Adaptive reference signal mapping in wireless multi-access communication networks

Embodiments provide systems and methods for adaptive reference signal mapping in wireless multi-access communication networks, including LTE, WLAN, WiMAX, Bluetooth, etc. In an embodiment, the reference signal mapping configuration is user equipment (UE) specific and can be configured semi-statically or dynamically according to one or more communication related parameters of the UE. The one or more parameters can include, without limitation, a modulation scheme used for communication with the UE, a modulation and coding scheme (MCS) used for communication with the UE, a distance of the UE relative to the base station, an antenna configuration at the UE, interference management capability of the UE, and a rank of the UE.

TECHNICAL FIELD

The present disclosure relates generally to reference signal mapping in wireless multi-access communication networks.

BACKGROUND

Background Art

Improving spectral efficiency continues to be an objective for future wireless multi-access communication network standards, such as Long Term Evolution (LTE), Wireless Local Area Network (WLAN), WiMAX, etc. One area where a potential for improvement in spectral efficiency exists is small cell environments, such as femtocells, for example. In a small cell, the path loss between a base station and a user equipment (UE) is smaller than in a large cell, and as a result the UE generally observes a higher Signal-to-Interference-and-Noise Ratio (SINR) than in the large cell.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of this discussion, the term “module” shall be understood to include at least one of software, firmware, and hardware (such as one or more circuits, microchips, processors, or devices, or any combination thereof), and any combination thereof. In addition, it will be understood that each module can include one, or more than one, component within an actual device, and each component that forms a part of the described module can function either cooperatively or independently of any other component forming a part of the module. Conversely, multiple modules described herein can represent a single component within an actual device. Further, components within a module can be in a single device or distributed among multiple devices in a wired or wireless manner.

Improving spectral efficiency continues to be an objective for future wireless multi-access communication network standards, such as Long Term Evolution (LTE), Wireless Local Area Network (WLAN), WiMAX, etc. One area where a potential for improvement in spectral efficiency exists is small cell environments, such as femtocells, for example. In a small cell, the path loss between a base station and a user equipment (UE) is smaller than in a large cell, and as a result the UE generally observes a higher Signal-to-Interference-and-Noise Ratio (SINR) than in the large cell.

The higher SINR allows for higher order modulation schemes, such as 256-QAM (256-Quadrature Amplitude Modulation), to be used for communication between the base station and the UE, which directly increases spectral efficiency. However, higher order modulation schemes require greater channel estimation accuracy and an increased reference signal overhead, which is directly at odds with the objective of improving spectral efficiency.

Existing approaches use a fixed reference signal mapping configuration for allocating resource elements (REs) to reference signals. As understood by a person of skill in the art, reference signals are known a priori at the UE and are used for channel estimation. For example,FIG. 1illustrates a reference signal mapping configuration100used in LTE for UE specific reference signals. The UE specific reference signals are known as Demodulation Reference Signals (DMRS) and occupy REs of the data bearing Physical Downlink Shared Channel (PDSCH). The grid shown inFIG. 1corresponds to a pair of consecutive (back to back in time) Physical Resource Blocks (PRBs) of the PDSCH. As shown, each PRB corresponds to 7 timeslots over 12 subcarriers. In an embodiment, a PRB pair corresponds to one Orthogonal Frequency Division Multiplexing (OFDM) sub-frame and is 1 milliseconds long.

For a UE with a rank equal to 1 or 2 (i.e., a UE being served up to two simultaneous data streams by the base station), LTE's current reference signal mapping configuration100assigns a total of 12 REs for DMRS pilots in a given PRB pair. These REs are illustrated using grey shadingFIG. 1. For higher UE ranks (e.g., 3 to 8), 24 REs are used for DMRS pilots in each PRB pair. However, LTE does not currently account for other UE communication related parameters in the mapping of REs to reference signals. For example, QPSK (Quadrature Phase Shift Keying) and 64-QAM have different channel estimation accuracy requirements. Yet, currently, the same fixed reference signal mapping configuration100is used for both modulation schemes, resulting in unnecessary reference signal overhead for QPSK and inadequate channel estimation performance for 64-QAM.

Embodiments, as further described below, provide systems and methods for adaptive reference signal mapping in wireless multi-access communication networks, including LTE, WLAN, WiMAX, Bluetooth, etc. In an embodiment, the reference signal mapping configuration is UE specific and can be configured semi-statically or dynamically according to one or more communication related parameters of the UE. The one or more parameters can include, without limitation, a modulation scheme used for communication with the UE, a modulation and coding scheme (MCS) used for communication with the UE, a distance of the UE relative to the base station, an antenna configuration at the UE, interference management capability of the UE, and a rank of the UE. In other embodiments, the one or more parameters can include any parameter, at the UE and/or the base station, that can affect directly or indirectly the spectral efficiency of communication with the UE.

FIG. 2illustrates an example base station200according to an embodiment. Example base station200is provided for the purpose of illustration only and is not limiting of embodiments. Example base station200can be used to perform embodiments of the present disclosure as further described below. As shown inFIG. 1, example base station200includes, without limitation, a processor202, a memory204, a symbol mapper210, a multi-carrier modulator212, a radio frequency (RF) transmitter214, and a plurality of antennas216.1,216.2, . . . ,216.n.

In an embodiment, memory204is configured to store reference signal mapping configurations206and user device configurations208. In another embodiment, memory204further stores logic instructions that when executed by processor202enable processor202to perform the functionality described herein.

Reference signal mapping configurations206include a plurality of reference signal mapping configurations according to embodiments. Each of the reference signal mapping configurations corresponds to a particular allocation of REs of the PDSCH to reference signals. In an embodiment, the allocation of REs of the PDSCH to reference signals is defined on a PRB pair basis, thereby determining the number and specific locations of REs of the PDSCH reserved for reference signals within the PRB pair. In other embodiments, the allocation can be defined on a smaller or a larger RE block than a PRB pair. In one embodiment, the reference signals are UE specific reference signals. In another embodiment, the reference signals are DMRS.

User device configurations208include communication related parameters associated with one or more UEs associated with base station200. The one or more UEs can correspond to UEs currently being served by base station200and/or to UEs previously having been served by base station200. In an embodiment, the user device configuration of a UE can include communication related parameters, such as, without limitation, a modulation scheme used for communication with the UE (in the downlink and/or uplink), an MCS used for communication with the UE (in the downlink and/or uplink), a distance of the UE relative to base station200(e.g., a rough estimate of the distance), an antenna configuration at the UE (e.g., number of receive antennas, degree of correlation of receive antennas, etc.), interference management capability of the UE (e.g., availability of advanced interference rejection or suppression at the UE), and a rank of the UE (number of simultaneous data streams being transmitted to the UE from base station200). Other communication related parameters can also be included in the user device configuration of the UE. For example, in an embodiment, any communication related parameter associated with the UE that can affect the spectral efficiency of communication with the UE can be included in the user device configuration of the UE stored in user device configurations208. As would be understood by a person of skill in the art, some of the communication related parameters can be fixed, while others can change over time. In embodiments, base station200tracks and updates any changing communication related parameters in user device configurations208.

Base station200is configured to determine and to dynamically adapt the reference signal mapping configuration used for a particular UE based on the communication related parameters associated with the UE. As mentioned above, the reference signal mapping configuration includes locations of REs for carrying reference signals within a PRB pair of the PDSCH allocated to the UE. In an embodiment, processor202is configured to determine one or more communication related parameters associated with the UE from user device configurations208stored in memory204. Based on the one or more communication related parameters, processor202identifies a first reference signal mapping configuration from among reference signal mapping configurations206stored in memory204.

For example, processor202may identify the first reference signal mapping configuration based on the modulation scheme used for communication with the UE. Alternatively or additionally, processor202may identify the first reference signal mapping configuration based on the distance of the UE from base station200. For example, in an embodiment, processor202assumes that the downlink channel from base station200to the UE is more stable, and therefore requires less reference signaling for channel estimation, when the UE is closer to base station200. Alternatively or additionally, processor202may identify the first reference signal mapping configuration based on the antenna configuration at the UE and/or the interference management capability of the UE, both of which can affect the required channel estimation accuracy at the UP and by consequence the amount of required reference signaling to the UE.

In another embodiment, base station200can further determine and dynamically adapt the reference signal mapping configuration for a particular UE based on parameters associated with base station200. In an embodiment, the parameters taken into account are parameters that can affect the spectral efficiency of communication with the UE. For example, in an embodiment, base station200can account for traffic load at base station200in determining the reference signal mapping configuration for the UE. For instance, if base station200is heavily loaded such that it is serving more than one UE using the same PRB pair of the PDSCH (e.g., using Multi-User Multiple Input Multiple Output (MU-MIMO)), then base station200may use a reference signal mapping configuration with more REs reserved to reference signals. This can result in a better channel estimation accuracy at the UE in view of anticipated increased interference. In another embodiment, base station200can determine the reference signal mapping configuration for the UE based on the number of transmit antennas at base station200used for communication to the UE. In an embodiment, as more transmit antennas are used, a higher SINR can be achieved at the UE and a reference signal mapping configuration with lower reference signaling overhead can be used.

After identifying the first reference signal mapping configuration, processor202embeds a plurality of reference signal bits within a user data bit stream for the UE in accordance with the first reference signal mapping configuration to result in a combined bit stream. In an embodiment, processor202embeds the plurality of reference signal bits at specific locations within the user data bit stream such that when the combined bit stream is loaded into the PRB pair the reference signal bits occupy those REs of the PRB pair indicated by the first reference signal mapping configuration. For example, with reference to the example configuration ofFIG. 1, the plurality of reference signal bits are embedded within the user data bit stream such that they occupy the grey shaded REs when the combined bit stream in loaded into the PRB pair. In an embodiment, the combined bit stream is loaded into the PRB pair subcarrier by subcarrier (e.g., starting with the lowest frequency subcarrier of the PRB pair).

Subsequently, processor202provides the combined bit stream to symbol mapper210. In an embodiment, processor202further indicates a modulation scheme to symbol mapper210by means of a control signal218. Symbol mapper210uses the modulation scheme indicated by control signal218to generate a plurality of symbols (e.g., constellation points) from the combined bit stream. The plurality of symbols are then provided to multi-carrier modulator212, which modulates the plurality of symbols onto respective subcarriers of the PRB pair to generate a multi-carrier subframe. In an embodiment, multi-carrier modulator212includes an Inverse Fast Fourier Transtorm (IFFT) module. In an embodiment, the plurality of symbols include a plurality of reference symbols corresponding to the plurality of reference signal bits. Multi-carrier modulator212modulates the reference symbols such that they occupy the REs of the PRB pair indicated by the first reference signal mapping configuration.

The output of multi-carrier modulator212is then provided to RF transmitter214. RF transmitter214can perform analog functions such as frequency up-conversion, power amplification, and filtering on the output of multi-carrier modulator212before transmitting the resulting signals via one or more of antennas216to the UE. As such, RF transmitter214transmits the multi-carrier subframe to the UE in the PRB pair allocated to the UE. The plurality of reference symbols are transmitted at the REs reserved for carrying reference signals within the PRB pair according to the first reference signal mapping configuration.

In another embodiment, processor202is configured to detect a change in the one or more communication related parameters associated with the UE and/or parameters associated with base station200and to identify a second reference signal mapping configuration from among reference signal mapping configurations206responsive to the detected change. For example, processor202can be configured to detect a change in a modulation order or the MCS of the UE, and to identify the second reference signal mapping configuration responsive to the change in the modulation order. For example, if the change corresponds to an increase in the modulation order or MCS of the UE (e.g., from QPSK to 64-QAM, from 16-QAM to 256 QAM, etc.), the second reference signal mapping configuration can be selected to include a larger number of REs for carrying reference signals than the first reference signal mapping configuration. This improves the channel estimation accuracy at the UE enabling better performance for higher order modulation schemes. Alternatively, the second reference signal mapping configuration can include a lower number of REs for carrying reference signals than the first reference signal mapping configuration when an increase in the modulation order or MCS is detected. Similarly, when a decrease in the modulation order or MCS is detected, processor202can identify a second reference signal mapping configuration with a lower or a larger number of REs for carrying reference signals than the first reference signal mapping configuration.

In another embodiment, processor202can be configured to detect a change in the distance of the UE relative to base station200, and to identify the second reference signal mapping configuration responsive to the distance change. For example, in an embodiment, processor202can detect that the distance between the UE and base station200has increased beyond a defined threshold. In response, processor202can select the second reference signal configuration to include a larger number of REs for carrying reference signals than the first reference signal mapping configuration.

As described above, in embodiments, base station200can be configured to determine and to dynamically adapt the reference signal mapping configuration used for a particular UE based on a variety of communication related parameters associated with the UE and/or parameters associated with base station200itself. Examples of such parameters are provided above for the purpose of illustration only and not limitation. As would be understood by a person of skill in the art, various other parameters can also be used.

In the following, example reference signal mapping configurations according to embodiments are provided. These example configurations are also provided for the purpose of illustration only and are not limiting of embodiments. In an embodiment, the reference signal mapping configurations can be grouped into various patterns, with each pattern being suited for a particular communication scenario with the UE. In another embodiment, each pattern includes multiple reference signal mapping configurations for various modulation schemes (and/or other communication related parameter), for example. The base station can dynamically adapt the reference signal mapping configuration for the UE by varying the configuration within a given pattern and/or across the various patterns. To signal a selected reference signal mapping configuration to the UE, in one embodiment, the base station signals an index corresponding to the pattern containing the selected reference signal mapping configuration. The UE can then determine the selected reference signal mapping configuration from the signaled pattern based on the used modulation scheme (and/or the other communication related parameter).

FIG. 3Aillustrates a first example pattern of reference signal mapping configurations according to an embodiment. As shown inFIG. 3A, the first example pattern includes two reference signal mapping configurations302aand302b. In other embodiments, the first example pattern can include more than two configurations. In an embodiment, configurations302aand302bare used for UEs with ranks less than or equal to 2. For higher ranks, configurations with additional reference signal REs can be employed. In an embodiment, configuration302ais used for QPSK and 16-QAM, and configuration302bis used for 64-QAM and 256-QAM.

It is noted that configuration302bis equivalent to LTE's current fixed reference signal mapping configuration, described with reference toFIG. 1above. In contrast, configuration302auses less REs for reference signals and thus results in a lower reference signal overhead for QPSK and 16-QAM compared to LTE's current approach. In another embodiment, to maintain backward compatibility with LTE's current approach, the first pattern can include configuration302bfor QPSK, 16-QAM, and 64-QAM and another configuration with more than 12 REs in the PRB pair (not shown inFIG. 3A) for 256-QAM.

FIG. 3Billustrates a second example pattern of reference signal mapping configurations according to an embodiment. As shown inFIG. 3B, the second example pattern includes two reference signal mapping configurations304aand304b. In an embodiment, configurations304aand304bare used for UEs with ranks less than or equal to 2. For higher ranks, configurations with additional reference signal REs can be employed. In an embodiment, configuration304ais used for QPSK and 16-QAM, and configuration304bis used for 64-QAM and 256-QAM.

Like configuration302b, configuration304bis also equivalent to LTE's current fixed reference signal mapping configuration. Configuration302auses less reference signal REs than configuration304aand thus results in a lower reference signal overhead for QPSK and 16-QAM. In fact, as shown inFIG. 3B, configuration304ais obtained by eliminating 4 reference signal REs from configuration304b. As such, configuration304acan be readily obtained by modifying LTE's current fixed reference signal mapping configuration.

FIG. 3Cillustrates a third example pattern of reference signal mapping configurations according to an embodiment. As shown inFIG. 3C, the third example pattern includes two reference signal mapping configurations306aand306b. In an embodiment, configurations306aand306bare used for UEs with ranks less than or equal to 2. For higher ranks, configurations with additional reference signal REs can be employed. In an embodiment, configuration306ais used for QPSK and 16-QAM, and configuration306bis used for 64-QAM and 256-QAM.

Configuration306bis identical to configurations302band304bdescribed above. Configuration306ais obtained by eliminating 4 reference signal REs from configuration306b. As such, like configuration304a, configuration306acan be readily obtained by modifying LTE's current fixed reference signal mapping configuration. In contrast, reference signal REs in configuration306aare less spread apart in frequency than the reference signal REs of configuration304a. As such, configuration304acan be more suitable than configuration306afor changing channel conditions, which can be due to movement of the UE, for example.

FIG. 3Dillustrates a fourth example pattern of reference signal mapping configurations according to an embodiment. As shown inFIG. 3D, the fourth example pattern includes two reference signal mapping configurations308aand308b. In an embodiment, configurations308aand308bare used for UEs with ranks less than or equal to 2. For higher ranks, configurations with additional reference signal REs can be employed. In an embodiment, configuration308ais used for QPSK and 16-QAM, and configuration308bis used for 64-QAM and 256-QAM.

Configuration308bis identical to configurations302b,304b, and306bdescribed above. Configuration308ais obtained by eliminating 6 reference signals REs from configuration308b. As such, like configuration304a, configuration308acan be readily obtained by modifying LTE's current fixed reference signal mapping configuration. However, configuration308auses less reference signal REs than configuration304a, and as such has lower reference signal overhead than configuration304a. In an embodiment, to augment the number of reference signal measurements obtained using configuration308a, the reference signal measurements in each PRB are reused for the subsequent PRB. For example, inFIG. 3D, the REs shown with hatching correspond to REs for which measurements are reused from equivalent REs (same timeslot/frequency subcarrier) of the previous PRB. As such, within any given PRB pair, measurements corresponding to 12 REs can be generated.

FIG. 4illustrates an example process400according to an embodiment. Example process400is provided for the purpose of illustration only and is not limiting of embodiments. Example process400can be performed by a base station, such as example base station200, to determine and use a reference signal mapping configuration in communication with a UE. In another embodiment, some of the steps of process400can be performed by the UE.

As shown inFIG. 4, process400begins in step402, which includes determining one or more communication related parameters associated with the UE. In an embodiment, the one or more parameters can include, without limitation, a modulation scheme used for communication with the UE, a modulation and coding scheme used for communication with the UE, a distance of the UE relative to the base station, an antenna configuration at the UE, interference management capability of the UE, and a rank of the UE. In other embodiments, the one or more parameters can include any parameter, at the UE and/or the base station, that can affect directly or indirectly the spectral efficiency of communication with the UE. In an embodiment, step402further includes retrieving the one or more communication related parameters from a memory of the base station. In another embodiment, step402can be performed by the UE itself, and the determined parameters are sent to the base station.

Subsequently, step404includes identifying a reference signal mapping configuration from among a plurality of reference signal mapping configurations based on the determined one or more communication related parameters. As mentioned above, the reference signal mapping configuration includes locations of REs for carrying reference signals within a PRB pair of the PDSCH allocated to the UE. In an embodiment, the plurality of reference signal mapping configurations are pre-defined and shared between the base station and the UE. In another embodiment, the reference signal mapping configuration can be identified by the UE and then signaled to the base station.

Process400terminates in step406, which includes embedding a plurality of reference signal bits within a user data bit stream for the UE in accordance with the identified reference signal mapping configuration. In an embodiment, step406includes embedding the plurality of reference signal bits at specific locations within the user data bit stream such that when a combined bit stream is loaded into the PRB pair the reference signal bits occupy those REs of the PRB pair indicated by the identified reference signal mapping configuration.

FIG. 5illustrates another example process500according to an embodiment. Example process500is provided for the purpose of illustration only and is not limiting of embodiments. Example process500can be performed by a base station, such as example base station200, to signal a reference signal mapping configuration to a UE.

As shown inFIG. 5, process500begins in step502, which includes configuring the UE with a plurality of patterns of reference signal mapping configurations. In an embodiment, each pattern includes multiple reference signal mapping configurations for various conditions of one or more particular communication related parameters. For example, each pattern may include multiple reference signal mapping configurations for various modulation schemes, as described above with respect to the example patterns illustrated inFIGS. 3A-3D. The UE stores the plurality of patterns received from the base station in a memory of the UE.

Subsequently, step504includes identifying a reference signal mapping configuration from a plurality of reference signal mapping configurations for use in communication to the UE. In an embodiment, step504further includes identifying a pattern from the plurality of patterns based one or more communication related parameters associated with the UE.

Process500terminates in step506, which includes signaling an index corresponding to a pattern of the plurality of patterns to the UE, where the signaled pattern contains the reference signal mapping configuration identified in step504. In an embodiment, log2(M) bits are used to signal the index, where M corresponds to the total number of possible patterns.

The UE uses the index to identify the pattern and then identifies the reference signal mapping configuration being used by the base station based on the one or more communication related parameters associated with the UE. For example, the UE may use its knowledge of the modulation scheme to determine the reference signal mapping configuration being used from the multiple configurations contained in the pattern.

In an embodiment, the base station can use process500to implement a semi-static mode and a dynamic mode. In the semi-static mode, the reference signal mapping is configured by higher layer signaling and can be typically valid for hundreds of milliseconds. The semi-static mode is suitable when the communication related parameters based on which the configuration is determined are relatively stable. For example, the UE may not be moving, or the configuration may be determined based on fixed parameters of the UE (e.g., antenna configuration, interference management capability, etc.). In the dynamic mode, the reference signal mapping configuration is determined and signaled at a higher rate (e.g., every1millisecond) to the UE. In an embodiment, the reference signal mapping configuration is indicated in the Downlink Control Information (DCI) of the Physical Downlink Control Channel (PDCCH) to the UE.

The breadth and scope of embodiments of the present disclosure should not be limited by any of the above-described exemplary embodiments as other embodiments will be apparent to a person of skill in the art based on the teachings herein.