Base Station Antenna Arrangement

Europaisches Patentamt European Patent Office © Publication number: 0 593 822 A1 Office europeen des brevets

EUROPEAN PATENT APPLICATION

@ Date of filing: 19.10.92

@ Date of publication of application: © Applicant: NORTHERN TELECOM LIMITED 27.04.94 Bulletin 94/17 World Trade Center of Montreal, 380 St. Antoine Street West, 8th Floor © Designated Contracting States: Montreal, Quebec H2Y 3Y4(CA) AT BE CH DE DK ES FR GB GR IE IT LI LU MC NL PT SE @ Inventor: Searle, Jeffrey Graham 5 Langdon Lane, Galmpton Brixham, Devon TQ5 OPQ(GB) Inventor: Dean, Stuart James 20 Southfield Road Paignton, Devon TQ3 2SU(GB) Inventor: Broome, Keith Roy 8 Seaton Close, Babbacombe Torquay, Devon TQ1 3UH(GB) Inventor: Chrystie, Peter John 32 Stoke Gabriel Road Galmpton, Brixham TQ5 ONQ(GB) Inventor: Cox, Christopher Richard "Channel View", Rickham, East Portlemouth Salcombe, Devon TQ8 PJ(GB)

CM CM © A base station antenna arrangement comprising control means for controlling the switching matrix 00 a plurality of antenna arrays each capable of forming means whereby a particular transceiver is connected 00 a multiplicity of separate overlapping narrow beams to a particular array via the beamforming means to Oi in azimuth, the arrays being positioned such that the exchange r.f. signals with a remote station located in m totality of beams formed by the arrays provides a the area covered by one of the narrow beams. substantially omni-directional coverage in azimuth, azimuth and elevation beamforming means for each array, a plurality of r.f. transceivers, switching matrix means for connecting each transceiver with one or other of the arrays via the beamforming means,

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This invention relates to a base station antenna cells. The extent of the advantage of a narrow arrangement for use in a Cellular Radio commu- beam antenna over an omni-directional antenna is nication system. a function of the beamwidth. The narrower the Cellular radio systems are currently in wide- beamwidth the greater the advantage, but this must spread use throughout the world providing tele- 5 be traded off against the increased size and com- communications to mobile users. In order to meet plexity of the antenna. the capacity demand, within the available frequen- Although the narrow beam is formed at radio cy band allocation, cellular radio systems divide a frequencies (typically in the 900 or 1800 MHz geographic area to be covered into cells. At the bands) it can usefully be visualised as analogous to centre of each cell is a base station, through which io a laser beam which emanates from the base station the mobile stations communicate. The available and tracks the mobiles. When contrasted with an communication channels are divided between the omni-directional antenna this clearly creates a high cells such that the same group of channels are quality transmission path with minimal interference. reused by certain cells. The distance between the Some of the potential benefits of narrow beam reused cells is planned such that co-channel inter- 75 antennas for cellular radio have been recognised in ference is maintained at a tolerable level. the literature, see for example "A Spectrum Effi- When a new cellular radio system is initially cient Cellular Base Station Antenna Architecture", deployed, operators are often interested in max- S.C. Swales and M.A. Beach, Personal & Mobile imising the uplink (mobile station to base station) Radio Communications Conference, Warwick, 1991 and downlink (base station to mobile station) range. 20 and "Proposed Advanced Base Station Antennas The ranges in many systems are uplink limited due for Future Cellular Mobile Radio Systems", W.S. to the relatively low transmitted power levels of Davies, R.J. Long and E. Vinnal, Australian hand portable mobile stations. Any increase in Telecomms Research, Vol 22, No. 1, pp 53-60. range means that less cells are required to cover a Within current systems the manner in which direc- given geographic area, hence reducing the number 25 tive antennas are used allows relatively small bene- of base stations and associated infrastructure fits to be obtained. The use of directive antennas in costs. current cellular radio systems is based on the When a cellular radio system is mature the principle of sectorisation as illustrated in Fig. 2. capacity demand can often increase, especially in The main sources of interference in a cellular sys- cities, to a point where more smaller size cells are 30 tern come from the so called first tier reuse cells. needed in order to meet the required capacity per An omni-directional base station antenna will re- unit area. The process used to create these smaller ceive interference from all six first tier reuse cells cells is known as cell splitting. Any technique (Fig. 2a). If an antenna with nominally 120° beam- which can provide additional capacity without the width is used, corresponding to a tri-sectored con- need for cell-splitting will again reduce the number 35 figuration, interference will be received from only of base station sites and associated infrastructure two first tier reuse cells (Fig. 2b). If an antenna with costs. 60° beamwidth is used, corresponding to a hex- The antenna used at the base station site can sectored configuration, interference will be received potentially make significant improvements to the from only one of the first tier cells (Fig. 2c). In range and capacity of a cellular radio system. The 40 sectorised cells the cellular radio transceivers at ideal base station antenna pattern is a beam of the base station are only connected to one sector narrow angular width as shown in Fig. 1a. The (or antenna) and cannot be used in other sectors narrow beam is directed at the wanted mobile, is within the same cell. narrow in both the azimuth and elevation planes, The sectorised approach to the use of directive and tracks the mobiles movements. When com- 45 antennas has reached its useful limit at 60° beam- pared to an omni-directional antenna, such a beam width and can go no further. There are two key will have the dual benefits of having high gain, disadvantages of the approach: leading to increased range in thermal noise limited a) The cellular radio transceivers are dedicated initial deployments, and rejecting interference from to particular sectors which leads to significant co-channel reuse cells allowing higher capacity 50 levels of trunking inefficiency. In practice this without cell splitting in mature deployments. The means that many more transceivers are needed narrow beam reduces interference in a balanced at the base station site than for an omni-direc- manner on the uplink and downlink. On the uplink tional cell of the same capacity. the base station receiver is protected from interfer- b) Each sector is treated by the cellular radio ence generated by mobile station transmitters in 55 network (i.e. the base station controller and mo- the co-channel reuse cells, Fig. 1b. On the down- bile switches) as a separate cell This means that link the mobile is unlikely to be in the beams of the as the mobile moves between sectors, a consid- base station transmitters in the co-channel reuse erable interaction is required between the base

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station and the network to hand off the call tion beamforming network combines the elements between sectors of the same base station. This of a column to a single feed point. The amplitude interaction, comprising signalling and processing and phase relationships of the r.f. signals coupled at the base station controller and switch, repre- to the elevation beamformer determine the eleva- sents a high overhead on the network and re- 5 tion beam pattern of the antenna for both transmit duces capacity. and receive. The transmit and receive signals for According to the present invention there is each elevation beamformer are coupled to the provided a base station antenna arrangement com- beamformer via individual diplexers 46. Filters prising a plurality of antenna arrays each capable which cover just the transmit or receive frequency of forming a multiplicity of separate overlapping io bands respectively can be used for this purpose. In narrow beams in azimuth, the arrays being posi- the transmit path the diplexers 46 are fed from tioned such that the totality of beams formed by separate linear power amplifiers 48, one for each the arrays provides a substantially omni-directional elevation beamformer. These amplify the r.f. sig- coverage in azimuth, azimuth and elevation beam- nals up to the power levels required for transmis- forming means for each array, a plurality of r.f. 15 sion. The power amplifiers need to have high lin- transceivers each for transmitting and receiving r.f. earity since the signals from every transmitter pass signals for one or more calls, switching matrix through the amplifiers simultaneously without pro- means for connecting each transceiver with one or ducing significant inter-modulation products. In the other of the arrays via the beamforming means, receive path the diplexers 46 feed separate sub- control means for controlling the switching matrix 20 stantially identical low noise amplifiers 50, one for means whereby a particular transceiver is con- each elevation beamformer. The low noise amplifi- nected to a particular array via the beamforming ers are required to amplify the weak received r.f. means to exchange r.f. signals with a remote sta- signals prior to any system losses to establish a tion located in the area covered by one of the low noise figure (high sensitivity) in the subsequent narrow beams, the arrangement further comprising 25 receive path. separate amplifying means for each beam. The linear power amplifiers are in turn con- Embodiments of the invention will now be de- nected to the outputs of azimuth beamformers 52, scribed with reference to the accompanying draw- one for each array. The azimuth beamformers have ings, in which: multiple output ports, one for each elevation beam- Figs. 1a and 1b illustrate schematically the use 30 former, via the relevant linear power amplifier. The of a narrow beam antenna to communicate be- phase and amplitude relationship of the outputs to tween a base station and a mobile station, the beamformers control the azimuth beam pattern Figs. 2a-2c illustrate schematically the principle from the array. The beamformer has multiple input of sectorisation of a base station, ports each of which provides a different azimuth Fig. 3 is a block diagram of the main elements 35 beam in space. Likewise the receive path has a of a base station, corresponding azimuth beamformer 54 for each Fig. 4 is a diagram of the constituents of a array. This combines the multiple inputs from the multiple narrow beam base station, elevation beamformers via the low noise amplifiers Fig. 5 illustrates the basic principle of a switch- to provide multiple outputs each for a different ing matrix, and 40 azimuth beam in space. The phase and amplitude Fig. 6 illustrates the operation of a multiple relationships used in the combination process con- narrow beam base station. trol the azimuth beam shapes. The transmit and The main elements of a base station antenna receive azimuth beamformers are substantially arrangement as shown in Fig. 3 comprise a mast, identical circuits used in a reciprocal manner. One tower or building 10 supporting the antenna array- 45 well known type of beamformer is the Butler matrix. (s) 12 and associated antenna electronics unit 14, Signals are passed to and from the azimuth which includes beamformers, diplexers and amplifi- beamformers by transmit and receive switch ma- ers. The antenna electronics unit 14 is connected trices 56 and 58. Each switch matrix comprises an via a cabin electronics unit 16 to the base station r.f. cross-bar switch which allows any of its inputs 18 which is under the control of a base station 50 to be connected to any of its outputs. The switch controller 20. matrix design is such that any number of transmit- The detailed constituents of the base station ters or receivers can be connected simultaneously antenna arrangement are shown in Fig. 4. Only one to any one beamformer port. Thus, if necessary, all of the antenna arrays is depicted. Each antenna the transmitters can be connected to one beam array 40 comprises a conventional array of individ- 55 port at a given time. Likewise all the receivers can ual antenna elements 42 arranged in rows and be connected, if necessary, to the same beam port columns. Each column of elements is energised via at the same time. The switch matrices are operated an elevation beamforming network 44. Each eleva- under the control of a control processor 60. A

4 5 EP 0 593 822 A1 6 typical switch matrix structure is illustrated in Fig. significant load from the network but allow the 5. A bank of parallel receivers 62, one for each antenna system to utilise effectively narrower beam, allow every receive channel to be monitored beamwidths than would otherwise be possible. on every beam simultaneously. For each channel The position of the amplifiers 48,50 at the top the receivers measure the quality of the wanted 5 of the mast or building is the key to the whole mobile station signal present on each beam. The architecture. Firstly the concept of switching the information on which is the 'best' beam is passed transmitters to any beam is impractical unless it to the control processor. The quality measure used can be achieved without generating intermodulation by the receivers will vary depending on the particu- products, or at least maintaining them at a very low lar cellular system concerned. In simple cases the io level. This is not possible if one were to attempt to measure will be the highest power level in other switch the power levels, which can be as high as cases carrier to interference ratio will be used. The 50 watts, at the transceiver outputs. It is necessary basic function of the control processor 60 is to to switch before power amplification. Secondly if control the transmit and receive switch matrices power amplification takes place at the foot of the such that the best beam (normally the one pointing is mast or building, the r.f. feeder cables must be at the mobile stations geographic position) for a very low loss and become large and expensive. given channel is selected. The inputs to the control This would be a significant practical limitation on processor are the beam quality data from the par- the number of beams one could have in a system. allel receivers and in some cases data from the By situating the amplifiers at the top of the transceiver control bus within the base station. The 20 mast or building the above problems are solved. latter allows the control processor to monitor a However, the precise position in the architecture given mobile station's assignment to various con- within the antenna electronics unit is still critical. trol and traffic channels in the system during the Other factors which must be taken into account are progress of a call. Knowledge of which channel the that since the individual amplifiers now pass the mobile is being moved to allows a prompt and non- 25 signals from all transmitters simultaneously, inter- disruptive assignment to the best beam. The con- modulation products must once again be at a very trol algorithms used will fall into two basic classes, low level. Also since the amplifiers are at the top of one for initial acquisition of the best beam for a the mast they must be extremely reliable and fail- new call and one for tracking of the best beam ures should produce gradual rather than cata- when a call is in progress. It is anticipated that due 30 strophic degradation in system performance. to different multipath conditions the parameters The positioning of the linear power amplifiers within the control algorithms will vary for rural and 48 between the transmit azimuth beamformer 52 urban cells. The determination of beam selection and the diplexers 46 provides an excellent com- on the uplink is used to select the corresponding promise between the above factors and cost. If a beam for the downlink. The switch matrices are 35 complete linear power amplifier were to fail (which coupled by r.f. bus paths to the bank of tran- is unlikely because of their highly redundant de- sceivers 64, one for each channel to be provided sign) the main effect would be a slight degradation by the base station. The transceivers are operated in the sidelobe level of the beam patterns. If, by under the control of the base station controller 66, comparison, the linear power amplifiers had been which also provides overall control for the switch 40 placed at the input to the transmit azimuth beam- matrix control processor 60. former a failure would mean the loss of an entire The key features of the invention can now be beam and the corresponding loss of coverage with- considered in more detail and contrasted to the in the cell. Because the linear power amplifiers are conventional sectorised base station. It is not a distributed, one for each elevation beamformer, this single feature of the invention but rather the overall 45 means that the power of each amplifier is relatively architecture (the functions and their precise dis- small, the final combination being done in space by position) which provides a practical and economic the antenna array 40. The low power of operation realisation of the narrow beam concept. of the linear power amplifiers allows the inter- Considered from the network viewpoint, the modulation requirements to be met. Still lower pow- narrow beam antenna system appears as an omni- 50 er of operation could be achieved if the linear directional cell site. Since any transceiver can be power amplifiers were placed on each antenna switched to any beam and hence look in any element. Whilst this in itself would be practical the direction, there are no sectors. Thus, within the necessary diplexer per antenna element would not network all signalling and processing associated be. with sector to sector hand-offs is eliminated. Also 55 A potential disadvantage of the invention is that the fact that transceivers can be used in any direc- a relatively large antenna aperture, in terms of tion eliminates the trunking inefficiency of sec- wavelengths, is needed to produce the narrow torised sites. These factors not only eliminate a beams. If the antenna aperture were very large this

5 7 EP 0 593 822 A1 8 could create aesthetic and structural problems, due 4. An arrangement according to any preceding to wind loading etc., in some sites. This potential claim including means for monitoring the beam disadvantage is overcome by using the same an- quality of each of the beams, the switch matrix tenna array 40 for transmit and receive. In this way control means being responsive to the beam the outline of the antenna, for reasonable beam- 5 monitoring means to control switching of calls width, is less than that of many conventional cell during the progress of said calls. sites. Thus the diplexing of transmit and receive through the common aperture is a key feature of 5. An arrangement according to any preceding the architecture. claim wherein the antenna arrays comprise Fig. 6 illustrates the system operation. Fig. 6a io rows and columns of antenna elements, each shows the concept of a multiplicity of narrow, over- array being provided with separate elevation lapping beams covering the cell area surrounding beamforming means for each column of ele- the base station. The beams are referenced b1- ments and separate transmit and receive azi- b24. Fig. 6b shows how, at time ti four mobile muth beamforming means being coupled to all stations ms1-ms4 are served by beams b2, b10 is the elevation beamforming means via diplexer and b21 . Beam b2 serves two mobile stations ms2 means. and ms3 at this time. As the mobile stations move geographically in relation to the base station, at 6. An arrangement according to claim 5 wherein time t2 beam b22 now serves mobile station ms1, the amplifying means are situated between the b4 serves ms3 and b8 serves ms4. Mobile station 20 azimuth beamforming means and the diplexer ms2 has, at time t2 moved out of the cell coverage means. of this base station and will now be served by an adjoining base station (not shown). 7. A base station antenna arrangement substantially as described with reference to Figs. 3-6 Claims 25 of the accompanying drawings.

1. A base station antenna arrangement comprising a plurality of antenna arrays each capable of forming a multiplicity of separate overlapping narrow beams in azimuth, the arrays be- 30 ing positioned such that the totality of beams formed by the arrays provides a substantially omni-directional coverage in azimuth, azimuth and elevation beamforming means for each array, a plurality of r.f. transceivers each for 35 transmitting and receiving r.f. signals for one or more calls, switching matrix means for connecting each transceiver with one or other of the arrays via the beamforming means, control means for controlling the switching matrix 40 means whereby a particular transceiver is connected to a particular array via the beamforming means to exchange r.f. signals with a remote station located in the area covered by one of the narrow beams, the arrangement 45 further comprising separate amplifying means for each beam.

2. An arrangement according to claim 1 further comprising separate amplifying means for 50 each beam coupled between the switching matrix means and each array.

3. An arrangement according to claim 1 or 2 wherein transmission and reception are effec- 55 ted through a common antenna aperture.

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13 European Patent Number EUROPEAN SEARCH REPORT Application Office EP 92 30 9520

DOCUMENTS CONSIDERED TO BE RELEVANT Citation of document with indication, where appropriate, Relevant CLASSIFICATION Or IHK Category of relevant passages to claim APPUCATION ant. CI.5 ) TRANSACTIONS OF THE INSTITUTE OF 1-7 H01Q25/00 ELECTRONICS, INFORMATION AND COMMUNICATION H0iq3/24 ENGINEERS OF JAPAN vol. E74, no. 6, June 1991, TOKYO JP pages 1547 - 1555 YAMADA ET AL. 'Base and Mobile Station Antennas for Land Mobile Radio Systems' * page 1548, paragraph 3. - page 1552; figures 1-6 *

39TH IEEE VEHICULAR TECHNOLOGY CONFERENCE 1-7 vol. I, May 1989, SAN FRANCISCO, CALIFORNIA pages 341 - 348 SWALES ET AL. 'MULTI-BEAM ADAPTIVE BASE-STATION ANTENNAS FOR CELLULAR LAND MOBILE RADIO SYSTEMS' * page 345, paragraph 4.0 - page 346; figures 3,4 *

US-A-4 882 589 (REISENFELD) 1-7 * the whole document * TECHNICAL FIELDS SEARCHED (Int. C1.5 ) WO-A-8 808 623 (HUGHES AIRCRAFT) 1-7 * abstract; figure 1 * HOiq H04Q

The present search report has been drawn up for all claims Place of search Date of completion of the search Examiner THE HAGUE 21 JUNE 1993 ANGRABEIT F.F.K. CATEGORY OF CITED DOCUMENTS theory or principle underlying the invention earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another : document cited in the application document of the same category document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document