CA2244366A1 - Velocity assisted handoff system and method for cellular mobile radio systems - Google Patents
Velocity assisted handoff system and method for cellular mobile radio systems Download PDFInfo
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- CA2244366A1 CA2244366A1 CA002244366A CA2244366A CA2244366A1 CA 2244366 A1 CA2244366 A1 CA 2244366A1 CA 002244366 A CA002244366 A CA 002244366A CA 2244366 A CA2244366 A CA 2244366A CA 2244366 A1 CA2244366 A1 CA 2244366A1
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- 238000000034 method Methods 0.000 title claims abstract description 57
- 230000001413 cellular effect Effects 0.000 title description 10
- 238000005259 measurement Methods 0.000 claims abstract description 67
- 238000010295 mobile communication Methods 0.000 claims abstract description 18
- 230000008859 change Effects 0.000 claims abstract description 14
- 230000003044 adaptive effect Effects 0.000 claims description 11
- 230000007246 mechanism Effects 0.000 claims description 7
- 238000004891 communication Methods 0.000 claims description 4
- 230000005754 cellular signaling Effects 0.000 claims 1
- 230000008569 process Effects 0.000 description 9
- 238000001228 spectrum Methods 0.000 description 8
- 238000005562 fading Methods 0.000 description 4
- 230000006978 adaptation Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 230000000694 effects Effects 0.000 description 3
- 239000013598 vector Substances 0.000 description 3
- 230000005540 biological transmission Effects 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000011664 signaling Effects 0.000 description 2
- 230000001934 delay Effects 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
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- VJYFKVYYMZPMAB-UHFFFAOYSA-N ethoprophos Chemical compound CCCSP(=O)(OCC)SCCC VJYFKVYYMZPMAB-UHFFFAOYSA-N 0.000 description 1
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Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S11/00—Systems for determining distance or velocity not using reflection or reradiation
- G01S11/02—Systems for determining distance or velocity not using reflection or reradiation using radio waves
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W24/00—Supervisory, monitoring or testing arrangements
- H04W24/10—Scheduling measurement reports ; Arrangements for measurement reports
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/08—Reselecting an access point
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W64/00—Locating users or terminals or network equipment for network management purposes, e.g. mobility management
- H04W64/006—Locating users or terminals or network equipment for network management purposes, e.g. mobility management with additional information processing, e.g. for direction or speed determination
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Abstract
An improved method of selecting a basestation for handoff within a mobile communications system is provided. In addition to signal quality information, a basestation is selected based upon velocity parameter information which indicates which basestation or basestations that the mobile station is moving towards or away from. A basestation is selected such that it has an acceptable signal quality, and such that it is a basestation which the mobile station is moving generally towards. In this manner, spurious handoffs which might happen at an intersection for example, are avoided. The velocity parameter might be an absolute velocity measurement, or a Doppler shift measurement which indicates only the component of the velocity in the direction of each basestation. A convenient way to compute the Doppler shift measurement in a multipath environment is to examine the rate of change of the phase of a lowest delay equalization weight.
Description
VELOCITY ASSISTED HANDOFF SYSTEM AND METHOD FOR
CELLULAR MOBILE RADIO SYSTEMS
Field of the Invention The invention relates to an improved system and method for selecting a basestation for handoff in mobile cellular radio systems.
Background of the Invention In mobile radio systems there may be handoff between radio units. That is, an active call involving a user's mobile station within a cellular system may be transferred from one radio basestation to another to improve or maintain the radio link either as a result of motion of the mobile station or as a result of changing radio transmission or interference conditions.
In many handoff situations, a choice must be made as to which is the "best" basestation to select from a number of possible candidates. That is, given that somehow the mobile station and the serving basestation have decided that a handoff is needed, a new basestation must be selected that will give the best performance for continuing the call. In traditional methods of handoff basestation selection, this choice is simply based on the radio signal strength. The basestation which is selected is the one which provides the strongest signal. These signal strength measurements may be made at either the mobile station or at the basestations.
It is a common objective of the handoff process to keep any interruptions to a call as short as possible. Ideally there should be no interruption and there should be as few handoffs as possible. Many systems make provision for a make-before-break handoff in which the signal is never interrupted. To achieve this, two connections (or more) are maintained to two (or more) base~tations while the call is being handed over from one basestation to another. DECT (digital enhanced cordless telephone)l and CDMA (code division multiple access) 2 are two such systems.
Conventional handoff methods which employ signal strength measurements alone in making handoff decisions have at least two difficulties associated with them. Firstly, signal strength varies rapidly with time depending on the motion of the mobile station, proximity to nearby objects and the radio propagation conditions. This means that the signal strength measurements must be averaged or filtered and the handoff decision must be based on the average signal strength and not on the immediate measurements. This introduces delay in the process which may lengthen the time taken for handoff so that it may be more obtrusive than desired.
The second difficulty is sometimes referred to as the "cornerN problem. A mobile station may be travelling down a city street where it receives service from a basestation at the end of the street, but is shielded by buildings along the street from other basestations which serve cross streets. At an intersection, the mobile station may be strongly illuminated by another basestation serving the cross street as well as its current serving basestation. The strong signal strength from the new basestation may cause the mobile station and its current IREF:ETSISt~d~dETS300-175 2REF:TIASt~d~d~S-95 serving basestation to consider doing a handoff to the new basestation. This handoff would be the wrong thing to do if the mobile station is continuing along its original street as it will be shadowed from the new basestation when it leaves the intersection. Such a handoff would likely cause the call to be lost (or dropped) briefly when it becomes shadowed from the new basestation while the call is switched back to the original serving basestation. However, a handoff would be the right thing to do if the mobile station turns the corner onto the new street as it will become shadowed from its original basestation after it leaves the intersection. Without the handoff to the new basestation, the call will likely be lost (or dropped) briefly when it becomes shadowed from the original basestation while the mobile station connects to the new basestation. The decision to do the handoff should be made while the mobile station is in the intersection, but the equipment cannot tell from the signal strength alone if the mobile station is turning the corner and thus if the handoff should be done, or if the mobile station is going to continue along the same street and thus no handoff is needed.
There are a number of techniques which attempt to enhance the existing methods by also examining the speed of the mobile station. Several of these focus on the problem of choosing between upper and lower tier cells in areas where there is a two-tier (or more) cellular system.
Furthermore, U.S. patent numbers 5,140,695 to Yasuda et al. titled "Channel Assignment System in Mobile Communication System" which issued on August 18, 1992 and 5,239,667 to Kanai titled "Method of Controlling Handoff in Cellular Mobile Radio Communication~ System" which i~sued August 24, 1993 make use of measurements of the fading frequency of the received radio signal to estimate the speed of motion of the mobile station, and use this speed information as part of the handoff decision. In the second of these patents, the speed information is used to adjust a threshold in relation to the mean signal strength which triggers the handoff.
In these patents, the fading frequency is referred to as the "Doppler Frequency". This may be used to determine the speed of the mobile station. However, this is not a measurement of the true Doppler effect as the sign of the fading frequency will always be positive and as such it is not possible to determine the direction of motion of the mobile station.
Summary of the Invention It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
In the handoff methods provided by the invention, additional information about the motion and direction of the mobile station is used to assist the handoff process. In a general sense, the basestation selected for the handoff should be not only the one with a good signal quality but also the one the mobile station is moving towards. The basestations from which the mobile station is moving away should be avoided for handoff even though they may have as good a signal quality as others. By measuring the velocity (speed and direction) of the mobile station with respect to the basestations, a basestation for handoff can be selected which meets the above objectives.
According to a first broad aspect, the invention provides a method for selecting a handoff basestation in a mobile communications system having a plurality of basestations in which a mobile station has an active connection to a first of the basestations, the handoff method comprising the steps of:
determining for one or more basestations a signal quality measurement of a signal transmitted from the mobile station to the basestation or vice versa; determining velocity parameter information for the mobile station; determining on the basis of the signal quality measurements and velocity parameter information whether a handoff is necessary, and if so, which of the one or more basestations is to be the handoff basestation.
According to a second broad aspect, the invention provides a hand off selection system for use in a mobile communications system in which a plurality of basestations are available to provide communications service to a mobile station, the handoff ~election system comprising: means for determining the signal strength of signals transmitted between the mobile station and one or more of the basestations; means for determining velocity parameter information for the mobile station; a basestation selection controller for selecting on the basis of the signal strengths and the velocity parameter information whether or not a handoff is required, and if so, to which of the one or more basestations.
Brief Description of the Drawings Preferred embodiments of the invention will now be described with reference to the attached drawings in which:
Figure 1 is a block diagram of a conventional mobile communications system;
Figures 2a and 2b are block diagrams of basestation and mobile station functionality respectively according to an embodiment of the invention;
Figure 3 is a detailed view of a basestation selection controller according to an embodiment of the invention;
Figure 4a is a schematic showing various propagation paths in a mobile communications environment;
Figure 4b is a plot of an example spectrum for a Doppler frequency; and Figure 5 is a block diagram of an equalizer adapted for use in determining Doppler shift information.
Detailed Description of the Preferred Embodiments Referring firstly to Figure 1, a conventional mobile communications system is comprised of a plurality of basestations (BS) 10,12,14,16 each connected through respective communications 15 link~ 18,20,22,24 to a mobile communications network 26. A
system coverage area (not shown) is defined by the combined coverage areas of the basestations. A mobile station (MS) 28 is shown moving through the system coverage area. A velocity vector v indicates the velocity of the mobile station 28 with respect to the location of a particular basestation 16. This velocity v includes the speed s of motion of the mobile station 28 and the angle ~ between the mobile station direction of motion and the direction from the mobile station to the particular basestation 16. Such a velocity vector exists for each of the basestationY
in the system. In conventional systems these velocity vectors are neither computed nor used in the process of determining when a handoff is to be performed, or in the process of determining to which of several potential basestations handoff is to be performed. Typically to determine the signal quality of the channel between each basestation and the mobile station, each ba~estation 10,12,14,16 transmits a pilot signal whose strength is measured by the mobile station 28. The mobile station 28 periodically broadcasts these pilot signal strength measurements.
In conventional systems, a basestation selection controller 32 (implemented in conjunction with or as part of the mobile communications network) determine~ when handoff is to be performed on the basis of the pilot signal strength measurements only.
An embodiment of the invention will now be described with reference to Figures 2a,2b and 3 in which a novel basestation selection controller is employed which utilizes both signal quality measurements and velocity parameter information.
In this embodiment, the velocity parameter information comprises a velocity parameter for each basestation which represents the speed and direction at which the mobile station is approaching (or leaving) the particular basestation, and the signal quality measurements comprise the above-described signal strength measurements of pilot signals.
The speed and direction of the mobile station in this embodiment are measured by the ba~estations by making use of the Doppler effect. The motion of the mobile station imparts a shift in the radio frequencies received by the basestations from the radio transmitter in the mobile station that is proportional to the speed and direction of the mobile station with respect to the basestation. This shift in received frequency is a few Hertz for a mobile station moving a few miles per hour. The following table gives several examples of shifts in frequency and the corresponding speed of a mobile station moving directly toward~ a basestation as calculated from standard physical formulae.
Speed (km/hr) Doppler Shift (Hz) 1 +2 +18 +88 100 +176 200 +352 The Doppler shift does not provide an absolute measurement of the speed and direction of the mobile station.
Rather, it provides a measure of the rate of increase or decrease of a mobile station's distance from the basestation. Thus a mobile station which is moving in a perfect circle centred at a base~tation, at any speed, will produce a Doppler shift of zero.
If the Doppler shift of the mobile station is measured by each of the potential new basestations, preferably at the same time the signal strengths are being measured, then this can be used to assist the handoff basestation selection controller in selecting a basestation for handoff. The basestation should be selected that has a suitable signal strength and which has the highest positive Doppler shift of the received signals from the mobile station. This is the basestation towards which the mobile station is moving the fastest.
In this example, it is assumed that a channel exists between one of the basestations and the mobile station, and that the channel carrier frequency is Fref. It is further assumed that all of the basestations potentially involved in a handoff have or are capable of receiving or generating an accurate replica of Fref. This may be done through GPS (global positioning system) receivers or through the mobile communications network for example.
Referring firstly to Figures 2a and 2b, an example of an arrangement for measuring the velocity of a mobile station with respect to a particular basestation using the Doppler shift technique will be described. Only the portions of the basestation and mobile station which are related to determining the velocity are shown. The arrangement at the basestation is shown in Figure 2a and includes a basestation reference frequency unit 50 which produces a reference frequency Fref in the case that the particular basestation has a channel established with the mobile station and is transmitting a signal with carrier frequency Fref through the basestation transmitter 52, or a replica of Fref in the case that the particular base~tation does not have a channel established with the mobile station. The base~tation has a receiver 54 for receiving a signal with receive carrier frequency Frx from the mobile station. A frequency detector 56 is connected to compare both the basestation reference frequency Fref and the received signal having carrier frequency Frx. The frequency detector 56 determines the difference between the frequencies of its two inputs, namely the difference between Fref and Frx. More particularly, it~0 determines a Doppler frequency defined by:
FD = [Frx - Fref]
A Doppler frequency analyzer 58 is connected to the frequency detector 56 for producing a velocity parameter consisting of a signal 60 representing the component of the velocity of the mobile station in the direction of the basestation as a function of the Doppler frequency FD. More specifically it produces a Doppler voltage VD which is a function of the spectrum of the Doppler frequency, as discussed in detail below. The received signal is also passed to a signal strength measurement circuit 62 which outputs signal strength measurements 64, and to the remaining parts of the basestation receiver (not shown).
A portion of a mobile station is shown in Figure 2b including a receiver 70 for receiving a signal from the baseqtation, a transmitter 72 for transmitting a signal to the baseqtation, a MS automatic frequency control unit 74 and a MS
local oscillator 76. Together, the frequency control unit 74 and the local oscillator 76 make the carrier frequency transmitted by the mobile station Ftxm equal to (or another known fixed relation to) the carrier frequency Frxm received by the mobile station from the basestation. The frequency received by the mobile station Frxm is defined by the equation:
Frxm = Fref(c/(c-s Cos(~)) where c is the speed of light, s is the speed of the mobile station, and ~ is the angle between the direction of motion of the mobile station and the direction of the basestation from the mobile station (see Figure 1). Since the mobile station transmits on the same frequency as it receives (or another known fixed relation), the mobile station transmit frequency is defined by the same equation, namely Ftxm = Frxm = Fref(c/(c-s Cos(~)).
Referring again to Figure 2, the ~ignal received by the base~tation has a frequency Frx which is again a Doppler shifted version of the signal transmitted by the mobile station, namely Frx = Ftxm(c(c-s Cos(~))).
The frequency detector 56 determines a Doppler frequency representing the difference between Frx and Fref, namely FD = -[Fref - Frx]
= -[Fref - Ftxm(c/(c-sCos(~)))]
= -[Fref - Fref(c/(c-sCos(~))(c/(c-sCos(~)))]
- -[Fref (1-c/(c-2sCos(~)))]
where the approximation made in the last line of the above equation holds true because the speed of the mobile station s is much less than the speed of light c. It can be seen that the Doppler frequency is zero for ~ equals to 90~, this repre~enting the case when the mobile station is moving along a portion of a circle centred at the basestation. This is true independent of the speed s. When the Doppler frequency is positive, the mobile station is moving towards the respective basestation, while if the Doppler frequency is negative, the mobile station is moving farther away from the respective basestations.
Depending on the propagation environment, the Doppler frequency will have different frequency spectrum characteristics.
In a simple propagation environment, the spectrum of the Doppler frequency will have a single component at the Doppler frequency corresponding to the speed and direction of motion. In a typical cellular mobile environment, however, multipath propagation combines multiple Doppler components in the measurement. The radio signal may be reflected from various objects in the environment and thus arrive at the basestation receiver over different paths from many directions for example as shown in Figure 4a which shows both a direct path 100 between a mobile station 28 and a basestation 10, and a reflected path 102. These different signals arrive at the basestation with different relative Doppler shifts with the result that the Doppler shift is not a discrete value, but instead has a spectrum, for example as shown in Figure 4b, which has several peaks only one of which corresponds with the part of the received signal that arrived on the direct path. In some cases the average Doppler shift can be measured and this can be used to assist in selection of the appropriate basestation for handoff. However, in other cases, a strong radio signal may be reflected from a structure such as a building which the mobile station is approaching while it is proceeding away from the basestation. This will give a false positive Doppler frequency shift and this component of the Doppler-Spectrum should not be used for assisting the hand off deci~ion. The Doppler frequency analyzer is provided for determining which peak in the Doppler frequency spectrum corresponds to the part of the received signal that arrived on the direct path.
There are many ways to construct such a Doppler frequency analyzer. For example, it may consist of a sampler which sample~ the output of the frequency detector with an A/D
converter, followed by a Fourier Transform operator which determines the Doppler frequency spectrum components, and a processor for determining which of the components is the earliest arriving component this being the component corresponding with the direct path. This apparatus/method would work particularly well if the mobile station transmits pulsed signals. The analysis would be more difficult for continuous signals. The Doppler frequency analyzer produces a velocity parameter (in the form of a Doppler voltage) which is the direct path component of the Doppler frequency. A positive velocity parameter indicates the mobile station is moving closer to the respective basestation, while a negative velocity parameter indicates the mobile station is moving away from to the respective basestation.
The signal strength measurements 64 produced by each of the signal strength measurement circuits 62 (one for each basestation) and the velocity parameters 60 produced by the Doppler frequency analyzers 58 (one for each basestation) are passed through the mobile communication network to a basestation selection controller 32 as shown in Figure 3. The basestation selection controller 32 then selects the basestation which satisfies two separate criteria, the first being that the mobile station is moving generally towards the selected basestation as indicated by a positive velocity parameter 60, and the second being that the selected basestation has a strong enough signal strength measurement 64. The invention is not concerned with the subsequent enactment of the handoff.
Preferably, the basestation having the largest positive velocity parameter 60 is selected if this has an acceptable signal strength measurement 64, as this is the basestation that the mobile station is moving towards the fastest.
Another embodiment of the invention provides a convenient technique for determining the direct component of the Doppler frequency shift in the cellular mobile environment. This technique provides a mechanism which replaces both the above-described frequency detector and Doppler frequency analyzer functions.
As indicated previously, in the multipath environment the receiver will receive various versions of the information signal with various delays. Typically, an adaptive equalizer is provided which attempts to capture the signal in not only the direct path component between the basestation and the receiver, but also in the other reflected/delayed components. It then attempts to combine the components such that the energy in the reflected components is neither wasted nor combined destructively with the energy in the direct path component. An example of an adaptive equalization structure is shown in Figure 5. The received signal is sampled and passed through a tapped delay line 110 having a number of delay line locations D1, D2,...D12 Each ,, .. ., , .. . . . . . . . , .. .. ., . ... , . .,. ., , . , . , , .. .,, ~ . . , ~. . ... ... .... . ..
delay line location Dl, D2,...D12 in the tapped delay line 110 has a re~pective complex equalization weight W1, W2...W12 which is multiplied by the value in the delay line location. These weighted values are summed with adder 120 to form an equalized signal 122 consisting of a weighted sum of all the samples in the tapped delay line. An equalizer weight adaptation processor 124 receives the equalized signal 122 and continuously adapts the equalization weights 114. This type of adaptive equalization is a well known step in the digital demodulation of signals in fading environments and will not be described in any further detail. Advantageously however, the equalizer can also be used to produce a velocity parameter estimate. The rate of change of the phase (or the frequency) of the lowest delay equalization weight Wl in an equalizer is a measure of the Doppler frequency of the component of the received signal having the shortest (i.e.
the direct) path. As described previously, the Doppler frequency of the direct path may be used to form a velocity parameter estimate.
The rate of change of phase of the lowest delay equalization weight Wl may be determined using any convenient technique, for example, by taking the difference between the phase of the last two values of the phase of the lowest delay equalization weight and dividing this by the adaptation period.
This may be performed by the equalizer weight adaptation processor 124 for example.
There are a number of equalization algorithms which use various techniques to select the equalization weights, but as the Doppler frequency mea~urement only looks at the rate of change of one the weights, the mathematical details of the algorithm by which the weight is selected is not a concern. However, the rate at which the algorithm updates the weights will determine the maximum Doppler frequency that can be detected. For example, for a vehicle speed of 100 km/h, the Doppler frequency would be 176 Hertz and thus the weight would need to be updated more frequently than every 5.6 milliseconds to be able to observe this frequency. Typically the radio channel changes in times of this order, and so the typical equalizer algorithm will be designed to update its weights at rates of this order. In using this technique to measure the Doppler frequency, it is necessary to use an equalization process that updates the weights at a rate sufficient that the highest expected Doppler frequency can be observed.
According to another aspect of the invention, in addition to aiding in the selection of the appropriate basestation for handoff, the velocity of the mobile station may be used to rank the importance or urgency of a handoff request.
If the mobile radio system is asked to perform several handoffs at the same time (or nearly the same time), the most urgency or priority should be given to the mobile station with the highest velocity away from the current basestation as that is the one that is moving the fastest and thus the one that will move out of range of its current basestation the soonest. Similarly, a mobile station with a very low or zero velocity may have its handoff deferred as it will remain within the range of its existing basestation for some time and a handoff may not be necessary. The Doppler frequency method is particularly suited to this application because it does not apply an undue significance to a component of a mobile station's velocity which is perpendicular to the direction of the basestation, this representing circular motion about the basestation.
CA 02244366 l998-07-29 In cellular systems that use a hierarchy of cells, some small ones for local coverage and some larger ones for wide area coverage, the motion of the mobile station, as determined by its velocity can be used to assist the process of handoff to the local or wide area cells. These classes of cell coverage are some times referred to as macro cells and micro cells. The macro cells are intended for wide area coverage for mobile stations that are moving quickly or which are outside the limited coverage areas of the micro cells. The micro cells are intended to provide coverage for "hot spots" where there is a concentration of traffic, but which might only be moving at pedestrian speed or not moving at all. The velocity information can be used to assist the selection between the micro cell and the macro cell.
A fast moving mobile station, as measured by its high velocity should be handed over to a wide area macro cell. A slow moving mobile station, with a low velocity should be handed over to a micro cell if one is available. If a fast moving mobile station stops moving, that is its velocity decreases to zero ~or thereabouts), then if it is in a macro cell, it should be considered for handoff to a micro cell as the mobile station is now stationary.
Many basestations now include in their design equipment with the needed accuracy to measure the mobile station's carrier frequency and to compare the measurements between the basestations. The u~e of, for example, GPS receivers to provide timing information to the basestations also provides the opportunity to make accurate frequency measurements which allow the Doppler shift of the mobile station to be determined.
In general, a mobile station's carrier frequency will not be constant or particularly accurate, as the mobile stations typically include only low stability oscillators and are usually designed to adjust their transmission to match the frequency of the basestations. Thus the mobile station's operating frequency will depend on the basestation's frequency and will be in error by some amount that depends on the mobile station's accuracy and its tuning capability and resolution. A group of subtending basestations, however, can still determine the relative Doppler shift of the mobile station and thus its speed and direction, by making their measurements at the same time. A11 of the Doppler frequency measurements made of the mobile station at the same time can be compared to determine the relative Doppler shift of the mobile station. The absolute frequency of the mobile station is not needed to determine the relative motion. Thus the uncertainty and instability in the mobile station's operating frequency can be compensated for by synchronizing the measurements at the basestations.
The process of the mobile station adjusting its transmit frequency to that of the basestation has the effect of doubling the apparent Doppler shift seen by the basestations. If the mobile station is moving towards the basestation it is working with, it will tune its transmitter based on the apparent received frequency which is Doppler shifted as a result of the motion. The transmitted signal from the mobile station is thus that of the basestation plus the Doppler shift seen at the same mobile station. Similarly, the frequency of the mobile station's signal will be blue shifted (increased in frequency) again as it travels from the mobile station to the basestation. Thus the size of the Doppler frequency shift (as exemplified in Table 1 above) will be doubled for those mobile stations which tune their local transmitter to the received basestation and which are moving with respect to the basestation.
The synchronization needed between basestations to assure that measurements may, for example, be made at the same time may be achieved through the GPS synchronization receiver already mentioned. The measurements may be made and time stamped with the GPS time and then measurements made at the same time can be compared by the basestation selection control process to determine the relative Doppler shift and the best basestation to select. An alternate means to achieve this synchronization of measurements can be achieved through the timing of the T1 signal used to transmit data from the basestation to the fixed network.
Alternatively, the basestation measurements may be synchronized by means of known over-the-air techniques.
An alternative means to measure the velocity of the mobile station would be to include a GPS receiver in the mobile station unit. This GPS unit can measure the velocity of the mobile station and report the measurements to the cellular system basestation using the appropriate signalling channel. This will give reliable velocity information with respect to all the basestation locations.
The methods provided by embodiments of the invention may be used to supplement most existing handoff methods including break-before-make, and the above mentioned make-before-break methods.
In the illustrated embodiments, a velocity parameter has been determined by comparing a transmitted and received signal at the basestations. Of course, other workable methods exist. For example, the velocity measurements might be made at the mobile station and transmitted to the basestation selection controller on a signalling channel.
,. ., ~.. . .
While the velocity parameters discussed above represent only the components of the velocity of a mobile station in a direction directly towards or away from a basestation, alternatively absolute velocity measurements may be made (with a GPS system for example). This requires knowledge of the geographic location of the basestations, and post-processing to determine which basestation(s) the mobile station is moving towards.
In the above-described embodiment, the signal strengths at the various basestations were determined by measuring the strength of pilot signal transmissions of the basestations and received by the mobile station. Of course, the signal strengths may be determined in any number of ways. For example, the basestations could all measure the strength of a signal tran~mitted by the mobile station.
As is well known in the art, there are many ways of selecting a set of basestations from within a mobile communications system to be candidates for handoff, for example by determining when the signal quality for each basestation exceeds a predetermined threshold. This might be referred to as the "Candidate Seta for example. In the instant application, the velocity parameter information may be determined for a set of basestations selected in any suitable manner, but preferably for the set of basestations already satisfying the signal quality criteria, for example, belonging to the Candidate Set.
In the illustrated embodiment a handoff decision is made on the basis of signal strength measurements in combination with velocity parameter information. Of course, measurements or values other than the signal strength may be used to determine the quality of a particular channel. A signal-to-noise ratio or channel error rate might be used for example. In general, some sort of signal quality measurement is required and this may consist of a single measurement for each channel or a combination of measurements.
In the illustrated embodiment, a basestation is selected which has a good signal quality and which the mobile station is moving generally towards. More intelligent selection criteria may be employed to effectively handle the aforementioned "corner problem" for example. To handle this case, the criteria could be established that the velocity parameter information indicates ~Q~h that the mobile station is moving away from the previous basestation and that the mobile station is moving towards the new basestation. This handover decision may be assisted by the knowledge that the two basestations serve streets where turns are possible.
Different criteria may be used to allow the handoff from any particular basestation to any other particular basestation.
Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein.
CELLULAR MOBILE RADIO SYSTEMS
Field of the Invention The invention relates to an improved system and method for selecting a basestation for handoff in mobile cellular radio systems.
Background of the Invention In mobile radio systems there may be handoff between radio units. That is, an active call involving a user's mobile station within a cellular system may be transferred from one radio basestation to another to improve or maintain the radio link either as a result of motion of the mobile station or as a result of changing radio transmission or interference conditions.
In many handoff situations, a choice must be made as to which is the "best" basestation to select from a number of possible candidates. That is, given that somehow the mobile station and the serving basestation have decided that a handoff is needed, a new basestation must be selected that will give the best performance for continuing the call. In traditional methods of handoff basestation selection, this choice is simply based on the radio signal strength. The basestation which is selected is the one which provides the strongest signal. These signal strength measurements may be made at either the mobile station or at the basestations.
It is a common objective of the handoff process to keep any interruptions to a call as short as possible. Ideally there should be no interruption and there should be as few handoffs as possible. Many systems make provision for a make-before-break handoff in which the signal is never interrupted. To achieve this, two connections (or more) are maintained to two (or more) base~tations while the call is being handed over from one basestation to another. DECT (digital enhanced cordless telephone)l and CDMA (code division multiple access) 2 are two such systems.
Conventional handoff methods which employ signal strength measurements alone in making handoff decisions have at least two difficulties associated with them. Firstly, signal strength varies rapidly with time depending on the motion of the mobile station, proximity to nearby objects and the radio propagation conditions. This means that the signal strength measurements must be averaged or filtered and the handoff decision must be based on the average signal strength and not on the immediate measurements. This introduces delay in the process which may lengthen the time taken for handoff so that it may be more obtrusive than desired.
The second difficulty is sometimes referred to as the "cornerN problem. A mobile station may be travelling down a city street where it receives service from a basestation at the end of the street, but is shielded by buildings along the street from other basestations which serve cross streets. At an intersection, the mobile station may be strongly illuminated by another basestation serving the cross street as well as its current serving basestation. The strong signal strength from the new basestation may cause the mobile station and its current IREF:ETSISt~d~dETS300-175 2REF:TIASt~d~d~S-95 serving basestation to consider doing a handoff to the new basestation. This handoff would be the wrong thing to do if the mobile station is continuing along its original street as it will be shadowed from the new basestation when it leaves the intersection. Such a handoff would likely cause the call to be lost (or dropped) briefly when it becomes shadowed from the new basestation while the call is switched back to the original serving basestation. However, a handoff would be the right thing to do if the mobile station turns the corner onto the new street as it will become shadowed from its original basestation after it leaves the intersection. Without the handoff to the new basestation, the call will likely be lost (or dropped) briefly when it becomes shadowed from the original basestation while the mobile station connects to the new basestation. The decision to do the handoff should be made while the mobile station is in the intersection, but the equipment cannot tell from the signal strength alone if the mobile station is turning the corner and thus if the handoff should be done, or if the mobile station is going to continue along the same street and thus no handoff is needed.
There are a number of techniques which attempt to enhance the existing methods by also examining the speed of the mobile station. Several of these focus on the problem of choosing between upper and lower tier cells in areas where there is a two-tier (or more) cellular system.
Furthermore, U.S. patent numbers 5,140,695 to Yasuda et al. titled "Channel Assignment System in Mobile Communication System" which issued on August 18, 1992 and 5,239,667 to Kanai titled "Method of Controlling Handoff in Cellular Mobile Radio Communication~ System" which i~sued August 24, 1993 make use of measurements of the fading frequency of the received radio signal to estimate the speed of motion of the mobile station, and use this speed information as part of the handoff decision. In the second of these patents, the speed information is used to adjust a threshold in relation to the mean signal strength which triggers the handoff.
In these patents, the fading frequency is referred to as the "Doppler Frequency". This may be used to determine the speed of the mobile station. However, this is not a measurement of the true Doppler effect as the sign of the fading frequency will always be positive and as such it is not possible to determine the direction of motion of the mobile station.
Summary of the Invention It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
In the handoff methods provided by the invention, additional information about the motion and direction of the mobile station is used to assist the handoff process. In a general sense, the basestation selected for the handoff should be not only the one with a good signal quality but also the one the mobile station is moving towards. The basestations from which the mobile station is moving away should be avoided for handoff even though they may have as good a signal quality as others. By measuring the velocity (speed and direction) of the mobile station with respect to the basestations, a basestation for handoff can be selected which meets the above objectives.
According to a first broad aspect, the invention provides a method for selecting a handoff basestation in a mobile communications system having a plurality of basestations in which a mobile station has an active connection to a first of the basestations, the handoff method comprising the steps of:
determining for one or more basestations a signal quality measurement of a signal transmitted from the mobile station to the basestation or vice versa; determining velocity parameter information for the mobile station; determining on the basis of the signal quality measurements and velocity parameter information whether a handoff is necessary, and if so, which of the one or more basestations is to be the handoff basestation.
According to a second broad aspect, the invention provides a hand off selection system for use in a mobile communications system in which a plurality of basestations are available to provide communications service to a mobile station, the handoff ~election system comprising: means for determining the signal strength of signals transmitted between the mobile station and one or more of the basestations; means for determining velocity parameter information for the mobile station; a basestation selection controller for selecting on the basis of the signal strengths and the velocity parameter information whether or not a handoff is required, and if so, to which of the one or more basestations.
Brief Description of the Drawings Preferred embodiments of the invention will now be described with reference to the attached drawings in which:
Figure 1 is a block diagram of a conventional mobile communications system;
Figures 2a and 2b are block diagrams of basestation and mobile station functionality respectively according to an embodiment of the invention;
Figure 3 is a detailed view of a basestation selection controller according to an embodiment of the invention;
Figure 4a is a schematic showing various propagation paths in a mobile communications environment;
Figure 4b is a plot of an example spectrum for a Doppler frequency; and Figure 5 is a block diagram of an equalizer adapted for use in determining Doppler shift information.
Detailed Description of the Preferred Embodiments Referring firstly to Figure 1, a conventional mobile communications system is comprised of a plurality of basestations (BS) 10,12,14,16 each connected through respective communications 15 link~ 18,20,22,24 to a mobile communications network 26. A
system coverage area (not shown) is defined by the combined coverage areas of the basestations. A mobile station (MS) 28 is shown moving through the system coverage area. A velocity vector v indicates the velocity of the mobile station 28 with respect to the location of a particular basestation 16. This velocity v includes the speed s of motion of the mobile station 28 and the angle ~ between the mobile station direction of motion and the direction from the mobile station to the particular basestation 16. Such a velocity vector exists for each of the basestationY
in the system. In conventional systems these velocity vectors are neither computed nor used in the process of determining when a handoff is to be performed, or in the process of determining to which of several potential basestations handoff is to be performed. Typically to determine the signal quality of the channel between each basestation and the mobile station, each ba~estation 10,12,14,16 transmits a pilot signal whose strength is measured by the mobile station 28. The mobile station 28 periodically broadcasts these pilot signal strength measurements.
In conventional systems, a basestation selection controller 32 (implemented in conjunction with or as part of the mobile communications network) determine~ when handoff is to be performed on the basis of the pilot signal strength measurements only.
An embodiment of the invention will now be described with reference to Figures 2a,2b and 3 in which a novel basestation selection controller is employed which utilizes both signal quality measurements and velocity parameter information.
In this embodiment, the velocity parameter information comprises a velocity parameter for each basestation which represents the speed and direction at which the mobile station is approaching (or leaving) the particular basestation, and the signal quality measurements comprise the above-described signal strength measurements of pilot signals.
The speed and direction of the mobile station in this embodiment are measured by the ba~estations by making use of the Doppler effect. The motion of the mobile station imparts a shift in the radio frequencies received by the basestations from the radio transmitter in the mobile station that is proportional to the speed and direction of the mobile station with respect to the basestation. This shift in received frequency is a few Hertz for a mobile station moving a few miles per hour. The following table gives several examples of shifts in frequency and the corresponding speed of a mobile station moving directly toward~ a basestation as calculated from standard physical formulae.
Speed (km/hr) Doppler Shift (Hz) 1 +2 +18 +88 100 +176 200 +352 The Doppler shift does not provide an absolute measurement of the speed and direction of the mobile station.
Rather, it provides a measure of the rate of increase or decrease of a mobile station's distance from the basestation. Thus a mobile station which is moving in a perfect circle centred at a base~tation, at any speed, will produce a Doppler shift of zero.
If the Doppler shift of the mobile station is measured by each of the potential new basestations, preferably at the same time the signal strengths are being measured, then this can be used to assist the handoff basestation selection controller in selecting a basestation for handoff. The basestation should be selected that has a suitable signal strength and which has the highest positive Doppler shift of the received signals from the mobile station. This is the basestation towards which the mobile station is moving the fastest.
In this example, it is assumed that a channel exists between one of the basestations and the mobile station, and that the channel carrier frequency is Fref. It is further assumed that all of the basestations potentially involved in a handoff have or are capable of receiving or generating an accurate replica of Fref. This may be done through GPS (global positioning system) receivers or through the mobile communications network for example.
Referring firstly to Figures 2a and 2b, an example of an arrangement for measuring the velocity of a mobile station with respect to a particular basestation using the Doppler shift technique will be described. Only the portions of the basestation and mobile station which are related to determining the velocity are shown. The arrangement at the basestation is shown in Figure 2a and includes a basestation reference frequency unit 50 which produces a reference frequency Fref in the case that the particular basestation has a channel established with the mobile station and is transmitting a signal with carrier frequency Fref through the basestation transmitter 52, or a replica of Fref in the case that the particular base~tation does not have a channel established with the mobile station. The base~tation has a receiver 54 for receiving a signal with receive carrier frequency Frx from the mobile station. A frequency detector 56 is connected to compare both the basestation reference frequency Fref and the received signal having carrier frequency Frx. The frequency detector 56 determines the difference between the frequencies of its two inputs, namely the difference between Fref and Frx. More particularly, it~0 determines a Doppler frequency defined by:
FD = [Frx - Fref]
A Doppler frequency analyzer 58 is connected to the frequency detector 56 for producing a velocity parameter consisting of a signal 60 representing the component of the velocity of the mobile station in the direction of the basestation as a function of the Doppler frequency FD. More specifically it produces a Doppler voltage VD which is a function of the spectrum of the Doppler frequency, as discussed in detail below. The received signal is also passed to a signal strength measurement circuit 62 which outputs signal strength measurements 64, and to the remaining parts of the basestation receiver (not shown).
A portion of a mobile station is shown in Figure 2b including a receiver 70 for receiving a signal from the baseqtation, a transmitter 72 for transmitting a signal to the baseqtation, a MS automatic frequency control unit 74 and a MS
local oscillator 76. Together, the frequency control unit 74 and the local oscillator 76 make the carrier frequency transmitted by the mobile station Ftxm equal to (or another known fixed relation to) the carrier frequency Frxm received by the mobile station from the basestation. The frequency received by the mobile station Frxm is defined by the equation:
Frxm = Fref(c/(c-s Cos(~)) where c is the speed of light, s is the speed of the mobile station, and ~ is the angle between the direction of motion of the mobile station and the direction of the basestation from the mobile station (see Figure 1). Since the mobile station transmits on the same frequency as it receives (or another known fixed relation), the mobile station transmit frequency is defined by the same equation, namely Ftxm = Frxm = Fref(c/(c-s Cos(~)).
Referring again to Figure 2, the ~ignal received by the base~tation has a frequency Frx which is again a Doppler shifted version of the signal transmitted by the mobile station, namely Frx = Ftxm(c(c-s Cos(~))).
The frequency detector 56 determines a Doppler frequency representing the difference between Frx and Fref, namely FD = -[Fref - Frx]
= -[Fref - Ftxm(c/(c-sCos(~)))]
= -[Fref - Fref(c/(c-sCos(~))(c/(c-sCos(~)))]
- -[Fref (1-c/(c-2sCos(~)))]
where the approximation made in the last line of the above equation holds true because the speed of the mobile station s is much less than the speed of light c. It can be seen that the Doppler frequency is zero for ~ equals to 90~, this repre~enting the case when the mobile station is moving along a portion of a circle centred at the basestation. This is true independent of the speed s. When the Doppler frequency is positive, the mobile station is moving towards the respective basestation, while if the Doppler frequency is negative, the mobile station is moving farther away from the respective basestations.
Depending on the propagation environment, the Doppler frequency will have different frequency spectrum characteristics.
In a simple propagation environment, the spectrum of the Doppler frequency will have a single component at the Doppler frequency corresponding to the speed and direction of motion. In a typical cellular mobile environment, however, multipath propagation combines multiple Doppler components in the measurement. The radio signal may be reflected from various objects in the environment and thus arrive at the basestation receiver over different paths from many directions for example as shown in Figure 4a which shows both a direct path 100 between a mobile station 28 and a basestation 10, and a reflected path 102. These different signals arrive at the basestation with different relative Doppler shifts with the result that the Doppler shift is not a discrete value, but instead has a spectrum, for example as shown in Figure 4b, which has several peaks only one of which corresponds with the part of the received signal that arrived on the direct path. In some cases the average Doppler shift can be measured and this can be used to assist in selection of the appropriate basestation for handoff. However, in other cases, a strong radio signal may be reflected from a structure such as a building which the mobile station is approaching while it is proceeding away from the basestation. This will give a false positive Doppler frequency shift and this component of the Doppler-Spectrum should not be used for assisting the hand off deci~ion. The Doppler frequency analyzer is provided for determining which peak in the Doppler frequency spectrum corresponds to the part of the received signal that arrived on the direct path.
There are many ways to construct such a Doppler frequency analyzer. For example, it may consist of a sampler which sample~ the output of the frequency detector with an A/D
converter, followed by a Fourier Transform operator which determines the Doppler frequency spectrum components, and a processor for determining which of the components is the earliest arriving component this being the component corresponding with the direct path. This apparatus/method would work particularly well if the mobile station transmits pulsed signals. The analysis would be more difficult for continuous signals. The Doppler frequency analyzer produces a velocity parameter (in the form of a Doppler voltage) which is the direct path component of the Doppler frequency. A positive velocity parameter indicates the mobile station is moving closer to the respective basestation, while a negative velocity parameter indicates the mobile station is moving away from to the respective basestation.
The signal strength measurements 64 produced by each of the signal strength measurement circuits 62 (one for each basestation) and the velocity parameters 60 produced by the Doppler frequency analyzers 58 (one for each basestation) are passed through the mobile communication network to a basestation selection controller 32 as shown in Figure 3. The basestation selection controller 32 then selects the basestation which satisfies two separate criteria, the first being that the mobile station is moving generally towards the selected basestation as indicated by a positive velocity parameter 60, and the second being that the selected basestation has a strong enough signal strength measurement 64. The invention is not concerned with the subsequent enactment of the handoff.
Preferably, the basestation having the largest positive velocity parameter 60 is selected if this has an acceptable signal strength measurement 64, as this is the basestation that the mobile station is moving towards the fastest.
Another embodiment of the invention provides a convenient technique for determining the direct component of the Doppler frequency shift in the cellular mobile environment. This technique provides a mechanism which replaces both the above-described frequency detector and Doppler frequency analyzer functions.
As indicated previously, in the multipath environment the receiver will receive various versions of the information signal with various delays. Typically, an adaptive equalizer is provided which attempts to capture the signal in not only the direct path component between the basestation and the receiver, but also in the other reflected/delayed components. It then attempts to combine the components such that the energy in the reflected components is neither wasted nor combined destructively with the energy in the direct path component. An example of an adaptive equalization structure is shown in Figure 5. The received signal is sampled and passed through a tapped delay line 110 having a number of delay line locations D1, D2,...D12 Each ,, .. ., , .. . . . . . . . , .. .. ., . ... , . .,. ., , . , . , , .. .,, ~ . . , ~. . ... ... .... . ..
delay line location Dl, D2,...D12 in the tapped delay line 110 has a re~pective complex equalization weight W1, W2...W12 which is multiplied by the value in the delay line location. These weighted values are summed with adder 120 to form an equalized signal 122 consisting of a weighted sum of all the samples in the tapped delay line. An equalizer weight adaptation processor 124 receives the equalized signal 122 and continuously adapts the equalization weights 114. This type of adaptive equalization is a well known step in the digital demodulation of signals in fading environments and will not be described in any further detail. Advantageously however, the equalizer can also be used to produce a velocity parameter estimate. The rate of change of the phase (or the frequency) of the lowest delay equalization weight Wl in an equalizer is a measure of the Doppler frequency of the component of the received signal having the shortest (i.e.
the direct) path. As described previously, the Doppler frequency of the direct path may be used to form a velocity parameter estimate.
The rate of change of phase of the lowest delay equalization weight Wl may be determined using any convenient technique, for example, by taking the difference between the phase of the last two values of the phase of the lowest delay equalization weight and dividing this by the adaptation period.
This may be performed by the equalizer weight adaptation processor 124 for example.
There are a number of equalization algorithms which use various techniques to select the equalization weights, but as the Doppler frequency mea~urement only looks at the rate of change of one the weights, the mathematical details of the algorithm by which the weight is selected is not a concern. However, the rate at which the algorithm updates the weights will determine the maximum Doppler frequency that can be detected. For example, for a vehicle speed of 100 km/h, the Doppler frequency would be 176 Hertz and thus the weight would need to be updated more frequently than every 5.6 milliseconds to be able to observe this frequency. Typically the radio channel changes in times of this order, and so the typical equalizer algorithm will be designed to update its weights at rates of this order. In using this technique to measure the Doppler frequency, it is necessary to use an equalization process that updates the weights at a rate sufficient that the highest expected Doppler frequency can be observed.
According to another aspect of the invention, in addition to aiding in the selection of the appropriate basestation for handoff, the velocity of the mobile station may be used to rank the importance or urgency of a handoff request.
If the mobile radio system is asked to perform several handoffs at the same time (or nearly the same time), the most urgency or priority should be given to the mobile station with the highest velocity away from the current basestation as that is the one that is moving the fastest and thus the one that will move out of range of its current basestation the soonest. Similarly, a mobile station with a very low or zero velocity may have its handoff deferred as it will remain within the range of its existing basestation for some time and a handoff may not be necessary. The Doppler frequency method is particularly suited to this application because it does not apply an undue significance to a component of a mobile station's velocity which is perpendicular to the direction of the basestation, this representing circular motion about the basestation.
CA 02244366 l998-07-29 In cellular systems that use a hierarchy of cells, some small ones for local coverage and some larger ones for wide area coverage, the motion of the mobile station, as determined by its velocity can be used to assist the process of handoff to the local or wide area cells. These classes of cell coverage are some times referred to as macro cells and micro cells. The macro cells are intended for wide area coverage for mobile stations that are moving quickly or which are outside the limited coverage areas of the micro cells. The micro cells are intended to provide coverage for "hot spots" where there is a concentration of traffic, but which might only be moving at pedestrian speed or not moving at all. The velocity information can be used to assist the selection between the micro cell and the macro cell.
A fast moving mobile station, as measured by its high velocity should be handed over to a wide area macro cell. A slow moving mobile station, with a low velocity should be handed over to a micro cell if one is available. If a fast moving mobile station stops moving, that is its velocity decreases to zero ~or thereabouts), then if it is in a macro cell, it should be considered for handoff to a micro cell as the mobile station is now stationary.
Many basestations now include in their design equipment with the needed accuracy to measure the mobile station's carrier frequency and to compare the measurements between the basestations. The u~e of, for example, GPS receivers to provide timing information to the basestations also provides the opportunity to make accurate frequency measurements which allow the Doppler shift of the mobile station to be determined.
In general, a mobile station's carrier frequency will not be constant or particularly accurate, as the mobile stations typically include only low stability oscillators and are usually designed to adjust their transmission to match the frequency of the basestations. Thus the mobile station's operating frequency will depend on the basestation's frequency and will be in error by some amount that depends on the mobile station's accuracy and its tuning capability and resolution. A group of subtending basestations, however, can still determine the relative Doppler shift of the mobile station and thus its speed and direction, by making their measurements at the same time. A11 of the Doppler frequency measurements made of the mobile station at the same time can be compared to determine the relative Doppler shift of the mobile station. The absolute frequency of the mobile station is not needed to determine the relative motion. Thus the uncertainty and instability in the mobile station's operating frequency can be compensated for by synchronizing the measurements at the basestations.
The process of the mobile station adjusting its transmit frequency to that of the basestation has the effect of doubling the apparent Doppler shift seen by the basestations. If the mobile station is moving towards the basestation it is working with, it will tune its transmitter based on the apparent received frequency which is Doppler shifted as a result of the motion. The transmitted signal from the mobile station is thus that of the basestation plus the Doppler shift seen at the same mobile station. Similarly, the frequency of the mobile station's signal will be blue shifted (increased in frequency) again as it travels from the mobile station to the basestation. Thus the size of the Doppler frequency shift (as exemplified in Table 1 above) will be doubled for those mobile stations which tune their local transmitter to the received basestation and which are moving with respect to the basestation.
The synchronization needed between basestations to assure that measurements may, for example, be made at the same time may be achieved through the GPS synchronization receiver already mentioned. The measurements may be made and time stamped with the GPS time and then measurements made at the same time can be compared by the basestation selection control process to determine the relative Doppler shift and the best basestation to select. An alternate means to achieve this synchronization of measurements can be achieved through the timing of the T1 signal used to transmit data from the basestation to the fixed network.
Alternatively, the basestation measurements may be synchronized by means of known over-the-air techniques.
An alternative means to measure the velocity of the mobile station would be to include a GPS receiver in the mobile station unit. This GPS unit can measure the velocity of the mobile station and report the measurements to the cellular system basestation using the appropriate signalling channel. This will give reliable velocity information with respect to all the basestation locations.
The methods provided by embodiments of the invention may be used to supplement most existing handoff methods including break-before-make, and the above mentioned make-before-break methods.
In the illustrated embodiments, a velocity parameter has been determined by comparing a transmitted and received signal at the basestations. Of course, other workable methods exist. For example, the velocity measurements might be made at the mobile station and transmitted to the basestation selection controller on a signalling channel.
,. ., ~.. . .
While the velocity parameters discussed above represent only the components of the velocity of a mobile station in a direction directly towards or away from a basestation, alternatively absolute velocity measurements may be made (with a GPS system for example). This requires knowledge of the geographic location of the basestations, and post-processing to determine which basestation(s) the mobile station is moving towards.
In the above-described embodiment, the signal strengths at the various basestations were determined by measuring the strength of pilot signal transmissions of the basestations and received by the mobile station. Of course, the signal strengths may be determined in any number of ways. For example, the basestations could all measure the strength of a signal tran~mitted by the mobile station.
As is well known in the art, there are many ways of selecting a set of basestations from within a mobile communications system to be candidates for handoff, for example by determining when the signal quality for each basestation exceeds a predetermined threshold. This might be referred to as the "Candidate Seta for example. In the instant application, the velocity parameter information may be determined for a set of basestations selected in any suitable manner, but preferably for the set of basestations already satisfying the signal quality criteria, for example, belonging to the Candidate Set.
In the illustrated embodiment a handoff decision is made on the basis of signal strength measurements in combination with velocity parameter information. Of course, measurements or values other than the signal strength may be used to determine the quality of a particular channel. A signal-to-noise ratio or channel error rate might be used for example. In general, some sort of signal quality measurement is required and this may consist of a single measurement for each channel or a combination of measurements.
In the illustrated embodiment, a basestation is selected which has a good signal quality and which the mobile station is moving generally towards. More intelligent selection criteria may be employed to effectively handle the aforementioned "corner problem" for example. To handle this case, the criteria could be established that the velocity parameter information indicates ~Q~h that the mobile station is moving away from the previous basestation and that the mobile station is moving towards the new basestation. This handover decision may be assisted by the knowledge that the two basestations serve streets where turns are possible.
Different criteria may be used to allow the handoff from any particular basestation to any other particular basestation.
Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein.
Claims (27)
1. A method for selecting a handoff basestation in a mobile communications system having a plurality of basestations in which a mobile station has an active connection to a first of the basestations, the handoff method comprising the steps of:
determining for one or more basestations a signal quality measurement of a signal transmitted from the mobile station to the basestation or vice versa;
determining velocity parameter information for the mobile station;
determining on the basis of the signal quality measurements and velocity parameter information whether a handoff is necessary, and if so, which of the one or more basestations is to be the handoff basestation.
determining for one or more basestations a signal quality measurement of a signal transmitted from the mobile station to the basestation or vice versa;
determining velocity parameter information for the mobile station;
determining on the basis of the signal quality measurements and velocity parameter information whether a handoff is necessary, and if so, which of the one or more basestations is to be the handoff basestation.
2. A method according to claim 1 wherein to be selected as the handoff basestation, the handoff basestation must have an acceptable signal quality measurement, and the velocity parameter information must indicate that the mobile station is moving generally towards the handoff basestation.
3. A method according to claim 1 wherein the velocity parameter information is used to determine which basestation or basestations the mobile station is generally moving towards and to select the one of these with the best signal quality measurement to be the handoff basestation.
4. A method according to claim 1 wherein the signal quality measurements are used to determine which basestations have an acceptable signal quality for handoff and to select the one of these that the mobile station is approaching the fastest, as indicated by the velocity parameter information.
5. A method according to claim 1 wherein the step of determining velocity parameter information comprises the steps of determining a velocity parameter for each respective basestation representing the direction of motion of the mobile station with respect to the basestation or the rate at which the mobile station is approaching the basestation.
6. A method according to claim 1, wherein to measure each velocity parameter, the method further comprises the steps of:
measuring a Doppler shift in a signal transmitted between the mobile and the respective basestation or vice versa;
converting the Doppler shift into the velocity parameter.
measuring a Doppler shift in a signal transmitted between the mobile and the respective basestation or vice versa;
converting the Doppler shift into the velocity parameter.
7. A method according to claim 6 wherein the step of measuring the Doppler shift comprises the step of:
measuring the difference between a transmitted frequency and a received frequency.
measuring the difference between a transmitted frequency and a received frequency.
8. A method according to claim 6 wherein the basestation has an adaptive equalizer for performing channel equalization, the adaptive equalizer having a lowest delay tap with a complex equalization weight which is adaptively adjusted depending on the characteristics of the signal received over the channel, and the step of measuring the Doppler shift comprises:
determining the rate of change of phase of the equalization weight for the lowest delay tap and using this as a measure of the Doppler shift.
determining the rate of change of phase of the equalization weight for the lowest delay tap and using this as a measure of the Doppler shift.
9. A method according to claim 8 wherein the rate of change is approximated by computing the difference between two successive values of the equalization weight for the lowest delay tap.
10. A method according to claim 5 further comprising the step of the mobile communications system ranking the urgency of handoff requests according to the rate at which the mobile station is approaching the basestation, wherein a mobile station with a higher rate away from the basestation is given a higher priority for handoff.
11. A method according to claim 1 wherein the velocity parameter information is derived from a global positioning system unit in the mobile station.
12. A method according to claim 11 wherein the mobile station communicates the velocity parameter information over a cellular signalling channel.
13. A method according to claim 6 wherein the velocity parameters for each of the basestations are determined substantially simultaneously.
14. A method according to claim 13 further comprising the step of using global positioning system receivers to provide timing information to the basestations such that the velocity parameters for the basestations may be determined substantially simultaneously.
15. A method according to claim 13 further comprising the step of using a T1 signal connected to each basestation to provide timing information to the basestations such that the velocity parameters for the basestations may be determined substantially simultaneously.
16. A method according to claim 1 wherein the step of determining a signal quality measurement comprises the steps of:
each basestation transmitting a pilot signal;
the mobile station measuring the signal strength of each pilot signal; and the mobile station broadcasting the signal strengths to the basestations.
each basestation transmitting a pilot signal;
the mobile station measuring the signal strength of each pilot signal; and the mobile station broadcasting the signal strengths to the basestations.
17. A method according to claim 1 wherein the signal quality measurements are one or a combination of signal strength measurements, signal to noise ratio measurements, and channel error rate measurements.
18. A method according to claim 1 further comprising the step of the mobile communications system using the velocity parameter information to determine whether a hand off is necessary between a micro cell and a macro cell, wherein mobile stations with higher velocities are connected to a basestation of the macro cell and mobile stations with lower velocities are connected to a basestation of the micro cell.
19. A method according to claim 1 wherein:
the step of determining velocity parameter information comprises the mobile station determining its own absolute position and velocity using a GPS receiver; the step of determining the handoff basestation comprises using the position and velocity determined by the mobile station to determine which basestation or basestations the mobile station is generally moving towards and selecting the one of these with the highest signal strength to be the hand off basestation.
the step of determining velocity parameter information comprises the mobile station determining its own absolute position and velocity using a GPS receiver; the step of determining the handoff basestation comprises using the position and velocity determined by the mobile station to determine which basestation or basestations the mobile station is generally moving towards and selecting the one of these with the highest signal strength to be the hand off basestation.
20. A method according to claim 1 further comprising the step of requiring further criteria to be satisfied before deciding a handoff is to occur from a particular first basestation to another particular basestation.
21. A method according to claim 20 wherein the further criteria comprise the requirement that the mobile station is moving towards the another particular basestation and away from the particular first basestation.
22. A handoff selection system for use in a mobile communications system in which a plurality of basestations are available to provide communications service to a mobile station, the hand off selection system comprising:
a signal quality measurement mechanism for determining the quality of signals transmitted between the mobile station and one or more of the basestations;
a velocity parameter information measurement mechanism for determining velocity parameter information for the mobile station;
a basestation selection controller for selecting on the basis of the signal quality measurements and the velocity parameter information whether or not a handoff is required, and if so, to which of the one or more basestations.
a signal quality measurement mechanism for determining the quality of signals transmitted between the mobile station and one or more of the basestations;
a velocity parameter information measurement mechanism for determining velocity parameter information for the mobile station;
a basestation selection controller for selecting on the basis of the signal quality measurements and the velocity parameter information whether or not a handoff is required, and if so, to which of the one or more basestations.
23. A handoff selection system according to claim 22 wherein the signal quality measurement mechanism comprises a signal strength measurement circuit in each of the basestations for determining a respective signal quality measurement, wherein each basestation sends its respective signal quality measurement to the basestation selection controller through a mobile communications network.
24. A handoff selection system according to claim 22 wherein the velocity parameter information measurement mechanism comprises:
in each basestation, a doppler shift measurement circuit for determining doppler shift in a signal transmitted by the mobile unit and converting this to a velocity parameter indicative of the rate of change of the distance between the mobile unit and the basestation, wherein each basestation sends its respective velocity parameter to the basestation selection controller through a mobile communications network.
in each basestation, a doppler shift measurement circuit for determining doppler shift in a signal transmitted by the mobile unit and converting this to a velocity parameter indicative of the rate of change of the distance between the mobile unit and the basestation, wherein each basestation sends its respective velocity parameter to the basestation selection controller through a mobile communications network.
25. A handoff selection system according to claim 24 wherein each doppler shift measurement circuit comprises:
an adaptive equalizer for equalizing a signal received from the mobile unit by the basestation, the adaptive equalizer having a weighted delay tap line with a lowest delay location and having weights which are adjusted adaptively, including a weight for the lowest delay location;
a rate of change calculator for calculating a rate of change of the phase of the lowest delay weight in the adaptive equalizer, this rate of change serving as the velocity parameter.
an adaptive equalizer for equalizing a signal received from the mobile unit by the basestation, the adaptive equalizer having a weighted delay tap line with a lowest delay location and having weights which are adjusted adaptively, including a weight for the lowest delay location;
a rate of change calculator for calculating a rate of change of the phase of the lowest delay weight in the adaptive equalizer, this rate of change serving as the velocity parameter.
26. A basestation comprising:
a signal quality measurement mechanism for determining a signal quality measurement of a channel between the basestation and a mobile unit; and a velocity parameter information measurement mechanism comprising a doppler shift measurement circuit for determining doppler shift in a signal transmitted by the mobile unit and converting this to a velocity parameter indicative of the rate of change of the distance between the mobile unit and the basestation.
a signal quality measurement mechanism for determining a signal quality measurement of a channel between the basestation and a mobile unit; and a velocity parameter information measurement mechanism comprising a doppler shift measurement circuit for determining doppler shift in a signal transmitted by the mobile unit and converting this to a velocity parameter indicative of the rate of change of the distance between the mobile unit and the basestation.
27. A handoff selection system according to claim 26 wherein each doppler shift measurement circuit comprises:
an adaptive equalizer for equalizing a signal received from the mobile unit by the basestation, the adaptive equalizer having a weighted delay tap line with a lowest delay location and having weights which are adjusted adaptively, including a weight for the lowest delay location;
a rate of change calculator for calculating a rate of change of the phase of the lowest delay weight in the adaptive equalizer, this rate of change serving as the velocity parameter.
an adaptive equalizer for equalizing a signal received from the mobile unit by the basestation, the adaptive equalizer having a weighted delay tap line with a lowest delay location and having weights which are adjusted adaptively, including a weight for the lowest delay location;
a rate of change calculator for calculating a rate of change of the phase of the lowest delay weight in the adaptive equalizer, this rate of change serving as the velocity parameter.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US93474697A | 1997-09-22 | 1997-09-22 | |
| US08/934,746 | 1997-09-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2244366A1 true CA2244366A1 (en) | 1999-03-22 |
Family
ID=25465996
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002244366A Abandoned CA2244366A1 (en) | 1997-09-22 | 1998-07-29 | Velocity assisted handoff system and method for cellular mobile radio systems |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2244366A1 (en) |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2001030090A2 (en) * | 1999-10-18 | 2001-04-26 | Telefonaktiebolaget Lm Ericsson (Publ) | Method and system for the wireless transmission of loss sensitive data |
| US8467359B2 (en) | 2010-05-13 | 2013-06-18 | Research In Motion Limited | Methods and apparatus to authenticate requests for network capabilities for connecting to an access network |
| US8594061B2 (en) | 2010-06-22 | 2013-11-26 | Blackberry Limited | Methods and apparatus to access network connectivity information using predicted locations |
| US8644276B2 (en) | 2010-05-13 | 2014-02-04 | Research In Motion Limited | Methods and apparatus to provide network capabilities for connecting to an access network |
| US8665842B2 (en) | 2010-05-13 | 2014-03-04 | Blackberry Limited | Methods and apparatus to discover network capabilities for connecting to an access network |
| US8935754B2 (en) | 2009-04-24 | 2015-01-13 | Blackberry Limited | Methods and apparatus to discover authentication information in a wireless networking environment |
| EP3025543A4 (en) * | 2013-07-26 | 2017-02-22 | LG Electronics Inc. | Method for measuring mobility of ue for multi-antenna beamforming in wireless communication system and apparatus therefor |
-
1998
- 1998-07-29 CA CA002244366A patent/CA2244366A1/en not_active Abandoned
Cited By (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2001030090A2 (en) * | 1999-10-18 | 2001-04-26 | Telefonaktiebolaget Lm Ericsson (Publ) | Method and system for the wireless transmission of loss sensitive data |
| WO2001030090A3 (en) * | 1999-10-18 | 2001-11-15 | Ericsson Telefon Ab L M | Method and system for the wireless transmission of loss sensitive data |
| US8935754B2 (en) | 2009-04-24 | 2015-01-13 | Blackberry Limited | Methods and apparatus to discover authentication information in a wireless networking environment |
| US8943552B2 (en) | 2009-04-24 | 2015-01-27 | Blackberry Limited | Methods and apparatus to discover authentication information in a wireless networking environment |
| US9572030B2 (en) | 2009-04-24 | 2017-02-14 | Blackberry Limited | Methods and apparatus to discover authentication information in a wireless networking environment |
| US9820149B2 (en) | 2009-04-24 | 2017-11-14 | Blackberry Limited | Methods and apparatus to discover authentication information in a wireless networking environment |
| US10136319B2 (en) | 2009-04-24 | 2018-11-20 | Blackberry Limited | Methods and apparatus to discover authentication information in a wireless networking environment |
| US8467359B2 (en) | 2010-05-13 | 2013-06-18 | Research In Motion Limited | Methods and apparatus to authenticate requests for network capabilities for connecting to an access network |
| US8644276B2 (en) | 2010-05-13 | 2014-02-04 | Research In Motion Limited | Methods and apparatus to provide network capabilities for connecting to an access network |
| US8665842B2 (en) | 2010-05-13 | 2014-03-04 | Blackberry Limited | Methods and apparatus to discover network capabilities for connecting to an access network |
| US8594061B2 (en) | 2010-06-22 | 2013-11-26 | Blackberry Limited | Methods and apparatus to access network connectivity information using predicted locations |
| EP3025543A4 (en) * | 2013-07-26 | 2017-02-22 | LG Electronics Inc. | Method for measuring mobility of ue for multi-antenna beamforming in wireless communication system and apparatus therefor |
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| Date | Code | Title | Description |
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| FZDE | Discontinued |