Adaptive frequency planning in a cellular network

Radio frequencies of a base station in a cellular network are chosen adaptively so that when choosing the uplink frequency the base station measure at all frequencies or at some frequencies of the reception band used in the system what is the total power of the signal received at the frequency in question. The frequency at which the lowest power is received is chosen as uplink frequency. When choosing the downlink frequency, a tuning signal is transmitted at all frequencies or at some frequencies of the transmission band. Mobiles located in areas of other cells and operating at the same frequency experience the effect of the tuning signal as an interference. The base station is tuned to use that downlink frequency at which the interference caused by itself to other traffic is sufficiently low.

This invention concerns selection of frequencies in a cellular network
 comprising several base stations and wherein data transmission between a
 base station and a mobile in radiocommunication with it takes place at
 some frequency of a number of frequencies established for the base station
 in question.
 BACKGROUND OF THE INVENTION
 A basic idea of the cellular telephone system is to use the system's
 limited frequency band in such a way that despite limited frequencies it
 is possible to obtain the required capacity. This is achieved by forming
 cells. All the frequencies of the system are not available to the cell,
 but a certain group of frequencies only. The adjacent cell for its part
 can not use the frequencies of the frequency group of this cell in
 question, but these frequencies are available in such a cell only, which
 is located far enough from the cell in question. Signal strengths have
 hereby dropped sufficiently between cells using the same frequencies,
 whereby the interference of the same channel is also low enough and will
 not cause interferences in the radio channel. The allocation of
 frequencies on the described principle is called reuse of frequencies.
 The matter presented above is essential in cellular planning intended to
 select cell size and system parameters, such as frequency allocation and
 cell capacity and number, so that such a continuous coverage is achieved
 economically which will support the required traffic density. Thus,
 factors to be taken into account in cellular planning are, among others,
 traffic density in different areas and the maximum transmission power and
 interference of mobiles.
 The term `reuse factor` relating to the use of frequencies depends on the
 operator's cellular planning, and interference limitations also set up a
 limit for the reuse factor. The reuse factor has a decisive importance for
 the efficiency of the spectrum. The smaller the reuse factor, the more
 efficient is the use of the frequency spectrum. The reuse factor is
 determined by relative interference levels, C/I levels, wherein C is the
 level of the received carrier and I is the interference level. Each factor
 is affected by the used handover strategy, the power regulation of
 mobiles, discontinuous transmission DTX and frequency jumping.
 In a completed network a separate frequency set is allocated for the cell,
 that is, a certain number of carriers at a certain frequency, and the
 reuse factor indicates how far from this cell the same frequencies are
 reused. Even though a certain number of carriers has been allocated for
 the cell, this does not always mean that all carriers are in use. The
 reuse factor is then "loose", whereby if the cell capacity must be
 increased, it may be increased by introducing more carriers from the
 cell's frequency set.
 If the location and number of base stations are seen as constant, the
 frequency spectrum allocated for the system establishes an upper limit for
 the maximum capacity which the network may achieve. To illustrate this,
 reference is made to FIG. 1, which is a diagrammatic view of a
 geographical area covered by the cells in a cellular network. It is
 assumed for the sake of clarity, that the cells are of the same size and
 they may be presented by circles having the same radius, while base
 stations BTS shown by black dots are in the center of the cell and the
 distance between base stations is d. It is assumed that the reuse factor
 is 4, which means that four frequency sets are required: frequency set A,
 frequency set B, frequency set C and frequency set D. Thus, the same
 frequency set may reoccur so that a cell of one frequency set is between
 the cells of another frequency set, for example, in the manner shown in
 the figure. The carrier frequency is thus reused at distance D. Assuming
 that there are two carriers for each cell, that is, there are two carriers
 in each frequency set, whereby the total number of carriers would be 8,
 and assuming that a carrier requires a 200 kHz band as in a GSM system,
 the system shown in the figure as an example would require a frequency
 band of 1.6 MHz. This illustrates the problem concerning known networks
 that the network's frequency band sets an upper limit for the network's
 capacity. To increase the capacity the number of carriers must be
 increased, and this is possible only by making the frequency band bigger.
 Interference can be taken into account as one criterion when the base
 station controller selects the frequency to be allocated for the
 connection. The base station may take this information into account when
 allocating those channels from free channels, which have as low a noise
 level as possible and when deciding on the cell's internal handover when
 it has been noticed that some channel of those which are used in traffic
 suffers from a higher uplink interference level than the ones which exist
 in any free channels. However, this can hardly be done in practice,
 because the operator wants the system to have as high a utilization rate
 as possible so that all channels would be in use. In practice, such a
 situation can hardly exist, especially in a network with interference
 limitation: if all channels would be in use in all cells, then the quality
 of connections would not be acceptable due to the rising interference
 level. For this reason, the network would be jammed on account of
 congestion, even if free channels might still be allocated. In such a
 situation the said interference level of uplink channels can be taken into
 account. In this way a kind of automatic channel planning between cells is
 achieved: the use of a certain channel in the first cell leads to an
 interference level of some magnitude in some other cell, thus preventing
 from taking into use any such channel which is interfered in this cell and
 which at the same time interferes with the first cell. This is basically a
 dynamic method of channel allocation, that is, if a cell is overloaded but
 cells wherein interfering channels are used are not, then the cell may use
 these channels temporarily.
 According to the above presentation, a separate frequency set is allocated
 for each cell in a completed cellular network and the reuse factor
 indicates how far from this cell the same frequencies are reused. To
 achieve this, much frequency planning work has been needed. Frequency
 planning is fixed, that is, once frequencies have been allocated to the
 cells they are permanent. A drawback of fixed frequency planning is that
 it requires much work and it is not able to adapt itself to changes
 occurring in traffic volumes. When the network is complemented with new
 base stations, then a new frequency plan must be made. If a new base
 station is placed on the margin of the network, then the quantity of
 necessary work is reasonable, but if the base station or several base
 stations are located within the network between existing base stations so
 that their cell size is reduced, then frequency planning will require much
 work.
 U.S. Pat. No. 5,212,831, Chuang and Sollenberger, presents a method and
 equipment for independent adaptive allocation of frequencies in FDMA and
 TDMA systems. According to the adaptive method, a frequency is allocated
 for each base station based on signal strength measurements performed by
 the base station. The measurements are done so that the base station turns
 off its own transmitters and listens to the downlink frequencies of other
 base stations and measures their signal powers. The received frequency
 with the lowest power is allocated temporarily as the downlink frequency
 of this base station. Each base station repeats this procedure
 independently and asynchronically in relation to other base stations. When
 all base stations have performed the procedure, then each has selected one
 downlink frequency. Then the same measurement cycle is performed again so
 many times that the downlink frequencies chosen by the base stations will
 no longer change or a predetermined number of iteration cycles has been
 performed.
 In the method according to this invention there are at least two obvious
 drawbacks. Firstly, the base stations have to interrupt their transmission
 for the time of measurements, which is difficult because during that time
 and at least at that frequency connections with mobiles can not be kept
 up. This is harmful for network subscribers. Secondly, the strength of the
 downlink interference signal of other base stations is measured at the
 base station performing the measurements, whereby it is possible that the
 base station will not detect big interferences, but the serving mobiles
 will nevertheless suffer big interferences.
 This second drawback is illustrated in FIG. 2. It shows a situation where
 base stations BS1 and BS2 are located in the landscape so that a signal
 between them will not proceed directly but strongly reflected. Such a
 situation will arise in urban conditions at crossings where buildings
 prevent the signal from proceeding straightly or in the country where high
 points in the landscape prevent a straight progress. Hereby, when using
 the method according to the US patent, BS1 would measure the transmission
 frequency of BS2 and would find that its interferences are low at this
 frequency. Correspondingly, BS2 would measure the frequency of BS1 and
 would find it small. The outcome would be that each base station could
 tune in on the same downlink frequency. It would result from this that
 when a car is at point A, it is in traffic connection with base station
 BTS1 at frequency f, whereby the connection would be of a good quality,
 but when the car has arrived at a crossing it would receive the frequency
 transmitted by base station BTS2, whereby it would experience the signal
 from base station BTS2 at the same frequency as a strong interference.
 Under these circumstances, at the crossing there would always be a strong
 interference in the mobile's reception, irrespective of which base station
 the mobile MS is in connection with.
 An objective of the present invention is to bring about adaptive frequency
 planning without the drawbacks of known methods. The frequency planning
 must be able to work at the same time as the base station is in normal
 traffic use, and the planning must be able to take into account any real
 interferences which the mobile may experience.
 The established objectives are achieved through the methods defined in the
 independent claims. The dependent claims present various ways of
 implementing the method.
 BRIEF SUMMARY OF THE INVENTION
 The invention is based on the insight that when choosing uplink frequencies
 of a tuning base station, the dispatch, that is, the real traffic of
 transmitter/receivers operating in the cells is listened to, and when
 choosing downlink frequencies the interference caused by the dispatch of
 the tuning base station in any transmitter/receiver in the area of other
 cells is taken into account. The transmitter/receiver may be a test unit,
 but preferably it is a mobile in the normal traffic operation of the
 system. Measurements may be performed while the tuning base station is in
 normal operation in traffic.
 Uplink frequencies are chosen so that the base station doing the frequency
 plan, that is, the tuning base station, with a subset of frequencies of
 the reception band used in the system or at all frequencies of the
 reception band measures which is the total power of the signal received at
 the frequency in question. Depending on the system, the measurement may be
 performed in different ways. Firstly, the measurement may be performed by
 measuring the frequency constantly. This manner must be used in FDD
 systems. Secondly, if the system is a TDM system, the measurement may be
 performed constantly by seeing if the same frequency is used by several
 channels, wherein the power may be at different time slots, that is, it
 may vary between channels, or the measurement may also be performed
 separately for each time slot in the frame either for each received time
 slot or just for any desired time slots of the frame. In practice, the
 first-mentioned way of measuring the entire carrier constantly is
 preferable.
 When the total power received at each frequency has been measured, the
 frequencies are placed in an order of quality. If powers have also been
 measured for different time slots, then such time slots of the frequency
 in question are also placed in an order of quality. A set of uplink
 frequencies, which may comprise one frequency or several frequencies, is
 chosen according to this order.
 Downlink frequencies are chosen so that at all frequencies of the
 transmission band or with a subset of frequencies the tuning base station
 sends a signal which may be an unmodulated carrier or a carrier modulated
 with noise or with another tuning signal. It is especially preferable to
 modulate the carrier with some identifiable information. If a test unit
 located in the area of other cells is in connection with the said other
 cell at the same signal frequency and at the some moment of time as those
 at which the tuning base station is transmitting at the moment, it will
 experience this signal as an interference, whereby the interference level
 will rise in the receiver. The test unit relays measurement results
 concerning the quality of the connection through its own base station to
 the network, which informs the base station making the frequency plan how
 big an interference it has caused, whereby the base station will deduce
 whether it may put the frequency/channel in use or whether the following
 frequency will be examined.
 The network preferably controls the entire tuning process. A suitable
 network element for controlling is, for example, the base station's
 controller, which gives the tuning base station instructions to transmit
 an interference signal and, having received the measurement results of the
 caused interference, it analyses whether the tuning base station may put
 the said frequency or channel into use or whether measurements will be
 continued at another frequency. It then commands the tuning base station
 to operate in accordance with the results of the analysis.
 Preferable test units are mobiles operating in the network which even
 otherwise perform measurement routines required in a mobile telephone
 system and transmit them at regular intervals to the network. The tuning
 base station then receives a lot of interference measurement results.
 Those frequencies are chosen as the set of downlink frequencies, which may
 comprise one frequency or several frequencies, at which the interference
 caused by the tuning base station to other traffic is sufficiently low.

DESCRIPTION OF A PREFERABLE EMBODIMENT
 In FIG. 3, cell A is assumed to be a tuning base station which has to
 choose both uplink and downlink frequencies for itself. It is assumed that
 the other cells use certain frequencies which are mentioned later and that
 there is a different frequency in each direction, whereby the system is a
 Frequency Division Duplex (FDD) or a Frequency Division Duplex/Time
 Division Duplex (FDD/TDD) system, whereby the latter uses time division,
 besides frequency division, as in, for example, a GSM system. It is also
 assumed for the sake of simplicity that each cell uses only one frequency
 both in the uplink direction and in the downlink direction. Thus, base
 station BTS of cell G transmits information at the frequency f of a mobile
 in the cell's area (downlink frequency) and receives its dispatch at
 frequency f.sub.2 (uplink frequency). The corresponding frequencies used
 by cell B are f.sub.3 and f.sub.4 while the frequencies of cell F are
 f.sub.5 and f.sub.6. The frequencies of other cells are not presented.
 The method is described with the aid of FIGS. 3 and 4. Base station A
 starts the tuning by listening to the system's uplink frequencies, that
 is, the transmissions of mobiles, however, without paying any attention to
 their information contents but taking into account the received signal
 power only. This is stage 1 in FIG. 4. In some cases it may listen only to
 a part of the system's uplink frequencies, but it can listen to all
 frequencies of the uplink band. The receiver of the base station of cell A
 is tuned from one frequency to another frequency of the transmission band,
 and at each frequency it measures the total power received, stage 4 in
 FIG. 4.
 If the system is a FDD system, the total power received at this frequency
 is measured.
 If the system is a FDD/TDD system, only the total power may be measured, if
 desired, by finding out whether the frequency is divided into time slots.
 Hereby the base station takes into use all the time slots of the uplink
 frequency which it has chosen. It is also possible to perform a
 measurement based on time slots. This requires that the base stations of
 the network are synchronized or almost synchronized in relation to one
 another or that they know each other's timing, whereby each base station
 knows the frame synchronization of the others. Hereby the tuning base
 station may take into use just a part of the frame's time slots, while the
 other time slots may be used by other base stations. Of course, a base
 station may also use all time slots.
 Since the powers of real mobile transmissions are measured in the
 measurements and since traffic density varies in the network according to
 the hour and weekday, the measurements must last long enough, so that by
 averaging the results a sufficiently reliable picture is obtained of
 uplink frequency powers. This means that the measurement process may last
 for hours, even for days, although the individual measurement may be
 short. On the other hand, if individual measurements are short bursts,
 they may be sent in cycles, for example, for a few hours, and decisions
 may be made based on the material thus obtained. The use of bursts also
 allows sending them one after another for a short time only, for example,
 for a few minutes, and some frequency is then chosen for use. After a
 short time, for example, after half an hour, new measurements of a short
 duration are made and the results they give will show how to act. This
 allows one to react very quickly to changes in traffic density occurring
 in the network.
 If the base station is returning and certain transmission/reception
 frequencies have already been chosen for it, then all other reception
 frequencies, except the measurement frequency being used, are available
 for mobile connections and all transmission frequencies of the base
 station are available in a normal manner. This means that measurements
 will hardly disturb the operation of the base station and of the mobiles
 which are in traffic connection with it. A separate test
 transmitter/receiver may also be used at the base station.
 When all uplink frequency powers have been measured, that frequency is
 chosen as uplink frequency at which the received total power is as low as
 possible, stage 6 in FIG. 4.
 Referring to the example shown in FIG. 3, the measurements of base station
 A comprise the total power of reception frequency f.sub.2 of cell G, the
 total power of reception frequency f.sub.4 of cell B and the total power
 of reception frequency f.sub.6 of cell F. Thus, the frequencies are those
 which the mobiles in the cell, which are indicated by numbers 31, 32 and
 33 in the cells, use in their transmission in the base station direction,
 and the powers are long-term average powers at the frequencies in question
 measured by the tuning base station. It is obvious that the longer the
 distance is between mobiles and tuning base station the less is their
 effect on the measured long-term power value. Thus, after base station A
 has arranged all powers of its measured signal frequencies in an order of
 magnitude, it could find out by studying the list, for example, that the
 medium power of frequency f.sub.2 as measured at base station A has fallen
 so much that it would not cause any exceeding of the permissible
 interference level at the base station of cell A, were the base station to
 choose it as its uplink frequency. Thus, the base station chooses
 frequency f.sub.2 as its uplink frequency. This frequency is also used as
 uplink frequency in cell G, even though base station A is not able to
 distinguish different frequency sources from one another. The uplink
 frequency may of course be chosen by any criterion, for example, one may
 choose a frequency corresponding to the lowest measured signal power. Base
 station A could find out from the list that the power measured from signal
 frequency f.sub.4 is so high that should the base station choose it as its
 uplink frequency, the interference level would be higher than permissible
 at the base station. Thus, the frequency in question may not be chosen.
 The same result concerns the frequency f.sub.6 used by cell F and in the
 same way all frequencies of immediately adjacent cells. The frequency
 f.sub.4 is the same as the uplink frequency used by mobile 33 in cell B.
 However, the base station does not know this, because it is not able to
 distinguish different frequency sources from each other.
 If the cellular system according to FIG. 3 is a TDM system, the method
 according to the invention makes it possible to choose only certain time
 slots from the uplink frequency to be chosen, whereby in terms of time the
 tuned base station will use only a part from the chosen frequency leaving
 the remaining part to other base stations. As is known, in time-divided
 systems, the traffic channel is formed of frequency and of time slots so
 that the carrier frequency is divided into time slots and a number N of
 successive time slots forms a frame. Hereby N mobiles may use the same
 carrier at the same time. According to the invention, long-term medium
 power measurements may also be performed on a time slot basis, whereby of
 a chosen uplink frequency only certain time slots may be put to use, while
 the remaining time slots may be left to other base stations. This is
 especially advantageous when the assumed traffic density is small in the
 tuning cell. A measurement based on time slots requires synchronization
 between base stations.
 After the uplink frequency has been chosen, the tuning base station of cell
 A chooses a downlink frequency, which must be such that it will not cause
 too much interferences in the connections of mobiles operating in other
 cells.
 The downlink frequency is chosen so that the tuning base station sends an
 interference signal at all carrier frequencies or at their subset of the
 transmission band of the system's base stations, stage 2 in FIG. 4. In a
 frequency-divided system, the tuning signal is sent constantly, whereas in
 time-divided systems the tuning signal may be sent either constantly, if
 the intention is to put the whole carrier in use, or the tuning signal may
 be sent only in some time slots of the frame.
 If the tuning signal is sent constantly, the transmission first begins with
 a low power and then rises to a value which is the same as the one
 calculated in advance as the maximum transmission power for the tuning
 cell.
 If the tuning signal is transmitted in time slots, the signal envelope
 during rise and fall will follow a shape determined in the system's
 specifications, for example, a cos.sup.2 curve in a GSM system. Any burst
 maximum power may be chosen between a very low power and the maximum
 power, so by choosing in the examined frame time slot, that is, the
 channel, the transmission power to change, for example, from one time slot
 to another in a purposeful manner, much information can be obtained about
 the effect of the interference in other cells. For example, the tuning
 signal may vary between two values in terms of time. A tuning signal which
 is sent in time slots also allows measurements of very short duration. For
 example, the measurement may last for only one frame, in GSM for 4.615 ms
 or for a few frames or for a multiframe lasting 6.12 seconds. When the
 power is measured on a time slot basis, tuning bursts can be sent in some
 time slots only.
 The tuning signal causing interference in the receiver may be a pure
 unmodulated carrier, it may be modulated with noise or with some
 identifiable information. In digital systems, some suitable identifiable
 bit pattern may be sent in the tuning burst. It is especially advantageous
 to send such an identifier in the burst which will individualize the
 tuning base station in some way. In other cells the receivers will then
 decode the tuning information and report to the network not only the
 interference effect but also the source, from which the interference
 derives. The power variation of the tuning signal mentioned in the
 preceding paragraph may be used for the same purpose. By varying the power
 in a certain manner, for example, according to the square wave, the base
 station causing the interference can be easily identified. When the
 interference to be identified is fast, short tuning cycles may be used:
 for example, tuning signal bursts are sent for 10 ms at intervals of 10
 minutes. Fast interferences may thus be corrected through coding in the
 receiver, so the interference will not harm their operation.
 A possible procedure is such that the measuring receiver is located in some
 cell and a base station of this cell is in connection with a receiver at
 the transmission frequency of the base station. When the measuring
 receiver receives, besides the serving base station signal, a signal of
 the same frequency sent by the tuning base station, this will cause
 interference in the reception: the receiver's interference level will rise
 in proportion to the tuning signal strength. The receiver performs
 measurements concerning signal quality and relays the results of the
 measurement to the network. When such measurements are performed in a
 sufficient number of cells at the downlink frequencies which they use, the
 caused interference is found out at each frequency. The network relays
 this information to the base station, stage 3 in FIG. 4, and based on this
 information the base station will choose that frequency as downlink
 frequency at which the interference caused to other traffic and found by
 measurements will be smallest, stage 7 in FIG. 4.
 However, it is not necessary to send measurement results all the time. For
 example, they may be sent only when a mobile clearly identifies an
 interference as caused by some base station (as there may be some other
 reason for the rise of the interference level) or only when the network
 has informed the mobiles of a beginning tuning measurement and commanded
 them to send measurement results, if interferences occur in their
 reception. The mobiles may send measurement results on a normal traffic
 channel by using a suitable mechanism, for example, by taking some bits of
 the traffic burst for this purpose.
 According to a preferable embodiment, the network attends to the entire
 tuning process. The base station controller is a suitable network element
 for this task. The base station controller hereby starts the tuning
 process of the individual base station, receives the measurement results
 and reports to the base station which frequencies or channels
 (frequency/time slot) it must use.
 In practice, the use of the above-mentioned separate measuring receiver is
 difficult and slow, and it is in fact preferable to use as measuring
 receivers the real mobiles operating in the network. In all cellular
 networks mobiles constantly perform various measurements to do with signal
 quality, on which they report at regular intervals to the network. For
 example, in a GSM system the mobile sends a measurement report twice a
 second. When the tuning base station sends a tuning signal at different
 frequencies, the mobiles which receive information at these frequencies
 from their base station will experience interferences in their reception
 on which they will report to the network. Thus, the network gets a lot of
 information at the same time about interferences caused by the signal
 transmitted by the tuning base station. If a sufficient number of these
 measurements is made, sufficient statistical information is obtained for
 concluding at which downlink frequency of the tuning base station least
 interference will be caused to the traffic of the other cells. The tuning
 signal may be constant, but it is very advantageous to use short bursts
 lasting for a few milliseconds, because the interference caused by these
 in the reception of mobiles can be corrected through channel decoding.
 The above presentation is further illustrated with the aid of FIG. 3. A
 tuning base station in cell A sends a tuning signal i.a. at downlink
 frequencies f.sub.1, f.sub.3 and f.sub.5 of cells G, B and F. The downlink
 direction information of the traffic channel of mobiles operating in these
 cells, which are indicated generally and jointly by reference numbers 31,
 33 and 32, is correspondingly relayed at frequencies f.sub.1, f.sub.3 and
 f.sub.5. In its reception each mobile experiences the signal sent by the
 tuning base station at the same frequency as an interference, which is
 stored in those parameters describing the quality of the connection which
 the mobile is measuring. The mobile sends the information to the base
 station controller (not shown in the figure) in the network, which will
 inform the tuning base station about the magnitude of the interference
 which it has caused. If the interference of the tuning base station is so
 big that it threatens to cut off the connection of some mobile, the
 network may interrupt the test for a moment and change the tuning
 frequency or channel, stage 4 in FIG. 4.
 When many tuning tests are made at different hours of the day, the average
 interference caused to the other traffic by the use of each frequency is
 found out. This interference is compared with a threshold value. If the
 interference caused at a certain frequency is too high, then the tuning
 base station will not use this frequency. If no such frequency is found at
 which the interference caused to the reception of mobiles served by base
 stations of other cells would be sufficiently low, then either the tuning
 is discontinued or a higher interference is accepted, the threshold values
 are changed and the search is begun from the start. In the latter case the
 information may be relayed to the base station controller that the maximum
 load level of the network should be reduced. In a GSM network this is
 done, for example, by an admission control procedure.
 Upon completion of measurements, the base station of cell A is tuned to use
 that transmission frequency at which the interference to other traffic is
 sufficiently low. Let us assume that such a frequency would be downlink
 frequency f.sub.1, of cell G. After application of the method according to
 the invention, the base station would hereby have been tuned to use the
 same frequencies as cell G.
 The response of uplink and downlink frequencies to each other has not been
 taken into account in the foregoing. In most systems, frequencies are in
 couples so that a certain reception band frequency corresponds to a
 certain transmission band frequency, whereby the difference between
 frequencies is always the same. If applying the method in such a system,
 the following should be done:
 a) the power received from different uplink frequencies is measured in the
 way described above and the frequencies are arranged in an order of
 quality based on the measurement,
 b) the couple corresponding to the best uplink frequency is chosen as
 tuning signal frequency in the downlink direction,
 c) the effects of the tuning signal in the reception of mobiles are
 examined in the manner presented above,
 d) if the effects are acceptable, the base station begins using this
 frequency couple,
 e) the search is finished or the chosen uplink and downlink frequencies are
 removed from the lists of quality and the next step is item b for finding
 the following frequency couple,
 f) if the effects are not acceptable, the couple corresponding to the next
 best uplink frequency is chosen as tuning signal in the downlink direction
 and its effect is studied and the steps according to items c, d and e are
 taken etc.,
 g) if no suitable frequency couple is found, the tuning of the base station
 is interrupted or the limit of acceptable interferences is raised and the
 search is started again from item a.
 Any desired number of frequency couples can be chosen for the base station
 by using this procedure.
 All base stations in the network may tune by the method according to the
 invention and tuning may be a continuous process, one performed at
 suitable intervals or by a special order. It is not essential from the
 viewpoint of the invention who gives the order. It is advantageous to let
 the network attend to the entire tuning process, whereby the base station
 performs measurements of the uplink power, sends a tuning signal under
 control by the network and tunes to use frequencies stated by the network
 and the base station may also start tuning independently. In all cases the
 network's frequency plan adapts automatically to the traffic in the
 network and the base stations use the frequencies at which they receive as
 little interference as possible in the uplink direction and at which they
 cause as little interference as possible to the traffic in other cells in
 the downlink direction.
 When the base stations have tuned, the network or the base stations begin
 collecting information on normal traffic interferences. If due to
 installation of new base stations or for any other reason the chosen
 frequency or frequencies are no longer suitable, the base stations will
 tune to a new frequency by the method according to the invention. Tuning
 will hardly impede other traffic, if the system is a time-divided one,
 because some time slots may relay normal traffic and other time slots are
 used for uplink and downlink measurements. In a purely frequency-divided
 system the other frequencies are of course available, except the tuning
 frequencies.
 The above presentation has mainly studied one frequency at a time. It is
 known that time-divided mobile networks use frequency jumping, wherein the
 frequency/time slot couples of the traffic channel change in accordance
 with some predetermined frequency jumping pattern. The presented method
 makes it possible to choose the best possible frequency jumping pattern
 when the interference caused at different frequencies and at their
 different time slots is known or the power received in the uplink
 direction is known based on measurements. Conversely, it is possible in
 tuning to examine desired frequency jumping patterns only and to choose
 such a pattern in the use of which the caused interference is acceptably
 low or the received power is acceptably low.
 The work required in frequency planning is reduced considerably by using
 the presented method. Base stations may easily be added to the network and
 they will independently seek such frequencies or channels in the use of
 which they will interfere as little as possible with connections in other
 cells. Hereby the adding of new base stations will not require any new
 frequency planning. In addition, the operation of a base station in normal
 use need not be interrupted while it is in the tuning stage. When choosing
 a suitable tuning signal for use when looking for the downlink frequency,
 the tuning will not at all disturb normal operation in the network,
 because any increase of the interference level in the mobile reception can
 be compensated for by a power adjustment of the base station in connection
 with the mobile. If the tuning signal is of a short duration, that is, if
 bursts are transmitted, any information that may be destroyed can be
 replaced with the aid of error correction. Decisions on the selection of
 uplink frequencies are based solely on the measurement of the total power
 received at different frequencies, whereas decisions on the selection of
 downlink frequencies are based on chosen criteria, which may be Bit Error
 Rate (BER), relative interference level C/I and Frame Error Rate (FER) or
 detection of variation in the interference level or other measured
 quality/power occurring in different channels. For this reason, the method
 may be implemented with simple and advantageous technology.