Method device base station and site for reducing the number of feeders in an antenna diversity diversity system

The invention generally relates to tower-mounted amplifiers (TMA) and antenna diversity. In particular the invention relates to a method and arrangement for reducing the number of feeders from an antenna diversity system to a radio base station.

Antenna diversity is used in order to improve reception (or transmission) of transmitted radio signals. There are many kinds of diversity, such as time diversity, space diversity, polarisation diversity and combinations thereof. Space and polarisation diversity reduces the effects of fading received radio signals.

An antenna diversity systems comprises at least two antennas arranged at a distance from each other or with different polarisation. In case of receive diversity the signal (RX signal) from a distant transmitter is received on the two or more antennas. In case of transmitter diversity the transmit signal (TX signal) is transmitted on the two or more transmit antennas to which the transmitter is connected. In the following the antennas of a diversity arrangement are called diversity antennas. It is possible to send and receive on the same antenna by connecting a duplex filter to the antenna, the duplex filter separating the TX and RX signals from each other. Signals are transported between an antenna and a transceiver (TRX) on a feeder. In prior art diversity arrangements a feeder and its associated antenna is referred to as a diversity branch or simply branch. In the context of the present invention a diversity branch comprises a diversity antenna and the devices the signal received on this antenna passes.

In the following diversity in connection with reception will be discussed. The RX signals from the diversity antennas are subjected to diversity processing in order to obtain an enhanced signal. Diversity processing may for example be to select the antenna signal which is strongest, or to add the signals and further process the resulting signal.

Each RF signal received at an diversity antenna arrangement is connected to respective tower-mounted amplifiers (TMAs) from which the amplified signal is transported in an individual diversity branch containing a feeder, typically a coaxial cable, which is connected to a transceiver TRX in the radio base station. Several branches are connected to one TRX. The TRX may be provided with a diversity receiver for demodulation and diversity signal processing.

A tower mounted unit is sometimes called a mast head amplifier. It should be noted that these units need not be mounted in a tower, but may be mounted on poles, walls of buildings, building roofs etc. The same goes for the diversity antennas. The invention is therefore not restricted to amplifiers mounted in towers. A tower mounted amplifier (TMA) is just a name under which a device of this kind is known to the man skilled in the art.

FIG. 1 discloses a prior art site comprising a radio base station (RBS) 1 , feeders 2 - 5 extending between the radio base station and TMAs 6 - 9 . Each TMA is connected to a respective diversity antenna 10 - 13 . The TMAs are all identical and in the following reference is therefore only made to TMA 6 . The antenna 10 is connected to a duplex filter 14 comprising a transmitter part (TX) 15 and a receiver part (RX) 16 . An RF amplifier 17 amplifies the received filtered RX signal and supplies it to another duplex filter 18 which comprises a transmitter part (TX) 19 and a receiver part (RX) 20 . receiver part (RX) 20 . The function of a duplex filter is to separate the TX signal from the RX signal and prevent the TX signal from leaking over into the receiving chain 21 . Signals received on antenna 10 , 11 , 12 and 13 respectively and processed in TMA 6 , 7 , 8 and 9 respectively follow a diversity branch A, B, C and D respectively.

The radio base station 1 comprises duplex filters 22 - 25 and low noise amplifiers 26 - 29 (LNA), one for each TMA. The heart of the radio base station is the transceiver units 30 , 31 (TRX 1 , TRX 2 ). Depending on the capabilities a transceiver has and the traffic capacity a radio base station is designed for, there may be just one TRX or many more transceivers than the two shown.

The arrangement shown in FIG. 1 is called 4 way diversity with 4 feeders.

In FIG. 2 a frequency spectrum illustrating the range of frequencies that can pass through the RX filter 22 , this range being the full RX band 32 allotted the service. The radio channel on which the RX signal to a transceiver is received and the TX signal is transmitted from the same transceiver is called a user channel. Since there are two transceivers shown in FIG. 2 there are two user channels, each shown with a small rectangle. The frequency band the user channels occupy is called a user band and this is indicated at brace 33 . In each one of the four branches a respective user band is present and it is the same (in terms of kHz) in all branches. A feeder, however, is capable to transport signals on all the frequencies up to several GHz including the full RX band 32 . It is thus apparent that the feeder in each branch is utilized with poor efficiency.

In principle one TRX is sufficient to diversity process the four RX signals and obtain the enhanced RX signal. A radio base station is however designed to handle large traffic volumes and therefore, and also for safety reasons, it comprises many TRXes. The output of each LNA is for this reason connected to all transceivers of the radio base station, as is shown by the various arrows, collectively shown at 34 .

U.S. Pat. No. 6,505,014 discloses a base station with an antenna diversity system connected to a multi coupler from which the antenna signals are fed to the respective receivers in individual feeders.

A drawback with the prior art is that each branch requires its own feeder. A diversity antenna system with many antennas will thus require as many feeders as there are antennas. Feeders are expensive. They are also heavy. Antennas are less expensive. Therefore, systems comprising many diversity antennas are prohibitive from economical point of view although they would be beneficial from reception quality point of view.

One object of the invention is to reduce the number of feeders compared with prior art and provide a method device, radio base station and system in accordance with claims 1 , 7 , 11 and 12 .

A characteristic feature of the invention is to move/frequency translate an RX signal received on one diversity antenna to a non used frequency and to consolidate/combine the frequency translated signal with an RX signal, that has not been frequency translated, and to forward the resulting composite signal on a single feeder to the radio base station.

Depending on the radio system in which the invention is used the meaning of “frequency” and “signal frequency” may be different. A preferred implementation of the invention is cellular mobile radio systems such as WCDMA, GSM, AMPS, NMT. The bandwidth of an WCDMA signal is 5 MHz, in GSM 200 kHz, in AMPS 30 kHz and in NMT 25 kHz. It is the signal with these respective bandwidths that is frequency translated to another non-used (by this RBS) part of the frequency band. In WCDMA the 5 MHz signal comprises voice and/or data from several users, in the GSM system the 200 kHz signal comprises voice and/or data from up to eight (8) users, in AMPS and NMT the 30 and 25 kHz signals comprise voice and/or data from one (1) user. Thus, in practice, the signal frequency is a frequency band.

FIG. 1 . is a block diagram of a prior art 4 way diversity system

FIG. 2 . is a frequency diagram illustrating the prior art diversity system of FIG. 1 ,

FIG. 3 . is a block diagram of a first embodiment of a 4 way diversity system in accordance with the invention,

FIG. 4 . is a frequency diagram illustrating the first embodiment of the invention,

FIG. 5 . is a block diagram of a second embodiment of a 4 way diversity system in accordance with the invention, and

FIG. 6 . is a frequency diagram illustrating the second embodiment of the invention,

FIG. 7 . is a block diagram of a third embodiment of the invention,

FIG. 8 . is a frequency diagram illustrating the third embodiment of the invention,

FIG. 9 . is a modification of the first embodiment,

FIG. 10 . is a frequency diagram associated with modification in FIG. 9 ,

FIG. 11 . is a block diagram of a fourth embodiment of the invention,

FIG. 12 . is a frequency diagram illustrating the fourth embodiment,

FIG. 13 . is a block diagram of a fifth embodiment comprising a separate frequency converter unit, and

FIG. 14 . is a frequency diagram associated with the frequency converter unit.

FIG. 3 illustrates an embodiment of the invention. Elements similar to those in FIGS. 1 and 2 bear the same reference designations. A novel tower mounted amplifier 35 comprises frequency converters 36 , 37 , 38 , each one connected to a respective antenna 11 , 12 and 13 in order to move the frequency of the received RF antenna signal to a respective non-used frequency by mixing the RF antenna signal with a respective reference signal f 1 , f 2 , and f 3 of a predetermined frequency. The signal received on antenna 10 is not moved in frequency. The reference signal may be a continuous wave signal, CW signal, a signal from a local oscillator or any equivalent.

The reference signal does not occupy a frequency band. Accordingly, the signal definition given above does not apply to the reference signal. A frequency converter is a device which as input receive the RX antenna signal and mixes it with the reference signal in order to obtain a frequency translated signal on an intermediate frequency (IF). The output of each frequency converter is connected to a respective band pass filter IF 1 , IF 2 and IF 3 . The amplified non-converted, original, radio signal received on antenna 10 in diversity branch A, and each IF signal in the respective branches B, C and D enters a combiner 39 wherein they are combined into a composite signal which is passed to a band pass filter RX 2 in duplex filter 18 . The duplex filter is connected to feeder 2 . Accordingly the composite signal comprising the signals in the respective diversity branches A-D is forwarded to the radio base station 1 on a single feeder.

Filter IF 1 prevents IF signals from converters 37 and 38 to leak into diversity branch B. Filters IF 2 and IF 3 have similar functions.

FIG. 4 is associated with FIG. 3 and is a frequency spectrum illustrating signals at different locations in the TMA and in the feeder. The user band is shown at 33 and the three intermediate frequencies are shown at 40 , 41 and 42 . This time the feeder is utilized with three times the efficiency provided with the prior art arrangement shown in FIGS. 1 and 2 .

In the shown embodiment the received radio signals are up converted to IF frequencies above the user band 33 . The IF frequencies are mutually different, i.e. the three IF signals at the output of the frequency converters lie at different IF frequencies.

The frequency range passed by filters RX 1 is shown at 43 . The frequency range passed by filter RX 2 is denoted the full RX band. Note that the IF frequencies must fall within a non-used part of the full RX band. In the shown embodiment the up converted IF signals should fall to the right of the frequency range 43 passed by filters RX 1 . In a situation where the frequency range 43 falls more to the right in FIG. 4 , as shown by the dashed brace 44 the IF signals should fall to the left of the dashed brace in order not to interfere with the user band 33 . The RX signals are thus down converted to an IF below the user band 33 . Situations may occur in which the IF signals fall on either side of the user band.

The arrangement in FIG. 3 provides 4 way diversity with 1 feeder.

At the radio base station the signals carried by the feeder pass the duplex filter 22 , the low noise amplifier 26 and enter the transceiver 30 in which they are split into four RX chains. The signals associated with diversity branches B, C and D are frequency converted. The signals in each of the diversity branches A-D are subjected to diversity processing. Diversity processing may take place in a diversity receiver.

The arrangement shown in FIG. 5 is similar to FIG. 3 , but this time the tower mounted amplifier 45 comprises only one frequency converter 36 . In combiner 39 the original signal in diversity branch A is combined with the IF signal in branch B into a composite signal which is forwarded to the radio base station on the single feeder 2 . This arrangement provides 2 way diversity with 1 feeder. The frequency spectrum in the single feeder 2 is shown in FIG. 6 .

An arrangement providing 4 way diversity with 2 feeders is shown in FIG. 7 . This arrangement is achieved by doubling the arrangement shown in FIG. 5 and uses two TMAs 45 . The feeders 2 , 4 from the two TMAs are connected to a respective duplex filter in the radio base station. As indicated above it is in principle possible to use one TRX only, but in practice the radio base station comprises several transceivers. It should be understood that both feeders 2 and 4 carry the same two user bands 33 and the same intermediary frequencies 40 - 42 The frequency diagram shown in FIG. 8 is thus identical for the two feeders.

An advantage with the TMA embodiments in which the direct antenna signal is consolidated with the IF signals and the resulting composite signal is forwarded on a single feeder to the radio base station is that the existing radio base stations need not be modified, since they already comprise frequency converters by which the IF signals plus the direct RF signals are transformed into the second IF frequency.

With the arrangement in accordance with the invention is achieved that the diversity signals are forwarded to the radio base station on a reduced number of feeders as compared to prior art where there are as many feeders as diversity antennas and each diversity signal is forwarded on a feeder of its own.

FIG. 9 illustrates a modification of the embodiment shown in FIG. 3 wherein all antenna signals are frequency converted. An additional frequency converter 46 is used and is inserted in diversity branch A of antenna 10 . In this embodiment four IF signals are consolidated into composite signal which is forwarded to the radio base station on the single feeder 2 . This provides 4 way diversity on 1 feeder. In this embodiment the radio base station comprises only one duplex filter, one low noise amplifier and one transceiver, although this has not been shown in the drawing. Since only one transceiver is used, only one user channel is used on the user band and therefore there no double rectangles are shown in the frequency diagram in FIG. 10 . In this embodiment the signals passed by filters RX 1 may be on a level of some GHz (10 9 ), and the IF signals on the feeder on a level of some MHz (10 7 ). The reference frequencies f 1 -f 4 fall in a frequency range in-between.

A modification of the arrangements shown in FIGS. 5 and 7 is to frequency convert all diversity antenna signals and use an additional frequency converter similar to frequency converter 46 in the embodiment of FIG. 9 .

In FIG. 11 the RX signals the signals on antennas 11 , 12 and 13 are subjected to a first frequency translation in frequency converters 36 , 37 , 38 (f 1 , f 2 , f 3 could be on the same frequency, but in that case f 5 , f 6 , f 7 must have different frequencies) which are connected to filters IF 1 , IF 2 and IF 3 . The RX signal on antenna 10 , however, is not subjected to any frequency translation in the TMA. A feature of the FIG. 11 embodiment is that the frequency translated IF signals on the outputs of filters IF 1 , IF 2 and IF 3 are subjected to a second frequency translation in frequency converters 47 , 48 , 49 by mixing them with a second set of reference signals f 5 , f 6 and f 7 and filter the resulting signals in IF filters RX 3 , RX 4 and RX 5 connected to the frequency converters 47 - 49 . The direct, original, antenna signal passes filter RX 1 in the duplex filter 14 , the low noise amplifier 17 and a second filter RX 1 , similar to RX 1 in the duplex filter 14 and is combined, in combiner 39 , with the twice frequency converted signals at the output of filters RX 3 -RX 5 . The composite signal in all diversity branches is forwarded on the single feeder 2 to the radio base station.

The reference signals f 5 -f 7 are so selected that the signals at the output of filters RX 3 -RX 5 fall on frequencies, shown at brace 50 in FIG. 12 , adjacent to the frequency, shown at brace 51 , of the filtered direct signal. This embodiment allows for use of SAW (surface acoustic wave) filters IF 1 -IF 4 with steep characteristics and accurate pass band frequencies.

FIG. 13 illustrates an embodiment comprising a frequency converter unit 52 connected between the tower-mounted amplifier 35 and the radio base station 1 . The frequency converter unit is used together with radio base stations that do not have a sufficient amount of frequency converters to frequency convert the split up signals into one and the same frequency so as to provide for diversity processing of the split up signals.

In FIG. 13 the TMA 35 is similar to the one shown in FIG. 9 but the notations used for filters and reference frequencies differ. The composite signal comprising the IF signals on the different IF frequencies are fed to the frequency converter unit 52 on the single feeder 2 .

The frequency converter unit 52 comprises a duplex filter 53 with a filter TX and a filter 54 (RX 2 - 5 ). The duplex filter is connected to frequency converters 55 - 58 of which 55 is connected to a duplex filter 59 and 56 - 58 are connected to a respective filter 60 - 62 . The RX filter of the duplex filter 59 and filters 60 - 62 are all similar to filters RX 1 in TMA 35 . The reference signals f 5 -f 8 are selected so that the resulting frequency converted signals at the outputs of the frequency converters 55 - 58 all are of the same frequency. The filtered frequency converted signals appear at the outputs of the frequency converter unit and are adapted for diversity processing.

A frequency diagram of the signals appearing in the feeder and the frequency converter unit 52 is shown in FIG. 14 . The frequency diagram for the composite signal in feeder 2 is the same as the one shown in FIG. 9 . Note that the RX 1 signal in the diagram illustrates the four output signals from the frequency converter unit.

Although not shown in the drawings it should be understood that a non-shown noise reducing filter is inserted after the low noise amplifier 17 in diversity branch A, that is the branch carrying the original, non-frequency translated RX signal.

In many of the above described embodiments of the invention the filter RX 2 and RX 2 - 5 respectively may be omitted provided the RX 2 -RX 5 filters at the outputs of the frequency converters prevent their respective signals from leaking over into an adjoining diversity branch.