METHOD OF IMPROVING AIR INTERFACE PERFORMANCE IN RADIO TELEPHONE SYSTEM

Abstract: The invention relates to a method of improving performance of an air interface (170) that exists between a base station (100) and a subscriber terminal (150) in a radio telephone system. In accordance with the invention, parameter values of an environment database (180) controlling the operation of the air interface (170) are adjusted to correspond to the operating conditions of the base station (100). The parameters comprise, for example: a selection between the base station (100) receiver sensitivity limitation and co-channel interference limitation, a maximum accepted speed of the subscriber terminal (150) communicating with the base station (100) via the radio connection (170), the diameter of the cell of the base station (100), a maximum delay spread, information indicating whether neighbouring base stations are synchronous.

Description: METHOD OF IMPROVING AIR INTERFACE PERFORMANCE IN RADIO TELEPHONE SYSTEM FIELD OF THE INVENTION The invention relates to a method of improving air interface per- formance in a radio telephone system in which an air interface implemented by a radio connection exists between a base station and a subscriber terminal. BACKGROUND OF THE INVENTION In the prior art, air interface performance is improved by installing supplementary devices in a base station. The operating environment of a base station located in rural areas, for example, is taken into account by disposing a base station antenna on the top of a special mast construction so as to attain better audibility and a wider coverage area. In addition, antenna amplifiers disposed on the top of the mast can be used, for example. A problem in the above described arrangement is that it brings about extra expenses. In addition, the described solution requires careful planning so as to have the necessary parts available at the site for carrying out the installation at the scheduled time. BRIEF DESCRIPTION OF THE INVENTION An object of the invention is thus to provide a method and equip- ment implementing the method so as to solve the above mentioned problems. This is achieved by a method of the type presented in the introduction, which is characterized by adjusting parameter values of an environment database controlling the operation of the air interface to correspond to the operating conditions of the base station. The invention also relates to a base station in a radio telephone system, having an air interface implemented by a radio connection to a subscriber terminal. According to the invention, the base station is characterized in that in the base station, the operation of the air interface is arranged to be controlled by an environment database whose parameter values are to be set to correspond to the operating conditions of the base station. The preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on the idea that the operation of a base station is improved by optimizing some of the parameters affecting the operation of the air interface, even at the expense of some other parameters, provided, however, that the overall performance of the air interface in the operating envi- ronment of the base station can be improved. The method and system of the invention provide many advantages. The invention does not require installation of supplementary devices at the base station. The operation of the base station can be optimally tuned according to the conditions at the installation site. It is not necessary to put the invention to use immediately in connection with the installation, but it can also be done later on when the base station is in use. The base station of the invention fulfils the necessary type-approval requirements when the environment database is not used. In certain special conditions, some properties need not even reach the required standard to ensure the best possible performance when the environment database is in use. If this is not acceptable, the operation of the base station can still be optimized within the limits of the standard required in the type approval. BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in more detail by means of preferred embodiments with reference to the accompanying drawings, in which: Figure 1 shows an example of a structure of the radio telephone system of the invention; Figure 2 shows the structure of a transceiver. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a typical structure of the radio telephone system of the invention. Only blocks essential to the description of the invention are included in Figure 1 , but it is obvious to those skilled in the art that a conventional radio telephone system also comprises other operations and structures which do not need to be described in more detail in this context. The examples show a digital radio telephone system using TDMA, but the invention is suitable for use in all types of radio telephone systems, in FDMA, CDMA and different types of hybrid systems and also in analog radio systems. A radio telephone system typically comprises a fixed network infra- structure, i.e. a network part, and subscriber terminals 150 that may be fixedly placed, vehicle mounted or portable terminal equipments. The network part includes base stations 100. Several base stations 100 are controlled in a centralized manner by a base station controller 102 communicating with them. The base station 100 comprises transceivers 114. Typically, the base station 100 comprises 1 to 16 transceivers 114. In TDMA systems, for example, one transceiver 114 provides radio capacity for one TDMA frame, in other words, typically for eight timeslots. The base station 100 comprises a control unit 118 controlling the operation of the transceivers 114 and a multiplexer 116. The multiplexer 116 assigns traffic channels and control channels used by several transceivers 114 to one transmission connection 160. The base station 100 transceivers 114 communicate with an antenna unit 112 that implements a bi-directional radio connection 170 to the subscriber terminal 150. The structure of the frames to be transmitted on the bi-directional radio connection 170, which is called an air interface, is accurately defined. Figure 2 gives a more detailed description of the structure of a transceiver 114. A receiver 200 comprises a filter that blocks frequencies beyond the desired frequency band. Next, a signal is converted into an interme- diate frequency or directly to a baseband, after which the signal is sampled and quantized in an analog-to-digital converter 202. An equalizer 204 compensates for interference caused by multipath propagation, for example. A demodulator 206 extracts a bit stream from the equalized signal, and the bit stream is transmitted to a demultiplexer 208. The demultiplexer 208 demulti- plexes the bit stream from different timeslots into distinct logical channels. A channel codec 216 decodes the bit stream of the distinct logical channels, in other words, it concludes whether the bit stream consists of signalling information to be transmitted to a control unit 214, or whether it is speech to be transmitted to a speech codec 122 of the base station controller 102. The channel codec 216 also carries out error correction. The control unit 214 carries out internal control tasks by controlling different units. A burst generator 228 adds a training sequence and a tail to the data coming from the channel codec 216. A multiplexer 226 assigns a specific timeslot to each burst. A modulator 224 modulates digital signals to a radio frequency carrier. This is an analog opera- tion required to be executed by a digital-to-analog converter 222. A transmitter 220 comprises a filter for narrowing down the bandwidth. Furthermore, the transmitter 220 controls transmission output power. A synthesizer 212 generates the required frequencies for different units. The synthesizer 212 includes a clock that can be controlled locally or in a centralized manner from elsewhere, from the base station controller 102, for example. The synthesizer 212 generates the required frequencies by a voltage-controlled oscillator, for example. The base station controller 102 comprises a group switching field 120, a transcoder 122 and a control unit 124. The group switching field 120 is used for speech and data switching and for connecting signalling circuits. The transcoder 122 converts different digital speech coding formats used between the public switched telephone network and the radio telephone network so that they are suitable for one another, for example from the 64 kbit/s format of the fixed network to some other format (for example 13 kbit s), and vice versa. The control unit 124 carries out call control, mobility management, gathering of statistical information, and signalling. In the example of Figure 1 , a connection is established via a mobile exchange 132 from the subscriber terminal 150 to a telephone 136 connected to a PSTN 134 (Public Switched Telephone Network). An OMC (Operations and Maintenance Centre) controlling and monitoring the operation of the radio telephone system communicates with the mobile exchange 132. The OMC 138 is typically a relatively powerful computer including special software. The control may also focus on individual parts of the system, since data transmission connections between different parts of the system can be provided with control channels required for transmitting control information. In addition, the personnel installing the network and monitoring its use can utilize for example a portable computer including management software 140 (EM = Element Manager) for management of individual network elements. In the example of the figure, the device 140 is connected to a data transmission port in the control unit 118 of the base station 100 and it can monitor and control the operation of the base station 100. It can examine and change the values of the parameters that control the operation of the base station, for example. In accordance with the invention, the base station 100 includes an environment database 180 arranged to control the operation of the air interface 170. The environment database 180 includes various parameters whose values can be set to correspond to the operating conditions of the base station 100. The parameters can be adjusted during both the installation stage and the operation stage of the base station 100. The air interface 170 operates normally for environment database 180 parameters whose values have not been adjusted, i.e. the default values are used. During the installation stage of the base station 100, the environment database 180 may not exist or it is not in use, whereby the base station 100 operates in normal mode. Later on, the environment database 180 can be added either as a whole or incrementally, whereby the parameters of the environment database 180 begin to control the operation of the air interface 170 of the base station 100. The environment database 180 parameters are set via the OMC 138 and/or the network element management computer 140 described above, for example. The invention is preferably implemented by software, whereby the invention requires relatively simple software modifications in an accurately specified section in the control unit 118 of the base station 100 and possibly in the control unit 214 of each transceiver. Furthermore, the invention naturally requires the environment database 180, and its size and complexity may vary greatly, the simplest parameters being words that are easy to use and under- stand. The environment database 180 can be technically implemented by utilizing general save, search and update services that are provided by database software and that may already exist in the base station 100 for other purposes. In the following, let us study by way of simple examples the use of the method of the invention under different conditions. Let us assume that the base station 100 is located in a rural area. In that case, the operation of its receiver is limited by its sensitivity. The antenna 112 is most likely placed on a mast to improve audibility. It is most likely that interfering signals caused by other radio transmitters and specified in the GSM specifications do not exist. In that case, the sensitivity of the receiver can be improved by increasing the amplification of an analog Low Noise-type antenna amplifier that is disposed in the antenna 112, and, consequently, the blocking of the interfering signals is reduced. It is then possible to reach a satisfactory compromise that is suitable for the base station 100 environment and that improves the overall performance of the base station 100. When the base station 100 is located in an urban area, the operation is most likely limited by co-channel interference, and the sensitivity of the receiver is less important than in rural conditions. In that case, various types of interference elimination methods improving the performance of the base station 100 receiver may be used in the presence of the co-channel interference in digital signal processing. On the other hand, the interference elimination methods usually reduce the sensitivity of the base station 100 receiver. In other words, co-channel interference elimination is supported at the expense of the sensitivity of the receiver, provided, however, that the overall performance is again improved. In urban conditions the accepted speed of the mobile station 150 is often limited. This information can be utilized in the base station 100 in controlling frequency correction, e.g in controlling the adaptive equalizer 204 according to the speed. The maximum delay spread can be limited in the base stations 100 operating in small cells or in inner cells. This means that the time duration of the channel impulse response can be limited, whereby the receiver performance can, again, be improved (as regards sensitivity and interference tolerance). If the base station 100 has synchronous neighbouring base stations, it is then possible to utilize interference elimination methods which re- quire that the training sequence in the signal transmitted by the interfering base station overlaps with the training sequence in the desired signal. When the operation of the base station 100 transmitter is controlled by the environment database 180, the operation of the air interface 170 can also be improved in the downlink transmission path towards the subscriber terminal 150. An example thereof is the control of an intelligent antenna solution, since randomizing the phase and setting the delay optimally to separate antenna signals depend on the operating environment of the base station 100 and they are controlled by the measurements made in said environment. Although the invention is described above with reference to an ex- ample according to the accompanying drawings, it is obvious that the invention is not restricted thereto, but it can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.

Claims: CLAIMS 1. A method of improving air interface (170) performance in a radio telephone system in which an air interface (170) implemented by a radio connection exists between a base station (100) and a subscriber terminal (150), characterized by adjusting parameter values of an environment database (180) controlling the operation of the air interface (170) to correspond to the operating conditions of the base station (100). 2. A method as claimed in claim 1, characterized in that the air interface (170) operates normally for parameters whose values have not been adjusted. 3. A method as claimed in claim 1, characterized in that the parameters are presented by guide words that are easy to use. 4. A method as claimed in claim 1, characterized in that the parameters comprise a selection between the base station (100) receiver sen- sitivity limitation and co-channel interference limitation. 5. A method as claimed in claim ^characterized in that the parameters comprise an accepted maximum speed of the subscriber terminal (150) having the radio connection (170) with the base station (100). 6. A method as claimed in claim ^characterized in that the parameters include the diameter of the cell of the base station (100). 7. A method as claimed in claim ^characterized in that the parameters comprise a maximum delay spread. 8. A method as claimed in claim ^characterized in that the parameters comprise information indicating whether neighbouring base stations are synchronous. 9. A method as claimed in claim ^ characterized in that at least some of the parameters are adjusted during the installation stage of the base station (100). 10. A method as claimed in claim 1, characterized in that at least some of the parameters are adjusted during the operation stage of the base station (100). 11. A base station (100) in a radio telephone system, having an air interface (170) implemented by a radio connection to a subscriber terminal (150), characterized in that in the base station (100), the operation of the air interface (170) is arranged to be controlled by an environment database (180) whose parameter values are to be set to correspond to the operating conditions of the base station (100). 12. A base station as claimed in claim 11, characterized in that at the air interface (170), the base station (100) is arranged to operate normally for parameters whose values have not been adjusted. 13. A base station as claimed in claim 11, characterized in that the parameters are arranged to be presented by guide words that are easy to use. 14. A base station as claimed in claim 11, characterized in that the parameters comprise a selection between the base station (100) receiver sensitivity limitation and co-channel interference limitation. 15. A base station as claimed in claim 11, characterized in that the parameters comprise an accepted maximum speed of the subscriber terminal (150) having the radio connection (170) with the base station (100). 16. A base station as claimed in claim 11, characterized in that the parameters include the diameter of the cell of the base station (100). 17. A base station as claimed in claim 11, characterized in that the parameters comprise a maximum delay spread. 18. A base station as claimed in claim 11, characterized in that the parameters comprise information indicating whether neighbouring base stations are synchronous. 19. A base station as claimed in claim 11, characterized in that at least some of the parameters are adjusted during the installation stage of the base station (100). 20. A base station as claimed in claim 11, characterized in that at least some of the parameters are adjusted during the operation stage of the base station (100).

Inventors: PIIRAINEN OLLI, PIIRAINEN, OLLI
Applicants: NOKIA TELECOMMUNICATIONS OY, PIIRAINEN OLLI, PIIRAINEN, OLLI
Application year: 1998
Main classification: H04Q
IPC codes: H04B7, H04W88