WO2008097142A1

WO2008097142A1 - A method for improved random access in a cellular wireless access system

Info

Publication number: WO2008097142A1
Application number: PCT/SE2007/050060
Authority: WO
Inventors: David Astely; Hannes Ekström; Tobias Tynderfeldt; Alireza Nejatian
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-02-05
Filing date: 2007-02-05
Publication date: 2008-08-14

Abstract

A method for use in a cellular wireless system (100) with a base station (120) which controls traffic to and from a cell (110) with a number of user terminals (130) which can receive traffic from the base station during a down link interval and transmit traffic to the base station during an up link interval. There are guard periods between the up link and the down link intervals, and between the down link and the up link intervals. The system comprises a random access channel for unscheduled transmissions by the user terminal, and the method comprises letting the base station determine and transmit information to user terminals for calculating an offset time for initiating random access transmissions relative to a nominal starting time for random access transmissions, said offset time also being used by the base station to initiate (530) reception of said random access transmissions.

Description

TITLE

A method for improved random access in a cellular wireless access system.

TECHNICAL FIELD The present invention discloses a method for use in a cellular wireless access system in which there is at least one base station which controls the traffic to and from a cell in the system. The cell can accommodate a number of user terminals, and the user terminals can be scheduled for receiving traffic from the base station during a first time interval, the down link interval, and for transmitting traffic to the base station during a second time interval, the up link interval.

In the system, there is a first guard period between the transition from the up link interval to the down link interval, and a second guard period between the transition from the down link interval to the up link interval, and the system also comprises a random access channel for unscheduled transmissions from the user terminal to the base station.

BACKGROUND In future cellular wireless access systems as well as in some contemporary systems, a principle which may be used is the so called TDD principle, Time Division Duplex. In TDD systems, transmissions from the base stations to the user terminals, the "down link" as well as transmissions from the user terminals to the base stations, the "up link", are carried out on the same fre- quency, but with a division in time.

In TDD operation, pauses in the form of silent periods, so called "guard periods", need to be inserted at the transition from downlink to uplink periods due to propagation delays and also to allow the user terminals to switch from re- ception to transmission and vice versa for the base stations. Currently, it is intended to create these guard periods by removing or silencing a number of down link symbols at the part of the downlink periods which immediately ad- join the uplink period. In this way, a guard time at the transition from downlink to uplink will be created.

In addition, a guard time is also needed at the transition from uplink to downlink in order to allow the base stations to switch from reception to transmission, and to allow the user terminals to switch from transmission to reception. Since terminals have their transmission timing controlled, this second guard period may be created by advancing the terminal transmission time relative to a nominal starting time.

Thus, the magnitude of the guard times mentioned need to take into account a number of factors, such as the cell range or size and terminal and base station switch times, as well as possible network synchronization inaccuracies.

However, an additional problem in the context of guard periods with a view to cell range is caused by the so called random access channel, which is a channel in which the user terminals in a cell may freely initiate unscheduled transmissions to the base station.

The transmissions of random access bursts are typically carried out in an unsynchronized fashion, i.e. the random access bursts transmitted by different user terminals are not time aligned when they arrive at the base station. The main reason for this is that, for an initial access to the base station, a user terminal may not know the round trip propagation delay towards the base station, which in turn means that it cannot compensate for the propagation delay when determining when to start to transmit the random access burst. Instead, the user terminal typically determines this based on the timing of the down link signals. This means that random access bursts from user terminals with a large propagation delay arrive later than random access bursts from user terminals with a short propagation delay. In order to take the varying arrival time of random access bursts from user terminals with short and large propagation delay into account, the random access detection window in the base station is sufficiently large to handle the maximum expected roundtrip propagation delay. In other words the size of the random access detection window should match the length of the random access bursts transmitted by the user terminals plus the maximum expected roundtrip propagation delay in the cell. The "additional" time in the random access detection window, corresponding to the roundtrip propagation delay, may be viewed as a guard period for random access bursts.

Thus, even though the guard period at the transition from down link to up link may be increased by adding more idle symbols at the end of the down link period, the cell range may still be limited by the guard period of the random access burst.

SUMMARY

In conclusion, as explained above, there is a need for a mechanism by means of which a random access channel may be adapted to varying parameters such as cell size, propagation delays etc.

This need is addressed by the present invention in that it discloses a method for use in a cellular wireless access system which comprises at least one base station which controls the traffic to and from a cell in the system.

The cell mentioned is able to accommodate at least a number of user terminals, and in the system a user terminal can be scheduled for receiving traffic from the base station during a down link interval, and a user terminal can be scheduled for transmitting traffic to its base station during an up link interval, with a first guard period between the transition from the up link interval to the down link interval, and a second guard period between the transition from the down link interval to the up link interval. In the system for which the invention is intended, there is also a random access channel for unscheduled transmissions from the user terminal to the base station, and according to the method of the invention, the base station determines and transmits information to user terminals in the cell of the base station which allows the user terminals to calculate an offset time for initiating random access transmissions relative to a nominal starting time for random access transmissions. The offset time is also used by the base station to initiate reception of said random access transmissions.

In one embodiment of the invention, said information is transmitted explicitly, i.e. the base station transmits a time interval to the user terminals in the cell, with the interval being the offset from the nominal starting time for random access transmissions.

As an alternative, said information is transmitted from the base station as information relative to an amount of idle, i.e. "silent", symbols which are comprised in the down link and/or the up link transmissions. Using this information, the user terminals calculate the offset.

Thus, as will also be explained in more detail in the following detailed description of some embodiments of the invention, by means of the invention a solution to the problem mentioned above is obtained, which makes it possible to match the guard period of the random access burst with the guard period of the other uplink channels. The invention also makes it possible to relax the requirements on the base station switch time from receive to transmit without sacrificing cell range for the random access channel.

The invention also makes it possible to effectively, in terms of overhead, support variable cell ranges for TDD with a rather fine resolution or granular- ity. The invention also comprises a radio base station and a user terminal for use in a system which functions as explained above.

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 system in which the invention may be applied, and Fig 2 shows a random access structure in a known system, and Fig 3 shows a problem which may be addressed by the invention, and Fig 4 shows examples of solutions according to the invention, and Fig 5 shows a flowchart of some steps of the invention, and Fig 6 shows a block diagram of a base station of the invention, and Fig 7 shows a block diagram of a user terminal of the invention.

DETAILED DESCRIPTION

Fig 1 schematically shows a system 100 in which the invention may be applied. The system 100 is a cellular wireless access system, and as such comprises a number of cells, one of which is shown in fig 1 with the reference number 110.

The cell 110 comprises at least one radio base station, an RBS, shown as 120 in fig 1. The RBS 120 serves, inter alia, to control the traffic to and from users in the cell 110. The cell 110 can accommodate at least one user termi- nal, with two user terminals being shown in fig 1 , with the reference numbers 130 and 140.

The system 100 is shown as a cellular telephony system, and the invention will be described with reference to such a system, but it should be pointed out that this is by way of example only, the invention may be applied to a number of different wireless access systems. Also, the terminology used when describing the invention with reference to the system 100 is merely intended to facilitate the reader's understanding of the invention, and is not intended to restrict the scope of protection sought by this application. For example, the term base station or radio base station, RBS, should be interpreted as meaning a node in the system with the function of an RBS. In some systems, for example, a function essentially corresponding to that of the RBS is performed by a node called Node B. Such systems are naturally also encompassed by the invention.

Similarly, it should be realized that the term user terminal or UE is merely an example intended to facilitate the reader's understanding of the invention. For example, in some systems, the terms UT, User Terminal, or MS, Mobile Station, are used. Naturally, such systems are also encompassed by the scope of the present invention.

In addition, although the UEs 130, 140, are shown as cellular telephones in fig 1 , it should be realized that this is merely to facilitate the understanding of the invention, the UEs may be many other kinds of devices, portable or stationary, such as, for example, computers.

The system 100 for which the invention is intended is one in which communication to the UEs 130, 140, from the RBS 120 can be scheduled to take place during a first interval in time, usually referred to as the down link interval. DL, and the traffic from the UEs 130, 140, to the RBS 110 can be sched- uled to take place during a second interval in time, the up link interval, UL.

Taking LTE (Long Term Evolution) as an example, a scheduler in the RBS 120 controls when the different UEs in the cell will transmit and receive data. The time unit that the scheduler works with will in the following be referred to as Transmission Time Interval, TTI. The invention is especially suitable for a so called TDD system, "Time Division Duplex", in which the UL and the DL transmit on the same frequency but are divided in time. However, the invention can also be applied to FDD systems, Frequency Division Duplex.

In the system 100, there are silent or idle periods, so called 'guard periods" inserted at the transitions from down link to up link, as well as at the transitions from up link to down link. These guard periods are inserted in order to account for a number of factors, as will be explained in the following.

In TDD systems, guard periods need to be inserted between downlink and uplink periods due to propagation delays between the RBS and the UEs, and also to allow the UEs and the RBS to switch from receive to transmit and vice versa.

One way of creating these guard periods while maintaining a high degree of commonality with FDD systems is by silencing or in effect "removing" a number of DL symbols at the end of the DL period, i.e. DL symbols which immediately precede the uplink period. In that way, a guard time T_Du at the transi- tion from downlink to uplink is created. These "removed" symbols are also called idle symbols.

Also, a guard time TUD is needed at the transition from uplink to downlink to allow the RBS to switch from reception to transmission and to allow the UE to switch from transmission to reception. Since scheduled UEs are subjected to control of transmission timing, the guard period TUD may be created by advancing the terminal transmission time, i.e. by moving it forward in time relative to a nominal starting time for the UL, which is done within the "slack" created by idle DL symbols. In other words, the sum of both guard periods, T_Du and TUD, matches the time of the idle DL symbols. The guard period between downlink and uplink, T_Du, should be chosen taking into account the maximum expected roundtrip propagation delay from the RBS to a cell edge UE₁ in addition to which there should also be time allocated to allow cell edge terminals to switch from reception to transmission. To allow for different cell sizes, the length of the guard period may be varied and made greater in a larger cell. This can be done by increasing the number of silent or "idle" DL symbols.

Thus, by varying, increasing or decreasing, the number of idle DL symbols, the guard periods T_Du and TUD rnay be varied, and cells of different sizes may be supported. However, as touched upon previously, the system 100 also comprises a so called random access channel, i.e. a channel where the UEs may initiate unscheduled transmissions to the RBS.

In order to take into account the different propagation delays for UEs at different distance from the RBS, the random access detection window in the RBS is intended to comprise a guard period which is sufficiently large to cover the uncertainty in arrival time of random access bursts at the RBS. This guard period should therefore cover the maximum roundtrip propagation de- lay for a cell of a certain size.

The same random access burst is expected to be used both in FDD and TDD systems, and the burst is depicted in fig 2. As can be seen in fig 2, the RACH (random access channel) burst comprises a Cyclic Prefix, CP, and the pre- amble as such. The difference in length between the random access detection window (T_CP + TPRE) in the RBS and the random access bursts transmitted by UEs is also shown in fig 2 as a random access guard period TGT- It should be pointed out that the random access burst could contain some information instead of / in addition to the preamble.

With renewed reference to the guard periods TDU and TU_D, these may be increased by, inter alia, adding more idle DL symbols. However, there is no corresponding mechanism for making such variations, for example for the cell range, when it comes to extending the guard period T_Gτ of the random access preamble. Furthermore, TUD is intended to be created by using a time offset for the scheduled UL signals. Since this time offset will not be known by UEs transmitting RACH bursts, the actual random access detection window will be reduced by TUD. i.e., the time required to switch from UL reception to DL transmission in the RBS. Thus, the cell range may become limited by the random access detection window.

In order to avoid interference between different TTIs, the random access burst format is typically designed so that the random access window in the RBS conforms to the TTI length of the other (i.e. scheduled) UL and DL channels. As an example, for the FDD mode of LTE, with 1 ms sub frames, the random access detection window in the RBS should be 1 ms. For TDD, on the other hand, when the scheduled UL TTIs are time shifted by TUD, the random access detection window will be 1 ms - TUD-

One conceivable way of obtaining larger RACH guard periods and to thus increase the cell range could be to use several consecutive RACH "bursts", which will give a corresponding increase in the random access guard period. However, this would result in an unacceptable overhead due to the large increase in the random access detection window, inter alia, the coarse granularity which would result.

A further option, which has been considered, is to not schedule users in the UL TTI immediately following the RACH, and to thereby increase the random access guard period by one TTI. However, this method would provide a very coarse granularity in the random access window size. Another problem with the RACH burst in a TDD system is that in a multi-cell network, there may be RBS-RBS interference due to transmissions from distant base stations that are still "on the air" at the DL to UL transition, and also due to transmissions from nearby base stations that have started to transmit too early due to synchronization errors at the UL to DL switch. This may significantly interfere with parts of the random access detection window, effectively making initial access e.g. at handover degrade. This is depicted in fig 3, which shows an UL period flanked by two DL periods, and also shows the interference RBS-RBS / in an RBS of the system as a function of time

According to the invention, the problem of varying the transmission timing of the RACH burst according to cell size will be addressed by letting the RBS determine and transmit information to user terminals in the cell of the base station which allows the user terminals to calculate an offset time for initiating random access transmissions, i.e. the RACH "bursts". The offset is relative to a nominal starting time for random access transmissions, and will allow the UEs to adjust their RACH transmission timing so that the RACH burst, in particular the random access preamble, is received at the RBS within the actual detection "window" used at the RBS for the RACH. This window is also ex- tended by the same offset. It should be noted that it would in principle be possible to set the window size so that the cyclic prefix is not included.

The information about time offset for the RACH transmitted from the RBS to the UEs in the cell can be sent in a number of different ways. One such way is to transmit the offset explicitly, i.e. that the RBS transmits a time interval to the UEs in the cell, said interval being the offset. This could e.g. be done as an exact time offset, or as an index in a predefined look up table. In the case that a look up table is used, this look up table will be known by the UE.

As an alternative, the RBS can transmit the information as being relative to the amount of idle symbols which are comprised in the down link transmissions. In fig 4, some of the effects as well as the purpose of the timing advance offset TT_A is illustrated. In effect, for different offsets, varying cell ranges may be supported by letting the random access detection window (RACH slot) in the RBS increase and by shifting it in time to start earlier at the UEs as well.

Fig 4 shows some versions of the timing advance offset, with the offset being shown as T_A. Three examples are shown in fig 4:

• TTA = 0. This is intended to show the nominal starting time for the

RACH transmissions, relative to which the offset is employed to advance the RACH transmission.

• TTA=TUD- Here, the RACH transmission from the UE has been advanced in time by an offset corresponding to the guard period TUD- The RBS has also extended the window for RACH transmissions by

"opening" the window TUD earlier, with respect to the nominal starting time of TTA⁼0.

• TTA=TUD + TDU- In this example, the RACH transmission from the UE has been advanced in time by an offset corresponding to both of the guard period T_UD and T_Du- The RBS has also extended the window for

RACH transmissions by "opening" the window TUD ⁺ T_Du earlier, with respect to the nominal starting time of TJ_A=0.

Although figure 4 describes signalling of positive Tχ_A values, it is also possi- ble to let the RBS signal a negative value for TT_A- This would mean that the UEs should start to transmit RACH bursts later with respect to the nominal starting time of T_TA=0. A negative TTA could either be combined with a shorter random access burst, so that the RACH burst doesn't interfere with the TTI which would follow the RACH window, or with no scheduled data in one or more TTI immediately following the RACH. In the latter alternative, the random access detection window would thus be allowed to extend into the un- scheduled TTI, which could be useful if there is significant interference at the beginning of the scheduled UL transmission period.

The offset can be transmitted to the UEs as, for example, the number of si- lent or idle DL symbols which constitute the total sum of T_Du + T_UD, and the UEs will then calculate how much earlier the RACH transmissions should start, based on a standardized rule which is programmed in the UEs, for example when the system is set up, or when the UE is manufactured.

Turning now to how the offset is determined by the RBS, this can be done in a number of ways, some examples of which will be given below.

The offset TTA can be calculated by RBS as, or based on, the guard period, TUD between the UL and the DL.

As an alternative, TTA can be determined by the RBS based on cell range or size, which will tell the RBS how much timing advance is necessary for the RACH, since the propagation delay can be calculated from this.

Alternatively, T_TA can be determined by the RBS based on the interference in the cell, suitably the interference in the up link period. In that version, the interference in the UL is measured by measuring means in the RBS or calculated in the RBS based upon such parameters as BER, BLER, signal throughput, received power etc. The interference level will allow the RBS to calculate how much the RACH transmissions need to be "moved in time" to avoid interference.

In a system which uses the present invention, it would also be possible to adapt T_DU. and/or the number of idle DL symbols (which determines TDU+TUD) based on the T_TA value selected by the RBS. This would be done if it is determined that the system needs a T_TA value that is close to the total number of DL idle symbols, or perhaps even larger than this number, in which case it might be desirable to increase the number of DL idle symbols.

The description so far of the invention has been with reference to a case in which idle symbols are "inserted" at the end of the DL transmission period. However, within the scope of the present invention, the principle of adapting the transmission time offset of the RACH channel is equally applicable for a number of other cases where idle symbols are "inserted", such as:

• at the beginning of the UL time period. In such a case, the scheduled UEs transmitting in the UL will still be advanced by TUD, and therefore the RACH bursts should be advanced by T_TA in similarity with the description above.

• at the end of the UL time period. For this case, the scheduled UEs transmitting in UL will be delayed by Tis - TUD, where Tis corresponds to the time of the idle symbols. For this case, the RACH time offset should be advanced by TTA - Tis from the nominal starting point.

• at the start of the DL time period. Also for this case, the scheduled UEs transmitting in UL will be delayed by T|_S - TUD, and therefore the RACH should be advanced by TTA - Tis from the nominal starting point.

Turning now to fig 5, there is shown a flow chart 500 of some of the more essential steps of the invention, with alternatives being indicated by means of dashed lines: as shown in step 510, the base station determines the offset time TT_A, i.e. information which will allow the UEs in the cell to calculate an offset time for initiating the RACH bursts. As has been pointed out previously, the offset can be either negative or positive.

The time TT_A can be determined in a number of ways, e.g. based on the first guard period, TU_D, or based on the cell range or size, as an alternative to which it can be based on the interference in the cell, suitably but not necessarily the interference in the up link period.

The information regarding the time TJA is transmitted to the UEs in the cell, either explicitly, step 530, as a time interval, or "implicitly", step 540. "Implicitly" here means that the information about the time TJA is transmitted as information relative (540) to an amount of idle symbols which are comprised in the down link transmissions.

Step 550 shows that regardless of how the time T_TA is transmitted to the UEs, the RBS uses the time TJA to intitate reception of the random access transmissions from the UEs in the cell.

Fig 6 shows a schematic block diagram of an RBS 600 according to the in- vention. As shown in fig 6, the RBS 600 comprises scheduling means 610, which serve to schedule a user terminal for receiving traffic from the RBS 600 during the down link interval and for scheduling a user terminal to transmit traffic to the RBS 600 during the up link interval.

The scheduling means 610 of the RBS 600 also serve to schedule the guard period, TUD. between the transition from the up link interval to the down link interval and the guard period, TDU_> between the transition from the down link interval to the up link interval,

In addition, the RBS 600 comprises means 620 for determining and means 630 for transmitting to the UEs in the cell the information which allows the user terminals to calculate the offset time TTA-

The offset time Tγ_A is used by reception means 640 in the RBS 600 to initiate reception of the RACH bursts. In fig 7, a block diagram of some of the components of a user terminal, UE, 700 are shown. As can be seen in fig 7, the UE 700 comprises means 710 for being scheduled by an RBS for the DL₁ the UL and the guard periods T_Du and TUD- In addition, the UE 700 comprises means 720 for receiving informa- tion from an RBS which allows calculating means 730 in the UE 700 to calculate the time offset TJA-

The invention is not limited to the examples of embodiments shown above, but may be freely varied within the scope of the appended claims.

For example, it has been mentioned above that the information regarding the idle symbols by means of which the offset time is created are comprised in the down link transmissions. Naturally, they may also be comprised in the up link transmissions. Suitably, if the idle symbols are comprised in the down link transmissions, they are inserted at the end of the down link transmission period, and if they are comprised in the up link transmissions, they are suitably inserted at the start of the up link period.

Claims

1. A method (500) for use in a cellular wireless access system (100) comprising at least one base station (120, 600) for the control of traffic to and from a cell (110) in the system, said cell being able to accommodate at least a number of user terminals (130), in which system (100) a user terminal (130) can be scheduled for receiving traffic from said base station during a first time interval, the down link interval (DL), and in which system said first user terminal can be scheduled for transmitting traffic to said base station during a sec- ond time interval, the up link interval (UL), and in which system there is a first guard period, TUD, between the transition from the up link interval to the down link interval, and a second guard period, TDU, between the transition from the down link interval to the up link interval, the system also comprising a random access channel for unscheduled transmissions from the user terminal to the base station, the method being characterized in that it comprises letting the base station determine (510) and transmit (520) information to user terminals in the cell of the base station which allows the user terminals to calculate an offset time for initiating random access transmissions relative to a nominal starting time for random access transmissions, said offset time also being used by the base station to initiate (530) reception of said random access transmissions.

2. The method (500) of claim 1 , according to which said information is transmitted explicitly (530), i.e. the base station (120, 600) transmits a time inter- val to the user terminals in the cell, said interval being the offset.

3. The method (500) of claim 1 , according to which said information is transmitted by the base station (120, 600) as information relative (540) to an amount of idle symbols which are comprised in the down link transmissions.

4. The method (500) of claim 1 , according to which said information is transmitted by the base station (120, 600) as information relative (540) to an amount of idle symbols which are comprised in the up link transmissions.

5. The method (500) of any of the previous claims, according to which said information is determined (510) by the base station (120, 600) based on said first guard period, TUD-

6. The method (500) of any of claims 1-4, according to which said information is determined (510) by the base station (120, 600) based on cell range or size.

7 The method (500) of any of claims 1-4, according to which said information is determined (510) by the base station (120, 600) based on the interference in the cell.

8. The method (500) of claim 7, according to which said interference is the interference in the up link period.

9. The method (500) of any of claims 1-4, according to which said information is determined by the base station (120, 600) based on the amount of idle symbols in the down link transmissions.

10. The method (500) of any of claims 1-4, according to which said informa- tion is determined by the base station (120, 600) based on the amount of idle symbols in the down link transmissions minus the time necessary for the base station (120, 600) to switch from transmission to reception.

11. The method (500) of any of claims 1-10, according to which the offset time can be either a positive or a negative value.

12. The method (500) of any of the previous claims, according to which the length of the random access transmissions from the user terminals is maintained regardless of the offset time.

13. The method (500) of any of claims 1-11 , according to which the length of the random access transmissions are adapted according to the offset time.

14. The method (500) of any of claims 3-11 , according to which the number of idle symbols comprised in the down link transmissions are adapted ac- cording to the offset time..

15. A node for use as a Radio Base Station, an RBS (120, 600), in a cellular wireless access system (100) for the control of traffic to and from a cell (110) in the system, said cell being able to accommodate at least a number of user terminals (130), the RBS comprising means (610) for scheduling a user terminal (130) for receiving traffic from said base station during a first time interval, the down link interval (DL), the RBS (120, 600) also comprising means (610) for scheduling said first user terminal to transmitting traffic to the RBS during a second time interval, the up link interval (UL), with the RBS compris- ing means (120) for scheduling a first guard period, TUD_> between the transition from the up link interval to the down link interval and a second guard period, T_0U, between the transition from the down link interval to the up link interval, there also being a random access channel for unscheduled transmissions (Tcp, TPRE) from the user terminal (130) to the RBS (120, 600), the RBS (120, 600) being characterized in that it comprises means (620) for determining and transmitting (630) to the user terminals in the cell (110) information which allows the user terminals (130) to calculate an offset time (TT_A) for initiating random access transmissions relative to a nominal starting time for random access transmissions, said offset time also being used by reception means (640) in the RBS (120, 600) to initiate reception of said random access transmissions.

16. The RBS (120, 600) of claim 15, which transmits said information explicitly, i.e. the RBS transmits a time interval (TTA) to the user terminals (130) in the cell (110), said interval being the offset (TT_A)-

17. The RBS (120, 600) of claim 15, which transmits said information (T_TA) as information relative to an amount of idle symbols which are comprised in the down link transmissions.

18. The RBS (120, 600) of any of claims 15-17, in which said information is determined based on said first guard period, TU_D-

19. The RBS (120, 600) of any of claims 15-17, in which said information is determined based on cell range or size.

20. The RBS (120, 600) of any of claims 15-17, in which said information is determined based on the interference in the cell.

21. The RBS (120, 600) of claim 20, in which said interference is the interference in the up link period.

22. The RBS (120, 600) of any of claims 15-17, in which said information is determined based on the amount of idle symbols in the down link transmissions.

23. The RBS (120,600) of any of claims 15-17, in which said information is determined based on the amount of idle symbols in the down link transmissions minus the time necessary for the RBS to switch from transmission to reception.

24. The RBS (120, 600) of any of claims 15-23, in which the offset time can determined to be either a positive or a negative value.

25. The RBS (120, 600) of any of claims 17-24, which adapts the number of idle symbols comprised in the down link transmissions according to the offset time which is calculated.

26. A user terminal, UE (130, 700), for use in a cellular wireless access system (100), the UE comprising means (710) for being scheduled by a Radio Base Station, RBS, (120, 600), for:

• receiving traffic from said RBS during a first time interval, the down link interval (DL), and • for transmitting traffic to the RBS during a second time interval, the up link interval (UL),

• a first guard period, T_UD, between the transition from the up link interval to the down link interval, and

• a second guard period, TD_U> between the transition from the down link interval to the up link interval, in which system there is also a random access channel for unscheduled transmissions (T_Cp, T_PRE) from the UE (130) to the RBS (120, 600), the UE (130) being characterized in that it comprises means (720) for receiving information from the RBS which allows calculating means (730) in the UE (130) to calculate an offset time (TTA) for initiating random access transmissions relative to a nominal starting time for random access transmissions.

27. The UE (130, 700) of claim 26, which maintains the length of the random access transmission regardless of the signalled offset time (TTA )•

28. The UE (130, 700) of claim 26, which adapts the length of the random access transmissions according to the signalled offset time (TT_A).