GB2347584A - Passive signal distribution system for indoor radio telephony - Google Patents

Abstract

A radio transceiving arrangement for a radio communications system comprises a plurality of antennas A<SB>1</SB>-A<SB>2N</SB>, each having a different coverage area, at least one RF transmitter, at least a first and a second RF receiver from which uplink signals are combined using an intelligent signal combining device, and a passive signal distribution system arranged to deliver downlink signals to the antennas and to deliver uplink signals from the plurality of antennas to the receivers. The passive distribution system includes a plurality of passive combining stages 100 in the downlink signal path and a plurality of passive combining stages 106 in the uplink signal path, the system including at least one less passive combining stage in the uplink path than in the downlink path. The antennas may be split into two sets A<SB>1</SB>-A<SB>N</SB>, A<SB>N+1</SB>-A<SB>2N</SB> wherein signals received by the first set of antennas are sent to the first input of the combining device and those from the second set are sent to the second, diversity input of the combining device.

Description

Radio Transceiving Arrangement This invention relates to a radio transceiving arrangement for a radio communications system, in particular but not exclusively to one including a passive signal distribution system for distributing radio frequency signals to and from multiple antenna locations distributed within an indoor environment.

In a conventional cellular communications network, such as a GSM (Global System for Mobile Communications) network, the network coverage area is split into cells in which mobile stations are serviced. Each cell has at least one base transceiver station (BTS) connected to at least one antenna from which radio frequency (RF) signals are transmitted as downlink signals to mobile stations receiving service in the cell, and at which uplink RF signals are received from those mobile stations.

A BTS includes an RF transmitter feeding RF signals to a transmit antenna and a normal RF receiver receiving RF signals from a receive antenna, which is often the same antenna as used for transmission.

A second RF receiver, referred to as a"diversity receiver"may be provided in the BTS and connected to a second receive antenna. The first receive antenna and the second receive antenna have the same coverage area but may have different polarizations or may be separated by a distance sufficient to achieve space diversity between the uplink signals received at the two receive antennae respectively. Fading effects are thus experienced differently at the two receive antennae and the uplink signals respectively received at normal RF receiver and the diversity receiver are combined using an intelligent combiner, commonly referred to as a"diversity combiner". The diversity combiner may use one of a number of intelligent signal combining techniques, such as selection combining (e. g. dynamically selecting the stronger of the two signals received at the different receivers) or maximal ratio combining (e. g. adding the two signals with weighting factors depending on a detected bit error rate in each signal), under the control of a signal processor monitoring the signals received at each of the receive antennae. The effect is to achieve a higher gain in the uplink at the BTS.

In some cellular environments, especially indoor environments, it is difficult to achieve sufficient coverage throughout a cell using a single antenna location. In such cases, it is common to use multiple antenna locations, distributed within the total coverage area of the cell. A signal distribution system is used to distribute the uplink and downlink RF signals to/from the BTS from/to the multiple antenna locations. The distribution system splits the downlink signals delivered from the BTS transmitter and feeds them to each of the antenna locations, and combines the uplink signals from each of the antenna locations and delivers the signals to the BTS receiver.

Signal distribution systems may be either active or passive. In an active distribution system, the signals may be amplified at different points in the distribution system. However, the amplification stage or stages can introduce interference problems. Such interference problems are particularly severe when more than one network operator uses the same signal distribution system to convey the uplink and downlink signals to and from their respective BTSs from and to antenna locations distributed throughout a shared communication cell.

Passive distribution systems on the other hand introduce an unavoidable path loss at each combining and splitting stage, but by using a purely passive distribution system, the risk of interference problems can be minimised.

A problem with purely passive distribution systems is therefore that there is a limit to the number of combining and splitting stages and therefore antenna locations which may be used within a single cell. This is particularly the case in relatively large cellular environments, such as large buildings, in which the signal must be distributed, commonly using coaxial cables, over relatively large distances from the BTS to the antenna locations.

One way to overcome the limits in the number of antenna locations and size of a cell using a passive signal distribution system is to split the cell into sectors, each sector being serviced by a different BTS. However, the use of additional BTSs increases costs and complexity.

A further difficulty with conventional purely passive signal distribution systems is that a scheme using multiple antennae at a single antenna location to implement diversity reception is generally not efficient to implement, and therefore not used. This leads to a loss of diversity gain in the uplink. In order to maintain a balanced link budget, the BTS downlink power must be reduced.

This reduces the coverage that could otherwise be achieved using a purely passive distribution system if the full downlink power were available.

In accordance with the present invention there is provided a radio transceiving arrangement for a radio communications system, said arrangement comprising a plurality of antennae, each having a different coverage area, at least one RF transmitter, at least a first and a second RF receiver from which uplink signals are combined using an intelligent signal combining device, and a passive signal distribution system arranged to deliver downlink signals to each of said antennae from said transmitter, to deliver uplink signals from only selected ones of said plurality of antennae to said first receiver and from other ones of said plurality of antennae to said second receiver.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 illustrates selected elements of a conventional BTS; Fig. 2 illustrates a conventional cell site equipment arrangement including four BTS units and a passive signal distribution system; Fig. 3 illustrates a cell site equipment arrangement used in accordance with an embodiment of the invention; Fig. 4 illustrates cell site equipment in accordance with an embodiment of the invention, installed in an in-building environment ; and Fig. 5 illustrates a signal mixing arrangement used in an embodiment of the invention whereby a common distribution system may be used by two independent network operators.

Referring now to Fig. 1, selected elements of a conventional GSM BTS 10 are illustrated. The BTS 10 includes an RF transmitter 12 providing a plurality of downlink channels from a transmit output port 14. The transmit output port 14 supplies RF signals to one or more transmitter antennae.

The BTS 10 also includes two RF receivers 16,18, which are substantially identical in structure and functionality, receiving a plurality of uplink channels input via receiving input ports 20 and 22. The first input port 20 is referred to herein as a"normal receive port", and the second receive port 22 is referred to herein as a"diversity receive port", in accordance with conventional terminology referring to a BTS with an in-built intelligent combiner, such as the diversity combiner 24.

In prior art arrangements where a single antenna, or a passive signal distribution system, is used, only the normal receive port 20 is used, and the diversity receive port 22 is unconnected. However, in prior art arrangements where two antennae are used at the same antenna location in order to achieve signal diversity, such as space diversity or polarization diversity, the uplink RF signal from each of the two antennae is input into each of the normal receive port 20 and the diversity receive port 22, respectively, and the signals output from the two receivers 16,18 are combined to improve the received uplink signal strength in the diversity combiner 24. The diversity combiner 24 is conventionally referred to as"diversity combiner"and may use any of a number of known diversity combining techniques. These techniques include selection combining, maximal ratio combining and equal gain combining, although various other diversity combining techniques are known in the art.

Fig. 2 illustrates a prior art arrangement of cell site equipment used to arrange a plurality of BTSs to service a single coverage area (i. e. a cell). The arrangement uses a purely passive signal distribution system to provide indoor coverge. The signal distribution system distributes RF signals to/from multiple antennae Al-A2N from/to an arrangement of one or more BTS units 10, in this example four BTS units 10. Where more than one BTS unit is used at the cell site, as in this example, the different BTSs may belong to a single network operator, or different network operators, in this example any of two to four network operators.

A 4-way combiner 30 collects the RF signals output from each transmit port 14 of the four BTSs 10, combines the signals and delivers along a combined downlink feed path to a duplexer 32. From the duplexer 32, combined uplink and downlink signals are transmitted along a common feed path to a 2N-way splitter/combiner arrangement 34, which splits the downlink signals equally between the 2N antennae Al-A2N.

The uplink signals received from the mobile stations receiving service in the cell, exemplified here by mobile stations MSl and MS2, at one or more of the antennae A,-A2N are combined in the splitter/combiner arrangement 34, delivered along a common feed path to the duplexer 32 and to a four-way splitter 36, which divides the received signals evenly between each of the four BTS units 10. The signals received from the splitter 36 are input at each of the normal receive ports 20 of the BTS units 10. Although each of the BTS units 10 is provided with a diversity receive port 22, these diversity receive ports are unused.

Referring now to Fig. 3, an embodiment of the invention is illustrated in which, similar to the arrangement of Fig. 2, four conventional GSM BTS units 10, identical to those described in relation to Fig. 1, are utilised which deliver and receive RF signals from each of 2N antennae Al-A2N- The antennae A,-A2N is located at various different antenna locations within the coverage area of the cell serviced by the four BTS units 10.

Although the arrangement may be used in an outdoor environment, it is likely that the arrangement of the invention will be most useful in indoor environments. For example, the antenna locations may be respectively located at different floors of a single building, or at regularly spaced intervals along a vehicular tunnel, these being examples of indoor environments where acceptable levels of coverage are difficult to achieve using a single antenna location.

The purely passive distribution system used in the embodiment of the invention illustrated in Fig. 3 will be described below.

The downlink signals taken from each transmit port 14 of each of the four BTSs 10 are delivered to a four-way combiner 100, and the combined signals are delivered along a common feed path to a two-way splitter 102, having outputs connected to a first duplexer 104 and a second duplexer 108.

The downlink signals from the first duplexer are delivered to a first N-way splitter/combiner 106, which splits the downlink signals received from duplexer 104 between approximately half, in this embodiment exactly half, of the total antennae servicing the cell, antennae Al-AN. The signals received at duplexer 108 are passed along a common feed path to a second N-way splitter/combiner 110, which splits the downlink signals received from duplexer 108 between the remaining approximate half, in this embodiment exactly half, of the antennae servicing the cell, antennae AN+I-A2N As will be appreciated, since splitter 102 splits the combined downlink signals equally between its two output ports, and the remaining parts of the distribution system perform similar equal splitting of the signal path, the downlink signals transmitted from each of the antennae A,-A2N are substantially identical, both in power and in content.

Uplink signals received at any or each of the antennae A,-AN are combined in the N-way splitter/combiner 106 and delivered, via duplexer 104, to a first four-way splitter 112, which divides the received signal equally between four output ports, each respectively connected to a normal receive port 20 of the four BTS units 10.

On the other hand, the uplink signals received at any or all of the antennae AN+,-A2N are delivered, via duplexer 108, to a second four-way splitter 114. The combined signal received at splitter 114 is divided equally between each of its four output ports, which are each respectively connected to the diversity receive ports 22 of the four BTS units 10.

It is to be understood that the antennae Al-AN are preferably located within a first common area, making up approximately one half of the entire cell coverage area, which is separate from a second common coverage area, making up approximately the other half of the cell coverage area, in which antennae ANIS-AN are located.

With this arrangement, a mobile station located within the first common area will transmit uplink signals which are received mainly by one or more of the antennae A,-AN. Hence, the uplink signals from mobile station MS, signals will be delivered primarily to the normal receive ports 20 of each of the BTS units 10. Conversely, a mobile station MS2 located within the second common area will transmit uplink signals which are received mainly by one or more of the antennae AN+I-A2N. Hence, the uplink signals received from mobile station MS2 will be delivered primarily to the diversity receive ports 22 of each of the BTS units 10.

The signal distribution arrangement in this embodiment of the invention uses one less passive combining stage in the uplink signal feed path than that of the prior art arrangement illustrated in Fig. 2. This reduces the path loss in the uplink signal path by at least 3dB. The intelligent combiner within each of the BTS units 10, using an intelligent (diversity) combining technique (such as selection combining), will predominantly favour the signal received on one of the receive ports, for each uplink channel respectively, since the signal for a given uplink channel will generally be received at a far higher strength on one of the normal receive port or the diversity receive port (depending on the location of the mobile station within the cell) than the other.

On the other hand, all mobiles within the cell receive all downlink signals originating from the same transmitters, and may therefore monitor all common control channels broadcast by those transmitters independent of their location within the cell.

Fig. 4 illustrates, by means of a further embodiment of the invention, how the antennae connected to a signal distribution system are preferably located at antenna locations distributed throughout the cell coverage area, and the separation of the antennae in two separate common areas within the cell.

In this embodiment, a passive signal distribution system is used to distribute signals to antennae Al-AS each respectively located on different floors of an 8-storey building. Each of antennae A,-A4 are located within the four upper floors of the building, and deliver uplink signals, via the signal distribution system, to the normal receive port of a single GSM BTS unit 10.

Each of the antennae A-A8 are located in the four lower floors of the building, and deliver uplink signals to the diversity receive ports 22 of the BTS unit 10.

As shown in Fig. 4, each of the N-way splitter/combiners 106 and 110 described in relation to the arrangement of Fig. 3 may in practice be implemented using a hierarchical network of passive two-way splitter/combiners 200, in the form of 3dB hybrid couplers or other suitable types of passive coupler, connected by sections of coaxial cable. This embodiment also includes, iri a similar fashion to the embodiment of Fig. 3, a passive two-way splitter 202, which may in practice be a 3dB hybrid coupler or a filter coupler, and two duplexers 204 and 206.

The uplink signals from an exemplary mobile station MS ; located on the seventh floor of the building are received primarily at antenna A2, although lower power signals may also be received at other antennae such as antennae A, and A3. These uplink signals from mobile station MS, are delivered, via the upper hierarchical network of splitter/combiners 200 and the duplexer 204 to the normal receive port of the BTS unit 10. On the other hand, the uplink signals from an exemplary mobile station MS2 located on the second floor of the building are received primarily at antenna A7, although lower power signals may be received at other antennae such as antennae A6 and A8. These uplink signals from mobile station MS2 are delivered, via the lower hierarchical network of splitter/couplers 200 and duplexer 206, to the diversity receive port 22 of the BTS unit 10.

The downlink signals received by each of the mobile stations MS, and MS2 (and other mobile stations receiving service in the cell within the building) each originate from the single transmit port 14 of the BTS unit 10, since the two-way splitter 202 splits the downlink signals equally between the four-way splitter/combiner arrangement located in the upper half of the building and the four-way splitter/combiner arrangement located in the lower half of the building.

In the embodiment of Fig. 4, only a single BTS unit 10 is illustrated, although it will be appreciated that other numbers of BTS units 10 may be used, by means of an arrangement similar to that shown in Fig. 3.

Fig. 5 illustrates a portion of a signal distribution system used in accordance with a further embodiment of the invention. In this embodiment of the invention, RF signals are distributed to and from BTS units operated by two or more different network operators sharing parts of a passive signal distribution system and a common set of antennae A,'-A4N'-In this embodiment, one or more network operators operate a GSM1800 network (using frequencies of approximately 1800 MHz) and one or more network operators operate a GSM900 network (using frequencies of approximately 900 MHz).

As illustrated in Fig. 5, on the antenna side the signal feed paths between the BTS units of the different network operators and the 4N antennae are split using 3dB hybrid junctions Jl-J2N. On the BTS side, 2N inputs are provided along lines LI-L2N from the BTS equipment of each operator using an arrangement similar to that illustrated in Fig. 3, except that the inputs from the N-way splitter/combiners 106 and 110 are delivered to the input lines L ;-L instead of directly to antennae A,-A2N-Using such an arrangement, the uplink signals received at antennae A,'-A2N'are delivered to the normal receive ports of one or more BTS units 10 of each of the network operators, whereas the uplink signals received at antennae A2N+I'-A4N'are delivered to the diversity receive ports of the one or more BTS units 10 of each network operator.

Advantages of the present invention will be apparent from the above. In a passive signal distribution system, each passive combining stage introduces a loss of at least three dB and by use of the present invention, a significantly reduced uplink path loss is achieved. This allows the downlink transmit power used in each of the BTSs 10 to be increased, relative to that which may be used in the arrangement of Fig. 2, whilst maintaining a balanced link budget. As a result, the number of antenna locations and/or the distance over which the distribution system delivers RF signals may be increased. This is particularly useful in relatively large indoor environments, such as multi-floor buildings (having from 8 to 50 floors and above) and relatively long indoor tunnels.

It is to be appreciated that the present invention is not limited to arrangements in which a two-way intelligent combiner is in-built in a BTS unit 10. Such an in-built two-way intelligent combiner is preferred, since such many commercially-available BTS units include such a device. Where the BTS unit 10 used is not provided with an in-built intelligent combiner, such an intelligent combiner may be provided separate from the BTS unit 10. Furthermore, the intelligent combiner need not necessarily be a two-way intelligent combiner. For example, a four-way intelligent combiner may be used, in which case the signal distribution system may be arranged such that collected from antennae situated in four separate common areas within the cell coverage are respectively delivered to separate ones of the intelligent combiner input ports.

Furthermore, the invention is not limited to purely passive signal distribution systems. Where the risk of interference is relatively low, one or more amplification stages may be included in the signal distribution system.

Although the above description relates to a GSM cellular communications system, it will be appreciated that the invention may be applied to other cellular communications systems, such as CDMA, W-CDMA, and various third-generation cellular communications systems.

Moreover, the invention is not limited to use in cellular communication systems, but may be used in other radio access systems, such as fixed radio access networks and wireless LAN and wireless WAN systems such as the next generation HIPERLAN 1, HIPERLAN 2 and HIPERACCESS systems.

It is envisaged that further modifications and variations may be employed by the skilled person in relation to the embodiments described above, without departing from the scope of the present invention.

Claims (11)

CLAIMS:

1. A radio transceiving arrangement for a radio communications system, said arrangement comprising a plurality of antennae, each having a different coverage area, at least one RF transmitter, at least a first and a second RF receiver from which uplink signals are combined using an intelligent signal combining device, and a passive signal distribution system arranged to deliver downlink signals to each of said antennae from said transmitter, to deliver uplink signals from only selected ones of said plurality of antennae to said first receiver and from other ones of said plurality of antennae to said second receiver.

2. An arrangement according to claim 1, wherein said selected ones are arranged to have a first combined coverage area, said other ones are arranged to have a second combined coverage area, and said first and second combined coverage areas are generally non-overlapping.

3. An arrangement according to claim 2, wherein said first and second coverage areas are generally separate.

4. An arrangement according to claim 2 or 3, wherein said first coverage area includes a plurality of first internal spaces in a construction and said second coverage area includes a plurality of second internal spaces in said construction.

5. An arrangement according to claim 4, wherein said first and second internal spaces are floors of a building.

6. An arrangement according to any preceding claim, wherein said intelligent combiner is in-built in a base station unit, and said first receiver is a "normal"receiver and said second receiver is a"diversity"receiver on said base station unit.

7. An arrangement according to any preceding claim, wherein said transmitter transmits control signals on a common channel, and said passive distribution system delivers said control signals to each of said plurality of antennas.

8. An arrangement according to any preceding claim, wherein said passive distribution system includes a plurality of passive combining stages in the downlink signal path and a plurality of passive combining stages in the uplink signal path, the system including at least one less passive combining stage in the uplink path than in the downlink path.

9. An arrangement according to any preceding claim, wherein said passive signal distribution arrangement is purely passive.

10. A radio transceiving arrangement for a radio communications system, said arrangement comprising a plurality of antennae, each having a different coverage area, at least one RF transmitter, at least a first and a second RF receiver from which uplink signals are combined using an intelligent signal combining device, and a passive signal distribution system arranged to deliver downlink signals to said antennae and to deliver uplink signals from said plurality of antennae to said receivers, wherein said passive distribution system includes a plurality of passive combining stages in the downlink signal path and a plurality of passive combining stages in the uplink signal path, the system including at least one less passive combining stage in the uplink path than in the downlink path.

11. A radio transceiving arrangement for a cellular communications system, in accordance with any preceding claim.