CN1085031C - Mobile Station Assisted Handoff Scheme for Digital Cellular Telephone Network - Google Patents

Abstract

A digital cellular telephone network is provided with a mobile telephone handover arrangement in which a mobile telephone provides signals relating to the strength of downlink signals. These signals are signals from the first base station with which the telephone is communicating, and downlink signals from other selected base stations in the network. Based on this information, the central controller calculates the carrier-to-interference ratio C/I and determines when and to which other base station the mobile telephone should be handed over.

Description

Mobile Station Assisted Handoff Scheme for Digital Cellular Telephone Network

The present invention relates to handover arrangements for digital cellular telephone networks.

Mobile telephone networks are well established in many geographical areas, enabling relatively small and lightweight telephones to be used universally. Smallness and lightness depend primarily on the use of relatively low power transmissions between the telephone and fixed communication transmitters and receivers. The communication transmitters and receivers in the base stations are generally dispersed evenly around the network area mainly geographically in an array and are connected to a central network control station for controlling the communication between all base stations and mobile phones. At the beginning of each telephone call for each communication, the central control station assigns frequencies and channels and selects the appropriate base station. As is often the case, the central controller provides a switching scheme during each call to change or change channels and frequencies to connect phones to communicate with different base stations. The central controller also monitors and records calls for user billing.

Typically, base stations are designed to provide multiple areas of suitable transmission and reception through a number of hexagonal side-by-side areas, or so-called "cells". The carrier frequency used by one cell is different from any frequency used in a closely adjacent cell, but is the same frequency used by another cell a little further away in the network and usually separated by two other cells of the network . In this way, the cellular type of the communication cell can be easily formed by using 7 different carrier frequency sets. Of course, if the mobile phone moves from one cell to another during a call, the control station can and is designed to transfer or switch the mobile phone to communicate on a different frequency as is known.

This is presently determined simply by comparing the reported signal strength between the cell with which the phone is communicating at any time and the neighboring cells. If the signal strength of an adjacent cell increases by a predetermined amount beyond the signal strength of the communicating cell, the central controller will handover the mobile telephone to this adjacent cell.

In fact such a solution is not reliable enough, especially for networks near coastal areas that need to work in completely open spaces, difficult terrain near mountains and/or in densely populated areas with tall buildings.

In the article "Handover Techniques for Third Generation Mobile Systems, CHIA Mobile Radio Conference, November 13-15, 1991, Nice, France, pp. 243-249 in XP000444200", various applications are discussed. General guidelines for switching schemes. In WO-A-9217953 a switching scheme is disclosed in which a specific C/I ratio is always measured when switching conditions appear to be necessary.

It is an object of the present invention to provide a network and a mobile telephone which work more satisfactorily in difficult terrain and also provide better service in less difficult terrain, even when the density of use is high.

According to one aspect of the present invention, there is provided a handover scheme for a digital cellular telephone network having a central controller and a plurality of mobile telephones, wherein each mobile telephone is designed to measure and transmit a first base station on a control channel of the network The downlink signal strength of the base station and the measurement results of the downlink signal strength of other base stations, and where all these signals are passed to the central controller, which determines the C/I ratio and whenever this ratio drops below the predetermined A value of , then initiates a handover of the mobile phone from the first base station to another base station, where C is equal to the downlink signal strength of the first base station and I is the sum of the downlink signal strengths of the other base stations.

The controller preferably initiates handover of the mobile telephone from the first base station to another base station only if the C/I ratio of the other base station is above a predetermined value.

The downlink strength of the base station may preferably be modified by a selected coefficient or adjustment factor related to previous calculations or measurements of the network before summing.

According to another aspect of the present invention, there is provided a mobile telephone for use in a digital cellular telephone network having a handover scheme as defined above, comprising means for continuously periodically measuring downlink signals received from a first base station and means for downlink signal strength received from a plurality of other base stations on a control channel, and means for transmitting these measurements to the first base station.

The measuring means may be designed to measure the strength of downlink signals from surrounding base stations operating on the same carrier frequency. The measuring means may also be designed to measure the strength of downlink signals from immediately surrounding base stations operating at different carrier frequencies.

Handover schemes for digital cellular telephone networks and telephones in these networks according to the invention will now be described by way of example with reference to the accompanying schematic diagrams, wherein:

Figure 1 is an idealized planning diagram of a cellular network;

Fig. 2 is another idealized planning diagram of a cellular network;

Fig. 3 is a plan diagram of a part of a typical cellular network;

Figure 4 is a diagrammatic side view of another cellular network;

Figure 5 is an idealized planning diagram of a cellular network with cells with super reused carrier frequencies; and

Figure 6 is a glossary of terms for Figures 1 to 5.

Digital cellular mobile telephone networks are known and typically each cell is designed to transmit on 3 different carrier frequencies each providing 8 channels (time division multiplexed). Two channels of one carrier frequency are used for control signals, and the remaining channels of one carrier frequency and all channels of the other two carrier frequencies are used for voice or data transmission, so-called "traffic". Channels are designed in pairs and one channel of each pair is used for transmission to a mobile phone (downlink signal) and the other channel of the pair is used for transmission from the mobile phone (uplink signal).

A mobile phone is usually first connected to communicate with the cell closest to it, and is assigned a pair of channels for traffic communication by the central network controller. This is done by exchanging messages on a control channel according to known and well practiced procedures. In a particular embodiment of the invention, each mobile phone is designed to periodically respond to signals from other cells and to relay signals received from other cells (downlink link signal) to the central controller. The purpose of receiving and measuring the strength of various downlink signals, including its own downlink signal, is to provide information that the central controller can use to "handover" a particular mobile phone to another cell. This ensures that satisfactory communication quality is maintained as conditions change, typically due to the mobile-telephone moving closer to an adjacent cell or possibly entering an area of poor local reception in the cell with which the mobile-telephone is currently communicating.

In Figure 1, the control channel frequency is reused every 7th cell (C1 to C7) in the cellular network. Normal traffic channel frequencies are reused every 4 cells (V1 to V4). In Figure 1, the reuse level for the traffic channels is much higher than that for the control channels, but it will be noted that "possible interferers" cause the control channels to use different frequencies. "Possible interferer" is used to identify cells or base stations that use the same set of traffic channel frequencies in the system, such that the signal from the likely interferer is likely to be strong enough to cause interference in the area of the cell or base station with which the mobile phone is currently communicating .

During the course of a call, the mobile phone periodically measures the strength of the downlink signals of the serving base station and those of possible sources of interference. All measurements are reported to the central controller, which calculates the C/I ratio of the radio link being used according to the following method:

L _s = strength of the downlink signal of the serving base station

L _is1 = Strength of the downlink signal of the first possible interferer

L _is2 = Strength of the downlink signal of the second possible interferer ...................... ................................................... ...................................

L _ism = strength of the downlink signal of the mth possible interferer

The C/I ratio of the wireless link being used is calculated as:

C/I＝L _s /(L _is1 +L _is2 +...+L _ism )

When the C/I ratio drops below a predetermined level, the central controller will handover the mobile phone to another cell. Such measurements are sometimes referred to as "direct C/I ratio measurements".

In Fig. 2, the level of traffic channel frequency reuse is the same as that of control channel frequency reuse. In the figure, the same traffic channel and control channel are reused every 7 cells.

When using the network in Figure 2, the mobile-telephone periodically measures the downlink signal strength of the serving base station as well as those "reference" base stations during the course of a call. "Reference" base stations are used to identify a series of adjacent base stations on different control channel frequencies and are specifically selected for estimating the signal levels of possible interferers. All these measurements are reported to the central controller, which calculates the approximate C/I ratio of the radio link being used according to the following method:

_Lisk = strength of the downlink signal of the kth possible interferer (to be estimated)

L _s = strength of the downlink signal of the serving base station

L _rk1 = downlink signal strength of the first reference base station for the kth possible interferer

L _rk2 = strength of the downlink signal of the second reference base station for the kth possible interferer.......................... ................................................... ...................................................

L _rkx = downlink signal strength of the xth reference base station for the kth possible interferer

The interference level of the kth possible interferer is approximately:

L _isk = MAX[(L _rk1 ^* a ₁ ), (L _rk2 ^* a ₂ ), . . . , (L _rkx ^* a _x )]

Here, a ₁ , a ₂ , . . . , a _x are fixed coefficients or adjustment factors. This adjustment factor is preset from measurements previously made for the network and provides the necessary adjustments so that the profile of the combined signal at the reference base station after adjustment is approximately the same as the profile of the Kth possible interferer in the service area of the serving base station same. This is illustrated in Figure 3.

The C/I ratio of the wireless link being used is therefore calculated as:

C/I＝L _s /(L _is1 +L _is2 +...+L _ism )

Switching is arranged by the central controller when the C/I ratio drops below a predetermined level. Such criteria are sometimes referred to as being based on "approximate C/I ratios".

Certain special circumstances arise for certain locations or terrains as shown in FIG. 4 . In this way, when the boundary of a planned service area is crossed, there may be a plurality of neighboring base stations whose signal strength varies relative to the serving cell. A typical situation is depicted in Figure 4.

Base station A is planned to cover or serve part of the road surface on a mountain road but depending on the position of the antenna, the base station also covers part of the ground area. The road surface is affected by radio signals from many other base stations, so only a few frequencies are assigned to base station A cleanly enough. Traffic from the ground layer must be prevented from being captured by base station A, otherwise it will go into blocking. To determine where the traffic is coming from, signals from selected remotely located base stations B and C are used as reference. On the hill, the mobile phones are in line-of-sight of base stations B and C, so their downlink signal strength is high relative to that of the serving base station. In the ground area, the situation is reversed because the signals from base stations B and C are blocked by buildings.

Thus, at runtime, the mobile phone is designed to monitor signal strength and pass this information on to a central controller, which will determine the relative location of the mobile phone and decide whether handover is in fact necessary in this particular embodiment.

Signal strength information is processed as follows:

L _s = strength of the downlink signal of the serving base station

L _cs1 = downlink signal strength of the first selected base station

L _cs2 = downlink signal strength of the second selected base station ...................... ................................................... ...................................................

L _csx = downlink signal strength of the Xth selected base station

if and only if:

positive logic negative logic

L _cs1 -L _s >d ₁ L _cs1 -L _s <d ₁ or L _cs2 -L _s >d ₂ and L _cs2 -L _s <d ₂ ...................... ..................................................... or L _csx -L _s > d _x and L _csx -L _s < d _x means that the mobile phone is in the planned service area, where d ₁ , d ₂ , ... d _x are constants whose values are based on the serving base stations and those within and between the planned service area determined by the predicted or measured signal strength of the out-of-selected base stations. Such switching is subject to what may be termed "service area control".

A typical logic program for determining switching is generally automatically executed by the central controller as follows:

(a) Establishing a radio traffic channel between a mobile telephone and a serving base station.

(b) Based on information received from the network, the mobile phone periodically measures the strength of the downlink signal of the serving base station and those of other base stations. All measurements are reported to the central controller by the serving base stations. Measurement results include:

For serving base stations:

L _u - Uplink signal strength

L _d - downlink signal strength

Q _u - Uplink quality

Q _d — Downlink quality

D - the distance between the mobile phone and the serving base station

For other base stations

C _n1 , C _n2 , ..., C _nk - base station identification

L _n1 , L _n2 , ..., L _nk - downlink signal strength

(c) When a new measurement is received, the central controller checks whether a switchover is necessary by looking at the trigger function:

T(L _u , L _d , Q _u , Q _d , D, C _n1 , C _n2 ,..., C _nk , L _n1 , L _n2 ,..., L _nk )

If T>0, handover is necessary, go to step (d). Otherwise go back to step (b).

(d) When it is determined that handover is necessary, from the K non-serving base stations in the measurement report, the central controller selects multiple base stations as possible handover targets by using the corresponding evaluation function:

Q _ni (L _u , L _d , Q _u , Q _d , D, C _n1 , C _n2 ,..., C _nk , L _n1 , L _n2 ,..., L _nk )

Q _ni is a function parameterized to determine whether the base station identified by C _ni is eligible as a handover target. If Q _ni >0, the base station is qualified.

(e) The central controller determines the priority of eligible handover targets by using the priority function:

P _ni (L _u , L _d , Q _u , Q _d , D, C _n1 , C _n2 ,..., C _nk , L _n1 , L _n2 ,..., L _nk )

P _ni is a function parameterized to determine the relative priority of the handover targets identified by C _ni .

(f) The central controller commands the mobile station to handover to the target base station with the highest priority.

For the switching procedure according to the specific embodiment of the invention and using the system described with reference to Figure 1, the central controller is programmed to operate as follows:

When a radio traffic channel has been established between a mobile phone and a serving base station, all possible interference sources of the serving base station are selected from K non-serving base stations in the signal strength measurement report.

L _is1 =the strength of the downlink signal of the first possible interferer of the serving base station.

L _is2 = strength of the downlink signal of the second possible interferer of the serving base station. ................................................... ................................................... ...................................

L _ism = strength of the downlink signal of the mth possible interferer of the serving base station

The C/I ratio of the serving base station can then be calculated as:

CI _s =L _d /(L _is1 +L _is2 +...+L _ism )

The trigger function T is defined as:

T = ci_threshold - _CIs

Here ci_threshold is a predetermined minimum acceptable C/I ratio.

T>0 means that the C/I ratio is worse than the minimum acceptable level, so handover to another better base station is necessary.

For the possible handover targets identified by _Cnj , the relevant possible interference sources are selected from the K non-serving base stations in the measurement report.

L _inj1 =strength of the downlink signal of the first possible interferer of the base station identified by C _nj .

L _inj2 = strength of the downlink signal of the second possible interferer of the base station identified by C _nj . ................................................... ................................................... ...................

L _injm =strength of the downlink signal of the mth possible interferer of the base station identified by C _nj .

Then the C/I ratio of the base station identified by _Cnj can be calculated as:

CI _nj ＝L _nj /(L _inj1 +L _inj2 +...+L _injm )

The relevant check function is defined as:

Q＝CI _nj -ci_threshold

Q > 0 means that the C/I ratio is better than the minimum acceptable level, so the base station is qualified to be a handover target.

In fact, the priority function of base stations identified by _Cnj is defined as:

P _nj = CI _nj

The priority of possible handover targets is only determined according to the corresponding C/I ratio. The better the C/I ratio and thus the better the signal quality, the higher the priority.

It is also possible to determine switching based on an approximate C/I ratio, as explained earlier with reference to FIG. 2 . This is performed by picking all reference base stations for estimating the C/I ratio of the serving base station out of the K non-serving base stations in the measurement report.

L _rs1 = downlink signal strength of the first reference base station used to estimate the C/I ratio of the serving base station

L _rs2 = Strength of the downlink signal of the second reference base station used to estimate the C/I ratio of the serving base station ...................... ................................................... ................................................... ....

L _rsx = strength of the downlink signal of the xth reference base station used to estimate the C/I ratio of the serving base station

Then in the program of the central controller, the C/I ratio for the serving base station can be calculated as:

CI _s = L _d /MAX[(L _rs1 ^* a _s1 ), (L _rs2 ^* a _s2 ), . . . , (L _rsx ^* a _sx )]

The trigger function T for switching is defined as:

T = ci_threshold - _CIs

Here ci_threshold is the predefined minimum acceptable C/I ratio.

T>0 means that the C/I ratio is worse than the minimum acceptable level, so handover to another better base station is necessary.

Possible switching targets are identified by _Cnj . This is selected among the relevant reference base stations among the K non-serving base stations in the measurement report.

_Lrnj1 = Strength of the downlink signal of the first reference base station used to estimate the C/I ratio of the base station identified by _Cnj .

_Lrnj2 = Strength of the downlink signal of the second reference base station used to estimate the C/I ratio of the base station identified by _Cnj . ................................................... ................................................... ...................................

_Lrnjx = Strength of the downlink signal of the xth reference base station used to estimate the C/I ratio of the base station identified by _Cnj .

Then the C/I ratio of the base station identified by _Cnj can be calculated as:

CI _nj = L _nj /MAX[(L _rnj1 ^* a _nj1 ), (L _rnj2 ^* a _nj2 ), . . . , (L _rnjx ^* a _njx )]

The relevant check function is defined as:

Q＝CI _nj -ci_threshold

Q > 0 means that the C/I ratio is better than the minimum acceptable level, so the base station is qualified to be a handover target. In other words, assuming that the C/I ratio for possible handover target stations is higher than a predetermined value, handover can be performed.

The priority function of the target base station is identified by _Cnj and is defined as:

P _nj = CI _nj

The priority of possible handover targets is only determined according to the corresponding C/I ratio. The better the C/I ratio and thus the better the signal quality, the higher the priority.

As mentioned earlier with reference to Figure 4, more difficult terrain or locations may require some special solution. Where switching is determined by location, the following procedures are provided:

From the K non-serving base stations in the measurement report, all selected remote base stations for estimating whether the mobile station is within the planned service area of the active base station are selected.

L _cs1 = strength of the downlink signal of the first selected base station for estimating whether the mobile station is within the planned service area of the base station being used.

L _cs2 = Downlink signal strength of the second selected base station used to estimate whether the mobile station is within the planned service area of the base station being used. ................................................... ................................................... ...................................

L _csx = strength of the downlink signal of the xth selected base station for estimating whether the mobile station is within the planned service area of the base station being used.

A trigger function T > 0 or a toggle function is provided if:

Positive logic Negative logic L _cs1 -L _d >d _s1 L _cs1 -L _d <d _s1 or L _cs2 -L _d >d _s2 and L _cs2 -L _d <d _s2 ......... .. ........................................................... or L _csx -L _d >d _sx and L _csx -L _d <d _sx

T>0 means that the mobile station is outside the planned service area of the base station being used, so handover to another base station is necessary.

Possible handover targets are identified by _Cnj , and the relevant reference base station is selected from the K non-serving base stations in the measurement report.

L _cnj1 = Downlink signal strength of the first selected base station used to estimate whether the mobile station is within the planned service area of the base station identified by C _nj .

L _cnj2 = Downlink signal strength of the second selected base station used to estimate whether the mobile station is within the planned service area of the base station identified by C _nj . ................................................... ................................................... ...................................

L _cnjx = Downlink signal strength of the xth selected base station used to estimate whether the mobile station is within the planned service area of the base station identified by C _nj .

The evaluation function Q is greater than 0, if:

Positive logic Negative logic L _cnj1 -L _nj >d _nj1 L _cjn1 -L _nj <d _nj1 or L _cnj2 -L _nj >d _nj2 and L _cnj2 -L _nj <d _nj2 ......... ..................................................................................... ..or L _cnjx -L _nj ＞d _njx and L _cnjx -L _nj ＜d _njx

Q > 0 means that the mobile station is within the planned service area of the base station identified by _Cnj , so it is qualified to be a handover target.

For the priority function, factors such as signal strength, traffic load and C/I ratio are used.

The described handover scheme can be applied to over-under supported cellular networks. Referring to FIG. 5, the cellular network has an up-down support configuration in which some cells have super-reuse carriers. The working spectrum of the network is split into two groups, a regular reuse group and a super reuse group. Regular reuse frequencies are reused at safe distances, such as distances in a 7-cell reuse pattern, and are intended to serve mobile phones near the borders of cells where the C/I ratio is usually the worst. As the name implies, super-reuse frequencies can be reused very intensively to produce the required scaling.

Some common base stations are equipped with both types of frequencies. The super-reuse frequencies allocated to base stations are divided into several groups, each with different sources of interference. Based on the distribution of interference encountered by each mobile phone, the central controller determines the most suitable set of frequencies to be assigned to carry the traffic.

Standalone microcells with antenna heights well below roof level can only be equipped with super reuse frequencies. By establishing suitable handover connections, a micro cell in a good location can effectively absorb the traffic of more than one normal base station in its vicinity. With such a solution, the surrounding base stations are generally called "mother base stations", while the micro cells are called "child base stations".

Conventional frequency selection mechanisms used with upper-lower support structures are based solely on the signal strength of the serving base station. A strong signal means that the mobile station is close to the base station and the system will allocate a super reused frequency for a call. Otherwise, a regular reuse frequency is used. This scheme works better on flat, uniform ground. But in an interference-limited environment, signal strength alone has no practical indication of sensitivity to interference. In open areas, signal levels are usually higher, but so is interference. When the signal is weak, the mobile phone may be inside a building and better shielded from interference.

So it is more efficient to use the frequency selection mechanism of the specific embodiment of the present invention. The improved handover scheme of the present invention can be applied to frequency selection. The process is as follows:

(a) At call setup, the central controller assigns a regular reuse frequency.

(b) during an ongoing call, when signal strength measurements are available, the central controller checks whether there are sufficiently clean super-reuse frequencies available (including those in the associated sub-base station) by using a check function according to the following criteria :

- Direct C/I ratio measurement (Figure 1)

- Approximate C/I ratio (Figure 2)

- Service area control (Fig. 4)

(c) If a clean super reuse frequency is found, the central controller sorts the usable super reuse frequency according to the priority according to the number of each idle traffic channel. The more idle traffic channels, the higher the priority.

(d) The central controller switches the mobile phone to the super reuse frequency with the highest priority.

(e) For super reuse frequencies, the central controller constantly monitors the radio link in use to see if it is necessary to return to the normal reuse frequency. A handover trigger function based on the following criteria can be used:

- Direct C/I ratio measurement

- Approximate C/I ratio

- Service area control

(f) If a return to regular reuse frequency indicated by the handover trigger function is necessary, the central controller evaluates each of the serving base station and those base stations of its neighbors by using a merit function and a priority function based on the following criteria: Regular reuse frequency:

- Direct C/I ratio measurement

- Approximate C/I ratio

- Service area control

(g) The central controller switches the mobile phone to the regular frequency with the highest priority.

(h) Return to step (b).

In the described scheme, the strength of the downlink signal of the control signal is measured and used to determine the handover criteria and the position of the mobile phone in FIG. 4 . It can be seen that the strength of the traffic channel downlink signal can also be used additionally or alternatively.

It can be seen that, in particular embodiments of the invention, the handover criterion is based on the C/I ratio and is determined when this ratio falls below a predetermined value for the serving base station, or conversely, a possible handover The C/I ratios of the target base stations are monitored to identify and select a suitable one of these base stations for handover.

Legend

A- The first layer of the base station of the common control channel

B-First layer of base station for common traffic channel

C - The first layer of the base station for common control channel and common traffic channel

D-Possible handover target base station

E-reference base station A

F-interfering base station

G-Service Territory Boundary

H - reference base station B

I- Signal level contours of interfering base stations

J - Interference zone within the service area

K - Contours of adjusted signal levels of reference base stations A and B

L - Unintended (-ve logical) service area of base station A / Intended (+ve logical) service area of base station A (L _B2 -L _A1 )＞d _B or (L _C1 -L _A1 )＞d _C

M - Unintended (+ve logical) service area of base station A/Intended (-ve logical) service area of base station A (L _B2 -L _A1 )＞d _B and (L _C2 -L _A2 )＞d _C N - base station AO - reference base station BP - reference base station CQ - cell layer for first co-channel of regular reuse frequency R - cell layer for first co-channel of super reuse frequency in parent base station S - for The cell layer of the first co-channel of the sub-base station

Claims (4)

1, be used to have the handover scheme of the digital cellular telephone networks of a central controller and a plurality of mobile phones, it is characterized in that, each mobile phone is designed to measure and to send the measurement result of the downlink signal strength of the intensity of down link signal of first base station and other base station on the control channel of network, and be that all these signals are passed to central controller, it is determined a C/I ratio and no matter when drops to when being lower than a predetermined value when this C/I ratio, then start the switching of mobile phone from first base station to another base station, here C equals the intensity of the down link signal of first base station, and I is the summation of the downlink signal strength of other base station.

2, according to the handover scheme of claim 1, it is characterized in that having only when the C/I of other base station ratio is higher than predetermined value, controller starts the switching of the mobile phone from first base station to another base station.

3, according to the handover scheme of claim 1 or 2, it is characterized in that the base station down link intensity before the addition can by with network before calculating or measure relevant selected coefficient or regulatory factor and revise.

4, according to the handover scheme of claim 1, the mobile phone that it is characterized in that described digital cellular telephone networks have the down link signal that is used for periodically measuring the intensity of the down link signal that receives from first base station continuously and on control channel, receives from a plurality of other base stations intensity device and be used to send the device of these measurement results to first base station.

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