WO2013040749A1

WO2013040749A1 - Method for transmission of control signals in a wireless communication system

Info

Publication number: WO2013040749A1
Application number: PCT/CN2011/079852
Authority: WO
Inventors: George Koudouridis; Peter Legg; Gunnar Hedby; Henrik Lundqvist; Tomas Edler
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2011-09-20
Filing date: 2011-09-20
Publication date: 2013-03-28
Also published as: CN103782651A

Abstract

The present invention relates to a method in a wireless communication system for transmitting control signals, said wireless communication system employing at least one first radio access technology (RAT1) and at least one second radio access technology (RAT2), said first (RAT1) and second radio access technologies (RAT2) using radio access technology (RAT) specific control signals for controlling transmission to or from mobile stations (MSs); said method comprising the step of: transmitting control signals belonging to said first radio access technology (RAT1) using said second radio access technology (RAT2). Furthermore, the invention also relates to a computer program, a computer program product, a mobile station device, and a wireless communication system.

Description

METHOD FOR TRANSMISSION OF CONTROL SIGNALS IN A WIRELESS

COMMUNICATION SYSTEM

Technical Field

The present invention relates to a method for transmission of control signals in a wireless communication system. Furthermore, the invention also relates to a computer program, a computer program product, a mobile station device, and a wireless communication system.

Background of the Invention

Energy saving is one of the key requirements in future wireless communication networks. A typical need is to meet expected Quality of Service (QoS) of data delivery whilst minimising the energy consumption of radio network equipment. Energy may be saved by a number of different techniques, including different sleep modes, e.g. in which Base Station (BS) hardware is deactivated, and by switching off a cell completely.

A step towards this involves the switching off a radio cell in which there are no active mobile users. When there is only a single layer of cells, where by layer it is meant cells of one Radio Access Technology (RAT) on one frequency; when a cell is switched off this usually introduces a radio coverage hole into the network. When a User Equipment (UE) is switched on within a coverage hole it is unable to make a call. This problem can be circumvented by deploying a network with more than one layer, and ensuring that at every geographical location there is at least one layer providing coverage at the moment.

For example, a network could comprise a GSM layer at 900 MHz, and a LTE layer at 2600 MHz. If the GSM layer provides universal coverage it is safe to switch off LTE cells - if a UE connects to GSM in a location with no LTE coverage, the management system can determine to re-energise an LTE cell to provide service to the UE.

Drawbacks of this approach are the complexity of the interaction between separate RATs and the delay incurred. The delay is large because the UE must undertake an inter-RAT handover (in the above example it would be from GSM to LTE), in addition to any delays in the signalling and decision taking in the two different RATs. The coverage hole issue in a single layer network has been addressed in prior art. It has been considered in the context of Heterogeneous Networks (HetNets) consisting of a mixture of macro cells and low power nodes. One proposal is called Beyond Cellular Green (BCG) which employs a cell with wide area coverage to provide signalling connectivity (only) to a UE - this is called a Signalling Base Station (SBS). Other cells provide data only connections, and are called Data Base Station (DBS). Hence, a UE will maintain two radio links, one to a SBS and one to a DBS. When a UE initiates a call, it signals to the SBS, and then if necessary a DBS is switched on to provide data coverage to the UE. Advantages of this approach are that there are no signalling coverage holes and there is no reliance upon another layer to achieve this. Bearer establishment is consequently more rapid.

However, the above proposed solution requires new hardware in the network (e.g. in the base station) and in the handsets. Additionally, there are many operational aspects of the BCG architecture which have yet to be fully evaluated, such as interference between DBS transmissions to SBS transmissions (and vice versa), and handoff from one SBS to another SBS. Furthermore, another important issue to mobile operators is how to leverage the multiple radio networks that they have rolled out. Many have many different RATs, such as GSM, HSPA and LTE, coexisting in the same geographical area. Each individual RAT represents a large capital investment and will have taken many years to tune and optimise. Therefore, the addition of another RAT is not an attractive proposition to the operators. Furthermore, legacy networks such as GSM must be maintained in operation to support older handsets that are not capable of connecting to the later technologies, such as UMTS or LTE.

Summary of the Invention

The object of the present invention is to provide a method which mitigates and/or solves the disadvantages of prior art solutions, and more specifically to a method which facilitates integration of different radio access technologies in the same wireless communication system. Another object of the invention is to provide a solution which facilitates integration of different radio access technologies in the same wireless communication system without costly investment in new communication devices for the mobile operators. Yet another object of the invention is to provide energy saving mechanisms for wireless communication systems employing multiple different radio access technologies.

According to an aspect of the invention, the objects are achieved with a method in a wireless communication system for transmitting control signals, said wireless communication system employing at least one first radio access technology (RATI) and at least one second radio access technology (RAT2), said first (RATI) and second radio access technologies (RAT2) using radio access technology (RAT) specific control signals for controlling transmission to or from mobile stations (MSs); said method comprising the step of: transmitting control signals belonging to said first radio access technology (RATI) using said second radio access technology (RAT2).

The invention also relates to a computer program and a computer program product adapted to execute any of the methods according to the present invention. According to another aspect of the invention, the objects are achieved with a wireless communication system adapted for employing at least one first radio access technology and at least one second radio access technology, said first and second radio access technologies using control signals for controlling transmission to or from mobile stations, said control signals being specific for each radio access technology; said wireless communication system comprising means for transmitting control signals belonging to said first radio access technology using said second radio access technology.

According to yet another aspect of the invention, the objects are achieved with a mobile station device adapted for communicating in a wireless communication system, said wireless communication system being adapted for employing at least one first radio access technology and at least one second radio access technology, said first and second radio access technologies using radio access technology specific control signals for controlling transmission to or from mobile stations; said mobile station device comprising communication means for receiving and processing control signals belonging to said first radio access technology, wherein said control signals are sent using transmissions to or from said mobile station device using said second radio access technology. This present invention achieves many of the benefits of the mentioned BCG system without needing new base station hardware - existing base stations may be re-used with only software modifications.

Additionally, it provides a rapid bearer establishment. Thus the invention provides a mechanism to save energy by allowing base stations to be switched off unless they are required to provide data delivery to a MS. For example, these could be UMTS or LTE nodes.

The wide area signalling connectivity of a traditional network can be provided by another legacy network (for example, GSM), and this allows the data delivery base stations (for example, UMTS or LTE) to be switched off. Additionally, it provides a rapid bearer establishment since the mobile station is able to directly use the signalling messages of the

RAT to which it would like to connect (for example, signalling messages of UMTS or LTE).

New mobile station handsets are required if the full benefits is to be achieved however in this respect. The invention describes the means for an older RAT to actively assist a new RAT by providing transportation mechanisms for signalling information. This means that fewer LTE/BCG nodes need to be deployed in such systems. Since the introduction of BCG requires new handsets anyway, the additional functionality at the handset to support the signalling transfer over a legacy network, for example GSM, can be included.

Further applications and advantageous of the invention will be apparent from the following detailed description.

Brief Description of the Drawings

The appended drawings are intended to clarify and explain different embodiments of the present invention in which:

Figure 1 illustrates a conventional RAT architecture for GSM (2G), HSPA (3G) and LTE (4G); Figure 2 illustrates an embodiment of RAT architecture according to the present invention;

Figure 3 illustrates an example of UE implementation: control signalling delivered over user plane of UMTS;

- Figure 4 illustrates an example of UE implementation: control signalling delivered over control plane of UMTS; and

Figure 5 illustrates an embodiment demonstrating energy saving aspects of the invention. Detailed Description of the Invention

From the above description of prior art and the drawbacks related to mentioned prior art, the present invention therefore relates to a method for transmission of control signals in wireless communication systems so as to mitigate or solve the above mentioned drawbacks. To achieve the above mentioned objects, the inventors have realised that the transmission of control signal should be performed in such a way that control signals belonging to a first RAT is transmitted by a second RAT. The first and second RATs being different RATs and both being employed in the same wireless communication system. This general idea can be extended so that the system comprises a number of different RATs and the control signals of one RAT is transported via one or more other RATs in the system to Mobile Stations (MS). Examples of RATs that may be considered without limiting the scope of the invention are: GSM, CDMA, UMTS, LTE, LTE-Advanced, WiMax and WiFi. The present method realises dynamic signalling assignments among different RATs so that the transport of signalling traffic can be shared dynamically between them. The control signalling sharing/assignment can be done based on the coverage area of each RAT, the traffic load, the application layer services in use in the MSs and the service demands, RAT-related energy efficiency, UE capabilities, operation failures like node failure and/or halted operation, etc.

Thereby, energy may be saved by enabling the deployment of a radio network of a data- centric RAT in which base stations are only switched on when there is a MS that needs to be served with data. This is possible because a second network of another different RAT covering wide/large areas can provide the wide-area signalling connectivity that enables a MS to receive or make a call anywhere within the wide-area. Furthermore, the MS is able to employ the same signalling messages as those used within the data centric RAT, but these messages are transferred to the data-centric RAT using the wide-area RAT. This results in rapid call setup directly to the data-centric RAT. An example of a data-centric network could be UMTS, whilst the wide-area RAT could be GSM. The wide-area RAT can at the same time support MSs that use its control and data plane in a conventional manner. These could e.g. be legacy GSM only terminals.

Therefore, at least one of the RATs in the system should support large area coverage cells or a large area coverage using multiple cells, according to an embodiment of the invention. Generally, large area coverage cells are called macro cells. Mega cells provided by satellite systems are even larger. Large area coverage (preferably, both indoor and outdoor) may be achieved by deploying a combination of macro-, micro-, pico- and femto-cells to cover whole cities or city districts.

Further, if a set of radio access networks is considered using multiple RATs, e.g., GSM, UMTS, LTE, etc, each one with each own control plane and data plane implementation. In some RATs, e.g. GSM, UMTS, and LTE, the control plane from the user plane are clearly distinguished in terms of different protocol stacks. The control plane stack is used to carry both Radio Resource Control (RRC) messages and Non-Access Stratum (NAS) messages. The user plane stack is used for user data transmission. The control plane is used for RAT specific signalling. This means that the signalling is concerned with radio connection of the UE (including mobility) within a specific RAT and the management of the bearer(s) over that specific RAT.

Signalling in current wireless communication system generally takes the following forms: • Broadcast signalling - this (downlink only) signalling carries system parameters for the cell from which it is sent, and is read by UEs in idle and active mode. Examples are paging intervals, mobility parameters for idle mode, base station transmit power, parameters governing random access; • Dedicated signalling - this is bidirectional signalling to individual UEs. It is employed with UEs in active mode (or those transitioning to active mode). Radio Resource Control (RRC) signalling is concerned directly with the radio interface. Examples are for radio resource connection establishment, active mode handover, reconfiguration of UE settings in active mode, bearer establishment/reconfiguration. Non-access Stratum

(NAS) signalling takes place between the UE and the core network, and runs over the RRC protocol - this means that RRC carries NAS messages. NAS signalling is used for session management and mobility management. Mobility management is concerned with user authentication and IP connectivity establishment. Session management is concerned with bearer establishment.

Hence, according to another embodiment of the invention the different RATs uses a control plane protocol stack and a user plane protocol stack specific for each RAT, and control signals belonging to one RAT are transferred by means of a control plane protocol stack or a user plane protocol stack belonging to another RAT.

According to yet another embodiment, the control signals are dedicated to individual MSs, and preferably any of NAS or RRC messages. It should also be noted from the above description that the control signals may be related to any network aspects in the group comprising: radio resource connection establishment, active mode handover, reconfiguration of a MS in active mode, session management, mobility management, and bearer establishment/reconfiguration.

Regarding signalling traffic this can be categorised in terms of content and transport. In terms of content, the signalling under consideration is RAT- specific control signalling traffic. In terms of transport, the signalling can be divided into: signalling traffic carried by the control plane of the originating RAT (as in the prior and conventional art) ; or signalling traffic typically carried by the data plane and/or control plane of another different RAT.

Separating content from the transport allows for the implementation of:

• A set of radio access networks using multiple RATs, wherein the RAT-specific signalling traffic of one RAT can be carried by the data plane of another RAT; and • A set of radio access networks using multiple RATs, wherein the RAT-specific signalling traffic of one RAT can be carried by the control plane of another RAT.

The utilisation of the above mechanism can among other things eases the migration from past radio generations to future radio generations by facilitating control signalling over any RAT used in the communication system.

The present method allows efficient service assignment and distribution among RATs e.g. GSM could stand for all GSM/UMTS signalling, while UMTS can be used to serve voice and data traffic. Further, it allows energy savings by powering off nodes within a RAT e.g., GSM can be used for UMTS signalling so that UMTS nodes may be switched off. Another advantage is that it enables resilient operation where signalling survives network and/or node failures. It should be noted that the terms user plane and data plane are used interchangeably and are used to refer to the same radio plane implementation which carries user traffic of any type e.g., voice, data, video, etc. Further, the use of the term mobile station refers to any mobile communication device adapted for communicating in wireless communication systems, e.g. a UE.

Figure 1 illustrates a set of RAT architectures according to prior art. From left to right there are 2G (GSM), 3G (UMTS) and 4G (LTE) RATs. The control plane of each RAT is used to transfer radio control signalling (RRC) to/from the BSC/RNC/eNB and non-access stratum signalling to/from the Core Network (CN). This signalling is RAT-specific, as explained above.

Figure 2 on the other hand illustrates an embodiment of the present invention. In this case the HSPA (UMTS) signalling is sent over the GSM user plane. The transfer of signalling occurs between the RNC and the BSC, although other points of transfer could be envisaged, such as between base stations or between core network nodes. Control signalling for the GSM RAT is not shown in figure 2 for clarity. However, it uses the GSM control plane as in previous figure 1. In figure 2 HSPA NAS signalling and RRC signalling is delivered over the GSM user plane. Figures 3 and 4 schematically show how a UE can be implemented to work with the above described RAT architecture. In the first case, i.e. figure 3, control signalling is delivered over/via the user plane of UMTS and the data is sent over the user plane of LTE. In the second case, i.e. figure 4, control signalling is delivered over the control plane of UMTS. In both cases, the control plane of LTE is not used below the RRC protocol, instead signalling messages are received on UMTS and then passed over to the RRC instance in the LTE part of the handset. NAS messages are sent further by the RRC instance of the LTE part of the handset, as indicated in the figures. It shall be understood that in the reverse direction (i.e. the uplink) the reverse paths are followed.

Figure 5 illustrates an embodiment demonstrating energy saving aspects of the invention. A UMTS network is deployed using base stations (e.g. Node Bs or NBs) that can operate without providing signalling connectivity. Indeed NBs are only switched on when there is a need to deliver data to a MS. This gives substantial energy savings in the UMTS network. The wide-area signalling connectivity is in this example offered by a GSM network. When a MS need to make a call on the UMTS network, it can initiate the call by sending the signalling to the UMTS network over the GSM network. The MS uses standard UMTS signalling messages but these are tunnelled over the GSM network to the UMTS network nodes, e.g. to the RNC. By tunnelling it is meant that the signalling messages are passed transparently, i.e. without reading their contents, over the GSM network to the UMTS network. If the UMTS network recognizes that the MS lies in a hole of the UMTS data coverage it can then energise a NB to serve the MS. Thereafter data is delivered to the MS over the UMTS network, whilst signalling can proceed over the GSM or over the UMTS networks in a conventional fashion now that a NB to serve the MS is switched on.

In figure 5 it is assumed that:

• NB that lies nearest to the UE is in dormant mode (powered off), and there is no UMTS coverage from other more distant NBs;

• The UE has adequate coverage from the GSM network;

· The UMTS network is Packet Switched (PS), with a SGSN connected to the RNC

The scenario in figure 5 is as follows: • A UE wishes to establish a radio bearer in the UMTS network. For example, this could be a video call that is not supported by the GSM network.

The procedure in figure 5 is as follows:

· The UE enters into active mode and initiates the establishment of a radio bearer over

UMTS (1). Since no UMTS cell is selected, the UE establishes a radio bearer via its GSM connection to send the UMTS signalling messages. The UMTS signalling messages are sent (2) over the user plane between UE and BS, via the base station controller (BSC) (3), all the way (3)-(8) to the UMTS radio network controller (RNC) by following one of the two illustrated paths (3,4,5,6) or (3,7) illustration (a). The

RNC acts upon the signalling message (for an initial "Attach" message it would forward this to the SGSN), and wakes up the NB (8). The RNC sends its response to the UE either by following reversibly (8,7, 3,2)/(8,6, 5,4,3,2) the same path via the user plane or by activating the control plane of the UMTS air interface (9), illustration (b).

The present invention also relates to a wireless communication system corresponding to the methods according to the invention and may therefore be modified in accordance with the different embodiments of the present methods. The system is arranged such that control signals belonging to a first RAT are transmitted using a second different RAT. This can be achieved by means of communication mechanisms. These may include but are not limited to message transfer between BSC nodes of different RATs, between base stations of different RATs, between core network nodes of different RATs, message encapsulation and transfer using the existing user or control plane of a second RAT, and message interception and rerouting to another RAT.

With reference to fig. 2, wireless communication systems according to the invention may comprises one or more Base Station Controllers (BSCs) specific for each RAT, and the transmission of control signals to MSs is performed from a BSC of a first RAT via a BSC of at least one second different RAT. Since the BSC terminates the Radio Resource Control (RRC) protocol, this linkage provides the shortest communications path between RRC entities, thereby minimising latency, complexity and loading on the transport network layer. Also in this case, the concept can be extended so that control signals are transmitted via more than one different RAT to MSs in the system.

The invention further relates to a mobile station device having communication means for receiving and processing control signals belonging to a first RATI. The control signals are sent using transmissions to or from the mobile station device using a second different RAT2.

The MS device needs to have the capability to transmit and receive on two different RATs at the same time. It also needs to be capable/adapted of intercepting signalling messages sent on a second RAT and passing these to the control stack of a first different RAT (e.g. to the RRC of the first RAT). This is illustrated in Figures 3 and 4. Additionally, the MS device may also be capable of operating in a conventional manner, such as signalling and data delivered over the first RATI. When the MS device connects to the network, it may check whether the signalling interface is available for the first RAT. If the signalling interface is available the MS device proceeds as usual. Otherwise, the MS device determines which RAT (in this example, the second RAT) is set to carry the signalling. The MS device would then proceed by establishing a radio bearer on the second RAT to deliver the signalling content applicable to the first RAT. Furthermore, as understood by the person skilled in the art, any method according to the present invention may also be implemented in a computer program, having code means, which when run in a computer causes the computer to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product. The computer readable medium may consist of essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.

Finally, it should be understood that the present invention is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims.

Claims

1. Method in a wireless communication system for transmitting control signals, said wireless communication system employing at least one first radio access technology (RATI) and at least one second radio access technology (RAT2), said first (RATI) and second radio access technologies (RAT2) using radio access technology (RAT) specific control signals for controlling transmission to or from mobile stations (MSs); said method being characterised by the step of:

- transmitting control signals belonging to said first radio access technology (RATI) using said second radio access technology (RAT2).

2. Method according to claim 1, wherein said second radio access technology (RAT2) is adapted for using a control plane protocol stack and a user plane protocol stack specific for each radio access technology (RAT); and

- said control signals are transmitted by means of said control plane protocol stack or said user plane protocol stack belonging to said second radio access technology (RAT2).

3. Method according to claim 2, wherein said wireless communication system further comprises one or more base station controllers (BSCs) specific for each radio access technology (RAT); and said step of transmitting involves:

- transferring said control signals from a first base station controller (BSC1) belonging to said first radio access technology (RATI) via a second base station controller (BSC2) belonging to said second radio access technology (RAT2).

4. Method according to claim 3, wherein said control signals are dedicated to individual mobile stations (MSs).

5. Method according to claim 4, wherein said control signals are non-access stratum (NAS) or radio resource control (RRC) messages.

6. Method according to claim 5, wherein said control signals are related to any in the group comprising: radio resource connection establishment, active mode handover, reconfiguration of a mobile station (MS) in active mode, session management, mobility management, and bearer establishment/reconfiguration.

7. Method according to claim 1, wherein said first and second radio access technologies (RATs) are any in the group comprising: GSM, UMTS, LTE, LTE-Advanced, WiMax and

WiFi.

8. Method according to claim 7, wherein at least one of said first (RATI) or second radio access technologies (RAT2) supports large area coverage cells or a large area coverage using multiple cells.

9. Method according to claim 1, wherein said control signals are transmitted based on one or more aspects of said wireless communication system such as: coverage areas of said first (RATI) and second radio access technologies (RAT2), traffic load, services provided, service demands, energy saving, communication node capabilities, communication node failure, and communication node status.

10. Computer program, characterised in code means, which when run in a computer causes said computer to execute said method according to any of claims 1-9.

11. Computer program product comprising a computer readable medium and a computer program according to claim 10, wherein said computer program is included in the computer readable medium.

12. Mobile station device adapted for communicating in a wireless communication system, said wireless communication system being adapted for employing at least one first radio access technology (RATI) and at least one second radio access technology (RAT2), said first (RATI) and second radio access technologies (RAT2) using radio access technology (RAT) specific control signals for controlling transmission to or from mobile stations (MSs) ; said mobile station device being characterised in that comprising communication means for receiving and processing control signals belonging to said first radio access technology (RATI), wherein said control signals are sent using transmissions to or from said mobile station device using said second radio access technology (RAT2).

13. Wireless communication system adapted for employing at least one first radio access technology (RATI) and at least one second radio access technology (RAT2), said first (RATI) and second radio access technologies (RAT2) using radio access technology (RAT) specific control signals for controlling transmission to or from mobile stations (MSs); said wireless communication system being characterised in that comprising communication mechanisms for transmitting control signals belonging to said first radio access technology (RATI) using said second radio access technology (RAT2).

14. Wireless communication system according to claim 13, comprising at least one mobile station device according to claim 12.