WO1992002104A1

WO1992002104A1 - Cellular radio

Info

Publication number: WO1992002104A1
Application number: PCT/GB1991/001240
Authority: WO
Inventors: Si Tak Stanley Chia
Original assignee: British Telecommunications Public Limited Company
Priority date: 1990-07-25
Filing date: 1991-07-25
Publication date: 1992-02-06
Also published as: GB9016331D0; AU8232391A

Abstract

A cellular radio system is described in which a plurality of base stations (1, 5) each having a radio transceiver, provide radio coverage over a respective radio coverage area. Identification transmitters (8) repeatedly transmit an identification signal over an identified radio coverage area (10) which is at least partially within one of the said radio coverage areas, so that a mobile station (6) having a radio transceiver can communicate with one of the base stations, and upon receipt of the identification signal the mobile station can be determination to be within the said identified radio coverage of the identification transmitter.

Description

CELLULAR RADIO

The present invention relates to cellular radio, and in particular, although not exclusively, to a cellular radio system having overlapping acrocells and microcells. A conventional cellular radio system has a number of radio base stations each serving a respective radio coverage area or cell, with which a mobile radio station can communicate over a radio link. As a mobile station moves from one cell to the next, the communication link is transferred from the present base station to the next base station using a procedure known as hand-over or hand-off. The need for hand-over is usually determined on the basis of one or more criteria. Commonly used criteria are received signal strength indication (RSSI) of the mobile at the base station, and the distance from the current base station as determined by the round trip time for signals to and from the mobile station. In addition, in digital systems, bit error rate (BER) may be used.

Taking RSSI as an example, a base station monitors the RSSI of the mobile station as the mobile station travels through a cell and when the RSSI falls below a certain threshold, hand-over of the radio link from the present base station to another base station with which the mobile station can have an improved radio link takes place. The cells of conventional cellular radio systems are relatively large, typically several kilometres across, and this allows time for data to be averaged for even a fast moving mobile station and a handover decision made on the basis of trends in that data. Recently there have been moves towards having cellular radio systems with relatively small cells of up to a few kilometres in diameter, and typically less than one kilometre in diameter, and these are often referred to as "microcells" while the relatively large, sometimes called wide area or conventional cells are often referred to as "macrocells". These terms do not indicate absolute size limitations but rather reflect the relative size of these two cell types. Microcellular radio systems should provide a better frequency re-use and hence greater user density. Proposals for microcellular systems suggest their application to road networks, for example motorways where high speed mobile stations will pass through a microcell very quickly, or to pedestrian precincts or in-building environments where there many be a high density of mobile stations. This means that the time for acquisition, let alone averaging of data, e. g. RSSI, for hand-over is limited. Furthermore, the radio coverage of microcells is subject to large variations in signal level over relatively short distances, in for example in an urban environment.

In combined macrocellular and microcellular systems, sometimes termed integrated cellular radio systems, the problems of determining the need for handover are further exacerbated. In such a system microcells might well only provide duplicate coverage over a small part of a macrocell. Thus, a number of different types of handover are possible: macro-macrocell handover between adjacent macrocells, macrocell to microcell handover when a mobile station enters a microcell coverage area within a macrocell, micro-microcell handover between adj cent microcells, and finally microcell to macrocell handover when a mobile station leaves a microcell coverage area. The last of these handover conditions typifies the problems in an integrated cellular radio system. A microcell can have quite abrupt boundary coverage compared to the relatively flexible boundary coverage of macrocells. Thus when, a mobile station leaves a microcell the need for handover to the macrocell can occur quite abruptly, and if not carried out successfully the radio link can be lost. The previously described criteria for handover decisions have limitations in this context. The present invention relates to the provision of other criteria for handover, particularly, though not exclusively, in microcellular systems. According to a first aspect the invention provides a cellular radio system for providing communication with a mobile station, comprising: first and second base stations, each for providing communication with the mobile station over respective first and second radio coverage areas; a plurality of identification transmitters within the first radio coverage area, each for transmitting an identification signal receivable by the mobile station within a separate identified coverage area; control means for monitoring progress of a mobile station through the first radio coverage area by assessing a sequence of at least two received identification signals, to determine any requirement for handover of communication with the mobile station from the first base station to the second base station.

In the present context, a cell served by a base station is defined as the area over which radio communications with a mobile is intended or expected to be maintained, i. e. the area beyond which handover will be required. Thus a cell boundary may coincide, in the limiting case, with the respective radio coverage area of a base station, but may, and probably usually will be within that radio coverage area. Bearing this in mind, the cellular radio system of the present invention can use the determined position of the mobile station as an indication of an immediate requirement for handover or as an indication of the impending requirement for handover, depending on the positioning of the location identifying transmitters relative to the cell boundary. The location identifying transmitters could for example be positioned at locations within the cell so that passage of a mobile station through a cell could be tracked, so that the need for handover can be anticipated.

The location identifying transmitters may be located so that receipt of the identification signal is indicative of the entry or exit of a mobile .station into or from a particular base station radio coverage area. For example a location identifying transmitter could be placed adjacent to or just at the intended limit of the cell, i. e. the cell boundary, to provide an indication of the imminent entry or exit, of a mobile station to or from that cell. Also, for example, the location identifying transmitters may be located so that receipt of the identification signal is indicative of a mobile station having entered a base station coverage area.

The identifiers can each have a unique identification code or some location identifying transmitters may share a common identification code. In the latter case receipt of the identification signal may be taken to indicate a particular type of impending occurrence, for example a particular type of impending handover, e. g. microcell to macrocell handover. In applications of cellular radio systems to road networks the common identification signals can be divided into categories, for example

(a) indicating a major traffic entry or exit point to a cell,

(b) indicating a minor traffic exit point from a cell,

(c) indicating an intermediate point within the given area,

(d) indicating the boundary between one given area and another given area.

Preferred embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: -

Figure 1 shows a cellular radio system with a number of overlapping macrocells and microcells overlaid on a road network;

Figure 2 shows the location of a microcell identifier (MI) with respect to a mobile station, with inset construction of a MI in block schematic form; Figure 3 shows a MI located at an entry or exit point of a microcellular network i. e. a primary node;

Figure 4 shows a minor road crossing a microcell and has two examples of each of a secondary and an intermediate node;

Figure 5 shows a boundary network of two microcells;

Figure 6 is a simplified flow schematic for the MI controlled handover from a macrocell to a microcell;

Figure 7 and Figure 8 show typical arrangements of microcells at major road junctions;

Figure 9 is a simplified flow schematic for the MI controlled handover from a microcell to a macrocell;

Figure 10 shows the arrangement of MI' s used in a queued handover system;

Figure 11 is a flow schematic for queued MI controlled handover from macrocell to microcell;

Figure 12 shows a typical microcellular network infrastructure;

Figure 1 shows a cellular radio system comprising a plurality of base stations (BS) (1,5) each having a radio transceiver for providing radio communication coverage over a respective radio coverage area (2, 3), which area may constitute a macrocell (2) or a microcell (3). These macrocells and microcells are overlaid on a road network (4). Referring now to figures 1, 2 and 3, identification transmitters or Microcell Identifiers (Mis) (8 - marked by an asterisk) are provided for repeatedly transmitting an identification signal over an identified radio coverage area (10) which is at least partially within one of the said radio coverage areas (2) or (3). In use a mobile station (MS) (6) having a radio transceiver and being able to communicate with one of the BS' s (1) and (5) can be determined to be within the identified radio coverage area (10) of one of the identification transmitters (8) upon receipt of that transmitters identification signal. Referring to figure 2, a radio Ml comprises a power supply (13) for powering the unit, a ROM (14), a modulator (15), a CPU (14a), RF driver (16a), and an antenna (16). Under the control of the CPU the ROM (14) is used to store a particular code sequence for the MI which is read by the modulator (15), and passed to the RF section (16a) which drives the antenna (16) to transmit this code sequence. The antenna (16) is a unity gain omni-directional monopole antenna.

A typical radio MI is mounted at a height of 6m, for example from a lamp-post (11), and would have a transmitter power of approximately 4.8 x 10^'12 , to provide, assuming a MS antenna (12) at a height of 1.5m, a receiving threshold for the MS of -120 dBm and an inverse 4^th power propagation law, a typical MI coverage distance of approximately 50m. For an average MS speed of 64 kph (17.78 ms^'1), a MI coverage area of 50m will give a mobile unit 2.8 seconds to detect the broadcast. The coverage area of the Mis can be extended by increasing the transmitter power according to the local vehicular traffic speed or geographical requirements.

In the first embodiment of the invention Mis are positioned within a microcell at points where the location of a MS passing through the microcell needs to be identified.

Microcells have very well defined boundaries and the flow of mobile stations within a microcell is generally not random, consequently a finite number of entry and exit points of a MS from or to a microcell can be identified. Specifically the position within the microcell of a MS when control of its communication link with a BS needs to be handed over for example from one microcell to another, from a microcell to a macrocell or vice versa needs to be identified.

These positions within the microcell can conveniently be categorised with reference to figure 1, into primary nodes (17), secondary nodes (18), intermediate nodes (19) and boundary nodes (20).

Primary nodes (17) are the points at which a MS will enter or exit the trailing (first or last) microcell of a microcellular network. There may be more than two primary nodes per microcellular network, if for example the network includes complex road junctions.

Referring to figure 4, points within the microcell, other than primary nodes (17), at which a MS may enter or leave the microcellular network for example side roads (21) are called secondary nodes (18). Intermediate MS' s (19) are positioned along the length of the microcell and particularly either side of secondary nodes. Boundary nodes (20) occur at the intersection (22) of two microcells, as shown in figure 5.

Each MI stores in ROM (14) one of 4 possible code sequences dependant on its position. MI' s at primary nodes transmit entry/exit code sequences which are unique, within the macrocell, to each primary node. Intermediate code sequences transmitted by intermediate MS' s are the same for all intermediate MI' s within the same microcell but are unique to each microcell within the macrocell. Mis at secondary nodes transmit an exit code which is common to all microcells within the macrocell. The fourth type of code sequence is transmitted by a MI at a boundary node. These codes are unique, within the microcell to each boundary node.

MS' s will continuously monitor transmissions from MI' s and will retransmit a version of the received code sequence to the BS' s which will take the appropriate handover (HO) decision.

There are four possible handover scenarios which may occur in a micro/macro cellular communications systems. We will now describe how the present embodiment allows handover decisions to be taken rapidly and reliably in each of these scenarios. The MI - initiated handover in all cases is performed irrespective of the signal levels or the bit error rate performance and allows the target cell and the moment when transfer will occur to be rapidly, reliable and simply identified.

1. Macrocell - Macrocell handover

This occurs as normal without the involvement of Mis in figure 5, for example by received signal strength indication (RSSI), BER or, distance measurement. This type of HO does not need to be MI - initiated since cells are larger and therefore decision and measurement times can be longer.

2. Macrocell to microcell Handover

There are two entry points from a macrocell to a microcell, via a primary node or via a secondary node. In both cases if the macro/micro cellular system is to work most efficiently HO from the macrocell to the microcell should occur so as to reserve spare, capacity for the Macrocell BS to communicate with MSs in the macrocell beyond the coverage of microcells. However if a MS is merely crossing a microcell for example on a minor road (21) (see figure 4) HO from macro to microcell immediately following by HO from micro to Macrocell should be avoided, as this places an excessive switching overhead on the system.

Dealing with entrance to a microcell via a primary node first, and referring to figure 3. As the MS (6) approaches the primary node (17) it is continuously monitoring for transmissions from MI' s, upon receipt of the primary node code sequence the MS retransmits this to the macrocell base station. Since this code is unique within the macrocell the BS is rapidly able to identify both the MS position and the target microcell and requests HO to the appropriate microcell BS (5). If a free channel is available within the microcell, HO is effected, if not no HO occurs until the next MI is encountered. This procedure is shown in figure 6 in a simplified flow schematic. If a MS enters a microcell via a secondary node (see figure 4) it will, on passing the secondary node (18), receive an exit code sequence, however no action will be taken and HO will not occur unless the MS subsequently receives an intermediate code sequence at an intermediate node (19). Hence if the MS simply crosses the microcell it will remain under the control of the Macrocell BS, whereas if it enters and progresses along the microcell it will be handed over to the microcell BS. Intermediate Mis also serve to initiate HO from the macrocell to the microcell BS of a MS which switches on only when it is well within the microcell.

3. Micro-Cell to Mi cro-cell HO

In passing from one microcell to another a MS (24), in figure 5, will pass a boundary MI (20) from which it will receive a boundary code sequence. The current microcell BS (23), from the boundary code sequence, will identify the microcell the MS is about to enter and will offer HO to the target BS (24) in the way previously described. This microcell to microcell HO process occurs without involving the macrocell BS. A microcell may have more than two boundary nodes as shown in figures 7 and 8.

4. Microcell to Macrocell HO

There are two possible exit points from a microcell to a macrocell, via a primary node or via a secondary node. If a MS is currently in a microcell the detection of a primary node code sequence will be identified by the microcell BS as signifying an exit to a macrocell, and the microcell BS will request HO to the macrocell BS in the way shown schematically in figure 9.

As soon as a MS leaves the microcell via a secondary node (18) it will receive an exit code sequence from the secondary node MI, see figure 4, and the microcell BS will rapidly be able to request HO to the macrocell BS.

This is an important advantage of using MI' s since generally radio signal levels drop off rapidly when a MS leaves a microcell via a secondary node (e. g. side road) allowing very little time for any measurement of, for example, RSSI, BER, distance etc..

In a second alternative embodiment of the invention there is provided a cellular radio system comprising a queued MI controlled HO process. This system is substantially as previously described, except that at primary, secondary and boundary nodes, two, rather than one, Mis are provided separated by a small physical distance typically 1 to 20m as shown in figure 10. The position of the first MI (25) or (26) encountered gives the point within the microcell at which the HO process is to be started. The position of the second MI (26) or (25) encountered marks the absolute limit of the microcell.

Thus, in use when a MS encounters the first MI, say (25), a handover request will be initiated. If no channel is immediately available, the base station will continue to attempt the handover until the second MI (26) is encountered where a forced termination of the call or an emergency handover to a macrocell base station will be effected. A flow diagram for this process is shown in Figure 11. If the MS is still travelling along a microcellular cluster, the macrocell base station will attempt to re-direct the call back to the next available microcell base station when the MS encounters an intermediate MI in that microcell.

Mis are preferably radio transmitters, but could alternatively be infra-red or ultra-sound transmitters or induction loops or piezo-electric transducers, the latter two conveniently being buried beneath the road surface.

As Mis are very low power transmitters they can be driven by mains power supply, backed up with storage batteries. A charging device for the Ml could derive its power from street lighting power supplies either by direct connections or by an induction coil. Alternatively, solar cells may be used. In addition to their application to road networks, MI' s can usefully be used in pedestrian precincts or in buildings. They are particularly useful for indoor microcell environments as these environments have very well defined exits and entrances to specific areas. For instance, a building will have a well defined entrance to the lobby. From the lobby, there will be a defined entrance to lifts and corridors. Rooms along the corridors have doors at their entrances. Thus if a microcellular system is to be deployed within a building, cells will be implemented at specific areas and the microcell identifiers are ideal for assisting the handover process without ambiguity. Microcell identifiers may be installed at all doorways or entrances where handovers are desired. As mobile units are moving at slow speed, the criterion of fast handover initiation may not be as critical as in an outdoor area. Furthermore, there will be sufficient time for the mobile unit to listen to the broadcast of the microcell identifiers. Shielding from obstacles will not be a problem as the microcell identifiers could be installed overhead or at prominent positions along the side of a corridor.

The preferred embodiment of the claimed cellular radio system has been described thus far with reference only to the micro and macro cellular structure. It will of course be realised that the system also requires a supporting infrastructure, and by way of example, one such infrastructure will now be described.

Conventionally, clusters of macrocell base stations (1) are under the control of a mobile switching centre (27) which is connected to the public switched telephone network (PSTN). Likewise, clusters of microcell BSs (5) can also be linked together by a microcell control unit (28). The distribution of these microcell control units may be as few as one per macrocell, as presented in Figure 13. Calls originated from a microcell base station (5) will be routed through a microcell control unit (28) to the PSTN directly without involving a mobile switching centre (27). This will reduce the loading of a mobile switching centre and its associated communication network. To be precise, a mobile control unit should have the same function as a mobile switching centre (27) except that the former serves* microcells (3) while the latter serves macrocells (2). For the convenience of handover and assignment of microcells to a microcell control unit, the boundary of the last microcell within an overlaying macrocell should coincide with the boundary of that macrocell.

In use when a MS is within a macrocell beyond any microcell, the file for the MS is usually kept within the mobile switching centre and the macrocell base station.

However, when a MS enters a macrocell which contains microcells, the file for the MS is still initially kept within the mobile switching centre and the macrocell base station. A copy of the file is also sent to the microcell control unit. As the MS enters a microcell, the MS will be de-registered from the mobile switching centre and re-registered with the microcell base station.

Registration information is, however, retained by the macrocell BS until the MS proceeds to a microcell located beyond the current macrocell. When the MS proceeds directly from a series of microcells in one macrocell to another series of microcells in the next macrocell, the file of the MS will be routed through to the next microcell base station via the links between the two microcell control units. The file for this MS which is kept within the current macrocell base station will also be transferred to the new macrocell base station.

It will be understood that the current invention may be implemented under many different transmission protocols and modulation methods. The transmission of the code sequence could be organised on a signalling channel using frequency division multiple access (FDMA) mode. In this case the carrier carrying the code sequence should be continuously transmitted without interruption in order to guarantee the reception of the broadcast by the MS. The modulation can be simple non-coherent frequency shift keying, for example. Alternatively the invention may be used within a digital cellular system operating with time division multiple access (TDMA) such as the Group Special Mobile (GSM) system. In this case the code sequence broadcast by MI' s could conveniently be, for example, 8 bit long digital code words. The MS could monitor for MI broadcasts during a currently idle time slot within the TDMA time frame, for example between transmit and receive time slots. After the code sequence has been demodulated by the MS the information can be conveyed back to the serving BS via the Slow Associated Control Channel. In this implementation of the current invention the receiver within the MS must be programmed to tune to a MI Broadcast Channel after tuning to the Broadcast Control Channel carrier.

Although microcell identifiers are well suited for TDMA digital cellular radio systems, they may also be used in analogue cellular radio systems (e. g. TACS). A simple approach is to implement a second simple receiver (microcell identifier receiver) incorporated into the existing mobile unit. The front end can be shared with the existing transceiver unit. With suitable decoding logic in the microcell identifier receiver, the necessity of handover can be signalled back to the base station via in-band supervisory signalling which is required for maintaining the communication link. Naturally the signalling protocol within the base station has to be modified to interpret the information returned by the mobile unit.

An alternative approach is to force the mobile unit to periodically switch over to the specific frequency to listen to the microcell identifier broadcast. Interruption of conversation may be necessary but this will only be a few milliseconds and will not cause any perceivable impairment to the speech. During this period, the frequency synthesizer re-tunes to a specific channel for the microcell identifier broadcast, decodes the message, and then re-tunes back to the original channel. If a handover message is detected, the information can be transferred back to the base station via in-band signalling. Similar modification of the signalling protocol is also necessary.

Obviously alternative modulation and transmission implementation of the invention are possible.

Claims

CLAI MS .

1. A cellular radio system for providing communication with a mobile station, comprising: first and second base stations, each for providing communication with the mobile station over respective first and second radio coverage areas; a plurality of identification transmitters within the first radio coverage area, each for transmitting an identification signal receivable by the mobile station within a separate identified coverage area; control means for monitoring progress of a mobile station through the first radio coverage area by assessing a sequence of at least two received identification signals, to determine any requirement for handover of communication with the mobile station from the first base station to the second base station.

2. A cellular radio system as claimed in claim 1 wherein the first radio coverage area is a relatively small radio coverage area hereinafter a microcell; and the second radio coverage area is a relatively large coverage area, hereinafter a macrocell.

3. A cellular radio system as claimed in claim 2 wherein there is: a third base station for providing communication with the mobile station over a third relatively small radio coverage area, hereinafter microcell; and the control means is for determining any requirement for handover of communication with the mobile station from the first base station to either the second base station or the third base station.

4. A cellular radio system as claimed in claim 3 wherein the two microcells are within the macrocell.

5. A cellular radio system as claimed in claim 4 wherein the control means has means for assessing whether a sequence of received identification signals is of a first class indicative of a requirement for handover of communication with the mobile station from the first base station to the second base or a second class indicative of a requirement for handover of communication with the mobile station form the first base station to the third base station.

6. A cellular radio system as claimed in any preceding claim wherein the identification transmitter transmits unique identification signals.

7. A cellular radio system as claimed in claims 4 to 5 wherein at least some of the identification transmitters transmit the same identification signal.

8. As claimed in claim 7 wherein the identification signals are indicative of the location of the identification transmitters within the given area.

9. As claimed in claim 7 or 8 wherein the identification signal is selected from a plurality of categories.

10. As claimed in claim 9 wherein said categories include one or more of the following:

(a)indicating a major traffic exit or entry point, (b)indicating a minor traffic exit point, (c)indicating an intermediate point within the given radio coverage area,

(d)indicating the boundary between the said given area and another given radio coverage area.

11. A mixed cell cellular radio system for providing a radio communications link between a base station a mobile station, comprising a first base station able to provide radio coverage over a relatively large cell, hereinafter called a macrocell, and a second base station able to provide radio coverage over a relatively small cell, hereinafter called a microcell, at least partly co¬ extensive with the macrocell, characterised by a plurality of classes of identification transmitters each class for repeatedly transmitting an identification signal over a different part of the microcell, so that in use the receipt of each class of identification signal by the mobile station is used singularly or in combination to assess any requirement for handover of communication with the mobile station.

12. A system as claimed in claim 11 wherein a criterion for the requirement of handover is that any communication link with the mobile station is preferentially formed with the second, rather than the first, base station.

13. A system as claimed in claim 11 wherein each class of identification transmitter is associated with a different control procedure for assessing any requirement for handover of communication with the mobile station.

14. An integrated cellular radio system for providing a radio communications link between a base station and a mobile station, comprising a first base station able to provide radio coverage over a relatively large cell, hereinafter called a macrocell, and a second base station able to provide radio coverage over a relatively small cell, hereinafter called a microcell, at least partly co¬ extensive with the macrocell, characterised by a plurality of identification transmitters each for repeatedly transmitting an identification signal over a different part of the microcell, so that m use the receipt of more than one identification signal by the mobile station is used to identify the location and direction of motion of the mobile station within the mixed cell system so that the need for handover of the communications link from the first base station to the second, or from the second to the first, can be assessed.

15. A method of providing communication with a mobile station within a cellular radio system, the method comprising the steps of: providing communication with the mobile station over first and second radio coverage areas, served by respective first and second base stations; transmitting a plurality of identification signals within the first radio coverage area, each receivable by the mobile station within a separate identified coverage area; and assessing a sequence of at least two received identification signals, and thereby monitoring the progress of a mobile station through the first radio coverage area and determining any requirement for handover of communication with the mobile station from the first base station to the second base station.