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OFDMA (Orthogonal Frequency Division Multiple Access) in an architecture that uses OFDM techniques to distribute users along a frequency by exploiting the spectrum arrangement in subcarriers. It allows a user to use only a subgroup of the spectrum, permitting to other users to transmit simultaneously and thus maximizing the frequency reuse. This architecture requires the assignment of subcarriers to users by using dynamic frequency assignment algorithms. Fur- thermore, this subdivision allows for a more efficient spectrum and interference management in two layer networks. Due to its high efficiency, OFDM is the multicarrier technology selected by WiMAX and LTE and is a candidate to re- place UMTS technology. However, UMTS use of CDMA copes with interference is a better way as for OFDMA femtocell a self-organizing network is required to cope with interference, either my FBS measurements of the environment or by collected UE data. Generally, multicarrier modulations like OFDM have robustness against multipath and narrowband interference from close stations, high spectral efficiency and frequency diversity for multiple user access. 3.2.3.1 WiMAX technologies

Designed for last mile connectivity, WiMAX was published in 2004 under IEEE 802.16d standard and in 2005 evolved to IEEE 802.16e which incorporates mo- bile connectivity. The working group of WiMAX designed an end to end IP network architecture which makes it especially suitable for femtocell deploy- ment [16]. WiMAX is also suitable for mobile centric femtocell applications as it can support up to 70 Mbps symmetric rates which ensure high QoS. The industrys interest in WiMAX is high due to the fact that the emission licenses have already been auctioned for Europe and USA and the standards have been published since 2004 and 2005, making WiMAX femtocells a feasible alterna- tive to UMTS and HSPA. There are some cases where companies purchased frequency bands to be used solely by the WiMAX [17] femtocells and elimi- nating this way the cross-layer interference. However such strategies luck in spectrum reuse efficiency and add cost of the operator.

3.2.3.2 LTE

3GPP release 8 in December 2008, commonly known as LTE introduces higher throughputs, more flexible spectrum management and spectral bandwidth. It is expected that legacy GSM and UMTS mobile networks worldwide will upgrade to LTE [18], making it the more widely used mobile access technology. LTE evolves the HNB to Home eNodeB (HeNB) making it the main point for radio access. Since May 2008 PicoChip, developed and manufactured LTE femtocells based on the technical specifications of that time. There is still a long way before the marketability of femtocells is certain as the auctioning of LTE license has not taken place yet.

3.3

Interference

With the introduction of femtocells some changes occur in a macro cellular network, a new layer is added and thus the architecture is composed by two

18 Chapter 3. Background separated layers: the macro layer and the femtocell layer. A network architec- ture like the one described is called two-tier or two-layer network, with the one layer being composed of traditional macro base stations and the second layer by several shorter range base stations which are randomly located inside the emit- ting range of the first layer. This design however introduces new problems and design challenges. Transmitters from the two layers, when using the same RF in the same geographic area, will cause confusion to receiving systems making it harder to distinguish the source of the signal. This is the interference problem and is one of the main challenges of telecommunication systems that femtocell deployment needs to face. System like CDMA which are interference limited in order not be greatly affected will need the introduction of interference avoidance techniques like power control and time hopping while capacity limited systems as OFDMA will need intelligent frequency planning technology to adapt with the femtocell interference presence. If the fore mentioned techniques are not applied dead zones can be created which will disrupt the macro layer service near a working femtocell.

3.3.1

Co-layer Interference

As the femtocell deployment is random and opportunistic is highly possible that several femtocells will be installed in proximity with other either horizontally or vertically in apartments and houses causing interference one to another. There are two types of interference source: the downlink (originating from FBS) and the uplink (originating from UE). This is co-layer interference and only affects the femtocell layer architecture and the unwanted signalling received by a fem- tocell reduces the communication quality. The scenario gets worst when signals from nearby femtocells at any location are concentrated and overpower the fem- tocell power levels causing dead zones problems and connectivity issues to the user.

The access mode any given femtocell operates greatly impacts the co-layer interference with the OAM (Open access mode) reducing the dead zone problem and CAM (Closed Access mode) increasing it. In addition, based on measure- ments made in [1], it is shown that as QoS increases so does the dead zone problem.In TDD systems if all femtocells are synchronised then the transmis- sion of femtocell A will cause downlink interference to users of femtocell B and in the same way the uplink transmission of UE A will be sensed as uplink interfer- ence to UE B. In case however where the femtocells are not synchronised, then the emitting time is random, causing transmissions to overlap between uplink and downlink making the interference much harder to cope with. Therefore, timing is important aspect of TDD-based FBS which at the same time is tricky problem.

3.3.1.1 CDMA

In CDMA systems, 3GPP in [19] suggests the use of interference management techniques to cope with the high power UE transmission levels that reduces the coverage area of a victim femtocell. Power limit to be imposed by femtocell to subscribed UE is one solution in which the noise rise in the uplink can be controlled. To achieve this, the FBS scans and gather information about the environment and the received power by nearby UE and then set a maximum

3.3. Interference 19 UE transmit power for the desired CINR. In order to cope with the downlink interference to a victim UE caused by nearby FBS, it is recommended that FBS strictly control their emission power by relying on adaptive power techniques. Especially in CAM scenarios where the UE is served by the FBS it is subscribed to and not by the one that receives the best signal from. These two solutions are for UMTS and HSUPA systems.

3.3.1.2 OFDMA

For OFDMA systems, it is not required to sense the full transmission band for emissions in proximity. According to the QoS, only some sub channels are allocated to the user by the FBS, which in this case selects the ones that are not subject to interference. The uplink interference is more important to cope with than with the downlink one, as the uplink interference affects all the users of a femtocell while as the downlink only the interfered user. When compared with CDMA systems where the transmission uses the entire available band, the selection of sub channels by OFDMA systems makes the interference more manageable to handle with. Thus the allocation of sub channels frequencies has an important role in the interference impact in OFDMA systems and the creation of dead zones.

3.3.2

Cross-layer Interference

Cross-layer interference happens when the source of a signal and the receiver belong in two different layers, for example a femtocell transmission affecting the downlink quality of UE at the edge of a MBS. On the contrary, uplink interference can also occur when a macro layer UE is interfering with the uplink quality of a FBS user. CDMA networks are mostly affected by cross layer interference and the spectrum used is the same for both the macro and femtocell layer. In [20] and [21] it is suggested that an operator uses the spectrum splitting technique in which a RF band X for example is divided to X1 and X2, with X1 = X-X2. Then the X1 is used only be the macro layer and X2 in dense femtocell deployment regions. On the one hand splitting the spectrum will eliminate cross-layer interference but on the other hand due to the high price and scarcity of electromagnetic spectrum this would lead to inefficient spectrum usage. In OFDMA systems, with the usage of subcarriers cross layer and co layer interference is mitigated and handled in a more efficient way, making this way OFDMA femtocells a welcome solution. Still, OFMDA systems are subject to different problems as time and frequency synchronization which in severe cases can cause loss of orthogonality among subcarriers and disruption of the whole network.

3.3.2.1 CDMA

In CDMA networks there are two scenarios under which cross-layer uplink in- terference can occur. In the first scenario the femtocell users interfere with the Macro node B. If the femtocell is working under CAM then the transmissions originating from femtocell layer UE are the source of interference to the macro layer, requiring this way power control measures from an operator on the femto- UE to reduce the noise production. If however, the femtocell is working under

20 Chapter 3. Background OAM, then the UE can camp freely on the base station with the highest signal either it be a femtocell or a macro cell, requiring the minimum transmission power and in this way causing the less interference in the network. On the opposite scenario, the macro layer users transmit in high power to reach the MBS and at the same time interfere with the femtocell operation. Since the MBS is unaware of the exact location of the user and if there is a femtocell in proximity then considering the worst case interference scenario an appropriate upper limit must be set to a macro layer UE. Just like in the uplink scenario, downlink interference can occur when a FBS is transmitting in proximity with macro layer users and in case the signal is strong enough then dead zones can appear in the macro layer. A 3GPP proposed solution for UMTS and HSDPA networks is to limit the FBS maximum transmit power. However a fixed limit on the transmit power is not an efficient solution as the circumstances are not always the same, requiring adaptive power control on femtocells which can in- crease their cost. In cases where the femtocell is located at the edge of a macro cell, then with good indoor isolation the interference is minimised. If on the other hand, the femtocell is located to close to a MBS, the coverage of a FBS is greatly reduced resulting even in corruption of service. Noticeably, a study made by Femto Forum in [22] shows that if a user is located at least 250 meters from the closest microcell and 1000m from the closest macro cell then 14.4 Mbps throughputs can be achieved in HSDPA networks.

3.3.2.2 OFDMA

With the division of the spectrum into OFDMA sub channels and the assign- ment to each network layer, cross layer interference is practically avoided with only adjacent channel interference remaining. In co-channel femtocell opera- tion, when a FBS is at the edge of a MBS coverage area, the passing of a UE that is connected with a MBS can cause uplink interference to a FBS user that is using the same sub channel, especially if required by the MBS to increase the transmission power due to the long distance. It is therefore important that femtocells at the edge of a macro base station to plan their uplink sub channel allocation taking into consideration the any spectral occupancy. If the femtocell is located to close to the MBS and requires from a user to increase the transmit power then the users of the macro layer that are on the same sub channel with the femtocell user, loose their connectivity with the macro cell. Just like with CDMA, it is required in this case for the FBS to limit the power control of its users.For downlink interference to appear an operating FBS and a macro cell user walking down the road are on the same sub channels, for example a fem- tocell and a macro cell serving both their users in sub channels 1 to 4. In this case the noise ratio of the macro cell user will increase dramatically and heavy interference will occur. The femtocell user will also experience some interference but reduced due to the wall attenuation and the better signal from the short distance to the FBS.

3.4

Mobility Manager

Mobility management is a challenging issue in femtocell deployment especially for mobile power consumption and signalling load. The initial assumption is

3.4. Mobility Manager 21 that no specific MM (Mobility Management) is required for femtocells as the operation takes place in the same network. However this is far from truth as implementation falsifies many aspects of macro layer network assumptions. To begin with in high density femtocell areas it is possible that the 512 PSC ( Primary Scrambling Codes) in UMTS and 504 PCI (Physical Cell Identities) in LTE will not be sufficient to distinguish all the FBS identities and information in a network. In addition the neighbouring cell list is dynamic and custom to random changes when compared with the static macro layer neighbouring cell list. Furthermore, FBS do not operate all in the same access mode and those in CAM can affect the cell measurements of a large number of UE.

3.4.1

Femtocell characterization

As described above femtocells cannot be treated as macro base station in mo- bility management with implications to the network performance. The intro- duction of identifiers and mechanism that can describe the aspects of femtocells is necessary to improve the mobility procedures for femtocell support. In this way the network is divided into two layer architecture and the introduction of new techniques can reduce the signalling overhead among the two layers. 3GPP proposed layer 1 and layer2 methods are described in the following sections. 3.4.1.1 Hierarchical Cell Structure

Based on [23], HSC (Hierarchical Cell Structure) can be used to distinguish more efficiently femtocells from macro cells as it allows operators to set priorities from 0 to 7 to different customised categories of network cells(macro, micro, pico). HSC is considered to be one of the most effective ways of solving conflicts in UMTS and GSM hot spots and high traffic demand areas.

3.4.1.2 Different Femtocell PLMN ID

3GPP pre-release 8 introduced another way for distinguishing femtocells from macro cells which is adopted by the majority of femtocell manufacturers. By using different PLMN ID there is less signalling towards CN and better power consumption on UE as the selection of the right base station is straight forward according the settings assigned to each UE by an operator [23]. However some compatibility issues may rise with old SIM/UICC cards and also the operator might be lacking the required additional PLMN ID for the separation of the two networks.

3.4.1.3 Reverse Frequency and PSC/PCI

3GPP release 8, simplifies the problem for femtocell identification by taking into account only CAM cells. CAM femtocells and macrocells broadcast information regarding CAM deployment which can be used by UE which are not allowed access on CAM femtocell to avoid cell measurements and power consumption. In deployments with shared carrier frequency all CAM femtocells broadcast their reserved PSC information while the OAM femtocells may sporadically broadcast this information as well. This information must be noted that is only effective in the UARFCN (UTRA Absolute Radio Frequency Channel Number) for the PLMN where the user is receiving this information. Any PSC information

22 Chapter 3. Background regarding CAM femtocells is considered valid and used by the UE for the next 24 hours.

3.4.1.4 CAM Indicator

According to [24] and [25], a layer 2 approach in 3GPP is the usage of a CAM indicator which is transmitted with the MIB (Master Information Block) in UMTS and LTE network architectures and its purpose is to indicate the access mode a FBS is operating in.

3.4.2

Femtocell identification

The use of NCL (Neighbouring Cell List) makes it possible and easy for a UE to identify surrounding macro cells. The NCL is created locally at a FBS with self-executed algorithms and in case of outbound mobility from femto layer to macro layer can inform a UE of near MBS and FBS. However the opposite is not as simple due to the high density and number of FBS. For this reason, release of 3GPP introduced the UE autonomous search for CAM cells [26] to camp on. The UE which is not configured for CAM cell can disable the search function [27]. Intra-frequency and inter-frequency measurements can assist the search function for reserved PSC/PCI. The autonomous search from release 9 will support open and hybrid access modes.