Patent Application
20070153742
The present invention relates generally to wireless communications, and more particularly to handovers in a wireless communications network.
The telecommunications industry is in the process of developing a new generation of flexible and affordable communications that includes high-speed access while also supporting broadband services. Many features of the third generation mobile telecommunications system have already been established, but many other features have yet to be perfected.
One of the systems within the third generation of mobile communications is the Universal Mobile Telecommunications System (UMTS) which delivers voice, data, multimedia, and wideband information to stationary as well as mobile customers. UMTS is designed to accommodate increased system capacity and data capability. Efficient use of the electromagnetic spectrum is vital in UMTS. It is known that spectrum efficiency can be attained using frequency division duplex (FDD) or using time division duplex (TDD) schemes. Space division duplex (SDD) is a third duplex transmission method used for wireless telecommunications.
As can be seen in
The UTRAN consists of a set of Radio Network Subsystems 128 (RNS), each of which has geographic coverage of a number of cells 110 (C), as can be seen in
Each Radio Network Subsystem 128 (RNS) includes a Radio Network Controller 112 (RNC) and at least one Node B 114, each Node B having geographic coverage of at least one cell 110. As can be seen from
LTE, or Long Term Evolution (also known as 3.9G), refers to research and development involving the Third Generation Partnership Project (3GPP) aimed at identifying technologies and capabilities that can improve systems such as the UMTS. The present invention is related to LTE work that is taking place in 3GPP.
Generally speaking, a prefix of the letter “E” in upper or lower case signifies LTE, although this rule may have exceptions. The E-UTRAN consists of eNBs (E-UTRAN Node B), providing the E-UTRA user plane (RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs interface to the access gateway (aGW) via the S1, and are inter-connected via the X2.
An example of the E-UTRAN architecture is illustrated in
In the example of
The eNB may host functions such as radio resource management (radio bearer control, radio admission control, connection mobility control, dynamic allocation of resources to UEs in both uplink and downlink), selection of a mobility management entity (MME) at UE attachment, routing of user plane data towards the user plane entity (UPE), scheduling and transmission of paging messages (originated from the MME), scheduling and transmission of broadcast information (originated from the MME or O&M), and measurement and measurement reporting configuration for mobility and scheduling. The MME/UPE may host functions such as the following: distribution of paging messages to the eNBs, security control, IP header compression and encryption of user data streams; termination of U-plane packets for paging reasons; switching of U-plane for support of UE mobility, idle state mobility control, SAE bearer control, and ciphering and integrity protection of NAS signaling.
The present invention is related to handovers in LTE, although the solution of the present invention may also be applicable to present and future systems other than LTE. Because the physical layer cannot accommodate all possible service data unit (SDU) sizes, SDUs have to be segmented before transmission over the radio link.
The main problem is how to ensure a lossless handover (HO) when SDUs are segmented in the base station (BS). According to related art, in order to ensure a lossless handover, it has been proposed to introduce some control messages, for example related to medium access control (MAC) automatic repeat request (ARQ) and MAC segmentation, in order to facilitate communication between the source and target BS. For instance, the target BS would be given information about the correctly received segments and the missing ones.
The problems of such a prior art solution are at least twofold. First, if the PDU size on which the segmentation is based is not fixed, it may not be possible to retransmit the exact same missing segments at the target BS. Re-segmentation will be required. Second, this prior art solution increases not only traffic in the network but also the complexity of the system. MAC entities in different BSs need to exchange control messages.
From a user/application viewpoint, performing handover during the MAC SDU (IP packet) transmission just introduces the handover processing latency to the SDU delivery if the HO can wait for a while. If there are intervals between IP packet arrivals, the impact becomes clear.
According to an embodiment of the present invention, the handover (HO) is limited at the service data unit (SDU) boundary. For handovers in LTE, an exemplary embodiment of the invention provides for segmentation to take place at the base station. In order to meet the established LTE requirements in terms of latency and data rate, the present invention provides that such a segmentation would take place right before radio transmission, in the base station (BS), as opposed to in a central node as is the case in the pre-LTE UTRAN (segmentation at RLC layer in RNC). Therefore, SDUs (e.g. IP packets) would be segmented at the medium access control (MAC) layer or radio link control (RLC) layer in the base station before transmission over the radio link.
A primary improvement here is that a lossless handover is ensured without segment forwarding or HARQ/ARK status information exchange between the source base station and the target base station. This invention has the advantage of being a simple system which does not increase traffic. Also, from a user/application point of view, delay introduced by a handover (HO) can be reduced.
In the downlink, segmentation of a new SDU is stopped when a handover decision is made, and the base station waits for all pending segments to be transmitted before issuing the handover command to the user equipment (UE). In the uplink, segmentation of a new SDU is stopped when the handover command is received and the UE waits for all pending segments to be correctly received by the source BS before executing the HO command and moving to the target BS. According to an exemplary embodiment of this invention, SDU segmentation takes place right before radio transmission, in the base station (e.g. in the eNB).
A preferred embodiment of the present invention will now be described. This is merely to illustrate one way of implementing the invention, without limiting the scope or coverage of what is described elsewhere in this application.
As mentioned, the invention includes two principles, for the downlink (DL) and uplink (UL) respectively. First, in the downlink, the segmentation of a new SDU is stopped when a HO decision is taken and the BS waits for all pending segments to be transmitted before issuing the HO command to the UE. Second, in the uplink, the segmentation of a new SDU is stopped when the HO command is received and the UE waits for all pending segments to be correctly received by the source BS before executing the HO command and moving to the target BS. The second principle is exemplified by the UE in
An illustrative message flow is shown in
Depending on the urgency of the HO, the network and the UE may decide not to apply these two principles just described. When they decide not to apply their own principle, the whole SDU would be normally retransmitted at the target BS. Note that a second, less-preferred option is this: If MAC or RLC (i.e. MAC/RLC) segment retransmission is supported, it is possible that the source BS delivers the MAC/RLC ARQ and segmentation information to the target BS, and the target BS transmits the MAC/RLC segment as the same way as in the prior art.
In any event, at least in the acknowledged mode, all MAC/RLC SDUs remaining in the buffers are tunnelled/transferred from the source BS to the target BS. This is because all acknowledged MAC/RLC SDUs are already removed from the buffer, and all other SDUs will require transmission or may require retransmission at the target BS.
Alternatively a threshold could be configured to limit the two principles described above to the cases where only a given percentage or number of segments are missing.
Of course, a person skilled in the art will understand that the UL and DL principles may not be applied together. For instance, only the DL part could be used.
The advantages of this present invention include the fact that it is a relatively simple system. It does not increase the traffic, and from the user/application viewpoint, the delay introduced by HO can be reduced.
As seen in the embodiment shown in
Turning now to
For the case where the apparatus 600 is located at the network side, the present invention includes a method of handover from the source base station to a target base station, which comprises segmenting a plurality of service data units in the source base station, at a medium access control layer, transmitting segments produced by the segmenting step, and issuing a handover command to a user device after the segments are transmitted. This exemplary embodiment of the method also includes stopping the segmenting when a handover decision is taken (i.e. made). After stopping the segmenting, transmission of the pending segments is completed, and then the handover command is issued. A threshold may be configured that limits the method to cases in which only a given percentage or number of segments are missing.
The embodiments described above can be implemented using a general purpose or specific-use computer system, with standard operating system software conforming to the method described herein. The software is designed to drive the operation of the particular hardware of the system, and will be compatible with other system components and I/O controllers. The computer system of this embodiment includes a CPU processor comprising a single processing unit, multiple processing units capable of parallel operation, or the CPU can be distributed across one or more processing units in one or more locations, e.g., on a client and server. A memory may comprise any known type of data storage and/or transmission media, including magnetic media, optical media, random access memory (RAM), read-only memory (ROM), a data cache, a data object, etc. Moreover, similar to the CPU, the memory may reside at a single physical location, comprising one or more types of data storage, or be distributed across a plurality of physical systems in various forms. In the context of
It is to be understood that the present figures, and the accompanying narrative discussions of best mode embodiments, do not purport to be completely rigorous treatments of the method, system, mobile device, and software product under consideration. A person skilled in the art will understand that the steps and signals of the present application represent general cause-and-effect relationships that do not exclude intermediate interactions of various types, and will further understand that the various steps and structures described in this application can be implemented by a variety of different sequences and configurations, using various different combinations of hardware and software which need not be further detailed herein. Of course, as mentioned above, the solution of the present invention may also be applicable to present and future systems other than LTE.
The present invention is based upon and claims priority to Provisional U.S. patent application Ser. No. 60/756,118 titled “Boundary for Handover” which was filed on Jan. 3, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60756118 | Jan 2006 | US |
