WO2009002243A1

WO2009002243A1 - Varied cell size in a time division duplex system

Info

Publication number: WO2009002243A1
Application number: PCT/SE2007/050468
Authority: WO
Inventors: Magnus Johansson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-06-26
Filing date: 2007-06-26
Publication date: 2008-12-31

Abstract

A method (500) for use in a cellular system (100) with cells (110) and Mobile Stations, MSs (112), with at least one Base Station (111) for each cell. The system utilizes the TDD principle, with a Down Link, DL, period (320) and an Up Link, UL, period (350), a guard period, TTG (340), at the transition from DL to UL, and a guard period, RTG (360), at the transition from UL to DL, each of the DL and UL periods comprising a certain amount of time slots. The method (500) comprises at least a first Base Station (111) of a first cell (110) in the system (100) being given information about the geographical size of the first cell, and using this in order to adapt (330, 430) the number of time slots used in at least one of the DL (320) and UL (350) periods according to the cell size.

Description

TITLE

Varied cell size in a Time Division Duplex system.

TECHNICAL FIELD The present invention discloses a method and a device for varying the cell size in a Time Division Duplex system.

BACKGROUND

A cellular wireless access system is usually divided into cells, with a number of users in each cell, and a base station per cell, through which the traffic to and from the users in the cell is routed. A user, or rather the terminal of the user, is often referred to as a Mobile Station, MS. The traffic from the base station to the users is referred to as Down Link, DL, traffic, and the traffic from the users to their base station is referred to as Up Link, UL, traffic.

In some cellular systems, the Time Division Duplex, TDD, principle is used. According to the TDD principle, UL and DL traffic share the same frequency or frequencies, but are divided in time.

One example of a system which utilizes the TDD system is the WiMAX system, which uses so called "frames" which comprise both an UL and a DL part. In a WiMAX frame, there is also a so called guard period at the transition from DL to UL, in order to give the base station and the MSs time to switch between reception and transmission, and to compensate for the propagation delay between the base station and the MSs. This guard period is referred to as TTG, transmit/receive transition gap, and there is also a guard period at the transition from UL to DL, known as RTG, receive/transmit transition gap.

In, for example, the WiMAX system, the guard periods are fixed, which means that it is not possible for an operator to expand the cell size beyond that which is made possible by the maximum (cell edge) round trip delay, RTD, allowed by the TTG. This is an obvious disadvantage for operators who wish to expand the cells of their systems more than that.

SUMMARY Thus, as explained above, there is a need for a solution by means of which an operator of a TDD system such as, for example, a WiMAX system, would be able to vary the size of the cells in his system in a more flexible manner than that which is possible today.

Such a solution is offered by the present invention in that it discloses a method for use in a cellular wireless access system in which there can be a number of cells with a number of Mobile Stations, MSs, in each cell, and at least one Base Station for each cell.

The traffic to and from the MSs in a cell in the system is routed via the Base Station of that cell, and the system in which the invention may be applied is one which utilizes the Time Division Duplex, TDD, principle. Thus, there is a Down Link, DL, period with communication from the Base Stations to the MSs and an Up Link, UL, period with communication from the MSs to the Base Stations, and there is also a first guard period, TTG, at the transition from DL to UL, and a second guard period, RTG, at the transition from UL to DL.

In the system in which the invention may be applied, each of the DL and UL periods comprise a certain amount of time slots, and according to the method of the invention, at least a first Base Station of a first cell in the system is given information about the geographical size of the first cell, which the Base Station uses in order to adapt the number of time slots used for transmission in at least one of the DL and UL periods according to the cell size.

Thus, since, according to the invention, the cell size can be used in order to adapt the number of time slots used for transmission in the DL and/or the UL period, the size of a cell in a system in which the invention is applied can be varied in a flexible manner, particularly as opposed to that which is possible at present.

In a particular embodiment of the invention, the Base Station adapts the number of time slots used for transmission in the DL period by not using one or more of the time slots at the end of the DL period for transmission, so that a longer TTG guard period is obtained, which means that a larger cell size can be used.

In another embodiment of the invention, the Base Station adapts the number of time slots used for transmission in the UL period by not using one or more of the time slots at the beginning of the UL period for transmission, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used.

The invention also discloses a Base Station which functions according to the method of the invention.

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 part of a system in which the invention may be applied, and Fig 2 shows prior art, and

Figs 3 and 4 show concepts according to the invention, and Fig 5 shows a flow chart of a method of the invention, and Fig 6 shows a block diagram of a base station of the invention.

DETAILED DESCRIPTION

Fig 1 schematically shows a part of a system 100 in which the invention can be applied. The invention will be described below with reference to a WiMAX system, but it should be pointed out that the invention can be applied in a variety of systems which use the TDD principle, such as, for example, the LTE, Long Term Evolution, system. Thus, although the terminology used in the following will be WiMAX terminology, the man skilled in the field will realize which components in other systems, such as the LTE system, that correspond to the components shown in the drawings and described in the following.

Returning now to fig 1 , there is shown a part of a cellular wireless access system 100, in which there can be a number of cells, one of which is shown as 110 in fig 1. In each cell 110 of the system 100, there can be a number of users with terminals, so called Mobile Stations, MSs, one of which is shown as 112 in fig 1. In addition, the system comprises a number of Base Stations,

BSs, one of which is shown in fig 1 as 111. The Base Stations can have slightly different roles in different systems, but a basic role for the Base

Station is to route all traffic to and from the MSS in a cell to and from the system as such.

As mentioned previously, the system in fig 1 uses the so called TDD, Time Division Duplex, principle. According to the TDD principle, there are periods with transmission from the BSs in the system to their MSs, the so called Down Link, DL, periods, which alternate with periods with transmission from the MSs in the system to their BSs, the so called Up Link, UL, periods.

In order to allow the BSs/MSs in the system to switch between transmit and receive, and also in order to take into account propagation delays, the so called RTD, Round Trip Delay, between the BSs and the MSs, there needs to be an "intermission" at the transition between the UL and DL periods. In the WiMAX system, this need is addressed by means of a guard period during which no transmission are made, the WiMAX system having a first guard period, the so called TTG (transmit/receive transition gap), at the transition from DL to UL, and a second guard period, the so called RTG (receive/transmit transition gap), at the transition from UL to DL.

The TDD principle used in WiMAX is shown in fig 2: one WiMAX frame comprises one DL sub frame and one UL sub frame. As is indicated in fig 2, the UL and the DL sub frames do not need to be of equal duration. Each sub frame comprises a number of time slots, i.e. shorter periods of time. As is also shown in fig 2, at the transition from DL to UL, there is the so called TTG guard period, i.e. a period during which no transmission may be made, and at the transition from UL to DL there is the so called RTG guard period, during which transmission is also prohibited.

As mentioned previously, the duration of the TTG and RTG periods are fixed in WiMAX, which places restrictions and the size of a cell, since the guard periods must allow for the propagation delays from cell edge MSs.

Thus, in the WiMAX system, there are specified values of TTG and RTG for each supported channel bandwidth, which will put limits on the maximum distance between BS and MS, i.e. the maximum cell radius.

As an example, for a channel with 7 MHz bandwidth, WiMAX specifies the following:

For 7 MHz, the TTG and RTG values are 376 PS and 120 Physical Slots, PS, respectively, and one PS is 4 samples long for OFDMA.

TTG includes RTD and SSRTG, where the SSRTG is the RX/TX switching time in the MS. SSRTG is specified as 50 μs in the WiMAX standard. For a 7 MHz bandwidth, the sampling frequency is 8 MHz, which means that TTG is 376 ^* 4 / 8e6 = 188e-6, that is 188 μs. The RTD is 2 ^* r / c, where r is the distance between BS and MS, and c is the speed of light. The maximum distance between BS and MS is RTD * c / 2 = (TTG - SSRTG) * c / 2. For 7 MHz bandwidth, the maximum cell radius thus becomes ((188e-6 - 50e-6) ^* 3e8) / 2 = 20700 meters.

Thus, the use of fixed TTG and RTG implies a fixed upper limit to the cell size in WiMAX, which is a disadvantage for a system operator who wishes to be able to design his system in a flexible manner, and perhaps also wishes to have the ability to redesign his system at a later point in time.

This disadvantage is addressed by the present invention in the following manner: at least a first Base Station of a first cell in the system is given information about the geographical size of the first cell, i.e. the cell of that Base Station, and uses this in order to adapt the number of time slots used for transmission in at least one of the DL and UL periods according to the cell size.

This concept of the present invention can be explained more closely with reference to figs 3 and 4: fig 3 shows a WiMAX frame 310, which, as explained in conjunction with fig 2, comprises a DL period 320, a TTG period 340, an UL period 350 and an RTG period 360. However, as opposed to the prior art frame shown in fig 2, the Base Station of the cell in which the frame 310 is used does not utilize a certain number of time slots at the end of the DL period, which thus in effect creates a longer TTG period, the increase in TTG being shown as "Δt", with the resulting TTG being shown as TTG'.

The time slots at the end of the UL period 320 which are not used are thus "silenced", i.e. no transmissions are made by the Base Station during a certain period 330 at the end of the DL period. The larger TTG obtained in this way will thus enable longer RTDs, which in turn means that the cell radius can be expanded by a distance which corresponds to the extra time "Δt". The increase in cell radius "Δr" can be expressed as Δr=(c ^* Δt)/2, where c is the speed of light.

Fig 4 shows another alternative according to the invention: instead of silencing a number of time slots at the end of the DL, the same effect can, in principle be obtained by means of silencing a number of time slots at the beginning of the UL, which would then have to be applied by all of the MSs in the cell in question. In fig 4, items which have already been shown in fig 3 have been given the same reference numerals as in fig 3. As explained, the embodiment of fig 4, as opposed to that of fig 3, shows a "silent period" 430 at the beginning of the UL period 350.

The increase in cell radius "Δr" obtained by means of the silent period 430 can be expressed as above, i.e. Δr = (c ^* Δt)/2. However, since the silent period 430 implies a period of no transmission from the MSs in the cell in question, the MSs would have to be notified by the Base Station of this in advance. This is suitably done by means of a broadcast message from the BS of the cell in question, said message comprising information regarding the amount of silent time slots, and when the use of the "silent period" Δt 430 is to begin.

The expansion of the cell size which has been described above in connection to figs 3 and 4 can be carried out in various ways with respect to the role of the Base Station. In one embodiment of the invention, the Base Station receives information about the desired cell radius from the system, and comprises a function for using this information in order to determine how many time slots that should be encompassed by the "silent period" Δt, and if the silent period Δt should be taken from the DL or the UL period. This decision is then transmitted by the Base Station to its MSs, at least in the case where the decision is to place all or part of the silent period in the UL, while it may not be necessary if the silent period is placed in its entirety in the DL period. The function for deciding on where the silent period should be placed, in the UL or in the DL may use the traffic intensity as input to its decision, In other words, if there is a high traffic intensity in the UL, it may be suitable to have a silent period in the DL, and vice versa with a high traffic intensity in the DL.

In an alternative embodiment, the information received by the Base Station comprises information about how many time slots that should be encompassed by the silent period Δt, and if they should be taken from the DL or the UL period, said information then being transmitted by the Base Station to its MSs. In this embodiment, the decision regarding the silent period, i.e. its duration, and if it should be taken from the DL or the UL period, has been taken by another node in the system, or possibly manually by an operator.

The expansion of the cell size which has been described above can also, according to the invention, be "reversed" if the need should arise. Thus, if there is a need or a desire to revert to the "original" cell radius, the Base

Station can, according to the invention, adapt the number of time slots used for transmission time in the DL period by using one or more time slots at the end of the DL period for transmission, if those time slots have previously been silenced.

Similarly with the UL period, the Base Station can adapt the number of time slots used for transmission in the UL period by using one or more of the time slots at the beginning of the UL period for transmission, if those time slots have previously been silenced, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used

The use of previously silenced time slots can also be done by means of a function in the Base Station which is informed of a change on the cell radius, and then takes the corresponding decision regarding the "activation" of previously silenced time slots, or the Base Station can simply be ordered by the system to activate previously silenced time slots. In both cases, the Base Station transmits the necessary information to the MSs in its cell.

Fig 5 shows a flow chart of a method 500 of the invention. Steps which are options or alternatives are indicated with dashed lines. As shown in step 510, the method comprises giving at least a first Base Station of a first cell in the system information about the geographical size of the first cell, and as shown in step 520, the Base Station then uses this information in order to adapt the number of time slots used for transmission in at least one of the DL and UL periods according to the cell size.

As shown in step 525, and as has also been described previously in this text, the decision on how to adapt the number of time slots in the DL or the UL can be taken either by a function for this in the Base Station, or the Base station can simply be ordered by a central function in the system to relay such instructions to the MSs in its cell, hence the text "central/BS" in step 525..

As indicated in steps 530 and 540, the adaptation can involve either silencing time slots at the end of the DL, step 530, or silencing time slots at the beginning of the UL, step 540.

As indicated in steps 550 and 560, either of steps 530 and 540 can be "reversed", if it is desired to reduce the cell radius again, so that previously silenced time slots are used again for transmission.

The invention also discloses a Base Station which functions basically according to the method of the invention as described above. A schematic block diagram of such a Base Station is shown in fig 6.

As can be seen in fig 6, the Base Station 600 of the invention comprises an antenna 610 for reception and transmission of information from/to the MSs in the cell of the Base Station, as well as also comprising an interface towards other nodes in the system, said interface being shown together with the antenna

The Base Station also comprises a receiver part, Rx, 620, a transmitter part Tx 630, a computer such as a microprocessor 640, and a memory 650.

The antenna/interface 610 can, together with the receiver 620, be used for receiving information about the geographical size of the first cell, which information can then be processed by the computer 640 in conjunction with the memory 650 in order to arrive at the number of time slots used for transmission in either the DL 320 or the UL 350 periods according to the cell size. The result of the processing can then be transmitted by the transmitter 630 and the antenna/interface 610 to the MSs. Thus, the computer 640, the memory 650, the transmitter 630 and the antenna/interface 610 can be seen as adaptation means.

As stated previously in this text, the adaptation means may adapt the number of time slots used for transmission in the DL period by not using one or more of the time slots at the end of the DL period for transmission, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used. This may be "reversed" so that the adaptation means subsequently adapt the number of time slots used for transmission time in the DL period by using one or more time slots at the end of the DL period for transmission, if those time slots have previously been silenced, in order to adapt to a smaller cell size.

As an alternative, the adaptation means can adapt the number of time slots used for transmission in the UL period by not using one or more of the time slots at the beginning of the UL period for transmission, so that a longer TTG guard period is obtained, by means of which a larger cell size can be used. This can also be "reversed", so that the adaptation means adapt the number of time slots used for transmission in the UL period by using one or more of the time slots at the beginning of the UL period for transmission, if those time slots have previously been silenced, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used

In addition, the Base Station of the invention may use the computer 640 and the memory 650 to use the information about the cell size in order to determine how many time slots that should be encompassed by the adaptation, and if they should be taken from the DL or the UL period. This can then be transmitted to the MSs by means of the antenna/interface 610 and the transmitter 630.

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. It should, for example, be pointed out that it is possible according to the invention to silence time slots in both the beginning of the UL and the end of the DL in one and the same time frame, in order to obtain a larger guard period TTG.

Claims

1. A method (500) for use in a cellular wireless access system (100) in which system there can be a number of cells (110) with a number of Mobile Stations, MSs (112), in each cell, and at least one Base Station (111) for each cell, with the traffic to and from the MSs in a cell being routed via the Base Station of that cell, which system utilizes the Time Division Duplex, TDD, principle, so that there is one Down Link, DL, period (320) with communication from the Base Stations to the MSs and one Up Link, UL, period (350) with communication from the MSs to the Base Stations, with a first guard period, TTG (340), at the transition from DL to UL, and a second guard period, RTG (360), at the transition from UL to DL, each of the DL and UL periods comprising a certain amount of time slots, the method (500) being characterized in that (510) at least a first Base Station (111 ) of a first cell (110) in the system (100) is given information about the geographical size of the first cell, and uses this in order to adapt (330, 430) the number of time slots used for transmission in at least one of the DL (320) and UL (350) periods according to the cell size.

2. The method (500) of claim 1 , according to which the Base Station (111 ) adapts (520) the number of time slots used for transmission in the DL period by not using one or more of the time slots (330) at the end of the DL period (320) for transmission, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used.

3. The method (500) of claim 1 , according to which the Base Station (111 ) adapts (550) the number of time slots used for transmission time in the DL period (320) by using one or more time slots (330) at the end of the DL period for transmission, said time slots having previously been silenced, in order to adapt to a smaller cell size.

4. The method (500) of claim 1 , according to which the Base Station (111 ) adapts (540) the number of time slots used for transmission in the UL period (350) by not using one or more of the time slots (430) at the beginning of the UL period for transmission, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used.

5. The method (500) of claim 4, according to which the Base Station (111 ) adapts (560) the number of time slots used for transmission in the UL period (350) by using one or more of the time slots (430) at the beginning of the UL period for transmission, said time slots having previously been silenced, in order to adapt to a smaller cell size.

6. The method (500) of any of claims 1-5, according to which the Base Station (111 ) comprises (525) a function for using the information about the cell size in order to determine how many time slots that should be encompassed by said adaptation, and if they should be taken from the DL or the UL period, which is then transmitted by the Base Station to its MSs (112) if the time slots in question are to be taken from the UL period.

7. The method (500) of any of claims 1-5, according to which the information received by the Base Station (111) comprises (525) information about how many time slots that should be encompassed by said adaptation, and if they should be taken from the DL or the UL period, said information being transmitted by the Base Station to its MSs (112).

8. A Base Station (111 , 600) for use in a cellular wireless access system (100) in which system there can be a number of cells (110) with a number of Mobile Stations, MSs (112), in each cell, the Base Station (111 , 600) being intended for routing the traffic to and from the MSs in a cell, which system utilizes the Time Division Duplex, TDD, principle, so that there is one Down Link, DL, period (320) with communication from the Base Station (111 , 600) to the MSs and one Up Link, UL, period (350) with communication from the MSs to the Base Station, with a first guard period, TTG (340), at the transition from DL to UL, and a second guard period, RTG (360), at the transition from UL to DL, each of the DL and UL periods comprising a certain amount of time slots, the Base Station (600) being characterized in that it comprises means (610, 620) for receiving information about the geographical size of the first cell, and means (640, 650, 630, 610) for using this information in order to adapt the number of time slots used for transmission in at least one of the DL (320) and UL (350) periods according to the cell size.

9. The Base Station (111 , 600) of claim 8, in which the adaptation means (640, 650, 630, 610) adapt the number of time slots used for transmission in the DL period by not using one or more of the time slots (330) at the end of the DL period (320) for transmission, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used.

10. The Base Station (111 , 600) of claim 8 or 9, in which the adaptation means (640, 650, 630, 610) adapt the number of time slots used for transmission time in the DL period (320) by using one or more time slots (330) at the end of the DL period for transmission, said time slots having previously been silenced, in order to adapt to a smaller cell size.

11. The Base Station (111 , 600) of claim 8, in which the adaptation means (640, 650, 630, 610) adapt the number of time slots used for transmission in the UL period (350) by not using one or more of the time slots (430) at the beginning of the UL period for transmission, by means of which a longer TTG guard period is obtained, so that a larger cell size can be used.

12. The Base Station (111 , 600) of claim 11 , in which the adaptation means (640, 650, 630, 610) adapt the number of time slots used for transmission in the UL period (350) by using one or more of the time slots (430) at the beginning of the UL period for transmission, said time slots having previously been silenced, in order to adapt to a smaller cell size.

13. The Base Station (111 , 600) of any of claims 8-12, comprising means (640, 650) for using the information about the cell size in order to determine how many time slots that should be encompassed by said adaptation, and if they should be taken from the DL or the UL period, and means (610, 630) for transmitting this to the MSs (112) whose traffic is routed via the Base Station.

14. The Base Station (111 , 600) of any of claims 8-12, in which the adaptation means (640, 650, 630, 610) forward the information received to the MSs (112) whose traffic is routed via the Base Station, the information which is received already comprising information about how many time slots that should be encompassed by said adaptation, and if they should be taken from the DL or the UL period.