HK1113239A - Interference estimation in the presence of frequency errors - Google Patents

Description

Interference estimation in the presence of frequency errors

Technical Field

The present invention relates to electronic digital communication systems, and more particularly to receivers in wireless communication systems.

Background

Digital communication systems include Time Division Multiple Access (TDMA) systems, such as cellular radiotelephone systems that conform to the GSM telecommunications standard and its enhancements, such as GSM/EDGE, and Code Division Multiple Access (CDMA) systems, such as cellular radiotelephone systems that conform to the IS-95, CDMA2000, and wideband CDMA (wcdma) telecommunications standards. Digital communication systems also include "hybrid" TDMA and CDMA systems, such as cellular radiotelephone systems that conform to the Universal Mobile Telecommunications System (UMTS) standard, which specifies a third generation (3G) mobile system developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunications Union (ITU) IMT-2000 framework. The third generation partnership project (3GPP) promulgates UMTS and WCDMA standards. For simplicity, the present application focuses on WCDMA systems, but it should be understood that the principles described in the present application may be implemented in other digital communication systems.

WCDMA is based on direct sequence spread spectrum techniques, where the base station and physical channel (terminal or user) are separated in the downlink (base-to-terminal) direction by a pseudo-noise scrambling code and an orthogonal channelization code, respectively. Since all users in a CDMA system share the same Radio Frequency (RF) resource, it is important that each physical channel does not use more power than necessary. This is achieved by a Transmit Power Control (TPC) mechanism, where the base station sends TPC commands to the users in the Downlink (DL) direction, and the users implement these commands in the Uplink (UL) direction, and vice versa. The TPC commands cause the users to increase or decrease their transmitted power levels by multiple increments, thereby maintaining a target signal-to-interference ratio (SIR) of the Dedicated Physical Channel (DPCH) between the base station and the users. The DPCH includes a Dedicated Physical Data Channel (DPDCH) and a Dedicated Physical Control Channel (DPCCH) in the UL and DL. The DPDCH carries higher layer network signaling and possibly also voice and/or video services, while the DPCCH carries physical layer control signaling (e.g., pilot symbols/signals, TPC commands, etc.). WCDMA terminology is used herein, but it should be understood that other systems have corresponding terminology. Scrambling and channelization codes and transmission power control are well known in the art.

Fig. 1 shows a communication system, such as a WCDMA system, comprising a Base Station (BS)100, which base station 100 handles in this example connections with four Mobile Stations (MS)1, 2, 3, 4. In the downlink, the BS 100 transmits to each mobile station at a corresponding power level, and spreads (spreads) a signal transmitted by the BS 100 using orthogonal code words. In the uplink, MS 1-MS 4 transmit to BS 100 at respective power levels. Each BS (referred to as a node B in 3GPP parlance) in the system serves a geographical area that may be divided into one or more cells. These BSs are connected to corresponding radio network controllers (RNCs, not shown in fig. 1) by dedicated telephone lines, optical fiber links, microwave links, etc. The RNC directs the MS or User Equipment (UE) to make calls through the appropriate BS, and the RNC connects to external networks, such as the Public Switched Telephone Network (PSTN), the internet, etc., through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).

WCDMA is designed to operate with a low signal-to-noise ratio (SNR), and therefore WCDMA algorithms such as an SIR estimator and an Automatic Frequency Control (AFC) algorithm are designed for this case. For example, an SIR estimation algorithm used in a Transmission Power Control (TPC) scheme to obtain sufficient quality of service (QoS) is designed to be used at a low SIR. QoS is typically quantified by block error rate (BLER). It will be appreciated that in WCDMA systems (and other communication systems employing Direct Sequence (DS) spread spectrum techniques), the noise (N) includes thermal noise and interference, since the interference signal behaves like noise (i.e., spread in frequency and level with a noise floor) due to the spreading of the signal by the "wrong" spreading code of the interference signal.

The SIR is used for inner loop power control because it is assumed to have an almost one-to-one mapping for BLER. Outer loop power control, which operates at a low response rate, is also included in WCDMA to compensate for the remaining mismatch between SIR and BLER. Power Control and SIR-to-BLER mapping are known in the art and are described, for example, in "estimation and Closed-Loop Power Control for 3G", IEEE pp.831-835(2003) by Louay m.a.

In such a communication system, a BS transmits predetermined pilot symbols on a DPCH of a UE. The BS also transmits pilot symbols on a common pilot channel (CPICH), which the UE typically uses when estimating the impulse response (impulse response) of the wireless channel to the BS. It should be appreciated that due to the typically high SNR of the CPICH, the UE uses the CPICH pilots for channel estimation and does not use the DPCH pilots, but the UE still uses primarily the DPCH pilots for SIR estimation, i.e., for DL power control.

It is also known that a better SIR estimator provides better receiver performance, measured as the amount of power required for a given BLER target, and that the lower the required power the better. To improve the SIR estimator in WCDMA, the CPICH may be used for I estimation, while the DPCH pilots are used only to estimate the S part of the SIR. This is described, for example, in U.S. patent application publication No.2005/0094816 "Interference timing in CDMASystems Using Alternative scanning Codes" to Lindoff et al. The following five equations represent such SIR estimators.

For S, the desired signal estimates S_i ^DPCHGiven by:

equation 1

Wherein the content of the first and second substances,

equation 2

And n is_pIs a DPCH pilot symbol u per slot_k ^PThe number of the (c) component(s),y_DPCH，i(k) is the despread DPCH pilot symbol for rake finger (rake finger) i at time k, and denotes the complex conjugate.

For I, interference signal estimation I_i ^DPCHGiven by:

equation 3

Wherein SF_CIs directed to the spreading factor of the channel (e.g., CPICH) used to calculate the I estimate, and SF_DIs to estimate the spreading factor of the channel to which to switch (e.g., DPCH) for I (in the case where the two are different channels), and:

equation 4

Wherein u is_k ^CPICHIs the CPICH pilot symbol k and,is the CPICH channel estimate, y, for tap i_CPICH，i(k) Is a despread CPICH pilot symbol for rake finger i at time k, and N_CIs the number of pilot symbols per slot for the channel used to obtain the I estimate. In WCDMA communication systems, SF_CTypically 256, and the CPICH has ten pilot symbols per slot. In this example, CPICH symbols (i.e., ten symbols) in one slot are used to determine the I estimate. It should be understood that different numbers of symbols may be used and that different communication systems may have different numbers of symbols in a slot.

Estimating SIR for SIR_EST：

Equation 5

Wherein n is_fIs the number of rake fingers.

In the case of laboratory tests and standard checks (benchmark), good signal quality is generally assumed, that is to say the terminal operates with a good SNR. Furthermore, in this case, good terminal behavior is required, which means that the required downlink power should be small if the SNR of the CPICH is high. The following describes "poor" terminal behavior, including long power control loop transitions. In this case, the residual frequency error (the frequency error left after AFC corrects the tuning of the receiver) has a greater effect on the I estimate than on the BLER. It will be appreciated that the SIR to BLER mapping, which is heavily dependent on the interference level, changes the SIR reference value for the outer loop power control and, due to the slow response of the outer loop power control, creates a long transition where the downlink power level is set too high. Therefore, erroneous SIR estimates are obtained in these cases.

Disclosure of Invention

In the presence of residual frequency errors, it is desirable to avoid the behavior of current SIR estimation algorithms by better algorithms that estimate interference I (and SIR). The inventors have observed that the I estimation process is dominated by residual frequency error in case of low interference. This will affect the SIR estimation but not the BLER, so when a situation with low interference is detected, the I estimation strategy (which is part of estimating the SIR) can be changed to compensate for the effect of the residual frequency error.

According to one aspect of the invention, a method of estimating an interference level of a received signal in a receiver is provided. The method comprises the following steps: detecting an interference level of the received signal; determining whether the detected interference level is low; and if the detected interference level is low, estimating the interference level by at least one of estimating only in a radial direction and de-rotating (de-rotating) the received signal before estimating the interference level.

According to another aspect of the present invention, there is provided an apparatus for estimating an interference level of a signal received in a receiver. The apparatus includes a detector configured to detect an interference level of a received signal; and a processor configured to determine whether the detected interference level is low, and if the detected interference level is low, estimate the interference level by at least one of estimating only in a radial direction and de-rotating the received signal before estimating the interference level.

According to another aspect of the invention, a computer readable storage medium containing a computer program for estimating an interference level of a signal received in a receiver is provided. The computer program performs the steps of: detecting an interference level of the received signal; determining whether the detected interference level is low; and estimating the interference level by at least one of estimating only in a radial direction and de-rotating the received signal before estimating the interference level, if the detected interference level is low.

Drawings

The various aspects, features and advantages of the present invention will be understood by reading the specification in conjunction with the drawings, in which:

FIG. 1 illustrates a communication system;

FIG. 2 is a block diagram of an exemplary user equipment in a communication system;

FIG. 3 is a flow chart of a method of estimating interference levels; and

FIG. 4 is a flow chart of a modified method of estimating interference levels.

Detailed Description

Fig. 2 is a block diagram of a portion of a receiver 200 (e.g., a mobile terminal in a WCDMA communication system) in accordance with aspects of the present invention. The radio signal is received by a suitable antenna 202 and down-converted and sampled to a baseband signal by a front end receiver (FeRX) 204. The down-conversion assumes a carrier frequency of f_C. The samples of the baseband signal are then input to a path searcher 206, which path searcher 206 correlates the received samples with a known pilot signal and estimates a path delay profile, which is input to a rake combiner 208 and a channel estimator and SIR estimator 210. rake combiner 208 and channel estimator 210 despread the pilot channel, estimate the impulse response of the wireless channel, and despread and combine the received echoes (echo) of the received data with the control symbols. Other blocks in fig. 2 are described below, it being understood that the receiver may be implemented with other arrangements of the functional blocks shown in fig. 2.

rake combining and channel estimation are well known in the art. Various aspects of rake receivers are described in the following documents: "Introduction to Speed-Current amplification Techniques and the theory Application to Urban Digital Radio" by Turin, proc. IEEE, vol.68, pp.328-353 (3 months 1980); U.S. Pat. No.5,305,349 entitled "Quanticoded coherent Rake Receiver" to Dent; U.S. Pat. No.6,363,104 to Bottomley, entitled "Method and Apparatus for Interference Cancellation in a RakeReceiver"; patent No.6,801,565 entitled "Multi-Stage Rakecombining Methods and Apparatus" to Wang et al; U.S. patent application publication No.2001/0028677 entitled "Apparatus and Methods for Finger Delay Selection in Rake Receivers" to Wang et al. Channel estimation is described, for example, in the following documents: U.S. patent application publication No.2005/0105647 entitled "Channel Estimation by adaptive interpolation" to Wilhelmsson et al.

As shown in the flow chart of fig. 3, the method of estimating the interference level may include or may be improved by including the step of detecting the interference level I (step 302) and then determining (step 304) whether the detected I level is lower. For example, the quality of the received signal (e.g., chip energy) to interference energy ratio E may be estimated_C/I₀) It is then determined whether the quality exceeds a (cross) threshold to detect a situation with low interference. For example, a suitable estimate of the quality of the received signal is the code power (RSCP) of the received signal divided by the Received Signal Strength Indicator (RSSI), e.g., E_C/I₀RSCP/RSSI, wherein RSCP is E_CIs the signal code power of the CPICH. The signal quality estimate and RSCP values and RSSI values are preferably generated by the path searcher 206 and one or more of them are provided to higher layer processing (e.g., for handover measurements) and to a Control Unit (CU)212 according to an embodiment of the present invention. The comparison of the signal quality estimate with the threshold, which may be set by software programmed operation of the control unit, may be performed by a suitably configured or programmed processor CU 212 or even by a suitable comparator. With respect to a suitable value or range of values for the threshold, it is presently believed that E is the same for WCDMA communication systems_C/I₀The signal level starts to be good enough at about-8 dB.

It should be understood that situations with low interference levels may also be detected in other ways. For example, rather than considering the E of the CPICH as described above, the SIR of the CPICH may be considered according to the following expression_C/I₀：

Equation 6

Using SIR instead of E_C/I₀Has the advantage that the SIR measurements do not include quadrature interference, which does not affect the performance of the terminal.

It will also be appreciated that in a WCDMA communication system, the SIR estimation and other steps of the methods described herein are preferably performed once per slot if SIR is used, or if E is used_C/I₀Then every 30-100 milliseconds. In other communication systems, these methods are performed in a system-dependent manner.

If the detected I level is not low, an I estimate may be generated in a conventional manner using equations 3 and 4 above (step 306). If I is lower, i.e. E_C/I₀Higher (e.g., greater than-8 dB), the information (represented as a yes/no signal in fig. 2) is input to the channel and SIR estimator 210, and the channel and SIR estimator 210 generates an I estimate by performing an I estimation method corresponding to the information described in more detail below (step 308). These estimators 210 also generate estimates of channel filter taps h in any of a number of ways known in the art. Rake combiner 208 then decodes the received signal using the h and I estimates and generates an SIR estimate for use in subsequent processing included in the power control loop in a manner well known in the art.

I estimation method

The residual frequency error is typically 0-50Hz and is currently considered to be the main contributor to the interference quantified by the I estimate. In general, when there is a (small) residual frequency error between the frequency of the received carrier signal and the frequency of the receiver's Local Oscillator (LO)214, the despread CPICH symbol can be written as:

equation 7

That is, the frequency error can be viewed as a phase shift Δ ═ 2 π f between consecutive symbols k, k +1_e/R_CIs rotated by the sign of (a), wherein f_eIs the residual frequency error, R_CIs the symbol rate and y-bar is the zero error symbol. For WCDMA systems, R_C(chip rate)/SF_C＝(3.84MHz)/SF_CThus, for example, SF_CThe CPICH symbol rate of 256 is R_C15000 symbols per second, and N_C10CPICH pilot symbols per slot. For low E_C/I₀The S and I estimates are conventionally made according to equations 1-4 above, but for high E_C/I₀And thus interfere with the case where the residual frequency error is controlled, either or both of the following modified methods are preferably used for I estimation.

Method 1: estimation in radial direction only I

In one embodiment of the present invention, a modified method of estimating interference I (excluding interference due to frequency error) for small residual frequency errors (i.e., less than 50Hz in WCDMA systems) uses the following equation:

equation 8

And

equation 9

Wherein phi isRe (x) represents the real part of the complex number x, the other quantities being as defined above. Thus, the modified method (step 308 in fig. 3) includes calculating the I estimate according to equations 8 and 9. With this I estimate, the SIR estimate can be calculated according to equation 5 above.

It should be appreciated that the "radial" direction is parallel to the real coordinate axis due to the angle of the channel estimate being compensated. It will also be appreciated that in many receivers, due to quantization, the residual frequency error is in the range of 10-60 Hz. In a WCDMA communication system, the residual frequency error should be less than about 100Hz in order to get a good estimate using method 1.

Method 2: derotation of signals prior to calculating I

According to another embodiment of the present invention, a modified method of estimating interference I includes compensating received symbols or samples based on an estimate of a corresponding residual frequency error. An estimate of the residual frequency error is readily obtained from AFC device 216 in receiver 200 operating in a manner well known in the art. For example, U.S. patent No.6,606,363 to Atarius et al describes a method and apparatus for estimating a frequency offset by combining pilot symbols and data symbols, and international publication No. wo 02/29978a2 to Dent et al describes a method and apparatus for automatic frequency control in a CDMA receiver. The steps of this modified method are described in the flow chart of fig. 4 and include:

the residual frequency error f is estimated, for example, by obtaining the estimate from the AFC device 216_e(step 402); and

for each symbol, the received CPICH symbol is de-rotated with a corresponding phase shift according to the following expression:

equation 10

WhereinIs a derotated symbol, Δ ═ 2 π f_e/R_CAnd other parameters are as described above.

The derotated symbol may then be usedThe interference level I and SIR are estimated according to equations 1-5 above. It should be understood that equation 10 may be used for channels other than CPICH (e.g., DPCH).

In general, method 2 is "better" than method 1 from a performance perspective, but method 2 may be more difficult to implement, because the (estimated) residual frequency error is corrected before the interference is calculated. Method 1 estimates noise in only one direction (radial); since the noise in the orthogonal direction is assumed to be the same, the total interference is estimated to be twice as much as the interference in the radial direction. Furthermore, method 1 has a good approximation for small residual frequency errors. It is presently believed that in some implementations, method 1 is simpler than method 2, but in other implementations, method 2 is simpler than method 1. Of course, it should be understood that in other implementations, methods 1 and 2 may be used in combination.

It will be appreciated that the above-described process is performed iteratively as necessary, for example, to respond to the time-varying nature of the communication channel between the transmitter and receiver. Further, the description is written with respect to channels such as DPCH and CPICH, but it should be understood that other channels may be suitable. The use of CPICH pilot symbols is advantageous because the CPICH covers the entire area of a cell in a WCDMA system and the pilots are transmitted continuously. However, I may be estimated on another channel (e.g. directly on the DPCH), in which case equation 8 uses the DPCH parameters and ignores the conversion of equation 9.

To facilitate understanding, many aspects of the invention are described in terms of sequences of actions that can be performed by, for example, elements of a programmable computer system. It will be recognized that various actions could be performed by specialized circuits (e.g., discrete logic gates interconnected to perform a specialized function or application-specific integrated circuits), by program instructions being executed by one or more processors, or by a combination of both. A wireless receiver implementing embodiments of the present invention may be included in, for example, mobile telephones, pagers, headsets, laptop computers and other mobile terminals, and the like.

Moreover, the invention can additionally be considered to be embodied entirely within any form of computer-readable storage medium having stored therein an appropriate set of instructions for use by or in connection with an instruction-execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch instructions from a medium and execute the instructions. As used herein, a "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium include an electrical connection having one or more wires, a portable computer diskette, a Random Access Memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), and an optical fiber.

Thus, the invention may be embodied in many different forms, not just all of the forms described above, and all such forms are contemplated to be within the scope of the invention. For each of the various aspects of the invention, any such form may be referred to as "logic configured to" perform a described action, or alternatively as "logic that" performs a described action.

It should be emphasized that the terms "comprises" and "comprising," when used in this application, specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

The particular embodiments described above are illustrative only and should not be considered as limiting in any way. The scope of the invention is determined by the following claims, and is intended to cover all modifications and equivalents that fall within the scope of the claims.

Claims (20)

1. A method of estimating an interference level of a received signal in a receiver, the method comprising the steps of:

detecting an interference level of the received signal;

determining whether the detected interference level is low; and

if the detected interference level is low, estimating the interference level by at least one of estimating only in a radial direction and de-rotating the received signal before estimating the interference level.

2. The method of claim 1, wherein the interference level is detected by estimating a quality of the received signal, and the step of determining whether the detected interference level is low comprises determining whether the quality exceeds a predetermined threshold.

3. The method of claim 2, wherein the mass is a ratio of chip energy to interference energy.

4. The method of claim 3, wherein the quality is a code power of the received signal divided by a strength indicator of the received signal.

5. The method of claim 1, wherein the radial direction is parallel to a real coordinate axis.

6. The method of claim 5, wherein the interference level is estimated only in a radial direction according to the following equation:

and

wherein, I_i ^DPCHIs an estimate of the interference level on the first channel DPCH for the receiver tap I, I_i ^CPICHIs an estimate of the interference level on the secondary channel CPICH for this tap i, k is the time index, SF_CIs a spreading factor, SF, for said second channel_DIs a spreading factor, N, for said first channel_CIs the number of symbols per slot on said second channel, phi isAngle of (a) y_CPICH，i(k) Is the despread pilot symbol of said second channel at time k for tap i,is a channel estimate of said second channel for tap i, and u_k ^CPICHIs a pilot symbol of the second channel at time k.

7. The method of claim 1, wherein the received signal is de-rotated by estimating a residual frequency error and de-rotating symbols in the received signal with a corresponding phase shift for each symbol, the corresponding phase shift given by the equation:

wherein the content of the first and second substances,is the de-rotated symbol of the channel CPICH, k is the index, f_eIs the residual frequency error, N_CIs the number of symbols per slot on the channel, and R_CIs the symbol rate on the channel and uses de-rotated symbolsTo estimate the interference level.

8. The method of claim 1, wherein the receiver operates in a wideband code division multiple access wireless communication system.

9. An apparatus for estimating an interference level of a received signal in a receiver, the apparatus comprising:

a detector configured to detect an interference level of the received signal; and

a processor configured to determine whether the detected interference level is low, and if the detected interference level is low, estimate the interference level by at least one of estimating only in a radial direction and de-rotating the received signal before estimating the interference level.

10. The apparatus of claim 9, wherein the detector detects the interference level by estimating a quality of the received signal, and the processor determines whether the quality exceeds a predetermined threshold.

11. The apparatus of claim 10, wherein the mass is a ratio of chip energy to interference energy.

12. The apparatus of claim 11, wherein the quality is a code power of the received signal divided by a strength indicator of the received signal.

13. The apparatus of claim 9, wherein the radial direction is parallel to a real coordinate axis.

14. The apparatus of claim 13, wherein the processor is configured to estimate the interference level only in a radial direction according to the following equation:

and

wherein, I_i ^DPCHIs an estimate of the interference level on the first channel DPCH for the receiver tap I, I_i ^CPICHIs an estimate of the interference level on the secondary channel CPICH for this tap i, k is the time index, SF_CIs a spreading factor, SF, for said second channel_DIs a spreading factor, N, for said first channel_CIs the number of symbols per slot on said second channel, phi isAngle of (a) y_CPICH，i(k) Is the despread pilot symbol of said second channel at time k for tap i,is a channel estimate of said second channel for tap i, and u_k ^CPICHIs a pilot symbol of the second channel at time k.

15. The apparatus of claim 9, wherein the processor is configured to de-rotate the received signal by estimating a residual frequency error and de-rotating symbols in the received signal with a corresponding phase shift for each symbol, the corresponding phase shift given by the equation:

wherein the content of the first and second substances,is the de-rotated symbol of the channel CPICH, k is the index, f_eIs the residual frequency error, N_CIs the number of symbols per slot on the channel, and R_CIs the symbol rate on the channel and uses de-rotated symbolsTo estimate the interference level.

16. The apparatus of claim 9, wherein the receiver operates in a wideband code division multiple access wireless communication system.

17. A computer-readable storage medium containing a computer program for estimating an interference level of a signal received in a receiver, wherein the computer program performs the steps of:

detecting an interference level of the received signal;

determining whether the detected interference level is low; and

if the detected interference level is low, estimating the interference level by at least one of estimating only in a radial direction and de-rotating the received signal before estimating the interference level.

18. The medium of claim 17, wherein the interference level is detected by estimating a quality of the received signal, and the step of determining whether the detected interference level is low comprises determining whether the quality exceeds a predetermined threshold.

19. The medium of claim 17, wherein the interference level is estimated only in a radial direction according to the following equation:

and

wherein, I_i ^DPCHIs an estimate of the interference level on the first channel DPCH for the receiver tap I, I_i ^PICHIs an estimate of the interference level on the secondary channel CPICH for this tap i, k is the time index, SF_CIs a spreading factor, SF, for said second channel_DIs a spreading factor, N, for said first channel_CIs the number of symbols per slot on said second channel, phi isAngle of (A) Y_CPICH，i(k) Is the despread pilot symbol of said second channel at time k for tap i,is a channel estimate of said second channel for tap i, and u_k ^CPICHIs a pilot symbol of the second channel at time k.

20. The medium of claim 17, wherein the received signal is de-rotated by estimating a residual frequency error and de-rotating symbols in the received signal with a corresponding phase shift for each symbol, the corresponding phase shift given by the equation:

wherein the content of the first and second substances,is the de-rotated symbol of the channel CPICH, k is the index, f_eIs the residual frequency error, N_CIs the number of symbols per slot on the channel, and R_CIs the symbol rate on the channel and uses de-rotated symbolsTo estimate the interference level.