HK1167049A - Multi-carrier transmitter back-off estimation - Google Patents

Description

Multi-carrier transmitter compensation estimation

Technical Field

The present invention relates generally to multicarrier transmitters and, in particular, to estimating a compensation metric for reducing a maximum transmit power of a multicarrier signal.

Background

In practice, a wireless communication transmitter transmits energy in a frequency region different from the frequency region intended for transmission. If not limited in some way, the unwanted emitters interfere significantly with other communication systems operating in these frequency regions. Thus, wireless communication standards impose limitations on out-of-band (OOB) emissions of transmitters.

The OOB emission requirements typically specify a minimum Adjacent Channel Leakage Ratio (ACLR) for each channel near the intended transport channel. ACLR is the ratio of the power transmitted in the expected channel to the power leaked to some nearby channel. In general, leakage into nearby channels is directly related to the operating point of the transmitter's power amplifier. For example, when a power amplifier operates in its non-linear region at a higher power level, the amount of power leaking into nearby channels increases due to intermodulation distortion. Accordingly, an effective method for achieving the predetermined OOB transmission requirements requires reducing or "back off" the maximum transmit power on the intended transmit channel from its nominal value.

The amount of maximum transmit power that must be compensated while also taking into account amplifier efficiency depends on the nature of the transmit waveform (e.g., modulation, spreading code, spreading factor, gain factor, etc.). For certain signals, these properties can be well quantified in terms of Cubic Metric (CM) or peak-to-average power ratio (PAPR), as described in 3GPP technical specification TS 25.101 release 8. However, such quantification can be costly in terms of fast computation of processing resources, making estimation of the required compensation particularly problematic when the nature of the transmitted waveform dynamically changes.

By a number of known ways of addressing this problem for single carrier transmitters, the PAPR or CM can be pre-calculated for all possible configurations of the transmitted waveform and the corresponding required compensation stored in the look-up table. However, as the number of configuration possibilities increases, this approach proves increasingly impractical due to the size of the look-up tables required. For example, a multi-carrier transmitter simultaneously transmits two or more separately modulated carriers, each occupying a different frequency region. See, for example, multicarrier operation, which is proposed by 3GPP rel.9 "Dual-Cell HSUPA" (3GPP work item description, RP-090014). When the configuration on each carrier is independent of the configuration of the other carriers, the number of possible configurations of the composite waveform (and thus the size of the look-up table required) may be several orders of magnitude larger than in a single carrier transmitter.

Disclosure of Invention

The methods and apparatus taught herein estimate the amount by which the maximum transmit power of a multicarrier signal is to be reduced based on metrics that are specific to each carrier of the multicarrier signal. That is, instead of determining the metrics of the composite waveform of the multi-carrier signal, the present invention obtains the metrics for each carrier of the multi-carrier signal and, in at least one embodiment, combines those metrics to estimate the required compensation.

More specifically, the inventive transmitter comprises a multi-carrier transmitter compensation metric estimation circuit that obtains carrier-specific metrics for each carrier of the multi-carrier signal. The metric obtained for each carrier directly or indirectly indicates the amount by which the maximum transmit power on that carrier should be reduced from a predetermined nominal maximum transmit power to achieve a predetermined out-of-band emission requirement when transmitted individually. For example, in one embodiment, the carrier specific metrics for a carrier include PAPR, CM, or any other similar metrics for a signal on a carrier. In an alternative embodiment, the carrier-specific metric for that carrier includes a maximum power reduction value (e.g., MPR) determined for that carrier as it was transmitted in a single carrier transmitter in the past. This MPR value may be based, for example, on the calculation of PAPR, CM or similar metrics, directly or indirectly, or on their approximation. Regardless of the type of carrier-specific metrics obtained, the multi-carrier transmitter compensation metric estimation circuit thereafter estimates the compensation metrics based on those carrier-specific metrics.

When the estimation of the compensation metric is based on these carrier-specific metrics, the transmitter typically requires less computational complexity and/or memory usage than when estimating the compensation metric based on a similar metric obtained for the multicarrier signal as a whole. In fact, the number of possible configurations of the multicarrier signal as a whole increases exponentially with the number of carriers, by defining the configurations on each carrier independently of those on the other carriers. Therefore, the look-up table mapping the configuration parameters of the multi-carrier signal to its PAPR, CM, or MPR may be too large to implement, and any calculation thereof may be too complex to implement. Instead, the transmitter in some embodiments maintains a smaller look-up table that maps configuration parameters of signals on individual carriers to their carrier-specific metrics, and obtains the carrier-specific metrics for all carriers by retrieving each of them from the same look-up table. By using the same small look-up table to obtain each carrier specific metric, the transmitter requires lower memory usage.

Having obtained the carrier-specific metrics, the compensation metric estimation circuit in some embodiments bases the estimation of the compensation metric on both the carrier-specific metrics and the power ratio between the power on one carrier and the power on the other carrier. This may entail, for example, basing the estimation of the compensation metric on a weighted sum of the carrier-specific metrics, where the weight applied to each carrier-specific metric depends on that power ratio. The weighted values of the different power ratios may be expressed as a function of the power ratio or stored in a look-up table. Accordingly, the compensation metric estimation circuit may require only a few smaller look-up tables or simple calculations: one for obtaining carrier-specific metrics based on configuration parameters of each carrier and the other for obtaining weights based on power ratios.

The present invention is of course not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

Drawings

Fig. 1 is a block diagram illustrating a multi-carrier transmitter according to an embodiment of the present invention.

Fig. 2 is a process flow diagram illustrating a method performed by the multi-carrier transmitter of fig. 1.

Fig. 3 is a block diagram illustrating a multi-carrier transmitter compensation metric estimation circuit in accordance with one embodiment of the present invention.

Fig. 4 is a graph showing a least squares fit of carrier specific metrics to required compensation metrics for different power ratios on the carriers of a multi-carrier signal.

Fig. 5 is a graph showing weights derived from the least squares fit of fig. 4.

Detailed Description

Fig. 1 illustrates one embodiment of a wireless communication transmitter 10 configured to transmit a multicarrier signal via one or more antennas 50. The transmitter 10 includes a plurality of signal generators 20, a transmitter front end 30, and a multi-carrier transmitter compensation metric estimation circuit 40.

Each signal generator 20_kData of different Data signals_kSpread and modulated onto a different one of the N carriers, where each carrier k occupies a different frequency region, (1 · k · N) than the other carriers. More specifically, the Data signal Data_kIndicating all data to be transmitted on a certain carrier k, including both user information and control information. Signal generator 20_kConfig according to configuration parameters specific to that carrier k_k(e.g., the number of physical channels present, their gain factors, and their spreading factors and spreading codes) to perform Data signal Data_kChannelization and scrambling. Signal generator 20_kThe carrier k is then modulated with the spread signal in a modulation process that is independent of the modulation process of the other signal generators 20 (e.g., by a plurality of separately modulated local oscillators). The resulting modulated carrier x_kAnd thereafter provided to the transmitter front end 30 such that the front end 30 receives the N separately modulated carriers occupying different frequency regions.

Combining circuitry 32 included in the transmitter front end 30 aggregates the N carriers to form a multi-carrier signal. In doing so, the combining circuit 32 follows the predicted transmit power P on each carrier_kFor carrier x₁...x_NCommon weighting (where the predicted transmit power P on each carrier is_kThe sum is less than or equal to a predetermined nominal maximum transmission power P_max). Notably, such aggregation can cause the multicarrier signal to exhibit certain undesirable properties (e.g., high PAPR or CM). These properties, in turn, cause amplifier 34 to generate more intermodulation products (i.e., unwanted frequencies) in the multicarrier signal when operating at a given power level in the non-linear region. These unwanted frequencies of course contribute to the out-of-band emissions of the transmitter.

However, according to the invention, the combining circuit 32 reduces the maximum transmission power of the multicarrier signal by a certain amount; i.e. the amount of compensation metric provided to combining circuit 32 by multicarrier transmitter compensation metric estimation circuit 40. FIG. 1 will do thisThe compensation metric is shown as maximum power reduction, referred to herein as "MPR". When the maximum transmit power reduction compensation measure MPR is to be reduced, the combining circuit 32 aims at suppressing the generation of intermodulation products and thereby achieving a predetermined out-of-band emission requirement (hereinafter referred to as OOB)_req)。

The multi-carrier transmitter compensation metric estimation circuit 40 estimates the compensation metric MPR according to the method shown in fig. 2. As shown in fig. 2, the compensation metric estimation circuit 40 obtains carrier-specific metrics for each of the N carriers (block 100). The metric obtained for each carrier k indicates, directly or indirectly, that it should be transmitted from P when transmitted separately_maxReducing the maximum transmit power on that carrier to achieve OOB_reqThe amount of (c). For example, in one embodiment, the carrier-specific metrics for carrier k include: the maximum power reduction value (e.g., MPR) determined for that carrier is as if it were transmitted in a single carrier transmitter. In this case, the maximum power reduction value directly indicates on which carrier k OOB is implemented_reqThe need for this is reduced. In an alternative embodiment, the carrier-specific metrics for that carrier k include: PAPR, CM, or any other similar measure of the signal on the carrier. Since PAPR or CM may be specifically used to determine the maximum power reduction value described above, these metrics indirectly indicate that OOB is implemented_reqThe need for this is reduced. See, for example, 3GPP technical specification TS 25.101 release 8, (provisions can easily determine MPR from CM as MPR ═ Max (CM-1, 0)). Again, regardless of the type of carrier-specific metrics obtained, the circuitry 40 thereafter estimates compensation metrics based on those carrier-specific metrics (block 110).

When the estimation of the compensation metric is based on these carrier-specific metrics, the transmitter 10 advantageously requires less computational complexity and/or memory usage than would be required if the compensation metric were estimated based on similar metrics obtained for the multi-carrier signal as a whole. In fact, the number of possible configurations of the multicarrier signal as a whole increases exponentially with N, by defining the configuration on each carrier k independently of those on the other carriers. Therefore, the look-up table mapping the configuration parameters of the multi-carrier signal to its PAPR, CM, or MPR may be too large to implement, and any calculation thereof may be too complex to implement. In contrast, the transmitter 10 in one embodiment maintains a smaller look-up table that maps configuration parameters of signals on individual carriers to their carrier-specific metrics, and obtains their carrier-specific metrics by retrieving each of all N carriers from the same look-up table. By using the same small look-up table to obtain each carrier specific metric, the transmitter 10 requires lower memory usage. Alternative embodiments that calculate carrier-specific metrics based on configuration parameters also require less computational complexity.

In the embodiment shown in fig. 3, for example, the multi-carrier transmitter compensation metric estimation circuit 40 comprises a plurality of carrier-specific metric generation circuits 42. Individual carrier specific metric generation circuit 42_kIs configured to: by configuring based on configuration parameters specific to that carrier k_kThat metric is calculated or retrieved from a look-up table to obtain the carrier specific metric M_k. Such configuration parameters may vary depending on whether the signal on carrier k is formatted according to the standards of WCDMA, LTE, etc. For example, the configuration parameters may include: gain coefficients of physical channels in a signal when formatted according to WCDMA, or resource block allocation and modulation scheme when formatted according to LTE. Of course, as described above, the look-up table mapping these configuration parameters to carrier-specific metrics may be common to each carrier-specific metric generation circuit 42.

Fig. 3 also provides additional details of basing the compensation metric estimation on the obtained carrier-specific metrics. Specifically, the compensation metric estimation circuit 40 of fig. 3 also includes a compensation metric generation circuit 44 that receives each carrier-specific metric M_kAnd the expected transmit power P on that carrier k_k. Thus, in one embodiment, the compensation metric generation circuit 44 bases the estimation of the compensation metric MPR on the carrier specific metric M₁...M_NAnd the power ratio PR between the power on each carrier k and the power on the other carriers_k：

Or, alternatively

P_kAnd P_jCan represent one of the total power on the carrier (i.e., the power of all channels on each carrier) or less than the power of all channels on the carrier (e.g., the power of a single control channel on each carrier). This may entail, for example, basing the estimation of the compensation metric MPR on the carrier-specific metric M₁...M_NWherein the weight a applied to a particular metric for each carrier depends on the power ratio PR_kOr otherwise as a function thereof:

MPR＝α₀(PR₁，...，PR_N)+α₁(PR₁)·M₁+α_N(PR_N)·M_N(2)

note that in this embodiment, the weight α is₀Depending on all power ratios PR_k. For example, weighting alpha₀May be the power ratio PR_kThe largest one of those, the sum of the squares of those power ratios, or any function reflecting the power distribution between carriers. Of course, there are several possibilities for how to parameterize the power relationship between the carriers, the parameterization chosen in equation (1) is only one example. In practice, only N-1 parameters are needed to uniquely characterize the power relationship between the N carriers. Thus, for a dual carrier signal (i.e., N-2), only one of the power ratios (e.g., power ratio PR of carrier 1) is used₁) It is sufficient to characterize the power relationship between the two carriers. Instead of using the parameterization in equation (2), for example, one can use:

wherein the content of the first and second substances,furthermore, there is symmetry between carriers in the sense that the result does not depend on the count of carriers, and therefore there is a relationship that can be established between the parameters. For example, in (3), it is considered thatAnd

fig. 4 and 5 provide a simple example of the above-described embodiment using equation (3) to characterize the power relationship between two carriers in a dual carrier signal (i.e., N-2). FIGS. 4 and 5 also illustrate a method for determining weightingAndto power ratio PR_kThe method of (1). In this example, the required compensation metric MPR is determined empirically for different configurations of the dual-carrier signal (e.g., via offline calculation of MPR of the composite waveform of the dual-carrier signal, measurement of actual RF circuitry, or simulation using a real RF front-end model). Carrier-specific metrics M for those different configurations of dual carrier signals₁And M₂Is determined as CM₁And CM₂. MPR, M in equation (3) are determined for different configurations₁And M₂Weighted, weightedAndvia different power ratios PR₁M of (A)₁、M₂And MPR. FIG. 4 shows for P₁＝P₂And P₁Ratio P₂Weighting 10dB greaterAndis approximated as a linear plane. FIG. 5 shows the power ratio PR for all power ratios₁Weights derived from this least squares fitAndthe value of (c).

Regardless of how the power relationship between the carriers is characterized, for the corresponding power ratio PR_kBy different values of₀...α_NThe value of (d) can be expressed in the estimation circuit 40 as a power ratio PR_kOr stored in a look-up table. For example, in one embodiment, the compensation metric generation circuit 44 also includes a weight generation circuit 46. The weight generation circuit 46 is configured to generate a weight based on the power ratio PR corresponding to that weight_kTo calculate the weight alpha₀...α_NOr retrieve them from a look-up table.

In view of the above discussion, the estimation circuit 40 may require only a few smaller look-up tables or simple calculations: a configuration parameter config for each carrier k_kTo obtain carrier specific metrics M₁...M_NAnd the other for the power-based ratio PR_kTo obtain the weight alpha₀...α_N. Having determined these values, the estimation circuit 40 may estimate the compensation metric MPR (again based on calculations or retrieval from a look-up table) according to equation (2). Of course, while the above-described implementations advantageously require less computational complexity and/or memory usage, further simplifications and/or approximations are possible.

For example, in one embodiment, based on the corresponding power ratio PR_kRather than that power ratio PR_kTo determine the weighting alpha₀...α_N. Continuing with the example of FIG. 5, if the power ratio PR₁Falls anywhere below 5dB, the weighting is then appliedIt can be determined that the signal is 1.8,is determined to be 0.5, andwas determined to be 0.25. Phase (C)Conversely, if the power ratio PR₁Falls anywhere above 10dB, the weighting is then appliedIt can be determined that the signal is 0.5,is determined to be 1.0, andis determined to be 0. Power ratio PR_kIs quantized and the resulting weight alpha₀...α_NFurther reducing the computational complexity and/or memory usage of the transmitter 10.

The above examples show that it is possible to do without calculating or retrieving one or more carrier specific metrics M in certain situations₁...M_NTo further simplify the conveyor 10. For example when the power ratio PR_kBeyond a certain value, to one or more carrier-specific metrics M₁...M_NMay become insignificant (e.g., in fig. 5, when the power ratio PR₁In excess of 10dB, to M₂Weighting ofNear 0). In this case, the estimation circuit 40 may obtain carrier-specific metrics for less than all N carriers, or at least base the estimation of the compensation metric MPR on less than all N carrier-specific metrics obtained.

By yet another approximation, the weighting α in one embodiment₀...α_NFor example, by an assumed or worst-case operating condition of the transmitter 10 (e.g., where the power on all carriers is equal such that all carriers contribute equally to driving the power amplifier 34 into its non-linear region). In this case, the weight α₀...α_NIs practically not dependent on the power ratio PR_kChanging or otherwise not depending on the power ratio PR_kIs constant.

Of course,although the embodiments described above with reference to fig. 3-5 base the estimation of the compensation metric MPR on the power ratio PR_kHowever, the present invention is not limited thereto. In practice, the compensation metric generation circuit 44 of fig. 3 may receive other values (e.g., configuration parameters for the carriers) instead of or in addition to the expected transmit power on each carrier. In this case, the compensation metric generation circuit 44 bases the estimation of the compensation metric MPR on the carrier-specific metric M in a similar manner as described for fig. 4 and 5₁...M_NAnd one or more of these configuration parameters. On the other hand, the configuration parameters may vary depending on whether the signal on carrier k is formatted according to the standards of WCDMA, LTE, etc. For example, the configuration parameters may include gain coefficients of physical channels in a signal when formatted according to WCDMA, or resource block allocation and modulation scheme when formatted according to LTE.

Accordingly, although embodiments of the present invention are described herein primarily with respect to multi-carrier operation in wideband code division multiple access (W-CDMA) systems, those skilled in the art will recognize that the inventive techniques disclosed and claimed herein are not so limited, but may be advantageously applied to a number of multi-carrier wireless systems, such as multi-carrier Long Term Evolution (LTE) systems, multi-carrier enhanced data rates for GSM evolution (EDGE) systems, and so forth.

Accordingly, it should be understood that the foregoing description and accompanying drawings represent non-limiting examples of the methods and individual devices taught herein. Accordingly, the invention is not limited by the foregoing description and drawings. Instead, the invention is limited only by the following claims.

Claims (22)

1. A method in a wireless communication transmitter configured to transmit a multi-carrier signal, the multi-carrier signal being an aggregate of two or more separately modulated carriers occupying different frequency regions, the method comprising:

obtaining, for each of the two or more carriers, a carrier-specific metric indicating an amount by which, when transmitted individually, the maximum transmit power on that carrier should be reduced from a predetermined nominal maximum transmit power to achieve a predetermined out-of-band emission requirement; and

estimating a multi-carrier transmitter compensation metric for reducing a maximum transmit power of the multi-carrier signal based on the obtained two or more carrier specific metrics.

2. The method of claim 1, wherein obtaining carrier-specific metrics comprises: the carrier-specific metric is calculated or retrieved from a look-up table based on one or more configuration parameters of the signal on that carrier.

3. The method of claim 2, wherein the one or more configuration parameters comprise: gain coefficients of physical channels in wideband code division multiple access, WCDMA, signals, or resource block allocation and modulation schemes in long term evolution, LTE, signals.

4. The method of claim 1, wherein obtaining carrier-specific metrics comprises: a cubic metric, a peak-to-average power ratio, or a maximum power reduction value is obtained.

5. The method of claim 1, wherein estimating the multi-carrier transmitter compensation metric is further based on a power ratio between the power of one or more channels on one of the carriers and the power of the corresponding one or more channels on the other carriers.

6. The method of claim 5, wherein the power ratio comprises:

a ratio between the power of all channels on one of the carriers and the power of all channels on the other carrier; or

A ratio between the power of a single control channel on one of the carriers and the power of a corresponding control channel on the other carrier.

7. The method of claim 1, wherein estimating the multicarrier transmitter compensation metric is further based on one or more configuration parameters of a signal on at least one of the carriers.

8. The method of claim 7, wherein the one or more configuration parameters comprise: gain coefficients of physical channels in WCDMA signals, or resource block allocation and modulation schemes in long term evolution, LTE, signals.

9. The method according to claim 1, wherein estimating the multi-carrier transmitter compensation metric is based on a weighted sum of the obtained two or more carrier-specific metrics.

10. A method according to claim 9, wherein the weighting applied to each carrier-specific metric is constant or alternatively depends on at least one of:

a power ratio between the power of one or more channels on one of the carriers and the power of corresponding one or more channels on the other carrier; or

One or more configuration parameters of a signal on at least one of the carriers.

11. The method of claim 9, wherein the weighting applied to each carrier-specific metric depends on a power ratio between a power of one or more channels on one of the carriers and a power of corresponding one or more channels on the other carriers, and wherein the method further comprises: the weights are calculated or retrieved from a look-up table based on the values of the power ratios or alternatively based on which of a plurality of ranges the power ratios fall within.

12. A wireless communication transmitter configured to transmit a multi-carrier signal, the multi-carrier signal being an aggregate of two or more separately modulated carriers occupying different frequency regions, the wireless communication transmitter comprising:

a multi-carrier transmitter compensation metric estimation circuit configured to:

obtaining, for each of the two or more carriers, a carrier-specific metric indicating an amount by which, when transmitted individually, the maximum transmit power on that carrier should be reduced from a predetermined nominal maximum transmit power to achieve a predetermined out-of-band emission requirement; and

estimating a multi-carrier transmitter compensation metric for reducing a maximum transmit power of the multi-carrier signal based on the obtained two or more carrier specific metrics.

13. The wireless communication transmitter of claim 12, wherein the multi-carrier transmitter compensation metric estimation circuit is configured to: the carrier-specific metrics are obtained by calculating them based on one or more configuration parameters of the signal on that carrier or retrieving them from a look-up table.

14. The wireless communication transmitter of claim 13, wherein the one or more configuration parameters comprise: gain coefficients of physical channels in wideband code division multiple access, WCDMA, signals, or resource block allocation and modulation schemes in long term evolution, LTE, signals.

15. The wireless communication transmitter of claim 12, wherein the multi-carrier transmitter compensation metric estimation circuit is configured to: a carrier specific metric is obtained as a cubic metric, a peak-to-average power ratio, or a maximum power reduction value.

16. The wireless communication transmitter of claim 12, wherein the multi-carrier transmitter compensation metric estimation circuit is configured to: the multi-carrier transmitter compensation metric is also estimated based on a power ratio between the power of one or more channels on one of the carriers and the power of the corresponding one or more channels on the other carriers.

17. The wireless communication transmitter of claim 16, wherein the power ratio comprises:

a ratio between the power of all channels on one of the carriers and the power of all channels on the other carrier; or

A ratio between the power of a single control channel on one of the carriers and the power of a corresponding control channel on the other carrier.

18. The wireless communication transmitter of claim 12, wherein the multi-carrier transmitter compensation metric estimation circuit is configured to: the multi-carrier transmitter compensation metric is also estimated based on one or more configuration parameters of a signal on at least one of the carriers.

19. The wireless communication transmitter of claim 18, wherein the one or more configuration parameters comprise: gain coefficients of physical channels in WCDMA signals, or resource block allocation and modulation schemes in long term evolution, LTE, signals.

20. The wireless communication transmitter of claim 12, wherein the multi-carrier transmitter compensation metric estimation circuit is configured to: basing the multi-carrier transmitter compensation metric on a weighted sum of the obtained two or more carrier-specific metrics.

21. The wireless communication transmitter of claim 20, wherein the weighting applied to each carrier-specific metric is a constant or alternatively depends on at least one of:

a power ratio between the power of one or more channels on one of the carriers and the power of the corresponding one or more channels on the other carrier; or

One or more configuration parameters of a signal on at least one of the carriers.

22. The wireless communication transmitter of claim 20, wherein the weighting applied to each carrier-specific metric depends on a power ratio between the power of one or more channels on one of the carriers and the power of the corresponding one or more channels on the other carriers, and wherein the multi-carrier transmitter compensation metric estimation circuit is further configured to: the weights are calculated or retrieved from a look-up table based on the values of the power ratios or alternatively based on which of a plurality of ranges the power ratios fall within.