WO1994011957A1 - A method for improving radio coverage in a cellular radio system and a cellular radio system - Google Patents

A method for improving radio coverage in a cellular radio system and a cellular radio system Download PDF

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Publication number
WO1994011957A1
WO1994011957A1 PCT/FI1993/000372 FI9300372W WO9411957A1 WO 1994011957 A1 WO1994011957 A1 WO 1994011957A1 FI 9300372 W FI9300372 W FI 9300372W WO 9411957 A1 WO9411957 A1 WO 9411957A1
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WO
WIPO (PCT)
Prior art keywords
channel
station
repeater
base station
mobile
Prior art date
Application number
PCT/FI1993/000372
Other languages
French (fr)
Inventor
Martin William Greenwood
Original Assignee
Nokia Telecommunications Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nokia Telecommunications Oy filed Critical Nokia Telecommunications Oy
Priority to AU49633/93A priority Critical patent/AU4963393A/en
Publication of WO1994011957A1 publication Critical patent/WO1994011957A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
    • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
    • H04B7/2603—Arrangements for wireless physical layer control
    • H04B7/2606—Arrangements for base station coverage control, e.g. by using relays in tunnels
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
    • H04W16/24—Cell structures
    • H04W16/26—Cell enhancers or enhancement, e.g. for tunnels, building shadow

Definitions

  • the present invention relates to a method for imp- roving radio coverage in a cellular radio system, comp ⁇ rising at least one base station and a plurality of mobile stations communicating with said base station, and at least one repeater station retransmitting messages re ⁇ ceived from the base station to at least one mobile stati- on.
  • the invention also concerns a cellular radio system operating according to this method.
  • Radio base stations communicate by means of radio signals with the mobile terminal, which may be a handportable radio telephone or may be a telephone in a car, or may be some other kind of equipment which is gain ⁇ ing access to the radio network.
  • the mobile terminal which may be a handportable radio telephone or may be a telephone in a car, or may be some other kind of equipment which is gain ⁇ ing access to the radio network.
  • the mobile terminal may be a handportable radio telephone or may be a telephone in a car, or may be some other kind of equipment which is gain ⁇ ing access to the radio network.
  • a possible alternative to installing more base sta- tions is to install a mixture of base stations and radio repeaters.
  • a radio repeater takes the radio signal from the base station, amplifies it, and re-transmits it to the mobile. It also receives signals from the mobile and re ⁇ transmits them to the base station. If a repeater is in- stalled at some distance from the base station, it can improve local coverage in an area where coverage would otherwise be poor. Repeaters are commonly used in this matter for TV and other radio systems, and to some extent in cellular radio.
  • a major problem with repeaters in cellular radio which is solved by this invention, is the need to maintain isolation between the receiving antenna and the transmit ⁇ ting antenna.
  • the repeater As radio signals from the base station are received by the repeater by an antenna, the repeater then amplifies the signals, typically by 60dB. The amplified signals are re-transmitted by the repeater by another an ⁇ tenna to the mobile. If, though poor design or incorrect installation, signals from the second transmitting antenna are received by the first receiving antenna, it is possi- ble that a feedback loop occurs, whereby the repeater re- amplifies the same signals until it becomes saturated. In such a condition the repeater will cease to function as a repeater and will merely transmit a maximum-power signal, causing interference to other network users. This condition may be difficult to avoid, whilst still having sufficient amplification for the repeater to be effective.
  • the two antennas of a repeater station will have to be physically well separated, perhaps with one antenna atop a tower, and the other at the bot- tom, the receiving antenna being highly directional in favours of signals transmitted from the base station.
  • the practical problem of installing antennas with perhaps 80 dB isolation between them makes the installation expen ⁇ sive.
  • the physical separation requires towers, feed cables and so on, and then the radio frequency testing and com ⁇ missioning must be conducted by highly skilled techni ⁇ cians.
  • the object ot the present invention is to overcome the drawbacks and difficulties explained above.
  • the inventive method is character ⁇ ized by the steps of
  • the repeater re-transmits on frequencies different from those it receives.
  • the second is that the base station uses a different frequency from that which it commands the mobile to use.
  • the repeater receives signals on one or more carrier frequency and re-transmits essentially similar signals on one or more different RF carrier frequency. This eliminates the risk of the re ⁇ peater suffering instability as a result of feedback from one antenna to the other.
  • a base station will transmit messages to the mobile, both during call set-up and while the call is in progress, which instruct the mobile to use a particular radio frequency. If a frequency-shifting re ⁇ peater were used in conjunction with a conventional base station, then there would be a problem with base station to mobile communications via the repeater. The mobile would be instructed by the base station to use a particu ⁇ lar radio frequency but when base station and mobile use that frequency, the repeater would introduce a frequency offset into the communication which would disrupt the link. So far as the mobile was concerned, the signals it was receiving would be on the wrong frequency.
  • the present invention overcomes this problem by causing the base sta ⁇ tion to offset the frequency of its transmission by a cer ⁇ tain amount, so that the repeater shifts the transmission back to a frequency which is consistent with the control signals that are being sent to the mobile.
  • the invention enables the use of repeaters with low installation cost, having eliminated the risk of feedback instability in the repeater.
  • the antenna installation is much simpler, reducing the total infrastructure cost, whilst enabling a higher amplification to be used in the repeater.
  • the changes are transparent to the mobile, so that no modification needs to be made to the mobile.
  • it can be shown that in the preferred embodiment, it is possible to make the base station modifications without changes to the hardware or site visits.
  • All radio links, whether direct or via the repeater use the normal frequency bands and protocols so that the invention may be employed without having additional frequency bands allocated for the net ⁇ work and without change to the regulatory certification of the equipment.
  • figure 1 shows the general situation of downlink transmission via a repeater
  • figure 2 shows the downlink transmission situation according to the present invention
  • figure 3 shows a preferred embodiment of a base station according to the present invention
  • figure 4 shows a preferred embodiment of a repeater according to the present invention
  • figure 5 is a frequency allocation scheme for the network.
  • the basic situation in a cellular network with repeater links shown.
  • Radio signals from the base station 1 are received by the repeater 2 by an antenna A.
  • the repeater 2 amplifies the signals, typi ⁇ cally by 60dB.
  • the amplified signals are re-transmitted using the same frequency by the repeater with an antenna B to a mobile 3.
  • This prior art system is afflicted with all the drawbacks and problems explained above.
  • the present invention is illustrated in Figure 2.
  • the base station 4 instructs the mobile 6 to use Channel X.
  • the base station 4 is, however, transmitting on Channel Y.
  • the repeater 5 however, translates Channel Y into Channel X, whilst also amplifying it, and the mobile re ⁇ ceives a signal which is on Channel X.
  • This method applies both to calling (signalling) channels, which a.o. things may instruct the mobile to use a certain voice channel (i.e. channel X) , and to the voice channel itself.
  • a mobile without modifications in the mobiles it is not possible for a mobile to estab ⁇ lish a call via the radio frequency that is intended for the base station to repeater part of the link.
  • a preferred embodiment of the base station is il ⁇ lustrated in Figure 3, which shows the signal processing in the base station. Again, for clarity, only the downlink path is shown.
  • an extra soft- ware module CCS is inserted into the channel selection control of the base station, between the Link Control Software LCS and the TRX unit, which is the sub-system which transmits and receives radio signals, converting between RF signals and baseband signals.
  • the extra soft- ware module CCS is a Channel Conversion Software, and it translates the channel number that is requested by the link control software into a new channel number that is consistent with the operation of the repeater.
  • the TRX unit is primarily hardware and is respon- sible for putting voice and data information onto the air interface, and for receiving the air interface and ex ⁇ tracting voice and data information.
  • the LCS is primarily software and is responsible for control of the transmis ⁇ sion links.
  • the software module CCS may be down-loaded into the equipment remotely and is simple to design because may be an independent part of the link control software. Messages and voice traffic are carried out in the normal way through the TRX unit.
  • figure 4 which shows the signal processing in the repeater, the signals from the base station are re ⁇ ceived at antenna A. The signals are filtered and ampli ⁇ fied in a filter 7 and an amplifier 8, respectively. The signal is then mixed by a mixer 9 using a local oscillator 10.
  • the SAW filter filters a fixed band, and by making the local oscillators 10 and 14 to have different frequencies, it is possible to shift the frequency of the in-band signals between reception at an- tenna A and transmission at antenna B.
  • the local oscillators 10 and 14 are both programmable, which enables the amount of frequency shift and the centre fre ⁇ quency of the re-transmitted band to be dynamically al ⁇ tered, according to the traffic load and other circum- stances.
  • the programming is preferably done by remote control, e.g. from a base station or a switching center of the cellular system.
  • Figure 5 shows a typical frequency allocation plan which might be used in connection with the present inven- tion.
  • the link control software of the base station allo ⁇ cates channels to links in the ranges 11 - 18 and 22 - 50.
  • the Channel Conversion Software (see figure 3) translates the TRX frequency control programming so that channels 11 to 18 are converted to channels 1 to 8.
  • Channels 22 to 50 are unaffected.
  • the repeater converts channels 1 to 8 back to channels 11 to 18.
  • the base station may communi ⁇ cate with a mobile on any of channels 22 to 50 if the com ⁇ munication is direct, or on any of channels 11 to 18 if via a repeater.
  • Channels 9, 10 and 19 to 21 are unused to avoid possible edge of band problems.
  • the repeaters can be daisy- chained, whereby the frequency allocation plan is used to provide different channels for communication between each pair of repeater stations, e.g. in order to cover with one single base station a greater and/or a topographically very demanding area, than has been possible with prior art solutions.

Abstract

The invention discloses a method for improving radio coverage in a cellular radio system and a cellular radio system, which comprises at least one base station and a plurality of mobile stations communicating with said base station, and at least one repeater station for retransmitting messages received from the base station to at least one mobile station. According to the inventive method a first communication channel to a mobile station (6) is converted in the base station (4) into a second communication channel having a frequency offset from the first channel, in order to send a message to a repeater station (5) on the second channel. Correspondingly, in the repeater station (5), the message is received on the second channel, the second channel is converted back to the first channel, and finally, the message is transmitted to the mobile station (6) on the first channel.

Description

A method for improving radio coverage in a cellular radio system and a cellular radio system
The present invention relates to a method for imp- roving radio coverage in a cellular radio system, comp¬ rising at least one base station and a plurality of mobile stations communicating with said base station, and at least one repeater station retransmitting messages re¬ ceived from the base station to at least one mobile stati- on.
The invention also concerns a cellular radio system operating according to this method.
Cellular radio systems rely on a network of radio base stations. The base stations communicate by means of radio signals with the mobile terminal, which may be a handportable radio telephone or may be a telephone in a car, or may be some other kind of equipment which is gain¬ ing access to the radio network. In principle, it should be possible for a subscriber to make or receive a call anywhere within the supposed region in which the network operator is offering service.
In practice, there are always places where coverage is poor. For instance, indoor coverage is generally worse than outdoor and built-up areas tend to be worse than areas of open terrain. Usually, a network operator will compensate for this by installing more base stations in regions where the coverage is poor. However, there are new cellular radio systems under development which operate at a substantially higher radio frequency, which pose a greater problem in establishing good coverage. For in¬ stance, services based on the new DCS1800 standard will be operating in the region of 1800 MHz, double the frequency of existing cellular services. At these higher frequen¬ cies, the coverage area from a single base station is re- duced by a factor of at least four. To achieve the same degree of coverage, the number of base stations must be greatly increased, which means that the infrastructure cost will also be greatly increased.
A possible alternative to installing more base sta- tions is to install a mixture of base stations and radio repeaters. A radio repeater takes the radio signal from the base station, amplifies it, and re-transmits it to the mobile. It also receives signals from the mobile and re¬ transmits them to the base station. If a repeater is in- stalled at some distance from the base station, it can improve local coverage in an area where coverage would otherwise be poor. Repeaters are commonly used in this matter for TV and other radio systems, and to some extent in cellular radio. A major problem with repeaters in cellular radio, which is solved by this invention, is the need to maintain isolation between the receiving antenna and the transmit¬ ting antenna. As radio signals from the base station are received by the repeater by an antenna, the repeater then amplifies the signals, typically by 60dB. The amplified signals are re-transmitted by the repeater by another an¬ tenna to the mobile. If, though poor design or incorrect installation, signals from the second transmitting antenna are received by the first receiving antenna, it is possi- ble that a feedback loop occurs, whereby the repeater re- amplifies the same signals until it becomes saturated. In such a condition the repeater will cease to function as a repeater and will merely transmit a maximum-power signal, causing interference to other network users. This condition may be difficult to avoid, whilst still having sufficient amplification for the repeater to be effective. Normally, the two antennas of a repeater station will have to be physically well separated, perhaps with one antenna atop a tower, and the other at the bot- tom, the receiving antenna being highly directional in favours of signals transmitted from the base station. The practical problem of installing antennas with perhaps 80 dB isolation between them makes the installation expen¬ sive. The physical separation requires towers, feed cables and so on, and then the radio frequency testing and com¬ missioning must be conducted by highly skilled techni¬ cians.
The object ot the present invention is to overcome the drawbacks and difficulties explained above. In order to achieve this object, the inventive method is character¬ ized by the steps of
- converting in the base station a first communi¬ cation channel to a mobile station into a second com¬ munication channel having a frequency offset from said first channel, in order to send a message to a repeater station on said second channel;
- receiving in the repeater station said message on said second channel;
- converting in said repeater said second channel back to said first channel; and
- transmitting said message to said mobile station on said first channel.
There are two essential components in the method according to the present invention. The first is that the repeater re-transmits on frequencies different from those it receives. The second is that the base station uses a different frequency from that which it commands the mobile to use. Consider first the repeater. The repeater receives signals on one or more carrier frequency and re-transmits essentially similar signals on one or more different RF carrier frequency. This eliminates the risk of the re¬ peater suffering instability as a result of feedback from one antenna to the other. There is indeed some possibility of re-amplification of the signals transmitted by the re- peater, but such re-transmission will now be on a differ- ent frequency and can be arranged to lie outside the oper¬ ational bandwidth of the equipment.
Now consider the base station and its communication with the mobile. Normally, a base station will transmit messages to the mobile, both during call set-up and while the call is in progress, which instruct the mobile to use a particular radio frequency. If a frequency-shifting re¬ peater were used in conjunction with a conventional base station, then there would be a problem with base station to mobile communications via the repeater. The mobile would be instructed by the base station to use a particu¬ lar radio frequency but when base station and mobile use that frequency, the repeater would introduce a frequency offset into the communication which would disrupt the link. So far as the mobile was concerned, the signals it was receiving would be on the wrong frequency. The present invention overcomes this problem by causing the base sta¬ tion to offset the frequency of its transmission by a cer¬ tain amount, so that the repeater shifts the transmission back to a frequency which is consistent with the control signals that are being sent to the mobile.
The invention enables the use of repeaters with low installation cost, having eliminated the risk of feedback instability in the repeater. The antenna installation is much simpler, reducing the total infrastructure cost, whilst enabling a higher amplification to be used in the repeater. Despite the complication in the channel alloca¬ tion at the base station end of the link, the changes are transparent to the mobile, so that no modification needs to be made to the mobile. Furthermore, it can be shown that in the preferred embodiment, it is possible to make the base station modifications without changes to the hardware or site visits. All radio links, whether direct or via the repeater, use the normal frequency bands and protocols so that the invention may be employed without having additional frequency bands allocated for the net¬ work and without change to the regulatory certification of the equipment.
The characteristics of the preferred embodiments of the method and the cellular radio system according to the present invention are set forth in the attached claims.
The invention is hereinafter explained in detail by means of examples referring to the attached drawings, in which figure 1 shows the general situation of downlink transmission via a repeater, figure 2 shows the downlink transmission situation according to the present invention, figure 3 shows a preferred embodiment of a base station according to the present invention, figure 4 shows a preferred embodiment of a repeater according to the present invention, figure 5 is a frequency allocation scheme for the network. Considering now figure 1, the basic situation in a cellular network with repeater links shown. Radio signals from the base station 1 are received by the repeater 2 by an antenna A. The repeater 2 amplifies the signals, typi¬ cally by 60dB. The amplified signals are re-transmitted using the same frequency by the repeater with an antenna B to a mobile 3. This prior art system is afflicted with all the drawbacks and problems explained above.
The present invention is illustrated in Figure 2. The base station 4 instructs the mobile 6 to use Channel X. The base station 4 is, however, transmitting on Channel Y. The repeater 5, however, translates Channel Y into Channel X, whilst also amplifying it, and the mobile re¬ ceives a signal which is on Channel X.
This method applies both to calling (signalling) channels, which a.o. things may instruct the mobile to use a certain voice channel (i.e. channel X) , and to the voice channel itself. On the other hand, without modifications in the mobiles it is not possible for a mobile to estab¬ lish a call via the radio frequency that is intended for the base station to repeater part of the link. This im¬ plies that certain frequencies will have to be used for the base station to repeater link, and different frequen¬ cies for direct communication between the base station and the mobile (cf. fig. 5) . A preferred embodiment of the base station is il¬ lustrated in Figure 3, which shows the signal processing in the base station. Again, for clarity, only the downlink path is shown. The principle of operation is the same for the uplink path. Within the base station, an extra soft- ware module CCS is inserted into the channel selection control of the base station, between the Link Control Software LCS and the TRX unit, which is the sub-system which transmits and receives radio signals, converting between RF signals and baseband signals. The extra soft- ware module CCS is a Channel Conversion Software, and it translates the channel number that is requested by the link control software into a new channel number that is consistent with the operation of the repeater.
The TRX unit is primarily hardware and is respon- sible for putting voice and data information onto the air interface, and for receiving the air interface and ex¬ tracting voice and data information. The LCS is primarily software and is responsible for control of the transmis¬ sion links. The software module CCS may be down-loaded into the equipment remotely and is simple to design because may be an independent part of the link control software. Messages and voice traffic are carried out in the normal way through the TRX unit. In figure 4, which shows the signal processing in the repeater, the signals from the base station are re¬ ceived at antenna A. The signals are filtered and ampli¬ fied in a filter 7 and an amplifier 8, respectively. The signal is then mixed by a mixer 9 using a local oscillator 10. This reduces the signal to a frequency of typically 150 MHz. A SAW (Surface Acoustic-Wave) filter 11 band- limits the signal, a second amplifier 12 amplifies it, whereafter it is up-converted in a second mixer 13 with a second local oscillator 14. Finally the signal is filtered with a filter 15 and put through a power amplifier 16 for transmission from antenna B. The SAW filter filters a fixed band, and by making the local oscillators 10 and 14 to have different frequencies, it is possible to shift the frequency of the in-band signals between reception at an- tenna A and transmission at antenna B. Preferably, the local oscillators 10 and 14 are both programmable, which enables the amount of frequency shift and the centre fre¬ quency of the re-transmitted band to be dynamically al¬ tered, according to the traffic load and other circum- stances. The programming is preferably done by remote control, e.g. from a base station or a switching center of the cellular system.
Figure 5 shows a typical frequency allocation plan which might be used in connection with the present inven- tion. The link control software of the base station allo¬ cates channels to links in the ranges 11 - 18 and 22 - 50. The Channel Conversion Software (see figure 3) translates the TRX frequency control programming so that channels 11 to 18 are converted to channels 1 to 8. Channels 22 to 50 are unaffected. The repeater converts channels 1 to 8 back to channels 11 to 18. Thus the base station may communi¬ cate with a mobile on any of channels 22 to 50 if the com¬ munication is direct, or on any of channels 11 to 18 if via a repeater. Channels 9, 10 and 19 to 21 are unused to avoid possible edge of band problems. Other frequency plans are also possible to use, and if the band is split into three or more sections, the repeaters can be daisy- chained, whereby the frequency allocation plan is used to provide different channels for communication between each pair of repeater stations, e.g. in order to cover with one single base station a greater and/or a topographically very demanding area, than has been possible with prior art solutions.
In order to provide the maximum flexibility for the repeater as a product, and to enable a network to be re¬ configured during service, it is important for the re¬ peater to be remotely configurable. Such dynamic control of the frequency offset and centre frequency is effected using an operations and maintenance interface, over a ra- dio modem link.
It is clear for one skilled in the art that the various embodiments of the present invention are not re¬ stricted to the examples described above, but that they may vary within the scope of the attached claims.

Claims

Claims
1. A method for improving radio coverage in a cel¬ lular radio system, comprising at least one base station and a plurality of mobile stations communicating with said base station, and at least one repeater station retrans¬ mitting messages received from the base station to at least one mobile station; characterized by the steps of: - converting in the base station (4) a first com¬ munication channel to a mobile station (6) into a second communication channel having a frequency offset from said first channel, in order to send a message to a repeater station (5) on said second channel; - receiving in the repeater station (5) said messa¬ ge on said second channel;
- converting in said repeater said second channel back to said first channel; and
- transmitting said message to said mobile station (6) on said first channel.
2. A method for improving radio coverage in a cel¬ lular radio system according to claim 1, characterized by that the base station (4) simultaneously uses the first channel for direct communication with a mobile station (6) and the second channel for communication with the same mo¬ bile station (6) via said repeater station (5) .
3. A method for improving radio coverage in a cel¬ lular radio system according to claim 1 or 2, character¬ ized by that the repeater station (5) is dynamically prog- rammed to set the frequency offset and/or the centre fre¬ quency of the re-transmitted band.
4. A method for improving radio coverage in a cel¬ lular radio system according to any one of claims 1 - 3 , characterized by using a frequency allocation plan (Fig. 5) in the base station (4) for splitting up the frequency band into sections in order to allocate channels between repeater stations (5) and mobile stations (6) .
5. A method for improving radio coverage in a cel¬ lular radio system according to claim 4, characterized by that said frequency allocation plan is used to provide different channels for for communication between each pair of repeater stations (5) in a row of daisy-chained re¬ peater stations.
6. A cellular radio system comprising: - at least one base station (4) set to communicate with a plurality of mobile stations; and
- at least one repeater station (5) set to retrans¬ mit signals received from the base station to mobile sta¬ tions; characterized by that
- the base station (4) includes means for conver¬ ting a first communication channel to a mobile station (6) into a second communication channel having a frequency offset from said first channel, in order to send a message to a repeater station (5) ,
- the repeater station (5) includes means (A,7,8) for receiving said message on said second channel, means (9-14) for converting said second channel back to said first channel, and means (15,16,B) for transmitting said message to a mobile station (6) on said first channel.
7. A cellular radio system according to claim 6, characterized by that the base station (4) is set up for simultaneous use of the first channel for direct com¬ munication with a mobile station (6) and of the second channel for communication with the same mobile station via a repeater station (5) .
8. A cellular radio system according to claim 6 or 7, characterized by that the repeater station (5) is prog¬ rammable for a dynamic control of the frequency offset and/or the centre frequency of the re-transmitted band.
PCT/FI1993/000372 1992-11-12 1993-09-17 A method for improving radio coverage in a cellular radio system and a cellular radio system WO1994011957A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU49633/93A AU4963393A (en) 1992-11-12 1993-09-17 A method for improving radio coverage in a cellular radio system and a cellular radio system

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9223703.1 1992-11-12
GB9223703A GB2272599A (en) 1992-11-12 1992-11-12 A method of cellular radio communication and a cellular radio system for use in such method

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Publication Number Publication Date
WO1994011957A1 true WO1994011957A1 (en) 1994-05-26

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