A dividing cascade network for a support station in a radio telephone network

Abstract: The invention relates to a dividing network in the support station in a radio telephone network, wherein the received multi-channel antenna signal (ANT) is divided among sepa­ rate receivers (CH1-CHn) by means of a dividing unit. The dividing unit is made up of a filter (2), an am­ plifier (3) and a dividing network (1). According to the invention it has, in a cascade (11; 21), N directional couplers (H1-Hn; B1-Bn), there being branched out from the cascade in succession to each of the N receivers (CH1-CHn) a share, acording to the coupling (K1-kn) of the direc­ tional coupler, of the filtered and amplified antenna signal (4).

Description: A dividing cascade network for a support station in a radio telephone network The invention relates to a dividing network in the support station in a radio telephone network, wherein a received multi-channel antenna signal is divided among separate receivers by means of a dividing unit, the dividing unit consisting of a filter, an amplifier and a dividing network. In radio telephone networks, and especially in cellular telephone systems, the support stations comprise a plurality of frequency channels, which may be time channeled and/or frequency channeled. Owing to limited antenna space and to cabling costs, the aim is to connect several parallel receivers and transmitters to the same antenna. For example, when the capacity of a cellular telephone network is increased, the number of separate cells will increase, the radius of the cells will decrease, and the number of radio channels per one cell will increase. In such use it will be necessary to couple simultaneously a number of separate receivers/transmitters to one receiver/transmitter antenna in a multi-channel support station. The term receiver or reception used hereinafter is to be understood as also meaning transmission or a transmitter. A multi-channel reception can be implemented by using a so-called dividing unit. All of the connections of the dividing unit must be well matched within the entire reception band, and faults in individual receivers must not disturb the other receivers in operation. The dividing unit must be easy to extend and also small in size. Figure 1 shows a typical prior-art block diagram of such a dividing network. In this, the dividing unit is made up of a filter 2, an amplifier 3, and the actual dividing network 1 which divides the antenna signal. The multi-channel antenna signal ANT arrives, filtered and amplified, at the dividing network 1 at point 4. The dividing network 1 is typically made up of several parallel passive dividers, dividing by three or the like; by means of these dividers the received signals are divided equally among the separate receivers, outputs CH1, CH2-CHn. For extension of the dividing network, one unused output EXT is often left in the network. Figure 2 illustrates the structure of the dividing network in this prior-art system. The problem involved in the dividing unit described is how to implement the dividing so that the result will be a small-sized system requiring little external cabling. The extension of the network may also involve problems, as may electrical deficiencies in it, such as weak output matchings and possibly limited insulation between the outputs. At least some of the foregoing problems and disadvantages are addressed by the following invention, which provides a divider including a filter, an amplifier and a dividing network for a filtered and amplified signal, the dividing network comprising a cascade of N directional couplers for distributing the filtered and amplified signal in proportions according to respective coupling (k1-kn) of respective directional couplers, and respective receivers or transmitters coupled to respective directional couplers, characterised in that respective receivers or transmitters are coupled to respective directional couplers via respective controllable attenuators (ATT1-ATTn) for compensating for differences between the proportions of the signal distributed by respective directional couplers (H1-Hn;B1-Bn). The advantage of the present invention is that it facilitates the manufacture of equipment and the assembly, planning and cabling of the system. Furthermore, the attenuators compensate for variation in signal power output from respective couplers. In a preferred embodiment the filtered and amplified signal is derivable from a received multi-channel antenna signal and is divided among separate receivers by the dividing network, or the filtered and amplified signal is derivable from a summation by the dividing network of signals from transmitters for forming a multi-channel antenna signal, or a combination of both systems is provided. Such a preferred embodiment has an advantage in that it is suitable for telecommunications equipment where space and weight is a premium, since it does not require separate transmit and receive channel dividers. Preferably, the respective coupling (k1-kn) of the respective directional couplers in the cascade are substantially equal and the respective coupling ki = 10xlog 10 (1/N) dB, where i=1...n. This has the advantage that all the couplers can be made substantially the same thereby simplifying their design and implementation, and reducing production costs. In an alternate embodiment the directional couplers have couplings of different magnitude such that the respective coupling (ki) increases towards an end of the cascade in accordance with the formula ki = 10*log 10 (1/(N-i+1)))dB where i = 1.... n. This has the advantage that the signal power output from respective couplers is equal. If the attenuators are controllable automatically in a test state of the support station, the attenuators are always appropriately preset and do not have to be adjusted during operation of the divider. The invention is described below in greater detail by way of example only and with reference to the accompanying drawings, in which Figure 1 is a block diagram of a prior-art dividing unit; Figure 2 depicts in greater detail the dividing network of the dividing unit of Figure 1; Figure 3 is a block diagram of a cascade dividing network according to the invention, in which the couplings of the directional couplers are equal; Figure 4 depicts another embodiment of the cascade dividing network according to the invention, in which the couplings of the directional couplers are unequal; Figure 5 depicts, in the form of a block diagram, the structure according to the invention, applied to a transmitter system. Figure 3 depicts a cascade dividing network in accordance with the invention, and one embodiment of it, as a block diagram. A filtered and amplified multi-channel antenna signal 4 is led to matched directional couplers H1-Hn, by means of which the signals coming to receivers CH1-CHn of the channel units are branched. The receivers can be connected either directly or via controllable attenuators ATT1-ATTn. The cascade is terminated in an impedance I. In this embodiment the couplings k of the directional couplers are always equal. The optimal coupling k of the directional coupler is: k i = k opt = 10*log 10 (1/N) dB, wherein i= 1...n, whereby the minimum attenuation is attained. Thus, in this case: k 1 = k 2 = .... =k 1 = k n dB. Since the coupling in each directional coupler is the same, the signal level is attenuated after each directional coupler so that, after the i th output, the level has been attenuated: A (output i) = 10* (i-1)*log 10 (1-10 k/10 )+k dB as compared with input 4. For this reason the signal level is different in each output of the dividing network. The different values of the output-specific attenuations, i.e. in practice the signal levels, can be compensated for either by means of controllable attenuators ATT1-ATTn or by taking into account, when measuring the strength differences of the field, the attenuation difference in the receivers coupled to the dividing unit. The supplementary attenuation required when using controllable attenuators can easily be measured automatically when the support station is in the so-called RF-loop state judged on the basis of the strength reading of the field. The control can be automated by using, for example, voltage-controlled PIN diode attenuators. In Figure 3 the controls are indicated by reference numerals C1-Cn. The directional couplers H1-Hn of the dividing network 11 may be separate or be disposed directly in connection with the receivers inside the channel units, in which case it will be easy to attain a large electrical resolution between the directional couplers. In Figure 3 this situation is illustrated by unit 13, which, containing the possibly attenuator ATT, may be located in connection with the channel unit. Since an actual separate dividing unit is in such a case no longer necessary, the overall structure will be small, inexpensive and simple. In addition, the cabling will be simple to plan and to implement. In the case depicted in Figure 3, it is easy, by means of directional couplers, to attain a great electrical resolution also for the reason that the coupling necessary is of a small magnitude. For example, in an 8-channel system it is -9 dB, and for example in a 64-channel system it is -18 dB. A supplementary resolution of over -20 dB is easily achieved using a balanced structure of the directional coupler. Furthermore, the directional couplers H1-Hn are identical, in which case their design and implementation even in large production runs is inexpensive and precise. Further extension of a cascade dividing network 11 from the design length will be simple, since the attenuation from the input 4 of the dividing unit to the output is smaller than the attenuation from the input in question to the input of the channel units. The cascade can be continued by placing an amplifier in the place of the terminal impedance I. The increasing effect of the amplifier, which extends the cascade, on the overall noise level of the system is in such a case minimal (in the previous example 0.04 dB, if the noise level of the amplifier is 2 dB). The cascade dividing network 11 according to Figure 3 can, of course, be applied for use also in connection with a transmitter or transmitters; this situation is depicted schematically in Figure 5. The cascade dividing network according to the invention can also be implemented in a variable-coupling form. In this case the couplings of the directional couplers B1-Bn are not equal (Figure 4). In such a case the coupling k grows stronger as the channel number increases, according to Formula 4 ki = 10*log 10 (1/(N-L+1)) dB, where i varies within the range 1-n. In this case k1<k2<...<ki<...<kn is in force. It can be seen from the above formula that, when i approaches the value n, the coupling becomes kn = 0 dB, i.e. in a variable-coupling dividing network 21 dividing into n branches, only n-1 directional couplers are needed, and the n th directional coupler is replaced by direct coupling kn. The special features of this second embodiment example include, not only the above-mentioned absence of the n th directional coupler, but also the fact that channel-specific controllable attenuators are not at all necessary. Also in this case the directional couplers can be implemented as separate couplers directly inside the channel units, which is indicated by unit 23, depicted by broken line. Also in this case a high electrical resolution can be simply achieved between the directional couplers. Likewise, the cabling in the construction of the support station can be implemented in a simple form. Also in this case, the dividing network according to Figure 4 can be used in connection with a transmitter, for example in block 31 (in Figure 5). The cascade dividing networks according to the invention can be used most advantageously in multi-channel diversity support stations having more than 16 channels. In this case, in accordance with the above, the dividing network can be implemented within the channel units CH1-CHn. The preferred embodiment provides for a very simple antenna cabling between the channels and for good possibilities for extending the dividing network. In its electrical properties the cascade dividing network is easier to implement in accordance with the requirements than is a dividing network in accordance with the state of the art. Figure 5 shows schematically the use of a dividing network according to the invention in a transmitter system. In it, signals coming from separate transmitters T1-Tn are fed to the dividing network 31, from which they are fed via the filter 2 to the amplifier 3 in order to be fed further as a multi-channel antenna signal to the antenna ANT. In the case of a transmitter, the dividing network can be implemented either in a constant-coupling or variable-coupling form in accordance with the embodiment examples described above. In practice, however, the dividing networks used in transmitter coupling are such that the carrier-wave specific power levels must remain low in order that the total power loss should not be high. Although the dividing network according to the present invention has been dscribed above only as applied to a radio telephone network, it is clear that it can also be applied in other corresponding cases. Uses for the dividing network according to the present invention are found particularly at support stations of cellular telephone systems.

Claims: A divider including a filter (2), an amplifier (3) and a dividing network (1) for a filtered and amplified signal (4), the dividing network comprising a cascade (11;21) of N directional couplers (H1-Hn;B1-Bn) for distributing the filtered and amplified signal (4) in proportions according to respective coupling (k1-kn) of respective directional couplers (H1-Hn;B1-Bn), and respective receivers (CH1-CHn) or transmitters (T1-Tn) coupled to respective directional couplers (H1-Hn;B1-Bn), characterised in that respective receivers (CH1-CHn) or transmitters (T1-Tn) are coupled to respective directional couplers via respective controllable attenuators (ATT1-ATTn) for compensating for differences between the proportions of the signal distributed by respective directional couplers (H1-Hn;B1-Bn). A divider according to claim 1, wherein the filtered and amplified signal (4) is derivable from a received multi-channel antenna signal (ANT) and is divided among separate receivers (CH1-CHn) by the dividing network (1). A divider according to claim 1, wherein the filtered and amplified signal is derivable from a summation by the dividing network (1) of signals from transmitters (T1-Tn) for forming a multi-channel antenna signal (ANT). A divider according to any of claims 1 to 3, wherein the respective coupling (k1-kn) of the respective directional couplers (H1-Hn) in the cascade (11;21) are substantially equal and the respective coupling ki = 10xlog 10 (1/N) dB, where i=1...n. A divider according to any of claims 1 to 3, wherein the directional couplers (B1-Bn) have couplings of different magnitude such that the respective coupling (ki) increases towards an end of the cascade in accordance with the formula ki = 10*log 10 (1/(N-i+1))) dB where i = 1....n. A divider according to any preceding claim, in a radio telephone network support station. A divider according to claim 6, wherein the attenuators (ATT1-Attn) are controllable (C1-Cn) automatically in a test state of the support station. A divider according to any preceding claim, wherein the attenuators (ATT1-ATTn) are voltage controlled PIN diode attenuators. A divider according to any of the preceding claims, wherein the directional couplers (H1-Hn;B1-Bn) are incorporated into the receivers (CH1-CHn) or the transmitters (T1-Tn).

Inventors: HULKKO JAAKKO, HULKKO, JAAKKO
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1989
Main classification: H03H7
IPC codes: H03H7