GB2465031A

GB2465031A - Notifying a subscriber station whether it should communicate with a target base station using a new protocol

Info

Publication number: GB2465031A
Application number: GB0820639A
Authority: GB
Inventors: Yanling Lu; Kevin Power; Rajni Agarwal; Masato Okuda
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 2008-11-11
Filing date: 2008-11-11
Publication date: 2010-05-12
Anticipated expiration: 2028-11-11
Also published as: GB0820639D0; GB2465031B

Abstract

A method of facilitating handover of a dual-mode subscriber station, which can communicate using a first and a second protocol, from a serving base station to a target base station in a wireless communications system, e.g. IEEE 802.16 (WiMAX). The serving base station is operable to communicate with the subscriber station using the first protocol, e.g. IEEE 802.16e, and the target base station is operable to communicate with the subscriber station using both the first protocol and the second protocol, e.g. IEEE 802.16e and IEEE 802.16m, one of the two protocols being selected for use with the subscriber station at any one time. The method further comprises, during the handover and before ranging, the subscriber station receiving a notification indicating whether the subscriber station should communicate with the target base station using the second protocol. Hence, the invention aims to provide backwards compatibility for legacy subscriber stations whilst providing the enhanced features of new protocols for new subscriber stations.

Description

WIRELESS COMMUNICATIONS SYSTEM

Introduction

The present invention relates to procedures during handover of a subscriber station (or mobile station) from a serving base station to a target base station. More particularly, the present invention relates to the situation in which there are two protocols currently in use in a wireless communications system and to handover procedures in such circumstances.

As technologies such as IEEE 802.16 evolve, backwards compatibility becomes an important issue for operators as they seek to maintain their current customer base whilst also providing newly developed terminals and devices which support enhanced features and protocols. The current 802.16m project is no different, where one of the key design requirements is full backwards compatibility with the legacy IEEE 802.16e system[1J. From a network point of view, a certain deployment may consist of both 16m and 16e (or 16j) BSs (base stations) which may complicate mobility related issues such as handover. In this deployment scenario, the 16e (or 16j) BSs will support only 16e (16j) protocols whereas the 16m BSs may support, for example, both 16e (16j) and 16m protocols. As the 16m BS can support two modes of operation, then with respect to the frame structure of such a BS, a zone used to support legacy terminals is TDM with a zone for the new 16m MSs (see figure 1), or with a window if the whole symbol is not necessarily taken up with this signalling. In the following text, this type of 16m BS is referred to as a 16m "mixed mode" BS with legacy support. In such a deployment, there will also be a mix of MSs (mobile stations), both 16e and 16m (or 16j and 16m). The new 16m MSs will support both modes of operation (16e and 16m) but not simultaneously, therefore, throughout the remaining text, this type of MS will be referred to as a "dual-mode" MS (it can only operate in one mode at any given time).

The same cases (and the remainder of this text) apply accordingly to a 16m/16j situation and no further reference is made specifically to the 16j legacy equipment.

It is clear that the above mentioned deployment complicates many operating mechanisms of the system. From the dual mode MS perspective, handover between base stations is one of these mechanisms that must be addressed carefully. Shown below are 9 possible handover cases with respect to the serving and target BS type.

These cases are also demonstrated in Figure 2.

Serving BS Target BS (1) 16m without legacy support -16m without legacy support (2) 16m without legacy support -16m with legacy support (3) 16m without legacy support 3 Legacy 16e (4) 16m with legacy support 3 16m without legacy support (5) 16m with legacy support 3 16m with legacy support (6) 16m with legacy support 3 Legacy 16e (7) Legacy 16e 3 16m without legacy support (8) Legacy 16e 3 16m with legacy support (9) Legacy 16e 3 Legacy 16e In Figure 2, the first six cases can be considered as a 16m Cell Initiated Handover. For the last three cases they can be referred to as a 16e Cell Initiated Handover.

For the case in which handover is of a subscriber station from a legacy base station, such as a WiMAX 16e base station, there is some increased latency associated with handover to a base station using a different protocol (for example WiMAX 16m) even if that target base station can also support the first or legacy protocol.

This can lead to delays in handover as well as more complex signalling requirements.

Embodiments of the present invention aim to overcome or at least mitigate some of the difficulties in handover between base stations, at least one of which is uses a first protocol for communications and the other of which selectably uses the first protocol and a second protocol. * 3

The invention is defined in the independent claims, to which reference should now be made. Advantageous embodiments are set out in the sub claims. Features of different aspects are freely combinable.

The wireless communications system may comprise any collection of nodes or communication apparatuses which are capable of communicating wirelessly with one another, for example by forming a wireless network. For example, the wireless communications system may comprise a WIMAX network, based on the IEEE 802.16 standards. In another example, the wireless communications system may comprise a telecommunications network, for example based on the standards developed by 3GPP or 3GPP2.

The base station may comprise any apparatus which serves directly or indirectly to control access by other stations to network resources, which may include, for example, permitting access to the resources, allocating bandwidth, monitoring connections, and soon.

The subscriber station (SS) may comprise any apparatus which is used by a user to access network resources. The subscriber station may or may not be portable, and may be intended for location indoors or outdoors. Examples of subscriber stations may include handsets, mobile telephones, smartphones, PC peripherals or components, consumer electronics devices, game terminals and mp3 players. The subscriber station may comprise a mobile station (MS) or user equipment, for example. It is to be understood that the terms SS and MS are used interchangeably, and that the term MS when used does not limit the invention to stations which are mobile.

A wireless protocol may comprise a set of rules governing communications between the stations in the wireless communications system, and may be referred to as an air interface and may conform to a standard. The first wireless protocol may be a first air interface and the second wireless protocol may be a second air interface. The first and second wireless protocols may conform respectively to first and second (sets of) standards, e.g. new and legacy standards. In one arrangement, the first wireless protocol conforms to the 16e (legacy) standard, and the second wireless protocol conforms to the 16m (new) standards. A station which is operab'e to communicate using a new standard may also be operable to communicate using a legacy standard, whereas another station may be operable to communicate using only the legacy standard as set out in more detail herein.

Detailed description

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:-Figure 1 * an example of 16m frame structure with legacy support; Figure 2 * handover scenarios with and without legacy support, Figure 3 * legacy Handover process, Figure 4 * The MS's Behaviour flow chart for HO scenario 8 of Figure 2, Figure 5 * high-level flow for a general MS Behaviour during HO Scenario #8 of Figure 2, and Figure 6 * an example of network re-entry of 16e Cell Initiated Handover (For HO scenario #8 of Figure 2).

Regarding the previously mentioned Handover (HO) issues with such a deployment scenario, prior to the MS switching to the target BS from its current serving BS, it can obtain the required information (from Target BS) to perform the initial steps of the HO procedure. In WIMAX such information may include (but is not restricted to): * Network topology information, such as neighbour BSID (base station identification), * Channel information(Fragmentation index, Frequency Assignment Index, Preamble Index) * HO (handover) optimization information (By which the MS can omit some sub procedure of HO re-entry), * System configuration information (Superframe Header (SFH)-similar to DCD/UCD in 16e)) Similar information is provided under different systems.

The above information may be transmitted through a broadcast messages such as, for example, MOB-NBR_ADV. More importantly, the target BS MAC version which may be included within the system configuration information is crucial for performing HO in such a mixed deployment Upon receipt of this MAC message which includes the MAC version of the target BS, in the 16e system, the MS would then perform the HO process, which consists of the following procedures (see also Figure 3) 1. Cell reselection: the MS may use neighbour BS information(channel information, system configuration information and so on) acquired from a decoded MOB_NBR-ADV message or similar type message and may make a request to schedule scanning intervals or sleep intervals to scan, and possibly range neighbour BSs for the purpose of evaluating MS interest in HO to a potential target BS.

2. HO Decision and Initiation: A HO begins with a decision for an MS to HO from a serving BS to a target BS. The decision may originate either at the MS or the serving BS by transmitting HO request from MS and BS respectively. On reception of MS HO request message, the BS should respond it with a response message. Therefore, the message transmitted by BS, as a response in the MS initiation case or a request in the BS initiation case, should include the recommended HO target BSs accompanied with their neighbour BSID, Preamble index/Subchannel, HO process optimization and so on.

3. Synchronization to target BS and obtain DLJUL parameters: The MS shall synchronize to the DL transmissions of the Target BS and obtain DL and UL transmission parameters. If the MS had previously received a MOB_NBR-ADV message including Target BSID, Frequency Assignment and Superframe Header contents then this process may be shortened.

4. Ranging, UL parameters adjustment and network re-entry: MS and target BS shall conduct initial ranging or HO ranging. In the ranging procedure, the MS's UL parameters (Time, frequency and power offset) are adjusted to match the UL channel characteristics. Based on the HO process optimization obtained in HO Initiation phase/ MOB-NBR_ADV, the following network re-entry phase could be omitted in some sub-phase, for instance: authorization and registration. Finally, the IP connectivity should be re-established.

5. Termination of MS Context: The final step in HO. Termination of MS Context is defined as serving BS termination of context of all connections belonging to the MS and the context associated with them (i.e., information in queues, ARQ state machine, counters, timers, header suppression information, etc., are discarded).

Please refer to P802.16 Rev2ID5 in particular Figure 147.

In a mixed deployment, when a dual mode 16m MS performs a handoff (or handover) to a 16m mixed mode BS (with legacy support), as mentioned before, it could be served in the legacy zone (operating in iGe or 16j mode) or in the new zone (operating in 16m mode). Based on some criteria such as resource management or load balancing the new BS may decide which zone the MS shall be served in. For example, if in the target BS, the 16m zone is fully occupied then the mixed mode BS could inform the incoming MS to operate in the legacy 16e mode, that is: be served in the legacy zone.

As mentioned previously, the HO can take place as a 16m Cell Initiated HO or 16e Cell Initiated HO. When HO takes place as a 16m Cell Initiated HO, within the serving BS (16m BS), new signalling can be defined to inform the MS of the target BS related information, in order to facilitate the HO procedure.

However, when the HO takes place as a 16e Cell Initiated HO (Scenario 7, 8 & 9 in Figure 2), the messages of the legacy system may be difficult to modify. Consequently, some channel and system configuration information of neighbour 16m BSs may not be obtained from the legacy system. However, it is important to note, it may be possible to include the 16m BS features within the legacy message, but this will totally depend on the capabilities of the 16e BS. On the other hand, it should most certainly be possible to acquire the MAC version of the target BS through the legacy message, but in order to reduce the latency associated with the HO procedure, then new signalling mechanisms and procedures are required for supporting dual mode MSs in a mixed deployment of 16e BSs and 16m mixed mode BSs.

In the HO scenario 8 in Figure 2, from the dual mode MS perspective (operating in 16e mode), it will reside in the initial HO zone (16e) of the mixed mode target BS and the MS will then acquire DL related control information (DL-MAP and DCD). It will also acquire information related to the UL channel (UL-MAP, UCD) and thus continue network re-entry as per the protocols defined in IEEE 802.16e as shown in Figure 3. At this stage (when HO and network re-entry is complete), for the purposes of data transfer, the BS may decide to switch the dual mode MS from the legacy zone to the 16m zone and consequently change its mode of operation. This procedure may be inefficient and will introduce an unnecessary latency as a result of the legacy IEEE 802.16e network re-entry protocols. This problem may be solved, for instance, if the mixed mode BS has prior knowledge that it will switch the MS from the legacy to the 16m zone. In this case, it may be possible for the MS to perform DL synchronisation in 16e mode and then perform the remaining network re-entry procedure in 16m mode thus reducing the overall latency related to handover and network re-entry. The reason for this reduced latency is that we assume network re-entry in 16m mode will be significantly reduced when compared to that of 16e (as per the 16m system requirements).

Note, the MSs mentioned in the following text are 16m dual mode MSs except when explicitly indicated as 16e MSs.

Embodiments of this invention solve or at least mitigate the problems related to the HO procedure when handoff is from a single protocol (or legacy) RS to a BS which has the capability of using two protocols. Such a situation occurs, for example, when the dual mode MS (operating in 16e mode) will handoff to a 16m mixed mode BS(case 8 in Figure 2).

In essence, the legacy handover process shown in Figure 3 is supplemented with an extra notification step which takes place before ranging. For example, the notification maybe in the form of a control message following downlink synchronisation.

In particular, we focus on reducing the latency where we suggest the following detailed procedure (see also Figure 4 for flow chart): Target BS behaviour (16m mixed mode): * To inform the incoming MS of the target mode/zone of operation (16e or 16m), the target 16m BS should broadcast the zone permission information by which the MS can be informed of its mode of operation when handing over. The signalling could be in bitmap/flag or zone identifier form. If the signalling is in bitmap/flag form, by switching the bit value between 0 and 1, the BS can inform the MS of its mode/zone of operation based on the predefined description of bit value. If the signalling is in zone identifier form, the MS can acquire the information explicitly.

* Additional information can be broadcast within each zone which will inform the MS on how to access each of the DL zones. For instance, if the MS performs DL synchronisation and MAP acquisition in 16e mode, and the signalling in step 1 (above) indicates the MS should change to 16m mode of operation (thus operating in 16m zone) then additional signalling will be required in order to access this zone. Such information may include (but not restricted to); * Zone offset (in units of symbols from legacy preamble) * Preamble location * Superframe Header location * * Cell ID (as it may be different from 16e zone) * Available subchannels or Resource Blocks * Subcarrier allocation scheme Serving BS behaviour (16e): * The Serving BS may or may not have capabilities to signal new features and relevant parameters of a 16m Target BS.

If the serving 16e BS has signalling capabilities to support HO to a 16m BS, the extent of such capabilities may be variable. As an example, the Serving 16e BS may have some or all of the following capabilities.

o The legacy messages may contain new features of the target 16m BS where we can define new TLV (s) to serve this purpose, and can be included in MOB-NBR_ADV, MOB-HOBS-REQIRSP or some similar MAC messages.

o New features of 16m BS may include the 16m preamble type/index.

For example, if the 16m preamble is non-hierarchical (one preamble) or hierarchical (primary and secondary preambles).

o It may also be possible to include the system configuration information of the target BS but this will depend on the capabilities of the 16e BS. Most importantly, the zone permission information from the target 16m BS should be included as well.

16m/16e Dual Mode MS/Terminal Behaviour during HO from 16e Serving BS to 16m Target BS for Scenario 8 of Figure 2 Figure 4 demonstrates the behaviour in the form of a flowchart.

-ASO1) The MS starts with currently being served in Legacy Cell i.e. Serving 1 6e BS. Depending on whether the MS is allocated Scanning opportunities and/or whether Neighbour BS information is available directly (from say Serving BS), The MS may follow the flow-chart Branch starting -ASO2 or -ASO8 or both.

-ASO2) Begin HO sub-procedure (for acquisition of Network Topology) based on Scanning opportunities.

-ASO3) Check if current frame has Scanning opportunity. If yes, go to -ASO5, else go to -ASO4.

-ASO4) Increment frame and go to -ASO3.

-ASO5) If the Scan Mode for the MS is "16m", go to -ASO7, if the Scan Mode is "16e", go to -ASO6.

-ASO6) Perform Scanning to look for suitable nearby BSs for handover and (optionally) Association process with the potential Target 16m BS using legacy 16e procedures and go to -AS10.

-ASO7) Perform Scanning to look for suitable nearby BSs for handover and (optionally) Association process with the potential Target 16m BS using the 16m procedures and go to -ASIO.

-ASO8) Begin HO sub-procedure (for acquisition of Network Topology) based on Network Topology Advertisement by Serving 16e BS by means of legacy messages like MOB_N BR-AD V. -ASO9) If the Serving 16e BS is not capable of providing 16m related information; in particular, the Mode of Operation (16e or 16m) in Target 16m BS as well as parameters to access the Zone of Target Mode, then goto-ASII.

-AS1O) Update the Mode of Operation (16e or 16m) assumed for use with the Target BS based on the information acquired from -ASO6 or -ASO7 or -ASO9, whichever arrives last. The Mode of Operation at this stage is denoted by "TM1".

-AS 1 1) Decide if the HO should be performed. If not, go to -ASO1.

-AS12) Obtain the DL/UL Parameters for Target 16m BS from the zone indicated by Mode of Operation "TMI". At this stage the notification of whether the MS should use 16m or 16e is received.

-AS13) Update the Mode of Operation (16m or 16e) in Target BS based on information acquired from -AS12. The Mode of Operation at this stage is denoted by "TM2". The protocol (or mode) indicated for use by the Target BS may have changed as a result of changes in loading or fluctuating QoS.

-AS14) IfTM2'= 16m,goto-AS21.

-ASI 5) Perform DL Synchronization in 1 6m zone of Target BS.

-AS16) Update the Mode of Operation (16e or 16m) again for the Target BS based on information acquired from -AS15 (the notification information as before, but from a later frame). The Mode of Operation at this stage is denoted by "TM3". This is another opportunity for flexibility and adaptation to current conditions.

-AS17) If TM3!= 16m,goto-AS2O.

-AS18) Perform UL Ranging & Synchronization procedure in 16m mode.

-AS19) Perform the remaining HO procedure in 16m mode and go to -AS22.

The procedure may involve but not limited to the following: * Capabilities negotiation * Authorization and authentication * Registration and service flow re-establishment -AS2O) Switch Mode of Operation in Target 16m BS from 16m to 16e and perform the remaining HO procedure in 16e mode; go to -AS22.

-AS21) Continue the remaining HO procedure in 16e mode and go to -AS22.

-AS22) HO to Target 1Gm BS complete.

It is pertinent to note that at various stages during the HO, the MS may be able to acquire information regarding Mode of operation and associated parameters in the Target 16m BS from either the Serving BS (subject to capabilities) or from Target 16m BS. These information may include but not limited to: * Zone offset (in units of symbols from legacy preamble) * Preamble location * Superframe Header location * Cell ID (as it may be different from 16e zone) * Available subchannels or Resource Blocks * Subcarrier allocation scheme Note, all legacy MSs will simply ignore this information.

Figure 5 highlights the high level HO procedure. As the TM is determined by the target BS based on certain criterion, for example, radio resource utility ratio, the TM is relatively stable in comparison with the HO execution time; thus, in most cases the TMI is the same as TM2. However, it might happen that TM1 is changed during the HO execution, so the MS should switch the operation mode during the HO execution, as demonstrated in -BSO6 and -BSO7.

Figure 6 is an example where the MS switches mode after acquiring the 16m DL parameters in the 16e mode and thus completes the remaining network re-entry procedures in 16m mode. As the 16m procedures/protocols will be enhanced/optimized when compared to 16e, then this mechanism will improve the handover related latency when a 16m dual mode MS performs a handoff from a 16e BS to a 16m mixed mode BS.

Summary & benefits

Embodiments of this invention propose a solution for mobility management procedure for systems such as IEEE 802.16m with legacy support. In particular, preferred embodiments focus on reducing handover related latencies in a deployment scenario that consists of 16e BSs and 16m mixed mode BSs. In the case where the handover is originated from a 16e only cell, the target 16m BS shall supply additional information to the dual mode MS (operating in 16e mode) so as to notify if a mode switch is required during the handover procedure. If a mode switch is required (from 16e to 16m), the MS shall perform HO in the required mode. Moreover, in the case where Target Mode of operation (TM) is changed by the Target 16m BS during the HO procedure, the proposed scheme allows the MS to switch directly to the newly updated Target Mode (TM) during the network re-entry procedure, thereby saving the resource and time required for the network re-entry procedure.

The key benefits of invention embodiments are as follows: ** Minimises the impact on existing legacy design as the number of modifications required are minimal and legacy devices that are unaware of the signalling mechanism can coexist in the same system with those that are aware.

** Improves the handover and network re-entry related latencies for a 16m dual mode MS when handing off from a 16e cell to a 16m mixed mode cell ** Can increase efficiency of radio resource management within the 16m mixed mode BS In any of the above aspects, the various features may be implemented in hardware, or as software modules running on one or more processors. Features of one aspect may be applied to any of the other aspects.

The invention also provides a computer program or a computer program product for carrying out any of the methods described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein.

A computer program embodying the invention may be stored on a computer-readable medium, or it could, for example, be in the form of a signal such as a downloadable data signal provided from an Internet website, or it could be in any other form.

Claims

CLAIMS1. A method of facilitating handover of a dual-mode subscriber station, which can communicate using a first and a second protocol, from a serving base station to a target base station in a wireless communications system, the wireless communications system comprising at least the serving base station, the target base station and the subscriber station, wherein the serving base station is operable to communicate with the subscriber station using the first protocol and the target base station is operable to communicate with the subscriber station using both the first protocol and the second protocol, one of the two protocols being selected for use with the subscriber station at any one time, the method comprising: during the handover and before ranging, the subscriber station receiving a notification indicating whether the subscriber station should communicate with the target base station using the second protocol.
2. A method according to claim 1, wherein the notification is broadcast by the target base station so that it can be used by any subscriber station in the process of handover to the target base station.
3. A method according to claim 1 or 2, wherein the notification is transmitted repeatedly so that the subscriber station can receive it and change between the first and second protocols more than once during the handover.
4. A method according to any of the preceding claims, wherein the notification is zone permission information setting out which zone of a frame structure that the subscriber station can use for communication with the target base station.
5. A method according to claim 4 wherein the zone permission information is in the form of a zone identifier.
6. A method according to claim 4 wherein the zone permission information is in the form of a predefined bit map.
7. A method according to any of the preceding claims, wherein the notification includes information relating to use of the second protocol.
8. A method according to any of the preceding claims, wherein the serving base station transmits a notification indicating whether the subscriber station should communicate using the second protocol.
9. A method according to any of the preceding claims, wherein the serving base station transmits a control message identifying features of the target base station.
10. A method according to any of the preceding claims wherein a change to use of the second protocol takes place during the handover, preferably immediately after the notification and preferably before ranging.
11. A method according to any of the preceding claims wherein a change to use of the second protocol takes place after a downlink synchronisation with the target base station.
12. A method according to any of the preceding claims, wherein the first protocol communications take place in one window of a downlink or uplink subframe and the second protocol communications take place in another window of the same or a different downlink or uplink subframe.
13. A method according to any of the preceding claims, wherein the wireless communications system is a frame-based system, such as a CDMA-based system, or an OFDM or OFDMA system.
14. A wireless communications system facilitating handover of a dual-mode subscriber station, which can communicate using a first and a second protocol, from a serving base station to a target base station in the wireless communications system, the wireless communications system comprising at least the serving base station, the target base station and the subscriber station, wherein the serving base station is operable to communicate with the subscriber station using the first protocol and the target base station is operable to communicate with the subscriber station using both the first protocol and the second protocol, one of the two protocols being selected for use with the subscriber station at any one time, the wireless communications system further comprising transmission means which are operable during the handover and before ranging, to transmit a notification indicating whether the subscriber station should communicate with the target base station using the second protocol.
15. A dual-mode subscriber station, which can communicate using a first and a second protocol, which is operated in use as part of a wireless communications system facilitating handover of the subscriber station, from a serving base station to a target base station in the wireless communications system, the wireless communications system comprising at least the serving base station, the target base station and the subscriber station, wherein the serving base station is operable to communicate with the subscriber station using the first protocol and the target base station is operable to communicate with the subscriber station using both the first protocol and the second protocol, one of the two protocols being selected for use with the subscriber station at any one time, the dual-mode subscriber station further comprising receiving means which are operable during the handover and before ranging, to receive a notification indicating whether the subscriber station should communicate with the target base station using the second protocol.
16. A method in a dual-mode subscriber station, which can communicate using a first and a second protocol, and which is operated in use as part of a wireless communications system facilitating handover of the subscriber station, from a serving base station to a target base station in the wireless communications system, the wireless communications system comprising at least the serving base station, the target base station and the subscriber station, wherein the serving base station is operable to communicate with the subscriber station using the first protocol and the target base station is operable to communicate with the subscriber station using both the first protocol and the second protocol, one of the two protocols being selected for use with the subscriber station at any one time, the method comprising, during the handover and before ranging, receiving a notification indicating whether the subscriber station should communicate with the target base station using the second protocol and subsequently changing its mode to that indicated by the second protocol if necessary.
17. A target base station in a wireless communications system facilitating handover of a dual-mode subscriber station, which can communicate using a first and a second protocol, from a serving base station to the target base station, the wireless communications system comprising at least the serving base station, the target base station and the subscriber station, wherein the serving base station is operable to communicate with the subscriber station using the first protocol, wherein the target base station is operable to communicate with the subscriber station using both the first protocol and the second protocol, one of the two protocols being selected for use with the subscriber station at any one time, and wherein the target base station further comprises transmission means which are operable during the handover and before ranging, to transmit a notification indicating whether the subscriber station should communicate with the target base station using the second protocol.
18. A method in a target base station in a wireless communications system facilitating handover of a dual-mode subscriber station, which can communicate using a first and a second protocol, from a serving base station to the target base station, the wireless communications system comprising at least the serving base station, the target base station and the subscriber station, wherein the serving base station is operable to communicate with the subscriber station using the first protocol, wherein the target base station is operable to communicate with the subscriber station using both the first protocol and the second protocol, the method comprising: Selecting one of the two protocols for use with the subscriber station at any one time; and during the handover and before ranging, transmitting a notification indicating whether the subscriber station should communicate with the target base station using the second protocol.
19. A computer program which when executed on one or more computing devices of a wireless communications system carries out the method of any of the preceding method claims.
20. A base station, subscriber station, wireless communications system, method or computer program as set out in the description and/or shown in the figures.