WO2014029041A1

WO2014029041A1 - Method and access point to enhance downlink transmission through uplink scheduling

Info

Publication number: WO2014029041A1
Application number: PCT/CN2012/001114
Authority: WO
Inventors: Zhan Zhang; Huaisong Zhu
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2012-08-20
Filing date: 2012-08-20
Publication date: 2014-02-27

Abstract

The embodiments disclose a method and an Access Point (AP) to enhance downlink transmission through uplink scheduling taking the downlink transmission into account in a Time Division Duplex cellular network. The method obtains a downlink scheduling for transmissions from an AP to a User Equipment (UE) that is served by the AP, and schedules an uplink transmission from the UE to the AP at frequency resource blocks which are preferred by the DL scheduling for the UE. Several different ways to fits to different cases to carry out the UL scheduling and transmissions are suggested to be implemented.

Description

METHOD AND ACCESS POINT TO ENHANCE DOWNLINK TRANSMISSION THROUGH

UPLINK SCHEDULING TECHNICAL FIELD

The present technology generally relates to wireless communication, particularly to a method and Access Point (AP) to enhance downlink transmission through uplink scheduling.

BACKGROUND

Nowadays, there is increasing interest in utilizing reciprocity of Time Division Duplex

(TDD) system to further enhance the system capacity, such as the DL performance. In a radio air-interface of cellular networks, the more the Channel Status Information (CSI) is available to the Access Point (AP) on the transmission side, the better downlink (DL) performance in the system can achieve. Although UL and DL transmissions are facing different interferences, the channel reciprocity of TDD systems enables effective and accurate UL/DL channel information sharing between the access point (AP) and user equipments (UEs). Thus, by taking this advantage, TDD system can employ the shared comprehensive channel information to further improve the downlink transmission performance.

However, owing to the contradiction between a large number of UEs and the limited feedback capacity or sounding capacity of the existing TDD cellular system, only partial or coarse CSI for the DL transmission is available to the AP and utilized to configure the DL beam-forming (BF) and link adaptation (LA), which constrains the improvement of the overall downlink transmission performance in TDD system. SUMMARY

Therefore, it is an object to solve at least one of the above-mentioned problems.

According to an aspect of the embodiments, there is provided a method for uplink (UL) scheduling by an Access Point (AP) in a Time Division Duplex (TDD) cellular network. The method obtains a downlink (DL) scheduling for transmissions from a serving AP to a User Equipment (UE) that is served by the serving AP and schedules an UL transmission from the UE to the serving AP at frequency resource blocks which are preferred by the DL scheduling for the UE. Optionally, the method may further estimates the UL channel status information (CSI) based on at least a portion of the information obtained from the scheduled UL transmission, such as the UL Demodulation Reference Signal (DRS) and the UL payload data, and calculates the DL BF, specifically the DL BF matrix, used for the DL transmission to the UE by utilizing the estimated CSI

According to another aspect of the embodiments, there is provided an Access Point (AP) in a Time Division Duplex (TDD) cellular network. The AP comprises an obtaining unit, which is adapted to obtain a downlink (DL) scheduling for transmissions from a serving AP to a User Equipment (UE), and a scheduling unit, which is adapted to schedule an UL transmission from the UE to the serving AP at frequency resource blocks preferred by the DL scheduling for the corresponding UE.

Optionally, the AP may further comprises an estimating unit, which is adapted to estimate the UL CSI based on at least a portion of the information obtained from the scheduled UL transmission from the UE, such as the UL Demodulation Reference Signal (DRS) and the UL payload data, and a calculating unit which is adapted to calculate the DL BF for the DL transmission to the UE by utilizing the estimated CSI.

It is advantageous that the UL resource assignment is adjusted through a dynamic UL scheduling, which takes the DL transmission scheduling from the AP to the UE into account. In this way, the DL transmission performance is beneficially guaranteed or even further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The technology will now be described, by way of example, based on embodiments with reference to the accompanying drawings, wherein:

Fig. 1 illustrates a schematic view of a wireless communication network environment suitable for implementing an embodiment;

Fig. 2 illustrates a flowchart of a method for UL scheduling by an AP in a TDD cellular network in accordance with an embodiment;

Fig. 3 illustrates a flowchart of a method for UL scheduling by an AP in a TDD cellular network in accordance with another embodiment;

Fig. 4 illustrates a block diagram of the AP in a TDD cellular network according to an embodiment; and

Fig. 5 illustrates a block diagram of the AP in a TDD cellular network according to another embodiment.

DETAILED DESCRIPTION

Embodiments herein will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments are shown. This embodiments herein may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Like numbers refer to like elements throughout.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" "comprising," "includes" and/or "including" when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The present technology is described below with reference to block diagrams and/or flowchart illustrations of methods, apparatus (systems) and/or computer program products according to the present embodiments. It is understood that blocks of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, may be implemented by computer program instructions. These computer program instructions may be provided to a processor, controller or controlling unit of a general purpose computer, special purpose computer, and/or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer and/or other programmable data processing apparatus, create means for implementing the functions/acts specified in the block diagrams and/or flowchart block or blocks.

Accordingly, the present technology may be embodied in hardware and/or in software

(including firmware, resident software, micro-code, etc.). Furthermore, the present technology may take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code embodied in the medium for use by or in connection with an instruction execution system. In the context of this document, a computer-usable or computer-readable medium may be any medium that may contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

Although the technology herein is described with reference to the LTE communication network in the context, it should understand that the embodiments are not limited to this, but may indeed be applied to all suitable wireless communication networks involved in UL/DL scheduling. Although specific terms in some specifications are used here, such as eNB, it should be understand that the embodiments are not limited to those specific terms but may be applied to all similar entities, such as macro base station, femto base stations and Core Network (CN).

Take the Time Division - Long Term Evolution (TD-LTE) system as example, there are usually the following two options for the current system design to support DL BF:

One is to capture the frequency-selective instantaneous CSI through the sounding and fully utilize it for a relatively high DL performance. To support flexible DL frequency selective scheduling and the BF at the sub-bands preferable to DL, a full-bandwidth sounding using UL periodic pilot is required and can usually be supported by sounding resources. It is known that the sounding signal (i.e. sounding reference signal (SRS)) has been standardized to facilitate CSI acquisition in TD-LTE system. However, in such a manner, each UE costs a lot of sounding resource in terms of time and frequency. As such, due to limited sounding resources in LTE system, it is challenging to serve a large number of UEs.

The other is to capture the long-term CSI, which is usually statistically averaged non-frequency-selective information. DL BF based on such information results in a relatively low performance though this option costs less sounding resources and is suitable for non-frequency-selective scheduling and BF. For this option, one possible DL CSI acquisition is from the sources such as UL data transmission based on channel pilot such as UL DMRS or the payload data.

Embodiments herein will be described below with reference to the drawings.

Fig. 1 illustrates a schematic view of a wireless communication network environment suitable for implementing an embodiment.

As shown in Fig.l, the wireless communication network 100 comprises the AP 110 and three UEs 120, 130 and 140. In operation, the UL transmission and the DL transmission are performed between the AP and the UE. The UL transmission may contain the information such as UL Demodulation Reference Signal (DRS), the payload data, the pseudo data etc. The term "AP" used herein may indicate any type of communication node, such as base station, eNB, NodeB and so on. The term "UE" used herein may indicates all forms of devices enabling the user to communicate via wireless communication network, such as, smart phones, cellular phone, Personal Digital Assistant (PDA), and the like. The wireless communication network comprises, but not limited to, the TD-LTE network. For simplicity and clarity, only one AP and three UEs are shown in the wireless communication network 100, it will be appreciated that one or more APs may exist in the wireless communication network, and each AP may serve one or more UEs in the mean time.

Fig. 2 illustrates a flowchart of a method for UL scheduling by an AP in a TDD cellular network in accordance with an embodiment.

In step 210, the AP 110 may obtain a DL scheduling from the AP 110. The DL scheduling comprises the scheduling information of the DL transmission, such as the timing to perform the DL transmission, the frequency resource blocks (RBs) to be used in the DL transmission, the DL transmission priority etc. Here, the DL scheduling can be the DL scheduling for the DL transmission to be performed by the AP 110. In this case, the AP 110 may have determined the DL scheduling from the AP 110 and the UE 120 by itself in advance. Furthermore, the DL scheduling also can be the DL scheduling for the DL transmission performed in history by the AP 110. In this case, the AP 110 can retrieve the DL scheduling in the DL scheduling database, such as a text file, recording all the related information on the DL scheduling performed by the AP 110.

In step 220, the AP 1 10 schedules an UL transmission from the UE (e.g. UE 120) to the AP 110 at frequency resource blocks preferred by the DL scheduling for the corresponding UE 120. Here, the UL transmission comprises, but not limited to, the only UL Demodulation Reference Signal (DRS) transmission, the DRS transmission in an UL DRS Multi-User Multiple-Input and Multiple-Output (MU-MIMO) transmission fashion, the UL payload data transmission along with DRS and the UL pseudo-data transmission along with DRS, which will be described in detail later.

Specifically, the DL scheduling is performed by the AP 110, the frequency resource blocks preferred by the DL scheduling may refer to the frequency resource blocks to be used in the upcoming DL transmission. In this way, after the UE 120 performs the UL transmission to the AP 110 at the specific frequency resource blocks, the AP 110 can receive and obtain the information in the UL transmission, such as the UL DRS, the payload data, and the like. Then this information can be used to estimate UL channel status information (CSI) at the specific frequency resource blocks, thereby the upcoming scheduled DL transmission that also makes use of these frequency resource blocks can be improved due to its DL beam-forming optimization by utilizing the accurately estimated UL CSI as known to the skilled person in the art.

It will be appreciated that the ways to determine the frequency resource blocks preferred by the DL scheduling described above merely are by way of example, other suitable means can readily conceived by the skilled in the art.

As a whole, the UL transmission scheduling takes into account the DL scheduling for the

DL transmission to be performed from the AP to the UE. In other words, for example, at least portion of the frequency resource blocks in the UL transmission scheduling covers the frequency resource blocks preferred by the DL scheduling. In this manner, the DL transmission performance is beneficially guaranteed or even further improved.

Fig. 3 illustrates a flowchart of a method for UL scheduling by an AP in a TDD cellular network in accordance with another embodiment.

In the embodiment, the step 310 and 320 simply work in the similar way to the step 210 and 220 mentioned above in Fig. 2 respectively, which will not be repeated for purpose of simplicity.

After UL transmission using frequency resource blocks preferred by the DL scheduling is scheduled in step 320, the AP 110 notifies the corresponding UE 120 of the UL transmission scheduling. When for example there is data in the buffer of the UE 120 waiting to be transmitted to the AP 110, the UE 120 will transmit the data to the AP 100 at the frequency resource blocks designated by the UL transmission scheduling. Then, the AP 110 receives the UL transmission and retrieves the information included in the UL transmission. The information may comprise the UL DRS and payload data.

Subsequently, the AP 110 estimates the UL channel status information (CSI) based on at least a portion of the information obtained from the scheduled UL transmission (step 330). In other words, the information obtained from the scheduled UL transmission such as the UL DRS, the payload data or the combination thereof can be taken to estimate the UL CSI. Taking the UL DRS as example, the AP 110 may estimate the UL CSI by comparing the obtained UL DRS with the corresponding original UL DRS, which UL DRS may be kept by the AP in advance. Here, the Channel State Information (CSI) may refer to known channel properties of a communication link. This information describes how a signal propagates from the transmitter to the receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI estimation process is known to the skilled in the art, which will not be described in more detail here. In practice, the CSI may be represented as channel matrices.

After estimating the CSI, the AP 110 may then calculate the DL Beam Forming (BF) weights with the estimated CSI in step 330 (step 340). Specifically, the DL BF weights may be represented as a matrix, which can be calculated with the CSI channel matrix by the matrix transformation. Since the matrix transformation from the CSI channel matrix to the DL BF matrix is known in the art, it will not be described in more detail here. Then the DL BF is utilized to perform the DL transmission. Since the CSI estimation is based on the adequately refresh channel information, the DL BF preferably is the frequency-selective DL BF

In the embodiment, the UL scheduling directed UL transmission to obtain the channel information for CSI estimation can be done in a time interval comparable to a few of milliseconds, in a contrast, as well known, it takes much more time to perform the conventional signal sounding to serve the similar purpose. As seen, the embodiment provides a higher efficient way to deal with the CSI estimation.

Optionally, if there are data waiting for both UL and DL transmission between the UE (e.g. UE 120) and the AP (e.g. AP 110), the AP 110 will employ the UL transmission in the form of one of the following options : (I) the only UL DRS transmission, (II) the UL DRS transmission in the UL DRS MU-MIMO transmission fashion, and (III) the UL payload data transmission along with DRS. Specifically, when there is data waiting to be transmitted to the UE 120 in the buffer of the AP 110, it means that a upcoming DL transmission will be or has been scheduled for the buffered data. At this point, the AP 110 will be triggered to initiate an UL transmission scheduling at the frequency resource blocks to be used in the upcoming DL transmission.

If the UL transmission is the only UL DRS transmission (Option I), the UL transmission scheduling will request the UE to transmit a signal with DRS and a dummy data to the AP, the dummy data is a randomly generated data used to filling in the data portion instead of the actual payload data, where payload data may indicate the data waiting to be transmitted to the AP 110 in the buffer of the UE 120. In this case, only the DRS can be used to estimate the UL CSI.

The UL transmission also can be the UL DRS transmission in the UL DRS MU-MIMO transmission fashion (Option II), which is more suitable for the scenario that a number of upcoming DL scheduling for different UEs, for example the DL scheduling for the UE 120 and the DL scheduling for the UE 130, intend to utilize the same frequency resource blocks in the respective DL transmission. In this case, it will be advantageous that the AP 110 schedules an UL DRS Multi-User Multiple-Input and Multiple-Output (MU-MIMO) transmission for both UEs (UE 120 and UE 130) at DRS resource blocks. That is, the DRS sent by the UE 120 and the DRS sent by the UE 130 share the DRS resource blocks in the UL transmission, and they are transmitted to the AP 110 together. On the AP side, the different cyclic shifts of the two DRSs can help distinguish them from each other. Then the respective DRSs are used to estimate the UL CSI for the UE 120 and the UE 130. It will be appreciated that the UL DRS MU-MIMO transmission can be scheduled for more than two UEs each time. As can be seen, the UL DRS MU-MIMO transmission scheduling can serve two or more UEs at the same time, which embody the higher efficiency than the UL transmission that serves only one UE at a time as in option I.

Moreover, if the UL transmission is the UL payload data transmission along with DRS (Option III), the UL transmission scheduling will request the UE to transmit a signal with DRS and a payload data (i.e. the data waiting to be transmitted to the AP 110 in the buffer of the UE 120) to the AP. In this case, Both the DRS and the payload data can be used to estimate the UL CSI. Specifically, the DRS and the payload data are retrieved from the UL transmission by the AP. It should be appreciated that the channel transmission surely results in the data errors in the retrieved DRS and payload data more or less. Then, as known in the art, the AP derives the original payload data sent by the UE from the retrieved payload data signal based on a first-step CSI through comparison between the retrieved DRS and the corresponding original DRS kept by the AP in advance. Subsequently, the more accurate UL CSI re-estimation can be done by comparing the retrieved DRS with the original DRS and comparing the retrieved payload data with the payload data signal. Since both the DRS and the payload data involve in the UL CSI estimation, the UL CSI can be estimated more accurately than that only using the DRS as in option I.

It should be appreciated that the above UL transmission simply are described by way of examples, and any suitable UL transmission forms can be used for the UL scheduling in this embodiment. It should also be appreciated that the individual UL transmission forms can be UE specific. For example, it is possible that the UL scheduling for the UE 120 employs the only UL DRS transmission (Option I), while the UL scheduling for the UE 130 instead employs the UL payload data transmission along with DRS (Option III). Optionally, in the case that there is only data waiting for DL transmission from the AP (e.g. AP 110) to the UE (e.g. UE 120), and there is no data waiting for an UL transmission from the UE to the AP, the AP will employ the UL transmission in the form of one of the following options: (A) the only UL DRS transmission, (B) the UL DRS transmission in the UL DRS MU-MIMO transmission fashion, and (C) the UL pseudo-data transmission along with DRS. Specifically, when there is data waiting to be transmitted to the UE 120 in the buffer of the AP 110, it means that a upcoming DL transmission will be or has been scheduled for the buffered data. At this point, the AP 110 will be triggered to initiate an UL transmission scheduling at the frequency resource blocks to be used in the upcoming DL transmission.

In such case, the AP 110 still can employ the only UL DRS transmission (Option A) and the UL DRS transmission in the UL DRS MU-MIMO transmission fashion (Option B), which simply work in the same way as the option I and option II respectively as mentioned above and will not be repeated for brevity.

Furthermore, since there is no data waiting for an UL transmission from the UE 120 to the AP 110, it is not possible to schedule an UL transmission with payload data. However, the AP 110 may add a new pseudo-data mode indicator in UL scheduling, specifically the Physical Uplink Shared Channel (PUSCH) scheduling, and instruct the UE 120 to send the pseudo-data, i.e. the specified UL sequences, in the UL transmission (Option C). This option fits into the case that the UL transmission is low and the UL frequency resource blocks are available to allow the pseudo-data transmission.

In one embodiment, the AP 110 further determines the UL scheduling priority for the UE 120 among all the UEs served by the AP 110 based on at least one of the following factors:

- The DL transmission priority for the UE,

- The number of data to be transmitted from the UE to the AP,

- The UL transmission priority of the UE configured by the upper protocol layer,

- The timing of the DL transmission for the UE.

Specifically, if the UE 120 has higher DL transmission priority, the DL transmission directed to the UE will be prioritized to perform, thus the AP 110 will determine that the UE 120 has a higher UL scheduling priority.

If the UE 120 for example has a larger number of data waiting to be transmitted to the AP

110, the AP 110 will grant the UE 120 with higher UL scheduling priority considering the need to alleviate the data buffering load on UE side. Furthermore, the UL scheduling priority for the UE also can be set in advance, for example, the upper layer (e.g. application layer) has configured the UL scheduling priority for all the UEs. Moreover, the timing of the DL transmission for the UE may assist in determining the corresponding UL scheduling priority. For instance, if the timing of DL transmission (e.g. time slot 1) directed to the UE 120 is prior to the timing of DL transmission (e.g. time slot 9) directed to the UE 130, then it can be determined that the UE 120 has higher UL scheduling priority than the UE 130 so as to obtain the UL transmission information used to estimate the UL CSI for the UE 120 firstly.

It should be appreciated that the above factors are described by way of examples, and any other suitable factors can be applied to determine the UL scheduling priority in this embodiment. It should also be appreciated that all the factors can be used separately or in combination in the UL scheduling priority determination.

After determining the UL scheduling priority for the UE, the AP 110 may obtain UL scheduling order of all the UEs, and thereby schedule the UL transmission for the UE according to the ordering. In this way, the UL scheduling can be optimized among all the UEs.

In another embodiment, the AP may compare the UL traffic status information with the DL traffic status information between the AP and the UE. If the UL traffic and the DL traffic are asymmetric, the AP may determine to perform the UL scheduling process as describe above.

Here, the UL/DL traffic status information comprises the UL/DL traffic load and the frequency resource blocks utilities for UL/DL transmission. The UL/DL traffic load can be statistically averaged traffic load between the AP and the UE in a predetermined period, so do the frequency resource blocks utilities for UL/DL transmission.

If the UL/DL traffic status information is indicated by the UL/DL traffic load, the asymmetry means the UL traffic load is less than the DL traffic load, in other words, the UL transmission traffic from the UE to the AP is less than the DL transmission traffic from the AP to the UE in average. In this case, the normal UL transmission (i.e. the UL transmission with payload data) can not satisfy the requirement to provide enough information to estimate the UL CSI at all the frequency resource blocks preferred by the DL transmission scheduling, thus the AP may determine to perform the UL scheduling described above, for example, scheduling the only DRS UL transmission for the UE 120 at the frequency resource blocks to be used by the upcoming DL transmission directed to the UE 120.

If the UL/DL traffic status information is indicated by the frequency resource blocks utilities for UL/DL transmission, the asymmetry means the frequency resource blocks utility for UL transmission is less than the frequency resource blocks utility for DL transmission. For example, provided that the frequency resource blocks (RBs) used in the UL transmission from the UE 120 to the AP 1 10 include the RB 1, RB 3, and the frequency resource blocks used in the DL transmission from the AP 110 to the UE 120 include the RB 1, RB 3 and RB 4, as can be seen, RB 1 and RB 3 are utilized in both the UL transmission and the DL transmission, while RB 4 is only utilized in the DL transmission. Thus, in order to obtain the UL transmission information for estimating the UL CSI at the RB 4, the AP 110 may schedule an only DRS UL transmission from the UE 120 to the AP 110 at the RB 4.

Optionally, if the UL traffic and the DL traffic are symmetric, the UL scheduling at the frequency resource blocks preferred by the DL scheduling may not always necessary, or feasible for example due to the fact that the frequency resource blocks preferred by the DL transmission scheduling may have been fully utilized such that a new scheduling is not possible. In this case, the AP may determine whether to perform the UL scheduling described above based on the predetermined strategies.

One strategy is to only schedule the UL transmission when the frequency resource blocks preferred by the DL scheduling are available. That is, the AP will not schedule the UL transmission at a frequency resource block preferred by the DL transmission scheduling, unless it is the time that the frequency resource block is not used in the normal UL transmission, which ensures that the normal UL transmission will not be impacted.

Another strategy is to adjust normal UL data transmission by scheduling the UL transmission under the precondition of minimizing its influence to the normal UL transmission or without impairments to the normal UL transmission. For example, the DL scheduling is intended to utilize the RB 1 in the DL transmission (e.g. TX1) from the AP 110 to the UE 120, and the RB 1 has been scheduled to be utilized in the UL transmission (e.g. TX2) from the UE 130 to the AP 110. In this case, in order to obtain the UL transmission information at the RB 1 to estimate the UL CSI between the AP 110 and the UE 120, which UL CSI will be used to calculate the DL BF matrix of the DL transmission TX1, the AP 110 may change the scheduling for the UE 130 utilize another frequency resource block (e.g. RB 2) to perform the UL transmission TX2, then schedule a only DRS UL transmission from the UE 120 to the AP 110 at the RB 1.

It should be appreciated that the above strategies are described by way of examples, and any other suitable strategies can be applied to determining whether to perform the UL scheduling.

In a further embodiment, if the UE is not assigned sounding resources or enough sounding resources for DL BF matrix calculation, the AP may determine to perform the UL scheduling as described above. Specifically, if the AP has assigned the UE with adequate sounding resources for estimating the UL CSI between the AP and the UE accurately, it is not necessary to schedule such an UL transmission taking the DL transmission into account. Conversely, if the UE is not assigned the sounding resources, or the assigned sounding resources are not adequate to accurately estimate the UL CSI used in the DL BF matrix calculation, it will be advantageous if the AP schedule such an UL transmission, the information in which can be used to estimate the UL CSI accurately.

Fig. 4 illustrates a block diagram of the AP in a TDD cellular network according to an embodiment.

As illustrated in the Fig 4, the AP may comprise the obtaining unit 410 and scheduling unit 420. It should be appreciated that the AP is not limited to the shown elements, and can comprise other conventional elements and the additional elements for other purposes.

In the AP (e.g. 110), the obtaining unit 410 may obtain a DL scheduling from the AP 110. The DL scheduling comprises the scheduling information of the DL transmission, such as the timing to perform the DL transmission, the frequency resource blocks (RBs) to be used in the DL transmission, the DL transmission priority etc. Here, the DL scheduling can be the DL scheduling for the DL transmission to be performed by the AP 110. In this case, the AP 110 may have determined the DL scheduling from the AP 110 and the UE 120 by itself in advance. Furthermore, the DL scheduling also can be the DL scheduling for the DL transmission performed in history by the AP 110. In this case, the obtaining unit 410 can retrieve the DL scheduling in the DL scheduling database, such as a text file, recording all the related information on the DL scheduling performed by the AP 110.

The scheduling unit 420 schedules an UL transmission from the UE 120 to the AP 110 at frequency resource blocks preferred by the DL scheduling for the corresponding UE 120. Here, the UL transmission comprises, but not limited to, the only UL Demodulation Reference Signal (DRS) transmission, the DRS transmission in an UL DRS Multi-User Multiple-Input and Multiple-Output (MU-MIMO) transmission fashion, the UL payload data transmission along with DRS and the UL pseudo-data transmission along with DRS.

As a whole, the UL transmission scheduling takes into account the DL scheduling for the DL transmission to be performed from the AP to the UE. In other words, for example, at least portion of the frequency resource blocks in the UL transmission scheduling covers the frequency resource blocks preferred by the DL scheduling. In this manner, the DL transmission performance is beneficially guaranteed or even further improved.

In the embodiment, the obtaining unit 510 and the scheduling unit 520 simply work in the similar way to the obtaining unit 410 and the scheduling unit 420 mentioned above in Fig. 4 respectively, which will not be repeated for purpose of simplicity.

After UL transmission using frequency resource blocks preferred by the DL scheduling is scheduled by the scheduling unit 520, the AP 110 notifies the corresponding UE 120 of the UL transmission scheduling. When for example there is data in the buffer of the UE 120 waiting to be transmitted to the AP 110, the UE 120 will transmit the data to the AP 100 at the frequency resource blocks designated by the UL transmission scheduling. Then, the AP 110 receives the UL transmission and retrieves the information included in the UL transmission. The information may comprise the UL DRS and payload data.

Subsequently, the estimating unit 530 estimates the UL channel status information (CSI) based on at least a portion of the information obtained from the scheduled UL transmission. In other words, the information obtained from the scheduled UL transmission such as the UL DRS, the payload data or the combination thereof can be taken to estimate the UL CSI. Take the UL DRS as example, the estimating unit 530 may estimate the UL CSI by comparing the obtained UL DRS with the corresponding original UL DRS, which UL DRS may be kept by the AP in advance. Here, the Channel State Information (CSI) may refer to known channel properties of a communication link. This information describes how a signal propagates from the transmitter to the receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI estimation process is known to the skilled in the art, which will not be described in more detail here. In practice, the CSI may be represented as channel matrices.

After the CSI estimation, the calculating unit 540 may then calculate the DL Beam Forming (BF) with the estimated CSI by the estimating unit 530. Specifically, the DL BF weights may be represented as a matrix, which can be calculated with the CSI channel matrix by the matrix transformation. Since the matrix transformation from the CSI channel matrix to the DL BF matrix is known in the art, it will not be described in more detail here. Then the DL BF is utilized to perform the DL transmission. Since the CSI estimation is based on the adequately refresh channel information, the DL BF preferably is the frequency-selective DL BF

While the embodiments have been illustrated and described herein, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present technology. In addition, many modifications may be made to adapt to a particular situation and the teaching herein without departing from its central scope. Therefore it is intended that the present embodiments not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the present technology, but that the present embodiments include all embodiments falling within the scope of the appended claims.

Claims

1. A method for uplink, UL, scheduling by an Access Point, AP, in a Time Division Duplex, TDD, cellular network (100), comprising:

- Obtaining (210) a downlink, DL, scheduling for transmissions from a serving AP (110,

400, 500) to a User Equipment, UE, (120-140);

- Scheduling (220) a UL transmission from the UE (120-140) to the serving AP (110, 400, 500) at frequency resource blocks preferred by the DL scheduling for the corresponding UE (120-140).

2. The method of the claim 1, the method further comprises:

- Estimating (330) the UL channel status information, CSI, based on at least a portion of the information obtained from the scheduled UL transmission;

- Calculating (340) the DL beam-forming, BF, for the DL transmission to the UE (120-140) by utilizing the estimated CSI.

3. The method of the claim 1 or 2, wherein the UL transmission comprises at least one of the followings:

- Only UL Demodulation Reference Signal, DRS, transmission;

- UL DRS transmission in a UL DRS Multi-User Multiple-Input and Multiple-Output, MU-MIMO, transmission fashion;

- UL payload data transmission along with DRS;

- UL pseudo-data transmission along with DRS.

4. The method of any one of the claims 1-3, wherein if there are data waiting for both UL and DL transmission between the UE (120-140) and the serving AP (110, 400, 500), the UL transmission is one of the only UL DRS transmission, the UL DRS transmission in the UL DRS MU-MIMO transmission fashion, and the UL payload data transmission along with DRS.

5. The method of the claim 4, wherein if there is only data waiting for DL transmission from the serving AP (110, 400, 500) to the UE (120-140) without data waiting for a UL transmission from the UE (120-140) to the serving AP (110, 400, 500), the UL transmission is one of the only UL DRS transmission, the UL DRS transmission in a UL DRS MU-MIMO transmission fashion and the UL pseudo-data transmission along with DRS.

6. The method of the claim 1, the method further comprises:

- Determining the UL scheduling priority for the UE (120-140) based on at least one of the options including the DL transmission priority for the UE (120-140), the number of data to be transmitted from the UE (120-140) to the serving AP (110, 400, 500), the UL transmission priority of the UE (120-140) configured by the upper protocol layer, and the timing of the DL transmission for the UE (120-140);

Wherein the scheduling step (220) comprises:

- Scheduling the UL transmission from the UE (120-140) at frequency resource blocks preferred by the DL scheduling according to the determined UL scheduling priority for the UE (120-140).

7. The method of the claim 1, the method further comprises:

- Comparing a UL traffic status information with a DL traffic status information between the serving AP (110, 400, 500) and the UE (120-140);

- If the UL traffic and the DL traffic are asymmetric, determining to perform the UL scheduling as claimed in claim 1.

8. The method of the claim 7, the method further comprises:

- If the UL traffic and the DL traffic are symmetric, determining whether to perform the

UL scheduling as claimed in claim 1 based on one of the predetermined strategies.

9. The method of the claim 8, wherein the predetermined strategies comprises:

- Only scheduling the UL transmission when the frequency resource blocks preferred by the DL scheduling are available;

- Adjusting normal UL data transmission by scheduling the UL transmission under the precondition of minimizing its influence to the normal UL transmission or without impairments to the normal UL transmission.

10. The method of any one of the claims 7-9, wherein the UL/DL traffic information comprises at least one of the UL/DL traffic load and the frequency resource blocks utilities for UL/DL transmission.

11. The method of the claim 10, wherein if the UL/DL traffic information comprises the UL/DL traffic load, then the asymmetry means the UL traffic load is less than the DL traffic load.

12. The method of the claim 10, wherein if the UL/DL traffic information comprises the frequency resource blocks for UL/DL transmission, then the asymmetry means the frequency resource blocks utility for UL transmission is less than the frequency resource blocks utility for DL transmission.

13. The method of the claim 1, the method further comprises:

- If the UE (120-140) is not assigned sounding resources or enough sounding resources for DL BF calculation, determining to perform the UL scheduling as claimed in claim 1.

14. The method of the claim 2, wherein the DL BF is the DL frequency-selective BF.

15. The method of claim 1, wherein the TDD cellular network (100) is a Time Division - Long Term Evolution, TD-LTE, network.

16. An Access Point, AP, (1 10, 400, 500) in a Time Division Duplex, TDD, cellular network (100), comprising:

An obtaining unit (410, 510) adapted to obtain a downlink, DL, scheduling for transmissions from a serving AP (110, 400, 500) to a User Equipment, UE, (120-140);

A scheduling unit (420, 520) adapted to schedule a UL transmission from the UE (120-140) to the serving AP (110, 400, 500) at frequency resource blocks preferred by the DL scheduling for the corresponding UE (120-140).

17. The AP of the claim 16, the AP further comprises:

An estimating unit (530) adapted to estimate the UL CSI based on at least a portion of the information obtained from the scheduled UL transmission from the UE (120-140);

A calculating unit (540) adapted to calculate the DL beam-forming, BF, for the DL transmission to the UE (120-140) by utilizing the estimated CSI.

18. A computer program product, comprising instructions for implementing the steps of the method according to any one of the claims 1-15.

19. A recording medium which stores instructions for implementing the steps of the method according to any one of the claims 1-15.