Method for determining multiple interference in a mobile radio system

The invention relates to a method for determining the total downlink or uplink interference percentage in the coverage area of a designated base station for a mobile radio system with a given channel plan. The invention provides for the need to carry out, after a conventional channel allocation has been carried out, a check of the extent to which total interference requirements are met. The method consists in that field strengths from all base stations are measured on all traffic routes within the geographical area of the mobile radio system. The interference is calculated at each point within the coverage area of a base station for each relevant pair of base stations, preferably base stations which are located in the same channel and base stations which are located in an adjacent channel. The interferences of all the relevant pairs of base stations are added in order to form the total interference at each point. The invention makes it possible to calculate the total interference percentage which constitutes the ratio between the length of road distance with disturbed coverage and the length of total road distance with coverage. The interference percentage is calculated suitably within a contour which limits the coverage area of the base station concerned.

FIELD OF THE INVENTION 
The present invention relates to a method for determining the total degree 
of downlink or uplink interference in the coverage area of a designated 
base station for a mobile radio system with given channel allocation. The 
invention is directly related to the problems set in Swedish Patent 
Applications 8900742-1, 800743-9, 8900744-7 and 8900745-4 and especially 
to the invention "Method for radio cell planning" (8900744-7). This method 
utilizes field strength measurements and an allocation algorithm which 
provide the possibility of a simple adaptation of the cell system, that is 
to say increase or decrease of the number of cells with changed traffic 
demand. If a computerized channel allocation based on an exclusion matrix 
has been carried out, a certain minimum quality with respect to the 
interferences in the system is ensured in a first approximation. The 
quality is ensured in the coverage area of a designated base station for 
each of the interference sources but not for the combined interference 
from all co-channel and adjacent-channel interference sources. It is 
necessary either to use allocation algorithms which take account of the 
sum of interferences, see Patent Application 8900742-1 "Method for 
resource allocation in radio systems", or to use methods for carrying out, 
after a conventional channel allocation has been carried out, a check of 
to what extent total interference requirements are met. The present 
invention relates to the latter case and provides a method for carrying 
out a control relating to the total interference situation. 
DISCUSSION OF BACKGROUND 
The problem with multiple interferences in cell systems has been known for 
some time. However, what has not been known previously is how interference 
between two cells in a metropolitan environment can be described with 
sufficient accuracy in a quantitative way. A new measuring technique 
developed at Televerket Radio, however, provides a detailed pattern of the 
complicated wave propagation in a city environment and has created the 
prerequisites for very detailed interference studies and thereby also for 
studies of multiple interference between cells. 
SUMMARY OF THE INVENTION 
According to the invention, a method is provided for checking for a given 
channel plan in a mobile radio system that multiple interferences in the 
coverage areas of the different base stations meet the required limit 
values. 
The main characteristics of the method can be obtained from the patent 
claims following.

DETAILED DESCRIPTION OF THE INVENTION 
The background for the present invention is a measuring technique newly 
developed at Televerket Radio, which opens up quite new possibilities for 
describing the wave propagation in a metropolitan environment. 
The prerequisites are that there is a number of base stations each with its 
coverage area and a frequency band for the system with a limited number of 
channels. There is also a requirement for the C/I carrier to interference 
noise ratio which is needed for good reception and how much interference a 
receiver can tolerate in adjacent channels. A set of for example base 
stations using the same or adjacent channels can be considered a "relevant 
pair" of stations. 
Using a specially calibrated receiver equipment, the received power from 
all base stations is measured on relevant traffic routes in the 
geographical area which is covered by the mobile radio system. For these 
measurements, the measured field strengths form mean values over sections 
of 20 m (approximately 30 wavelengths) and each section is tied to a 
co-ordinate information. The field strength values are represented in dBm 
in the measurement data of the received signal power. The measurements are 
not as extensive as it sounds since field strengths from up to twelve base 
stations can be registered at one time as a matter of fact. It is quite 
possible to make all necessary measurements for one cell including 
coverage and interference area in one night. This type of measurement has 
already been carried out with success in the Stockholm area. 
The measurements provide knowledge about which potential power a receiver 
in a mobile unit would receive from different cells wherever the mobile 
unit is located within the geographical area. The potentially received 
power originating from mobile units within the coverage area can also be 
easily calculated at an arbritary base station. Thus, the interference 
situation both for the mobile units and the base stations are known. 
Since the interferences can be described on the one hand with respect to 
the base station receivers and on the other hand with respect to the 
receiver of the mobile units, there exists an uplink interference and a 
downlink interference. 
The downlink situation is shown in FIG. 1. Assume that all the base 
stations together with the corresponding service areas are numbered from 1 
to N. In FIG. 1, two stations i and j are shown with associated service 
areas. A mobile unit M in the ith coverage area receives a wanted power Pi 
from its own base station and an unwanted interference power Pj from the 
base station number j. There is a small difference between the terms 
"service area" and "coverage area". By coverage area is here meant all 
measured roads which, with respect to a given base station, have a 
sufficiently high received power to allow satisfactory reception. In a 
service area, there can be unmeasured points with good reception. 
The minimum allowable C/I (carrier to interfrence) noise ratio for 
acceptable co-channel quality is LPl and the minimum allowable C/I for 
acceptable quality with interference in the first adjacent channel is LP2 
and so forth. For the (k-1)th adjacent channel, C/I must be greater than 
LPk, k.ltoreq.M. A diagram plot can be produced in which geographical 
points, for which it holds true that 
EQU Pi/Pj&lt;LPk 
EQU k=1,2, . . . M 
are marked by the symbol "0" and the remaining points are marked by the 
symbol ".". This provides a pattern where noisy points are marked by "0" 
and points with acceptable reception are marked by ".". 
M is the number of necessary co-channel and adjacent-channel limit values. 
FIG. 2 shows the uplink situation. In the figure are shown two base 
stations i and j with associated service areas. In this case, the base 
station i is exposed to interference Qj from a mobile unit Mj in the 
coverage area of base station j. The base station i receives a wanted 
power Qi from a mobile unit Mi in its coverage area. The coverage areas 
are defined in the same way as earlier or possibly adjusted for any 
imbalance uplink in the power budgets for up- and downlink. 
The uplink interference is calculated in the following way. When the mobile 
unit Mj in FIG. 2 passes through the entire coverage area of base j, a 
noise power i is generated in the base i. The minimum allowable C/I noise 
ratio for acceptable co-channel quality is designated by LQl and the 
minimum C/I for acceptable quality for the first adjacent channel is 
designated by LQ2 and so forth. For the (k-1)th adjacent channel, C/I must 
be greater than LQk, k.ltoreq.M, in the same way as earlier. The noise 
power varies in dependence on the interfering mobile unit's instantaneous 
position and its different noise power production can be statistically 
characterized by means of a distribution function. 
a) The distribution function is calculated with a starting point from the 
measured field strength values. The noise values are generated by 
randomization according to said distribution, which can be implemented, 
for example, by letting all cases of noise production be represented in 
table form and carrying out a uniform designation of all numerical values 
of the table. All the values are thus stored in their own memory location 
and selection is carried out uniformly over all the addresses of the 
memory locations. 
Assuming that a mobile unit Mi passes through the coverage area i and in 
doing so receives coverage field strength Qi and a random noise field 
strength Qj at a given point in the coverage area. In the same way as 
above, a diagram plot of the interference situation can be produced by 
marking geographical points for which it holds true that Qi/Qj&lt;LQk. 
Due to the fact that the noise field strength Qj is randomized, the Qi/Qj 
ratio becomes a stochastic variable. The consequence is that the diagram 
plot also becomes stochastic and assumes a new appearance each time the 
calculation is carried out. In practice, it is found that diagram plots 
produced in this way are gathered well around their "mean value" and that 
a single occurrence can be considered as representative. If one is not 
satisfied with this, there is always the possibility of estimating the 
mean value of the diagram plot by simulating the interfering mobile unit's 
effect several times in the manner described above. 
b) The distribution function is approximated by a logarithmically normal 
distribution. It is well-known from the literature that noise field 
strengths originating from mobile units located the same distance from the 
base have an almost logarithmically normal distribution. This also applies 
with good approximation to noise field strengths in a base from mobile 
units in an adjoining coverage area. The log-normal distribution is 
completely determined by mean value and spread, which parameters can be 
easily calculated from the given measured noise field strengths. Compared 
with case a), it is not the distribution function which is calculated but 
only the mean value and spread of the true distribution of noise values. 
The true distribution is further approximated by a logarithmically normal 
distribution. The median for the true logarithmated noise field strengths 
can be used very well as mean value in the log-normal distribution. The 
simulated noise powers are generated with the aid of a generator of 
normally-distributed numerical values and knowledge of the mean value and 
spread as above. 
The power values Q in the base stations from transmitting mobile units can 
be directly related to the power values P from transmitting base stations 
due to the fact that the transmission loss between base and mobile unit 
does not depend on the direction of transmission. Since P-values are 
simply obtained from measured wave propagation data, this also applies to 
Q-values. 
FIGS. 3 and 4 show an illustration of the diagram plots where there can be 
seen, on the one hand, the road network where the home signal strength 
from the base station is sufficiently high for satisfactory reception, 
which we call coverage, and, on the other hand, positions in the coverage 
area where the reception is disturbed by the base stations in Skarholmen 
and Odenplan, respectively. An interference percentage is defined which is 
the percentage of coverage with disturbed reception. The diagram plots 
show the noise received by the mobile unit (downlink interference) but it 
is equally when possible to describe in the same way where the mobile unit 
is located close to the base station and is disturbed by a mobile unit in 
the adjoining coverage area (uplink interference). The interference 
percentage is in both cases 2.6% and 1.9%, respectively. The present 
invention has application both in the downlink and uplink but since the 
principles of the application of the invention are the same regardless of 
the direction of communication, only the downlink is used as an example in 
the text which follows. 
In FIG. 5, an example of calculated interference from both Skarholmen and 
Odenplan is shown. The noise contributions are virtually additive which is 
due to the fact that the interferences from the two base stations arrive 
at different locations in the coverage area and are therefore independent 
of one another. The diagram plots can be easily added by setting up simple 
mathematical rules: 0+0=0 
EQU 0+.=.+0=0 
EQU .+.=. 
After that, the interference percentage is calculated as above. As shown, 
the total interference is 4.4%. The uplink interferences in a 
corresponding situation have a quite different interference pattern but, 
assuming that the interferences are calculated in a correct manner by 
Monte-Carlo-simulation, for example by means of the random technique 
described above, the additivity principle also applies here for small 
noise contributions. With computerized channel allocation, the individual 
noise contributions can be easily taken into consideration and care can be 
taken that pairs of cells do not interfere with one another. It is more 
difficult, though possible, to ensure during the channel allocation 
procedure that the total disturbance of the coverage in one cell is kept 
below a given limit value. With conventional computerized channel 
allocation, there is reason to check that a proposal for a channel plan 
does not have disadvantageous characteristics with respect to the sum of 
the individual noise contributions in each cell. 
FIG. 5 shows multiple interference in the coverage area of the base station 
in Vallingby from only two interfering base stations, but the 
contributions from all remaining base stations which share the channel 
with Vallingby will naturally be studied, and also base stations close to 
Vallingby which use channels adjacent to the base in Vallingby. In the 
noise pattern shown in FIG. 5, it can be seen that the interferences in 
just this case are located at the outer edge of the coverage area. As a 
matter of fact, the interference from Skarholmen has no practical 
significance since a mobile unit in this part of the coverage comes under 
quite a different cell. However, interference can arise with high traffic 
if a mobile unit is not successful in achieving handover to an adjoining 
cell. 
FIG. 6 shows a technique which can be used if it is to be defined in 
greater detail which part of the coverage area is of significance with 
respect to interference. An arbritary closed contour can be used but in 
most cases it is sufficient to define the cell coverage with the aid of a 
circle. Generally, the circle is not centered around the base station 
site. In calculating the interference percentage, only the disturbances 
falling within the circle are now taken into consideration. The 
interference percentage within this circle is 0.8%. 
The present invention relates to a method for determining the appearance of 
the total interference pattern with respect to all possible noise sources 
in the coverage area of a station. The invention is applicable both for 
interference at the mobile unit and for interference at the base station.