WO2009075622A1

WO2009075622A1 - An improved broadcast channel.

Info

Publication number: WO2009075622A1
Application number: PCT/SE2007/050982
Authority: WO
Inventors: Jie Mao; Jingyue Nie; Huaisong Zhu
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-12-12
Filing date: 2007-12-12
Publication date: 2009-06-18
Also published as: CN101897130A

Abstract

A method (700) for use in a wireless cellular communications system (100, 300), in which (710) there is a Radio Base Station (110), RBS, which serves to control traffic to and from users equipments (120), UEs, within a cell (130) in the system. Transmissions from the RBS to the UEs comprise (715) transmissions on one or more broadcast channels, and a UE can assume (720) one of at least two states, one such state being an 'idle' state. A UE which is in the idle state can transmit synchronization messages to its RBS in a random access channel, and the RBS uses such synchronization messages as 'beacons' for beam forming transmissions of broadcast channels to the UE, and for directing such formed beams towards the UE.

Description

TITLE

An improved broadcast channel.

TECHNICAL FIELD The present invention concerns the field of wireless cellular communications system, in which a Radio Base Station, RBS, serves to control traffic to and from user equipments, UEs, within a cell in the system, with transmissions from the RBS to the UEs comprising one or more broadcast channels.

BACKGROUND

In present day cellular wireless systems such as, in particular, Time Division Synchronous Code Division Multiple Access, TD-SCDMA systems, certain data is sent from Radio Base Stations, RBSs, to users, UEs in the system on so called broadcast channels.

Examples of broadcast channels in TD-SCDMA systems are the Primary Common Control Channel, P-CCPCH, or the Secondary Common Control Channel, S-CCPCH. Examples of data which is transmitted on the broadcast channels in TD-SCDMA systems is system information such as the so called system information block 1 , 3, 5 etc, as well as paging messages and traffic data for MBMS, Multimedia Broadcast Multicast Services.

A problem in systems with broadcast channels is that there is a high degree of inter-cell interference caused by the broadcast channels.

SUMMARY

Thus, as stated above, there is a need for a solution by means of which the degree of inter-cell interference caused by broadcast channels in a cellular wireless systems can be reduced.

Accordingly, it is an object of the present invention to obviate, at least to some extent, the problem mentioned above. This object is met by the present invention in that it discloses a method for use in a wireless cellular communications system in which there is a Radio Base Station, an RBS, which controls the traffic to and from user equipments, UEs, in a cell in the system.

In a system in which the invention may be applied, transmissions from an RBS to its UEs comprise transmissions on one or more broadcast channels, and the UEs in the system can assume one of at least two states, one such state being an "idle" state.

A UE which is in the "idle" state can transmit synchronization messages to an RBS in an uplink channel, and according to the method of the invention, the RBS uses such synchronization messages as "beacons" for beam forming transmissions of broadcast channels to the UE and for directing such formed beams towards the UE.

Thus, in a system in which the invention is applied, broadcast channels can be transmitted to idle UEs with an increased degree of directivity, instead of being transmitted as more or less omni-directional signals, which has of necessity been the case hitherto, since an RBS has not been able to know the direction to UEs which are in the idle state. By means of the invention, the degree of inter-cell interference caused by transmissions of broadcast channels may accordingly be reduced.

In a preferred embodiment, the UE transmits the synchronization messages when it receives a transmission on a broadcast channel, in other words the beacon is "triggered" by the broadcast channel from the RBS.

In a another embodiment, the UE transmits the synchronization messages when it wants to receive a transmission on a broadcast channel, in other words the broadcast channel to the UE from the RBS is "triggered" by the beacon. In another embodiment, the UE transmits the synchronization messages with a certain periodicity, and this periodicity can be controlled by messages from the RBS. The control messages, although they are transmitted to the UE by the RBS may also be from another source in the system, such as, for example, a Radio Network Controller, an RNC, which is a node that, inter alia, has as its purpose to control one or more RBSs in the system.

The method of the invention may in principle be applied to any wireless cellular system in which one or more broadcast channel(s) is (are) used, but in a preferred embodiment, the inventive method is applied to a TD-SCDMA system, a Time Division Synchronous Division Multiple Access system.

The invention also discloses a transceiver for use as a Radio Base Station, an RBS, in a cellular wireless communications system. The inventive RBS comprises means for controlling traffic to and from user equipments, UEs₁ in a cell in the system, and the inventive RBS also comprises means for:

• transmitting to the UEs on one or more broadcast channel,

• beam forming of transmissions to the UEs, • receiving synchronization messages from UEs on a random access channel.

According to the invention, the RBS comprises means for using the synchronization messages as "beacons" for beam forming transmissions of broadcast channels to the UE.

Thus, the RBS can use the beacon signal for finding the angle to the UE, and can use this angle as a direction in which to form and direct the beam of the broadcast channel when making broadcast transmissions to the UE. In this way, the beacon signal resembles a "lighthouse", i.e. an aid for finding a direction. In addition, the invention also discloses a transceiver for use as a User Terminal, a UE, in a wireless cellular communications system in which there is a Radio Base Station, RBS, which serves to control traffic to and from the UE.

The inventive UE comprises means for receiving transmissions from the RBS on one or more broadcast channels, as well as comprising means for assuming one of at least two states, one such state being an "idle" state.

When the UE of the invention is in the idle state, it can transmit a synchronization message to the RBS on a random access channel, and the UE of the invention transmits this synchronization message when it receives a transmission from the RBS on a broadcast channel.

Further details and advantages of the present invention will become apparent from the appended detailed description.

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 a schematic overview of a system in which the invention may be applied, and Fig 2 shows a subframe of the invention, and

Fig 3 shows the invention applied in the system of fig 1 , and

Fig 4 shows a first transceiver of the invention for use as an RBS, and

Fig 5 shows an antenna for use in the RBS of fig 5, and

Fig 6 shows a second transceiver of the invention for use as a UE, and Fig 7 shows a rough flow chart of a method 700 of the invention.

DETAILED DESCRIPTION Fig 1 shows an overview of a system 100 in which the invention may be applied. The invention may be applied in a variety of wireless cellular systems in which broadcast channels are used and cause inter-cell interference, but the invention will in the following be described with reference to a TD-SCDMA system, Time Division Synchronous Code Division Multiple Access. However, it should be pointed out that the use of a TD-SCDMA system in the description is as an example only, and should not be seen as restricting the scope of the present invention.

Returning now to fig 1 , it can be seen that the system 100 comprises a first transceiver 110, a so called Radio Base Station, RBS, which serves to, inter alia, control traffic to and from user equipments, UEs, in a cell 130 in the system. One UE 120 is shown in the cell 130 in fig 1 , but it should be pointed out that this is as an example only, a cell in the system 100 may comprise a large number of UEs.

The RBS 110 transmits certain information to the UEs in the cell 130 on so called broadcast channels. Examples of broadcast channels in a TD-SCDMA system are the Primary Common Control Channel, P-CCPCH. and the Secondary Common Control Channel, S-CCPCH. Among the information transmitted on the broadcast channels, mention may be made of system information such the so called system information block 1 , 3, 5, as well as paging messages, MBMS (Multimedia Broadcast Multicast Service) traffic data and certain signalling data.

In a system such as the TD-SCDMA system shown in fig 1 , the UEs may assume a number of different states, one of which is a so called "idle" state, which is a state in which the UE restricts its transmissions to the RBS. If a UE is in the idle state and still needs to access information in the broadcast channels, the RBS will not have any information as to where the UE is in the cell, due to the UE's "silence". As shown in fig 1 , the only way for the RBS to ensure that a transmission reaches a UE which is in the idle state is thus to transmit the broadcast channel in a very wide antenna beam, as indicated by the wide beam 140 in fig 1.

The antenna beam 140 will reach the UE, but due to the width of the beam, it may also "spill over" into adjacent cells, and cause interference in those cells.

The invention is intended to address the problem of interference from broadcast channels due to wide beams used to reach idle UEs. This is accomplished by letting UEs which are in the idle mode transmit a "beacon" signal to the RBS, i.e. a signal which may be used by the RBS to locate the UE and to transmit the broadcast channel or channels in a narrower beam, which will still ensure good reception at the UE but which will reduce or entirely eliminate the problem of interference in other cells in the system 100.

The invention proposes the use of synchronization messages from the UE to its RBS in a random access channel for use as such "beacon" messages. If the system in which the invention is applied has more than one random access channel, one of these may be chosen to send beacon messages in, but in the TD SCDMA system, the UpPCH, Up Link Synch Channel proposed for this use.

As background regarding the UpPCH, fig 2 shows a sub frame of the LCR (Low Chip Rate) frame used in TD-SCDMA, in which the UpPCH is comprised. As shown in fig 2, the LCR sub frame has an extension in time of 5 milliseconds, and comprises seven time slots, indicated with arrows in each slot. Arrows which point "up" (towards the top of the paper) are up link time slots and arrows which point "down" (towards the bottom of the paper) are down link time slots.

The LCR sub frame also comprises one up link pilot tone slot, UpPTS, one down link pilot tone slot, DwPTS, and one Guard Period, GP. The position or time slot in the sub frame allocated to the DwPTS, GP and UpPTS is shown enlarged separately in fig 2, as indicated by the arrow with dashed lines. The UpPCH is transmitted in the time slot allocated to the UpPTS.

In TD-SCDMA, there are 256 different SYNC-UL codes for an entire system, and the SYNC-UL codes are divided into 32 code groups, with 8 SYNC-UL codes in each code group.

In a preferred embodiment of the invention, only one of the SYNC-UL codes will be allocated for use as the uplink "beacon" signal for use by all the UEs in a cell, since the RBS does not need to distinguish between different UEs for the present purpose; the RBS can beam form the transmissions of the broadcast channel or channels towards each UE that sends the SYNC-UL. This SYNCJJL code can be chosen more or less arbitrarily from the codes which are available, and in other embodiments more than one code can be chosen.

Fig 3 shows a system 300 with the present invention applied. The reference numbers from fig 1 have been retained for corresponding components in the system 300 in fig 3. As can be seen in fig 3, the beam 340 emitted towards the UE 120 is now a narrow beam, a beam which has been shaped towards the location of the origin of the "beacon" message, i.e. the UE. Thus, the "spill over" effect of the wide beam 140 of fig 1 is reduced to a large extent, or in some cases entirely eliminated. In this way, the use of the beacon message can be said to resemble the use of a "lighthouse", i.e. the beacon message is used by the RBS in order to find the direction of the direction towards the UE, which is the used in order to direct the formed beam 340 (or the centre of the formed beam 340) towards the UE which transmits the beacon message.

In one embodiment of the invention, idle UEs transmit synchronization messages (i.e. "beacon" messages) when they receive a transmission on a broadcast channel, so that a transmission on a broadcast channel will serve as a trigger for an idle UE to initiate the process by means of which the RBS transmits broadcast channels towards the UE in a focused (shaped) beam.

In another embodiment of the invention, idle UEs transmit synchronization messages (i.e. "beacon" messages) when they want to receive a transmission on a broadcast channel, so that synchronization messages received by the RBS will serve as "trigger" for the RBS to start to transmit broadcast channels towards the UE in a focused (shaped) beam.

In one aspect of the embodiment, idle UEs transmit the synchronization messages with a certain periodicity, which is suitably controlled by messages from the RBS, so that the RBS may control how frequently it receives the beacon messages which the RBS uses to perform the beam forming. In one embodiment, the control messages can be the so called system information blocks. UE could suitably have a default value of the periodicity before it has received any system information block, and the UE can then update the value after receiving the value from system information.

Before the mechanism which the RBS may employ to perform the beam shaping is described in detail, an RBS of the invention will be described with the aid of fig 4. Thus, fig 4 shows an RBS 400 of the invention, with the major "building blocks" of the RBS also being shown in fig 4.

As can be seen, the RBS 400 of the invention comprises an antenna 410 for transmissions to and from the UEs in a cell, and the RBS also comprises a receiver 420 and a transmitter 430, which serve to receive and transmit information to and from the UEs. The receiver and the transmitter thus interface with the antenna 410, as shown in fig 4.

The RBS 400 also comprises a control component 440, such as a microprocessor or other computer means, and also comprises a memory 450. The control component 440 is shown as interfacing all of the other components in the RBS 400, since the control component controls the function of the other components.

Another component in the RBS 400 is an interface "Int" 460, which is meant to signify the interface that the RBS has towards the higher nodes in the system, i.e. the nodes in the system to and from which traffic and possibly also control information from the UEs is transmitted/received. One example of control information to a UE from a node other than the UE could be "hand over" instructions. This interface is usually a landline interface.

In the RBS 400, it will be the control component 440 which is responsible for controlling traffic to and from user equipments, UEs, in a cell in the system, while the transmitter 630 and the receiver 620 together with the antenna 610 will be used by the RBS to transmit and receive to the UEs on one or more broadcast channel. The synchronization messages from UEs on a random access channel will thus be via the antenna 610 and the receiver 620.

The beam forming, as will be explained in more detail in the following, will be controlled by the control component 640, and carried out in the antenna 610.

If the RBS 400 is to control the UEs regarding a periodicity for transmitting the beacon messages, this will suitably be controlled via the control component 640, preferably with the aid of the memory 650, and control messages for this will be transmitted via the antenna 610 by the transmitter 630.

Suitably but not necessarily, the RBS 600 is an RBS in a TD-SCDMA system, Time Division Synchronous Division Multiple Access, and the broadcast channels used will in a TD-SCDMA system comprise the Primary Common Control Channel, P-CCPCH and the Secondary Common Control Channel, S-CCPCH. Turning now to the issue of how the RBS 400 performs the beam shaping as employed in the present invention, it can be pointed out that beam shaping as such is a well known technology, and a wide variety of beam forming techniques may be used by the RBS 400 in order to achieve the desired effect, i.e. a narrow (at least in the horizontal plane) antenna beam directed towards a certain UE. Naturally, if two or more UEs are closely located to each other, it may be advantageous to use one and the same beam for those UEs. Since beam forming as such is well known to those skilled in the art, it will only be described briefly here.

In order to obtain the beam forming effect, the antenna 410 of the RBS 400 should comprise two or more antenna radiation elements, the term "radiation element" here being used as a generic term for an antenna element which may be used both for reception and transmission. Examples of known beam forming techniques which may be used with respect to the antenna elements of the antenna 410 in the RBS 400 are phase or frequency controlled antenna arrays, the term "array" here being used to include the antenna elements of the antenna of the RBS, or digital beam forming at the baseband level. Other examples include a beam-forming network at RF-level using passive devices such as 3 dB hybrid couplers, fixed phase shifters and delay lines, or a Butler matrix.

In order to briefly illustrate the concept of beam forming, an example will be described with reference to fig 5, which shows an example of an embodiment of the antenna 410 of the RBS 400

The antenna 410 of fig 4 comprises three radiation elements 520, 530, 540, arranged symmetrically in a column in the antenna 410. The antenna 410 also comprises a beam forming controller 560, which could, for example, be the controlling device 440 of the entire RBS 400 as shown in fig 4, or at least comprised in that component. In a preferred embodiment, the beam forming controller 560 carries out its task by means of a beam forming algorithm. The beam forming device is connected to the radiation elements 520, 530, 540, via a feeder network 550. As indicated in fig 5, the beam forming in the antenna 410 in fig 5 is carried out by means of phase control, i.e. the signal to the different antenna elements in the antenna are given different phase shifts or phase delays φ-i , ψ₂, ψ₃, which are calculated by the beam forming controller 560 in order to achieve the desired shape of the antenna beam. However, it should be pointed out again that phase control is merely one of a variety of known techniques for achieving beam forming.

It can also be pointed out that the one-dimensional antenna array or column antenna 410 of fig 5 is merely an example of an antenna which is suitable for beam forming. In other embodiments, the antenna 410 may comprise two or more radiation elements, which may also be arranged as a two dimensional matrix instead of the one dimensional vector shown in fig 5. In addition, the antenna of fig 5 only permits the use of beam forming in the direction indicated by means of the arrow "S", since the antenna elements are arranged along an axis parallel with that arrow. In order to obtain beam forming in two directions, the two dimensional antenna mentioned above would be necessary, as long as the antenna elements in the antenna are of a single polarization.

Regarding the use of the beacon messages for beam shaping, the use of a signal in order to find the angle or direction to the transmitter of the signal, i.e. the "lighthouse" function mentioned previously, is as such a well known technique which can be carried out in a wide variety of ways. However, one way of implementing this is to let the antenna shown in fig 5 be used for this purpose, i.e. to use an antenna with a plurality of radiation elements, so that, for example, signal strength measurements may be compared between the different elements of the antenna in order to find the direction to the transmitter, in this case the UE. Fig 6 shows an example of a transceiver 600 for use as a UE of the invention. As can be seen, the UE 600 of the invention comprises an antenna 610 for transmissions to and from an RBS, and the UE also comprises a receiver 620 and a transmitter 630, which serve to receive and transmit traffic to and from an RBS. The receiver 620 and the transmitter 630 thus interface with the antenna 610. as shown in fig 6.

The UE 600 also comprises a control component 640, such as a microprocessor or other computer means, and also comprises a memory 650. The control component 640 is shown as interfacing all of the other components in the UE 600, since the control component controls the function of the other components.

Thus, in a UE 600 of the invention, the antenna 610 and the receiver 620 will serve as means for receiving transmissions from an RBS on one or more broadcast channels, while the control component 640 would serve as means for assuming one of at least two states, one such state being an "idle" state.

By using the transmitter 630 and the antenna 610 the UE will be able to, when it is in the idle state, transmit a synchronization message to an RBS on a random access channel, and the UE will transmit the synchronization message when it receives a transmission from the RBS on a broadcast channel, said transmission being received by the antenna 610, the receiver 620 and detected by the control component 640.

If the synchronization message is to be transmitted with a certain periodicity as controlled by the RBS, the antenna 610, the receiver 620 and the control component 640 will serve as means in the UE 600 for letting the periodicity be controlled by messages from the RBS.

Fig 7 shows a rough flow chart of a method 700 of the invention. Steps which are options or alternatives are shown with dashed lines. As has also emerged from the description above, the inventive method 700 is intended for use in a wireless cellular communications system such as the one 300 shown in fig 3, and as indicated in step 710, in the system there will be a Radio Base Station, an RBS, such as the one 110 of fig 3, which serves to control traffic to and from user equipments such as the one 120 of fig 3 within a cell 130 in the system.

As indicated in step 715, transmissions from the RBS to the UEs will comprise transmissions on one or more broadcast channels, and as shown in step 720, a UE can assume one of at least two states, one of which is an "idle" state.

Step 725, "synch", shows that a UE which is in the idle state can transmit synchronization messages to its RBS in a random access channel, and as shown in step 730, "BF", the RBS uses such synchronization messages as "beacons" for beam forming (BF) transmissions of broadcast channels to the UE.

Step 735 shows that in one embodiment of the invention, a UE transmits synchronization messages when it receives a transmission on a broadcast channel.

As shown in step 740, "f(RBS)", in one embodiment of the invention, the UE transmits synchronization messages with a certain periodicity, which can be controlled by messages from the RBS.

Step 745 shows that the inventive method 7+00 may suitably but not necessarily be applied to a TD-SCDMA system. Time Division Synchronous Division Multiple Access. In such an embodiment, as shown in step 750, the broadcast channels will comprise the Primary Common Control Channel, P- CCPCH, and the Secondary Common Control Channel, S-CCPCH, and as shown in step 755, the synchronization messages transmitted by a UE in a random access channel in a TD-SCDMA system will be the SYNCJJL messages in the Up Link Synch channel, the UpPCH channel.

The method of the invention may be implemented in hardware or software or in a combination of hardware and software. A software implementation may be stored on a computer readable medium such as, for example, a floppy disk, a CD or DVD disc, magnetic tape, E²-memories etc.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims. For example, the forming of the beam which is directed towards a UE which transmits the beacon message may also comprise power control of the beam, i.e. if the beacon message is used by the RBS to measure the distance of the UE from the RBS, the distance can then be used by the RBS to calculate how much output power should be used in the transmissions to the UE. In order to measure the distance to the UE, the RBS may use delay times in the messages received from the UE, or the power level in the received messages.

Claims

1. A method (700) for use in a wireless cellular communications system (300), in which (710) there is a Radio Base Station (110), RBS, which serves to control traffic to and from users equipments (120). UEs, within a cell (130) in the system, in which system transmissions from the RBS to the UEs comprise (715) transmissions on one or more broadcast channels, and in which system a UE can assume (720) one of at least two states, one such state being an "idle" state, and where (725) an UE which is in the idle state can transmit synchronization messages to an RBS in a random access channel, the method being characterized in that (730) the RBS uses such synchronization messages as "beacons" for beam forming transmissions of broadcast channels to the UE and for directing such formed beams towards the UE.

2. The method (700, 735) of claim 1 , according to which the UE transmits synchronization messages when it receives a transmission on a broadcast channel.

3. The method (700, 740) of claim 1 or 2, according to which the UE transmits synchronization messages with a certain periodicity, said periodicity being controlled by messages from the RBS.

4. The method (700, 745) of any of the previous claims, applied to a TD- SCDMA system, Time Division Synchronous Division Multiple Access.

5, The method (700, 750) of claim 4, according to which the broadcast channels comprise the Primary Common Control Channel, P-CCPCH, and the Secondary Common Control Channel, S-CCPCH.

6. The method (700, 755) of claim 4 or 5, according to which the synchronization messages transmitted by a UE in a random access channel are SYNCJJL messages in the Up Link Synch channel, the UpPCH channel.

7. The method (700, 755) of claim 6, according to which there is a plurality of codes available for use as SYNCJJL messages, and one of these is chosen as the "beacon" message for beam forming in a cell.

8. A transceiver (400) for use as a Radio Base Station, an RBS, in a cellular wireless communications system (300), the RBS comprising means (440,

430 ,410) for controlling traffic to and from user equipments (120), UEs, in a cell in the system, the RBS comprising means for:

• transmitting (430, 410) to the UEs on one or more broadcast channel,

• beam forming (410, 440) of transmissions to the UEs, • receiving (410, 420, 440) synchronization messages from UEs on a random access channel, the RBS (400) being characterized in that it comprises means (440) for using the synchronization messages as "beacons" for beam forming transmissions of broadcast channels to the UE and for directing such formed beams towards the UE.

9. The RBS (400) of claim 8, in which the beam forming means comprise a plurality of radiation elements (520, 530, 540), a beam forming feeder network (560), and a beam forming controller (560).

10. The RBS (400) of claim 8 or 9, in which the beam forming means comprise a beam forming algorithm.

11. The RBS (400) of any of claims 8-10, comprising means (440, 430, 410) for sending control messages to one or more UEs (120) for controlling a periodicity with which the UEs transmits their synchronization messages.

12. The RBS (400) of any of claims 8-11 being an RBS in a TD-SCDMA system, Time Division Synchronous Division Multiple Access.

13. The RBS (400) of claim 12, in which the broadcast channels comprise the Primary Common Control Channel, P-CCPCH, and the Secondary Common Control Channel, S-CCPCH.

14. A transceiver (600) for use as a User Terminal (120), a UE, in a wireless cellular communications system (300) in which there is a Radio Base Station (110), RBS, which serves to control traffic to and from the UE, the UE comprising means (610, 620, 640) for receiving transmissions from the RBS on one or more broadcast channels, the UE (600) also comprising means (640) for assuming one of at least two states, one such state being an "idle" state, which UE when it is in the idle state can transmit a synchronization message to the RBS on a random access channel, the UE being characterized in that it transmits the synchronization message when it receives a transmission from the RBS on a broadcast channel.

15. The UE (600) of claim 14, which transmits the synchronization message with a certain periodicity, the UE comprising means (610, 620, 640) for letting the periodicity be controlled by messages from the RBS.

16. The UE (600) of claim 14 or 15, being a UE in a TD-SCDMA system, Time Division Synchronous Division Multiple Access.

17. The UE (600) of claim 16, which transmits the synchronization message on a random access channel as SYNC_UL messages in the Up Link Synch channel, the UpPCH channel.

18. The UE (600) of claim 17, which uses one of a plurality of codes which are available for use as SYNC_UL messages as the synchronization message.