Femtocells, first called Access Point Base Station, are cellular base stations, which are used indoors for example homes and offices. They connect to mobile operators' networks using DSL, cable broadband and optical fibres. As an important component of LTE (long term evolution), it has drawn many mobile operators' attention.
Although it has great potential and many advantages, this technique is not mature. Interference is one of the drawbacks. This project concentrates on interference avoidance. Researchers have proposed some methods to resolve this problem. Software named OPNET will be used to simulate network environment using these methods and make some improvements.
Some backgrounds of 4G, LTE and femtocell have been acknowledged. A book named 'Femtocells Technologies and Deployment' written by Jie Zhang and an article named 'OFDMA Femtocells: A Roadmap on Interference Avoidance have been read. Use of OPNET simulator is being studied so far.
According to recent surveys , 50% of phone calls and 70 % of data services will take place indoors in next few years . As a result, indoor service quality will be improved by mobile operators. However, macrocell coverage becomes expensive and customers may suffer from weak signal power, some new solutions of serving indoor users are demanded.
Femtocell, as the latest technique, has drawn many mobile operators' attention. It can improve quality of indoors users' phone calls and date service by enhancing signal coverage. It integrates 2G, 3G and wifi as a whole and can connect to mobile networks through DSL, cable broadband and optical fibres. An advantage is that it enlarges signal coverage to where macrocell can not reach.
However, as more and more femtocells will be used, performance and capacity of macrocell will be impacted. New methods to reduce these impacts will be developed. On the other hand, femtocells will also have influence on each other. Because of unknown number and locations of fentocells, traditional ways of planning and optimizing network structure will be useless. So how to mitigate interference between femtocells and macrocells and also between femtocells becomes a big issue for engineers to solve.
Interference just between femtocells is called co-layer interference because it is caused by femtocells in the same network layer. Most co-layer interferences occur in neighbors because of limited signal range of femtocell. In terms of downlink, main problems are femtocell coverage holes in both CDMA and OFDMA and noise rising at femtocell user equipment (UEs) in CDMA. As to uplink, problems are noise rising at the Femtocell Access Point (FAP) in CDMA and intercarrier interference at the FAP in OFDMA.
Interference between different layers, for example, between femtocells and macrocells, is called cross-layer interference because two network layers lead to this kind of interference. Problems caused by cross-layer interference are more complicated than those caused by co-layer interference. In uplink, problems of CDMA are noise rising at both the FAP and macro base station. Problems of OFDMA are intercarrier interference at the FAP and risk of invalidation of macrocell subchannels. While in downlink, femtocell coverage hole happens both in CDMA and OFDMA. Indoors and outdoors macrocell coverage hole and intercarrier interference at the macrocell users occur in CDMA and OFDMA respectively.
So far there are some main approaches solving these problems. In CDMA engineers propose two main methods including power limits in femtocell user equipment (UEs) and adaptive power control. Two main solutions in OFDMA are orthogonal channels assignment and intelligent subchannels allocation.
This project aims to simulate femtocell network in different models and environment with OPNET software and analysis performances of different simulation results. Then make some improvement of exist approaches. So far a paper named 'OFDMA Femtocells: A Roadmap on Interference Avoidance' and a book named 'Femtocells Technologies and Development' written by Jie Zhang have been read already. The guidebook of the software called OPNET is being studied. Remaining job is to operate this software skillfully to simulate different femtocell network environments. Understand existing approaches to these problems deeply and try to make some improvement of these methods. Furthermore, may propose a better means to resolve these problems.
3. Background (Literature Review)
The technology of femtocell first came out in Mobile World Congress (MWC) in 2008. It is accepted by mobile operators such as Orange and T-mobile. At first, femtocell was famous in Europe. It is a small cellphone base station. Its signal coverage ranges from 50 meters to 200 meters in order to satisfy users in small spaces like homes and offices. According to surveys , 70% phone calls and data service happen indoors, 20% - 40% in Europe, 40% - 50% in US and 60% in China. Although they are mobile businesses, they happen in regular places. It is estimated that by 2012, there could be around 70 million FAPs installed in homes or offices around the world, serving more than 150 million customers . Consequently, the co-channel deployment of such a large femtocell layer will impact existing macrocell networks, affecting their capacity and performance . Problems caused by interference between co-layer and cross-layer need to be solved to provide customers better service. Interference avoidance becomes a big issue for engineers.
A femtocell access point (FAP) looks like a wifi access point (WAP), but they are totally different. WAP implements wifi technologies such as IEEE 802.11b, 802.11g, and 802.11n. FAP implements cellular technologies such as GSM/GPRS/EDGE, UMTS/HSPA/LTE and mobile WiMAX (IEEE 802.16e). It also provides 2G, 3G and wifi signals and gets access to the Internet through DSL, cable broadband or optical fibres and connects to mobile operators' networks. It does not need a cellular core network, see Figure 3.1 .
Figure 3.1 Typical femtocell and macrocell scenario
Femtocell needs cellular technologies. The aim of femtocell is to provide higher indoors data rates. Nowadays the main trend is UMTS and HSPA, while it also uses GSM, GPRS and EDGE. Providers have developed 2G/3G femtocells and equipment based on LTE is being developed.
Because femtocell access point (FAPs) can be installed by users rather than professional workers, it requires strong ability of self-optimization. When it is first installed, it can download configuration information and software itself to make it run properly. When it is running, it will detect surrounding radio environment and reset its configuration in order to minimize interference which can impact outdoor macrocells and femtocells in neighborhood. As a result, the ability of self-configuration is one of the most important indexes to assess the performance of a femtocell.
A large deployment of femtocell is expected in 2012  (see Figure 3.2 and Figure 3.3).
Figure 3.2 Global femto station infrastructure equipment market forecast (Data from www.wirelessweek.com)
Figure 3.3 Total 3G femtocell deployment market in 2012 (Date from www.fwdconcepts.com/femtocell)
Some reasons why femtocell is very important are as followings:
1) Cost of femtocell is low.
2) It provides better signal quality indoors.
3) It provides higher data rates.
4) It releases pressure from macrocells and make capacity of macrocell better.
5) Its ability of self-configuration makes it easy to install and run.
6) Operators can control it through Internet.
On the other hand, the topology of conventional cellular networks is changed by the deployment of femtocells. Now the architecture of new network is consist of two layers including macrocell layer and femtocell layer. The macrocell layer is planned and built by mobile operators, while the femtocell layer is distributed randomly by users. Macrocells are like umbrellas sited on the ground covering most area and femtocells extends signal coverage to spaces which cannot be covered by macrocells.
The topology of two-layer networks brings, however, new problems and creates new design challenges. When several transmitters emit their signals in the same frequency band and within the same geographic location, a receiving system sensing that frequency band will not be able to distinguish which transmitter it is listening to. This is a very elementary description of the problem of interference in telecommunications systems and it is one of the main challenges that the deployment of femtocells will face . Both Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiple Access (OFDMA) systems will be affected seriously by femtocells. So interference avoidance techniques need to be developed in order to decrease the negative influences.
A kind of interference is co-layer interference, which is produced in the same layer. Most co-layer interference happens between femtocells in neighborhood because they are installed close to each other. A diagram summarizing the main problems originating from co-layer interference is presented in Figure 3.4 .
Figure 3.4 Main problems caused by co-layer interference
Location in which a femtocell is installed is based on users; several femtocells may locate close to each other. So a femtocell may affect its neighboring femtocells. For a femtocell using Global System for Mobile communication (GSM), if its neighboring femtocell use the same frequency with it, its performance may decrease. Furthermore if many femtocells use the same frequency, the interference is very huge, may be higher than the transmitting power. The area suffering from severe interference and having low value of the Carrier to Interference and Noise Ratio (CINR) in which communication cannot be set up is called dead zone. Size of dead zone depends on CINR. Figure 3.5  gives an example of dead zones based on different values of CINR.
Figure 3.5 Downlink coverage areas of femtocells in a residential environment with absence of macrocell coverage. The dark squares indicate the location of the FAPs and the black areas are regions where the CINR requisite is not met by any femtocell. (a) Coverage areas if CINR>0dB.
(b) Coverage areas if CINR>10dB
Co-layer interference in femtocell consists of downlink and uplink. If some femtocells are installed in the same area and begin downlink at the same time, one femtocell may cause interference to user equipment of other neighboring femtocells. It is the same in uplink as in downlink.
The main problem in neighboring CDMA uplinks is the rise in the noise level due to the spatially distributed nature of the interferers . If user equipments need to transmit with high power, this may result in rising noise level in neighborhood, thus size of dead zone becoming bigger. The use of interference management technique is recommended in  by 3GPP to cope with this problem.
Because of huge amount of femtocells in some area, power limits on user equipment is a good way to reduce interference. The noise level can be controlled in femtocell environment. Transmit power of user equipment should be controlled by its femtocell rather than itself. Femtocell will gather information by detecting signal power transmitted by neighboring femtocells.
In uplink OFDMA, femtocells will choose subchannels which are not interfered by other femtocells to transmit signals. Figure 3.6  indicates why femtocell access point need to choose transmit subchannels instead of user equipment.
Figure 3.6 Co-layer uplink interference in an OFDMA femtocell network
In this OFDMA femtocell network, user1 and user2 transmit in the same subchannels. User2 is in coverage of femtocell1 and femtocell2 while user1 is just in coverage of femtocell1. As a result, user2 will know which subchannel is interfered by user1 while user1 will not. User2 can connect to femtocell2 and user1 will be interfered.
In downlink, high transmit power and transmitting at the same time will cause interference in CDMA. In OFDMA, interference will appear when femtocells use same subchannels to transmit data.
Cross-layer interference is described as interference happens in two layers and equipment which is affected belongs to two different layers. Figure 3.7  illustrates the main problems caused by cross-layer interference.
Figure 3.7 Main problems caused by cross-layer interference
In uplink CDMA, two situations will cause cross-layer interference. Firstly, a user is in coverage of a macro-NodeB and a femtocell which is Closed Subscriber Group. The user cannot joint the femtocell group so he connects to macro-NodeB to create communication. The power of femtocell is greater than that of macro-NodeB in this area. So the macro-NodeB is interfered. Another case is that when a user in edge of macro-NodeB coverage, it needs high transmit power to connect to marcrocell, this may interfere femtocells near the user.
In uplink OFDMA, when users belonging to femtocells or macrocells use same subchannels to get communication, this may cause interference. If they use different subchannels to transmit, they will be free from interference.
Situations of downlink are similar to those of uplink. Noise level affects performance of CDMA and subchannels allocation is key point of reducing interference in OFDMA.
In terms of CDMA time-hopping is thought of an appropriate way of reducing interference in uplink of cross-layer. The period of CDMA transmission is T which is divided into N hopping slots, and femtocell user equipment choose one of N slots randomly to transmit. This method reduces the interference between different femtocells (co-layer interference) and between femtocells and macrocells (cross-layer interference) by a factor of N . Another method called joint hopping is that all users belong to the same femtocell transmit in the same time because there are a lot of user equipments. These approaches may succeed because each femtocell transmits just in its time slot without disturbing other femtocells. If time slots are not enough, two femtocells may transmit in the same time slot as long as they are far from each other. Even though there is a risk of producing interference between these two femtocells, long distance between them will not make this happen.
Another method suggested in  is using antennas with N sectors in femtocell access points in order to reduce cross-layer interference. This approach can enhance reception powers in uplink. However complicated antenna will raise the cost of a femtocell access point.
Compared with CDMA, OFDMA systems have the advantage of providing two dimensions (time and frequency) for the management of radio resources. They therefore provide a much higher versatility for the design of interference avoidance techniques . It is easier to sensing radio environment around because of limited coverage of femtocell. And femtocells will choose more appropriate subchannels to transmit. In order to reduce cross-layer interference in two-layer networks, some companies  have opted for a split spectrum approach (see Figure 3.8). This assumes the division of the available spectrum between the two layers, i.e. all of the operator's macrocells will use one set of subchannels, while the femtocell layer will use the remaining ones .
Figure 3.8 Assignment of F frequency subchannels in a two-layer OFDMA network. FM subchannels will be used by the macrocells, while Ff subchannels will be assigned to the femtocell network layer 
In terms of macrocells,  analyses and presents two schemes of subchannels allocation: Round Robin (RR) and Proportional Fair (PF). In the Round Robin scheme, the users of the macrocells are allocated the same length and same order transmission time slots. After the last user's transmission time slot, the transmission round goes to the first user and the cycle starts over again. In the Proportional Fair (PF) scheme, the transmission order of users depends on their throughput needs. Users who have a higher transmission rate will thus transmit first. A lot of simulations and analytical results indicate that the subchannel throughput in macrocell layer of the Proportional Fair (PF) scheme doubles.
From the point of view of femtocells, it has been shown that at a certain femtocell density, the Area Spectral Efficiency of a femtocell will reach a maximum value. The throughput of a femtocell depends on the femtocell density in certain area.
4. Research methods
This project involves several aspects. First is literature review. Books, papers and many materials should be read to know the background of LTE and femtocells. The latest knowledge of femtocells can be acquired from these materials. The second step is to get information from the 3rd Generation Partnership Project (3GPP) which is collaboration between groups of telecommunications associations, known as the Organizational Partners . New technologies and trends of femtocells can be learnt from 3GPP. Some simulation will be done and then focus on interference avoidance. Teamwork also is recommended in order to help each other and make better understanding about this project.
Get info from 3GPP
The technology of femtocell and LTE becomes more and more popular. Some mobile operators such as T-mobile have released their LTE equipment for business and many mobile operators will sell their femtocell access points. As thousands of femtocell access point will be settled, it is a challenge to solve problems caused by interference. This project aims at finding approaches to resolve problems caused by interference between femtocells and between femtocells and macrocells. It is quite a useful project that will contribute to users and mobile operators.