WO2002060197A1

WO2002060197A1 - Method and system for selecting an optimal antenna location in a telecommunication system

Info

Publication number: WO2002060197A1
Application number: PCT/SE2002/000131
Authority: WO
Inventors: Jan Arie Van Houwelingen; Olof Erik Grimlund; Kjell Arne Ljungdahl
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2001-01-26
Filing date: 2002-01-25
Publication date: 2002-08-01
Also published as: SE0100222D0

Abstract

In a telecommunication system employing at least one fixed radio terminal 10, 15 and at least one base station 100, where the downlink and uplink is transmitting on different frequencies, an acceptable location for an antenna 80 is determined. A lo-cation is selected for the antenna 80, and at least one signal is exchanged between the base station 100 and the antenna 80 at the selected location. A signal strength of the signal is measured, and an interference level of the signal is estimated. A deter-mination is made whether the selected location is acceptable based on the measured signal strength and the estimated interference level, with regards to both the down-link and uplink measurement values separately.

Description

Method and system for selecting an optimal antenna location in a telecommunication system

Technical field of the invention

This invention generally relates to a method and system for selecting an antenna location for a fixed radio terminal in a telecommunication system. More in particular, this invention relates to a method and a system for selecting an optimal antenna location where physical radio transmission properties require improved and adapted measurements.

Background of the invention

Fixed radio terminals have been used for several years in various communication systems, such as for instance the so-called Radio in the Local Loop (RLL), which is cellular systems employing fixed cellular radio terminals, e.g. any of the standardised GSM/TDMA/WCDMA systems. Fixed radio terminals also have been applied in satellite communication systems and in Wireless LAN systems. One of the major driving forces behind many of the communication systems available on the market is low line cost and one way of achieving this, is utilisation of fixed radio communication terminals.

In order to achieve low line cost, it is important that the fixed communication terminals are installed quickly and in a manner such that later reinstallation will not be necessary. Technically qualified staff for installation of terminals is costly, and must therefore be used as efficiently as possible. Since antennas are installed outdoors in most cases, it may be unpleasant and even dangerous for the installing staff to be exposed to sometimes harsh weather conditions for a longer period than needed, and perhaps they will have no other choice than to work on a slippery and hazardous roof. It is also important that the terminals are installed with as low a path loss as possible to provide a good link budget between the fixed terminals and the handling radio base station, and thereby minimising the number of base station sites required.

Typically, the optimal position for mounting an antenna, that is connected to one or several fixed terminals, is found by making signal strength measurements of the received signal at various locations. After having measured signal strengths, the location is selected having the strongest received signal strength. These signal strength measurements are often made by the terminal.

A problem with this approach is that the terminals, which are produced in a very large number of units, are typically not calibrated and may thus lead to inexact measurements. Assume, for example, that the terminals have a tolerance of ±4 dB in measured received signal strength, as required for instance by the GSM specification. A margin of 4 dB has to be added to compensate for terminals reporting signal strengths higher or lower than the actual received signal strength. Then, if the actual received signal strength measured by a terminal to be installed is 4 dB too low, the indication from the terminal is 8 dB too low. Thus, the signal strength indicated by the terminal might be too low, though the location is acceptable. It might, therefore, be hard, tedious and even hazardous for the installation staff to find an acceptable location for installing the antenna.

Moreover, the measured signal strength alone does not always provide a reliable es- timate of the link quality. Interference also should be taken into account. Interference can be estimated by measuring the bit error rate (BER). A problem with BER measurements is that they generally require a long period of time to perform, thus adding significantly to the installation time. Also seasonal changes like humidity and temperature variations affect measurements. For example, a 10^"3 BER target with a 6 dB margin for seasonal change means that a BER below 10^"6 should be measured. It may take up to 10 hours to collect enough measurements to ensure that the BER is below 10^"6, and such a long time for measuring transmission quality is unacceptable.

Prior art, like for example US Patent No. 6,035,183 discloses a way of providing signal quality information to fixed wireless access terminals. The system consists of a first and a second link, and further a switch for alternating signal measurements between the links. However, the system has several drawbacks and it does not provide a solution to the problem of obtaining sufficiently accurate measurements in communication systems using different frequencies on the downlink in comparison with the uplink.

The problem associated with measurements in frequency division duplex (FDD) systems is that measurements are not as simple and straight forward is they are in time division duplex (TDD) systems. This is due to the fact that in FDD-systems, the frequencies used for downlink and uplink are slightly different, which is not the case for the corresponding TDD-systems. Physical properties associated with radio transmission, like for example path attenuation and fading characteristics, are closely related to the transmission frequency, and thus measurements of the optimal antenna location is systems with separated downlink and uplink frequencies have to be more sophisticated than for systems with equal downlink and uplink frequencies.

There is thus a need for an improved technique for selecting an antenna location in a telecommunication system, using different downlink and uplink frequencies, that is easy, quick and accurate.

Summary of the invention

It is therefore an object of this invention to provide an improved technique for selecting an antenna location in a telecommunication system that is easy, quick and accurate. The technique here disclosed is a lot more effective and sophisticated than corresponding techniques of prior art, although not more complicated. The present invention alleviates the problems associated with prior art technology by means of a telecommunication system employing at least one fixed radio terminal and at least one base station, using different frequencies for downlink and uplink, a method and a system for determining an acceptable location for an antenna, the method comprising the steps of and the system comprising means for: selecting a location for the antenna; exchanging at least one signal between the base station and the antenna at the selected location; measuring a signal strength of the signal; estimating an interference level of the signal; and determining whether the selected location is acceptable based on the measured signal strength and the estimated interference level, with regards to both the downlink and uplink measurement values separately.

According to exemplary embodiments, the terminal may be mounted indoors or outdoors. The antenna may be mounted indoors, e.g. if the antenna is close to the base station, or outdoors. This is advantageous as no separate outdoor installation of the antenna is necessary unless the conditions for radio transmission requires any outdoor installation of an antenna.

According to one embodiment, the signal strength may be measured at the terminal. According to another embodiment, the signal strength may be measured at the base station and reported to the terminal. It is beneficial to have the option of choosing whether to measure signal strength at the base station or at the terminal, and the present invention admits this option.

According to exemplary embodiments, the estimation of the interference level may be performed by measuring an error rate of the signal at the terminal or at the base station. In order to speed up measurements, the signal may be attenuated by decreasing the output power from the base station or from the terminal. Hereby is achieved a much faster measurement, whereby an installation can be made not only faster, but also safer and less costly. This is a tremendous advantage of the present invention, which is beneficial for all involved parties.

Brief description of the drawings

The features, objects, and further advantages of this invention will become apparent by reading this description in conjunction with the accompanying drawings, in which like reference numerals refer to like elements and in which:

Fig 1 schematically illustrates a fixed radio communication system according to the present invention.

Fig 2 illustrates an alternative embodiment of the fixed radio communication system according to the invention.

Fig 3 is a diagram showing the dependency of signal to noise ration versus bit error rate in a system according to the invention.

Fig 4 depicts a flow chart for finding an acceptable location for an antenna in a communication system according to the invention.

Fig 5 illustrates how measurements are performed with regards to the downlink and uplink transmission quality separately. Detailed description

The following description is of the best mode presently contemplated for practising the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be ascertained with reference to the issued claims.

For illustrative purposes, the following description is directed to a cellular radio communication system, but it will be understood that this invention is not so limited and applies to other types of communication systems.

According to exemplary embodiments, signal strength and signal interference are considered in determining where to install an antenna for a fixed radio terminal. The signal strength and the signal interference may by derived from several measurements averaged together or from a maximum measured value. However, many other state of the art techniques for measuring and analysing signals and transmission properties are conceivable and those techniques are well-known to any person skilled in the art.

According to a first aspect of the present invention, a fixed radio telecommunications terminal mounted indoors can be used together with an antenna unit mounted outdoors. Fig 1 illustrates a communication system with a fixed radio terminal, e.g. a fixed cellular terminal 10, mounted indoors and an antenna unit 80 mounted outdoors. The fixed cellular terminal 10 receives signals from and transmits signals over an air interface 90 to a base station 100, for example via the antenna unit 80. The fixed cellular terminal 10 may be connected to at least one telephone 30 and/or personal computer 50. The fixed cellular terminal 10 includes radio parts, digital parts and other parts, e.g. a power supply. Of course, state of the art communication capabilities like Bluetooth and infrared (IR) transmission may be included in the fixed radio terminal 10 and its related parts. Also, the fixed cellular terminal 10 may include a data adapter 20 for fax and data to facilitate communication with a per- sonal computer 50 via a modem 40. The fixed cellular terminal 10 is connected to the antenna unit 80 via, for instance a coaxial cable 82. It should be appreciated that the antenna unit 80 instead may be mounted indoors, in particular if the terminal is placed close to the base station. If the measurements of signal strength and transmission quality allows for indoor mounting of the antenna unit 80, it is a preferred sim- plification of the outdoors installation procedure, which otherwise will be needed.

Fig 2 illustrates an exemplary communication system according to another aspect of the invention. In the system shown in Fig 2, the radio parts 15 are located outdoors. Other devices, e.g. a data adapter 25 and a power supply 27, 29, 35, 37 may be lo- cated indoors to facilitate simplified installation and replacement.

According to an exemplary embodiment, received signal strength may be measured at the fixed terminal 10, 15 or at the antenna 80. To increase the signal strength measurement accuracy, external calibrated measurement equipment can be used, e.g. equipment connected to the external antenna 80. Alternatively, the communication terminal 10, 15 can be calibrated, e.g. in the factory, or hardware/software can be added in the terminal 10, 15 so that the terminal 10, 15 is able to measure with higher accuracy.

According to another embodiment, the signal strength may be measured by measuring the received signal strength in a signal transmitted from the radio terminal 10, 15 to the base station 100. The transmitted signal from the terminal 10, 15 has a relatively high accuracy, e.g. ±1 dB, thus affording an accurate measurement. The measurement equipment in the base station 100 can be calibrated with a low relative increase in cost. Usually, components used in base stations are of higher quality than components used in high volume terminals. Components of higher quality and performance are affordable which may not be the case for terminals produced in very high volumes, where the cost for the end consumer is a crucial parameter. As a consequence of this, the accuracy in signal transmission from base stations are usually better than from terminals. The measured signal strength can be reported by the base station 100 to the terminal 10, 15, preferably as a short message service (SMS) message over one of the control channels.

According to an exemplary embodiment, a BER measurement may be made at the terminal 10, 15. Alternatively, the BER measurement may be made at the base sta- tion 100, and the result of the measurement may be sent to the terminal 10, 15, e.g. as an SMS message. As yet another alternative, the terminal 10, 15 may transmit a known bit pattern to the base station 100, the base station 100 may receive and retransmit the pattern back to the terminal 10, 15, and the terminal 10, 15 may then measure the BER in the received re-transmitted pattern.

With reference to Fig 3, illustrating a diagram of signal to noise ratio versus BER, the BER measurement can be made faster by attenuating the transmitted signal. Assume for example, that the target BER for an acceptable amount of interference is 10^"6 (including margin). If the received signal is attenuated by 6 dB, the BER target of 10^"6 translates into a target of about 10^"3. Only a few minutes are required to make sure the BER is below 10^"3. Thus, by attenuating the received signal, the BER measurement time is reduced from several hours to only a few minutes. Hereby, the time required for installing the terminal, or its antenna, with sufficient transmission performance is greatly reduced.

Further with reference to Fig 3, another embodiment of the invention is that attenuation may be achieved by reducing the output power at the base station 100, if the BER measurement is made at the terminal 10, 15. However, it is also possible by reducing the power of the terminal 10, 15, if the BER measurement is made at the base station 100 or if the BER measurement is made at the terminal 10, 15 using a re-transmitted pattern received from the base station 100. In the GSM system, for example, attenuation may be achieved using the downlink power control mechanism in which the radio terminal 10, 15 reports a measured signal strength to the base station 100 which uses the reported measured signal strength to control the downlink output power. If the radio terminal 10, 15 purposely reports too high a meas- ured signal strength during the measurement period, the base station 100 responds by reducing the downlink output power, thus attenuating the signal received by the terminal 10, 15.

Based on the measured signal strength and the BER, the terminal 10, 15 determines whether the selected location for the antenna 80 is acceptable. This determination may be made, e.g. by a microprocessor in the terminal 10, 15. For example, if the measured signal strength is within a predetermined range, and the BER is less than a predetermined target, the location is determined to be acceptable. The range for the measured signal strength depends, e.g. on the technology, environment, and the distance from the base station 100. For a GSM terminal in general, an example of an acceptable measured signal strength range may be -80 dBm to -90 dBm.

Fig 4 is a flowchart depicting a method for selecting a location for mounting an antenna according to an exemplary embodiment. The method begins (S10) with an initial location of an antenna 80, which is selected (S20), based for instance on the direction of the base station 100. At the next step (S30), a signal is exchanged between the base station 100 and the antenna 80 at the selected location. Afterwards, a signal strength of the signal is measured (S40). The signal strength may be measured, at the terminal 10, 15 or may be measured at the base station 100 and reported to the terminal 10, 15. Furthermore, an interference level of the signal is estimated

(S50), based e.g. on BER measurements made at the terminal 10, 15 or made at the base station 100 and reported to the terminal 10, 15. The signal quality estimation or measurement value is displayed to a user, or installation staff, at the terminal 10, 15 by means of for instance a set of diodes or by a connected external measurement (not shown) instrument indicating the present measurement value. At the next step in the sequence, a determination is made whether the selected location is acceptable (S60), based on the measured signal strength and the estimated interference level. If the location is not acceptable, the process starts (S10) again, and another location is selected (S20). If the location is acceptable, the antenna 80 and terminal 10, 15 are mounted (S70), whereby the measurement procedure and installation sequence ends (S80).

Fig 5 illustrates the attenuation pattern of two different transmission signals in an FDD-system with different frequencies for the downlink (DL) and uplink (UL) respectively. The striped attenuation pattern depicts the downlink signal and the full attenuation pattern depicts the uplink, which is slightly shifted. The striped line depicts a predetermined limit for the downlink and the full line depicts a predetermined limit for the uplink. If any of the measured values go below either of its respective limiting lines, the received signal strength and quality is insufficient, and hence another position for mounting the antenna must be found. When measuring the signal strength and quality, for instance by installation staff on a roof, the downlink can either be measured as an absolute value or a value relative to the predetermined limit. It is preferable to find the optimum position with regards to both downlink and uplink, and therefore, if the maximum value (M) is found for either of the downlink or uplink transmission, a small adjustment can be made of the antenna position in order to find an acceptable position. The corresponding measurement value (A), in an acceptable position for placing the antenna, where both of the transmission directions are considered has also been pointed out in the figure.

It will be appreciated by those of ordinary skill in the art that this invention can be embodied in other specific forms without departing from its essential character. The embodiments described above should therefore be considered in all respects to be illustrative and not restrictive. For example, although described above with reference to a GSM system employing a fixed cellular terminal, the invention is also applicable in other types of communication systems.

Claims

1. In a telecommunication system employing at least one fixed radio terminal (10, 15) and at least one base station (100), using different frequencies for downlink and uplink, a method for determining an acceptable location for an antenna (80), the method comprising the steps of: selecting a location for the antenna (80); exchanging at least one signal between the base station (100) and the antenna (80) at the selected location; measuring a signal strength of the signal; estimating an interference level of the signal; and determining whether the selected location is acceptable based on the measured signal strength and the estimated interference level, with regards to both the downlink and uplink measurement values separately.

2. The method of claim 1, wherein the signal strength and the interference level are measured at the terminal (10, 15).

3. The method of claim 1, wherein the signal strength is measured at the base sta- tion (100) and reported to the terminal (10, 15).

4. The method of claim 1, wherein the step of estimating an interference level comprises measuring an error rate of the signal.

5. The method of claim 4, further comprising attenuating the transmitted signal before measuring the error rate.

6. The method of claim 4, wherein the error rate is measured at the terminal (10, 15).

7. The method of claim 5, wherein the error rate is measured at the base station (100) and reported to the terminal (10, 15).

8. The method of claim 7, wherein the step of attenuating comprises decreasing the output power of the base station ( 100).

9. The method of claim 7, wherein the step of attenuating comprises: reporting to the base station (100) a signal strength that is higher than the measured signal strength; and attenuating the signal transmitted by the base station (100) based on the reported signal strength.

10. The method of claim 5, wherein the step of attenuating comprises decreasing the output power of the terminal (10, 15), and the error rate is measured at the base station ( 100) and reported to the terminal (10,15).

1 1. The method of claim 4, wherein the terminal (10, 15) sends a known bit pattern to the base station (100), the base station (100) re-transmits the bit pattern to the terminal (10, 15) without changes, and the error rate is measured at the terminal (10, 15).

12. The method of claim 5, wherein the terminal (10, 15) sends a known bit pattern to the base station (100), the base station (100) retransmits the pattern without changes, and the error rate is measured at the terminal (10, 15).

13. The method of claim 12, wherein the step of attenuating comprises decreasing the output power of the terminal (10, 15).

14. The method of claim 12, wherein the step of attenuating comprises decreasing the output power of the base station (100).

15. A system for determining an acceptable location for an antenna (80) in a communication system comprising at least one fixed radio terminal (10, 15) and at least one base station (100), using different frequencies for downlink and uplink, the system comprising: means for measuring a signal strength of at least one signal exchanged between the base station (100) and the antenna (80) at a selected location; means for estimating an interference level of the signal; and means for determining whether the selected location is acceptable based on the measured signal strength and the estimated interference level, with regards to both the downlink and uplink measurement values separately.

16. The system of claim 15, wherein the means for measuring the signal strength is included in the terminal (10, 15).

17. The system of claim 15, wherein the means for measuring the signal strength is included in the base station (100), and the base station (100) includes means for reporting the measured signal strength to the terminal (10, 15).

18. The system of claim 15, wherein the estimating means estimates an interference level by measuring an error rate of the signal.

19. The system of claim 18, further comprising means for attenuating the transmitted signal before measuring the error rate.

20. The system of claim 18, wherein the error rate is measured at the terminal (10,

15).

21. The system of claim 19, wherein the error rate is measured at the base station (100) and reported to the terminal (10, 15).

22. The system of claim 21, wherein the means for attenuating decreases the output power of the base station (100).

23. The system of claim 22, wherein the means for attenuating comprises: means for reporting to the base station (100) a signal strength that is higher than the measured signal strength; and means for attenuating the signal transmitted by the base station (100) based on the reported signal strength.

24. The system of claim 18, wherein the means for attenuating decreases the output power at the terminal (10, 15), and the error rate is measured at the base station (100) and reported to the terminal (10, 15).

25. The system of claim 17, wherein the terminal (10, 15) sends a known bit pattern to the base station (100), the base station (100) retransmits the bit pattern to the terminal without changes, and the error rate is measured at the terminal (10, 15).

26. The system of claim 18, wherein the terminal sends a known bit pattern to the base station (100), the base station (100) retransmits the bit pattern to the termi- nal without changes, and the error rate is measured at the terminal (10, 15).

27. The system of claim 26, wherein the means for attenuating decreases the output power of the terminal (10, 15).

28. The system of claim 26, wherein the means for attenuating decreases the output power of the base station (100).