TITLE

Timesharing between different cellular telephony systems.

TECHNICAL FIELD The present invention relates to the use of multiple cellular telephony standards within one and the same system.

BACKGROUND

An operator of a wireless cellular communications system of a certain standard, who wishes to make a slow transition from his present system to a system of another standard may be faced with a problem if he only has access to a certain limited bandwidth. For example, an operator of a GSM system may have access only to a bandwidth of 4,6 - 7 MHz on the 900 MHz band and wishes to make a transition of his system from the GSM standard to the WCDMA standard, the operator needs to allocate at least 4.6 MHz for WCDMA use, that being the minimum bandwidth in a WCDMA system.

The allocation of bandwidth to the WCDMA system needs to be done either by allocating additional bands of unused frequencies, or by freeing up approximately 23 GSM frequencies in a continuous band. The latter will be the case if there is no unused bandwidth available to the operator of the GSM system, and will most likely result in unwanted restrictions being placed on both systems.

SUMMARY

As has become evident from the explanation above, there exists a need for an operator of a wireless cellular system such as, for example, a GSM system, to be able to make a transition to another system, such as, for example a WCDMA system, within a limited bandwidth.

This need is addressed by the present invention in that it discloses a method for use in a wireless cellular telecommunications system, in which system

there is at least one cell in which there can be a plurality of users with user terminals and at least one base station which controls the traffic to and from users in the cell.

The method of the invention comprises the step of letting the base station alternate between operating according to a first and a second cellular communications standard, so that the base station alternates between operating according to the first standard during a first predetermined period of time and according to the second standard during a second predetermined period of time.

In addition, the method of the invention comprises the step of letting the first cellular communications standard be the WCDMA standard.

Suitably but not necessarily, the first predetermined period of time is chosen to essentially be equal to the length of a multiple of a frame in the WCDMA system, one frame having a duration of 10 ms in WCDMA. In one embodiment, the first and second predetermined periods of time are chosen to be essentially equal.

Thus, by means of the invention, it has been made possible for operators of cellular wireless communications systems to make a smooth transition from one system to another within a limited bandwidth.

This, and other advantages of the invention, will become even more apparent form the following detailed description.

The present invention also discloses a radio base station which functions essentially according to the method described above, and also discloses a user terminal for use in a system in which the invention is applied.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows an overview of a system in which the invention may be applied, and

Fig 2 shows a flow chart of a method of the invention, and

Fig 3 shows a block diagram of a base station of the invention, and

Fig 4 shows a block diagram of a user terminal of the invention.

DETAILED DESCRIPTION

Fig 1 shows an overview of a system 100 in which the present invention may be applied. As shown in fig 1 , the system 100 comprises at least one cell 110, in which there can be a plurality of users with user terminals 130, and there is also at least one base station 120 which controls the traffic to and from the user terminals 130 in the cell 120.

In a traditional system 100, the base station and the user terminals operate according to a defined system standard, such as, for example, the GSM standard. If the operator of the system wishes to upgrade his system so that it instead operates according to another standard, such as, for example, the WCDMA standard, he may have a problem if he has restrictions on the amount of bandwidth that he can use, since both systems, both the present system, e.g. a GSM system, and the system, e.g. a WCDMA system, to which he wishes to make a transition, must operate within the bandwidth available to the operator.

Thus, if there are bandwidth limitations which the operator needs to follow, the new system, in the present example the WCDMA system, may be introduced at the expense of the existing system, the GSM system in the present example, which is not particularly attractive for the operator, since this probably will place unwanted restrictions on both systems.

In addition, at such a transition, users will need to buy new terminals 130 which can function according to the standard of the new system, which is also an unattractive solution.

The present invention offers a solution to these problems, by means of a method which is shown in fig 2, which shows a rough flow chart of some of the steps of the method of the invention. Steps which are options or alternatives are shown with dashed lines.

The method 200 is, as has emerged from the description above, intended for use in a wireless cellular telecommunications system 100, such as the one shown in fig 1. As shown in fig 1 , in the system 100 there can be at least one cell 110 with a number of user terminals 130 in it, and there is also at least one base station 120 which, inter alia, controls the traffic to and from the user terminals 130 in the cell 110.

The method 200 of the invention comprises the step 210 of letting the base station alternate between operating according to a first and a second cellular communications standard, so that the base station operates according to the first standard during a first predetermined period of time and according to the second standard during a second predetermined period of time, said periods alternating with each other.

As shown in step 220, the method 200 also comprises the step of letting the first cellular communications standard be the WCDMA standard, so that the base station 120 alternates between being a WCDMA base station and a base station of one other system.

In a particular embodiment of the invention, as shown in step 230 in fig 2, the first predetermined period, which is the period during which the base station

120 functions according to the WCDMA standard, is chosen to essentially be equal to the length of a dual frame in the WCDMA system due to interleaving,

i.e. at present 20 ms. Suitably, as shown in step 240, the first and second predetermined periods of time are chosen to be essentially equal, i.e. so that the base station 120 functions according to both standards during essentially equal periods of time.

As shown in step 250, the second cellular communications standard according to which the base station 120 functions is chosen to be a GSM standard or an LTE standard. The phrase "a GSM standard", and "an LTE standard" are used here as generic examples of TDMA and OFDM standards, respectively.

The BCCH channel, Broadcast Control Channel, in the GSM system is a channel which at present is broadcast constantly. Preferably, the standard should be changed to broadcast only in the GSM time periods. In order to make the invention function better, as shown in step 260, this may be addressed as follows: either the base station 120 ceases to transmit the BCCH signals during the periods of time that the first cellular communications standard, i.e. the WCDMA system, is used. As an alternative, if the available bandwidth permits, the base station is made to transmit the BCCH signals of the GSM system on a frequency band which is set aside or allocated for those signals. Approximately 2.4 MHz or more will be needed for this purpose.

With respect to "WCDMA part" of the base station of the invention, a function known as IPDL, Idle Period Down Link, in the WCDMA standard may suitably be used in order to enable the base station to alternate between its "WCDMA mode" and its "non WCDMA mode".

Fig 3 shows a block diagram of a radio base station 300 of the invention. As has merged from the description above, and as is also indicated in fig 3, the base station 300 of the invention is equipped with means for functioning both according to a first and a second cellular communications standard, and is

also equipped with means for alternating between the first and a second cellular communications standards, so that the base station will operate according to the first standard during a first predetermined period of time and according to the second standard during a second predetermined period of time.

The exact nature of the means for functioning according to both of said standards, as well as alternating between the two standards, can be designed in a large number of ways, which will be realized by the man skilled in the field. Accordingly, only some of many possible examples will be described below, one of which is shown in fig 3.

As shown in fig 3, the base station 300 is equipped with an antenna 310 for receiving and transmitting signals to and from the base station. Naturally, separate send and receive antennas are entirely possible.

The antenna 310 is connected to the rest of the base station via feeder means 315.

The base station 300 is equipped with means 330 and 340 for acting as a base station according to the GSM standard (GSM being given only as an example of the second standard). Again, the exact nature and design of a GSM base station is known to those skilled in the field, so it will not be described in detail here. However, the "GSM part" of the base station 300 is connected to the antenna 310 via a Transceiver, Trx, which is shown separately as block 330.

Apart from the GSM means 330, 340, the base station 300 is also equipped with similar means 330', 350, for acting as a base station according to the WCDMA standard. The TRX of the "WCDMA part" of the base station is also shown separately, as block 330'.

In order to enable the base station 300 to alternate between the two standards, it is equipped with a switch 320, suitably a hardware switch, which controls which of the TRXs 330, 330', that is connected to the feeder 315 at the moment. Suitably, the switch is controlled by a processor or computer of some kind, which can be integrated in some other part of the base station 300, or which can be an integral part of the switch 320.

As mentioned previously, the means for enabling the base station 300 to function both according to a first cellular communications standard and according to a second cellular communications standard can be designed in a large number of ways, as can the means for switching between the two "modes", with the design of fig 3 being an example only.

Other examples of designs which would achieve the desired purpose and which can be mentioned are:

• The base station can be equipped with one and the same antenna for receive and transmit purposes, as well as a common feeder to the antenna, and a Multi Carrier Power Amplifier, an MCPA, with a switch connected to the input of the MCPA, said switch switching between components for the two systems,

• Different TX antennas and feeders for the two standards, with the switching being performed by turning on or off the power of the WCDMA/GSM component of the base station.

• One and the same TX and RX antenna and feeder, and a splitter combiner, with the switching being performed by turning on or off the power of the WCDMA/GSM component of the base station.

One area which has so far not been addressed in this description is how a time reference could be achieved which would allow the base station 300 to operate on the respective standard for the intended periods of time, and to

switch between the two standards at the intend points in time. Examples which can be mentioned of how this can be achieved are:

• Timing synchronization information from the GPS system, • A transmission line which would connect the base stations in the network to a synchronization source,

• Synchronization via RF long-wave,

• Synchronization via the IP-net,

• Possibly via the power grid, with knowledge of the frequency of the grid, e.g. 50 Hz.

Turning now to the user terminal's role in the present invention, the user terminals in the system also need to be synchronized to the base station's alternations between the two standards. Users may have terminals which are adapted to either of the two systems between which the base station alternates, i.e. in the present example the WCDMA system and the GSM system.

Suitably, the synchronization is done by means of the base station of the invention transmitting a message to the terminals in the system indicating the alternations. The base station should transmit a message to the user terminals in the cell informing them of an alteration in the standard which the base station will follow during a coming period of time This can be done in a number of ways, but suitable examples of which mention can be made are:

• The base station transmits the message during the period in question.

• The base station transmits the message prior to the beginning of the period in question.

In other words, in order to exemplify the first bullet point above, if the base station at the moment is acting as a WCDMA base station, it can send a

message to the WCDMA user terminals in the cell that it will switch to GSM ₁ said message being transmitted either during the WCDMA period, suitably at its end, or at the beginning of the "GSM period". The same principle will be valid for the GSM terminals in the system.

The message to the WCDMA terminals needs to comprise information regarding when the GSM period will begin and end. The same is valid for the GSM UEs, which must know when the WCDMA periods begin and end.

Suitably, the message or messages is/are transmitted on a broadcast channel of the system.

With respect to the user terminals which should be used in a system which utilizes the invention, such a user terminal should thus be equipped with means for receiving and acting on the message from the base station as described above, i.e. a message comprising information regarding a period of time during which the terminal may not make any transmissions which would interfere with a base station or user terminals of a system which is defined by the second standard.

In addition, if the user terminal is a GSM terminal, it should also be adapted so that it does not need the BCCH channel of GSM during the "WCDMA periods", in other words it should be able wait during the WCDMA periods before it needs or expects transmissions from the base station on a certain channel of the GSM standard, such as the BCCH channel of GSM.

Naturally, the user terminal which is in a system of the invention can also be a user terminal which is adapted to the WCDMA standard.

A user terminal 400 which may be used together with the system of the invention is schematically shown in fig 4. The user terminal 400 comprises an antenna 410 which is used both for transmission and for reception, and also

comprises a transmitter part 420 and a receiver part 430. In addition, the terminal 400 comprises a processor, such as a microprocessor 440, which inter alia controls the operation of the transmitter 420 and the receiver 430.

As has been explained, the user terminal 400 can be a user terminal intended for either of the two standards which the base station of the system alternates between. If the standard which the user terminal 400 is referred to as a first standard, then the user terminal 400 will also be equipped with means for receiving and acting on a message from the base station, said message comprising information regarding a period of time during which the user terminal 400 may not make any transmissions which would interfere with a base station or user terminals of a system which is defined by the second standard which the base station of the invention can alternate to.

These means are suitably the antenna 410 and the receiver 420, which receive the message, and the processor 440, which controls the transmissions made by the user terminal 400.

If the user terminal 400 is a GSM terminal, it should also be equipped with means for enabling it to wait during the WCDMA periods before it needs or expects transmissions from the base station on a certain channel of the GSM standard, such as the BCCH channel of GSM. Suitably, these means are the processor 440.

The invention is not limited to the examples of embodiments described above and shown in the drawings. For example, the second system or standard according to which the base station may function is not limited to being the GSM standard, this is merely an example of a system which may be used in the invention.

In addition, it should be pointed out that the method of the invention may be implemented in hardware or software, or a suitable combination of hardware

or software. Those parts of the method which are implemented as software may be stored as a computer program on a computer readable storage medium.