Downlink PDSCH transmission mode selection and switching algorithm for LTE

A method, system, and base station for deterministically selecting a downlink transmission mode and rank in a Long Term Evolution (LTE) configured wireless communication system. The Transmission Mode Selection (TMS) utility receives from a wireless device a request for downlink physical downlink shared channel (PDSCH) service using a particular transmission mode and specific rank. The TMS utility determines an estimate of the throughput corresponding to the request. In addition, the TMS utility obtains throughput estimates of other distinct pairings of transmission mode and rank to compare with the throughput estimate corresponding to the request. The TMS utility utilizes device feedback information and HARQ error information that are already existing/available in order to determine the best transmission mode and rank pair, according to the best Error-Adjusted-Throughput.

BACKGROUND

1. Technical Field

The present invention generally relates to wireless communication systems and in particular to transmission mode selection in wireless communication systems.

2. Description of the Related Art

Multiple-Input Multiple-Output (MIMO) systems are a primary enabler of the high data rate sought to be achieved by Long Term Evolution (LTE), an emerging 4G wireless access technology. Closed-Loop-Spatial-Multiplexing and Open-Loop-Spatial-Multiplexing are the two primary MIMO Transmission Modes used in the LTE downlink. In order to achieve satisfactory throughput performance, the choice of the most suitable MIMO Transmission Mode should not only depend on the measured signal quality at the mobile but also on additional factors such as the channel correlation and mobile speed.

In wireless communication systems, a base station selects a particular transmission mode and rank based on several factors, including the precoder and the Channel Quality Indicator (CQI) information reported by the mobile. Some of the other factors which are not reported by the mobile device and upon which the transmission mode and rank selection depends include (a) the relative speed between the mobile and the base station (Doppler) and (b) the channel correlation (multipath) between the base station and mobile antennas. Conventional approaches utilize complicated algorithms for heuristic estimation of Doppler and Multi-path to adjust the downlink transmission modes. Two particular approaches for solving the transmission mode and rank selection problem are the following: (a) at the mobile: CQI/PMI/RI (precoder matrix index/rank indicator) reporting is modified to be based not only on the measured Carrier to Interference (C/I) (ratio) but also on the observed channel correlation and Doppler; and (b) at the eNodeB: the correlation and Doppler are estimated at the eNodeB and the estimated information is used together with the reported CQI/PMI/RI to select a suitable transmission mode and rank. However, these solutions have the following problems: (1) the LTE standards do not impose any requirement on the mobile device to estimate the channel correlation and Doppler, so the mobile device based solution described in (a) above is not workable; (2) Estimation of correlation and speed require (a) fine measurements of the channel between the mobile and the eNodeB and (b) a significant level of computational and logical complexity; and (3) The most significant obstacle is the fact that there is no deterministic way to map the correlation and speed to the performance of specific transmission modes and rank. Additionally, the performance of transmission modes and rank also depends on the C/I operating point. Consequently, any such heuristic map would be very sensitive to at least three (3) input parameters, Doppler, channel correlation (multipath) and measured C/I. There can be other additional mobile device and eNodeB specific conditions that could influence the performance of the downlink Transmission Modes and Rank. Consequently, a scheme at the eNodeB (or the mobile device) to determine all the major causes affecting performance of Transmission Modes and selecting the most suitable Transmission Mode (TxMode) and Rank based on the estimated value for each of the major causes, would be complex and a heuristic guess at best.

Through empirical data, simulations and analysis, it has been observed that if an eNodeB automatically follows/fulfills the request by a mobile/wireless device and provides downlink communication transmission to the wireless device via the requested transmission mode and rank, then the achieved throughput would be suboptimal. There can even be regions of throughput inversion, in which regions the measured throughput reduces with an increase in C/I. Thus, suboptimal throughput is likely achieved if the eNodeB blindly follows wireless devices' inputs/indications for the best Transmission Mode and Rank pairing.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

The illustrative embodiments provide a method, system, and base station for deterministically selecting a downlink transmission mode in a Long Term Evolution (LTE) configured wireless communication system. The Transmission Mode Selection (TMS) utility receives from a wireless device a request for downlink physical shared channel (PDSCH) service using a particular transmission mode and specific rank. The TMS utility determines an estimate of the throughput corresponding to the request. In addition, the TMS utility obtains estimates of Hybrid Automatic Repeat Request (HARQ) error adjusted throughput corresponding to other distinct pairings of transmission mode and rank to compare with the throughput estimate corresponding to the request. The TMS utility utilizes device feedback information and HARQ error information that are already existing/available in order to determine the best transmission mode and rank pair, according to the best Error-Adjusted-Throughput.

Within the descriptions of the figures, similar elements are provided similar names and reference numerals as those of the previous figure(s). Where a later figure utilizes the element in a different context or with different functionality, the element is provided a different leading numeral representative of the figure number (e.g., 1xx forFIG. 1and 2xx forFIG. 2). The specific numerals assigned to the elements are provided solely to aid in the description and not meant to imply any limitations (structural or functional) on the invention.

With reference now toFIG. 1, a Long Term Evolution (LTE) configured wireless communication system is illustrated, according to one embodiment. In the described embodiments, wireless communication system100may support several standards/networks, such as third generation (3G) and fourth generation (4G) networks, which include Long Term Evolution (LTE) standard.

Wireless communication system100comprises a number (“L”) of wireless communication devices, for example, first and second wireless devices, mobile station (MS)/user equipment (UE)102and MS/UE104. One or more of the wireless communication devices may be associated with a mobile user/subscriber. Thus, in certain instances, a wireless communication device may be interchangeably referred to herein as a user device, user equipment (UE), mobile user device, mobile station (MS), subscriber or user, as a general reference to the association of the device(s) to a device user. These references are however not intended to be limiting on the applicability of the invention to devices not directly associated with individual/human users. Within the description which follows, the example wireless device is primarily referred to as MS/UE102or simply UE102.

Wireless communication system100comprises at least one base-station (BS)/Evolved Universal Terrestrial Radio Access Network Node B (eNodeB/eNB)106, illustrated with its various functional components in block diagram representation. In one embodiment, BS/eNodeB106may represent a base transceiver station (BTS), an enhanced/evolved node B (enodeB), or a base station. BS/enodeB106comprises controller (Cntl)108. Controller108comprises memory114, digital signal processor (DSP)/Processor110and RF transceiver112. Although the logic of RF transceiver112is illustrated within controller108, it should be appreciated that with other implementations, RF transceiver112may likely be positioned external to controller108. Also, while shown as a single module/device, DSP/Processor110may be one or more individual components communicatively coupled to each other, and controlling different functions within Cntl108. The wireless communication devices all connect to BS/eNodeB106via an antenna array comprising one or more antenna elements, of which BS antenna116is illustrated.

Wireless communication system100is further illustrated with second BS/eNodeB150. BS/eNodeB106and second BS/eNodeB150are interconnected by means of an X2 interface (e.g., X2152). Furthermore, BS/eNodeB106and second BS/eNodeB150are connected by means of an S1 interface (e.g., S1154) to an evolved packet core (EPC), more specifically, to mobility management entity (MME)/serving gateway (S-GW)160.

In addition to the above described hardware components of BS/eNodeB106within wireless communication system100, various features of the invention may be completed/supported via software (or firmware) code or logic stored within memory114(or other storage) and executed by DSP/Processor110. Thus, for example, illustrated within memory114are a number of software/firmware/logic components/modules, including operating system (OS)140which includes Transmission Control Protocol (TCP)/Internet Protocol (IP) module142and Media Access Control (MAC) protocol module144. Memory114also includes scheduler124, precoder126, CQI reports/values128, error adjusted throughput values127and HARQ feedback information/histories134. BS/eNodeB106schedules via scheduler124downlink communication of data between BS106and wireless device102. Also included within memory114are counter136, configurable adjustment constant125, rank indicator (RI) reports132and precoding matrix indicator (PMI) reports130. In addition, memory114comprises transmission mode selection (TMS) logic/utility120. In actual implementation, TMS logic120may be combined with scheduler124as well as with one or more of the other components/modules to provide a single executable component, collectively providing the various functions of each individual component when the corresponding combined utility is executed by processing components of BS/eNodeB106. In the descriptions which follow, TMS utility120is illustrated and described as a stand-alone or separate software/firmware component, which provides specific functions, as described below. In the described embodiment, TMS utility120provides certain functions that are executed by components within communication architecture/system100, specifically MS/UE102and/or BS/eNodeB106. Additional detail of the functionality associated with TMS logic/utility120is presented below with reference toFIG. 2and subsequent figures.

During wireless communication, MS/UE102and MS/UE104wirelessly communicates with BS/eNodeB106via the antenna array. As one wireless device, e.g., MS/UE102, receives data from another electronic device (e.g., MS/UE104), BS/eNodeB106schedules, via scheduler124, downlink communication of data between BS/ENodeB106and MS/UE102. Furthermore, scheduler124schedules both the uplink and downlink TCP data transmission pertaining to wireless devices, MS/UE102and MS/UE104.

With reference now toFIG. 2, there is depicted a block diagram representation of a communication architecture for communication between user equipment (UE) and Evolved Universal Terrestrial Radio Access Network Node B (eNodeB). As illustrated, communication architecture100comprises UE102and eNodeB106. UE102comprises packet data convergence protocol (PDCP) A223, radio link control (RLC) A224, medium access control (MAC) A225, and physical layer (PHY) A226. In communication with UE102is eNodeB106. Included within eNodeB106are: transmission mode selection (TMS) logic/utility120, packet data convergence protocol (PDCP) B233, radio link control (RLC) B234, medium access control (MAC) B235, and physical layer (PHY) B236. Additionally, communication system/architecture100includes network260and server280.

In one embodiment, eNodeB106transmits and receives data and/or information from UE102. PDCP A223, RLC A224, MAC A225, and PHY A226of UE102enable UE102to communicate (e.g., configuration data) via respective connections/layers with eNodeB106, which similarly comprises PDCP B233, RLC B234, MAC B235, and PHY B236. Network260also receives and transmits data to and from UE102and eNodeB106. Network260may send and receive data/information from one or more components (not shown) via server280. Server280represents a software deploying server and communicates with eNodeB106via network260. TMS utility120may be deployed from/on the network, via server280. With this configuration, the software deploying server performs one or more functions associated with the execution of TMS utility120.

Those of ordinary skill in the art will appreciate that the hardware components and basic configuration depicted inFIG. 1andFIG. 2may vary. The illustrative components within wireless communication system/architecture100and BS/eNodeB106are not intended to be exhaustive, but rather are representative to highlight essential components that are utilized to implement the present invention. For example, other devices/components may be used in addition to or in place of the hardware depicted. The depicted example is not meant to imply architectural or other limitations with respect to the presently described embodiments and/or the general invention.

Certain of the functions supported and/or provided by TMS utility/logic120are implemented via processing logic (or code) executed by processor110and/or other device hardware. Among the software code/instructions/logic provided by TMS utility120, and which are specific to the described embodiment, are: (a) logic for receiving from a wireless device indications of a best particular transmission mode and rank for downlink transmission service, based on one or more reports including channel quality indicator (CQI) reports, PMI reports and RI reports; (b) logic for using the HARQ error history at the enodeB to calculate a first error-adjusted-throughput corresponding to the transmission mode and rank (e.g. rank2) indicated by the wireless device; (c) logic for providing estimates of HARQ error adjusted throughput corresponding to other distinct pairings of transmission mode and rank; and (d) logic for selecting the transmission mode and rank with the best error adjusted throughput based on a comparison between the first error-adjusted-throughput and the error-adjusted-throughput corresponding to other distinct pairings of transmission mode and rank. According to the illustrative embodiment, when processor110executes TMS utility120, eNodeB106initiates a series of functional processes that enable the above functional features as well as additional features/functionality. These features/functionalities are described in further detail below within the (continuing description ofFIGS. 1-2and) description ofFIGS. 3-5.

In LTE, the downlink transmission modes that provide the highest data rates are (a) Closed Loop Spatial Multiplexing (CLSM) (i.e., Mode4) that is also referred to as Closed Loop Multiple Input Multiple Output (CL-MIMO) and (b) Open Loop Spatial Multiplexing (OLSM) (i.e., Mode3) that is also referred to as Open Loop MIMO. Both transmission modes consist of sending either one or two concurrent data streams from eNodeB/BS106to the mobile/MS102.

Multiple Input Multiple Output (MIMO) transmission modes form an essential part of LTE by enabling the LTE configured system, for example, wireless communication system100to achieve ambitious requirements for throughput and spectral efficiency. MIMO refers to the use of multiple antennas at the transmitter and receiver side. For the LTE downlink, a 2×2 configuration for MIMO is assumed as the initial baseline configuration, i.e. two transmit antennas at the base station and two receive antennas at the terminal side. In addition, wireless communication system100may be configured with four transmit or receive antennas, and the particular antenna arrangement is reflected in LTE specifications. Different gains are achieved, according to the MIMO transmission mode that is used.

Spatial multiplexing enables transmission of different streams of data simultaneously on the same resource block(s) by exploiting the spatial dimension of the radio channel. These data streams can belong to one single user (single user MIMO/SU-MIMO) or to different users (multi user MIMO/MU-MIMO). While SU-MIMO increases the data rate of one user, MU-MIMO provides an increase in overall capacity. Spatial multiplexing is possible if allowed by the mobile radio channel.

In wireless communication system (WCS)100/BS106, transmission mode selection (TMS) utility120receives one or more feedback reports from a wireless device (e.g., MS102) indicating the wireless device's “choice” for physical downlink shared channel (PDSCH) service via a particular transmission mode (TxMode) and a specific rank (i.e., via a particular pairing of transmission mode and rank). For example, given that the current transmission mode is 4 (i.e., Closed Loop Spatial Multiplexing (CLSM)), the mobile device may report the best Rank2-precoder (i.e., the wireless/mobile device requests Rank2) and the Channel Quality Indicators (CQIs) corresponding to the 2 layers of the Rank2precoder. The LTE system allows the wireless device to provide feedback (reports) consisting of channel quality indicator (CQI) reports, Precoder Matrix Index (PMI) reports and Rank Indicator (RI) reports. In CL-MIMO, the mobile device reports (a) the best Rank (i.e., either 1 or 2) via RI, (b) the best precoder at that rank via the PMI, and (c) the estimated max supportable data-rate (i.e., based on the CQI) given that rank and precoder to the eNodeB. The eNodeB may then schedule data for the mobile on the physical downlink shared channel (PDSCH) using the reported channel information on the requested Rank, Precoder and CQIs.

TMS utility120/enodeB106receives an indication of a choice of the best rank via rank indicator132as well as a corresponding precoder (i.e. a candidate precoder) and signal-to-noise+interference power ratio (C/I) in Precoding-Matrix-Indicator (PMI)130and Channel Quality Indicator (CQI)128respectively. TMS utility120may select an Open Loop Spatial Multiplexing (OLSM) transmission mode when channel conditions do not enable a particular CLSM transmission mode and rank pairing to achieve effective optimal/effective performance. OLSM consists of sending one (1) or two (2) concurrent data streams to the mobile. OLSM is similar to the closed loop transmission mode, but unlike CLSM, OLSM has no PMI feedback. OLSM feedback is limited to a CQI and a Rank-Indicator (RI). In LTE, Rank1OLSM (i.e., a Rank1OLSM pairing) implements Transmit Diversity via Space-Time-Block-Codes or Space-Frequency-Block-Codes. Rank2OLSM (i.e., a Rank2OLSM pairing) is implemented as Large-Delay-Cyclic-Delay-Diversity (LD-CDD). However, since LD-CDD does not involve a PMI feedback, LD-CDD is more resilient in high speed mobile environments than is Rank2CLSM. According to LTE, Mode3consists of Rank1and Rank2OLSM. Mode4consists of Rank1and Rank2CLSM as well as Transmit Diversity (Rank1OLSM). Switching the mobile from Rank2CLSM to Transmit Diversity does not require any radio resource control (RRC) involvement that may cause procedural overheads, since per the LTE standard, Transmit Diversity is part of both Transmission Mode4and Mode3.

In wireless communication system100, TMS utility120utilizes device feedback information and HARQ error information that are already existing/available in order to determine the best transmission mode and rank pair. In particular, TMS utility120utilizes feedback reports received from the wireless device as well as HARQ error history to determine an estimate of the throughput corresponding to the first pairing of transmission mode (e.g., CLSM) and rank (e.g., rank2) requested/indicated by the wireless device (e.g., MS102). In addition, TMS utility120obtains estimates of HARQ error adjusted throughput corresponding to other distinct pairings of transmission mode and rank to compare with the throughput estimate corresponding to the request. TMS utility120compares HARQ error adjusted throughputs corresponding to the first pairing and the other distinct pairings, respectively, in order to schedule transmission service based on the highest HARQ Error-Adjusted-Throughput. Furthermore, even after selecting a particular transmission mode and rank, TMS utility120periodically evaluates the performance of other transmission modes and rank, and switches to the (Transmission Mode, Rank) pair that TMS utility120estimates is able to support the highest data rate to the mobile.

FIG. 3is a block diagram illustrating LTE downlink baseband signal generation at the BS/eNodeB, including the functional components/processes relevant for MIMO transmission (i.e., layer mapping and precoding), according to one embodiment. ENodeB downlink system300comprises several downlink signal generation blocks of which layer mapper304, precoder block126and resource element mapper block(s)308are illustrated.

In LTE spatial multiplexing, up to two code words can be mapped onto different spatial layers via layer mapper304. One code word represents an output from the channel coder (not explicitly shown). The number of spatial layers available for transmission is equal to the rank of a corresponding channel matrix. TMS utility120utilizes precoding on the transmitter side to support spatial multiplexing. In particular, TMS utility120/BS/enodeB106multiplies the signal with a specific precoding matrix at precoding block126before transmission. TMS utility120selects the optimum precoding matrix (i.e., the matrix which provides maximum capacity) from a predefined “codebook” which is known at BS/eNodeB106and at MS102.

FIG. 4is a flow chart which illustrates the process of selecting a transmission mode and rank by using available HARQ feedback error history, according to one embodiment. Although the method illustrated inFIG. 4may be described with reference to components shown inFIGS. 1-2, it should be understood that this is merely for convenience and alternative components and/or configurations thereof can be employed when implementing the method. Key portions of the method may be completed by TMS utility120executing within eNodeB106(FIGS. 1-2) and controlling specific operations of/on enodeB/BS106, and the method is thus described from the perspective of either/both TMS utility120and eNodeB106.

The process begins at initiator block402and proceeds to block404, at which, TMS utility120receives one or more feedback reports and, in particular, a CQI report, a PMI report and an RI report from a mobile/wireless (subscriber) device (e.g., UE/MS102). Based on receipt of these feedback reports, TMS utility120/eNodeB106receives indication of the mobile/wireless device's choice of transmission mode T and a particular rank “A” (e.g., A=2). TMS utility120initiates throughput estimation corresponding to the indicated Tx mode and particular rank, as shown at block406. At block408, TMS utility120utilizes a number (corresponding to the particular rank indicated by the mobile) of CQI reports to obtain the mobile's estimate of maximum data rate that is supportable via the downlink transmission. At block410, TMS utility120utility obtains an estimate of the maximum data rate by using HARQ error feedback to adjust the mobile's estimate. TMS utility120begins estimating throughput values for distinct pairings of transmission mode and rank including a pairing with a same transmission mode T with a rank “B” (e.g., B=1) that is different from mobile's choice of rank, as shown at block412. The best rank “B” precoder and the corresponding CQI values are not known to eNodeB106, since the mobile has reported a rank “A” precoder and corresponding CQI values.

In one embodiment, TMS utility120uses one or more CQI values from a pairing of a specific transmission mode and higher rank (i.e., the higher Rank pairing) to provide one or more adjusted CQI values for a (different) pairing of the (same) specific transmission mode and lower rank (i.e., the lower Rank pairing). Obtaining the adjusted CQI value(s) from one or more CQI values from the higher Rank pairing is feasible if the CQI value(s) from the higher Rank pairing is appropriately adjusted to compensate for interference from additional layers that are present for the higher Rank pairing but absent for the lower Rank pairing. If, for example, Rank A represents Rank2and Rank B represents Rank1, TMS utility120obtains an adjusted layer1CQI value (corresponding to a lower/Rank1pairing) by compensating for the interference from the second Layer of Rank2. A Rank1transmission has only one (1) layer and does not have interference from a second layer. As a result, Rank1CQI would be higher than the maximum Rank2CQI. Based on simulations that demonstrate that the Rank1CQI is on average about 5 dB higher than the Rank2CQI for both CLSM and OLSM, TMS utility120utilizes a preset/configurable value to compensate for interference. However it can be shown that the performance of the TMS utility does not depend on the specific value of the configurable constant, specifically if the preset value is different from the actual difference in the Rank1CQI and the maximum Rank2CQI.

Returning toFIG. 4, at block414, to obtain an adjusted layer “B” CQI, TMS utility120adds (a) the higher (one) of the unadjusted rank “A” CQIs and (b) the rank “B” HARQ error feedback stored at the eNodeB. At block416, to obtain an error adjusted layer “B” CQI (i.e., an error adjusted throughput) estimate, TMS utility120adds a configurable adjustment constant to the adjusted layer B CQI. TMS utility120obtains the configurable adjustment constant from an average of an amount by which a CQI value of rank “B” is larger than a maximum CQI value for rank “A”, according to experimentation and previous measurements. TMS utility120evaluates an estimate of the data rate corresponding to the error adjusted layer B CQI estimate, as shown at block418. At block420, TMS utility120compares the Tx mode T and rank B (pairing) estimate of data rate (as well as estimates corresponding to other distinct pairings) to the Tx mode T and rank A estimate of data rate.

TMS utility120determines whether the estimate of data rate corresponding to rank “A” is larger than the estimate of data rate of rank B, as shown at decision block422. If at decision block422TMS utility120determines that the estimate of data rate corresponding to rank A is larger than the estimate of data rate of TxMode T and rank B (and other estimates corresponding to the other distinct pairings), TMS utility120serves (i.e., transmits data to) the mobile by accepting/selecting the mobile's choice/indication of a pairing of Tx mode T and rank A, according to the request/report(s), as shown at block424. However, if at decision block422, TMS utility120determines that the estimate of data rate corresponding to rank A is smaller than the estimate of data rate of rank B, TMS utility120rejects the mobile's indication of Tx mode T and rank “A” and switches to service via the same Transmission mode (i.e., TxMode T) and rank B or via another/Transmission mode and rank (e.g., Transmission Mode3, Rank1or Rank2), as shown at block426. The process ends at block428.

In one embodiment, following a rejection of a particular transmission mode and/or rank, TMS utility120/eNodeB106periodically checks whether or not conditions have sufficiently improved to provide a re-evaluation/re-consideration of the particular transmission mode and/or rank that was previously rejected. Re-evaluation/re-consideration of the particular transmission mode and/or rank allows TMS utility120to avoid an overly conservative TxMode and Rank selection. The re-evaluation/re-consideration may include or may accompany an adjustment procedure that factors improvement in transmission conditions to modify HARQ error adjustments.

To enable potential feedback adjustments based on a re-evaluation, if TMS utility120rejects the transmission mode (i.e., TxMode T) and rank (i.e., Rank A) that the wireless device requests, TMS utility120initiates a counter136(FIG. 1) (i.e., set counter value to 1) (block506/FIG. 5) if the current rejection (i.e., of the transmission mode and rank) of the user's current request was accepted when last requested by the wireless device. In addition, TMS utility120may initiate the counter if the rejection of the request (a) occurs at the first time that the request is presented (in a particular time period) and/or (b) occurs for a request for which there is not yet any stored HARQ feedback history. If the same/identical request (e.g., for service via transmission mode T and Rank A pairing) by the wireless device is currently rejected and was also rejected when last requested, TMS utility120increments the counter by 1. Thus, the counter is incremented if a current rejection continues or forms a consecutive pattern of similar rejections, and the counter is reset to 0 or 1 if a previous rejection pattern is currently broken.

If the counter value reaches a pre-determined threshold value, TMS utility120modifies the HARQ adjustment values corresponding to the previously rejected TxMode T and Rank A (e.g., by dividing the HARQ adjustments by 2). In other words, TMS utility120determines whether the counter reaches a configurable threshold value (e.g., 100 successive TxMode T and Rank A override decisions). If the counter reaches the configurable threshold value, then TMS utility120/eNodeB106modifies the TxMode T and Rank “A” HARQ adjustments. For example, TMS utility120divides these HARQ adjustments by 2 (or by some other preconfigured integer), and uses these updated HARQ values to re-calculate the estimated data-rate for TxMode T and Rank “A”.

The HARQ modification for the given (TxMode, Rank) pair is needed for the following reason: If TMS utility120/eNodeB106has been continuously rejecting the mobile's choice of TxMode T and Rank A, then the HARQ adjustment values for this (TxMode T, Rank A) pair are not normally/generally updated at eNodeB106. The fact that the particular HARQ adjustment values are not generally adjusted during transmission service via a different/chosen pair results from the fact that eNodeB106serves data to the mobile in the different/chosen TxMode and Rank pair, and the HARQ adjustments (based on ACK/NAK from the mobile) corresponds to that chosen TxMode and Rank pair. As a result, the HARQ adjustment values of the particular (TxMode T, Rank A) pair that are/were reported by the mobile are “stuck at some earlier point in the history”. However, channel conditions may have changed at the mobile/wireless device to favor the selection of TxMode T and Rank A, but the historical HARQ error adjustments maintained at the eNodeB for this (TxMode T, Rank A) pair would still have a negative bias. TMS utility120provides a procedure for periodically reducing/modifying the HARQ adjustments for the overruled (TxMode T, Rank A) pair that allows TMS utility120/eNodeB106to reduce the negative historical HARQ bias when TMS utility120periodically checks the estimated datarate for this transmission mode and rank pair.

FIG. 5is a flow chart which illustrates the process of periodically evaluating transmission conditions and using a threshold value to provide a particular HARQ error adjustment, according to one embodiment. The process begins at initiator block502and proceeds to block504, at which, TMS utility120rejects the mobile's request for a particular Tx mode and rank “A” following comparison with Tx mode and rank B. TMS utility120initiates a counter to track the number of consecutive rejections of a particular pairing of Tx mode and rank A, as shown at block506. At block508, TMS utility120receives indication that the current channel conditions and other factors affecting data transmission are suitable for transmission service via an intermediate service pairing.

In one embodiment, the TMS utility120(eNodeB) performs the selection of the (TxMode, Rank) pair by testing the performance of each (TxMode, Rank) pair, using the 2-way cascading fallback, and ultimately selecting the pair that gives the best data-rate. With this methodology, each of the (TxMode, Rank) pairs are tested and the best pair is selected based on the measured error performance, rather than by using a heuristic map to select an “appropriate” TxMode and Rank from a given estimate of the speed and correlation. Thus, given a Transmission Mode T and Rank A reported by the mobile, TMS utility120ultimately performs one or more of the following: (a) the utility honors the mobile's request and selects Transmission Mode T and Rank A for serving data to the mobile; (b) the utility selects a pairing comprising the same Transmission Mode T but some other Rank B (that is different from A) for serving the mobile; (c) the utility selects a different Transmission Mode T1and same rank A; and (d) the utility selects a different Transmission Mode T1and different rank B. The selection is based on the eNodeB's estimate of the Transmission Mode and Rank pair that is likely to provide the highest data-rate to the mobile, given the eNodeB's estimate of the channel conditions and the mobile's HARQ history. The HARQ history is maintained for each Transmission Mode and Rank pair. In one embodiment, TMS utility120provides for a cascading fallback from the most/very aggressive and potentially highest data rate mode (e.g., Transmission Mode4, Rank2) to the most/very conservative and potentially lowest data rate mode (e.g., Transmission Mode3, Rank1which is also called Transmit Diversity) based on the measured performance of these modes.

In another embodiment, TMS utility120executes a switch from service via a first pairing to service via a second pairing in several steps. TMS utility120may first switch to an intermediate service via a third pairing. A change to the intermediate service may provide a more moderate/gradual change (in aggressiveness or conservativeness) than a change to the second service. Secondly, TMS utility120may switch to service via the second pairing if the intermediate service does not achieve satisfactory performance. Illustrating the preceding (description of the action that TMS utility120performs based on a set of transmission mode and rank pairing options) is the following example described via enumerated steps: (1) The mobile reports/requests Transmission Mode4and Rank2, but TMS utility120/eNodeB106determines that the performance at the mobile's requested/reported mode to be suboptimal based on the HARQ error history at this reported/requested mode; (2) As a result, TMS utility120/eNodeB106switches the mobile to another mode (e.g., Transmission Mode3, Rank2), that potentially provides the next highest data rate. This switch may solve a current performance issue if degraded performance at the previous mode is due to higher mobile speed; (3) If TMS utility120determines that performance remains suboptimal, then TMS utility120/eNodeB106is able to switch the mobile to another mode (e.g., Transmission Mode4, Rank1) that potentially provides the next highest data rate (i.e., lower and more conservative than the previous 2 modes). This mode selection may solve the problem if the performance issue at the previous modes was due to a high correlation (low rank) channel; and (4) If all these intermediate mode and rank switches fail, then TMS utility120/eNodeB106is able switch the mobile to Transmission Mode3and Rank1, which pairing of transmission mode and rank is known as Transmit Diversity. In one embodiment, Transmit Diversity is the most conservative transmission mode. In addition, Transmit Diversity is the most robust and has the best error performance.

Referring again to the flow chart ofFIG. 5, at block510, TMS utility120switches service for UE/MS102by selecting an intermediate pairing comprising a different/second transmission mode with either same rank A or some other rank. In one embodiment, when a preconfigured maximum value for a level of change in performance aggressiveness for the wireless device is less than a potential level of change (in the level of aggressiveness) from a previously selected pairing to the current best (calculated) pairing, TMS utility120schedules transmission service via an intermediate pairing, according to the preconfigured maximum value. In one embodiment, TMS utility120determines the throughput performance level via the second/different transmission mode and rank and TMS utility120switches from the second/intermediate mode to a third transmission mode and rank only if the performance level via the second transmission and rank is unsatisfactory/sub-optimal.

TMS utility120switches to a third pairing of Transmission Mode T and rank B (instead of the mobile's (repeated) request for Transmission Mode T and rank A) based on sub-optimal performance of the second pairing of Transmission mode T1and rank B, as shown at block512. At block514, TMS utility120increments the counter by one (1) to begin/continue tracking a number of consecutive rejections of the mobile's request (i.e., the TxMode T and Rank A pair). TMS utility120determines that conditions have improved to merit re-evaluation of other/specific transmission modes and ranks based on information (e.g., a trend of improvement in CQI reports or other feedback) received that indicates an improvement in channel conditions, as shown at block516.

In one embodiment, TMS utility120triggers a period check (by switching to the TxMode and Rank pairing under evaluation) following receipt of indications of improved channel conditions. In another embodiment, TMS utility120initiates the periodic check by evaluating higher quality feedback and/or by evaluating other feedback that is adjusted to minimize information bias. In another embodiment, TMS utility120performs the switch to complete/perform a practical evaluation if the type and quality of information required for effective/efficient evaluation is unavailable and/or if the type of information required is provided only by a particular transmission mode and/or Rank. According to the periodic check, TMS utility120allows periodic switching back from a lower/conservative Transmission mode and rank to a higher/aggressive Transmission Mode and rank to determine if conditions have improved at the particular higher Transmission Mode and rank. As an illustration, TMS utility120/eNodeB106may have previously chosen to serve UE102on Transmission Mode3, Rank1. However, TMS utility120also periodically checks the performance of the other Transmission Modes and Ranks (e.g., [Transmission Mode3, Rank2], [Transmission Mode4, Rank1], [Transmission Mode4, Rank2]) that can potentially provide higher data rates. In order to perform the periodic check, TMS utility120may switch to any of these other Transmission Modes and Rank.

Returning toFIG. 5, at block518, TMS utility120initiates a comparison of the mobile's choice (i.e., TxMode T, Rank A pair) with other transmission mode and rank pairs including Tx mode and rank B pair, according to the periodic check. TMS utility120compares the counter value (that tracks consecutive rejections of the TxMode T and Rank A pair) with the threshold value, as shown at block520. At decision block522, TMS utility120determines whether the counter value is larger than the threshold value. If at decision block522TMS utility120determines that the counter value is not larger than the threshold value, TMS utility120obtains an estimate of the actual maximum data rate corresponding to TxMode T and rank A by using a (non-threshold based) HARQ adjustment, as shown at block524. However, if at decision block522TMS utility120determines that the counter value is larger than the threshold value, TMS utility120obtains the estimate of the actual maximum data rate by using a HARQ adjustment divided by 2 (or other integer/value), as shown at block526. TMS utility120re-initiates/continues the comparison of Tx mode and rank pairs, as shown at block528. The process ends at block530.

Simulation data indicates that incorporating the functionality of TMS utility120into the TxMode and Rank selection process of an eNodeB improves the downlink throughput by 110% in some cases and by 25-50% on an average compared to systems which do not employ TMS utility120. In addition, an eNodeB equipped with TMS utility120is able to match the theoretical “best case” throughput over a wide range of signal quality and channel models. Furthermore, TMS utility120shrinks the C/I range over which the measured throughput differs from the “ideal” value.

TMS utility120provides a solution that performs well even in cases where other more complex methods that select the best Transmission Mode and Rank based on explicit measurements of the mobile speed and channel correlation fail. For example, the actual delivered performance for downlink CL-MIMO may also depend on the radio frequency (RF) performance of the mobile. While all mobiles are expected to meet the minimum performance criteria, some mobiles may be better at handling high-spectrally-efficient schemes than others. In other words, for the same relative speed (Doppler) and channel correlation, the actual downlink performance may have mobile specific variations. Algorithms that explicitly consider only the Doppler and channel correlation factors may end up selecting downlink transmission modes for all mobiles in the same way. However, by providing a solution based on the Error-Adjusted-Throughput, TMS utility120identifies higher/lower errors in the selected transmission modes and selects a mode that best suits a mobile, given the HARQ error history. Thus, TMS utility120provides a low complexity low MIPS (i.e., millions of instructions per second) solution that reuses HARQ information that already exists at the eNodeB.

In the flow chart above, certain processes of the methods are combined, performed simultaneously or in a different order, or perhaps omitted, without deviating from the spirit and scope of the invention. Thus, while the method processes are described and illustrated in a particular sequence, use of a specific sequence of processes is not meant to imply any limitations on the invention. Changes may be made with regards to the sequence of processes without departing from the spirit or scope of the present invention. Use of a particular sequence is therefore, not to be taken in a limiting sense, and the scope of the present invention extends to the appended claims and equivalents thereof.

The illustrated and described embodiments provide, in an eNodeB, a mechanism and system for selecting a downlink transmission mode in a Long Term Evolution (LTE) configured wireless communication system. The Transmission Mode Selection (TMS) utility/logic receives from a wireless device a request for downlink physical downlink shared channel (PDSCH) service using a particular transmission mode and specific rank. The TMS utility determines an estimate of the throughput corresponding to the request. In addition, the TMS utility obtains estimates of HARQ error adjusted throughput corresponding to other distinct pairings of transmission mode and rank to compare with the throughput estimate corresponding to the request. The TMS utility utilizes device feedback information and HARQ error information that are already existing/available in order to determine the best transmission mode and rank pair, according to the best Error-Adjusted-Throughput. Furthermore, even after selecting a particular transmission mode and rank, TMS utility120periodically evaluates the performance of other transmission modes and rank, and switches to the (Transmission Mode, Rank) pair that TMS utility120estimates is able to support the highest data rate to the mobile.

As will be further appreciated, the processes in embodiments of the present invention may be implemented using any combination of software, firmware or hardware. As a preparatory step to practicing the invention in software, the programming code (whether software or firmware) will typically be stored in one or more machine readable storage mediums such as fixed (hard) drives, diskettes, optical disks, magnetic tape, semiconductor memories such as ROMs, PROMs, etc., thereby making an article of manufacture (or computer program product) in accordance with the invention. The article of manufacture containing the programming code is used by either executing the code directly from the storage device, by copying the code from the storage device into another storage device such as a hard disk, RAM, etc., or by transmitting the code for remote execution using transmission type media such as digital and analog communication links. The methods of the invention may be practiced by combining one or more machine-readable storage devices containing the code according to the present invention with appropriate processing hardware to execute the code contained therein. An apparatus for practicing the invention could be one or more processing devices and storage systems containing or having network access to program(s) coded in accordance with the invention.

Thus, it is important that while an illustrative embodiment of the present invention is described in the context of a fully functional base station/eNodeB with installed (or executed) software, those skilled in the art will appreciate that the software aspects of an illustrative embodiment of the present invention are capable of being distributed as a computer program product in a variety of forms, and that an illustrative embodiment of the present invention applies equally regardless of the particular type of media used to actually carry out the distribution. By way of example, a non exclusive list of types of media, includes recordable type (tangible) media such as floppy disks, thumb drives, hard disk drives, CD ROMs, DVDs, and transmission type media such as digital and analogue communication links.