WO2002104063A1

WO2002104063A1 - Method and system of channel allocation

Info

Publication number: WO2002104063A1
Application number: PCT/SE2002/001219
Authority: WO
Inventors: Peter Lundh
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2001-06-18
Filing date: 2002-06-18
Publication date: 2002-12-27
Also published as: SE0102200D0

Abstract

The present invention describes channel allocation in a communications system. For communication links involving a central station or base station, time-slots are allocated to the user equipments depending on signal propagation time or distance to the station.

Description

Method and system of channel allocation

TECHNICAL FIELD OF THE INVENTION

The present invention relates to channel allocation in a communications system, and more especially it relates to a cellular mobile radio system, particularly to systems using time division duplex, TDD, e.g. a Universal Mobile Telecommunications System, UMTS or WCDMA system, in TDD-mode.

BACKGROUND AND DESCRIPTION OF RELATED ART

Time division duplex, transmitting uplink and downlink in- formation separated in time, frequently on a common carrier, is previously known. Downlink and uplink transmissions occur in time-slots. Commonly, a plurality of time- slots for downlink transmissions are grouped together in downlink time-segments, and time-slots for uplink transmis- sions are, correspondingly, grouped in uplink time- segments. The time-segments for uplink, «Up», and downlink, «Down», transmissions respectively, each of one or more time-slots, are separated in time by guard periods, G_u,Gd, containing no information and preventing data trans- mitted in uplink time-slots and downlink time-slots to overlap in time at the receiving end due to propagation delay. This is illustrated in figure 1 using the timing of the Central Station as a time-reference. Due to propagation time-delay an «Up» segment transmitted from a User Equipment is received at a point in time later than the time-point for transmission. The time-difference, Δt, corresponds to the (one-way) propagation time-delay. Correspondingly, transmissions of «Down» segments from the Central Station to the User Equipment are also delayed by Δt. To illustrate the concept of timing advance in relation to figure 1, consider a situation when a user is allocated all time-slots of a time-segment. A timing advance, TA, in figure 1 advances the «Up» time-slots in relation to the «Down» time-slots at the user equipment. Such a timing advance corrects for the propagation time delay, Δt, between a central station, e.g. a base station or Node B of a mobile communications system, and a user equipment. Hence, the timing advance is twice the (one-way) propagation time delay. At least in principle, the timing advance is capable of correcting for the propagation time delay between two parties once a connection has been established and the round-trip delay has been correctly estimated. With no or insufficient timing advance, the guard period G_u will overlap the «Up»-segment, or worse (for large propgagation time delays) the data segments of the «Down» and «Up» segments will overlap. If colliding data is not otherwise protected it will be destroyed upon collision. Prior to connection establishment, without any means to estimate the appropriate timing advance, the parties rely solely on a predefined guard period. In the sequel, the points (or more correctly intervals) in time when the usage of time-slots are switched from up to down or vice versa are referred to as Switching Points, SPs.

When a time-segment comprises more than one time-slot and a user is allocated only a fraction of the time-slots, the timing advance of that user only applies for the one or more time-slots allocated by the user and not the entire time-segment. Timing advance for time-slots of a time- segment allocated to other respective users is determined in relation to their distances to the central station.

European Patent Application EP0979017 identifies a dilemma of interfering parties in a multiple access system, by means of interference between users in different cells. The system uses asymmetric time-slot allocation for up- and downlinks. Users in the interior of a cell are allocated time-slots close to a switching point, whereas users close to a cell border are allocated time-slots far from a switching point.

U. S . Patent No . US5959982 provide for guard bands between transmit and receive portions in a TDD wireless system to accommodate propagation delay.

3^r Generation Partnership Project (3GPP) : Technical Specification Group Radio Access Network, Physical Channels and Mapping of Transport Channels onto Physical Channels (TDD) , 3G TS 25.221 v3 . 6. 0 , France, March 2001 , describes in chapter 5 physical channels in TDD. The physical channel is a burst, transmitted in a particular time-slot of radio frames. A radio frame comprises a multiple of time-slots. Each TDMA frame is allocated 15 time-slots. A time-slot comprises 2560 time-chips for spreading. Each of the 15 time-slots is allocated to either the uplink or the downlink. Section 5.1 describes examples of configurations with one switching point per frame or multiple switching points per frame. It also describes symmetric as well as asymmetric exemplary channel allocations for uplink and downlink directions within the frame. Depending on which out of a number of predefined time-slot formats, the number of chips during the guard period of a time-slot is 96 or 192.

Subsection 5.2 of the 3GPP Technical specification describes spreading of data prior to mapping to the physical channel. The spreading consists of two steps:

- channelization, and - scrambling. The channelization transforms data symbols into a number of chips by means of spreading. For this purpose a spreading code is used. A plurality of users can use the same time- slot if they use orthogonal spreading codes, without inter- fering. Scrambling is a method for rearranging the chips of the spread signal. Details on spreading and scrambling in UMTS are revealed in 3^rd Generation Partnership Project (3GPP) : Technical Specification Group Radio Access Network, Spreading and modulation (TDD) , 3G TS 25.223 v3 . 5. 0 , France, March 2001.

None of the cited documents reveals a method and system for alleviating interference between users due to propagation delay within one single cell, by allocating users to different time-slots within a time-segment of more than one slot.

SUMMARY OF THE INVENTION

Cited prior art references describe use of guard space or guard period to circumvent the adverse effect of overlapping Up and Down time-slots. However, a strictly limited guard period, and the observation that timing alignment only solves the problem of alleviating overlapping time- slots between two users, imposes strong limits on the cell sizes. Though, some protection is achieved by using different spreading sequences, interference effects will still remain due to substantial differences between transmitted and received power. The smaller differences in power level of time-slots for the same direction (up/down) is one reason for which overlapping of time-slots for the same direction is less detrimental from a system aspect.

Consequently, it is an object of the present invention to allocate channels to different users such that the number of time-slots, where uplink data and downlink data overlaps can be reduced.

It is also an object to present a method and system of allocating channels allowing for cell sizes excessive of that allowed by guard period and/or timing advance alone.

A further object is to achieve a system capable of determining channel allocation aspects on the basis of measured data concerning only parameters of the user for which a channel is allocated and possible other users allocated to time-slots of the same time-segment.

Finally, it is an object to achieve a method and system of channel allocation with relaxed requirements for guard period and timing advance.

These objects are met by a method and system allocating time-slots depending on the propagation time-delay between the user and the base station.

The invention is particularly well suited for TDD mode of a universal mobile telecommunications system.

Preferred embodiments of the invention, by way of examples, are described with reference to the accompanying drawings below.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 displays uplink and downlink time-slots at a transmitter and receiver.

Figure 2 illustrates channel allocation, according to the invention.

Figure 3 schematically depicts radio coverage around an omnidirectional base station antenna. Figure 4 presents a flow chart illustrating channel allocation according to the invention.

Figure 5 shows a flow chart illustrating in further detail channel allocation according to the invention.

Figure 6 shows an ordered time-slot allocation in a high- load situation with all time-slots allocated according to a preferred embodiment of the invention.

Figure 7 shows an ordered time-slot allocation with channels represented by channelization spreading-codes .

Figure 8 shows a network element according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 2 illustrates channel allocation, according to the invention. In figure 2 no guard periods are illustrated. However, according to prior art guard periods are allowed for in each time-slot. Additionally, an extra guard period between up- and down-segments, with one or more time-slots in each segment, can be provided. A drawback of using extensive guard periods is the waste of capacity introduced by freeing time-intervals of data transmissions.

In UMTS the guard period of each time-slot corresponds to 25 μs or 50 μs depending on slot format. This corresponds to 7.5 or 15 km respectively. As for timing advance, the guard period reflects the roundtrip delay. Correspond- ingly, the largest radial distance from a base station using no timing advance is approximately 3.8 or 7.5 km, for colliding data of adjacent time-slots to be avoided. If there are more than two time-slot in each of the up and down segments, this can be achieved by allocating users far from the base station, e.g. more than the distance rl al- lowed for by the guard period alone, to time slots with at least one time-slot in between to the nearest switching point as shown in figure 2. Due to the interference suppression obtained by spreading, the impact of time-slot overlap is reduced whenever the time-overlap is reduced.

The larger the time-separation of the time-slot and the nearest switching point, the larger the distance between radio base station and user equipment allowed for. As the range between the user equipment and the base station is increased other channel properties, such as transmission power, need to be considered as well.

If timing advance were used, cell-size could be extended as regards the base station to which the user equipment is connected, virtually with no guard period. However, timing advance can cause severe disturbances of connections of other user equipments. Timing advance of transmissions from a user equipment can introduce severe interference due to collisions with earlier slots.

Collisions of time-slots are particularly severe when col- liding slots are not of the same type (up/down) . When they are of the same type the interference is suppressed by spreading. However, when the slots are not of the same type the power difference is substantial and the suppression achieved by spreading is likely to be insufficient for non-orthogonal spreading codes.

Figure 3 schematically shows radio coverage around an omnidirectional base station antenna BS . A first maximum distance rl corresponds to a roundtrip propagation time delay equal to the duration of a guard period of a time-slot. A second maximum distance r2 corresponds to the cell size. The interior of hexagonal Hi, with a radius not larger than rl, defines a «Near» area and the intermediary of Hi and a hexagonal H2 , with a radius not larger than r2 , defines a «Far» region.

However, freeing one or more entire time-slots, to be used for guard period between the allocated time-slot and the nearest switching point, on a regular basis, would reduce channel capacity more than can generally be accepted. However, due to the suppression of interference from time- slots for the same direction (up/down) , collisions between time-slots for the same direction can be allowed to occur. A significant gain can be achieved, by eliminating collisions between time-slots for different directions (up/down) .

With reference to figure 3 User Equipments within the «Near» area can be allocated any time-slot within a frame, as regards propagation time delay, whereas users in the «Far» area should not be allocated time-slots close to a switching point, i.e. where the nearest time slot is a time-slot for opposite direction (up/down) , because of the substantial propagation time delay. In order to have time- slots available for distant users in the «Far» region, users within the «Near» region should preferably be allocated time-slots close to a switching point, even if they could be allocated any time slot between two switching points, from viewpoint of their individual delay properties. As the users move within a cell there might be need for real- location of time-slots. Another reason for time-slot real- location might be a time-slot heavily interfered from a user equipment or base station in another cell. There can also be other reasons for channel reallocation according to dynamic channel allocation of prior art, not excluded by this invention. Consequently, this invention also includes dynamic time-slot reallocation in accordance with the propagation delay time of the users as a measure to be combined with other measures for channel allocation or reallocation. Preferably, such reallocation can occur at se- lected points in time, with a time-space within a range of minutes or less, not loading the system by continuous real- locations .

As an immediate alternative to estimate the distance between the base station and the user equipment from propaga- tion delay time measurements, this invention also covers distance estimates from propagation path loss measurements . Received signal strength decreases when distance increases.

In an exemplary situation with three users in the «Near» area, using one time-slot each for uplink and downlink di- rections, respectively, one user in the «Near» area using one time-slot for uplink direction and two time-slots for downlink direction, and three users in the «Far» area each using one time-slot for uplink and downlink directions, respectively, a channel allocation within one frame could be as depicted in figure 2.

Figure 4 illustrates in a flow chart channel allocation according to the invention. Roundtrip delay or, equiva- lently, distance between a base station and a user equipment is estimated. The users are classified according to the propagation time delay as being «Near» or «Far», depending on their location as described in relation to figure 3. Optionally, they are sorted in ascending/descending order according to propagation time delay. The closer the user is to the base station the less the likelihood is that he will be far from the base station for the next frame.

Consequently, there is a gain achieved by sorting the users according to their propagation delay, since a need for time-slot reallocation will arise less frequently. Finally, the users are allocated time-slots depending on their propagation time delay. Users in the «Near» region are primarily allocated time-slots adjacent or close to a switching point for up and down directions respectively, whereas time-slots in the «Far» area (c.f. figure 3) are allocated time-slots in the middle of an «Up» and a «Down» time-segment respectively (c.f. figure 2). The channel allocation is load dependent. In highly loaded situations some users might be barred, or not allowed the number of time-slots requested.

Out of the users allowed (or more correctly their allowed requests for time-slots) , the allocation is preferably performed as illustrated in figure 5. In the figure only user data for which there are time-slots available in a time- segment are considered. In this example there are time- slot requests of M users to allocate. The limitation of number of users to allocate corresponds to prior art considerations . Only users for which there are channel re- sources available can be allocated channel resources. As explained above, when spreading is used more than one user can be allocated to the same time-slot, as long as the users of the same time-slot use orthogonal channelization spreading codes or channelization spreading codes with suf- ficiently small correlation. In this sense a time-slot is free to be allocated to a user as long as there is at least one channelization spreading code available, designed not to interfere with channelization spreading codes already in use by other users in the same time-slot. However, accord- ing to the 3GPP specification parallel physical channels shall be transmitted using different channelization codes. Therefore, users of parallel channels are preferably allocated time-slots with sufficient capacity rather than multiple time-slots each of insufficient channel capacity. This can be achieved by, at the time of allocation, only considering time-slots of sufficient or maximal channel capacity to be free in the sense of figure 5. As regards how the channel resources are allocated a preferred method of allocation is illustrated in figure 5. Distance between each user to be allocated one or more time-slots and base station is determined. Preferably, the distance is estimated from signal roundtrip delay. Signals used for this purpose can be, e.g., signals on a random access channel or a traffic channel. As an alternative to the propagation time delay, the distance can be estimated from the signal propagation path loss. The distances of the users of the various time-slot requests d (i) are sorted in ascending or descending order. In figure 5 a descending order is as- sumed. These time-slot requests are allocated free time- slots as requested with as many intermediary time-slots to a switching point, SP, as possible. When time-slots of the first, most distant, user have been allocated for both directions (up/down) as need be, a counter i is increased by one, if any slot was allocated. The process is iterated with the updated counter, picking the next user/time-slot request of the list, until all M users/time-slot requests have been allocated one or more time-slots, remembering the prerequisite of figure 5. E.g. with 8 time-slots in «Up» direction (uplink) of a frame and 16 channelization codes for each of the time-slots, there is a maximum of 120 users that each can be allocated an uplink time-slot of the frame .

If the distance of user i in the «Near» region is denoted d (Ni) and the distance of user j in the «Far» region is denoted d (Fj) and

d (Nl ) ≤d (N2) ≤d (N3 ) ≤---≤d (F3 ) ≤d (F2) ≤d (Fl) , a resulting time-slot distribution for only one channelization code is illustrated in figure 6. Figure 6 assumes the same distribution of users as for the example above, resulting in the allocation of figure 2, but now illustrated in relation to figure 5.

As already mentioned, an immediate alternative to the descending order of the sorted distances in figure 5 is to use distances sorted in ascending order, initially allocating «Near» region users .

In another exemplary alternative in accordance with the invention, short delay («Near») time-slots are allocated starting in direction from the two switching points of a time-segment towards the center of the time-segment, allocating time-slots by alternating between time-slots closer to either of the two switching points. Great delay («Far») time-slots are allocated starting from the center time-slot of the time-segment alternating outwardly towards the switching points.

The allocation of time-slots to «Far» region users and «Near» region users, respectively, can be performed in parallel or sequentially.

In general, it is beneficial not to allocate time slots closer than necessary to any switching point. However, time-slots can be kept available for potential «Far» region users if «Near» region users are allocated time-slots maximally close in time to a switching point.

In figure 6 the use of various spreading codes for each time-slot is not exploited in detail. Depending on the spreading factor used a time slot can simultaneously be al- located to one or more users. Figure 7 illustrates a situation with parallel channels due to spreading. The amount of data capacity of a time-slot channelized by spreading can be expressed in terms of a spreading factor,

SF. The larger the spreading factor of a channelization spreading code the less the amount of data in the time slot. In figure 7, only two out of 16 exemplary parallel channels are shown. The users N1-N4 and F1-F are the same as those of figure 6. They all use channels of the same spreading factor, e.g. SF=16. In addition, a «Near» user

N5 in the «Near» zone of figure 3 allocates two spreading codes (parallel channels) of the same spreading factor

(SF=16) in down-link direction and a spreading code of lower spreading factor (SF=8) in uplink direction.

As regards the allocation of a «Down» time-slot to user N4, the time-slot channel later allocated to user N3 is first considered as a candidate. However, as user N4 is allocated two parallel channels and there is only one available (the other already being allocated to user F3) at the first time-slot considered, this will not be considered to be "free," in accordance with figure 5, and another time-slot needs to be considered. The time-slot with two parallel channels (or channelization codes) free and not allocated to another user is allocated to user N . In the next iteration user/time-slot request N3 is allocated the time- slot of free capacity corresponding to its requirement.

In figure 7, two time-slots in «Up» direction and three time-slots in «Down» direction are not allocated. When, e.g., not all time-slots of a time-segment («Up» or «Down») are requested by users, only a preferred subset of time- slots of the time-segment needs to be considered for chan- nel allocation according to the invention.

Figure 8 shows a network element according to the invention. Preferably, the network element is a radio network controller of a radio communications system. The network element comprises means 1 for allocating time-slots on the communication link depending on the distance between the user equipment and a central station. Preferably, the cen- tral station is a base station or Node B of a UMTS system. For determining the distance it comprises means 2 for estimating the one-way propagation time delay between the user equipment and the central station, the corresponding round- trip delay, the one-way propagation path loss between the user equipment and the central station or the corresponding propagation path loss of both directions to/from the user equipment. The network optionally comprises means 3 for sorting user equipments to allocate channels according to their respective distances to the central station. Conse- quently, means 1 allows for allocation of time-slots in distance-ascending and distance-descending order as preferred examples . The network element means 1 also allows for channel reallocation.

•A person skilled in the art readily understands that the receiver and transmitter properties of a BS or a UE are general in nature. The use of concepts such as BS, UE or RNC within this patent application is not intended to limit the invention only to devices associated with these acronyms. It concerns all devices operating correspondingly, or being obvious to adapt thereto by a person skilled in the art, in relation to the invention. As an explicit nonexclusive example the invention relates to mobile stations without a subscriber identity module, SIM, as well as user equipment including one or more SIMs. Further, protocols and layers are referred to in close relation with UMTS terminology. However, this does not exclude applicability of the invention in other systems with other protocols and layers of similar functionality. The invention is not intended to be limited only to the embodiments described in detail above. Changes and modifications may be made without departing from the invention. It covers all modifications within the scope of the following claims.

Claims

1. A method of allocating time-slots in a communications system avoiding interference between user equipments connected to a central station on a time-division duplex communication link between user equipments and the central station, the method c h a r a c t e r i z e d i n that one or more user equipments ' requests for time-slots are allocated time-slots depending on the distance between the one or more user equipments and the central station.

2. The method according to claim 1 c h a r a c t e r - i z e d i n that available time-slots define the maximum number of user equipments to allow for channel allocation and that the user equipments to allow for channel allocation are sorted according to their respective distances to the central station.

3. The method according to claim l or 2 c h a r a c t e r i z e d i n that time-slots free for allocation are selected from a subset of time-slots of a time-segment, the subset comprising a number of time-slots corresponding to the number of user equipments to allow for channel alloca- tion.

4. The method according to any of claims 1-3 c h a r a c t e r i z e d i n that a user equipment is allocated one or more free time-slots close in time to a switching point if the user equipment is at a small distance from the central station and one or more free time-slots distant in time to its nearest switching point if the user equipment is at a large distance from the central station.

5. The method according to any of claims 1-4 c h a r a c t e r i z e d i n that the most distant user equip- ment is allocated one or more free time-slots of a time- segment or subset thereof maximally distant in time from switching points, between which time-slots are being allocated in the time-segment or subset thereof, and successively less distant user equipments are allocated one or more free time-slots of the time-segment or subset thereof not being more distant in time from its nearest switching point than time-slots of the time-segment or subset thereof previously allocated.

6. The method according to any of claims 1-4 c h a r - a c t e r i z e d i n that the closest user equipment is allocated one or more free time-slots of a time-segment or subset thereof maximally close in time to a switching point, being one of two switching points between which time-slots are being allocated in the time-segment or sub- set thereof, and successively more distant user equipments are allocated one or more free time-slots of the time- segment or subset thereof not being closer in time to its switching point than time-slots of the time-segment or subset thereof previously allocated.

7. The method according to any of claims 1-6 c h a r a c t e r i z e d i n that the time-slots comprise one or more channelization spreading code channels.

8. The method according to any of claims 1-7 c h a r a c t e r i z e d i n that time-slots of a bi-directional communication link are transmitted on one carrier frequency.

9. The method according to any of claims 1-8 c h a a c t e r i z e d i n that the distance of user equipments is determined based upon the one-way propagation time delay between the user equipment and the central station or the corresponding roundtrip propagation time-delay.

10. The method according to any of claims 1-8 c h a r a c t e r i z e d i n that the distance is determined based upon the one-way propagation path loss between the user equipment and the central station or the corresponding propagation path loss in both uplink and downlink directions.

11. The method according to claim 9 or 10 c h a r a c t e r i z e d i n that the propagation time-delay or path-loss is estimated from signals transmitted on a traf- fie channel or a random access channel.

12. The method according to any of claims 1-11 c h a r a c t e r i z e d i n that time-slots are dynamically reallocated to the user equipments.

13. The method in any of claims 1-12 c h a r a c t e r - i z e d i n that it is a method of allocating time-slots avoiding interference from a user equipment at a large distance from the central station.

14. The method in any of claims 1-13 c h a r a c t e r i z e d i n that it is a method of allocating time-slots avoiding interference between time-slots transmitted on the same carrier frequency to or from the same central station.

15. The method in any of claims 1-14 c h a r a c t e r i z e d i that it is a method of allocating time-slots avoiding data from one or more user equipments transmitted in uplink direction to interfere with data destined for one or more other user equipments transmitted in downlink direction.

16. The method according to any of claims 1-15 c h a r a c t e r i z e d i n that the central station is a base station of a radio communications system.

17. The method according to any of claims 1-16 c h a r a c t e r i z e d i n that the channel allocation is performed in a radio network controller of a radio communications system.

18. A method of channel allocation of time-slots in a communications system, the method c h a r a c t e r i z e d i n that it includes the method in any of claims 1—17 in a weighted measure of deciding which time-slot to allocate to which user equipment.

19. A network element of a communications system for allocating time-slots avoiding interference between user equipments connected to a central station on a time-division duplex communication link between user equipments and the central station, the network element c h a r a c t e r - i z e d b y means for allocating time-slots on the communication link depending on the distance between the user equipment and the central station.

20. The network element according to claim 19 c h a r a c t e r i z e d i n that time-slots free for allocation are selected from a subset of time-slots of a time-segment, the subset comprising a number of time-slots corresponding to the number of user equipments to allow for channel allocation.

21. The network element according to claim 19 or 20 c h a r a c t e r i z e d b y means for sorting user equipments to allocate one or more time-slots according to the respective distances between the user equipments and the central station.

22. The network element according to any of claims 19-21 c h a r a c t e r i z e d b y means for allocating a user equipment a free time-slot close in time to a switching point if the user equipment is at a small distance from the central station and a time-slot distant in time to its nearest switching point if the user equipment is at a large distance from the central station.

23. The network element according to any of claims 19-22 c h a r a c t e r i z e d b y means for allocating the most distant user equipment one or more free time-slots of a time-segment or subset thereof maximally distant in time from switching points, between which time-slots are being allocated in the time-segment or subset thereof, and for successively allocating one or more free time-slots of the time-segment or subset thereof, the time-slots not being more distant in time from its nearest switching point than time-slots of the time-segment or subset thereof previously allocated, to user equipments in consecutively distance- descending order.

24. The network element according to any of claims 19-22 c h a r a c t e r i z e d b y means for allocating the closest user equipment one or more free time-slots of a time-segment or subset thereof maximally close in time to a switching point, being one of two switching points between which time-slots are allocated in the time-segment or subset thereof, and for successively allocating one or more free time-slots of the time-segment or subset thereof, the time-slots not being closer in time to its nearest switching point than time-slots of the time-segment or subset thereof previously allocated, to user equipments in consecutively distance-ascending order.

25. The network element according to any of claims 19-24 c h a r a c t e r i z e d i n that the time-slots comprise one or more channelization spreading code channels.

26. The network element according to any of claims 19-25 c h a r a c t e r i z e d i n that time-slots of a bidirectional communication link are transmitted on one carrier frequency.

27. The network element according to any of claims 19-26 c h a r a c t e r i z e d b y means for determining the distance of user equipments based upon the one-way propagation time delay between the user equipment and the central station or the corresponding roundtrip propagation time- delay.

28. The network element according to any of claims 19-26 c h a r a c t e r i z e d b y means for determining the distance of user equipments based upon the one-way propagation path loss between the user equipment and the central station or the corresponding propagation path loss in both uplink and downlink directions.

29. The network element according to claim 27 or 28 c h a r a c t e r i z e d i n that the propagation time- delay or path loss is estimated from signals transmitted on a traffic channel or a random access channel.

30. The network element according to any of claims 19-29 c h a r a c t e r i z e d b y means for reallocating time-slots to user equipments.

31. The network element according to any of claims 19-30 c h a r a c t e r i z e d i n that it is a network element of allocating time-slots avoiding interference from a user equipment at a large distance from the central station.

32. The network element according to any of claims 19-31 c h a r a c t e r i z e d i n that it is a network element of allocating time-slots avoiding interference between time-slots transmitted on the same carrier frequency to or from the same central station.

33. The network element in any of claims 19-32 c h a r a c t e r i z e d i n that it is a network element of al- locating time-slots avoiding data from one or more user equipments transmitted in uplink direction to interfere with data destined for one or more other user equipments transmitted in downlink direction.

34. The network element according to any of claims 19-33 c h a r a c t e r i z e d i n that the central station is a base station in a radio communications system.

35. The network element according to any of claims 19-34 c h a r a c t e r i z e d i n that the network element is a radio network controller in a radio communications sys- tem.

36. Radio communications system c h a r a c t e r i z e d b y means for carrying out the method in any of claims 1-18.