Patent Application
20110176466
The present invention generally relates to wireless communication receivers, and more particularly relates to techniques for reducing power consumption in wireless receivers by selectively deactivating receiver circuits during operation.
In a wireless packet-switched data network employing Orthogonal Frequency Division Multiplexing (OFDM), modulated symbols are constructed from a data packet and inserted into a rectangular grid of symbols in the time-frequency domain. In the so-called Long-Term Evolution (LTE) wireless systems developed by members of the 3rd-Generation Partnership Project, this rectangular grid is divided into “resource blocks,” such that each resource block includes consecutive sub-carriers in the frequency domain and consecutive OFDM symbols in the time domain. An LTE resource block includes twelve consecutive sub-carriers in the frequency domain and a “slot” of seven consecutive OFDM symbols in the time domain (in the normal case; an LTE transmitter can be configured to use only six OFDM symbols in a resource block to combat large delay spreads). Each element (called “resource element” in LTE) of the resource block represents a basic unit in which a complex-valued symbol can be transmitted.
To coherently demodulate the symbols included in these resource blocks, an OFDM receiver must estimate the channel over which the resource blocks are transmitted. To facilitate this estimation, known reference symbols, commonly referred to as pilot symbols, are transmitted in each resource block. In LTE, these reference symbols are jointly referred to as reference signals, and three different types of reference signals are defined. First, common reference signals, or “cell-specific” downlink reference signals, are transmitted in every downlink resource block, and thus span the entire downlink bandwidth for the cell. These can be used by an receiver to estimate the channel. In some cases, an additional reference signal, known in LTE as a “UE-specific” reference signal, is dedicated to a particular user for certain transmission modes, such that only the targeted receiver can process the reference symbols. When these dedicated reference symbols are used, the same transmission methods (e.g., multi-antenna precoding) used for the data symbols are also used for the known reference symbols. A third type of reference signal, the MBSFN reference signal, is used for transmissions in accordance with the 3GPP specifications for Multi-Media Broadcast over a Single Frequency Network (MBSFN).
The response of the wireless channel in an OFDM system is a slow-varying, two-dimensional function of time and frequency. Accordingly, reference symbols need not be placed in every subcarrier nor in every OFDM symbol interval. Instead, reference symbols are distributed across each resource block (an exemplary arrangement is illustrated in
According to various embodiments of the methods and apparatus disclosed herein, a “micro-sleep” functionality is selectively enabled in a wireless receiver, based on an evaluation of channel conditions, traffic characteristics, or both. When micro-sleep operation is appropriate, such as when an estimated signal-to-interference ratio is higher than a pre-determined threshold, one or more receiver circuits in a mobile station can be de-activated for a portion of a sub-frame (or other transmission-time interval) that generally carries traffic data but is not currently carrying data targeted to the mobile station. In this manner, significant power savings can be achieved, independently of or in addition to any power savings provided by existing discontinuous-receive (DRX) technologies.
In an exemplary embodiment of a method of controlling a wireless receiver, a receiver circuit is activated for a first portion of a first transmit-time interval, the first portion comprising control channel data and one or more first reference symbols. In an LTE system, for example, this first portion may comprise the control channel portion of an LTE downlink subframe, i.e., the first one, two, or three OFDM slots of the subframe. The exemplary method further comprises evaluating a channel condition, a data-traffic characteristic, or both, and selectively de-activating the receiver circuit for a second portion of the first transmit-time interval, based on said evaluating.
In some embodiments, one or more channel conditions are evaluated against pre-determined criteria. In these embodiments, then, evaluating a channel condition may comprise estimating a channel condition based on the first reference symbols and comparing the estimated channel condition to a suitable pre-determined threshold (where the threshold level in any particular case depends on the channel condition being evaluated). The estimated channel condition may be, for example, an estimated signal-to-noise ratio, in which case the receiver circuit is selectively de-activated if the estimated signal-to-noise ratio exceeds a corresponding pre-determined threshold. In other embodiments, the estimated channel condition is an estimated delay spread and the receiver circuit is selectively de-activated for the second portion of the transmit-time interval if the estimated delay spread is less than a suitable pre-determined threshold. In still others, the estimated channel condition is an estimated Doppler spread, and the receiver circuit is selectively de-activated if the estimated Doppler spread exceeds a different pre-determined threshold.
In still other embodiments, the selective de-activation of the receiver circuit is based, at least in part, on evaluating a data-traffic characteristic. In some of these embodiments, evaluating the data traffic characteristic comprises determining a current type of service, and may further comprise estimating a required data speed for the current type of service, such that selectively de-activating the receiver circuit comprises de-activating the receiver circuit if the estimated required data speed is less than a pre-determined threshold level for the estimated required data speed.
The above-summarized methods, and variations thereof, may be implemented in a wireless receiver, including wireless receivers configured for operation in LTE networks. An exemplary wireless receiver thus comprises a receiver circuit that is configured to be selectively disabled and a control circuit, wherein the control circuit is coupled to the receiver circuit and is configured to activate the receiver circuit for a first portion of a first transmit-time interval, the first portion comprising control channel data and one or more first reference symbols, to evaluate a channel condition, a data-traffic characteristic, or both, and to selectively de-activate the receiver circuit for a second portion of the first transmit-time interval, based on said evaluating. As with the various embodiments of the methods summarized above, in some embodiments of this wireless receiver, the first transmit-time interval comprises an LTE subframe, and the first portion of the LTE subframe comprises at least the first OFDM symbol of the LTE subframe.
In some embodiments, the control circuit is configured to evaluate one or more channel conditions against pre-determined criteria. In these embodiments, the control circuit is configured to evaluate a channel condition by estimating the channel condition based on the first reference symbols and comparing the estimated channel condition to a suitable pre-determined threshold level for the channel condition. In some of these embodiments the estimated channel condition is an estimated signal-to-noise ratio, and the control circuit is configured to selectively de-activate the receiver circuit if the estimated signal-to-noise ratio exceeds a suitable pre-determined threshold level for signal-to-noise ratio. In others, the estimated channel condition is an estimated delay spread and the control circuit is configured to selectively de-activate the receiver circuit if the estimated delay spread is less than a corresponding pre-determined threshold. In still others, the estimated channel condition is an estimated Doppler spread and the control circuit is configured to selectively de-activate the receiver circuit if the estimated Doppler spread exceeds a pre-determined threshold level for Doppler spread. Selective de-activation based on a combination of these and/or other factors is also possible, in some embodiments, as is selective de-activation based on evaluation of a data-traffic characteristic, such as a current type of service, and/or a required data speed for the current type of service.
Of course, the present invention is not limited to the above features and advantages. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.
Although the following techniques are generally described in the context of a Long Term Evolution (LTE) wireless system, as specified by the 3rd-Generation Partnership Project (3GPP), those skilled in the art will appreciate that these techniques may be readily adapted to other wireless standards and systems in which reference symbols are distributed across time and/or frequency resources. Thus, the description of various details of the present invention in the context of LTE should be viewed as illustrative, and not limiting.
Reference symbols are transmitted in the first and fifth OFDM symbols of each slot (one-half of a subframe)—thus, OFDM symbol numbers 0, 4, 7 and 11 in each subframe contain reference symbols, denoted with an “R” in
However, if no data is to be received by the terminal for that subframe, then there is a potential for portions of the receiver, such as one or more analog radio components, to be turned off, to save power, albeit for a short interval. This use of very short sleep intervals is hereinafter called “micro-sleep”. As suggested by the example of
The use of micro-sleep techniques is independent of the use of conventional Discontinuous Reception mode (DRX), and thus provides power-savings potential even in scenarios where DRX functionality (in the network) is not used, or in scenarios where DRX is not used very effectively. Indeed, those skilled in the art will appreciate that there are significant differences between the micro-sleep techniques described herein and conventional DRX techniques. One difference is the difference in timing: a conventional DRX cycle involves multiple frames or sub-frames, from as few as ten in some cases, to as many as 10,000 in extreme cases. The micro-sleep techniques described herein may be performed within a single subframe. Another difference is that conventional DRX require coordination between the transmitter and receiver nodes—this may be accomplished, for example, via shared assumptions about DRX cycle durations (e.g., as specified in a standards document or other specification), or via explicit signaling between the transmitter and receiver nodes, or via a combination of both. The micro-sleep techniques described herein, on the other hand, may be carried out by a receiver node autonomously.
The preceding discussion of the basic micro-sleep approach implicitly assumes that the demodulation and decoding of the CCH can be successfully carried out without the use of the reference symbols provided in OFDM symbols 4, 7, and 11 of the subframe of interest. In practical implementations of LTE, it has turned out that the ability to successfully decode CCH is often a critical point in an LTE system's planning. One reason for this is that the CCH does not use hybrid-automatic-repeat-request (HARQ) techniques, thus no retransmissions of the CCH are possible. As a result, the terminal needs to reliably decode the CCH in one pass, to avoid throughput loss. Good channel estimates are generally needed for reliable CCH decoding, thus reference/pilot symbols transmitted in the portion of the OFDM symbol that contains data (i.e., symbols 4, 7 and 11) in the current subframe, the previous subframe, or even both, might be needed to produce channel estimates that are sufficiently accurate for CCH decoding. This may be the case even if no data was transmitted to the terminal in the previous slot.
If micro-sleep is used, however, then only a subset of the total reference symbols, e.g., the reference symbols transmitted in the CCH OFDM symbols, are systematically available for CCH channel estimation. Depending on the prevailing system conditions, this can lead to bad CCH performance, i.e., unreliable demodulating of the CCH.
In the discussion that follows, several techniques for using micro-sleep without impairing receiver performance are presented. For example, by adaptively enabling the use of micro-sleep from subframe to subframe, based on the channel conditions, one or more traffic characteristics, or both, receiver performance may be maintained at a sufficiently high level. This is accomplished by using micro-sleep only when channel conditions and/or traffic characteristics permit. For instance, depending on the current channel conditions, the mobile station can determine whether the channel estimator needs reference symbols transmitted in the data portion of each LTE subframe for good CCH channel estimation. If not, then the mobile station can de-activate all or part of its receiver for the latter portion of each subframe that doesn't carry traffic data for the mobile station. Otherwise, the mobile station's receiver is kept active throughout each subframe (or at least through each subframe for which the CCH is demodulated) so that reference symbols in the traffic data portion of the subframe may be included in the channel estimation process.
In other words, the mobile station can determine, depending on the channel conditions, whether reliable CCH decoding is possible without the use of reference symbols transmitted in the data portion of the LTE subframes. If so, micro-sleep is enabled (for subframes in which no data is transmitted to the terminal. Otherwise, micro-sleep is disabled, so that the channel estimator has access to a larger set of reference symbols.
Channel condition information that may be used in determining whether micro-sleep should be enabled may include, for example, one or more of a current signal-to-interference ratio (SIR); delay spread, and Doppler spread. In some receivers, one or more traffic characteristics, such as a current type of data service, may be used to determine whether to enable micro-sleep or not. In various embodiments this traffic characteristic information may be used instead of, or in addition to, channel condition information.
For example, a mobile station according to some embodiments of the present invention may include control processing circuits configured to determine that channel conditions are sufficient to allow for micro-sleep by evaluating whether the current SIR is larger than a corresponding pre-determined threshold, or whether a current type of service has a low required data speed, such as for voice transmissions, or whether the delay spread is lower than a suitable threshold, e.g., when the channel is substantially flat in its frequency response, and/or whether the Doppler spread is larger than a threshold level for Doppler spread. Some embodiments may be configured to evaluate two or more of the above channel conditions or traffic characteristics, or similar conditions and characteristics.
In LTE systems, the modulated symbols are inserted into one or more resource blocks defining a rectangular area in the time-frequency domain, as shown in
In more detail, the communication device 200 of
The communication device 200 further comprises a baseband & control processing circuit 250, which, in the exemplary embodiment of
At least one element of RX radio circuit 240 may be de-activated (i.e., powered off), under the control of baseband & control processing circuit 250, via control/disable interface 290. (This interface may consist of a single digital input, in some embodiments, a single- or multi-wire serial interface, in others, or a parallel interface in still others. Once more, those skilled in the art of radio design and control will be familiar with the details of such interfaces; these details are not necessary to a complete understanding of the present invention and are thus not included herein.) Thus, the overall power consumption of the device 200 may be improved by selectively disabling all or part of RX radio circuit 240, via control/disable interface 290, when circumstances permit. In particular, one or more elements of RX radio circuit 240 may be selectively de-activated for a portion of an LTE subframe (or other transmission-time interval, for another wireless protocol), based on a prevailing channel condition, traffic condition, or both.
The process illustrated in
The radio circuit activation indicated at block 310 is for a “first portion” of a sub-frame, this first portion including control channel data and one or more first reference symbols. As discussed earlier, the control channel may include scheduling information indicating whether traffic data targeted to the mobile station is included in the current sub-frame. To demodulate and decode the control channel, an estimate of the current channel propagation conditions is needed—thus, the reference symbols in the first portion of the sub-frame are measured, as shown at block 320, and used to calculate a channel estimate. The channel estimate for a given resource block may be based, in some embodiments, on reference symbols from frequency-adjacent resource blocks and/or from time intervals immediately preceding the current sub-frame.
As shown at block 330, channel conditions, traffic characteristics, or both, are evaluated. As will be explained in further detail below, this evaluation will be used to determine whether or not micro-sleep should be utilized. Of course, a decision to use micro-sleep is unnecessary if traffic data for the mobile station is present in the current block. Accordingly, if data is scheduled (e.g., as determined at block 340 of
As shown at block 410, the evaluation of a channel condition may comprise the estimate (calculation) of a channel condition based on the reference symbols from the first portion of the sub-frame. (In some embodiments, this evaluation might occur before the sub-frame begins, and may thus be based on previous reference symbols. In others, the reference symbols from the first part of the sub-frame may be combined with prior symbols, e.g., using a filtering function, to estimate the channel condition.) The estimated channel condition might be, for instance, an estimate of the signal-to-interference ratio for the received signal. Techniques for estimating channel coefficients, signal-to-noise ratios, and other channel metrics, by comparing received reference symbols to known values for those symbols are well known to those skilled in the art and are not detailed here.
As shown at block 420, the estimated channel condition is compared to a pre-determined threshold (which may be, for example, a factory-configured parameter stored in the wireless device's memory). The particular threshold level used in a given embodiment will depend, of course, on the particular channel condition being evaluated. In the illustrated process, if the estimated channel condition is greater than the corresponding pre-determined threshold, then the receiver circuit is de-activated for a second portion of the sub-frame, as shown at block 430. Otherwise, the receiver circuit is left on, and reference symbols provided in the remainder of the sub-frame are measured and used for calculating channel estimates, as shown at block 440. Under these circumstances, the use of additional reference symbols in calculating the channel estimates will generally provide for improved estimates.
Those skilled in the art will appreciate that the “greater-than-threshold” evaluation shown in block 420 is appropriate for some channel conditions, but not for others. For instance, if the estimated signal-to-interference ratio (SIR) for the received signal is sufficiently high, then fewer reference symbols are needed to estimate the channel coefficients to an accuracy sufficient for reliably decoding the control channel. Thus, if the SIR is higher than a pre-determined threshold, micro-sleep can be enabled, and the reference signals in the remaining portion of the sub-frame ignored. Other channel conditions might also be subjected to a “greater-than-threshold” evaluation, alone or along with SIR. For instance, a Doppler spread for the received signal may be estimated. In some cases, the Doppler spread may be so high that additional reference symbols are unlikely to improve the accuracy of the channel coefficients symbols, since the channel is too rapidly changing. In such a case, micro-sleep may be enabled (and the receiver circuit disabled for part of the sub-frame) if the Doppler spread exceeds a pre-determined threshold.
Other channel conditions might be evaluated using a “less-than-threshold” evaluation. For instance, propagation channels having a relatively low delay spread, i.e., having a relatively “flat” fading channel response, are more likely to be accurately characterized using relatively few reference symbols. Thus, an estimated delay spread can be compared to a pre-determined threshold, in some embodiments of the present invention—if the delay spread is less than the pre-determined threshold, then micro-sleep is enabled and the receiver circuit de-activated for a portion of the sub-frame.
Those skilled in the art will appreciate that traffic characteristics may also be used to determine when the selective de-activation of the receiver circuits is appropriate. For example, some types of service, such as voice-over-IP, may involve relatively low data rates and require relatively low signal-to-interference ratios (e.g., 3-4 dB) for successful reception of the encoded data. Furthermore, in some cases, the CCH may be more robustly encoded, so that reliably decoding the CCH is not an issue. In such a scenario, channel estimates might be regularly calculated based only on the reference symbols appearing in OFDM symbol 0 of the current subframe, for example, or based only on OFDM symbol 0 of the previous subframe. On the other hand, other traffic types may require much more accurate channel estimates, and thus require use of all the reference symbols from the preceding subframe, regardless of whether traffic data for the mobile was included in that previous subframe. Thus, in some embodiments of the invention, an evaluation of the traffic type may be used to determine whether micro-sleep should be enabled. For traffic types that require only minimal accuracy of channel estimates, reference symbols from the traffic portion of the preceding subframe may be safely ignored, and the receiver disabled (if no traffic data for the mobile is present). Otherwise, the receiver must remain enabled so that more reference symbols can be incorporated into the channel estimation process.
Those skilled in the art will appreciate that a good trade-off between CCH performance and power saving in an LTE mobile station may be achieved using variations of the techniques disclosed herein. Those skilled in the art will also appreciate that the inventive techniques disclosed herein are not limited to application in LTE mobile stations, but may be also applied to other devices and/or other wireless systems in which control channels and reference symbols are defined in similar ways. Finally, those skilled in the art will appreciate that the inventive techniques disclosed herein may be implemented by modifying conventional receiver circuits and receiver processing circuits, and that several embodiments may comprise one or more microprocessors, microcontrollers, or the like, configured with appropriate stored program instructions for carrying out the techniques discussed above.
Those skilled in the art will appreciate that terms such as “first”, “second”, and the like, as used herein, are generally used merely to distinguish between various elements, regions, sections, etc., and not necessarily to indicate a particular order or priority. As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. Like terms refer to like elements throughout the description.
With the above range of variations and applications in mind, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims, and their legal equivalents.
This application claims priority under 35 U.S.C. §119(e) to provisional application Ser. No. 61/296,977, filed 21 Jan. 2010 and titled “Methods for Micro Sleep in LTE.” The entire contents of this related provisional application are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61296977 | Jan 2010 | US |
