FI101032B - Extended cell system - Google Patents

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

1 101032

Extended cellular system

The present invention relates to digital time division multiple access (TDMA) radio systems and in particular to the allocation of 5 voice channels in an extended cellular system.

In Digital Time Division Multiple Access (TDMA) radio systems, a number of mobile radio stations may use the same frequency (radio channel) to communicate in a base station folder sa. The radio channel is sequentially divided into repetitive frames, which are further divided into time slots, e.g., eight time slots, which are allocated to users as needed. Short data bursts are sent in time slots. The mobile station synchronizes with the signal from the base station and transmits a burst according to this synchronization so that the burst of the mobile station is received at the base station in the time slot reserved for this particular mobile station. However, the mobile stations may be at different distances from the base station, in which case the transmission time of each mobile station must be synchronized to the base station taking into account the propagation delay due to this distance so that the signal is received at the base station in the correct time slot. To do this, the base station measures the time difference between its own transmission and the: w transmission received from the mobile station, and on this basis determines a • Y 25 suitable timing advance for the mobile station. With this timing prediction, the mobile station promotes its transmission time relative to the base time provided by the synchronization received from the base station.

Various internal factors in the system limit the maximum possible timing advance to a certain maximum • ·· *. * · 30 minvalue. This maximum value of the timing advance, in turn, determines the maximum cell size that the base station of the system can. ···. serve. For example, in the pan-European mobile phone system, the GSM (Global System for Mobile Communicati- * 'on) timing advance can have values between 0 and 233 μβ, which means a cell size with a radius of up to 35 km.

101032 2

A cell in a radio system usually provides the same level of service throughout the cell. In some cases, however, there may be a need to target a portion of a cell's radio capacity, either permanently or temporarily, to a limited area within a cell. Temporary concentration of capacity may be required, for example, in emergency situations, disaster situations, or in the service of an important traffic area (eg an airport) during peak hours. Intra-cell radio capacity has previously been sought to be shared to some extent by cell sectoring and directional antennas, but these do not provide a sufficiently flexible, efficient and accurate allocation of capacity to a particular geographic target.

EP patent application 0 564 429 also discloses an annular expanded cell with an additional offset between the time of transmission of the base station and the time of reception. This allows communication with a mobile station that is at a distance from the base station greater than the maximum distance set by said timing advance.

20 Applicant's previous Finnish patent applications FI933091 and FI933092 disclose an extended cell system in which an extended cell formed around a normal cell, the so-called long-distance cell by changing the time between reception and transmission:. 25 in those base station location transceivers that serve the remote cell. Some of the base station transceivers • · · operate with normal timing. . normal cell size, the so-called. lähisolua. Near and far cells • ♦ · * «[·’ are separate cells for mobile stations, each with *. * * 30 has its own broadcast frequency (radio channel), which is ·; ··; mobile stations can measure a cell in the normal way. * ··. for selection. The mobile station primarily selects and locks in the broadcast frequency that is strongest based on the measurements.

• · ·. * ·: 35 In an extended cellular system involving a near and far cell, there are problems with this. At the base station location, the transmission power of the transceiver serving the remote cell at the broadcast frequency is higher (or at least equal) than the transmission power of the transceiver 5 serving the near cell at the broadcast frequency. A mobile station close to a base station location (i.e., in a nearby cell) receives the strongest signal at the remote cell broadcast frequency and is always locked to it. If the transmission powers on the far and near cell broadcast frequencies are equal, interlocking can occur with equal probability to the far or near cell regardless of the location of the mobile station. In this way, the mobile station can be locked to a near or far cell, even if it is located at such a distance (too far or too close, respectively) from the base station location that it cannot communicate through the cell due to incorrect timing.

A mobile station locked to the wrong cell broadcast frequency may interfere with a call on the same radio channel in a situation where it transmits a random access burst on the broadcast frequency uplink (RACH), for example, to perform a location update, initiate a call, send a short message, or answer a call. In this case, the random access burst arrives at the base station location either too early or too late: ... 25 late, thus on the broadcast frequency over the previous or • · · "j. * Next time slot. The collision of the random access burst * · · * and the second time slot burst causes a short-term interference in the second time slot (traffic) • o * · '· "to the current call. The time slot disturbed in the remote cell is the time slot • · · V * 30 before the communication channel and the time slot following it in the local cell. Also, the mobile station must not connect to the wrong cell because the timing of the bursts is not correct. The mobile station performs the number of retries determined by the parameters of the mobile network and then moves to try at the next highest Λ 101032 4 broadcast frequency, which is typically an intracellular carrier. If the wrong cell is a remote cell, the next cell is typically a nearby cell with the correct timing of the random access burst.

It is an object of the invention to mitigate interference caused by a mobile station locked to a near or far cell to calls taking place in the cell.

This is achieved by a timing advance digital time division multiple access (TDMA) radio network with an extended cellular system comprising a near-cell in which transmission and reception timing is normal; a remote cell in which the relative timing of reception and transmission has been changed from normal timing so that the remote cell extends from the common base station location 15 beyond the maximum distance between the base station equipment and the mobile station in the radio network determined by the maximum allowable timing preset value; the near-cell broadcast frequency and the far-cell broadcast frequency, each of which has one slot allocated in the uplink direction in each of the 20 frames as a communication channel for mobile stations.

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and for the transmission of access bursts. According to the invention, the system is characterized in that the time slot immediately preceding the uplink access time slot at the remote cell universal frequency has the lowest priority. 25 cosol calls in allocation.

• · · * «. * Because interference caused by access bursts from a mobile station locked to the wrong cell is always •. . for a certain time slot in this cell, interference can be reduced by avoiding allocating this time slot to V * 30 calls whenever possible. When time slots other than the time slot preceding the time slot reserved for the uplink communication channel are available on the remote cell broadcast frequency, the call is established for the other time slot. If the call has had to be allocated to the time slot preceding the slot allocated to the uplink downlink channel, the call is transferred via the remote handover within the remote cell to another slot as soon as it is released. If the transmission powers of the near and far cells on the broadcast frequencies are the same or almost the same, even in the near cell, the calls 5 are primarily allocated to a time slot other than the time slot reserved for the non-uplink communication channel, when one is free. If the call has had to be allocated to the next time slot allocated to the uplink access channel, the call is transferred via the handover within the near-10 cell to another time slot as soon as it is free. Such slot prioritization can minimize the use of interference-prone slots and thus interference in calls.

The invention will be described in more detail with reference to the first embodiments with reference to the accompanying drawing, in which Figure 1 shows an extended cellular system according to the invention, Figure 2 shows an extended cellular system of the base station of Figure 2, Figure 3 shows the base station transmission and reception timing in a remote cell, and Figures 4 and 5 are flowcharts illustrating a prioritized slot allocation in a remote cell:. 25 and in a nearby cell, respectively.

The present invention is intended to be applied to any digital time division multiple access (TDMA) radio network using Timing Advance. to shift the moment of transmission of the mobile radio station · · · V: 30 relative to the moment set by the sync signal transmitted by the base station so that the timing. ···. the advance compensates for the transmission delay caused by the distance between the base station and the mobile station, and the transmission of the mobile station is received at the base station in the correct TDMA time slot. The invention is particularly suitable for use in GSM and 6101032 DCS1800 mobile telephone systems.

GSM (Global System for Mobile Communications) is a pan-European mobile phone system that is becoming a global standard. Figure 1 shows very briefly the basic components of the GSM system, without going into more detail on their characteristics or other aspects of the system. For a more detailed description of the GSM system, reference is made to the GSM Recommendations and to the book "The GSM System for Mobile Communications", M. Mouly & 10 M-B. Pautet, Palaiseau, France, 1992, ISBN: 2-9507190-0-7.

The mobile switching center MSC takes care of the connection of incoming and outgoing calls. It performs the same types of tasks as a public switched telephone exchange. In addition to these, it also performs functions specific to mobile-only voice traffic, such as subscriber location management. The mobile stations MS connect to the exchange MSC by means of the base station systems BSS. The base station system consists of a base station controller BSC and base stations BTS. One base station controller BSC is used to control several base stations BTS. The tasks of the BSC typically include e.g. handovers in cases where the handover is made within a base station or between two base stations, both of which are under the control of the same BSC. Figure 1 shows one base station system in which two base stations BTS1 and BTS2 are associated with a base station controller BSC. BTS1 • · · \ * j. ’Is a conventional base station whose radio coverage area forms * * a normal GSM radio cell C1. Base station BTS2 serves the so-called

an extended cellular system with a so-called proximal cell C3 and *. *. * distant cell C2. An example of such an extended cell system • · · V · 30 is shown in Figure 2.

In Fig. 1, for the sake of simplicity, one transceiver TRX1 serving a nearby cell C3 and one transceiver TRX2 serving a remote cell C2 are shown in the base station BTS2. Of course, the base station can have any number of transceivers for each cell.

The timing of the transmitter TX1 and the receiver RX1 of the transceiver TRX1 of the proximity cell C3 is normal, e.g. according to GSM recommendations, therefore the proximity cell C3 is practically a normal size cell which can extend up to the maximum distance BMAx determined by the timing advance ADMAX2 In the GSM system, the max is approx. 35 km. The distance cell C2 extends beyond this maximum distance to a distance r2 greater than rmax. This is achieved in that in the transceiver TRX2 of the remote cell C2, the relative timing of the transmitter TX2 and the receiver RX2 is changed by an additional offset which compensates for the propagation delay caused by the excessive distance. In this case, an annular cell is formed whose maximum radius r2 = max + the distance corresponding to the propagation delay of the offset and the minimum radius r1 = the distance corresponding to the propagation delay of the offset. A burst transmitted from a mobile station MS2 with a distance from the base station BTS2 between max and r2 from the base station BTS2 is received at the base station in time with respect to the timing of the receiver RX2, which enables remote cell traffic.

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On the other hand, a mobile station in a nearby cell less than a distance r1 from the base station is unable to communicate with the transceiver TRX2 of the remote cell C2. The timing of the remote • · · • · · \ 'j / lun C2 can be dimensioned so that the near and far **' cells have an overlapping area, the so-called handover area.

*. *. * An example of the implementation of an extended cellular system is presented in the applicant's previous patent applications FI933091 and FI933092. The implementation of a remote cell can also be used from the EP patent application

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‘·’ 0564429 known principles. It should be noted, however, that the invention is not limited to these particular embodiments of the expanded cell.

8 101032

In Digital Time Division Multiple Access (TDMA) radio systems, a number of mobile radio stations may time division use the same radio channel to communicate with a base station. In full dup-5 lex traffic, the transmission direction MS-BTS is generally referred to as the uplink direction and the transmission direction BTS-MS as the downlink direction. The radio channel usually consists of a pair of carriers between which there is a constant frequency difference, the so-called duplex interval, e.g. 45 or 75 MHz. In this application, the term frequency refers to a radio channel consisting of a carrier pair.

In the proximal cell C3 the frequency F1 of the transceiver TRX1 and in the remote cell the frequency F2 of the transceiver TRX2 are both so-called. a broadcast frequency on which common control channel information is transmitted in the downlink (BTS-MS) direction, such as the Frequency Correction Channel (FCCH), the SCH (Synchronization Channel) and the Broadcast Control Channel (BCCH) of the GSM system, e.g. in the TSO slot. C3 and remote cell C2 actually form two independent cells, each of which is, for example, 20 cells according to the GSM specification with all the parameters belonging to the cell.Mobile stations can measure these broadcast frequencies F1 and F2 according to the GSM specifications for cell selection. according to, the mobile station primarily selects the cell,. ^ 25 whose broadcast frequency is the strongest based on the measurements, and locks into it.

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* · * * In an extended cell system with near and far cell, there are problems with this. At the base station location • o V. · The transmission power of the transceiver TRX2 ♦ * t V * 30 serving the remote cell C2 at the broadcast frequency F2 is greater than or at least equal to the transmission power of the transceiver TRX1 serving the remote cell C3 at the broadcast frequency F1. The mobile station MSI near the base station location (i.e. in the nearby cell C3) receives the strongest signal at the broadcast frequency F2 of the remote cell and is always locked to it.

9 101032

If the transmission powers at the far and near cell broadcast frequencies F2 and F1 are equal, locking can occur with equal probability to the far cell C3 or the near cell C2 regardless of the location of the mobile station. This allows the mat-5 cavities to be locked to a near or far cell, even if located at such a distance (too far or too close, respectively) from the base station location that it cannot communicate through the cell due to incorrect timing.

A mobile station 10 locked to the wrong cell broadcast frequency may interfere with a call in the second time slot on the same frequency in a situation where it transmits a random access burst on the broadcast frequency uplink (RACH), for example, to perform a location update, initiate a call, or send a short message. In this case, the random access burst arrives at the base station location either too early or too late, thus going on the general transmission frequency over the previous or next time slot transmission. A collision between a random access burst and a second slot burst 20 causes a short-term interference to a call in progress in the second slot (traffic channel). The time slot disturbed in the remote cell is the time slot before the contact channel and in the local cell the time slot following it. Also, the mobile station cannot establish a connection to the wrong cell because the timing of the * '.t 25 pur s is not correct. The mobile station performs • · · the number of retries ♦ · · * · ’* determined by the parameters of the mobile network and then moves to try at the next strongest broadcast frequency, which is *. ·. * Typically an intracellular carrier. If the wrong cell is a long-distance cell, the next cell is typically a neighboring cell where .... · the timing of the random access burst is correct.

, ···, In the example case of Fig. 3, the TDMA frame period of the transceiver TRX2 of the remote cell C2 on the broadcast frequency F2 comprises 8 time slots TS0 to TS7, but the time slots · *. '; Depending on the system, the number may be higher or lower, e.g. 4. There is a timing offset between the start of the frame periods of the transmitter TX2 and the receiver RX2, which allows communication in the remote cell C2. According to the GSM recommendations, the slot number-ti is such that on the receiving side a certain slot number occurs three slots later than the corresponding slot number on the transmitting side. The uplink communication channel is in the time slot TSO, while the other time slots TS1 to TS7 are dedicated traffic and / or control channels.

It is assumed that the mobile station MSI (Fig. 2) is assigned to the remote cell C2 at the broadcast frequency F2, even though it is in the service area of the neighboring cell C3. However, since the MSI is within the minimum radius r1 of the remote cell C2 (i.e., outside the service area of the remote cell C2), it cannot cell at frequency F2. This is because the transmission and reception timing of the transceivers TRX2 of the base station BTS2 has been changed at F2 as shown in Figure 3 so that the MSI locked to the broadcast frequency F2 can only communicate at a distance r1-r2 from the base station 20 to the base station TR2. In Figure 3, the bottom line depicts the timing of the transmission of the mobile station MSI locked to frequency F2. It is further assumed that, for example, the MSI wants to perform si- j. 25 update the call, start a call, send a short message, • · «*”. * Man, or answer the call. In all cases, the MSI initiates the connection by transmitting the remote cell broadcast frequency F2 on the uplink access channel in a time slot *. *. * In the TSO random access burst. Because the MSI is too close to the base station BTS2, the random access burst it transmits arrives at the base station too early relative to the timing of the receiver RX2 • · and partially goes over the previous time slot of frequency F2, as illustrated in Figure 3 in the shaded area 30. The second mobile station MS2, which is in the area of the remote cell C2, precedes the burst burst transmitted by the MS1 in the burst transmitted by the MS2 in the time slot 7, and causes a short-term interference in the communication of the MS2. The MSI also cannot establish a connection on the frequency of the remote cell C2 because the ha-5 access burst does not arrive at the receiver RX2 in the time slot TSO. The MSI performs the number of retries determined by the parameters of the mobile network and then proceeds to try to establish a connection on the next best heard frequency, which in the exemplary case is typically the broadcast frequency F1 of the neighboring cell C3. The connection establishment attempt to the transceiver TRX1 of the nearby cell C3 is successful because the MSI is in the service area of the cell C3 and the access burst arrives at the receiver RX1 in time in the time slot TSO. Attempts of the type described above and retries on the broadcast frequency F1 of the remote cell C3 briefly interfere with calls in the remote cell time slot 7. If there are many mobile stations in the local cell C3, communication in the time slot TS7 may be completely interrupted when multiple mobile stations MS attempt to connect to the base station BTS almost simultaneously.

A similar type of interference occurs if the mobile station MS2 is locked in the nearby cell C3 on the broadcast frequency F1 when it is at a distance from the base station. 25 higher than rmax. This may occur if the general transmission power of the remote cell C3 and the • · · 'Γ.Υ remote cell C2 at the frequencies F1 and • · «' ** 'F2 are seins or nearly the same. In this case, the access burst transmitted by the MS2 on the frequency F1 in the time slot TSO arrives at the receiver RX1 too late in relation to the timing of the receiver and partially overlaps at the receiver with the next time slot TS1. If a mobile station • Melie MSI is simultaneously making a call in time slot TS1, the bursts of the mobile stations collide and there is a short-term interruption in the communication of the mobile station MSI. Also, the connection of the mobile station MS2 35 is not successful and it will, after retries, switch to the next best broadcast frequency, which is probably F2.

These interferences caused by a mobile station locked to the wrong cell can be mitigated by the prioritized allocation according to the invention. According to the basic principle of the invention, the aim is to avoid the time slots of the broadcast frequencies F1 and F2 in which interference is caused. Figures 4 and 5 are flow charts illustrating the allocation of time slots for calls in a remote cell and a neighboring cell, respectively. The allocation according to the invention can be performed as an additional measure in the network element of the mobile communication network, which otherwise performs channel management in the cell. This network element can be a base station, a base station controller or a mobile switching center.

Referring to Fig. 4, in the remote cell C2, a channel request is received on the broadcast frequency F2 (step 401). In step 402, it is checked whether there are free time slots in cell C2 other than time slot TS7 of frequency F2. It should be noted that the problem underlying the invention does not occur at other frequencies in the cell that are not broadcast frequencies and that do not transmit random access bursts. Thus, on these other frequencies in the cell, the time slot TS7 can also be used.

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... If at least one time slot is free on frequency F2 or another frequency I: 25 on the remote cell C2, this time slot is allocated to the call (step 403). If no other time slots are available in cell C2, the time slot TS7 of frequency F2 is temporarily allocated to the call (step 404). After this, however, • · \ V is constantly monitored to see if ··· V ϊ 30 in the remote cell C2 frees up some other time slot to which the call can be "*. · Transferred (step 405). If such a slot is freed, • the call is transferred to the remote cell's internal handover

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from the time slot TS7 of F2 to this released time slot i on the frequency F2 or on another frequency of the cell C2 (item 35 406).

* «· 13 101032

The slot allocation illustrated in Fig. 5 in the near cell C3 can be applied when the transmission powers of the near cell and the far cell at the broadcast frequencies F1 and F2 are equal to or nearly equal. In this case, the travel-5 stations can also be incorrectly locked to the frequency F1 of the nearby cell. The flowchart of Fig. 5 follows the flowchart of Fig. 4, except that an attempt is now made to avoid allocating the time slot TS1 of the frequency F1. When a channel request is received for the neighboring cell C3 on the broadcast frequency F1 (step 10 501), it is checked whether there are free time slots in the cell other than the time slot TS1 of the frequency F1 (step 502). If another time slot is free on frequency F1 or another frequency in cell C3, this time slot is allocated to the call (step 503). The time slot TS1 of the frequency F1 must be allocated if there are no other free time slots in the adjacent cell C2 (step 504). In this case, however, the release of the other time slots is monitored (step 505), and the call is transferred from the time slot F1 of the frequency F1 by an intra-cell handover to another time slot as soon as one is released (step 506).

Although the invention has been described with reference to certain embodiments, it is to be understood that the description is given by way of example only and that changes and modifications may be made therein without departing from the spirit and scope of the invention as defined by the appended claims; ; 25 protection areas.

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Claims (4)

14 101032 A timing advance digital time division multiple access (TDMA) radio network having an extended 5 cell system comprising a proximal cell in which transmission and reception timing is normal, a remote cell in which the relative timing of reception and transmission is changed from normal timing 10 so that the remote cell extends from the common base station location beyond the maximum distance (rmax) determined by the maximum allowed timing advance value between the base station equipment and the mobile station in the radio network, , characterized in that the time slot immediately preceding the time slot reserved for the transmission of random ac-20 cess bursts at the remote cell: *: ': broadcast frequency has the lowest priority for long-distance cell calls all okoitaessa. System according to Claim 1, characterized in that • I | ^ 25 The time slot immediately following the time slot allocated for the transmission of random ac- • · · **. * Cess bursts on the near-cell • · · * · ”broadcast frequency has the lowest priority in the allocation of near-cell calls. • · A system according to claim 1 or 2, characterized in that the radio network is adapted to perform intra-cell handover out of a time slot immediately preceding the time slot reserved for the transmission of random access bursts at a remote cell broadcast frequency, to another time-35 as soon as one is released. is 101032 A system according to claim 1, 2 or 3, characterized in that the radio network is adapted to perform intra-cell handover out of a time slot which immediately follows the time slot reserved for the transmission of random access bursts at a neighboring cell broadcast frequency, to another time slot as soon as it is released. • · • · · • · • · 1 ♦ • · · • ♦ e ^ • · · • • • o • · A • · · Φ • 101032 16