/2(z) can be estimated by an interference estimation unit 92 of interference cancellation control unit 74, which multiplies the interference channel 11(2) by the pilot symbol estimates pt2O) to simulate the effect of the interference channel on the transmitted pilot symbols. The resulting interference symbol estimates are subtracted from the received signal on a per antenna element basis, prior to data demodulation, to at least partially cancel interference in the data signal received from the transmitter of the cell serving the receiver that is caused by the pilot signal transmitted by a transmitter of a neighbouring cell.

In addition to the symbol rate interference cancellation unit 70, the receiver architecture 60 also includes a bit rate interference cancellation unit 72. The symbol rate interference cancellation process described above will likely not remove all interference from the received signal. Thus, the bit rate interference cancellation unit is operative to apply a scaling factor to log likelihood ratios (LLR5) generated by the symbol rate processor 66 that are affected by interference caused by the pilot signal of the transmitter of an adjacent cell. The individual LLRs that are affected by the intcrfcrcncc will bc known, as thc REs in a given rcsourcc block that arc affcctcd by interference can be determined once the cell ID, bandwidth and number of antennae of the interfering transmitter are known, and the bit rate interference cancellation unit 72 is configured to scale the value of these affected LLRs by a constant scaling factor that is derivcd from thc channel estimate used by the symbol rate interference calculation unit.

It will be appreciated that in some circumstances symbol rate interference cancellation or bit rate interference cancellation will not be required. For example, it has been found in simulation that the effectiveness of the symbol rate interference cancellation is reduced when the relative delay between the serving cell and the interfering cell is around half the duration of an OFDM signal. In such circumstances the symbol rate interference cancellation performed by the symbol rate interference cancellation unit (and indeed the bit rate interference cancellation performed by the bit rate interference cancellation unit 72) should be disabled to reduce processing overhead.

Accordingly, the operation of the symbol rate interference cancellation unit 70 and the bit rate interference cancellation unit 72 is controlled by the interference cancellation control unit 74, as will now be described with reference to Figure 6.

Figure 6 is a flow diagram illustrated steps performed by the interference cancellation control unit 74 to control the operation of the symbol rate interference cancellation unit 70 and the bit rate cancellation unit 72.

The interference cancellation control unit 74 continually monitors the signal quality of the signals received by the receiver from an interfering cell, and determines, based on the signal quality, whether interference cancellation should be applied.

In a first step 100, an interfering neighbouring cell is detected by the receiver. The interference cancellation control unit 74 calculates the power PREF NEIGHBOUR of a reference signal of the interfering neighbouring cell and the power PREF SERV of a reference signal of the serving cell, at step 102, in the manner described above.

At step 104, the interference cancellation unit 74 performs a calculation PREF NEIGHBOUR -PREF SERV, to determine thc difference in the power of thc received reference signals for the interfering cell and the serving cell. The result of this calculation is compared to a first predetermined threshold X, to establish whether the level of interference from the interfering cell is sufficient to warrant interference cancellation.

If the result of the calculation falls below the threshold X (i.e. ifPREFNEIGHBOIJR -PREF SERV < X), then the level of interference from the interfering neighbouring cell is insufficient to warrant interference cancellation, and so at step 106 the interference cancellation control unit 74 deactivates the symbol rate interference cancellation performed by the symbol rate interference cancellation unit 70 and the bit rate interference cancellation performed by the bit rate interference cancellation unit 72, if they are active.

If the result of the calculation meets or exceeds the threshold X (i.e. if PREFNEIGFIBOUR -PREF SERV ? X), then the level of interference may be sufficient to warrant interference cancellation, and so in one embodiment, the interference cancellation control unit 74 activates, at step 108, the symbol rate interference cancellation performed by the symbol rate interference cancellation unit and the bit rate interference cancellation performed by the bit rate interference cancellation unit 72 such that interference cancellation is performed on received signals.

In the description above, it has been assumed that the serving cell and the interfering cell arc time aligned, or that any small timing offset that exists between the serving cell and the interfering cell is smaller than the cyclic prefix length of a transmitted pilot signal, such that the interference from the pilot signal of the interfering cell is aligned with a data RE in the serving cell, as shown in Figure 2. This is a valid assumption for many deployment scenarios.

It will be appreciated that the method used by the interference cancellation control unit 74 to control the operation of the interference cancellation scheme helps to minimise the power consumption of the receiver, as it can deactivate interference canccllation whcn thc lcvcl of intcrfcrcncc is not high cnough to havc a scrious detrimeiltal effect on the reception of the signal from the serving cell, thereby reducing power consumption, but can activate the interference cancellation scheme when interference from neighbouring cells becomes detrimental to reception of the signal from thc serving cell.

Claims

CLAIMS1. A method of controlling the operation of an interference cancellation scheme in a receiver, the method comprising the steps of: receiving a signal containing a desired signal portion and an interfering signal portion; estimating the power in the desired signal portion; estimating the power in the interfering signal portion; calculating a difference between the estimated power in the interfering signal portion and the desired signal portion; and if the difference so calculated betters a threshold, commencing operation of the interference cancellation scheme.
2. A method according to claim 1 further comprising estimating a bandwidth of a transmitter of an interfering cell used to transmit the interfering signal portion of the received signal.
3. A method according to any claim 1 or claim 2 further comprising estimating a number of antennas of a transmitter of an interfering cell used to transmit the interfering signal portion of the received signal.
4. A method according to any one of the preceding claims wherein the interference cancellation scheme comprises a method for cancellation of adjacent cell interference in a signal received by a receiver of a cellular communication system comprising: generating a channel estimate for a propagation channel between a transmitter of an interfering cell and the receiver; receiving information relating to operating parameters of the transmitter of the interfering cell; generating, using the information so received, a pilot symbol estimate for a pilot symbol transmitted by the transmitter of the interfering cell; calculating an estimated interference symbol by multiplying the pilot symbol estimate by the channel estimate; and subtracting the estimated interference symbol from a signal received at the receiver.
5. A method according to claim 4 wherein generating the channel estimate for the propagation channel comprises generating dc-patterned pilot symbols from the received signals and performing a moving average filtering operation on the de-patterned pilot symbols.
6. A method according to claim 4 or claim 5 wherein receiving information relation to operating parameters of the transmitter comprises performing a cell search to detect an identifier of the transmitter of the interfering cell.
7. A method according to claim 6 wherein receiving information relating to operating parameters of the transmitter further comprises estimating a bandwidth and a number of antennas of the transmitter.
8. A method according to claim 7 wherein estimating the bandwidth of the transmitter comprises calculating a power value of the channel estimate.
9. A method according to any one of claims 2 to 8 further comprising generating log likelihood ratios (LLRs) for data symbols contained in the received signal and scaling the value of the LLRs so generated.
10. A method according to claim 9 wherein the LLR values are reduced by a constant scaling factor.
11. A receiver for a cellular communication system, the receiver being configured to: receive a signal containing a desired signal portion and an interfering signal portion; estimate the power in the desired signal portion; estimate the power in the interfering signal portion; calculate a difference between the estimated power in the interfering signal portion and the desired signal portion; and if the difference so calculated betters a threshold, commence operation by the receiver of an interference cancellation scheme.
12. A receiver according to claim 11 wherein the receiver is further configured to estimate a bandwidth of a transmitter of an interfering cell used to transmit the interfering signal portion of the received signal.
13. A receiver according to 11 or claim 12 wherein the receiver is further configured to estimate a number of antennas of a transmitter of an interfering cell used to transmit the interfering signal portion of the received signal.
14. A receiver according to any one of claims 11 to 13, the receiver being further configured to: generate a channel estimate for a propagation channel between a transmitter of an interfering cell and the receiver; receive information relating to operating parameters of the transmitter of the interfering cell; generate, using the information so received, a pilot symbol estimate for a pilot symbol transmitted by the transmitter of the interfering cell; calculate an estimated interference symbol by multiplying the pilot symbol estimate by the channel estimate; and subtract the estimated interference symbol from a signal received at the receiver.
15. A receiver according to any one of claims 11 to 14 comprising an interference cancellation unit which comprises: a dc-patterning unit configured to generate dc-patterned pilot symbols from a signal received by the receiver; and a moving average filter configured to perform a moving average filtering operation on the dc-patterned pilot symbols.
16. A receiver according to any one of claims 11 to 15 wherein the receiver is configured to perform a cell search to detect an identifier of the transmitter of the interfering cell.
17. A receiver according to claim 16 wherein the receiver is further configured to estimate a bandwidth and a number of antennas of the transmitter.
18. A receiver according to claim 17 further comprising a power calculation unit, the power calculation unit being configured to estimate the bandwidth of the transmitter by calculating a power value of the channel estimate.
19. A receiver according to any one of claims 11 to 18 wherein the receiver further comprises a processor configured to generate log likelihood ratios (LLRs) for data symbols contained in the received signal and to scale the value of the LLRs so generated.
20. A receiver according to claim 19 wherein the processor is configured to reduce the LLR values by a constant scaling factor.