METHOD AND DEVICE CONCERNING FADING RELATED ACCESS CONTROL IN A DIGITAL RADIO SYSTEM

Abstract: The method implies that one for the signal quality optimal channel selection is predicted on the basis of a measured momentary fading of the transmitted signal. The mobile unit or the base unit performs signal strength measurements on the channels which are correlated with the carrier for which time access is to be performed. The obtained measurement values are averaged over a short period of time, whereafter the fading is calculated as the difference between the momentary signal strength value and the measured average. Channel selection for the next, following packet transmission is after that decided in a way that a channel with negative fading margin is avoided. The decision is based on prediction of unsuitable channel.

Description: TITLE OF THE INVENTION Method and device concerning fading related access control in a digital radio system TECHNICAL FIELD The present invention relates to a new method and device at a stationary or mobile radio communication system comprising devices for time shared channel access and which makes possible reduced influence of fading and time dispersion at transmission of radio signals between a transmitting and a receiving equipment in the mentioned system. STATE OF THE TECHNOLOGY In radio systems, preferably digital radio systems, it it previously known by among other things the patent document US,A, 5 020056 to reduce influence from synchronous fading in a digital radio system the transmission of which is based on TDMA-technology. The document relates to a method to reduce influence of existing fading with a certain periodicity by the time slot, which relates to a certain user, being moved around in a predefined way in one in the system included transfer frame. The system consequently does not take into account the momentary time period of the fading when measurements are not made on the transmitted signal. Another patent document US,A,4 032884 relates to a radio system for transmission of data and where momentary measurements of the transmission quality of the connection link is performed. If the measurement indicates that unacceptable high fading exists, the transmission in the following time slots will be stopped by the receiving unit informing about this to the transmitting unit until the the receiving unit again informs that the transmission quality is acceptable. Further it is previously known by the patent document US,A,4 901 319 to, at a radio commmunication system, calculate the fading characteristic for a channel regarding the time period of the fading, i.e. the time interval between signal strength dips of a predetermined depth. By the presented documents it is consequently previously known, at a radio system, to have the possiblility to change frequency for a time slot if the transmission quality becomes too bad, alternatively to stop the transmission if the transmission signal quality is not acceptable. It is further known to use the dip frequency of the fading to influence the transmitted signal. What on the other hand is not previously known and which accordingly constitues a new method is that in a digital radio system have the possibility to change frequency for one or more time slots at predicted momentary fading dips which are calculated on the basis of a measured fading frequency of the transmitted signal. DESCRIPTION OF THE INVENTION TECHNICAL PROBLEM Modern radio communication systems of to-day implies for a faultless signal transmission that the received signal has not been subject to any real disturbing moments during the transmission between the transmitting and the receiving equipment. Interference due to for instance fading is, however, a physical phenomenon which is difficult to cope with, so we will also henceforth concentrate ourselves on developing the technology which restricts the consequences of the fading. Fading consists of variations in the strength of the received signal, in time or space, and is caused by among other things interference which occurs between different wave propagation paths, as for instance between direct and earth reflected signal, or between different signal paths in the atmosphere. The different wave propagation paths therefore displays different transit times between the transmitting and receiving equipment wich causes time dispersion. A high time dispersion implies risk for a worse quality of the signal transmission and by that also an increased probability of interference. The invention aims at solving this problem. A problem at the planning of a new radio communication system is that those in the radio communication system included transmitting and receiving equipments must be located within a comparatively restricted geographical area, otherwise the time dispersion would imply that the signal transmission quality would be too poor. If therefore the consequences of the fading could be restricted, the surface coverage of the system could be increased with maintained transmission quality. The invention also intends to solve this problem. Time dispersion is often well correlated with fading which implies that a reduced influence of time dispersion would considerably improve the transmission quality in environments with extensive multipath propagation, as in for instance highly populated dwelling areas, city centres etc. The invention also intends to solve this problem. In an embodiment of the subject of invention influences of the radio communication system are used, in which influences the traffic shall be possible to be directed over to a suitable channel in one for the system optimal and flexible way. The invention also intends to solve this problem. THE SOLUTION What can principally be regarded as significant for the invention is that it is based on an optimal channel selection, i. e. channel selection in the time plane, which is predicted on the basis of measured momentary fading for the stationary or mobile unit so the influence of fading is influenced in wished direction. One further characteristic is that the invention makes use of the fact that the fading for neighbouring points in time and space are correlated. Besides the invention makes use of the fact that the fading for not correlated points are in the main uncorrelated which implies that the channel selection of different users will be uncorrelated, i.e. different users will not have any of the invention caused tendency to compete for the same channel in the time plane. In one embodiment of the invention idea the mobile unit or the base unit performs signal strenght measurements on channels which are correlated with the carrier for which time shared access is to be performed. The signal strenght values are averaged over a short time, for instance 50 ms, whereafter the fading is calculated as the difference between signal strength value and average. In one further embodiment of the invention idea the mobile unit or base unit calculates the average period of the fading, i.e. the average for a fading cycle as for instance the distance between two minima. In one further embodiment of the invention idea the unit supposes that the fading follows a predetermined distribution, as for instance Rayleigh-distribution, at which the average distance is a half wavelenght. The average period is at that decided by knowlewdge of the speed of the mobile, which can be determined by means of for instance speedometer, doppler frequency or fading measurement. The margin of the fading to wanted minimum values are after that calculated as the difference between the fading and the decided minimum value according to the following: Fading = signal strength - average. Channel selection for the next packet transmission is after that decided in a way that the at the moment strongly fading influenced channel or channels are avoided, i.e. the channels which shows a negative fading margin. The decision is based on prediction of unsuitable channel. The prediction is made on the basis of that the unit has performed measurements considerably shorter than the average period, whereafter the unit calculates the momentary phase position and start- respective end phase for the minimum value. Unsuitable channel selection in relation to momentary phase position is then calculated according to the following: Unsuitable channel = 360°n + (start phase, end phase) ,n = 0,1,2,3... In another embodiment of the invention idea when the unit can not decide a definite momentary phase position, for instance if the fading decision can be made only once per average period, the unit has two alternative phase positions. Possible momentary phase positions and start- and end phase for the mimimim value is then calculated by means of fading and accepted fading distribution stored in a table. Unsuitable channel interval is then calculated according to the following: 360°*n + (start phase, end phase) Unsuitable channel = 360°*n - momentary phase + (start phase, end phase) N = 0,1,2,3.... ADVANTAGES Reduced influence from fading implies that a radio communication system can offer surface coverage with higher transmission quality and capacity to a lower cost than today's. A reduced influence of time dispersion also implies an increased transmission quality in environments with extensive multipath propagation. The invention should also become of great importance as access technology for the 3rd generation's mobile radio communication systems, especially if they are developed for ATM-similar functionality, such as for instance "band width on demand" . The invention can in such a connection be used for deciding optimal time interval in consultation with other service demands etc. The invention can beside that also be implemented at existing radio systems which use access technologies based on channel division in the time plane, such as for instance DECT, DCS 1800 etc. The present invention is a further development of existing technologies for dynamic channel allocation, which essentially are based on measurements of the average attenuation of the transmitted signal. The invention implies a considerable improvement because also momentary conditions are included at the channel allocation with the aim to reduce the negative influence of rapid fading. DESCRIPTIONS OF FIGURES One at present suggested embodiment of a device which shows the for the invention significant characteristics shall be described below while referring to the enclosed drawings where Figure 1 shows the relation of the fading and the channel structure in the time plane at a certain speed. T expresses the average period of the fading, and Figure 2 shows an example of table for decision of phase position for Rayleigh-distributed fading. DETAILED EMBODIMENT The invention is based on that optimal channel selection (selection of channel in the time plane) is predicted with starting point from measured and/or estimated "momentary" fading for the radio unit (stationary or mobile unit) , so that influence from the fading is influenced in wanted direction. Normally this implies that one aims at mimizing the depth of the fading dips (see Figure 1). In certain cases it can also be desirable to minimize the fading maximum in order to reduce interference etc. Often the fading is regarded to be random, i.e. one can not predict the fading in a certain point. This is however not quite correct. The invention thus makes use of the overlooked fact that the fading for closely located points in space and time are correlated. By closely located is meant tenths of a second, and a wavelenght's separation in space. The invention also makes use of the fact that the fading for not closely located points are on the whole uncorrelated. The latter fact implies that different users selection of channels will be uncorrelated, i.e. different users will not have any by the invention caused tendency to compete for the same channel in the time plane. The invention can be embodied as below: Mobile- or base unit performs signal strength measurements on channels which are correlated with the carrier for which time shared access shall be performed. The signal strength values are averaged over a "short" time, for instance 50 ms. The fading is calculated as the difference between signal strength value and average: fading = signal strength - average The mobile- or base unit calculates the average period (T) of the fading, i.e. the average time for a fading cycle (for instance the distance between two minima). Alternatively the unit assumes that the fading follows a predetermined distribution. This can for instance be Rayleigh-distribution, at which the average distance is a half wave-lenght. The average period (T) is in that case decided by knowledge of the mobile's speed, which for instance can be decided by means of a speedometer, measurement of the doppler frequency or fading measurement. The marginal (fmarg) of the fading to wanted minimum values (min) shall be calculated: fmarg = fading - min Channel selection for the next transmission is decided in a way that unsuitable channel is avoided, i.e. channel with negative fmarg (see Figure 1). The decision is based on prediction of unsuitable channel. The prediction of unsuitable channel is made according to two alternative methods: One method implies that the unit has performed measurements with interval considerably shorter than the average period (T). This implies that the unit can calculate momentary phase position, as well as start- and end phase for minimum value. Unsuitable channel interval (Ochannel) in relation to momentary phase position can then be simply calculated: Ochannel = 360°n + (start phase, end phase), n=0,l,2,3.. The method demands for higher measurement capacity, calculation capacity and memory capacity in relation to the following method. On the other hand it is more robust and secures better knowledge of unsuitable channels in the time plane. Should be especially advantageous at true packet transmission. The other method is used when the unit can not decide a definite momentary phase position. For instance if the fading decision only can be made once per average period. In that case the unit has two alterntive phase positions, because it does not know whether the signal level is increasing och decreasing. Possible momentary phase positions as well as start- and end phase for minimum value is calcualted by means of fading and ac¬ cepted fading distribution in stored table, see Fig. 2. Unsuitable channel interval (Ochannel) can then be calculated for the other method: 360°*n[start phase, end phase] Ochannel = . 360°*n - momentary phase + [start phase, end phase] for n = 0,1,2,3... Notice that the prediction in most cases can be valid a number of periods (i.e. for n=2,3,..). The validity depends on speed, environment, band width etc. The invention requires various degrees (depending on selected method) of possibility to perform signal strenght measurements on channels which are correlated with the carier for which time shared access is to be performed. In CDMA-system this is guaranteed due to the pilot channels which are necesssary for the controlling of the effect. On the whole demands for control of effect brings about a possible measurement channel which is fairly correlated. In order to make the invention usable the system must allow a fairly free channel allocation in the time. Systems with statistic channel allocation, which is avantageous from a capacity point of wiev, more than well meets the demands. Packet radio is on the whole well suitable. DECT is an existing system which meets the demands. The invention is not restricted to the above, as an example shown embodiment, but can also be subject to modifications within the frame of the following patent claims and idea of invention.

Claims: PATENT CLAIMS 1. Device for, at a stationary or mobile digital radio communication system which makes use of dynamic channel allocation, reducing the influence of fading and time dispersion at transmission of signals between a transmitting and receiving equipment, c h a r a c t e r i z e d in that one to a digital radio system belonging mobile unit or base unit is allotted device which performs signal strength measurements on channels which are correlated with the carrier for which time shared access shall be performed, that the mobile unit or base unit calculates, on the basis of the signal strength measurement, the average period of the fading, and that the mentioned fading is calculated as the difference between the signal strength and the calculated average, that prediction of unsuitable channel is made from the f ct that the unit has performed measurements which are considerably shorter than the mentioned average period, whereafter the unit calculates the momentary phase position as well as start- and end phase for the mimimum value, whereupon unsuitable channel selection in relation to momentary phase position is calculated. 2. Device according to patent claim 1, c h a r a c t e r i z e d in that the digital radio communication system is arranged to transmit information in time slots divided into time frames and that selected time slots are arranged for transmitting the mentioned information to, respective from, a transmitting and receiving unit in the mentioned system. 3. Device according to patent claim 1 and 2, c h a r a c t e r i z e d in that the channel selection function is arranged to influence mentioned allocation of channels depending on the fading condition in the area where the transmitting respective receiving unit exists. 4. Device according to any of the previous patent claims, c h a r a c t e r i z e d in that the mentioned radio communication system is arranged with one or more units for reception respective transmission of information, which are possible to call from, respective to, one in the system included mobile or stationary unit. 5. Device according to any of the previous patent claims, c h a r a c t e r i z e d in that the unit measures one or more signal strength values and that they are averaged over a time period of milliseconds. 6. Device according to any of the previous patent claims, c h a r a c t e r i z e d in that the unit calculates the average period of the fading on the basis of the distance between at least two minima. 7. Device according to any of the previous patent claims, c h a r a c t e r i z e d in that the fading follows a predetermined distribution at which the average distance is a half wavelenght at which the average distance is calculated on the basis of knowledge of the speed of the mobile. 8. Device according to any of the previous patent claims, c h a r a c t e r i z e d in that channel selection for the next, following transmission is decided on the basis of the negative fading margin of the channel. 9. Device according to any of the previous patent claims, c h a r a c t e r i z e d in that prediction of unsuitable channel selection is made by the momentary phase position of the signal. 10. Device according to any of the previous patent claims, c h a r a c t e r i z e d in that the momentary phase position of the signal as well as start- and stop phase is calculated by using the fading and supposed fading distribution stored in a table. 11. Method, at a stationary or mobile digital radio communication system which use dynamic channel allocation, to reduce influence of fading and time dispersion at transmission of signals between a transitting and a receiving equipment, c h a r a c t e r i z e d in that one to a digital radio system belonging mobile unit or base unit is allotted device which performs signal strenght measurements on channels which are correlated with the carrier for which time shared access is to be performed, that the mobile unit or the base unit calculates, on the basis of the signal strenght measurement, the average period of the fading, and that the mentioned fading is calculated as the difference between the signal strength and the calculated average, that prediction of unsuitable channel is made on the basis of the unit having performed measurement which are considerably shorter than mentioned average period, whereafter the unit calculates the momentary phase position as well as start- and end phase for the minimum value, whereupon unsuitable channel selection in relation to momentary phase position is calculated.

Inventors: ANDERSSON TORBJOERN, WICKMAN JOHAN, ANDERSSON, TORBJOERN, WICKMAN, JOHAN
Applicants: ANDERSSON TORBJOERN, TELIA AB, WICKMAN JOHAN, ANDERSSON, TORBJOERN, WICKMAN, JOHAN
Application year: 1995
Main classification: H04B7
IPC codes: H04B17, H04W72