GB2281663A

GB2281663A - Filters for radio base stations

Info

Publication number: GB2281663A
Application number: GB9317192A
Authority: GB
Inventors: Martin W Greenwood
Original assignee: Nokia Telecommunications Oy
Current assignee: Nokia Oyj
Priority date: 1993-08-18
Filing date: 1993-08-18
Publication date: 1995-03-08
Anticipated expiration: 2013-08-18
Also published as: GB9317192D0; GB2281663B

Abstract

A radio base station for a cellular system has a filter sub-assembly comprising a filter 36 in the main receive path 34, a filter 36a in the diverse receive path 46, a duplex filter 26 for operating transmit and receive signals into a common antenna 14 and a combiner 23 for combining two radio frequency transmit signals into a single signal for feeding to the duplex filter 26. This way of packaging filters in a single sub-assembly uses the minimum of space in the base station, and offers flexibility of configuration by connection of appropriate links 48, 50 and antennas 12, 14 at a front panel (54, Fig. 4) to allow e.g. duplex or non-duplex operation, receive diversity or non-diversity, single or multiple sector operation. <IMAGE>

Description

Title: Filters for Radio Base Stations Field of the Invention This invention relates to radio frequency filters for radio base stations, and to radio base stations incorporating such filters.

Background to the Invention Within a radio base station, such as for GSM (Groupe Systeme Mobile) cellular systems, there are several radio frequency (RF) filters. These filters serve different purposes. There is normally at least a receive RF filter and a transmit RF filter. The receive filter is primarily responsible for meeting the blocking performance and the main function of the transmit filter is to avoid out of band transmission. Additional filters are often required, such as an additional receive filter to implement receive diversity and extra filtering in both directions to implement diversity (where both transmitter and receiver share the same antenna).

RF filters for base stations, in the 900 to 1800 MHz range, tend to be quite bulky. Filters which are physically large generally have lower power loss, which is important in obtaining both optimum receive sensitivity and transmitter output power. A further consideration is that if multiple RF carriers are combined, the lower field strengths in the larger filter result in lower intermodulation products. A typical receiver filter, operating at 1800 MHz, might have dimensions 45x45x250mm and use a series of quarter-wave resonant co-axial cavities with air dielectric.

The present invention is concerned with a means for packaging filters so as to use a minimum of space within the base station, whilst maintaining optimum manufactured cost and flexibility of configuration.

Disclosure of the Invention According to one aspect, the invention provides a plurality of radio frequency filters for a radio base station, the filters being combined in a single subassembly.

Preferably, the sub-assembly comprises a front panel having accessible connections such as sockets for electrical connection of the sub-assembly to the remainder of the base station.

The filters may include a main receive filter and a diverse receive filter and the two receive filters may be accommodated in a single unitary body, preferably of aluminium. If desired, this single unitary body may also incorporate the transmit filters.

According to another aspect the invention provides a radio base station comprising radio frequency filters for the transmit and receive paths which serve a particular region of coverage, wherein the filters are combined into a single sub-assembly. There will normally be one sub assembly per region of coverage, called a cell.

All of the RF filters, for a given base station with a particular antenna set, may be fabricated as a single unit, although in the preferred embodiment to be described the two transmit filters are incorporated in a diplexer and the main and receive filters are fabricated as a separate unit. Ancillary functions, such as VSWR measurement, combiners, and low noise amplifiers may be built into the same sub-assembly. The sub-assembly provides the following advantages: - the receive filters (or all the filters) can be manufactured in a single process, such as casting as a single block of metal or machining from solid.

This greatly reduces the number of manufacturing operations and produces a more compact assembly. The parts inventory is reduced and testing can be accomplished within a single test process at one test station; - the additional filtering needed for antenna diversity can be included at little extra cost because it does not require an extra unit to be fitted; - the total number of connectors is reduced. This reduces power losses, cost and intermodulation products; - the sub-assembly provides exactly the right degree of modularity to configure different base station variants from a common kit of parts.

The principle may be extended to incorporate a narrow-band combiner. In this instance, the filtering which is implicit in the narrow-band combiner filters enables the specification for the transmit roofing filter to be relaxed. A narrow-band combiner normally includes a set of tunable cavities in a single block of metal, so fabricating the other filters into the same unit saves manufacturing cost and makes for a compact assembly.

The unit described interfaces all of the antenna feeds, which serve a particular cell, to all of the transmitter/receiver (TRX) units which serve that same cell. For instance, a base station in a single-cell configuration would have one or more TRX units (depending on the number of carriers) connected to a single subassembly . The sub-assembly would be connected via feed cables to one antenna, if the installation used duplex operation and no diversity, two antennas if there were receive diversity and duplex operation, and three antennas if there were receive diversity without duplex operation.

If there were two or three cells, there would be two or three sub-assemblies and up to nine antennas.

The preferred sub-assembly contains four distinct RF modules integrated into a single unit. These are: - a wide band combiner for combining two RF signals into a single signal; - a duplexer (including transmit filters) for operating transmit and receive signals into a common antenna, a VSWR monitor circuit for monitoring the state of the antenna and a coupler for injecting a test signal; - a main receive path including a receive filter, a low noise amplifier, a power splitter to provide signals for two receivers, and coupler to inject a test signal; - a diverse receive path similar to the main receive path.

These RF modules have all their RF connections brought out to the front panel where different connections can be made to suit different applications.

For single sector duplex operation the transmit/receive antenna is connected to the common port of the duplexer and the receive port of the duplexer is linked directly to one receive path which is then connected to the receiver.

The transmitter is connected to the transmit port of the duplexer. Two transmitters can be accommodated by using the wide-band combiner. A diverse receive antenna and receiver can be used with the diverse receive path of the sub-assembly.

For single sector non-duplex operation the duplex port of the sub-assembly is used as a transmit filter to connect the transmitter to the transmit antenna. Two transmitters can be accommodated by using the wide-band combiner. The receive port of the duplexer is terminated in the characteristic impedance. The receive antenna is connected directly to the receive path of the subassembly. Similarly a diverse antenna and receiver can be connected to the diverse receive path.

For multiple sector operation a separate sub-assembly is used for each sector. Each sub-assembly can be configured independently for either duplex or non-duplex operation, single or two carrier and may or may not have a diverse receiver.

In the case of multi sector operation a spare transceiver can be accommodated. The normal transmitter is connected to one input port of the wide-band combiner of the subassembly for that sector and the spare transmitter is connected to the other input port by way of a multi-way switch. Each output of the multi-way switch goes to an input port of the sub-assembly in a different sector.

Thus the spare transmitter can be switched to any one sector. A similar multi-way switch is used to connect one receive output of the sub-assembly in each sector to the spare receiver. Similarly a diverse receiver can be switched between sectors.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figures 1 to 3 show an antenna filter sub-assembly according to the invention in three alternative configurations, Figure 4- is a perspective view of the sub-assembly with the components in exploded view, and Figure 5 shows a front panel of the sub-assembly.

Referring to Figure 1, the sub-assembly 10 is illustrated in a two transceiver/diplexer configuration for incorporation in a base station of a cellular radio station communicating with mobile handsets in a radio telephone system. The sub-assembly is connected to two antennas 12, 14, has two pairs of outputs 16, 18 feeding Rx signals to the processing circuitry of the base station and two inputs 20, 22 receiving Tx signals. The subassembly 10 has the following functional blocks: - a wide band combiner 23 for combining the two radio frequency transmit signals (Tx) into a single signal; - a diplexer 24 including a duplexer 26 providing duplex operation of receive signals (Rx) and transmit signals (Tx) into a common antenna, in this case antenna 14. The duplexer 26 includes the two transmit filters 25.The diplexer 24 also has a VSWR monitoring circuit 28 for monitoring the state of the transmit antenna 14, and a connection 30 for injecting a test signal; - a main receive path 34 including a receive filter 36, a low noise amplifier 38, a power splitter 40 to provide the two main Rx signals on outputs 18, a coupler connection 42 to inject a test signal, and an alarm output 44 to indicate a fault; - a diverse receive path 46 duplicating the main receive path. Components similar to those of the main receive path bear the same reference numerals with the suffix a. The power splitter 40a provides the two diverse Rx signals on output 16.

Hence, the sub-assembly of Figure 1 includes the filter 36 in the main receive path 34, the filter 36a in the diverse receive path 46 and the duplexer 26 (including filters 25), together with other functions, in a single unit.

In the configuration shown in Figure 1, a first link 48 is in place and connects the output of the combiner 23 to the input of the duplexer 26. A second link 50 interconnects the Rx output of the duplexer 26 with the input to the main receive path 34.

Figure 2 shows a one transceiver/diplexer configuration in which the link 48 is omitted and the combiner 23 is redundant, the single Tx signal on input 20 being fed directly into the input of the duplexer 26. A single Rx signal forms the output 18 of the main receive path 34 and a single Rx signal forms the output 16 of the diverse receive path 46.

Figure 3 shows a one transceiver/three antenna configuration. The additional antenna 52 is connected to the input of the main receive path 34 and both links 48 and 50 are omitted.

Figure 4 shows the components of the sub-assembly in exploded view. The sub-assembly is in the form of a unit having a front panel 54 behind which are mounted: an aluminium body 56 carrying the receive filters 36, 36a and machined with a recess 57 to accommodate a printed circuit board carrying the components 38, 38a, 40, 40a and 44; a subframe 58 carrying the combiner 23; the diplexer 24; and a housing 60 accommodating an interface board.

The sockets shown on the right and left hand sides of the circuit diagrams of Figures 1 to 3 project through apertures in the front panel 54, rendering the sockets accessible from the front of the sub-assembly, as best seen in Figure 5.

The two optional links 48, 50 are shown in Figures 4 and 5 as handles terminating in plugs insertable into the appropriate sockets on the front panel 54.

The base station into which the sub-assembly is fitted takes the form of a cabinet accommodating all the base station components. When the sub-assembly is fitted into the base station cabinet, the front panel 54 is accessible at the front of the cabinet for ease of connection of the sub-assembly to the antennas and to the remainder of the base station, and for ease of insertion and removal of the links 48, 50.

Claims

1. A plurality of radio frequency filters for a radio base station, the filters being combined in a single subassembly.

2. A plurality of filters according to claim 1, wherein the sub-assembly comprises a front panel having accessibile connections such as sockets for electrical connection of the sub-assembly to the remainder of the base station.

3. A plurality of filters according to claim 1 or 2 wherein the filters include a main receive filter and a diverse receive filter and the two receive filters are accommodated in a single unitary body.

4. A plurality of filters according to claim 3, wherein the single unitary body is of aluminium.

5. A radio base station comprising radio frequency filters for the transmit and receive paths which serve a particular cell or region of coverage, wherein the filters are combined into a single sub-assembly.

6. A radio base station according to claim 5, wherein the filters include a main receive filter and a diverse receive filter and the two receive filters are accommodated in a single unitary body.

7. A radio base station according to claim 5 or 6, wherein the sub-assembly accommodates units performing additional functions, for example: VSWR measurement, power feed for mast-head pre-amplifier, passive combiners, splitters, isolators, or low noise amplifiers.

8. A radio base station according to any of claims 5 to 7, wherein the sub-assembly includes a narrow-band combiner to combine multiple transmit signals.

9. A radio base station according to any of claims 5 to 8, wherein the sub-assembly forms one of a plurality of sub-assemblies each serving a corresponding cell or region of coverage.

10. A radio base station according to claim 9, wherein a spare transmitter and receiver are switchable to any one of the sub-assemblies to provide additional capacity in the corresponding cell or to replace the capacity of the damaged transmitter or receiver used for that cell.

11. A radio base station constructed and arranged substantially as herein particularly described with reference to any one of the alternative embodiments shown in the accompanying drawings.