The radio and performance requirements and the necessary signaling to support the operation of a Wideband Code Division Multiple Access (WCDMA) home base station (BS) were successfully completed in Third Generation Partnership Project (3GPP) release 8. The corresponding home BS requirements are specified in 3GPP Technical Specification (TS) 25.104. The WCDMA home BS is also interchangeably termed as home NodeB(HNB) or more specifically frequency division duplex (FDD) HNB. The home BS requirements have also been specified for universal terrestrial radio access (UTRA) time division duplex (TDD) (i.e. TDD HNB), see 3GPP TS 25.105, “Base Station (BS) radio transmission and reception (TDD)”
The radio and performance requirements for the evolved UTRA (E-UTRA) home BS have also been standardized in release 9. There were two main variants of the E-UTRA home BS standardization work: Long Term Evolution (LTE) FDD Home eNode B (FDD HeNB) described in 3GPP Technical Report (TR) 36.921, “Evolved Universal Terrestrial Radio Access (E-UTRA); FDD Home eNodeB (HeNB) Radio Frequency (RF) requirements” and LTE TDD Home eNode B (TDD HeNB) described in 3GPP TR 36.922, “Evolved Universal Terrestrial Radio Access (E-UTRA); LTE TDD Home eNodeB RF Requirements”.
Home base stations (HBS) are already operational in other technologies such as Global System for Mobile communication (GSM) and 3GPP2 CDMA technologies (e.g. CDMA2000 1xRTT and HRPD).
Home base stations (e.g. FDD/TDD HNB, TDD/FDD HeNB, GSM HBS, CDMA2000 1x HBS, HRPD HBS etc) are intended to be deployed in home or other private premises such as office or corporate environment. A large building may house several tens or even hundreds of home base stations. Hence a large number of users can be served by a home base station in a large building environment. An operator may choose to share the same carrier between home base stations and macro/mico/pico base stations (i.e. non home base stations covering larger areas than a home base station) or alternatively assign a dedicated carrier only for home base stations. Particularly, in the former scenario the users served by the home base station may also generate significant interference towards the macro/micro/pico base stations or relay nodes. Therefore the transmit power of a User Equipment (UE) served by the home base station needs to be properly regulated.
The terms femto base station, home base station, home Node B or home eNode B refer to the same type of base station in principle. For simplicity and consistency the term home base station (HBS) will be used in the rest of this application.
The following sections describe various concepts and technological aspects, which are used or are related to the present description.
Home Base Station
In existing Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) specifications, three classes of base stations are defined:                Wide area BS, serving macro cell deployment;        Medium range BS, serving micro cell deployment; and        Local area BS, serving pico cell deployment.        
In E-UTRAN specifications, two classes of base stations are defined:                Wide area BS, serving macro cell deployment; and        Local area BS, serving pico cell deployment.        
In the E-UTRAN, the wide area BS is also called as the general purpose BS.
Home base stations are being developed to serve even smaller and more localized areas than the pico cells. Home base stations operate under licensed frequency band and for LTE FDD and TDD they are currently under standardization within 3GPP. For UTRAN FDD and TDD, the HBS requirements are already specified.
In WCDMA, the home base station maximum output power is limited to 20 dBm for the non multiple input multiple output (MIMO) case or 17 dBm per antenna port in case of MIMO (2×2). Same power level is also agreed to be specified for HBS in LTE FDD and TDD.
In general these different base station classes differ due to different minimum coupling loss in different environments. Secondly they typically operate at different maximum output power levels. For instance wide area BS operates at higher maximum output power than the medium range and so on. These factors in turn lead to different performance requirements for different base station classes. These requirements are defined in.
One main difference compared to other base station classes is that the Home base station is owned by a private subscriber, who is at the liberty to install at any location. The operating carrier frequency of the HBS is typically configured by the operator. However the owner of the HBS may have the option to allow or disallow the access of its HBS to the external users. This mechanism is called as the closed subscriber group (CSG). Nonetheless the strict network planning is not possible in case of home base station network. The HNB deployment becomes even more cumbersome from the perspective of interference in environment comprising of large number of HNB subscribers located in large complex or building e.g. 200 HNB housed in 200 closely packed apartments in the same residential complex. This is in contrast with other base station classes, which are deployed by an operator according to some well defined principles. The lack of precise network planning of home base stations may cause interference to other base stations.
Home Base Station Implementation Aspects
A home base station comprises of normal base station functions such as transmitting and receiving signals to and from multiple UE. In addition it could contain a normal receiver circuitry, which is similar to the UE receiver. For simplicity this will be referred to as a HNB measurement unit herein. The purpose of the HNB measurement unit is to perform different types of measurements, which in turn can be used for adjusting its output power level. The measurements are typically done over the signals received from other HBS or non-home BS such as macro/micro/pico BS or relay nodes. Relays are used to improve the coverage of macro/micro/pico base stations in cell coverage borders.
Home Base Station Deployment Scenarios
Mixed Carrier Scenario:
In this scenario the home BS operates on the same frequency channel as that of the surrounding macro network belonging to the operator which deploys the home BS. An operator with a limited spectrum may be obliged to have a mixed carrier deployment scenario if it wishes to offer home base station coverage. The mixed carrier scenario is typically more challenging in terms of co-channel interference between:                Home base stations, and        Home base stations and the non-Home BS e.g. macro network.        
The interference situation becomes even worse in UTRAN TDD and Long term evolution (LTE) TDD home BS deployment scenario. This stems from the fact that any difference in uplink and downlink slot or sub-frame configurations in HBS and non-HBS or within different HBS results in severe cross-slot (or cross-sub-frame) interference. Even if the same TDD sub-frame configurations are used in all network nodes, due to the imperfect sub-frame timing due to practical constraints, there will be interference leakage.
Dedicated Carrier Scenario:
In this scenario the home BS operates on a different frequency channel compared to that of the surrounding non-home BS network (e.g. macro network) belonging to the operator which deploys the home BS. This scenario is typically less severe in terms of interference between the home network and the macro network. However, there would still be an impact of adjacent channel interference due to the out of band emissions. This is due to the fact that practical filters used in HBS or any other network nodes or UE cannot completely suppress the emissions outside their operating carrier frequency channel.
Interference Scenarios
Table 1 shows the possible HeNB related interference scenarios, which are mainly reproduced from 3GPP TR 36.922. The listed interference scenarios are applicable for both TDD and FDD deployments. The main difference may exist in how to model the interference since FDD network is typically not synchronized. On the other hand in TDD are network nodes operating over the same carrier frequency should be synchronized.
TABLE 1Important and Typical Interference ScenariosNumberAggressorVictim1UE attached to HomeMacro eNode B UplinkeNode B2Home eNode BMacro eNode B Downlink3UE attached to MacroHome eNode B UplinkeNode B4Macro eNode BHome eNode B Downlink5UE attached to HomeHome eNode B UplinkeNode B6Home eNode BHome eNode B Downlink7UE attached to HomeOther SystemeNode B and/or HomeeNode B8Other SystemUE attached to Home eNode Band/or Home eNode BUE Mobility Measurements
The UE connected to HBS or to any other type of BS (e.g. macro BS) performs the same types of neighbor cell measurements for mobility purposes.
If the UE is served by the HB then it reports the neighbor cell measurements to its serving HBS, which uses them for taking the mobility decisions e.g. for performing handovers.
Similarly if the UE is served by the macro BS then it reports the neighbor cell measurements to its serving macro BS, which in turn uses them for taking the mobility decisions e.g. for performing handovers.
In WCDMA the following three downlink radio measurements are specified primarily for the mobility purpose, see 3GPP TS 25.215, “Physical layer measurements (FDD)”.                CPICH RSCP        CPICH Ec/No; CPICH Ec/No=CPICH RSCP/carrier RSSI        UTRA Carrier RSSI        
The Received Signal Code Power (RSCP) is measured by the UE on cell level basis on the common pilot channel (CPICH). The UTRA carrier Received signal strength indicator (RSSI), i.e. the total received power and noise from all cells, including serving cells is measured over the entire carrier. The above CPICH measurements are the main quantities used for the mobility decisions.
In E-UTRAN the following downlink radio measurements are specified also primarily for mobility purpose, see 3GPP TS 36.214, “Evolved Universal Terrestrial Radio Access (E UTRA); Physical layer measurements”.
:                Reference symbol received power (RSRP)        Reference symbol received quality (RSRQ): RSRQ=RSRP/carrier RSSI        
The RSRP or RSRP part in RSRQ in E-UTRAN is solely measured by the UE on cell level basis on reference symbols. There is no specific carrier RSSI measurement rather it is part of RSRQ definition.
The neighbour cell measurements are typically averaged over long time period in the order of 200 ms or even longer to filter out the effect of fast fading.
There is also a requirement on the UE to measure and report the neighbour cell measurements (e.g. RSRP and RSRQ in E-UTRAN) from certain minimum number of cells. In both WCDMA and E-UTRAN, see 3GPP TS 25.133, “Requirements for support of radio resource management (FDD)” and 3GPP TS 36.133, “Evolved Universal Terrestrial Radio Access (E UTRA); Requirements for support of radio resource management”, this number is 8 cells (comprising of one serving and seven neighbour cells) on the serving carrier frequency (or commonly termed as intra-frequency).
In GSM system the following measurement for mobility is specified.                GSM Carrier RSSI        
In cdma2000 1 x RTT system the following quality measurement for mobility is specified; see 3GPP2 CS.0005-D v1.0 “Upper Layer (Layer 3) Signaling Standard for CDMA2000 Spread Spectrum Systems Release D”.
.                CDMA2000 1x RTT Pilot Strength        
In cdma2000 HRPD system the following quality measurement for mobility is specified, see 3GPP2 CS.0024-A v3.0 “cdma2000 High Rate Packet Data Air Interface Specification”.
.                CDMA2000 HRPD Pilot StrengthMaximum Allowed Transmit Power Adjustment        
The maximum allowed transmit power of both home BS and UE served by the home BS can be adjusted to minimize interference to other home BS or to other types of base stations such as macro or micro base stations. This will be described below:
Adjustment of Home BS Maximum Transmit Power
Unlike other base station classes, home base stations are owned by the subscriber and will be deployed in homes, flats and other private premises. This means their physical location is not under the control of an operator. As they operate under licensed band, they may cause interference to each other but as well as to the outdoor base stations specifically the macro networks or to other network nodes such as relays. The interference will be worse in case home and outdoor base stations operate on the same carrier frequency. Due to this potential risk of interference, the maximum output power of the home base station should be regulated to minimize the impact on other cell applications e.g. macro. Typically the maximum output power will be varied slowly i.e. in the order of several seconds or even longer.
Adjustment of UE Maximum Transmit Power
For the sake of clarity the following definitions will be used:                HUE is a UE served by a home base station        MUE is a UE served by a radio network node which is not a home base station e.g. MUE is the UE served by macro BS.        
In general a UE in terms of its maximum output power capability belongs to a particular UE power class (PC). For instance in WCDMA, there are several UE power classes namely:                PC4=21 dBm        PC3bis=23 dBm        PC3=24 dBm        
For instance a WCDMA UE belonging to the PC3, which is the most commonly used power class, can operate at maximum output power equal to 24 dBm.
In LTE, hitherto there exists only one UE power class namely:                PC3=23 dBm        
In general existing mechanisms allow the network to configure the UE to the maximum output power level, which is lower than its power class capability. For instance a LTE UE can be configured to a maximum output power of 10 dBm i.e. 13 dBm lower than its nominal value (i.e. power class capability).
This configuration at a lower than the maximum possible output power is typically achieved by using higher signaling such as radio resource control (RRC) signaling.
In existing systems the UE maximum output power is typically lowered in small cells such as in pico or micro network deployment scenarios.
In existing systems the maximum output power of a HUE can also be lowered by the serving home BS.
Home BS Measurements for Maximum Adaptive Power Setting
As stated above a home base station can have a measurement unit for performing measurements over signals received from other HBS and from non-HBS network nodes such as macro BS or relay node. This means that a home base station can in principle perform the same measurements which are performed by a UE. However the terminology used for the measurements done at the HBS may differ with those done by the UE. For instance, the WCDMA HBS measurement, “CPICH Ec”, is the equivalent of the WCDMA UE measurement CPICH RSCP. In both cases they can be regarded as signal strength measurements.
These measurements are used by the home base station to adaptively set the maximum transmit power of the home BS and also that of the HUE, i.e. setting of maximum output power as explained elsewhere in this application. For setting the maximum output power of the HUE, the HBS may also use the UE reported measurements (e.g. mobility measurements as explained elsewhere in this application) as well the internal HBS measurements or simply the former measurements.
Depending upon the access technology of the home base station one or more radio measurements specific to that access technology will typically be used by the home base station to adjust its transmit power level. This means in WCDMA that the measurements that can be used by the HBS are similar to the WCDMA UE measurements: CPICH RSCP, CPICH Ec/No and UTRA carrier RSSI.
The cell specific measurements (i.e. equivalent to UE measurements CPICH RSCP or Ec/No in WCDMA) are performed on neighboring base stations, which may be home base stations, macro/mico/pico (i.e. non home base station) or combination thereof. In any case according to the current WCDMA requirements only up to 8 cells can be measured on the intra-frequency carrier or 6 cells on inter-frequency carriers. These measurements need to be combined and processed in an adequate manner to make sure that the adjusted power leads to reduction in interference to the non home base stations. At the same time whenever possible, that is when relatively shielded from the non home base stations, the home base station is able to operate at relatively higher output power so that home base station resources are fully exploited.
The HUE served by HBS may cause uplink interference to other radio network nodes such as non-HBS radio network nodes (e.g. macro BS) or other HBS operating in a carrier frequency, which may be the same or adjacent to that of the HBS serving the HUE. The uplink interference towards these radio network nodes becomes even more severe with the increase in the number of HUEs served by the HBS. The interference problem is further accentuated when there is a large number of HBSs operating in the same area such as in a large complex with several apartments. In the latter case, the cumulative effect of the interference may in particular significantly deteriorate the reception quality of the signal at the other radio network nodes (e.g. macro BS or relay node). This situation is shown in FIG. 1.
In FIG. 1, the HNB and the macro BS may operate in the same carrier frequency or in a different carrier frequency e.g. HNB and Macro BS operate in adjacent carrier frequencies. The HNB and macro BS may belong to the same or different access technologies. For instance both HNB and macro BS can be based on WCDMA. Alternatively HNB and macro BS are based on WCDMA and LTE respectively. In the latter case typically the HNB and LTE don't operate over the same carrier frequency; however they may very well operate in adjacent carrier frequencies. The adjacent carriers and the corresponding network (HBSs or macro BS s) may also belong to different operators.
It is desirable to reduce the interference from the HUE served by HBS towards other radio network nodes such as non-HBS radio network nodes.
Several solutions have been proposed to adaptively set the maximum output power of the HUE so as to minimize the co-channel or neighbor cell interference towards other radio network node e.g. macro BS. The other radio network node may belong to the same or different technologies compared to that of the MUE (and home BS) under consideration.
In existing systems typically the serving HBS uses one or more UE reported mobility measurements such as CPICH RSCP or path loss to lower the maximum output power of the UE below its nominal output power level.
In accordance with another pre-existing solution the HBS measures the signal strength or signal quality from one or more strongest macro base station and determine the UE maximum allowed UE output power. Another solution is to use the total interference received at the HBS as a metric to determine the UE maximum output power.
However, all the above UE and HBS measurements suffer from inadequate accuracy levels. For instance carrier RSSI and CPICH RSCP in WCDMA or RSRP in LTE have very coarse measurement accuracies e.g. ±7-9 dB. The RSRQ in LTE and CPICH Ec/No in WCDMA have relatively better accuracy. But their accuracies also deteriorate at lower SNR levels. Furthermore the use of only CPICH Ec/No or RSRQ for setting the maximum output power is not appropriate due to the fact that the interference component in these measurements does not fully incorporate and depict the overall interference on a carrier. Indeed these measurements are primarily tailored for the mobility purpose.
Another drawback is that these existing solutions don't enable home base station to identify whether HUEs are in the proximity of other radio network nodes such as macro BS. Thus these solutions on the one hand are unable to fully protect the macro network as the HUE serving by the home base stations may be operating at higher output power than desired. One the other hand the maximum output power of the HUE may be conservatively set causing performance loss of the HUE operating in a home base station environment.
Hence the current solutions don't fully ensure the protection of non home base stations especially the macro base stations from the interference generated by the HUEs under the control of home BSs.