Method in a mobile radio system

(54)

PATENT HOLDER Telefonaktiebolaget LM Ericsson, 126 25 Stockholm SE

INVENTOR

AGENT

Carl Magnus Frodigh, Kista SE, Knut Magnus Almgren, Sollentuna SE, Håkan Gunnar Olofsson, Stockholm SE Norin K (56) (57)

NAME Procedure for handover in a mobile radio system, mobile radio system, as well as mobile terminal and radio base station included in such a system

CALLED PUBLICATIONS: - - SUMMARY:

The present invention relates to a method for performing handover in a mobile radio system. The mobile radio system performs quality measurements on signals transmitted (300) between a mobile terminal and previously determined radio base stations. Handover is performed (301,302,303) when any of the measured signal strengths exceeds the signal strength of the radio base station to which the mobile terminal is currently connected added to a handover value. The handover value is dependent on measured signal strengths and neighbor cell relation values. The grid cell relation values ​​depend on a probability of handover from one radio base station to another radio base station in the mobile radio system.

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

510 052

TECHNICAL FIELD

The present invention relates to a mobile radio system comprising radio base stations for transmitting control information and message information in the form of radio information to mobile terminals. In particular, the invention relates to a method in which the transmission of message information to a mobile terminal is transmitted from one radio base station to another radio base station, i.e. a so-called handover.

BACKGROUND OF THE ART

It is desirable that a mobile radio system has high traffic management capacity and high coverage. The traffic management capacity of a mobile radio system depends, among other things, on the number of available radio channels and how efficiently these channels can be utilized. It is known to arrange several radio base stations with small coverage areas, also called cells, close to each other within a mobile radio system. Available radio channels can then be utilized more efficiently for managing traffic peaks within a limited geographical area than if radio base stations with large coverage areas are arranged far apart in the mobile radio system. The coverage area is the area within which a call is established. The arrangement of many radio base stations close to each other can thus increase the capacity of a mobile radio system. However, two radio base stations with such overlapping coverage areas cannot normally use the same radio channel for communication with different mobile terminals, as is the case for, for example, mobile radio systems implemented in TDMA technology and FDMA technology.

When a mobile terminal has a communication link connected to a first radio base station (also referred to below as active

510 052 radio base station and its associated cell are, by analogy, called active coverage of said cell) and moving from the area of ​​the first radio base station to another area covered by a second radio base station, a new communication link must be established between the mobile terminal and the second radio base station.

This is automatically controlled by the mobile radio system and the procedure is called handover or handoff.

Comparatively much signaling is required in the mobile radio system in connection with handover. This means that the radio system is loaded every time handover is to be performed. It is therefore desirable to avoid unnecessary handover.

Due to the presence of radio shadows, caused by, for example, variations in terrain, in a cell belonging to a radio base station, there may be islands where the radio signal of another cell's radio base station is stronger. If sufficient signal strength can be offered without handover to such an island, it is disadvantageous to perform such handover. Avoiding handover to said islands reduces the signaling in a radio system and thus creates a lower load on the mobile radio system.

US 4,718,081 describes a device and method for performing handover. The purpose of the procedure is to avoid handover to radio base stations, which would not be optimal to handover. Signal strength measurement is performed on handover candidate radio base stations as well as their neighbor radio base stations. Candidate radio base stations are the radio base stations to which a mobile has the opportunity to handover if needed. Neighboring cells of a cell are the cells that surround this cell. Calculation of a weighted average of

510 052 measured signal strength for candidate radio base stations and its neighboring radio base stations is performed for all candidate radio base stations. The signal strengths of the neighboring radio stations are multiplied by a weight factor preferably equal to 0.5 in the calculation. The calculated weighted averages of the candidate radio base stations are compared. The radio base station that has the highest weighted average is selected to handle an ongoing call.

DISCLOSURE OF THE INVENTION

The present invention is based on the realization that, by considering how a second radio base station, which is likely to take over a mobile telephone call (handover) from a first radio base station, is related to the (other) radio base stations which are neighboring base stations, is possible to avoid unnecessary handover when there is an island located so that it is not appropriate to carry out handover to the base station that has given rise to this island.

It is thus an object of the present invention to avoid carrying out handover in areas where radio communication signals are received with high signal strength (or where any other measure of signal quality used to assess when handover is to be of high value) from a radio base station which is far from said areas. Fewer handovers reduce the load on the radio system and also the risk of ongoing calls being switched off. This also causes the system to be less susceptible to interference as the number of mobile terminals connected to the radio station and which are at a distance from it decreases.

510 052

This is accomplished by the present invention by performing handover of a mobile terminal from a first radio base station to a second radio base station when a first quality value corresponding to a measured quality measure of a radio signal transmitted between the second radio base station and the mobile terminal exceeds the same quality measure of the signal of the first radio base station. added to a handover value, which is dependent on a measured quality value of a radio signal transmitted between an additional radio base station and the mobile terminal and a value of a neighbor relation (hereinafter referred to as relational value).

An advantage of the present invention is that the number of handovers in the average mobile radio system decreases, which reduces the load on the mobile radio system.

The invention will now be described in more detail with the aid of preferred embodiments and with reference to the accompanying drawings.

DESCRIPTION

Figure 1 shows a three-cell mobile radio system.

Figure 2 shows a mobile radio system.

Figure 3 shows a flow chart for a handover.

PREFERRED EMBODIMENTS

Figure 1 shows a mobile radio system comprising mobile terminals and radio base stations with transmitters and receivers for radio signals. For the sake of simplicity, only one mobile terminal 101 has been drawn. When the quality of a connection link between a radio base station and a mobile terminal is less than one

510 052, the mobile radio system has the opportunity to assign a new connection link between another radio base station and the mobile terminal 101. This can be done, for example, during an ongoing call and is referred to as handover or handoff.

With the aid of Figure 1, a method of handover according to the present invention is explained, which method aims to avoid calls being transmitted to a radio base station which, in an area (island), has a high signal quality value but which is not yet for any reason. suitable to be the radio base station that handles the call. Said areas are islands in cells where radio signals are received with higher quality from a radio base station located outside and relatively far from the area where the island in question is located.

In order to clarify the explanation of the present invention, a known method of handover will also be described with reference to Fig. 1.

Fig. 1 shows a mobile radio system 100 comprising three radio base stations RBS1, RBS2, RBS3. The radio base station RBS1 has an associated cell C1, the radio base station RBS2 a cell C2 and the radio base station RBS3 a cell C3. Due to, for example, terrain variations, a first island 102 is also formed in cell C2 where for a mobile terminal 101 there is better transmission and reception ratio relative to the radio base station RBS1 than to the radio base station RBS2, although the latter radio base station RBS2 is closer to the first island 102 .

As the mobile terminal 101 moves from its current position A in a direction 104 toward a position B, the mobile terminal 101 eventually enters the first island 102. At

510 052 passage into the first island 102, in a known method, a handover is performed to the radio base station RBS1 due to better transmission and reception ratio of the mobile terminal 101 relative to this radio base station RBS1 than to the radio base station RBS2. The mobile terminal 101 moves further in the direction 104, leaving the first island 102 and again entering the region of cell C2 where there is better reception conditions relative to RBS2, whereby handovers to this base station are performed. As the mobile terminal 101 passes into cell C3, handover to the radio base station RBS3 takes place.

Handover causes a strain on the mobile radio system because it requires comparatively much signaling. By utilizing the present invention to avoid handover to islands of the type described above, that is, to islands that are far from the radio base station that has given rise to the respective island, the load on the system is reduced. Since each handover involves an increased risk of losing an ongoing call, the present invention also reduces this risk. The interference and path loss can greatly increase when a mobile terminal is connected to a radio base station that is far from it, and since this is avoided by the present invention, further advantages are thus obtained.

There is another island 103 which is closer to radio base station RBS2 than RBS1 but in which there is better transmission and reception relationship with radio base station RBS1 than with radio base station RBS2. However, this island is relatively close to the radio base station RBS1 (as compared to the island 102) and when the mobile terminal 101 enters the second island 103 is also performed using a preferred embodiment of

510 In the present invention, a handover to the radio base station RBS1.

This type of handover is performed by the present invention because of the proximity of the island to the radio base station in which the island originates.

The quality measurement that a mobile radio system uses to decide on handover performance can be of several types, but the most common are: signal strength, bit error content and signal / interference ratio (C / I). For ease of description, reference will only be made to a preferred case using signal strength as a quality metric, but it will be appreciated by those skilled in the art that there are a wide variety of quality metrics that can be used in the present invention without this in some way changes the basic conditions of the present invention.

When the mobile terminal 101 is in contact with the mobile radio system 100, it performs signal strength measurements on signals transmitted between the mobile terminal 101 and all radio base stations RBS1, RBS3 which are recorded in a neighbor cell list assigned to the mobile terminal 101, and between the mobile terminal 101 and the radio base station RBS2 forming the mobile terminal 101. cell. Radio base stations in the neighbor cell list are referred to as neighbor radio base stations or neighbor cells. For example, if cell C2 is active cell for mobile terminal 101, cells C1, C3 are included in said neighbor cell list as candidates for handover. In a digital mobile radio system, for example, the signal strength measurement is carried out with respect to the neighboring cells of the mobile terminal on radio signals transmitted from the radio base stations, ie on downlink signals, and for the active cell of the mobile terminal, the measurement of the radio base station in the active cell is performed.

510 052 radio signals transmitted from the mobile terminal, ie on the uplink signal. The measured signal strengths obtained in mobile terminal 101 are transmitted by the mobile terminal to the mobile radio system via the radio base station of the active cell. Thus, the mobile radio system has information on all signal strengths of the radio base stations that are candidates for a handover of the mobile terminal 101. In an analog mobile radio system, the system obtains corresponding signal strengths by measuring the signal strength from the mobile terminal to all radio base stations, i.e. on uplink signals, which are included in the neighbor cell list. The present invention does not depend on how the measurement of the signal strengths is performed, but only on the fact that it is performed.

A known algorithm for evaluating the signal strength and deciding on a possible handover to a particular candidate cell is to compare the signal strength of the candidate cell with the signal strength of the active cell added to a hysteresis value as follows: SS cand&gt; SSakiiv + hyst \. Said signal strengths are measured, for example, in decibels in relation to 1 mW, that is, the unit of the measured signal strengths is dBm. The comparison is performed at regular intervals for all candidate cells. A hysterical value cough! is used to avoid handover back and forth between two radio base stations as a mobile terminal moves along the boundary of the two cells in question. Handover is only performed when the mobile terminal is safely located in the candidate cell and will therefore probably not have to directly handover back to the previously used radio base station. For mobile radio system 100 where cell C2 is active cell and C1 and C3 respectively are neighboring cells to C2 and thus are handover candidates, the following signal strengths are obtained in the signal strength measurement SS_C1, SS_C2, SS_C3. Each

510 052 signal strength derives from a respective radio base station RBS1, RBS2, RBS3. With these measured signal strengths, the following comparisons are performed: SSci&gt; SSc2 + hysti and SSc3&gt; SSc2 + hysti, at regular intervals to decide on a possible handover.

According to the present invention, a handover value is taken into account according to SSkand&gt; SSaknv + handover value. The handover value is a value that is mainly dependent on the measured signal strengths from the different candidate radio base stations as well as the relationship values ​​between the candidate cells. The dependence is such that high values ​​of signal strength and low values ​​of relational value increase the handover value and thus more refined demands are placed on the signal strength transmitted by the candidate radio base station for handover to be performed than is the case with known technology. In one embodiment of the invention, one can add the hysteria value according to the above known hysteresis value.

Here the concept of neighbor cell relationship will now be briefly explained. The neighbor cell relationship is a value that depends on the probability of handover between two radio base stations. This probability, for example, can be defined as the average number of handouts performed over a specific unit of time, which can be, for example, 24 hours or a month. This value can be changed when the number of handovers performed from a first radio base station to a second radio base station during the specified time period changes. The likelihood of handover from the first radio base station to the second radio base station need not be equal to the probability of handover from the second radio base station to the first radio base station. A high value corresponds to a high probability of handover between two radio base stations, that is, many handovers have previously been made between these

510 052

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t

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<td>one</td><td>low value</td><td>corresponds to one</td><td>low</td>

<td>to</td><td>handover</td><td>shall</td><td>happen.</td>

<td>can</td><td>e.g.</td><td>summarized</td><td>in a</td>

<td>for</td><td colspan="2">the mobile radio system 100 which</td><td>is shown</td>

in Figure 1:

Qs = q₂iQ₃₁ qi2 qu q₂₃ q₃₂

Each element in a row in the matrix

Q₃ corresponds to the probability of handover from one radio base station to another radio base station in the mobile radio system 100. For example, row 2 in the matrix Q₃ corresponds to a first neighbor cell relation value q₂₁ the likelihood of handover from the radio base station RBS2 to the radio base station

RBS1, and a second neighbor cell relation value q₂₃ corresponds to the probability of handover from the radio base station RBS2 to the radio base station RBS3. A neighbor cell relation value, in row 3, q₃₂ which corresponds to the probability of handover from the radio base station RBS3 to the radio base station RBS2 is not necessarily as high as the neighbor cell relation value q₂₃. For example, for some reason more handover may have been performed from the radio base station RBS2 to the radio base station RBS3 than in the opposite direction.

For a mobile radio system with n radio base stations, the probability of handover between all mobile radio system stations with the following matrix is ​​described: Qn = qi₂q_2i .

qin qm

The values ​​in the matrix Q_n can be utilized in different ways, one way is to convert the matrix to a matrix Q 'where the input values ​​are calculated as follows: q'1 = aq_{in: j} + (la) q_3i, where 0 &lt;a &lt;1.

510 052

The definition of relationship value reported above is only one example of a possible relationship value, there are a large number of different definitions of relationship value that can be used without affecting the basis of the present invention. It is also not necessary to describe the relationships in the form of a matrix without other descriptions being used.

Another way of determining relational values ​​is to utilize the geometrical relationship between the different radio base stations. the easiest possible case is to simply measure the distance between the various radio base stations and use this distance as a starting point to calculate the relationship value. There are, of course, a large number of ways of using the system's geometry to determine the relationship values, which should appear obvious to one of ordinary skill in the art when reading the present patent application. Of course, different ways of calculating the relationship values ​​can give rise to different values.

Another way of measuring neighbor affiliation is that the mobile radio system calculates the attenuation of radio signals received by the mobile terminal which have been transmitted from the radio base stations. The attenuation is defined as the ratio between the power of a received radio signal and the power transmitted by the radio base station. Then the inverse of the damping is compared. If a mobile receives radio signals where the inverse of the attenuation is large for two compared radio signals, it is likely that these two are neighbors. By determining that neighbor relationship between two radio base stations prevails when the inverse of the attenuation of two radio signals emitted from said radio base stations exceeds, for example, 80 dB, a measure of neighbor cell relations in

510 052 analogy with the above and a Q_nmatrix as above can thus be formed.

Neighbor cell relationships as a concept have previously been proposed and described in US patent application with application number US08 / 609422 and PCT application PCT / SE97 / 00328.

The relationship values ​​can also be defined as being constant, that is, determined to certain values.

Here, a process according to the present invention will now be described in more detail with the aid of an exemplary embodiment. In the example, we will assume that the active cell is cell C2. Thus, when the mobile terminal 101 is in contact with the mobile radio system 100 and is in the cell C2, signal strength measurements are made between the radio base stations RBS1, RBS2, RBS3 and the mobile terminal 101 and by signal strengths according to:

SSci&gt; SSc2 + hyst} + handover value and

SSci&gt; SSc2 + hyst} + handover value.

For the sake of simplicity, the comparison of signal strengths is explained only by analyzing the differences

SSci&gt; SSc2 + host \ + handover value as mobile 101 approaches the first island 102.

Since the mobile terminal 101 is in the cell C2, the handover value depends on the difference SSc \&gt; SSc2 + hyst} + handover value of

SSc3 a ratio ---, where SS_C3 is the signal strength of the radio base station q₃in

RBS3 and q₃₁ is the neighbor cell relationship between cell C3 and cell C1, that is, the probability of handover from the radio base station RBS3 to the radio base station RBS1.

510 052

Suppose mobile terminal 101 approaches the first island 102.

SSC 3

The ratio --- becomes relatively high since SS_C3 has a qsi relatively high value and q₃₁ has a relatively low value. Thus, the handover value becomes relatively high, on SSc3 due to the ratio --- being relatively high. Since q₃in the handover value is high, thus no handover is performed to the radio base station RBS1 as the mobile terminal moves into the first island 102.

When the same mobile terminal 101 is at the edge of the second island 103, in this case, too, is the difference

SSci&gt; SSc2 + hyst} + handover value that determines whether any handover should occur. The handover value is also now determined by the SSC3 ratio ---. Since the signal strength SS_C3 from the radio base station qsi

RBS3 in this case has a low measured value and also the relation value q₃₁ is relatively low, the handover value becomes relatively low. A handover is thus performed to the radio base station RBS1 as the mobile terminal moves into the second island 103. The reasoning for the value of the handover value is analogous to situations where the mobile terminal 101 moves into the cell C1 or the cell C3.

Figure 2 shows part 200 of a mobile radio system in accordance with Figure 1. Figure 2 shows a cell 'C2 with 6 neighboring cells C1, C3, C4, C5, C6, C7. Each cell has a respective radio base station RBS1, RBS2, RBS3, RBS4, RBS5, RBS6, RBS7 intended for the radio communication in each corresponding cell. For convenience, so-called omnicelles are shown. The present

510 The invention of course also applies in mobile radio systems with so-called sector cells.

When the mobile terminal 101 is in the cell C2 is taken during the analysis to determine if any possible handover should take place all measured signal strength SS_C1, SS_C2, SS_C3, SS₄, SS_cs, SS_C6, SS_C7and all corresponding neighbor cell relations in consideration.

As an example, it will now be shown how the analysis is performed as the mobile terminal 101 passes into the first island 102 in cell C2. The difference SSc \&gt; SSci + hysti + handover value in this scenario is the condition that must be fulfilled in order for a handover to be performed to the radio base station RBS1. In calculating the handover value, the following quotas are taken between signal strengths and. SSc3 SSc4 SScs SScs SSC7 relation values ​​in consideration: ---, ---, ---, --- and ---. The qai q $ in qsi qei q7i reasoning above, regarding the size of the quotas in the mobile radio system 100, also applies here.

To obtain a well-defined and controllable range within which the above quotas correspond to a value, a so-called saturation function is used. An example of such a function is a tangent hyperbolic function and then, for example, the following (SS - xoi - x tanhl ------- k dxi xq where the function assumes values ​​between 0 and fltnax »Xoi OCh dx₃ are parameters that define the slope of the linear part of the function and its midpoint coordinates.

Example "

on typical values ​​of the parameters x₀₁ and dx_x is 80 dBm and the mean of all elements except the diagonal, where q is in Q_nthe matrix and a are the desired range of the function. A suitable value can

0 For example, be 5, which means that said function has a minimum value of -2.5 and a maximum value of +2.5.

All five quotas mentioned above give a functional value of f_x, which values ​​are summed and the sum obtained is multiplied by a norming factor equal to 1 / (the number of terms in the sum). This can be summarized by the following terms:

<img file="SE510052C2_D0008.tif" />

To obtain a well-defined and controllable range within which the above-mentioned summation and standardization corresponds to a value, for example the same saturation function as above, which gives:

handover value = - x tanh

X _ / 2max J

J) assumes values ​​between 0 and f2_max, x₀₂ and dx₂ are parameters that define the slope of the linear part of the function and its midpoint coordinates.

Examples of typical values ​​of parameters f2_max, x₀₁ and dx_x are 3 - 6,

0.5 and 1, respectively.

When a mobile terminal is located in a cell k, which has a corresponding radio base station k, with neighboring cells having the corresponding n radio base stations, in the neighbor cell list and wants to compare measured signal strength from radio base station k (active cell) with a measured signal strength from a radio base station j can be a handover candidate, the following comparison is performed: SSj&gt; SSk + host \ + handover value; where the handover value is calculated according to:

510 052

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where i * k and i * j and f_x is defined as above (2) transfer value =

1 - + - x tanh

2 k

¹ . We

---- X Λ - X02 n-1 _______ dX2

J)

X / ~ 2max t where f2_max, x₀2 and dx₂ are defined as above.

Figure 3 is a flow chart illustrating one

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an embodiment

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101 and the radio base station RBS2 is connected to the mobile terminal

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101

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all signal strengths

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is measured

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active

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neighbor cells

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RBS 1, RBS3, RBS4, RBS5, RBS6

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RBS7 as well

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RBS2

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101,

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signal strengths SS_C1, SS_C2, SS_C3, SS₄, SS_C5, SS_C6 and SS_C7 obtained. Signal strength of all neighboring cells SS_C1, SS_C3, SS₄, SS_C5, SS_C6 and SS_C7is compared with the signal strength of the active cell radio base station SS_C2 in a step 3001 according to the principle explained above. In step 302, it is checked whether any of the conditions are met. According to an alternative no, no handover is performed and a new signal strength measurement is performed in step 300 after a predetermined time interval has passed. In a step 303, handover is performed according to a known method to a new radio base station or new cell and in a step 304 the mobile radio system and the mobile terminal 101 are updated. For example, the mobile terminal gets a new neighbor cell list. When a handover is performed in the mobile radio system, the value of the relational value is also updated, in which value the newly performed handover is calculated.

510 052

The method can also be used when the mobile phone is in idle mode, that is, switched on and in contact with a mobile radio system but not connected for calls. The mobile terminal then has information on which cell it prefers.

The inventive method can also be used in mobile radio systems built with Code Division Multiple Access (CDMA) technology. Each mobile terminal is connected to one or more radio base stations. These radio base stations are called active sets in a CDMA system. The method according to the invention is thus applicable in selecting which radio base stations to be included in the active set of a mobile terminal.

The invention is, of course, not limited to the embodiments described above and shown in the drawing, but can be modified within the scope of the appended claims.

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