WO2008085095A1

WO2008085095A1 - A method and a device for adaptive control signalling

Info

Publication number: WO2008085095A1
Application number: PCT/SE2007/050014
Authority: WO
Inventors: Tobias Tynderfeldt; Erik Westerberg
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2007-01-12
Filing date: 2007-01-12
Publication date: 2008-07-17
Anticipated expiration: 2009-07-12

Abstract

The invention discloses a method for use in a wireless access system, in which system at least a first user terminal can be assigned a first contiguous frequency spectrum for the exchange of data and control signals with the base station, and in which system the exchange of control signals between the second user terminal and the base station can be placed in connection to said first spectrum for the first user terminal. According to the method, measurements of the level of interfering signals in said first spectrum are made, and in the exchange of control signals between the second user terminal and the base station is placed in or adjacent to the first spectrum, depending on the measured interference level.

Description

- - -

TITLE

A method and a device for adaptive control signalling.

TECHNICAL FIELD The present invention relates to a method for use in a wireless access system which comprises at least one base station for the control of traffic to and from a cell in the system, the cell being able to accommodate at least a first and a second user terminal. In the system for which the invention is intended, at least the first user terminal can be assigned a first contiguous frequency spectrum for the exchange of data and control signals with the base station, and the exchange of control signals between the second user terminal and the base station can be placed in connection to the spectrum of the first user terminal.

The invention also describes a base station and a user terminal for use in a system of the type mentioned above.

BACKGROUND

In future wireless access systems which are at present envisioned, so called Single Carrier principles may be used. This principle means that user terminals will only transmit data in a contiguous spectrum, or in other words that a continuous part of the frequency spectrum will be allocated to a single user terminal, with no interruptions.

The Single Carrier principle puts specific requirements on the control signals which are sent from the user terminals to the base station of the cell that they are in. If a user terminal has been scheduled to transmit data to the base station, the control signals of that user terminal may be multiplexed or "interwoven" with the data of the user terminal as it is transmitted to the base station. . - -

If, on the other hand, a user terminal has not been scheduled to transmit any data, its control signals may be scheduled for transmission to the base station at the outer edges of a frequency spectrum which has been assigned to another user terminal which has data to transmit.

With a single carrier (SC) scheme, e.g. SC FDMA (Frequency Division Multiple Access), where a user terminal may transmit over only a small part of the frequency carrier available to the system, the system becomes more sensitive to large variations in the interference level between different parts of the frequency carrier, as compared to, for example, a CDMA based scheme like WCDMA, where each signal is always transmitted over the entire frequency carrier.

In future systems which utilize the SC principle, particularly FDD (frequency Division Duplex) systems and TDD (Time Division Duplex) systems, two interference sources which may be bothersome are base station to base station interference, and user terminal to user terminal interference, both of which could potentially become very strong. In particular in the uplink, i.e. transmission from the user terminal to the base station, with transmit and receiver filters with very steep slopes in the frequency domain in the base stations, the base station to base station interference level may vary significantly over the frequency carrier, with particularly high interference levels at the edges of the frequency carrier.

It is worth noting that this co-existence related interference, with higher interference at the frequency carrier edges, can also appear in an "ordinary" co-existence scenario between two FDD carriers, as well as between two synchronized TDD carriers, although with lower interference peaks, and with less steep interference variations over the frequency carrier.

As described above, the control signals for a first user terminal may be placed at one or both of the frequency edges of the spectrum for a second / - -

user terminal which has data to transmit, which is beneficial from a peak data rate perspective. However, this principle makes the control signalling more sensitive to co-existence related interference of the kind described above, i.e., interference from adjacent frequency carriers.

The control signalling is critical for good system performance. Detection errors in the control signalling could, for example, lead to unnecessary retransmissions on the down link data channel, or to incorrect link adaptation decisions, and thus reduced cell and user throughput. Errors in scheduling request signalling may also lead to increased access times.

The invention also discloses a radio base station which works essentially according to the principles of the system described above, as well as a user terminal for use with such a radio base station.

SUMMARY

Thus, as described above, there is a need for a method and a device by means of which interference may be prevented from "jamming" control signals in a system in which it is desired to let user terminals utilize a contiguous frequency spectrum for data and control signal transmission.

This need is addressed by the present invention in that it discloses a method for use in a wireless access system which comprises at least one base station for the control of traffic to and from a ceil in the system. The cell is able to accommodate at least a first and a second user terminal, and in the system for which the invention is intended, at least the first user terminal can be assigned a first contiguous frequency spectrum for the exchange of data and control signals with the base station, and the exchange of control signals between the second user terminal and the base station can be placed in connection to the spectrum for the first user terminal. According to the invention, measurements of the level of interfering signals in the first spectrum are made, and the exchange of control signals between the second user terminal and the base station is placed in or adjacent to the first spectrum, depending on the measured interference level.

Suitably, if the measured interference level is below a certain predefined threshold, the control signals of the second user terminal are placed at one or both of the edges of the first spectrum, so that the first user can utilize as much as possible of the first spectrum contiguously.

Preferably, if the interference level is above a certain predefined threshold, the control signals of the second user terminal are placed inside said first spectrum, so as to protect them from the interfering signals

Thus, by means of the invention, a method is obtained by the use of which control signals from one UE may be adaptively positioned so that they are afforded maximum "protection" from interfering signals, whilst at the same time preserving as much as possible of a contiguous spectrum for another UE which wishes to exchange data with the base station.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig 1 shows a schematic view of a system in which the invention may be applied, and

Figs 2a, 2b and 3 show transmission of data in a known system, and Fig 4 shows a problem to be solved by the invention, and Figs 5 and 6a-6c show alternative solutions according to the invention, and Fig 7 shows a component in a system according to the invention, and Fig 8 shows a flow chart of some major steps of the invention.

DETAILED DESCRIPTION . - -

Fig 1 very schematically shows a part of a system 100 in which the invention may be applied, which will be described briefly here in order to facilitate the understanding of the invention.

The system 100 in which the invention is intended to be applied is a wireless access system, i.e. a system which comprises at least one cell, such a cell being shown in fig 1 as 110. The system comprises at least one radio base station, RBS, shown as 120 in fig 1 , which serves to control traffic to and from user terminals in the cell 110.

Accordingly, fig 1 shows two user terminals, UE, with the reference numbers 130 and 140.

The system 100 for which the invention is intended is a system which utilizes the so called single carrier, SC, principle, which, inter alia, means that each

UE 130, 140, is assigned a contiguous part of the spectrum available to the system in order to exchange data with the RBS 110. The exchange of data between the UE 130, 140, and the RBS 110 will be described in the following as being the up link, UL, i.e. traffic from the UE to the RBS, but in principle the invention may be equally well applied in the down link direction, DL, i.e. from the RBS to the UEs, or it can also be applied in both directions simultaneously.

A number of cases can be discerned for UL data transmission: in a first case, each UE 130, 140, only has control signals to transmit to the RBS. The exact nature of the control signals are not central to the invention, but the so called L1/L2 control signals can serve well as an example. Signals which are comprised in L1/L2 are, for example, HARQ, ACK/NACK or CQI.

Thus, in a first case, each UE 130, 140, in the cell 110 only has control signals to transmit to the RBS 120. Fig 2a has a diagram 200 which schematically shows how the signalling is carried out in this case, for one of ^uu ( -u 1-

the UEs: within the total bandwidth 205 that would be allocated to the UE in the case that the UE had data to transmit to the RBS, the control signals 210 are scheduled at one or both of the ends of the spectrum 205. The frequencies in between those used by the control signals 210 are left empty.

The control signals can be transmitted in some different ways:

1. If there is no constraint on SC for the control signalling, the control signalling could possibly be transmitted simultaneously at both edges of the spectrum 205. 2. If there is an SC constraint on the control signalling, it can only be transmitted at one of the spectrum edges at a time. In this case, there are still two options: a) The entire control frame is transmitted at one of the edges b) For the purpose of frequency diversity, a first part of the control frame is transmitted at the first spectrum edge, while a second part of the control frame is transmitted at the second spectrum edge.

For alternative 2b), which will be used in the LTE uplink, the SC property of the transmitted signal is maintained also for the control signalling, while at the same time providing frequency diversity for the narrow band control signalling. The frequency diversity will provide significantly better performance, in terms of coverage, BLER, etc, and is therefore desirable to use for the control signalling. In this alternative, the UE will transmit control signals at both edges of the frequency carrier. This is shown in fig 2b, with the control signals indicated as 210'.

In fig 3, a diagram 300 illustrates the case when one of the UEs 130, 140, has data to transmit: in this case, data frames 310 are interwoven or interspersed with control frames 320 over the bandwidth or spectrum 305 available to that UE. /.UU/-U I -

As can be seen in fig 3, at either end of the spectrum 305 of the UE, there is a small band 340, 350, left empty. The reason for this is as follows: in a case where one of the UEs 130 has data to transmit, and the other UE 140 only has control signals to transmit, the control signals for the UE 140 may be placed at the bands shown as 340, 350 in fig 3, i.e. the control signals of UE 140 will be transmitted at the edges of the spectrum which is assigned to UE 130. As an alternative, which is not shown here, the control signals of UE 140 could be transmitted at only one of the ends 340, 350, of the spectrum of the UE 130.

The positioning of the control signals of UE 140 at the outer ends of the spectrum 305 available to UE 130 is beneficial when it comes to the peak data rate possible for the UE 130. However, placing control signals at the perimeter of the spectrum also causes a potential problem, due to the fact that it is at the perimeter of the spectrum that interference will be the most bothersome, and also due to the fact that the control signals are crucial for the performance of the system.

Future wireless access systems, for which the invention is envisioned, will use principles known as FDD, Frequency Division Duplex, and TDD, Time Division Duplex. The basic principles of FDD and TDD are well known to those skilled in the field, and will thus not be elaborated upon here, but the following problems with the positioning of signals shown in fig 3 can be envisioned:

For co-existence between a TDD and an FDD system, as well as between two uncoordinated TDD networks (unsynchronized and/or with different asymmetry), UL signals and DL signals may be transmitted in adjacent frequency carriers or even on the same frequency carrier. The problem may also occur if there is an FDD DL in the vicinity of an FDD UL. - -

8

In such a scenario, two new interference sources appear that are negligible between e.g. two FDD UL carriers or two FDD DL carriers: RBS-RBS interference, and UE-UE interference, i.e. interference between adjacent RBSs or interference between two UEs in adjacent cells.

Such interference can potentially become quite strong. In particular in the UL, with transmit and receiver filters with very steep slopes in the frequency domain, the RBS-RBS interference level may vary significantly over the frequency carrier, with particularly high interference levels at the edges of the frequency carrier.

It can be pointed out that this co-existence related interference, with higher interference at the frequency carrier edges, can also appear in an "ordinary" co-existence scenario between two FDD carriers, although with lower interference peaks, and with less steep interference variations over the frequency carrier.

As described above, placing the control signals at the frequency edges is beneficial from a peak rate perspective, but it does, however, make the control signalling more sensitive to co-existence related interference, i.e., interference from adjacent frequency carriers.

The control signalling is critical for good system performance, since, for example, detection errors in controls signals such as HARQ, ACK/NAK, could lead to problems in the system performance, such as unnecessary retransmissions on the DL data channel, or incorrect link adaptation decisions, and thus reduced cell and user throughput. Errors in scheduling request signalling may also lead to increased access times.

Fig 4 attempts to illustrate a problem which may arise with control signals at the edges of the spectrum: shown in fig 4 are two spectra employed by two adjacent cells in a wireless access system, the first spectrum 410 being the " -

down link spectrum of the first cell, having a relatively high power level since it is the downlink, and the second spectrum 420 being the up link spectrum of the second cell. In addition, in the example shown, the first cell is thought of as an FDD cell, and the second cell as a TDD cell.

As can also be seen in fig 4, in the TDD cell, as shown above, control signals 440 from a first UE to the RBS of the cell are positioned at the edges of the spectrum 420, with data and control signalling 430 from a second UE to the RBS being positioned between the control parts 440 of the spectrum 420. As can be seen, the spectrum 410 of the first cell will interfere with the spectrum 420 of the second cell, particularly at the edges of the spectrum 420, which is where the sensitive control signals 440 from the first UE have been placed.

A basic principle according to the method of the present invention is that measurements of the level of interfering signals in the spectra in which the invention is applied are made, i.e. in the case of fig 4 the measurements would be made in the spectrum 420. However, the measurements may also be divided into smaller sub blocks of the spectra 420.

Also according to the invention, the control signals 440 between the first UE and the RBS are placed in or adjacent to the "data spectrum" 430 of the second UE, depending on the measured interference level. The term "adjacent" should here be taken to mean at one or both of the edges of the data spectrum 430, as shown in fig 4. Similarly, the term "in" should be taken to mean non-adjacent, in other words at a, suitably, predefined distance from the edge or edges of the data spectrum.

Thus, since the invention comprises monitoring the interference level in the spectrum, it is possible to adaptively place the control signals 440 where it is the most suitable according to the interference situation: if there is a high degree of interference, the control signals 440 will be placed inside the spectrum 420, at a distance from the edges of the spectrum 440 which will zuu /-in -

10

suffice to protect them from interference. This will affect the peak data rate available to the UE which exchanges data and control signals in the part 430 of the spectrum, but will not affect its transfer capacity to a harmful extent.

Conversely, if the degree of interference is low, the control signals 440 may be placed at the edges of the spectrum 420, as shown in fig 4, i.e. adjacent to the part 430 of the spectrum which is used for data and control traffic from the other UE.

In a preferred embodiment, a threshold is defined, and depending on where the interference level is with respect to that threshold, the positioning of the control signals 440 from the first UE will be altered to protect the control signals form the first UE.

The way that the interference level is measured will be elaborated upon in more detail later in this text.

Turning now to fig 5, a diagram 500 is shown in order to illustrate a case where the interference level at the edges of the spectrum 540 of a first UE has become such that the control signals 520 from a second UE which have been located in the spectrum 540 of the first UE are at risk.

Thus, the entire spectrum 540 is in principle available to the first UE for transmitting data and control signals to the RBS, and the control signals 520 of the second UE have been placed in that spectrum 540. Due to a high degree of interference at the edges of the spectrum 540, the control signals 520 of the second UE have been moved to a position inside the spectrum 540 where they will not be at risk from the interfering signals.

As shown in fig 5, the control signals of the second UE have been placed essentially at the middle of the spectrum 540, which is preferred but not necessary, as will be shown later in this text. - -

1 1

As has been described previously in this text, the part 510 of the spectrum which is utilized by the first UE for data and control signals should be contiguous over the spectrum 540. Thus, in fig 5, it can be seen that one part 530 of the spectrum 540 is left unused, since the contiguity of the data spectrum 510 has been interrupted by the control signals 520.

Naturally, depending on where in the spectrum that the signals 520 are placed, the unused part will vary in size, and thus leave more or less "data spectrum" 510 for the first UE.

With continued reference to fig 5, it should be pointed out that the unused part 530 of the spectrum 540 in principle could be used by a third UE for data and/or control signals, while maintaining the SC principle for the third UE as well.

In one embodiment of the invention, it could be envisioned to have an "adaptive" placement of the control signals 520: if the interfering signals are measured at a particular "distance" into the spectrum 540, the signals 520 could be placed at a distance from the spectrum edges which depends on where the interference level is sufficiently low.

Fig 6a shows another way of utilizing the spectrum 650 available to a first UE for data exchange with the RBS if the interference level exceeds a certain threshold: In the example shown in fig 6a, only the upper part 610 of the spectrum 650 suffers from interfering signals. Thus, only the control signals 620 which belong to a second UE and which were placed at the upper edges of the spectrum 650 in a situation without interference need be altered with respect to the upper part 610 of the spectrum 650.

Accordingly, it is only the control signals 620 which have been moved from the position at the upper edge of the spectrum 650, while if the control - -

12

signals 620 had been positioned at the lower edge of the spectrum 650, they could have been maintained there.

In addition, the control signals 620 which were moved from the upper edge of the spectrum 650 have been moved to a position where the spectrum 650 is partitioned into two parts, 610 and 630. One part, 610, is data from the original first UE. but the second part, 630, is data traffic which comes from a third UE, which has a defined bandwidth for its data transmissions which is within the bandwidth 630.

Thus, the control signals at either or both of the edges of the spectrum 650 of a first UE may be moved so that room is made in the spectrum for data traffic from other UEs with a smaller need for bandwidth.

Fig 6b shows an alternative embodiment, in which the control signals from the second UE have been divided into a first 620 and a second 640 part which have been placed at respective first and second locations inside the spectrum 650. The location of the first part 620 is inside the spectrum 650, in between the "data spectrums" 610, 630, and the location of the second part 640 is at the edge of the spectrum 650. Thus, in the example shown in fig 6b, it is at the "upper" part of the spectrum 650 that the interference level is the most problematic, which is why the control signals 620 have been moved from there.

Fig 6c shows yet another alternative embodiment in which the control signals from the second UE have been divided into a first 620 and a second 640 part which have been placed at respective first and second locations inside the spectrum 650. However, in the embodiment of fig 6c, the control signals 620, 640, have been placed at respective first and second locations inside the spectrum 650 with both of said first and second locations being inside the spectrum 650. This can be used in a case where there is a high level of interference at both edges of the spectrum, but it is still desired to obtain - -

13

frequency diversity in the control signalling. In fig 6c, the part of the spectrum 650 that is adjacent to the lower edge has been utilized for data 660 from a third UE.

Turning now to the measurement of interfering signals, these can be made in a multitude of ways, which are well known to those skilled in the field, and will thus only be touched upon briefly here: the interference measurements are suitably carried out by the RBS, and can comprise both direct interference measurements, as well as more indirectly by measuring the quality of the traffic received from the UE or UEs in the cell of the RBS. This traffic quality can for example be BER (bit error rate), BLER (block error rate) or the throughput speed of the traffic.

The direct interference measurements could be measured by the RBS by straight forward signal strength measurements on unused parts of the frequency carrier, or alternatively, from SINR (Signal-to-lnterference-and- Noise Ratio) measurements in the parts of the frequency carrier that are used for data and/or control frames.

It should not be excluded that the interference measurements for e.g. DL transmissions can be made by one or more of the UEs, which would then report the measured interference level to the RBS.

Regarding the signalling of information about the placement of the control signals from the RBS to the UEs, this could be carried out as system information on the broadcast channel, or as dedicated signalling to each UE from the RBS. Thus, a UE which is capable of receiving such dedicated signalling is also comprised in the invention, - - -

14

In another embodiment of the invention, in order to inform moving UEs which are due for hand over to another cell of the placement of the control signals in the target cell, information about this could be transferred from the target cell to the source cell over the interface between the RBSs, and then forwarded to the moving UE prior to the handover. In this way, the UE would know the placement of the control signals prior to entering the target cell, and would be prepared to send data on these control channels. Accordingly, a UE which is capable of receiving such signals is also comprised in the invention.

Fig 7 schematically shows some components for use in an RBS 700 of the invention: the RBS 700 needs to comprise means 710 for assigning to a first UE a first contiguous frequency spectrum for the exchange of data and control signals with the RBS.

Also, the RBS 700 needs to comprise means 720 for scheduling the exchange of control signals between a second UE and the RBS 700 in connection to the spectrum of the first UE. In addition, the RBS needs to comprise means 730 for utilizing the results of measurements of the level and possibly also variations of interfering signals in the first spectrum, as well as means 740 for scheduling the exchange of control signals between the second UE and the RBS in or adjacent to the spectrum of the first UE depending on the measured interference level.

In addition, if the RBS is to carry out the interference measurements as discussed above, the RBS needs to comprise means 750 for carrying out the interference measurements.

On the other hand, if the interference measurements are made by the UEs, the RBS needs to comprise means 760 for receiving the results of the interference measurements from at least one UE. - -

15

It should be pointed out that although the measurement of the interference level can be carried out at sub-ranges of the first spectrum, in a preferred embodiment the measurements are carried out over the entire spectrum. With reference to, for example fig 5, the interference measurements would then be made in the entire spectrum 540.

Finally, fig 8 is a flowchart 800 which illustrates some of the major steps of the method of the invention: In step S1 , 810, measurements of the level of interfering signals in the spectrum which has been assigned to a first UE for data transmission are made, and in step S2, 820, the exchange of control signals between a second UE and the RBS is placed in or adjacent to the spectrum of the first UE, depending on the measured interference level. The measurements in step S1 can be made either by the RBS or by one of the UEs.

In step 3, 830, the interference level is compared to a predefined threshold, and depending on whether the interference level is above or below the threshold, in step S4, 840, the control signals of the second UE is placed at one or both of the edges of the spectrum of the first UE, so that the first UE can utilize as much as possible of its first spectrum contiguously, or in step S5, 850, the control signals of the second user terminal are placed inside the spectrum of the first UE.

The comparison with the threshold could be carried out in a number of ways, for example:

1. Absolute, i.e. if the measured interference exceeds a certain level in a part of the frequency spectrum, the control signalling will not be scheduled in that part of the frequency carrier.

2. Relative, i.e. if the measured interference is, for example, five times higher in one part of the frequency spectrum than the average interference level in the spectrum, the control signalling is not scheduled in that part of the spectrum. 16

3. A combination of 1 ) and 2) above.

In step S6, 860, the method is applied to the up link traffic direction, and as an alternative, in step S7, 870, it is applied to the down link direction. Naturally, although this is not shown as a separate step, the method can be applied to both directions simultaneously.

Claims

1. A radio base station, an RBS (120, 700), for use in a wireless access system (100) for the control of traffic to and from user terminals (130, 140) in a cell (110) in the system (100) and for the exchange of data and/or control signals with said user terminals (130, 140), said RBS (120, 700) being able to control at least a first (130) and a second (140) user terminal in said cell (110), the RBS (120, 700) comprising means (710) for assigning at least said first user terminal (130) a first contiguous frequency spectrum (305, 420, 540. 650) for the exchange of data (310) and control (320, 330) signals with the RBS, the RBS further comprising means (720) for scheduling an exchange of control (440, 520, 620, 640) signals between the second user terminal (140) and the RBS in connection to said first spectrum for the first user terminal, the RBS being characterized in that it comprises means (730) for utilizing the results of measurements of the level of interfering signals in said first spectrum, and in that it also comprises means (740) for scheduling the exchange of control signals between the second user terminal and the RBS in or adjacent to the first spectrum depending on said measured interference level.

2. The RBS (120, 700) of claim 1 , in which the scheduling means (740), if the interference level is below a certain predefined threshold, places the control signals (340, 350, 440) of the second user terminal (140) at one or both of the edges of the first spectrum, so that the first user can utilize as much as possible of the first spectrum (305, 420) contiguously.

3. The RBS (120, 700) of claim 1 , in which the scheduling means (740), if the interference level is above a certain predefined threshold, places the control signals (520, 620, 640) of the second user terminal inside said first spectrum (540, 650).

4. The RBS (120, 700) of claim 3, in which the scheduling means (740) places the control signals (520, 620, 640) of the second user terminal inside said first spectrum(540, 650), at a certain predefined distance from the edges of the first spectrum.

5. The RBS (120, 700) of claim 3 or 4, in which the scheduling means places the control signals (520, 620,640) of the second user terminal contiguously inside said first spectrum (540, 650).

6. The RBS (120, 700) of claim 3 or 4, in which the scheduling means places the control signals (520, 620,640) of the second user terminal non- contiguously at a first (620) and a second (640) location inside said first spectrum (540, 650).

7. The RBS (120, 700) of any of claims 1-6, further comprising means (750) for carrying out said interference measurements, the results of which are utilized.

8. The RBS (120, 700) of any of claims 1-6, further comprising means (760) for receiving the results of the interference measurements from at least one of said user terminals (130, 140).

9. The RBS (120, 700) of any of claims 1-8, which signals the scheduling of the control signals of the second user terminal (140) to the second user terminal in a broadcast message.

10. The RBS (120, 700) of any of claims 1-8, which signals the scheduling of the control signals of the second user terminal (140) to the second user terminal in one or more dedicated messages. Δ\JV I -U I - I Z

19

11. The RBS (120, 700) of any of claims 1-10, in which said exchange of data and control signals between the RBS and the first and second user terminals is the uplink direction, i.e. from the user terminals to the RBS.

12. The RBS (120, 700) of any of claims 1-10, in which said exchange of data and control signals between the RBS and the first and second user terminals is the downlink direction, i.e. from the RBS to the user terminals.

13. The RBS (120, 700) of any of the previous claims, comprising means for receiving information from a second RBS about the placement of the control signals in the cell of the second RBS, the RBS (120, 700) also comprising means for transmitting said information to a UE in the cell of the RBS, said UE being intended for handover to the cell of said second RBS.

14. A user terminal, UE, (130, 140) for use with the RBS of claim 13, the UE comprising means for receiving information from the RBS about the scheduling of the control signals in an adjacent cell to which the UE is intended for handover.

15. A user terminal, UE (130, 140) for use with the RBS of any of claims 1-

14, the UE comprising means for receiving data regarding the scheduling of its control signals in one or more dedicated messages from the RBS (120, 700).

16. A method (800) for use in a wireless access system (100) comprising at least one base station (120, 700) for the control of traffic to and from a cell (110) in the system, said cell being able to accommodate at least a first (130) and a second (140) user terminal, in which system (100) at least said first user terminal (130) can be assigned a first contiguous frequency spectrum (305, 420, 540, 650) for the exchange of data (310, 510, 610) and control (320, 522, 620) signals with the base station, and in which system an exchange of control signals between the second user terminal (140) and the -iUU f -U 1 - 1

20

base station (120, 700) can be placed in connection to said first spectrum for the first user terminal, the method (800) being characterized in that measurements of the level of interfering signals in said first spectrum are made (810) and in that the exchange of control signals between the second user terminal and the base station is placed in or adjacent to the first spectrum depending on the measured interference level (820).

17. The method (800) of claim 16, according to which (830), if the interference level is below a certain predefined threshold, the control signals of the second user terminal (140) are placed at one or both of the edges of the first spectrum, so that the first user can utilize as much as possible of the first spectrum contiguously.

18. The method (800) of claim 16, according to which, if the interference level is above a certain predefined threshold, the control signals of the second user terminal are placed inside said first spectrum.

19. The method (800) of claim 18, according to which the control signals of the second user terminal are placed inside said first spectrum, at a certain predefined distance from the edges of the first spectrum.

20. The method (800) of claim 18 or 19, according to which the control signals of the second user terminal are placed contiguously inside said first spectrum, at a certain predefined distance from the edges of the first spectrum.

21. The method (800) of claim 18 or 19, according to which the control signals of the second user terminal are placed non-contiguously inside said first spectrum, at a first and a second location from the edges of the first spectrum. - -

21

22. The method (800) of any of claims 16-21 , according to which the interference measurements are carried out by the base station.

23. The method (800) of any of claims 16-21 , according to which the interference measurements are carried out by at least one of the user terminals, and the results are communicated to the base station.

24. The method (800) of any of claims 16-23, according to which the scheduling of the control signals of the second user terminal is signalled to the user terminals by the base station in a broadcast message.

25. The method (800) of any of claims 16-23, according to which the scheduling of the control signals of the second user terminal is signalled to the user terminals by the base station in one or more dedicated messages.

26. The method (800) of any of claims 16-25, according to which said exchange of data and control signals between the base station and the first and second user terminals is the uplink direction, i.e. from the user terminals to the base station.

27. The method (800) of any of claims 16-25, according to which said exchange of data and control signals between the base station and the first and second user terminals is the downlink direction, i.e. from the base station to the user terminals.

28. The method 120, 700) of any of claims 16-27, according to which information from a second RBS about the placement of the control signals in the cell of the second RBS is received by the base station (120, 700), and according to which the RBS (120, 700) transmits said information to a UE in the cell of the RBS, said UE being intended for handover to the cell of said second RBS.