Field of the Invention

The present invention relates to an apparatus, a method and a computer program product for performing an optimized synchronous RA-less handover without explicit handover confirmation message.

Related background Art

The following meanings for the abbreviations used in this specification apply:

5G Fifth generation of mobile networks

ACK Acknowledgment

CN Core Network

CQI Channel quality indicator

DL Downlink

eNB e-NodeB

HO Handover

LTE Long term evolution

MAC Medium access control

MS Milliseconds

NACK Negative acknowledgement

PDCP Packet data convergence protocol

PHY Physical layer

RA Random access

RF Radio frequency

RACH Random access channel

RLC Radio link control RRC Radio resource control

RRM Radio resource management

TA Time advance

UE User equipment

UL Uplink

Embodiments of the present invention relate to a synchronous RA-less handover procedure.

The synchronous (or RA-less) handover is a procedure on which the source and target cell negotiate the time instant T at which the handover should take place. After the negotiation, the source cell sends to the UE the handover command, including the handover time instant T. Additionally, the handover command may include an UL allocation for the UE in the target cell . From the beginning of the procedure until the handover time instant T, the UE receives data from the source cell.

At the handover switching time instant T, the source cell stops scheduling data to the UE, and the UE starts reconfiguring towards the target cell. When the reconfiguration finishes, the UE sends a handover confirmation message to the target cell indicating that it is ready to receive or send data. Once the target cell receives the handover confirmation, it starts scheduling data to the UE. The reception of the handover confirmation by the target cell is also used for triggering the data path switching and user plane update in the CN and the release of the UE context in the source cell.

From the instant the source cell stops scheduling data until the time instant the target cell starts scheduling data, there is a period of time on which the UE is unable to send or receive any data. In the following, this period of time during which the UE is unable to send or receive data is defined as interruption time. Thus, there it is desirable to shorten this interruption time in order to speed up the handover procedure.

Summary of the Invention

Embodiments of the present invention address this situation and aim to overcome the above-described problem and to reduce the interruption time during which the UE is unable to send or receive any data .

According to a first aspect of the present invention an apparatus is provided which comprises at least one processing circuitry, and at least one memory for storing instructions to be executed by the processing circuitry, wherein the at least one memory and the instructions are configured to, with the at least one processing circuitry, cause the apparatus at least: to calculate a handover time instant for a handover of a terminal device based on terminal device capabilities information, and to determine that the handover of the terminal device is successful when a first uplink transmission of the terminal device on a control or user plane is received .

According to a second aspect of the present invention a method is provided which comprises :

calculating a handover time instant for a handover of a terminal device based on terminal device capabilities information, and

determining that the handover of the terminal device is successful when a first uplink transmission of the terminal device on a control or user plane is received .

The first aspect and the second aspect may be modified as follows:

For example, the time it will take for the terminal device to complete a reconfiguration during the handover may be estimated based on the terminal device capabilities information .

The first uplink transmission of the terminal device may comprise an acknowledgement/non-acknowledgement sent from the terminal device to the apparatus after receiving downlink data from the apparatus, and/or user data sent from the terminal device to the apparatus for forwarding to the core network. The first uplink transmission of the terminal device may be a message sent on the radio link control (RLC) or medium access control (MAC) layer.

A data path switch and/or a user plane update in a core network for the terminal device may be switched upon receiving the first transmission of the terminal device.

A path switch for the terminal device may be initiated at the handover time instant or at a time determined based on the handover time instant.

The reception of the first uplink transmission may be notified to a layer controlling data transmission to/from the terminal device.

The layer controlling data transmission to/from the terminal device is the radio resource control (RRC) layer.

According to a third aspect of the present invention an apparatus is provided which comprises at least one processing circuitry, and at least one memory for storing instructions to be executed by the processing circuitry, wherein the at least one memory and the instructions are configured to, with the at least one processing circuitry, cause the apparatus at least: to receive a handover command including information regarding a handover time instant and uplink allocation in a target cell, to stop sending and/or receiving data to and/or from the source cell, to reconfigure for the target cell, and to send a first uplink transmission which is a control or user plane transmission data to the target cell after reconfiguration .

According to a fourth aspect of the present invention a method is provided which comprises :

receiving a handover command including information regarding a handover time instant and uplink allocation in a target cell,

stopping sending and/or receiving data to and/or from the source cell,

reconfiguring for the target cell, and

sending a first uplink transmission which is a control or user plane transmission to the target cell after reconfiguration.

The third aspect and the fourth aspect may be modified as follows:

For example, the first uplink transmission may comprise an acknowledgement/non-acknowledgement sent to the target cell after receiving downlink data from the target cell, and/or user data sent from the apparatus to the target cell for forwarding to the core network.

The first uplink transmission may be a message sent on the radio link control (RLC) or medium access control (MAC) layer.

In case of a handover failure, a connection re-establishment towards the target cell may be performed .

According to a fifth aspect of the present invention an apparatus is provided which comprises at least one processing circuitry, and at least one memory for storing instructions to be executed by the processing circuitry, wherein the at least one memory and the instructions are configured to, with the at least one processing circuitry, cause the apparatus at least: to calculate a handover time instant for a handover of a terminal device and to estimate the time it will take for the terminal device to complete a reconfiguration during the handover based on terminal device capabilities information, and to send a handover command to the terminal device including the handover time instant.

According to a sixth aspect of the present invention a method is provided which comprises :

calculating a handover time instant for a handover of a terminal device and to estimate the time it will take for the terminal device to complete a reconfiguration during the handover based on terminal device capabilities information, and

sending a handover command to the terminal device including the handover time instant.

According to a seventh aspect of the present invention a computer program product is provided which comprises code means for performing a method according to the second aspect and/or fourth aspect and/or sixth aspect and/or their modifications when run on a processing means or module. The computer program product may be embodied on a computer-readable medium, and/or the computer program product may be directly loadable into the internal memory of the computer and/or transmittable via a network by means of at least one of upload, download and push procedures.

According to an eight sixth aspect of the present invention an apparatus is provided which comprises

means for calculating a handover time instant for a handover of a terminal device based on terminal device capabilities information, and means for determining that the handover of the terminal device is successful when a first uplink transmission of the terminal device on a control or user plane is received . According to a ninth sixth aspect of the present invention an apparatus is provided which comprises

means for receiving a handover command including information regarding a handover time instant and uplink allocation in a target cell, means for stopping sending and/or receiving data to and/or from the source cell,

means for reconfiguring for the target cell, and

means for sending a first uplink transmission which is a control or user plane transmission to the target cell after reconfiguration.

According to a tenth sixth aspect of the present invention an apparatus is provided which comprises

means for calculating a handover time instant for a handover of a terminal device and to estimate the time it will take for the terminal device to complete a reconfiguration during the handover based on terminal device capabilities information, and

means for sending a handover command to the terminal device including the handover time instant.

Brief Description of the Drawings

These and other objects, features, details and advantages will become more fully apparent from the following detailed description of embodiments of the present invention which is to be taken in conjunction with the appended drawings, in which :

Fig . 1 illustrates source and target eNBs and a UE according to an embodiment of the present invention,

Fig . 2A illustrates a synchronous handover procedure according to an embodiment of the present invention in case of an ongoing DL transmission, Fig . 2B illustrates a synchronous handover procedure according to an embodiment of the present invention in case of an ongoing UL transmission,

Fig . 3 illustrates a reuse of existing MAC or RLC signaling for indicating a successful handover according to an embodiment of the present invention, and Fig . 4 illustrates a synchronous handover procedure according to the prior art.

Detailed Description of embodiments

In the following, description will be made to embodiments of the present invention . It is to be understood, however, that the description is given by way of example only, and that the described embodiments are by no means to be understood as limiting the present invention thereto.

Before describing embodiments in detail, it is again referred to the problem underlying the present application .

Fig . 4 illustrates the current synchronous RA-less handover procedure [1]. From step 301 to step 305, the UE receives data from the access point (source eNB) in the source cell. In particular, in step 301 the UE performs RRM measurements and estimates the TA. In step 302, the UE sends a measurements report to the source eNB. In step 303, handover decision, admission control and negotiation of the handover time instant T between source and target eNBs are carried out. In step 304, the source eNB sends a handover command to the UE including the handover time instant T. In step 305, the UE continues receiving data from the source eNB until the handover instant T. In step 306, the handover takes place at the time instant T, wherein the source eNB stops scheduling data to the UE and the UE starts listening to the target cell. Meanwhile, the source eNB forwards its buffered data to the target eNB in step 308. In step 309, the target eNB allocates resources to the UE and buffers incoming data. In step 310, the target eNB sends an UL allocation to the UE.

Thus, at the handover time instant T, the source eNB stops sending data to the UE, and the UE initiates the reconfiguration towards the target cell (step 307). During this step the UE is unable to send or receive any data. When the UE finishes the reconfiguration, it informs the target eNB that is ready to receive data again with the handover confirmation message (step 311). Thereafter, the target eNB resumes the data transmission. That is, in step 312, the target eNB schedules data to the UE, and in step 313, the UE confirms the reception of the data with an ACK/NACK.

Therefore, there is a data interruption at each synchronous handover between steps 306 and 312.

The data interruption at each synchronous handover becomes an important issue in scenarios where the UE has to perform a large number of cell changes (e.g . in ultra-dense networks, or when the UE is moving at high speeds). Moreover, it becomes critical when considering the upcoming ultra-low latency applications for 5G.

From Fig. 4 it can be observed that in a RA-less handover the interruption time, i.e., the duration of the data interruption, may depend on the following factors (among others) :

a) The UE reconfiguration time.

b) The time it takes for the source cell to forward data to the target. c) The time it takes for the UE to send the handover confirmation, and for the target eNB to process it. d) Other processing times at the UE and at both cells.

At step 306, the source eNB, the target eNB and the UE initiate the handover. Although the handover time instant T is known, the network is unaware of the time instant when the UE will finish the reconfiguration and will be ready to receive or send data from/to the target eNB.

The UE reconfiguration time varies with the type of handover. For instance, in case of an intra-frequency handover, it may last only a few milliseconds. However, in an inter-frequency handover the UE reconfiguration takes more time to be completed, as the UE has to perform the RF retuning towards a different frequency band . According to [2], the UE reconfiguration with the RF retuning typically takes 20ms. However, this time may differ between UEs with different hardware and different processing capabilities.

Hence, there is a problem regarding the data interruption that occurs at each synchronous handover and the uncertainty for the network to know when the UE will complete the reconfiguration .

In the following, some prior art documents are listed concerning the background as described above :

References:

[1] Barbera, S., et al, "Synchronized RACH-less Handover Solution for LTE Heterogeneous Networks". International Symposium on Wireless Communication Systems (ISWCS), Brussels, August 2015, pp. 755-759.

[2] 3GPP TR 36.881. Study on latency reduction techniques for LTE (Release 14). V.14.0.0 [3] Pedersen, K.I., et al, "Techniques for RACH (Random Access Channel)-Less Synchronized Handover for Wireless Networks" WO 2015/127987 Al . [4] 3GPP R2-072799. Samsung and NTTDoCoMo. UL Time synchronized handover.

[5] 3GPP R2-072476. Nortel. Inter eNB handover in a synchronous network.

[6] 3GPP TS 36.300. Evolved Universal Terrestrial Radio Access (E- UTRA) and Evolved Universal Terrestrial Radio Access Network (E- UTRAN); Overall description; Stage 2. Reference [1] describes an implementation of the synchronous RA-less handover for LTE.

Reference [2] proposes the synchronous RA-less handover as a solution for reducing the data interruption time in LTE handovers. It proposes to include the UL allocation in the handover command, eliminating step 310 in Fig . 4.

Reference [3] describes techniques for computing the TA used by the UE in a synchronous RA-less handover.

Reference [4] proposes dedicated UL control signaling instead of using an RRC handover confirmation message in a synchronous UL handover. This solution requires an explicit and additional UL message (e.g . CQI signaling) for indicating the handover confirmation.

Reference [5] describes call flows for inter-eNB handover in a synchronous network. Reference [6] describes the procedures of a legacy LTE handover. As this procedure is conceived for asynchronous networks, it considers a handover complete message. As mentioned above, according to embodiments of the present invention, the interruption time (the time during which a UE cannot receive and send data) during a synchronous or RA-less handover is to be reduced .

In the following, a general overview of an embodiment of the present invention is described by referring to Fig. 1.

In particular, Fig . 1 shows an eNB 1 (source eNB) as an example for an access point in a mobile network providing access to a source cell according to the present embodiment, an eNB 2 (target eNB) as an example for an access point in a mobile network providing access to a target cell according to the present embodiment, and an UE 3 as an example for a terminal device according to the present embodiment.

The eNB 2 (access point in target cell) comprises a processor/controller 21 as an example for at least one processing circuitry and a memory 22 as an example for at least one memory for storing instructions to be executed by the processing circuitry.

The memory 22 and the instructions are configured to, with the processor 21, cause the eNB 2 to calculate a handover time instant for a handover of a terminal device based on terminal device capabilities information, and to determine that the handover of the terminal device is successful when a first uplink transmission of the terminal device on a control or user plane is received .

The UE 3 (terminal device) comprises a processor/controller 31 as an example for at least one processing circuitry and a memory 32 as an example for at least one memory for storing instructions to be executed by the processing circuitry.

The memory 32 and the instructions are configured to, with the processor 31, cause the UE 3 to receive a handover command including information regarding a handover time instant and uplink allocation in a target cell, to stop sending and/or receiving data to and/or from the source cell, to reconfigure for the target cell, and to send a first uplink transmission which is a control or user plane transmission data to the target cell after reconfiguration .

Thus, no dedicated handover confirmation message is sent from the UE 3 to the eNB 2 (access point in target cell), but the first uplink transmission of the terminal device is considered as an indication that the handover was successful.

The first uplink transmission of the terminal device may be include an acknowledgement/non-acknowledgement sent from the terminal device to the apparatus after receiving downlink data from the apparatus, and/or user data sent from the terminal device to the apparatus for forwarding to the core network, and/or may be is a message sent on the radio link control (RLC) or medium access control (MAC) layer.

The eNB 1 (access point in source cell) comprises a processor/controller 11 as an example for at least one processing circuitry and a memory 12 as an example for at least one memory for storing instructions to be executed by the processing circuitry.

The memory 12 and the instructions are configured to, with the processor 11, cause the eNB 1 to calculate a handover time instant for a handover of a terminal device and to estimate the time it will take for the terminal device to complete a reconfiguration during the handover based on terminal device capabilities information, and to send a handover command to the terminal device including the handover time instant. Moreover, the eNBs 1 and 2 and the UE 3 may further comprise input/output (I/O) units or functions (interfaces) 13, 23 or 33 connected to the processor 11, 21 or 31. The I/O units 13 and 23 may be used for performing communication between the eNBs 1 and 2 and the core network and may also be used for communicating with other entities or functions not shown in Fig. 1. Moreover, the I/O units 13, 23 and 33 may comprise a transmitter/receiver for radio communication between the eNBs 1 and 2 and the UE 3.

The handover described above may be a synchronous or RA-less handover.

In the following, some more details of the present embodiment are described .

1. During the HO decision, the UE capabilities information is used in the source cell and the target cell for computing the handover time instant T and estimate the time it will take for the UE to complete the reconfiguration .

2. The access point in the target cell (target eNB) deduces that the handover has been successfully completed when it receives the first control or user plane transmission from the UE (e.g . MAC/RLC ACK/NACK or any other first UL transmission).

3. As a consequence of 2, the UE does not transmit a handover confirmation message indicating that the reconfiguration is complete (like the RRC_Reconfiguration_Complete). As the network computed the time instant T including the UE capabilities, the handover procedure by default relies on the UE being able to comply and reconfigure on the estimated time. 4. The reception of the first control or user plane transmission from the UE by the access point of the target cell (e.g. MAC/RLC ACK/NACK or any other first UL transmission) is used for triggering the data path switch and the user plane update in the CN . The path switch may be initiated at time T, or some time derived from this, as an example the earliest possible time that it is estimated that the access point of the source cell (source eNB) has sufficient buffered data to transmit until the time T, to make the switch happen as early after T as possible, since after the time T the target cell has implicitly taking the role of the source cell as being the primary serving cell. UE context must be available in the source cell for some time after T, for example by the existing handover procedure this is until a UE Context Release message is received from the target cell indicating that the handover is complete, to support successful RRC Connection Re-establishment towards the source cell if the UE chooses to initiate this procedure after detecting a handover failure.

5. In case of a handover failure because the UE was unable to comply with the handover time instant T, the UE should perform the RRC Connection Re-establishment towards the target cell that is now considered the primary serving cell. However, it is noted that the embodiment does not preclude any UE behavior in accordance with the existing re-establishment procedure, in particular performing it towards the source cell. To explain the merits of the present embodiment, in the following some of the delays during the handover procedure are explained . It typically takes 6 ms for the UE to encapsulate and transmit the handover confirmation message ([1], [2]). Additionally, it may take 5 ms more in the target cell to process the handover confirmation message sent by the UE [1] . In total, the process of sending a handover confirmation message (step 311 in Fig . 4 described above) delays the overall handover procedure by 11ms. In case of an inter-frequency handover, the UE reconfiguration may take a typical time of 20ms to be completed . However, for an intra-frequency handover, it may take only a few milliseconds, being larger the time it takes for the UE to indicate that it is ready to receive data, than the time that it spends in the reconfiguration. By avoiding sending an explicit handover confirmation message, the handover process can be sped up by an average of 11 ms, reducing the interruption time by the same amount. Moreover, the amount of control signaling exchanged between the UE and the network is also reduced .

Nevertheless, if the handover confirmation message is not sent, it should be ensured in the target cell and the source cell that the computation of the handover time instant T allows for the UE to finish the reconfiguration on time. One solution could be to consider a large value of T. However, large values of T will unnecessarily increase the overall handover time.

Currently, the UE informs the network about its capabilities and category through the existing RRC UE Capability Information message. The network can use this information to derivate the handover time instant T and estimate how fast the UE may perform the handover. In this way, the handover time instant T can be adapted to different types of UEs with different processing delays. As the handover time instant T is calculated taking into account the UE processing delays, the UE does not have to inform explicitly when it will be ready to receive or send data. Therefore, after the handover time instant T, scheduling of data to/from the UE can immediately start in the target cell, without waiting for an explicit handover confirmation message.

After receiving DL data, the UE has to confirm to the network the reception of the data via an ACK/NACK. These confirmations can be reused by the network for detecting a successful handover. When the MAC or RLC layers at the target cell receive the first ACK/NACK, a notification can be sent to its RRC layer to indicate that the handover has been successfully completed . This procedure can be extended to the reception of any UL control or data transmission (e.g. user plane data, scheduling requests, ACK, NACK). The reception of the first UL message in the target cell (received at any layer) should be notified to the RRC layer controlling data transmission to/from the UE in the target cell, confirming that the ongoing handover has been completed (see also Fig. 3 described later). As this procedure reuses the first control or data UL transmission for detecting a successful handover, it is not required to send additional messages that increase the amount of signaling and overall delays.

If the UE (target cell) does not receive any control or user plane transmission from the target eNB (UE), a handover failure is declared . If the handover failure happened because the UE was not ready to perform the handover at the time instant T, it is proposed for the UE to initiate the connection re-establishment directly towards the target cell. This scheme is faster than the implementations where the UE should perform the re-establishment towards the source cell and then, initiate again the handover process. This can be done as the handover failure happened because the UE was unable to be ready at the handover time instant, rather than due to a poor radio-link performance at the target cell.

Figs. 2A and 2B show the proposed synchronous RA-less handover procedure without explicit handover confirmation. This proposal reduces the interruption time and the amount of signaling exchanged between the UE and the network as an explicit handover confirmation is avoided .

Fig . 2A illustrates the proposed synchronous handover procedure for an ongoing DL transmission, whereas Fig. 2B illustrates the proposed synchronous handover procedure for an ongoing UL transmission.

In particular, in Fig. 2A, in step 101 the UE performs RRM measurements and estimates the TA. In step 102, the UE sends a measurements report to the access point (source eNB) in the source cell. In step 103a, average UE processing delays are estimated taking into account the reported RRC_UE Capability_Information message, and in 103b, handover decision, admission control and negotiation of the handover time instant T are carried out considering step 103a. Steps 103a and 103b may be carried out at both the target cell and the source cell, or at only one of them. In step 104, the access point of the source cell (source eNB) sends a handover command to the UE including the handover time instant T and UL allocation in the target cell . In step 105, the UE continues receiving data in the source cell until the handover instant T.

In step 106, the handover takes place at the time instant T, wherein scheduling of data to/from the UE is stopped in the source cell and the UE starts listening to control signaling in the target cell, and in step 107, the UE performs reconfiguration for the target cell. Meanwhile, buffered data are forwarded from the source cell to the target cell in step 108, and in step 109 incoming data for the UE are buffered for transmission in the target cell. In step 110, data transmission to/from the UE is scheduled in the target cell. In step 111, the UE sends in the target cell a first UL transmission, which may be in this case an ACK/NACK regarding the data sent in step 110. In step 112, the reception of the first UL transmission from the UE by the access node (target eNB) of the target cell is understood as successful handover (HO) from the source cell to the target cell.

Fig . 2B illustrates the handover procedure for an ongoing uplink transmission and is similar to Fig . 2A, so that in the following, only the differences are described. In particular, steps 201 to 204 are identical to steps 101 to 104 of Fig . 2B. Since the present handover procedure is for the uplink case, the UE continues sending data the access point (source eNB) in the source cell in step B5 until the handover time instant T. After the handover time instant T (step B6) and UE reconfiguration (step B7), the UE sends data to the access point (target eNB) in the target cell in step BIO, wherein sending of the data is the first UL transmission of the UE. In step Bl l reception of the data at the access point (target eNB) of the target cell, i.e., reception of the first UL transmission from the UE is understood as successful handover from the source cell to the target cell . In step B12, the access point (target eNB) of the target cell sends its first DL transmission to the UE, which may be in this case an ACK/NACK regarding the data sent in step BIO.

Fig . 3 illustrates an example of how existing MAC or RLC ACK/NACKs are notified to the RRC layer to indicate a successful handover. That is, when an ACK/NACK is transmitted either on the RLC layer (dotted line) or on the MAC layer (solid line), then this is notified to the RRC layer in both the UE and network entities of target cell (target eNB), so that it can be concluded that the handover was successful.

The proposed synchronous handover scheme has the following advantages:

- The estimation of the handover time instant T in a synchronous handover is improved by including the UE capabilities and average UE processing delays. - Synchronous RA-less handover without explicit handover confirmation as is assumed by default, speeding up the handover process.

- The amount of the exchanged RRC signaling between the UE and the network is decreased compared to the original implementation of the synchronous handover.

- DL transmission in the target cell can be resumed earlier compared to the original implementation of the synchronous handover. - The UE can resume the UL transmission earlier compared to the original implementation of the synchronous handover. - The handover data interruption is reduced compared to the original implementation of the synchronous handover.

For the purpose of the present invention as described herein above, it should be noted that

- method steps likely to be implemented as software code portions and being run using a processor at a network element or terminal (as examples of devices, apparatuses and/or modules thereof, or as examples of entities including apparatuses and/or modules therefore), are software code independent and can be specified using any known or future developed programming language as long as the functionality defined by the method steps is preserved;

- generally, any method step is suitable to be implemented as software or by hardware without changing the idea of the invention in terms of the functionality implemented;

- method steps and/or devices, units or means likely to be implemented as hardware components at the above-defined apparatuses, or any module(s) thereof, (e.g., devices carrying out the functions of the apparatuses according to the embodiments as described above, eNode-B etc. as described above) are hardware independent and can be implemented using any known or future developed hardware technology or any hybrids of these, such as MOS (Metal Oxide Semiconductor), CMOS (Complementary MOS), BiMOS (Bipolar MOS), BiCMOS (Bipolar CMOS), ECL (Emitter Coupled Logic), TTL (Transistor-Transistor Logic), etc., using for example ASIC (Application Specific IC (Integrated Circuit)) components, FPGA (Field-programmable Gate Arrays) components, CPLD (Complex Programmable Logic Device) components or DSP (Digital Signal Processor) components; - devices, units or means (e.g . the above-defined apparatuses, or any one of their respective means) can be implemented as individual devices, units or means, but this does not exclude that they are implemented in a distributed fashion throughout the system, as long as the functionality of the device, unit or means is preserved;

- an apparatus may be represented by a semiconductor chip, a chipset, or a (hardware) module comprising such chip or chipset; this, however, does not exclude the possibility that a functionality of an apparatus or module, instead of being hardware implemented, be implemented as software in a (software) module such as a computer program or a computer program product comprising executable software code portions for execution/being run on a processor;

- a device may be regarded as an apparatus or as an assembly of more than one apparatus, whether functionally in cooperation with each other or functionally independently of each other but in a same device housing, for example.

It is noted that the embodiments and examples described above are provided for illustrative purposes only and are in no way intended that the present invention is restricted thereto. Rather, it is the intention that all variations and modifications be included which fall within the spirit and scope of the appended claims.