WO1997001254A1

WO1997001254A1 - An extended cell system

Info

Publication number: WO1997001254A1
Application number: PCT/FI1996/000361
Authority: WO
Inventors: Jukka Suonvieri
Original assignee: Nokia Telecommunications Oy
Priority date: 1995-06-22
Filing date: 1996-06-20
Publication date: 1997-01-09
Also published as: FI953148A0; FI953148A; FI101032B; AU6225696A

Abstract

The present invention relates to digital time division multiple access (TDMA) radio systems employing timing advance. An extended cell system comprises an inner cell (C3) employing a standard timing of transmission and reception, an outer cell (C2) in which the relative timing of reception and transmission is offset with respect to the standard timing. Both the outer cell and the inner cell have an individual broadcasting frequency. The problem is that the mobile station may lock into the outer cell or into the inner cell with the same probability regardless of its actual location area. The mobile station cannot, however, communicate in the wrong cell, but it only causes interference to the other users in the wrong cell. For minimizing these interferences, the time-slot immediately preceding or following the time-slot allocated for transmitting random access bursts on the broadcasting frequency of the outer cell has the lowest priority in allocating the calls in the outer cell or the inner cell, respectively.

Description

An Extended Cell System

Field of the Invention

The present invention relates to digital time division multiple access (TDMA) radio systems and allocating speech channels in an extended cell system, in particular.

Background of the Invention

In digital time division multiple access (TDMA) radio systems, a group of mobile radio stations may use a same frequency (radio channel) for communication with a base station in accordance with a time division principle. A radio channel is divided into successive, repeated frames, which are further sub-divided into time-slots, e.g. eight time-slots, which are allocated to the users on demand. In the time-slots, short data bursts are transmitted. A mobile station synchronizes itself to a signal received from the base station and transmits a burst in accordance with the synchronization so that the burst will be received at the base station within a specific time- slot allocated to mobile station. The mobile stations may, however, be located at the different distances from the base station, and consequently, the transmitting time of each mobile station must be synchronized with the base station taking into account the propagation delay caused by this distance, so that the signal will be received at the base station in a correct time-slot. For this purpose, the time delay between its own transmission and the transmission received from the mobile station is measured by the base station, and on the basis of the time delay, the base station determines a proper timing advance for the mobile station. The timing advance enables the mobile station to advance its transmitting time with respect to a reference timing provided by the synchronization from the base station. Various inherent factors of the radio system limit the highest possible value of the timing advance to a specific maximum value. This maximum value of the timing advance, in turn, determines the maximum cell size that can be served by a base station. E.g. in the pan-European mobile communications system GSM (Global System for Mobile Communication) , the values of the timing advance may vary within the range 0 - 233 μs, which represents a cell size having a radius of up to 35 km.

A cell of the radio system usually provides the same level of service within the entire cell. In some cases, however, a need may arise to concentrate part of the radio capacity of the cell, either permanently or temporarily, only to a certain area inside the cell. Temporary concentration of the capacity may be required e.g. in an emergency situation, in catastrophes or for the service of an important traffic area (e.g. an airport) during the rush hours. It has earlier been endeavoured to distribute the internal radio capacity of a cell by means of sectorized cells, as well as directional antennas, but these approaches do not allow sufficiently flexible, effective and accurate concentration of the capacity on a certain geographical target area.

European Patent Application No 0,564,429 also discloses a ring-shaped extended cell having an additional offset between the transmission and the reception times of the base station. This enables communication with such a mobile station whose distance from the base station is longer than the maximum distance determined by said timing advance. The applicant's previous patent applications FI-933,091 and FI-933,092 disclose an extended cell system in which an extended cell, a so-called outer cell, has been formed around a standard cell at a same base station site by changing the timing between reception and transmission at those transceivers of the base station site that are serving the outer cell. Part of the transceivers of the base station site operate using standard timing, serving a standard cell size, a so-called inner cell. From the point of view of the mobile stations, inner and outer cells are separate cells each of them having a dedicated broadcasting frequency (radio channel, which may be measured in a normal way by the mobile stations for selecting the cell. A mobile station is arranged to primarily select and lock onto the broadcasting frequency which has the highest signal level on the basis of the measurements.

The cell selection described above causes problems in an extended cell system comprising an inner cell and an outer cell. The transmitting power on the broadcasting frequency of a transceiver serving the outer cell at the base station site is higher than (or at least equal to) the transmitting power on the broadcasting frequency of the transceiver serving the inner cell. The mobile station located close to the base station site (i.e. in the inner cell) will receive the strongest signal on the broadcasting frequency of the outer cell and inevitably lock onto it. If the transmitting power is the same on the broadcasting frequencies of the outer cell and the inner cell, locking may take place with the same probability either into the outer cell or into the inner cell regardless of the location of the mobile station. The mobile station may thus lock into the inner cell or the outer cell, although it is located at such a distance (too far or too near) from the base station site that it cannot communicate via the cell due to incorrect timing.

A mobile station that is locked onto the broadcasting frequency of the wrong cell may also cause interference to an ongoing call on the same radio channel in a situation in which the mobile station is transmitting a random access burst on an uplink RACH channel (random access channel) of the broadcasting frequency e.g. when it wishes to carry out location updating, set up a call, send a short message or answer a call. In such a case, the random access burst arrives at the base station site either too soon or too late, thus overlapping with the transmission of the previous or the following time-slot on the broadcasting frequency. The collision of the random access burst and a burst of another time-slot causes a short-term interference to an ongoing call in the other time-slot (traffic channel) . In the outer cell, the interfered time-slot is the time-slot preceding the random access channel, and in the inner cell it is the time-slot following the random access channel. It is not possible for the mobile station either to establish a connection to the wrong cell, since the timing of the bursts is not correct. The mobile station will make a number of re-attempts, determined by the parameters of the mobile communication network, and will then make a new attempt to lock onto the next strongest broadcasting frequency, which is typically the carrier wave of the inner cell. If the wrong cell is an outer cell, the next best cell is typically an inner cell, in which the timing of the random access burst is appropriate. Disclosure of the Invention

The object of the present invention is to reduce interference caused to ongoing calls in the cell by a mobile station locked into an inner or outer cell. The object is achieved by means of a time division multiple access (TDMA) radio network comprising an extended cell system comprising an inner cell employing a standard timing of transmission and reception; an outer cell in which the relative timing of reception and transmission is offset with respect to the standard timing in such a way that the outer cell extends farther from a common base station site than the maximum distance between a base station equipment and a mobile station in a radio network, said distance being determined by the highest value allowed for the timing advance; a broadcasting frequency of the inner cell and a broadcasting frequency of the outer cell both having in each frame one uplink time-slot reserved as the random access channel for transmitting random access bursts of the mobile stations. In accordance with the invention, the method is characterized in that the time-slot immediately preceding the uplink random access channel on the broadcasting frequency of the outer cell has the lowest priority in allocating the calls of the outer cell.

Since a mobile station locked into the wrong cell will always interfere with a preset time-slot of the cell, the interference may be reduced by avoiding the allocation of this time-slot for calls when possible. In case there are other time-slots available on the broadcasting frequency of the outer cell than the one preceding the time-slot reserved for the uplink random access channel, the call will be established in some other time-slot than said time-slot. In case it has been necessary to allocate the call into the time- slot preceding the time-slot reserved for the uplink random access channel, the call will be handed over by an intracell handover to another time-slot in the outer cell immediately when there is one available. Provided that the transmitting power of the inner cell and the outer cell on the broadcasting frequency is the same or almost the same, the calls of the inner cell will also be primarily allocated to some other time-slot than the one following the time-slot reserved for the uplink random access channel, when there is one available. If it has been necessary to allocate a call to the time- slot following the time-slot reserved for the uplink random access channel, the call will be immediately handed over to another time-slot by an intra-cell handover in the inner cell when such a time-slot becomes available. This kind of priority system of time-slots enables minimizing the use of time-slots sensitive to interference, and thus the interferences occurring in the calls.

Brief Disclosure of the Drawings In the following, the invention will be disclosed in closer detail by means of the preferred embodiments with reference to the attached drawings, in which:

Figure 1 shows a radio system of the invention having an extended cell system,

Figure 2 shows an extended cell system of a base station shown in Figure 2, Figure 3 shows the timing of the transmission and reception of the base station in an outer cell, and

Figures 4 and 5 are flow charts illustrating prioritized allocation of time-slots in an outer cell and in an inner cell, respectively. The Preferred Embodiment of the Invention

The present invention may be applied in any radio network employing time division multiple access (TDMA) , and timing advance for shifting the transmitting time of a mobile radio station with respect to a time set by a synchronization signal transmitted by the base station, so that the timing advance will compensate the propagation delay caused by the distance between the base station and the mobile station and the transmission of the mobile station will be received in the correct TDMA time-slot at the base station. The invention is especially suited for the GSM and DCS1800 mobile communications systems.

GSM (Global System for Mobile Communications) is a pan-European mobile communication system, which is becoming a worldwide standard. Figure 1 shows briefly the basic elements of the GSM system, not paying closer attention to their features or other aspects of the system. A more detailed description of the GSM system is disclosed in the GSM specifications and "The GSM System for Mobile Communications", M. Mouly, M-B. Pautet, Palaiseau, France, 1992, ISBN:2-9507190-0-7, which is incorporated herein by reference.

A mobile services switching centre MSC handles switching of incoming and outgoing calls. It carries out functions similar to those of an exchange of a public switched telephone network PSTN. In addition, it also carries out tasks typical of mobile telecommunication only, such as subscriber location management. Mobile stations MS are connected to the MSC by means of base station systems BSS. A base station system is composed of a base station controller BSC and base stations BTS. One BSC is used for controlling a plurality of base stations BTS. The tasks of the BSC typically include handovers in cases where a handover is performed within a base station (intra-BTS handover) or between two base stations that are controlled by the same BSC (intra-BSC handover) . Figure 1 shows one base station system in which a BSC is connected to two base stations, BTSl and BTS2. BTΞ1 is a standard base station whose radio coverage area forms a normal GSM radio cell Cl. BTS2 serves a so-called extended cell system having a so-called inner cell C3 and an outer cell C2. An example of an extended cell system is shown in Figure 2. For the sake of simplicity, Figure 1 only illustrates one transceiver TRX1 serving the inner cell C3 and one transceiver TRX2 serving the outer cell C2 at the base station BTS2. There may naturally be any number of transceivers for both cells at the base station BTS2.

The timing of a transmitter TX1 and a receiver RX1 of the TRX1 serving the inner cell C3 is normal, that is, in accordance with the GSM specifications, and the inner cell C3 is therefore practically a normal- sized cell, which may extend up to the maximum distance rmax from the base station BTS2, said maximum distance being determined by the maximum value ADMAX allowed for the timing advance in the mobile communications system. In the GSM system, rmax is about 35 km. The outer cell C2 extends beyond this maximum distance to a distance r, which is longer than rmax. This has been achieved by changing the relative timing of a transmitter TX2 and a receiver RX2 in the transceiver TRX2 of the outer cell C2 with an additional offset, which compensates the propagation delay caused by an over-long distance. Thus, a ring-shaped cell is formed, having a maximum radius r2 = rmax + a distance corresponding to the propagation delay of the offset, and a minimum radius rl = a distance corresponding to the propagation delay of the offset. A burst that is transmitted from a mobile station MS2, whose distance from the base station BTS2 is between rmax - r2, is received at the base station BTS2 at the correct time with respect to the timing of receiver RX2, which enables the traffic in the outer cell. On the other hand, a mobile station whose distance from the base station in the inner cell is shorter than rl is not able to communicate with transceiver TRX2 of the outer cell C2. The timing of the outer cell C2 may be dimensioned in such a way that the inner cell and the outer cell have an overlapping area, that is, a so-called handover area.

An example of a practical implementation of an extended cell system is disclosed in the applicant's former patent applications FI-933,091 and FI-933,092. The outer cell may also be implemented in accordance with the principles disclosed in European patent application No. 056,4429. It must be noted, however, that the invention is not limited to these specific embodiments of an extended cell. In digital time division multiple access

(TDMA) radio systems, a group of mobile radio stations may use the same radio channel for communicating with a base station in accordance with the time division principle. In full duplex traffic, the transmission direction MS-BTS is usually termed as an uplink direction, and the transmission direction BTS-MS as a downlink direction. A radio channel is usually composed of a pair of carrier frequencies with a constant frequency offset, a so-called duplex spacing, which may be e.g. 45 or 75 MHz. The term frequency herein refers to a radio channel which is composed of a pair of carrier frequencies.

The frequency Fl of transceiver TRX1 in the inner cell C3 and the frequency F2 of transceiver TRX2 in the outer cell C2 are both so-called broadcasting frequencies, which are used in the downlink direction (BTS-MS) for transmitting common control channel information, such as the FCCH (Frequency Correction Channel) , SCH (Synchronization Channel) and BCCH (Broadcast Control Channel) of the GSM system e.g. in a time-slot TSO. The inner cell C3 and the outer cell C2 of the base station BTS2 thus actually establish two independent cells, e.g. GSM cells with all the cell- specific parameters. In accordance with the GSM specifications, mobile stations may measure these broadcasting frequencies Fl and F2 for selecting the cell. In accordance with the GSM specifications, a mobile station selects a cell whose broadcasting frequency is the strongest according to the measurements, and locks onto that frequency.

This causes problems in an extended cell system comprising an inner cell and an outer cell. At base station site BTS2, the transmitting power on broadcasting frequency F2 of transceiver TRX2 serving the outer cell C2 is stronger than or at least equal to the transmitting power on broadcasting frequency Fl of transceiver TRX1 serving the inner cell C3. A mobile station MSI located close to the base station site (i.e. in the inner cell C3) receives the strongest signal on the broadcasting frequency F2 of the outer cell, and always locks onto that frequency. If the transmitting power of the outer cell and that of the inner cell are equal on the broadcasting frequencies F2 and Fl, locking may take place to the outer cell C3 or to the inner cell C2 with the same probability regardless of the location of the mobile station. The mobile station may thus lock into the inner cell or the outer cell, although it is located at such a distance

(too far or too near) from the base station site that it cannot communicate via the cell due to inappropriate timing.

A mobile station that is locked onto the broadcasting frequency of the wrong cell may also cause interference to an ongoing call on the same radio channel in a situation in which it is transmitting a random access burst on an uplink RACH channel (random access channel) of the broadcasting frequency e.g. when it wishes to carry out location updating, set up a call, send a short message or answer a call. In such a case, the random access burst arrives at the base station site either too soon or too late, thus overlapping with the transmission of the previous or the following time-slot on the broadcasting frequency. The collision of the random access burst and a burst of another time-slot causes a short-term interference to the ongoing call in the other time-slot (traffic channel) . In the outer cell, the interfered time-slot is the time-slot preceding the random access channel, and in the inner cell the time-slot following the random access channel. It is not possible for the mobile station either to establish a connection to the wrong cell, since the timing of the bursts is not correct. The mobile station makes a number of re- attempts determined by the parameters of the mobile communication network, and then makes a new attempt to lock onto the next strongest broadcasting frequency, which is typically the carrier wave of the inner cell. If said wrong cell is an outer cell, the next best cell will typically be an inner cell, in which the timing of the random access burst is correct.

In the exemplary case shown in Figure 3, the TDMA frame period of the transceiver of the outer cell C2 on broadcasting frequency F2 consists of 8 time¬ slots TS0-TS7, but the number of time-slots may be larger or smaller depending on the system, e.g. 4. Between the frame periods of transmitter TX2 and receiver RX2 there is a timing offset which enables communication in the outer cell C2. In accordance with the GSM recommendations, the time-slots have been numbered in such a way that a certain time-slot number occurs three time-slots later on the receiving side than the respective time-slot number on the transmitting side. The uplink access channel is located in time-slot TSO, whereas the other time-slots TSl - TS7 are dedicated traffic and/or control channels.

Let us assume that a mobile station MSI (Figure 2) is locked onto the broadcasting frequency F2 of the outer cell C2 although it is located in the service area of the inner cell C3. In addition, since MSI is within the area determined by the minimum radius rl of the outer cell C2 (i.e. outside the service area of the outer cell C2), it cannot, however, communicate on frequency F2. This is due to the fact that the timing of reception and transmission of the transceivers TRX2 of base station BTS2 has been changed on frequency F2 in accordance with Figure 3 so that MSI which is locked onto broadcasting frequency F2 can operate only when it is located within the distance rl- r2 from the base station, whereby a signal transmitted by it arrives at the base station at the right moment with respect to the timing of TRX2. The bottom line in Figure 3 illustrates the timing of the transmission of MSI which is locked onto frequency F2. Let us further assume that MSI wishes to carry out location updating, set up a call, send a short message or answer a call, for instance. In all cases, MSI starts call establishment by transmitting a random access burst on the uplink random access channel of the broadcasting frequency F2 of the outer cell in time-slot TSO. Since MSI is located too close to BTS2, the random access burst transmitted by it arrives at the base station too early with respect to the timing of RX2, and partly overlaps with the previous time-slot of frequency F2, as illustrated in Figure 3 with a shadowed area 30. In case another mobile station MS2 which is located within the area of the outer cell C2 has transmitted in this time-slot, the random access burst transmitted by MSI will collide in time-slot 7 with the- burst transmitted by MS2, and cause a short-time interference in the communication of MS2. Neither does MSI succeed in establishing a connection on the frequency of the outer cell C2, since the random access burst does not arrive at the receiver RX2 in time-slot TSO. MSI makes a number of re-attempts determined by the parameters of the mobile communication network, and goes then, for establishing a connection, onto the next strongest frequency, which frequency is typically the broadcasting frequency Fl of the inner cell C3. The attempt to establish a connection to the transceiver TRX1 of the inner cell C3 is successful, since MSI is located within the service area of cell C3, and the random access burst arrives at the receiver RX1 at an appropriate time in time-slot TSO. The attempts and re- attempts on the broadcasting frequency Fl of the outer cell C2 as disclosed above temporarily interfere with the calls in time-slot TS7 of the outer cell. If there is a large number of mobile stations in the inner cell C3, the communication in time-slot TS7 may be completely blocked when several mobile stations are trying to establish a connection to the base station BTS by means of random access bursts almost simultaneously.

A similar interference will occur if a mobile station MS2 is locked onto the broadcasting frequency Fl of the inner cell C3 and located within the distance more than rmax from the base station. This is possible if the transmitting power of the inner cell C3 and the outer cell C2 on the broadcasting frequency is the same or almost the same. In such a case, the random access burst which is transmitted by MS2 on frequency Fl in time-slot TSO arrives at the receiver RX1 too late with respect to the timing of the receiver, and partly overlaps in the receiver with the following time-slot TSl. If the mobile station simultaneously has an ongoing call in time-slot TSl, the bursts of the mobile stations will collide, a short-term interference will occur in the communication of MSI. MS2 does not succeed either, in establishing a connection, and it will make a new attempt to lock onto the next best broadcasting frequency, which is probably F2.

These interferences caused by a mobile station that is locked into the wrong cell may be reduced with prioritized allocation according to the invention. In accordance with the basic principle of the invention, an attempt is made for avoiding those time-slots in which interference is caused. Figures 4 and 5 show flow charts illustrating allocation of time-slots to calls in an outer cell and in an inner cell, respectively. The allocation in accordance with the invention may be carried out as an additional procedure in that network element of the mobile communication network that carries out channel management in the cell in other cases, too. This network element may be a base station, a base station controller or a mobile services switching centre.

With reference to Figure 4, a channel request is received in the outer cell C2 on broadcasting frequency F2 (step 401) . In step 402 it is checked whether there are other time-slots available in the cell besides time-slot TS7 on frequency F2. It must be noted that the problem underlying the invention does not occur on the other frequencies of the cell, which frequencies are not broadcasting frequencies, and no random access burst are transmitted along them. On these other frequencies of the cell, time-slot TS7 may thus also be used. Provided that there is at least one time-slot available on frequency F2 or on any other frequency of the outer cell C2, this time-slot will be allocated to a call (step 403) . Provided that there are no other time-slots available in cell C2, the time-slot TS7 of frequency F2 will be temporarily allocated to the call (step 404) . Thereafter, an attempt is made for continuously monitoring whether any other time-slot becomes available in the outer cell C2, allowing the handover of the call (step 405) . In case such a time- slot becomes available, the call will be handed over by an intracell handover in the outer cell from time-slot TS7 on frequency F2 to the available time-slot on frequency F2 or any other frequency of cell C2 (step 406) .

Allocation of time-slots in the inner cell C3 illustrated in Figure 5 may be applied when the transmitting power of the inner cell and that of the outer cell on the broadcasting frequencies Fl and F2 are the same or almost the same. In this case, the mobile stations may also incorrectly lock onto the frequency Fl of the inner cell. The flow chart of Figure 5 is similar to the flow chart shown in Figure 4, with the exception that in Figure 5, allocating time-slot TSl on frequency Fl is avoided. Once the channel request to the inner cell C3 has been received on the broadcasting frequency Fl (step 501) , it is checked whether there are other time-slots available on frequency Fl besides time-slot TSl (step 502) . If there is any other time-slot available on frequency Fl or on any other frequency of cell C3, this time-slot will be allocated to the call (step 503) . Time-slot TSl on frequency Fl must be allocated in case there are no other available time-slots in the inner cell C2 (step 504) . In that case, however, releasing of other time- slots is monitored (step 505), and the call is transferred by means of intracell handover of the inner cell from time-slot TSl on frequency Fl to another time-slot immediately when such a time-slot becomes available (step 506) .

Although the invention has been disclosed above with reference to specific embodiments, it is obvious that the explanation has only been made by way of example, allowing alterations and modifications without deviating from the scope and the spirit of the invention set forth in the attached claims.

Claims

Claims :

1. A time division multiple access (TDMA) radio network employing timing advance and comprising an extended cell system comprising: an inner cell employing a standard timing of transmission and reception; an outer cell in which the relative timing of reception and transmission is offset with respect to the standard timing in such a way that the outer cell extends farther from a common base station site than the maximum distance between a base station equipment and a mobile station in a radio network, said distance being determined by the highest value allowed for the timing advance; a broadcasting frequency of the inner cell and a broadcasting frequency of the outer cell both having in each frame one uplink time-slot reserved as the random access channel for transmitting random access bursts of the mobile stations, c h a r a c t e r i z e d by the time-slot immediately preceding the time- slot allocated for transmitting random access bursts on the broadcasting frequency of the outer cell having the lowest priority in allocating the calls of the outer cell.

2 . A system as claimed in claim 1 , c h a r a c t e r i z e d by the time-slot immediately following the time- slot allocated for transmitting random access bursts on the broadcasting frequency of the inner cell having the lowest priority in allocating the calls of the inner cell.

3. A system as claimed in claim 1 or 2, c h a r a c t e r i z e d by the radio network being arranged to perform an intracell handover from the time-slot immediately preceding the time-slot allocated for transmitting random access bursts on the broadcasting frequency of the outer cell to another time-slot as soon as such a time-slot becomes available.

4. A system as claimed in claim 1, 2 or 3, c h a r a c t e r i z e d by the radio network being arranged to perform an intracell handover from the time-slot immediately following the time-slot allocated for transmitting random access bursts on the broadcasting frequency of the inner cell to another time-slot as soon as such a time-slot becomes available.