GB2509984A

GB2509984A - Selection of a base station carrier channel

Info

Publication number: GB2509984A
Application number: GB1301098.8A
Authority: GB
Inventors: Gbenga Salami; Pankaj Uplenchwar; Colin Kellett
Original assignee: Ubiquisys Ltd
Current assignee: Ubiquisys Ltd
Priority date: 2013-01-22
Filing date: 2013-01-22
Publication date: 2014-07-23
Anticipated expiration: 2033-01-22
Also published as: GB201301098D0; GB2509984B

Abstract

A method of operating a base station, such as a femtocell, in a network comprising a plurality of other base stations, each operating on a respective carrier channel, is disclosed. The method comprises detecting a first channel used by at least one other base station, scanning carrier channels at least partially overlapping the detected channel, measuring interference caused on the detected channel by use of each of the scanned channels, and selecting a channel for use based on the interference measurements. A second channel used by other base stations adjacent to the first channel may be detected. The scanned channels may be between the first and second channels, and interference caused on the second channel by the scanned channels may be measured. The channel may be selected such that interference on the first and second channels is balanced according to a relative operating priority. A method of setting the maximum downlink or uplink power for a base station on the basis of frequency offsets between its carrier channel and those of at least two other base stations is disclosed. Its carrier channel is between those of the other base stations, with a greater frequency offset from one than the other.

Description

SELECTION OF A BASESTATION CARRIER CHANNEL

The present invention relates to a basestation for use in a cellular mobile communications network, and to a method of operation of a basestation in which a carrier channel for the basestation is selected.

Small cell basestations are known and used in many cellular networks. A small cell basestation forms an access point that provides mobile coverage in areas where such coverage is problematic. Small cell basestations may for example be deployed indoors in residential or business premises, or in remote outdoor locations. The small cell basestation connects to the core network of a cellular network operator by means of an existing network connection. The device then provides cellular network coverage for subscribers within a coverage area of the device. Small cell basestations are intended to complement existing macro layer network coverage such that user equipment devices may attach to and use either a macro layer basestation or a small cell basestation, depending on their location. Small cell basestations are typically intended to run autonomously, and thus have many self-configuration capabilities. One important self configuration capability for a small cell basestation is the selection of a carrier channel on which the small cell basestation will operate.

Factors influencing carrier channel selection for a small cell basestation may vary according to operator deployment scenarios. Some cellular network operators maintain three carrier channels, including a non-opeiational or clear carrier, which may be used for small cell deployment. Other operators employ only two carrier channels, with one typically used as a camping carrier and the second as a capacity carrier. If operators having two channels wish to employ small cells, small cell basestations must be deployed either co-channel with the camping or capacity carrier, or straddled between the two carriers. Co-channel deployment offers advantages in mobility between the macro layer and the small cell layer but causes considerable interference to the macro channel on which the small cell operates. Straddling the small cell base station channels evenly between the macro channels reduces the downlink interference on the macro channel by up to 50% as measured by the size of the dead zone on the macro network. However, the interference is then shared between the two macro channels, and interference on a high usage channel such as a camping channel may be undesirable. Still other operators use only a single carrier channel, and thus typically deploy small cell basestations co-channel with the single macro carrier channel.

According to the present invention, there is provided a method of operation of a basestation in a mobile communications network, wherein the network comprises a plurality of other basestations, each operating on a respective carrier channel, the method comprising: detecting a channel in use by at least one of the other basestations; scanning carrier channels at least partially overlapping the detected channel; checking a measure of interference caused on the detected channel by use of each of the scanned channels; and selecting a channel from among the scanned carrier channels for use in the basestation, based on the checked interference measures.

According to another aspect of the present invention, there is provided a method of operation of a basestation in a mobile communications network, wherein the network comprises at least two other basestations, each operating on a respective carrier channel, and wherein the basestation operates on a carrier channel which is positioned between the carrier channels of the at least two other basestations with a greater frequency offset from one of the carrier channels than the other of the carrier channels, the method comprising: setting at least one of a maximum downlink power for transmissions from the basestation and a maximum uplink power for transmissions from user equipments attached to the basestation based on the frequency offsets between the carrier channel of the basestation and the carrier channels of the at least two other basestations.

According to another aspect of the present invention, there is provided a basestation adapted to operate in accordance with the method of the first or second aspects of the invention.

For a better understanding of the present invention, and to show more clearly how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-Figure 1 illustrates a part of a cellular communications network operating in accordance with an aspect of the present invention; Figure 2 is a schematic representation of a basestation operating in accordance with an aspect of the present invention; Figure 3 is a flow chart illustrating steps in a method in accordance with the present invention; Figure 4 illustrates a frequency allocation of a channel in accordance with the UMTS cellular standard; Figure 5 is a flow chart illustrating in more detail aspects of the method of Figure 3 Figure 6 illustrates a frequency allocation of a channel in accordance with an aspect of the present invention; and Figures 7 and 8 are further flow charts illustrating in more detail aspects of the method of Figure 3.

The following description illustrates aspects of the present invention with reference to a small cell basestation. However it will be appreciated that the invention may be applied to other types of basestation including for example macro layer basestations micro basestations, pico basestations or any other type of basestation unit.

Figure 1 shows a part of a cellular communications network. In the illustrated cellular network, a small cell basestation, or small cell access point (SAP) 10 has been deployed. The SAP 10 is in the vicinity of three macro layer cellular basestations 12, 14, 16 within the same cellular network. It will be appreciated that a practical network will include many more macro layer basestations, but the present invention can be described sufficiently without illustrating additional basestations. The SAP 10 and macro basestations 12, 14 and 16 communicate with user equipment devices (U Es) via wireless links. Signals are transmitted via the wireless link from the SAP or macro basestation to the UEs (downlink) or from the UEs to the SAP or macro basestation (uplink).

Figure 2 illustrates in more detail the SAP 10. At this level of generality, the SAP 10 is known, and will be described only so far as is necessary for an understanding of the present invention. The SAP 10 operates under the control of a processor 20, which monitors, and controls the operation of, the other components of the SAP 10. The communication with the core network of the cellular network operator is typically over the internet, by means of an interface 22. Signals to and from the interface 22 for communication over the wireless interface to mobile devices or other user equipment are passed to a modem 24, which puts the signals into the appropriate format, based on the relevant cellular standard. The invention will be described further with reference to a SAP 10 operating in accordance with the UMTS cellular standard, but it will be apparent that the invention can be applied to any other appropriate standard.

Signals received by the SAP 10 over the wireless interface are passed to conventional receive (Rx) circuitry 26, operating at a frequency that is derived from a signal received from an oscillator 28. Signals for transmission over the wireless interface are passed to conventional transmit (Tx) circuitry 30, operating at a frequency that is derived from a signal received from an oscillator 32. The oscillators 28, 32 are shown here as separate, but there may instead be a single oscillator, with the Rx circuitry 26 and the Tx circuitry 30 deriving the relevant receive and transmit frequencies from that single oscillator. Connected to the Rx circuitry 26 and the Tx circuitry 30 is an antenna 34.

Figure 3 is a flow chart illustrating a process performed by the SAP 10 under the control of the processor 20 during installation of the device. The same process can be carded out at regular intervals while the SAP 10 is in operation, to take account of changes in the signals transmitted by macro layer basestations in the vicinity.

Referring to Figure 3, in step 50 of the process, the SAP 10 attempts to detect downlink transmissions from macro layer basestations within the cellular network and in the vicinity of the SAP 10.

In the UMTS system, frequency carrier channels are spaced apart by 200kHz. Each channel is given a number, the UTRA Absolute Radio Frequency Channel Number (UARFCN), where the frequency of a channel is its UARFCN multiplied by 200kHz.

Carrier channels have a nominal width of 5MHz. Each channel is identified by the UARFCN that corresponds to the centre frequency of the carrier, although the carrier will in fact contain components over a frequency range of 5MHz centred on that centre frequency.

Figure 4 illustrates a situation where a basestation is transmitting on channel number 10590. That is, it is generating signals across a bandwidth of approximately 5MHz centred on 2118MHz, which is the product of the channel number, 10590, and 200kHz.

Macro layer basestations are typically operating on carrier channels that are determined by the cellular network operator as part of its network planning. For example, and as discussed above, two carrier channels may be allocated to all of the macro layer basestations in the network. In a typical arrangement, the first channel is known as the camping carrier', and is used to cover the complete legion with multiple cell sites. The second channel is often known as the capacity carrier', and is only deployed in certain areas, being used for capacity offload and/or High Speed Download Packet Access (HSDPA). The SAP 10 may thus be able to detect one or more macro layer basestations operating on a first channel or one or more macro layer basestations operating on a second channel. Small cell basestations are often deployed in areas of high usage and the SAP 10 may thus be able to detect one or more macro layer basestations operating on each of the first and second channels.

Referring again to Figure 3, in step 60 of the process, the SAP 10 selects a carrier channel for operational use by the SAP 10, based on information obtained in step 50.

The process by which the SAP selects a carrier channel is illustrated in the flow chart of Figure 5, which shows a breakdown of process steps which may be conducted as part of step 60.

Referring to Figure 5, in a first step 61, the SAP determines whether only one macro channel has been detected at step 50, or whether more than one channel has been detected at step 50. If only one channel has been detected at step 50, ("No" at step 61), the SAP 10 proceeds at step 62 to scan all channels, that is all UARFCNs between the detected macro channel and the guard band separating the detected channel from an adjacent channel. As discussed above, each UARFCN is separated by 200kHz, and the scanned channels between the macro channel and the guard band are thus all overlapping to a greater or lesser degree with the macro channel. At step 63, for every scanned channel the SAP 10 checks a measure of interference caused on the detected macro channel as a result of use of the scanned channel. The measure of interference may be either one or both of the Received Signal Code Power (RSCP) or the Received Signal Strength Indicator (RSSI). Either of these measurements provides an indication to the SAP 10 of the level of interference that would be caused on the detected macro channel as a result of use by the SAP 10 of the currently scanned carrier channel.

Having scanned all channels between the detected macro channel and the guard band the SAP 10 identifies, at step 64, the scanned channel associated with a minimum of interference caused on the macro channel. This will be the channel having the minimum RSCP or RSSI measurement on the macro channel during scanning. Having identified this channel, the SAP 10 selects the identified channel as the operational channel for the SAP 10 in step 65. Having selected a channel, the SAP 10 proceeds to step 70, as illustrated in Figure 3.

It is noted that for network operators having a single macro carrier channel, the step 61 of checking for detection of more than one carrier channel may be unnecessary, and the step 60 of selecting a carrier channel for the SAP 10 may be implemented only by steps 62 to 65 illustrated in Figure 5.

Referring again to Figure 5. if more than one channel has been detected at step 50 (Yes" at step 61), and continuing the example of a cellular network operator employing two macro channels, the SAP 10 proceeds at step 66 to scan all channels, that is all UARFCNs, between the two detected macro channels. Typically the detected channels are adjacent, meaning that all the scanned UARFCNs overlap to a greater or lesser extent with both of the detected macro channels. At step 67, for every scanned channel the SAP 10 checks a measure of interference caused on each of the detected macro channels as a result of use of the scanned channel. As in step 63, the measure of interference may be either one or both of the Received Signal Code Power (RSCP) or the Received Signal Strength Indicator (RSSI). Either of these measurements provides an indication to the SAP 10 of the level of interference that would be caused on each of the macro channels as a result of use by the SAP 10 of the currently scanned carrier channel.

Having scanned all channels between the two detected macro channels the SAP 10 identifies, at step 68, the scanned channel that causes interference on the detected macro channels which is distributed in accordance with a relative operating priority of the channels. This process is discussed in further detail below with reference to Figures 7 and 8. Having identified a channel in step 68, the SAP 10 selects the identified channel as the operational channel for the SAP 10 in step 69 and proceeds to step 70, as illustrated in Figure 3.

Checking interference measures on the two detected marco channels during scanning allows the SAP 10 to select a channel according to the interference caused on the adjacent macro channels. Operating priorities for the two macro channels may dictate for example that one channel be protected from interference to a greater extent than the other. One channel may be judged sufficiently important that it is to be protected from interference regardless of the cost in terms of interference to the other channel.

However, it is likely that distributing interference effects between the two channels will involve a balancing process of protecting one channel within limits of acceptable interference caused on the other channel. The closer the SAP channel is in frequency to a macro channel, the greater the interference that will be caused on the macro channel by use of the SAP channel. Protecting a macro channel thus involves increasing the frequency separation between the SAP channel and the protected macro channel. As the SAP channel is positioned between the two macro channels, increasing separation, and hence reducing interference relative to one channel inevitably reduces separation, and hence increases interference on the other channel.

Hence the potential requirement for a balancing process, as discussed above.

The balancing process is illustrated in Figure 6, which shows a SAP carrier channel 2 straddled between two macro carrier channels, a first carrier channel 4, and a second carrier channel 6. The SAP channel 2 is offset towards the first carrier channel 4, thus preferentially protecting the second carrier channel 6 from interference. However, as illustrated by arrows 5 and 7, the SAP channel may be selected to have any offset relative to the two macro carrier channels 4, 6, such that the interference caused by the SAP channel 2 is distributed between the macro channels 4, 6 in accordance with a relative operating priority of the macro channels 4, 6.

The process of identifying a channel for the SAP 10 that satisfies operating priorities for interference caused on the adjacent macro channels (step 68 of Figure 5) may be conducted via the steps illustrated in Figure 7. Thus in a first step 68a, the SAP 10 designates one of the detected macro channels as a priority channel and in a second step 68b, the SAP identifies the scanned channel which causes a minimum of interference on the priority macro channel. As discussed above, this may include selecting a channel of minimum interference on the priority macro channel regardless of the impact on the other detected macro channel. However, step 68b may also involve checking that interference caused by a scanned channel on the non priority macro channel does not exceed a certain threshold, before identifying a scanned channel as satisfying operating priorities.

Steps involved in designating one of the detected macro channels as the priority channel are illustrated in Figure 8. As discussed above, most operators using two macro channels designate one channel as a camping carrier and the other as a capacity carrier. It is often the case that network operators wish to preferentially protect the camping carrier from interference, rather than the capacity carrier, although in the present specification no assumption is made as to which channel might be protected. In situations where one channel is to be preferentially protected, details of the channel to be protected may be programmed into the SAP 10 by a manufacturer.

The SAP 10 may then consult a memory to determine which channel is to be designated a priority channel.

In some cases, even where an operator has a confirmed preference for protecting one channel over the other, there may be circumstances in which the particularities of a deployment require a change in the established priority status of the two macro carriers. It is generally assumed that for similarly loaded first and second detected macro carriers, a magnitude of the RSCP or RSSI that is measured as an indication of interference caused will be substantially similar. However, it is possible that a first macro basestation operating on a first macro channel may be positioned much closer geographically to the SAP 10 than a second macro basestation operating on the other channel. The smaller path loss from the first macro basestation to the SAP 10 results in much higher values of RSCP or RSSI being registered at the SAP 10 for the first channel than for the second channel. This lesser geographic separation between the SAP 10 and the first macro basestation renders the first macro channel, on which the first macro basestation is operating, much more susceptible to interference from the SAP 10 than the second channel. In such cases it may therefore be necessary to protect the more susceptible channel by increasing frequency separation between the susceptible channel and the SAP channel, regardless of which of the first or second channels is considered of greater priority by the network operator in a more standard environment.

Referring to Figure 8, in a first step 68a(i), the SAP 10 compares magnitudes of RSCP or RSSI measurements for each of the macro channels. Measurements may be compared for a single scanned carrier, a plurality of scanned carriers or for all scanned carriers. At step 68a(ii), the SAP 10 checks whether or not a difference in magnitude between the RSCP or RSSI values on the two macro carriers exceeds a certain threshold. If the difference does not exceed the threshold ("No" at step 68a(ii)), the SAP 10 concludes that the geographical deployment situation is reasonably balanced, and proceeds, at step 68a(ih) to refer to a memory in order to determine which of the macro channels should be designated as the priority channel, and at step 68a0v) to designate the channel specified in the memory as the priority channel. If however the difference in magnitude in RSCP or RSSl values between the macro channels does exceed the threshold, the SAP 10 assumes a skewed geographical deployment situation and proceeds to step 68a(v), in which the channel having the higher RSCP or RSSI value is designated as the priority channel.

The result of the process steps illustrated in Figures 7 and 8 is that a SAP 10 checks for a geographical deployment situation rendering one of the adjacent macro channels highly susceptible to interference. On identifying such a situation, the SAP 10 designates the susceptible channel as a priority channel, and identifies for selection and use in the SAP 10 the scanned carrier channel causing a minimum of interference to the priority, highly susceptible channel. If the SAP 10 does not identify a particular geographical deployment situation, the SAP 10 refers to a memory to read operator instructions as to which of the macro channels should be preferentially protected. The SAP 10 designates the instructed channel as the priority channel and proceeds to identify for selection the scanned carrier channel which causes a minimum of interference on the priority channel. As discussed above, the search for a minimum interference channel may be balanced by maximum acceptable interference values on the other, non priority macro channel. Figures 5 to 8 thus illustrate how the SAP 10 selects a carrier channel based upon interference caused on the detected macro channel or channels.

Referring again to Figure 3, the process then continues to step 70, in which the SAP 10 informs the adjacent macro basestations of the newly selected SAP channel and updates the neighbour list accordingly.

The process then proceeds to step 80 in which a downlink (DL) transmit power for the SAP 10 is set. The maximum downlink transmit power of the SAP 10 determines the coverage area of the SAP 10, and is set at a level to ensure that neighbouring basestations are not adversely affected by operation of the SAP 10. Continuing the above example of a network operator having two macro carrier channels, both of which are detected by the SAP 10, the initial DL transmit power is calculated on the basis of macro layer RSCP or RSSl measurements from both of the adjacent carriers.

The initial DL transmit power is calculated according to the following formula: initial DL TX Power = Target RSCP -Macro RSCP o/ftet -IOlogyo (% Total SAP power allocated to CPICH,.) + Minimum indoor loss Equation (1) Where: Macro RSCP offset is applied to account for the lower interference compared to a co-channel situation. This parameter is used to adjust the initial DL power and is set at 6dB to account for roll off filtering effects; 101og10 (% total SAP power allocated to CPICH) is typically equal to 10 dB when 10% of total power is allocated to the common pilot channel (CPICH); and Minimum indoor loss is an estimate of the path loss between the SAP 10 and connected UEs.

In the case of an evenly straddled SAP channel, the target RSCP is calculated according to the following formulae: Target RSCP = max (Average RSCP, Minimum RSCP) Equation (2) Average RSCP = IOlogio (Linear average (95" percentile Carrier], 95th percentile Carrier 2)) Equation (3) Where: Average RSCP is the average of the largest measured RSCPs of both the adjacent carriers being straddled by the SAP carrier; and Minimum RSCP is the minimum RSCP level the power algorithm aims to achieve within the SAP coverage area.

According to aspects of the present invention, the SAP channel is not evenly straddled between the adjacent macro carriers but is offset in order to protect one or other of the carriers from interference. Equations (2) and (3) are thus replaced by Equations (4) and (5) below: Target RSCP = max (Target offset RSCP, Minimum RSCP) Equation (4) Target u//set RSCP = lOloglo ({wI 1j0(95th percentile Garrierl/lO)} + {w_2*I(/95th percentile GarrieO/1O)1, Equation (5) Where: Wi and W2 are weighting factors applied to the RSCP values of the adjacent carriers in order to account for the frequency offset of the SAP carrier relative to the adjacent carriers. Wi and W2 are calculated according to the following formulae: WI = 1-{(OJjset carrier UARFCN-Carrierl tL4RECfV /(N-12/ Equation (6) W2 = I -((Carrier2 UARFCN-Of/set carrier UARFCN,) /(N-I)) Equation (7) Where: N is the number of UARFCNs between the two adjacent carriers, Carrier 1 and Carrier 2; and WI + W2 = I It will be appreciated that in the standard UMTS deployment, Carrier 1 UARFCN is the lower channel number, such that: Carrier 1 UARFCN < Offset carrier UARFCN, and Carrier 2 UARFCN> Offset carrier UARFCN.

By applying appropriate weighting factors reflecting the frequency offset of the SAP channel with respect to the adjacent macro carriers, the process balances the effect of the SAP 10, minimising the overall disruption to both of the neighbouring basestations.

After setting the initial DL transmit power of the SAP 10, the process continues to step 90, in which an initial maximum uplink (UL) power for UEs connected to the SAP 10 is set. The uplink power of connected UEs is capped to prevent unwanted uplink noise rise at adjacent macro basestations. The maximum uplink power is capped based on the path loss between a UE connected to the SAP 10 and the relevant macro basestation. The macro basestation is referred to in the following formulae as a NodeB in accordance with the UMTS cellular standard.

The initial UL transmit power is set according to the following formula: Ut Power = fVodeB noise floor -Straddled Ut noise rise + Smallest NodeB to SAP path loss Equation (8) Where: NodeB noise floor is a database parameter defining the NodeB noise floor at the antenna, which is typically -lO4dBm; Straddled UL noise rise is a database parameter defining the imposed back off margin, which is typically 15 dB; and Smallest NodeB to SAP path loss is the 1St percentile of the path loss histogram derived from downlink monitor mode (DLMM)/sniff of the adjacent carriers by the SAP 10.

In order to calculate the smallest NodeB to SAP path loss, weighting factors are again applied to account for SAP channel offset from the adjacent macro carriers. The weighting factors are applied as follows: Smallest NodeB to SAP path loss = IOlogjo ((WI *10(181 percentile Carrierl_PL1!lO)1 + percenlile Carrier2_PLZI1O)B Equation (9) Where: Carrierl_PL1 and Carrier2_PL2 are histograms derived from the DLMM/sniff of the adjacent carriers by the SAP 10; and Wi and W2 are weighting factors calculated according to equations (6) and (7).

Having set the initial DL and UL powers for the SAP 10, the SAP 10 can begin operation at step 100 as illustrated in Figure 3.

During operation, at step 110, continual adaptations are made to the DL and UL powers to reflect current conditions detected at the SAP 10.

Ongoing DL power is calculated as follows: DL TX Power = Major/tv ZL path loss -I-Target Offset RSCP -Macro RSCP offset -I-Fixed loading o/jet + Loading margin -lOloglo (% Total SAP power allocated to CPIH,) Equation (10) Where: Majority ZL path loss is the 951h percentile of the SAP to UE path loss. These measurements are gathered in a histogram when the UE is considered to be mobile and not stationary over a window of time; Macro RSCP offset is applied to account for the lower interference compared to a co-channel situation. This parameter is used to adjust the initial DL power and is set at between 3 and 6dB; Fixed loading offset is a database parameter that can be used to bias the maximum DL transmit power; Loading margin is an additional power added to account for variability of the macro layer interference within a building and inter cell interference caused by network loading; and 10log (% total SAP power allocated to CPICH) is typically equal to 10 dB when 10% of total power is allocated to the common pilot channel (CPICH).

Target Offset RSCP is calculated according to equation (5), reproduced below, and equations (6) and (7): Target ojftet RSCP = IO/ogio U wi 10(95th percentile Carrierl/1011 + jw2*/O(o5th percentile Garrier2f 10))) Equation (5) The RSCP measurements and weighting factors are continually updated by the SAP 10 to allow for updated calculation of Target offset RSCP.

It may be the case that RSCP measurements are available from UE5 operating in compressed mode. These measurements are collated in separate histograms and the smallest 1st percentile is used in calculation of Target offset RSCP. If compressed mode UE measurements are available then they replace the SAP 10 measurements in equation (5), resulting in Target offset RSCP calculated according to the following formula: Target oJjet R5CP = lOfogj0 ((ff1 *10(lst percentile Carrierl!10)} + 1jy2-*10l8t percentile Carrier2/10)}, Equation (11) Ongoing UL power is calculated as follows: UL Power = NodeB noise floor -L'plink noise rise mat gin + Minority ML path loss Equation (12) Where: Uplink noise rise margin is a database parameter defining the back off margin which is typically 15dB; and Minority ML path loss is the smallest 1S percentile of the long term NodeB to SAP UE path losses on adjacent carriers.

In order to calculate the Minority ML path loss, the weighting of the adjacent carriers is again taken into account and Equation (9) is used: JtIinoritv ML path loss = IOlog;o (/1*7 *10(lst percentile Garrierl_PL1!lO)1 + {1y2*]($lSt percentile Carrier2_PL2J1O))) Equation (9) Where: Wi and W2 are calculated using Equations (6) and (7).

As noted above, measurements from UEs operating in compressed mode may be available. Separate histograms may therefore be maintained including NodeB to SAP UE path loss measurements reported by UEs in compressed mode. It may be desirable to maintain separate histograms for SAP DLMM/sniff statistics and UE compressed mode measurements, resulting in two histograms (one for each macro carder) of DLMM/sniff statistics and two histograms (one for each macro carrier) of UE compressed mode measurements. This arrangement allows flexibility in selecting suitable path loss percentile statistics for the SAP DLMM/sniff separate from the UE path loss percentile measurements.

Aspects of the present invention thus allow a network operator the flexibility to deploy a basestation, for example a small cell basestation on a carrier located anywhere between two adjacent macro carriers, or between an adjacent macro carrier and the guard band. The small cell carrier is selected in a self configuration process conducted by the small cell basestation, involving scanning all channels between the adjacent macro channels, and selecting a channel for use according to the interference caused by the channel on the adjacent macro channels. In one example, the channel selected may be the channel causing minimum interference on one or both of the adjacent macro channels.

Further aspects of the invention enable the setting of appropriate downlink power and uplink power limit for a small cell basestation operating on a carrier which is offset between two adjacent macro carriers. The power setting process minimises overall interference on both of the adjacent carriers. Measurements from connected UEs in compressed mode may be used in the power setting process.

Claims

CLAIMS1. A method of operation of a basestation in a mobile communications network, wherein the network comprises a plurality of other basestations, each operating on a respective carrier channel, the method comprising: detecting a channel in use by at least one of the other basestations; scanning carrier channels at least partially overlapping the detected channel; checking a measure of interference caused on the detected channel by use of each of the scanned channels; and selecting a channel from among the scanned carrier channels for use in the basestation, based on the checked interference measures.
2. A method as claimed in claim 1, further comprising: detecting a second channel in use by at least one of the other basestations which is substantially adjacent to the first detected channel; and checking a measure of interference caused on the second detected channel by use of each of the scanned channels; wherein scanning carrier channels at least partially overlapping the detected channel comprises scanning carrier channels between the detected channel and the second detected channel; and wherein selecting a channel for use in the basestation comprises selecting a channel such that interference caused on the detected channel and the second detected channel is balanced in accordance with a relative operating priority of the detected channel and the second detected channel.
3. A method as claimed in claim 2, wherein selecting a channel for use in the base station comprises: designating one of the detected channel or the second detected channel as a priority channel; and selecting a scanned carrier channel that causes a minimum of interference on the priority channel for use in the basestation.
4. A method as claimed in claim 3, wherein designating one of the detected channel or the second detected channel as a priority channel comprises: comparing a magnitude of checked interference measures on each of the detected and second detected channels; and on identification of a difference in magnitude exceeding a threshold value, designating the channel associated with the higher magnitude interference measures as the priority channel.
5. A method as claimed in any one of claims 2 to 4, further comprising setting a maximum downlink power for transmissions from the basestation, wherein the maximum downlink power is set based on a frequency offset between the selected carder channel and the detected carrier channel and a frequency offset between the selected carrier channel and the second detected carrier channel.
6. A method as claimed in claim 5, wherein the maximum downlink power is set based on a target value calculated as a weighted combination of values on the detected and second detected carrier channels, wherein the weighting values are set according to a frequency offset between the selected carrier channel and the respective one of the detected and second detected carrier channels.
7. A method as claimed in any one of the preceding claims, further comprising setting an ongoing maximum downlink power based on measurements received from user equipment devices attached to the basestation and operating in compressed mode.
8. A method as claimed in any one of claims 2 to 7, further comprising setting a maximum uplink power for transmissions from user equipment devices attached to the basestation, wherein the maximum uplink power is set based on a frequency offset between the selected carrier channel and the detected carrier channel and a frequency offset between the selected carrier channel and the second detected carder channel.
9. A method as claimed in claim 8, wherein the maximum uplink power is set based on a path loss calculated as a weighted combination of values relating to the detected and second detected carrier channels, wherein the weighting values are set according to a frequency offset between the selected carrier channel and the respective one of the detected and second detected carrier channels.
10. A method as claimed in any one of the preceding claims, further comprising setting an ongoing maximum uplink power for transmissions from user equipment devices attached to the basestation based on measurements received from user equipment devices aftached to the basestation and operating in compressed mode.
11. A method as claimed in claim 1, wherein scanning channels at least partially overlapping the detected channel comprises scanning channels between the detected channel and a guard band.
12. A method as claimed in claim 11, wherein selecting a channel for use in the basestation comprises selecting a channel causing a minimum of interference on the detected channel.
13. A method as claimed in any one of the preceding claims, wherein the measure of interference comprises at least one of Received Signal Code Power or Received Signal Strength Indicator.
14. A method of operation of a basestation in a mobile communications network, wherein the network comprises at least two other basestations, each operating on a respective carrier channel, and wherein the basestation operates on a carrier channel which is positioned between the carrier channels of the at least two other basestations with a greater frequency offset from one of the carrier channels than the other of the carrier channels, the method comprising: setting at least one of a maximum downlink power for transmissions from the basestation and a maximum uplink power for transmissions from user equipments attached to the basestation based on the frequency offsets between the carrier channel of the basestation and the carrier channels of the at least two other basestations.
15. A basestation, for use in a cellular mobile communications network, wherein the cellular mobile communications network comprises a plurality of other basestations, each operating on a respective carrier channel! and wherein the basestation is adapted to operate in accordance with the method according to any preceding claim.