GB2498698A - Setting user equipment maximum uplink power in a basestation coverage area - Google Patents

Abstract

Setting a maximum uplink power for user equipment devices (26) in a coverage area of a first basestation (18) located in a coverage area of one or more second basestations (12) in a cellular communications network. The maximum uplink power is set for user equipment devices based on a measure of the path losses between the one or more second basestations and user equipment devices in the coverage area of the first basestation. The measure of the path losses between the one or more second basestations and the user equipment devices is formed from a measure of the path loss between the one or more second basestations and the first basestation and an estimate of the excess of the path losses between the one or more second basestations and the user equipment devices over the path loss between the one or more second basestations and the first basestation.

Description

INTERFERENCE MANAGEMENT FOR STRADDLED CARRIER DEPLOYMENTS

This invention relates to interference management, and in particular to methods and systems for setting power levels in a basestation of a cellular communications network, in order to manage levels of interference between devices in the network. The invention is particularly, though not necessarily exclusively, relevant to setting power levels in a basestation in a small cell, or in a femtocell basestation.

In many regions around the world, WCDMA (Wideband Code Division Multiple Access) mobile network operators fall broadly into two groups. The first group are those operators who have three UMTS (Universal Mobile Telecommunications System) carriers. When deploying femtocells, these operators tend to use a carrier that is either not operational or is only used in hot spot' or border regions. In this manner, the interference effects of deploying femtocells are considered to be minimal, assuming that the femtocells have some basic form of interference management.

The second group of operators are those that may only have two UMTS carriers and do not have the luxury of assigning a relatively unused carrier to the femtocell population. Interference management is a relevant concern for this group of operators.

Operators that fall within this group might for example have a spectrum policy whereby a single UMTS Rel 99 camping carrier is deployed throughout the operator's territory, and a Rel 99 and/or HSDPA (High Speed Downlink Packet Access) capacity carrier is deployed in certain regions of the territory. In general, HSDPA enabled phones are handed over to the HSDPA carrier when a call is established, and may remain on this carrier after the call is concluded. Alternatively, in certain deployments, the user may return to the camping carrier after call termination. A 2G GSM (Global System for Mobile communications) voice layer that is also deployed throughout the region and is used to provide coverage when there is no 3G coverage.

In this spectrum configuration, there are three options available to an operator deploying femtocells, namely (a) deploying the femtocells on the Rel 99 camping carrier (b) deploying the femtocells on the HSDPA carrier or (c) deploying the femtocells straddling (i.e. midway between) the Rel 99 and HSDPA carriers. This straddling carrier deployment effectively creates a third carrier (the UMTS standard can support up to three different Universal Terrestrial Radio Access (UTRA) Absolute Radio Frequency Channel Numbers (UARFCNs) in the Idle mode neighbour cell lists, including the serving UARFCN).

It is then desirable to set transmit powers in the femtocell that manage interference to levels that enable femtocell deployments in either straddled carrier or co-channel carder configurations.

According to a first aspect ot the present invention, there is provided a method of setting a maximum uplink power for user equipment devices in a coverage area of a first basestation in a cellular communications network, wherein the first basestation is located in a coverage area of a second basestation, the method comprising: setting the maximum uplink power for user equipment devices based on a measure of the pathlosses between the second basestation and user equipment devices in the coverage area of the first basestation.

For a better understanding of the present invention, and to show 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 cellular communications network, in accordance with an aspect of the invention; Figure 2 illustrates a femtocell basestation, in accordance with an aspect of the invention; Figure 3 is a flow chart, illustrating an RRM configuration algorithm in accordance with an aspect of the present invention; Figure 4 illustrates the setting of a maximum uplink power; and Figure 5 further illustrates the method in accordance with the invention.

Figure 1 shows a part of a cellular communications network 10.

Figure 1 shows a macrolayer basestation 12, having a dedicated connection in to a core network (CN) 14 of the cellular communications network. The macrolayer basestation 12 provides cellular service across a coverage area. Located in the coverage area is a building 16, which contains a femtocell basestation 18, which in this example is connected in to the core network (CN) 14 of the cellular communications network by means of an existing broadband internet connection 20. Of course, it will be appreciated that there will be many such buildings and femtocell basestations in an operational network, but the illustrated devices are sufficient for an explanation and understanding of the present invention.

User equipment (UE) devices 22, 24, such as mobile phones, smartphones, internet access devices, portable computers or the like, are located in the coverage area of the network outside the building 16, while a user equipment device 26 is located within the building 16. Of course, it will be appreciated that there will be many such UE devices in an operational network, but the illustrated devices are sufficient for an explanation and understanding of the present invention.

Figure 2 shows the femtocell basestation, or femto access point (AP), 18 in more detail. Specifically, the femtocell basestation 18 includes radio transceiver (TRX) circuitry 22. The TRX circuitry 22 detects signals received by an antenna 24. The TRX circuitry 22 is able to receive signals transmitted by tiE devices on assigned system uplink (UL) frequencies, but is also able to receive signals transmitted by other basestations on assigned system downlink (DL) frequencies. The TRX circuitry 22 also converts signals into a suitable form for transmission over the radio interface.

The femtocell basestation 18 also includes an internet interface 26, for establishing the connection over the internet to the core network 14 of the cellular network.

The femtocell basestation 18 operates under the control of a processor 28, which is responsible for various functions. The operation of the processor 28 will be described herein only to the extent required for an understanding of the present invention.

In traditional cellular networks, the Mobile Network Operator (MNO) is responsible for Radio Access Network (RAN) designs which include configuration of each basestation's SF parameters such as carrier frequency. transmit power levels. DL primary scrambling codes, neighbour cell lists etc. However, for a network of femtocells that is expected to be deployed by the MNO customers in an ad hoc manner, with a deployment population that will more than likely outnumber macrolayer basestations, a different approach to deployment is required, such as automatic configuration. In addition, the femtocells are expected to coexist with the existing MNO networks, with the possibility of sharing radio resources such as carrier frequencies, and this presents a whole new set of challenges. One of the key challenges is controlling Radio Frequency (SF) interference impacts from the femto network, and this requires careful control of femtocell RE parameters, so that the positive benefits of femtocells can be realised without causing impacts to the existing macro network.

The inputs to the Radio Resource Management (RRM) auto-configuration algorithms come mainly from measurements and information derived from the femto APs network scanning function known as the Network Listen Mode. The Network Listen Mode has two operational states, namely the Down Link Monitor Mode (DLMM) and the Fast Sniff Mode (ESM). The DLMM is typically invoked at power on to scan the surrounding 3G WCMDA and 2G GSM basestations, including neighbouring femto APs. This involves measuring signals levels, decoding broadcast channels and extracting radio frequency (SF) parameters associated with the surrounding macro network. The FSM is periodically invoked after power on when the AP is idle, to collect short samples off the downlink (WCDMA and OSM), and in the process continuously refreshing information held about macro neighbours and neighbouring APs detected during previous DLMM runs! while also building up information on any newly detected macro neighbours or neighbouring APs. In addition, the inputs to the RRM algorithms are complemented by UE measurements collected as users move around the femto AP coverage area, to help fine tune the femto APs SF parameters.

Figure 3 is a flow chart, showing a summary of the RSM process in the femtocell basestation 18, which may for example be a residential femtocell AP. On powering up, the femto AP connects to an access point management system (AP-MS) in the core network 14, runs through a series of diagnostic functions (i.e. self check, register with the management system, download most recent software load etc) and sets up its IP network configurations. Following the successful hardware and system configuration, the femto AP goes through a radio frequency (SF) auto-configuration. This auto-configuration occurs in several stages.

In a first stage, the femto AP downloads (step 50 in Figure 3) from the management system RF parameters, such as the Universal Terrestrial Radio Access (UTRA) Absolute Radio Frequency Channel Numbers (UARFCNs), the maximum and minimum allowed total DL/UL transmit power levels, and the femto AP DL primary scrambling codes. From the range of UARFCNs the femto AP uses the Downlink Monitor Mode (DLMM) of the Network Listen Mode function to detect and decode the 3G WCDMA and 2G OSM broadcast channels (BCH) of all surrounding base stations and Femto APs. The APs neighbour cell list is created from these detected cells.

In a second stage, the UARFCN carrier and primary scrambling code selection is completed, as per the RRM processes described below. This is followed by setting of the initial total downlink and uplink transmit power levels. Finally, the cell is brought up and the AP's System Information Blocks (SIBs) are updated with the 3G WCDMA and 2G GSM neighbour cell lists, as well as idle mode mobility parameters that encourage UEs to stick to the femto AP once camped on.

In a third stage, after power up, RF optimisation takes place, as described in more detail below.

Down Link Monitor Mode (DLMM) is typically activated (step 52 in Figure 3) at power up, once per day, or when a change to the surrounding 3G WCDMA environment is detected by the FSM. In the DLMM, the femto AP can determine whether RF conditions are poor (step 54 in Figure 3), and, if so, can raise an alarm in step 56. In the DLMM, the femto AP performs a cell search and measurements on all possible carriers that it is allowed to operate on, as indicated by the management system, in order to detect the nearby 3G WCDMA or 2G GSM rnacrocells and any collocated femto AP primary scrambling codes. Thus, the capabilities of DLMM include: detection of surrounding WCDMA basestations, including other femto APs and GSM basestations; Common Pilot Channel (CPICH) Received Signal Code Power (RSCP) or Relative Common Pilot Channel Energy per chip versus Noise (CPICH Ec/lo) measurements on detected surrounding WCDMA basestations and carrier received signal strength indication (RSSI) measurements; RSSI measurements on detected surrounding GSM basestations; extraction of cell system information from Broadcast Channels of detected WCDMA and GSM basestations, such as neighbour cells lists, cell ID, mobility parameters, CPICH transmit (Tx) Power, etc. calculation of frequency offsets from detected surrounding 30 WCDMA and 20 OSM macrocells, which are used to correct for frequency drift in the femto AP's local frequency reference.

The detections and measurements made in the DLMM are used as inputs to the initial auto-configuration of the femto AP's RF parameters such as UARFCN carrier, primary scrambling codes, neighbour cell lists and DL/UL power setting (step 58 in Figure 3).

When this has been completed, the access point can enter service in step 60 of Figure 3.

When the access point is active, that is, there is at least one call active, the femtocell access point can obtain information from UE measurement reports. This allows further control of RE parameters.

For example, downlink and uplink powers can be optimised (step 62 in Figure 3), based on the femto AP UE measurement reports. The optimisation aims to minimise radio frequency (RF) leakage, and dead zones, outside the intended AP coverage area, and to avoid an uplink noise rise to the surrounding macrolayer (ML) node Bs.

As another example, dynamic High Speed Downlink Packet Access (HSDPA) power allocation can be performed (step 64 in Figure 3) for active sessions, aiming to allocate as much power as possible to HSDPA sessions, depending on the current cell loading, hence ensuring optimal use of total available power.

As another example, dynamic management of a noise rise target for High Speed Uplink Packet Access HSUPA can be performed (step 66 in Figure 3), aiming to limit the HSUPA power whenever HSUPA sessions could impact Release 99 services in the uplink.

As another example, dynamic dedicated channel (DCH) power management can be performed (step 68 in Figure 3) for DCHs, aiming to ensure that total DCH power available is available to all DCHs as needed, while also ensuring fair distribution of resources across DCHs.

As another example, DL/UL radio access bearer (RAB) rate adaption can be performed (step 70 in Figure 3) for Release.99 packet switched (PS) sessions that will reconfigure RABs to a lower data rate, for example when experiencing interference, and will restore the RAB to a higher data rate at an appropriate time, for example when no longer experiencing interference.

UE measurements relating to basestations that it can detect can also be used (step 72 in Figure 3) to monitor for new macrolayer basestations on the same frequency as the femtocell.

After successful power on, Fast Sniff Mode (FSM) is activated (step 80 in Figure 3) whenever the AP is idle (i.e. there is no active voice or data session in progress). In Fast Sniff Mode, the femto AP periodically (for example, every 90 seconds) tunes to the 3G WCDMA and 2G GSM DL frequency band, and samples the DL for about 20 milliseconds. From the DL samples, it is possible to: refresh previous signal measurements such as CPICH RSCP, CPICH Ec/lo and RSSI made on previously detected 3G WCDMA cells, 2G GSM cells and nearby femto APs; refresh previously read WCDMA cell system information such as mobility parameters, by comparing current value tags of SIBs to their last known values; detect the presence of new WCDMA primary scrambling codes or their disappearance; calculate the frequency offsets from surrounding 3G WCDMA and 2G GSM macro cells, and use these to correct for frequency drift in the femto AP's local frequency reference.

The detections, measurements made and information gathered by the FSM are used to update the femto AP's RF parameters, as indicated by step 82 in Figure 3. For example! neighbour cell lists are updated if a new WCDMA macro cell is detected. If changes, or significant events, are detected in the RE environment using the East Sniff Mode (FSM) of the Network Listen Mode function, this could indicate that that the initial RF parameters selected are no longer optimal, and could be used to trigger a rerun of the DLMM process for auto-configuration of the parameters in question.

Typically, a femtocell makes use of UE measurements to calculate the path loss from the femto UE to the macro nodeB. These measurements are then used to determine an indoor macro layer to femto UE path loss finger print of the building that the femto cell is deployed within. In straddled carrier deployments the UE measurements would require compressed mode to be activated. It has been found that regular UE compressed mode measurements often do not report the macro layer RSCP. This is dependant on UE vendor implementation. Furthermore compressed mode measurements tend to make the radio link unreliable as the UE needs to switch UARFCNs.

There is therefore described an up link power capping algorithm, of particular use in a straddled carrier deployment, that does not require compressed mode measurements, and that makes use of the down link load margin calculation to estimate the path reduction back to the macro network in the building.

In femtocell deployments, either on the same carrier as the macrolayer or straddled across two carriers, the femto AP UE transmit powers should be controlled so as to ensure that the population ot femto AF UEs do not cause an uplink noise rise to surrounding macro basestations, which can cause their cell to shrink.

Given an UL operating power range by the AP MS, and also given measurements of the path loss to macro cells provided by DLMM and FSM and by UE measurements, the femto AP sets a maximum allowed transmit power. This is set such that, if all its UEs were transmitting at this maximum power level, they will not cause a significant UL noise rise to the macro Node B. Figure 4 shows the principle behind the setting of the femto AP UE maximum allowed Tx power. The aim is to ensure that the interference at the macro cell from a femto AP UE is always below the macro noise floor. The maximum allowed AP UE power is calculated by summing the pathloss to the nearest macro cell, a typical macro noise floor value and a back off margin (to cater for multiple AF UEs transmitting simultaneously at maximum power). The WCDMA basestation path loss is calculated by subtracting the measured RSCP of the basestation from the OPICH Tx Power of the basestation extracted from its SIBs by the Network Listen Mode function. The typical noise floor and the back off margin are parameters provided by the AP-MS. This approach keeps the noise rise caused by the Femto AP UE well below the up link noise caused by the macro cell traffic. Finally, if there are no macro signals or other APs detected by the Network Listen Mode on the AP's operating carrier or adjacent carriers, the maximum femto AP UE power is set to a value provided by the AP MS.

Typically the uplink power cap is calculated as follows when the femtocell is deployed on the same carrier as the macro layer: UE_Uplink_Power = NodeB_Noise_Floor -Uplink_Noise_Rise_Margin + Minority_ML_Pathloss Where: NodeB_Noise_Floor = database parameter defining the nodeB noise floor at the antenna, typically -lO4dBm Uplink_Noise_Rise_Margin = database parameter defining the back off margin, typically 15dB Minority_ML_Pathloss = smallest of 1St percentiles of the long term NodeB to femto cell UE pathlosses.

When the femto cell is deployed in a straddled carrier configuration, the Minority_ML_Pathloss histogram is replaced by the algorithm below.

Firstly, the straddled carrier uses the FSM to estimate the UE to macro node-B path-loss. For every FSM the measurements are stored for every available PSC on both UARFCNs. For the two UARFCNs the smallest minority Macro Layer to Femto path-loss is obtained by finding the 151 percentile and subsequently the linear average of the percentile after every FSM: Minority_ML_Pathloss = (Average(1 st Percentile Cairierl PL1 1st Percentile Carrier2 PL2)) -Delta_U L_Fath_Loss_Margin Secondly, the Delta_UL_Path_Loss_Margin is an estimate of the ML path loss variance in the building over and above the nodeB to Femto path loss measured at the femto cell by averaging FSM. This can be calculated for example as follows: The Femto Cell DL power is calculated as a function of measured ML RSCP on the adjacent carriers. The measured adjacent carrier ML RSCP measurements are captured through ESMs and stored in a histogram.

If the ML interference level (i.e. RSCP) is constant throughout the building then it would be expected that the DL power would converge to a value of average fast sniff ML RSCP adjacent channel measurements + 95% UE to femto indoor path loss measurements.

If the ML interference level in the house is higher than the level detected by fast sniff at the femto (e.g. femto is installed down stairs where the ML RSCP levels are about - 1 OOdBm while upstairs they can be approximately -85dBm on first floor due to height gain) the running averaged down link load margin increases to adjust the DL power (by 15dB) to accommodate the additional interference level.

Hence Delta_UL_Path_Loss_Margin = running average DL_Load_Margin.

Thus, an ongoing downlink load margin calculation is used to determine the level of macro layer interference within the building. Although applicable to all spectrum deployments (clear carrier, occupied carrier and straddled carrier) this solution is of particular importance for a straddled carrier deployment as there is no macro layer RSCP measurement to provide an interference reference. The calculation uses an X/Y clustering type algorithm, which excludes the effect of intra-cell loading and only reacts to the inter-cell interference. Intra-cell interference is essentially negated by the Orthogonal Variable Spreading Factor (OVSF) code orthogonality on the down link of a small cell.

As mentioned above, the load margin calculation uses a XJY cluster algorithm which is based on a combination of the 90% Transmit Code Power (referred to below as the majority_Tx_code_power) and the CPICH Ec/No (referred to below as the Majority_ZL_EcNo) measurements to determine the appropriate load margin. In this embodiment, there are thiee load margins (low, medium or high) that can be used.

Specifically, this always ensures that a higher value is used for the load margin when the majority femtocell CPICH Ec/No drops below the specified target CPICH Ec/No, and the transmit code power saturates above the target allowed transmit code power for a particular service or an average over a range of services.

The target CPICH Ec/No and the target transmit code power are typically set as database parameters. Alternatively, the target transmit code power could also be calculated adaptively as a function of femtocell loading. The Transmit Code Power histogram (from whence the 90% is calculated) is gathered from layer 1 DL DCH code power measurements gathered across all calls -irrespective of service.

As an example, if there is a large variance in the average ML interference relative to the FSM interference levels measured at the femto cell, then it is expected that the femto cell CPICH Ec/lo would be poor, and additional DL DCH code power would be required to achieve the 90% coverage target. Furthermore, if the macro layer cell loading increases then this could also result in an increase in required DCH code power and degraded CPICH Ec/lo.

The behaviour of the X/Y load margin cluster algorithm follows this set of rules: if (Majority_ZL_EcNo <Min_EcNo) and (majority_Tx_code_power> target transmit code power); Load_margin = High_Loadmargin; elseif (Majority_ZL_EcNo <Min_EcNo) and (majority_Tx_code_power < target transmit code power); Load_margin = Medhim_Loadmargin; elseif (Majority_ZL_EcNo > Min_EcNo) and (majority_Tx_code_power) <target transmit code power); Load_margin = Low_Loadmargin; elseif (Majority_ZL_EcNo >Min_EcNo) and (majority_Tx_code_power >= target transmit code power); Load_margin = Low_Loadmargin; end; An extracted simulation result of the UL power capping with the Delta_UL_Path_Loss_Margin used in estimating the additional back-off on the uplink is shown in Figure 5. The use of the Delta_UL_Path_Loss_Margin clearly provides extra protection for the macro Node B against harmful ZAP layer uplink transmissions. The UL power converges to -lldBm after 12 minutes of operation.

There is thus described a method for setting the uplink power in such a manner as to ensure good service for UEs in the coverage area of the basestation, while avoiding undue interference with the macrolayer.

Claims (1)

1. <claim-text>CLAIMS1. A method of setting a maximum uplink power for user equipment devices in a coverage area of a first basestation in a cellular communications network, wherein the first basestation is located in a coverage area of a second basestation, the method comprising: setting the maximum uplink power for user equipment devices based on a measure of the pathlosses between the second basestation and user equipment devices in the coverage area of the first basestation.</claim-text> <claim-text>2. A method of setting a maximum uplink power for user equipment devices in a coverage area of a first basestation in a cellular communications network, wherein the first basestation is located in a coverage area of a second basestation, the method comprising: setting the maximum uplink power for user equipment devices based on a measure of the pathlosses between the second basestation and user equipment devices in the coverage area of the first basestation, comprising forming the measure of the pathlosses between the second basestation and user equipment devices in the coverage area of the first basestation from: a measure of the pathloss between the second basestation and the first basestation and an estimate of the excess of the pathlosses between the second basestation and user equipment devices in the coverage area of the first basestation over the pathloss between the second basestation and the first basestation.</claim-text> <claim-text>3. A method of setting a maximum uplink power for user equipment devices in a coverage area of a first basestation in a cellular communications network, wherein the first basestation is operating on a carrier that overlaps two other carriers in use in the network, the method comprising: setting the maximum uplink power for user equipment devices based on a measure of the pathlosses between other basestations and user equipment devices in the coverage area of the first basestation, comprising forming the measure of the pathlosses between other basestations and user equipment devices in the coverage area of the first basestation based on measurements taken on the two other carriers.</claim-text> <claim-text>4. A method as claimed in claim 3, wherein the step of forming the measure of the pathlosses between other basestations and user equipment devices in the coverage area of the first basestation comprises: calculating pathlosses between a basestation operating on a first of the other carriers and user equipment devices in the coverage area of the first basestation; calculating pathlosses between a basestation operating on a second of the other carriers and user equipment devices in the coverage area of the first basestation; and forming an average of said calculated pathiosses.</claim-text> <claim-text>5. A computer program product, comprising a computer readable medium, and code defning instructions for performing the method as claimed in any of claims 1 to 4.</claim-text> <claim-text>6. A basestation, configured to operate in accordance with a method as claimed in any of claims 1 to4.</claim-text> <claim-text>7. A cellular communications network, comprising a basestation as claimed in claim 6, and at least one user equipment device.</claim-text> <claim-text>8. A user equipment device, when used in a network as claimed in claim 7.</claim-text>