The invention relates to a method and an arrangement for monitoring the condition of base station antennas in cellular radio systems. Such systems provide voice and data communication services. Several standards such as TACS, AMPS, D-AMPS, NMT and GSM have been established.
One problem encountered in such systems is to determine the integrity of radio base stations used in the systems when they have been installed. Different parts of the base station equipment may be damaged from vandalism or natural causes such as, for instance, moisture. The operator of the system must detect any occurring error promptly in order to achieve sufficient coverage and service level.
One way of determining the functionality of a radio base station is to perform a manual inspection. Service personnel then inspect the base station visually and perform on-site tests. Cellular systems, however, normally require large numbers of base stations in order to achieve sufficient coverage, and therefore manual inspection becomes very cumbersome and expensive.
A more feasible method to determine the integrity of a base station is to let the base station perform various self-tests and report any incorrect behaviour to the mobile network which the base station is part of. This may be performed regularly or when traffic is low. Means for performing such self-tests may be easily integrated into most circuitry and software and are therefore widely used today.
When it comes to antennas of base stations, however, particular solutions have to be considered in order to achieve self-test functionality. It is nevertheless vital to monitor the states of the antennas since they normally extend from the base station structures and therefore are more exposed to the environment.
One method for monitoring the condition of an antenna is to perform a voltage standing wave ratio (VSWR) measurement. This method involves feeding a radio frequency signal towards an antenna. The incidental and reflected powers are measured and compared. If the incidental power is much larger than the reflected power the antenna is likely to be functioning properly.
This method is suitable for transmitter antennas and for most duplex antennas used for both transmitting and receiving. In these cases the transmitter emits a test signal and the incidental and reflected powers are measured. These measurements may be performed with a directional coupler and power meter.
When it comes to receiver antennas, however, the task is more complicated. No transmitter circuitry is present and therefore a high frequency transmitter capable of emitting at least around 1 mW must be introduced. This adds not only cost but also complexity, especially if multiple receiver antennas are used to provide antenna diversity. Moreover, if the receiver is provided with a tower-mounted amplifier (TMA), which is used to improve the receiver range, yet a further complicating element has to be considered. The reverse attenuation of the TMA is very high and therefore VSWR measurements become very difficult.
Some attempts have nevertheless been made to develop arrangements for monitoring receiver antennas in base station systems where tower mounted amplifiers are used. These systems still perform VSWR measurements. One such arrangement is disclosed in U.S. Pat. No. 5,507,010. A signal, having a frequency outside the frequency band of the receiver is then emitted from a test unit in the base station and sent towards the receiver antenna. At the site of the tower mounted amplifier this signal is picked up by a filter. The frequency of the signal is then shifted into the frequency band of the receiver and the signal is coupled directly to the receiver antenna. Such an arrangement allows a VSWR measurement to be made from the base station at the cost of introducing additional filters and frequency changing means in the system. Another possibility is to place a separate VSWR measurement unit on top of the tower mounted amplifier, which also requires a substantial number of additional components to be used, including a powerful transmission circuit.
Another disadvantage with the VSWR approach is that the receiver and transmitter antennas of the system have to be tested separately, one at a time.
There are, however, methods described in the art for simultaneously testing two antennas in a mobile radio system. One such method and a corresponding apparatus are described in WO 97/00586. A test signal is then emitted from a transmitter unit and coupled via a transmission branch of a first duplex filter to a first antenna. The signal is picked up by a second antenna and is coupled via a transmission branch of a second duplex filter, coupling means and frequency shifting means to a receiver part of the system. Such systems function properly, however, only for testing duplex antennas, and require additional frequency shifting means.
It is to be noted that VSWR measurements on duplex antennas also may be difficult. This is particularly relevant when it comes to so called dual duplex tower mounted amplifiers. In such cases up to three resonant filters are connected between the transmitter unit and the antenna. The reverse attenuation of each filter may be around 20 dB. When three filters are series connected the total attenuation make measurements difficult.
One object of the present invention is to provide a simple but efficient arrangement for testing receiver antennas in radio base stations.
Another object of the invention is to provide a simple but efficient arrangement for testing receiver antennas in radio base stations, provided with multiple receiver antennas.
Another object of the invention is to provide a simple and inexpensive arrangement for testing receiver antennas provided with tower mounted amplifiers.
Yet another object of the invention is to provide an arrangement that allows simultaneous tests of at least one receiver antenna and at least one transmitter antenna.
Yet another object of the invention is to provide an arrangement that facilitates monitoring of duplex antennas provided with dual duplex tower mounted amplifiers.
These objects are achieved, according to one aspect of the invention in a radio base station where a receiver unit is connected to a first antenna and a transmitter unit to a second antenna. The receiver receives signals within a first frequency band and the transmitter transmits signals within a second frequency band. A radio frequency test signal having a frequency within the frequency band of the receiver is coupled to the antenna of the transmitter and thereby also to the antenna of the receiver unit. An evaluation unit connected to the receiver unit evaluates the signal received in order to determine the integrity of the receiver antenna. Since the frequency of the signal received lies within the frequency band of the receiver it may be evaluated with precision even if the signal originally emitted has a power much lower than, for instance, a signal used for VSWR measurements.
According to one embodiment of the invention the test signal emitter emits a signal within a spare channel, not regularly used by the receiver. As a result the testing procedure may be performed regularly without disturbing the ongoing traffic.
According to another embodiment of the invention the antenna connected to the transmitter is a duplex antenna also used to receive signals. This antenna is connected to a duplex filter and the above mentioned test signal is coupled to the antenna at a point between the duplex filter and the antenna. This allows testing to be performed on receiver antennas when the antenna coupled to the transmitter is a duplex antenna.
According to yet another embodiment of the invention, a coupling means used to couple the test signal to the transmitter antenna may also be used to perform voltage standing wave ratio measurements on the transmitter antenna. This arrangement allows the invention to be implemented at a low cost in installations where such VSWR measurements are performed.
According to yet another embodiment of the invention, the test signal emitter may also be used to implement separate tests of multicouplers and receivers in a receiving branch of the base station. This arrangement allows the invention to be implemented at a low cost in installations where such measurements are performed.
An arrangement according to the invention is then characterised as it appears from the characterising portion of claim 1.
A method according to the invention is then characterised, as it appears from the characterising portion of claim 10.