CA2174761A1 - Radio antenna arrangement - Google Patents

Abstract

A radio receiver is provided comprising at least a first antenna (10) and a second antenna (11) each having different directional antenna patterns; a variable delay (18) for delaying signals received at the antennas with respect to signals received at the other antennas;
a combiner (19) for combining the signals received at each antenna and outputting a received signal; and an equalizer (20) for combining components of the received signal.

Description

` WO 96/08088 ~ 21 7 4 7 G 1 PCr/US9S/10046 DIVERSITY RECEIVER WITH COMBINER FOR EQUALIZATION AND DIVERSITY
TRANSMIl~ER WITH SPLITTER AND DELAY
S
Fi~ld of the Inv~ntion This invention concerns a radio receiver and transmitter antenna arrangement providing irregular coverage area. It is, 10 specifically, applicable to radio tr~nsmi~sion systems which use ~1igit~l modula~ion and which incorporate eql~li7-ers for the reduction of multi-path propagation effects. An example of such a system is the GSM ni~it~; mobile radio teiephone system. The invention also concerns a radio providing diversity reception and 15 transmission.

R~k~rollnd to the Inv~n~ion Antenna diversity is a te~hnique whereby two receiving 20 antennas are used which are physically spaced apart by several wavelengths. The radio receiver conventionally has two separate parallel amplifying paths to which each antenna is connected.
Towards the end of the receiver proc.qs~in~ chain the two si~n~l~ are fed into a processor which conventionally either selects the best 25 sign~l or phase shifts and then coherently adds the two si~n~l~. The purpose is that for fading si~n~ (e.g. for mobile radio systems), the sign~ls on the two antennas are statistically much less likely to be in a faded condition simultaneously. Thus the diversity combined si~n~l will exhibit reduced f~rlin~ effects. The conventional double 30 receiver and diversity combiner add signifir~nt extra complexity, however, and it would be desirable to provide a simpler solution.
For fiigit~lly modulated radio systems incorporating multi-path equalizers (e.g. GSM), European Patent Application publication No. 0430481 describes a diversity arrangement in which each of 35 two parallel receiver ~h~in~ is connected to one of two ports of a special two-port eq l~li7~r, which acts as an integrated WO 96/08088 PCrlUS95/10046 2 ~ 7 ~ 2 equalizer/diversity combiner. The arrangement nevertheless it still requires a double receiver ~-h~in British Patent Applic~tion publication No. 2237706 describes a space-diversity system in which sign~l~ to or from one antenna are 5 delayed with respect to sign~ls to or from a second antenna and a Viterbi eq~li7~r is used to combine the delayed and undelayed si~n~ls.
A problem with prior art arrangements is that circumstances could be such that the delay of the delay element is, at least 10 occasionally, virtually equal to the actual multi-path separation for different propagation paths, thereby m~kin~ reception worse rather than better.
Radio tr~nsmitting and receiving stations provide coverage of their $ign~1s in their int.ont1e~ area of service. An example is the 15 cell sites used to provide service coverage for cellular mobile radio telephone systems.
The desired area of coverage is determined by the radiated power of the tr~n~mitting system, the sensitivity of the receiving system, the shape of the antenna radiation patterns, the direction 20 and height of the installed antenn~s, as well as intervening terrain between the cell site and the subscriber to the radio service. (e.g.
the mobile or portable station for the subscriber of a cellular mobile telephone system). Cellular mobile telephone systems usually operate under interference conclitions and therefore the effective 2~ boundary of the cell (best server) is determine~l in a complex way by the coverage of the interfering cells and the neighbour cells.
Cell site antennas can employ omni-directional antennas or directional ant~nn~s. Over flat, even terrain and in the absence of interference, the shape of the coverage area will largely follow the 30 radiation pattern of the antenna. This will normally be a regular symmetrical shape.
It is sometimes desirable to achieve irregular coverage, for example to follow a particular terrain or boundary feature. This would require a cell site antenna with an irregular pattern.

3~ Conversely however, very often, terrain features will modify the coverage of a cell site antenn~, compared to the coverage that it ~ WO 96/08088 PCTIUS95110046 ~7~761 would have over regular terrain. The result of such modi~led coverage may be poor service insome areas and interference to subscribers using a different cell in other areas. Thus, it is desirable to compensate for the effect of modi~led coverage by means of an antenna with a compensatory irregular r~ tion pattern.
It is very difficult to ~lesi~n a single antenna having a complex irregular radiation pattern. Furthermore, it would be prohibitively costly to tailor the lesi~n of antenn~ to particular cell sites.
Antenn~ with complex and irregular radiation patterns can be made by means of the phased array ~nt~nn~ terhnique.
However, such antennas are costly, difficult to set up to the desired pattern, sPn~itive, delicate, require regular maint~n~nce and are usually large.
Simple combination of multiple antenn~ with different radiation patters in order to produce a combined pattern of the desired shape have not been successful. Simple combination (i.e.
addi~ions) of the sign~l~ from multiple antenn~s causes severe interference lobes, particularly at the boundary between the patterns of the individual antennas.
A method of combination is desired that o~/elcollles the problem and provides reliable irregular shaped coverage areas.

.Sllmm~ry of the Invon~ion In accordance with a first aspect of the present invention, a radio receiver is provided co~ ising first and second ant~nn~s, physically spaced apart to provide diversity, and an eq~l~li7~r for combining components of a received symbol which are separated in time, a combiner for combining the si~n~ls received at the first and second antennas and coupling the combined sign~l to the eqll~li7~r and a variable delay means in the receive path of one of the ant~nn~ or delaying sign~ls received at that antenna with respect to sign~l~
received at the other antenna, so as to si~nifi~ntly reduce the probability of destructive interference between the si~n~l~ from the first and second ant~nn~s.
In accordance with a second aspect of the invention, a correspon~ling tr~n~mitter is provided as l~fine~i in the t~l~im~.

WO 96/08088 ~ CT/US95/10046 ~--In accordance with a third aspect of the invention, a radio receiver is provided comprising first and second antennas each having different directional antenna patterns; a variable delay means for delaying si~n~l~ received at the antennas with respect ~o 5 sign~ls received at the other antenn~, a combiner for combini~g the !;ign~ls received at each antenna and outputting a received sign~l;
and an eq-l~li7er for combining components of the received sif~n~l.
In accordance with a fourth aspect of the invention, a corresponding tr~n~mitter is provided as defined in the ~l~ims.
The invention in all its aspects provides a very cheap and simple arrangement in which diversity and irregular coverage can be provided. A particular advantage is that diversity can be provided by simply adding a combiner, a delay element and an extra antenna, for example in the RF stage. No additional processing is essen~i~l, because the ~ign~ls from the two antenn~ are additionally combined by the eq l~li7er operating in its normal manner.
Two antenna sign~l~ may be processed by (different) delay line means or a plurality of antenna and delay lines may be used. A
phased array can be provided comprising parallel receiver ~h~in~ for diversity and combinations of delay elements in one or more o~ the receiver c h~in~ for further diversity.
Multi-path diversity can be provided through more than two ~ntenn~ provided that the si~n~l~ from the antennas are separated from each other by a delay.
The delay means may be analog or ~iigit:~l and may exist in the IF
or RF stages. In the case of RF processing, the delay means can be in the form of an external unit which may incorporate its own frequency converters, IF amplifiers and delay means.
The delayed and added signal may be enabled, disabled or modified according to some detected characteristic of the sign~l, as is described below.
The problem addressed by the invention also arises in a repeater or cell enh~ncer which incorporates a delay, where the delay in a signal from the repeater can, at least occasionally, virtually equal the acrual multi-path delay separation for signals from the repeated an a main cell transmitter, thereby m~king reception worse rather than better.

` WO 96/08088 2 1 7 ~ 7 6 1 PCI/US95/10046 Rri~f nesrr~tion of ~he nr~winp~

Figure 1 shows a first embodiment of a multi-path enh~ncement diversity receiver in accordance with the invention.
Figure 2 shows a phasor diagram for explanation of the invention.
Figure 3 shows an embodiment of a diversity receiver in accordance with the invention, in which the delay means are provided in the RF stage.
Figure 4 shows an add-on RF diversity unit in accordance with another embodiment of the invention.
Figure 5 shows a tr~n~mitter providing diversity in accordance with aspects of the invention.
Figure 6 illustrates a repeater employing the invention.
Figure 7 shows the repeater of Figure 6.
Figures 8 ,9 and 10 show r~ tion patterns of antennas.
Figure 11 shows a receiver in accordance with an embodiment of the present invention.
Figure 12 shows a tran~mitter in accordance with an embodiment of the present invention.

net~ilr~ esrription of the ~?r~ferred omhorlim~nts ~ Referring to Figure 1, there is shown a part of a typical GSM radio receiver com~lising a main receive ?(nt~nn~ 10 and a diversity antenna 11. The main antenna 10 provides a sign~l to a main receive pa~ 12 comprising a front end RF amplifier unit 13 including a frequency converter unit (not shown) and an IF stage 14. A number of frequency converter units and IF stages may be used. The diversity antenna provides a sign~l to a diversity receive path 15 comprising a front end amplifier unit 16 and an IF stage 17. The diversity receive path 15 also comprises a delay element 18 which may be in the form of a long transmission line, surface acoustic wave delay line, or one or more filters. The output of the IF stage 14 on the main receive path 12 and the delay element 18 on the diversity receive path are combined in a 3~ simple combiner 19 and passed to a multi-path eqll~li7~r 20. The multi-path eq1l~li7.or is in accordance with the GSM sperific~tion and WO 96/08088 PCI~/US9~/10046 '--7~

may, for example, be that described in EP-A-0318685 or ~P-A-0343189. The equalizer 20 is in digital form, i.e. the input to the equalizer 20 is an A/D converter. The eql1~li7~r uses four-times oversampling. After eq~ 7in~, the sign~l is subjected to error coding 5 in an error coder 21, in accordance with the GSM specification, and the result is passed to a speech decoder for extracting speech information and synthesi7in~ speech. Analog equalizers could equally be used.
The operation of the receiver of Figure 1 is as follows. A GMSK
si~n~l (or other binary modulated si~n~l) is received at an antenna 10 10 from a mobile tr~n~mitter. At the same time, a si~n~l is received from the tr~nsmitter at antenna 11 via a different path. Each symbol of the si~n~l has a duration of approximately 4 microseconds. The si~n~ls received at the antennas are ampliflled and down-converted in front end units 13 and 16 and IF stages 14 and 17 respectively. The signal 15 from ~ntenn~ 11 is delayed in delay element 18. The delay ele~ent delays the si~n~l by at least a sufficent fraction of a bit period that when combined in the combiner 19, the probability of destructive interference between the si~n~l~ from the paths 12 and 15 is si~nific~ntly reduced. In order to further reduce the probability of 20 destructive interference, the delay introduced by the delay element is made variable. The combiner 19 sums the si~n~l~ and passes the summed combination to the multi-path eql-~li7~r 20. The equalizer 20 performs rli~it~1-to-analog conversion and applies a~ro~.iate delays and phase shifts to different portions of the si~n~l so as to re~ n the 25 various components of a symbol in time and phase, in a manner known in the art in respect of inter-symbol interference equalization. The eqll~li7.or 20 acts on the separate components of the signal received on paths 12 and 15 (and any components introduced by actual multipath reflections before reaching the antenn~ 10 and 11) and corrects the 30 time error and any phase error therebetween. The resultant e~ualized sign~l is demodulated to extract the symbols and subjected to error coding in the error coder 21.
To reduce the probability of destructive interference at the combiner 19, a delay of at least 1/4 bit period is preferred (though 35 smaller delays may suffice). A 1/2 bit period is consi-l~red a useful delay. A limitin~ factor is the bandwidth of the filters in the signal ` WO 96/08088 2 ~ 7 4 7 6 1 p~ 9~,l0046 path after the combiner. In the GSM system, these filters will remove the distinction between two si~n~l~ separated by less than about one bit period. Accordingly, a particularly preferred delay is in the 1 to 2 bit-period range. The GSM eqll~li7.or is typically designed to equalize 5 delays of up to 16 microseconds and in theory a delay of 10-16 microseconds can be used (i.e. up to 5 bit periods), but some benefit will be lost if the sign~l.s them~elves are subject to multi-path delay.
Use of greater overs~mplin~ in the equalizer may permit shorter delays The principle of operation of the equalizer 20 is further illustrated with reference to Figure 2. The figure shows a time axis with phasors representing multiple sign~l~ illustrated rotating in phase around the ~ime axis. The time axis is shown in stereoscopic view so that both the time delays and the phases of the different phasors can 15 be seen. Two symbols Q1 and Q2 are illustrated separated in phase. If these symbols were to be summed, they would provide the result Q1 +
Q2 illustrated. It can be seen that these symbols can either be summed together or can cancel each other out with equal likelihood. In contrast, the symbols Q3 and Q4 are illustrated which have been 20 separated in time by delay d. When sl~mmin~ these in summer ~, being wide-band si~n~ , they cannot cancel each other out. The eqll~li7~r performs a phase shift on symbol Q4, bringing it into phase with symbol Q3 and delays symbol Q4 so that it coincides with symbQl Q3. Thus the two symbols are added and will always provide a larger 25 resultant sign~l illustrated as Q3 and Q4.
The principle is that the multi-path eql-~li7~r will coherently combine two or more .si~n~l~ of any phase which arrive at a receiver antenna, provided that they exhibit a different time delay. This is the normal int~n(1e-1 function of the equali_er in ~nh~ncing sign~l~ which 30 suffer multi-path propagation. In this application, the diversity sign~l is made to look like a multi-path delayed sign~l by the added delay line and it is, therefore, coherently combined by the conventional multi-path eqll~li7-or. The delay is made variable in order to remove the possibility lthat the deliberately introduced delay is equal but 35 opposite to the natural propagation delay of the sign~l between ~he ~wo ant~nn~s. No additional phase correction is required and provided the WO 96/08088 ~ ,35/10046 ~--~747~ 8 equalizer has been optimized, the diversity improvement could be a minimum of 3 dB and typically 6 dB for f~lin~ si~ . The delay line means could be an analog delay line, a .lif~ l delay line, an IF delay line or an RF delay line (see below). Transmission line, lumped circuit, S surface acoustic wave or ~ it~l circuitry can be used (not necessarily exdusively) for the delay line.
In a linear receiver, such as used for the GSM mobile telephone system, the delay line and simple combiner can be implemented at the front end of the RF section of the receiver. This is illustrated in Figure 10 3. In this figure, elements of Figure 3 have the same reference numerals as in Figure 1. An RF delay line 25 is connected to the diversity antenna 11 and the other end of the delay line is connected to a simple RF combiner 26, together with an RF connector from the main antenna 10. The output of the combiner 26 is passed to the front 15 end unit 13 and other elements as shown. The arrangement is possible because two or more indepenflent .si~n~ls can be processed by a linear receiver without mutual interference. This means that the double receiver chain can be dispensed with. A conventional non-diversity receiver (with eql-~li7~r) can be used with the diversity components 20 (delay line means and simple combiner) contained within a external, add-on unit.
The combiner can be a simple signal adder or a hybrid. The delay line can operate at RF frequencies, or the RF diversity unit ca~ .
incorporate its own frequency converters so that the delay line can 25 operate at an intermediate frequency, as illustrated in Figure 4.
Referring to Figure 4, elements of Figure 3 are shown with the same reference numerals. Between the diversity antenna 11 and the combiner 26 are provided a first RF filter 30, a mixer 31, an IF flter 32, a delay element 33, a second mixer 34 and a second RF filter 35.
30 Coupled with the mixers 31 and 34 is a local oscillator 36. The mixer 31 and generator 36 operate to down-convert the si~n~l to an IF
frequency such as 100-200MHz. At such a frequency, a cheap and compact delay element 33 can be implemented, for example in the form of a surface acoustic wave filter. The IF frequency is up-35 converted in mixer 34 and the operation of the invention is as for theFigure 3 embodiment.

W096/08088 ~ 1 7 4 7,6 1 P~,,U~5~10046 Items 11, 25 and 26 could be supplied as an add-on unit.
Under certain conditions, when one or more of the diversity sign~l~ suffers actual multi-path propagation, it will be advantageous to alter the time delay of the delay line for each frame of the ~ligi~lly modulated sign~l This is achieved by using switchable delay elements, for example as shown in dotted outline in Figure 4, where delay element 40 has a longer delay than that of element 33 and is swit~h~hle into the IF path in place of element 33. Alternatively, in the embodiment of Figure 3, the delay element 25 could be switched from the diversity receive path to the main receive path - i.e. between points A-A' and B-B'. Combinations of these arrangements could be used.
The switching of the delay element is in response to dispersion information taken from the eqll~li7~r 20. As an alternative, the swit~hing is repetitive or pseudo-random.
In a further embodiment of the invention, a small frequency s~ift of about 3 to 5 kHz is applied to the diversity si~n~l and/or a phase shift of 0-360 is applied. This feature provides improvements when the sign~l~ are Doppler shifted. In the embodiments of Figures 1 and 4, the frequency shift is applied in the lF stage, for example by adjusting the injection frequency from oscillator 36. The frequency shift can be made at the RF level in a m~nn~r readily implemented by one skilled in the art. A phase shift is implemented by a variable capacitor/diod~
network in the RF sign~l path, or by a series of transmi~sion line phase shift elements. This te~hnique of swit~hing the delay element operates in conjunction with the error coder 21 in a manner simil~r to the principal of frequency hopping in the GSM art.
As for the switching of the delay element, the frequency shi~t and/or phase shift is varied on a frame-by-frame basis.
The delay, frequency shift and/or phase shift can be enabled, disabled or modified according to characteristics of the received sig~al such as sign~l strength, interference or delay spread. In this way, an adaptive multi-path enh~ncement diversity arrangement is provided.
In principle, multi-path ~nh~ncement diversity could be used at the receivers at either end of a two-way radio link. For mobile/portable radio telephone systems, it is rarely attractive to have W096/08088 ~ 2174161 P~ 95/10046 ~

a second ~nt~nn~ system on the mobile or portable unit (i.e. diversity for the downlink path - base-to-mobile path).
Figure S illustrates an arrangement in accordance with another aspect of the invention in which a delay element is introduced within the tr~ncmitter chain enabling the use of two tr~nsmitting antennas at the base station to provide downlink diversity, instead of two receiving antennas on the mobile or portable unit.
The arrangement comprises a tr~ncmitter 50, a simple splitte~
51, a main tr~n~mitter antenna 52, a diversity tr~nsmitter antenna 53 10 and a delay element 54 connected between the splitter 51 and the diversity antenna 53. The tr~n.~mitt~r 50 comm~lni~tes with a mobile unit 55 which incorporates a multi-path eqll~1i7.~r. The a~ itional features of delay swit~hin~, frequency shifting and/or phase shifting described above can also be applied. In practice, the delay element 54 15 is preferably included in the tr~ncmit path prior to power amplification of the ci~n~l and two power amplifllers are provided for amplifying the primary sif~n~l and the delayed si~n~l The tr~ncmitter 50 can be a tr~n~mitter only and the mobile unit 55 can be a receiver only, provided with a multi-path equalizer 20 specifically added for the task of eqll~li7ing the si~n~l.c from the main and diversity antenn~s 52 and 53.
The principal of the invention can be applied to a cell enh~ncer to ovelcol.le problems at the boundary of the cell enh~ncer arising from natural multiple paths (from the main cell and the ~nh~ncer). This is 25 illustrated in Figure 6, in which a main cell tr~ncmitter 60 is shown having a tr~ncmit radius 61 and, within the main cell tr~ncmit radius there is a cell ~nh~ncer 62 having a tr~n.cmit radius 63. The cell enh~ncer is cypically used to cover a problematic area or "hole" in the cell. It has a tr~ncmit radius smaller than that of the main cell. The 30 cell enh~ncer simply retransmits the si~n~l 65 it receives from the main cell tr~n~mitter~ with shielding provided to prevent it from retr~n~mittin~ its own si~n~l At a point 64 on the boundary of the cell ~nh~ncer transmit area, the signal 66 from the cell ~nh~ncer may arrive almost simultaneously with the signal 67 from the main cell 3 ~ transmitter. In accordance with this aspect of the invention, the ce!l ~nh~ncer introduces a delay in the signal 66. This delay is periodically wog6/08088 ~l7~61 ~ 5"0046 switched in order to accomodate possible fading at different points on the cell ~nh~ncer tr~nsmit boundary.
As shown in Figure 7, the cell enh~nc~r 62 comprises a receive antenna 70, a transmit antenna 75 and a receive/tr~n~mit path 5 comprising preampli~ler 72 and power ~m~lifier 74. A delay element is introduced on the receive side at position 71 or before the power amplifier at position 73. The delay is switched frame-by frame by timing circuitry 76. It will be understood that variations described above in relation to receive diversity and tr~n~mit diversity can be 10 implemented. For example there can be more than one receive antenna or more than one tr~n~mit antenn~. In each case the sign~l through one antenna is delayed with respect to the sign~l through the other(s).
A further aspect of the present invention provides the means 15 by which two or more antennas may be combined to produce a complex radiation pattern which is the superposition of the simpler radiation patterns of the individual ant~nn~s. Thus, irregular coverage may be achieved in order to follow a particular terrain or boundary feature. This is done according to the present invention 20 without causing the interference lobes which would result from simple additive combination. It is applicable to radio systems which use ~ligit~l modulation and which incorporate equalisers for the reduction of multi-path ~lo~ tion effects.
In practice, when terrain features modify the coverage of a 25 cell site antenn~, compared to the coverage that it would have over regular terrain, the present invention may be used to compensate for the effect by means of an antenna with a compensatory irregular radiation pattern. This situation is shown in Figures 8 and 9. In Figure 8, line 83 represents the pattern of an omni-directional 30 antenna 81 and the coverage it would provide over flat, even terrain. However in direction x, the coverage is reduced due to hilly terrain in that direction. In direction y, coverage is extended due to ducting along a valley. In direction Z, coverage is as expected, over regular terrain. The overall irregular coverage boundary is shown 3 5 as line 84 in Figure 8.

W 0 96/08088 ~ ~7~7 ~1 1 2 Pc~rruS95/10046 ~

The reduced coverage in direction x provides poor service quality for users in that area, while the extended radiation in direction y is undesirable because it could cause interference to subscribers using a distant cell which shares the same frequ~n~ies.
In order to compensate, an antenna with the inverse irregular r~ tion pattern is required, as shown in Figure 2. Such an antenna arrangement may be provided according to the present invention so that increased radiation is provided in direction X and reduced radiation is provided in direction Y. The overall radiation pattern (i.e. measured over flat terrain or in a test ~h~mher) would need to approximate to line 91 in Figure 9.
An ~ nt~ n n ~ arrangement, according to the present invention, produces the desired effect of such a combination process is shown in Figure 10, whereby three ~ nte n n ~ with individual patterns shown by lines 104, 105 and 106 combine to produce an overall radiation pattern shown by line 107. This approximates to the required irregular pattern required by the example shown in Figure 9.
An antenna arrangement, according to the above aspect of the present invention is shown in Figure 11. A radio receiver 111 is provided including two or more antennas 112-114 each having different directional anntena patterns, an equaliser 116 for combining components of a received ~i~n ~1, a combiner 118 is provided for combining the sign~ls received at each of the ~ntenn~s and coupling the combined signal to the equaliser 116 and separate delay means 121-122 are provided in the receive path of each of the antenn~ for delaying sign~l~ at that antenna differently with respect of sign~l~ received at the other antennas so as to ~limin~te the probability of interference between the si~n~ls from the different antenn~s.
In accordance with a further aspect of the invention, as shown in Figure 12, a radio tr~n~mitter 200 is provided for communication with a receiver 201 having an equaliser 202 for combining components of a received symbol which are separated in time wherein that the transmitter comprises two or more antenn~c having different directional antenn~s. Splitter means 209 for WO 96/1)8088 2 1 7 4 7 6 1 Pcr~S95/10~46 splitting the si~n~l to be tr~n~mitted and coupling it to the different antennas 203-205 and delay means 206-208 provided in the transmit path of each of the ant~nn~s for delaying the sign~l~
transmitted by that antenna by more than the pre-determined minimum delay with respect to si~n~ls tr~n~mitted by any of the other antennas so as to ~limin~te the probability of interference between the ~ign~ls from the different antenn~.
The present invention provides a cheap and simple method in which multiple antenna sets (tr~n~mit and receive) can be combined without mutual interference in order to provide irregular cell site coverage or alternatively to compensate for the irregular cell site coverage which would otherwise occur for a single antenna set due to terrain features. A partic~ r advantage is that the irregular cell coverage, so produced, is a single cell rather than separate cells, as would be the case if separate transmitt~r/receivers operating on different frequencies were connected to each antenna. The arrangement described herein is much simpler in terms of equipment and the requirement for con~lol ~h~nnels than the alternative of a site having separate multiple cells.
According to the present invention, the shape and amplitude of the individual antenna patterns required to make up the pattern of the combined cell site coverage of the irregular cell sites may be controlled by many factors. These include:-The horizontal beam width of the individual antenn~;
25 2 The gain of the individual ~ntenn~c;
3 The direction of poindng of each antenna;

4 The height of each antenna;

5 The down tilt of each ant~nn~.

Path loss prediction programs exist to determine the coverage pattern of the individual antennas for the actual terrain area over which they radiate (taking into account terrain features, free space antenna pattern, antenna height and down tilt etc.) Such programmes can be used to determine the combined coverage for the actual terrain.

WO 96/08088 1 ~ J'.,55/10046 2174~6~
. ~ 14 The method of operation of the antenna combining scheme is as described above with reference to Figures 1-7. The difference in embodiments is that in the case of combining two (or more) ~ntf~nn~ to provide diversity reception, the antennas would 5 normally have the same radiation patterns and direction of radiation but, are spaced apart by at least several wavelengths. In the present invention, the antenn~ have dfflerent radiation patterns and/or different directions of radiation in order to produce an irregular radiation pattern when combined and the individual 10 antennas may or may not be co-located.
In both embodiments, for a cellular mobile telephone system, normally both tr~n~mitter combining and receiver combining would be used at a site. The antenna arrangements may be used particularly in either TDMA or CDMA communications systems.

Claims (10)

Claims

1. A radio receiver comprising:
at least a first antenna and a second antenna each having different directional antenna patterns;
a variable delay means for delaying signals received at each antenna with respect to signals received at the other antennas;
a combiner for combining the signals received at each antenna and outputting a received signal; and an equalizer for combining components of the received signal.

2. A radio receiver according to claim 1 wherein the signals received at the antennas are divided into discrete frames and the delay means vary the delay from frame to frame.

3. A radio receiver according to any one of the preceding claims wherein the delay means further comprises frequency shift means for shifting the frequency of the signal received at one antenna with respect to the frequency of the signal received at the other antenna.

4. A radio receiver according to any one of the preceding claims wherein the delay means further comprises phase shift means for shifting the phase of the signal received at one antenna with respect to the phase of the signal received at the other antenna.

5. A radio transmitter for communication with a receiver having an equaliser for combining components of a received symbol which are separated in time, the transmitter comprising:
a plurality of antennas each having different directional antenna patterns to provide a desired combined antenna pattern;
splitter means for splitting a signal to be transmitted and coupling it to each of the antennas; and delay means provided in the transmit path of each of the antennas for delaying signals transmitted by that antenna by more than the pre-determined minimum delay with respect to signals transmitted by the other antennas so as to significantly reduce the probability of destructive interference between the signals from the different antennas.

6. A radio receiver comprising:
at least a first antenna and a second antennas physically spaced apart to provide diversity;
variable delay means in the receive path of each of the antennas for delaying signals received at that antenna with respect to signals received at the other antenna;
a combiner for combining the signals received at each antenna and and outputting a combined signal;
an equalizer for combining components of the combined signal which are separated in time.

7. A radio receiver according to claim 6 further comprising means for receiving signals at each antenna which is divided into discrete frames and means for varying the delay is from frame to frame.

8. A radio receiver according to claim 7, wherein the equalizer comprises means for indicating a characteristic of the combined signal and the means for varying the delay varies the delay in response to the characteristic.

9. A radio receiver according to claim 8 wherein the characteristic is the dispersion of the combined signal.

10. A radio transmitter for communication with a receiver having an equalizer for combining components of a received symbol which are separated in time, the transmitter comprising at least a first antenna and a second antenna physically spaced apart to provide diversity, splitter means for splitting a signal to be transmitted and coupling it each antenna and a variable delay means provided in the transmit path of at least one of the antennas for delaying signals transmitted by that antenna by more than a predetermined minimum delay with respect to signals transmitted by the other antennas.