WO1994029973A1

WO1994029973A1 - Process for improving the coverage of mobile telephones in cellular mobile radio systems

Info

Publication number: WO1994029973A1
Application number: PCT/EP1994/001538
Authority: WO
Inventors: Werner Schmidt
Original assignee: Detecon Deutsche Telepost Consulting Gmbh
Priority date: 1993-06-16
Filing date: 1994-05-12
Publication date: 1994-12-22
Also published as: DE4319694A1; AU6927794A; DE4319694C2

Abstract

The description relates to the improvement of the coverage of mobile telephones in cellular mobile radio systems in which at least a first fixed station (BTS) with at least one aerial is arranged in each cell (1) and covers at least the range of this cell (1) with radio frequencies of a channel group. There is at least one second fixed station (Y) in the range of the cell (1) which covers at least a part of the cell (1) with radio frequencies of the same channel group as that of the first fixed station (BTS), and the carrier frequencies of the channels of the channel group of the second fixed station (Y) differ by a given frequency shift from the carrier frequencies of the channels of the channel group of the first fixed station (BTS). It is possible for a second additional fixed station (Z) to be arranged in the region of the cell (1) and cover at least one part of the cell (1) with the radio frequencies of the seame channel group as those of the first fixed station (BTS). The transmission of this further fixed station (Z) is phase-shifted as against that of the first fixed station (BTS) in order to attain such a time that the two signals reach the receiver of the mobile station within a time slot which is solved and used by an equaliser in the mobile station.

Description

METHOD FOR IMPROVING THE SUPPLY OF MOBILE PHONES IN

CELLULAR MOBILE RADIO SYSTEMS

The effect of fading phenomena is known in mobile radio systems. It arises from the superimposition of radio waves that reach the receiving antenna via different propagation paths, such as. B. both on the direct path of the line of sight as well as usually several reflectors, such as houses, mountains vehicles or other objects, the dimensions of which are large compared to the wavelength of the carrier frequency. If these signal components have comparable individual levels, depending on the receiving location, there may be strong fluctuations in the reception level, up to extinction, that is to say the mutual cancellation of the individual components. The distribution of the levels measured during the movement through such a radio field is generally well approximated by the so-called Rayleigh distribution, which is why this effect is often also referred to as Rayleigh fading or Rayleigh fading.

These fading effects, which are unavoidable in mobile telephony, have a negative impact in several ways. If the field strength drops below a certain value, voice or other user data can no longer be successfully transmitted. When the subscriber device moves through the local wave field, situations with a high field strength, ie good transmission quality, alternate with such a low field strength, ie low quality with strong interference. If the average reception field strength is high enough, there are such dips however, only for a short time and do not have a disturbing effect in the subjective speech impression. Therefore, cellular networks are dimensioned with a corresponding reserve in the radio coverage.

Such a power reserve is only of limited effect for handhelds that are operated at low speeds or stationary. There is still a certain likelihood that they will remain in so-called fading holes for a longer period of time, in which telephoning is difficult or impossible. Therefore, in operating instructions there is often a hint that the user should take a few steps in such situations to find a point with better care. In addition to the inconvenience, however, this is not a real remedy, since the wave field is not only changeable locally, but also in time, for example by moving reflectors. Therefore, the same effect can appear again after a short time.

In modern mobile radio systems, the subscriber devices also measure the level of the station currently supplying the best as well as the neighboring station in the ready state ("stand-by mode" or "idle mode"), that is to say when there is no conversation to be able to assign yourself to the best station. This process? takes place, unnoticeable by the participant, even during a connection. This supports the "handover" process of switching a connection from one base station to another base station by the switching devices of the mobile radio system. In this way it should be ensured that the connection is always made via the best base station. This process is Required so that the connection does not break off when switching between radio zones with devices that move quickly, such as car telephones.

The fading described, however, leads to incorrect assignments and undesired connection switching because of the level fluctuations in portable devices in stationary operation. With the increasing spread of small and light pocket telephones, this effect is increasingly becoming a problem in modern networks.

Various methods have been developed to reduce this effect. This includes

- adaptive power control

- Interleaving and channel coding

- Diversity reception

- adaptive equalization

- spread spectrum transmission - frequency hopping method.

These methods are at least included as options in the new GSM standard for digital mobile communication systems and are briefly described below. It shows that all these methods are unable to adequately solve the problem described for hand-held telephones that are used stationary or at low speeds.

The adaptive control of the transmission power allows the

Attenuation of the actual transmission power compared to the maximum power by up to 30 dB in 2 dB steps and at intervals of about 0.5 seconds. With a reduced transmission power, the performance can be increased again if a fading drop occurs will. However, this process is too slow to correct the fading fluctuations.

The interleaving of bits and blocks (interleaving) spreads the transmission errors occurring during radio transmission due to fading in the receiver and thus distributes them evenly, so that these errors can be recognized and corrected by the channel coding. However, this presupposes that firstly the bit error rate is relatively low on average and secondly that the duration of a fading dip is not too long. If the disturbance lasts, for example, longer than approx. 10 ms, the errors can generally no longer be corrected.

Another measure to reduce the fading effect is diversity reception, in which signals from two or more independent antennas are received, processed separately and then combined to form an overall signal. The required distance between the antennas and the additional effort make it difficult to use diversity in pocket telephones.

Through adaptive equalization, signal components that reach the receiver via different propagation paths can be resolved in time and the reception quality can thus be improved. In the GSM system, multi-way routes with a detour length of between approximately 900 meters and approximately 4.8 km can be used. Longer detours act like interference signals, while shorter detours still contribute to fading, as is particularly common in urban areas supplied with small cells, the most important area of application for pocket telephones. Here, adaptive equalization has little effect on reducing fading. The slow frequency hopping method causes a switch in the transmitter and in the receiver between different carrier frequencies according to a fixed scheme.

Because different instantaneous levels are present on each carrier frequency due to fading with the same transmission power, the result is that a connection is only briefly affected by a fading dip on one frequency. In this way, the effectiveness of interleaving and channel coding at low speeds is supported, although the frequency hopping method itself does not change the mean error rate during transmission. The disadvantage here is the need for frequencies and the high level of implementation in the fixed stations. In addition, it should be noted that this method only works during the connection, but not during the establishment of the connection and other signaling or in the level measurements for the radio zone assignment or the connection switching, since there is nothing on the frequencies used for this frequency hopping is applied.

The relationships between these methods and results are e.g. in W. Koch, J. Petersen; Diversity and frequency hopping in D-Ntz; PKI Techn. Mitt. 2/1990, pp. 13-19 and in A. Baier, G. Heinrich, U. Wellens; Adaptive equalization in digital TDMA mobile radio systems; PKI Techn. Mitt. 1/1989, pp. 75-82 detailed.

In the following, methods and devices are specified which largely eliminate the described effects of fading in pocket telephones without additional effort in these devices. The transmission Improved quality and also avoided the risk of incorrect assignments and unnecessary connection switching. The methods of interleaving and channel coding as well as adaptive equalization, which are basically implemented in every device according to the GSM standard, are hereby optimally brought into effect even at low speeds and with small cells. The elaborate frequency hopping method (optionally in the GSM standard) can thus be dispensed with.

The starting point is a radio cell that is supplied by a base station (BTS), which contains the transmitters and receivers for the channels used in the cell. This can be a sector cell supplied with a directional antenna from the edge or a cell supplied from the center by an omnidirectional antenna.

According to a first method, at least one further device (Y) is arranged in the area of the cell, characterized in that it supplies at least part of the cell with the same channels as the BTS, the carrier frequencies of these channels being a specific frequency offset against the BTS. This frequency offset can be fixed or variable over time.

The frequency offset should be between 0.05 and 0.2 ppm (parts per million) from the carrier frequency of the BTS, i.e. in a 900 MHz system it should be between about 45 and 180 Hz, i.e. in the order of magnitude of the speed-dependent Doppler frequency with moving mobile stations.

The operation of the method is that at the location of the receiver, when the high-frequency signals transmitted by the BTS and the device (Y) are superimposed, the reception level changes over time in the rhythm of the frequency storage (this is the difference between the carrier frequency of the BTS and the carrier frequency of the further device). This means that even without moving the mobile station, fading dips are short-lived and their influence can be eliminated by the measures of interleaving and channel coding, similar to the case with rapidly moving mobile stations or when using the frequency hopping method the case is.

The principle of the invention is based on the following:

In the case of a fixed (currently not moving) mobile station and a single fixed station BTS present in the radio cell, it can happen that at the location of the mobile station there is a "fading hole" (drop in field strength) which exists over a longer period of time.

In order to avoid such a fading hole, or to shorten at least the duration of the existence of the fading hole, two slightly different carrier frequencies are sent from at least two base stations arranged in the radio cell to the mobile station. The superimposition of the carrier frequency of the one base station with the carrier frequency of the second base station which deviates slightly therefrom results in interference between the two carrier frequencies which carry the same information, so that the thickness in the fading hole begins to fluctuate with the difference between the two carrier frequencies . Instead of the previous (total) field strength drop, the field strength now changes in rhythm with the Difference between the two carrier frequencies, so that this also results in higher field strength amplitudes in the fading hole, which suddenly enable reception on the part of the mobile station again. Because of these short-term drops in field strength in the fading hole, it is now possible to use the equalization and correction measures mentioned in the introduction to the description, because according to the invention the field strength in the fading hole is, so to speak, stimulated to vibrate. Channel coding can be carried out using the bit interleaving or block interleaving method. All correction methods can therefore also be used in the fading hole, where these correction methods normally do not work at all due to the lack of field strength.

According to the second method, at least one further device (Z) is arranged in the area of the cell, characterized in that it has at least one

Part of the cell is supplied with the same channels as the BTS, the transmissions of the device (Z) on each channel being offset in time from those of the BTS in such a way that the two signals reach the receiver of the mobile station within the time window, which is resolved and used by the equalizer can be. This time offset can either be uniform for all channels or can be set individually for each channel, depending on the size of the cell, distance between BTS and (Z), and the current distance of the mobile station to BTS and to (Z).

According to the above explanations, the time delay with which the signals reach the receiver of the mobile station should be in a system according to the GSM standard. are between about 3 and 16 microseconds.

This ensures that a signal that can be used by the adaptive equalizer is present in the receiver even without the presence of multipath propagation-related propagation. The likelihood that the two signals received by the BTS and the device (Z) are subject to a fading dip is correspondingly lower than with only one signal.

Both methods can also be combined, i.e. that a device (X) emits a signal on each channel, which is both offset in time from the transmission signal of the BTS and has a frequency offset.

Furthermore, both methods can also be combined in such a way that a device (YZ) emits a signal on each channel, which is composed of a first partial signal which has the same time position as the signal of the BTS, but a frequency offset with respect to this and a second partial signal which has a time offset with respect to the BTS signal and which can additionally have a frequency offset with respect to the BTS signal.

The frequency offset of the second partial signal can differ in magnitude and sign from the frequency offset of the first partial signal. If, for example, the offset of the first partial signal is +90 Hz, that of the second partial signal can also be +90 Hz, or +63 Hz, or 0 Hz (no offset), or -90 Hz. In contrast to the frequency projection method, the above methods can also be used on the GSM signaling channels (BCCH, CCCH).

As an extension of the method, the frequency offset on each channel can be set individually as a function of the frequency offset determined in the associated BTS receiver from the signal received by the mobile station.

For example, the frequency offset can be reduced if it can be concluded from the frequency offset caused by the Doppler effect and measurable in the receiver of the BTS that the mobile station is moving sufficiently quickly. Another application can be that in the case of very fast moving mobile stations, e.g. those that are used in high-speed trains, the Doppler shift is partially compensated for by an appropriately set frequency offset, so that the overall resulting frequency offset remains within the range that can be processed by the adaptive phase tracking in the receivers of the mobile station and the BTS. This can prevent an increase in the bit error rate at high speeds, as reported in the cited publications.

In an additional extension of the method, the time offset on each channel can be set individually as a function of the impulse response of the channel determined in the associated receiver in the BTS during adaptive equalization.

There are various options for realizing the described methods. In a first embodiment (U), the modulating signal (baseband signal) is decoupled in the BTS, possibly delayed, modulated onto a carrier frequency derived from the frequency generation of the BTS and shifted by the frequency shift, and fed to a transmitting antenna after power amplification.

This embodiment is suitable for installation at the location of the BTS, so that the entire cell area covered by the BTS can be supplied by one device.

In a second embodiment (V), said devices are implemented in the form of a relay transmitter (repeater). For this purpose, the signal transmitted by the BTS is received, converted into an intermediate frequency position with a first mixed frequency, amplified, if necessary delayed, and then with a second mixed frequency which differs from the first mixed frequency by the desired frequency offset. converted into the original frequency position and transmitted again after amplification.

As with all direct frequency relay transmitters (on-frequency repeater), when setting up this device, care must be taken to ensure sufficient decoupling between the receiving and transmitting antennas so that no tendency to oscillate begins.

In order to avoid this danger, the device can be developed in a third embodiment (W) in such a way that the BTS also transmits the transmission signal in a different frequency range, for example as a directional radio signal, and the relay station transmits this signal receives, processed as described above, and then transmits again with the defined frequency offset on the frequencies used by the BTS.

The methods shown can also be used to improve the transmission conditions in the transmission direction from the mobile stations to the fixed station. Relay transmitters, which receive the signals from the mobile stations, provide them with a frequency offset, a time offset, or a combination of these features, and transmit them to the BTS in the original frequency position or via a directional radio link can serve this purpose.

Some special arrangements of the devices in radio cells according to the inventive method are shown below.

The subject matter of the present invention results not only from the subject matter of the individual patent claims, but also from the combination of the individual patent claims with one another. All of the information and features disclosed in the documents, including the summary, in particular the spatial configuration shown in the drawings are claimed to be essential to the invention, insofar as they are new to the prior art, individually or in combination.

The invention based on several

Drawings illustrating execution paths explained in more detail. Further features and advantages of the invention which are essential to the invention emerge from the drawings and their description. Show it:

FIG. 1: shows a radio cell illuminated from the center by a BTS with an omnidirectional antenna, where a further device E, preferably in the version (U), is set up at the location of the BTS, which additionally supplies approximately the same area as the BTS ( Service area 3).

FIG. 2 shows a radio cell 1 which is likewise supplied with a center, within which two devices (E), e.g. after the execution (V), are arranged, each supplying a certain part of the cell with superimposed supply areas 3. It can e.g. are areas in which the operation of pocket telephones is increasingly expected, such as pedestrian zones, sports facilities, train stations, suburban housing complexes, parks or the like.

FIG. 3 relates to a sector cell 4 supplied by a BTS with a directional antenna, which can be part of an extensive sector cell network, on the edge of the cell opposite the BTS of which a device (E) is arranged, e.g. after the version (W), which additionally supplies the said sector cell (4) with the same channels as the BTS from this location with a directional antenna opposite the BTS (sector coverage area 5).

With this arrangement, the additional time offset of the signal can be dispensed with if the distance between BTS and (E) already results in a corresponding delay due to the transit time of the signal. FIG. 4 shows another cell, supplied by a BTS with a directional antenna from the edge, as part of a radio network, in which devices (E), for example in the version (W), are arranged at intersections with neighboring cells, which in turn are associated with Sector antennas additionally supply the cell. The arrangement is chosen so that appropriate facilities for additional supply to neighboring cells can also be accommodated at the same locations. With the same reasoning as in FIG. 3, the additional time offset of the transmission signals can also be dispensed with here.

The radio network structure that results when this arrangement is continued on a regular basis is outlined in FIG. With such an arrangement, the co-channel interference distance and thus the repeatability of the channels in the area are improved at the same time because the BTS can transmit with lower power.

FIG. 6 shows a cell supplied from the center by a BTS, on the edge of which several devices (E), for example according to version (W), are arranged, which in turn provide parts of the cell with directional antennas with the same channels as the BTS.

With this arrangement too, relay transmitters for the neighboring cells can be arranged at the same locations, and better channel repeatability is also made possible. Drawing legend

1 radio cell BTS = base station

2 first base station (BTS) Y ^{**** •} second additional base station 3 coverage area of the Z = third additional base station)

4 sector cell (BTS) X = fourth further fixed station

5 sector cell (further facility)

U - first version V = second version W = third version E = further equipment

Claims

Patent claims

1. A method for improving the supply of mobile telephones in cellular mobile radio systems, with at least one first fixed station (BTS) with at least one antenna being arranged in each cell (1), which at least covers the area of this cell (1) with radio frequencies Channel group sweeps, characterized in that at least one second further base station (Y) is arranged in the area of the cell (1), which supplies at least a part of the cell (1) with radio frequencies of the same channel group as that of the first base station (BTS), and that the carrier frequencies of the channels of the channel group of the second base station (Y) differ by a certain frequency offset from the carrier frequencies of the channels of the channel group of the first base station (BTS).

2. The method according to claim 1, that the frequency offset has a constant value over time.

3. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the frequency offset is variable in time.

4. The method according to any one of claims 1-3, characterized in that the frequency offset between the carrier frequency of the first and the second base station in the order of magnitude the speed-dependent Doppler frequency of a moving (54-240 km / h) mobile station.

5. Device for carrying out the method according to any one of claims 1-4, that the frequency offset is between 0.05 and 0.2 ppm (parts per million) from the carrier frequency of the first base station.

6. The device according to claim 5, characterized in that the first base station (BTS) is arranged on the edge of a sector cell (4) and that the further base station (E) of the first base station (BTS) with respect to the sector cell (4) is arranged opposite , (Fig. 3).

7. A method for improving the supply of mobile telephones in cellular mobile radio systems, wherein at least one first fixed station (BTS) with at least one antenna is arranged in each cell (1), which has at least the area of this cell (1) with radio frequencies Channel group sweeps, characterized in that in the area of the cell (1) at least one further base station (Z) is arranged, which supplies at least a part of the cell (1) with radio frequencies of the same channel group as that of the first base station (BTS), and that The other base station (Z) is transmitted out of phase with the transmission of the first base station (BTS) by such a time offset that the two signals reach the receiver of the mobile station within a time window which is resolved and used by an equalizer arranged in the mobile station .

8. Device for carrying out the method according to claim 7, that the time offset for all channels of the channel group is set uniformly.

9. Device for carrying out the method according to claim 7, so that the time offset is set individually for each individual channel of the channel group.

10. Device according to one of claims 8 or 9, so that the time offset with which the signals of the first base station (BTS) and the further base station (Z) reach the receiver of the mobile station is in the range of approximately between 3 and 16 microseconds.

11. The method according to claim 1 and 7 and one or more of claims 2-4, characterized in that the further Fest¬ station (X) emits a signal on each channel of the channel group, which is both temporally compared to the transmission signal of the first base station (BTS) is offset, as well as has a frequency offset.

12. The method according to claim 1 and 7 and one or more of claims 2-4, characterized in that the further base station (YZ) emits a signal on each channel of the channel group, which is composed of a first partial signal, which is the same time ¬ layer has like the signal of the first base station (BTS), but has a frequency offset with respect to it and a second partial signal which is opposite the signal of the first base station (BTS) has a time offset and which additionally has a frequency offset compared to the signal of the first base station (BTS).

13. Device for carrying out the method according to

Claim 12, that the frequency offset of the second partial signal is different in magnitude and sign from the frequency offset of the first partial signal.

14. The method according to any one of the preceding claims 1-13, that the frequency offset on each channel is set individually as a function of the frequency storage determined in the associated receiver of the first fixed station (BTS) from the signal received by the mobile station.

15. The method according to any one of the preceding claims 1-13, so that the time offset on each channel is set individually as a function of the impulse response of the channel determined in the assigned receiver in the first fixed station (BTS) during adaptive equalization.

16. Device according to one of the preceding claims 1-15, characterized in that from the signal of the first base station (BTS), the modulating signal (baseband signal) is coupled out, possibly delayed, one derived from the frequency generation of the first base station (BTS) and around the frequency offset shifted carrier frequency is modulated and is fed to the further base station (Y or Z or YZ) after the gain of a transmitting antenna.

17. The device according to one of claims 1-15, characterized in that there is a relay connection between the first base station (BTS) and the further base station (Y or Z or YZ), with the further base station that of the first base station (BTS ) received signal is received, converted into an intermediate frequency position with a first mixing frequency, amplified, possibly delayed, and then with a second mixing frequency, which differs from the first mixing frequency by the desired frequency offset, converted into the original frequency range and transmitted after amplification by the other base station (Y or Z or YZ).

18. The apparatus of claim 17, d a d u r c h g e k e n n z e i c h n et that the first base station (BTS) its transmit signal in addition in a different frequency range, e.g. sends out as a radio signal and the relay station processes this signal according to the technical teaching of claim 17 and then sends it again with the defined frequency offset on the frequencies used by the first base station (BTS).

19. Device according to one of the preceding claims 1-18, characterized in that the first base station (BTS) supplies its sector cell (4) as part of a radio network from the edge and that the other base stations (E) in the intersection points with adjacent sector cells (4) are arranged (Fig. 4).

20. Device according to one of the preceding Claims 1-18, characterized in that the first fixed station (BTS) supplies its sector cell (4) from the center and that further fixed stations (E) are arranged on the edge of this sector cell (4), which in turn have parts of the sector cell (with directional antennas) 4) provide the same channels as the first base station (BTS).