GB2359452A - Allocation of channels in a pico-cell system - Google Patents

Abstract

A pico-cell situated within a building 1 shares a number of traffic channels with the external maco-cells 2, 3, 4, 5, 6. Upon establishing a communication link with the pico-cell base station the interference present on the broadcast channels BCCHs 92, 94, 96, 98, 100 of the surrounding cells is measured by a mobile 13 inside the building and compared against a threshold. A traffic channel 102, 104, 106, 108, 110 associated with a cell whose interference level is considered to meet the threshold is assigned to the mobile station. If none of the of the cells are considered to meet the threshold the pico-cell's own BCCH 90 is assigned to the mobile. Carrier to interference ratio may be used to determine the interference.

Description

t,' 2359452 CELLULAR COMMUNICATIONS SYSTEM FOR BUILDINGS AND METHODS OF

CHANNEL ALLOCATION This invention relates to in-building cellular communications systems and particularly to means of channel allocation in such systems.

Cellular communications systems generally include a switch controller coupled to the public-switched telephone network (PSTN) and a plurality of base-stations. One or more mobile stations communicate, over communication channels, with a basestation that facilitates a call between the mobile station and the PSTN.

A particular type of cellular communications system, to which this invention is particularly applicable, is the GSM system (Global System for Mobile Communications), a description of which can be found in the book "The GSM System for Mobile Communications" by M. Mouly and M. Pautet.

GSM networks generally include mobile services switching centres, base-stations and mobile stations. Each of the plurality of base stations generally defines a geographic area or "cell" proximate to other base-station to produce coverage areas. Cell sizes can be as large as 70km. in diameter in rural areas or typically 200m in urban areas. As a mobile station moves from one cell to the next, the communication link is transferred from its current serving base-station to a neighbouring base-station using a procedure known as handover or handoff.

In GSM the total available frequency spectrum is divided up into a plurality of 20OkHz channels. For example, the GSM 900 frequency spectrum extends between 890MHz and 960MHz.

A single absolute radio frequency channel number (ARFCN) or RF carrier is actually a pair of frequencies, one used in each direction (transmit and receive) This allows information to be passed in both directions. For GSM 900 the paired frequencies are separated by 45MHz. For each cell in a GSM network, at least one ARFCN must be allocated and more may be allocated to provide greater capacity- 2 The use of multiple access techniques, eg TDMA (Time Division Multiple Access) permits the simultaneous transmission from several mobile stations to a single base-station on the same carrier frequency. The RF carrier in GSM can support up to eight TDMA time slots, ie in theory, each RF carrier is capable of supporting up to eight simultaneous telephone calls. Standard GSM has a total of 124 frequencies available for use in a network but most network providers are unlikely to be able to use all of these frequencies and are generally allocated a small sub-set of the 124.

As the maximum size of a cell is approximately 70km in diameter, the number of frequencies allocated may not be sufficient to cover the whole of the network provider's operating area. To overcome this limitation the network provider must re-use the same frequencies over and over again in what is termed a frequency re-use pattern. When planning the frequency re-use pattern, the network planner must take into account how often to use the same frequencies and determine how close together the cells are, otherwise co-channel and/or adjacent channel interference may occur. Co-channel interference occurs when RF carriers of the same frequency are transmitting in close proximity to each other and the transmission from one RF carrier interferes with the other RF carrier. Adjacent channel interference occurs when an RF source of a nearby frequency interferes with the RF carrier.

The communications channels of the GSM system are designated as either control channels or traffic channels. For example. control information is relayed from a base-station to mobile stations located in its cell over a broadcast control channel (BCCH). Speech and data information are carried between the base- station and the mobile stations by a plurality of traffic channels (TCH). Each mobile station is adapted to be able to monitor not only the BCCH transmitted by its serving base-station (in whose cell it is located) but also the BCCHs of neighbouring base-stations. These measurements can be reported back to the serving base-station and a value for C/I can be calculated. C/I is a measure of the relative interference 3 levels caused by the environment during a communication and thereby a measure of call quality. These levels are usually expressed as the ratio of the received signal level from the wanted source (carrier level C) to the interference received level (interference level I), or C/I and are expressed in dB. C/I values in a given cell depend on the locations of the mobile stations and on the locations of the interfering sources. Hence, they depend on cellular planning and on frequency re-use.

Capacity and coverage requirements of a GSM network can require the use of indoor cellular coverage. This can be provided by a so- called pico cells. For example, each floor of a multi-storey building may have its own cell. Served by an RF head (transceiver) located on each floor or by a distributed antenna system. This arrangement allows a mobile station to establish a call anywhere in a building and to maintain it whilst moving around the building or between floors. Inbuilding systems must be carefully balanced so that they can coexist with the outside "macro" cells.

In the known distributed antenna system, the RF transceivers are distributed throughout the whole building and linked to one base-station. The indoor system is configured as a unique cell with one BCCH carrier serving the whole coverage area. A gain in capacity can be achieved easily by increasing the number of RF transceivers in the building. In spite of these advantages, a drawback appears in the practical implementation which is the necessity of finding interferencefree carrier frequencies in all places where provided. For a satisfactory performance of distributed antenna system, it is necessary deployment phase that the frequencies inside coverage is an indoor to ensure in the the building are not being interfered with by the external network. This requirement is often difficult to accomplish especially when the number of available indoor carrier frequencies is restricted. This invention aims to provide a means for efficiently deploying a given spectral resource in an in-building cellular communications network.

4 In a first aspect, the present invention consists of a method for allocating a frequency channel to a call between a mobile station and a base-station in a cellular communications network, the network being configured to include an inner cell and a plurality of external cells surrounding the inner cell, each cell being configured to provide a broadcast control channel (BCCH) having a frequency unique to each cell, each external cell being configured to provide a traffic channel (TCH), and the inner cell being configured to provide a plurality of TCH's having frequencies in common with at least one of those provided by the external cells, in which the method includes the steps of; establishing a communications link between a mobile station located in the inner cell and a base-station-serving the inner cell, measuring interference levels of signals received at the mobile station located in the inner cell from at least one of the external cells, identifying external cells whose interference levels do not meet a pre-set criterion, and allocating the call to a channel not associated with the soidentified cells.

In a second aspect, the present invention consists of apparatus for allocating a frequency channel to a call between a mobile station and a base-station in a cellular communications network, the network being configured to include an inner cell and a plurality of external cells surrounding the inner cell, each cell being configured to provide a broadcast control channel (BCCH) having a frequency unique to each cell, each external cell being configured to provide a traffic channel (TCH), and the inner cell being configured to provide a plurality of TCH's having frequencies in common with at least one of those provided by the external cells, in which the apparatus includes; means for establishing a communications link between a mobile station located in the inner cell and a base station serving the inner cell, means for measuring interference levels of signals received at the mobile station located in the inner cell from at least one of the external cells, means for identifying external cells whose interference levels do not meet a pre-set criterion, and means for allocating the call to a channel not associated with the so-identified cells.

Thus, by virtue of the present invention, a call to be established in a cell inside a building can be assigned to a carrier which is free of interference (or at least subjected to an acceptably low level of interference) from carriers of the same frequency generated by the external (macro) cells.

In a first embodiment, the step of measuring the interference levels takes the form of a C/I measurement performed at a mobile station or at a base station. For example, the strengths of the BCCH signal received from the serving base station and the BCCH signals received from each base- station in each surrounding external cell are measured and reported to the serving base-station. At the serving base- station, a C/I value for each surrounding cell is computed and compared with a pre-set first threshold. Those below the threshold, ie showing a high interference level are discarded. From those remaining, the serving base-station allocates to the mobile station the TCH channel associated with one of the remaining external cells, preferably the one having the highest C/I value. If all C/I measurements are below the first threshold, then the serving base-station allocates to the mobile station its own BCCH for communication. This first embodiment can be used for allocating a new incoming call.

In a second embodiment, the invention can be further extended to identify an intra-cell handover candidate as follows. Either a mobile station located in the inner cell or its serving base-station is adapted to monitor the C/I values for neighbouring cells and when a potential source of interference is detected (by comparing the C/I value with a second threshold) a handover condition is reached and a new channel with better communications quality needs to be found.

6 This new channel is identified by means of the process described with reference to the first embodiment mentioned above. once identified the call is re-allocated to the new channel within the building.

The first threshold may have a single value for the network or it may be assigned different values for every carrier. The same applies to the second threshold.

The invention thus can provide dynamic allocation of frequencies for calls and, by the continuous monitoring of the interference levels, prevent the appearance of critical situations by changing the frequency assigned to any particular call.

The invention makes use of the standard process of reporting the signal strength of neighbours in order to evaluate the degree of interference at a particular external is causing to an indoor system. For this purpose, the potential external interfering cells are defined as neighbours.

Another function that the invention can provide is the ability to estimate the interference that a particular call can be suffering in order to predict quality degradation. The invention can be configured to use the signal strength values reported for the serving and neighbour cells and calculate the difference in signal level that the mobile station is receiving from both serving and interfering cells. A particular frequency can be considered suitable for use by the mobile station if the level from the interfering cell is weaker than the level from the indoor antenna within a certain protection margin.

The indoor system is configured as a single cell and the interference will come from different sources at the different locations within the building. It will affect the indoor traffic channels differently. The invention estimates interference in a particular frequency and assigns or reallocates the call to another frequency that has been previously verified as non-interfering. In this way, the call always stays within the indoor system on the best possible frequency depending on the location of the mobile station.

7 Some embodiments of the invention will now be described, by way of example only, with reference to the drawings of which; Figure 1 is a schematic diagram of cellular network configured in accordance with the present invention, and Figure 2 is a schematic diagram of the network of figure 1 illustrating carrier frequency coverage areas.

In figure 1, a building 1 is configured as one communications cell and is surrounded by five neighbouring cells 2, 3, 4, 5 and 6. Each cell is served by a respective base- station 7-12 and in the case of the building 1, multiple leaky feeders (not shown) from the base-station 1 are distributed throughout the building. The cell comprising the building 1 has a single BCCH 90 and each external cell 2-6 has its own unique BCCH designated BCCH 15 92, 94, 100, 98 and 96 respectively. Thus, there are six separate BCCH frequencies existing in the example of figure 1. Each external cell 2-6 also has its own unique TCH designated TCH 102, 104, 110, 108 and 106 respectively. The cell comprising the building 1 is configured to provide all five 20 traffic channels TCH 102, 104, 110, 108 and 106. Any mobile station 13 located anywhere in the building 1 can therefore receive one BCCH (90) and can be allocated any one of the five TCH's in accordance with the allocation procedure of the invention. The leaky feeders allow all five TCH frequencies and the BCCH 90 to be received anywhere in the building. In this embodiment, the base station 7 transmits its BCCH 90 carrier at a power level sufficient to provide good coverage inside the entire building 1 and transmits the 5 TCH carriers at a lower power compared with the BCCH 90 so that their signal level will not interfere with the external system comprising cells 2-6 with which it shares these common TCH frequencies. The external system will detect only the BCCH 90 from the building 1, but as the frequency of the BCCH 90 is unique to the network illustrated, it will not adversely affect the external system.

As TCH carrier frequencies are re-used in the building 1 from the external cells 2-6, channel allocation must be done in 8 a way so that adjacent channel interference and co-channel interference, to which a mobile station 13 communicating in the building is susceptible are minimised.

In order to manage call allocation in the indoor system in the building 1, each TCH carrier has those neighbour cells which contain the same TCH carrier frequency as itself are defined as potential interferes (for instance cell 4 will be a potential source of interference to TCH 110).

The decision to handle calls within the building 1 are based on measurements of C/I ratios.

In a first example, channel allocation for a new call is performed as follows. C/I measurements are made by comparing the signal strength of BCCH 90 received at the mobile station 13 from its serving base-station with each BCCH-frequency received at the mobile station 13 from each neighbouring cell which contains a potentially interfering TCH.

Each C/I value obtained is then compared with a predetermined first threshold and the cell associated with the highest C/I value above this threshold is identified. This cell will be the lowest potential interferer and so the TCH assigned to the call example. if is assigned If no is assianed is that contained in the identified cell. For BCCH 98 gives the highest C/I ratio, then the call to TCH 108.

C/I value exceeds the first threshold then the call to BCCH 90 (of the serving cell).

In a second example a handover or re-allocation of a call already in progress on a TCH to another carrier within the cell is performed as follows.

In order to assist an intra-cell handover process, C/I measurements are performed by comparing the signal strength of BCCH 90 received at the mobile station 13 with each BCCH received at mobile station 13 from all neighbouring cells which contain potentially interfering TCH's.

The C/I measurements are each compared with a second pre- set threshold. Cells associated with a C/I value which fall below this second threshold are identified as potential interferers. Of the remaining cells, the C/I measurements are 9 then compared with the first threshold and the TCH associated with the cell having the highest value above the threshold is identified as the best handover candidate carrier.

If all values fall below the first threshold (ie no TCH is suitable) then the best handover candidate is identified as the indoor BCCH 90.

Say, for example, that a call is currently being conducted on TCH 110. C/I values for neighbouring cells 26 are constantly being monitored at the base-station 7 and at some particular point during the call, the C/I values for cells 3 and 4 drop below the second threshold. This signifies that a source of interference exists which could result in a dropped call (particularly as cell 4 contains the same TCCH frequency as that in use). Therefore the base station 7 has to find a suitable handover candidate out of the remaining TCHs in common with cells 2, 5 and 6. It compares the C/I values for cells 2, 5 and 6 with the first threshold and re-allocates the call to the TCH in common with the cell having the highest C/I value above the first threshold. Intra- cell handover can also be performed for calls already existing on the BCCH 90. When a C/I value exceeding the first threshold is detected, the TCH associated with the relevant cell is identified as a handover candidate carrier and the call is re-allocated accordingly. 25 The above processes ensure that the mobile station 13 is assigned to a carrier frequency free of interference from the external cells 2-6. Handovers between radiation points where the main interfering cells change can be performed directly, when possible and only when no other non-BCCH (ie TCH carrier) is available will they take place through the BCCH. The calls are allocated when possible to TCHs in order to minimize the occupancy of the BCCH and to avoid congestion on this carrier. The BCCH is needed as a rescue layer and the resources that this carrier provides are usually limited.

A call can be transferred to a non-BCCH carrier as soon as the protection defined in the first threshold (which can be specified on a per carrier basis) is guaranteed for all the neighbours that may interfere with the frequency allocated in the carrier. In a similar way, a call will stay in a non-BCCH carrier until protection defined in the second threshold (which can be specified by per-carrier basis) is not satisfied for any of the neighbours that may interfere with the frequency allocated in the carrier.

For every location within the building, the BCCH works as shell, transmitting at a higher power than the TCH carriers.

only when and where the level of interference is acceptable is the TCH carrier used.

Figure 2 shows the coverage areas of each TCH contained within the inbuilding system. It can be seen that regions close to a particular external cell avoid using the TCH associated with that cell.

11

Claims (1)

1 A method for allocating a frequency channel to a call between a mobile station and a base-station in a cellular communications network, the network being configured to include an inner cell and a plurality of external cells surrounding the inner cell, each cell being configured to provide a broadcast control channel (BCCH) having a frequency unique to each cell, each external cell being configured to provide a traffic channel (TCH), and the inner cell being configured to provide a plurality of traffic channels having frequencies in common with at least one of those provided by the external cells, in which the method includes the steps of; (a) establishing a communications link betwe(n a mobile station located in the inner cell and a base-station serving the inner cell, (b) measuring interference levels of signals received at the mobile station located in the inner cell from at least one of the external cells, (c) identifying external cells whose interference levels do not meet a pre-set criterion, (d) and allocating the call to a channel not associated with the so-identified cells.

2 A method according to claim 1 in which the step of measuring interference levels includes the step of monitoring a C/I (carrier to interference) ratio.

3 A method according to claim 2 in which the step of identifying external cells includes comparing a C/I ratio with first threshold.

4 A method according to claim 3 in which the first threshold has a plurality of values, each value being peculiar to each traffic channel provided in each external cell.

12 A method according to any of claims 2-4 in which the step of allocating includes the step of allocating the call to a traffic channel associated with an external cell having the highest C/I ratio above the first threshold.

6 A method according to either of claims 3 or 4 in which the step of allocating includes the step of allocating the call to the inner cell's BCCH when no C/I ratios exceed the first threshold.

7 A method according to any of claims 1-4 including after step (b) and prior to step (c) the further steps of, (e) detecting external cells whose interference levels do not meet a further pre-set criterion, (f) and eliminating said detected cells from subsequent steps.

8 A method according to claim 7 in which the step of detecting includes comparing a C/I ratio with a second threshold.

9 A method according to claim 8 in which the second threshold has a plurality of values, each value being peculiar to each traffic channel provided in each external cell.

Apparatus for allocating a frequency channel to a call between a mobile station and a base-station in a cellular communications network, the network being configured to include an inner cell and a plurality of external cells surrounding the inner cell, each cell being configured to provide a broadcast control channel (BCCH) having a frequency unique to each cell, each external cell being configured to provide a traffic channel (TCH), and the inner cell being configured to provide a plurality of traffic channels having frequencies in common with at least one of those provided by the external cells, in which the apparatus includes;

13 means for establishing a communications link between a mobile station located in the inner cell and a base station serving the inner cell, means for measuring interference levels of signals received at the mobile station located in the inner cell from at least one of the external cells, means for identifying external cells whose interference levels do not meet a pre-set criterion, and means for allocating the call to a channel not associated with the soidentified cells.

11 A method for allocating a frequency channel substantially as herein before described with reference to the drawings.

12 Apparatus for allocating a frequency channel substantially as herein before described with reference to th drawings.