TITLE

A method, an apparatus or a computer program for estimating a signal to noise ratio.

FIELD OF THE INVENTION

Embodiments of the present invention relate to a method, an apparatus or a computer program for estimating a signal to noise ratio.

BACKGROUND TO THE INVENTION

A signal to interference ratio (SIR) for a wireless channel gives a measure of the effectiveness of that channel at reliably transferring data. If the SIR is high then the wireless channel is typically reliable and may be able to transfer large amounts of data without error. If the SIR is low then the wireless channel is unreliable and may be unable to transfer data without error.

The SIR may consequently be used to measure a wireless channels capability of carry data.

In some implementations such as those defined in the cdma2000 High Rate Packet Data Air Interface Specification, the data rate used in the wireless channel may be adapted in dependence on the SIR of the data channel as measured at the mobile station.

In other implementations such as those defined in the WCDMA Specification for inner loop (fast) power control, the transmit power of a base station, which uses the wireless channel to communicate with a mobile station, is increased via a feedback loop if the SIR measured at the mobile station decreases below a target value. The SIR is checked against the target value every 0.667μs.

If the mobile station uses an equaliser to improve the signal gain of the wireless channel, then this gain should be taken into account in the SIR estimate. Thus the 'signal' used to estimate SIR should be the equalised signal. However, there is an inherent delay associated with equalisation. This delay arises because equalisation

^■ typically involves estimating the channel impulse-response (CIR) of the wireless channel using a received signal, converting the CIR into taps for a finite impulse response filter (FIR) and then using the FIR to filter the received signal.

BRIEF DESCRIPTION OF THE INVENTION

It would be desirable to provide a new method for reliably estimating SIR.

It would be desirable if this new method were fast, for example, suitable for use within a WCDMA inner loop (fast) power control loop.

According to one aspect of the invention there is provided a method of estimating a signal to interference ration (SIR) comprising: estimating the signal power of a signal received at a first time; estimating the interference power of a signal received at a second time preceding the first time; and combining the signal power estimate at the first time and the interference power estimate at the second time to produce an estimate of the signal to interference ratio at the first time.

The inventors have realised that there may be frequent fast signal power variations because, for example, of fast power control but that channel conditions are typically more slowly varying. This assumption about slowly varying channel conditions may become invalid if the mobile station is travelling at high speeds but is otherwise valid.

According to another aspect of the invention there is provided a method of estimating received signal power comprising: receiving from a wireless channel at a first time a first signal; equalising the first signal using a channel estimate for the wireless channel at a third time, preceding the first time; and estimating signal power using the equalised first signal.

A fast reasonably reliable signal power estimate can thus be obtained by using an outdated channel estimate in the equalisation of the first signal.

Instead of waiting for the channel estimate to be calculated from the first signal and updating the equaliser, the current equaliser may be used to equalise the first signal.

Thlas-the-"first=signaHs=eqb'a!ised using an equaliser that is deliberately not matched to the channel at the time at which the first signal is received but is equalised using an equaliser that is matched to the channel at an earlier time i.e. the equaliser coefficients are derived from a previously received signal.

A subset of first signal may be used for signal power estimation. This advantageously reduces processing overhead. The subset may be the pilot field in downlink dedicated physical control channel (DPCCH).

The estimation of the interference power may comprise: receiving from the wireless channel at a second time that precedes the first time, a second signal; equalising the second signal using an estimate for the wireless channel at the second time; and estimating interference power using the equalised second signal.

Whereas the signal power is estimated using an outdated equaliser to equalise a current signal to which the equaliser is not matched, the interference power is estimated using an outdated equaliser to equalise an outdated signal to which the outdated equaliser is matched.

The common pilot channel (CPICH) may be used as second signal.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 schematically illustrates a method of estimating a signal to interference ratio

(SIR); and

Fig. 2 schematically illustrates an apparatus that is suitable for performing the method.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Rg. T schematically illustrates a method of estimating a signal to interfereFiεe ratio

(SIR).

Time flows in this figure from left to right.

At Time t, signals 110, 112 are received via a wireless channel. In this example, the wireless channel is the downlink channel between a WCDMA base station and a mobile terminal. The method illustrated in Fig. 1 in this example occurs at the mobile terminal.

It should be appreciated that the described method of estimating SIR is not limited for use in the mobile terminal but may also be used in any wireless receiver including base stations. Furthermore, although the example is described in the context of WCDMA, the method may be used in conjunction with other wireless communication protocols.

The signal 1 10 is a data channel r(t). In this example it is the downlink dedicated physical control channel (DPCCH), at time t. This signal is transmitted as a series of packets each of which occupies a time slot. Each packet comprises a pilot field comprising pilot symbols.

The signal 112 is a pilot channel p(t) used for channel estimation. In this example it is the common pilot channel (CPICH). This is a continuous signal, so the channel estimation may be continuously updated.

The signal p(t) 112 is provided to block 100, where it is used to estimate the channel impulse response (CIR) of the wireless channel at the time when the pilot signal p(t) was received. This value of the channel impulse response, CIR(t) 101 , is provided to block 102. The estimation of a channel impulse response from a pilot signal of known composition is well known to persons skilled in the art and will not consequently be described.

At block 102, the CIR(t) is used to determine the equaliser coefficients w(t) valid at time t. For example, if the equaliser is a finite impulse response (FIR) filter, the block 102 determines the tap values for the FIR. The determination of equaliser coefficients

from a channel impulse response is well known tα persons skilled in the act and will not consequently be described. A reference to equalising a signal using a channel estimate should be understood to mean equalising the signal using an equaliser solved using the channel estimate.

The equalizer coefficients w(t) are used to program the equaliser. The equaliser is therefore programmed to equalise a signal received at time t, this is indicated by referring to the equaliser as E(t).

At block 130, the equaliser E(t) is used to equalise the signal r(t) 110. The equaliser E(t) is correctly matched to this signal and the equalised signal 131 may be used to acquire the data d(t) carried by signal r(t). The equalised signal 131 is also provided to block 132 where it is used to estimate the interference power of the received signal r(t) 110. At block 132, the chip estimates of the equalised signal 131 are despread using the CPICH spreading code and multiplied by the complex conjugate of the known CPICH symbol. After that the interference power can be estimated using known methods, such as the differential method.

Differential method:

H is the ratio of dedicated channel spreading factor and CPICH spreading factor M is the number of CPICH symbols in one slot u is the despread CPICH sample multiplied with complex conjugate of the known CPICH symbol

At block 120, the equaliser E(t) is used to equalise a signal r(t+T) 210. The signal 210 is a data channel, in this example it is the downlink dedicated physical control channel (DPCCH), at time t+T. This signal is transmitted in a series of packets each of which occupies a time slot. Each packet comprises a pilot field comprising pilot symbols. The block 120 does not equalise the whole signal 210 but only the pilot field of the signal 210. The equalised pilot field of the signal r(t+T) 210 is provided to block

122 where it is used to estimate the signal power S(t+T) 123 of the signal_r(t+T) 2.10.

At block 122 the equalised pilot field is despread and the symbol level signal S is used to estimate the signal power either coherently (conj(S)*S) or non-coherently.

The use of a subset of the signal r(t+T) i.e. the pilot field for signal power estimation reduces processing overhead.

The signal power estimate is therefore based upon the most recently received signal 210. A fast reasonably reliable signal power estimate can thus be obtained by using an 'outdated' channel estimate CIR(t) in the equalisation of the most recently received signal r(t+T).

Instead of waiting for the channel estimate to be calculated from the signal r(T+t) and updating the equaliser to E(t+T), the current equaliser E(t) is used to equalise the signal r(t+T) 210. Thus the signal r(t+T) 210 is equalised using an equaliser E(t) that is deliberately not matched to the channel at the time T+t at which the signal r(T+t) is received but is equalised using an equaliser E(t) that is matched to the channel at an earlier time t i.e. the equaliser coefficients w(t) are derived from a previously received signal p(t).

It is of course desirable that the most recently determined equaliser coefficients are used to program the equaliser before equalisation of the signal r(t+T). The separation, in time, between t and t+T may be substantially the delay inherent within estimating the wireless channel i.e. the delay introduced by blocks 100 and 102.

The block 140 receives the estimate of the signal power S (t+T) 123 and the estimate of the interference power T(t) 133 and combines them to produce a signal to noise value SIR(t+T) 141 for time t+T. The value SIR(t+T) can thus be computed quickly and is available very quickly after time t+T, so that it can be used in the WCDMA inner (fast) power loop control.

The interference power l(t) is calculated when data d(t) is acquired and the signal power S(t+T) is calculated using the current ('outdated' or mismatched) equaliser

E(t). Therefore, no additional calculation of equaliser coefficients w is required far

SIR estimation.

Although in the embodiment described above the interference power is calculated for each slot i.e. every T, this may not be necessary. As the interference is assumed to be slowly varying, the interference power T(t) 133 may be used, for example, at times t+T, t+2T, ..t+(m-1)T before a new interference power T(t+mT) is estimated.

In the embodiments described above, the equalised signal 131 is used only to estimate the interference power. In an alternative embodiment, the equalised signal 131 is used to estimate both the 'outdated' interference power and the 'outdated' signal power to produce an 'outdated' SIR estimate. The 'current' signal power estimate 123 is then used to correct the 'outdated' SIR estimate to produce a current SIR estimate

Thus, the method of estimating a signal to interference ratio (SIR) comprises: a) estimating 122 the signal power S(t+T) of a signal r(t+T) 210 received at a first time t+T; b) estimating 132 the interference power l(t') of a signal r(t') 110 received at a second time t' preceding the first time t+T; and c) combining 140 the signal power estimate S(t+T) at the first time t+T and the interference power estimate l(t) at the second time t to produce an estimate of the signal to interference ratio SIR(t+T) at the first time t+T.

The estimation of the received signal power S(t+T) may comprise:

(i) receiving from a wireless channel at a first time t+ T the first signal r(t+T) 210; (ii) equalising 120 the first signal 210 using an equaliser solved for the channel estimate CRT(t) 101 for the wireless channel at a time t, preceding the first time T+t; and (iii) determining a signal power estimate S(t+T) using the equalised first signal 121.

The estimation of the interference power l(t') may comprise:

(i) receiving from the wireless channel at the second time t' that precedes the first time t+T, a second signal r(t');

equalising the second signal r(t') using an estimate w(-t') for the wireless channel at the second time (f); and determining an interference power estimate T(t') using the equalised second signal.

The second time t' may be the same as or precede the time t.

Therefore, the signal power is estimated using an outdated equaliser to equalise a current signal to which the equaliser is not matched. However, the interference power is estimated using an outdated equaliser to equalise an outdated signal to which the outdated equaliser is matched.

Fig. 2 schematically illustrates an apparatus 300 that is suitable for performing the method illustrated in Fig. 1. The apparatus is capable of wirelessly receiving signals. It may be a transceiver such as a mobile cellular telephone, a module for a transceiver or circuitry such as a chip or chipset for a transceiver. The blocks in Fig 1 may be carried out using any suitable combination of hardware, firmware, and software. For example, a programmable microprocessor and memory combination may be used to perform the blocks or application specific integrated circuits (ASICs) or similar dedicated circuitry may be used.

Fig 2 illustrates a simple microprocessor implementation. The apparatus 300 comprises: an antenna 302 that is connected to radio frequency receiver or transceiver circuitry 304. The antenna 302 receives radio frequency electromagnetic waves and the radio frequency circuitry converts the radio frequency electromagnetic waves to a digital signal. The digital signal is processed by baseband circuitry 306. The baseband circuitry comprises a processor 308 that is connected to write to and read from a memory 310. The memory 310 comprises computer program instructions 312 that control operations of the apparatus when loaded into the processor 308. The computer program instructions 312 provide the logic and routines that enables the apparatus 300 to perform the methods illustrated in Fig 1.

The computer program comprising computer program instructions which when loaded in the processor 308 provide: means for estimating the signal power of a signal received at a first time e.g. blocks 120, 122 in Fig 1 ; means for estimating the

interference power of a srgnal received at a second time preceding the first time e.g. blocks 130, 132 in Fig 1; and means for combining the signal power estimate at the first time and the interference power estimate at the second time to produce an estimate of the signal to interference ratio at the first time e.g. block 140 in Fig 1.

The computer program instructions 312 may arrive at the apparatus 300 via an electromagnetic carrier signal or be copied from a physical entity 320 such as a computer program product, a memory device or a record medium such as a CD- ROM or DVD.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

I/we claim: