METHOD FOR RESOURCE MANAGEMENT IN A CELLULAR COMMUNICATION NETWORK AND RESOURCE MANAGEMENT SYSTEM
A method for resource management in a cellular communication network, wherein the network includes at least one base station per cell for a plurality of cells that operates in TDD (Time Division Duplex) transmission mode, wherein the base stations implement a respective UL/DL configuration mode in which frames are composed of a specific sequence of downlink DL, uplink UL and special S subframes, is characterized in that the base stations are operated such that neighboring base stations with adjacent and/or overlapping coverage areas employ different UL/DL configuration modes, wherein interference between neighboring base stations is taken account of by implementing power control on a subframe basis. Furthermore, a resource management system for use in a cellular communication network is described.
The present invention relates to a method for resource management in a cellular communication network, wherein said network includes at least one base station per cell for a plurality of cells that operates in TDD (Time Division Duplex) transmission mode, wherein said base stations implement a respective UL/DL configuration mode in which frames are composed of a specific sequence of downlink DL, uplink UL and special S subframes.
Furthermore, the present invention relates to a resource management system for use in a cellular communication network, wherein said network includes at least one base station per cell for a plurality of cells that operates in TDD (Time Division Duplex) transmission mode, wherein said base stations are configured to implement a respective UL/DL configuration mode in which frames are composed of a specific sequence of downlink DL, uplink UL and special S subframes.
Time Division Duplex (TDD) is a transmission mode supported both in 3GPP UMTS, LTE (Long Term Evolution) and IEEE 802.16 that utilizes the same radio access scheme as the Frequency Division Duplex (FDD), i.e. in case of LTE OFDMA (Orthogonal Frequency-Division Multiple Access) in the downlink and the SC-FDMA (Single Carrier Frequency-Division Multiple Access) in the uplink, CDMA in case of UMTS and OFDMA in case of IEEE 802.16 in both uplink and downlink. Furthermore, TDD uses the same subframe format as well as the same configuration protocols as FDD. The main difference compared with FDD is that TDD macro cellular base stations, or evolved Node B (eNBs) in 3GPP terminology, support an unpaired frequency band, where downlink and uplink are separated in time domain, with each frame being composed by downlink (DL), uplink (UL) and special (S) sub-frames.
Special sub-frames are used to switch from downlink to uplink and they are included at least once within each frame. In particular, the special sub-frame consists of the following three special fields, a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). In 3GPP LTE, the UL/DL portion of each frame may be configured according to the specification provided in document 3GPP TS 36.300, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Rel 10), April 2011, which defines 7 different UL/DL configuration modes as shown in the following table:
Despite the provision of flexibility in resource allocation, such asymmetric UL/DL brings new challenges in admission control and load balancing and introduces certain limitations due to interference reasons. In the case that two adjacent eNBs apply different UL/DL configuration models, it may happen that eNBs directly interfere each other, or UEs (User Equipment) are receiving direct interference from each other as illustrated in
In particular, although UL/DL configuration may be configured according to traffic demand and changed dynamically to reflect traffic variations by altering the UL/DL frame portion, conventional interference control methods require neighbor eNBs to be synchronized following the same UL/DL configuration mode, which is a major barrier to flexibility. It can thus be noted that there is generally a trade-off between flexibility and interference, as also acknowledged in document Peter W. C. Chan, et. al., “The Evolution Path of 4G Networks: FDD or TDD?”, IEEE Communication Magazine, Vol. 44, No. 12, December 2006. However, the dynamic UL/DL configuration mode is a desired feature of TD-LTE, as discussed in the document cited above as well as in document D. Astely, E. Dahlman, A. Furuskär, S. Parkvall, “TD-LTE—The radio-access solution for IMT-Advanced/TDD”, CHINACOM, August 2010.
Previous efforts to provide cell-independent asymmetric UL/DL configuration, in which neighboring cells adopt a different UL/DL configuration, concentrate on evaluating loose UL/DL synchronization among neighboring cells like Adaptive TDD (as described, e.g., in G. Szladek, B. Heder, J. Bito, “Investigation of Interference conditions in BFWA System Applying Adaptive TDD”, IEC, Prague, 2005) and further methods that utilize opportunistic interference mitigation (as described, e.g., in E. Foutekova, P. Agyapong, H. Haas, “Channel Asymmetry and Random Time Slot Hopping in OFDMA-TDD Cellular Networks”, IEEE VTC Spring, Singapore, May 2008). Although such methods provide flexibility, their performance is subject to the degree of correlation of UL/DL configuration modes among neighboring cells as well as on the associated traffic load. Thus their use suit best cases where the UL/DL mode is similar with small sub-frame differences, providing only limited degree of flexibility.
An alternative method is the Hybrid Division Duplex (HDD), which combines both FDD and TDD schemes within each cell (described, e.g., in S. Yun, et.al., “Hybrid Division Duplex System for Next-Generation Cellular Services”, IEEE Transactions on Vehicular Technology, Vol. 56, No. 5, September 2007) splitting each cell into two regions, with the outer macro cell region operating in FDD mode and the inner in TDD. HDD may resolve cross-slot interference for UEs located at the edge of each cell due to the use of the FDD, while being capable to provide cell-independent asymmetric UL/DL configurations among adjacent cells via the TDD mode. Such a proposal provides a high degree of UL/DL configuration mode flexibility, but it is subject to the use of FDD mode, which might not be available for certain deployment scenarios that rely solely on TDD.
Furthermore, US 2009/0249153 A1 discloses a dynamic adjustment of the UL/DL configuration mode with the purpose of introducing alternations of selected sub-frames for UL to DL or vice versa. The means of sub-frame adjustment is the introduction of a new sub-frame referred to as “mute”-sub-frame, where the eNB is neither in DL nor UL mode. Such a sub-frame is used to provide synchronization among neighboring cells/eNBs and/or associated UEs in order to introduce a sub-frame change, i.e. a dynamic UL/DL configuration adjustment, smoothly without interruption and performance degradation avoiding cross-slot interference. Such a method provides the means of concurrent dynamic adjustment of the UL/DL configuration mode among a set of neighboring cells or for an individual cell but is not addressing the problem of providing a different UL/DL configuration mode among neighboring cells or different network regions.
It is therefore an object of the present invention to improve and further develop a method and a system of the initially described type for resource management in a cellular communication network in such a way that a high degree of UL/DL configuration mode flexibility is provided without introducing significant cross-interference.
In accordance with the invention, the aforementioned object is accomplished by a method comprising the features of claim 1. According to this claim, such a method is characterized in that said base stations are operated such that neighboring base stations with adjacent and/or overlapping coverage areas employ different UL/DL configuration modes, wherein interference between neighboring base stations is taken account of by implementing power control on a subframe basis.
Furthermore, the aforementioned object is accomplished by a system comprising the features of claim 23. According to this claim, such a system is characterized in that it comprises a management mechanism that controls the operation of base stations in such a way that neighboring base stations with adjacent and/or overlapping coverage areas employ different UL/DL configuration modes, wherein interference between neighboring base stations is taken into account by implementing power control on a subframe basis.
According to the invention it has first been recognized that the above mentioned trade-off between flexibility and interference can be resolved by allowing different individual base stations (or network regions, i.e. group of base stations) to operate at the same time in different access modes, i.e. UL or DL, while potential interference (or other performance degradations) is repressed by the way of controlling the transmission power for individual subframes. Specifically, a management mechanism is provided that controls the operation of neighboring base stations in such a way that they may be assigned different UL/DL configuration modes, while subframe power control allows for interference-decoupling between those base stations with different patterns. Thus, the method according to the invention reduces or even avoids cross-interference between base stations and UEs. Power control may also apply UL/DL patterns including phases with neither UL nor DL such that cross-interference is avoided.
The method according to the invention provides a higher degree of flexibility compared to adaptive TDD and to opportunistic interference mitigation, relies only on TDD deployment and its objective is not limited to adjusting the UL/DL configuration but to configure dynamically different UL/DL modes into neighboring cells and regions. Thus, the main advantage of this invention is that it allows for different UL/DL configurations among neighbor cells in TD-LTE enhancing the network performance. In this way the network resources are exploited more efficiently, e.g. by matching the traffic/resource demand with the UL/DL configuration mode, thereby capturing diverse and evolving traffic demands. Such solution should follow the Self-Organized Network (SON) paradigm, as described in document 3GPP TR 36.902, Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Self-configuring and self-optimizing network (SON) use cases and solutions, April 2011, being a part of the eNB self-configuration process upon a new eNB installation and may also re-configure an existing UL/DL mode based on dynamic traffic conditions.
According to a preferred embodiment base stations may be configured to identify those subframes which are potentially subject to interference. Hereinafter, such subframes will be briefly denoted “potentially interfering subframes”. Essentially, cross-interference will occur in situations in which a base station has two neighboring base stations with adjacent and/or overlapping coverage areas that employ a different UL/DL configuration mode. For instance, one of the neighboring base stations may operate in UL, while in the same subframe another of the neighboring base stations operates in DL. In such case, a base station would identify its respective subframe as potentially interfering subframe.
Advantageously, once potentially interfering subframes have been identified, a base station may perform power control for these subframes. In a specific embodiment it may be provided that a base station reduces its transmission power for such subframes, either dynamically or by a predefined amount. In certain situations, e.g. in cases of high traffic load, potentially interfering subframes can be made completely “silent”, which means that the base station does not at all transmit or receive in such subframes. In such case the respective base station functions as a kind of “buffer” base station between two neighboring base stations that operate in different UL/DL configuration modes.
According to a further preferred embodiment a base station, in case a potentially interfering subframe is a special S subframe, keeps using the DL part of said S subframe, provided a neighboring base stations with adjacent and/or overlapping coverage area also employs a DL subframe. The reason is the fact that a DL part of a special S frame is like a conventional DL subframe, while the same does not hold for the UL part since this contains additional synchronization information.
With respect to an efficient implementation of power control it may be provided that subframes with a fixed maximum power are introduced. The fixed maximum power may be specified on the basis of a preceding training phase. According to another embodiment power control may be implemented by introducing time-zones with different maximum power, wherein UEs are scheduled to these time-zones depending on their long-term Signal-to-Noise Ratio SNR (Signal-to-Noise Ratio). Again, a respective preceding training phase for specifying maximum power may be performed.
Additionally or alternatively, power control may include performing per-frame signaling among potentially interfering base stations and adjusting power sequentially for the purpose of interference cancellation.
According to another embodiment power control may include controlling the maximum power depending on the backhaul capacity and/or the number of UEs being served by the involved base stations. According to still another embodiment power control may include excluding parts of the spectrum in the DL. Sparing out parts of the spectrum in the DL reduces also the emitted power in time-domain and avoids the problem that the useful signal is either “drowned” in noise or the automatic gain-control reaches the saturation region, i.e. FR>1 at one base station to avoid drowning a UE at the other base station in the UL.
Beside the regulation of sub-frame transmission, the introduction of a different UL/DL configuration mode among certain neighbor base stations requires also a process of selecting the optimal UL/DL configuration mode for each base station. In a specific embodiment the UL/DL configuration mode selection may be performed cooperatively among base stations based on information provided by a central network management entity and/or by the base stations themselves. In particular, this information may include details about an expected resource demand, which may be determined for each base station on the basis of an estimation and/or forecasting of the traffic volume associated with the respective base station. Once a certain UL/DL resource demand is associated with each base station, different centralized or distributed methods may be applied to select the optimal UL/DL configuration mode, either to provide an initial solution part of a configuration phase or as a (dynamic) adjustment to an existing configuration.
In this regard it is important to note that the UL-DL ratio not only depends on the data/resource demand, but also on channel condition and therefore the UL-DL ratio may vary depending on location and geographical characteristics.
In order to enable dynamic managing of the UL/DL configuration mode and to allow for fast and effective adaptations a monitoring process may be implemented that keeps track of the evolving traffic demands to identify major alternations or QoS degradation. According to a preferred embodiment a monitoring mechanism is provided in which the base stations monitor traffic load in the UL and/or in the DL for locations within their respective coverage areas. This monitoring mechanism may be carried out using UE positioning techniques, e.g., the ones described in document 3GPP TS. 25.111, Technical Specification Group RAN; LMU performance specification; UE positioning in UTRAN, (Rel 9), December 2009. Upon a major change in traffic, which may be identified according to predefined criteria, a different UL/DL configuration mode may be computed. Oscillations among specific UL/DL configuration modes may be avoided by introducing a hold down timer within each UL/DL configuration mode, which prohibits changes of UL/DL configuration modes for predefined time durations. This would also reduce the minimum re-computation period.
Advantageously, the UL/DL configuration mode selection and adaptation may be performed by iterative adjustments. For instance, base stations may be allowed to iteratively adjust the UL/DL configuration depending on the QoS demands and how those demands were fulfilled. This process may be an iterative feedback-loop by allowing base stations to adjusting their UL/DL mode/pattern based on the selection of adjacent base stations. Currently selected UL/DL configuration modes may be communicated among base stations, in particular via the X2 interface in case of 3GPP, and/or towards an associated OAM (Operation, Administration and Maintenance), in particular via the N-Inf.
With respect to enhancing scalability a clustering step may be provided according to which base stations with identical or similar UL/DL resource demands and/or geographical properties are grouped to form a cluster. The UL/DL configuration mode selection (inclusive of the iterative improvement techniques described above) may then be performed on the level of the clusters rather than considering individual base stations. This could speed up the selection of UL/DL configuration patterns and may reduce the required overhead. The degree of similarity may be subject to preadjustment or may be changed dynamically depending on the current situations.
According to an embodiment varying the UL/DL Configuration Modes may be performed via the means of interference management. It should be noted that this approach is not as efficient as the previously described approaches, but may prove to be a good solution in certain scenarios, for instance in energy saving scenarios, where the unused spectrum helps base stations to switch off certain local components. Similarly to the previously described approaches, this scheme also assumes that certain base stations may have available resources to perform the re-scheduling of certain resources.
At off-peak times the present invention may also contribute to enhance the energy saving and the efficiency of resource utilization within TD-LTE by introducing a different UL/DL configuration mode among neighbor base stations. According to an embodiment an OAM (Operation, Administration and Maintenance) function is provided that is responsible for monitoring the network and for identifying off-peak period as well as keep track of the initial adjustment of the UL/DL configuration mode. It may be provided that during off-peak times a mode is selected with a higher ratio of UL-subframes in which base stations may turn off their RF frontend. Consequently, by increasing the share of UL frames and turning off RF frontends the overall energy consumption can be significantly reduced. This further allows for serving a higher number of devices with strong UL-demands, such as M2M applications.
The purpose of such UL/DL configuration mode adjustment process is to align base stations with similar traffic demands by assigning to them the same UL/DL configuration providing synchronization to avoid interference, to avoid interference among neighbor base stations with different traffic demands, and to switch off as many unused carriers and/or radio resource as possible to save energy. Thus, this embodiment is similar to the ones described above with an additional parameter being energy saving by switching-off unused carriers.
There are several ways how to design and further develop the teaching of the present invention in an advantageous way. To this end it is to be referred to the patent claims subordinate to patent claim 1 on the one hand and to the following explanation of preferred embodiments of the invention by way of example, illustrated by the figure on the other hand. In connection with the explanation of the preferred embodiments of the invention by the aid of the figure, generally preferred embodiments and further developments of the teaching will we explained. In the drawing
It should be noted that even though hereinafter practical details and embodiments of the present invention concentrate on 3GPP LTE scenarios, the same principles apply to any other TDD system.
The lower part of
In the scenario illustrated in
In order to mitigate this problem, in accordance with embodiments of the present invention, eNB B either controls the transmission power or completely restricts UL and DL transmission for subframes 3, 4, 6, 7, 8, and 9 that may create interference for eNB A and eNB C. In
A specific embodiment of the present invention is illustrated in subframe 6 of
Furthermore, indicated at step 304, neighbor eNBs having assigned the same or a similar UL/DL configuration mode are having similar geographical properties may be grouped to form clusters, which enhances scalability since improvement techniques for UL/DL configuration mode adaptation can be applied on a cluster basis rather considering individual eNBs.
Once the above steps are completed, centralized 306 or distributed 308 optimal UL/DL configuration mode selection techniques may be applied. For instance, a centralized approach may start from the highest load cluster or eNB (in case no clustering step 304 is carried out) and may select an optimal UL/DL configuration mode for this cluster/eNB. Next, neighbor clusters/eNBs may be examined, i.e. the next lower load neighbor cluster or eNB may be determined and the optimal UL/DL configuration mode for such cluster or eNB may be selected. This process may be repeated until an optimal UL/DL configuration mode is selected for each cluster/eNB under consideration. In a next step, the selected UL/DL configuration modes may be adjusted with respect to neighbors until all neighbors are examined. Then, the next highest load cluster or eNB may be selected, continuing until all of them are considered.
Distributed approaches may consider only a neighbor clusters or eNBs so the same process is applicable considering only a neighboring scope.
Embodiments that include the deployment of interference management methods UL/DL configuration mode selection may follow the same process. However, instead of selecting the optimal UL/DL configuration mode for clusters and eNBs, they may schedule the neighbor cluster or eNB resources within different spectrum bands.
For energy saving the same steps as described above may be considered, followed by an additional step that aims to switch off the carriers or other eNB component that are not in use.
It should be noted that the power control schemes may also be combined with the interference management mechanisms in order to maximize the resource utilization benefits.
Many modifications and other embodiments of the invention set forth herein will come to mind the one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing description and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
Claims
1. A base station of a cell that operates in TDD (Time Division Duplex) transmission mode, wherein the base station is configured with a plurality of UL/DL configurations which each include a DL subframe, an UL subframe and a special subframe, the UL/DL configurations vary between the cell and a neighboring cell, and the base station transmits and receives a UL/DL configuration to and from a base station of the neighboring cell.
2. A communication method in a base station of a cell that operates in TDD (Time Division Duplex) transmission mode, wherein the base station is configured with a plurality of UL/DL configurations which each include a DL subframe, an UL subframe and a special subframe, the UL/DL configurations vary between the cell and a neighboring cell, the method comprising:
- transmitting a UL/DL configuration to a base station of the neighboring cell; and
- receiving the UL/DL configuration from the base station of the neighboring cell.
Type: Application
Filed: Jun 23, 2016
Publication Date: Oct 13, 2016
Inventors: Konstantinos SAMDANIS (Dossenheim), Peter ROST (Dresden), Long LE (Dossenheim)
Application Number: 15/190,219
