Channel Allocation for Registering in a Cellular Radio Network

(71)

Application No 9526482.6

Date of Filing 22.12.1995 (51)

Applicant(s)

Spectronics Micro Systems Limited (Incorporated in the United Kingdom) (52) (56)

INT CL⁶

H04Q 7/38 7/32

UK CL (Edition 0 )

H4L LDSJ LDSL L1H10

Documents Cited

GB 2271486 A GB 2263608 A EP 0292182 A2 US 5214789 A

GB 2260050 A US 4667202 A

Bar Hill Business Park, Saxon Way, Bar Hill, CAMBRIDGE, CB3 8SL, United Kingdom (72) Inventor(s)

John Clinton Worsnop David Russell Griffiths (58) Field of Search

UK CL (Edition 0 ) H4L LDSH LDSJ LDSL INTCL⁶ H04Q7/32 7/38

ON LINE : WPI, JAPIO, INSPEC (74) Agent and/or Address for Service

Reddie & Grose

Theobalds Road, LONDON. WC1X 8PL, United Kingdom (54) Channel Allocation for Registering in a Cellular Radio Network (57) A mobile unit for use in a cellular radio network which includes apparatus for registering the mobile unit with a radio channel. The mobile unit includes means (34) for storing data relating to the frequencies of radio channels operated by base sites in the network and to the geographical areas in which the mobile units are capable of using each radio channel. A location system (40) is used to determine the current position of the mobile unit in the network. A comparing means (35) compares the current position of the mobile unit with the stored geographical data and a selecting means (36) selects a radio channel frequency for registration in dependence on the comparison.

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- 1 RADIO COMMUNICATION SYSTEM

The present invention relates to a radio communication system, particularly but not exclusively to apparatus for registering a mobile unit with a radio channel offered by a base site in a cellular radio network.

In a radio communication network where multiple radio channels are available to mobile units for communication, a key issue is registration - the technique of switching channels to provide optimum service levels to the mobile units .

Suppose it is necessary to develop a radio system to meet the following requirements:

Regional coverage area (eg greater London or

Singapore Island); and a large mobile fleet (in excess of 1000 vehicles) There are two distinctive approaches to this kind of situation. The traditional solution requires a relatively small number of base sites, each covering a good proportion of the total area of interest, and possibly with each base site offering a number of channels for communication. In contrast, the cellular solution requires a relatively large number of base sites, each covering a cell which is a fraction of the total coverage area and offering few channels possibly just one.

Although both solutions have their advantages the cellular solution becomes increasingly attractive as traffic load increases .

-2As a simplified example, consider the following radio system where, because of traffic loading and channel capacity considerations, twenty radio channels may be needed at any one time. In a traditional scheme, they might be split between four base sites with, say five radio channels per site. Between any pair of base sites there will be a substantial coverage overlap, and therefore all the radio channels must be on different frequencies. Hence, twenty different radio frequencies will be required in this scheme. In a cellular scheme, the twenty radio channels could be provided by as many as twenty separate base sites. For simplicity, assume that each base site covers an identical area - one cell - and that base sites can be located at will. If there is no appreciable radio coverage overlap between non-adjacent cells, the area can be served by as few as seven different radio frequencies, with each radio frequency being reused two or three times. The most obvious benefit of the cellular scheme over the traditional scheme lies in the 3:1 reduction in the number of different radio frequencies required.

In a system requiring mobile units to switch channels automatically, a mobile unit roaming between one coverage area and the next must re-register with a new radio channel. The decision when to de-register from the existing radio channel, and on which new radio channel to register, may be made either in the mobile unit (eg in MPT 1327 trunk systems) or by a central channel controller (eg in TACS analog cellular telephone systems). However, the information on which this decision is based is always some measure of quality of service, either at the mobile unit or at the base site. The quality of service determination

-3usually involves an indication of received signal strength (a measure of the radio frequency carrier level) and sometimes an additional quality measure such as BIT error rate. If the quality of service drops below a given threshold level for more than a certain time period, the mobile unit will switch to a new radio channel.

Although such systems can work satisfactorily, they suffer from certain disadvantages. Firstly, a system of this type is necessarily non-deterministic, particularly in regions where coverage areas overlap. Therefore, the particular channel on which a mobile unit in a given position is registered cannot be predicted, because it depends on factors such as the previous route followed by the mobile unit and local variations in radio frequency signal strengths. Secondly, the exact boundaries of the coverage areas of base sites may be difficult to define because they depend in an unpredictable way on the quality of service threshold. This may mean that a mobile unit experiences a period of poor quality service just before it re-registers. In addition, when a mobile unit does switch channels, the choice of channel on which to reregister is determined by an instantaneous measurement of quality of service on each available channel. This may not be the best choice and within a few seconds, it may be necessary to switch channels once more when the mobile unit moves to a new location.

The fundamental problem with traditional registration is simply that neither the system or any mobile unit has sufficient information on which to make the best registration decision.

-4In accordance with the present invention, there is provided apparatus for registering a mobile unit with a radio channel provided by a base site in a cellular radio network, comprising: means for storing data relating to the frequencies of radio channels operated by base sites in the network, and to the geographical areas in which mobile units are capable of using each radio channel; means for determining the current position of the mobile unit in the network; means for comparing the current position of the mobile unit with the stored geographical data; and means for selecting a radio channel frequency for registration in dependence on the comparison.

A wide range of different basic techniques exist for automatically determining vehicle location. These include satellite based location systems such as, for example, the Omega system and also systems which use local radio beacons. A mobile unit operating in one of these systems monitors phase differences in signals received from different satellites or beacons and uses these to derive the current position of the mobile unit. Typically a positional accuracy of approximately 100 metres may be obtained for a single derivation of position.

As the mobile unit moves through the network, it periodically updates its current position using some kind of automatic vehicle location system. The new positional information is compared with the stored geographical information to determine whether the mobile unit needs to re-register. Re-registration is only necessary when the mobile unit has moved outside the stored geographical area covered by the radio channel on which it was operating.

-5Preferably, any overlapping geographical coverage areas are divided by imaginary boundaries in the storage means, with the region on one side of each imaginary boundary being allocated a different radio channel frequency for communication from the region on the opposite side. The network is effectively represented by a virtual coverage map divided into cells by the imaginary boundaries. Each cell on the map is covered by a single base site such that any mobile unit in a given cell is registered to the radio channel provided by that cell's base site.

In a preferred arrangement, the imaginary boundaries have a predetermined width which must be traversed fully by the mobile unit before re-registration is permitted. In order to reduce unnecessary re-registration at the imaginary boundaries, and to take into account the inaccuracy inherent in any automatic vehicle location system, a buffering arrangement can be built into the imaginary boundaries. Once a mobile unit is registered on a given radio channel, the buffering arrangement delays reregistration until the mobile unit has travelled across the entire width of the boundary. In an especially preferred form, the width of the imaginary boundaries is of the same order of magnitude as the imprecision in the mobile unit's positional determination.

These and other aspects of the present invention, are defined in the appended claims to which reference should now be made .

To make efficient use of the radio channels, the registration decisions are preferably made by the mobile units, since otherwise they would need to report their

-6positions very frequently. To be able to achieve this, the mobile unit must be aware of the frequency and geographical coverage areas of the radio channels provided by the base sites at least in its vicinity. It is conceivable that this information can be transmitted to the mobile units via the radio network itself.

An embodiment of the invention will now be described by way of example, with reference to the accompanying figures, in which:

Figure 1 shows schematically part of a radio coverage map for a cellular radio network; and

Figure 2 shows a block diagram of circuitry provided at a mobile unit.

Figure 1 shows part of a radio coverage map for a cellular radio network. The three base sites A, B, C each offer a radio channel for communication with a mobile unit using frequencies F_A, F_B, F_c respectively. The limits of the geographical areas in which a mobile unit is capable of communicating on each frequency are shown by boundary lines 10, 11, 12. The geographical areas overlap each other and so to avoid interference, the three frequencies F_A, F_b, F_c are all different.

In geographical areas where there is actual radio coverage overlap, imaginary boundaries 20, 21, 22 are superimposed on the radio coverage map, giving a virtual coverage map. The imaginary boundaries divide the overlapping areas in such a way that regions on opposite sides are allocated different base sites and therefore different radio

-7 frequencies. For example, imaginary boundary 20 divides part of the coverage overlap between bases A and B into two regions. The first region, on the left hand side of the imaginary boundary 20, is allocated to base site A and frequency F_A and the second region on the right handside is allocated to base site B and frequency F„. The imaginary boundaries divide the virtual coverage map into cells.

In figure 2, a mobile unit 30 is shown which comprises a transmitter/receiver 31 which is coupled to an antenna 32 for transmitting signals to and receiving signals from a base site.

The transmitter/receiver 31 is also coupled to a microprocessor 33 which stores in store 34 a map showing the positions of the imaginary boundaries between the base sites in the vicinity of the mobile unit (GEOGRAPHICAL DATA), together with the frequencies of the radio channels provided by the base sites in the areas defined by the imaginary boundaries (FREQUENCY DATA). The mobile unit 30 also comprises a location system 40 responsive to signals from local beacons or from satellites or from any other form of positioning system to derive the position of the mobile unit 30. This is typically derived to within an accuracy of the order of 100 metres. The location system 40 repeatedly calculates the position of the mobile unit 30 and supplies it to the microprocessor 33. The location system 40 updates the positional information sufficiently quickly for the microprocessor effectively to monitor the position of the mobile unit continuously.

In operation, the mobile unit 30 initially derives its position using the location system 40. This is supplied

-δύο the microprocessor 33 where it is compared in comparator 35 to the map stored in store 34. The comparator 35 decides which coverage area or cell includes the mobile unit's position within its imaginaryboundaries. Having ascertained which particular coverage area the mobile unit 30 falls within, a selector 36 selects a radio channel for communication by reading off the radio channel frequency in store 34 which is allocated to that coverage area. The transmitter/receiver 31 is then tuned by the microprocessor 33 to the particular frequency of the radio channel which is provided for communicating with the base site. Every time the location system calculates a new position for the mobile unit 30, the routine for determining the radio channel frequency which should be used for communication is repeated to see whether re-registration is necessary.

Suppose the mobile unit 30 starts at a place adjacent base site B where the mobile unit is tuned to frequency F_B. If the mobile unit 30 moves towards base site A, it will eventually reach imaginary boundary 20. Once it crosses the imaginary boundary 20, the mobile unit 30 will tune into frequency F_A operated by base site A. In store 34, the imaginary boundaries 20, 21, 22 are given a width of, say, 100 metres which is comparable to the imprecision of the location system 40. The microprocessor 33 is programmed to allow re-registration only once the mobile unit 30 has crossed the full width of the imaginary boundaries. Giving the imaginary boundaries such a width serves as a buffer against unnecessary or premature reregistration. Therefore, the mobile unit 30 remains tuned to frequency F_B until it emerges from the imaginary boundary zone, at least 100 metres along from where it

-9first encountered the imaginary boundary 20. Of course, if the mobile unit turns back towards base site B before it has crossed the full width of imaginary boundary 20, re-registration will not be necessary. The width of the imaginary boundaries 20, 21, 22 is much less than the average width of the overlapping regions.

Registering a mobile unit with a radio channel in accordance with the present embodiment offers a number of advantages including:

1) Predictable behaviour. If the position of the mobile unit is known, it is possible to predict with reasonable certainty the radio channel on which the mobile unit is registered. This is not always the case if the mobile unit is located in a buffer region covered by an imaginary boundary. However, these instances are rare since the width of the boundaries are typically very narrow compared to the size of the network and the area served by each base site .

2) Increasing re-use of radio channel frequencies.

In a conventional cellular network, it is essential to ensure that there is no physical coverage overlap between two base sites operating on the same radio frequency. By contrast, such physical overlap in a scheme in accordance with the present embodiment is not a problem provided that, on the virtual coverage map, the overlapping area is assigned to a base site(s), operating on a different frequency.

3) Providing a consistent quality of service across the entire radio network. For example, on crossing

- 10an imaginary boundary, a mobile unit will reregister well before degredation of service sets in.

4) Improving system reliance. In a worst case scenario, for example if the frequency and geographical coverage area information is temporarily unavailable, a mobile unit will fallback to using quality of service as a basis for registration. In this case, the mobile unit is no worse off than in a traditional registration scheme. However, in the case of partial failures, such as failure of a single base site, a system in accordance with the present embodiment will still offer some advantages over a traditional registration scheme. In the first place, a mobile unit which is in the locality of the failed base site can determine, by a simple geometrical calculation, which functional geographical coverage area is closest and hence which base site is most suitable. This will serve to spread the load from the failed base site onto adjacent base sites in a predictable and controlled manner. In the second place, once the base site failure has been detected by the network controller, a revised geographical coverage map can be brought into effect, possibly via the mobile radio data system.

5) Aiding traffic load balancing between base sites. The traffic load carried by each base site can be controlled by defining the position of each imaginary boundary so that the resulting cells match the geographical distribution of traffic. In

- 11 principle, this could even be done to adapt to dynamically varying loads.

6) Making it easier to introduce smaller cells in high traffic areas. When it is determined that a particular area has an exceptionally high radio traffic density, the solution is to provide multiple cells, each with a much smaller coverage area than is typical for the scheme as a whole. This applies equally whether the scheme was originally planned in this way or whether it is a modification of an existing scheme in response to traffic growth. In accordance with the present invention, this cell splitting can be accommodated by operating additional radio channels from an existing base site, sharing the same antenna system and hence the same physical coverage area. However, each radio channel would be given an appropriate virtual coverage area limited by imaginary boundaries which divide the existing cell to give the desired service level.

7) Reducing the traffic overhead caused by registration. Since a mobile unit knows which base site to use at anytime, the overhead of message dialogue between mobile units and base sites is reduced. Typically, a mobile unit need only send a single short message each time it changes base site, and no more overhead is required. Indeed, for some applications even this level of exchange is unnecessary. Of course, the registration overhead is also reduced because a mobile unit will make a

- 12better choice as to when to re-register and which new base site to use.